THE CLOSEST KNOWN FLYBY OF A STAR TO THE SOLAR SYSTEM

Perturbations by passing stars on Oort cloud comets have previously been proposed as the source of long-period comets visiting the planetary region of the solar system (Oort 1950; Biermann et al. 1983; Weissman 1996; Rickman 2014), and possibly for generating Earth-crossing comets that produce biological extinction events (Davis et al. 1984). Approximately 30%, of craters with diameters <10 km on the Earth and Moon are likely due to long-period comets from the Oort Cloud (Weissman 1996). Periodic increases in the flux of Oort cloud comets due to a hypothetical substellar companion have been proposed (Whitmire & Jackson 1984); however, recent time series analysis of terrestrial impact craters are inconsistent with periodic variations (Bailer-Jones 2011), and sensitive infrared sky surveys have yielded no evidence for any wide-separation substellar companion (Luhman 2014). A survey of nearby field stars with Hipparcos astrometric data (Perryman et al. 1997) by García-Sánchez et al. (1999) identified only a single candidate with a pass of within 0.9 pc of the Sun (Gl 710; 1.4 Myr in the future at ∼0.34 pc); however, it is predicted that ∼12 stars pass within 1 pc of the Sun every Myr (García-Sánchez et al. 2001). A recent analysis by Bailer-Jones (2014) of the orbits of ∼50,000 stars using the revised Hipparcos astrometry from van Leeuwen (2007), identified four Hipparcos stars whose future flybys may bring them within 0.5 pc of the Sun (however, the closest candidate HIP 85605 has large astrometric uncertainties; see discussion in Section 3).

A low-mass star in the solar vicinity in Monoceros, WISE J072003.20-084651.2 (hereafter W0720 or "Scholz's star") was recently discovered with a photometric distance of ∼7 pc and initial spectral classification of M9 ± 1 (Scholz 2014). This nearby star likely remained undiscovered for so long due to its combination of proximity to the Galactic plane (b = +23), optical dimness (V = 18.3 mag), and low proper motion (∼01 yr⁻¹). The combination of proximity and low tangential velocity for W0720 (V_tan ≃ 3 km s⁻¹) initially drew our attention to this system. If most of the star's motion was radial, it was possible that the star may have a past or future close pass to the Sun. Indeed, Burgasser et al. (2014) and Ivanov et al. (2014) have recently reported a high positive radial velocity. Burgasser et al. (2014) resolved W0720 as a M9.5+T5 binary and provided a trigonometric parllax distance of 6.0^+1.2_−0.9 pc. Here we investigate the trajectory of the W0720 system with respect to the solar system, and demonstrate that the star recently (∼70,000 years ago) passed through the Oort Cloud.

We obtained medium-resolution spectroscopy of W0720 on UT dates 2013 November 17 and 19 with the Southern African Large Telescope and Magellan telescopes, in the optical and near-infrared, respectively. As summarized in Ivanov et al. (2014), we estimated the spectral type of W0720 to be L0 ± 1, and measured a radial velocity of 77.6 ± 2.5 km s⁻¹. Recent adaptive optics imaging and high-resolution spectroscopy by Burgasser et al. (2014) indicated that the star is actually a low-mass binary classified as M9.5+T5, with projected separation 0.8 AU, and a multi-epoch mean radial velocity of 83.1 ± 0.4 km s⁻¹. Given the radial velocity accuracy, number of reported epochs (11), and improved astrometric solution provided by the Burgasser et al. (2014) study, we simply adopt their parameters for our calculations: α = 07:20:03.21 (ICRS; epoch 2014.0), δ = −08°46'51  83 (ICRS; epoch 2014.0), μ_α = −40.3 ± 0.2 mas yr⁻¹, μ_δ = −114.8 ± 0.4 mas yr⁻¹, ϖ = 166 ± 28 mas, d = 6.0^+1.2_−0.9 pc, v_radial = 83.1 ± 0.4 km s⁻¹, v_total = 83.2 ± 0.4 km s⁻¹ (all 1σ uncertainties).

Using the Galactic velocity ellipsoids and population normalizations from Bensby et al. (2003), we estimate that W0720 has probabilities of 77.3, 22.5, and 0.2% of belonging to the thin disk, thick disk, and halo Galactic populations, respectively. The spectral classification indices are consistent with a thin disk star of approximately solar composition (Burgasser et al. 2014; classified "dM" based on ζ = 1.034 ± 0.018; Lépine et al. 2007). Classification as a metal-poor subdwarf is ruled out, and given the correlation between metallicity and kinematic properties for M-type stars (Savcheva et al. 2014), it is unlikely that W0720 belongs to either the thick disk or halo. Among 890 nearby solar-type stars from a chromospheric activity-velocity catalog (Jenkins et al. 2011), only three have Galactic velocity components within ±10 km s⁻¹ of W0720's velocity (HIP 51500, 63851, 117499), and all three are less chromospherically active than our Sun (log R'_HK ≤ −4.95) and have inferred chromospheric ages in the range 4–8 Gyr (Mamajek & Hillenbrand 2008). W0720's velocity is similar to that of the Hercules dynamical stream, a kinematic group of stars of heterogenous ages and composition, likely perturbed to the solar circle from smaller Galactocentric radii due to dynamical interactions with the Galactic bar (Bensby et al. 2007). Based on its status as an old thin disk star, consideration of the ages of Sun-like stars of similar velocity, and the isochronal ages for other Hercules stream members (Bensby et al. 2014), we adopt an age for W0720 of 3–10 Gyr (2σ range). Interpolating the mass–age estimates for the W0720 components from Burgasser et al. (2014) for solar composition, this age range maps to component masses of M_A = 86 ± 2 M_Jup and M_B = 65 ± 12 M_Jup (2σ uncertainties). The hydrogen-burning mass limit for stars is near ∼75 M_Jup (Saumon & Marley 2008; Dieterich et al. 2014), hence W0720A is probably a low-mass star, and W0720B is probably a brown dwarf.

We integrated the orbit of W0720 and the Sun with a realistic Galactic gravitational potential using the NEMO Stellar Dynamics Toolbox (Teuben 1995; Barenfeld et al. 2013). With the current velocity data, we simulated 10⁴ orbits of W0720 and the Sun, sampling the observed astrometric values and observational uncertainties for W0720 using Gaussian deviates. From these 10⁴ simulations which take into account the observational uncertainties, we find that W0720 passed as close as Δ = 0.252^+0.111_−0.068 pc (1σ; ^+0.317_−0.110 pc 2σ) or 52.0^+22.9_−13.9 kAU (1σ; ^+65.5_−22.7 kAU 2σ) of the Sun. The time of closest approach was 70.4^+14.7_−9.8 (1σ; ^+36.2_−17.6 2σ) kya⁹ . Figure 1 shows the distributions of the closest approach separations and times for W0720 and the Sun for the 10⁴ orbit simulations. The median nearest pass position from the simulations was (X, Y, Z) = (−0.122, +0.120, +0.185) pc (here quoted in a comoving frame centered on the solar system barycenter, where X is in the current direction of the Galactic center, Y is in the current direction of Galactic rotation, and Z is toward the current north Galactic pole), with approximate 1σ uncertainties of (σ_X, σ_Y, σ_Z) = (0.047, 0.045, 0.068) pc, corresponding to celestial position (α_ICRS, δ_ICRS ≃ 170° ± 29°, +68° ± 14°; 1σ unc.), in the vicinity of Ursa Major. As a check, we also calculated a linear trajectory that ignores the Galactic potential, finding that W0720's closest approach was 70.7 kya at 0.25 parsec at a barycentric position in Galactic coordinates of (X, Y, Z = −0.125, +0.119, +0.185 pc). Note that given the short timespan, the predictions of the time and position of the W0720–Sun minimum pass from the linear trajectory agrees with the more accurate orbit integration to better than 2.5%. At its closest pass W0720 would have had proper motion exceeding any known star: 70'' yr⁻¹ (i.e., capable of traversing a full Moon in 26 yr; cf. the current highest proper motion star, Barnard's star, with 10  73 yr⁻¹).

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The predicted Galactic coordinates of the nearest pass of the binary system (ℓ, b = 135° ± 15°, 47° ± 13°) is near one of the two strong peaks in the longitude of aphelia distribution of new class I comets from the Oort Cloud (ℓ = 135° ± 15°) (Matese et al. 1999). However, this appears to be coincidence, as any comets on eccentricity ∼1 orbits from the vicinity of the nearest pass ∼70 kya would have periods of ∼4 Myr, and hence require ∼2 Myr to reach the inner solar system. Also, the two primary peaks in the distribution of longitudes of aphelia for long-period comets are reasonably explained by the effects of the solar motion and Galactic tide (Feng & Bailer-Jones 2014). Among 10⁴ simulations, the nearest past separation between W0720 and the Sun was Δ = 0.087 pc (18.0 kAU), and this was the only simulation that brought W0720 within the classical boundary of the dynamically active inner Oort Cloud ("Hills Cloud"; Hills 1981; Weissman 1996) at a < 20 kAU. Approximately 79% of the simulations brought W0720 within 0.337 pc of the Sun, the previously closest estimated pass of a known star to the solar system (Gliese 710; 0.337 pc, 1.4 Myr in the future; García-Sánchez et al. 2001). Approximately 99.96% of the simulated trajectories brought the star within the Sun's tidal radius of 1.35 pc (Mamajek et al. 2013), and 98% of the simulated trajectories brought the star within the maximal range of semimajor axes for retrograde orbiting Oort Cloud comets (∼120 kAU; García-Sánchez et al. 1999). The rarity of stellar passes with such a small impact parameter can be assessed from the analysis of García-Sánchez et al. (2001). Extrapolating the power-law distribution of minimum separations versus cumulative number of encounters per Myr from García-Sánchez et al. (2001), one estimates that encounters by stellar systems within 0.25 pc of the Sun occur with a frequency of 0.11 Myr⁻¹ or once every ∼9.2 Myr. For comparison, flybys this close (0.25 pc) are statistically rare (∼2.4%) among encounters by all stellar systems that penetrate the Sun's tidal radius of ∼1.35 pc (Mamajek et al. 2013), of which ∼4.5 occur per Myr (García-Sánchez et al. 2001).

The flyby's gravitational interaction is in the regime where the Sun's influence on the star's trajectory (and vice versa) is negligible (Collins & Sari 2010): $G{{M}_{\odot }}/bv_{*}^{2}\simeq {{10}^{-5.6}}\ll 1,$ where G is the Newtonian constant, M_⊙ is the Sun's mass, b is impact parameter (minimum Sun–star separation), and v_* is the velocity of the star with respect to the solar system barycenter. Major comet showers, where the flux of long-period comets increases by factors of >10 are likely limited to cases of high mass interlopers passing within ∼10 kAU of the Sun (Heisler & Tremaine 1986), and are exceedingly rare (<10⁻³ Myr⁻¹; extrapolating from results by García-Sánchez et al. 2001). A perturbed comet with new aphelion similar to that of the Sun-W0720 minimum separation, but with perihelion in the planetary region of the solar system, will have a ≃ 26 kAU and P ∼ 4.2 Myr. Although comets throughout the Oort cloud may be perturbed by an interloper star onto trajectories that would bring them to the inner solar system (Weissman 1996), the highest density of outer Oort cloud comets should exist at smaller orbital distances (Heisler 1990), so most of the perturbed comets should originate from the vicinity of the star's closest pass to the Sun. Hence, any enhancement on the long-period comet flux should become manifest ∼2 Myr in the future. A proxy indicator of the encounter-induced flux of Oort Cloud comets is defined as γ = M_*/v_*b (Feng & Bailer-Jones 2014 using the variables as defined earlier, and with M_* being the mass of the interloper binary W0720). Using the 10⁴ simulations, we estimate that the W0720 flyby induced γ ∼ 10^{−7.48 ± 0.15(1σ)} M_⊙ km⁻¹ s AU⁻¹. Simulations by Feng & Bailer-Jones (2014) suggest that encounters with γ <10^−5.3 are unlikely to generate an enhancement in the distribution of longitudes for long-period comets compared to that predicted to be generated by Galactic tidal effects. All of the 10⁴ simulated orbits had γ < 10^−7.0, hence the pass of the W0720 system should have a negligible statistical impact on the flux of long-period comets during the coming millenia.

The proximity of W0720's flyby to the solar system can be compared to those of Hipparcos stars recently studied by Bailer-Jones (2014). He identified one star (HIP 85605) with a flyby closer than the median minimum separation that we estimated for W0720 (0.25 pc); however, two future flybys (HIP 89825 [Gl 710] and HIP 63721) both have Δ ≃ 0.27 pc (0.27^+0.17_−0.17 pc and 0.27^+0.40_−0.23 pc, respectively; 90% CL). Bailer-Jones (2014) predicts that HIP 85605 will pass within Δ = 0.10^+0.10_−0.06 pc (90% CL) of the Sun ∼332 kyr in the future. Unfortunately, as Bailer-Jones (2014) points out, HIP 85605 is a visual binary, and there is considerable dispersion in its published proper motions and its Hipparcos parallax is of low accuracy. The solution which brings HIP 85605 close to the Sun relies on the Tycho-2 proper motion (${{\mu }_{\alpha }}$, ${{\mu }_{\delta }}$ = +4.0, −7.6 ± 2.0, 1.9 mas yr⁻¹; Høg et al. 2000), the revised Hipparcos parallax of ϖ = 146.84 ± 29.81 mas, and the Pulkovo radial velocity of −21.0 ± 0.3 km s⁻¹ (Gontcharov 2006 as reported in the XHIP compiled catalog; Anderson & Francis 2012). If any of these values are substantially in error, then the proximity of the flyby solution is likely to be spurious. Both the original and revised Hipparcos astrometric solutions (Perryman et al. 1997; van Leeuwen 2007) had much larger proper motions than the Tycho-2 solution. Bailer-Jones (2014) mentions that a reanalysis of the Hipparcos astrometric solution by F. van Leeuwen suggests "that the Hipparcos-2 parallax of HIP 85605 and its (relatively large) uncertainty are valid, but that on account of the large residuals and the complex nature of this system the solution should be treated with caution."

Unfortunately, the Hipparcos parallax for HIP 85605 leads to some astrophysical inconsistency. As Bailer-Jones (2014) note, HIP 85605's B-V color (1.1 mag) is consistent with a K-type star, whereas the absolute magnitude calculated using its revised Hipparcos parallax (M_V = 11.86 ± 0.45 mag) is more consistent with an M dwarf. We find that HIP 85605's colors from Hipparcos and 2MASS ($B-V$ = 1.10 ± 0.11, $V-{{K}_{s}}$ ≃ 2.66 ± 0.07, $J-H$ ≃ 0.53 ± 0.03, $H-{{K}_{s}}$ ≃ 0.12 ± 0.03) are all consistent with a ∼K4 dwarf (Pecaut & Mamajek 2013), in agreement with the spectral template fitting analysis of Pickles & Depagne (2010; which also yielded K4V). D. Latham (2015, private communication) has visually confirmed that five spectra of HIP 85605 taken with the CfA Digital Speedometer (as reported in García-Sánchez et al. 1999) are indeed consistent with a typical K dwarf. As HIP 85605 is spectrophotometrically a K dwarf, the Hipparcos—derived absolute magnitude (M_V ≃ 11.9) places the star nearly five magnitudes fainter than the main sequence (M_V^MS ≃ 7.0) for a star of its color—too faint to be simply a metal-poor dwarf, yet too red and luminous for a white dwarf¹⁰ . Hence, most likely HIP 85605 is a ∼K4V at d ≃ 60 pc and the Hipparcos parallax is erroneous. At d ≃ 60 pc, HIP 85605 has a slightly different velocity (U, V, W = −10.7, −14.3, −11.6 km s⁻¹) compared to that used in the simulation by Bailer-Jones (2014), and its "flyby" of the Sun correspondingly moves further into the future and further away (∼2.8 Myr in future, ∼10 pc away). We conclude that HIP 85605 is unlikely to penetrate the Sun's Oort Cloud, and that W0720 now appears to have the closest flyby of any known star.

Given its current visual magnitude of V $\simeq$ 18.3 (Ivanov et al. 2014), at its closest approach of ∼0.25 pc, Scholz's star (W0720) would have had an apparent magnitude of V $\simeq$ 10.3, brighter than the current nearest star (Proxima; V = 11.2) but still much dimmer than the faintest naked eye stars (V ∼ 6). However, W0720 is an active M dwarf star (Burgasser et al. 2014), and V-band flares have been observed to exceed 9 mag on timescales of minutes among such stars, and brief flares exceeding 12 mag may be possible (Schmidt et al. 2014). Flares among the coolest M dwarfs have been witnessed with energies of ∼10³⁴ ergs (Schmidt et al. 2014) and luminosities of ∼10²⁹-10³⁰ erg s⁻¹. If W0720 experienced occasional flares similar to those of the active M8 star SDSS J022116.84 + 194020.4 (Schmidt et al. 2014), then the star may have been rarely visible with the naked eye from Earth ($V\lt 6$; ${\Delta }V\lt -4$) for minutes or hours during the flare events. Hence, while the binary system was too dim to see with the naked eye in its quiescent state during its flyby of the solar system ∼70 kya, flares by the M9.5 primary may have provided short-lived transients visible to our ancestors.

Improved astrometry for W0720 via ground-based telescopes (Dieterich et al. 2014) or Gaia (de Bruijne 2012), and further radial velocity monitoring, should help reduce the uncertainties in the flyby timing and minimum separation between Scholz's star and the solar system. Past systematic searches for stars with close flybys to the solar system have been understandably focused on the Hipparcos astrometric catalog (García-Sánchez et al. 1999; Bailer-Jones 2014); however, it contains relatively few M dwarfs relative to their cosmic abundance. Searches in the Gaia astrometric catalog for nearby M dwarfs with small proper motions and large parallaxes (i.e., with small tangential velocities) will likely yield addition candidates.

E.E.M. acknowledges support from NSF grant AST-1313029. A.Y.K. and P.V. acknowledge the support from the National Research Foundation (NRF) of South Africa. We thank Dave Latham, Alice Quillen, Kevin Luhman, Cameron Bell, Dave Cameron, Peter Teuben, Segev Ben-Zvi, Adam Burgasser, Ralf-Dieter Scholz, Matt Multunas, and Richard Sarkis for discussions. We thank the referee for a very timely and useful report.