Embedded Young Stellar Object Candidates in the Active Star-Forming Complex W51: Mass Function and Spatial Distribution

Abstract

We present 737 candidate Young Stellar Objects (YSOs) near the W51 Giant Molecular Cloud (GMC) over an area of 1.25 • × 1.00 • selected from Spitzer Space Telescope data. We use spectral energy distribution (SED) fits to identify YSOs and distinguish them from main-sequence or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses > 5 M ⊙ (corresponding to mid-to early-B main-sequence spectral types) 46 of which are located in the central active star forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M ⊙ and 18 M ⊙ . The derived slopes of the MF are −1.26 and −2.36 in the active starforming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

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

“…This resulted in the identification of 737 YSO candidates. Although there is a slight difference between the cataloged region in Kang et al (2009) study and in this paper, they have 91% of the sources in common; and among them, 54% of the sources are YSO candidates in both studies. We compare our results with this earlier work in Section 2.3.1.…”

“…In Table 5 we summarize the final source classification results. Kang et al (2009) used Spitzer and 2MASS data and classified 737 YSO candidates in W51 by using a combination of color criteria (Simon et al 2007) and SED slope criteria (Lada 1991;André et al 1993;Greene et al 1994). The class identification for the slope of log(λF λ ) vs. log(λ) between 2 and ∼ 20 µm is:…”

“…However, Kang et al (2009) classified the sources based on their SED slopes of the four IRAC bands or the SED slope of log(λF λ ) vs. log(λ) between 2 and ∼ 24 µm, without a constraint on the number of data points. 17% (119/680) of Kang et al (2009) sources are classified as contaminants in our study, such as AGBs, PAH galaxies, PAH-dominated sources, or faint or possibly background sources (classified as YSOs in our first classification phase but then eliminated based on the Koenig & Leisawitz (2014) criteria). Finally, 12% (80/680) of the Kang et al (2009) YSOs were classified as photospheres in our catalog since they do not have the infrared color excess that is described above for Class I or Class II candidates.…”

“…There are studies based on maser emissions and velocity measurements showing the interaction between the star-forming region W51B and the supernova remnant W51C (i.e., Green et al 1997;Ginsburg et al 2015). Kang et al (2009) studied the YSO population of W51 by using Spitzer and 2MASS data and combining both the color selection criteria from Simon et al (2007) and the spectral energy distribution (SED) slope classification (Lada 1991;André et al 1993;Greene et al 1994), in addition to SED models from Robitaille et al (2006). This resulted in the identification of 737 YSO candidates.…”

Evidence for Delayed Massive Star Formation in the M17 Proto-Ob Association

“…For this mass range, the premain-sequence evolutionary tracks are horizontal (Bernasconi & Maeder 1996;Siess et al 2000), and YSOs follow the zero-age main-sequence mass-luminosity relation (Kang et al 2009). …”