Behavior of Blue Straggler Stars in Dwarf Spheroidal Galaxies

【日時】6月1日(水) 10:30~12:00
【場所】国立天文台・三鷹 中央棟(北)1階 講義室
【発表者(敬称略)】Zhao Zhengshi(総研大 D2・三鷹、指導教員 : 有本 信雄)
【タイトル】Behavior of Blue Straggler Stars in Dwarf Spheroidal Galaxies
Blue straggler stars (BSSs) are located above and blue-ward of main sequence turn off (MSTO) in color-magnitude diagram (CMD) of stellar systems. They have been identified in many stellar systems during past 60 years, since discovered by Sandage (1953) in globular cluster (GC) M3. The locations of BSSs imply that they are more massive than MSTO stars if they are normal single stars, and considering all stars in a cluster had formed at the same time, according to standard theory of stellar evolution, BSSs should have evolved to white dwarfs long ago.
However, they appear along the MS and lazy to evolve as other stars in the same cluster. Two different mechanisms of them are commonly believed nowadays: mass transfer in binary systems and stellar merger occurred through a direct stellar collision. The former might be dominant in lower density environments, whereas the later might be considered to mainly occur in high-density environments.
Relative frequency of bimodal distributions, concentrating in central regions and decreasing at intermediate radius and rising again in the outskirts, are a characteristic phenomenon of BSSs in GCs. Dynamical simulation of Mapelli et al (2004) showed that these distributions can be reproduced by requiring the central BSSs formed mainly by CL
mechanism, and the outskirt BSSs formed by MT mechanism. On the contrary, radial distributions of BSSs in three dSphs Draco, Ursa Minor and Sculptor, shows quite flat distribution and that are consistent with model predictions for BSS formation by MT mechanism (Mapelli et al 2007 & 2009). However, in dwarf galaxies both of young
MSs and old BSSs are located on the blue ward of current MSTO. Momany et al. (2007) derived a statistically significant anti-correlation between BSSs’ relative frequency and absolute magnitude of dSphs which had been found in GCs and OCs, and mentioned that the anti-correlation can be used as a discriminator: galaxies obeying the anti-correlation are more likely to possess genuine primordial BSS rather than young
main sequence stars.In view of small range of luminosity of their samples (-10