Dust and glowing hydrogen obscure the Carina complex at visible wavelengths. An X-ray study, combined with infrared surveys, provides knowledge of newly formed stellar associations and past supernova explosions in this system.
An association of young stars and its nascent environment is a chaotic region driven by massive stellar winds and ejecta from explosions known as supernovae. Whereas the region's diffuse core may glow with light from the reformation of hydrogen that had been ionized by the hottest stellar members, its dusty cloud hides the cooler stellar membership. At visible wavelengths, a full inventory of such a recently formed (within the past 10 million years) stellar group is nearly impossible. Multi-wavelength studies, from radio to infrared to ultraviolet and even X-rays, are essential to gaining a complete picture of how a stellar association forms. A large team of astronomers has now published a string of sixteen papers, including four1,2,3,4 discussed here, describing an X-ray study that sheds light on one of the most enigmatic of such chaotic regions — the Carina complex. The papers form an entire issue of the Astrophysical Journal Supplement Series.
Picture an area in the sky, located in the Milky Way, that is three times the diameter of the Moon. Viewing the Galaxy's Sagittarius–Carina spiral arm from Earth, astronomers find within the Carina region an extraordinary complex of massive stars, recently formed stellar associations and large regions of ionized hydrogen bounded by dusty clouds that have ongoing stellar birthplaces buried within. The obscuring dust limits astronomers from obtaining a full understanding of everything that is going on in this region. But as the Galaxy is relatively transparent to X-rays, the X-ray spectral region provides a means of peering through much of this dust and gaining a more complete census.
In their study, known as the Chandra Carina Complex Project, the team of astronomers obtained a deep X-ray survey of the Carina complex (Fig. 1) using the Chandra X-ray Observatory, one of the four great observatories launched by NASA in the past two decades. The team's focus was on point-like X-ray sources, as well as diffuse X-ray emission, from recent and ongoing star-formation regions within the Carina complex. The Chandra Observatory provided unsurpassed X-ray sensitivity and spatial resolution, allowing separation of extended diffuse emission from point-like sources. Over 14,000 point sources were detected within a 1.5-square-degree region of sky observed in the allocated 1.2 megaseconds of observatory time. In comparison, the entire catalogue of X-ray sources found by the Einstein Observatory in the 1980s, obtained with much lower spatial resolution and X-ray sensitivity, listed only 5,000 sources scattered over an extensive portion of the celestial sphere. Within the Carina complex, only a few dozen sources were identified.
The extended team compared the X-ray data to visible and infrared data obtained from ground and space observatories. Positional correlations between the point sources in the several surveys, along with their combined spectral energy distributions, led to the identification of the X-ray sources. Gagné et al.1 demonstrated that, of the 200 known O- and B-type stars (the two brightest and most massive class of stars), 68 of the 70 O-stars and about half of the earlier (more massive, hotter) B-type stars were detected at X-ray wavelengths.
There are at least four mechanisms of X-ray emission from hot massive stars in general. First, single O-stars have such massive winds that they develop instabilities, leading to density clumps within the winds. High-speed gas moves past slow-moving gas clumps, resulting in shocks and X-ray emission. Second, the massive winds of binary O-star systems in close, elliptical orbits collide, leading to strong X-ray emission, which is modulated by the stars' orbital period (the famous Eta Carinae binary is a classic example of this phenomenon). Third, a few O-stars have high, large-scale magnetic fields (with a strength of about 1 kilogauss) resulting in gas flows from their polar regions that collide at their magnetic equators, thus leading to strong X-ray emissions. Finally, some B-stars seem to have lower-mass companions at the 'pre-main-sequence' stage of stellar evolution; sometimes, their X-ray emissions are the only indications of the unseen companions. More results from the Chandra Carina Complex Project will lead to greatly increased knowledge of these mechanisms and the possible discovery of new causes of X-ray emission in massive stars.
Povich et al.2 compared the X-ray point sources to the Two Micron All-Sky Survey and the Spitzer Space Telescope Vela-Carina survey and identified 94 candidate O- or B-stars that now await confirmation by subsequent studies, thus potentially increasing the number of known OB stars in the Carina region by 50%. Preibisch et al.3 made a partial study of the Carina complex at sub-arcsecond angular resolution in the near infrared using the European Southern Observatory Very Large Telescope. Of the more than 7,000 Chandra X-ray sources within the surveyed field, nearly 90% have an identified near-infrared stellar counterpart. (Preibisch and colleagues' study is sensitive to near-infrared stellar counterparts down to 0.5 to 1.0 solar masses.) Many more results will be published in the near future, most notably on the stellar initial mass function (how many stars formed for each mass interval) of the Carina stellar associations.
Perhaps the most important result of the Chandra Carina Complex Project is the determination of the diffuse X-ray contribution by Townsley and colleagues4. The team carefully subtracted out all point-like (stellar) sources and found that the majority of the X-ray emission is truly diffuse. Much of the X-ray emission can be associated with the edges of cold, neutral clouds where fast-moving gas in the winds and supernova ejecta collide. The massive wind of a single O-star is known to create an interstellar bubble that grows as the star evolves. Eventually the O-star explodes as a supernova that increases the size of the bubble. With time, multiple bubbles from many O- and B-star winds and supernovae overlap and combine into a superbubble. Indeed, the Carina nebula — the extended region of emission — is a superbubble, being the combination of multiple stellar winds and supernova ejecta. Across the surveyed region, Townsley and colleagues' analysis of the diffuse X-ray emission allowed them to identify portions of the X-ray nebula as caused by stellar winds (low-energy X-rays) and supernova ejecta (high-energy X-rays).
This impressive X-ray survey and its initial correlations with visible and infrared surveys open a treasure trove that will lead to much new understanding on how star associations form and evolve with time. We anticipate many, even more exciting, results to appear in the journals soon.
Gagné, M. et al. Astrophys. J. Suppl. Ser. 194, 5 (2011).
Povich, M. S. et al. Astrophys. J. Suppl. Ser. 194, 6 (2011).
Preibisch, T. et al. Astrophys. J. Suppl. Ser. 194, 10 (2011).
Townsley, L. K. et al. Astrophys. J. Suppl. Ser. 194, 15 (2011).
Townsley, L. K. et al. Astrophys. J. Suppl. Ser. 194, 1 (2011).
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Astronomy & Astrophysics (2018)