Indicators of Stellar Mass in the Photometric H-band

Abstract

Extensive infrared spectral surveys, such as the APOGEE survey in the H-band, are now being conducted, many targeting the Galactic Bulge and recording observations of primarily red giant stars. However, because stars of different masses converge to the red giant region, the masses of single red giant stars are poorly constrained. These surveys are now using spectral resolving powers that are high enough to measure the equivalent widths of individual spectral lines, which are mostly from molecular species. Because other observations can constrain or determine the star’s luminosity and radius, we have computed spherical stellar atmospheres for a fixed luminosity and radius but for a range of masses. We then computed the H-band flux spectrum for each model and searched for spectral lines that are sensitive to mass. Our synthetic spectra reveal many lines of CO that become weaker with increasing stellar mass. To explore this, we created a ratio of equivalent widths using a representative, unblended CO line and an unblended OH line that did not vary with mass. We found that this ratio varied about 30% over the mass range from to . We repeated the spectral analysis using spherical model stellar atmospheres computed with a composition solar and found that the ratio displayed a very similar dependence on mass. The presence in the H-band of spectral features sensitive to the masses of red giant stars opens up the potential of constraining more tightly the physical properties of the stars making up the galactic bulge and globular clusters.