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Register a new userThe extraordinary composition of the blue comet C/2016 R2 (PanSTARRS)
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We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). This comet was observed on 23-24 January 2018 with the IRAM 30m telescope, and in January to March 2018 with the Nanc{c}ay radio telescope. Visible spectroscopy was performed in December 2017 and February 2018 with small amateur telescopes. We report on measurements of CO, CH3OH, H2CO and HCN production rates, and on the determination of the N2/CO abundance ratio. Several other species, especially OH, were searched for but not detected. The inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of about 2.8 AU. The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N2 and CO and dust poor. This suggests that this comet might belong to a very rare group of comets formed beyond the N2 ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers.
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Early observations of comet C/2016 R2 (PanSTARRS) have shown that the composition of this comet is very peculiar. 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. We detect strong emissions from the ions $mathrm{N_2^+}$ and $mathrm{CO^+}$, but also $mathrm{CO_2^+}$, emission from the CH radical, and much fainter emissions of the CN, $mathrm{C_2}$, and $mathrm{C_3}$ radicals which were not detected in previous observations of this comet. We do not detect OH or $mathrm{H_2O^+}$, and derive an upper limit of the $mathrm{H_2O^+/CO^+}$ ratio of 0.4, implying that the comet has a low water abundance. We measure a $mathrm{N_2^+/CO^+}$ ratio of $0.06pm0.01$. The non-detection of $mathrm{NH_2}$ indicates that most of the nitrogen content of the comet lies within $mathrm{N_2}$. Together with the high $mathrm{N_2^+/CO^+}$ 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 $mathrm{CO_2^+/CO^+}$ ratio is $1.1pm0.3$. We do not detect $mathrm{^{14}N^{15}N^+}$ lines, and can only put a lower limit on the $mathrm{^{14}N/^{15}N}$ ratio measured from $mathrm{N_2^+}$ of about 100, compatible with measurements of the same isotopic ratio for $mathrm{NH_2}$ 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.
Recent observations of the long period comet C/2016 R2 (PanSTARRS) indicate an unusually high N2/CO abundance ratio, typically larger than 0.05, and at least 2-3 times higher than the one measured in 67P/Churyumov-Gerasimenko. Another striking compositional feature of this comet is its heavy depletion in H2O, compared to other comets. Here, we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. We first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. We then alternatively investigate the possibility that the building blocks of the comet C/2016 R2 (PanSTARRS) agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. We show that N2/CO ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the CO and N2 icelines. Moreover, high CO/H2O ratios (>100 times the initial gas phase value) can be found in grains condensed in the vicinity of the CO iceline. If the building blocks of a comet assembled from such grains, they should present N2/CO and CO/H2O ratios consistent with the measurements made in comet C/2016 R2 (PanSTARRS)s coma. Our scenario indicates that comet C/2016 R2 (PanSTARRS) formed in a colder environment than the other comets that share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.
We present mid-infrared observations of comet P/2016 BA14 (PANSTARRS), which were obtained on UT 2016 March 21.3 at heliocentric and geocentric distances of 1.012 au and 0.026 au, respectively, approximately 30 hours before its closest approach to Earth (0.024 au) on UT 2016 March 22.6. Low-resolution ($lambda$/$Delta lambda$~250) spectroscopic observations in the N-band and imaging observations with four narrow-band filters (centered at 8.8, 12.4, 17.7 and 18.8 $mu$m) in the N- and Q-bands were obtained using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the 8.2-m Subaru telescope atop Maunakea, Hawaii. The observed spatial profiles of P/2016 BA14 at different wavelengths are consistent with a point-spread function. Owing to the close approach of the comet to the Earth, the observed thermal emission from the comet is dominated by the thermal emission from its nucleus rather than its dust coma. The observed spectral energy distribution of the nucleus at mid-infrared wavelengths is consistent with a Planck function at temperature T~350 K, with the effective diameter of P/2016 BA14 estimated as ~0.8 km (by assuming an emissivity of 0.97). The normalized emissivity spectrum of the comet exhibits absorption-like features that are not reproduced by the anhydrous minerals typically found in cometary dust coma, such as olivine and pyroxene. Instead, the spectral features suggest the presence of large grains of phyllosilicate minerals and organic materials. Thus, our observations indicate that an inactive small body covered with these processed materials is a possible end state of comets.
The N$_2$ and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) (hereafter `C/2016 R2) is a unique comet for detailed spectroscopic analysis. We aim to explore the associated photochemistry of parent species, which produces different metastable states and forbidden emissions, in this cometary coma of peculiar composition. We re-analyzed the high-resolution spectra of comet C/2016 R2, which were obtained in February 2018, using the UVES spectrograph of the European Southern Observatory (ESO) Very Large Telescope (VLT). Various forbidden atomic emission lines of [CI], [NI], and [OI] were observed in the optical spectrum of this comet when it was at 2.8 au from the Sun. The observed forbidden emission intensity ratios are studied in the framework of a couple-chemistry emission model. The model calculations show that CO$_2$ is the major source of both atomic oxygen green and red-doublet emissions in the coma of C/2016 R2 (while for most comets it is generally H$_2$O), whereas, CO and N$_2$ govern the atomic carbon and nitrogen emissions, respectively. Our modelled oxygen green to red-doublet and carbon to nitrogen emission ratios are higher by a factor {of 3}, when compared to the observations. These discrepancies can be due to uncertainties associated with photon cross sections or unknown production/loss sources. Our modelled oxygen green to red-doublet emission ratio is close to the observations, when we consider an O$_2$ abundance with a production rate of 30% relative to the CO production rate. The collisional quenching is not a significant loss process for N($^2$D) though its radiative lifetime is significant ($sim$10 hrs). Hence, the observed [NI] doublet-emission ratio ([NI] 5198/5200) of 1.22, which is smaller than the terrestrial measurement by a factor {1.4}, is mainly due to the characteristic radiative decay of N($^2$D).
The recent observations show that comet C/2016 R2 (Pan-Starrs) has a unique and peculiar composition when compared with several other comets observed at 2.8 au heliocentric distance. Assuming solar resonance fluorescence is the only excitation source, the observed ionic emission intensity ratios are used to constrain the corresponding neutral abundances in this comet. We developed a physico-chemical model to study the ion density distribution in the inner coma of this comet by accounting for photon and electron impact ionization of neutrals, charge exchange and proton transfer reactions between ions and neutrals, and electron-ion thermal recombination reactions. Our calculations show that CO2+ and CO+ are the major ions in the inner coma, and close to the surface of nucleus CH3OH+, CH3OH2+ and O2+ are also important ions. By considering various excitation sources, we also studied the emission mechanisms of different excited states of CO+, CO2+, N2+, and H2O+. We found that the photon and electron impact ionization and excitation of corresponding neutrals significantly contribute to the observed ionic emissions for radial distances smaller than 300 km and at larger distances, solar resonance fluorescence is the major excitation source. Our modelled ion emission intensity ratios are consistent with the ground-based observations. Based on the modelled emission processes, we suggest that the observed ion emission intensity ratios can be used to derive the neutral composition in the cometary coma only when the ion densities are significantly controlled by photon and photoelectron impact ionization of neutrals rather than by the ion-neutral chemistry.
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