月別アーカイブ: 2011年5月

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

Exploring galaxy clusters using weak lensing and spectroscopic redshift

【日時】5月25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)】内海 洋輔(総研大 D3・三鷹、指導教員 : 宮崎 聡)
【タイトル】Exploring galaxy clusters using weak lensing and spectroscopic redshift
我々は弱重力レンズによる質量分布と分光観測結果を比べ,奥行き構造について調べた.今回はさらに質量分布の構築法を改善し,より銀河団らしく,よりノイズを落とすような “Optimal” フィルタを採用しテストした.その結果について報告し,将来的な HSC を使った弱重力レンズによる銀河団カタログ構築と,それを使った宇宙論パラメータの制限についての展望を述べる.
また今回の観測では偶然にも多数の $z=0.5$ の比較的赤方偏移の大きい銀河団を発見した.本観測領域の特徴や銀河団プロファイルに対する観測的制限も行ったので合わせて報告する.


【日時】5月11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)】富田 賢吾(総研大 D3・三鷹、指導教員 : 富阪 幸治)
星形成過程では様々な物理が重要になるが、多次元の輻射流体計算に基づく研究は始まったばかりである。これまで、低質量星の形成においては輻射輸送の効果は比較的重要ではなく、バロトロピック近似でも定性的に尤もらしい結果が得られると考えられてきた。しかし最近の研究では、原始星コアが形成される際に莫大なエネルギーが輻射で放出されて周囲のガスを加熱し、アウトフローが放出される(Bate 2010)、ファーストコアが破壊される(Schoenke & Tscharnuter 2011)といったこれまでの理解とは全く異なる現象が起こり得ることが報告されている。