Interstellar Visitors: A Physical Characterization of Comet C/2019 Q4 (Borisov) with OSIRIS at the 10.4 m GTC

Julia de Leon, Javier Licandro, Miquel Serra-Ricart, Antonio Cabrera-Lavers, Joan Font Serra, Riccardo Scarpa, Carlos de la Fuente Marcos,.and Raúl de la Fuente Marcos

Draft version September 13, 2019
preprint, AAS20106

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AV data

“Comet C/2019 Q4 (Borisov) was discovered on August 30, 2019 by G. Borisov observing from the Crimean Astrophysical Observatory, when the object was nearly 3 au from the Sun. The discovery was announced by the Minor Planet Center on September 11, 2019 (MPEC 2019-R106), giving a value of the heliocentric orbital eccentricity of 3.07. The orbit determination (as of September 12, 2019) publicly available from JPL’s Small-Body Database or SBDB puts the value of the eccentricity (heliocentric or barycentric) at e = 3.64 ± 0.15, i.e. hyperbolic at the 18σ level. Such a high barycentric eccentricity makes C/2019 Q4 the second interstellar minor body ever found, after 1I/2017 U1 (‘Oumuamua) that was observed for 80 d (e.g. ‘Oumuamua ISSI Team et al. 2019). N -body calculations carried out as described by de la Fuente Marcos & de la Fuente Marcos (2019) using the orbit determination available from SBDB and the covariance matrix show that C/2019 Q4 entered the Solar system from insterstellar space at
−33.8−+00..65 km s−1 (median and 16th and 84th percentiles, ‘Oumuamua had 26 km s−1 , e.g. Mamajek 2017) from a radiant in equatorial coordinates of (2h.25−+00..0203 , +59◦.04+0.15 −0.13 ), close to open star cluster Stock 2. It will reach its apex at (18h.14−+00..0910 , −53◦.3 ± 0.5) and 33.6 ± 0.6 km s−1 . All the computations indicate that it came from interstellar space and it will return to it. Contrary to what happened with ‘Oumuamua, C/2019 Q4 is currently entering the Solar System, so it will be observable for a longer period. At the time of this writing, the object is at low solar elongation and is visible only at low elevation, making observations difficult. We observed comet C/2019 Q4 on the night of September 12, 2019, using the 10.4 m Gran Telescopio Canarias (GTC), located at the El Roque de Los Muchachos Observatory (La Palma, Canary Islands, Spain), managed by the Instituto de Astrof´ısica de Canarias (IAC). We obtained 3 images of 30 seconds in the Sloan r’ to identify the target in the field of view, confirming its cometary nature (see Fig. 1). We also obtained a total of three visible spectra of 300 seconds in the 0.49-0.92 µm wavelength range, using the OSIRIS camera-spectrograph (Cepa 2010). To get the reflectance spectrum, we also observed three solar analog stars from the Landolt catalogue (Landolt 1992) at the same airmass as that of the object. Data reduction followed the procedure described in Licandro et al. (2019), and consisted of bias and flat-field correction, background subtraction and extraction of the 1D spectrum from 2D images,and wavelength calibration. The spectrum of the comet was divided by the spectra of the three solar analogs, and the resulting spectra were finally averaged. The final spectrum was normalized to unity at 0.55 µm and is shown in Figure 1. We also plot the template spectrum of the D-type class as defined by DeMeo et al. (2009). As it can be seen, the visible spectrum of C/2019 Q4 resembles those of typical cometary nuclei. We computed the spectral slope in the 0.55–0.90 µm range, S’ = 10 ± 1 %/1000 ˚A, which is roughly in the middle of the range of visible spectral slopes observed for cometary nuclei and asteroids in cometary orbits (see Licandro et al. 2018). This suggests that comets formed in other stars can have a similar composition to those formed in the Solar System and thus be likely produced by similar processes.”