Late-Type Stars in M31. I. Photometric Study of AGB Stars and Metallicity Gradients
Late-Type Stars in M31. I. Photometric Study of AGB Stars and Metallicity Gradients
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M31 中的晚型恒星。
DOI：
10.1086/117466
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
D. Crabtree
中科院分区：
文献类型：
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作者：
J. Brewer;H. Richer;D. Crabtree
我们在 M31 中拍摄了 5 个 7\arcmin \x 7\arcmin\ 场，沿 SW 长轴跨越了 4 到 32 kpc 的半乳心半径。通过两个宽带（\V\ 和 \I）和两个窄带（\CN\ 和 \TiO）滤光片观察场。宽带数据被用来构建\IvsVI\颜色星等图（CMD），并且在我们的一些领域中，我们在造父变星不稳定带中发现了大量的恒星。发现中场造父变星的距离模量与文献值中的其他值非常吻合。研究了所有 5 个区域的 IvsVI CMD 中巨分支 (GB) 的宽度，我们表明在其中四个区域中，GB 宽度的可能解释是金属丰度和质量变化的组合。使用宽带数据，在五个视场中测量了渐近巨分支（AGB）光度函数（LF），我们发现这些 LF 之间存在差异。我们推测区域中不同的恒星形成历史如何导致观测到的 AGB LF 和 GB 宽度。使用窄带数据和宽带数据，我们将 AGB 恒星分为富碳 (C) 和富氧 (M) 类型。碳星 LF 用于获得 M31 距离模量的估计值，该值与来自造父变星的值一致。 C 星与 M 星的比率 (C/M) 被认为是这些恒星形成时气态化学丰度的指标。我们发现，C/M 比随着星系中心距离的增加而平稳增加，这表明与金属丰度呈负相关。这是在单个河外系统中首次演示这种效应。我们发现 GB 和 AGB LF 宽度的差异不会显着影响 C/M 比。我们考虑了增加的 C/M 比率对 M31 中 ISM 的影响，并引用了支持模型的证据，其中 M31 中的颗粒成分是半乳心距离的函数。
We have imaged five 7\arcmin \x 7\arcmin\ fields in M31 spanning galactocentric radii from 4 to 32 kpc along the SW-major axis. The fields were observed through two broad-band (\V\ and \I) and two narrow-band (\CN\ and \TiO) filters. The broad-band data were used to construct \IvsVI\ color-magnitude diagrams (CMDs) and, in some of our fields, we found significant numbers of stars in the Cepheid instability strip. A distance modulus for the Cepheids in the middle field was found that agreed well with other values in the literature values. The width of the giant branch (GB) in the \IvsVI\ CMD of all 5 fields was investigated, and we show that in four of the fields a likely explanation for the GB width is a combination of {\it both} metallicity and mass variations. Using the broad-band data, the asymptotic giant branch (AGB) luminosity functions (LFs) were measured in the five fields, and we show that differences exist between these LFs. We speculate on how the different star forming histories in the fields may lead to the observed AGB LFs and GB widths. Using the narrow-band data along with the broad-band data we separated the AGB stars into carbon-rich (C) and oxygen-rich (M) types. The carbon stars LFs were used to obtain an estimate for the distance modulus of M31 which agrees with the value derived from Cepheids. The ratio of C- to M-stars (C/M) is believed to be an indicator of gaseous chemical abundance at the time of formation of these stars. We show that the C/M ratio increases smoothly with galactocentric distance, suggesting an inverse correlation with metallicity. This is the first demonstration of this effect within a single extragalactic system. We find that differences in the width of the GB and the AGB LFs do not significantly affect the C/M ratio. We consider the effect of the increasing C/M ratio on the ISM in M31, and cite evidence in favor of a model where the grain composition in M31 is a function of galactocentric distance.