Asteroseismic analysis of the pre-main-sequence stars in NGC 2264

Abstract

NGC 2264 is a young open cluster lying above the Galactic plane in which six variable stars have previously been identified as possible pre-main-sequence (PMS) pulsators. Their oscillation spectra are relatively sparse with each star having from 2 to 12 unambiguous frequency identifications based on Microvariability and Oscillations of Stars satellite and multi-site ground-based photometry. We describe our efforts to find classical PMS stellar models (i.e., models evolved from the Hayashi track) whose oscillation spectra match the observed frequencies. We find model eigenspectra that match the observed frequencies and are consistent with the stars’ locations in the HR diagram for the three faintest of the six stars. Not all the frequencies found in spectra of the three brightest stars can be matched to classical PMS model spectra possibly because of effects not included in our PMS models such as chemical and angular momentum stratification in the outer layers of the star. All the oscillation spectra contain both radial and nonradial p-modes. We argue that the PMS pulsating stars divide into two groups depending on whether or not they have undergone complete mixing (i.e., have gone through a Hayashi phase). Lower mass stars that do evolve through a Hayashi phase have oscillation spectra predicted by classical PMS models, whereas more massive stars that do not, retain mass infall effects in their surface layers and are not well modeled by classical PMS models.