An x-ray tour of massive-star-forming regions with Chandra

Abstract

The Chandra X-ray Observatory is providing fascinating new views of massive star-forming regions, revealing all stages in the life cycles of massive stars and their effects on their surroundings. I present a Chandra tour of some of the most famous of these regions: M17, NGC 3576, W3, Tr14 in Carina, and 30 Doradus. Chandra highlights the physical processes that characterize the lives of these clusters, from the ionizing sources of ultracompact HII regions (W3) to superbubbles so large that they shape our views of galaxies (30 Dor). X-ray observations usually reveal hundreds of pre-main sequence (lower-mass) stars accompanying the OB stars that power these great HII region complexes, although in one case (W3 North) this population is mysteriously absent. The most massive stars themselves are often anomalously hard X-ray emitters; this may be a new indicator of close binarity. These complexes are sometimes suffused by soft diffuse X-rays (M17, NGC 3576), signatures of multi-million-degree plasmas created by fast O-star winds. In older regions we see the X-ray remains of the deaths of massive stars that stayed close to their birthplaces (Tr14, 30 Dor), exploding as cavity supernovae within the superbubbles that these clusters created. Revealing the Life Cycle of a Massive Stellar ClusterHigh-resolution X-ray images from the Chandra X-ray Observatory and XMM-Newton elucidate all stages in the life cycles of massive stars -from ultracompact HII (UCHII) regions to supernova remnants -and the effects that those massive stars have on their surroundings. X-ray studies of massive star-forming regions (MSFRs) thus give insight into the massive stars themselves, the accompanying lower-mass cluster population, new generations of stars that may be triggered by the massive cluster, interactions of massive star winds with themselves and with the surrounding neutral medium, and the fate of massive stars that die as cavity supernovae inside the wind-blown bubbles they created.In the era of Spitzer and excellent ground-based near-infrared (IR) data, why are X-ray studies of MSFRs important? Chandra routinely penetrates A V > 100 mag of extinction with little confusion or contamination, revealing young stellar populations in a manner that is unbiased by the presence of inner disks around these stars. These vast pre-main sequence (pre-MS) populations are easily seen by Chandra and the 17 ′ ×17 ′ imaging array of its Advanced CCD Imaging Spectrometer (ACIS-I); a typical ACIS-I observation of a nearby (D < 3 kpc) MSFR, lasting 40-100 ksec, finds 500-1500 young stars, tracing the initial mass function (IMF) down to ∼ 1M ⊙ . These observations can increase the number of known cluster members by as much as a factor of 50 in poorly-studied regions (e.g. NGC 6357, J. .In the first 7 months of 2006, at least 18 refereed papers on X-ray observations of MSFRs (using Chandra or XMM) were published or submitted. These include studies of 30 Doradus (Townsley et al. 2006a,b), NGC 6334 (Ezoe et al.

Understanding the Links among the Magnetic Fields, Filament, Bipolar Bubble, and Star Formation in RCW 57A Using NIR Polarimetry

“…This implies that cloud material has been either eroded or blown out by the expanding I-fronts, outflowing gas, or strong stellar winds from the embedded early type protostars. Therefore, CO and Spitzer maps suggest that bipolar bubble likely blown by the massive stars emanated stellar winds, outflowing gas, and expanding ionization fronts (see Townsley 2009).…”

“…The observed ratios of the infrared fine-structure ionic lines (Ne ii, Ar iii, and S iv) (Lacy et al 1982) indicate that at least eight O7.5V stars are necessary to account for the ionization of the region. However, even these stars may not be sufficient to account for the Lyα ionizing photons inferred from radio data (Figuerêdo et al 2002;Barbosa et al 2003;Townsley 2009). Based on the newly discovered cluster of stars, using X-ray data, Townsley et al (2014) suggested that an additional cluster of OB stars might be deeply embedded that were not known before because of heavy obscuration.…”

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“…The review is not comprehensive and much of the relevant research, both observational and theoretical, is still in progress. Other important topics include characterizing accretion shocks in PMS stars (8) and studying local templates for starburst galaxies (9). Discussion X-Rays from Protostellar Outflow Shocks.…”