Chandra Detection of an Evolved Population of Young Stars in Serpens South

Abstract

We present a Chandra study of the deeply embedded Serpens South starforming region, examining cluster structure and disk properties at the earliest stages. In total, 152 X-ray sources are detected. Combined with Spitzer and 2MASS photometry, 66 X-ray sources are reliably matched to an IR counterpart. We identify 21 class I, 6 flat spectrum, 16 class II, and 18 class III young stars; 5 were unclassified. Eighteen sources were variable in X-rays: 8 exhibiting flarelike emission, and one periodic source. The clusters X-ray luminosity distance was estimated: the best match was to the nearer distance of 260 pc for the front of the Aquila Rift complex. The N H vs. A K ratio is found to be ∼0.68x10 22 , similar to that measured in other young low mass regions, but lower than that measured in the ISM and high mass clusters (∼1.6-2x10 22 ). We find the spatial distribution closely follows that of the dense filament from which the stars have formed, with the class II population still strongly associated with the filament. There are four sub-clusters in the field, with three forming knots in the filament, and a fourth to the west, which may not be associated but may be contributing to the distributed class III population. A high percentage of diskless class IIIs ( upper limit 30% of classified X-ray sources) in such a young cluster could indicate that processing of disks is influenced by the cluster environment and is not solely time-scale dependent. the diskless class III stars (which do not exhibit excess emission in the IR) from field stars in combination with the IR photometry.In this paper we first discuss, in Sect. 2, the Chandra and Spitzer observations and the data reduction process. In Sect. 3 we will discuss the identification and evolutionary classification of the YSOs. A discusssion of the X-ray properties of the identified members, their spatial distribution, and the X-ray luminosity distance determination is presented in Sect. 4. Finally, a brief summary is presented in Sect. 5. Observations andData Reduction 2.1. Chandra ACIS-I Data The Serpens South cluster was observed by Chandra with the ACIS instrument on 7th June 2010, in a single epoch with a 97.49 ks/pix exposure time as part of OBSID 11013. The ACIS-I field was centered on the J2000 coordinates: 18 h 30 m 03 s , -02 o 01 ′ 58 ′′ , with a 17'×17' field of view. The observation was taken with ACIS-I in F AINT mode, and T E exposure mode with no grating, and included two of the ACIS-S chips adjacent to the ACIS-I array.The primary and secondary data were downloaded from the Chandra X-ray Center's archive and reprocessed with the CIAO v4.8 reproc tool to ensure that the latest CALDB 4.7.2 corrections were applied (Fruscione et al. 2006). The CIAO task f luximage was used to create an exposure map to correct for the photon energy dependent effective collecting area and the presence of chip gaps. Exposure maps were created to accurately represent the effective area across the imaging array. 1 The effective area is applied by CIAO to all further processing. ...

An Initial Overview of the Extent and Structure of Recent Star Formation within the Serpens Molecular Cloud Using Gaia Data Release 2

“…The similarity in velocities indicate that the entire region is located at the same distance (see discussion in Ortiz-León et al 2017). However, a recent analysis of the X-ray luminosity function of Serpens South placed the cluster at 260 pc (Winston et al 2018), consistent with the previous identificiation of an extinction wall at ∼ 250 pc to both Serpens Main and the Aquila Rift (Straižys et al 1996(Straižys et al , 2003. In the 3D structure of the Milky Way from the velocity analysis of Lallement et al (2014), the Aquila Rift is also placed at ∼ 250 pc.…”

“…For Serpens South and W40, very few sources that have been previously identified as candidate members can be evaluated with Gaia. The rejection rates from samples of disks and X-ray emitters (Kuhn et al 2010;Dunham et al 2015;Getman et al 2017;Winston et al 2018) with Gaia matches are 60%, while the rejection rate for Povich et al (2013) is 80%. However, many of the objects in this region are clustered in proper motion and are offset by 5 km s −1 from the proper motion centroid of the three main optical clusters.…”

Variability and Jet Activity in the YSO MHO 3252 Y3 in Serpens South

“…While this star is quite bright at short and mid-infrared wavelength, it does not show a long wavelenth excess and therefore was not listed as a Class 0, I, or II YSO by Gutermuth 2008. A search of the Chandra database showed that this star has not been detected at X-ray energies and it is not listed as a YSO in the X-ray selected list of (Winston et al 2018). The star is not associated with reflection nebulosity on deep UKIRT images, unlike many other of the YSOs in the Serpens South cluster.…”

A Census of Star Formation in the Outer Galaxy. II. The GLIMPSE360 Field

“…As we do not have ancillary data, such as X-ray observations, to separate young completely diskless Class III YSOs from field stars, we cannot reliably differentiate between the weak disk bearing Class IIa YSOs identified here and truly mid-IR diskless Class III YSOs (Winston et al 2011(Winston et al , 2018. For this reason, we list the class of all objects with slope α < −1.6 as being Class IIa/III in this paper.…”