Refine
Has Fulltext
- no (2)
Document Type
- Article (2)
Language
- English (2) (remove)
Is part of the Bibliography
- yes (2)
Keywords
- X-rays: stars (2) (remove)
Institute
- Institut für Physik und Astronomie (2) (remove)
We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-Ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H II region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low-and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the newborn stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low-and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.
The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.