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ANTARES neutrino detection: kinematic evidence that the Swift/XRT X-ray counterpart is a likely Upper Sco member

ATel #8124; Eric E. Mamajek (University of Rochester)
on 5 Oct 2015; 17:26 UT
Credential Certification: Eric Mamajek (emamajek@pas.rochester.edu)

Subjects: Radio, Infra-Red, Optical, X-ray, Neutrinos, A Comment, Star, Transient, Variables, Young Stellar Object, Pre-Main-Sequence Star

The star USNO-B1.0 0626-0501169 (2MASS J16260214-2718141) is the optical counterpart of a Swift/XRT X-ray source that was originally proposed to be the counterpart of an ANTARES neutrino detection (ATEL #7987). The ANTARES neutrino has triggered follow-up observations (ATEL #7992, #7993, #7994, #7995, #7996, #7998, #7999, #8000, #8002, #8003, #8006, #8027, #8034, #8097). ATEL #7993, #7994, #7996 noted that the star is chromospherically active, and de Ugarte Postigo et al. (ATEL #7994) noted strong Li absorption, hinting that the star could be young and associated with the Rho Oph region (hereafter "Oph"). Here I analyze the star's kinematics.

The star is at (l,b) = (350.76,+15.05 deg), and situated 2.8 deg from the Oph embedded cluster, which has centroid (RA,Dec) = (246.78, -24.48 deg; Mamajek 2008, AN, 329, 10). However, the star is within the box defining the Upper Sco subgroup of Sco-Cen (hereafter "US"; see Fig. 9 of de Zeeuw et al. 1999, AJ, 117, 354). US represents an older population of stars with mean age ~11 Myr (Pecaut, Mamajek & Bubar 2012, ApJ, 746, 154). Oph and US are roughly codistant (d ~ 140 pc) and comoving within <2 km/s (Mamajek 2008).

Using astrometry from UCAC4 (Zacharias et al. 2013, AJ, 145, 44) and SPM4 (Girard et al. 2011, AJ, 142, 15), I estimate an updated mean proper motion for the Oph cluster: = -6,-25 +- 1,1 mas/yr. Combined with the group's median RV, distance, and centroid from Mamajek (2008), I estimate revised UVW velocity of (-5.6,-13.6,-9.0 km/s; all +-~1 km/s). This revised velocity is within 3 km/s of US (U,V,W = -6.4,-15.9,-7.4 +- 0.5,0.7,0.2 km/s; Chen et al. 2011, ApJ, 738, 122).

Placing a hypothetical member of either Oph or US at 140 pc at the star's position yields the following predicted values:
Oph: pmRA,pmDec = -5.7,-23.9 mas/yr, RV = -5.6 km/s
US: pmRA,pmDec = -10.0,-24.9 mas/yr, RV = -5.6 km/s

The predicted RVs are essentially identical. There are several published proper motions (B1.0, UCAC2, PPMX, PPMXL, SPM4, UCAC4), and they generally agree within their uncertainties, however the best recent value comes from UCAC4: pmRA, pmDec = -10.3,-25.1 (+-1.2,1.1) mas/yr. The UCAC4 proper motion is remarkably close to that for a US member at 140 pc. At least 7 US members lie within 1 deg of the star (Hoogerwerf 2000, MNRAS, 313, 43).

One can also calculate a kinematic distance to the star using convergent point methodology (e.g. Mamajek 2005, ApJ, 634, 1385). Using the US velocity from Chen et al. (2011), and the UCAC4 proper motion, I estimate a parallax of 7.22+-0.65 mas (d = 139+-12 pc). The star's peculiar motion is only 0.2+-0.8 km/s. Both values are consistent with US membership.

Tucker et al. (ATEL #7996) report spectral type K5-K7. Using the pre-MS Teff sequence of Pecaut & Mamajek (2013, ApJS, 208, 9), this corresponds to Teff = 4020 K (approx. +-100 K). Using BV photometry from UCAC4, JHKs photometry from 2MASS, and the SpT/color tables from Pecaut & Mamajek (2013), and examining optical/near-IR color excesses, I estimate extinction of Av = 0.17+-0.1 mag. I calculate a luminosity using the star's 2MASS J magnitude (J = 9.907+-0.024), J-band extinction (A_J = 0.048+-0.028 mag), bolometric correction BC_J = 1.61+-0.03, and predicted parallax 7.22+-0.65 mas. The star's luminosity is log(L/Lsun) = -0.40+-0.08 dex. Using the pre-MS isochrones of Baraffe et al. (2015, A&A 577, 42), the star's HRD position is consistent with age ~5 Myr-old and mass ~0.75 Msun. Recent work suggests that pre-MS isochronal ages of low-mass stars are likely underestimated by factors of ~2x (Bell et al. 2013, MNRAS, 434, 806). Hence, the true age is probably more like ~10 Myr, similar to other US members. All indications thus far point to the star being a pre-MS member of Upper Sco.