Mid-infrared diagnostics of metal-rich H ii regions from VLT andSpitzerspectroscopy of young massive stars in W31

Abstract

We present near‐infrared Very Large Telescope/Infrared Spectrograph and Array Camera and mid‐infrared (mid‐IR) Spitzer/Infrared Spectrograph spectroscopy of the young massive cluster in the W31 star‐forming region. H‐band spectroscopy provides refined classifications for four cluster member O stars with respect to Blum et al. In addition, photospheric features are detected in the massive young stellar object (massive YSO) #26. Spectroscopy permits estimates of stellar temperatures and masses, from which a cluster age of ∼0.6 Myr and distance of 3.3 kpc are obtained, in excellent agreement with Blum et al. IRS spectroscopy reveals mid‐IR fine structure line fluxes of [Ne ii–iii] and [S iii–iv] for four O stars and five massive YSOs. In common with previous studies, stellar temperatures of individual stars are severely underestimated from the observed ratios of fine‐structure lines, despite the use of contemporary stellar atmosphere and photoionization models. We construct empirical temperature calibrations based upon the W31 cluster stars of known spectral type, supplemented by two inner Milky Way ultracompact (UC) H ii regions whose ionizing star properties are established. Calibrations involving [Ne iii] 15.5 μm/[Ne ii] 12.8 μm, [S iv] 10.5 μm/[Ne ii] 12.8 μm or [Ar iii] 9.0 μm/[Ne ii] 12.8 μm have application in deducing the spectral types of early to mid O stars for other inner Milky Way compact and UC H ii regions. Finally, evolutionary phases and time‐scales for the massive stellar content in W31 are discussed, due to the presence of numerous young massive stars at different formation phases in a ‘coeval’ cluster.

A long-period Cepheid variable in the starburst cluster VdBH222

“…However methanol maser emission − considered to be uniquely associated with massive star formation − has recently been detected within it (Green et al 2009), as have both H ii regions (Caswell & Haynes 1987) and IR-colour selected candidate massive young stellar objects (Busfield et al 2006), potentially challenging this orthodoxy. Sanna et al (2014) placed the massive star-forming region W31 in the 3 kpc arm, although the findings of Furness et al (2010) dispute this assertion, favouring a much smaller distance. Given this uncertainty, VdBH 222 provides the first compelling observational proof of significant recent star-formation activity within this region.…”

Determining the age of young embedded clusters

“…Such a high concentration of massive stars would suggest a large total number of stars. Blum et al (2001) and Furness et al (2010) also estimate the age of W31 using spectroscopic spectral‐typing.…”

Stellar clusters in the inner Galaxy and their correlation with cold dust emission

“…The EC ages compiled from the literature were estimated using a variety of methods, including: comparison with theoretical isochrones on a Hertzsprung-Russell diagram constructed after spectroscopic classification in the near infrared (e.g., Furness et al 2010), use of the relation between the circumstellar disk fraction in the cluster and its age (following Haisch et al 2001), and comparison with synthetic clusters constructed by Monte Carlo simulations (Stead & Hoare 2011), among others. We notice that from the 25 ECs with available age estimates, there are two objects that seem to be artificial outliers, with ages that are too old to be embedded, namely 7.5 ± 2.6 Myr and 25 ± 7.5 Myr (respectively, clusters VVV CL100 and VVV CL059 from Borissova et al 2011) 15 .…”