High resolution optical spectroscopy of the N₂-rich comet C/2016 R2 (PanSTARRS)

Abstract

Context. Early observations of comet C/2016 R2 (PanSTARRS) have shown that the composition of this comet is very peculiar. Radio observations have revealed a CO-rich and HCN-poor comet and an optical coma dominated by strong emission bands of CO + and, more surprisingly, N + 2 . Aims. The strong detection of N + 2 in the coma of C/2016 R2 provided an ideal opportunity to measure the 14 N/ 15 N isotopic ratio directly from N + 2 for the first time, and to estimate the N 2 /CO ratio, which is an important diagnostic to constrain formation models of planetesimals, in addition to the more general study of coma composition. Methods. We obtained high resolution spectra of the comet in February when it was at 2.8 au from the Sun. We used the UVES spectrograph of the ESO VLT, complemented with narrow-band images obtained with the TRAPPIST telescopes. Results. We detect strong emissions from the ions N + 2 and CO + , but also CO + 2 , emission from the CH radical, and much fainter emissions of the CN, C 2 , and C 3 radicals which were not detected in previous observations of this comet. We do not detect OH or H 2 O + , and derive an upper limit of the H 2 O + /CO + ratio of 0.4, implying that the comet has a low water abundance. We measure a N + 2 /CO + ratio of 0.06 ± 0.01. The non-detection of NH 2 indicates that most of the nitrogen content of the comet lies within N 2 . Together with the high N + 2 /CO + ratio, this could indicate a low formation temperature of the comet, or that the comet is a fragment of a large differentiated Kuiper Belt object. The CO + 2 /CO + ratio is 1.1 ± 0.3. We do not detect 14 N 15 N + lines, and can only put a lower limit on the 14 N/ 15 N ratio measured from N + 2 of about 100, compatible with measurements of the same isotopic ratio for NH 2 and CN in other comets. Finally, in addition to the [OI] and [CI] forbidden lines, we detect for the first time the forbidden nitrogen lines [NI] doublet at 519.79 and 520.04 nm in the coma of a comet.

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

“…The seeing was 0.9 during the observations of the [OI], [NI] lines (UVES setting 390+580) and 1.0 during the observations of the [CI] lines (UVES setting 437+860). More details about the observations and data reductions are given in Opitom et al (2019).…”

“…These factors were estimated by assuming that the full-slit fluxes in the bright lines are identical on Feb. 11, Feb. 13, and Feb. 14. The contamination by C 2 band emission, which generally can affect the [OI] 5577 observation (Decock et al 2015), is negligible in this comet due to the very small amount of C 2 in the coma of this comet (Opitom et al 2019). The measurements obtained on the different nights were averaged.…”

“…1). However, the [CI] line at 9824 Å was contaminated by telluric lines so that its surface brightness could not be accurately measured (Opitom et al 2019). We thus used the known line ratio [CI] 9824/[CI] 9850 = 0.34 (Nussbaumer & Rusca 1979) to compute the [CI] line ratio which is reported in Table 2.…”

“…Cometary optical spectra are usually dominated by various resonance fluorescence lines of radicals such as OH, NH, CN, C 2 , C 3 , and NH 2 . However, the observations of C/2016 R2 have shown that its coma is rich in N 2 and CO and depleted of H 2 O, HCN, and C 2 , which is unusual and makes this comet a unique case among the comets observed so far (Biver et al 2018;Opitom et al 2019;McKay et al 2019). Opitom et al (2019) have observed several atomic oxygen (5577, 6300, & 6364 Å), carbon (8727, 9824, & 9850 Å) and nitrogen (5197 & 5200 Å) forbidden emission lines in C/2016 R2.…”

“…Last_printer_version fore. Opitom et al (2019) recently observed comet C/2016 R2 and reported the first-ever atomic nitrogen forbidden emissions.…”