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.

Keywords: infrared: stars; stars: early-type; stars: formation; stars: pre-main sequence.

Figure 1.

The entire W51 is shown in Spitzer IRAC bands (blue: 3.6 μm, green: 4.5 μm, red: 8.0 μm).

Figure 2.

Left: the entire W43 is shown in Spitzer IRAC bands. W43-Main and W43-South are shown within dashed boxes. Right: W43-Main is shown in the upper right panel and W43-South is shown in the lower right panel (blue: 3.6 μm, green: 4.5 μm, red: 8.0 μm).

Figure 3.

Color–color diagrams used to identify contaminant objects among the sources with detection at all four IRAC bands following the criteria in (Gutermuth et al. 2009). The background logarithmic gray-scale indicates the overall source density in each color–color space. The upper panels show the sources in W51, and the lower panels show the sources in W43. In panels (a), (b), (d), and (e) PAH galaxies are marked with green circles. Panels (c) and (f) show knots of shocked emission (orange circles) and PAH-contaminated sources (blue circles).

Figure 4.

Color–color and magnitude-color diagrams for the W51 region. Upper panel shows [4.5]−[5.8] vs. [3.6]−[4.5] and [4.5]−[8.0] vs. [3.6]−[5.8] IRAC color–color diagrams. Lower panel shows [3.6]−[8.0] vs. [3.6] and [3.6]−[4.5] vs. [3.6] IRAC magnitude-color diagrams. Red dots: class I, green: class II, blue: transition disk candidates, black: class III and photospheres, gray: AGB star candidates, orange triangles: background contaminants.

Figure 5.

Color–color and magnitude-color diagrams for W43 region. The upper panels show [4.5]−[5.8] vs. [3.6]−[4.5] and [4.5]−[8.0] vs. [3.6]−[5.8] IRAC color–color diagrams. The lower panels show [3.6]−[8.0] vs. [3.6] and [3.6]−[4.5] vs. [3.6] IRAC magnitude-color diagrams. Red dots: class I; green dots: class II; blue dots: transition disk candidates; black dots: class III and photospheres; gray dots: AGB star candidates; orange triangles: background contaminants.

Figure 6.

[4.5]−[24.0] vs. [3.6]−[4.5] IRAC color–color diagram for W51 (left) and W43 (right). Red dots: Class I; green dots: Class II; blue dots: transition disk candidates; black dots: Class III and photospheres; red asterisk: embedded protostars.

Figure 7.

Distribution of the SED spectral index α calculated from the original photometric data without correcting extinction in W51 (left panel) and W43 (right panel). The three vertical lines mark the division between the YSO regions based on their SED slopes (from left to right: Class III, Class II, Flat Spectrum, and Class I).

Figure 8.

Number of groups containing 10 or more stars identified by the MST algorithm. The left panel is plotted for the W51 region, and the right panel is plotted for the W43 region.

Figure 9.

Left: the nine MST groups identified using $d_c={82}_{·}^{″}5$ are shown with group numbers overlaid on the IRAC 4.5 μm image. Right: the 16 MST subgroups identified by d_c = 51″ are shown with a cluster number and a letter (see Table 6).

Figure 10.

K–S statistic distribution of random MST groups, between each of the 1000 random realizations and the mean random cluster distribution. The green histogram shows the groups identified with SLF branch length $d_c={82}_{·}^{″}5$ in the W51 region (left), and d_c = 81″ in the W43 region (right). The blue histogram shows the groups identified with branch length d_c = 51″ in the W51 region (left), and d_c = 59″ in the W43 region (right). Dashed lines in both figures represent the K–S statistic value between the mean CDF of all random clusters and the CDF of observed MST groups, with the same color code of histograms. For both regions, the inferred probability that the observed MST groups are drawn from a random distribution is less than 10⁻³.

Figure 11.

Left: the 51 MST groups determined using the SLF method with a branch length of 81″ (2.20 pc for objects at the distance of W43). Right: the 95 MST subgroups determined with a branch length of 59″ (1.61 pc). The YSOs are plotted in colors according to the identified clusters.

Figure 12.

YSO model SEDs that fit best (black line) to the data points (filled circles) in the W51 region. Gray lines show the fits satisfying $({\chi }^2-{\chi }_{\text{best}}^2)/n_{\text{data}}<3$ criteria. This figure shows those sources with 24 μm data listed in Table 10.

Figure 13.

YSO model SEDs that fit best (black line) to the data points (filled circles) in the W43 region. Gray lines show the fits satisfying $({\chi }^2-{\chi }_{\text{best}}^2)/n_{\text{data}}<3$ criteria. This figure shows those sources with 24 μm data listed in Table 11.

Figure 14.

Top: the distribution of MYSO candidates is shown in the RGB image (blue: 3.6 μm, green: 8.0 μm, red: 24.0 μm). White circles represent the MYSO candidates, and cyan circles represent the MYSO candidates with 24.0 μm detections. Bottom: RGB images of W51A (as shown by the dashed box in the upper panel) and W51B (as shown by the dashed box in the upper panel). Magenta diamonds show methanol masers, and black dashed boxes show the main H II regions. Subgroups of YSOs are shown with different colored circles, and MYSO candidates are shown with white circles. YSO candidates that do not belong to subgroups are not shown here.

Figure 15.

Distribution of MYSO candidates in W43 is shown in the left RGB image. The distribution is shown in the giant H II region in W43-Main on the right, where Motte et al. (2003) studied the compact sub-millimeter fragments that are shown with white Xs (blue: 3.6 μm, green: 8.0 μm, red: 24.0 μm). Subgroup 3 and subgroup 62 are shown in order to illustrate the positional correspondence with the sub-millimeter fragments. White circles represent the MYSO candidates, and cyan circles represent the MYSO candidates with 24.0 μm detections.