New X-ray observations of IQ Aurigae and alpha2 Canum Venaticorum - Probing the magnetically channelled wind shock model in A0p stars

Astronomy and Astrophysics (Impact Factor: 4.48). 05/2011; 531.
Source: arXiv

ABSTRACT We present new X-ray observations of the A0p stars alpha^2 CVn (log Lx < 26.0
erg/s) and IQ Aur (log Lx = 29.6 erg/s) and find that their X-ray luminosities
differ by at least three orders of magnitude. IQ Aur possesses a strong cool
plasma component with X-ray emitting regions located well above the stellar
surface, but also significant amounts of hot plasma. Further, a large X-ray
flare is detected from IQ Aur, implying the presence of magnetic reconnection.
Our comparison study of similar stars indicates that the occurrence of X-ray
emission generated by magnetically channelled wind shocks (MCWS) strongly
depends on stellar properties. X-ray emission is preferably generated by more
luminous and massive objects such as IQ Aur. The MCWS scenario can consistently
describe the X-ray emission of these A0p stars, assuming that the very strong
magnetic confinement of the stellar wind has led to the build up of a rigidly
rotating disk around the star, where magnetic reconnection and centrifugal
breakout events occur.

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    ABSTRACT: We present a comprehensive statistical investigation of the evolution of magnetic CP stars, aimed at providing constraints to the theories that deal with the origin of the magnetic field in these stars. We have collected from the literature data for 150 magnetic CP stars with accurate Hipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1 observations of circularly polarized spectra for 100 stars. From these spectra we have measured the mean longitudinal magnetic field, and discovered 48 new magnetic CP stars (five of which belonging to the rare class of rapidly oscillating Ap stars). We have determined effective temperature and luminosity, then mass and position in the H-R diagram for a final sample of 194 magnetic CP stars. We found that magnetic stars with M>3Msun are homogeneously distributed along the main sequence. Instead, there are statistical indications that lower mass stars (especially those with M<=2Msun) tend to concentrate in the centre of the main sequence band. We show that this inhomogeneous age distribution cannot be attributed to the effects of random errors and small number statistics. Our data suggest also that the surface magnetic flux of CP stars increases with stellar age and mass, and correlates with the rotation period. For stars with M>3Msun, rotation periods decrease with age in a way consistent with the conservation of the angular momentum, while for less massive magnetic CP stars an angular momentum loss cannot be ruled out. The mechanism that originates and sustains the magnetic field in the upper main sequence stars may be different in CP stars of different mass. Comment: Accepted by A&A; 13 pages, 10 figures
    Astronomy and Astrophysics 01/2006; DOI:10.1051/0004-6361:20054596 · 4.48 Impact Factor
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    ABSTRACT: Context.Low-mass stars are known to exhibit strong X-ray emission during their early evolutionary stages. This also applies to classical T Tauri stars (CTTS), whose X-ray emission differs from that of main-sequence stars in a number of aspects.Aims.We study the specific case of RU Lup, a well known accreting and wind-driving CTTS. In comparison with other bright CTTS we study possible signatures of accretion and winds in their X-ray emission.Methods.Using three XMM-Newton observations of RU Lup, we investigate its X-ray properties and their generating mechanisms. High-resolution X-ray spectra of RU Lup and other CTTS are compared to main-sequence stars. We examine the presence of a cool plasma excess and enhanced plasma density in relation to X-rays from accretion shocks and investigate anomalous strong X-ray absorption and its connection to winds or circumstellar material.Results.We find three distinguishable levels of activity among the observations of RU Lup. While no large flares are present, this variability is clearly of magnetic origin due to the corresponding plasma temperatures of around 30 MK; in contrast the cool plasma component at 2–3 MK is quite stable over a month, resulting in a drop of average plasma temperature from 35 MK down to 10 MK. Density analysis with the $\ion{O}{vii}$ triplet indicates high densities in the cool plasma, suggesting accretion shocks to be a significant contributor to the soft X-ray emission. No strong overall metal depletion is observed, with Ne being more abundant than Fe, that is at solar value, and especially O. Excess emission at 6.4 keV during the more active phase suggest the presence of iron fluorescence. Additionally RU Lup exhibits an extraordinary strong X-ray absorption, incompatible with estimates obtained at optical and UV wavelengths. Comparing spectra from a sample of main-sequence stars with those of accreting stars we find an excess of cool plasma as evidenced by lower $\ion{O}{viii}$/$\ion{O}{vii}$ line ratios in all accreting stars. High density plasma appears to be only present in low-mass CTTS, while accreting stars with intermediate masses (${\gtrsim}2~M_{\odot}$) have lower densities.Conclusions.In all investigated CTTS the characteristics of the cooler X-ray emitting plasma are influenced by the accretion process. We suspect different accretion rates and amounts of funnelling, possibly linked to stellar mass and radius, to be mainly responsible for the different properties of their cool plasma component. The exceptional X-ray absorption in RU Lup and other CTTS is probably related to the accretion flows and an optically transparent wind emanating from the star or the disk.
    Astronomy and Astrophysics 10/2007; 473(1). DOI:10.1051/0004-6361:20077644 · 4.48 Impact Factor
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    ABSTRACT: Context. Herbig Ae/Be stars (HAeBe) are pre-main sequence objects in the mass range 2 $M_{\odot}$ $<$ $M_*$ $<$ 8 $M_{\odot}$. Their X-ray properties are uncertain and, as yet, unexplained.Aims. We want to elucidate the X-ray generating mechanism in HAeBes.Methods. We present an XMM-Newton observation of the HAeBe HD 163296. We analyse the light curve, the broad band and the grating spectra, fit emission measures and abundances and apply models for accretion and wind shocks.Results. We find three temperature components ranging from 0.2 keV to 2.7 keV. The $\ion{O}{vii}$ He-like triplet indicates an X-ray formation region in a low density environment with a weak UV photon field, i.e. above the stellar surface. This makes an origin in an accretion shock unlikely; instead we suggest a shock at the base of the jet for the soft component and a coronal origin for the hot component. A mass outflow of $\dot M_{\rm shock} \approx 10^{-10}~M_{\odot}$ yr$^{-1}$ is sufficient to power the soft X-rays.Conclusions. HD 163296 is thought to be single, so this data represent genuine HAeBe X-ray emission. HD 163296 might be prototypical for its class.
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