Chandra's First Decade of Discovery

Session 2: High resolution Stellar Spectroscopy

X-ray emission processes in stars and their immediate environment

Paola Testa, Harvard-Smithsonian Center for Astrophysics

A decade of X-ray stellar observations with Chandra and XMM-Newton has led to significant advances in our understanding of the physical processes at work in hot (magnetized) plasmas in stars and their immediate environment, providing new perspectives and challenges, and in turn the need for improved models. The wealth of high-quality stellar spectra has allowed us to investigate, in detail, the characteristics of the X-ray emission across the HR diagram. Progress has been made in addressing issues ranging from classical stellar activity in stars with solar-like dynamos (such as, flares, activity cycles, spatial and thermal structuring of the X-ray emitting plasma, evolution of X-ray activity with age,...), to X-ray generating processes (e.g. accretion, jets, magnetically confined winds,..) that were poorly understood in the pre-Chandra/XMM-Newton era. I will discuss the progress made in the study of high energy stellar physics and its impact in a wider astrophysics context, focusing on the role of spectral diagnostics now accessible.

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The Shocking Truth about Massive Stars

Lidia Oskinova, University of Potsdam
W.-R. Hamann (Uni Potsdam), A. Feldmeier (Uni Potsdam)

X-ray emission from massive stars manifests the dynamic nature of their powerful stellar winds. Observations made by Chandra and XMM-Newton in the last decade, and follow up discoveries revolutionized our perception of stellar winds. I will show that Chandra spectroscopy is a perfect tool to answer the most pressing problems in wind research, such as inferring the parameters of wind clumping. This is pre-requisite to obtain empirically correct stellar mass-loss rate - a key ingredient of stellar feedback. X-rays also provide the best means to study the physics of stellar wind. Shocks, magnetic fields, rotation, which are typically ignored in classical models, must be accounted for to understand X-rays in massive stars. I will briefly review our current work based on the analyses of the latest Chandra and XMM-Newton observations of stars with spectral types from B to WO. We have fairly good ideas how X-rays are produced in O stars via wind shocks. We have the suspicion that some additional mechanisms should be at work in B stars, but we have little understanding yet about X-rays in WR stars. The latter are immediate SN and GRB progenitors. I will consider a possible scenario to explain the X-rays from WR stars, and discuss its implication for the physics of progenitors and their circumstellar environment.

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Collisionless plasmas in WR140, η Carinae and other colliding-wind systems.

Andy Pollock, XMM-SOC European Space Agency
Michael F. Corcoran (USRA-CRESST/NASA-GSFC)

The several-year orbits of the most X-ray brilliant colliding-wind systems WR140 (WC7+O5) and η Carinae (LBV) up to their nearly simultaneous periastron passages in 2009 January have given the opportunity to make direct high-resolution measurements of the collisionless plasma at different orbital phases. The clear differences are compared with the predictions of analytical and numerical models of the physics and dynamics involved. They also provide outstanding elemental abundance estimates for stellar evolutionary purposes and inform the shock physics at work in the binary systems in which the majority of massive stars are formed.

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Discovery of an Accretion-Fed Corona in an Accreting Young Star

Nancy Brickhouse, Smithsonian Astrophysical Observatory
S. R. Cranmer (SAO), A. K. Dupree (SAO), G. J. M. Luna (SAO), S. Wolk (SAO)

A deep (489 ks) Chandra High Energy Transmission Grating spectrum of the classical T Tauri star TW Hydrae shows a new type of coronal structure that is produced by the accretion process. In the standard model for a stellar dipole, the magnetic field truncates the disk and channels the accreting material onto the star. The He-like diagnostic lines of Ne IX provide excellent agreement with the shock conditions predicted by this model, with an electron temperature of 2.5 MK and electron density of 3 x 1012 cm-3 (see also Kastner et al. 2002). However, the standard model completely fails to predict the post-shock conditions, significantly overpredicting both the density and absorption observed at O VII. Instead the observations require a second “post-shock” component with 30 times more mass and 1000 times larger volume than found at the shock itself. We note that in the standard model, the shocked plasma is conveniently located near both closed (coronal) and open (stellar wind) magnetic structures, as the magnetic field connecting the star and disk also separates the open and closed field regions on the stellar surface. The shocked plasma thus can provide the energy to heat not only the post-shock plasma, but also adjacent regions (i.e. an “accretion-fed corona”) and drive stellar material into surrounding coronal structures. These observations provide new clues to the puzzling soft X-ray excess found in accreting systems, which depends on both the presence of accretion and the level of coronal activity (Guedel and Telleschi 2007). This Large Program with Chandra demonstrates the value of high signal-to-noise, high resolution spectroscopy for understanding the complex interaction of magnetic and accretion processes in late-type star formation.

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