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Register a new userInitial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), using Hubble, Spitzer, ZTF, Keck, APO and GROWTH Visible & Infrared Imaging and Spectroscopy
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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.
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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.
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.
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.
We observed Episodically Active Asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and have found that it is no longer active after its recent outbursts at the end of 2018 and start of 2019. The inactivity during this apparation allowed us to measure the absolute magnitude of Gault of H_r = 14.63 +/- 0.02, G_r = 0.21 +/- 0.02 from our secular phasecurve observations. In addition, we were able to constrain Gaults rotation period using time-series photometric lightcurves taken over 17 hours on multiple days in 2020 August, September and October. The photometric lightcurves have a repeating $lesssim$0.05 magnitude feature suggesting that (6478) Gault has a rotation period of ~2.5 hours and may have a semi-spherical or top-like shape, much like Near-Earth Asteroids Ryugu and Bennu. The rotation period of ~2.5 hours is near to the expected critical rotation period for an asteroid with the physical properties of (6478) Gault suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spun up by the Yarkovsky-OKeefe-Radzievskii-Paddack effect.
We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2019 November 29 using the GROWTH, the APO ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery observations of 2I obtained by ZTF from 2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System comets shows an object very similar to mildly active Solar System comets with an out-gassing rate of $sim$10$^{27}$ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of $sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H$_2$O sublimation with the comet being active with super volatiles such as CO at heliocentric distances $>$6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2Is nucleus of $lesssim$1.4 km. We use the size estimates of 1I/Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size-distribution resulting in a slope of $sim$3.4$pm$1.2, similar to Solar System comets and bodies produced from collisional equilibrium.
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