Leveraging the ALMA Atacama Compact Array for Cometary Science: An Interferometric Survey of Comet C/2015 ER61 (PanSTARRS) and Evidence for a Distributed Source of Carbon Monosulfide

Abstract

We report the first survey of molecular emission from cometary volatiles using standalone Atacama Compact Array (ACA) observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2015 ER61 (PanSTARRS) carried out on UT 2017 April 11 and 15, shortly after its April 4 outburst. These measurements of HCN, CS, CH3OH, H2CO, and HNC (along with continuum emission from dust) probed the inner coma of C/2015 ER61, revealing asymmetric outgassing and discerning parent from daughter/distributed source species. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for each molecule and a stringent upper limit for CO. HCN is consistent with direct nucleus release in C/2015 ER61, whereas CS, H2CO, HNC, and potentially CH3OH are associated with distributed sources in the coma. Adopting a Haser model, parent scale lengths determined for H2CO (L p ∼ 2200 km) and HNC (L p ∼ 3300 km) are consistent with previous work in comets, whereas significant extended source production (L p ∼ 2000 km) is indicated for CS, suggesting production from an unknown parent in the coma. The continuum presents a point-source distribution with a flux density implying an excessively large nucleus, inconsistent with other estimates of the nucleus size. It is best explained by the thermal emission of slowly moving outburst ejectas, with total mass 5–8 × 1010 kg. These results demonstrate the power of the ACA for revealing the abundances, spatial distributions, and locations of molecular production for volatiles in moderately bright comets such as C/2015 ER61.

Emission from HCN and CH$_3$OH in comets. Onsala 20-m observations and radiative transfer modelling

“…Our observations on 9 May were disregarded due to an inaccurate set of ephemeris. Saki et al (2021) report a water production rate around 1 × 10 29 mol s −1 in mid-april 2017 and an HCN rotational temperature near 70 K. This is consistent with the Atacama Large Millimeter/Submillimeter Array (ALMA) compact array HCN observations in mid-April by Roth et al (2021b).…”

A SUBLIME 3D Model for Cometary Coma Emission: the Hypervolatile-Rich Comet C/2016 R2 (PanSTARRS)

“…This assumes that the nucleus can be divided into two different activity regimes: (1) an ambient outflow from the majority of the (thermally-activated) sublimating area of the nucleus, and (2) enhanced gas production from a (set of) spatiallyconfined vent(s) or jet(s), in particular, on the sunward-facing side of the nucleus. Similar 2-component models (involving a sunward jet and ambient/isotropic coma, or a hemispherically asymmetric outflow) have been invoked previously to explain asymmetries in high-resolution spectral line profiles from several comets, including 29P/SW1 (Festou et al 2001), O1/Hale-Bopp (Gunnarsson 2003), 19P/Borelly (Bockelée-Morvan et al 2004b), 2I/Borisov (Cordiner et al 2020), 46P/Wirtanen (Roth et al 2021b) and C/2015 ER61 (Roth et al 2021a). The increased production rate and outflow velocity measured on the sunward side of these comets is consistent with the results of fluid dynamic and Monte Carlo coma models (Crifo et al 1999;Fougere et al 2016), and arises as a result of elevated temperatures in both the coma and the nucleus, due to increased solar insolation.…”

“…Fitting the same three transitions simultaneously using SUBLIME (allowing only the CH 3 OH abundance to vary) gave Q(CH 3 OH) = (6.7 ± 2.2) × 10 26 s −1 , which corresponds to a CH 3 OH/CO abundance ratio of 1.3 ± 0.4% at the nucleus. For this model, we adopted CH 3 OH-CO collisional transition rates based on the CH 3 OH-H 2 rates from Rabli & Flower (2010), with solar pumping rates from Roth et al (2021a). Uncertainties of a factor of five in the CH 3 OH-CO collision rates lead to at most a 7% error on the CH 3 OH abundance.…”

“…Threesigma upper limits were derived by comparing the spectrallyintegrated noise level (from −0.8 to 0.5 km s −1 ) to SUB-LIME model line intensities for each molecule (again, using the same coma physical parameters derived in Section 4.3). For HCN we used the same collision and pumping rates as Cordiner et al (2020), for H 2 CO we used the Roth et al (2021a) rates, and for 13 CO we used the same rates as for CO. Production rate 3σ upper limits are Q(HCN) < 8.3 × 10 25 s −1 , Q(H 2 CO) < 2.3 × 10 26 s −1 and Q( 13 CO) < 2.7 × 10 27 s −1 , corresponding to abundance ratios HCN/CO 13 CO (J = 3 2)…”

“…The resulting (spectral-spatial-temporal) dataset is analyzed using a new, time-dependent, three dimensional radiative transfer code (SUBLIME), which is an evolution of the steady-state model used previously by Paganini et al (2010), Bøgelund & Hogerheijde (2017), de Val-Borro et al (2018), and Roth et al (2021a), and includes excitation via collisions with CO and electrons, as well as radiative processes. In Section 5.4, we briefly discuss the conditions for which a time-dependent solution of the molecular excitation produces more accurate results than the steady-state treatment.…”