Detections of Massive Stars in the Cluster MCM2005b77, in the Star-forming Regions GRS G331.34−00.36 (S62) and GRS G337.92−00.48 (S36)

Massive stars form continuously in the Galactic disk, chemically enriching the interstellar medium by losing mass at high rates and by exploding. They are usually detected in groups or clusters of stars and make good tracers of Galactic structure (e.g., Georgelin & Georgelin 1976; Russeil 2003). More than 3000 candidate stellar clusters have been detected in large near-infrared and mid-infrared surveys of the Galactic plane (e.g., Dutra et al. 2003; Mercer et al. 2005; Froebrich et al. 2007; Glushkova et al. 2010; Borissova et al. 2011; Camargo et al. 2012; Solin et al. 2012). The most concentrated and populous candidate clusters are usually studied first—with about 20 young clusters known to be more massive than ≳10,000 M_⊙—but there is a plethora of sparse groups of stars or extended regions of increased stellar counts in the direction of nebulosities.

In the course of the Galactic Legacy Infrared Mid-plane Survey Extraordinaire (GLIMPSE), marvelous nebulae were detected along the Galactic plane, making it a "bubbling" environment (e.g., Watson et al. 2008; Churchwell et al. 2009; Watson et al. 2009). A typical bubble is marked by a shell of mid-infrared emission from dust and polycyclic aromatic hydrocarbon (PAH) bands (e.g., at 5.8 and 8.0 μm) surrounding a H ii region. Indeed, GLIMPSE bubbles are filled with radio continuum emission (e.g., Watson et al. 2008; Deharveng et al. 2010; Richards et al. 2012). The bubbles are created by winds, explosions, and UV radiation from massive stars (O-type and early B-type stars). To investigate their origin, it is of primary importance to study loose candidate clusters found in the direction of H ii regions, to detect the ionizing stars and to characterize the continuum photons and the associated bubbles. The types of the dominant stars determine the number of Lyman photons (N_lyc) of a system. About 90% of bubbles are expected to contain massive OB stars. Undoubtedly, the Westerlund 2 cluster sustains the RCW49 nebula (e.g., Povich et al. 2008), but, so far, massive stars have remained undetected in most bubbles. For several bubbles associated with H ii regions, candidate ionizing stars (photometrically identified) are listed in the works of, for example, Watson et al. (2008), Watson et al. (2009), and Sidorin et al. (2014). Interstellar extinction toward bubbles is highly patchy, and some candidate clusters may simply be zones of lower interstellar extinction (e.g., Dutra & Bica 2001; Gonzalez et al. 2012; Messineo et al. 2015). Typically, more than 50% of the detected candidate clusters are found to be spurious when followed up with spectroscopic studies of the brightest stars (e.g., Borissova et al. 2005; Froebrich et al. 2007; Messineo et al. 2014b).

In this paper, we report on low-resolution infrared spectroscopy of several stellar overdensities that are projected toward Galactic bubbles. The candidates MCM2005b72 (Mercer et al. 2005) and DBS2003−157 (Dutra et al. 2003) are both located in the direction of GRS G331.34−00.36 (S62; Churchwell et al. 2006; Culverhouse et al. 2011; Simpson et al. 2012). The candidate cluster DBS2003−172 (Dutra et al. 2003) is projected onto GRS G337.92−00.48 (S36; Churchwell et al. 2006; Culverhouse et al. 2011). The candidate cluster MCM2005b77 (Mercer et al. 2005) is located at $(l,b)=(332\buildrel{\circ}\over{.} 780,+00\buildrel{\circ}\over{.} 022)$ near the H ii region (and far-infrared source) IRAS 16137−5025. The spectroscopic observations are described in Section 2 and the data analysis in Section 3. In Sections 4 and 5, we review the properties of the confirmed stellar clusters and massive stars. A summary is provided in Section 6.

Targets were selected among candidate clusters identified in GLIMPSE and Two Micron All Sky Survey (2MASS) data, with bright near-infrared stars (e.g., Dutra et al. 2003; Mercer et al. 2005), observable with the Son of ISAAC (SofI) spectrograph (see Table 1 and Figure 1), and possibly located in the direction of bubble H ii regions.

Zoom In Zoom Out Reset image size

Observations were taken with the SofI spectrograph on the ESO NTT/La Silla Telescope, under program 089.D-876(A) (P.I. Messineo), over three nights, from 2012 May 31 to June 2.

The spectroscopic observations were carried out using a 1'' × 290'' slit. Typically, the slit orientation was adjusted to observe two targets per slit. For every target, a spectrum was taken with the high-resolution grism and the K_s filter, at a resolving power R ∼ 2200. At least four exposures per target were taken in an ABBA cycle, nodding along the slit.

Data reduction was performed with IDL scripts and with the Image Reduction and Analysis Facility (IRAF⁵ ) software. Exposures close in time were subtracted from one another and flat-fielded with spectroscopic lamp flats. Up to five stellar traces were extracted in a single exposure. Atmospheric transmission and instrumental response curves were obtained with spectra of B-type stars observed in the same manner as the science targets, typically within a variation of 0.2 in air mass. Stellar lines (Br_γ and He i) were removed from the standard spectra with a linear interpolation, and the resulting spectra were multiplied by blackbody curves at the stellar effective temperatures.

Positions and spectral types of stars observed in GRS G331.34−00.36 (S62), MCM2005b77, and GRS G337.92−00.48 (S36) are listed in Tables 2–4. Spectra are shown in Figures 2 and 11.

Table 4.  List of Late-type Stars in the Direction of the Candidate Stellar Clusters MCM2005b72/DBS2003−157 (GRS G331.34−0.36), MCM2005b77, and DBS2003−172 (GRS G337.92−00.48)

Field	ID	α(J2000)	δ(J2000)	EW(CO)	Sp(RGB)	KS	${A}_{{K}_{S}}$	BCk
		[hh mm ss]	[° ' '']	Å		[mag]	[mag]	[mag]
G331.34−0.36	9	16 12 34.94	−51 49 01.5	20.54 ± 1.39	M6	5.79 ± 0.02	0.67 ± 0.02	3.04
G331.34−0.36	10	16 12 25.17	−51 55 56.4	15.20 ± 3.18	M1	6.60 ± 0.02	0.95 ± 0.01	2.73
G331.34−0.36	11	16 12 40.06	−51 45 29.5	20.69 ± 1.65	M6	7.21 ± 0.03	0.62 ± 0.02	3.04
G331.34−0.36	12	16 12 36.47	−51 50 19.3	19.76 ± 2.12	M5	7.44 ± 0.03	0.83 ± 0.02	2.96
G331.34−0.36	13	16 12 33.00	−51 45 19.9	22.90 ± 2.17	M7	8.65 ± 0.02	2.10 ± 0.02	3.13
G331.34−0.36	14	16 12 02.22	−51 49 25.1	10.02 ± 1.62	K1	8.91 ± 0.03	0.17 ± 0.02	2.45
G331.34−0.36	15	16 12 42.35	−51 44 23.6	04.82 ± 1.00	≲K0	9.00 ± 0.02	0.14 ± 0.02	2.40
G331.34−0.36	16	16 12 27.95	−51 46 35.7	13.77 ± 1.36	K5	9.21 ± 0.03	0.62 ± 0.02	2.64
G331.34−0.36	17	16 12 37.14	−51 47 07.6	14.14 ± 0.37	K5	10.00 ± 0.03	0.22 ± 0.02	2.64
MCM2005b77	13	16 17 07.51	−50 31 06.4	27.17 ± 0.88	>M7 Mira	5.32 ± 0.03	0.65 ± 0.02	3.17
MCM2005b77	14	16 17 27.40	−50 29 46.5	31.52 ± 1.25	>M7 Mira	6.61 ± 0.03	1.26 ± 0.02	3.17
MCM2005b77	15	16 17 42.12	−50 30 03.3	20.03 ± 1.24	M5	6.79 ± 0.02	0.69 ± 0.02	2.96
MCM2005b77	16	16 17 50.70	−50 27 53.6	20.51 ± 1.22	M6	6.96 ± 0.02	0.35 ± 0.02	3.04
MCM2005b77	17	16 17 08.65	−50 27 36.0	20.38 ± 1.19	M6	7.04 ± 0.02	0.82 ± 0.02	3.04
MCM2005b77	18a	16 17 06.87	−50 30 45.6	11.06 ± 0.47	K2	7.21 ± 0.03	0.18 ± 0.02	2.50
MCM2005b77	19	16 17 28.15	−50 33 40.4	20.31 ± 1.60	M6	7.29 ± 0.02	1.12 ± 0.02	3.04
MCM2005b77	20	16 17 03.38	−50 28 33.4	13.22 ± 1.05	K5	7.34 ± 0.02	0.26 ± 0.01	2.64
MCM2005b77	21	16 17 19.95	−50 27 59.2	21.32 ± 1.58	M7	7.48 ± 0.02	0.83 ± 0.02	3.13
MCM2005b77	22b	16 17 25.79	−50 27 20.8	16.48 ± 0.37	M2 Mira	7.59 ± 0.03	3.60 ± 0.05	2.80
MCM2005b77	23	16 17 45.95	−50 34 05.1	19.78 ± 1.41	M5	7.61 ± 0.03	0.88 ± 0.02	2.96
MCM2005b77	24	16 17 10.36	−50 33 59.3	21.67 ± 1.53	M7	7.83 ± 0.02	1.26 ± 0.02	3.13
MCM2005b77	25	16 17 21.30	−50 26 43.4	16.54 ± 1.43	M2	7.99 ± 0.02	1.10 ± 0.02	2.80
MCM2005b77	26	16 17 26.00	−50 30 23.0	09.77 ± 1.03	K1	10.51 ± 0.03	0.17 ± 0.04	2.45
MCM2005b77	27	16 17 29.89	−50 32 41.9	10.86 ± 1.87	K2	10.70 ± 0.03	0.60 ± 0.02	2.50
MCM2005b77	28	16 17 23.97	−50 30 47.3	10.15 ± 1.03	K2	11.00 ± 0.03	0.31 ± 0.02	2.50
MCM2005b77	29	16 17 36.20	−50 30 34.0	15.84 ± 0.27	M1	11.39 ± 0.03	1.28 ± 0.03	2.73
MCM2005b77	30	16 17 23.92	−50 31 26.8	20.99 ± 1.13	M6	11.63 ± 0.05	1.30 ± 0.05	3.04
DBS2003−172	7	16 41 08.56	−47 08 25.7	04.32 ± 2.36	≲K0	8.51 ± 0.02	0.08 ± 0.02	2.40
DBS2003−172	8	16 41 06.24	−47 10 03.7	21.36 ± 0.50	M7	8.63 ± 0.02	2.54 ± 0.02	3.13
DBS2003−172	9	16 41 06.13	−47 11 14.8	07.49 ± 1.18	≲K0	9.07 ± 0.03	0.18 ± 0.02	2.40
DBS2003−172	10	16 41 06.02	−47 07 32.4	21.76 ± 0.20	M7	9.49 ± 0.80	2.16 ± 0.44	3.13
DBS2003−172	11	16 41 07.70	−47 08 15.3	23.63 ± 3.99	M7	10.45 ± 0.02	3.13 ± 0.04	3.13

Notes.

^aMCM2005b77-18 = CD-50 10319. ^bMCM2005b77-22 = IRAS 16136−5020.

Download table as:  ASCIITypeset image

3.1. Spectral Classification of Early-type Stars

Spectral classification was performed by comparing the shapes of detected spectral lines from H, He, and other heavier atoms (e.g., C, Na, Mg, and Fe) with those in atlases of known early-type stars (e.g., Hanson et al. 1996; Morris et al. 1996; Hanson et al. 2005).

G331.34−0.36 (S62): In the direction of the H ii region G331.34−0.36, we detected two B0-5 stars, one massive O4-6 star, and five other early types with less well determined spectral types. In Figure 2, stars #2 and #5 show the He i line at 2.1126 μm and Br_γ in absorption, as seen in the spectra of B0-5 stars. It could be argued that there is a possible He ii line at 2.1891 μm present in the spectrum of star #5 (O9-B0). The spectrum of the O4-6 star #3 shows two C iv lines (2.0705 and 2.0796 μm), N iii/C iii emission at ∼2.115 μm, the Br_γ line in absorption, and the He ii line at 2.1891 μm in absorption. The spectra of stars #1, #4, #6, #7, and #8 show only Br_γ in absorption; their spectral types may therefore range from O to F types. For stars #3 (O6V), #5 (O9V), and #6 (B0), spectra in the bands J, H, and K were acquired and analyzed by Pinheiro et al. (2012). By having added the detection of C iv lines, we confirm that star #3 is an early O-type star.

Zoom In Zoom Out Reset image size

The feature at 2.183 μm that appears in some spectra is a glitch, at the juncture of the lower and upper quadrants (pixels 510–512) of the SofI 1024 × 1024 array.

MCM2005b77: In the cluster MCM2005b77, the spectra of stars #1, #3, #4, #5, #6, #7, #8, #9, and #11 are characterized by Br_γ lines in absorption, He i at 2.1126 μm in absorption, and possible He i at 2.0587 μm. The latter He i line appears in emission in the spectra of stars #1, #4, and #5, indicating a supergiant luminosity class (Hanson et al. 1996; Davies et al. 2012).

The spectra of stars #2 and #10 show only Br_γ in absorption, which is compatible with spectral types from B to F. In Section 4.2, we refine their spectral types photometrically to a G-F and a B0-5 star, respectively. The spectrum of star #12 has a strong Br_γ line in emission.

DBS2003−172 in GRS G337.92−00.48 (S36): We detected seven early-type stars in the direction of DBS2003−172. The spectra of stars #1, #2, #4, and #6 display only the Br_γ line in absorption. Their spectral types are therefore poorly constrained and could range from O to F. The spectrum of star #3 shows a strong Br_γ line in emission.

On the spectroscopic exposure, two stellar traces are seen at the location of 2MASS J16410805−4706466 (stars #5A and #5B). Indeed, in the SofI acquisition image taken with the filter NB_2.248 (0.030 μm wide) and shown in Figure 3, there are two stars, separated by about 14 (5 pixels), which were unresolved in 2MASS images. We extracted two spectra from the rectified traces, using narrow apertures of 2 pixels centered on the two maxima. Knots of the He i line at 2.058 μm and Br_γ are clearly seen in the traces of both components, but appear to be stronger in the B component (the source with weaker continuum). Diffuse nebular emission was also detected. For the B component, the ratio between the equivalent width (EW) of the He i line at 2.058 μm and that of the Br_γ line is found to be equal to ∼1.2 (with no extinction correction). For the A component, this ratio is ∼0.9. The B component displays an additional weak He i line at 2.112 μm, [Fe iii] at 2.2178 μm, and possible [Fe ii] at 2.241 μm (see Table 3).

Zoom In Zoom Out Reset image size

3.2. Spectral Classification of Late-type Stars

3.3. Available Photometry

We searched for counterparts in the near-infrared catalogs from 2MASS (Skrutskie et al. 2006) and the Deep Near-infrared Survey (DENIS; Epchtein et al. 1999), using a search radius of 20. Matches with 2MASS data points were visually checked by superimposing 2MASS data points on the acquisition charts (astrometrically aligned to 2MASS).

We also searched the mid-infrared catalogs of the GLIMPSE (Benjamin et al. 2003), the Midcourse Space Experiment (MSX; Egan et al. 2003; Price et al. 2001), and the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010), using search radii of 20, 50, and 20, respectively. We searched for BVR measurements in the Naval Observatory Merged Astrometric Data set (NOMAD) by Zacharias et al. (2005) and for data in the second release (DR2) of the GAIA catalog (Gaia Collaboration 2018).

For all but three detected stars, infrared charts (provided in the Appendix) were available from the VISTA Variables in the Via Lactea survey (VVV; e.g., Soto et al. 2013). VVV K_s-band sources are in the regime of linearity up to ≈11.5 mag. Detected targets have K_s from 5.57 to 11.63 mag, so 2MASS measurements are adopted for most of the targets.

In Tables 5 and 6, we show the collected photometric measurements of the early-type and late-type stars detected in GRS G331.34−0.36, MCM2005b77, and GRS G337.92−00.48.

Table 5.  Photometric Data of Early-type Stars in MCM2005b72/DBS2003−157, MCM2005b77, and DBS2003−172

		2MASS			DENIS			GLIMPSE				MSX	WISE		NOMAD			GAIA
Field	ID	J	H	KS	I	J	KS	[3.6]	[4.5]	[5.8]	[8.0]	A	W1	W2	B	V	R	pmRa	pmDec	G	plx	var
		[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mas yr−1]	[mas yr−1]	[mag]	[mas]
G331.34−0.36	1	10.02	9.08	8.67	12.68	10.02	8.61	8.38	8.32	8.18	8.25	⋯	8.36	8.25	⋯	16.28	⋯	−4.15 ± 0.16	−3.84 ± 0.10	14.26	−0.11 ± 0.10	0
G331.34−0.36	2	9.74	9.18	8.89	11.70	9.71	8.87	8.72	8.63	8.60	8.83	⋯	8.63	8.48	15.88	14.30	12.82	−3.76 ± 0.61	−1.39 ± 0.42	13.13	0.07 ± 0.36	0
G331.34−0.36	3	11.70	10.27	9.57	16.67	11.62	9.47	9.01	8.88	8.80	8.87	⋯	⋯	⋯	⋯	⋯	⋯	−2.44 ± 0.42	−2.93 ± 0.28	17.87	−0.15 ± 0.28	0
G331.34−0.36	4	11.20	10.14	9.72	14.14	11.24	9.65	9.36	9.32	9.28	⋯	⋯	9.29	9.03	15.74	15.17	⋯	−3.94 ± 0.19	−4.45 ± 0.13	15.77	−0.06 ± 0.13	0
G331.34−0.36	5a	11.78	10.60	9.99	15.84	11.75	9.98	9.53	9.48	9.48	⋯	⋯	9.35	8.91	17.53	⋯	17.35	−3.98 ± 0.36	−3.50 ± 0.21	17.45	−0.12 ± 0.21	0
G331.34−0.36	6a	11.73	10.69	10.15	15.36	11.75	10.07	9.76	9.51	9.17	7.46	⋯	9.45	9.29	⋯	⋯	19.10	−4.19 ± 0.26	−3.68 ± 0.16	16.84	0.08 ± 0.17	0
G331.34−0.36	7b a	13.64	11.68	10.61	⋯	13.47	10.65	⋯	⋯	⋯	⋯	2.35	7.89	6.90	⋯	⋯	⋯	−1.21 ± 4.14	−4.24 ± 3.75	20.90	−0.44 ± 2.29	0
G331.34−0.36	8	11.61	11.26	11.08	12.62	11.59	11.05	10.96	10.79	10.57	⋯	⋯	10.93	10.57	14.34	13.72	13.83	−2.88 ± 0.05	−3.84 ± 0.03	13.51	0.51 ± 0.03	0
MCM2005b77	1	7.99	7.03	6.49	11.39	7.91	6.46	6.74	⋯	5.96	5.93	5.86	6.20	5.92	18.63	16.22	13.14	−3.08 ± 0.20	−5.18 ± 0.14	12.86	1.02 ± 0.15	0
MCM2005b77	2	12.89	9.51	7.80	⋯	12.83	7.78	6.82	⋯	5.91	6.04	5.92	6.69	6.07	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
MCM2005b77	3	10.24	8.91	8.22	14.85	10.15	8.14	7.61	7.52	7.42	7.39	⋯	7.41	7.20	⋯	⋯	17.30	−3.49 ± 0.30	−4.34 ± 0.20	16.30	0.84 ± 0.19	0
MCM2005b77	4	9.81	8.90	8.41	13.12	9.73	8.30	8.03	7.94	7.82	7.86	⋯	8.00	7.85	⋯	⋯	⋯	−3.94 ± 0.19	−3.95 ± 0.13	14.67	0.51 ± 0.12	0
MCM2005b77	5	9.99	9.09	8.59	13.19	10.01	8.55	8.17	8.03	7.89	7.96	⋯	8.11	7.94	16.20	15.78	14.92	−4.25 ± 0.20	−4.16 ± 0.13	14.85	0.39 ± 0.13	0
MCM2005b77	6	10.40	9.26	8.71	14.49	10.40	8.70	8.33	8.13	7.99	8.05	⋯	8.24	7.98	⋯	⋯	17.71	−4.47 ± 0.24	−4.03 ± 0.16	15.89	0.25 ± 0.16	0
MCM2005b77	7	10.52	9.49	8.99	14.09	10.54	8.95	8.59	8.54	8.36	8.43	⋯	8.62	8.46	⋯	⋯	16.79	−4.40 ± 0.20	−4.05 ± 0.13	15.57	0.40 ± 0.14	0
MCM2005b77	8	10.57	9.71	9.30	13.44	10.49	9.33	8.99	8.92	8.85	8.97	⋯	9.02	8.95	18.08	17.71	15.37	−4.51 ± 0.16	−3.76 ± 0.10	14.97	0.54 ± 0.10	0
MCM2005b77	9	10.80	9.87	9.43	13.91	10.81	9.36	9.04	8.94	8.88	8.80	⋯	8.72	8.49	16.11	15.40	⋯	−4.38 ± 0.20	−3.89 ± 0.13	15.74	0.39 ± 0.13	0
MCM2005b77	10	11.69	10.68	10.13	15.39	11.78	10.14	9.85	9.73	9.69	9.78	⋯	⋯	⋯	17.83	⋯	16.90	−7.73 ± 0.54	−7.14 ± 0.32	17.06	−0.54 ± 0.33	0
MCM2005b77	11	11.94	10.82	10.28	⋯	11.96	10.22	9.89	9.74	9.66	9.80	⋯	⋯	⋯	⋯	⋯	⋯	−4.53 ± 0.33	−4.09 ± 0.20	17.54	0.29 ± 0.21	0
MCM2005b77	12	14.34	12.31	10.97	⋯	14.36	10.94	9.41	8.93	8.48	8.27	⋯	9.56	8.83	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
DBS2003−172	1	10.45	9.25	8.55	14.59	10.25	8.58	8.33	8.15	8.09	⋯	⋯	7.77	7.14	⋯	⋯	17.97	−1.00 ± 0.46	−2.86 ± 0.34	16.40	0.13 ± 0.27	0
DBS2003−172	2	10.45	9.43	8.95	13.90	10.44	8.91	8.60	8.41	⋯	⋯	⋯	⋯	⋯	⋯	⋯	16.58	−1.30 ± 0.21	−3.59 ± 0.18	15.32	−0.43 ± 0.14	0
DBS2003−172	3	11.63	10.38	9.59	15.42	11.50	9.64	8.89	8.53	8.08	⋯	⋯	⋯	⋯	⋯	⋯	19.65	−1.79 ± 0.34	−2.97 ± 0.24	17.05	0.01 ± 0.20	0
DBS2003−172	4	11.46	10.45	9.98	14.99	11.50	9.95	9.70	9.55	9.51	9.47	⋯	⋯	⋯	⋯	⋯	18.74	−1.61 ± 0.27	−3.15 ± 0.21	16.45	0.36 ± 0.18	0
DBS2003−172	5Ab	14.26	11.75	10.78	⋯	12.99	9.42	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	1
DBS2003−172	5Bb	⋯	13.49	11.11	⋯	12.99	9.42	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	1
DBS2003−172	6	11.65	11.25	11.05	12.84	11.76	11.06	10.98	10.88	10.82	⋯	⋯	⋯	⋯	14.99	13.82	13.79	−3.34 ± 0.05	−2.40 ± 0.04	13.84	0.41 ± 0.03	0

Notes.

^a2MASS upper limits were replaced with photometric measurements from Pinheiro et al. (2012). (VVV magnitudes were above linearity.) ^bMagnitudes are estimated from VVV images. The H magnitude of 5 A is not in the linear regime (within 0.4 mag). The K_s magnitudes are also above linearity. However, estimates from the SofI acquisition image yielded values within 0.12 mag. (a) Star #6 in GRS G331.34−0.36 (2MASS J16123324−5146173) was previously photometrically classified as a young stellar object (Robitaille et al. 2008). The detection of the Br_γ line in absorption indicates a later stage (a dwarf).

Download table as:  ASCIITypeset image

Table 6.  Photometric Data of Late-type Stars in MCM2005b72/DBS2003−157, MCM2005b77, and DBS2003−172

		2MASS			DENIS			GLIMPSE				MSX	WISE		NOMAD			GAIA
Field	ID	J	H	KS	I	J	KS	[3.6]	[4.5]	[5.8]	[8.0]	A	W1	W2	B	V	R	pmRa	pmDec	G	plx	var
		[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[mas yr−1]	[mas yr−1]	[mag]	[mas]
G331.34−0.36	9	7.82	6.39	5.79	11.49	7.12	4.92	5.51	6.13	5.34	5.37	⋯	5.41	5.36	19.56	⋯	14.72	−1.38 ± 0.19	−4.23 ± 0.13	12.98	0.29 ± 0.13	0
G331.34−0.36	10	8.99	7.28	6.60	13.43	9.02	6.69	6.66	6.35	6.05	6.11	⋯	6.18	6.21	⋯	⋯	⋯	−3.04 ± 0.21	−2.69 ± 0.14	14.88	−0.09 ± 0.13	0
G331.34−0.36	11	9.32	7.83	7.21	13.02	9.31	7.22	6.92	7.01	6.76	6.83	⋯	6.82	6.97	⋯	14.99	⋯	−3.25 ± 0.21	−1.80 ± 0.14	14.65	−0.15 ± 0.14	0
G331.34−0.36	12	9.77	8.15	7.44	14.07	9.76	7.45	7.07	7.18	6.95	6.97	⋯	7.04	7.13	⋯	⋯	18.92	−4.88 ± 0.23	−5.33 ± 0.16	15.46	−0.24 ± 0.15	0
G331.34−0.36	13	13.56	10.19	8.65	⋯	13.50	8.61	7.57	7.59	7.21	7.26	⋯	7.68	7.53	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
G331.34−0.36	14	9.80	9.16	8.91	10.98	9.77	8.81	8.77	8.90	8.75	8.76	⋯	8.54	8.66	13.54	12.55	11.96	−6.83 ± 0.06	−2.96 ± 0.04	12.02	0.81 ± 0.04	0
G331.34−0.36	15	9.84	9.19	9.00	10.99	9.80	8.94	8.88	8.97	8.89	⋯	⋯	8.73	8.70	13.57	12.50	11.87	−2.85 ± 0.07	−5.64 ± 0.05	12.02	0.70 ± 0.04	0
G331.34−0.36	16	10.98	9.72	9.21	13.98	10.95	9.19	8.98	9.02	8.75	⋯	⋯	8.74	8.72	19.91	⋯	16.21	−3.16 ± 0.17	−3.91 ± 0.12	15.55	0.07 ± 0.12	0
G331.34−0.36	17	11.02	10.27	10.00	12.52	11.07	9.98	9.88	9.94	9.79	9.74	⋯	9.94	9.94	15.53	14.11	13.54	−1.27 ± 0.05	−4.55 ± 0.03	13.70	0.55 ± 0.03	0
MCM2005b77	13	7.60	6.17	5.32	12.80	8.09	4.63	⋯	⋯	4.07	4.02	3.32	4.46	3.52	14.97	14.99	14.56	−3.68 ± 0.28	−3.40 ± 0.19	14.23	0.35 ± 0.19	1
MCM2005b77	14	10.15	7.83	6.61	⋯	10.18	6.65	⋯	5.82	5.17	4.96	5.17	5.95	5.18	⋯	⋯	⋯	−4.93 ± 0.76	−5.47 ± 0.47	19.08	0.27 ± 0.50	0
MCM2005b77	15	8.85	7.43	6.79	12.69	8.77	6.72	6.67	6.62	6.36	6.29	⋯	6.45	6.55	⋯	⋯	16.15	−1.84 ± 0.22	−0.19 ± 0.15	14.11	0.53 ± 0.14	0
MCM2005b77	16	8.39	7.38	6.96	10.77	8.39	6.94	6.87	7.02	6.76	6.71	⋯	6.60	6.83	14.87	13.85	⋯	−5.50 ± 0.15	−2.46 ± 0.11	12.31	0.72 ± 0.10	0
MCM2005b77	17	9.37	7.76	7.05	13.66	9.26	7.14	6.90	6.93	6.64	6.57	6.02	6.74	6.85	16.33	⋯	16.82	−8.96 ± 0.23	−7.50 ± 0.15	14.99	0.47 ± 0.15	0
MCM2005b77	18	8.13	7.44	7.21	9.49	8.03	7.21	7.18	7.24	7.13	7.12	⋯	7.12	7.28	12.99	11.22	10.34	−2.75 ± 0.07	−0.19 ± 0.04	10.36	1.03 ± 0.04	0
MCM2005b77	19	10.16	8.13	7.29	15.63	10.13	7.35	6.69	6.95	6.62	6.64	⋯	6.72	6.80	⋯	⋯	⋯	−5.22 ± 0.35	−5.04 ± 0.20	16.85	0.44 ± 0.21	0
MCM2005b77	20	8.46	7.64	7.34	10.07	8.50	7.38	7.22	7.34	7.19	7.11	⋯	7.09	7.19	13.29	12.45	11.20	−2.33 ± 0.10	−1.82 ± 0.07	11.18	0.39 ± 0.07	0
MCM2005b77	21	9.99	8.24	7.48	14.71	9.99	7.43	7.04	7.26	6.93	6.89	⋯	6.92	6.95	⋯	⋯	⋯	−3.75 ± 0.27	−3.96 ± 0.17	16.01	0.61 ± 0.17	0
MCM2005b77	22	14.92	10.45	7.59	⋯	12.02	5.70	4.88	⋯	⋯	⋯	2.54	5.32	3.40	⋯	⋯	⋯	⋯	⋯	⋯	⋯	1
MCM2005b77	23	10.04	8.31	7.61	14.43	10.02	7.58	7.08	7.33	7.04	7.02	⋯	7.07	7.14	⋯	⋯	⋯	−1.81 ± 0.25	−3.26 ± 0.16	15.90	0.52 ± 0.15	0
MCM2005b77	24	11.14	8.84	7.83	⋯	10.98	7.80	7.12	7.27	7.02	6.97	⋯	7.28	7.28	⋯	⋯	⋯	−4.39 ± 0.69	−5.30 ± 0.42	18.67	0.97 ± 0.48	0
MCM2005b77	25	10.65	8.79	7.99	15.96	10.63	7.94	7.51	7.57	7.34	7.35	⋯	7.48	7.52	⋯	⋯	⋯	−3.32 ± 0.32	−3.10 ± 0.21	17.16	0.45 ± 0.21	0
MCM2005b77	26	11.40	10.77	10.51	12.92	11.43	10.43	10.38	10.42	10.28	10.05	⋯	⋯	⋯	14.99	14.36	13.19	−1.28 ± 0.04	−3.14 ± 0.03	14.07	0.39 ± 0.03	0
MCM2005b77	27	12.40	11.19	10.70	15.73	12.45	10.67	10.29	10.31	⋯	⋯	⋯	⋯	⋯	⋯	⋯	18.21	−4.40 ± 0.24	−3.89 ± 0.16	17.25	0.18 ± 0.16	0
MCM2005b77	28	12.16	11.29	11.00	14.00	12.15	11.01	10.82	10.86	10.88	10.93	⋯	10.68	10.59	⋯	15.61	⋯	−0.45 ± 0.08	−1.09 ± 0.05	15.33	0.29 ± 0.05	0
MCM2005b77	29	14.40	12.29	11.39	⋯	14.41	11.32	10.72	10.70	10.50	10.64	⋯	10.66	10.57	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
MCM2005b77	30	15.01	12.71	11.63	⋯	15.05	11.72	10.81	10.96	10.64	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
DBS2003−172	7	9.24	8.65	8.51	10.19	9.19	8.54	8.40	8.44	8.41	8.52	⋯	⋯	⋯	12.48	11.77	10.81	2.21 ± 0.08	1.27 ± 0.07	11.09	0.97 ± 0.04	0
DBS2003−172	8	14.32	10.42	8.63	⋯	14.11	8.59	7.46	7.45	7.00	7.00	⋯	7.86	7.47	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
DBS2003−172	9	10.00	9.36	9.07	11.42	9.91	9.09	8.94	9.04	8.95	8.82	⋯	8.81	8.84	13.91	12.84	11.99	−1.20 ± 0.07	−2.72 ± 0.06	12.57	0.33 ± 0.04	0
DBS2003−172	10	14.47	11.10	9.49	⋯	14.18	9.54	8.39	8.34	7.78	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0
DBS2003−172	11*	17.36	12.76	10.45	⋯	⋯	10.42	8.86	8.66	7.94	⋯	⋯	8.69	7.54	⋯	⋯	⋯	⋯	⋯	⋯	⋯	0

Note. (a) For the observed late-type stars, a few alias names are found in the SIMBAD database. MCM2005b77-18 coincides with CD-50−10319 and MCM2005b77-22 with IRAS 16136−5020. MCM2005b77-22 is listed as a possible carbon star by Skiff (2014). (**) Proper motion values are from GAIA DR2.

Download table as:  ASCIITypeset image

3.3.1. PSF Photometry from VVV Images

For fields GRS G331.34−0.36 (MCM2005b72/DBS2003−157), MCM2005b77, and GRS G337.92−00.48 (DBS2003−172), photometric catalogs of fainter stars were extracted from VVV images using the point-spread function (PSF) fitting routines by Stetson (1987). Images (stacks and tile stacks) were retrieved from the VISTA Science Archive (VSA)⁶ and reduced with the Cambridge Astronomy Survey Unit (CASU) pipeline.⁷ Stacks are created from two dithered exposures of the same chip. Tile stacks are obtained by combining six offset exposures per observation to yield contiguous coverage. For DBS2003−172, a comparison of measurements extracted from the tile stack and from individual stacks of a 3' × 3' field is given in Figure 4. For the DBS2003−172 field and for the GRS G331.34−0.36 field (8' × 8'), target measurements were analyzed in four stacks (EXPTIME = 10 s, 10 s, and 4 s), but eventually, for the surrounding field, the magnitudes measured on the tile stack were retained. For MCM2005b77, one stack that covered the whole cluster was available (6' × 6', EXPTIME = 10 s in the J, H, and K_s bands).

Zoom In Zoom Out Reset image size

Catalogs extracted from VVV data were combined with 2MASS data points. Absolute calibration was performed with 2MASS data points from 12.5 to 15 in J mag, from 12.5 to 14.5 in the H band, and from 11.5 to 13.0 mag in the K_s band. 2MASS magnitudes were retained for bright stars that were saturated in the VVV images.

For GRS G331.34−0.36, JHK_s photometry of the central ${2}^{{\prime} }\times {2}^{{\prime} }$ taken with a spatial scale of 048 pix⁻¹ is also available from Pinheiro et al. (2012).

3.3.2. Photometry of DBS2003−172 #5A and #5B

Stars #5 A and B are blended in 2MASS. The acquisition image taken with the narrowband filter NB2.248 (2.248 μm) on SofI shows two stars, DBS2003−172 #5A and #5B, surrounded by a nebular arc. By measuring their counts, and those of nearby stars, on the acquisition image, using a PSF model and scaling them by the 2MASS magnitudes, we estimated K_s ≈ 10.82 mag for component A and K_s ≈ 11.24 mag for component B. These K_s magnitude estimates are consistent with values from the VVV survey. In VVV images, component A is clearly detected in all bands, but component B is detected only in the K_s band. K_s magnitudes are above the linear regime (within 0.6 mag): we measured (J, H, K_s) = (14.257 ± 0.010, 11.750 ± 0.400, 10.785 ± 0.035 mag) for component A and (H, K_s) = (13.495 ±0.140, 11.115 ± 0.125 mag) for component B.

3.4. Estimates of Extinction

For early-type stars, estimates of reddenings (color excess) are obtained by assuming intrinsic colors per spectral type, as listed in Messineo et al. (2011). For late-type stars, intrinsic colors are taken from Koornneef (1983). Reddenings in J–K_s, H–K_s, and J–H are converted into ${A}_{{K}_{{\rm{s}}}}$ values using the coefficients provided by Messineo et al. (2005) for an index α = −1.9. This index is consistent with the recent determination by Wang & Jiang (2014), which is based on spectroscopic observations of K-type giants. Tables 4 and 7 list our estimates of ${A}_{{K}_{{\rm{s}}}}$. The sample mainly consists of "naked" early-type stars, as inferred by the consistent ${A}_{{K}_{{\rm{s}}}}$ values from multicolors and from the small values of $| Q1|$ and $| Q2|$ (Messineo et al. 2012). Star MCM2005b77 12 (Br_γ in emission) has an 8 μm excess. Stars DBS2003−172 5A and 5B are embedded in a nebula.

Table 7.  Infrared Color Properties of Early-type Stars in the Direction of MCM2005b72/DBS2003−157 (GRS G331.34−0.36), MCM2005b77, and DBS2003−172 (GRS G337.92−0.48)

Field	ID	Sp. Type	(J–Ks)o	(H–Ks)o	${A}_{{K}_{{\rm{s}}}}$ (JH)	${A}_{{K}_{{\rm{s}}}}$ (JKs)	${A}_{{K}_{{\rm{s}}}}$ (HKs)	Q1	Q2	Comment
G331.34−0.36	1	OBAF	−0.060	−0.010	0.829	0.760	0.637	0.191 ± 0.199	0.226 ± 0.037	⋯
G331.34−0.36	2	B0-5	−0.130	−0.030	0.554	0.524	0.471	0.048 ± 0.083	0.695 ± 0.060	FGa
G331.34−0.36	3	O4-6	−0.180	−0.040	1.315	1.242	1.112	0.165 ± 0.080	0.253 ± 0.041	⋯
G331.34−0.36	4	OBAF	−0.060	−0.010	0.929	0.830	0.652	0.292 ± 0.084	⋯	⋯
G331.34−0.36	5	O9-B0	−0.160	−0.040	1.096	1.047	0.960	0.104 ± 0.085	⋯	⋯
G331.34−0.36	6	B0-3	−0.130	−0.030	0.956	0.918	0.851	0.071 ± 0.124	−5.661 ± 0.008	⋯
G331.34−0.36	7	OBAF	−0.060	−0.010	1.686	1.656	1.602	0.053 ± 0.927	⋯	⋯
G331.34−0.36	8	OBAF	−0.060	−0.010	0.333	0.317	0.289	0.019 ± 0.080	⋯	FGa
MCM2005b77	1	B0-3	−0.080	−0.040	0.838	0.851	0.875	−0.021 ± 0.099	0.020 ± 0.005	Memberb
MCM2005b77	2	OBAF	−0.050	−0.040	2.843	2.763	2.621	0.302 ± 0.080	0.367 ± 0.006	BGc
MCM2005b77	3	B0-5	−0.050	−0.020	1.144	1.114	1.059	0.098 ± 0.091	−0.206 ± 0.010	BGc
MCM2005b77	4	B0-3	−0.080	−0.040	0.789	0.793	0.800	0.011 ± 0.083	−0.083 ± 0.011	Member
MCM2005b77	5	B0-3	−0.080	−0.040	0.787	0.794	0.806	0.002 ± 0.084	−0.312 ± 0.004	Member
MCM2005b77	6	B0-5	−0.050	−0.020	0.980	0.933	0.850	0.154 ± 0.080	−0.082 ± 0.010	Member
MCM2005b77	7	B0-5	−0.050	−0.020	0.890	0.851	0.782	0.127 ± 0.078	0.037 ± 0.014	Member
MCM2005b77	8	B0-5	−0.050	−0.020	0.746	0.709	0.645	0.120 ± 0.085	0.386 ± 0.009	⋯
MCM2005b77	9	B0-5	−0.050	−0.020	0.798	0.760	0.691	0.127 ± 0.139	−0.330 ± 0.005	Member
MCM2005b77	10	B0-5	−0.050	−0.020	0.874	0.867	0.854	0.021 ± 0.095	0.631 ± 0.014	⋯
MCM2005b77	11	B0-5	−0.050	−0.020	0.959	0.917	0.841	0.139 ± 0.800	⋯	Member
MCM2005b77	12	OB	−0.050	−0.020	1.726	1.837	2.035	−0.381 ± 0.084	−3.887 ± 0.012	⋯
DBS2003−172	1	OBAF	−0.060	−0.010	1.045	1.053	1.067	−0.067 ± 0.148	⋯	⋯
DBS2003−172	2	OBAF	−0.060	−0.010	0.894	0.836	0.732	0.155 ± 0.081	⋯	⋯
DBS2003−172	3	OB	−0.060	−0.010	1.088	1.126	1.192	−0.168 ± 0.094	⋯	⋯
DBS2003−172	4	OBAF	−0.060	−0.010	0.889	0.824	0.708	0.178 ± 0.088	0.099 ± 0.008	⋯
DBS2003−172	5A	OB	−0.060	−0.010	2.142	1.897	1.459	0.770 ± 1.122	⋯	⋯
DBS2003−172	5B	OB	−0.060	−0.010	⋯	⋯	3.575	⋯	⋯	⋯
DBS2003−172	6	OBAF	−0.060	−0.010	0.375	0.353	0.314	0.038 ± 0.074	⋯	FGa

Notes.

^aFG = object in the background of the H ii region. ^bMember = cluster member on the basis of the radial distance and extinction. ^cBG = object in the background of the H ii region.

Download table as:  ASCIITypeset image

We have detected several massive stars in the direction of each of the candidate clusters MCM2005b72/DBS2003−157 (GRS G331.34−0.36), MCM2005b77, and DBS2003−172 (GRS G337.92−0.48). In this section, we analyze the infrared properties of those stars, such as extinction and bolometric magnitudes, M_bol. We also estimate their spectrophotometric distances in order to investigate their association with the nearby H ii regions.

Detected late-type stars are not discussed further, because they appear to be giants that are unrelated to the star-forming regions analyzed. As shown in Table 4, for the assumed distances to the star-forming regions, the M_bol values for all but two of the detected late-type stars are ≳−5.27 mag, that is, typical of giants. The two bright stars are asymptotic giant branch stars (AGBs) with strong water absorption in their spectra (see also Messineo et al. 2014a, 2014b).

4.1. S62/GRS G331.34−00.36

The candidate clusters MCM2005b72 (Mercer et al. 2005), DBS2003−157 (Dutra et al. 2003), and Cl063 (Borissova et al. 2011) are projected onto the H ii region GRS G331.34−00.36. The H ii emission extends over an area of $3\buildrel{\,\prime}\over{.} 8\times 2\buildrel{\,\prime}\over{.} 0$, as measured by Culverhouse et al. (2011) at 100 and 150 GHz. At this location, a bubble (S62) is seen at mid-infrared wavelengths (Churchwell et al. 2006; Simpson et al. 2012); dust emission shapes and defines several subregions. The candidate cluster DBS2003−157 overlaps with a large portion of GRS G331.34−00.36; it was detected on 2MASS images as a region associated with nebular emission (Dutra et al. 2003). The candidate cluster MCM2005b72 was detected as an overdensity of stellar data points at 3.6 μm (Mercer et al. 2005) and is adjacent to DBS2003−157. The candidate cluster Cl063 corresponds to a bright clump of dust emission on the northeastern side of GRS G331.34−00.36 associated with IRAS 16089−5137; it was detected with VVV data (Borissova et al. 2011). These three candidate clusters are most probably subsets of the same young population associated with GRS G331.34−00.36.

The H ii region indicates the presence of massive stars. We assume a thermal emission from an optically thin plasma at a temperature of about 10,000 K and neglect dust contribution to the flux. Using the formula of Martín-Hernández et al. (2003), which is equivalent to that of Rubin (1968) when the recombination rate coefficients are taken from Storey & Hummer (1995), and using the H ii radio fluxes at 4.85 GHz (19.169 Jy), 100 GHz (7.05 ± 0.18 Jy), and 150 GHz (9.29 ± 0.27 Jy) listed in Kuchar & Clark (1997) and Culverhouse et al. (2011), we obtain log₁₀(N_lyc[photons s⁻¹]) = 49.25, 48.8, and 49.0, respectively. These values are consistent with that inferred from the flux at 22 GHz by Pinheiro et al. (2012). This amount can be generated by a single O4-6 star.

The region is still an active site of star formation. Several molecular clumps appear in the 870 μm ATLASGAL map of S62 (Schuller et al. 2009), which is shown in Figure 1. Sixteen compact molecular clumps, that is, 12 protostellar and four prestellar cores, were identified in the direction of S62 by Elia et al. (2017). Highly reliable radial velocities are available for nine clumps (V_lsr from −65.0 to −66.4 km s⁻¹; Urquhart et al. 2018). Details are provided in the Appendix. In summary, all clumps are likely at the close distance of about 4 kpc, with the exception of Hi-GAL #46272 (V_lsr = −97.3 km s⁻¹, 5.8 kpc; Urquhart et al. 2018) and #46184 (10.6 kpc; Elia et al. 2017), which are external to S62, as seen in the GLIMPSE 8 μm map. The blackbody temperatures of the clumps range from 11.9 to 30.2 K and, for a common distance of 3.9 kpc, individual masses are from ≈30 to ≈600 M_⊙, by having excluded Hi-GAL #46272 and #46184. Thirteen of these 14 clumps, equivalent to 3700 M_⊙, are above their critical mass threshold for virial equilibrium (Bertoldi & McKee 1992; König et al. 2017).

4.1.1. Properties of the Massive Stars Detected in S62/GRS G331.34−00.36

We detected one O4-6 star (#3) and one O9-B0 star (#5) in the direction of GRS G331.34−00.36 (S62). Pinheiro et al. (2012) identified the same two stars (#3 and #5) as the main ionizing sources. The O4-6 star has interstellar extinction ${A}_{{K}_{{\rm{s}}}}$(JH) = 1.3 mag, ${A}_{{K}_{{\rm{s}}}}$(JK_s) = 1.2 mag, ${A}_{{K}_{{\rm{s}}}}$(HK_s) = 1.1 mag, with an average value of 1.2 mag and σ = 0.1 mag. Assuming a dwarf class, as inferred in Pinheiro et al. (2012), and the M_K listed by Martins & Plez (2006), we derive a distance modulus, DM, of 12.66 ± 0.78 mag. If a giant class would be assumed, then DM = 13.30 ± 0.63 mag. The H ii region GRS G331.34−00.36 has a velocity of V_lsr = −64.8 ± 2.9 km s⁻¹ (Bronfman et al. 1996). Using this V_lsr and the Galactic rotation curve from Reid et al. (2009), we estimate a near-kinematic distance of 3.9 ± 0.3 kpc and DM = 12.98 ± 0.18 mag. We thus conclude that star #3 is associated with S62. The M_bol values derived using the near-kinematic distance are listed in Table 9.

Star #5 is 20'' away from star #3 and has ${A}_{{K}_{{\rm{s}}}}$ = 1.02 ± 0.05 mag. Similar estimates of extinction suggest that star #5 is physically associated with the bubble. In the color–magnitude diagram (CMD) in Figure 5, these two massive stars appear among the brightest stars. Assuming an O9-9.5 type for star #5, we obtain a DM between 12.20 ± 0.76 mag (dwarf) and 13.37 ± 0.65 mag (giant).

Zoom In Zoom Out Reset image size

Star #2 (B0-5) is located on the northeastern side of the region, outside the bubble, at a significantly lower extinction of ${A}_{{K}_{{\rm{s}}}}$ = 0.5 mag. A star that is foreground to the H ii region GRS G331.34−00.36 cannot be ruled out. Assuming a dwarf, we derived DM = 10.72 ± 1.27 mag (1.4 kpc); assuming a giant, we obtained DM = 11.81 ± 1.43 mag. Unfortunately, the spectra of stars #1, #4, #6, #7, and #8 have poor signal-to-noise ratio, and we can only conclude that they are not late-type stars. Star #8 (${A}_{{K}_{{\rm{s}}}}$ ≈ 0.3 mag) is likely to be foreground to the complex. From Figure 5, stars #4 (${A}_{{K}_{{\rm{s}}}}$ ≈ 0.8 mag) and #7 (${A}_{{K}_{{\rm{s}}}}$ ≈ 1.6 mag) are plausibly late-O stars related to S62. For star #6 (${A}_{{K}_{{\rm{s}}}}$ ≈ 0.9 mag), Pinheiro et al. (2012) report an early B type, which yields a DM = 12.66 ± 1.43 mag for a giant or 11.57 ± 1.27 mag for a dwarf.

By assuming that the stars are at the nebular distance of 3.9 kpc, star #3 (O4-6V) is able to produce log₁₀(N_lyc) from 49.0 to 49.5 photons s⁻¹, star #5 (O9-9.5V) from 47.6 to 47.9 photons s⁻¹, and star #6 (B0-B3V) from 43.9 to 47.6 (Panagia 1973; Martins et al. 2005). The O4-6 star is the main source of ionizing flux, as it accounts for the observed radio flux from this H ii bubble (see above).

By inspecting the H–K_s versus J–K_s colors of stars brighter than K_s = 11 mag, we find that a few other faint ionizing sources remain unobserved (K_s < 10.5 mag, see Figure 5).

4.2. MCM2005b77

MCM2005b77 was detected by Mercer et al. (2005) with GLIMPSE and 2MASS data. This stellar cluster shows a well-peaked surface density in the 2MASS K_s image, with a half-light diameter of 98'', and is rich in early-type stars. Our spectroscopic study revealed 12 early-type stars in the direction of this candidate cluster.

4.2.1. The CMD of MCM2005b77

A J–K_s versus K_s diagram of 2MASS data points in the direction of MCM2005b77 is shown in Figure 6. The diagram shows a dwarf foreground sequence at J–K_s ≈ 0.5 mag, a red clump sequence (not well defined) that crosses the CMD from K_s ≈ 10 mag and J–K_s ≈ 1 mag to K_s ≈ 12 mag and J–K_s ≈ 2.5 mag, and a tail of redder giants with J–K_s ≳ 2.5 mag. The cluster sequence, consisting of B-type stars, appears above the red clump sequence at K_s ≲ 10.5 mag and J–K_s ≈ 1.4 mag. On the CMD, it is difficult to identify this at lower magnitudes because of the high number of field clump stars. Late-type and early-type stars separate out in the J–K_s and H–K_s diagram shown in Figure 6. In the inner 19, we count a total of 16 photometric early-type stars with K_s < 11 mag, which include the eight spectroscopically confirmed early types (#1, #4, #5, #6, #7, #9, #10, and #11). Average proper motions in the right ascension and declination directions are μ_α and μ_δ, respectively, for all but star #10: μ_α = −4.15 ± 0.5 mas yr⁻¹ and μ_δ = −4.15 ± 0.5 mas yr⁻¹. Star #10 is more than 3σ off, with μ_α = −7.73 ± 0.54 mas yr⁻¹ and μ_δ = −7.14 ± 0.32 mas yr⁻¹. It could be unrelated to the cluster; however, star #10 (OBAF) is similar in colors and magnitudes to star #11 (B0-5), which, together with its location, indicates that it could be another B0-5 star with anomalous velocity. The average ${A}_{{K}_{{\rm{s}}}}$ and standard deviation of the eight spectroscopic early-type stars are 0.91 mag and 0.08 mag, respectively. The small dispersion of ${A}_{{K}_{{\rm{s}}}}$ values confirms that these stars are physically at the same distance and are members of the stellar cluster MCM2005b77 (see Table 7). Parallaxes from GAIA DR2 range from −0.54 to 1.0 mas, and the average is 0.3 ± 0.4 mas.

Zoom In Zoom Out Reset image size

There are four other spectroscopic early-type stars detected outside the cluster radius. Stars #2 (OBAF) and #3 (B0-5), with ${A}_{{K}_{{\rm{s}}}}$ values of 2.9 mag and 1.2 mag, are probably unrelated background stars. Red J–K_s and H–K_s colors indicate that star #2 (OBAF) is a G-F star (see Figure 6). Star #12 (Br_γ in emission) is still enshrouded in dust (${A}_{{K}_{{\rm{s}}}}$ = 1.8 mag). Star #8 (B0-5, K_so = 8.55 mag) has an ${A}_{{K}_{{\rm{s}}}}$ similar to that of the cluster (0.8 mag) and derived magnitudes consistent with those of cluster members, but it resides a few arcminutes away from the cluster (see Figure 1).

4.2.2. Spectrophotometric Distance of MCM2005b77

The H ii region G332.769−0.007 (Bronfman et al. 1996) coincides with IRAS 16137−5025 (MSX G332.7673−00.0069) and is ∼2' away from the cluster center. With V_lsr = −95.0 ± 2.8 km s⁻¹ (CS observations with a beam width of 50''; Bronfman et al. 1996), this H ii region is at a near-kinematic distance of 5.2 ± 0.1 kpc (DM = 13.60 ± 0.17 mag; Reid et al. 2009); a far-kinematic distance would yield 9.7 kpc. ¹³CO observations confirm the main component at −95.6 km s⁻¹ and a weaker component at −65.3 km s⁻¹ (Urquhart et al. 2007).

Along the line of sight to MCM2005b77, the model of Galactic dust distribution by Drimmel et al. (2003) predicts an ${A}_{{K}_{{\rm{s}}}}$ value of 0.8–0.9 mag (i.e., the extinction of MCM2005b77) for a heliocentric distance of 5.2 kpc. We thus conclude that the cluster MCM2005b77 (${A}_{{K}_{{\rm{s}}}}$ = 0.91 mag) is most probably at the same distance as the nearby IRAS 16137−5025.

To estimate a spectrophotometric distance to the cluster, we analyze the seven B-type members within the cluster radius, using the intrinsic colors and absolute magnitudes per spectral type tabulated in Messineo et al. (2011). The spectra of stars #1, #4, and #5 have the He i line at 2.058 μm in emission, which is common among early-B supergiants (e.g., Davies et al. 2012). Their dereddened K_s values, ${({K}_{{\rm{s}}})}_{{\rm{o}}}$, range from 5.65 mag to 7.81 mag. Star #6 (B0-5), with ${({K}_{{\rm{s}}})}_{{\rm{o}}}=7.73$ mag, is also assumed to be a supergiant. These four stars yield an average DM of 13.53 ± 1.08 mag (see Table 8), which is compatible with the gaseous near-kinematic distance of the nearby source IRAS 16137−5025.

Table 8.  Spectrophotometric Distance to the Cluster MCM2005b77

CLUSTER-ID	Sp. Group	(Ks)o	MKa	DM	Phot. Type	${M}_{K}$ b	DM (phot)b
		[mag]	[mag]	[mag]		[mag]	[mag]
MCM2005b77 1	B0-3	5.65	−6.27	11.92 ± 0.92	B3/B4	−6.70	12.35
MCM2005b77 4	B0-3	7.62	−6.27	13.89 ± 0.92	B0	−5.85	13.47
MCM2005b77 5	B0-3	7.81	−6.27	14.08 ± 0.92	B0	−5.85	13.66
MCM2005b77 6	B0-5	7.73	−6.49	14.22 ± 1.14	B0	−5.85	13.58
Average				13.53 ± 1.08

Notes.

^aM_Ks are average values for supergiants with types from B0 to B3 (or B5). ^bM_K and DM (Phot) are for photometrically inferred types (Phot. type) (Bibby et al. 2008).

Download table as:  ASCIITypeset image

Assuming a kinematic distance of 5.2 ± 0.1 kpc, the M_bol values of all detected cluster members range from −6.4 to −10.5 mag (see Table 9; these values are typical of evolved B-type stars, class I and III; Martins & Plez 2006).

Table 9.  Absolute Properties of Detected Early-type Stars in MCM2005b72/DBS2003−157, MCM2005b77, and DBS2003−172

ID	Sp	${A}_{{K}_{{\rm{s}}}}$	${{K}_{{\rm{s}}}}_{{\rm{o}}}$	MK	${{BC}}_{{K}_{{\rm{s}}}}$	DM	Mbol	log(L)
		[mag]	[mag]	[mag]	[mag]	[mag]	[mag]	[L⊙]
G331.34−0.36 2	B0−5	0.55 ± 0.03	8.34 ± 0.04	−4.64 ± 0.31	−3.12 ± 0.66	12.98 ± 0.31	−7.76 ± 0.73	${5.00}_{0.29}^{0.29}$
G331.34−0.36 3	O4−6	1.32 ± 0.03	8.25 ± 0.04	−4.72 ± 0.31	−4.55 ± 0.15	12.98 ± 0.31	−9.27 ± 0.35	${5.61}_{0.14}^{0.14}$
G331.34−0.36 5	O9−B0	1.10 ± 0.02	8.90 ± 0.04	−4.08 ± 0.31	−3.85 ± 0.13	12.98 ± 0.31	−7.93 ± 0.34	${5.07}_{0.14}^{0.14}$
G331.34–0.36 6	B0−3	0.96 ± 0.04	9.19 ± 0.05	−3.79 ± 0.31	−3.12 ± 0.66	12.98 ± 0.31	−6.91 ± 0.73	${4.66}_{0.29}^{0.29}$
MCM2005b77 1	B0−3	0.84 ± 0.03	5.65 ± 0.04	−7.95 ± 0.17	−2.50 ± 0.80	13.60 ± 0.17	−10.45 ± 0.82	${6.08}_{0.33}^{0.33}$
MCM2005b77 3	B0−5	1.14 ± 0.03	7.08 ± 0.04	−6.52 ± 0.17	−2.12 ± 1.17	13.60 ± 0.17	−8.64 ± 1.18	${5.35}_{0.47}^{0.47}$
MCM2005b77 4	B0−3	0.79 ± 0.03	7.62 ± 0.04	−5.98 ± 0.17	−2.50 ± 0.80	13.60 ± 0.17	−8.48 ± 0.82	${5.29}_{0.33}^{0.33}$
MCM2005b77 5	B0−3	0.79 ± 0.03	7.81 ± 0.04	−5.79 ± 0.17	−2.50 ± 0.80	13.60 ± 0.17	−8.29 ± 0.82	${5.21}_{0.33}^{0.33}$
MCM2005b77 6	B0−5	0.98 ± 0.03	7.73 ± 0.04	−5.87 ± 0.17	−2.12 ± 1.17	13.60 ± 0.17	−7.99 ± 1.18	${5.09}_{0.47}^{0.47}$
MCM2005b77 7	B0−5	0.89 ± 0.03	8.10 ± 0.03	−5.50 ± 0.17	−2.12 ± 1.17	13.60 ± 0.17	−7.62 ± 1.18	${4.94}_{0.47}^{0.47}$
MCM2005b77 8	B0−5	0.75 ± 0.03	8.55 ± 0.04	−5.05 ± 0.17	−2.12 ± 1.17	13.60 ± 0.17	−7.17 ± 1.18	${4.76}_{0.47}^{0.47}$
MCM2005b77 9	B0−5	0.80 ± 0.04	8.63 ± 0.06	−4.97 ± 0.18	−2.12 ± 1.17	13.60 ± 0.17	−7.09 ± 1.18	${4.73}_{0.47}^{0.47}$
MCM2005b77 10	B0−5	0.87 ± 0.03	9.26 ± 0.04	−4.35 ± 0.17	−2.12 ± 1.17	13.60 ± 0.17	−6.47 ± 1.18	${4.48}_{0.47}^{0.47}$
MCM2005b77 11	B0−5	0.96 ± 0.05	9.32 ± 0.80	−4.28 ± 0.82	−2.12 ± 1.17	13.60 ± 0.17	−6.40 ± 1.43	${4.46}_{0.57}^{0.57}$
MCM2005b77 12	OB	1.70 ± 0.03	9.26 ± 0.04	−4.34 ± 0.17	−2.87 ± 1.21	13.60 ± 0.17	−7.21 ± 1.22	${4.78}_{0.49}^{0.49}$
DBS2003−172 3	OB	1.05 ± 0.03	8.55 ± 0.04	−3.87 ± 0.50	−3.27 ± 0.86	12.41 ± 0.50	−7.14 ± 1.00	${4.75}_{0.40}^{0.40}$
DBS2003−172 5A	OB	1.90 ± 0.02	8.89 ± 0.04	−3.53 ± 0.50	−4.30 ± 1.00	12.41 ± 0.50	−7.83 ± 1.12	${5.03}_{0.45}^{0.45}$
DBS2003−172 5B	OB	3.58 ± 0.28	7.54 ± 0.31	−4.87 ± 0.59	−4.30 ± 1.00	12.41 ± 0.50	−9.17 ± 1.16	${5.57}_{0.46}^{0.46}$

Download table as:  ASCIITypeset image

4.2.3. Discussion of MCM2005b77

We found a concentration of eight early-type stars at the location of MCM2005b77. These have similar interstellar extinctions and appear along the same cluster sequence in the CMD. Assuming a distance of 5.2 kpc, the M_bol values of the members range from −6.40 mag to −10.45 mag, and the new rotating stellar models from the Geneva group yield stellar masses from 12 to 60 M_⊙ (Ekström et al. 2012). Within errors, all detected members except star #1 are consistent with a coeval population of about 8 Myr (Ekström et al. 2012). The maximum initial unexploded stellar mass in such a population has a mass of ≈28 M_⊙, while masses of 18 M_⊙ are already evolving after the main sequence.

We counted 16 photometric early-type stars, down to K_s = 11 mag, which is the limit at which completeness can be assumed. Assuming a Salpeter mass function, a minimum mass of ≈350 M_⊙ is derived, and by extrapolating to 1 M_⊙ we infer a cluster mass of about 3000 ± 900 M_⊙.

Star #1 has an ${A}_{{K}_{{\rm{s}}}}$ consistent with those of fainter members and infrared colors and spectral features consistent with a normal B star. It is exceptionally bright (M_K = −7.95 mag, M_bol = −10.45 mag) for an 8-Myr-old population. The second brightest star has M_K = −6.52 mag, and a companion could brighten it by only −0.75 mag. Assuming a single star, a mass of 60 M_⊙ is derived.

For a distance of 5.2 kpc, in the MCM2005b77 cluster, we counted four probable blue supergiants, one giant, and six dwarfs (<18 M_⊙; Martins et al. 2005). Assuming B0 types, we estimate log₁₀(N_lyc) = 49.2 photons s⁻¹; assuming B1 stars, 47.4 photons s⁻¹; and assuming B2 stars, 46.83 photons s⁻¹ (Panagia 1973). The cluster may be contributing ionizing photons for the nearby H ii region IRAS 16137−5025 (MSX G332.7673−00.0069). The H ii region has a flux density of 1.42 ± 0.15 Jy at 150 GHz, and log₁₀(N_lyc) ≈ 48.52 photons s⁻¹ are sufficient to power it. In the direction of IRAS 16137−5025, some molecular clumps are currently collapsing. The ATLASGAL clumps have integrated fluxes from 8.19 to 12.79 Jy (Urquhart et al. 2014; Wienen et al. 2015). Four compact clumps are detected with Hi-GAL data (numbers 47117, 47118, 47127, and 17129; Elia et al. 2017) and classified as protostellar clumps (with mid-infrared detections). Estimated dust temperatures range from 12.09 to 20.39 K, and, for an assumed distance of 5.2 kpc, masses from 500 to 1500 M_⊙ make up a total mass of about 3800 M_⊙ enclosed within a circle of 2 pc radius (see the Appendix). For Hi-GAL 47118 (AGAL332.767−00.019) and 47127 (AGAL332.774−00.009), similarly high masses of 1340 and 1982 M_⊙, respectively, are estimated by Heyer et al. (2016). Radial velocities confirm near-kinematic distances and association with IRAS 16137−5025 for the four Hi-GAL/ATLASGAL sources (from −94.8 to −95.6 km s⁻¹; Urquhart et al. 2018).

4.3. DBS2003−172

DBS2003−172 is reported as an infrared candidate stellar cluster by Dutra et al. (2003). Deeper infrared photometry confirmed a rich population of young stars in a star-forming region (Borissova et al. 2006).

DBS2003−172 appears to be the probable source of ionizing radiation for GRS G337.92−00.48, a H ii region of $2\buildrel{\,\prime}\over{.} 2\times 2\buildrel{\,\prime}\over{.} 0$ (Culverhouse et al. 2011), as shown in Figure 1. This H ii region coincides with the bulk of 8 μm emission from the bubble S36 (Churchwell et al. 2006). In the direction of this H ii region, Huang et al. (1999) measured three CO clouds at V_lsr −58.3, −40.5, and −33.4 km s⁻¹, respectively, which yield near-kinematic distances of 3.9, 3.0, and 2.7 kpc (or far-kinematic distances of 11.7, 12.5, and 12.9 kpc; Reid et al. 2009). The −40.5 km s⁻¹ cloud is far stronger than the other two clouds. Walsh et al. (1998) detected a site of active star formation with three methanol masers, whose velocities (V_lsr = −36.5, −38.0, and −38.9 km s⁻¹) indicate an association with the CO cloud at −40.5 km s⁻¹. We thus assume a V_lsr = −40.5 km s⁻¹ and a kinematic distance of 3.0 kpc (near) or 12.5 kpc (far) for DBS2003−172. We assume the near distance.

Considering the whole H ii region, the radio flux densities measured at 100 GHz and at 150 GHz of GRS G337.92−00.48 (Culverhouse et al. 2011) are 14.92 ± 0.14 Jy and 18.88 ±0.16 Jy, and Caswell & Haynes (1987) report 12.80 Jy at 5 GHz. Assuming thermal emission and a temperature of 10,000 K (5600 K) and using the formula by Martín-Hernández et al. (2003), we computed log₁₀(N_lyc) = 48.74 (48.85), 48.93 (49.05), and 49.05 (49.17) photons s⁻¹ with the 5 GHz, 100 GHz, and 150 GHz flux densities, respectively. This is equivalent to the log₁₀(N_lyc) emitted by a single O9-B0 supergiant or by a dozen B0 dwarfs (Panagia 1973).

The submillimeter emission detected by ATLASGAL nicely follows the mid-infrared bubble. Seven prestellar and 10 protostellar compact (<1 pc) clumps identified with Hi-GAL by Elia et al. (2017) are in the direction of this bubble. Their 870 μm fluxes range from 0.0 to 8.21 Jy (Elia et al. 2017). Temperatures were estimated by fitting a gray blackbody and range from 10.96 to 31.13 K. Assuming a common distance of 3.0 kpc, rescaled masses range from 50 to 3500 M_⊙, with two clumps above 1000 M_⊙. Elia et al. provide distances of about 3 kpc for 12 out of 17 clumps (see the Appendix). The just-released catalog of highly reliable velocities of ATLASGAL clumps (Urquhart et al. 2018) proposes close kinematic distances for seven Hi-GAL clumps (V_lsr from −37.6 to −40.0 km s⁻¹), and only the clumps 50264, 50313, and 50322 remain at the far distance. However, by looking at the bubble morphology and filaments, it appears possible to us that they are all associated with bubble S36.

In the ATLASGAL map, the candidate stellar cluster DBS2003−172 lies between two extended ATLASGAL clumps (AGAL 337.916−00.477 and AGAL 337.922−00.456) of 74.83 and 86.41 Jy, respectively (Urquhart et al. 2014). For a distance of 3.0 kpc, their angular sizes (${30}^{{\prime\prime} }\times {17}^{{\prime\prime} }$ and ${45}^{{\prime\prime} }\times {35}^{{\prime\prime} }$) correspond to $1.5\times 0.9$ and $2.1\times 1.7$ pc². AGAL 337.916−00.477 is one of the mid-IR bright sources studied by König et al. (2017), a high-mass protostar with a temperature of 34.4 ± 1.8 K and a mass of 1100 M_⊙ (for 3.0 kpc). With a velocity of −39.6 km s⁻¹, AGAL 337.916−00.477 is associated with GRS G337.92−00.48 (S36; Giannetti et al. 2014; Urquhart et al. 2018). The other extended clump, AGAL 337.922−00.456, breaks into two different Hi-GAL sources listed by Elia et al. (50279 and 50266). AGAL 337.922−00.456 is given at the far distance by V_lsr = −38.7 km s⁻¹ (Urquhart et al. 2014, 2018), but at the close distance by Kim et al. (2017): V_lsr = −43 km s⁻¹. The stellar cluster location and symmetry of the molecular structure around it suggest close distance.

4.3.1. Massive Stars in DBS2003−172

Six stars were spectroscopically detected as probable early types. Five spectra present only Br_γ in absorption, which does not allow for a precise classification. Their Q1 parameters, which combine H–K_s and J–K_s colors, are consistent with those of early-type stars (see Table 9 and Messineo et al. 2012). Indeed, in the K_s versus H–K_s diagram in Figure 7, a small overdensity of stars is found in the range 8 mag < K_s < 11 mag. Stars #1, #2, #3, and #4 yield an average ${A}_{{K}_{{\rm{s}}}}$ = 1.0 mag with σ = 0.1 mag and are likely to be members of the star-forming region. Spectral types have not been assigned, since the spectra have poor signal-to-noise ratio and only Br_γ is detected. With K_s from 8.55 to 9.98 mag and with detected Br_γ in absorption, which is the strongest absorption line in the spectrum of O-type stars, stars #1, #2, and #4 have photometry consistent with those of O-type stars. Star #3 (${A}_{{K}_{{\rm{s}}}}$ = 1.1 mag) has a strong Br_γ line in emission and JHK_s colors consistent with those reddened by interstellar extinction. Assuming a young dwarf, we estimate an O7–O8 type, which would be enough to make star #3 alone the main contributor to the total ${N}_{\mathrm{lyc}}$ necessary to excite this H ii region (see previous subsection). Star #6 is foreground to the star-forming complex (${A}_{{K}_{{\rm{s}}}}$ = 0.37 mag). Four other possible O-type stars remain unobserved (as shown in the color–color plot of Figure 7).

Zoom In Zoom Out Reset image size

Stars #5A and #5B are part of the star-forming complex DBS2003−172, as deduced by their colors and morphological interaction with the ISM. In VVV J, H, and K_s images, a comet-shaped nebula is seen to surround them. With ${A}_{{K}_{{\rm{s}}}}$ of 1.9 and 3.6 mag, stars #5A and #5B are likely embedded in the nebula. The infrared nebula coincides with an ultracompact H ii region at R.A. = 16^h41^m080 and decl. = −47^d06^m46 s, detected by Walsh et al. (1998). The spectra of DBS2003−172-5 (A and B) display stellar and circumstellar He i and Br_γ (see Figure 3). Owing to the presence of strong He i and Br_γ lines, they resemble typical spectra of ultracompact (UC) H ii regions (e.g., Doherty et al. 1994) and of H ii regions associated with evolved massive stars: in particular, IRS13, the remains of a stellar cluster close to the Galactic center, and the M1-78 nebula, which also have [Fe iii] at 2.2178 μm (Blum et al. 1995; Martín-Hernández et al. 2008; Eckart et al. 2013). Detections of [Fe iii] emission lines at 2.2178 μm have also been reported in the direction of planetary nebulae, but together with several H ii lines (Likkel et al. 2004). The ratio of the flux densities of the He i and Br_γ lines indicates stars hotter than an O7 star (e.g., Doherty et al. 1994). When this ratio is above unity and the star is a supergiant, it is typically a transitional object (e.g., Ofpe/WN9 stars and LBVs) or a WR star (e.g., Blum et al. 1995; Morris et al. 1996; Najarro et al. 1997; Messineo et al. 2009). The IRS13 E2 component (the brightest IR component of the IRS13 cluster) is a WN8 (Martins et al. 2007). The M1-78 nebula is most probably a combination of a H ii region and the ejecta from a central massive evolved star (O+WR; Martín-Hernández et al. 2008). The UC radio source (IRAS 16374−4701) detected at the location of stars #5A and #5B by Walsh et al. (1998) has flux densities of 452 mJy and 396 mJy at 8.64 GHz and 6.67 GHz (VIZIER), respectively. For a optically thin thermal nebula at 10,000 K and 3 kpc distant, we estimated log₁₀(N_lyc) = 47.3 photons s⁻¹ using the formula in Martín-Hernández et al. (2003). A star of O7 type or earlier produces log₁₀(N_lyc) ≳ 48.9 photons s⁻¹ for a dwarf class and ≳49.3 photons s⁻¹ for a supergiant class. On the other hand, if we could use the same formula for 10 known WR stars with available radio continuum measurements (Leitherer et al. 1997), we would obtain an average log₁₀(N_lyc) = 47.24 ± 034 photons s⁻¹. Assuming a distance of 3.0 kpc and a BC_K of −4.3 mag (typical of O7–O8 stars; hotter stars have a larger correction), with the ${A}_{{K}_{{\rm{s}}}}$ values listed in Tables 7 and 9, we inferred luminosities of 5.03 and 5.57 log(L/L_⊙). These values are consistent with those of evolved supergiants (see, for example, Hamann et al. 2006; Martins et al. 2007).

We emphasize that our main goal is to put at the disposal of the astronomical community ionizing stars detected in the studied H ii regions. These spectroscopically observed stars are precious benchmarks for stellar population studies. In this section we comment on the relation of the identified stars with their surrounding bubbles.

Bubbles are prototype laboratories of current triggers and sequential star formation. Given a molecular cloud and the first massive star forming within it, a H ii region forms and expands, creating a layer of compressed matter at its border, where molecular clumps condense, collapse, and interact. Bubbles are very common: about 5000 have already been identified, covering approximately 10% of the inner Galactic plane (Simpson et al. 2012). There is ionized gas in about 90% of the ≈600 bubbles studied by Churchwell et al. (2009) and Deharveng et al. (2010), but only 13% of them have been associated with stellar clusters. In the larger sample (1814) analyzed by Bufano et al. (2018), 60% of bubbles are associated with H ii regions excited by young O or B stars, 2% by blue supergiants, and about 38% are still of unknown origin. The underlying problem with classifying bubbles is the difficulty of detecting isolated massive stars and even stellar clusters. A cluster is defined as an overdensity of stars that are likely to be physically close and of similar age; see, for example, MCM2005b77 (see Section 4.2). For details on detection algorithms and associated problematics, we remind the reader of the summary by Schmeja (2011). However, lists of candidate clusters from different surveys provide several clusters with different sizes and centroids for the same bubble. These candidate clusters must be discussed in the context of the bubble morphology and extinction pattern, with reference to benchmark stars.

The bubbles S36 and S62 were selected because they are H ii regions with candidate stellar clusters where the brightest stars have K_s from 6 to 11 mag, suitable for a 4 m class telescope. The case of S62 is an exemplary one: S62 hosts several candidate clusters, which were identified with different data sets and methods (see Section 4.1). The bubble has a diameter of about 8 pc at 3.9 kpc. MCM2005b72 is a clump of bright stars (e.g., stars #4, #6, and #8) at mid- and near-infrared wavelengths, with a diameter of 72'' (1.3 pc at 3.9 kpc), detected with an automated algorithm. DBS2003−172 has a diameter of 126'' (2.3 pc at 3.9 kpc) and was visually detected and centered on an arc of a bright 8 μm knot of S62. Cl063 is a bright 8 μm nebula located on the border of the bubble S62, and it coincides with the ATLASGAL clump AGALG331.3873−0.360 (Hi-GAL 46282). We tried to redetect groups of bright GLIMPSE stars in S62 with the minimum spanning tree (MST) algorithm. The resulting maps depend on the chosen cutoff distance and on the sample stars analyzed, but have the advantage of being independent of centroids (e.g., Wright et al. 2014). When considering stars bright in K_s (<11. mag) and [3.6] mag, that is, OB stars at 3.9 kpc, one can nicely reproduce the detection of MCM2005b72 with a cutoff distance from 20'' to 40'' (0.4–0.8 pc at 3.9 kpc). A tree also appears at the location of the massive stars #3 and #5 (see Figure 8). CMDs of stars detected by 2MASS and GLIMPSE in selected areas of Figure 8 are shown in Figure 9. MCM2005b72 (box2) is located in the center of the bubble, so it may contain massive stars from the first generation of star formation, such as stars #4 and #6 (late-O-type dwarfs, ${A}_{{K}_{{\rm{s}}}}$ = 0.8 and 0.9 mag, M_K = −4.2 and −3.9 mag). In box3, star #5 has similar properties; it is an O9-B type (${A}_{{K}_{{\rm{s}}}}$ = 1.1, M_K = −4.1 mag). Star #3 is the main ionizing star since it is a massive O4-6 (${A}_{{K}_{{\rm{s}}}}$ = 1.3 mag) of about 40 M_⊙ and an age of 5–7 Myr (Ekström et al. 2012).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

In conclusion, naked OB stars are not concentrated, as one would expect for a mass-segregated population, but are detected inside the bubble in two groups, while condensing molecular clumps are currently located on the shell surrounding the bubble. In the direction of Cl63 (AGALG331.3873−0.360), which is on the shell, we detected another early type, star #7, which is likely to be contributing to the heating of the protostar AGALG331.3873−0.360. These findings are in line with the current literature. Indeed, bound stellar clusters are a rarity, and in more than 90% of the cases (infant mortality and cluster dissolution), we expect associations of massive stars. For example, the massive association Cygnus OB2 (about 7 pc in diameter) is at least 5–7 Myr old, is not associated with gas, and is made of discrete clumps of stars (1–2 pc in size, Wright et al. 2014, 2016). The authors conclude that Cygnus OB2 originated with discrete clumps, and there is no trace of dynamical evolution (e.g., mass segregation). Similar evidence is provided for the younger W33 complex by Messineo et al. (2015), 5 pc in diameter, which is made up of several condensing molecular clumps and has sparsely distributed exciting stars (2–4 Myr).

We performed a spectroscopic survey of bright infrared stars in the direction of GRS G331.34−00.36 (S62), GRS G337.92−00.48 (S36), and MCM2005b77, and for the first time we detected massive members in these regions:

• a.  
  A total number of 27 early-type and 32 late-type stars were detected.
• b.  
  We confirm one O4-6 star and one B0-5 star as the main sources of ionizing radiation of GRS G331.34−00.36 (S62), at a kinematic distance of 3.9 ± 0.3 kpc. The stars have ${A}_{{K}_{{\rm{s}}}}$ of 1.3 mag and 1.0 mag. With N_lyc = 49 photons s⁻¹, the massive O4-6 star can alone account for most of the energy needed to maintain this H ii region.
• c.  
  In GRS G337.92−00.48 (S36), at the distance of about 3.0 kpc, we detected four photometric O-type stars at ${A}_{{K}_{{\rm{s}}}}$ = 1.0 mag and two massive emission line stars surrounded by a nebula. The newly discovered massive stars can account for the bulk of radio continuum emitted by this H ii region.
• d.  
  We detected a cluster of B-type stars at the location of MCM2005b77. The cluster has an average ${A}_{{K}_{{\rm{s}}}}$ = 0.91 ± 0.08 mag and a spectrophotometric distance modulus of 13.53 ± 1.08 mag that agrees with the kinematic distance of the adjacent H ii IRAS 16137−5025. We inferred an age of 8 Myr and a mass of about 3000 M_⊙.
• e.  
  Star formation is currently ongoing in these bubbles, with new condensations detected by ATLASGAL with masses from 30 to 3500 M_⊙.

This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. DENIS is a joint effort of several institutes mostly located in Europe. It has been supported mainly by the French Institut National des Sciences de l'Univers, CNRS, and French Education Ministry, the European Southern Observatory, the State of Baden-Wuerttemberg, and the European Commission under networks of the SCIENCE and Human Capital and Mobility programs, the Landessternwarte, Heidelberg, and Institut d'Astrophysique de Paris. This research made use of data products from the Midcourse Space Experiment, the processing of which was funded by the Ballistic Missile Defense Organization with additional support from the NASA office of Space Science. This publication makes use of data products from WISE, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. We gratefully acknowledge use of data from the ESO Public Survey program ID 179.B-2002 taken with the VISTA telescope and data products from the Cambridge Astronomical Survey Unit. It is a pleasure to thank Dave Monet, who forwarded a copy of the NOMAD catalog to CDS. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research made use of Montage, funded by the National Aeronautics and Space Administration's Earth Science Technology Office, Computational Technologies Project, under Cooperative Agreement Number NCC5-626 between NASA and the California Institute of Technology. The code is maintained by the NASA/IPAC Infrared Science Archive. This work was partially funded by the ERC Advanced Investigator Grant GLOSTAR (247078). This work was partially supported by the Fundamental Research Funds for the Central Universities in China and USTC grant KY2030000054. A special thank you is for the great support offered by the European Southern Observatory. We thank Dr. Michelle Doherty for her careful reading of our manuscript, Dr. Ernesto Martins for his algorithms to solve for the minimum spanning three paths. We thank the anonymous referee for his constructive comments and for having suggested that we comment on the candidate cluster multiplicity.

In Figure 10, K_s charts from the VVV survey are shown for the detected stars. Charts are aligned with the Celestial axis: north is up, and east is to the left. Each chart has a field of view of 1' × 1'.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

In Table 10 we list the parameters of Hi-GAL clumps in the direction of the studied H ii regions. Mid-infrared sources detected with the Herschel satellite were matched with the ATLASGAL point sources to identify protostar and prestellar objects by Elia et al. (2017). Elia et al. (2017) estimate distances with available ¹²CO and ¹³CO observations and the Galactic rotation curve.

Highly reliable radial velocities of ≈8000 ATLASGAL clumps are now available from line observations of C¹⁸O(2-1), C¹⁷O(3-2), NH₃, and CS (Urquhart et al. 2018). Distances have been revised with the catalog of radial velocities by Urquhart et al. (2018) and masses rescaled to the adopted distances.