2020ApJ...889L..10M

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [O I] 6300 Å line in interstellar comet 2I/Borisov. We employ the observed flux in this line to derive an H₂O production rate of (6.3 ± 1.5) x 10²⁶ mol s^–1. Comparing to previously reported observations of CN, this implies a CN/H₂O ratio of ∼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. C₂/H₂O 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 H₂O active area of 1.7 km², which for current estimates for the size of Borisov suggests active fractions between 1% and 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 H₂O production rate.