Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): dislodged from the Oort Cloud or coming from interstellar space?

TL;DR

This study investigates the origin of comet C/2018 V1, using dynamical simulations and Gaia data, to determine whether it originated from the Oort Cloud or interstellar space, highlighting the potential for low-velocity interstellar comets.

Contribution

The paper applies N-body simulations and Gaia data analysis to assess the comet's origin, proposing that interstellar comets with low velocities may be more common than previously thought.

Findings

C/2018 V1 may originate from the Oort Cloud or interstellar space.

Low-velocity interstellar comets could be more frequent than assumed.

Two solar sibling candidates suggest possible interstellar origin.

Abstract

The chance discovery of the first interstellar minor body, 1I/2017 U1 (`Oumuamua), indicates that we may have been visited by such objects in the past and that these events may repeat in the future. Unfortunately, minor bodies following nearly parabolic or hyperbolic paths tend to receive little attention: over 3/4 of those known have data-arcs shorter than 30 d and, consistently, rather uncertain orbit determinations. This fact suggests that we may have observed interstellar interlopers in the past, but failed to recognize them as such due to insufficient data. Early identification of promising candidates by using N-body simulations may help in improving this situation, triggering follow-up observations before they leave the Solar system. Here, we use this technique to investigate the pre- and post-perihelion dynamical evolution of the slightly hyperbolic comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) to understand its origin and relevance within the context of known parabolic and hyperbolic minor bodies. Based on the available data, our calculations suggest that although C/2018 V1 may be a former member of the Oort Cloud, an origin beyond the Solar system cannot be excluded. If extrasolar, it might have entered the Solar system from interstellar space at low relative velocity with respect to the Sun. The practical feasibility of this alternative scenario has been assessed within the kinematic context of the stellar neighbourhood of the Sun, using data from Gaia second data release, and two robust solar sibling candidates have been identified. Our results suggest that comets coming from interstellar space at low heliocentric velocities may not be rare.

Tables

Table 1. Table 1: Heliocentric and barycentric Keplerian orbital elements of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto). Heliocentric values include the 1 σ 𝜎 \sigma uncertainty. The first orbit determination (2018 November 20) is referred to epoch JD 2458436.5, which corresponds to 0:00 on 2018 November 14 TDB, and it was produced by J. D. Giorgini (J2000.0 ecliptic and equinox). It is based on 625 observations with a data-arc span of 12 days. The second orbit determination (2019 May 15) is referred to epoch JD 2458438.5, which corresponds to 0:00 on 2018 November 16 TDB. It is based on 750 observations with a data-arc span of 37 days. Source: JPL’s SBDB.

		2018 November 20		2019 May 15
Parameter		heliocentric	barycentric	heliocentric	barycentric
Perihelion, $q$ (au)	=	0.386970$\pm$0.000011	0.394399	0.386954$\pm$0.000002	0.394183
Eccentricity, $e$	=	1.0006$\pm$0.0004	1.0107	1.00040$\pm$0.00002	1.01235
Inclination, $i$ (°)	=	143.989$\pm$0.002	144.206	143.9878$\pm$0.0005	144.2182
Longitude of the ascending node, $\Omega$ (°)	=	128.724$\pm$0.006	128.400	128.7222$\pm$0.0004	128.4031
Argument of perihelion, $\omega$ (°)	=	88.769$\pm$0.007	87.840	88.7749$\pm$0.0002	87.8495

Table 2. Table 2: Candidate insterstellar comets and 1I/2017 U1 (‘Oumuamua) as discussed by de la Fuente Marcos et al. ( 2018 ) . Data include the object’s designation, the heliocentric eccentricity and its 1 σ 𝜎 \sigma uncertainty, e h ± σ e h plus-or-minus subscript 𝑒 h subscript 𝜎 subscript 𝑒 h e_{\rm h}\pm\sigma_{e_{\rm h}} , the barycentric eccentricity, e b subscript 𝑒 b e_{\rm b} , the statistical significance of e b subscript 𝑒 b e_{\rm b} , ( e b subscript 𝑒 b e_{\rm b} - 1)/ σ e h subscript 𝜎 subscript 𝑒 h \sigma_{e_{\rm h}} that has been computed using all the decimal figures provided by the data source, the number of observations, and the data-arc span. The values of the eccentricity are referred to the epoch used by JPL’s SBDB, which is different for each object. Source: JPL’s SBDB.

Object	$e_h$	$e_b$	sig.	obs.	span
					(d)
C/1853 R1 (Bruhns)	1.000664$\pm$n/a	1.002029	n/a	17	90
C/1997 P2 (Spacewatch)	1.0279$\pm$0.0002	1.0182	86.98	94	49
C/1999 U2 (SOHO)	1.0$\pm$0.3	0.88	$-$0.39	41	1
C/2002 A3 (LINEAR)	1.00949$\pm$0.00002	1.00290	135.73	285	527
C/2008 J4 (McNaught)	1.008$\pm$0.010	1.001	0.11	22	15
C/2012 C2 (Bruenjes)	1.0034$\pm$0.0009	1.0001	0.13	246	16
C/2012 S1 (ISON)	1.00020$\pm$0.00002	0.99957	$-$21.50	6514	784
C/2017 D3 (ATLAS)	1.002434$\pm$0.000009	1.000301	32.07	520	773
C/2018 V1 (Machholz-Fujikawa-Iwamoto)	1.00040$\pm$0.00002	1.01235	510.93	750	37
1I/2017 U1 (‘Oumuamua)	1.20113$\pm$0.00002	1.20315	9673.71	207	80

Table 3. Table 3: Kinematic matches of C/2018 V1 (Machholz-Fujikawa-Iwamoto) from Gaia DR2 (I). Gaia DR2 designation, α 𝛼 \alpha , δ 𝛿 \delta , π 𝜋 \pi , σ π subscript 𝜎 𝜋 \sigma_{\pi} , μ α subscript 𝜇 𝛼 \mu_{\alpha} , σ μ α subscript 𝜎 subscript 𝜇 𝛼 \sigma_{\mu_{\alpha}} , μ δ subscript 𝜇 𝛿 \mu_{\delta} , σ μ δ subscript 𝜎 subscript 𝜇 𝛿 \sigma_{\mu_{\delta}} , V r subscript 𝑉 r V_{\rm r} , and σ V r subscript 𝜎 subscript 𝑉 r \sigma_{V_{\rm r}} from Gaia DR2.

Gaia DR2 designation	$\alpha$	$\delta$	$\pi$	${\sigma }_{\pi }$	${\mu }_{\alpha }$	${\sigma }_{{\mu }_{\alpha }}$	${\mu }_{\delta }$	${\sigma }_{{\mu }_{\delta }}$	$V_r$	${\sigma }_{V_r}$
	(°)	(°)	(mas)	(mas)	(mas yr-1)	(mas yr-1)	(mas yr-1)	(mas yr-1)	(km s-1)	(km s-1)
206710213246475648	75.58874228476	+45.93995550159	15.8998	0.8093	$-$1.921	1.275	1.612	1.111	1.13	3.66
1927143514955658880	358.74965337143	+46.26832670029	2.7573	0.0290	0.159	0.041	0.107	0.030	1.21	1.86
1966383465746413568	323.37898817647	+41.72653005266	3.2308	0.0236	0.382	0.037	0.135	0.037	0.56	0.93
5813389005667991808	259.80441797335	$-$66.38996438375	5.3887	0.2645	0.578	0.387	0.026	0.516	$-$0.61	0.25

Table 4. Table 4: Kinematic matches of C/2018 V1 (Machholz-Fujikawa-Iwamoto) from Gaia DR2 (II). Gaia DR2 designation, d 𝑑 d from Bailer-Jones et al. ( 2018b ) , heliocentric Galactic velocity components ( U 𝑈 U , V 𝑉 V , W 𝑊 W ) computed as described in the text.

Gaia DR2 designation	$d$	$U$	$V$	$W$
	(pc)	(km s-1)	(km s-1)	(km s-1)
206710213246475648	63${}_{-3}{}^{}{}_{}{}^{+4}$	$-$0.84$\pm$3.46	1.06$\pm$1.22	$-$0.11$\pm$0.40
1927143514955658880	359${}_{-4}{}^{}{}_{}{}^{+4}$	$-$0.75$\pm$0.70	0.98$\pm$1.65	$-$0.21$\pm$0.50
1966383465746413568	307${}_{-2}{}^{}{}_{}{}^{+2}$	$-$0.53$\pm$0.06	0.54$\pm$0.92	$-$0.31$\pm$0.13
5813389005667991808	185${}_{-8}{}^{}{}_{}{}^{+10}$	$-$0.42$\pm$0.34	0.63$\pm$0.37	$-$0.24$\pm$0.37

Full text

††pubyear: 2019††pagerange: Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): dislodged from the Oort Cloud or coming from interstellar space?–References

1 Introduction

The first unambiguous detection of an interstellar minor body,111https://minorplanetcenter.net//iau/mpec/K17/K17V17.html 1I/2017 U1 (‘Oumuamua), on 2017 October 19 by R. Weryk (Bacci et al., 2017; Meech et al., 2017; Williams, 2017) is becoming a puzzle in many respects (see e.g. the reviews by Hainaut et al. 2018; Bannister et al. 2019), but also a game changer in the study of how the Solar system interacts with interstellar debris. Finding this small object has opened a new window into our immediate neighbourhood: material from beyond the Solar system may eventually be studied without having to resource to interstellar travel (see e.g. Seligman & Laughlin 2018). ‘Oumuamua had a hyperbolic excess velocity of about 26 km s*-1* (see e.g. Mamajek 2017), but there must be interstellar comets and asteroids with lower hyperbolic excesses.

The distribution of the hyperbolic excess velocities of small bodies following hyperbolic paths with respect to the Solar system must match that of stars observed in the solar neighbourhood as interstellar minor bodies must likely come from exoplanetary systems (see e.g. Almeida-Fernandes & Rocha-Pinto 2018; Bailer-Jones et al. 2018a; Portegies Zwart et al. 2018). There are many examples of stars moving at low relative velocity with respect to the Sun (see e.g. Lépine et al. 2013). On the other hand, Valtonen & Innanen (1982) have shown that any interstellar minor body with a relative velocity not exceeding 0.5 km s*-1* can be captured by the Solar system; in other words, objects with relative velocity above 0.5 km s*-1* can probably enter and leave the Solar system at will. This implies that there should be a non-negligible number of interstellar interlopers with low hyperbolic excess velocities that were either missed altogether by past and present surveys or have already been discovered but not yet identified as such (see a more general discussion in Ashton et al. 2018).

De la Fuente Marcos, de la Fuente Marcos & Aarseth (2018) have suggested that the Solar system might have already been visited by interstellar comets, pointing out some suitable candidates to having an extrasolar provenance. However, it may be argued that the Bayesian prior probability of a comet being interstellar must be rather low, given the fact that there are no confirmed identifications of interstellar comets to date (other than ‘Oumuamua, if it is indeed a comet, see e.g. Micheli et al. 2018; Rafikov 2018; Sekanina 2019). Within this context, it may not be justified to speculate about past visits of interstellar comets on the basis of available data, but the fact is that, as of 2019 July 26, Jet Propulsion Laboratory’s (JPL) Small-Body Database (SBDB, Giorgini 2011)222https://ssd.jpl.nasa.gov/sbdb.cgi includes 2191 objects with nominal heliocentric eccentricity, $e\geq 1$. Out of this sample, 531 objects have data-arcs longer than 30 d and 364 longer than 80 d (like the one of ‘Oumuamua). In other words, the probability of finding an interstellar minor body among those with data-arcs as long as or longer than that of ‘Oumuamua is 0.00275; this estimate is probably lower than the one discussed by Do, Tucker & Tonry (2018). Orbit determinations based on data-arcs shorter than one month are sometimes regarded as unreliable and unsuitable to perform statistical analyses, but about 76 per cent of the known objects with $e\geq 1$ fall into this category. If the probability of finding an interstellar interloper (0.00275) is applied to the full sample (2191), six of them might have already been discovered. These numbers suggest that there is a strong bias against detecting interstellar minor bodies, which may not be correctly identified simply because they do not generate enough attention to be re-observed and their orbit determinations remain consistently poor during their relatively short observability windows.

As shown by de la Fuente Marcos et al. (2018), $N$-body simulations may help in singling out promising candidates to be re-observed before they leave the Solar system, never to return. Here, we present a detailed analysis of the pre- and post-perihelion dynamical evolution of C/2018 V1 (Machholz-Fujikawa-Iwamoto) —a slightly hyperbolic comet— aimed at identifying the most probable provenance of this object. This paper is organized as follows. In Section 2, we present the data and the tools used in this study, and discuss the scientific case. Section 3 investigates the orbital context of C/2018 V1. The dynamical evolution of C/2018 V1 is explored in Section 4. In Section 5 and as a practical exercise, we use data from the Gaia second data release (DR2) to investigate a putative kinematic link to stars in the neighbourhood of the Sun. Our results are discussed in Section 6 and our conclusions summarized in Section 7.

2 Data and methods

The source of most of the data used in this research is JPL’s SBDB and JPL’s horizons333https://ssd.jpl.nasa.gov/?horizons ephemeris system (Giorgini et al., 1996). This includes orbit determinations, covariance matrices, initial conditions (positions and velocities in the barycentre of the Solar system) for planets and minor bodies referred to epoch JD 2458600.5 (2019 April 27.0) TDB (Barycentric Dynamical Time) —which is the zero instant of time in the figures, J2000.0 ecliptic and equinox, unless explicitly stated— ephemerides, and other input data. Another data source is in Gaia DR2 (Gaia Collaboration, Prusti et al., 2016; Gaia Collaboration, Brown et al., 2018) that provides extensive astrometric and photometric data of stellar sources in the form of coordinates, $\alpha$ and $\delta$, parallax, $\pi$, radial velocity, $V_{\rm r}$, proper motions, $\mu_{\alpha}$ and $\mu_{\delta}$, and their respective standard errors, $\sigma_{\pi}$, $\sigma_{V_{\rm r}}$, $\sigma_{\mu_{\alpha}}$, and $\sigma_{\mu_{\delta}}$. Such data are used to perform a systematic search for stars with kinematic properties consistent with those of the pre-encounter trajectory of C/2018 V1 (in the unbound case) as discussed in Section 5.

2.1 Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): data

Using a 0.47-m reflector, D. E. Machholz reported the visual discovery of a comet on 2018 November 7 (observing from Colfax, CA, U.S.A.); the same object was independently discovered by S. Fujikawa (observing from Kan’onji, Kagawa, Japan) and M. Iwamoto (observing from Awa, Tokushima, Japan), and assigned the temporary name TCP J12192806-0211143.444http://www.cbat.eps.harvard.edu/unconf/followups/J12192806-0211143.html The Minor Planet Center (MPC) first listed the new object in its NEO Confirmation Page (NEOCP) as DM001 and then moved it to the Possible Comet Confirmation Page (PCCP) before issuing three MPECs (Buzzi et al., 2018; Hasubick et al., 2018; Machholz et al., 2018) with observations and orbit determinations under the new official name of C/2018 V1 (Machholz-Fujikawa-Iwamoto). Almost concurrently, the Central Bureau of Electronic Telegrams issued three CBETs (Gonzalez & Hale, 2018; Nakano, 2018; Soulier et al., 2018).

The orbit determination of C/2018 V1 initially available from JPL’s SBDB and computed by J. D. Giorgini on 2018 November 20, see Table 1, was based on 625 data points for a data-arc span of 12 days. The orbit of the comet was slightly hyperbolic, for this initial determination, at the 1.5$\sigma$ level when considering the heliocentric values (but 27.6$\sigma$ when considering the barycentric ones). The uncertainties in the orbit determination of C/2018 V1 were similar to those affecting the one of 1I/2017 U1 (‘Oumuamua) several weeks after discovery.

On 2019 May 15, a second orbit determination was made public that included additional observations that had surfaced over the previous months. The new orbit determination is shown in Table 1 and it is based on 750 data points for a data-arc span of 37 days. The orbit remains slightly hyperbolic, now at the 16.4$\sigma$ level when considering the heliocentric values (but 510.9$\sigma$ when considering the barycentric ones). The uncertainties are now comparable to those of the final orbit determination of ‘Oumuamua; therefore, we are confident that the new orbit determination is robust enough to produce sufficiently reliable initial conditions that can be used to make predictions regarding the past, present, and future orbital evolution of this interesting object.

2.2 Kinematic context: Gaia DR2 data

In Section 5, we use data from Gaia DR2 to investigate a putative kinematic link of C/2018 V1 to stars in the neighbourhood of the Sun. In order to interpret our results properly (e.g. by using colour-magnitude diagrams or CMDs), we focus on those sources with estimated values of the line-of-sight extinction $A_{G}$ and reddening $E(G_{\rm BP}-G_{\rm RP})$; Gaia DR2 includes 87 733 672 such sources,555https://www.cosmos.esa.int/web/gaia/dr2 all of them have strictly positive values of the parallax. Out of this sample, 4 831 731 sources have positions, parallax, radial velocity, and proper motions. This smaller sample is used to investigate a possible kinematic link of C/2018 V1 to neighbour stars. We have not performed any correction to address the issue of the zero-point offset in Gaia DR2 parallax data. There are several independent determinations of its value (see e.g. Riess et al. 2018; Stassun & Torres 2018; Bobylev 2019; Xu et al. 2019; Zinn et al. 2019) and most of them are larger than the value of 0.029 mas initially presented by Lindegren et al. (2018). The zero-point offset values quoted in the recent literature range from 0.029 to 0.082 mas that must be added to the original values in Gaia DR2 in order to perform the correction (i.e. in general the actual distances to the sources may be shorter than those computed directly from the catalogue data). Neglecting this correction has no significant effect on our conclusions as the uncertainties of interest here (see Section 5) are larger or of the same order as the value of the correction. Additional details on our Gaia DR2 software pipeline are described in de la Fuente Marcos & de la Fuente Marcos (2019).

2.3 Numerical integrations

In order to study the orbital evolution of C/2018 V1 and other previously detected minor bodies following hyperbolic paths, we have used a direct $N$-body code originally written by S. J. Aarseth that implements a fourth-order version of the Hermite integration scheme (Makino, 1991; Aarseth, 2003). The standard version of this code is publicly available from the Institute of Astronomy web site.666http://www.ast.cam.ac.uk/~sverre/web/pages/nbody.htm The numerical integrations of the orbit of C/2018 V1 consider the perturbations by eight major planets and treat the Earth–Moon system as two separate objects; they also include the barycentre of the dwarf planet Pluto–Charon system and the three most massive asteroids of the main belt, namely, dwarf planet (1) Ceres, (2) Pallas, and (4) Vesta. Additional details of the code used in our research and of our integrations and physical model are described in de la Fuente Marcos & de la Fuente Marcos (2012) and de la Fuente Marcos et al. (2018). Our calculations do not include non-gravitational forces. The orbit determinations in Table 1 did not require non-gravitational terms to fit the available astrometry; this suggests that any contribution due to asymmetric outgassing is probably a second order effect in this case. Therefore, we believe that neglecting the role of non-gravitational forces in our integrations is unlikely to have any major impact on our conclusions. The uncertainties in the values of the orbital elements, see Table 1, have been included in our simulations by applying the covariance matrix methodology discussed by de la Fuente Marcos & de la Fuente Marcos (2015). Calculations of heliocentric Galactic space velocities have been carried out as described by Johnson & Soderblom (1987) using the values of the relevant parameters provided by Schönrich, Binney & Dehnen (2010), and applying the Monte Carlo sampling procedure described by de la Fuente Marcos & de la Fuente Marcos (2019). Averages, standard deviations, and other statistics have been computed in the usual way (see e.g. Wall & Jenkins 2012).

2.4 A matter of orbital uncertainty?

As pointed out above, most minor bodies following putative nearly hyperbolic paths never attract enough attention to get good orbit determinations. In addition, it is customary that whenever discussing the possibility that an object has a hyperbolic orbit one should refer to the eccentricity of the barycentric orbital elements; most comets with values of the heliocentric eccentricity $\geq 1$ have barycentric values $<1$. For example, the orbit determination of C/2008 J4 (McNaught) is based on a data-arc span of 15 d with a heliocentric eccentricity of 1.008$\pm$0.010 but a barycentric value of 1.001 and C/2012 S1 (ISON) has a data-arc span of 784 d with a heliocentric value of 1.00020$\pm$0.00002 and a barycentric one of 0.99957 (see Table 2). However, all these values are osculating ones, often corresponding to the midpoint epoch of the available data-arc. These values may change over time as the objects interact with massive bodies in the Solar system such as the Sun or Jupiter. Besides C/2018 V1 (see Table 1), another rare example of hyperbolic comet with both hyperbolic heliocentric and barycentric orbit determinations is C/1997 P2 (Spacewatch) with a data-arc span of 49 d, a heliocentric eccentricity of 1.0279$\pm$0.0002, and a barycentric one of 1.0182 —i.e. it is hyperbolic at the 133$\sigma$ level heliocentrically and at the 87$\sigma$ level barycentrically (see Table 2).

Statistically, the bound orbit ($e<1$) is the null hypothesis and solid evidence needs to be provided if the claim is that the orbit is in fact hyperbolic. Osculating hyperbolic orbit determinations (heliocentric and barycentric) with a sufficiently high $\sigma$ level may be indicative of an extrasolar origin, but to actually show that their provenance is outside the Solar system $N$-body calculations must be used. In summary and in order to validate such theoretical expectations —i.e. that the null hypothesis can be rejected— a representative set of control orbits (statistically compatible with the available observations) must be integrated forward and backwards in time to confirm that the dynamical evolution of the candidate over a reasonable amount of time (a few hundred thousand years for relatively slow objects, tens of thousands for those as fast or faster than ‘Oumuamua) is consistent with not being bound to the Solar system —i.e. their relative velocity is above 0.5 km s*-1* when near the Hill radius of the Solar system (see e.g. Valtonen & Innanen 1982). This methodology has been previously used by de la Fuente Marcos & de la Fuente Marcos (2018b) to confirm independently that C/2017 K2 (PANSTARRS) is a bound and dynamically old Oort Cloud comet —see the works by Hui, Jewitt & Clark (2018) and Królikowska & Dybczyński (2018)— as well as to show that C/2018 F4 (PANSTARRS) could be a genuine representative of the average Oort Cloud comet population (Licandro et al., 2019).

3 Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): the current orbit in context

The overall orientations in space of the orbits of parabolic and hyperbolic minor bodies can be studied using the coordinates of their perihelia and poles. In heliocentric ecliptic coordinates, the longitude and latitude of the orbital pole are $(L_{\rm p},B_{\rm p})=(\Omega-90\degr,90\degr-i)$. The ecliptic coordinates of the perihelion, $(L_{q},B_{q})$, are given by the expressions: $\tan{(L_{q}-\Omega)}=\tan\omega\,\cos{i}$ and $\sin{B_{q}}=\sin\omega\,\sin{i}$ (see e.g. Murray & Dermott 1999). Figure 1 shows the distributions in the sky of the poles (top panel) and perihelia (middle panel) as well as the perihelion distances (bottom panel) of the known comets moving in parabolic (black empty circles) or hyperbolic (black filled circles) orbits (2191 objects as of 2019 July 26, 354 with $e>1$). The values of $i$, $\Omega$ and $\omega$ are practically independent of the nature, heliocentric or barycentric, of the orbit determination (see e.g. de la Fuente Marcos & de la Fuente Marcos 2017).

Some clusters are clearly visible in Fig. 1 and they are all associated with sungrazing comet groups (Marsden, 1967, 1989); they include the Kreutz group comets (Kreutz, 1888) and the Meyer, Marsden and Kracht (1 and 2) group comets (Sekanina, 2002; Marsden, 2005; Knight, 2008; Sekanina & Kracht, 2013, 2015, 2016). Kreutz sungrazers are found at $(L_{q},B_{q})\sim(282\degr,35\degr)$ and they could be the result of cascading fragmentation taking place during the last 1700 years (Sekanina & Chodas, 2004, 2007), Meyer group comets have $(L_{q},B_{q})\sim(98\degr,53\degr)$, Marsden group comets have $(L_{q},B_{q})\sim(102\degr,10\degr)$, and Kracht (1 and 2) group comets have $(L_{q},B_{q})\sim(102\degr,11\degr)$ —Marsden comets have $i\sim 26\degr$, but Kracht comets have $i\sim 13\degr$. The distribution in $q$ (bottom panels) is clearly different for parabolic and hyperbolic comets: parabolic comets tend to have shorter perihelia if we exclude those of the Kreutz family —Fig. 1, bottom panel, shows that the perihelion distances of Kreutz sungrazers are $<0.01$ au, but $>0.004$ au. This is to be expected if parabolic comets have experienced multiple perihelion passages.

Figure 1 shows that comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) —plotted as a green filled diamond, $(L_{\rm p},B_{\rm p})=(38\aas@@fstack{\circ}72,-53\aas@@fstack{\circ}99)$ and $(L_{q},B_{q})=(40\aas@@fstack{\circ}24,36\aas@@fstack{\circ}00)$— does not belong to any of the sungrazing comet groups. In fact, it does not appear to be dynamically coherent with any of the known parabolic or hyperbolic comets, which might indicate that it is not a first time visitor from the Oort Cloud (de la Fuente Marcos et al., 2018). Furthermore, the probability of finding another minor body within 15° of C/2018 V1 in both the $(L_{\rm p},B_{\rm p})$ and $(L_{q},B_{q})$ planes is 0.00137 (3/2190). The closest comets in terms of orbital geometry are C/1857 V1 (Donati-van Arsdale), C/1948 T1 (Wirtanen), and C/1997 S4 (SOHO), but they are several degrees away in terms of poles and perihelia; within 11° there are none and within 13° there are 2 comets, C/1857 V1 and C/1997 S4. Figure 1 also shows the location of other relevant objects to be discussed later; one of them, C/1999 U2 (SOHO) that is plotted as an cyan empty square, could be a bona fide Marsden comet as $(L_{q},B_{q})=(101.8\degr,9.6\degr)$ and $i=26.8\degr$, not a candidate interstellar comet as suggested by de la Fuente Marcos et al. (2018).

4 Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): dynamical evolution

Figure 2 shows the (past and future) short-term orbital evolution of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) —in green filled diamonds, new nominal orbit in Table 1— and those of a few hyperbolic minor bodies of probable or possible interstellar origin (de la Fuente Marcos et al., 2018) —namely, 1I/2017 U1 (‘Oumuamua) in pink filled squares (nominal heliocentric $e=1.20113\pm 0.00002$, 207 observations, data-arc span 80 d), C/1997 P2 (Spacewatch) in amber filled triangles ($e=1.0279\pm 0.0002$, 94 observations, data-arc span 49 d), C/1999 U2 (SOHO) in cyan empty squares ($e=1.0\pm 0.3$, 41 observations, data-arc span 1 d), C/2008 J4 (McNaught) in violet empty triangles ($e=1.008\pm 0.010$, 22 observations, data-arc span 15 d), and C/2012 S1 (ISON) in yellow empty diamonds ($e=1.00020\pm 0.00002$, 6514 observations, data-arc span 784 d), but these last two nearly overlap. The black thick line corresponds to the aphelion distance —$a\ (1+e)$, limiting case $e=1$— that defines the domain of dynamically old Oort Cloud comets (i.e. semimajor axis $a<40000$ au, see Królikowska & Dybczyński 2017) as opposed to those that may be dynamically new, or first time visitors from the Oort Cloud; the red thick line signals the radius of the Hill sphere of the Solar system (see e.g. Chebotarev 1965). Figure 2 shows that, besides C/2018 V1, C/1997 P2, C/2008 J4 and C/2012 S1 are suitable candidates to be interstellar comets (see Table 2 for additional details); as pointed out above, C/1999 U2 is a probable Marsden group comet and its orbit determination is perhaps too uncertain to make it a candidate, even if it is second to ‘Oumuamua in terms of dynamical evolution. For comparison, results from the old nominal orbit of C/2018 V1 in Table 1 are plotted as green empty diamonds.

Figure 3 shows the barycentric distance as a function of the velocity parameter for 1000 control orbits of C/2018 V1. The left-hand side set of panels shows results from integrations with input parameters consistent with the old solution in Table 1 (2018 November 20); the ones on the right-hand side show results from input data based on the new orbit determination (2019 May 15). In both cases, the left-hand side panel shows results at 1 Myr into the past and the right-hand side panel displays those at 1 Myr into the future. The velocity parameter is the difference between the barycentric and escape velocities at the computed barycentric distance in units of the escape velocity. Positive values of the velocity parameter identify control orbits that could have been followed by objects of interstellar provenance (left-hand side panel) or that lead to ejections from the Solar system (right-hand side panel). The initial conditions of the control orbits have been generated using the covariance matrix as described above.

For the old orbit determination (Fig. 3, left-hand side set of panels), the vast majority, $>99.9$ per cent, of the control orbits computed here placed C/2018 V1 close to or beyond the Hill sphere 1.3 Myr ago and 1.3 Myr into the future as well, i.e. under the gravitational influence of the Galactic tide. In summary and considering the old orbit determination in Table 1, our $N$-body simulations and statistical analyses suggest that C/2018 V1 came from interstellar space and it will return back to it, C/2018 V1 cannot be a dynamically new or old Oort Cloud comet (compare Figs 2 and 3 with fig. 5 in Licandro et al. 2019).

When input data based on the new orbital solution shown in Table 1 are used to compute the past and future orbital evolution of C/2018 V1 (Fig. 3, right-hand side set of panels, black points), about 43 per cent of the 1 Myr integrations into the past are compatible with an origin in interstellar space, and all the experiments performed predict that C/2018 V1 will be unbound from the Solar system, 1 Myr into the future. When longer integrations are carried out (Fig. 3, right-hand side set of panels, amber empty circles, 5 Myr into the past), the probability of having an extrasolar origin increases to about 73 per cent (72.6$\pm$0.5). This variation strongly suggests that this object, if extrasolar, may have remained weakly bound to the Solar system for a certain amount of time before returning now to interstellar space.

With these results in mind, C/2018 V1 may be a former member of the Oort Cloud that was perturbed a few million of years ago into the dynamically unstable path that now follows, but an origin beyond the Solar system cannot be excluded at any significant level by our analysis considering the orbit determination currently available for this object (see Table 1). However, if C/2018 V1 has an extrasolar origin, it arrived in the Solar system with a relative velocity barely above the minimum one to become unbound. With these facts laid on the table, in the following section we explore a plausible what-if scenario. This hypothetical scenario aims to respond to the question, can a very slow interstellar comet candidate still be compatible with an origin beyond the Oort Cloud?

5 Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto): an origin among the solar siblings?

The kinematic properties of minor bodies escaped from a planetary system hosted by another star must be consistent with those of stars in the solar neighbourhood; therefore, investigating the pre-perihelion trajectory of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) may shed some light on its true origin. The purpose of this section is not to single out the possible extrasolar source (i.e. the actual star) of C/2018 V1, but to investigate the plausibility of an origin among the known stars in the solar neighbourhood. In other words, if there are no known stars with a kinematic signature compatible with that of C/2018 V1, when the control orbit leads to an unbound state 1 Myr into the past, then the former Oort Cloud membership automatically becomes the preferred scenario for the origin of this comet. However, if there are stars that can be considered as kinematic analogues of C/2018 V1 in its unbound version, then an extrasolar origin cannot be fully excluded.

A statistical analysis of the results of the simulations shown in Fig. 3 indicates that at 0.31$\pm$0.08 pc from the barycentre of the Solar system and 1 Myr into the past, C/2018 V1 was moving inwards, at $-$0.30$\pm$0.14 km s*-1* —i.e. below the 0.5 km s*-1* critical value pointed out by Valtonen & Innanen (1982)— and projected towards $\alpha=13^{\rm h}~{}24^{\rm m}~{}36^{\rm s}$, $\delta=-48\degr~{}03\arcmin~{}00\arcsec$ $(201\aas@@fstack{\circ}2\pm 0\aas@@fstack{\circ}3,-48\aas@@fstack{\circ}05\pm 0\aas@@fstack{\circ}13)$ in the constellation of Centaurus (geocentric radiant or antapex) with Galactic coordinates $l=308\aas@@fstack{\circ}64$, $b=+14\aas@@fstack{\circ}43$, and ecliptic coordinates $\lambda=219\aas@@fstack{\circ}69$, $\beta=-35\aas@@fstack{\circ}92$. The components of its heliocentric Galactic velocity were $(U,V,W)=(-0.18\pm 0.08,0.22\pm 0.10,-0.08\pm 0.04)$ km s*-1*, which are compatible with an origin in a star with very small, but not zero, relative motion with respect to the Sun. If the 5 Myr-into-the-past calculations are used, the results are consistent with the previous ones: C/2018 V1 was located at 1.3$\pm$0.6 pc from the barycentre of the Solar system and moving inwards, at $-$0.4$\pm$0.2 km s*-1*, projected towards $\alpha=13\aas@@fstack{h}3\pm 0\aas@@fstack{h}7$ and $\delta=-48\degr\pm 2\degr$.

On the other hand, at 0.74$\pm$0.04 pc from the Sun and 1 Myr into the future, C/2018 V1 will be receding from us at 0.70$\pm$0.04 km s*-1* —this time exceeding the 0.5 km s*-1* critical value pointed out by Valtonen & Innanen (1982)— towards $\alpha=13^{\rm h}~{}33^{\rm m}~{}58^{\rm s}$, $\delta=-48\degr~{}45\arcmin~{}36\arcsec$ ($203\aas@@fstack{\circ}49\pm 0\aas@@fstack{\circ}09,-48\aas@@fstack{\circ}76\pm 0\aas@@fstack{\circ}03)$ also in the constellation of Centaurus (apex) with Galactic coordinates $l=310\aas@@fstack{\circ}12$, $b=+13\aas@@fstack{\circ}52$, and ecliptic coordinates $\lambda=221\aas@@fstack{\circ}78$, $\beta=-35\aas@@fstack{\circ}83$. Its post-perihelion heliocentric Galactic velocity will be (0.44$\pm$0.02, $-$0.52$\pm$0.03, 0.164$\pm$0.009) km s*-1*.

Considering the sample described in Section 2.2 and looking for stars with relative errors in the value of the parallax better than 20 per cent, we have found four with values of their heliocentric Galactic velocity components consistent —within 9$\sigma$— with those of the comet when inbound; their properties are shown in Tables 3 and 4—distances in Table 4 are from Bailer-Jones et al. (2018b)— and their kinematic signatures are plotted together with that of the comet in Fig. 4. Only one of them is from the solar neighbourhood (within 100 pc from the Sun). The stars in Tables 3 and 4 are relatively unstudied. A search for matching sources has been carried out using the tools provided by VizieR777http://vizier.u-strasbg.fr/viz-bin/VizieR (Ochsenbein, Bauer & Marcout, 2000) with a radius of 1$\aas@@fstack{\prime\prime}$0.

Given the what-if nature of the analysis performed in this section, only the most relevant additional data are discussed here. Gaia DR2 206710213246475648 has been observed by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectroscopic surveys (Cui et al., 2012; Zhao et al., 2012)888http://dr4.lamost.org as target J050221.30+455624.0; relevant data include a value of the heliocentric radial velocity of 0.23$\pm$4.67 km s*-1*, a metallicity [Fe/H] of $-$0.005$\pm$0.020, and a stellar sub-class of G3 (the Sun has a spectral type/luminosity class of G2V). Gaia DR2 1927143514955658880 has also been observed by LAMOST, its target identification is J235459.91+461605.9; its heliocentric radial velocity is given as 0.25$\pm$5.67 km s*-1*, its metallicity is $-$0.113$\pm$0.066, and its stellar sub-class is G3. Gaia DR2 1966383465746413568 is also known as TYC 3191-276-1 and Gaia DR2 5813389005667991808 is TYC 9064-2770-1; both stars are included in the Tycho-2 catalogue (Høg et al., 2000). Although it would have been desirable to have less uncertain values of the radial velocities of our sources, our search in VizieR suggests that these four sources remain poorly studied and the few available data are of inferior quality when compared with those available from Gaia DR2 (see values of the radial velocities in Table 3). This is to be expected, the stars of interest here have very low relative velocities with respect to the Sun; consistently, the values of their radial velocity are low and their relative errors large (see Table 3).

The fact is that if C/2018 V1 has an extrasolar provenance, it may have approached the Solar system at low relative velocity with respect to the Sun and this places an implicit connection between the interplanetary and interstellar environments. The Sun was born within a star cluster (see e.g. Adams 2010). It is still under debate whether this cluster was gravitationally bound —open cluster (see e.g. Portegies Zwart 2009)— or unbound —a stellar association (see e.g. Pfalzner 2013). Both stellar associations and open clusters eventually dissolve, contributing to the field stellar populations (see e.g. de la Fuente Marcos & de la Fuente Marcos 2008). The search for solar siblings (see e.g. Martínez-Barbosa 2016) or stars that formed together with the Sun has made steady progress during the last decade or so. In order to be classified as a solar sibling candidate, a nearby star must have age and chemical abundances (metallicity and isotopic ratios) consistent with those of the Sun (see e.g. Adibekyan et al. 2018).

In principle, a putative solar sibling should also have a small space motion relative to the Sun; however, if the Sun —together with many other physically unrelated stars— is trapped in a spiral corotation resonance (Lépine et al., 2017), having similar kinematics is no longer a robust condition to qualify as a solar sibling. Portegies Zwart (2009) has pointed out that a small number of solar siblings may still remain in the neighbourhood of the Sun, which triggered searches for suitable candidates (see e.g. Liu et al. 2015; Martínez-Barbosa et al. 2016). However, Mishurov & Acharova (2011) argued that this is unlikely when considering the long sequence of secular perturbations experienced by these stars in their journey throughout the Galactic disc. On the other hand, Valtonen et al. (2015) pointed out that less than 10 percent —and probably just about 1 per cent— of the true solar siblings could still remain within 100 pc of the present position of the Sun. In any case, stars with small space motions relative to the Sun, be they solar siblings or not, may host structures similar to our Oort Cloud (Oort, 1950) that may leak comets into the interstellar medium. Such minor bodies may experience hyperbolic encounters with the Solar system, entering from interstellar space, and be eventually detected from the Earth as slightly hyperbolic comets. None of the stars in Table 3 are listed as solar sibling candidates (Adibekyan et al., 2018). It is unclear whether C/2018 V1 might have had an origin in any of them (if we assume that it has an extrasolar provenance, which may be the most likely interpretation, statistically), but they are reasonably good kinematic analogues of C/2018 V1. Having been able to find several relatively good kinematic analogues of C/2018 V1 among those stars relatively close to the Sun only means that the predicted kinematic signature of C/2018 V1 prior to its recent perihelion passage (if originally unbound) is consistent with that of observed stars, i.e. it is not unphysical. On the other hand, the Solar system departure kinematics of C/2018 V1 shows that minor bodies can leave the sphere of influence of a planetary system at a very low relative speed, which may eventually become the approach velocity when the same object experiences a close encounter with another planetary system. Within this context, C/2018 V1 could be the first example of a new class of comets discussed by Torres et al. (2019), the transitional interstellar comets.

6 Discussion

The current heliocentric and barycentric orbit determinations of C/2018 V1 (Machholz-Fujikawa-Iwamoto) in Table 1 can be considered as compatible with this comet being unbound from the Solar system. However, considering all the available data and their associated uncertainties we have to conclude that they all appear to point in the same direction: rather than having come from interstellar space, C/2018 V1 seems to have been dislodged from the Oort Cloud in the recent past (a few Myr ago). Figure 1 can be interpreted as an indication that C/2018 V1 is part of the essentially isotropic Oort cloud background population, but the fact remains that material from interstellar space can also approach the Solar system from any direction (the stars in the solar neighbourhood are essentially isotropically distributed around the Sun). It may also be argued that comet astrometry can easily be noisy and biased and this may lead to incorrect results (see the different past orbital evolution in Fig. 3). The possibility that bad data may have corrupted the current orbit estimate and produce unreliable formal uncertainties cannot be fully neglected as C/2018 V1 was observed at low solar elongation. However, one has to assume that JPL’s SBDB has procedures in place to minimize these issues. In addition to being perhaps a first-time visitor from the Oort Cloud, recently perturbed by a stellar fly-by, it can also be argued that C/2018 V1 could have an interstellar origin.

De la Fuente Marcos et al. (2018) have shown that the distribution of geocentric equatorial coordinates of the radiants of known hyperbolic minor bodies is not isotropic but probably consistent with the one induced by one or more relatively recent stellar passages through the Oort Cloud (see e.g. Dybczyński 2002). One of such passages was that of the binary star WISE J072003.20-084651.2, also known as Scholz’s star (Mamajek et al., 2015). The distribution of radiants of known hyperbolic minor bodies in the sky plotted in figs 3 and 4 of de la Fuente Marcos et al. (2018) shows that the radiant of C/2018 V1 computed above —$(13\aas@@fstack{h}41\pm 0\aas@@fstack{h}02,-48\aas@@fstack{\circ}05\pm 0\aas@@fstack{\circ}13)$, if originally unbound— is well separated from the conspicuous overdensities of radiants present in the figures, but close to the projection of the Galactic disc on the sky, which outlines the Milky Way (the arrival directions of interstellar materials are expected to concentrate towards the Galactic plane, see e.g. Murray, Weingartner & Capobianco 2004). These facts suggest that C/2018 V1 could not possibly be a first-time visitor from the Oort Cloud, recently perturbed by a stellar fly-by. The overdensities of radiants identified by de la Fuente Marcos et al. (2018) could be consistent with the outcome of several stellar passages, but the radiant of C/2018 V1 is very far away from them.

The subject of a possible origin of C/2018 V1 within the stars that populate the Galactic neighbourhood of the Sun deserves further consideration. As pointed out in Section 2.2, our sample is made of sources with both line-of-sight extinction and reddening estimates in Gaia DR2; therefore, we can construct a CMD with the data to check for consistency and Fig. 5 shows the resulting CMD. This CMD has been obtained as the ones in fig. 5 of Gaia Collaboration, Babusiaux et al. (2018), fig. 19 of Andrae et al. (2018), or fig. 3 in de la Fuente Marcos & de la Fuente Marcos (2018a). As a reference, one PARSEC v1.2S + COLIBRI S_35 (Bressan et al., 2012; Marigo et al., 2017; Pastorelli et al., 2019)999http://stev.oapd.inaf.it/cgi-bin/cmd isochrone of age 4.568 Gyr and solar metallicity is also plotted (in red). The assumed value for the age of the Solar system is the one computed by Bouvier & Wadhwa (2010), 4568.2${}^{+0.2}_{-0.4}$ Myr. The value of the metallicity of the Sun used to obtain the isochrone is the one calculated by Vagnozzi, Freese & Zurbuchen (2017), Z${}_{\odot}=0.0196\pm 0.0014$. The uncertainties have been estimated using a Monte Carlo sampling approach similar to the one discussed by Bromley et al. (2018) and described by de la Fuente Marcos & de la Fuente Marcos (2019). The positions in the CMD of two of the entries in Table 3 —Gaia DR2 1927143514955658880 and 1966383465746413568— appear to be consistent with being robust solar sibling candidates as they are sufficiently close to the theoretical isochrone. The other two kinematic matches —Gaia DR2 206710213246475648 and 5813389005667991808— are unlikely to share age and metallicity with the Sun, and may be the result of trapping in a spiral corotation resonance as described by Lépine et al. (2017), but see below for a more detailed discussion on the reliability of their astrometric solutions.

One may also argue about the quality of the data collected from Gaia DR2. The astrometric solution of Gaia DR2 206710213246475648 has been computed using 12 visibility periods, the total number of field-of-view (FoV) transits matched to this source was 30, but the astrometric excess noise was 4.110 mas and its significance 6280, which strongly suggests that the source may not be single. In the case of Gaia DR2 1927143514955658880, the astrometric solution has been computed using 16 visibility periods, with 33 matches and the astrometric excess noise was 0.000 mas. The astrometric solution of Gaia DR2 1966383465746413568 has been computed using 17 visibility periods, the total number of FoV transits matched to this source was 39 and again the astrometric excess noise was 0.000 mas. In the case of Gaia DR2 5813389005667991808, the astrometric solution has been computed using 16 visibility periods, with 79 matches, however the astrometric excess noise was 1.132 mas and its significance 825, which suggests that the source might not be single. This analysis is consistent with the CMD in Fig. 5, sources Gaia DR2 1927143514955658880 and 1966383465746413568 appear to be single and astrometrically well-behaved; in addition, they match the isochrone of age 4.568 Gyr quite well. In sharp contrast, sources Gaia DR2 206710213246475648 and 5813389005667991808 may not be single and their positions in the CMD of Fig. 5 just confirm that their astrometry may not be reliable. If we apply the criteria discussed by Lindegren et al. (2018) to identify sources with poor astrometric solutions, Gaia DR2 206710213246475648 and 5813389005667991808 emerge as dubious, but Gaia DR2 1927143514955658880 and 1966383465746413568 are astrometrically well-behaved sources. We consider Gaia DR2 1927143514955658880 and 1966383465746413568 as bona fide solar sibling candidates that deserve further study.

Our analysis shows that both Gaia DR2 1927143514955658880 and 1966383465746413568 are robust kinematic analogues of C/2018 V1, but this does not necessarily mean that any of them could be the source of the comet studied here. Therefore, we can only state that the predicted past kinematic properties of C/2018 V1 may be consistent with those of some known stars located relatively close ($\sim$300 pc) to the Sun. Arguing for an actual origin implies that numerical simulations including the source star and the interstellar comet place both objects in close proximity at some time in the past. Such calculations have been performed using the approach described in de la Fuente Marcos & de la Fuente Marcos (2018c) and no positive results in the form of sufficiently close encounters within the last 200 Myr have been found. In any case, this section is merely an outline of how to proceed when testing the what-if scenario that motivates the analysis.

In addition to passing minor bodies like ‘Oumuamua, C/1997 P2 (Spacewatch), C/2008 J4 (McNaught), C/2012 S1 (ISON) or C/2018 V1, the Solar system may host a number of captured extrasolar minor bodies (Siraj & Loeb, 2019) as it might arguably be the case of comets C/1996 B2 (Hyakutake) (Mumma et al., 1996; Brooke et al., 1996; Irvine et al., 1996) and 96P/Machholz 1 (Langland-Shula & Smith, 2007; Schleicher, 2008) or Jupiter’s retrograde co-orbital asteroid (514107) 2015 BZ509 (Namouni & Morais, 2018), although 96P/Machholz 1 might eventually return to deep space (de la Fuente Marcos, de la Fuente Marcos & Aarseth, 2015). Slightly hyperbolic bodies like C/2018 V1 are primary candidates to be captured in heliocentric orbits; C/1996 B2, 96P/Machholz 1 or 514107 may have reached the Solar system at low relative speed —below the 0.5 km s*-1* critical value pointed out by Valtonen & Innanen (1982)— before being captured. This may have been the case of C/2018 V1 as well. The existence of low relative velocity interstellar interlopers also has strong implications on the effectiveness of the planetary seeding mechanism proposed by Pfalzner & Bannister (2019) and further developed by Grishin, Perets & Avni (2019); the presence of large numbers of low-relative-speed ejected planetesimals within an already evolved star-forming region may further accelerate the process of planet formation via captures.

Regarding the observability and even accessibility of interstellar interlopers (see e.g. the discussion in Eubanks 2019), those moving at low relative velocities with respect to the Sun are ideal targets not only because of their extended visibility windows compared to that of 1I/2017 U1 (‘Oumuamua) —that was 80 d— but also because they are far easier targets for in situ exploration (see e.g. Seligman & Laughlin 2018) via fast-response missions. Comet Interceptor,101010http://www.cometinterceptor.space/ a fast-class mission recently selected by the European Space Agency (ESA),111111http://www.esa.int/Our_Activities/Space_Science/ESA_s_new_mission_to_intercept_a_comet aims at visiting one interstellar interloper, starting its journey from the Sun-Earth Lagrange point L2. A slow interstellar comet is probably the most feasible target in terms of pre-encounter planning and accessibility for this future ESA’s mission. However, Table 1 and Fig. 3 clearly show that it could be difficult to make the right decision if the quality of the orbit determination is not good enough.

On the other hand, objects like C/2018 V1 can be natural probes into the resonant conditions that may surround the space just beyond the Oort Cloud and into the population of low-relative-velocity stars located near the Sun. In this regard, high resolution spectroscopy of the kinematic analogues of C/2018 V1 presented in Table 3 can help in confirming or rejecting any connection with the Sun and perhaps C/2018 V1.

7 Conclusions

In this paper, we have studied the pre- and post-encounter orbital evolution of C/2018 V1 (Machholz-Fujikawa-Iwamoto), a slightly hyperbolic comet first observed on 2018 November 7. This research has made use of the latest comet data, $N$-body simulations, Gaia DR2 data, and statistical analyses. Our conclusions can be summarized as follows.

1. (i)

We show that C/2018 V1 has little to no dynamical correlation with known parabolic or hyperbolic comets when considering its overall orbital orientation in space. 2. (ii)

We confirm that, after analyzing an extensive set of $N$-body simulations, C/2018 V1 may have come from the Oort Cloud but it will leave the Solar system aiming for interstellar space after its recent perihelion passage, never to return. It is however not possible to discard an extrasolar origin for this object using only the available data. 3. (iii)

If originally unbound, C/2018 V1 may have entered the Solar system nearly 1 Myr ago at very low relative velocity with respect to the Sun. We have carried out a search for nearby stars in Gaia DR2 that may have kinematics consistent with this scenario; two kinematic analogues of C/2018 V1 have been identified —Gaia DR2 1927143514955658880 and 1966383465746413568— and they could be solar sibling candidates. 4. (iv)

Our analysis shows that comets coming from interstellar space with relatively low velocities with respect to the Sun may not be uncommon.

Spectroscopic studies of C/2018 V1 may have been able to confirm if this comet could have an extrasolar origin by finding, or not, a chemical composition consistent with that of well-studied Solar system materials. Comet C/2018 V1 is already a Southern hemisphere object, and it was well positioned for observations from May to July in 2019, but its apparent visual magnitude will go above 25 mag by the end of its 2020 opposition. At 30 mag, the object will become virtually unobservable by the end of 2023.

Acknowledgements

We thank the referee for her/his constructive reports and helpful suggestions regarding the presentation of this paper and the discussion of our results, S. J. Aarseth for providing one of the codes used in this research, J. de Leon, J. Licandro and M. Serra-Ricart for discussions on the nature of hyperbolic minor bodies, and A. I. Gómez de Castro for providing access to computing facilities; RdlFM thanks L. Beitia-Antero for extensive discussions on Gaia DR2 data. This work was partially supported by the Spanish ‘Ministerio de Economía y Competitividad’ (MINECO) under grants ESP2015-68908-R and ESP2017-87813-R. In preparation of this paper, we made use of the NASA Astrophysics Data System, the ASTRO-PH e-print server, the MPC data server, and the SIMBAD and VizieR data bases operated at CDS, Strasbourg, France. This work has made use of data from the ESA mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.