Pre-discovery Activity of New Interstellar Comet 2I/Borisov beyond 5 au

Abstract

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in August 2019 at ∼ 3 au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey (CSS), Pan-STARRS and Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in Bolin et al. (2019). We identified comet Borisov in ZTF images taken in May 2019 and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as December 2018, when it was 7.8 au from the Sun. The comet was not detected in November 2018 when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H 2 O, such as CO or CO 2 , or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be < 7 km using the non-detection in November 2018, and estimate an area of ∼ 0.5-10 km 2 has been active between December 2018 and September 2019, though this number is model-dependent and is highly uncertain. The behavior of comet Borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two.

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 observed magnitude of r=20.5-21.0. As shown by Ye et al (2020) and confirmed by our work, the actual observed prediscovery r magnitudes are much closer to the brightness model predicted for a comet that has its activity dominated by CO than H 2 O (Fitzsimmons et al 2019). This is supported by the fact that H O is very weakly sublimating at temperatures 150 K at a heliocentric distance >3.5 au, while CO can become volatile much further from the Sun at heliocentric distances exceding 10-100 au (Meech & Svoren 2004).…”

“…However, the prediscovery photometry may also be compatible with CO 2 -driven activity where CO 2 can become active at >13 au (Womack et al 2017;Ye et al 2020). As discussed in Section 3.4, a significant production rate of H 2 O is inferred from the observed production of CN and C 2 gas (Fitzsimmons et al 2019;Kareta et al 2020) and is ∼100 kg s −1 comparable, though larger than the H 2 O∼20 kg s −1 production inferred from the detection of the [O I] 6300 Å line taken at further heliocentric distances (McKay et al 2020).…”

“…We used images that were taken with a 30 s r-filter exposure for the prediscovery images. We used two exposures taken on 2019 March 17 UTC, two exposures taken on 2019 March 18 UTC, six exposures taken on 2019 May 2 UTC, and four exposures taken on 2019 May 5 UTC (Ye et al 2020). In addition to the prediscovery detections, we identified additional postdiscovery detections of 2I in ZTF survey data between 2019 September 11 and 2019 December 20 UTC that were taken in SDSS-like g and r filters (Graham et al 2019).…”

“…We increased the range of our long-term light curve by using prediscovery observations of 2I found in r-filter images from the ZTF survey spanning 2019 March to May. Ye et al (2020) presented these measurements and analysis; here we repeat their extraction from raw data both as a cross-check and for methodological consistency reasons.…”

“…Therefore, it may be possible to distinguish between different compositional models of 2I by measuring its brightness at different heliocentric distances covering a wide span of times (e.g., Jewitt et al 2017a;Meech 2017). This indeed appears to be the case, with Zwicky Transient Facility (ZTF) precovery observations of 2I strongly favoring the activity of the comet being driven by more volatile species than H 2 0, such as CO or CO 2 (Ye et al 2020). In this paper, we build upon these ZTF results and present visible and nearinfrared (NIR) observations of 2I and its morphology; the null result for variability on short-term timescales; estimates of the comet's size, afρ, and dust mass-loss rate; strengthened evidence for activity driven by CO and H 2 O; and an estimate of the ISO cumulative size distribution slope.…”