从银河系轨道推断超高速恒星的性质
The nature of hypervelocity stars as inferred from their galactic trajectories
摘要：
我们计算了12颗超高速恒星（HVS）的银河系轨迹，假设它们起源于银河系中心。我们证明，这12颗恒星中有8颗与银河系有关。我们考虑了束缚星的后续轨道来计算它们的特征轨道周期，即2 Gyr。所有八颗恒星都远离银河系中心，这意味着恒星的寿命不到2 Gyr。因此我们推断观察到的HVS是大质量主序星，而不是蓝色水平支星。观测结果表明，大约每1500万年，蓝色HVS就会从银河系中心喷出。如果我们假设HVS是由双星的分裂产生的，那么这与观测到的围绕中心超大质量黑洞（SMBH）的极紧轨道上的蓝色恒星（即所谓的S星）的数量是一致的。在这样的双星系统中，其中一颗恒星以高速被抛出形成HVS;另一颗仍然作为S星与SMBH结合。我们进一步表明，观测到的一个高速系统正在向银河系中心移动，SDSS J172226.55 + 594155.9，不可能起源于银河系中心;相反，我们将其确定为晕天体。
英文摘要：
We have computed the galactic trajectories of twelve hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these twelve stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main sequence stars, rather than blue horizontal branch stars. The observations suggest that blue HVSs are ejected from the Galactic Centre roughly every 15 Myr. This is consistent with the observed population of blue stars in extremely tight orbits round the central super-massive black hole (SMBH), the so-called S-stars, if we assume that the HVSs are produced by the breakup of binaries. One of the stars in such a binary is ejected at high velocities to form a HVS; the other remains bound to the SMBH as an S-star. We further show that the one high-velocity system observed to be moving towards the Galactic Centre, SDSS J172226.55+594155.9, could not have originated in the Galactic Centre; rather, we identify it as a halo object.
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