Evolutionary models for metal-poor low-mass stars. Lower main sequence of globular clusters and halo field stars

TL;DR

This study presents detailed evolutionary models for very-low-mass, metal-poor stars, including their photometric properties, and compares these models with observations, improving understanding of the lower main sequence in globular clusters and halo stars.

Contribution

It provides new evolutionary models incorporating recent physics, accurate photometric transformations, and predictions for infrared colors, including the hydrogen-burning limit and stellar to substellar transition signatures.

Findings

Models accurately reproduce globular cluster main sequences.

Grey approximations yield inaccurate mass-magnitude relations near hydrogen-burning limit.

Predicted infrared colors show a blueshift indicating the stellar to substellar transition.

Abstract

We have performed evolutionary calculations of very-low-mass stars from 0.08 to 0.8 $\msol$ for different metallicites from [M/H]= -2.0 to -1.0 and we have tabulated the mechanical, thermal and photometric characteristics of these models. The calculations include the most recent interior physics and improved non-grey atmosphere models. The models reproduce the entire main sequences of the globular clusters observed with the Hubble Space Telescope over the afore-mentioned range of metallicity. Comparisons are made in the WFPC2 Flight system including the F555, F606 and F814 filters, and in the standard Johnson-Cousins system. We examine the effects of different physical parameters, mixing-length, $\alpha$-enriched elements, helium fraction, as well as the accuracy of the photometric transformations of the HST data into standard systems. We derive mass-effective temperature and mass-magnitude relationships and we compare the results with the ones obtained with different grey-like approximations. These latter are shown to yield inaccurate relations, in particular near the hydrogen-burning limit. We derive new hydrogen-burning minimum masses, and the corresponding absolute magnitudes, for the different metallicities. We predict color-magnitude diagrams in the infrared NICMOS filters, to be used for the next generation of the HST observations, providing mass-magnitudes relationships in these colors down to the brown-dwarf limit. We show that the expected signature of the stellar to substellar transition in color-magnitude diagrams is a severe blueshift in the infrared colors, due to the increasing collision-induced absorption of molecular hydrogen with increasing density and decreasing temperature.