DN Tauri - coronal activity and accretion in a young low-mass CTTS

We present a deep XMM-Newton observation of DN Tau, a young M0-type accreting CTTS and investigate its X-ray properties and X-ray generating mechanisms. Specifically we examine the presence of X-ray emission from magnetic activity and accretion shocks and compare our new X-ray data with UV data taken simultaneously and with X-ray/UV observations performed before. We find that the X-ray emission from DN Tau is dominated by coronal plasma generated via magnetic activity, but also clearly detect a contribution of the accretion shocks to the cool plasma component at about 2 MK as consistently inferred from density and temperature analysis. Typical phenomena of active coronae like flaring, the presence of very hot plasma at 30 MK and an abundance pattern showing the inverse FIP effect are seen on DN Tau. Strong variations in the emission measure of the cooler plasma components between the 2005 and 2010 XMM data point to accretion related changes. The UV light curve taken simultaneously is in general not related to the X-ray brightness, but exhibits clear counterparts during the observed X-ray flares. The X-ray properties of DN Tau are similar to those of more massive CTTS, but its low mass and large radius shift the accretion shocks to lower temperatures, reducing their imprint in the X-ray regime.Comment: 11 pages, 8 figures, accepted for publication in A&

X-ray activity cycle on the active ultra-fast rotator AB Dor A? Implication of correlated coronal and photometric variability

Although chromospheric activity cycles have been studied in a larger number of late-type stars for quite some time, very little is known about coronal activity-cycles in other stars and their similarities or dissimilarities with the solar activity cycle. While it is usually assumed that cyclic activity is present only in stars of low to moderate activity, we investigate whether the ultra-fast rotator AB Dor, a K dwarf exhibiting signs of substantial magnetic activity in essentially all wavelength bands, exhibits a X-ray activity cycle in analogy to its photospheric activity cycle of about 17 years and possible correlations between these bands. We analysed the combined optical photometric data of AB Dor A, which span ~35 years. Additionally, we used ROSAT and XMM-Newton X-ray observations of AB Dor A to study the long-term evolution of magnetic activity in this active K dwarf over nearly three decades and searched for X-ray activity cycles and related photometric brightness changes. AB Dor A exhibits photometric brightness variations ranging between 6.75 < Vmag < 7.15 while the X-ray luminosities range between 29.8 < log LX [erg/s] < 30.2 in the 0.3-2.5 keV. As a very active star, AB Dor A shows frequent X-ray flaring, but, in the long XMM-Newton observations a kind of basal state is attained very often. This basal state probably varies with the photospheric activity-cycle of AB Dor A which has a period of ~17 years, but, the X-ray variability amounts at most to a factor of ~2, which is, much lower than the typical cycle amplitudes found on the Sun.Comment: 10 page

A magnetic cycle of tau Bootis? The coronal and chromospheric view

Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter. During the last years spectropolarimetric observations led to the hypothesis that this star may host a global magnetic field that switches its polarity once per year, indicating a very short activity cycle of only one year duration. In our ongoing observational campaign, we have collected several X-ray observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to characterize tau Boo's corona and chromosphere over the course of the supposed one-year cycle. Contrary to the spectropolarimetric reconstructions, our observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29 (2012

The Chandra X-ray view of the power sources in Cepheus A

The central part of the massive star-forming region Cepheus A contains several radio sources which indicate multiple outflow phenomena, yet the driving sources of the individual outflows have not been identified. We present a high-resolution Chandra observation of this region that shows the presence of bright X-ray sources, consistent with active pre-main sequence stars, while the strong absorption hampers the detection of less luminous objects. A new source has been discovered located on the line connecting H_2 emission regions at the eastern and western parts of Cepheus A. This source could be the driving source of HH 168. We present a scenario relating the observed X-ray and radio emission.Comment: 7 pages, 6 figures, accepted for publication in A&

Spatially resolved X-ray emission of EQ Pegasi

We present an analysis of an XMM-Newton observation of the M dwarf binary EQ Pegasi with a special focus on the the spatial structure of the X-ray emission and the analysis of light curves. Making use of data obtained with EPIC (European Photon Imaging Camera) we were for the first time able to spatially resolve the two components in X-rays and to study the light curves of the individual components of the EQ Peg system. During the observation a series of moderate flares was detected, where it was possible to identify the respective flaring component.Comment: 6 pages, 11 figures, accepted by A&

Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths

We present new X-ray observations obtained with Chandra ACIS-S of the HD 189733 system, consisting of a K-type star orbited by a transiting Hot Jupiter and an M-type stellar companion. We report a detection of the planetary transit in soft X-rays with a significantly larger transit depth than observed in the optical. The X-ray data favor a transit depth of 6-8%, versus a broadband optical transit depth of 2.41%. While we are able to exclude several possible stellar origins for this deep transit, additional observations will be necessary to fully exclude the possibility that coronal inhomogeneities influence the result. From the available data, we interpret the deep X-ray transit to be caused by a thin outer planetary atmosphere which is transparent at optical wavelengths, but dense enough to be opaque to X-rays. The X-ray radius appears to be larger than the radius observed at far-UV wavelengths, most likely due to high temperatures in the outer atmosphere at which hydrogen is mostly ionized. We furthermore detect the stellar companion HD 189733B in X-rays for the first time with an X-ray luminosity of log LX = 26.67 erg/s. We show that the magnetic activity level of the companion is at odds with the activity level observed for the planet-hosting primary. The discrepancy may be caused by tidal interaction between the Hot Jupiter and its host star.Comment: 15 pages, accepted for publication in The Astrophysical Journa