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Authors
M. Guedel
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(abridged:) We study X-rays from jet-driving protostars and T Tau stars. We seek soft spectral components that may be due to shock emission, and shock-induced emission displaced from the stellar position. Two stellar samples are used, the first consisting of lightly absorbed T Tau stars with strong jets, the other containing protostars with disks seen nearly edge-on. The former sample was observed in the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), while Chandra archival data provided observations of the latter. We confirm the previously identified peculiar spectrum of DG Tau A and find similar phenomenology in GV Tau and DP Tau, suggesting a new class of X-ray spectra. These consist of a lightly absorbed, very soft component and a strongly absorbed, very hard component. The latter is flaring while little variability is detected in the former. The absorption of the harder component is about an order of magnitude higher than expected from the optical extinction assuming a standard gas-to-dust mass ratio. The flaring hard component represents active coronal emission. Its strong absorption is attributed to mass inflow from the accretion disk. The optical extinction is small because the dust has sublimated at larger distances. The weakly absorbed soft component cannot originate from the same location. Because the stars drive strong jets, we propose that the X-rays are generated in shocks in the jets. We find that for the three peculiar X-ray sources, the luminosity of the soft component roughly scales with the equivalent width of the [OI] 6300A line formed in the jets, and with the mass outflow rate. In the more strongly obscured protostars, the soft component is entirely absorbed, and only the hard, coronal component penetrates the envelope or the near-edge-on disk.
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We test models for the generation of X-rays in accreting T Tauri stars (TTS), using X-ray data from the classical TTS T Tau. High-resolution spectroscopy from the Reflection Grating Spectrometers on XMM-Newton is used to infer electron densities, element abundances and the thermal structure of the X-ray source. We also discuss the ultraviolet light curve obtained by the Optical Monitor, and complementary ground-based photometry. A high-resolution image from Chandra constrains contributions from the two companions of T Tau N. The X-ray grating spectrum is rich in emission lines, but shows an unusual mixture of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted by similar amounts of emission measure. The cool plasma confirms the picture of a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to that previously reported for other accreting TTS. Diagnostics from lines formed by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe abundance ratio is consistent with a trend in pre-main sequence stars in which this ratio depends on spectral type, but not on accretion. On the basis of line density diagnostics, we conclude that the density of the cool ``soft-excess plasma is orders of magnitude below that predicted for an accretion shock, assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue that loading of magnetic field lines with infalling material suppresses the heating process in a part of the corona. We thus suggest that the X-ray production of T Tau is influenced by the accretion process although the X-rays may not form in the bulk of the accretion footpoints.
We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the XMM-Newton Extended Survey of the Taurus Molecular Cloud. X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a soft excess in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams.
We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O {sc vii} triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.
We report the detection of X-ray emission from the jet-driving symbiotic star MWC 560. We observed MWC 560 with XMM-Newton for 36 ks. We fitted the spectra from the EPIC pn, MOS1 and MOS2 instruments with XSPEC and examined the light curves with the package XRONOS. The spectrum can be fitted with a highly absorbed hard X-ray component from an optically-thin hot plasma, a Gaussian emission line with an energy of 6.1 keV and a less absorbed soft thermal component. The best fit is obtained with a model in which the hot component is produced by optically thin thermal emission from an isobaric cooling flow with a maximum temperature of 61 keV, which might be created inside an optically-thin boundary layer on the surface of the accreting with dwarf. The derived parameters of the hard component detected in MWC 560 are in good agreement with similar objects as CH Cyg, SS7317, RT Cru and T CrB, which all form a new sub-class of symbiotic stars emitting hard X-rays. Our previous numerical simulations of the jet in MWC 560 showed that it should produce detectable soft X-ray emission. We infer a temperature of 0.17 keV for the observed soft component, i.e. less than expected from our models. The total soft X-ray flux (i.e. at < 3 keV) is more than a factor 100 less than predicted for the propagating jet soon after its birth (<0.3 yr), but consistent with the value expected due its decrease with age. The ROSAT upper limit is also consistent with such a decrease. We find aperiodic or quasi-periodic variability on timescales of minutes and hours, but no periodic rapid variability. All results are consistent with an accreting white dwarf powering the X-ray emission and the existence of an optically-thin boundary layer around it.
Hen 3-1341 is a symbiotic binary system consisting of a white dwarf and a red giant star that is one of about ten symbiotics that show hints of jets. The bipolar jets have been detected through displaced components of emission lines during its outburst from 1998 to 2004. These components disappeared when Hen 3-1341 reached quiescence. On February 23, 2012, Hen 3-1341 started a new outburst with the emergence of new bipolar jets on March 3, 2012. We observed Hen 3-1341 during quiescence with XMM-Newton in March 2010 with an effective exposure time of 46.8 ks and with Swift on March 8-11, 2012 as ToO observations with an effective exposure time of 10 ks in order to probe the interaction of the jet with the ambient medium and also the accretion onto the white dwarf. We fitted the XMM-Newton X-ray spectra with XSPEC and examined the X-ray and UV light curves. We report the detection of X-ray emission during quiescence from Hen 3-1341 with XMM-Newton. The spectrum can be fitted with an absorbed one-temperature plasma or an absorbed blackbody. We did not detect Hen 3-1341 during our short Swift exposure. Neither periodic or aperiodic X-ray nor UV variability were found. Our XMM-Newton data suggest that interaction of the residual jet with the interstellar medium might survive for a long time after outbursts and might be responsible for the observed X-ray emission during quiescence. Additional data are strongly needed to confirm these suggestions.
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