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arXiv 1407.6953astro-ph.GA
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在M32中看到红色:近红外和中红外观测对恒星成分的限制及在更远星系研究中的应用

Seeing Red in M32: Constraints on the Stellar Content from Near- and Mid-Infrared Observations and Applications for Studies of More Distant Galaxies

T. J. Davidge

摘要:

利用1 - 8 μ m波长间隔的地面和空间观测资料研究了本星系群M32中渐近巨星分支(AGB)星的性质,目的是证明红外观测作为恒星内容探测器的实用性。与等时线的比较表明,M32中最明亮的恒星的年龄只有几个Gyr,平均来说比M31外盘中具有相同内在亮度的AGB星年轻。说明恒星的变化是必不可少的AGB光度函数(LF)建模。假设Monachesi等人(2012,ApJ,745,97)测量的恒星形成历史和银河系AGB恒星的变化特性的模型LF与M32的K和[5.8] LF相匹配。模型还表明,[5.8] LF在M_[5.8] = -8.5和-10.0之间的斜率对恒星年龄的混合很敏感,M32中相当大一部分恒星的年龄必须大于7 Gyr,才能与[5.8] LF相匹配。研究了M32在红外波段的结构性质。根据近红外线和中红外线等照度线计算的有效半径与在可见光波长下测量的相似,这表明M32的恒星成分混合良好。然而,在近红外和中红外半径> 16弧秒(> 60秒差距)的等照度线比可见光波长的等照度线平坦。在[3.6]和[4.5]中,等照度线的傅立叶展开中四阶余弦项的系数从r < 16 arcsec时的“boxy”值变为r > 48 arcsec时的“disky”值。M32中心附近的中红外线颜色并不随半径而系统性地变化,这提供了在星系的这一部分中混合良好的恒星成分的证据。

英文摘要:

The properties of asymptotic giant branch (AGB) stars in the Local Group galaxy M32 are investigated using ground and space-based observations that span the 1 - 8um wavelength interval, with the goal of demonstrating the utility of infrared observations as probes of stellar content. Comparisons with isochrones indicate that the brightest resolved stars in M32 have ages of a few Gyr, and are younger on average than AGB stars with the same intrinsic brightness in the outer disk of M31. Accounting for stellar variability is shown to be essential for modelling AGB luminosity functions (LFs). Model LFs that assume the star-forming history measured by Monachesi et al. (2012, ApJ, 745, 97) and the variability properties of Galactic AGB stars match both the K and [5.8] LFs of M32. Models also suggest that the slope of the [5.8] LF between M_[5.8] = -8.5 and --10.0 is sensitive to the mix of stellar ages, and a sizeable fraction of the stars in M32 must have an age older than 7 Gyr in order to match the [5.8] LF. The structural properties of M32 in the infrared are also investigated. The effective radii that are computed from near-infrared and mid-infrared isophotes are similar to those measured at visible wavelengths, suggesting that the stellar content of M32 is well-mixed. However, isophotes at radii > 16 arcsec (> 60 parsecs) in the near and mid-infrared are flatter than those at visible wavelengths. The coefficient of the fourth-order cosine term in the fourier expansion of isophotes changes from `boxy' values at r < 16 arcsec to `disky' values at r > 48 arcsec in [3.6] and [4.5]. The mid-infrared colors near the center of M32 do not vary systematically with radius, providing evidence of a well-mixed stellar content in this part of the galaxy.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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