X-Ray and Infrared Point Source Identification and Characteristics in the Embedded, Massive Star-Forming Region RCW 38

Abstract

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.

Near-infrared variability in the star-forming region RCW 38

“…A good reference for typical T Tauri variability patterns is given in Herbst et al (1994). Forty-seven percent of our variable sources have already been identified as candidate YSOs by previous studies using X-ray or mid-infrared (MIR) emission (Wolk et al 2006;Winston et al 2011), corroborating the potential of identifying YSOs via their variability. On the other hand, we suggest that the majority of the remaining 53% of variable sources are also YSO candidates that have escaped detection by classical methods.…”

“…DeRose et al (2009) also identify 314 cluster members from a total of 336 JHK detections in a field of 56 × 56 around IRS 2. A similar number of cluster stars were previously inferred from Chandra observations, covering a field of view (FoV) of about 16 × 16 ; Wolk et al (2006) associated 360 X-ray sources out of 460 detections with the star-forming region. Recently, Winston et al (2011) have presented Spitzer observations from a 30 × 30 field and identified 624 YSOs on the basis of infrared photometry from 2MASS (1.2−2.2 μm) and IRAC (3−8 μm).…”

“…Nevertheless, Wolk et al (2006) find an additional 31 candidate OB stars (spectral type earlier than B9) within our field of view, based on JHK data and using the extinction-corrected K luminosity function. Obviously, pre-main sequence stars and/or circumstellar dust emission (K-excess) may simulate a K-brightness that is several magnitudes higher than the corresponding main sequence brightness.…”

“…Apart from the described low-mass population, Wolk et al (2006) suggest the presence of 31 OB candidates in a field of ∼10 × 10 , while Winston et al (2011) even report identifying 27 further O and a total of 604 OB star candidates in a field of ∼30 × 30 . Both studies used the extinction-corrected K band luminosity function to infer stellar types earlier than B9.…”

“…RCW 38 is an area of high-mass star formation at a distance of 1.7 kpc; for an overview of early observations, we refer to a review by Wolk et al (2006). The HII region is excited by the O5.5 binary RCW 38 IRS 2 (Frogel & Persson 1974;DeRose et al 2009).…”