Revealing the “missing” low-mass stars in the S254-S258 star forming region by deep X-ray imaging

Abstract

Context. X-ray observations provide a very good way to reveal the population of young stars in star forming regions avoiding the biases introduced when selecting samples based on infrared excess. Aims. The aim of this study was to find an explanation for the remarkable morphology of the central part of the S254-S258 star forming complex, where a dense embedded cluster of very young stellar objects (S255-IR) is sandwiched between the two H ii regions S255 and S257. This interesting configuration had led to different speculations such as dynamical ejection of the B-stars from the central cluster or triggered star formation in a cloud that was swept up in the collision zone between the two expanding H ii regions. The presence or absence, and the spatial distribution of low-mass stars associated with these B-stars can discriminate between the possible scenarios. Methods. We performed a deep Chandra X-ray observation of the S254-S258 region in order to efficiently discriminate young stars (with and without circumstellar matter) from the numerous older field stars in the area. Results. We detected 364 X-ray point sources in a 17 × 17 field (≈8× 8 pc). This X-ray catalog provides, for the first time, a complete sample of all young stars in the region down to ∼0.5 M . A clustering analysis identifies three significant clusters: the central embedded cluster S255-IR and two smaller clusterings in S256 and S258. Sixty-four X-ray sources can be classified as members in one of these clusters. After accounting for X-ray background contaminants, this implies that about 250 X-ray sources constitute a widely scattered population of young stars, distributed over the full field-of-view of our X-ray image. This distributed young stellar population is considerably larger than the previously known number of non-clustered young stars selected by infrared excesses. Comparison of the X-ray luminosity function with that of the Orion Nebula Cluster suggests a total population of ∼2000 young stars in the observed part of the S254-S258 region. Conclusions. The observed number of ∼250 X-ray detected distributed young stars agrees well with the expectation for the low-mass population associated to the B-stars in S255 and S257 as predicted by an IMF extrapolation. These results are consistent with the scenario that these two B-stars represent an earlier stellar population and that their expanding H ii regions have swept up the central cloud and trigger star formation (i.e. the central embedded cluster S255-IR) therein.

The nature of the high Galactic latitude O-star HD 93521: new results from X-ray and optical spectroscopy

“…By analogy with the two B0 stars investigated by Mucciarelli et al (2011), one expects about 300 low-mass pre-main sequence stars associated with HD 93521 if this stellar population follows a standard IMF 3 . We can now estimate what fraction of these low-mass objects would be detected with our X-ray observation and compare this to the actual number of sources in the field of view.…”

“…The brightness of very young low-mass stars in the X-ray domain makes this the ideal energy range to distinguish between foreground or background field stars and genuine cluster members coeval with the massive stars. This technique was successfully applied to reveal the population of low-mass pre-main sequence (PMS) stars associated with two rather isolated B0 stars in the diffuse H ii regions S 255 and S 257 (Mucciarelli et al 2011).…”

The Spatial Structure of Young Stellar Clusters. Ii. Total Young Stellar Populations

“…The probability distribution of L t,c , called the XLF, is associated with the IMF due to the statistical link between X-ray luminosity and stellar mass M (e.g., Feigelson et al 1993;Preibisch et al 2005a;Telleschi et al 2007). The assumption of a "universal XLF" has been used to estimate total populations in young stellar clusters, including Cep B (Getman et al 2006), M 17 (Broos et al 2007), NGC 6357 (Wang et al 2007), Rosette (Wang et al 2008(Wang et al , 2009(Wang et al , 2010, W 40 (Kuhn et al 2010), Trumpler 15 (Wang et al 2011), Trumpler 16 (Wolk et al 2011, Sh 2-254/255/256/257/258 (Mucciarelli et al 2011), NGC 1893(Caramazza et al 2012, and IC 1396 (Getman et al 2012). During PMS stellar evolution, there is a weak relation between X-ray luminosity and age (e.g., Preibisch & Feigelson 2005;Pandey et al 2014); however, L t,c does not rapidly decline during the first 5 Myr, unlike the rapid decrease in bolometric luminosity L bol during PMS evolution along the Hayashi track.…”

Sejong Open Cluster Survey (Sos) - V. The Active Star Forming Region Sh 2-255 - 257

“…Mucciarelli et al (2011) continued their search for lowmass YSOs in the quiescent phase, with a deep Chandra X-ray observation, and found that the total number of YSOs was consistent with that expected from the Kroupa IMF (Kroupa 2001) scaled to the num- ber of ionizing sources. On the other hand, sequential star formation scenarios within the SFR have been proposed (Howard et al 1997;Minier et al 2007;Chavarría et al 2008;Bieging et al 2009;Wang et al 2011;Mucciarelli et al 2011). Circumstantial evidence, such as the age difference among HII regions, and the number ratio of YSOs at different evolutionary stages, indicate that star formation activity propagated from Sh 2-255 and 257 into the molecular clouds behind the HII bubbles.…”

“…Excess emission at IR wavelengths, particularly the mid-IR, is a useful membership selection criterion because a large fraction of PMS stars in young open clus- PMS stars are also known as X-ray emitting objects (Flaccomio et al 1999;Caramazza et al 2012;Hur et al 2012;Sung et al 2013b;Lim et al 2014b;Hur et al 2015). Mucciarelli et al (2011) made deep X-ray observations of these SFRs down to 0.5 M ⊙ with the Chandra X-ray observatory. The observations covered a 17 ′ × 17 ′ field, and detected a total of 364 X-ray sources.…”