Reason to Doubt the Hipparcos Distance and the Close Flyby Scenario for the "Rogue Star" HIP 85605
Color-magnitude diagram for HIP 85605 compared to the known stars within 10 pc of the Sun (V-Ks color vs. absolute magnitude Mv).
HIP 85605 was recently identified by Bailer-Jones (2014; http://arxiv.org/abs/1412.3648) as having a Galactic orbit which may bring the star within 0.2 parsecs of the Sun 240-470 thousand years in the future. Bailer-Jones (2014) discusses the astrometric solution for HIP 85605 in detail (page 7 of their paper). The orbit which brings HIP 85605 so close to the solar system relies on the revised Hipparcos parallax for the star. Here we show that the Hipparcos parallax for HIP 85605 can most likely be ruled out on astrophysical grounds, and that the close flyby scenario reliant on this parallax is unlikely to be correct.
The filled black circle is HIP 85605, adopting apparent V magnitude of 11.03+-0.07 mag (converted from Tycho-2 BtVt photometry following Mamajek, Meyer, & Liebert 2006; Hog et al. 2000; Bt=12.49+-0.16, Vt=11.16+-0.07) and the revised Hipparcos parallax from van Leeuwen (2009) of 146.84+-29.81 mas (distance 6.8+1.7-1.2 pc). For the V-Ks color we adopt the same V magnitude and the Ks magnitude (8.371+-0.020) from 2MASS (Skrutskie et al. 2006). The original data and plot for the stars within 10 pc distance of the Sun come from the RECONS group at Georgia State (Todd Henry et al.; see http://www.recons.org/hrd.2010.0.html for original figure).
The take away message is that either 1) the revised Hipparcos parallax is severely in error, or 2) HIP 85605 must be some sort of exotic star nearly halfway between the main sequence and the white dwarf sequence. No other stars within 10 pc appear in the color/absolute magnitude space near HIP 85605 if the revised Hipparcos parallax is correct (i.e. the star is at distance ~7 parsecs).
The combination of color (V-Ks) and apparent magnitude (V) of the star could be consistent with a few different types of objects:
1) main sequence star at d ~ 60 pc (Mv ~ 7).
2) metal-poor main sequence star at d slightly closer than 60 pc.
3) white dwarf at d ~ 1 pc (Mv ~ 16).
4) giant star at d ~ 1600 pc (Mv ~ 0).
Options 1 and 2 seem most likely. Option 3 (white dwarf) is probably ruled out as the available astrometry (measured positions of the star) show no hint of an annual amplitude as large as ~1" (the parallax is almost certainly <100-200 mas). The star could possibly be a background K giant at ~1600 pc (Mv ~ 0). Metal poor main sequence stars typically appear somewhat below the main sequence, so the star could be somewhat closer than the main sequence photometric distance (~60 pc).
The optical and infrared colors of the star are all consistent with being a K-type star (V-Ks = 2.66+-0.07, J-Ks = 0.65+-0.03, J-W1 = 0.77+-0.03, B-V = 1.10+-0.11; adopting photometry from 2MASS, WISE, and Hipparcos). The V-Ks color is consistent with that of a K4V main sequence star, and indeed the best fit spectral template to the Tycho-2MASS photometry of HIP 85605 by Pickles & Depagne (2010) was also K4V. The V-Ks color is consistent with an effective temperature of ~4700K. In the kinematic and radial velocity study by Garcia-Sanchez et al. 1999), the best fit synthetic template spectrum for HIP 85605 had Teff=5000K (solar metallicity was assumed).
HIP 85605 is listed as the "B" component of a multiple system in the Washington Double Star catalog (WDS 17296+2439). Component "A" is HIP 85607, situated ~22" away. "A" has similar B-V color to "B", but is ~1.3-1.4 magnitudes brighter in the Vt, J, H, and Ks bands. The long-term Tycho-2 proper motion of "A" differs with that of "B" considerably (A: pmRA, pmDec = -10.9, 4.7 +- 1.2, 1.1 mas/yr; B: pmRA, pmDec = 4.0, -7.6 +- 2.0, 1.9 mas/yr). HIP 85607 was classified "K0" in the SAO catalog (this is an "HD" spectral type, no luminosity class was given). HIP 85607 is unlikely to be a main sequence dwarf given its negligible, indeed negative, Hipparcos parallax (-1.19+-1.83 mas). Based on the SAO spectral type and the small parallax, HIP 85607 is most likely close to having spectral type K0III. The properties of K0III standard stars are reviewed at: http://www.pas.rochester.edu/~emamajek/spt/K0III.txt, and a typical K0III or K0IIIb star has intrinsic colors (B-V)o ~ 1.01, (V-I)o ~ 0.98, and Mv ~ 1.0. Adopting its Hipparcos colors (B-V = 1.08+-0.03, V-I = 1.04+-0.02), and assuming HIP 85607 is a K0III star, its photometry is consistent with being lightly reddenened (E(B-V) ~ 0.07, E(V-I) ~ 0.06), suffering a little bit of interstellar extinction (Av ~ 0.19+-0.05 mag). Adopting an absolute magnitude of Mv ~ 1.0, the brightness of HIP 85607 corresponds to a photometric distance of d ~ 440 pc. Independently, Pickles & Depagne (2010) fit spectral templates to the BtVtJHK photometry for HIP 85607 and estimated it to be of spectral type K1III at photometric distance 383 pc. Based on the available evidence, it is most likely that WDS 17296+2439 is a "visual" binary, and not physical (i.e. component "A" HIP 85607 is unrelated to HIP 85605).
Spectroscopic and astrometric follow-up of HIP 85605 is warranted in order to clarify its nature and distance. Most likely, HIP 85605 is a typical K dwarf situated tens of parsecs away (60 pc?), but with a perturbed astrometric motion that affected its Hipparcos parallax. If the Hipparcos parallax is correct, then the star is astrophysically very interesting (no similar counterpart within 10 pc; it would need to be fairly exotic).
Many thanks to the RECONS group at GSU (Henry et al.) for providing the HR diagram plot showing the positions of the other known stars within 10 parsecs.