Ultraviolet and X-ray observations of NGC 2264.

Abstract

Pre-main-sequence stars in the galactic cluster NGC 2264 were observed with the IUE to search for ultraviolet chromospheric and transition region emission lines and with the Einstein Observatory to search for coronal X-ray emission. Fifteen cluster members (and four other stars) of spectral type B - K, ranging in age from 10⁶ to 10⁷yr, were observed in the ultraviolet. Chromospheric emission was detected in the faint Hα emission-line star W79 (NX Mon). The observations are consistent with the generally accepted idea that chromospheric activity of late-type stars subsides with age, but they are subject to uncertainties in circumstellar and interstellar extinction. Seven luminous X-ray sources (log L_x ⪆ 31.4) were found. Except for W189 and W107, the likely optical counterparts of the X-ray sources are inconspicuous stars fainter than 14th visual magnitude.