Mid-Infrared diagnostics of metal-rich HII regions from VLT and Spitzer Spectroscopy of Young Massive Stars in W31

Abstract

We present near-IR VLT/ISAAC and mid-IR Spitzer/IRS spectroscopy of the young massive cluster in the W31 star-forming region. H-band spectroscopy provides refined classifications for four cluster members O stars with respect to Blum et al. In addition, photospheric features are detected in the massive Young Stellar Object (mYSO) #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-infrared fine structure line fluxes of [Ne II-III] and [S III-IV] for four O stars and five mYSOs. 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) HII regions whose ionizing star properties are established. Calibrations involving [NeIII] 15.5um/[NeII] 12.8um, [SIV] 10.5um/[NeII] 12.8um or [ArIII] 9.0um/[NeII] 12.8um have application in deducing the spectral types of early- to mid- O stars for other inner Milky Way compact and UCHII regions. Finally, evolutionary phases and timescales 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.

16 pages, 13 figures, accepted for MNRAS

References in corpus (13)

• UBVJHK synthetic photometry of Galactic O stars
• Using the minimum spanning tree to trace mass segregation
• Radio Recombination Lines in Galactic HII Regions
• The ionizing radiation from massive stars and its impact on HII regions: results from modern model atmospheres
• Spitzer Observations of M83 and the Hot Star, H II Region Connection
• [Ne II] Observations of Gas Motions in Compact and Ultracompact H II Regions
• The Mid-Infrared [SIV]/[NeII] versus [NeIII]/[NeII] Correlation
• High Spatial Resolution Spectroscopy of W51 IRS2E and IRS2W: Two Very Massive Young Stars in Early Formation Stages
• The Ionizing Stars of the Galactic Ultra-Compact HII Region G45.45+0.06
• On the hardening of the ionizing radiation in HII regions across galactic discs through softness parameters
• M1-78: a nitrogen-rich Galactic compact HII region beyond the Perseus arm
• On the central ionizing star of G23.96+0.15 and near-IR spectral classification of O stars
• High Angular Resolution Imaging Spectroscopy of the Galactic Ultra-Compact HII Region K3-50A

Cited by in corpus (10)

• Spectrophotometric Distances to Galactic H\,{\sc{ii}} Regions
• The RMS Survey: Resolving kinematic distance ambiguities towards a sample of compact HII regions using HI absorption
• Stellar clusters in the inner Galaxy and their correlation with cold dust emission
• Broadband ESO/VISIR-Spitzer infrared spectroscopy of the obscured supergiant X-ray Binary IGR J16318-4848
• The Massive Star-Forming Regions Omnibus X-Ray Catalog, Second Installment
• Palomar/TripleSpec observations of {\it Spitzer}/MIPSGAL~24\mic\ circumstellar shells: unveiling the nature of their central sources
• Fine structure collision strengths and line ratios for [Ne V] in infrared and optical sources
• A long-period Cepheid variable in the starburst cluster VdBH222
• The ionizing sources of luminous compact HII regions in the RCW106 and RCW122 clouds
• Study of young stellar objects and associated filamentary structures in the inner Galaxy