# The Swift UVOT Stars Survey. III. Photometry and Color-Magnitude Diagrams of
103 Galactic Open Clusters

Michael H. Siegel  Pennsylvania State University, Department of Astronomy
525 Davey Laboratory
University Park, PA, 16802  Samuel J. LaPorte  Pennsylvania State University,
Department of Astronomy
525 Davey Laboratory
University Park, PA, 16802  Blair L. Porterfield  Pennsylvania State
University, Department of Astronomy
525 Davey Laboratory
University Park, PA, 16802  Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD, 21218  Lea M. Z. Hagen  Pennsylvania State University,
Department of Astronomy
525 Davey Laboratory
University Park, PA, 16802  Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD, 21218  Institute for Gravitation and the Cosmos
The Pennsylvania State University
University Park, PA 16802  Caryl A. Gronwall  Pennsylvania State University,
Department of Astronomy
525 Davey Laboratory
University Park, PA, 16802  Institute for Gravitation and the Cosmos
The Pennsylvania State University
University Park, PA 16802

###### Abstract

As part of the Swift/UVOT Stars Survey, we present near-ultraviolet (3000-1700
Å) point-source photometry for 103 Galactic open clusters. These data, taken
over the span of the mission, provide a unique and unprecedented set of near-
ultraviolet point-source photometry on simple stellar populations. After
applying membership analysis fueled mostly by GAIA DR2 proper motions, we find
that 49 of these 103 have clear precise CMDs amenable to investigation. We
compare the CMDs to theoretical isochrones and find good agreement between the
theoretical isochrones and the CMDs. The exceptions are the fainter parts of
the main sequence and the red giant branch in the $uvw2-uvw1$ CMDs, which is
most likely due either to the difficulty of correcting for the red leak in the
$uvw2$ filter or limitations in our understanding of UV opacities for cool
stars. For the most part, our derived cluster parameters – age, distance and
reddening – agree with the consensus literature but we find a few clusters
that warrant substantial revision from literature values, notably NGC 2304,
NGC 2343, NGC 2360, NGC 2396, NGC 2428, NGC 2509, NGC 2533, NGC 2571, NGC
2818, Collinder 220 and NGC 6939. A number of these are clusters in the third
Galactic quadrant where previous studies have mistaken the disk sequence for
the cluster. However, the GAIA DR2 proper motions clearly favor a different
sequence. A number of clusters also show white dwarf and blue straggler
sequences. We confirm the presence of extended main sequence turnoffs in NGC
2360 and NGC 2818 and show hints of it in a number of other clusters which may
warrant future spectroscopic study. Most of the clusters in the study have low
extinction and the rest are well fit by a “Milky-Way-like” extinction law.
However, Collinder 220 hints at a possible “LMC-like” extinction law. We
finally provide a comprehensive point-source catalog to the community as a
tool for future investigation.

open clusters:general; stars:general; stars: early-type

##  1 Introduction

Open clusters are gravitationally bound collection of $10^{2}$-$10^{4}$ stars
born in the same star formation event. They are, by far, the most abundant
family of clusters in the Milky Way with over 1500 cluster or cluster
candidates having been cataloged (Dias et al. 2002) Galactic open clusters are
predominantly young and metal-rich – most of the older clusters having long
since dissolved into the Galactic field. However, a number of old and metal-
poor clusters have been detected and serve as “fossils” for studying the early
Galaxy.

As compact collections of stars of similar age, abundance, distance and
reddening, individual star clusters present a snapshot of stellar evolution.
Studying large samples of clusters allows us to piece together the narrative
of stellar evolution and constrain the photometric properties of stars as
functions of mass, age and chemistry. Much of our understanding of stellar
evolution and photometry comes from the study of Galactic open and globular
star clusters. Cluster of all ages and chemistries are known to host stars
that are bright in the ultraviolet (UV), as shown in Siegel et al. (2014,
hereafter Paper I). These UV-bright stars in the nearby universe have
counterparts in the UV-bright stars seen in nearby galaxies and are,
ultimately, parallels to the stars contributing to the UV light of distant
unresolved stellar populations. Any understanding of luminous hot UV-bright
stars and stellar populations dominated by them will therefore have to lay its
foundation on the study of nearby star clusters. They are not only the best
repositories in which to find such stars, but having found them, the
properties can be immediately connected with stellar populations of known age,
distance, reddening and chemistry.

The study of the UV properties of star clusters is still in its early stages,
as detailed in Paper I. But there a number of recent results that indicate
this is a fertile field of study. For example, studies of open clusters in the
Magellanic Clouds have shown complex structure in the main sequence turnoff
(MSTO) – an extended MSTO (eMSTO) and split MSTO’s (Mackey & Broby Nielsen
2007; Milone et al. 2009; Goudfrooij et al. 2011). This split has also
recently been identified in a number of Galactic clusters (Marino et al. 2018.
Cordoni et al. 2018) This fine structure is thought to the be the result of
stellar rotation, which can affect the evolutionary lifetimes of stars, and
may show up more clearly in the UV. A recent comprehensive study of M67
(Sindhu et al. 2018) identified numerous blue straggler stars (BSS), white
dwarf binaries and stars that had excess far-UV emission consistent with
either unresolved binarism or chromospheric activity.

In addition to their utility for studying stellar evolution and the properties
of rare phases of stellar evolution, Galactic open clusters can be used as the
building blocks for studying larger astrophysical questions. Open clusters,
for example, can be used to probe the chemodynamical properties of the
Galactic disc (see, e.g., Friel 1995, Twarog et al. 1997, Yong et al. 2005,
2012; Frinchaboy & Majewski 2008). Old open clusters, with ages greater than 1
Gyr, are particularly suited to tracing the chemical evolution of the disk
through their age-metallicity relationship (AMR). Indeed, the similarity
between the AMR of a small group of open clusters and those of merging dwarf
galaxy systems has been key to confirming past Galactic mergers (Frinchaboy et
al. 2006). Open clusters may be particularly useful for probing the UV
extinction properties of nearby dust. As we have noted before (Paper I, Hagen
et al. 2017, 2019), the UV extinction curve is uncertain, both in slope and in
the strength of the “blue bump” at 2175 Å (Stecher 1965; Seaton 1979; Gordon
et al. 2003). Open clusters can serve as probes of dust properties on small
and large scales.

Of particular interest to recent scientific investigations is the utility of
clusters for studying the properties of complex stellar populations. Open
clusters are nearby, meaning they can be probed on a star-by-star basis to
determine age, abundance, foreground reddening and distance. However, they can
also be studied through their integrated light. By combining these two
measures, one can calibrate the study of more distant stellar populations that
cannot be resolved on a star-by-star basis but can only be studied though the
properties of their integrated light (see an illustration of this in the
context of the Magellanic Clouds in Searle et al. 1980). Studies of the
integrated light of unresolved stellar population have proven extraordinary
useful for untangling the star-formation histories and dust extinction
properties of both nearby and distance galaxies (see, e.g., Hoversten et al.
2011, Calzetti et al. 2015; Hagen et al. 2017, 2019). However, these studies
have a limitation: the UV portion of the spectral-energy distribution is not
as well constrained as the optical and infrared. The lack of empirical
information needed to calibrate synthetic spectra of unresolved stellar
populations was specifically cited by Bruzual (2009) as a limitation on the
utility of the models.

Despite the tremendous efforts made by investigators, most of the Milky Way’s
open clusters have not been studied in great detail at any wavelength. The
sheer number of open clusters in the Galaxy precluded most from being targeted
for individual study. Indeed, many of the parameters used by the WEBDA online
database of open clusters111http://www.univie.ac.at/webda/ come from global
optical-infrared (OIR) surveys of hundreds of clusters such as that of
Kharchenko et al. (2013 and 2016, hereafter K13 and K16, respectively). But
there are additional issues that complicate the study of open clusters, most
notably their preferred location within the Galactic midplane, with
concomitant heavy foreground reddening and field-star contamination.

This deficit of individual study is particularly acute in the UV where very
few clusters have been studied in detail (see discussion in Paper I). This
lacuna is the natural apotheosis of the factors that limit their study in the
optical: their location within the Galactic midplane makes study difficult
either due to crowding (most previous UV missions had coarse spatial
resolution), field star contamination, foreground extinction (which is much
stronger in the UV) or simple brightness constraints that preclude missions
like GALEX from studying objects in the Galactic midplane.

As a result of this, the spectral synthesis models used to study distant
unresolved stellar populations are still not well-constrained in the UV. And
this is particularly problematic because young stellar populations – those
most visible over the long distances in extragalactic astronomy and cosmology
– emit a large fraction of their rest-frame light in the UV. The lack of
thorough, systematic and empirical study of young UV-bright stellar
populations is a significant and limiting problem for extant and planned
ultraviolet surveys of the Milky Way, the Magellanic Clouds, Local Group
objects, nearby galaxies and distant galaxies. Understanding the star-by-star
and integral photometry of all these distant and extragalactic systems – from
the nearby to the deep universe – is critically dependent on studies of nearby
open clusters.

In this contribution, we address this deficit in our knowledge of Galactic
open clusters with a survey of 103 open clusters observed in the near-
ultraviolet (NUV) with the Neil Gehrels Swift Mission’s Ultraviolet-Optical
Telescope (UVOT). By combining Swift’s wide-field moderate resolution
photometry with precise astrometry from the recent GAIA DR2 catalog, we are
able to study these clusters in unprecedented detail, testing the utility of
theoretical isochrones and either confirming or revising the literature
parameters for these clusters.

We present the observational details of the program in §2. We then detail the
limitations on our analysis produced by saturation of the brightest stars and
contamination from the Galactic disk in §3. Using some radial velocities but
mostly GAIA DR2 proper motions to establish membership, we are able to
identify 49 clusters that have clear and distinct color-magnitude sequences,
many of which have never been the target of detailed study. We present these
color-magnitude diagrams (CMDs) along with comparison to theoretical
isochrones, with which we measure or revise their fundamental properties. The
resultant test of the theoretical isochrones shows excellent agreement,
indicating that the models perform well in the UV. We show that the isochrones
have a tendency to not match the photometry of cooler stars – red giant branch
(RGB) and late-type main sequence (MS) stars. We finally look ahead to other
uses for this dataset, which we will provide to the community.

##  2 Observations and Data Reductions

###  2.1 UVOT Data

UVOT is a modified Richey-Chretien 30 cm telescope that has a wide (17’
$\times$ 17’) field of view and a microchannel plate intensified CCD operating
in photon counting mode (see details in Roming et al. 2000, 2004, 2005) aboard
the Neil Gehrels Swift Gamma Ray Burst mission (Gehrels et al. 2004). The
instrument is equipped with a filter wheel that includes a clear white filter,
$u$, $b$ and $v$ optical filters, $uvw1$, $uvm2$ and $uvw2$ ultraviolet
filters, a magnifier, two grisms and a blocked filter. Although its primary
mission is to measure the optical/ultraviolet afterglows of gamma ray bursts,
the wide field, 2$\farcs$3 resolution, broad wavelength range (1700-8000 Å)
and ability to observe simultaneously with Swift’s X-Ray Telescope (XRT;
Burrows et al. 2005) allow a broad range of science, including the study of
hot or highly energetic stars. It is particularly ideal, in the context of hot
stars, for studying nearby star clusters. Its wide field can enclose most
nearby open clusters in a single pointing and its resolution allows
measurement of stars almost to the center of the most crowded fields. The NUV
filters – with effective wavelengths of 2600, 2246 and 1928 Å for $uvw1$,
$uvm2$ and $uvw2$, respectively, provide good coverage of the NUV wavelength
range and sensitivity to the 2175 Å blue bump.

The open clusters surveyed by UVOT are drawn from a variety of programs. The
bulk were observed as part of M. Siegel’s approved Cycle 12 GI and team fill-
in programs, observed optimally for 6 ks in UVOT’s three NUV filters
(although, in practice, the observing time tended to be closer to 5 ks).
Others were observed as calibration targets or coincidentally with other
studies. We have only included clusters in the sample if they were observed
for at least 0.8 ks in two of UVOT’s three NUV filters and where the field
center was within 5’ of the nominal cluster center, although some exceptions
have been made to expand the sample. The clusters in our GI and fill-in
program were generally selected to be comparable to the UVOT field-of-view,
with low reddening ($E(B-V)<0.5$) and locations away from Galactic Center
region, which is the most heavily oversubscribed part of the sky for Swift
observations.

The list of 103 clusters is given in Table 1. The table lists the exposure
time in UVOT’s three NUV passbands as well as its broadband $u$ passband. All
exposure times are given as the maximum exposure time on the combined image,
which removes deadtime losses. Most of the cluster images were created by
combining any image within 7$\farcm$5 of the cluster center. However, some of
the first images processed in the program used a much larger search radius in
an effort to provide data on the clusters with the richest history of prior
observations. All clusters were processed and photometered through the
pipeline described in Paper I, updated to the most recent calibrations
(Breeveld et al. 2011).

While this particular contribution focuses on the clusters with the cleanest
color-magnitude diagrams, we have created photometric catalogs for all 103
clusters. The photometric catalogs include calibrated PSF photometry in all
available passbands, including broadband $b$ and $v$ when available. They have
been filtered of objects with DAOPHOT (Stetson 1987, 1994) structural
parameters of $|SHARP|>0.5$ to remove blends and detections near bright
saturated stars (see Paper I for more information). The catalogs are currently
available on request but will eventually be made available through the MAST
archive.

Of the clusters in the sample, all but one have $uvw1$ data, all but one have
$uvm2$ data and all but three have $uvw2$ data. Only two clusters – Hogg 16
and Westerlund 2 – lack $uvm2$ and $uvw1$ data. Our sample includes few
clusters with very young ages ($t<100$ Myr). The reason for this is that very
young clusters tend to be either far too bright or far too enshrouded in dust
for UVOT study.

Table 1: Swift/UVOT Observations of Open Clusters

Cluster | u Exp | uvw1 Exp | uvm2 Exp | uvw2 Exp | Observation
---|---|---|---|---|---
| Time (ks) | Time (ks) | Time (ks) | Time (ks) | Dates
Blanco 1 | 0 | 3378 | 2461 | 3071 | 2010-11-10 – 2011-01-25
NGC 188 | 1195 | 2842 | 3111 | 4555 | 2005-06-29
$\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 2005-10-13
$\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 2007-10-30 – 2007-12-06
NGC 752 | 0 | 798 | 1032 | 1076 | 2011-07-31 – 2011-09-06
IC 1805 | 0 | 862 | 926 | 926 | 2008-04-06
NGC 1039 (M 34) | 0 | 2137 | 2206 | 2206 | 2011-03-08
$\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 2011-07-10 – 2011-07-14
NGC 1896 | 0 | 1717 | 1614 | 1614 | 2013-10-27
NGC 1996 | 0 | 3490 | 3428 | 3507 | 2013-10-20 – 2013-10-26

This table is available in its entirety in machine-readable and Virtual
Observatory (VO) forms.

##  3 Analysis and Results

###  3.1 Bright Star Issues

Most of our cluster fields are close to or within the Galactic midplane. They
also tend to be young ($t<1$ Gyr) and nearby ($m-M<14$). As a result, a number
of cluster fields have numerous bright saturated stars. “Saturated”, in the
case of a photon-counting instrument like UVOT, does not mean “saturated” in
the same sense as a CCD detector. What this means is that the incident count
rate exceeds 372 sec-1, the limit at which UVOT can no longer count incident
photons (corresponding to AB magnitudes of 12.52, 12.11 and 12.68 in the
$uvw1$, $uvm2$ and $uvw2$ filter, respectively). Thus, only crude lower limits
can be applied to the brightness of these stars. Even before reaching such a
high level, however, the coincidence loss becomes very strong and the dead-
time of the detector for incident photons will result in dark boxes around the
saturated cores of bright stars (Poole et al. 2010). This lowers the limit at
which useful photometry can be measured to well below 372 sec-1. This is
particularly noticeable with PSF photometry as very high coincidence loss both
changes the shape of the PSF for bright stars and can cause DAOPHOT to
underestimate the background sky level. In the discussions below, the phrase
“saturated” stars will be applied to both contexts and has the general meaning
of a star whose coincidence loss is too strong for reliable PSF photometry.
While Page et al. (2017) recently outlined a method to measure photometry from
saturated stars using readout streaks, this method was not appropriate for
most of our saturated stars as it would not produce the precision needed to
constrain theoretical isochrones. We have noted, within the text, several
clusters where the bright end of the MS is saturated and we can only provide
upper limits on the age of the clusters. Two clusters that were observed for
our program – NGC 2547 and NGC 2516 – are not included in the sample of 103
presented here because of bright star contamination problems that made
photometry of any kind impossible.

The other complication of bright stars is that they produce diffuse light
which may affect photometry (see extended discussion in the context of M 31 in
Hagen et al. 2019). This can include internal reflection, donuts and read-out
lines. However, while scattered light is a problem for the photometry of
extended sources, we have not found it to be a problem for point sources,
which are measured from a local sky. Our comparison of UVOT photon-counting
photometry to ground-based CCD photometry in Paper I showed that our methods
produces reliable linear photometry for all but the brightest stars.

###  3.2 Cluster Membership

While some of the clusters in our sample – such as M 67 – are above the
Galactic plane, the vast majority are within the plane ($\mid b\mid<10$). This
can create a problem in that the Galactic field star population is very
populous in these fields, potentially overwhelming the cluster population or
confounding analysis of its color-magnitude sequence. In some fields, the
correlation between distance/reddening/age causes the field stars to vaguely
resemble a MS (see, e.g., NGC 2360 in §3.4.13). And even in the most spartan
fields, it is difficult to identify rare phases of stellar of evolution such
as blue stragglers or white dwarfs, based on photometry alone.

We approached this problem on several fronts. Earlier proper-motion samples
proved too imprecise in most cases as the cluster proper motions were within
the field star distribution and the studies lacked the precision to tease them
out. Spatial analysis – using the parameters of K16 to select stars within the
core or half-light radii, proved effective for massive clusters but not for
small ones. Radial velocity surveys – such as that of Mermilliod et al. (2008,
hereafter MMU) proved useful but are rarely extensive enough to cover much of
the cluster and were primarily focused on bright red RGB stars. Such stars are
ideal for optical studies but are faint and poorly constrained in the UV.

Ultimately, we settled on using the proper-motion data provided in the recent
GAIA Data Release 2 (Gaia Collaboration 2016, 2018). The GAIA data are
uniform, precise and have a depth similar to that of UVOT. Cantat-Gaudin et
al. (2018, hereafter CG18) recently published a comprehensive membership
analysis of 1212 clusters – including almost all of our program clusters –
using five-dimensional phase-space information. We have used their membership
analysis for cluster membership in the UVOT data, identifying as members stars
with probabilities greater than 90%. For a handful of clusters, however, we
found that the CG18 study only identified a small number of member stars. For
these clusters, we did our own membership analysis, as detailed in the text.

Figure 1: GAIA DR2 vector-point (VPD) and Swift/UVOT color-magnitude diagrams
of the old open cluster M 67 compared the PARSEC-COLIBRI isochrones fit in
Paper I. The solid red squares show astrometric members selected from GAIA DR2
while the open red squares show radial-velocity members selected from Geller
et al. (2015).

Figure 1 demonstrates the utility of the GAIA data in the context of M 67. The
left panel shows a vector-point diagram (VPD) of the M 67 field. The dispersed
population toward the center is the Galactic disk while the concentrated
cluster of points off center is the cluster. Almost all of the radial-velocity
members from the recent survey of Geller et al. (2015) land within the offset
clump. For the majority of the open cluster the separation between cluster and
field was not quite so dramatic. More often, the cluster proper motion was
within the field star distribution. However, the GAIA DR2 parallaxes and
proper motions are so precise that the cluster members still show up a very
tight clump of stars within the more dispersed Galactic background and can be
easily disentangled from the field star population.

After extensive analysis, we found that 49 of our 103 target clusters could
both easily be distinguished from the field star population and showed a clear
sequence in the color-magnitude diagrams. We note that there are a number of
clusters that show clear clumps of cluster stars in the GAIA data itself but
fail to cross-match enough stars to the UVOT data for analysis, either because
the clusters are too faint or are too spatially dispersed. The photometry
catalogs included all photometric measures for both members and non-members.

###  3.3 Isochrone Fitting and Analysis

The goal of this paper is to compare the point-source photometry of the
clusters to theoretical isochrones in order to test the utility of the
isochrones (and, by extension, the atmospheric models underpinning them) in
the context of young to intermediate age stellar populations. The isochrones
chosen for this exercise are the PARSEC-COLIBRI isochrones of Marigo et al.
(2017, hereafter M17). These isochrones cover the entire range of ages and
metallicities of the open cluster sample. The most significant revisions of
this iteration are to the thermally pulsing asymptotic giant branch (TP-AGB)
which is not relevant to our analysis as these stars are too cool for good
constraint with UVOT. in fact, the detailed treatment of this phase cause the
isochrones to loop around as the theoretical stars’ UV emission (actually,
their red leak into the UV passbands, see Appendix to Paper I) waxes and wanes
during their TP-AGB phases. For the figures below, the TP-AGB phase has been
removed from the isochrones for the sake of clarity.

Isochrones were initially laid down using literature values - from studies
specific to the individual cluster if possible, from K16 where no previous
focused study had been made. The isochrones were then adjusted interactively
to better overlap the GAIA-selected sequences, with variation allowed in age,
metallicity, reddening, distance and reddening law (either Galactic, SMC or
LMC, based on the formulations of Pei 1992). The main degeneracy seen in the
UV isochrone fitting involved metallicity. With all the parameters allowed to
be free, the metallicity of any cluster became unconstrained. Any change in
metallicity could be accommodated by responsive changes in distance modulus
and reddening. We therefore fixed the metallicity of the clusters to
literature spectroscopic or photometric values, where available, and solar
metallicity otherwise. For the most part, we ignored the RGB stars since very
few are detected in the NUV and those that are tend to be faint and dominated
by red leak. We do note below a few clusters such as NGC 2477 (§3.4.24) where
the RGB is prominent.

Figure 2: A comparison of different isochrone fits to the photometry of the
globular cluster NGC 2360. The three lines are all set at a metallicity of
[M/H]=-0.1 and a distance modulus of $m-M=10.05$. However, they set at
different reddening values, with age adjusted to better match the MSTO. Note
that increasing or decreasing the reddening has opposite effects in the two
CMDs. A lower (higher) reddening moves the isochrone below (above) the
observed MS in the $uvm2-uvw1$ CMD and above (below) in the $uvw2-uvw1$ CMD.
This allows the reddening to be easily constrained by the observational data.

Other parameters, however, were easily constrained. The UV photometry proved
particularly adept at measuring foreground reddening. Due to the differing
orientations of the reddening vector – mostly blueward in the $uvm2-uvw1$
diagram and faintward in the $uvw2-uvw1$ diagram, an incorrect reddening
produces a discrepancy in the location of the MS for a given distance modulus.
With too much reddening, for example, the $uvm2-uvw1$ isochrone tends to end
up above the MS while the $uvw2-uvw1$ isochrone tends to end up below (Figure
2). Only the correct reddening value allows both isochrones to line up with
the MS. This allowed age and distance modulus to be varied independently to
match the MSTO. We are therefore confident in our measures of foreground
reddening, age and distance given the assumed metallicity.

Most of our program clusters were deliberately chosen to have low foreground
reddening, which precluded any analysis of the reddening law itself (set, by
default, to the Milky Way reddening law). However, for a few clusters, we were
able to explore this issue to a modest extent (see, e.g., Collinder 220,
§3.4.40)

As cataloged in Table 2 and in the notes on individual clusters, the
isochrones performed very well. In most cases, we found that the photometry
was consistent with the predictions of theoretical isochrones set to
literature values. For a number of clusters, however, we had to substantially
revise parameters to produce consistent fits. This was particularly common
among third quadrant clusters in which the literature values sometimes
described the foreground/background disk sequence, rather than the cluster. In
this region of the sky, the reddening along the line of sight causes the “blue
wall” of disk MSTO stars to veer redward as it gets fainter, making it
resemble a MS. However, applying the GAIA astrometric membership (as well as
radial velocity memberships, when available) made it clear that this was not
the cluster sequence, which was well-defined in the GAIA astrometry.

Notes on the individual clusters are detailed below, in order of ascending
Galactic longitude. Because this is a large survey program, we do not go into
great detail on our program clusters, many of which are worthy of a paper of
their own. However, we do comment on discrepancies between our derived values
and those found in the literature, the presence of unusual stellar types in
any cluster and any potential broadening in the MSTO.

###  3.4 Individual Clusters

Figure 3: Color-magnitude diagrams of the open clusters NGC 752, M 34, NGC
2192 and NGC 2204 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

####  3.4.1 NGC 752

NGC 752 is situated well below the Galactic midplane in the second Galactic
quadrant. It is very well-studied, with several cluster members established as
photometric standards and numerous thorough photometric and spectroscopic
surveys in the literature. Its age (1.4 Gyr), detailed abundance ratios and
projected orbit make NGC 752 a typical representative of the family of old
disk clusters (Carraro & Chiosi 1994; Maderak et al. 2013, Böcek Topcu et al.
2015, Twarog et al. 2015). Its similarity to disk red giants is evidence that
much of the disk population formed from disrupted clusters (Reddy et al.
2012a,b).

The UVOT field only encloses the central regions of this dispersed cluster
(K13 measure a radius of 1.4 degrees but a core radius of 3$\farcm$0).
However, the GAIA DR2 membership selection shows a clear MS (Figure 3, first
column). We cross-identify numerous spectroscopic members catalogued by Daniel
et al. (1994), all of which lie within the CG18 selection. The NGC 752 member
stars align almost perfectly with the isochrone predicted by the parameters
given in Twarog et al. (2015), which is expected given how well-studied and
catalogued this cluster is.

####  3.4.2 M 34

M 34 is second quadrant cluster situated well below the Galactic midplane. It
has been extensively studied and shown to be a solar or slightly super-solar
abundance intermediate age (200 Myr) moderately-reddened ($E(B-V)\sim 0.10$)
cluster (Jones & Prosser 1996; Schuler et al. 2003). K13 measure an outer
radius of 43$\farcm$8, indicating that M 34 fills the entire UVOT field and we
are only able to image the central regions of the cluster (K13 measure a core
radius of 9$\farcm$6).

The GAIA astrometry shows a clean separation from the field that contains the
spectroscopic members listed by Meibom et al. (2011) and traces a clear MS
(Figure 3, second column). Comparison to the M17 isochrones yields a distance
and reddening consistent with previous studies. We can not constrain the age
of the cluster as the upper main sequence is saturated on the UVOT exposures.
However, we can apply a lower limit to the age of 320 Myr, given the extent of
the MS we detect. We also show a number of cluster member stars blueward of
the primary sequence. These stars could represent the top of a faint white
dwarf sequence.

The lower end of the MS differs from the isochrones in the ($uvw2-uvw1$)
color-magnitude diagram (lower panel). As shall be seen, this discrepancy is
common in clusters that are close enough for UVOT to probe the lower MS. We
should note that this is near the point at which the red leak begins to take
over the isochrones as these stars are too cool ($T_{eff}<6300K$) to produce
significant UV light. Indeed, the only way to make the isochrone match the
data on the lower MS is to significantly increase the reddening (which causes
the isochrones to miss the upper MS in both panels). As the reddening
increases, these faint stars become “bluer” because the red leak flux in the
$uvw1$ band is extinguished faster than the red leak flux in the $uvw2$ band.
Given the known issues with fitting UV isochrones to faint red stars (§4), we
elected with this cluster (and other clusters demonstrating similar issues) to
be guided by the bright MS.

####  3.4.3 NGC 2192

NGC 2192 is a second quadrant cluster located above the Galactic midplane.
Photometry from Park & Lee (1999) and Tapia et al. (2010) indicate that it is
old (1.3 Gyr) and metal-poor ($[M/H]\sim-0.3$), an assessment shared by the
global surveys of Paunzen et al. (2010) and K16. It does not have a previous
spectroscopic survey.

The astrometric selection results in a clear MS in both CMDs (Figure 3, third
column). We find that the cluster is well-fit by the parameters derived by
previous investigators with a slightly older age of 1.6 Gyr, assuming the
photometric metallicity of $[M/H]=-0.3$. We identify one potential member star
brighter and bluer than the nominal MSTO. This star is identified as a 100%
likely member by CG18 and could represent a blue straggler. The MSTO of NGC
2192 does show some broadening beyond what would be expected from photometric
errors alone. That may indicate that NGC 2192 has an extended main-sequence
turnoff, although it is a bit at the older range for clusters that would have
potential eMSTOs. It could also represent either photometric scatter in an old
faint cluster or differential reddening in a moderately-reddened cluster. Our
analysis shows that differential reddening of even 0.05 magnitudes would be
enough to create the relevant scatter given the high sensitivity of the NUV
filters to reddening. However, there are not enough bright stars in NGC 2192
to measure any spatial variation. Spectroscopic study could resolve this
issue.

####  3.4.4 NGC 2204

NGC 2204 is situated 16 degrees below the Galactic plane in the third
quadrant. It is a slightly-metal-poor ($[M/H]=-0.20$) old (1.6 Gyr) cluster
(Kassis et al. 1997; Jacobson et al. 2011). While it has been surveyed
spectroscopically, these surveys have focused on old red giant stars, most of
which are faint in the UV. Our data do not include the $uvw1$ passband but do
include the redder $u$ passband.

The last column of figure 3 shows the CMDs compared to the M17 isochrones. The
astrometric selection overlaps the member stars from the radial velocity
surveys. Selecting stars as members based on either astrometry or radial
velocity reveals a clear sequence of stars in the CMD. Our isochrone fits to
the cluster favor a slightly older age (2.3 Gyr) than previous studies with
minimal reddening (although slightly larger reddening with an SMC-like
extinction law would also be consistent with the data). Note that a NGC 2204
has a prominent population of blue stragglers, which are all confirmed as
astrometric members. This confirms the detection of BSS by Frogel & Twarog
(1983).

####  3.4.5 NGC 2243

Figure 4: Color-magnitude diagrams of the open clusters NGC 2243, NGC 2251,
NGC 2281 and NGC 2287 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2243 is a third quadrant cluster located well below the Galactic midplane.
Previous studies by Twarog et al. (1997) and Jacobson et al. (2011) have found
it to be old (5 Gyr) and metal poor ($[M/H]=-0.42$). The radial velocity
survey of MMU only measured two member stars, both of which are faint RGB
stars that do not have UVOT counterparts.

Stars selected from CG18 form clean sequences in the CMDs (Figure 4, first
column) that are well-fit by the M17 isochrones with parameters similar to
previous studies, with slightly lower metallicity ($[M/H]=-0.5$) and minimal
reddening. We also identify a number of blue straggler stars that are
astrometric members.

####  3.4.6 NGC 2251

NGC 2251 is a third-quadrant cluster located near the Galactic midplane.
Parisi et al. (2005) and Reddy et al. (2013) have studied the cluster,
revealing it to be slightly metal-poor ($[M/H]=-0.1$), intermediate age (300
Myr) and with moderate reddening ($E(B-V)=0.20$).

The astrometric selection includes one spectroscopic member from MMU. The
member stars show a clear CMD sequence that corresponds almost exactly to the
consensus literature values, with slightly higher reddening ($E(B-V)=0.25$)
and age (Figure 4, second column). We do note that the MSTO is near the
saturation limit but we do not find a large number of saturated stars in the
image, so it likely this represents the true MSTO and thus the true age of the
cluster.

####  3.4.7 NGC 2281

NGC 2281 is positioned 16 degrees above the Galactic midplane in the second
quadrant. Studies by Glaspey (1987) and Netopil (2017) indicate that it is
intermediate age (600 Myr) and solar metallicity. NGC 2281 is also one of the
few clusters to have been previously studied in the UV, using FUV data from
GALEX (Smith 2018). The latter study found that the two-color FUV-optical
sequence of NGC 2281 was slightly offset from that of the Hyades and Coma Ber,
suggesting that it was younger than those two comparison clusters. However,
this conclusion was tentative given the uncertainty in reddening. K13 measure
a radius of 24$\farcm$3 so the UVOT images only cover the center of this low-
density cluster.

The astrometric membership includes one spectroscopic member from MMU and
defines a clear sequence in the CMDs (Figure 4, third column) that corresponds
almost exactly to the consensus literature values, with slightly higher
reddening ($E(B-V)=0.15$). That the reddening is slightly higher than the
value assumed in Smith (2018) would allow NGC 2281’s two-color sequence to
better overlap that of the Hyades and Coma Ber (see their Figure 5, which
assumes $E(B-V)=0.06$). This would be consistent with our estimate of an NGC
2281 age similar to that of the Hyades and Coma Ber.

####  3.4.8 M 41 (NGC 2287)

NGC 2287 (M 41) is a bright nearby second quadrant cluster situated below the
Galactic midplane. Extensive study has shown it to be young (200 Myr)
minimally reddened and with a high binary fraction (Harris et al. 1993, Dobbie
et al. 2012). The UVOT field only covers the central region of the cluster
(K13 measure a radius of 26$\farcm$4) and the data suffered from massive
saturation from its many bright stars, which prevented both adequate PSF
fitting and photometry of some of the brightest cluster members. It also
created numerous false detections around the image artifacts but these were
easily removed by matching to the GAIA DR2 catalog.

The astrometric selection includes a number of radial velocity members from
MMU. Selecting these stars and examining their aperture photometry shows a
clear narrow main sequence which is well-fit by the M17 isochrones with
parameters similar to those derived by previous investigators (Figure 4,
fourth column). The saturation limits our analysis, allowing us to only place
an upper limit (500 Myr) on the age of M 41 and making it impossible to
confirm the eMSTO report by Cordoni et al. (2018). We detect a number of
confirmed astrometric members along a probable white dwarf sequence and show
the deviation in the $(uvw1-uvw2)$ colors seen in several other clusters.

####  3.4.9 NGC 2301

Figure 5: Color-magnitude diagrams of the open clusters NGC 2301, NGC 2304,
NGC 2355 and NGC 2360 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2301 is a bright third quadrant cluster located within the Galactic
midplane. The catalogs of K16 and Paunzen et al. (2010) describe it as
slightly metal-rich ($[M/H]=+0.06$) and with a small amount of foreground
reddening ($E(B-V)=0.06$). It has been extensively studied for variable stars
(see, e.g., Wang et al. 2015). As with NGC 2287, which it bears a striking
similarity to, there is a plethora of bright saturated stars prevented PSF-
fitting photometry. The data presented is aperture photometry cleaned by
matching to the GAIA DR2 and corrected to the same aperture as the PSF
photometry.

The astrometrically-selected CMD (Figure 5, first column) shows clear loci for
the cluster stars. The one member star catalogued by MMU is well within the
proper motion locus and along the RGB, albeit significantly fainter than
expected. The M17 isochrones fit the photometry quite well, providing an upper
limit on the age of 400 Myr and favoring a slightly higher foreground
reddening ($E(B-V)=0.10$).

####  3.4.10 NGC 2304

NGC 2304 is a third quadrant cluster located above the Galactic midplane.
Multiple photometric studies have been done, producing a broad range of
parameters as shown in Table 2 (Ann et al. 2002, Hasegawa et al. 2008, Lata et
al. 2010, Oralhan et al. 2015). We were unable to identify a comprehensive
radial velocity catalog and the cluster was not surveyed by MMU.

The astrometric selection produces a clean CMD (Figure 5, second column). The
M17 isochrones favor a cluster that is intermediate age (900 Myr), moderately
metal-poor ($[M/H]=-0.3$) and slightly reddened ($E(B-V)=0.10$).
Interestingly, our fit is slightly improved if we use an SMC reddening law
(with no blue bump) as opposed to a Milky Way reddening law, although the
difference is very small at such a low level of foreground reddening.

####  3.4.11 NGC 2343

NGC 2343 is a third quadrant cluster situated near the Galactic midplane. Peña
& Martínez (2014) showed the cluster to be young (12 Myr) and metal-poor, with
$[Fe/H]\sim-0.4$ based on $uvby-\beta$ photometry. It would be unusual for
such a young cluster to be so metal-poor. Netopil et al. (2016), by contrast,
estimate a photometric metallicity if $[Fe/H]=-0.03$. Several catalogues,
including WEBDA, list the cluster at $[Fe/H]=-0.3$ but these appear to all
trace back to a photometric metallicity estimate of $[Fe/H]=-0.2$ from Claria
(1985).

Figure 6: Vector-point and color-magnitude diagrams of the cluster NGC 2343
compared to the best-fit isochrones of PARSEC-COLIBRI. The brightest stars in
the cluster are saturated, preventing us from measuring more than an upper
limit on the age of the cluster. The solid red squares show astrometric
members selected from GAIA DR2 while the open red square is a spectroscopic
member from MMU. The starred point marks the star used by Peña & Martínez
(2014) to estimate the metallicity of the cluster.

For unknown reasons, the CG18 catalog of NGC 2343 was among a few third-
quadrant clusters that had very few stars with high cluster membership
probabilities. We therefore created our own membership probability measures by
fitting two Gaussians to the distribution of proper motions in RA-DEC space.
Cluster membership was defined as the ratio of the two Gaussians and members
with ratios greater than 9.0 were identified as likely members. This method
was initially applied to all of our program clusters. For most, it produced
member catalog almost identical to CG18 and we chose to utilized the CG18
membership as it used more information and its membership lists were slightly
more conservative. NGC 2343 was one of the few where the methods disagreed.

Figure 6 shows the vector-point diagram of the cluster and shows a tight clump
of stars toward the edge of the field star distribution. Using these as
cluster members, we find that the metallicity estimate of and Peña & Martínez
are likely in error. The star they identify as 29 and use as their metallicity
estimator is almost certainly not a member of the cluster, being placed well
to the edge of the field star distribution and off of the MS. It is most
likely a metal-poor thick disk or halo star. HD 54387, used as the metallicity
estimator by Claria (1985), is saturated in the UVOT images. Its proper motion
does place it near the NGC 2343 proper motion, however.

The member stars trace a clear MS. Given the uncertainty in the metallicity,
we adopt solar metallicity. If the cluster is indeed slightly metal-poor, the
result would be a shorter distance modulus ($m-M=9.64$). While our photometry
is limited in its ability to measure the age of this cluster and we can only
give an upper limit (the bright star seen in the $uvw2-uvw1$ CMD is
saturated), we find it unlikely that NGC 2343 is a very young cluster, given
the small number of bright saturated stars in the images and the location of
the spectroscopically-confirmed RGB star from MMU. The location of the RGB
star, approximately three magnitudes brighter than the MS, is consistent with
an intermediate-age population. A young population would have a much hotter
and brighter RGB sequence. It is therefore likely that the cluster age is more
in the range of 100-500 Myr, as indicated by our photometry and Netopil et
al., than the 12 Myr measured by Peña & Martínez.

####  3.4.12 NGC 2355

NGC 2355 is a third quadrant cluster located above the Galactic midplane.
Previous investigations have revealed it to be slightly metal-poor
($[M/H]=-0.06$) and of intermediate age (900 Myr, Donati et al. 2015) with
typical disk abundance ratios (Jacobson et al. 2011).

The CMDs (Figure 5, third column) show a fit consistent with previous
investigations, but favoring a slightly higher age of 1.1 Gyr. The MSTO of the
cluster proved difficult to fit precisely with any combination of parameters.
Notably, we find that there is some broadening in the MSTO. This could
indicate an eMSTO but could also be the effect of differential reddening (to
which the NUV is particularly sensitive at the level of $E(B-V)=0.05$). The
cluster may be worthy of future spectroscopic study to determine the nature of
the MSTO broadening.

####  3.4.13 NGC 2360

NGC 2360 is third quadrant cluster located above the Galactic midplane. The
literature on this cluster contains widely different values for the parameters
as shown in Table 2. Reddy et al. (2012) and Sales-Silva et al. (2014) favor a
large distance ($m-M=11.72$) and young age (560 Myr) while Claria et al.
(2008) favors a closer distance ($m-M=10.09$) and older age (1.8 Gyr). This
discrepancy is likely due to the presence of two distinct color-magnitude
sequences in the field, something we have observed in numerous low-latitude
third quadrant clusters, as detailed in a number of following clusters. One of
these represents foreground disk; the other the cluster itself. WEBDA lists
the cluster at the longer distance and younger age.

Previous spectroscopic studies have been mostly focused on red giants, which
our data cannot use to constrain the properties of the cluster, due to their
lack of UV emission. However, the astrometric selection (Figure 5, last
column) clearly aligns with the brighter/redder main sequence. This sequence
is favored independently by the proper motions, parallaxes and spatial
distribution. Fitting the M17 isochrones to this sequence produces a fit
consistent with the Claria et al. result and inconsistent with the
Reddy/Sales-Silva result current listed in WEBDA. We conclude that the
closer/older solution is likely the correct one.

Figure 7: A close-up of the MSTO region of NGC 2360. The panel shows GAIA- and
spectroscopically-selected members. The MSTO region is very broad due to the
combination of eMSTO, convective hook and the beginning of the RGB.

NGC 2360 is one of the few clusters in our sample that is both bright enough
and close enough to explore the fine structure of the MSTO. Milone et al.
(2018) identified it as a cluster with an eMSTO. This region of the CMD is
tricky because it overlaps both the convective hook in the MSTO and the
beginning of the RGB. However, the $uvm2-uvw1$ diagram shows that the MSTO is
broader and more structured than one would expect from a simple stellar
population (the fit isochrone splits the difference between the blue and red
edges of the MSTO). Figure 7 shows a close-up of this turnoff region and it
appears to confirm the Milone et al. (2018) discovery, hinting at a
bifurcation in the MSTO, likely as a result of variation in stellar rotation.

####  3.4.14 Berkeley 37

Figure 8: Color-magnitude diagrams of the open clusters Berkeley 37, NGC 2374,
NGC 2396 and NGC 2420 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

Berkeley 37 is a faint third quadrant cluster situated above the plane. The
only photometric study to date is that Oralhan et al. (2015) who showed to be
distant ($m-M=13.60$) with light extinction ($E(B-V)=0.05$) and an
intermediate age (600 Myr). K16 derive a slightly shorter distance with more
reddening.

The CG18 selection did not pick out many cluster members, likely due to the
similarity of the cluster kinematics to that of the disk. We applied our own
selection criteria using the double-Gaussian method described for NGC 2343
(§3.4.11). The VPD shows a tight clump of stars within the dense field star
distribution. This clump corresponds to a faint intermediate-age main sequence
in the CMD (Figure 8, first column). We measure cluster parameters similar to
that of Oralhan. Our membership survey includes two stars that could be
potential blue stragglers. One of these in the core of the cluster while the
other is in the outskirts. This indicates that the former is likely a genuine
blue straggler while the latter may be a field star contaminant for a cluster
in a very dense region of the sky.

####  3.4.15 NGC 2374

NGC 2374 is third quadrant cluster situated near the Galactic midplane. It has
been poorly studied with no spectroscopic metallicity available. The only
recent photometric study is Carraro et al. (2015), who found it be young (250
Myr) and argued that its position near the Galactic warp makes it consistent
with either a thick disk cluster or an extended thin disk cluster.

MMU only identified one member star – a red giant that is faint and well-
removed from the main sequence. CG18 only identify a handful of astrometric
members. The VPD, however, shows a clear clump of stars well-removed from the
disk sequence that defines narrow photometric sequences in the CMDs (Figure 8,
second column). We derive parameters similar to that of Carraro et al. (2015)
albeit at slightly higher reddening ($E(B-V)=0.15$). Like many third quadrant
clusters, we also see the “second sequence” of the disk.

####  3.4.16 NGC 2396

NGC 2396 is a third quadrant positioned just above the Galactic midplane. It
has never been the focus of an individual study and the only parameters in the
literature come from the comprehensive survey of K16, who showed the cluster
to be nearby ($m-M=8.68)$, of intermediate age (300 Myr) and minimally
reddened ($E(B-V)=0.05$).

The CG18 astrometric selection produces a clear sequence in CMDs (Figure 8,
third column). However, the main sequence is over 2.5 mag fainter than that
predicted by the K16 parameters. The best fit shows a similar age (250 Myr),
slightly more reddening ($E(B-V)=0.15$) and a dramatically larger distance
($m-M=11.12$). Interestingly, the faint end of the $uvw2-uvw1$ sequence does
not show the deviation between data and isochrone that other bright clusters
show even though the underlying parameters of the stellar population are
similar.

####  3.4.17 NGC 2420

NGC 2420 is a third quadrant cluster positioned 19 degrees above the Galactic
midplane. Extensive investigation (Von Hippel & Gilmore 2000, Anthony-Twarog
et al. 2006, Souto et al. 2016) has revealed it to be old (2 Gyr) and slightly
metal-poor ($[Fe/H]\sim-0.14$). It is also one of the few clusters to be
extensively studied in the UV by GALEX (De Martino et al. 2008) and has
extensive radial velocity data in MMU.

Fitting the M17 isochrones to the astrometrically-selected stars using
literature values (Figure 8, last column) produces a reasonable fit but with a
slight discrepancy: the $uvm2-uvw1$ model colors are a little too red and the
$uvw2-uvw1$ colors are a little too blue. A detailed examination of the
photometry, including a direct comparison to the standard Swift/UVOT pipeline
photometry produced by UVOTSOURCE shows no error in the photometry. NGC 2420
give no indication of any chemical abundance anomalies, showing typical metal-
rich disk abundances with solar-type $\alpha$-abundances (Suoto et al. 2016).
Lowering the metallicity to [Fe/H]=-0.3 and decreasing the cluster distance
produces very good isochrones fits. However, the abundance of NGC 2420 is
very-well stablished through multiple spectroscopic studies. Previous research
has indicated some variation in the UV reddening law within external galaxies
and within the Galaxy, particularly the $R_{V}$ value and bump strength
(Siegel et al. 2014, Hagen et al. 2017) but the foreground reddening in NGC
2420 is low and modifying the reddening law produces only minor changes. We
are unable, at this point, to explain the slight discrepancy in NGC 2420
compared to other clusters that have better fits.

We do note, however, that the MSTO does show a bit of broadening, which would
be consistent with an eMSTO. NGC 2420’s 2 Gyr age is toward the lower end of
the range where eMSTO is expected to manifest before magnetic breaking evens
out the stellar rotation rates (Georgy et al. 2019). It would be worth further
investigation to determine if the MSTO stars shows differences in rotation
rates and potentially set the age limits of the eMSTO phenomenon on a more
empirical footing.

####  3.4.18 NGC 2422

Figure 9: Color-magnitude diagrams of the open clusters NGC 2422, NGC 2423,
NGC 2428 and NGC 2437 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2422 is a bright nearby third quadrant cluster positioned near the
Galactic midplane. Studies by Rojo Arellano et al. (1997) and Prisinzano et
al. (2003) have shown it to be very young (80 Myr) with moderate foreground
reddening ($E(B-V)=0.09$). Conrad et al. (2014) measure it as slightly metal
poor ($[Fe/H]=-0.03$) but within uncertainties of solar metallicity. The
cluster proved difficult to study due to the large number of saturated stars
and the concomitant number of false detections in the diffraction spikes. The
sample studied was only of stars the could be matched the GAIA DR2. The
cluster covers a large area of sky – 37$\farcm$8 in radius according to K13 –
and thus we have only imaged the core.

The CMDs (Figure 9, first column) shows a narrow young MS consistent with
literature expectations. The brightest stars in the cluster are badly
saturated, meaning we can only place an upper limit on the age. However, the
reddening and distance are well-constrained. As with other clusters, the faint
end of the $uvm2$-$uvw1$ sequence shows a deviation from the isochrone,
bending more sharply toward the blue than predicted.

####  3.4.19 NGC 2423

NGC 2423 is a third quadrant cluster positioned above the Galactic midplane.
Prior study has revealed it be a intermediate age (1 Gyr) and metal-rich
($[Fe/H]=+0.10$) cluster (Claria et al 2008, Santos et al. 2009, 2012, Paunzen
et al. 2010, Conrad et al. 2014).

Like NGC 2360 (3.4.13, the CMD shows a double sequence of the old thin disk
and cluster (Figure 9, second column). The astrometric selection picks the
brighter sequence and contains the radial velocity members identified by MMU.
The M17 isochrones produces a fit with parameters consistent with the
literature.

####  3.4.20 NGC 2428

NGC 2428 is a third quadrant cluster positioned 16 degrees below the Galactic
midplane. Conrad et al. (2014) show the cluster to be slightly metal-poor
($[Fe/H=-0.15$) with modest reddening ($E(B-V)=0.05$). The global survey of
K16 lists a shorter distance with heavier foreground reddening.

As with NGC 2360 and NGC 2423, the CMD (Figure 9, third column) shows a double
sequence and the WEBDA parameters, taken from Conrad et al, appear to trace
the bluer sequence. The member stars favor the redder sequence and the M17
isochrones give a fit more consistent with that of K16 than Conrad et al.,
indicating that this is the cluster sequence.

####  3.4.21 NGC 2437

NGC 2437 (M 46) is a bright cluster in the third quadrant. Davidge (2013)
shows it to be near solar metallicity, intermediate age (200 Myr) and
moderately reddened. The Conrad et al. (2014) survey identifies is as very
metal-poor, with [M/H] of -0.75, albeit with a very high uncertainty. It would
be unusual, however, for such a young cluster to be so metal-poor. K13 measure
a radius of 34$\farcm$2 indicating that UVOT only images the core of the
cluster.

The astrometric selection of CG18 includes the radial velocity members
identified by MMU and traces a long narrow MS in the CMDs (Figure 9, fourth
column). As noted above, the UV CMDs are not very adept at measuring
metallicity and NGC 2437 is a case in point. The properties of the cluster are
well-constrained at both low and solar-metallicity. At solar metallicity, we
derive fits close to the parameters of Davidge (2013) and K16, with a slightly
higher reddening ($E(B-V)=0.30$) and shorter distance ($m-M=10.84$). If the
cluster were indeed as a metal-poor as indicated in Conrad et al. (2014), an
isochrone with significantly shorter distances modulus ($m-M=9.96$) and
dramatically higher reddening ($E(B-V)=0.35$) would also fit the data quite
well. Given the high uncertainy in the latter measure and the lower likelihood
that such a young cluster would be so metal-poor, we list NGC 2437’s
parameters in Table 2 as derived for the higher metallicity.

####  3.4.22 NGC 2447

Figure 10: Color-magnitude diagrams of the open clusters NGC 2447, Berkely 39,
NGC 2477 and ESO 123-26 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2447 is a third quadrant cluster situated within the Galactic midplane.
Studies by Claria et al. (2005), Santos et al. (2009, 2012), Conrad et al.
(2014) and Reddy et al. (2015) describe as having slightly subsolar
metallicity ($[Fe/H]=-0.10$) and intermediate age (400 Myr). The K13 radius of
30$\farcm$0 indicates we observe only the core of the cluster.

Like many other third quadrant clusters, NGC 2447 shows two sequences in the
CMD (Figure 10, first column). The astrometric selection, however, favors the
bright sequence of young stars. The M17 isochrones fit this sequence with
parameters very similar to the literature values.

####  3.4.23 Berkely 39

Berkeley 39 is a third quadrant cluster situated 10 degrees above the Galactic
midplane. Previous studies (Kassis et al. 1997; Carraro et al. 1994, 1999;
Frinchaboy et al. 2006) have shown the cluster to be massive, old (5.5 Gyr)
and moderately metal-poor ($[Fe/H]=-0.20$) The extensive spectroscopic survey
of Bragaglia et al. (2012) identifies 30 radial-velocity cluster members with
very small star-to-star variations in abundance, consistent with a simple
stellar population. Unfortunately, only a handful of their program stars have
counterparts in the UVOT data due to their cool temperatures and subsequent
low UV emission.

Because the cluster is old, reddened and distant, we only detect the very top
of the MS in the $uvw2-uvw1$ CMD (Figure 10, second column) and do not detect
it all in the $uvm2-uvw1$ CMD (most likely due to the lack of red leak in
$uvm2$). We are unable to constrain its properties on so tenuous a basis.
However, overlaying isochrones from the consensus literature shows a rough
agreement. Moreover, we detect a number of likely cluster members that would
be along a potential blue straggler sequence.

####  3.4.24 NGC 2477

NGC 2477 is a third quadrant cluster located below the Galactic midplane.
Extensive previous investigation has shown it to be of intermediate age (1
Gyr) with moderate ($E(B-V)=0.25$) reddening (Jeffery et al. 2016) Metallicity
estimates range from [Fe/H]=+0.07 (Bragaglia et al. 2007) to -0.19 (Conrad et
al. 2014). We have taken the metallicity at an intermediate value of -0.10.
The cluster is large, with a K13 radius of 27$\farcm$0, indicating we only
observe the core.

The astrometric and spectroscopic selection identify a clear MS in the CMDs
(Figure 10, third column). The M17 isochrones produce a fit consistent with
the previous literature. However, we note that the MS is fairly broad and that
this breadth occurs along the entire observed MS. It is likely this represents
differential reddening as NGC 2477 is one of the few clusters we have that has
moderate foreground reddening ($E(B-V)=0.25$) and even small variations can
produce noticeable breadth in the CMD. We note at least three stars that are
likely blue stragglers.

As noted above, we mostly ignore the RGB stars in the clusters due to their
faintness and the dominance of the red leak in their photometric measures. NGC
2477, however, is one of the that has a prominent RGB. It is also well-
surveyed spectroscopically. One of the most noticeable aspects of its CMD is
that the RGB stars tend to land significantly blueward of the isochrone
prediction. This tendency of the isochrone to miss the few RGB stars can be
seen in other clusters but this the clearest example. This suggests that the
isochrones are not reproducing the NUV properties of the RGB accurately. The
reasons for this could be many but the two most likely are that the
atmospheric models do a poor job of predicting the intrinsically low UV flux
for such cool objects (likely due to opacity issues at low temperatures) or
that our accounting for the red leak is incorrect. Further NUV investigation
of bright field RGB stars would provide better constraint on both of these
inputs of the theoretical isochrones.

####  3.4.25 ESO 123-26

ESO 123-26 is a fourth quadrant cluster positioned well-below the Galactic
midplane. It has not been the subject of individual study, with the only
parameter estimates coming from global surveys such as K13 and K16.

This cluster is not included in the CG18 compilation and so we applied the
double Gaussian selection method used for NGC 2343 (§3.4.11). The VPD shows a
loose clumping of stars toward the outside of the field star distribution
which corresponds to a clear color-magnitude sequence (Figure 10, last
column). The best-fit isochrone is close to the K16 parameters but with a
longer distance ($m-M=10.20$) and a slightly older age (500 Myr). We caution
however, that the proximity of this cluster may mean that its brightest
members are saturated, so this age should be regarded as an upper limit.

####  3.4.26 NGC 2479

Figure 11: Color-magnitude diagrams of the open clusters NGC 2479, NGC 2482,
NGC 2506 and NGC 2509 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2479 is above the Galactic midplane in the fourth quadrant. A recent
photometric study by Piatti et al. (2010) revealed the cluster to be old (1
Gyr) and relatively distant ($m-M=11.00$) with minimal foreground reddening.

NGC 2479 is also not included in the CG18 study and we applied our own
astrometric selection (see §3.4.11). The VPD reveals a tight clump of stars
toward the edge of the field star distribution. Stars selected from this clump
trace a narrow sequence in the CMD (Figure 11, first column). The parameters
of Piatti et al. (2010) produce isochrones that overlay this sequence almost
exactly with a slightly older age of 1.1 Gyr.

####  3.4.27 NGC 2482

NGC 2482 is a fourth quadrant cluster positioned just above the Galactic disk.
Recent studies have shown it to be an intermediate age (400 Myr) near-solar
metallicity cluster (Reddy et al. 2013, Krisciunas et al. 2015, Conrad et al.
2014). The astrometric selection shows a clear sequence in the color-magnitude
diagram (Figure 11, second column). The M17 isochrones produce a fit similar
to previous studies but with lower reddening ($E(B-V)=0.05$).

####  3.4.28 NGC 2506

NGC 2506 is a third quadrant cluster located above the Galactic midplane.
Previous studies have shown it to be an old (2 Gyr) metal-poor ($[Fe/H=-0.3$)
cluster (Reddy et al. 2012b, Lee et al. 2012. Anthony-Twarog et al. 2016). The
cluster has also been the subject of extensive spectroscopic surveys which
have confirmed its low metallicity (MMU, Anthony-Twarog et al. 2018).

The CMD shows a dominant locus of cluster stars, which contains the radial
velocity members of MMU and astrometrically-selected members (Figure 11, third
column). The M17 isochrones produce a fit consistent with prior literature
given the assumed metallicity of [Fe/H]=-0.3. NGC 2506 also shows a very
prominent blue straggler sequence, the brightest of which is both a proper
motion and radial velocity member. There is quite a bit of breadth to the MS,
which is unexpected given the low reddening. The broadening occurs along the
entire length of the MS rather than just the MSTO, making the nature of it
unclear.

####  3.4.29 NGC 2509

NGC 2509 is an older cluster positioned above the Galactic disk. The
literature shows a number of very different solutions to NGC 2509’s
properties. Sujatha & Babu (2003) argue for a very old age of 8 Gyr while
Tadross (2055) and Carraro & Costa (2007) find a more intermediate age of
1.2-1.6 Gyr, but with a significant difference in distance modulus between
them ($m-M=11.50$ and 12.50, respectively). No spectroscopic metallicity is
available and those studies have assumed solar metallicity.

The CG18-selected CMD (Figure 11, last column) shows a faint MS. Assuming
solar metallicity, the best fit to the photometry is consistent with the
Tadross et al. (2005) results, showing an older age (1.7 Gyr) and a shorter
distance ($E(B-V)$=11.72).

####  3.4.30 NGC 2527

Figure 12: Color-magnitude diagrams of the open clusters NGC 2527, NGC 2533, M
48 and NGC 2567 (left to right). The solid lines are PARSEC-COLIBRI isochrones
set to the parameters in the text. Solid red squares show astrometric members
selected from GAIA DR2 by CG18 while open squares are spectrosopic members
from either MMU or sources given in the text.

NGC 2527 is a third quadrant cluster near the Galactic midplane. Reddy et al.
(2013) found it be of intermediate age (450 Myr) slightly metal-poor
($[Fe/H=-0.09$) and slightly reddened ($E(B-V)=0.04$), based on spectroscopy
and photometry. Conrad et al. (2014) find it to be slightly more metal-rich
($[Fe/H]=+0.06$) but based only on two stars.

The CMD (Figure 12, first column) shows a clean sequence, well-separated from
the disk. We find parameters similar to those of Reddy et al.. The age of the
cluster is a bit uncertain as the brightest MSTO stars are saturated. However,
the curvature of the upper parts of the CMD are inconsistent with an age older
than that found in Reddy et al. Note again that the $uvw2-uvw1$ colors at the
faint end of the MS are bluer than predicted by the isochrones.

####  3.4.31 NGC 2533

NGC 2533 is a third-quadrant cluster positioned above the Galactic midplane.
The cluster has never been targeted for individual photometric study. However,
basic parameters come from the blue straggler survey of Ahumada & Lapasset
(2007) and the global survey of K16.

The CG18 selection showed very few members stars so we applied our own proper-
motion analysis, as detailed in §3.4.11. The CMD (Figure 12, second column)
shows a rough faint main sequence and turnoff that are similar to the
expectations form the K16 parameters. We adjusted those parameters slightly to
a shorter distance ($m-M=12.20$) and older age (700 Myr), although we
emphasize that the analysis of this cluster is complicated by the somewhat
high ($E(B-V)=0.40$) reddening and red leak causing the RGB to cross over the
MSTO in the $uvw2-uvw1$ CMD.

####  3.4.32 M 48 (NGC 2548)

M 48 is one of the most well-studied open clusters in the Galaxy. While the
specific parameters of the various studies vary, there is a general consensus
that is close, of intermediate age (400 Myr) and with minimal reddening (Rider
et al. 2004; Balaguer-Núñez et al.2005; Wu et al. 2005; Sharma et al., 2006).
Most studies assume near solar metallicity although Balaguer-Núñez et al.
measure a photometric metallicity of [Fe/H]-0.24. The massive size of the
cluster (K13 measure a radius of 43$\farcm$2) indicates that we have only
surveyed the central regions.

The field contains many saturated stars and so only stars with GAIA DR2
counterparts are shown in the CMD (Figure 12). Assuming solar metallicity, we
derive similar parameters to the consensus literature, with slightly elevated
reddening ($E(B-V)=0.10$). We note that the brightest stars in M 48 are
saturated in the UVOT data and so we can only give an upper limit. However,
the curvature of the upper MS is consistent with an age of $\sim$ 300 Myr.

####  3.4.33 NGC 2567

NGC 2567 is a third-quadrant cluster positioned slightly above the Galactic
midplane. It has not been the subject of individual study. The K16 study
provides basic parameters, identifying the clusters as intermediate age (400
Myr) with modest foreground reddening ($E(B-V)=0.13$) while Conrad et al.
(2014) identify it as slightly metal-poor ($[Fe/H]=-0.08$).

The CG18 selection showed very few members stars. However, our analysis, using
the methods of §3.4.11, identifies a tight clump of stars in the VPD at the
center of the field star distribution. Lowering the probability threshold to
50% selects stars that form a clear MS and MSTO within the CMD (Figure 12,
last column). We find that the literature parameters exactly reproduce the
color-magnitude sequence.

####  3.4.34 NGC 2571

Figure 13: Color-magnitude diagrams of the open clusters NGC 2571, NGC 2627,
NGC 2658 and NGC 2669 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2571 is a third quadrant cluster positioned above the midplane. It has
been analyzed by Giorgi et al. (2002) and Özeren et al. (2014). The latter,
based on 2MASS photometry, shows moderate reddening ($E(B-V)=0.36$) while the
former shows a small amount of foreground reddening ($E(B-V)=0.10$. Both agree
that the cluster is quite young ($\sim 50$ Myr).

The CMD (Figure 13, first column) shows a clean MS well-separated from the
disk. We find, however, that the parameters derived by previous investigators
are somewhat inconsistent with the NUV photometry. The high reddening value
derived by Özeren et al. (2014) would be inconsistent with the color of the
upper MS for any age, predicting a $uvm2-uvw1$ color several tenths of a
magnitude redward of the photometry. The very young ages derived by the
previous studies are also inconsistent with the curvature of the upper MS, the
brightness of the main sequence and the color of the main sequence. While we
have multiple saturated stars in the field and our age thus represents an
upper limit, we note that Kilambia et al. (1978) derive an age of 175 Myr
based on photographic photometry, a result that is far more consistent with
the NUV photometry we present here. We therefore find it unlikely that this
cluster is very young.

####  3.4.35 NGC 2627

NGC 2627 is a third quadrant cluster situated just above the Galactic
midplane. Prior photometric studies have derived an old age of 1.6 Gyr (Piatti
et al. 2003; Ahumada 2005). There is no spectroscopic metallicity but Piatti
et al. estimate that is slightly subsolar ([Fe/H]$\sim-0.1$) based on
Washington indices.

MMU surveyed the cluster but none of their four RGB stars are detected in the
Swift data. The astrometric selection, however, produces a clear CMD with a
well-defined MS and MSTO (Figure 13, second column). The best-fit isochrone
roughly corresponds to the parameters from Ahumada. Note that because of the
age, distance, and reddening of this cluster, the RGB/AGB region overlaps the
MSTO in the $uvw2-uvw1$ region. The actual MSTO is the faintest of these
loops. Several astrometric members are in the blue straggler region of the
CMD, indicating a likely association.

####  3.4.36 NGC 2658

NGC 2658 is a third quadrant cluster located six degrees above the Galactic
midplane. The cluster has not been the target of spectroscopic study. Paunzen
et al. (2010) indicated that it was metal-poor but the updated survey of
Netopil (2016) indicate solar metallicity. Photometric studies (Ramsay &
Pollaco 1992, Ahumada 2003) describe it as intermediate age (300 Myr), solar
metallicity and with moderate reddening ($E(B-V)=0.40)$, making it one of the
few significantly reddened clusters in our sample.

The CMDs show a prominent MS and MSTO (Figure 13, third column). Fitting the
M17 isochrones to this sequence results in parameters similar to the
literature, with a slightly lower reddening value of $E(B-V)=0.30$. The MS
does show a significant breadth and some extension beyond the MSTO. This could
indicate that it has an eMSTO.

####  3.4.37 NGC 2669

NGC 2669 is a poorly-studied third quadrant cluster located six degrees below
the Galactic midplane. The last focused study of the cluster was Vogt & Moffat
(1973) but it has been included in the global surveys of Paunzen et al.
(2010), K13 and K16. These studies hint at a young (100 Myr) slightly metal-
poor ($[Fe/H]\sim-0.20$), moderately reddened ($E(B-V)=0.18$) cluster.

The astrometric selection includes one MMU radial velocity member and traces
the bluer sequence in the CMDs (Figure 13, last column). Assuming the Paunzen
et al. metallicity, we find parameters similar to those in the literature. We
note, however, that the brightest stars are saturated and so the age given is
an upper limit.

####  3.4.38 NGC 2818

Figure 14: Color-magnitude diagrams of the open clusters NGC 2818, NGC 2825,
Collinder 220 and NGC 3680 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 2818 is a distant third quadrant cluster situated above the Galactic
midplane. MMU has an extensive radial-velocity membership. This cluster was
recently studied in great detail by Bastian et al. (2018) who found that it
has a prominent eMSTO. Their complementary radial velocity study showed that
stars on the red side of eMSTO were fast rotators compared to stars on the
blue side, confirming the hypothesis of stellar rotation as the origin of the
eMSTO.

The CMDs (Figure 14, first column) show a well-defined MS with a prominent
MSTO. We find that the cluster is a bit older than the previous studies, with
the MSTO corresponding to an age of approximately 1 Gyr with slightly less-
reddening than Bastiat et al. (2018) of $E(B-V)=0.15$). One key difference may
be that Bastiat et al. assume solar metallicity while we assume a metallicity
of [M/H]=-0.2 based on the analysis of Mermilliod et al.(2001). Increasing the
[M/H] to solar metallicity would not affect the distance or reddening but
would reduce the age of the cluster slightly to 900 Myr. We identify one
member star beyond this turnoff, which could be either be a blue straggler or
a disk contaminant.

Figure 15: A close-up of the MSTO region of NGC 2818. The left panel shows
GAIA- and spectroscopically-selected members. The MSTO region is very broad
due to the combination of eMSTO, convective hook and the beginning of the RGB.
The right panel shows deviations from a linear fit to the MS region and shows
a broad pattern in the MSTO region, again consistent with an eMSTO in NGC 2818
likely resulting from differences in stellar rotation among the MSTO stars.

As noted, Bastian et al. identified an eMSTO in this cluster. As with NGC
2360, the overlap of convective hook, RGB and MSTO make analysis tricky.
However, the $uvm2-uvw1$ diagram shows that the MSTO is broader and more
structured (showing what looks like a bifurcated MSTO) than one would expect
from a simple stellar population. Figure 15 shows a close-up of this turnoff
region and it appears to confirm the broadening of the MSTO. The right panel
shows the deviation from a linear fit to the main sequence to further
highlight this feature.

####  3.4.39 NGC 2925

NGC 2925 is a fourth quadrant cluster situated just below the Galactic
midplane. The only focused study is that of Topaktas (1981) who concluded that
the cluster was moderately reddened ($E(B-V)=0.08$) based on photographic
photometry. K16 estimate an intermediate age of 500 Myr.

While the astrometric selection produces a clean sequence in the CMD (Figure
14, second column) we note that the three RGB stars cross-identified from MMU
lie outside of the astrometric selection (two of which are detected by UVOT).
In proper-motion space, they span a range of 5 mas yr-1, which is a stark
contrast to other clusters in which spectroscopically-confirmed members span
at most a few tenths of a mas yr-1. Applying our own analysis to the GAIA data
results in an almost identical membership selection to CG18 and confirms that
the three MMU members are outliers. We therefore find it likely that the MMU
stars are not members of NGC 2925 but, given the high proper motions, are halo
contaminants. The astrometric members favor the redder sequence in the field.
Fitting the M17 isochrones produces a fit very similar to K16 but with an
increase in reddening to $E(B-V)=0.15$.

####  3.4.40 Collinder 220

Collinder 220 is a fourth-quadrant cluster within the Galactic midplane. It
has not been the subject of any published individual study and our parameters
in Table 2 are taken from K16. UVOT obtained very deep imaging of this cluster
due to its proximity to 1E1024.0-5732 (WR21a), a colliding wind binary near
the Westerlund 2 cluster that was intensely monitored through periastron.

Analysis of this cluster proved challenging. The VPD shows a clear tight clump
of stars within the field star distribution. However, the field is so dense
that our double-Gaussian modelling failed. Picking out stars in this clump by
hand from within the clump showed (1) a tight sequence of stars in the CMD
(Figure 14, third column); (2) a concentration of stars at the nominal
position of the cluster; (3) a mean proper motion roughly consistent with the
K13 parameters ($(\mu_{\alpha},\mu_{delta})\sim(-7.3,2.6)$ compared to (-5.9,
2.5) of K13). The analysis of CG18 identifies most of the same stars as
members albeit with a more conservative selection. We are therefore confident
that this is indeed the cluster sequence.

The derived parameters are markedly different from those of K16. We measure a
much longer distance ($m-M=11.99$) and a younger age (200 Myr). What is really
interesting about this cluster, however, is that it the first to require a
change in the reddening law in order to properly fit the color-magnitude
sequences. Most of our clusters were picked to have low foreground extinction
and even those with more extinction are usually well-fit by the standard Milky
Way law. However, using the Milky Way law on Collinder 220 produces a
discrepancy in the colors of the two main sequences which no combination of
parameters could fit. Adopting the LMC extinction law, by contrast, results in
a consistent fit for both CMDs. Given the disagreement between our measured
parameters and those of K16, we put forward this interpretation with caution.
However, spectroscopic study or multi-wavelength study of Collinder 220 would
seem warranted to confirm or refute any changes in the UV extinction law
toward the cluster.

####  3.4.41 NGC 3680

NGC 3680 is a fourth quadrant cluster placed slightly above the Galactic
midplane. It has been the subject of detailed abundance studies (Santos et al.
2009; Pena Suarez et al. 2018) as well as photometric investigations (Anthony-
Twarog & Twarog 2004) which have described it as an old (1.75 Gyr) near-solar
metallicity disk cluster.

The astrometric selection produces a clear main sequence that contains all of
the MMU radial velocity members (Figure 14, last column). The isochrones,
however, favor a solution with minimal or no reddening, which produces a
consistent fit with a slightly shorter distance ($m-M=10.06$) and an older age
of 2.2 Gyr.

####  3.4.42 Lynga 2

Figure 16: Color-magnitude diagrams of the open clusters Lynga 2, NGC 6823,
NGC 6866, NGC 6939 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

Lynga 2 is a small obscured cluster situated toward the Galactic Center. The
cluster has been poorly studied, likely due to the high density of field stars
against a relatively spartan cluster. Alter et al. (1970) and Bica et al.
(2006) describe it as young (90 Myr), with moderate reddening ($E(B-V)=0.22$).

The astrometric and spectroscopic selection produces a spartan but clear MS in
the CMDs (Figure 16, first column). We find that an isochrone using the
parameters of Bica et al. follows the color-magnitude sequence quite well
(assuming solar metallicity). We note, however, that the brightest member
stars are saturated and therefore we are only able to apply an upper limit to
the age of the cluster.

####  3.4.43 NGC 6823

NGC 6823 is a distant obscured first quadrant cluster situated near the
midplane. It is the only very young ($t<$20 Myr) cluster in our program that
is well-observed although it still has a number of saturated stars. It has not
been the subject of individual study so parameters are taken from K16.

The astrometric selection produces a clean, if spartan, MS (Figure 16, second
column). Adjusted to lower reddening ($E(B-V)=0.70$) and longer distance
($m-M=11.49$) than given in K16, we get excellent agreement between the
isochrone and the sequence. The MS is a bit broader than the photometric
errors, possibly indicating differential reddening or the influence of stellar
rotation. The brightest stars in this cluster are saturated and so only an
upper limit on the age can be given. However, this upper limit is close to the
K16 age. Given the high-quality data, young age and foreground extinction,
this cluster would be a good candidate for future spectroscopic study.

####  3.4.44 NGC 6866

NGC 6866 is a first quadrant cluster situated within the Galactic midplane.
Previous studies have shown it to be of intermediate age (900 Myr), moderately
reddened ($E(B-V)=0.16$) and slightly metal-poor with $[Fe/H]=-0.10$ (Janes et
al. 2014, Bostanci et al. 2015)

The CG18 selection provided to be too conservative – identified very few
clusters members – and so we applied our own astrometric selection (§3.4.11).
The VPD shows a clear clump at the edge of the field star distribution that
includes the two MMU radial velocity members and produces a clear MS (Figure
16, third column). Fitting the M17 isochrones to the sequence produces
parameters similar to previous investigations.

####  3.4.45 NGC 6939

NGC 6939 is a second quadrant cluster positioned 12 degrees above the Galactic
midplane. Numerous studies have been done of the cluster and have found it to
be of solar abundance (Jacobson et al. 2007), heavily reddened ($E(B-V)=0.33$)
and old, with an age of 1.3-1.6 Gyr (Andreuzzi et al. 2004, Rosvick & Balam
2002). However, there are significant disagreements between the exact
parameters derived from the studies, as shown in Table 2.

The astrometric selection identifies a faint MS in the CMDs (Figure 16, last
column). We find that we can fit this sequence using parameters similar to
those of Andreuzzi et al. (2004) but only with some additional reddening
($E(B-V)=0.50$). This is one of the few clusters in our sample that has
substantial reddening and it is impossible to line up the main-sequence in
both color panels without increasing the reddening. At this level of
extinction, the $uvw2-uvw1$ isochrones show portions of the subgiant and giant
branch to be bluer than the MSTO because we are only measuring the degree of
red leak in these cool and heavily reddened stars. The CMD also shows at least
one star bluer and brighter than the MSTO. This is a proper-motion member and
crudely along the MS that a younger stellar population would follow. It is
likely that it is a blue straggler.

####  3.4.46 NGC 6991

Figure 17: Color-magnitude diagrams of the open clusters NGC 6991, NGC 7058,
NGC 7063 and NGC 7209 (left to right). The solid lines are PARSEC-COLIBRI
isochrones set to the parameters in the text. Solid red squares show
astrometric members selected from GAIA DR2 by CG18 while open squares are
spectrosopic members from either MMU or sources given in the text.

NGC 6991 is a poorly studied first quadrant midplane cluster. The only
estimates of age and distance come from the global survey of K16. Casamiquela
et al.(2016) studied the radial velocity but did not measure metallicity. The
cluster is quite large (K13 measure a radius of 26$\farcm$7) and thus we only
image the central regions.

Th first column of gigure 17 shows the CMDs of the cluster. The GAIA
astrometry matches a radial velocity member from Casamiquela et al. (2016).
These stars define a narrow CMD sequence. We measure parameter similar to K16,
but with an older age (1.6 Gyr).

####  3.4.47 NGC 7058

NGC 7058 is a nearby sparse second quadrant cluster that is within the
Galactic midplane. It has not been the target of individual study, with the
only estimates of its age, distance and reddening from the global survey of
K16.

The CG18 selection selected very few member stars and we therefore applied our
method (§3.4.11). The VPD shows a clear clump of stars well outside of the
field star distribution. This clump corresponds to a sequence in the CMD that
reveals NGC 7058 to be – under the assumption of solar metallicity –
parameters similar to K16 (Figure 17, second column). We estimate an age of
1.4 Gyr but this is likely an upper limit. NGC 7058 is sparse and the images
show a small number of bright saturated stars, at least two of which have the
same proper motion as the cluster. We note that NGC 7058 is another cluster
that shows deviation from the isochrones for faint stars in the $uvw2-uvw1$
color-magnitude diagram.

####  3.4.48 NGC 7063

NGC 7063 is a first quadrant cluster situated nine degrees below the Galactic
midplane. The only comprehensive study to date is that of Pena et al. (2007)
who characterize it as young (140 Myr) and with low foreground reddening
($E(B-V)=0.05$). The MMU survey observed two stars but classified both as non-
members.

The CG18 selection proved too conservative and we therefore applied our
methods (§3.4.11). The VPD shows a clear clump of stars outside of the field
star distribution. This clump corresponds to a sequence in the CMD that
reveals NGC 7063 to be – under the assumption of solar metallicity – roughly
consistent with the parameters of Pena et al 2007 (Figure 17, third column).
We caution that our estimated age is again an upper limit, given the presence
of several saturated stars and the curvature of the upper MS. As with other
clusters, NGC 7063 shows deviation from the isochrones for faint stars in the
$uvw2-uvw1$ color-magnitude diagram.

####  3.4.49 NGC 7209

NGC 7209 is a second quadrant cluster positioned seven degrees below the
Galactic midplane. No spectroscopic metallicity has been published but
photometric studies describe the cluster as solar metallicity, intermediate
age (500 Myr) and moderately reddened, with $E(B-V)=0.17$ (Vansevicius et al.
1997, Paunzen et al. 2010, Netopil et al. 2016, K16).

The astrometric and spectroscopic selection find a clear MS in the CMDs
(Figure 17, last column). Fitting the M17 isochrones to this sequence yields
parameters similar to the literature. However, we note that the CMD shows
significant structure that almost resembles a second turnoff. Increasing the
age of the cluster to 700 Myr would match this second turnoff albeit with some
slight discrepancy in the curvature of the upper MS. This would relegate the
brighter stars to being blue stragglers. If the brighter stars represent the
MSTO, then NGC 7209 would have an age of 500 Myr but an eMSTO. Spectroscopic
investigation of the rotation rates could determine if this is the case.

Table 2: Parameters of Clusters in this Study

Cluster | Distance | E (B-V) | log(Age) | [Fe/H] | Source
---|---|---|---|---|---
| m-M |  | (yr) |  |
NGC 752 | 8.30 | 0.03 | 9.15 | -0.03 | Böcek Topcu et al. (2015), Twarog et al. (2015)
NGC 752 | 8.30 | 0.05 | 9.15 | +0.00 | This Study
NGC 1039 | 8.38 | 0.10 | 8.35 | +0.07 | Jones & Prosser (1996)
NGC 1039 | 8.71 | 0.07 | 8.25 | +0.07 | Schuler et al. (2003)
NGC 1039 | 8.54 | 0.08 | 8.38 | 0.07 | Kharchenko et al. (2013)
NGC 1039 | 8.75 | 0.10 |  $<$8.20 | +0.10 | This Study
NGC 2192 | 12.70 | 0.20 | 9.04 | -0.31 | Park & Lee (1999)
NGC 2192 | 12.70 | 0.16 | 9.15 | -0.31 | Tapia et al (2010)
NGC 2192 | 12.70 | 0.15 | 9.20 | -0.30 | This Study
NGC 2204 | 13.00 | 0.13 | 9.20 | -0.23 | Kassis et al. (1997), Jacobson et al. (2011)
NGC 2204 | 13.16 | 0.00 | 9.35 | -0.20 | This Study
NGC 2243 | 12.78 | 0.11 | 9.67 | -0.42 | Twarog et al. (1997), Jacobson et al. (2011)
NGC 2243 | 12.91 | 0.00 | 9.70 | -0.50 | This Study
NGC 2251 | 10.60 | 0.20 | 8.50 | -0.10 | Parisi et al. (2005), Reddy et al. (2013)
NGC 2251 | 10.61 | 0.25 | 8.55 | -0.10 | This Study
NGC 2281 | 8.55 | 0.09 | 8.80 | 0.00 | Glaspey (1987), Netopil(2017)
NGC 2281 | 8.56 | 0.15 | 8.80 | 0.00 | This Study

aafootnotetext: Using LMC extinction law.

This table is available in its entirety in machine-readable and Virtual
Observatory (VO) forms.

##  4 Discussion

###  4.1 Isochrones and Cluster Fits

The purpose of this program was to test the utility of standard isochrones in
the UV. In that sense, we find that the isochrones perform quite well,
successfully reproducing the upper main sequence for cluster up to several Gyr
in age. Most of the CMDs are well-described using isochrones set to values at
or near those in the literature. For clusters with significant revisions, the
source of the discrepancy is either older studies or, in some cases, previous
studies mistaking the disk sequence for the cluster sequence.

The consistency between the models and the data shows that even for fairly
young stellar populations, the NUV emission is well-described by the existing
atmospheric models. There is little evidence of stochastic variation and
little evidence of the excess emission produced by chromospheric activity as
seen in the FUV (see discussion in Smith 2018). However, it is important to
note that our study is focused on only limited range of parameter space.
Almost all of our clusters are older than 100 Myr and the brightest stars in
the youngest are few and saturated. Younger clusters will host much more
massive stars, in which factors like binarism, rotation and magnetic field can
play a more important role and our understanding of convention and mixing are
poor (see, e.g., Viallet et al. 2013). In addition, our stars are primarily
metal-rich (or assumed to be) so we are unable to test how the isochrones
perform at lower metallicity levels. Finally, almost all of our clusters are
minimally reddened with only a handful having significant extinction.
Unfortunately, these limitations can not really be exceeded with the Milky
Way’s contingent of open cluster or with UVOT. We are currently investigating
the UV properties of old metal-poor populations in globular clusters. But
exploring very young ($<100Myr$) stellar populations or populations with
significant reddening would require surveys of the nearby stellar populations
in the Magellanic Clouds. Swift/UVOT has also surveyed these areas (Hagen et
al., 2017) but its reach may exceed its grasp in such dense star-forming
regions as 30 Doradus where the younger stars tend to reside. That is more
properly the domain of future missions that have the wide-field high-
resolution multi-filter FUV imaging.

The main deficiencies in the isochrones are for redder stellar types: the RGB,
AGB and lower MS stars that are cool and have little emission in the UV. This
is hardly a surprise given that for cool stars (those with $T_{eff}<7000K$),
the “UV” signal in two of Swift’s filters becomes dominated by the red leak.
This plays havoc with the effective wavelength (see, e.g., Siegel et al.
(2012)) and causes the reddening to have a non-linear effect (Paper I, Brown
et al. 2010). For later stellar types that are heavily extinguished, this will
cause them to move blueward in $uvw2-uvw1$ CMDs as the $uvw1$ ramps up the red
leak faster than the $uvw2$ filter.

However, it could also be produced by deficiencies in either the isochrones or
the underlying atmospheric models. Such discrepancies have been noted before
in NUV photometry of cool stars (see, e.g., Barker & Paust 2018) and are
likely the result of the underlying atmospheric models having an incomplete
treatment of UV absorption lines for very cool atmospheres. This problem has
been discussed in detail in the context of the UVBLUE project by Rodríguez-
Merino et al.(2005, 2009) and Chavez (2007) and is part of the motivatio for
the Hubble Space Telescope’s upcoming Hubble UV Legacy Library of Young Stars
As Essential Standards (ULLYSES) program.

Figure 18: A comparison of photoemtric distance from the fit isochrones for a
sample of the UVOT cluster stars, plotted against absolute $T_{eff}$ (bottom)
and observationsl $uvw2-uvw1$ color (top). The solid line shows agreement. The
vertical streak in the top panel is NGC 2658, where the MS is cut off slightly
fainter than the blue straggler sequence.

To determine the nature of these discrepancies, we examined the deviations of
stellar measures from the fit isochrones. This deviation was calculated by
measuring, for each star, the minimum quadrature color-magnitude distance from
the isochrone. Figure 18 shows the residuals plotted against observational
$uvw2-uvw1$ color and the effective temperature implied by the isochrone. If
the problem arises from the red leak, it should occur at a consistent observed
color, where the red leaks takes over the signal. However, it the problems
arises from atmospheric models, it should occur at a consistent effective
temperature, where the atmospheric models begin to fail. For clusters that
show the deviation at faint magnitudes, the discrepancies do not track
observed stellar color very well. The deviations do, however, track effective
temperature quite well, beginning to deviate toward bluer colors at
approximately $logT_{eff}\sim 3.8$ or 6300 K. This hints that the problem lies
within the stellar atmopshere models. However, for the bulk of stars that are
of interest in the UV, the atmospheric models and isochrones hold up quite
well.

###  4.2 Reddening Law

As noted above, most of our program clusters have low intrinsic reddening.
Those few that have substantial reddening seem to work fine with a “Milky-
Way”-like extinction law (i.e., one with a shallower slope and a strong 2175 Å
bump). Only one cluster – Collinder 220 – shows any sign of deviation from the
standard reddening law. This is somewhat surprising as previous investigations
have shown some variation in the extinction law at high latitudes within the
Galaxy (Peek & Schiminovich 2013), at low latitude within the Galaxy (Siegel
et al. 2012) and in nearby galaxies (Hagen et al. 2017, 2019). This suggests
that future endeavors should both look at resolved stellar population in
nearby galaxies where the reddening law is known to vary and at more reddened
clusters within our own Galaxy where variations in the law will be amplified
and more readily detectable. It might also be of benefit to make these studies
multi-wavelength, as in Hagen et al. (2017, 2019) where the more well-
understood OIR extinction law can be used to “nail down” the line-of-sight
extinction, freeing the UV to focus more on variation in the law instead of
variation in the total extinction level along the line of sight.

###  4.3 Extended MSTOs

Two of our clusters – NGC 2360 and NGC 2818 – have previously been identified
as having extended MSTOs based on photometric and spectroscopic exploration.
We confirm the photometric broadening and further note that a number of other
clusters (NGC 2355, NGC 2420 NGC 2658, NGC 6823, NGC 7209) show a similar
broadening of the MSTO beyond what is expected from photometric error. This is
unlikely to be the effect of differential reddening because most of our
clusters were selected to have low reddening and differential reddening would
broaden the entire MS. Spectroscopic investigation of rotation rates would
confirm the nature of these extended MSTOs, as it has for NGC 2360 and NGC
2818. If confirmed, this would support the contention of Cordoni et al. (2018)
that the feature is not unusual among Galactic open clusters and the UV may be
more sensitive to the effect than the optical or IR.

Considering that the eMSTO involves the brightest stars in any specific
population, this is an effect that may need to be accounted for in spectral
synthesis models. While the precise effect of stellar rotation on integrated
light is beyond the scope of this paper, the indications that stellar rotation
can increase both mass loss rates and main sequence lifetime makes their
potential ubiquity in young stellar populations both critical to confirm and
critical to incorporate into the models.

##  5 Conclusions

We have measured photometry of stars in 103 open clusters using the Swift/UVOT
telescope. We have analyzed 49 of these clusters, using GAIA DR2 and
spectroscopic studies to separate members stars from the field to study
precise CMDS and compared the CMDs to theoretical isochrones. Our main results
are:

$\bullet$ The theoretical isochrones reproduce the features of the CMDs very
well. The only consistent discrepancy is at faint magnitudes where cool late-
type stars are bluer in $uvw2-uvw1$ space than anticipated. This could be a
result of red leak or of inadequacies in the UV opacities in model atmospheres
for late-type stars. The discrepancies seem to be more closely related to
absolute magnitude than color, favoring a problem with the model atmospheres.

$\bullet$ Using these isochrones, we measure age, reddening and distance for
49 clusters with well-defined color-magnitude sequences. For those that have
been studied in detail before, we generally find agreement with literature
values. However, there are a number of clusters for which we find
significantly different values than the literature, particularly in the third
Galactic quadrant where some previous studies have mistaken the disk sequence
for the cluster. We catalog substantially revised parameters for the clusters
NGC 2304, NGC 2343, NGC 2360, NGC 2396, NGC 2428, NGC 2509, NGC 2533, NGC
2571, NGC 2818, Collinder 220 and NGC 6939.

$\bullet$ We confirm the presence of an extended MSTO in two previously
studied clusters – NGC 2360 and NGC 2818. We also identify broad MSTO features
in at least five other clusters that could warrant further spectroscopic
investigation.

$\bullet$ Most of our clusters have minimal reddening and are thus unsuited to
probe the UV properties of the foreground dust. However, one cluster –
Collinder 220 – shows significant improvement in the isochrone fits if an
“LMC-like” reddening law – one with a smaller red bump at 2175 Å and a steeper
extinction curve – is used.

Our investigation has only scratched the surface of what this remarkable data
set can yield. Measuring the integrated light would allow direct comparison
between isochrones and synthetic spectral models to test the validity of the
latter for unresolved stellar populations. Combining this database with extra
ground-based optical photometry or the broad-band photometry of GAIA DR2 would
allow the detection of unresolved white dwarf-main sequence binaries,
providing additional insight into the role of binarism in spectral synthesis
models and the integrated luminosity of stellar population (see, e.g., Buzzoni
et al. 2012, Hernández-Pérez & Bruzual 2013). A more thorough examination of
the MSTOs – especially in combination with spectroscopic study – would allow a
much more detailed exploration of eMSTOs and their connection to cluster age.

The authors acknowledge sponsorship at PSU by NASA contract NAS5-00136. This
research was also supported by the NASA ADAP through grants NNX13AI39G and
NNX12AE28G. The authors thank L. Girardi for useful discussions about the NUV
isochrones. The Institute for Gravitation and the Cosmos is supported by the
Eberly College of Science and the Office of the Senior Vice President for
Research at the Pennsylvania State University.

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