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

TL;DR

This study detects water vapor in interstellar comet 2I/Borisov using high-resolution spectra, estimating its water production rate and comparing its composition to Solar System comets, revealing similarities and differences.

Contribution

First detection of water vapor in interstellar comet 2I/Borisov using optical spectroscopy, providing key compositional and activity insights.

Findings

Water production rate of (6.3±1.5)×10^{26} mol s^{-1}

CN/H2O ratio between 0.3% and 0.6%

C2/H2O likely less than 0.1%

Abstract

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [OI]6300 A 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$\pm$1.5)$\times$10$^{26}$ mol s$^{-1}$. Comparing to previously reported observations of CN, this implies a CN/H$_2$O ratio of $\sim$0.3-0.6%. The lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for Solar System comets, but still within the range of observed values. C$_2$/H$_2$O is depleted, with a value likely less than 0.1%. The dust-to-gas ratio is consistent with the normal value for Solar System comets. Using a simple sublimation model we estimate an H$_2$O active area of 1.7 km$^2$, which for current estimates for the size of Borisov suggests active fractions between 1-150%, consistent with values measured in Solar System comets. More detailed characterization of 2I/Borisov, including compositional information and properties of the nucleus, is needed to fully interpret the observed H$_2$O production rate.