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arXiv 1608.06342astro-ph.GA
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内Milky Way的化学绝对结构:“倒置”盘形成的标志?

The Chemical Abundance Structure of the Inner Milky Way: A Signature of "Upside-Down" Disk Formation?

Jenna K. C. Freudenburg, David H. Weinberg, Michael R. Hayden, Jon A. Holtzman

摘要:

我们提出了一个内星系恒星[alpha/Fe]-[Fe/H]分布的模型,R=3-5 kpc,由Hayden等人测量为距中平面垂直距离|z|的函数。基于厚盘形成的“倒置”假设,即恒星形成气体层的厚度随着盘的恒星质量的增加而收缩,我们将单区化学演化与一个简单的公式相结合,即恒星分布的标度高度在时间尺度tc上从zh =0.8 kpc线性下降到zh =0.2 kpc,此后保持不变。我们假设一个线性指数的恒星形成历史,SFR ~ te^{-t/t_sf}。在恒星形成效率时标为2 Gyr,外流质量加载因子为1.5,tsf =3 Gyr,tc =2.5 Gyr的条件下,该模型再现了内盘星在[alpha/Fe]-[Fe/H]中的观测轨迹和H15在|z| =0-0.5 kpc,0.5-1 kpc,1-2 kpc时测量的金属量分布函数(MDF)。模型参数的实质性变化导致与H15数据的不一致;例如,t_c=1 Gyr或t_sf=1 Gyr的模型分别在高- |z|和低- |z|时无法匹配观测到的斜率。在大的回视时间内,z_h(t)下降到t_c ~ t_sf的时间尺度上,这表明相对于动力学加热,初始较薄的恒星星族的倒置形成是调节盘厚度的主要机制。我们的短t_c模型的失败表明,任何一个厚盘形成是一个离散事件的模型都不能再现H15所发现的ε对|z|的连续依赖性。我们的方案的内部磁盘的演变可以测试未来的测量|z|分布和年龄金属丰度分布在R=3-5千秒差距。

英文摘要:

We present a model for the [alpha/Fe]-[Fe/H] distribution of stars in the inner Galaxy, R=3-5 kpc, measured as a function of vertical distance |z| from the midplane by Hayden et al. (2015, H15). Motivated by an "upside-down" scenario for thick disk formation, in which the thickness of the star-forming gas layer contracts as the stellar mass of the disk grows, we combine one-zone chemical evolution with a simple prescription in which the scale-height of the stellar distribution drops linearly from z_h=0.8 kpc to z_h=0.2 kpc over a timescale t_c, remaining constant thereafter. We assume a linear-exponential star-formation history, SFR ~ te^{-t/t_sf}. With a star-formation efficiency timescale of 2 Gyr, an outflow mass-loading factor of 1.5, t_sf=3 Gyr, and t_c=2.5 Gyr, the model reproduces the observed locus of inner disk stars in [alpha/Fe]-[Fe/H] and the metallicity distribution functions (MDFs) measured by H15 at |z|=0-0.5 kpc, 0.5-1 kpc, and 1-2 kpc. Substantial changes to model parameters lead to disagreement with the H15 data; for example, models with t_c=1 Gyr or t_sf=1 Gyr fail to match the observed MDF at high-|z| and low-|z|, respectively. The inferred scale-height evolution, with z_h(t) dropping on a timescale t_c ~ t_sf at large lookback times, favors upside-down formation over dynamical heating of an initially thin stellar population as the primary mechanism regulating disk thickness. The failure of our short-t_c models suggests that any model in which thick disk formation is a discrete event will not reproduce the continuous dependence of the MDF on |z| found by H15. Our scenario for the evolution of the inner disk can be tested by future measurements of the |z|-distribution and the age-metallicity distribution at R=3-5 kpc.

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