Detection of a Water Tracer in Interstellar Comet 2I/Borisov

Abstract

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [O I]6300Å line in interstellar comet 2I/Borisov. We employ the observed flux in this line to derive an H 2 O production rate of (6.3±1.5)×10 26 mol s −1 . Comparing to previously reported observations of CN, this implies a CN/H 2 O ratio of ∼0.3-0.9%. The lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for Solar System comets, but still within the range of observed values. C 2 /H 2 O is normal or depleted depending on interpretation of recent results pertaining to C 2 . The dust-to-gas ratio is consistent with the normal value for Solar System comets. Using a simple sublimation model we estimate an H 2 O active area of 1.7 km 2 , which for current estimates for the size of Borisov suggests active fractions between 1-150%, consistent with values measured in Solar System comets. For the smallest size estimates our derived H 2 O production rates suggest Borisov may be hyperactive (> 100% of its surface active). However, these conclusions concerning active fraction are model dependent. More detailed characterization of 2I/Borisov, including compositional information and properties of the nucleus, are needed to fully interpret the observed H 2 O production rate.

Unusually high CO abundance of the first active interstellar comet

“…The presence of a gas and dust coma surrounding 2I/Borisov, on the other hand, has been confirmed by numerous observations, including the detection of cyanide (CN) and hydroxyl (OH) emission in the near-UV and optical [14][15][16] . The observations reported to-date have revealed a CN/H 2 O ratio and optical dust colours consistent with typical Solar System comets, while the C 2 abundance may be somewhat depleted 17,18 . The OH radical is believed to be a product of water photodissociation in the coma, but the origin of 2I/Borisov's CN is less certain, as cometary CN can be produced from the photodissociation of HCN and other nitriles, and from the degradation of organic-rich dust grains 19 .…”

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

“…One of the salient properties of 2I is that it contains significant amounts of volatiles such as CN and C 2 gas (Fitzsimmons et al 2019;Kareta et al 2020;Opitom et al 2019), and there is evidence in this work from the photometry presented in Section 3.3 that the comet also contains H 2 O, unlike the super-rich CO/N2/CH4, H 2 O-depleted comet C/2016 R2 (Cochran & McKay 2018;McKay et al 2020). Instead, it is acting like an Oort cloud comet on a megayearperiod orbit like C/1995 O1 (Hale-Bopp), C/2013 S1 (ISON), or C/2017 K2 (Jewitt et al 2017a;Meech et al 2017), which commonly demonstrate outgassing abundances of CO with respect to water in the 0.2%-20% range (Bockelée-Morvan et al 2004).…”

Mt. Wendelstein imaging of comet 41P/Tuttle-Giacobini-Kresak during the 2017 perihelion arc

“…Rumyantsev et al (2019) presented polarization measurements of 41P from a single epoch in April 2017. Other partly preliminary results were reported on volatile species (C 2 H 2 , HCN, C 2 H 6 and H 2 CO;McKay et al 2017McKay et al , 2018, the OH production rate and gas expansion velocity (Lovell et al 2017), and the radar size of the nucleus (Howell et al 2017).…”