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تسجيل مستخدم جديدMagnetic fields, winds and X-rays of massive stars: A spectropolarimetric survey of the Orion cluster
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In massive stars, magnetic fields are thought to confine the outflowing radiatively-driven wind, resulting in X-ray emission that is harder, more variable and more efficient than that produced by instability-generated shocks in non-magnetic winds. Although magnetic confinement of stellar winds has been shown to strongly modify the mass-loss and X-ray characteristics of massive OB stars, we lack a detailed understanding of the complex processes responsible. The aim of this study is to examine the relationship between magnetism, stellar winds and X-ray emission of OB stars. In conjunction with a Chandra survey of the Orion Nebula Cluster, we carried out spectropolarimatric ESPaDOnS observations to determine the magnetic properties of massive OB stars of this cluster. We found of two new massive magnetic stars in the Orion Nebula Cluster: HD 36982 and HD 37061, for which the estimated dipole polar strengths are 1150 (+320 -200) G and 620 (+220 -170) G, respectively. However, the apparent lack of clear correlation between X-ray indicator and the presence of a magnetic fields brings forth new challenges for understanding the processes leading to X-ray emission in massive stars.
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In massive stars, magnetic fields are thought to confine the outflowing radiatively-driven wind, resulting in X-ray emission that is harder, more variable and more efficient than that produced by instability-generated shocks in non-magnetic winds. Al
though magnetic confinement of stellar winds has been shown to strongly modify the mass-loss and X-ray characteristics of massive OB stars, we lack a detailed understanding of the complex processes responsible. The aim of this study is to examine the relationship between magnetism, stellar winds and X-ray emission of OB stars. In conjunction with a Chandra survey of the Orion Nebula Cluster, we carried out spectropolarimatric ESPaDOnS observations to determine the magnetic properties of massive OB stars of this cluster.
The origin of the magnetic fields in neutron stars, and the physical differences between magnetars and strongly magnetised radio pulsars are still under vigorous debate. It has been suggested that the properties of the progenitors of neutron stars (t
he massive OB stars), such as rotation, magnetic fields and mass, may play an important role in the outcome of core collapse leading to type II SNe. Therefore, knowing the magnetic properties of the progenitor OB stars would be an important asset for constraining models of stellar evolution leading to the birth of a neutron star. We present here the beginning of a broad study with the goal of characterising the magnetic properties of main sequence massive OB stars. We report the detection of two new massive magnetic stars in the Orion Nebula Cluster: Par 1772 (HD 36982) and NU Ori (HD 37061), for which the estimated dipole polar strengths, with 1 sigma error bars, are 1150 (+320,-200) G and 650 (+220,-170) G respectively.
One prediction of particle acceleration in the supernova remnants in the magnetic wind of exploding Wolf Rayet and Red Super Giant stars is that the final spectrum is a composition of a spectrum $E^{-7/3}$ and a polar cap component of $E^{-2}$ at the
source. This polar cap component contributes to the total energy content with only a few percent, but dominates the spectrum at higher energy. The sum of both components gives spectra which curve upwards. The upturn was predicted to occur always at the same rigidity. An additional component of cosmic rays from acceleration by supernovae exploding into the Inter-Stellar Medium (ISM) adds another component for Hydrogen and for Helium. After transport the predicted spectra $J(E)$ for the wind-SN cosmic rays are $E^{-8/3}$ and $E^{-7/3}$; the sum leads to an upturn from the steeper spectrum. An upturn has now been seen. Here, we test the observations against the predictions, and show that the observed properties are consistent with the predictions. Hydrogen can be shown to also have a noticeable wind-SN-component. The observation of the upturn in the heavy element spectra being compatible with the same rigidity for all heavy elements supports the magneto-rotational mechanism for these supernovae. This interpretation predicts the observed upturn to continue to curve upwards and approach the $E^{-7/3}$ spectrum. If confirmed, this would strengthen the case that supernovae of very massive stars with magnetic winds are important sources of Galactic cosmic rays.
A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075 sources (Feigelson et al. 2002), providing a uniquely large and well-defined sample to study the dependence of magnetic activity on bulk properties for stars descending the H
ayashi tracks. The following results are obtained: (1) X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with bolometric luminosity with <log L_t/L_bol> = -3.8 for stars with masses 0.7<M<2 Mo, an order of magnitude below the main sequence saturation level; (2) the X-ray emission drops rapidly below this level in some or all stars with 2<M<3 Mo; (3) the presence or absence of infrared circumstellar disks has no apparent relation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise as rotational periods increase from 0.4 to 20 days. This last finding stands in dramatic contrast to the strong anticorrelation between X-rays and period seen in main sequence stars. The absence of a strong X-ray/rotation relationship in PMS stars, and particularly the high X-ray values seen in some very slowly rotating stars, is a clear indication that the mechanisms of magnetic field generation differ from those operating in main sequence stars. The most promising possibility is a turbulent dynamo distributed throughout the deep convection zone, but other models such as alpha-Omega dynamo with `supersaturation or relic core fields are not immediately excluded. The drop in magnetic activity in intermediate-mass stars may reflect the presence of a significant radiative core. The evidence does not support X-ray production in large-scale star-disk magnetic fields.
Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. To investigate whether magnetic fields in massive st
ars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Spectropolarimetric observations were obtained during four different nights using the low-resolution spectropolarimetric mode of FORS2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. The presence of a magnetic field is confirmed in nine stars previously observed with FORS1/2: HD36879, HD47839, CPD-282561, CPD-472963, HD93843, HD148937, HD149757, HD328856, and HD164794. New magnetic field detections at a significance level of at least 3sigma were achieved in five stars: HD92206c, HD93521, HD93632, CPD-468221, and HD157857. Among the stars with a detected magnetic field, five stars belong to open clusters with high membership probability. According to previous kinematic studies, five magnetic O-type stars in our sample are candidate runaway stars.
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