根据双子座近红外光谱对1806-20星团和相关磁星距离的向下修正
A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini near-Infrared spectroscopy
摘要：
我们提出了银河系星团1806-20（G10.0-0.3）的OB和Wolf-Rayet（WR）成员的H-和K-波段光谱，以获得与2004年SGR 1806-20磁星巨大耀斑相关的修正的星团距离。从GNIRS光谱与双子座南，四个候选人OB星被确认为晚O/早B超巨星，而我们支持以前的中期WN和晚WC分类的两个WR星。基于B超巨星和WR星的绝对KS波段星等校准，以及来自双子座北的NIRI的近红外测光加上档案VLT/ISAAC数据集，我们获得了14.7+/-0.35 mag. 1806-20星团的已知恒星含量表明年龄为3-5百万年，从理论等时线拟合推断出距离模数为14.7+/-0.7 mag。总之，我们的结果支持距离模数为14.7+/-0.4 mag（8.7^+1.8_-1.5 kpc）到1806-20星团，这明显低于到磁星的标称距离15 kpc。对于我们所选择的距离，2004年12月巨大耀斑的峰值亮度降低了3倍，为7 × 10^46 erg/s，这样来自河外磁星巨大耀斑的BATSE短伽马射线暴（GRB）的污染就降低到了百分之几。我们推断磁星的前身质量为~48^+20_-8 Msun，这与最近对韦斯特隆德1号磁星获得的结果非常一致。
英文摘要：
We present H- and K-band spectroscopy of OB and Wolf-Rayet (WR) members of the Milky Way cluster 1806-20 (G10.0-0.3), to obtain a revised cluster distance of relevance to the 2004 giant flare from the SGR 1806-20 magnetar. From GNIRS spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous mid WN and late WC classifications for two WR stars. Based upon an absolute Ks-band magnitude calibration for B supergiants and WR stars, and near-IR photometry from NIRI at Gemini North plus archival VLT/ISAAC datasets, we obtain a cluster distance modulus of 14.7+/-0.35 mag. The known stellar content of the 1806-20 cluster suggests an age of 3-5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7+/-0.7 mag. Together, our results favour a distance modulus of 14.7+/-0.4 mag (8.7^+1.8_-1.5 kpc) to the 1806-20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the December 2004 giant flare is reduced by a factor of three to 7 X 10^46 erg/s, such that the contamination of BATSE short gamma ray bursts (GRB's) from giant flares of extragalactic magnetars is reduced to a few percent. We infer a magnetar progenitor mass of ~48^+20_-8 Msun, in close agreement with that obtained recently for the magnetar in Westerlund 1.
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