17 May 2024

The passage of interstellar comet 2I/Borisov in December 2019 offered a unique opportunity to acquire comprehensive data on an object originating from another planetary system, that left behind material in interplanetary space as it approached the Sun. Astronomers using a number of telescopes around the world, including the telescopes of the Isaac Newton Group (ING), monitored its behaviour spectroscopically and photometrically.

A paper recently published in the journal Monthly Notices of the Royal Astronomical Society reports on the data obtained using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), and the ISIS spectrograph on the William Herschel Telescope (WHT). The goal was to determine the dust production rate and the mass loss of the comet, to characterise its gas environment, and to compare these properties with other comets in the Solar System.

Overall, the observations show that 2I/Borisov has a similar appearance to other comets in our Solar System, with a reddish colour and homogeneous morphology. The comet's activity fluctuated around a constant level in October and November but decreased in December, when it started to lose less material after it passed closest to the Sun. Dust production and mass-loss calculations indicate approximately an average of 4 kg/s before perihelion while afterwards the net mass loss was about 0.6 kg/s.

To quantify the activity level of the comet, the astronomers estimated the effective scattering cross-sections in each spectral band. Then, they applied Mie scattering theory to find out what kind of dust particles can reproduce the observed behaviour. From models they estimated the size of the tiny dust particles in the coma to be around 100–200 nanometers and found that the terminal speed of these particles is higher than those observed in the tail.

The spectrum of the comet obtained using the WHT on 27th November 2019 exhibited several species like CN, NH2, and the forbidden lines of oxygen. The latter are present because of the high concentration of CO molecules. This itself is an interesting finding, taking into account that 2I/Borisov is one of the most pristine comets ever observed.

[Image]

(A) Composite image of 2I/Borisov using B, V and r filters obtained with the INT, in the early morning of 30th October 2019, and GNU Astro software. 

(B) The effective scattering cross-section over time, normalised to the median value in each band. The line connects the mean values corresponding to each night. The perihelion of comet 2I/Borisov is marked with a dashed line. TCS stands for Telescopio Carlos Sánchez. C

(C) Fluoresence spectrum of 2I/Borisov obtained using ISIS on the WHT on 27th November 2019. Credit: Figure extracted from Prodan et al. (2024).