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High resolution X-ray spectra of very young massive stars opened a new chapter in the diagnostics and understanding of the properties of stellar wind plasmas. Observations of several very young early type stars in the Orion Trapezium demonstrated that the conventional model of shock heated plasmas in stellar winds is not sufficient to explain the observed X-ray spectra. Detailed X-ray line diagnostics revealed extreme temperatures in some of the candidates as well as evidence for high plasma densities. It is also evident from high resolution spectra of more conventional early type stars, that not all show such extreme characteristics. However, the fact that some of the stars show hot and dense components and some do not requires more understanding of the physical processes involved in stellar wind emissions. The Orion Trapezium stars distinguish themselves from all the others by their extreme youth. By comparing the diverse spectral properties of theta Ori A and theta Ori E with those of theta Ori C, we further demonstrate that X-ray spectral properties of very young massive stars are far from understood.
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Strong winds from massive stars are a topic of interest to a wide range of astrophysical fields. In High-Mass X-ray Binaries the presence of an accreting compact object on the one side allows to infer wind parameters from studies of the varying properties of the emitted X-rays; but on the other side the accretors gravity and ionizing radiation can strongly influence the wind flow. Based on a collaborative effort of astronomers both from the stellar wind and the X-ray community, this presentation attempts to review our current state of knowledge and indicate avenues for future progress.
The cluster NGC 3603 hosts some of the most massive stars in the Galaxy. With a modest 50 ks exposure with the Chandra High Energy Grating Spectrometer, we have resolved emission lines in spectra of several of the brightest cluster members which are of WNh and O spectral types. This observation provides our first definitive high-resolution spectra of such stars in this nearby starburst region. The stars studied have broadened X-ray emission lines, some with blue-shifted centroids, and are characteristic of massive stellar winds with terminal velocities around 2000--3000 km/s. X-ray luminosities and plasma temperatures are very high for both the WNh and O stars studied. We conclude that their X-rays are likely the result of colliding winds.
The supersonic stellar and disk winds possessed by massive young stellar objects will produce shocks when they collide against the interior of a pre-existing bipolar cavity (resulting from an earlier phase of jet activity). The shock heated gas emits thermal X-rays which may be observable by spaceborne observa- tories such as the Chandra X-ray Observatory. Hydrodynamical models are used to explore the wind-cavity interaction. Radiative transfer calculations are performed on the simulation output to produce synthetic X-ray observations, allowing constraints to be placed on model parameters through comparisons with observations. The model reveals an intricate interplay between the inflowing and outflowing material and is successful in reproducing the observed X-ray count rates from massive young stellar objects.
Although the environments of star and planet formation are thermodynamically cold, substantial X-ray emission from 10-100 MK plasmas is present. In low mass pre-main sequence stars, X-rays are produced by violent magnetic reconnection flares. In high mass O stars, they are produced by wind shocks on both stellar and parsec scales. The recent Chandra Orion Ultradeep Project, XMM-Newton Extended Survey of Taurus, and Chandra studies of more distant high-mass star forming regions reveal a wealth of X-ray phenomenology and astrophysics. X-ray flares mostly resemble solar-like magnetic activity from multipolar surface fields, although extreme flares may arise in field lines extending to the protoplanetary disk. Accretion plays a secondary role. Fluorescent iron line emission and absorption in inclined disks demonstrate that X-rays can efficiently illuminate disk material. The consequent ionization of disk gas and irradiation of disk solids addresses a variety of important astrophysical issues of disk dynamics, planet formation, and meteoritics. New observations of massive star forming environments such as M 17, the Carina Nebula and 30 Doradus show remarkably complex X-ray morphologies including the low-mass stellar population, diffuse X-ray flows from blister HII regions, and inhomogeneous superbubbles. X-ray astronomy is thus providing qualitatively new insights into star and planet formation.
The X-ray emission from a simulated massive stellar cluster is investigated. The emission is calculated from a 3D hydrodynamical model which incorporates the mechanical feedback from the stellar winds of 3 O-stars embedded in a giant molecular cloud clump containing 3240 M$_{odot}$ of molecular material within a 4 pc radius. A simple prescription for the evolution of the stars is used, with the first supernova explosion at t=4.4 Myrs. We find that the presence of the GMC clump causes short-lived attenuation effects on the X-ray emission of the cluster. However, once most of the material has been ablated away by the winds the remaining dense clumps do not have a noticable effect on the attenuation compared with the assumed interstellar medium column. We determine the evolution of the cluster X-ray luminosity, L$_X$, and spectra, and generate synthetic images. The intrinsic X-ray luminosity drops from nearly 10$^{34}$ ergs s$^{-1}$ while the winds are `bottled up, to a near constant value of 1.7$times 10^{32}rm ergs s^{-1}$ between t=1-4 Myrs. L$_X$ reduces slightly during each stars red supergiant stage due to the depressurization of the hot gas. However, L$_X$ increases to $approx 10^{34}rm,ergs s^{-1}$ during each stars Wolf-Rayet stage. The X-ray luminosity is enhanced by 2-3 orders of magnitude to $sim 10^{37}rm ergs s^{-1}$ for at least 4600 yrs after each supernova, at which time the blast wave leaves the grid and the X-ray luminosity drops. The X-ray luminosity of our simulation is generally considerably fainter than predicted from spherically-symmetric bubble models, due to the leakage of hot gas material through gaps in the outer shell. This process reduces the pressure within our simulation and thus the X-ray emission. However, the X-ray luminosities and temperatures which we obtain are comparable to similarly powerful massive young clusters.
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