Chandra X-Ray Observations of Young Clusters. III. NGC 2264 and the Orion Flanking Fields

Chandra observations of solar-like pre-main-sequence (PMS) stars in the Orion flanking fields (FFs; age ~1 Myr) and NGC 2264 (~3 Myr) are compared with the results of the Chandra Orion Ultradeep Project survey of similar objects in the Orion Nebula cluster (ONC; ~0.5 Myr). The correlations between logL_X and mass found for PMS stars on convective tracks in these clusters are consistent with the relationships found for the ONC, indicating little change in the median values of either logL_X or logL_X/L_bol during the first ~3-5 Myr of evolution down convective tracks. The fraction of stars with extreme values of L_X, more than 10 times higher than the average for a given L_bol, or with logL_X/L_bol greater than the canonical saturation value of -2.9, is, however, larger by a factor of 2 in the younger ONC when compared with the Orion FFs and NGC 2264. PMS stars in NGC 2264 on radiative tracks have L_X/L_bol values that are systematically lower by a factor of ~10 than those found for stars of similar mass on convective tracks. The dramatic decrease in flux from convective to radiative phases of PMS evolution is likely related to major structural changes that influence the efficiency of magnetic field generation and thus the level of magnetic activity. As in the ONC, we find that stars with measured periods have, on average, higher X-ray luminosities. However, there is a wide range in L_X and L_X/L_bol for both periodic and nonperiodic stars of similar mass. Among stars with measured periods, the level of X-ray emission does not correlate with the rotation rate. For this data set, we find no statistically significant correlation between X-ray flux and (1) the presence or absence of circumstellar accretion disks or (2) disk accretion rates as assessed from ultraviolet excesses.