Young Stellar Objects in the Massive Star-forming Regions W51 and W43

Abstract

We present the results of our investigation of the star-forming complexes W51 and W43, two of the brightest in the first Galactic quadrant. In order to determine the young stellar object (YSO) populations in W51 and W43 we used color-magnitude relations based on Spitzer mid-infrared and 2MASS/UKIDSS near-infrared data. We identified 302 Class I YSOs and 1178 Class II/transition disk candidates in W51, and 917 Class I YSOs and 5187 Class II/transition disk candidates in W43. We also identified tens of groups of YSOs in both regions using the Minimal Spanning Tree (MST) method. We found similar cluster densities in both regions, even though Spitzer was not able to probe the densest part of W43. By using the Class II/I ratios, we traced the relative ages within the regions, and based on the morphology of the clusters, we argue that several sites of star formation are independent of one another in terms of their ages and physical conditions. We used spectral energy distribution-fitting to identify the massive YSO (MYSO) candidates since they play a vital role in the star formation process, and then examined them to see if they are related to any massive star formation tracers such as UCH II regions, masers, or dense fragments. We identified 17 MYSO candidates in W51, and 14 in W43, respectively, and found that groups of YSOs hosting MYSO candidates are positionally associated with H II regions in W51, though we do not see any MYSO candidates associated with previously identified massive dense fragments in W43.

Surveying the Giant H ii Regions of the Milky Way with SOFIA. I. W51A

“…Overall, we find 41 MYSO and potential MYSO candidates, many identified as such here for the first time. For sources SHA17 3 and SHA17 4, which were previously identified as potential MYSOs (Saral et al 2017), we find that with the added photometry at longer infrared wavelengths 1 , the absolute best fits yield masses of only 1 M , however we do have fits in the group of best fits that yield a stellar masses for these sources greater than 8 M .…”

“…It appears that there is a cluster of YSOs identified by Saral et al (2017) located interior to (or east of) this double arc (Figure 11), which we assume is most likely responsible for the shaping, heating, and ionizing source a. The four massive YSO candidates from Saral et al (2017) are shown in Figure 11, though we only detect sources in the SOFIA data at the locations of the sources labeled SHA17 3 and SHA17 4. (We will show in the section on SED model fitting that these sources are unlikely to be MYSOs).…”

A Census of Star Formation in the Outer Galaxy: The SMOG Field

“…They are within 1 kpc of the sun and can be studied in high resolution to substellar masses (Dunham et al 2015;Megeath et al 2016). More distant SFRs located towards the galactic center, such as W51 and W43 (Saral et al (2017), d∼6 kpc) or RCW 38 (Winston et al (2011), d∼1.7 kpc), may be the precursors for young massive clusters (YMCs; Bressert et al (2012); ). Spitzer observations of these regions can detect YSOs down to ∼0.5-1M , and are more similar to what we can achieve towards the outer galaxy.…”

Census of the Young Stellar Population in the Galactic H ii Region Sh2-242

“…In general, it is predicted from the spatial distribution of the YSOs and the Class I/II ratio that the S242 region is in its early stage of star formation. Comparing the ratio of Class I/II with the earlier reports (Chavarría et al 2008;Beerer et al 2010;Jose et al 2013;Saral et al 2017), the average age of the young members in the S242 region is estimated around 87.6°87.8°88.0°88.2°88.4°R…”