X-Ray Emission from Massive Stars

A composite image of the massive star forming region Cygnus OB2. Astronomers have determined that the X-ray emission from massive stars arises from shocks. The image shows X-ray emission from Chandra (blue), infrared from Spitzer (red), and optical data from the Isaac Newton Telescope (orange). Credit: X-ray: NASA/CXC/SAO/J.Drake et al, Infrared: NASA/JPL-Caltech, Optical: Univ. of Hertfordshire/INT/IPHAS

Massive young stars are known to emit strong X-rays. Unlike the X-ray emission from lower mass stars, however, which arises in stellar photospheres, the X-rays from massive stars are thought to result from powerful shocks. Several kinds of shocks can be responsible, produced either by very strong winds driven by the star’s radiation, by the head-on collision between winds that have been magnetically channeled by the star’s magnetic field, or by wind collisions in a binary stellar system in which each stars has a wind. Sorting out the mechanisms enables astronomers to identify the most active physical processes at work, and thereby decode additional information about the star’s physical makeup and evolutionary status.

CfA astronomers Nick Wright, Jeremy Drake, and Marcio Guarcello and their colleagues used the Chandra X-ray Observatory to study the emission from 106 massive stars in the relatively nearby Cygnus-OB2 cluster. This relatively large sample enabled the scientist to test their models by examining, for example, whether or not there are clear correlations between a star’s X-ray strength and its luminosity.

The astronomers find for their massive stars that there is a well-defined correlation between the X-ray and total stellar luminosity, with the X-ray strength being about sixteen million times less; indeed, their relation is similar to one previously reported for another massive star-forming region, and favors the first (radiatively driven) kind of shocks. For the most massive stars in the sample, however, the team does find evidence for colliding shocks. The new results help to constrain models of X-ray emission from massive stars. Because the relations are about the same as in other massive star clusters but now extended to different clusters and cluster environments, the new work also shows that the mechanisms are not very sensitive to the local conditions.

Reference(s):

"X-Ray Emission from Massive Stars in Cyg OB2," G. Rauw, Y. Nazé, N. J. Wright, J. J. Drake, M. G. Guarcello, R. K. Prinja, L. W. Peck, J. F. Albacete Colombo, A. Herrero, H. A. Kobulnicky, S. Sciortino, and J. S. Vink, ApJS 221, 1, 2015.

Source: Smithsonian Astrophysical Observatory (SAO)

Astronomers Watch Unfolding Saga of Massive Star Formation

Artist's conception of the development of W75N(B)-VLA-2.. At left, a hot wind from the young star expands nearly spherically, as seen in 1996. At right, as seen in 2014, the hot wind has been shaped by encountering a dusty, donut-shaped torus around the star and appears elongated. Credit: Bill Saxton, NRAO/AUI/NSF.  JPG image

VLA images of W75N(B)-VLA-2: Top, 1996; Bottom, 2014

Credit: Carrasco-Gonzalez, et al., NRAO/AUI/NSF.  JPG image

A pair of images of a young star, made 18 years apart, has revealed a dramatic difference that is providing astronomers with a unique, "real-time" look at how massive stars develop in the earliest stages of their formation.

The astronomers used the National Science Foundation's Karl G. Jansky Very Large Array (VLA) to study a massive young star called W75N(B)-VLA 2, some 4200 light-years from Earth. They compared an image made in 2014 with an earlier VLA image from 1996.

"The comparison is remarkable," said Carlos Carrasco-Gonzalez of the Center of Radioastronomy and Astrophysics of the National Autonomous University of Mexico, leader of the research team. The 1996 image shows a compact region of a hot, ionized wind ejected from the young star. The 2014 image shows that ejected wind deformed into an distinctly elongated outflow.

"We're seeing this dramatic change in real time, so this object is providing us an exciting opportunity to watch over the next few years as a very young star goes through the early stages of its formation," Carrasco-Gonzalez said.

The scientists believe the young star is forming in a dense, gaseous environment, and is surrounded by a doughnut-shaped, dusty torus. The star has episodes in which it ejects a hot, ionized wind for several years. At first, that wind can expand in all directions, and so forms a spherical shell around the star. Later, the wind hits the dusty torus, which slows it. Wind expanding outward along the poles of the torus, where there is less resistance, moves more quickly, resulting in an elongated shape for the outflow.

"In the span of only 18 years, we've seen exactly what we predicted," Carrasco-Gonzalez said.

There are theoretical models developed to explain why nearly-spherical expansion of such outflows had been seen with young stars much more massive than the Sun, when narrower, beam-like outflows were expected based on observations of less-massive, Sun-like stars at similar stages of development. W75N(B)-VLA 2 is estimated to be about 8 times more massive than the Sun. The more-uniform outflows are seen in massive young stars in the first few thousand years of their lives, the stage at which W75N(B)-VLA 2 is thought to be.

"Our understanding of how massive young stars develop is much less complete than our understanding of how Sun-like stars develop," Carrasco-Gonzalez said. "It's going to be really great to be able to watch one as it changes. We expect to learn a lot from this object," he added.

Carrasco-Gonzalez worked with an international team of astronomers from Mexico, the Netherlands, Sweden, Spain, Korea, and Japan. The scientists reported their discovery in the journal Science.

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Source: National Radio Astronomy Observatory (NRAO)