Pre-Main Stars. I. Light Variability, Shells, and Pulsation in NGC 2264

A light-variability study has been undertaken in the young cluster NGC 2264. Among the nonpulsating premain-sequence A and F stars, only about 25 percent have shown short-period light variability or light variations since 1953. The variables are generally faint for their color or are cooler stars. A link with the variabl cooler T Tauri stars is suggested. There exists a good correlation between light variability and (other) shell indicators. The variability data confirm previous suggestions that the fainter A/F stars are surrounded by thick shells and would be intrinsically much brighter. The general non- variability of the brighter A/F stars may be interpreted as an absence of thick circumstellar dust shells. The scatter in the pre-main-sequence band in the color-magnitude diagram can only partially be explained by shells, and an age spread between 1 and 3 X 108 years is indicated. Alternatively, should thick circumstellar shells be common, then these shells would have to be unusually stable. Shell expansion of less than 1 percent in radius within 15 years as well as timescales greater than 100 years between successive outbursts are required. W90, a star below the main sequence, has brightened by half a magnitude to V = 12.5 since 1953. The corresponding color variation suggests a ratio of total to selective extinction of about 9, somewhat incompatible with gray shell absorption and a proposed age of 2 X 106 years derived from the use of classical evolutionary tracks. Two Scuti variables with periods of 3 hours were detected. Arguments for cluster membership are given. If cluster membership is confirmed, then they would be the first known Cepheid-like pre-mainsequence pulsators. The periods are short compared with similar post-main-sequence stars. This could be interpreted either as an unusually high mass or as pulsation in a higher harmonic.