A&A 625, A133 (2019) https://doi.org/10.1051/0004-6361/201834902 Astronomy & © ESO 2019 Astrophysics Spectroscopic and dynamical properties of comet C/2018 F4, likely a true average former member of the Oort cloud? J. Licandro1,2, C. de la Fuente Marcos3, R. de la Fuente Marcos4, J. de León1,2, M. Serra-Ricart1,2, and A. Cabrera-Lavers5,1,2 1 Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain e-mail: [email protected] 2 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 3 Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain 4 AEGORA Research Group, Facultad de Ciencias Matemáticas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain 5 GRANTECAN, Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain Received 17 December 2018 / Accepted 15 March 2019 ABSTRACT Context. The population of comets hosted by the Oort cloud is heterogeneous. Most studies in this area have focused on highly active objects, those with small perihelion distances or examples of objects with peculiar physical properties and/or unusual chemical com- positions. This may have produced a biased sample of Oort cloud comets in which the most common objects may be rare, particularly those with perihelia well beyond the orbit of the Earth. Within this context, the known Oort cloud comets may not be representative of the full sample meaning that our current knowledge of the appearance of the average Oort cloud comet may not be accurate. Comet C/2018 F4 (PANSTARRS) is an object of interest in this regard. Aims. Here, we study the spectral properties in the visible region and the cometary activity of C/2018 F4, and we also explore its orbital evolution with the aim of understanding its origin within the context of known minor bodies moving along nearly parabolic or hyperbolic paths. Methods. We present observations obtained with the 10.4 m Gran Telescopio Canarias (GTC) that we use to derive the spectral class and visible slope of C/2018 F4 as well as to characterise its level of cometary activity. Direct N-body simulations are carried out to explore its orbital evolution. Results. The absolute magnitude of C/2018 F4 is Hr > 13:62 ± 0:04 which puts a strong limit on its diameter, D < 10:4 km, assuming a pV = 0:04 cometary-like value of the albedo. The object presents a conspicuous coma, with a level of activity comparable to those of other comets observed at similar heliocentric distances. Comet C/2018 F4 has a visible spectrum consistent with that of an X-type asteroid, and has a spectral slope S 0 = 4:0 ± 1.0%/1000 Å and no evidence of hydration. The spectrum matches those of well-studied primitive asteroids and comets. The analysis of its dynamical evolution prior to discovery suggests that C/2018 F4 is not of extrasolar origin. Conclusions. Although the present-day heliocentric orbit of C/2018 F4 is slightly hyperbolic, both its observational properties and past orbital evolution are consistent with those of a typical dynamically old comet with an origin in the Oort cloud. Key words. comets: individual: C/2018 F4 – comets: general – Oort cloud – techniques: spectroscopic – techniques: photometric – methods: numerical 1. Introduction 50 000 to 200 000 AU. The Oort cloud hosts a population of comets of heterogeneous nature (see e.g. Stern & Weissman In our solar system, a number of populations of small bodies 2001; Gibb et al. 2003; Fernández et al. 2004; Meech et al. 2016; are well studied and the notion of an average or typical mem- Bauer et al. 2017). Most authors consider the Oort cloud to have ber of such populations is well defined; good examples are the appeared very early in the history of the solar system, nearly near-Earth objects (NEOs; see e.g. Granvik et al. 2018) or the 4.6 Gyr ago, and to be made of fossil debris from the primor- Jupiter-family comets (see e.g. Fernández et al. 1999; Snodgrass dial protoplanetary disc. Importantly, the solar system was born et al. 2011). Unfortunately, the appearance of an average Oort within a star cluster (see e.g. Kaib & Quinn 2008). Based on this cloud comet remains unclear and this could be the result of most information, Levison et al.(2010) suggest that perhaps over 90% studies focusing on the extreme cases. of the material currently present in the Oort cloud is of extraso- The Oort cloud (Oort 1950) is a spherical structure that sur- lar origin, having been captured from the protoplanetary discs of rounds the solar system with an outer boundary located beyond other stars when the Sun was still part of the open star cluster or ? Based on observations made with the GTC telescope, in the stellar association where it was born. An analysis of a sufficiently Spanish Observatorio del Roque de los Muchachos of the Instituto de representative sample of objects from the Oort cloud should be Astrofísica de Canarias, under Director’s Discretionary Time (program able to either confirm or reject a dominant primordial extra- ID GTC2018-096). solar origin for the populations of small bodies hosted by the Article published by EDP Sciences A133, page 1 of6 A&A 625, A133 (2019) Table 1. Heliocentric and barycentric orbital elements and 1σ uncer- using the Optical System for Imaging and Low Resolution Inte- tainties of comet C/2018 F4 (PANSTARRS). grated Spectroscopy (OSIRIS) camera spectrograph (Cepa et al. 2000; Cepa 2010) at the 10.4 m Gran Telescopio Canarias (GTC). Orbital parameter Heliocentric Barycentric Two images, one with an exposure time of 180 s and the other of Perihelion distance, q (AU) = 3.4417 ± 0.0004 3.4355 30 s, were obtained between 0:33 and 0:43 UTC (at an airmass 0 Eccentricity, e = 1.00077 ± 0.00011 0.99771 of 1.32) using the SLOAN r filter. Three spectra, each one with Inclination, i (◦) = 78.160 ± 0.003 78.256 an exposure time of 900 s, were obtained between 0:48 and Longitude of the ascending node, Ω (◦) = 26.51923 ± 0.00004 26.46807 1:32 UTC (at an airmass of 1.29). OSIRIS has a mosaic of two Argument of perihelion, ! (◦) = 263.167 ± 0.004 263.321 Marconi 2048 × 4096 pixel CCD detectors, with a total unvi- Mean anomaly, M (◦) = −0.0020 ± 0.0004 359.9899 gnetted field of view of 7.8 × 7.8 arcminutes, and a plate scale of 0:127 00/px. In order to increase the S/N, we selected the 2 × 2 Notes. The orbit determination has been computed at epoch JD binning and the standard operation mode with a readout speed 2458227.5 that corresponds to 00:00:00.000 TDB, Barycentric Dynam- ical Time, on 2018 April 19, J2000.0 ecliptic and equinox. Source: JPL’s of 200 kHz (with a gain of 0:95 e−/ADU and a readout noise of SSDG SBDB. 4:5 e−). The tracking of the telescope matched the proper motion of the object during the observations. We found C/2018 F4 to be at 6.23 and 5.23 AU, heliocentric and geocentric distances, Oort cloud. Being able to clearly characterise the appearance of ◦ an average Oort cloud member may help in solving this difficult respectively, and its phase angle was α = 0:9 at the time of the and important problem. observations. The current sample of known Oort cloud comets is likely The spectra were obtained using the R300R grism in biased in favour of relatively active objects, those with short per- combination with a second-order spectral filter that produces ihelion distances, and those with unusual physical and/or chemi- a spectrum in the range 4800–9000 Å with a dispersion of cal properties; unremarkable comets tend to be missing, perhaps 32.25 Å/px for the used 2:500 slit width. The slit was oriented in neglected. Among the currently known minor bodies following parallactic angle to account for possible variable seeing condi- nearly parabolic or hyperbolic paths – which may have their ori- tions and to minimise losses due to atmospheric dispersion. The gin in the Oort cloud – about 75% have data-arcs spanning less three consecutive spectra were shifted in the slit direction by than a month and consistently uncertain orbit determinations; 1000 to better correct for fringing. In addition, two G2V stars – only a small subsample has been studied spectroscopically. This SA102-1081 and SA107-998 – from the Landolt catalogue suggests that our current perspective on the appearance of the (Landolt 1992) were observed immediately before and after average Oort cloud comet may be inaccurate. Comet C/2018 F4 the object, and at similar airmass (1.26 and 1.27, respectively) (PANSTARRS) is an object of interest in this regard. using the same spectral configuration. These stars are used as Comet C/2018 F4 was discovered by the Pan-STARRS sur- solar analogues to correct for telluric absorptions and to obtain vey (Kaiser 2004; Denneau et al. 2013) on 2018 March 17 relative reflectance spectra. at 6.4 AU from the Sun and with an apparent magnitude w Data reduction was carried out using standard Image Reduc- of 20.4 (Gilmore et al. 2018; S’arneczky et al. 2018; Tichy tion and Analysis Facility (IRAF1) procedures. The r0 images et al. 2018). It was initially classified as a hyperbolic asteroid, obtained were over-scan and bias corrected, flat-field corrected A/2018 F4 (Tichy et al. 2018), and was subsequently reclassified using sky-flats, and flux calibrated using standard stars observed as a comet (S’arneczky et al.