Searching for water ice in the coma of interstellar object 2I/Borisov

Abstract

Aims. Interstellar Objects (ISO) passing through our Solar System offer a rare opportunity to probe the physical and chemical processes involved in solid body and planet formation in extrasolar systems. The main objective of our study is to search for diagnostic absorption features of water ice in the near infrared (NIR) spectrum of the second interstellar object 2I/2019 Q4 (Borisov) and compare its ice features to those of the Solar system icy objects. Methods. We observed 2I in the NIR on three separate occasions. The first observation was made on 2019 September 19 UT using the SpeX spectrograph at the 3-m IRTF and again on September 24 UT with the GNIRS spectrograph at the 8-m GEMINI telescope and the last observation was made on October 09 UT with IRTF. Results. The spectra obtained from all three nights appear featureless. No absorption features associated with water ice are detected. Spectral modeling suggests that water grains, if present, comprise no more than 10% of the coma cross-section. The comet consistently exhibits a red D-type like spectrum with a spectral slope of about 6% per 1000 Å, which is similar to that of 1I/'Oumuamua and is comparable to Solar system comets.

Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov

“…For cometary dust, the optical color depends mainly on the particle size distribution, rather than on the dust composition 30 . The absence of temporal and spatial color variations indicates that no significant dust fragmentation and/or sublimation from grains occurred in the coma of 2I/Borisov, which is consistent with the lack of icy particles in the coma 31 .…”

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

“…The spectrum was adjusted to the photometric points. The IR color of 2I, as determined by the continuum slope of the prism spectra, was found to be neutral gray, in agreement with the rzJHK colors, in contrast with Yang et al (2019). No definitive absorption or emission lines were found within the errors of the measurements, similar to the lack of emission lines seen in the spectra of solar system comets and asteroids in the 0.7-2.5 μm range (Feldman et al 2004).…”

“…We converted the H abs magnitude determined with the 0 48 aperture into an effective cross section using Equation (2), resulting in an effective cross section of 2.65±0.39 km 2 , assuming an albedo equal to 0.1, typical for comet dust, and resulting in a value of 6.63±0.97 km 2 , assuming an albedo equal to 0.04, typical for comet nuclei (Fernández et al 2001;Bauer et al 2017). A higher albedo could also be used to calculate the cross section corresponding to an icy, more reflective composition (Yang et al 2009), but the NIR spectra presented here, as well as additional NIR spectra (Yang et al 2019), do not show strong evidence for the presence of ice in the coma of 2I.…”

Water Production Rates and Activity of Interstellar Comet 2I/Borisov

“…For Borisov, the water production rates derived from our large-aperture observations are in good agreement with those derived from narrow-slit observations (3.2 × 1.6 arcsec; apparent size ≈ 6 500 km; McKay et al (2020). However, this does not allow us to conclusively exclude the presence of icy grains, because this slit size is much larger than the expected lifetimes of icy grains Yang et al (2020). Icy grains would have sublimated within the slit used by McKay et al, which is thus too large to help us find different production rates with both methods.…”