# No Activity Among 13 Centaurs Discovered in the Pan-STARRS1 Detection
Database

Eva Lilly11affiliation: Planetary Science Institute, 1700 E. Fort Lowell,
Suite 106 Tucson, AZ 85719-2395, USA<EMAIL_ADDRESS>Henry Hsieh11affiliation:
Planetary Science Institute, 1700 E. Fort Lowell, Suite 106 Tucson, AZ
85719-2395, USA , James Bauer22affiliation: University of Maryland, College
Park, MD 20742-2421, USA , Jordan Steckloff11affiliation: Planetary Science
Institute, 1700 E. Fort Lowell, Suite 106 Tucson, AZ 85719-2395, USA
33affiliation: University of Texas at Austin, Austin, TX, 78712, USA , Peter
Jevčák44affiliation: Comenius University, Mlynská dolina, 842 48 Bratislava,
Slovakia , Robert Weryk55affiliation: Institute for Astronomy, University of
Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA , Richard J.
Wainscoat55affiliation: Institute for Astronomy, University of Hawaii, 2680
Woodlawn Drive, Honolulu, HI 96822, USA , Charles Schambeau66affiliation:
University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816,
USA

###### Abstract

Centaurs are small bodies orbiting in the giant planet region which were
scattered inwards from their source populations beyond Neptune. Some members
of the population display comet-like activity during their transition through
the solar system, the source of which is not well understood. The range of
heliocentric distances where the active Centaurs have been observed, and their
median lifetime in the region suggest this activity is neither driven by
water-ice sublimation, nor entirely by super-volatiles.

Here we present an observational and thermo-dynamical study of 13 Centaurs
discovered in the Pan-STARRS1 detection database aimed at identifying and
characterizing active objects beyond the orbit of Jupiter. We find no evidence
of activity associated with any of our targets at the time of their
observations with the Gemini North telescope in 2017 and 2018, or in archival
data from 2013 to 2019. Upper limits on the possible volatile and dust
production rates from our targets are 1-2 orders of magnitude lower than
production rates in some known comets, and are in agreement with values
measured for other inactive Centaurs.

Our numerical integrations show that the orbits of six of our targets evolved
interior to r$\sim$15 AU over the past 100,000 years where several possible
processes could trigger sublimation and outgassing, but their apparent
inactivity indicates their dust production is either below our detection limit
or that the objects are dormant. Only one Centaur in our sample – 2014 PQ70
experienced a sudden decrease in semi-major axis and perihelion distance
attributed to the onset of activity for some previously known inactive
Centaurs, and therefore is the most likely candidate for any future outburst.
This object should be a target of high interest for any further observational
monitoring.

Centaurs, TNOs, Comets

##  1 Introduction

Centaurs are small bodies most commonly defined as having perihelia ($q$) and
semimajor axes ($a$) between the orbits of Jupiter and Neptune, i.e., 5.2 AU
$<q,a<$ 30 AU (e.g. Jewitt, 2009), which are migrating towards the inner Solar
system from their source regions in trans-Neptunian space. Once a trans-
Neptunian object evolves to within Neptune’s orbit and becomes a Centaur, it
enters a chaotic, gravitational pin-ball machine where it is rapidly passed
between the gravitational influences of the giant planets. Centaurs eventually
leave the region via several routes - they collide with one of the planets,
they return back to the trans-Neptunian region or even get ejected from the
solar system, or about 20% of Centaurs enters an end state as a Jupiter-family
comet (JFC; Volk and Malhotra, 2008; Sarid et al., 2019). Due to the chaotic
gravitational environment, the typical dynamical lifetime of a Centaur is only
$\sim$10 Myr (Tiscareno and Malhotra, 2003; Di Sisto and Brunini, 2007). In
this way, the Centaur population delivers useful clues of the pristine
composition and physical properties of small trans-Neptunian objects (TNOs)
beyond our current observational limits right to our doorstep.

One of the most puzzling characteristics of Centaurs is comet-like activity
exhibited by $\sim$5% of the population (Chandler et al., 2020). Unlike in
most short-period comets, this activity is not water-driven because the
majority of active Centaurs resides at large heliocentric distances where the
surface temperatures are too low to allow for water ice sublimation (Meech and
Svoren, 2004; Womack et al., 2017). Active Centaurs also show a variety of
behavior, from long-term low-level outgassing such as in (2060) 95P/Chiron
(Luu and Jewitt, 1990; Foster et al., 1999), to sudden violent outbursts in
previously known dormant objects, as observed in 174P/Echeclus (Choi et al.,
2006; Bauer et al., 2008; Rousselot, 2008; Kareta et al., 2019; Seccull et
al., 2019), and unlike in regular comets, the timing of the activity onset is
not obviously centered on perihelion. Dust production rates often range by
orders of magnitudes from one object to another, and the source of this
activity remains unknown. Most known Centaurs have spent the majority of their
lifetime in region too warm to maintain surface supplies of super-volatile
ices such as CO and CO2 (Jewitt, 2009), for example if free CO ice was present
on all Centaurs, we should observe Centaurs with comae well beyond the orbit
of Neptune, just like in some long period comets (e.g. Szabó et al., 2008;
Meech et al., 2017), which is not the case. Multiple works have shown there
are no known active Centaurs beyond r$\sim$14 AU, and that the lack of
activity cannot simply be attributed to observational selection effects
(Jewitt, 2009; Cabral et al., 2019; Li et al., 2020). The fraction of active
objects among Centaurs with perihelia smaller than 14 AU is almost 20%, while
only 10% of Centaurs are active within the overall population. Moreover,
almost 27% of all active Centaurs has perihelia smaller than 6.5 AU, as can be
seen on Figure 1.

Therefore considering the wide range of heliocentric distances at which
Centaurs exhibit comae, the Centaur activity likely has more sources than just
sublimation of surface volatiles, and these sources may be triggered by
multiple mechanisms, some of which could be different than those acting in
regular comets.

It has been suggested that activity on Centaurs and some comets at large
heliocentric distances is driven by the crystallization of amorphous water ice
(AWI), which is porous and can contain molecules of super-volatile ices
trapped in the AWI during the condensation (Notesco and Bar-Nun, 1996; Jewitt,
2009; Shi and Ma, 2015). When the parent body enters a warmer environment, a
crystallization front starts propagating into the interior of the body,
releasing trapped gas molecules which then start to loft dust grains at
temperatures much lower than necessary for sublimation of crystalline water
ice. AWI has been detected by in situ observation of the Galilean moons of
Jupiter, (Hansen and McCord, 2004), however, the presence of AWI has never
been observationally confirmed on small bodies in the Solar system(Lisse et
al., 2013).

The phase transition of AWI to crystalline water ice has been proposed to
start at a heliocentric distance of r$\sim$7 AU Jewitt (2009); Womack et al.
(2017), while recent thermal modeling studies show it can be triggered up to
r$\sim$ 14 AU, with some extreme cases crystallizing as far as at r$\sim$ 16
AU (Guilbert-Lepoutre, 2012). Figure 1 shows that the known active Centaurs
indeed have perihelia within this heliocentric distance range, which could
support the idea of AWI crystallization as their main activity driver.
However, Figure 1 also shows that there are other apparently inactive bodies
with very similar orbital parameters. Thus, the mere presence of AWI, which
should be abundant in TNOs and Centaurs (Jenniskens and Blake, 1994), does not
appear to be enough on its own for an object to exhibit activity, indicating
that activity may require a certain trigger. Fernández et al. (2018) suggested
that a rapid change in the thermal environment of the body could jump-start
AWI crystallization in a dormant object. Such an orbital change could be
caused by a close encounter with a planet and subsequent decrease of
perihelion or semi-major axis, which is believed to have led to outbursts in
several Centaurs and the previously dormant quasi-Hilda P/2010 H2 (Vales)
(e.g. Mazzotta Epifani et al., 2006; Fernández et al., 2018; Jewitt and Kim,
2020).

Another plausible explanation for increased activity in Centaurs that have
been only recently scattered inward would be the sublimation of subsurface
pockets of CO2 and other super-volatile ices. CO2 in particular is
significantly more volatile than water ice, and is one of the most common
species found on cometary bodies (e.g. Bockelée-Morvan et al., 2004; A’Hearn
et al., 2011). Thermal models show that it can sublime vigorously interior to
$\sim$15 AU (Steckloff et al., 2015; Meech et al., 2017), which coincides
closely with the heliocentric distances of observed Centaur activity. Such
subsurface deposits could be forcefully exposed by some external force and
start to rapidly sublimate. We can rule out impacts as the predominant surface
activity trigger on Centaurs due to the very low collision rate in the region
(Durda and Stern, 2000), but other forces might play a role. For example,
Steckloff et al. (2016) has shown that topographic features on the surface of
comets, when steeper than $\sim$30 degrees, are prone to mass wasting events
that can excavate buried volatiles such as CO and CO2 to the surface.

An important aspect to consider is also the time an object has already spent
in the Centaur region, or the past changes in its orbit pushing its orbit
within the AWI crystallization limit or past the point where the sublimation
of other supervolatiles is possible. In general it is expected the active
Centaurs have young orbits - i.e.they have only recently entered the region,
since the timescale on which the crystallization front is able to propagate
before it depletes the AWI supplies is fairly small (in order of 105 years,
Guilbert-Lepoutre (2012)), and also the depletion of the CO-layers is rapid.
(Li et al., 2020).

CO has been detected in the comae of several Centaurs (e.g. Womack and Stern,
1997; Wierzchos et al., 2017; Womack et al., 2017), while CO2 that is
notoriously challenging to detect with ground-based assets, has only been
observed in comets so far (Ootsubo et al., 2012; Womack et al., 2017). It is
unclear if the observed production rates were caused by gases released during
the AWI phase transition or from sublimation of subsurface supervolatile ice
pockets. Therefore to better understand the types of activities displayed in
the Centaur region we need to gain a more informed and complex view of the
orbital and thermal histories of Centaurs and the current status of their
volatile content.

Observations for this work have been collected in the framework of a long-term
program aimed at searching for active Centaurs and analyzing their orbital and
thermal environment. Here we present the analysis of our pilot study on a
sample of 13 Centaurs that were discovered in the detection database of the
Panoramic Survey Telescope and Rapid Response System survey (Pan-STARRS1;
Weryk et al., 2016).

##  2 Methods

In this work we have observed and analyzed 13 Centaurs discovered in the Pan-
STARRS1 detection database (Weryk et al., 2016). All our targets have well-
defined orbits with orbital arcs spanning almost a decade. Our investigation
consisted of three steps: 1) analysis of images taken with Gemini-N to search
for comet-like activity, 2) a search of archival images from the Dark Energy
Camera (DECam) to look for traces of activity in the past, 3) thermal modeling
to examine surface temperatures as a function of orbital parameters and object
size to put upper limits on sublimative gas production from volatile species
potentially present on the surface, and 4) numerical integrations to inspect
the orbital histories of objects in our sample.

###  2.1 Observations and Data Reduction

We observed our targets with the GMOS-N (Gemini Multi-Object Spectrograph)
instrument on the Gemini North telescope on Maunakea, Hawaii between 8/1/2017
and 1/31/2018 (# GN-2017B-Q-69). GMOS-N has a 300 x 300 arcsec field of view
and a pixel scale of 0.0728 arcsec px-1. For our observations, we used the
imaging mode of GMOS N with 2x2 binning and a Sloan Digital Sky Survey (SDSS)
r’ filter (Fukugita et al., 1996), which is ideal for detecting dust (Meech
and Svoren, 2004). We employed non-sidereal tracking at the apparent rate and
direction of motion for each target, and acquired sequences of three 100 s
exposures, which were necessary to correctly identify each target based on
their motion. All our observations were done in queue mode, meaning each
object was observed under slightly different conditions throughout the
semester. Our observations are summarized in Table 1.

Standard bias subtraction, flatfield correction, and cosmic ray removal were
performed for all images using Python 3 code utilizing the ccdproc package in
Astropy111http://www.astropy.org (Astropy Collaboration et al., 2018) and the
L.A.Cosmic python module222Written for python by Maltes Tewes
(https://github.com/RyleighFitz/LACosmics) (van Dokkum, 2001a; van Dokkum et
al., 2012). Images from each night were aligned on the object and co-added
together to create composite images using IRAF software (Tody, 1986, 1993) in
order to maximize signal-to-noise (S/N). Absolute photometric calibration was
carried out using a photometry pipeline developed by Mommert (2017), standard
IRAF routines, and the all-sky ATLAS-REFCAT2 catalog (Tonry et al., 2018).

####  2.1.1 Target Selection

Our target list consisted of 13 Centaurs selected from a sample of 29 new
Centaur discoveries from the Pan-STARRS1 detection catalog (Weryk et al.,
2016) that were observable in the second half of 2017 from the Northern
hemisphere and were within Gemini-N observation limits. We included objects
with perihelia spanning the entire Centaur region and crossing the putative
AWI crystallization heliocentric distance interval (Jewitt, 2009; Guilbert-
Lepoutre, 2012). Most of the known active Centaurs were discovered close to
perihelion and were active at discovery. We therefore primarily selected
targets close to perihelion to ensure the best conditions for observing
possible activity. As of June 2021 none of our targets have been subject of
characterization by other works.

Figures 1 and 2 show that our targets cover a wide range of orbital elements
typical for Centaurs, and stay clear of the region often referred to as the
“JFC Gateway”, a temporary low-eccentricity region exterior to Jupiter through
which the majority of Centaurs bound to become JFCs pass through (Sarid et
al., 2019; Steckloff et al., 2020). In contrary, most objects in our sample
have perihelia near the upper limit of the AWI crystallization zone because
the data-mining algorithm applied to the Pan-STARRS1 detection database was
tuned to find objects beyond Neptune and the discovered Centaurs were mostly a
byproduct of that search (Weryk et al., 2016). It can also be seen that the
activity in Centaurs does not seem to be related to any preferential
eccentricity or inclination ranges, but instead their perihelion distances
appears to be the main constraint.

By definition, JFCs orbiting in the same region as the Centaurs (i.e., 5.2 AU
$<q,a<$ 30 AU) differ only by the fact they are active, and in terms of
Tisserand’s parameter with respect to Jupiter, $T_{J}$, which is given by

$T_{J}=a_{J}/a+2\cos i\sqrt{a/a_{J}(1-e^{2})},$ (1)

where $a_{J}$ is the semimajor axis of Jupiter, and (a,e,i) is the semimajor
axis, eccentricity, and inclination of the Centaur respectively. JFCs have
2$>T_{J}>$3 and perihelia below $\sim$6 AU, while Centaurs typically have
larger TJ and larger perihelia, signaling that their orbits are decoupled from
Jupiter (Fernández et al., 2018). To add to the confusion, the Gateway objects
are by definition Centaurs, but their orbits are not decoupled from Jupiter,
while there also are inactive Centaurs with $T_{J}<$3 in the population.
Objects with perihelia between 5.2 AU and 6 AU are thus on the border of these
two classes and they often make trips to within Jupiter’s orbit and back to
the Centaur region - the typical example being the famous Gateway Centaur 2019
LD2 (Steckloff et al., 2020),. Therefore we use the term “JFC Centaurs”
throughout this paper for addressing these borderline active objects. There
are a total of 18 active Centaurs and 21 JFCs Centaurs known in the region as
of 2021 February 10333https://physics.ucf.edu/ yfernandez/cometlist.html#ce.

We have also calculated Tisserand’s parameter with respect to Saturn and
Uranus - $T_{S}$ and $T_{U}$ respectively, and Table 2 shows that about half
of our targets are likely more influenced by the outer planets than Jupiter.
The current values of $T_{J}$ of our targets and other bodies in the region
are plotted against their perihelion distance, and are shown in Figure 3.
Again, we do not note any apparent correlation between activity and $T_{J}$.

###  2.2 Archival Search

We used the Canadian Astronomy Data Centre’s Solar System Object Image Search
(Gwyn et al., 2012) to search the Dark Energy Survey (DES; Abbott et al. 2018)
archive for serendipitous observations of our 13 Centaurs from the Dark Energy
Camera (DECam; Flaugher et al. 2015) installed on the Cerro Tololo Inter-
American Observatory’s Blanco 4-meter telescope. The DECam instrument consists
of 62 individual 2k$\times$4k CCDs for imaging in a hexagonal arrangement on
the focal plane. This results in a 2.2 degree diameter field of view and
0′′.263 per pixel plate scale. The combination of a relatively large field of
view and the deep imaging afforded by the Blanco telescope results in a
treasure trove of serendipitously imaged Solar System small bodies using
DECam. Such a search yielded the recent discovery of an active Centaur, 2014
OG392 (Chandler et al., 2020).

Since the archival data we used only cover observation epochs between
$\sim$2013 and 2019, they do not provide significant orbital coverage of a
typical Centaur orbit, as their orbital periods are several decades- to
several hundred years long. However, these archival data are still useful as
snapshot observations to search for possible activity. For reference, Figure 4
shows the orbital arc of one of our Centaur targets (2014 TV85) that is
covered by data from DECam.

Our DES search revealed numerous epochs of data for all but three of our
targets (2010 TU191, 2014 PQ70 and 2014 WX508). DECam’s archived multi-
extension FITS images have undergone basic reductions including bias
subtraction, flat field correction, and fitting of a World Coordinate System
as described in Morganson et al. (2018). We also applied our Python-based
DECam image processing pipeline to individual images to: (1) identify the FITS
extension containing coverage of each target based on its JPL Horizons
ephemeris coordinates, (2) save the identified individual CCD data and header
metadata as a new FITS image file, (3) perform cosmic ray removal utilizing
the LACosmic technique (van Dokkum, 2001b) as implemented in ccdpro (Craig et
al., 2017), and (4) perform photometric calibration based on Pan-STARRS field
stars (Tonry et al., 2018).

##  3 Results and Discussion

###  3.1 Photometric analysis and Nuclei sizes

In order to search for activity in our observational data, we first identified
the correct object in each star field by “blinking” the individual images, and
visually inspecting each one for extended surface brightness features moving
along with the source. We then stacked the images using standard IRAF routines
to increase the S/N. All our targets appeared stellar based on simple visual
inspection, as can be seen on Figure 5.

To reveal activity much fainter than could be discerned by a visual
inspection, we calculated surface brightness profiles (SBPs) of each target
using the method of Meech et al. (1997) and compared it to the average SBPs of
nearby stars. SBP analysis is a powerful method for detecting faint coma, and
by averaging the mean brightness values within a fixed distance from the
Centaur nucleus we can increase our sensitivity to coma many-fold. Fortunately
our targets were at heliocentric distance large enough that their sky-plane
motion was less than 0.1“/min, and the trailing of the stars used in the SBP
comparison was therefore negligible. In the most extreme case of the fastest
moving Centaur in our sample - 2014 NX65 the tracking elongation was about 21%
of the seeing FWHM values and hence undetectable. SBP analysis further
revealed no evidence of low level activity in any of the Centaurs in our
sample (e.g., 2014 NX65; Figure 6).

We calibrated our photometric data using sources from the GAIA (Gaia
Collaboration et al., 2016) and ATLAS REFCAT2 catalogues (Tonry et al., 2018)
as photometric references, and converted the apparent r’-band magnitudes,
$m_{r}$, measured for our targets to absolute r’-band magnitudes, $H_{r}$
(i.e. apparent magnitudes normalized to $r=\Delta=1$ AU and a solar phase
angle of $\alpha=0^{\circ}$) using:

$H_{r}=H_{r}(\alpha)+2.5\log[(1-G_{r})\cdot\Phi_{1}(\alpha)+G_{r}\cdot\Phi_{2}(\alpha)],$
(2)

where the reduced magnitude, $H_{r}(\alpha)$, i.e., the apparent magnitude
normalized to $r=\Delta=1$ AU, is given by

$H_{r}(\alpha)=m_{r}(r,\Delta,\alpha)-5\log(r\Delta)$ (3)

where $m_{r}(r,\Delta,\alpha)$ is the apparent magnitude at a given
heliocentric distance $r$, geocentric distance $\Delta$, and phase angle
$\alpha$ at the time of observation, with the average IAU slope-parameter
$G=0.13\pm 0.12$ measured in R-filter for Centaurs by Bauer et al. (2003). The
$\Phi_{i}$ functions are given by

$\Phi_{i}(\alpha)=\exp\left\\{-A_{i}\left(\tan\frac{\alpha}{2}\right)^{B_{i}}\right\\}$
(4)

where $A_{1}$ = 3.33, $A_{2}$ = 1.87, $B_{1}$ = 0.63 and $B_{2}$ = 1.22.

We then used our calculated absolute magnitudes and the average Centaur
geometric albedo of $p=0.08$ measured by (Bauer et al., 2013) from the
infrared WISE data to calculate effective nuclei diameters for our objects.
The resulting values are summarized in Table 3. However, Centaurs are a unique
small body population given their striking color bimodality and a wide span of
albedoes (e.g. Peixinho et al., 2004, 2012), where the measured mean albedoes
for the two distinct Centaur groups are 6$\pm$2% for the blue group, and
12$\pm$5% for the redder color group Bauer et al. (2013). Therefore, if our
targets belong to the blue group, they would be 15.4% larger on average, and
if they are in the red group, they would be 18.4% smaller than if they had the
mean albedo.

The photometric measurements reveal that our sample consists of an ensemble of
average-sized Centaurs with effective radii of $2.7<R<60$ km if we use the
mean albedo, where 2014 PQ70 is one of the smallest Centaurs ever observed
with a mean nucleus radius of 2.7 km, similar to a typical JFC (Bauer et al.,
2003; Fernández et al., 2013).

Table 4 summarizes the archival images from DECam that contain Centaurs in our
sample and were suitable for photometric calibration, along with observing
geometry parameters, filters used for the observations in question, and
measured apparent photometric magnitudes. These images were taken for other
research projects, and therefore they were taken using a variety of filters
and exposure times. Out of 13, only 3 objects, 2015 PQ70, 2010 TU191 and 2014
WX508, were not present in the archive. For the rest of our targets, we have
applied the SBP analysis following the procedure applied to the Gemini data,
which revealed no traces of a faint coma on the archival images similar to
what we have observed with Gemini.

Three objects from our target list — 2014 JD80, 2014 NX65 and 2013 MZ11 — were
also observed with the Hubble Space Telescope (HST) by Li et al. (2020), who
searched for activity among high-perihelion objects crossing the Centaur
region. None of the HST targets has exhibited signs of activity.

###  3.2 Thermal environment and its effect on activity levels

In our analysis of the thermal environment of the inspected Centaurs we were
focusing on two possible activity sources: The solar-driven sublimation of
surface volatiles and the crystallization of AWI capable of releasing trapped
gasses. Multiple volatile species have been observationally detected in active
Centaurs (e.g. Senay and Jewitt, 1994; Womack and Stern, 1997; Wierzchos et
al., 2017; Womack et al., 2017), while there is no direct evidence of the
presence of AWI (Lisse et al., 2013) on Centaur surfaces. It has to be noted
that due to their distance and their corresponding faintness, the detection of
any volatile species on Centaurs sized similarly to our sample is extremely
challenging (Kareta et al., 2021).

To asses the plausibility of activity driven by sublimation of surface
volatile deposits, we have computed the theoretical maximal sublimation rates
for the three most common volatiles found in comets, CO, CO2, and H2O for each
of our targets at their observed heliocentric distances. We used the thermal
model described by Steckloff et al. (2015), which assumes a spherical graybody
covered entirely in the pure volatile species of interest, with an albedo of
approximately 4 percent, and emissivity of 0.9. The model calculates the
dynamic sublimation pressure and the sublimative mass loss rate across the
surface of a planetary body by numerically solving for the equilibrium
temperature that balances incident solar energy with radiative and sublimative
heat loss. The model integrates over the entire surface of the spherical
nucleus and accounts for lower sunlight intensity toward the terminator due to
increasing solar incidence angles, assuming all sunlight is used to sublime
volatiles. This model thus estimates maximal production rates capable from
solar-driven activity.

Assuming comet-like dust-to-gas mass ratios $f_{dg}$, the calculated maximum
sublimation rates can then be used to compute an idealized maximal dust
production rate of the body under the assumption the whole surface is active.

Our simplified modeled sublimation rates are summarized in Table 3 and assume
only pure CO, CO2, or H2O surface ice as the major producer of gas molecules.
Our results show the CO-sublimation rate is many orders of magnitude larger
than CO2 and H2O-sublimation rates for our targets at their respective
heliocentric distances at the time of observation. CO thus has the strongest
volatility in the region we are inspecting and therefore we will consider it
as the most likely sublimating ice species, and we will use it to constrain
the active surface area and the dust production rates on our targets
throughout the rest of this paper.

It is unrealistic to expect that CO or other supervolatiles would cover the
whole surface at the analyzed heliocentric distances – we would observe strong
surface CO-driven activity much farther away from the Sun because CO starts to
sublimate at 25K. It also has been shown (Wierzchos et al., 2017) that known
Centaurs with detected CO-emissions (with the one excepion of 29P/Schwassmann-
Wachmann 1 (29P) (Womack et al., 2017)) are actually significantly CO-depleted
compared to comet Hale-Bopp, which could be considered an unprocessed nucleus
and had similar size and produced CO molecules at similar heliocentric
distance. However, objects that may appear depleted in volatile species due to
the lack of detected activity could still retain sizeable sub-surface volatile
deposits.

Idealized models of cometary nuclei depicting them as porous spheres composed
of a mixture of dust and ices (Capria et al., 2000; Capria, 2002) have shown
that both in the cases of pure CO ice and trapped CO gas, the source material
for CO-driven activity is likely stored under the surface where the
temperatures drop quickly, and in theory Centaurs should continuously release
CO along their orbits due to its low sublimation temperature and the shallow
depth of the deposits (Mazzotta Epifani et al., 2006), which has not been
observed. It is possible that during the residence in the Centaur region
objects gradually undergo surface devolatilization and develop a dusty crust
inhibiting the heat transfer and gas permeability of upper layer effectively
reducing the sublimation rate and gas flow to the surface (Wierzchos et al.,
2017). In the case of CO2, most of the volume is likely stored just under the
surface, but distant inactive Centaurs might have retained some surface
patches of CO2 since this supervolatile starts sublimating at r$\sim$15 AU
(Steckloff et al., 2015; Meech et al., 2017). However, the CO2 has not been
detected on Centaur surfaces yet, because it is even more challenging than
detecting CO with our current facilities, although it has been measured in
situ on comets (A’Hearn et al., 2011).

Therefore unless a Centaur is dynamically young in the region and still
retains surface CO/CO2 deposits, or has not developed dust crust yet, we can
expect the CO/CO2 sublimation from the subsurface to be relatively weak and
possibly go undetected until an external or internal force disturbs the
volatile-rich layers.

If subsurface pockets of amorphous water ice are present, crystallizing, and
releasing trapped volatiles to the surface, they could contribute to the gas
flow from the sub-surface. Laboratory experiments have shown trapped gas could
indeed be relatively abundant: AWI has the ability to retain gases up to a
gas-to-ice molecule ratio $f_{GI}=3.3$ (Laufer et al., 1987). However it
should be noted that the crystallization of AWI is not capable of heating
pockets of volatiles from below the surface because the phase transition
cannot be both exothermic (releasing heat) AND releasing trapped volatiles.
Kouchi and Sirono (2001) showed that AWI absorbs heat when it crystallizes and
releases trapped volatiles for concentrations of impurities (e.g. CO or CO2
molecules) above 2%, which are typical in the interstellar dust and comets.
The AWI phase transition itself therefore cannot produce enough heat to cause
rapid sublimation of subsurface volatiles resulting in outburst, but it can
act as a force opening the way to the surface and to provide additional gas
drag. For example the well known active Centaur 29P has been reported to have
CO and dust outbursts that are not correlated and could be associated with
multiple release mechanisms acting in parallel (Wierzchos and Womack, 2020).

All above implies the apparently inactive/dormant Centaurs orbiting within the
AWI crystallization zone have probably already depleted their surface
volatiles, since the average CO sublimation loss in the region is several
meters per orbit (Li et al., 2020), and the CO2-driven activity would be
visible, but they could still contain subsurface pockets of volatiles and AWI
buried under the dust crust. A sudden rapid orbital change pushing the
perihelion closer to the Sun could jump-start the crystallization front, which
will move rapidly releasing trapped gas until enough pressure builds up to
cause a landslide or opens a sinkhole exposing subsurface layers. This process
could lead to large-scale outgassing and/or outburst, possibly with a
significant lag, as was observed in several active Centaurs (Mazzotta Epifani
et al., 2006) and also in quasi-Hilda P/2010 H2 (Vales) (Jewitt and Kim,
2020). Such rapid release of gasses could also eject a large boulder or a
chunk of the parent body which can continue to disintegrate, as has been
likely observed during a dramatic ourburst of 174P/Echeclus in 2005, where the
the nucleus and coma brightness peaks were spatially separated (Rousselot,
2008; Bauer et al., 2008; Kareta et al., 2019). As the crystallization front
moves through the body, it could keep building the pressure pockets and create
openings throughout the surface even far away from perihelion, and in
subsequent orbital passages until the volatile deposits in each pocket are
depleted.

However, crystallization cannot be sustained for a time longer than 104–105
years (Guilbert-Lepoutre, 2012) further suggesting that active Centaurs have
young orbits, and have only recently been scattered inside to the warmer
regions of the Solar system.

Using 174P/Echeclus as an example, we can calculate the volume of AWI
necessary to release the observed dust volume. Given the dust production rate
observed during the 2005 outburst - 400 kg s-1 (Bauer et al., 2008), $f_{GI}$
for AWI, the density of water ice ($\rho_{\rm H_{2}O}=1000$ kg m-3), and the
range of dust-to-gas ratios measured for comets and estimated for active
Centaur 29P - $f_{dg}=$1-8 (Y. Fernández, personal communication; Fernández et
al., 2020; Wierzchos and Womack, 2020), it would require between $\sim$ 15 to
120 kg of AWI to crystallize every second and release trapped CO to match the
observed dust production if AWI is the single source. A $\sim$ 1 month long
outburst would require between $10^{7}-10^{8}$ kg of amorphous ice, or a
sphere of $\sim$ 20-40 m in diameter, which is an AWI pocket similar in size
to what Jewitt and Kim (2020) calculated for a Hilda asteroid undergoing an
outburst of similar magnitude.

It should be noted that our calculated values should only be used as an order-
of-magnitude estimates given there are no direct simultaneous measurements of
gas and dust mass loss from an active Centaur. The fdg ratio can change
depending on the Centaur and whether the mass loss is a product of a quiescent
activity, or an outburst (Fernández et al., 2020). A fresh sinkhole opening up
and causing an outburst could be much dustier than a steady-state outgassing
event diffusing through surface regolith. For example Wierzchos and Womack
(2020) has shown that in case of 29P several CO outbursts did not have a
corresponding dust outbursts. There is currently a real lack of knowledge of
the gas-to-dust ratio of Centaurs due to observational limitations, which will
be hopefully addressed and alleviated with future works using JWST (Kelley et
al., 2016), but such rigorous analysis is beyond the scope of this paper.

###  3.3 Mass loss rates and Fractional Active Surface area

We used the maximum CO sublimation production rates from our thermo-dynamical
model to derive the maximum surface sublimation-driven dust production rate,
$dM_{CO}/dt$, assuming CO to be the most likely volatile to drive the activity
at heliocentric distances of our targets, and $f_{dg}=$1 estimated for Centaur
29P during its quiescent activity phase (Fernández et al., 2020; Wierzchos and
Womack, 2020). This value most likely represents the volume of dust expected
to be lofted by steadily escaping gas from a weakly-active Centaur such as
targets of this study. The in situ $f_{dg}$ measurements on 67P revealed the
ratio being much higher closer to the nucleus - $f_{dg}=$4$\pm$2, compared to
ratios $f_{dg}=$0.1-2.0 typically measured remotely for comets(e.g. Weiler et
al., 2003; Choukroun et al., 2020), but the comet had an active water ice
sublimation, and was near its perihelion at the time the measurement were
taken, thus it could be expected the volume of dust being lofted was higher
than in our targets.

Then using the synthetic coma limit technique following Luu and Jewitt (1992),
we calculated the observed effective upper limit on the dust production rate.
To find quantitative limits on weak activity, we compared every Centaur’s PSF
with seeing-convolved models of weak coma. With the same image scale and
point-spread function as our data, models and data can be directly compared in
order to ascertain activity levels. We first created model point sources, then
generated a series of synthetic PSFs with varying coma levels and convolved
these with the seeing (Figure 7). Coma levels were parameterized by
$\eta=C_{c}/C_{n}$, where $C_{c}$ and $C_{n}$ are the scattering cross-
sections of the coma and nucleus, respectively, and the reference photometry
aperture radius used was $\Phi=4.04$ arcsec. For our analyses, we used coma
levels of $\eta=0.00$ to $\eta=0.25$ in $\eta=0.05$ increments.

Increasing coma levels generated this way generally has little effect on the
profile cores but does broaden the profile wings. We then noted at which coma
level were the synthetic profiles most similar to the profile of the target
object. Our comparison of the coma models to measured profiles of our sample
Centaurs produced no indication of activity within our detection limits.
However, we used the resulting limiting coma parameter $\eta_{lim}$ to convert
to the observed effective upper limit on the dust production rate $dM_{D}/dt$
via:

$dM_{D}/dt=\frac{(1.1\times
10^{-3})\cdot\pi~{}\rho~{}\bar{a}~{}\eta_{lim}R^{2}}{\Phi~{}r^{0.5}\Delta},$
(5)

where $\rho=2500$ kg m-3 is the assumed grain density consistent with
carbonaceous chondrites, which are associated with primitive C-type objects,
and are most likely the closest match to the Centaurs (Britt et al., 2002),
$\bar{a}=0.5\times 10^{-6}$ m is the assumed weighted mean grain radius (Bauer
et al., 2008), $R$ is the object’s effective radius, $\Phi$ is the angular
photometry radius in arcseconds, $r$ is the heliocentric distance in
astronomical units, and $\Delta$ is the geocentric distance in astronomical
units (Luu and Jewitt, 1992).

By comparing the maximal production rates of CO and CO2 derived from the
surface sublimative mass-loss and the upper limit on dust production rates
derived from our observations, we can calculate the effective fraction of
active surface area of the particular Centaur at the time of the observation,
and the actual active area as a function of object’s diameter. Since we are
using a photometric method to derive the upper limits on dust production, the
possible changes in dust-to-gas ratio affect the size of the effective
fraction of active surface area - the active area scales with $1/f_{dg}$,
because a larger active area is needed to lift the same amount of dust grains
at fixed gas flow, if the $f_{dg}$ is smaller. The resulting production rates
are in order of several kg with the effective active surface areas for each
target ranging from several- to several-thousand m2 in case of CO-driven
activity. Our values are 1-2 orders of magnitude smaller than values
calculated for other weakly active Centaurs (e.g. Bauer et al., 2003; Mazzotta
Epifani et al., 2014, 2017). Since the gas production rates from a CO2-driven
activity are several orders of magnitude lower than those calculated for CO,
and are strongly dependent on the heliocentric distance, the active surface
areas constrained by our coma-limit technique would be unrealistically large
for our most distant targets. However, patches only several hundred m2 of
sublimating surface CO2 would sustain the observed dust production limits on
two of our closest targets - 2014 PQ70 and 2015 BK518.

If this mass-loss volume is to be attributed to an escaped gas from AWI
crystallization, it would require a relatively modest subsurface pocket akin
to a sphere with several-m radius, which is in agreement with AWI pocket sizes
estimated for outbursting quasi-Hilda P/2010 H2 (Vales) (Jewitt and Kim,
2020). Resulting values are summarized in Table 3.

Our dust production upper limits can be well explained by a combined gas drag
from volatiles released from crystallized AWI, and from sublimating ice
patches under the surface exposed to the sun by the pressure of escaping
molecules.

Cometary nucleus models (e.g. Prialnik and Podolak, 1995; Capria et al., 2000)
predict that CO molecules should percolate through the outer layers of the
body and flow uniformly from anywhere on the surface on both the night and day
sides. This indicates there should also be an uniform associated dust release
due to the gas drag, unlike the dust mass loss caused by water sublimation
which tends to be released only from the day side. Also, it has been proposed
that sublimation of CO and CO2 and the associated dust production rate changes
very slowly with time after the activity onset because it originates in the
subsurface layers (Capria et al., 2000; Capria, 2002). This would mean if
there are large enough subsurface CO-deposits on Centaurs, most of the
dynamically younger population members should exhibit a low-scale outgassing
and dust mass-loss along their orbits similar to what has been observed in
(2060) 95P/Chiron, which would be in most cases fainter than our detection
limits (e.g. Kareta et al., 2021).

###  3.4 Orbital evolution

To track past orbital evolution and identify any recent potential activity-
triggering events such as significant orbital changes or close encounters with
giant planets, we ran several sets of backward numerical integrations for each
of our targets. We used the Bulrisch-Stöer integrator in the Mercury6 N-body
integration package (Chambers and Migliorini,, 1997), which is capable of
handling close planetary encounters with minimal loss of precision by using an
adaptive timestep once an object crosses massive body’s Hill sphere. All
integrations used a 30-day timestep, which is an sufficiently precise to map
the evolution of orbits with decades-long periods typical for Centaurs.
Particles were removed from the integrations when they collided with the Sun
or a planet, or reached semi-major axes of $a>100$ AU and were considered to
be ejected from the solar system. Dynamical clones were generated for each
object using covariance matrices from the JPL Small Body Database
444https://ssd.jpl.nasa.gov/sbdb.cgi. All of our targets have well defined
orbits with up to 10 opposition arcs and low residuals resulting in small
dispersion of clones from the nominal orbit.

For our first set of integrations, we produced 50 clones for each object, and
ran them as massless particles under the gravitational influence of eight
planets and the Sun for 100,000 years into the past, which is the the upper
time limit on amorphous water ice survival on Centaurs (Guilbert-Lepoutre,
2012). The orbits in the Centaur region are subjected to strong gravitational
influence from giant planets and their orbital dynamics becomes chaotic very
quickly. Therefore it is nearly impossible to know the system’s dynamical
evolution past the point in the history where the clones start to diverge
(Morbidelli et al., 2020).

After examination of these long-term integrations we ran two more sets of
integrations: a) using a time step of 10 days for past 5,000 years in order to
investigate clone evolution before the individual orbits started to diverge,
and b) using a 1-day timestep for 200 years into the past to search for
extremely recent orbit changes similar to other Centaurs observed in outburst
(e.g. Fernández et al., 2018).

Our analysis of the orbital histories revealed that most of the Centaurs in
our sample have relatively stable orbits with only minor orbital changes as
can be seen in Figures 9 and 10. Our 5 kyr integrations show that the nominal
orbit of each target and those of their clones start to diverge on $\sim$103
yr timescales where the chaotic environment makes any determination of the
evolution before this point statistical only, as shown on Figures 11 and 12.
Such a point of divergence is typically associated with a close encounter with
a planet.

We can divide the observed orbital behavior of our targets and their expected
activity levels into three groups based on perihelion distance:

  1. 1.

$q\gtrsim 15$ AU: Objects with perihelia outside of the AWI crystallization
zone (and CO2 sublimation limit) tend to have stable orbits on the considered
timescale, with changes in perihelion and semi-major axis of $\Delta a,\Delta
q\lesssim 1-3$ AU. Objects 2015 TV85, 2014 JD80, 2015 BH518, 2015 BD518, 2014
NX65, 2013 MZ11 and most clones of 2010 TU191 fall within this group. We
expect very limited activity driven by surface or subsurface volatiles due to
fairly long residence inside Neptune’s orbit and therefore the depletion of
surface sources, and we do not expect significant crystallization of AWI at
the distances they reside in.

  2. 2.

7 AU $\lesssim$q$\lesssim$ 15AU: Objects with perihelion distances in the AWI
crystallization zone. Centaurs 2014 YX49, 2010 OX393, 2014 WX508, and 2014
WW508 belong to this group, and it is entirely possible the AWI
crystallization/CO2 has been underway, but the released gasses remain under
the surface or remain below our detection limit. However, objects in this
group experienced only minor orbital changes in the past 105 yrs, and we would
expect only minimal gas-and dust loss driven by the crystallization since the
front should have stopped propagating inside the bodies or has propagated too
deep into the nucleus over the residence time in the zone (Guilbert-Lepoutre,
2012). As we noted before, temperatures in this region are too high for any
supervolatiles to have survived on the surface until present day.

  3. 3.

$q\lesssim 7$ AU: There are two Centaurs in our sample with perihelia and
semi-major axes below 7 AU: 2015 BK518 and 2014 PQ70. Both have current
$T_{J}>3$ and $T_{S}<3$, and their orbital evolution can be traced with a
reasonable precision $\sim 2000$ yr into the past, where the orbits of clones
start to turn chaotic under the strong gravitational influence from Saturn.
Our integrations show wide diffusion of possible semi-major axes while the
perihelia changed very little from the current value, similarly to what other
works have found on the evolution of low-perihelion Centaurs (Fernández et
al., 2018; Kareta et al., 2019; Steckloff et al., 2020). It is possible that
Centaurs in this group experienced periods of activity in the past, given
their proximity to Sun, with surface volatiles completely depleted and perhaps
some remaining pockets of subsurface ices still isolated under the surface. If
they entered the AWI crystallization zone within last $\sim$105 years, the AWI
crystallization front might be propagating under the surface, and with
favorable conditions can become active again. For example 2014 PQ70 in
particular would be good candidate for an outburst monitoring campaign because
it has experienced a decrease in semi-major axis $\sim$ 70 years ago (Figures
8 and 13) similar to what has been observed in other active Centaurs (Mazzotta
Epifani et al., 2006), and we could witness yet another bout of activity
triggered by this event in coming years.

There are three other notable Centaurs in our sample. 2010 TU191 may be a
fairly new addition to the Centaur region. Figure 10 shows that three of the
clones including the nominal orbit experienced a close encounter with Neptune
within 0.03 AU $\sim 38,000$ years ago, reducing the perihelion distance by 15
AU within a decade and illustrating some drastic orbital changes that are
common in the Centaur region. This object could represent a ‘fresh’ Centaur
with high likelihood of undisturbed surface volatile deposits, and it is
entirely possible it is weakly active below our detection limits, and might
become increasingly more active in the future.

Meanwhile, 2014 JD80 is in a 3:2 mean-motion resonance with Uranus, and
additional 0.5 Myr integration suggests it will stay resonant for next $\sim$1
Myr (K. Volk - personal communication).

Lastly, an apparently inactive object (2014 OX393) that we observed in this
work has a very similar ratio of perihelion distance and heliocentric distance
at the time of observation, and orbital elements as two known active Centaurs
— 167P/CINEOS (248835) and 2006 SX368 (Jewitt, 2009, 2015) — at the times of
their outbursts (Figure 1), which could indicate 2014 OX393’s inactivity could
be due to having a different orbital history compared to the other two. While
2014 OX393 has a stable orbit just inside the upper limit of the
crystallization zone, it either lacked an activity trigger in the form of a
sudden perihelion drop and rapid warming of the surface, or the past activity
has depleted surface volatiles.

With one exception, our fully-inactive Centaur sample has had relatively
stable orbits, adding support to the idea that sudden orbital changes might
trigger or re-start activity in the Centaurs.

##  4 Conclusions

We have observed 13 Centaurs with the Gemini North telescope with the goal of
searching for comet-like activity beyond the orbit of Jupiter. Our targets
were selected from objects newly discovered in the Pan-STARRS1 detection
database and thus have orbital arcs covering up to a decade. Most objects in
our sample have orbits crossing the range of heliocentric distances where the
temperatures allow for crystallization of AWI speculated to be the major
activity driver in Centaurs. We conclude that:

  1. 1.

None of the Centaurs in our sample showed signs of activity either in our
observations with Gemini, or in archival images from DECam dating back to
2013. However, we cannot rule out the presence of faint coma below our
detction limits.

  2. 2.

The thermo-dynamical model we used allowed us to calculate the theoretical
maximal sublimation rate of the three most common volatile species found on
comets - CO, CO2 and H2O, and our results suggests the majority of
sublimation-driven activity at the analyzed heliocentric distances should be
due to CO. Due to the heliocentric distances and orbital histories of
investigated Centaurs, it is unlikely that free CO or CO2 remains on the
surfaces, but it is possible our targets retain subsurface volatile deposits
and some of them could exhibit low-level gas and dust mass-loss below our
detection limits.

  3. 3.

The upper limit on dust production rates and fractional active surface areas
resulting from our coma modelling based on the non-detection of activity and
the maximum surface sublimation rates are in agreement with rates estimated
independently for other inactive Centaurs (e.g. Bauer et al., 2003; Jewitt,
2009; Li et al., 2020). If there are no surface super volatiles present and
sublimating, the estimated dust gas production rates could be sustained by a
modestly-sized (volume in order of several m2) exposed pockets of AWI
crystallizing and releasing trapped gas molecules.

  4. 4.

Our orbital history analysis revealed that most of our targets have relatively
stable orbits with no significant recent changes in perihelion distance. Our
results are consistent with earlier work suggesting that the activity on
Centaurs is most likely triggered by sudden drops in perihelion distance
and/or semi-major axis induced by close encounters with giant planets, which
change the thermal balance in the body. The rapid warming and crystallization
of AWI can then release trapped gas and build up pressure under the surface
which leads to exposure of more volatile material and subsequent outbursts.

  5. 5.

Our numerical integrations show that Centaur 2014 PQ70 has experienced a drop
in semi-major axis larger than 0.5 AU about 50 years ago, similar to what has
been seen in other active Centaurs in the past, and is therefore a good
candidate for monitoring of possible future outbursts of activity.

## Acknowledgments

This work was supported by NSF AST Grant #1910275.

We thank Kat Volk for helpful insights into Centaur dynamics and Yan Fernández
for insights into the dust-to-gas ratio of 29P.

The authors recognize and acknowledge the very significant cultural role and
reverence that the summit of Mauna Kea has always had within the indigenous
Hawaiian community. We are most fortunate to have the opportunity to conduct
observations from this mountain.

This project used public archival data from the Dark Energy Survey (DES).
Funding for the DES Projects has been provided by the U.S. Department of
Energy, the U.S. National Science Foundation, the Ministry of Science and
Education of Spain, the Science and Technology FacilitiesCouncil of the United
Kingdom, the Higher Education Funding Council for England, the National Center
for Supercomputing Applications at the University of Illinois at Urbana-
Champaign, the Kavli Institute of Cosmological Physics at the University of
Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State
University, the Mitchell Institute for Fundamental Physics and Astronomy at
Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos
Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho
Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da
Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft, and the
Collaborating Institutions in the Dark Energy Survey. The Collaborating
Institutions are Argonne National Laboratory, the University of California at
Santa Cruz, the University of Cambridge, Centro de Investigaciones
Energéticas, Medioambientales y Tecnológicas-Madrid, the University of
Chicago, University College London, the DES-Brazil Consortium, the University
of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi
National Accelerator Laboratory, the University of Illinois at Urbana-
Champaign, the Institut de Ciències de l’Espai (IEEC/CSIC), the Institut de
Física d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-
Maximilians Universität München and the associated Excellence Cluster
Universe, the University of Michigan, the National Optical Astronomy
Observatory, the University of Nottingham, The Ohio State University, the
OzDES Membership Consortium, the University of Pennsylvania, the University of
Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the
University of Sussex, and Texas A&M University.

Based in part on observations at Cerro Tololo Inter-American Observatory,
National Optical Astronomy Observatory, which is operated by the Association
of Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation.

This research has made use of data and/or services provided by the
International Astronomical Union’s Minor Planet Center.

This work has made use of data from the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing
and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

Eva Lilly would like to thank her children Betty and Sammy for being great
nappers so this paper could be written, and she wishes to express her
gratitude to PSI for having an excellent support system for families with
children.

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Table 1: Observing geometry, time of observation and photometry.

Name | UT Date | $\Delta^{a}$ | rb | $\alpha^{c}$ | FWHM | Airmass | $\Phi^{d}$ | $m_{r}^{e}$ |
---|---|---|---|---|---|---|---|---|---
2013 MZ11 | 2017 Sep 24 | 19.8 | 20.6 | 1.8 | 0.76 | 1.75 | 4.74 | 21.24 $\pm$ 0.02 |
2014 NX65 | 2017 Sep 24 | 17.7 | 18.7 | 0.8 | 0.62 | 1.21 | 3.37 | 22.00 $\pm$ 0.23 |
2014 TV85 | 2017 Dec 11 | 14.6 | 14.9 | 3.5 | 0.67 | 1.07 | 4.05 | 21.26 $\pm$ 0.02 |
2015 BD518 | 2017 Nov 14 | 16.5 | 16.3 | 3.4 | 0.73 | 1.31 | 2.00 | 21.93 $\pm$ 0.03 |
2015 BH518 | 2017 Dec 30 | 25.1 | 25.4 | 2.1 | 0.76 | 1.15 | 4.05 | 22.60 $\pm$ 0.07 |
2014 WW508 | 2017 Nov 17 | 13.3 | 14.0 | 3.4 | 0.67 | 1.02 | 3.37 | 22.17 $\pm$ 0.03 |
2010 TU191 | 2018 Jan 16 | 14.8 | 15.5 | 2.1 | 1.05 | 1.31 | 3.37 | 22.81 $\pm$ 0.04 |
2014 JD80 | 2017 Sep 19 | 19.2 | 19.8 | 2.2 | 1.54 | 1.70 | 4.00 | 21.83 $\pm$ 0.04 |
2014 OX393 | 2017 Nov 9 | 11.3 | 12.3 | 0.6 | 0.69 | 1.48 | 4.05 | 21.63 $\pm$ 0.02 |
2014 YX49 | 2017 Nov 11 | 19.3 | 19.6 | 2.7 | 0.62 | 1.04 | 2.50 | 21.66 $\pm$ 0.02 |
2014 WX508 | 2017 Dec 11 | 13.4 | 13.4 | 3.9 | 0.63 | 1.16 | 4.05 | 22.32 $\pm$ 0.10 |
2015 BK518 | 2018 Jan 17 | 7.9 | 8.1 | 6.8 | 1.13 | 1.25 | 8.16 | 23.71 $\pm$ 0.12 |
2014 PQ70 | 2017 Sep 23 | 7.2 | 8.1 | 6.6 | 1.11 | 1.39 | 7.47 | 24.02 $\pm$ 0.14 |

**footnotetext: Notes.
a Geocentric distance, AU.
b Heliocentric distance, AU.
c Solar phase angle, deg.
d Angular diameter of photometry aperture, arcseconds.
e Measured mean apparent r’-band magnitude.

Table 2: Orbital parameters.

Name | qa | ab | ec | id | Me | $T_{J}^{f}$ | $T_{S}^{g}$ | $T_{U}^{h}$ |
---|---|---|---|---|---|---|---|---|---
2013 MZ11 | 16.828 | 24.228 | 0.305 | 6.40 | 314.40 | 4.30 | 3.41 | 2.92 |
2014 NX65 | 18.386 | 22.868 | 0.196 | 11.40 | 16.30 | 4.26 | 3.39 | 2.94 |
2014 TV85 | 14.653 | 21.796 | 0.328 | 12.20 | 348.40 | 4.02 | 3.22 | 2.85 |
2015 BD518 | 16.277 | 23.383 | 0.304 | 17.20 | 359.20 | 4.08 | 3.25 | 2.83 |
2015 BH518 | 25.356 | 27.954 | 0.093 | 10.90 | 9.70 | 4.72 | 3.68 | 3.05 |
2014 WW508 | 13.259 | 28.219 | 0.53 | 9.50 | 8.20 | 4.08 | 3.21 | 2.71 |
2010 TU191 | 14.764 | 20.65 | 0.285 | 1.80 | 19.10 | 4.07 | 3.28 | 2.92 |
2014 JD80 | 19.012 | 25.318 | 0.249 | 39.10 | 337.90 | 3.52 | 2.82 | 2.48 |
2014 OX393 | 11.963 | 25.444 | 0.53 | 14.70 | 5.60 | 3.83 | 3.05 | 2.64 |
2014 YX49 | 13.825 | 19.113 | 0.277 | 25.60 | 78.10 | 3.59 | 2.95 | 2.73 |
2014 WX508 | 13.173 | 18.631 | 0.293 | 11.60 | 348.90 | 3.82 | 3.13 | 2.88 |
2015 BK518 | 6.393 | 14.258 | 0.552 | 7.00 | 16.50 | 3.10 | 2.69 | 2.77 |
2014 PQ70 | 6.25443 | 16.037 | 0.6 | 15.04 | 11.80 | 3.01 | 2.58 | 2.60 |

**footnotetext: Notes.
a Perihelion distance, AU.
b Semimajor axis, AU.
c Orbital eccentricity
d Inclination, degrees.
e Mean anomaly at the time of observation, degrees.
f Tisserand’sparameter with respect to Jupiter.
g Tisserand’s parameter with respect to Saturn.
h Tisserand’s parameter with respect to Uranus.

Table 3: Upper Limits on Sublimation and Dust Production Rates

Name | $H_{r}^{{\color[rgb]{0,0,1}a}}$ | $R^{{\color[rgb]{0,0,1}b}}$ | $\eta^{{\color[rgb]{0,0,1}c}}$ | $dM^{{\color[rgb]{0,0,1}d}}_{H_{2}O}/dt$ | $dM^{{\color[rgb]{0,0,1}e}}_{CO_{2}}/dt$ | $dM^{{\color[rgb]{0,0,1}f}}_{CO}/dt$ | $dM^{{\color[rgb]{0,0,1}g}}_{D}/dt$ | $f_{Act}^{{\color[rgb]{0,0,1}h}}$ | $A_{Act}^{{\color[rgb]{0,0,1}i}}$ | $V_{AWI}^{{\color[rgb]{0,0,1}j}}$
---|---|---|---|---|---|---|---|---|---|---
2013 MZ11 | 7.95 | 60.29 | 0.25 | $2.54\times 10^{-25}$ | $1.74\times 10^{-2}$ | $1.27\times 10^{7}$ | 10.80 | $8.50\times{-7}$ | $3.88\times 10^{4}$ | 27.14
2014 NX65 | 9.27 | 32.91 | 0.05 | $9.84\times 10^{-24}$ | $4.82\times 10^{-2}$ | $3.57\times 10^{6}$ | 0.76 | $2.10\times{-7}$ | $2.89\times 10^{3}$ | 11.19
2014 TV85 | 9.23 | 33.49 | 0.10 | $6.04\times 10^{-19}$ | $9.77\times 10^{0}$ | $5.23\times 10^{6}$ | 2.14 | $4.09\times{-7}$ | $5.76\times 10^{3}$ | 15.81
2015 BD518 | 9.45 | 30.30 | 0.05 | $8.66\times 10^{-21}$ | $1.24\times 10^{0}$ | $4.34\times 10^{6}$ | 0.74 | $1.70\times{-7}$ | $1.96\times 10^{3}$ | 11.10
2015 BH518 | 8.32 | 50.88 | 0.05 | $1.44\times 10^{-30}$ | $4.69\times 10^{-5}$ | $7.39\times 10^{6}$ | 1.09 | $1.48\times{-7}$ | $4.81\times 10^{3}$ | 12.65
2014 WW508 | 10.48 | 18.82 | 0.05 | $4.49\times 10^{-18}$ | $1.69\times 10^{1}$ | $2.60\times 10^{6}$ | 0.38 | $1.47\times{-7}$ | $6.53\times 10^{2}$ | 8.90
2010 TU191 | 10.75 | 16.61 | 0.05 | $3.81\times 10^{-20}$ | $1.59\times 10^{0}$ | $1.92\times 10^{6}$ | 0.25 | $1.32\times{-7}$ | $4.57\times 10^{2}$ | 7.77
2014 JD80 | 8.68 | 43.20 | 0.05 | $7.76\times 10^{-25}$ | $1.78\times 10^{-2}$ | $5.02\times 10^{6}$ | 1.17 | $2.33\times{-7}$ | $5.48\times 10^{3}$ | 12.94
2014 OX393 | 10.79 | 16.31 | 0.05 | $8.56\times 10^{-16}$ | $1.78\times 10^{2}$ | $2.34\times 10^{6}$ | 0.36 | $1.53\times 10^{-7}$ | $5.13\times 10^{2}$ | 8.72
2014 YX49 | 8.49 | 47.10 | 0.05 | $1.92\times 10^{-24}$ | $3.34\times 10^{-2}$ | $7.41\times 10^{6}$ | 1.40 | $1.88\times 10^{-7}$ | $5.26\times 10^{3}$ | 13.72
2014 WX508 | 10.68 | 17.15 | 0.05 | $2.60\times 10^{-17}$ | $3.61\times 10^{1}$ | $2.36\times 10^{6}$ | 0.32 | $1.36\times 10^{-7}$ | $5.02\times 10^{2}$ | 8.40
2015 BK518 | 14.16 | 3.46 | 0.10 | $3.15\times 10^{-10}$ | $4.65\times 10^{3}$ | $2.64\times 10^{5}$ | 0.06 | $2.17\times 10^{-7}$ | $3.27\times 10^{1}$ | 4.73
2014 PQ70 | 14.71 | 2.69 | 0.10 | $3.73\times 10^{-10}$ | $5.51\times 10^{3}$ | $3.13\times 10^{5}$ | 0.04 | $1.21\times 10^{-7}$ | $1.10\times 10^{1}$ | 4.13

**footnotetext: Notes.
a Absolute R-band total magnitude (at R=$\Delta$=1 AU and $\alpha$= 0 deg),
using IAU H, G phase-darkening, where G = 0.13 (Bauer et al., 2003).
b Effective nucleus radius, km.
c Coma limit parameter.
d Maximum H2O-sublimation rate, in kg s-1.
e Maximum CO2-sublimation rate, in kg s-1.
f Maximum CO-sublimation rate, in kg s-1.
g CO-driven dust production rate, calculated assuming $f_{dg}=6$, in kg s-1.
h Effective active surface fraction of CO-driven activity.
i Effective active surface area of CO-driven activity, in $m^{2}$.
j Effective radius of a spherical AWI volume if solely sustaining upper limit
on the CO gas flow, in $m$.

Table 4: DECam archival detections of or Centaur targets.

Name | UT Date | UT Time | Filter | $\Delta^{a}$ | rb | $\alpha^{c}$ | Airmass | $m^{d}$ |
---|---|---|---|---|---|---|---|---|---
2013 MZ11 | 2019-08-10 | 05:24:31.443 | i′ | 18.891 | 19.874 | 0.732 | 1.04 | 20.87 $\pm$ 0.04 |
2013 MZ11 | 2019-09-25 | 00:48:04.371 | VR | 18.976 | 19.826 | 1.572 | 1.15 | 21.38 $\pm$ 0.01 |
2014 JD80 | 2015-08-16 | 02:54:48.666 | VR | 19.315 | 20.24 | 1.193 | 1.01 | 22.02 $\pm$ 0.13 |
2014 JD80 | 2015-08-16 | 04:33:52.183 | VR | 19.316 | 20.24 | 1.196 | 1.03 | 22.05 $\pm$ 0.29 |
2014 JD80 | 2015-09-05 | 23:47:24.726 | VR | 19.493 | 20.227 | 1.993 | 1.14 | 22.39 $\pm$ 0.10 |
2014 JD80 | 2017-06-03 | 06:09:40.435 | i′ | 19.311 | 19.854 | 2.509 | 1.26 | 22.52 $\pm$ 0.34 |
2014 JD80 | 2017-07-02 | 07:23:43.929 | g′ | 18.975 | 19.838 | 1.588 | 1.0 | 22.16 $\pm$ 0.32 |
2014 JD80 | 2017-07-17 | 07:32:02.319 | i′ | 18.878 | 19.83 | 1.058 | 1.05 | 21.56 $\pm$ 0.10 |
2014 JD80 | 2017-08-27 | 02:08:09.002 | r′ | 18.932 | 19.808 | 1.488 | 1.04 | 21.33 $\pm$ 0.14 |
2014 JD80 | 2017-08-27 | 05:28:54.472 | r′ | 18.933 | 19.808 | 1.493 | 1.13 | 21.01 $\pm$ 0.10 |
2014 JD80 | 2017-08-28 | 01:49:49.905 | r′ | 18.939 | 19.808 | 1.524 | 1.05 | 21.04 $\pm$ 0.13 |
2014 JD80 | 2017-09-01 | 02:50:26.376 | r′ | 18.97 | 19.806 | 1.673 | 1.0 | 21.65 $\pm$ 0.24 |
2014 JD80 | 2017-09-01 | 03:51:49.346 | r′ | 18.971 | 19.806 | 1.674 | 1.02 | 21.36 $\pm$ 0.20 |
2014 NX65 | 2017-07-30 | 06:35:06.476 | g′ | 17.94 | 18.65 | 2.275 | 1.12 | 23.34 $\pm$ 0.81 |
2014 NX65 | 2017-07-30 | 06:36:37.613 | g′ | 17.94 | 18.65 | 2.275 | 1.11 | 23.05 $\pm$ 0.32 |
2014 NX65 | 2017-08-20 | 04:34:21.015 | r′ | 17.751 | 18.656 | 1.43 | 1.19 | 22.41 $\pm$ 0.22 |
2014 NX65 | 2018-08-12 | 05:09:17.383 | r′ | 17.981 | 18.771 | 1.982 | 1.26 | 22.36 $\pm$ 0.50 |
2014 NX65 | 2018-09-07 | 03:36:18.323 | g′ | 17.814 | 18.78 | 0.902 | 1.22 | 23.58 $\pm$ 0.24 |
2014 NX65 | 2018-09-12 | 03:13:44.044 | r′ | 17.803 | 18.782 | 0.737 | 1.23 | 22.02 $\pm$ 0.16 |
2014 NX65 | 2018-09-12 | 03:15:04.320 | g′ | 17.803 | 18.782 | 0.737 | 1.22 | 23.17 $\pm$ 0.28 |
2014 NX65 | 2019-09-26 | 05:30:12.957 | VR | 17.945 | 18.927 | 0.634 | 1.08 | 22.21 $\pm$ 0.03 |
2014 OX393 | 2014-10-03 | 07:06:38.523 | g′ | 11.047 | 12.032 | 0.863 | 1.34 | 23.04 $\pm$ 0.41 |
2014 OX393 | 2014-10-03 | 07:08:36.795 | r′ | 11.047 | 12.032 | 0.863 | 1.35 | 21.80 $\pm$ 0.11 |
2014 OX393 | 2014-10-03 | 07:10:35.396 | i′ | 11.047 | 12.032 | 0.863 | 1.36 | 21.87 $\pm$ 0.10 |
2014 OX393 | 2015-11-02 | 02:53:11.989 | r′ | 11.05 | 11.969 | 1.87 | 1.16 | 20.38 $\pm$ 0.16 |
2014 OX393 | 2015-12-28 | 01:43:08.750 | i′ | 11.764 | 11.974 | 4.637 | 1.4 | 21.50 $\pm$ 0.15 |
2014 OX393 | 2015-12-28 | 03:00:59.051 | i′ | 11.765 | 11.974 | 4.638 | 1.94 | 21.84 $\pm$ 0.15 |
2014 OX393 | 2016-07-22 | 09:53:40.451 | z′ | 12.005 | 12.022 | 4.848 | 1.29 | 21.77 $\pm$ 0.10 |
2014 OX393 | 2016-07-22 | 10:10:05.819 | z′ | 12.005 | 12.022 | 4.848 | 1.27 | 21.25 $\pm$ 0.10 |
2014 OX393 | 2017-11-13 | 04:18:13.011 | r′ | 11.337 | 12.306 | 0.963 | 1.39 | 21.80 $\pm$ 0.14 |
2014 TV85 | 2015-03-09 | 04:16:42.821 | VR | 14.884 | 15.59 | 2.626 | 1.68 | 21.83 $\pm$ 0.06 |
2014 TV85 | 2015-04-11 | 23:29:44.974 | z′ | 15.345 | 15.562 | 3.627 | 1.31 | 21.41 $\pm$ 0.11 |
2014 TV85 | 2016-01-10 | 05:46:06.736 | r′ | 14.455 | 15.349 | 1.582 | 1.26 | 21.29 $\pm$ 0.08 |
2014 TV85 | 2016-01-10 | 05:47:43.758 | g′ | 14.455 | 15.349 | 1.582 | 1.26 | 21.75 $\pm$ 0.13 |
2014 TV85 | 2017-03-01 | 00:50:23.512 | r′ | 14.173 | 15.073 | 1.622 | 1.51 | 21.17 $\pm$ 0.08 |
2014 TV85 | 2017-03-01 | 00:55:23.608 | g′ | 14.173 | 15.073 | 1.622 | 1.49 | 21.63 $\pm$ 0.10 |
2014 TV85 | 2018-01-28 | 06:50:34.844 | z′ | 13.981 | 14.898 | 1.431 | 1.21 | 20.88 $\pm$ 0.06 |
2014 TV85 | 2018-02-05 | 06:19:29.275 | i′ | 13.943 | 14.894 | 1.037 | 1.21 | 20.69 $\pm$ 0.08 |
2014 TV85 | 2018-12-13 | 06:21:48.624 | g′ | 14.489 | 14.772 | 3.694 | 1.64 | 21.62 $\pm$ 0.11 |
2014 WW508 | 2016-03-03 | 01:15:05.013 | VR | 13.159 | 13.47 | 4.051 | 1.49 | 21.10 $\pm$ 0.13 |
2014 WW508 | 2019-02-28 | 02:32:20.617 | VR | 13.839 | 14.57 | 2.697 | 1.38 | 22.34 $\pm$ 0.15 |
2014 WW508 | 2019-02-28 | 02:35:19.173 | VR | 13.839 | 14.57 | 2.697 | 1.39 | 22.55 $\pm$ 0.22 |
2014 YX49 | 2016-12-29 | 06:27:42.080 | g′ | 18.301 | 19.21 | 1.151 | 2.16 | 21.97 $\pm$ 0.11 |
2015 BD518 | 2014-12-29 | 08:26:36.064 | z′ | 15.716 | 16.466 | 2.264 | 1.73 | 21.62 $\pm$ 0.13 |
2015 BD518 | 2015-04-12 | 00:52:43.348 | r′ | 16.026 | 16.432 | 3.233 | 1.72 | 22.30 $\pm$ 0.14 |
2015 BD518 | 2015-04-12 | 00:54:11.319 | g′ | 16.026 | 16.432 | 3.234 | 1.72 | 22.48 $\pm$ 0.15 |
2015 BD518 | 2016-01-15 | 06:58:55.054 | r′ | 15.498 | 16.356 | 1.729 | 1.56 | 22.98 $\pm$ 0.25 |
2015 BD518 | 2016-01-15 | 07:00:25.880 | g′ | 15.498 | 16.356 | 1.729 | 1.56 | 23.12 $\pm$ 0.23 |
2015 BD518 | 2017-01-11 | 05:59:11.086 | z′ | 15.523 | 16.295 | 2.198 | 1.62 | 21.51 $\pm$ 0.06 |
2015 BD518 | 2017-03-30 | 01:02:15.573 | g′ | 15.539 | 16.287 | 2.381 | 1.57 | 22.01 $\pm$ 0.13 |
2015 BD518 | 2018-02-12 | 05:36:58.517 | g′ | 15.312 | 16.277 | 0.746 | 1.36 | 22.16 $\pm$ 0.07 |
2015 BH518 | 2013-04-01 | 02:24:49.422 | z′ | 24.537 | 25.345 | 1.351 | 1.14 | 19.46 $\pm$ 0.10 |
2015 BH518 | 2014-03-28 | 00:03:22.996 | VR | 24.479 | 25.35 | 1.118 | 1.49 | 22.12 $\pm$ 0.11 |
2015 BH518 | 2015-03-29 | 23:42:12.581 | z′ | 24.478 | 25.36 | 1.077 | 1.63 | 22.38 $\pm$ 0.11 |
2015 BK518 | 2017-03-02 | 05:30:37.098 | g′ | 6.34 | 7.307 | 1.826 | 1.11 | 23.30 $\pm$ 0.24 |

**footnotetext: Notes.
a Geocentric distance at the time of observation, AU.
b Heliocentric distance at the time of observation, AU.
c Phase angle at the time of observation.
d Apparent magnitude measured in given filter.

Figure 1: Perihelion distance vs. heliocentric distance of 13 Centaurs studied
in this work compared to known active Centaurs and JFCs Centaurs, and the
known Centaur population. All known active Centaurs reside within the interval
where the crystallization of AWI is possible. Our targets are distributed both
within and beyond the crystallization interval. A magenta star marks the
position of 167P/CINEOS which shares a similar orbit and heliocentric position
at the time of activity as one of Centaurs in our sample, 2014 OX393. Figure
2: Semi-major axis versus eccentricity (top panel) and semi-major axis versus
inclination (bottom panel) of our targets compared to known active Centaurs
and JFC Centaurs, and the known Centaur population. Dashed curves represent
the loci of orbits with perihelia equal to those of perihelia of giant
planets, dashed-dotted lines show semi-major axes of Jupiter and Neptune which
border the Centaur region. Solid grey triangle represent the JFC Gateway
region according to Sarid et al. (2019). Figure 3: Perihelion distance vs.
Tisserand Parameter with respect to Jupiter of 13 Centaurs studied in this
work compared to known active Centaurs and JFC Centaurs, and the known Centaur
population as of February 2021. Figure 4: The orbital image coverage of 2014
TV85 from the DECam archival images. The colored circles on the Centaur’s
orbit indicate the orbital locations for which there are DECam images. There
were a total of 19 archival DECam images of the object, out of which 8 were
suited for calibration. The calibrated data span 4 years (2014 – 2018). Figure
5: Postage stamp cutouts from stacked images of 13 Centaurs observed in this
work. The spatial dimensions of each stamp are 30 x 30 arcsec and each stamp
is centered on the object with North up and East to the left. None of the
objects showed obvious visual signs of activity, conclusion supported by the
SBP analysis (see Sec. 3.1). Figure 6: Radial surface brightness profile (SBP)
of 2014 NX65 (black line) compared to the average profile of background field
stars (red line). Both profiles match almost exactly, and therefore rule out
faint activity of our target down to the surface brightness of 24.0 mag
arcsec-2. Figure 7: Comparison of the PSF for object 2015 BH518 with a series
of synthetic PSFs with varying coma levels $\eta$. The coma parameter for this
object was 0.05. Figure 8: The evolution of semi-major axis of the recently
discovered active Centaur 2014 OG392, and two apparently inactive Centaurs
observed in this work. Centaur 2014 PQ70 has experienced decrease in semi-
major axis of similar amplitude as 2014 OG392 The semi-major axes of 2014 PQ70
and 2014 YX49 have been scaled down by 5 and 3 AU respectively for better
comparison on the plot. Figure 9: The evolution of semi-major axis of 12
Centaurs over the past 100,000 years. Figure 10: The evolution of perihelion
distance of 12 Centaurs over the past 100,000 years.

Figure 11: The evolution of semi-major axis of 12 Centaurs over the past 5000
years.

Hearn

Figure 12: The evolution of perihelion distance of 12 Centaurs over the past
5000 years. Figure 13: Orbital evolution of low-perihelion Centaur 2014 PQ70.
Three upper-most panels show the orbital evolution of semi-major axis (black
line) and perihelion distance (blue line) for 100 kyr, 5 kyr and 200 years
into the past (top to bottom). Two bottom panels show the evolution of
eccentricity and the orbital inclination in the last 100 kyr.