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Register a new userContemporaneous Multi-Wavelength and Precovery Observations of Active Centaur P/2019 LD2 (ATLAS
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Gateway Centaur and Jupiter co-orbital P/2019 LD2 (ATLAS) (Sarid et al. 2019) provides the first opportunity to observe the migration of a Solar System small body from a Centaur orbit to a Jupiter Family Comet (JFC) four decades from now (Kareta et al., 2020; Hsieh et al. 2020). The Gateway transition region is beyond where water ice can power cometary activity, and coma production there is as poorly understood as in all Centaurs. We present contemporaneous multi-wavelength observations of LD2 from 2020 July 2-4: Gemini-North visible imaging, NASA IRTF near-infrared spectroscopy, and ARO SMT millimeter-wavelength spectroscopy. Precovery DECam images limit the nucleus effective radius to <=1.2 km and no large outbursts were seen in archival Catalina Sky Survey observations. LD2s coma has g-r=0.70+/-0.07, r-i=0.26+/-0.07, a dust production rate of ~10-20 kg/s, and an outflow velocity between v~0.6-3.3 m/s. We did not detect CO towards LD2 on 2020 July 2-3, with a 3-sigma upper limit of Q(CO) < 4.4 * 10^27 mol/s (<200 kg/s). Near-infrared spectra show evidence for water ice at the 1-10% level depending on grain size. Spatial profiles and archival data are consistent with sustained activity. The evidence supports the hypothesis that LD2 is a typical small Centaur that will become a typical JFC, and thus it is critical to understanding the transition between these two populations. Finally, we discuss potential strategies for a community-wide, long baseline monitoring effort.
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The recently discovered object P/2019 LD2 (ATLAS) was initially thought to be a Jupiter Trojan asteroid, until dynamical studies and the appearance of persistent cometary activity revealed that this object is actually an active Centaur. However, the dynamical history, thermal environment, and impact of such environments on the activity of 2019 LD2 are poorly understood. Here we conduct dynamical simulations to constrain its orbital history and resulting thermal environment over the past 3000 years. We find that 2019 LD2 is currently in the vicinity of a dynamical Gateway that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets (JFC population; Sarid et al. 2019). Our calculations show that it is unlikely to have spent significant amounts of time in the inner solar system, suggesting that its nucleus is relatively pristine in terms of physical, chemical, and thermal processing through its history. This could explain its relatively high level of distant activity as a recently activated primordial body. Finally, we find that the median frequency of transition from the Gateway population into the JFC population varies from once every ~3 years to less than once every 70 years, if 2019 LD2s nucleus is ~1 km in radius or greater than 3 km in radius. Forward modeling of 2019 LD2 shows that it will transition into the JFC population in 2063, representing the first known opportunity to observe the evolution of an active Centaur nucleus as it experiences this population-defining transition.
We present visible and mid-infrared imagery and photometry of temporary Jovian co-orbital comet P/2019 LD$_2$ taken with HST/WFC3, Spitzer/IRAC, the GROWTH telescope network, visible spectroscopy from Keck/LRIS and archival ZTF observations taken between 2019 April and 2020 August. Our observations indicate that the nucleus of LD$_2$ has a radius between 0.2-1.8 km assuming a 0.08 albedo and a coma dominated by $sim$100$mu$ m-scale dust ejected at $sim$1 m/s speeds with a $sim$1 jet pointing in the SW direction. LD$_2$ experienced a total dust mass loss of $sim$10$^8$ kg at a loss rate of $sim$6 kg/s with Af$rho$/cross-section varying between $sim$85 cm/125 km$^2$ and $sim$200 cm/310 km$^2$ from 2019 April 9 to 2019 Nov 8. If the increase in Af$rho$/cross-section remained constant, it implies LD$_2$s activity began $sim$2018 November when within 4.8 au of the Sun, implying the onset of H$_2$O sublimation. We measure CO/CO$_2$ gas production of $lesssim$10$^{27}$ mol/s /$lesssim$10$^{26}$ mol/s from our 4.5 $mu$m Spitzer observations, $g$-$r$ = 0.59$pm$0.03, $r$-$i$ = 0.18$pm$0.05, $i$-$z$ = 0.01$pm$0.07 from GROWTH observations, H$_2$O gas production of $lesssim$80 kg/s scaling from our estimated $C_2$ production of $Q_{C_2}lesssim$7.5$times10^{24}$ mol/s from Keck/LRIS spectroscopy. We determine that the long-term orbit of LD$_2$ is similar to Jupiter family comets having close encounters with Jupiter within $sim$0.5 Hill radius in the last $sim$3 y, within 0.8 Hill radius in $sim$9 y. Additionally, 78.8$%$ of our orbital clones are ejected from the Solar System within $1 times 10^{6}$ years having a dynamical half-life of 3.4 $times 10^5$ years.
Comet P/2019 LD2 has orbital elements currently resembling those of a Jupiter Trojan, and therefore superficially appears to represent a unique opportunity to study the volatile content and active behavior of a member of this population for the first time. However, numerical integrations show that it was previously a Centaur before reaching its current Jupiter Trojan-like orbit in 2018 July, and is expected to return to being a Centaur in 2028 February, before eventually becoming a Jupiter-family comet in 2063 February. The case of P/2019 LD2 highlights the need for mechanisms to quickly and reliably dynamically classify small solar system bodies discovered in current and upcoming wide-field surveys.
P/2011 S1 (Gibbs) is an outer solar system comet or active Centaur with a similar orbit to that of the famous 29P/Schwassmann-Wachmann 1. P/2011 S1 (Gibbs) has been observed by the Pan-STARRS 1 (PS1) sky survey from 2010 to 2012. The resulting data allow us to perform multi-color studies of the nucleus and coma of the comet. Analysis of PS1 images reveals that P/2011 S1 (Gibbs) has a small nucleus $< 4$ km radius, with colors $g_{P1}-r_{P1} = 0.5 pm 0.02$, $r_{P1}-i_{P1} = 0.12 pm 0.02$ and $i_{P1}-z_{P1} = 0.46 pm 0.03$. The comet remained active from 2010 to 2012, with a model-dependent mass-loss rate of $sim100$ kg s$^{-1}$. The mass-loss rate per unit surface area of P/2011 S1 (Gibbs) is as high as that of 29P/Schwassmann-Wachmann 1, making it one of the most active Centaurs. The mass-loss rate also varies with time from $sim 40$ kg s$^{-1}$ to 150 kg s$^{-1}$. Due to its rather circular orbit, we propose that P/2011 S1 (Gibbs) has 29P/Schwassmann-Wachmann 1-like outbursts that control the outgassing rate. The results indicate that it may have a similar surface composition to that of 29P/Schwassmann-Wachmann 1. Our numerical simulations show that the future orbital evolution of P/2011 S1 (Gibbs) is more similar to that of the main population of Centaurs than to that of 29P/Schwassmann-Wachmann 1. The results also demonstrate that P/2011 S1 (Gibbs) is dynamically unstable and can only remain near its current orbit for roughly a thousand years.
Observations of active asteroid P/2017 S5 when near perihelion reveal the ejection of large (0.1 to 10 mm) particles at 0.2 to 2 m/s speeds, with estimated mass-loss rates of a few kg/s. The protracted nature of the mass loss (continuous over 150 days) is compatible with a sublimation origin, meaning that this object is likely an ice-bearing main-belt comet. Equilibrium sublimation of exposed water ice covering as little as 0.1 sq. km can match the data. Observations a year after perihelion show the object in an inactive state from which we deduce a nucleus effective radius 450(+100/-60) m (albedo 0.06+/-0.02 assumed). The gravitational escape speed from a body of this size is just 0.3 m/s, comparable to the inferred ejection speed of the dust. Time-series photometry provides tentative evidence for rapid rotation (lightcurve period 1.4 hour) that may also play a role in the loss of mass and which, if real, is a likely consequence of spin-up by sublimation torques. P/2017 S5 shares both physical and orbital similarities with the split active asteroid pair P/2016 J1-A and J1-B, and all three objects are likely members of the 7 Myr old, collisionally produced, Theobalda family.
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