改进极低质量恒星的PARSEC模型
Improving PARSEC models for very low mass stars
摘要：
许多恒星模型在再现极低质量星（VLMS）的基本观测关系，包括冷矮星的质量-半径关系和光学色-星等方面存在困难。在这里，我们在这些方面改进PARSEC模型。我们实现的T-tau关系从PHOENIX BT-Settl模型大气的外边界条件的PARSEC代码，发现这一变化单独减少的质量-半径关系的差异从8%到5%，我们比较的模型与多波段光度的集群Praesepe和M67，表明使用的T-tau关系明显改善了描述的光学颜色和星等。但无论如何，使用库鲁茨和凤凰模型光谱，模型的颜色仍然比观测结果更暗更蓝。然后，我们对上述T-τ关系作了一个位移，从T_eff = 4730 K时的0增加到T_eff = 3160 K时的~14%，再现了观测到的矮星质量-半径关系。用这个实验作为T-τ关系的校准，我们可以再现老的贫金属球状星团NGC 6397和47 Tuc，以及中、年轻的太阳金属度疏散星团M67和Praesepe中低质量恒星的光学和近红外CMD。因此，我们扩展PARSEC模型使用这种校准，提供VLMS模型更适合于较低的主序星在广泛的金属丰度和波长。这两套模型都可以在PARSEC网页上找到。
英文摘要：
Many stellar models present difficulties in reproducing basic observational relations of very low mass stars (VLMS), including the mass--radius relation and the optical colour--magnitudes of cool dwarfs. Here, we improve PARSEC models on these points. We implement the T--tau relations from PHOENIX BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass--radius relation from 8 to 5 per cent. We compare the models with multi--band photometry of clusters Praesepe and M67, showing that the use of T--tau relations clearly improves the description of the optical colours and magnitudes. But anyway, using both Kurucz and PHOENIX model spectra, model colours are still systematically fainter and bluer than the observations. We then apply a shift to the above T--tau relations, increasing from 0 at T_eff = 4730 K to ~14% at T_eff = 3160 K, to reproduce the observed mass--radius radius relation of dwarf stars. Taking this experiment as a calibration of the T--tau relations, we can reproduce the optical and near infrared CMDs of low mass stars in the old metal--poor globular clusters NGC6397 and 47Tuc, and in the intermediate--age and young solar--metallicity open clusters M67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models more suitable for the lower main sequence stars over a wide range of metallicities and wavelengths. Both sets of models are available on PARSEC webpage.
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