We present Spitzer and Chandra observations of the nearby ($260 pc) embedded stellar cluster in the Serpens cloud core. We observed, using Spitzer's IRAC and MIPS instruments, in six wavelength bands from 3 to 70 m, to detect thermal emission from circumstellar disks and protostellar envelopes and to classify stars using color-color diagrams and SEDs. These data are combined with Chandra observations to examine the effects of circumstellar disks on stellar X-ray properties. Young diskless stars were also identified from their increased X-ray emission. We have identified 138 YSOs in Serpens: 22 Class 0/ I, 16 flat-spectrum, 62 Class II, 17 transition disk, and 21 Class III stars; 60 of these exhibit X-ray emission. Our primary results are the following: (1) 10 protostars detected previously in the submillimeter are detected at k < 24 m, seven at k < 8 m;(2) the protostars are more closely grouped than more evolved YSOs (median protostar separation $0.024 pc); and (3) the luminosity and temperature of the X-rayYemitting plasma around these YSOs do not show any significant dependence on evolutionary class. We combine the infrared-derived values of A K and X-ray values of N H for eight Class III objects and find that the column density of hydrogen gas per magnitude of extinctions is less than half the standard interstellar value, for A K > 1. This may be the result of grain growth through coagulation and/or the accretion of volatiles in the Serpens cloud core.

We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded, and most massive young stellar clusters studied in X-rays: RCW 38. We detect 460 sources in the field, of which 360 are confirmed to be associated with the RCW 38 cluster. The cluster members range in luminosity from 10 30 to 10 33.5 ergs s À1 . Over 10% of the cluster members with over 100 counts exhibit flares, while about 15% of the cluster members with over 30 counts are variable. Of the sources identified as cluster members, 160 have near-infrared (NIR) counterparts either in the Two Micron All Sky Survey database or detected via Very Large Telescope observations. Of these, about 20% appear to have optically thick disks. An additional 353 members are identified through NIR observations, of which at least 50% possess optically thick disks. We fit over 100 X-ray sources as absorbed Raymond-Smith-type plasmas and find that the column to the cluster members varies from 10 21.5 to 10 23 cm À2 . We compare the gas to dust absorption signatures in these stars and find N H ¼ A V 2 ; 10 21 ð Þcm À2 . We find that the cluster contains 31 candidate OB stars and is centered about 10 00 (0.1 pc) west of the primary source of the ionization, the O5 star IRS 2. The cluster has a peak central density of about 400 X-ray sources pc À2 . We estimate that the total cluster membership exceeds 2000 stars.

We present spectral observations of 130 young stellar objects (YSOs) in the Serpens Cloud Core and NGC 1333 embedded clusters. The observations consist of near-IR spectra in the H and K-bands, from SpeX on the IRTF and far-red spectra (6000 -9000Å) from Hectospec on the MMT. These YSOs were identified in previous Spitzer and Chandra observations, and the evolutionary classes of the YSOs were determined from the Spitzer mid-IR photometry. With these spectra, we search for corroborating evidence for the pre-main sequence nature of the objects, study the properties of the detected emission lines as a function of evolutionary class, and obtain spectral types for the observed YSOs. The temperature implied by the spectral types are combined with luminosities determined from the near-IR photometry to construct HR diagrams for the clusters. By comparing the positions of the YSOs in the HR diagrams with the pre-main sequence tracks of Baraffe (1998), we determine ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks. The apparent isochronal ages of the YSOs in both clusters range from less than 1 Myr to 10 Myr, with most objects below 3 Myr. The observed distribution of ages for the Class II and Class III objects are statistically indistinguishable. We examine the spatial distribution and extinction of the YSOs as -2a function of their isochronal ages. We find the sources < 3 Myr to be concentrated in the molecular cloud gas while the older sources are spatially dispersed and are not deeply embedded. Nonetheless, the sources with isochronal ages > 3 Myr show all the characteristics of young stellar objects in their spectra, their IR spectral energy distributions, and their X-ray emission; we find no evidence that they are contaminating background giants or foreground dwarfs. However, we find no corresponding decrease in the fraction of sources with infrared excess with isochronal age; this suggests that the older isochronal ages may not measure the true age of the > 3 Myr YSOs. Thus, the nature of the apparently older sources and their implications for cluster formation remain unresolved.

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We present new Chandra X-ray data of the NGC 1333 embedded cluster and combine these data with existing Chandra data, Sptizer photometry and ground based spectroscopy of both the NGC 1333 and Serpens Cloud Core clusters to perform a detailed study of the X-ray properties of two of the nearest embedded clusters to the Sun.We first present new, deeper observations of NGC 1333 with Chandra ACIS-I and combine these with existing Spitzer observations of the region. In NGC 1333, a total of 95 cluster members are detected in X-rays, of which 54 were previously identified in the Spitzer data. Of the Spitzer identified sources, we detected 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of the Class II, and 50% of the Transition Disk young stellar objects (YSO). Forty-one -2 -Class III members of the cluster are identified, bringing the total identified YSO population to 178.The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters are compared to each other and the Orion Nebula Cluster. Based on a comparison of the XLFs of the Serpens and NGC 1333 clusters to the previously published ONC, we obtain a new distance for the Serpens cluster of 360 +22 −13 pc. Using our previously published spectral types, effective temperatures and bolometric luminosities, we analyze the dependence of the X-ray emission on the measured stellar properties. The X-ray luminosity was found to depend on the calculated bolometric luminosity as in previous studies of other clusters. We examine the dependence of L X on stellar surface area and effective temperature, and find that L X depends primarily on the stellar surface area. In the NGC 1333 cluster, the Class III sources have a somewhat higher X-ray luminosity for a given surface area. We also find evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types of M0 and K7, we speculate that this may result from the presence of radiative zones in the K-stars.The gas column density vs. extinction in the NGC 1333 parental molecular cloud was examined using the Hydrogen column density determined from the X-ray absorption to the embedded stars and the K-band extinction measured to those stars. In NGC 1333, we find N H = 0.89 ± 0.13 × 10 22 A K , this is lower than expected of the standard ISM but similar to that found previously in the Serpens Cloud Core.