Title: Discovery of a Weak CN Spectral Absorption Feature in Red Supergiant Stars in the Andromeda (M31) and Triangulum (M33) Galaxies Show Chinese title

Title: 发现仙女座（M31）和三角座（M33）星系红超巨星中的弱CN光谱吸收特征

Abstract: Using Keck DEIMOS spectra of stars in the Andromeda (M31) and Triangulum (M33) galaxies, selected from the large multi-band (near ultraviolet, visible light, and near infrared) Hubble Space Telescope surveys PHAT and PHATTER, respectively, we have identified a subset of stars that contain a previously unnoticed weak spectral absorption feature around 8000 Angstrom (0.8 micron). This absorption feature appears to be associated with the cyanogen (CN) molecule. Strong CN spectral absorption is a standard feature of carbon stars, which are thought to be intermediate mass (2-3 ~ M_sun) stars with C/O > 1 in the thermally-pulsating asymptotic giant branch phase of stellar evolution. However, the stars that are the focus of this paper are characterized by a weak version of this CN spectral absorption feature in a spectrum that is otherwise dominated by normal O-rich spectral absorption lines such as TiO and/or the Ca near infrared triplet. We have dubbed these stars "weak CN" stars. We present an automated method for identifying weak CN stars in M31 and M33, and examine their photometric properties in relation to model isochrones and stellar tracks. We find that weak CN stars tend to be fairly localized in color-magnitude space, and appear to be red supergiant stars with masses ranging from 5-10 M_sun, overall lifetimes of about 40-50 Myr, and currently in the core He burning phase of stellar evolution. Show Chinese abstract

Abstract: 利用分别从哈勃太空望远镜的大规模多波段（近紫外、可见光和近红外）巡天计划PHAT和PHATTER中挑选出的位于仙女座（M31）和三角座（M33）星系中的恒星的凯克DEIMOS光谱，我们识别出了一组包含以前未被注意到的约8000埃（0.8微米）附近弱吸收特征的恒星。这一吸收特征似乎与氰分子（CN）有关。 强CN光谱吸收是碳星的标准特征，碳星被认为是质量为2-3倍太阳质量的恒星，在热脉冲渐近巨星分支演化阶段具有C/O > 1的特性。 然而，本文所关注的恒星在光谱上表现出这种CN光谱吸收特征的弱版本，而其光谱主要由正常的氧丰富的吸收线（如TiO和/或Ca近红外三重线）主导。 我们将这些恒星命名为“弱CN”星。 我们提出了一种自动化方法来识别M31和M33中的弱CN星，并研究了它们的光度性质与模型等年龄曲线和恒星轨迹的关系。 我们发现弱CN星在色-星等空间中倾向于较为局部化，似乎是质量在5-10倍太阳质量之间的红超巨星，总寿命约为40-50百万年，并且目前处于核心氦燃烧的恒星演化阶段。