Seismology of pre-main-sequence stars in NGC 6530

Abstract

Nonradial p-mode oscillation spectra, computed from a dense grid of pre-main-sequence models, are fit to observed oscillation spectra of several stars in the young cluster NGC 6530. The five stars we consider, all previously identified as pulsating pre-main-sequence stars, each have from two to nine observed oscillation frequencies. For those stars with a more complete set of frequencies we are able to constrain the models using the oscillation spectra alone and confirm that the stars are in their pre-main-sequence and not post-main-sequence phase of evolution. For the stars with only two observed frequencies we are able to reduce the solution space of possible models. Comparing our model fits to the surface temperatures and luminosities derived from the observed colors and parallaxes, we find that the model fits are consistent with the cluster’s distance, i.e., the luminosities agree, but we discover that all of our models are systematically too cool.We attribute some of the discrepancy in the surface temperature to uncertainties in the surface boundary conditions of our models, but argue that most of the difference is a direct consequence of applying a single average color-dependent dereddening correction to all the stars when, in fact, it appears that the stars we selected are embedded in varying degrees of gas and dust. For one of the stars we identify a rotationally split l = 1 p-mode from which we derive a rotation period of 18 days.