# Outbursting Young Stellar Object PGIR 20dci in the Perseus Arm

Lynne A. Hillenbrand  Department of Astronomy, MC 249-17, California Institute
of Technology, Pasadena, CA 91125, USA<EMAIL_ADDRESS>Kishalay De
Department of Astronomy, MC 249-17, California Institute of Technology,
Pasadena, CA 91125, USA  Matthew Hankins Arkansas Tech University,
Russellville, AR 72801, USA  Mansi M. Kasliwal Department of Astronomy, MC
249-17, California Institute of Technology, Pasadena, CA 91125, USA  Luisa M.
Rebull IPAC, California Institute of Technology, Pasadena, CA 91125, USA  Ryan
M. Lau  Institute of Space & Astronautical Science, Japan Aerospace
Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210,
Japan  Roc M. Cutri IPAC, California Institute of Technology, Pasadena CA
91125  Michael C. B. Ashley School of Physics, University of New South Wales,
2052, Australia  Viraj R. Karambelkar Department of Astronomy, MC 249-17,
California Institute of Technology, Pasadena, CA 91125, USA  Anna M. Moore
Australian Nationa1 University, Research School of Astronomy and Astrophysics,
Mount Stromlo Observatory, Cotter Road, Weston Creek 2611 Australia  T.
Travouillon Australian Nationa1 University, Research School of Astronomy and
Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek 2611
Australia  A. K. Mainzer  University of Arizona, Department of Planetary
Science, Tucson, AZ 85721

###### Abstract

We report the discovery of a likely outbursting Class I young stellar object,
associated with the star-forming region NGC 281-W (distance $\sim 2.8$ kpc).
The source is currently seen only at infrared wavelengths, appearing in both
the Palomar Gattini InfraRed ($1.2~{}\mu$m) and the Near Earth Object
Widefield Infrared Survey Explorer ($3.4$ and $4.6~{}\mu$m) photometric time-
domain surveys. Recent near-infrared imaging reveals a new, extended scattered
light nebula. Recent near-infrared spectroscopy confirms the similarity of
PGIR 20dci to FU Ori type sources, based on strong molecular absorption in CO,
H2O, and OH, weak absorption in several atomic lines, and a warm wind/outflow
as indicated by a P Cygni profile in the He I 10830 Å line. This is a rare
case of an FU Ori star with a well-measured long term photometric rise before
a sharper outburst, and the second instance of an FU Ori star with a
documented two-step brightening in the mid-infrared.

††facilities: Palomar Gattini IR, PO:1.2m:ZTF, PO:1.5m:SEDM, Hale:TSPEC,
Keck/NIRES, Keck/DEIMOS, Spitzer, WISE, NEOWISE, IRSA

accepted to AJ on 5 February 2021

##  1 Introduction

Episodic accretion in young stellar objects is an important aspect of mass
accumulation in stars. It also plays a determining role in the evolution of
circumstellar disks, and likely influences the process of planet formation, as
well as planetary survival. Armitage (2015) provides a review of disk physics
in young stars, including nonsteady accretion and disk instabilities.

In “classical” T Tauri type young stars, the accretion process is controlled
mainly by physics in the inner disk and magnetospheric region that connects
the disk to the star. Observed time-variability in broad-band photometry and
in emission line profiles of these sources is dominated by stochastic
behavior, with timescales from hours to days (e.g. Findeisen et al., 2015;
Cody & Hillenbrand, 2018; Costigan et al., 2014). The empirical zoo of
accretion-related photometric behavior has been measured in unprecedented
detail for young stars by space missions including CoRoT, Spitzer, Kepler, and
now TESS, all of which sample timescales from minutes to a few months. Ground-
based surveys such as ASAS (Shappee et al., 2014), PTF (Law et al., 2009), ZTF
(Bellm et al., 2019; Graham et al., 2019), and ATLAS (Tonry et al., 2018) in
the optical, and VVV (Minniti et al., 2010) in the infrared, by contrast,
typically have lower precision and cadence (day to a few days), but the
advantage of long duration monitoring. Gaia and WISE$+$NEOWISE also provide
precise space-based photometric data, but at much lower cadence (months), with
total duration exceeding a decade now. Many of these surveys have all-
hemisphere or even all-sky coverage, enabling rare event detection – including
episodic accretion outbursts in very young stars.

Accretion-driven outbursts associated with young stellar objects have been
identified at an increasing rate over the past decade. The discoveries have
come from both the ground-based and the spaced-based platforms. Infrared
surveys have the advantage of being able to penetrate the high levels of
extinction towards the molecular clouds where young stars are born, and to
some extent the additional circumstellar extinction of the youngest self-
embedded sources.

Although young star outbursts appear to come in a continuum of amplitudes and
rise times, for historical reasons (Herbig, 1977), the labels EX Lup type
bursts (also known as EXors) and FU Ori type outbursts (also known as FUors)
are often applied. The former are more frequent, characterized by brightness
increases of a few magnitudes in amplitude, lasting a few months to one-to-two
years, and have emission line spectra. The latter are more infrequent, and can
be up to 5-6 mag in amplitude, lasting decades to centuries, and with
absorption line spectra except for P Cygni structure in some wind lines. An
increasing number of objects are unable to be cast neatly into either of these
categories, however. Our knowledge of the spectrum of amplitudes, durations,
and duty cycles for young star outbursts is still in its infancy.

The large-scale FU Ori outburst events are intrinisically rare. Furthermore,
those few that have been captured in-progress have not been particular well-
characterized111The situation can be compared to shorter timescale rare
occurences such as tidal disruption events and nearby supernovae, which garner
armies of dedicated followers.. The most recent FU Ori outburst discoveries
are improving the situation, e.g. Gaia 17bpi (Hillenbrand et al., 2018), Gaia
18dvy (Szegedi-Elek et al., 2020), and NWISE-F J213723.5+665145 (Stecklum,
2020; Connelley & Reipurth, 2020). The total census of FU Ori type stars
numbers $<30$ objects at present, with less than half of these having had
their outburst actually observed – rather than inferred from imaging at well-
separated pre-outburst and post-outburst epochs.

We report here on the discovery of substantial photometric brightening of an
object designated as PGIR 20dci. This source is located at R.A. = 00:52:20.21,
Dec. = $+$56:34:03.9, and we associate it with the NGC 281-W star forming
region. The object appears to be a bona fide FU Ori star that has just reached
a plateau phase in its brightening.

##  2 Source Environment

NGC 281, also known as Sh 2-184, is an H II region located in the Perseus
spiral arm. While the traditional distance in the literature on the stellar
population is $\sim$2.2 kpc, astrometry from Gaia DR2 (Gaia Collaboration et
al., 2018) indicates a somewhat further 2.8 kpc. This is also the distance
derived by Sato et al. (2008) from parallax studies at radio wavelengths of a
maser population associated with NGC 281-W (where PGIR 20dci is located).
These results place the region on the far side of the Perseus arm, and about
300 pc out of the Galactic plane.

IC 1590 ($<$2 Myr) is the main stellar cluster within the nebular region, and
contains the bright ionizing sources collectively designated as HD 5005. This
source is now a spatially resolved multiple star system, consisting of
components with spectral types O6.5, O8, and O9 (Guetter & Turner, 1997). The
cluster thus draws analogies in the literature as a Trapezium type system. The
stellar population of the IC 1590 cluster was investigated by Guetter & Turner
(1997) and by Sharma et al. (2012), the latter of whom find a size $r\approx
6.5$ pc.

To the southwest of the main cluster IC 1590, is a dark lane known as NGC
281-W. Another dark area just to the east of the nebulosity is designated NGC
281-E. These are the locations of molecular clouds at the same distance as the
large nebula, and presumably situated, at least in part, on its near side.
APOD provides an optical picture222https://apod.nasa.gov/apod/ap110825.html
that may clarify the geometry for some readers. CO maps of the overall region
can be found in Elmegreen & Lada (1978) and Lee & Jung (2003), who provide
evidence for cloud compression coming from the direction of the cluster and H
II region.

The NGC 281-W molecular core has a bright sub-millimeter and far-infrared
source, readily apparent in Herschel/SPIRE (Griffin et al., 2010) and SCUBA-2
(Holland et al., 2006) maps. This dust clump harbors a collection of mid-
infrared point sources to its east and southeast, as seen in WISE (Wright et
al., 2010) images.

The portion of this highly embedded cluster that is visible in the near-
infrared was discovered and studied by Carpenter et al. (1993). These authors
targeted the region with early near-infrared imaging capabilities based on the
position of IRAS 00494+5617. They found a cluster size r = 0.74 pc (corrected
here for differences in the distance assumption), with northeastern and
southwestern portions separated by a ridge of high extinction. The “typical”
extinction towards the embedded sources was estimated at $A_{V}\approx 10$
mag, with peak $A_{V}\approx 45$ mag. Carpenter et al. (1993) also derived a
mass of 340 $M_{\odot}$ (corrected here for differences in the distance
assumption), for the associated CS core333A distance-corrected mass in CO of
4000 $M_{\odot}$ was also reported by Carpenter et al. (1993), though the NGC
281-W cloud is somewhat larger than the area studied, with Lee & Jung (2003)
finding a total mass for the extended cloud of $1-3\times 10^{4}M_{\odot}$..
The same area of NGC 281-W was specifically targeted in a similar study by
Megeath & Wilson (1997) using more sensitive radio and infrared observations.

The object of interest here, PGIR 20dci, is located within the region of these
previous surveys, about 30″ west of the embedded cluster center. Figure 1
shows a portion of the field imaged by Megeath & Wilson (1997), with the
position of PGIR 20dci designated. Although faintly visible at
$K^{\prime}$-band in this archival image, PGIR 20dci has not been previously
cataloged, studied, or characterized.

Figure 1:  Left panel: Figure reproduced from Megeath & Wilson (1997, their
Figure 9) showing a deep near-infrared image taken in $K^{\prime}$
($2.2~{}\mu$m) along with contours of the $C^{18}O$ molecular line emission
intensity. Faint tick marks along the right axis indicate 10″ intervals, with
total field of view 2.′07 $\times$ 2.′07. Overlaid in bright green is a
marking for the position of PGIR 20dci, which does appear as a faint source in
this early 1990’s image. Right panel: $Ks$ ($2.2~{}\mu$m) image obtained in
2020 September, showing a much brighter and extended PGIR 20dci, even though
the image is shallower than the one on the left. In both panels, the data are
from infrared cameras on the Palomar 200” telescope (PFIR = Prime Focus
InfraRed, on the left, and WIRC on the right)

##  3 Outburst Discovery and Validation

In this section, we describe the initial photometric alert that drew our
attention to PGIR 20dci. We then detail the optical, near-infrared, and mid-
infrared photometric forensics work leading to establishment of the outburst
profile over time.

###  3.1 Gattini 1.2 $\mu$m ($J$-band) Lightcurve

The Palomar Gattini InfraRed survey (PGIR; De et al., 2020a; Moore & Kasliwal,
2019). observes the sky every two nights in the near-infrared $J$-band, to a
median depth of J = 15.7 mag (AB). The faint limit is much brighter than this
in crowded or confused regions, such as near the Galactic plane and in high
surface brightness regions, such as NGC 281-W.

On 2019, August 31, a “hostless” source was first detected in PGIR as a
positive image subtraction, and flagged (by co-author KD) for photometric and
spectroscopic follow-up. The initial source position required some refinement
due to the 8″ pixel size of PGIR. There are no counterparts in optical (e.g.
PanSTARRS Flewelling et al., 2016) or near-infrared (2MASS Skrutskie et al.,
2006) survey catalogs, though a hard stretch of the 2MASS images does reveal a
faint detection at $Ks$ and perhaps $H$. A nearby WISE catalog (Cutri et al.,
2013) source at 00:52:20.41 +56:34:03.3 was determined to be the source that
had brightened. The brightening is also recorded in NEOWISE (Mainzer et al.,
2014) data. The consequent increase in signal-to-noise likely yields better
astrometry, and hence a more accurate true source position of 00:52:20.21
+56:34:03.9, which we adopt. This position is 1.3″ away from the position
reported for the fainter WISE source, and 4-5″ from the PGIR position.

Figure 2:  Top panels: Discovery imaging sequence showing a 4′$\times$ 4′
region centered on PGIR 20dci; orientation is the standard north upward and
east to the left. From left to right are: the PGIR image from 2019-10-31 (MJD
= 58787), the static reference image of the field created from a series of
earlier images taken between 2018-10-28 and 2019-01-29, and the subtracted
difference image. PGIR 20dci is clearly visible in the last panel, and is
marked in all three. Bottom panel: $J$-band lightcurve of PGIR 20dci during
2019 and 2020. The observed upward slope in the data points at MJD $<$ 59000
corresponds to a source brightening rate of $-0.116\pm 0.011$ mag/month. The
lightcurve then flattens, and a fit for MJD $>$ 59000 is consistent with zero
slope ($-0.017\pm 0.022$ mag/month). We note that $J$-band observations from
much earlier indicate a significantly fainter source, with estimated limits
$J>17.1$ mag (2MASS) and $J>20$ mag (Megeath & Wilson, 1997); see text.
Vertical blue lines indicate the epoch of the infrared spectrum shown in
Figure 6 as well as a second spectral epoch discussed in the text.

Figure 2 shows the lightcurve produced from the PGIR forced difference
photometry pipeline. PSF-fit photometry is performed, with noise uncertainty
at the location of the transient estimated, as described in De et al. 2020b,
from difference images produced using the ZOGY algorithm (Zackay et al.,
2016).

Since its discovery at $J=14.97$ mag, PGIR 20dci has slowly increased in
brightness. The brightening trend observed during the 2019-2020 season (Figure
2) can be fit with a linear slope of $-0.116\pm 0.011$ mag/month. During the
latter part of 2020 the lightcurve appears to have plateaued, and a linear fit
is consistent with zero slope.

###  3.2 Pre-Discovery Photometric Information

In this section, we discuss the limited photometric record available for PGIR
20dci.

####  3.2.1 Optical and Near-Infrared

At the time of its PGIR detection, PGIR 20dci was invisible in optical time-
domain survey data, including ASAS and ZTF. Recently, however, a faint optical
source with $r=20-20.5$ mag is transiently present in ZTF alerts (Masci et
al., 2019). There is not a useful optical lightcurve, however, due to
infrequent detection and large error bars.

We also note that PGIR 20dci was undetected in deeper imaging of the NGC 281-W
region with HST in the optical F814W and F675W filters, from 2000, September,
accessible via the Hubble Legacy Archive. PGIR 20dci is also undetected in
archival Keck images with DEIMOS of the region from 2010, October 6,
accessible via the Keck Observatory Archive.

In the near-infrared, PGIR 20dci was undetected by 2MASS (Cutri et al., 2003),
as noted above. While the nominal 2MASS survey limit is $J<15.8$, we derive a
more accurate number for this exact environment by considering the error vs
magnitude distribution of sources within 4′ of PGIR 20dci. We find that
$10\sigma$ photometry is available to $J=16.1$ mag and $5\sigma$ photometry to
$J=17.1$ mag. While the diffuse background is high in the NGC 281-W region,
the dark cloud renders the point source background and confusion lower than
would be the case otherwise. Thus, the 2MASS magnitude limit is plausibly
deeper than nominal. We conclude that a reasonable flux upper limit for PGIR
20dci at epoch 2000, January 13 is approximately $J>17.1$ mag.

In the deeper $J$-band imaging of Megeath & Wilson (1997), PGIR 20dci is also
not apparent, except for perhaps a faint low surface brightness smudge near
the edge of detection. The claimed faint point source limit of these data is
$J\approx 21$ mag. PGIR 20dci does, however, have an apparent faint
counterpart in the K’-band imaging of Megeath & Wilson (1997), as indicated in
Figure 1. Although the sources from this study are not cataloged, the authors
report that the fainter stars in the displayed image have $K^{\prime}\approx
18.5$ mag (80% completeness level). PGIR 20dci thus appears to have
experienced a $\sim$5 mag brightening since the mid-1990’s epoch of the
Megeath & Wilson (1997) data acquisition.

####  3.2.2 Mid-Infrared Spitzer

In the mid-infrared, there is also a faint counterpart to PGIR 20dci in
Spitzer/IRAC (Werner et al., 2004; Fazio et al., 2004) imaging. The source
appears blue in comparison to the main embedded cluster, approximately 30″ to
the east of PGIR 20dci, perhaps due to somewhat lower extinction away from the
center of the molecular core (see contours in Figure 1).

There are three relevant Spitzer/IRAC AORs, one for P.I. Fazio in 2004 (AORKEY
4127744) and two for P.I. Wolk in 2009 (AORKEYs 34780160 and 34780416). The
former, from Spitzer’s cryogenic phase, includes all four IRAC bands (3.6,
4.5, 5.8, and 8 $\mu$m). The latter two were taken in the post-cryo phase of
Spitzer and thus include only IRAC1 (3.6 $\mu$m) and IRAC2 (4.5 $\mu$m). We
performed aperture photometry at the target’s location in the pipeline-
produced post-BCD mosaics using a radial aperture of 3 native pixels
(1.2$\arcsec$/pixel) and a background annulus from 3-7 native pixels. Aperture
corrections were applied as appropriate to the cryo and the post-cryo
observations, as described in the IRAC Instrument
Handbook444https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/iracinstrumenthandbook.
Specifically, we used values of 1.124, 1.127, 1.143, & 1.234 for the 4
channels in the cryogenic era, respectively, and 1.125 & 1.120 for the 2
channels in the post-cryo era, respectively.. The Instrument Handbook also has
the zero points555280.9, 179.7, 115.0, & 64.13 Jy for the four IRAC channels,
respectively. we used for converting the flux densities to magnitudes. Our
photometric results for Spitzer/IRAC are provided in the first three rows of
Table 1.

From some of these same Spitzer data, Sharma et al. (2012) found [S3.6] or
IRAC1 = 14.40 mag and [S4.5] or IRAC2 = 12.89 mag using a similar 3.″6 radial
aperture to us, but a smaller 3.″6–8.″4 sky annulus. These values are
consistent with our results for the first observation epoch, in 2004. We also
note that IRSA’s SEIP
catalog666http://irsa.ipac.caltech.edu/data/SPITZER/Enhanced/SEIP/overview.html
contains a source SSTSL2 J005220.20+563403.8 with IRAC1 = 14.57 mag and IRAC2
= 12.80 mag, adopting the 3.″8 aperture values and applying the flux-to-
magnitude conversions above, again consistent with our measurements for the
2004 epoch, within the reported errors.

Finally, we note that there are also Spitzer/MIPS observations of the region
available in the $24~{}\mu$m band. These MIPS1 data were taken in 2006,
February, which is between the two Spitzer/IRAC epochs. It would be valuable
to be able to compare the 2006 MIPS1 photometry to the 2010 W4 photometry, for
reasons that will become apparent below. Unfortunately, however, the MIPS1
imaging results in only flux upper limits for PGIR 20dci.

####  3.2.3 Mid-Infrared AllWISE

WISE images at the comparable bands WISE1 or $W1$ (3.4 $\mu$m) and WISE2 or
$W2$ (4.6 $\mu$m) have lower spatial resolution, but do show a source that
appears more distinct and pointlike in the unWISE image processing (Lang,
2014). The AllWISE catalog (Cutri et al., 2012) records a source WISEA
J005220.41+563403.3 with reported photometry given in Table 1. We took the
AllWISE profile fitting magnitudes and errors, as quoted. The measurements are
rated as “AAAA” in photometric quality, meaning SNR $>10$ in each of the four
bands, and they are unsaturated.

Table 1: Spitzer, WISE, and NEOWISE Photometry 888IRAC columns contain Spitzer/IRAC photometry derived here, with transformations made to the WISE photometric system appearing in the WISE columns as parenthetical entries. WISE columns contain photometry from several sources. The first WISE entry is taken directly from the IRSA AllWISE catalog. Subsequent entries are derived here from observations reported in the IRSA MEP and NEOWISE-R catalogs, combining individual measurements by “visit”; see text. for PGIR 20dci MJD | WISE1 | err | IRAC1 | err | IRAC2 | err | WISE2 | err | IRAC3 | err | IRAC4 | err | WISE3 | err | WISE4 | err
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  3.4 $\mu$m |  |  3.6 $\mu$m |  |  4.5 $\mu$m |  |  4.6 $\mu$m |  |  5.8 $\mu$m |  |  8.0 $\mu$m |  |  12 $\mu$m |  |  22 $\mu$m |
53214.65 | (15.01) |  $\cdots$ | 14.49 | 0.17 | 12.86 | 0.07 | (12.61) |  $\cdots$ | 11.77 | 0.30 | 11.56 | 1.60 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
55077.57 | (14.49) |  $\cdots$ | 13.99 | 0.12 | 12.42 | 0.07 | (12.18) |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
55080.44 | (14.45) |  $\cdots$ | 13.96 | 0.11 | 12.43 | 0.07 | (12.19) |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
(AllWISE) | 12.51 | 0.08 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.40 | 0.05 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 6.11 | 0.04 | 3.36 | 0.07
55222.15 | 12.63 | 0.25 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.41 | 0.09 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 6.04 | 0.22 | 3.27 | 0.32
55409.82 | 12.63 | 0.25 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.41 | 0.09 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 6.13 | 0.22 | 3.37 | 0.32
55586.45 | 12.62 | 0.25 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.43 | 0.09 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
56684.21 | 12.34 | 0.15 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.14 | 0.10 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
56873.87 | 12.56 | 0.22 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.14 | 0.03 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57045.96 | 12.40 | 0.13 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.07 | 0.10 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57235.95 | 12.43 | 0.21 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.12 | 0.07 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57408.47 | 12.26 | 0.26 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 11.00 | 0.01 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57602.68 | 12.31 | 0.26 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.97 | 0.07 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57769.55 | 12.20 | 0.20 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.89 | 0.05 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
57966.88 | 12.32 | 0.17 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.85 | 0.06 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
58129.81 | 12.02 | 0.17 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.68 | 0.06 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
58331.39 | 12.09 | 0.18 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.62 | 0.05 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
58491.38 | 11.83 | 0.18 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.50 | 0.03 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
58698.13 | 11.17 | 0.10 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 9.70 | 0.04 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
58858.12 | 10.35 | 0.09 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 8.97 | 0.03 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
59062.32 | 10.25 | 0.10 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 8.89 | 0.03 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
59222.09 | 10.20 | 0.08 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 8.93 | 0.03 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$

####  3.2.4 Mid-Infrared Colors

The available colors of the PGIR 20dci progenitor are $IRAC1-IRAC2=1.60$ and
$IRAC2-IRAC3=1.09$ mag from Spitzer, and $W1-W2=1.12$, $W2-W3=5.82$, and
$W3-W4=2.76$ mag from AllWISE.

These colors place the object firmly within the Class I category of young
stellar objects, meaning a steeply rising spectral energy distribution that is
best explained by the presence of a massive circumstellar envelope. The colors
are too red to be consistent with a geometrically flatter disk, as is
characteristic of Class II sources. PGIR 20dci is, however, even redder than
most Class I sources, which indicates substantial foreground extinction.

###  3.3 WISE$+$NEOWISE Lightcurve

Figure 3:  Top panels: Spitzer 2009 and WISE 2010 images of a 103″ $\times$93″
field centered on PGIR 20dci; orientation is the standard north upward and
east to the left. As described in the text, Spitzer photometry was derived
using an aperture radius of 3″6 (marked circle) while WISE photometry comes
from the profile fitting measurements. Although the WISE image is dominated by
brighter sources, there are no confusion flags set in any of the WISE or
NEOWISE catalog data for the profile fitting. Bottom panel: Mid-infrared
lightcurve assembled from available Spitzer, WISE, and NEOWISE data. Error
bars are indicated, though in some cases are smaller than the data points. The
Spitzer photometry is plotted both as observed (open squares) and color
corrected into the WISE photometric system (filled squares); see text. Each of
two large jumps in mid-infrared brightness is preceded by a shallow rise
phase. Gray bar marks the MJD coverage of the PGIR lightcurve shown in Figure
2. Vertical blue line indicates the epoch of the infrared spectrum shown in
Figure 6.

In this section, we present mid-infrared data that support the PGIR detection
of a significant brightening of the source.

We collected photometry from the WISE Multi-Epoch and NEOWISE (Cutri et al.,
2015) mission archives via
IRSA999https://irsa.ipac.caltech.edu/Missions/wise.html, rejecting
measurements with qi_fact=0 or qual_frame=0. We checked the WISE/NEOWISE
photometry
flags101010https://wise2.ipac.caltech.edu/docs/release/neowise/expsup/sec2_1a.html,
finding no entries in the w[12]cc_map_str (contamination and confusion)
column, and only non-event entries in the na (deblending) column. The nb
(multiple PSF) column indicates that a single profile is used in $>80$% of the
measurements; only $<20$% of the measurements (all towards the fainter end)
require a second PSF, perhaps to account for the faint star just to the north
of PGIR 20dci (see the Spitzer image panel in Figure 3).

For the WISE Multi-Epoch catalog and the subsequent, ongoing NEOWISE
monitoring data, we took the median of all profile fitting magnitude
measurements within each spacecraft “visit” covering the position, and we
calculated the error as the dispersion among these magnitudes. The median time
span per WISE or NEOWISE visit is about 1.25 days. Results are provided in
Table 1.

As illustrated in Figure 3, the mid-infrared lightcurve of PGIR 20dci
experienced an exponential rise phase over the past few years, that appears to
be plateauing during 2020. Considering only the three NEOWISE measurements at
MJD = 58491.8, 58698.2, and 58858.1, before the flattening, the rise rate was
$-0.121\pm 0.015$ mag/month or -1.40 mag/year during 2019-2020. We note that
this is identical, within the errors, to the slope derived for the $J$-band
lightcurve ($-0.116\pm 0.011$ mag/month; see discussion above regarding Figure
2). Before this steep recent rise, there was a much shallower but steady
upward trend in the WISE/NEOWISE lightcurve between 2010 and 2019, with a rise
slope of only -0.01 mag/month or -0.13 mag/year.

###  3.4 Overall Mid-Infrared Light and Color Evolution

As discussed in 3.2.2, prior to the 2010 WISE mission measurements, there are
data available in 2009 and 2004 from Spitzer. A shallow rise is also seen in
these data, though there are a limited number of data points. The steeper jump
that is implied between the Spitzer and WISE epochs occurs in a fairly
narrowly confined time frame, within 2009 September 6 (Spitzer) and 2010
January 26 (WISE), and warrants critical investigation.

The two shortest wavelength Spitzer and WISE filters are similar, and Jarrett
et al. (2011) find agreement in large-scale photometry comparisons to within
2-3%. However, PGIR 20dci is an extremely red source, beyond the range of
colors available to Jarrett et al. (2011), and thus we need to consider
potential color terms between the earlier Spitzer and later WISE photometry.
Similar considerations were also made for the lightcurve analysis of Gaia
17bpi (Hillenbrand et al., 2018) and PTF 14jg (Hillenbrand et al., 2019),
where color transformations were derived111111
$(W1-W2)_{corr}=1.62\times(IRAC1-IRAC2)-0.04$ mag, with rms = 0.24 mag. and
implemented, but not magnitude transformations. Here for PGIR 20dci, we have
tried to assess more carefully the impact of color terms on the lightcurve, by
looking at a large sample of young stellar objects observed with both Spitzer
and WISE. While there is a narrow correlation in the magnitudes themselves,
when considering the IRAC1$-W1$ and IRAC2$-W2$ transformations as a function
of IRAC1-IRAC2 color, the comparison data show a fanning out towards redder
IRAC1-IRAC2 color, which introduces significant uncertainty. Identical issues
were investigated by Antoniucci et al. (2014, see their section 2.1.1) and we
have adopted their corrections121212
$W1_{corr}=IRAC1-0.27489\times(IRAC2-IRAC1)+0.07146$ and
$W2_{corr}=IRAC2+0.1422\times(IRAC2-IRAC1)-0.01855$ . For the IRAC1-IRAC2
color of PGIR 20dci, this amounts to shifts of approximately 0.5 mag fainter
in going from IRAC1 to $W1$, and -0.25 mag brighter in going from IRAC2 to
$W2$. Figure 3 illustrates both the observed and the WISE-corrected Spitzer
magnitudes, which also appear in the WISE columns of Table 1.

In order order to negate the conclusion of a large jump in brightness between
the last Spitzer epoch and the first WISE epoch, the color corrections to the
photometry would have to be much larger than currently supposed (which is
certainly possible), but also in the opposite direction for the Spitzer IRAC1
channel. We also note that, even among the very red sources in this spatial
region, PGIR 20dci is an outlier in direct comparisons of Spitzer vs AllWISE
photometry, lending support to the reality of the apparent brightness jump.

We can further scrutinize the WISE/NEOWISE photometry by considering reported
aperture magnitudes. The aperture photometry exhibits the expected trends with
aperture size, with the 5″.5 aperture undersized and producing magnitude
overestimates, and other apertures oversized and thus having magnitudes that
well-underestimate the profile fitting magnitudes. Even if the undersized 5″.5
aperture photometry were used, the brightness jump between the Spitzer 2009
and WISE 2010 measurements would remain, albeit reduced by about 0.25 and 0.75
mag in $W1$ and $W2$, respectively. We note that the w[12]flg_1 (5.5″ aperture
confusion) column indicates that about 8% of the aperture photometry
measurements have some identified source of confusion; this is not significant
for the w1mpro magnitude measurements that we have adopted, however. Overall,
the profile fitting photometry is more robust to bad pixels, cosmic rays, and
the need to establish aperture corrections.

Finally, we can consider the evidence for color changes as the source has
brightened in the mid-infared. The recent steep brightening of PGIR 20dci has
been colorless, within the errors. The median color over the 2014-2020 NEOWISE
epochs is consistent with a constant value of $W1-W2=1.40$ mag and rms = 0.21
mag. Earlier, in 2010, however, the WISE colors were $W1-W2\approx 1.23$ mag
for each of the three epochs, so marginally bluer in the period just after the
2009-2010 jump in brightness. The even earlier 2009 and 2004 Spitzer colors
are both IRAC1-IRAC2 $\approx 1.60$, which after applying the Spitzer-to-WISE
color transformation quoted above, results in a corresponding $W1-W2=2.55$
mag. Applying the Antoniucci et al. (2014) magnitude corrections detailed
above results in fairly similar colors of 2.40, 2.31, and 2.26 mag for the the
three Spitzer epochs.

The above suggests that the color of PGIR 20dci became bluer by about 1 mag in
$W1-W2$ as it brightened during late 2009 and very early 2010 period, before
reddening by a small amount ($\approx 0.2$ mag) and then continuing to
brighten colorlessly. It is hard to know how to interpret this overall
behavior. If driven by an accretion event, one would expect some blueing
behavior. At the same time, there may be extinction clearing going on, which
would also imply a color change to the blue. The neutral color behavior that
is observed over the past five years as PGIR 20dci has brightened seems
anomalous with respect to these expectations.

In summary, the mid-infrared lightcurve of PGIR 20dci shows that the source
has undergone a long-timescale, two-step rise from a faint state to a bright
state over the past $\sim 15$ years. During the period 2004-2019, the source
increased its mid-infrared brightness by $>5$ mag in total. There was a
shallow brightening from 2004-2009, a first significant jump in brightness
during a 4 month time frame in late 2009 or early 2010, a second shallow rise
phase, and a more recent second significant brightness jump beginning in 2019
and plateauing in 2020.

##  4 Follow-up Observations and Data Analysis

In this section, we describe the imaging and spectroscopy that we collected on
PGIR 20dci.

###  4.1 Imaging

####  4.1.1 Data Acquisition

PGIR 20dci was flagged for follow-up with the Palomar 60” telescope and SEDM
(Spectral Energy Distribution Machine; Blagorodnova et al., 2018) in its
imaging mode. A sequence in the $gri$ filters was acquired on 2020 August 14
(UT) and reduced using the photometric reduction pipeline.

On 2020 August 28, we obtained near-infrared imaging of PGIR 20dci using the
Wide Field Infrared Camera (WIRC; Wilson et al. 2003) on the Palomar 200-inch
telescope. The data were acquired as a series of dithered exposures in $J$,
$H$, and $Ks$ bands for a total exposure time of 495, 330, and 300 seconds,
respectively. The data were reduced, stacked and photometrically calibrated
using the pipeline described in De et al. (2020a).

####  4.1.2 Findings

No counterpart to PGIR 20dci was detected in $g$ and $r$ bands, with an
approximate upper limit of $>21$ mag. However, we detect a faint extended
source in $i$ band, coincident with the bright nebula detected in the near-
infrared image. The final $Ks$-band image was shown in Figure 1. A close-up on
PGIR 20dci appears in Figure 4, which highlights the extended cometary
structure of the source in the near-infrared. Figure 5 shows the JHK color
composite image.

Taken together, the optical and infrared imaging suggests an extremely red
source, as well as a bright, extended, scattered light component associated
with the brightening of PGIR 20dci. The angular size of the extended structure
corresponds to a physical size $\sim$14,000 AU.

Figure 4:  Approximately 40″$\times$30″ cutout from the Palomar/WIRC $Ks$ (2.2
$\mu$m) image of Figure 1, centered near PGIR 20dci. Grid spacing is 5 time
seconds in R.A., and 5 arcseconds in Dec. The extended structure is oriented
southeast to northwest, with concentrated flux towards the southeast and
increasingly diffuse emission towards the northwest, including beyond the
lowest contour. The extent of the scattered light within the contours is
$\sim$14,000 AU.  Figure 5:  Color rendering of the Palomar/WIRC JHK data, in
standard orientation of north upward and east to the left. PGIR 20dci in
outburst is the extended source near center frame. Faintly visible are
extended “tails” of emission to the northwest, which may indicate the
orientation of the outflow axis.

In order to encompass the non-point source, extended nature of PGIR 20dci, we
measured photometry from the Palomar/WIRC images using a fairly large aperture
radius of 9 pixels (2″.25), whereas a 5-pixel (1″.25) radius would suffice for
point sources. Using the smaller radius would decrease the brightness of PGIR
20dci by about 0.1 mag. We calibrated by matching to 110 sources in common
between our final images and the 2MASS catalog. We find $J=14.83$, $H=13.08$,
and $Ks=11.77$ mag in 2020, September. The near-infrared colors of PGIR 20dci
in outburst are thus $J-H=1.75$ mag and $H-Ks=1.31$ mag. This location in a
$J-H$ vs $H-K$ diagram is consistent with that of a young stellar object that
is highly reddened.

###  4.2 Spectroscopy

####  4.2.1 Data Acquisition

On 2020 August 13, PGIR 20dci was observed with Keck II and NIRES (Near
Infrared Echellette Spectrometer), which is part of the family of similar
instruments described in Wilson et al. (2004). The detector records a prism-
dispersed simultaneous $YJHK$ spectrum at resolution $R\approx$ 2700\. Four
spectra were acquired using a 6″ ABBA nodding pattern, with integration time
of 90 seconds at each position. The data were reduced using a modified version
of spextool (Cushing et al., 2004), and flux-calibrated with the telluric A0V
standard star HIP 6002 using the xtellcor code (Vacca et al., 2003). Figure 6
shows the final extracted and combined spectrum. Due to the very red nature of
the source, there is little signal at the shorter wavelengths, and thus we
show only the $JHK$ spectral region.

Figure 6:  Comparison of PGIR 20dci (magenta) with the FU Ori stars V1057 Cyg
(green, from Connelley & Reipurth, 2018) and Gaia 17bpi (blue, from
Hillenbrand et al., 2018). For purposes of plotting, the comparison objects
have been scaled and shifted, with the corrections differing among the
different panels. The spectral match is apparent in molecular 12CO, H2O, and
OH, and possibly weak TiO and VO. Also present are atomic lines of H I
Pa$\beta$ (and possibly Br10), He I 10830 Å, and metals including Na I, Mg I,
Al I, Si I, and Ca I. While the hydrogen and helium lines likely have some
outflow aspect to their absorption, the metal lines originate in the
photosphore, and combined with the molecular line absorption indicate a mixed
temperature spectrum.

On 2020 September 4, a second spectrum was obtained at $R\approx 2700$ with
the Palomar 200” and the TripleSpec (Herter et al., 2008) near-infrared
spectograph. Although lower signal-to-noise, and not shown as a result, the
spectrum is broadly the same in its shape and presence of strong spectral
features as the Keck/NIRES spectrum from a month earlier. However, the flux
level is brighter. Considering Figure 2, the flux at this point in time could
have been several tenths of a magnitude brighter, though the error on this
estimate is large. A more likely explanation is that the TripleSpec slit
(1.0″) is wider than the NIRES slit (0.55″), and thus the spectrum would
include more of the bright extended structure of the source (see Figure 4).
Normalizing the two at $K$-band, the $K$-band slope is identical to that shown
in Figure 6, but there is a sharper break just before the CO region, resulting
in even deeper CO absorption. The $H$-band region is then slightly brighter
than the spectrum shown in Figure 6. This could mean that the source became
slightly bluer while temporarily brighter at this epoch. However, we attribute
the difference to the possibility that the two spectra may have included
different parts of the extended nebula.

####  4.2.2 Findings

Focussing on the Keck/NIRES spectrum, PGIR 20dci is highly reddened,
consistent with the extremely red mid-infrared colors measured by WISE. The
comparison objects131313Gaia 17bpi is a recent FU Ori outburst occurring in
2017 (Hillenbrand et al., 2018), which is still near its peak brightness.
V1057 Cyg is one of the originally defined FU Ori stars, estimated to have
outburst in 1970 (Herbig, 1977), and although it has faded considerably, the
spectrum maintains the general features of FU Ori stars (e.g. Connelley &
Reipurth, 2018). in Figure 6 are less reddened than PGIR 20dci. We can
estimate the relative reddening, and thus the extinction, by applying a
standard extinction law to find the the best spectral match to the V1057
spectrum. We find a differential extinction $\Delta A_{V}=16$ mag, which can
be added to a baseline $A_{V}$=4 mag for V1057 (Connelley & Reipurth, 2018).
The total extinction towards PGIR 20dci is thus estimated to be $A_{V}\approx
20$ mag.

Spectroscopically, the most salient feature of PGIR 20dci is the strong CO
absorption. The source has both the $\Delta\nu=2$ CO bands in the $K$-band,
and the $\Delta\nu=3$ bands CO in the $H$-band. There is also strong H2O in
$J$-band and $H$-band, probably in $K$-band as well, and likely $OH$ in
$H$-band. Less obvious is the expected TiO and VO absorption, though it may be
weakly present. PGIR 20dci shares with Gaia 17bpi and V1057 Cyg in having
clear $J$-band H2O absorption, and $H$-band and $K$-band CO absorption. The
H2O is weaker in PGIR 20dci, while the CO is stronger. We note that the CO in
V1057 Cyg is atypically weak for FU Ori stars (Connelley & Reipurth, 2018).

In terms of atomic lines, weak Na I, Mg I, Al I, Si I, and Ca I absorption are
all present. There is also weak H I Pa$\beta$ absorption at $1.28~{}\mu$m. The
metal line strengths are similar to those seen in the comparison object Gaia
17bpi, and somewhat stronger than in V1057 Cyg. PGIR 20dci and Gaia 17bpi also
have comparable Pa$\beta$, which is likely blueshifted and formed in a wind.

Notably, the He I 10830 Å line in PGIR 20dci shows a clear P Cygni profile
(see inset panel within Figure 6). This is evidence of a strong wind or
outflow, with blueshifted absorption in He I 10830 Å frequently seen in FU Ori
stars (Connelley & Reipurth, 2018). We estimate a maximum velocity around -450
kms-1.

In the nomenclature of Connelley & Reipurth (2018), PGIR 20dci appears to be a
bona fide FU Ori star. In addition to having had its eruption observed, it
satisfies all of the spectroscopic criteria: CO absorption, H2O absorption, VO
or TiO absorption (weakly), Pa$\beta$ absorption, lack of emission lines, weak
metal absorption, and He I 10830 Å absorption. The hydrogen and helium lines
likely have some outflow aspect to their absorption. The metal lines originate
in the source photosphere, however, and combined with the molecular line
absorption, indicate a mixed temperature spectrum.

##  5 Discussion

The dramatic brightening behavior of PGIR 20dci is detected most obviously in
the mid-infrared (Figure 3). The combined Spitzer, WISE, and NEOWISE data
provide a long duration lightcurve. As the source has brightened, it has
become detectable in the near-infrared as well (Figure 2). In the PGIR data,
which is higher cadence, the brightness of PGIR 20dci is currently near the
survey limit due to the high background in this field, and the lightcurve is
thus somewhat noisy, though well-sampled. Over only the few months prior to
manuscript submission, PGIR 20dci has appeared near the faint limit in the ZTF
alert stream, sampling the red optical. Given the evidence for a current
(second) plateau phase, the source may not become amenable to study in the
optical, however.

A two-step rise is indicated by the mid-infrared lightcurve, each step being
preceded by a shallow rise phase. The earliest available data for PGIR 20dci
show a shallow brightening between 2004 and 2009. Then PGIR 20dci exhibited a
rapid (-0.25 mag/month) blue brightening by 1.8 mag in $W1$ and 0.8 mag $W2$
between 2009 September 6 and 2010 January 26. This was followed by a shallower
slope (-0.01 mag/month), long-term colorless brightening over 2010-2018,
amounting to only another 0.5 mag in both W1 and W2. The slope of the
brightening increased during the latter part of 2018, and switched in 2019 to
a much steeper rise rate (-0.12 mag/month). At this point, the source became
detectable by PGIR in the near-infrared. PGIR (1.2 $\mu$m) and NEOWISE (3.4
and 4.6 $\mu$m) both record the same rise slope in this time frame, and both
show a transition to a plateau during 2020. The total brightening of PGIR
20dci over the past 16-25 years is $\sim 5-5.5$ mag.

Putting these lightcurve results into context, the recent rise slope compares
well to the typical rise rates that can be derived for the optical lightcurves
of historically documented FU Ori outbursts. These have a wide range, but are
most frequently about $-0.05$ to $-0.2$ mag/month (Hillenbrand, 2021, in
preparation). Regarding the two-step rise in the mid-infrared lightcurve, a
similar phenomenon was recorded for the confirmed FU Ori source Gaia 17bpi
(Hillenbrand et al., 2018). However, in that object the total duration of the
first plateau phase was shorter, only 1-2 years, instead of the 5-10 years
measured here for PGIR 20dci.

We can estimate a source luminosity near the outburst peak based on the
brightness level of the apparent plateau in the lightcurve ($J\approx 14.6$
mag from PGIR and $r\approx 20.5$ mag from ZTF), and the adopted distance
$d=2.81$ kpc. The bolometric correction is of course unknown, but adopting
values appropriate for a mid-F type star leads to a luminosity estimate of
10-12 $L_{\odot}$, with perhaps a factor of 3 uncertainty if the bolometric
correction is $\pm 1$ mag. This is on the far low side of FU Ori outburst peak
luminosities. Only Gaia 17bpi, the only other source known to have exhibited a
two-step mid-infrared brightening, like PGIR 20dci has, and HOPS 383, an
embedded young star outburster whose status as an FU Ori type outburst is
unclear, have luminosities this low.

Concluding our discussion, we note that the best studied FU Ori stars are the
optically visible portion of the population, especially those bright enough
for high dispersion spectroscopy. However, a sizable fraction of the known FU
Ori stars, about 1/3, are seen only at infrared wavelengths due to high
extinction (e.g. the $A_{V}=20-50$ mag population in Connelley & Reipurth,
2018). Indeed, theoretical predictions are that earlier stage young stellar
objects experience a higher rate of FU Ori type outbursts (Bae et al., 2014).
Most of the known optically invisible group have been designated as FU Ori
objects through source brightening at K-band that was noticed only post facto,
rather than while it was actually happening, along with spectroscopic
confirmation. Embedded FU Ori sources thus generally do not have quiescence-
to-rise lightcurves available.

Because it meets all applicable criteria for the FU Ori designation, notably
the spectral absorption signatures, lack of emission lines other than P Cygni
structure, and the total lightcurve amplitude, PGIR 20dci can be added to the
list of optically invisible FU Ori candidates. Unlike the other sources
141414We note that the VVV survey has produced some interesting candidates as
well, recently Guo et al. (2020)., PGIR 20dci has had its photometric rise
captured through wide-field continuous photometric monitoring programs.

##  6 Summary

We have discovered an outbursting young stellar object first detected at near-
infrared wavelengths, but also seen in long duration mid-infrared time series
data. PGIR 20dci is associated with the NGC 281-W molecular core, a known area
of recent and ongoing dense star formation activity. Its salient features are:

  * •

A Class I spectral energy distribution.

  * •

Foreground interstellar, molecular core, and circumstellar extinction
amounting to $A_{V}\approx 20$ mag.

  * •

A photometric rise that was detected in the PGIR time-domain survey, and
emphasized by comparing the recent $J$-band photometry with the lack of a
$J$-band detection at the same position more than two decades ago in the 2MASS
all-sky photometric survey or in the pointed work of Megeath & Wilson (1997),
implying a total near-infrared brightness increase of $>5$ mag.

  * •

Extended nebulosity in our new $Ks$-band imaging, that was not present in
1990’s images at the same wavelength, with a cometary type structure $\sim
14,000$ AU in size.

  * •

A measured 10-15 year photometric brightening trend exhibited in $3-5~{}\mu$m
Spitzer $+$ WISE $+$ NEOWISE data, accumulating a total mid-infrared
brightness increase of $>5$ mag.

  * •

Steepened mid-infrared brightening during 2019 that was coincident with source
detection at J-band.

  * •

Consistent rise rates during 2018-2020 in the mid-infrared NEOWISE lightcurve,
and 2019-2020 in the near-infrared PGIR lightcurve, amounting to $-0.12$
mag/month.

  * •

A flattening of the lightcurve from its rise phase to a plateau in 2020. The
peak luminosity is estimated at a modest 10-12 $L_{\odot}$.

  * •

A near-infrared absorption line spectrum showing hydrogen line, metal line,
and molecular band absorption, indicative of a “mixed temperature” spectrum.

  * •

Outflow as evidenced by a He I 10830 Å profile, with blueshifted absorption
seen to about -450 kms-1.

We conclude that PGIR 20dci is a bona fide FU Ori star. The object warrants
further study, especially at high spectral dispersion, as its outburst
continues to develop.

Palomar Gattini-IR (PGIR) is generously funded by Caltech, Australian National
University, the Mt Cuba Foundation, the Heising Simons Foundation, the Bi-
national Science Foundation. PGIR is a collaborative project among Caltech,
Australian National University, University of New South Wales, Columbia
University and the Weizmann Institute of Science. MMK acknowledges generous
support from the David and Lucille Packard Foundation.

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