Massive Stars in the W33 Giant Molecular Complex

Abstract

Rich in HII regions, giant molecular clouds are natural laboratories to study massive stars and sequential star formation. The Galactic star forming complex W33 is located at l =∼ 12. • 8 and at a distance of 2.4 kpc, has a size of ≈ 10 pc and a total mass of ≈ (0.8 − 8.0) × 10 5 M . The integrated radio and IR luminosity of W33 -when combined with the direct detection of methanol masers, the protostellar object W33A, and protocluster embedded within the radio source W33 main -mark the region out as a site of vigorous ongoing star formation. In order to assess the long term star formation history, we performed an infrared spectroscopic search for massive stars, detecting for the first time fourteen early-type stars, including one WN6 star and four O4-7 stars. The distribution of spectral types suggests that this population formed during the last ∼ 2 − 4 Myr, while the absence of red supergiants precludes extensive star formation at ages 6 − 30 Myr. This activity appears distributed throughout the region and does not appear to have yielded the dense stellar clusters that characterize other star forming complexes such as Carina and G305. Instead, we anticipate that W33 will eventually evolve into a loose stellar aggregate, with Cyg OB2 serving as a useful, albeit richer and more massive, comparator. Given recent distance estimates, and despite a remarkably similar stellar population, the rich cluster Cl 1813−178 located on the north-west edge of W33 does not appear to be physically associated with W33.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud–cloud collision triggering O star formation

“…In Figure 13(a), in which the case for G012.745-00.153 is presented, the evolution timescales which correspond to the measured rHII are about 0.2 Myr, 0.7 Myr, and 2 Myr for initial densities of 10 3 cm −3 , 10 4 cm −4 , and 10 5 cm −3 , respectively, suggesting that a quite high density of ∼ 10 5 cm −3 is required to be consistent with the age of 2-4 Myr estimated by Messineo et al (2015).…”

“…However, in the case of G012.820-00.238 shown in Figure 13(b), even with a quite high density of 10 5 cm −3 , the derived timescale is as short as 0.5 Myr, and it is inconsistent with the estimate by Messineo et al (2015) based on the comparison with the Arches cluster in the Galactic center, although their another estimate of less than 6 Myr, which is based on the stellar model by Ekstöm et al (2012), is consistent with the present results in Figure 13.…”

“…Contreras et al (2013), and Urquhart et al (2014). Orange and white circles indicate O and B-type star identified by Messineo et al (2015). White and pink contours show the VLA 90 cm and ATLASGAL 870 µm continuum image.…”

“…Messineo et al (2015) identified many high-mass stars in W33 based on the near infrared K-band observations using Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope (VLT). Positions of the OB stars identified by Messineo et al (2015) are indicated as circles on the three-color image of the Spitzer observations in Figure 1, where the circles colored in orange indicate the O-type stars, while those in white indicate early B-type stars. Two O4-6 are distributed in G012.…”

How can young massive clusters reach their present-day sizes?

“…Perhaps the most well studied example is the Galactic embedded region W3 Main that is continuing to form stars for at least 3 Myr (Feigelson & Townsley 2008;Bik et al 2014). Another more recently cited example is the W33 complex (Messineo et al 2015). In such regions, while there are individual ionized bubbles around each massive star, the overall stellar system still seems to be deeply embedded in molecular gas.…”