Context. The Taurus Molecular Cloud (TMC) is the nearest large star-forming region, prototypical for the distributed mode of lowmass star formation. Pre-main sequence stars are luminous X-ray sources, probably mostly owing to magnetic energy release. Aims. The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) presented in this paper surveys the most populated ≈5 square degrees of the TMC, using the XMM-Newton X-ray observatory to study the thermal structure, variability, and long-term evolution of hot plasma, to investigate the magnetic dynamo, and to search for new potential members of the association. Many targets are also studied in the optical, and high-resolution X-ray grating spectroscopy has been obtained for selected bright sources. Methods. The X-ray spectra have been coherently analyzed with two different thermal models (2-component thermal model, and a continuous emission measure distribution model). We present overall correlations with fundamental stellar parameters that were derived from the previous literature. A few detections from Chandra observations have been added. Results. The present overview paper introduces the project and provides the basic results from the X-ray analysis of all sources detected in the XEST survey. Comprehensive tables summarize the stellar properties of all targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus by about an order of magnitude and for the first time systematically accesses very faint and strongly absorbed TMC objects. We find a detection rate of 85% and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169 surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios, and show evidence for lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis (e.g., variability, rotation-activity relations, influence of accretion on X-rays) will be discussed in a series of accompanying papers.

We investigate the long-term evolution of X-ray coronae of solar analogs based on high-resolution X-ray spectroscopy and photometry with XMM-Newton. Six nearby main-sequence G stars with ages between ≈ 0.1 Gyr and ≈ 1.6 Gyr and rotation periods between ≈ 1 d and 12.4 d have been observed. We use the X-ray spectra to derive coronal element abundances of C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measure distribution (EMD). We find that the abundances change from an inverse-First Ionization Potential (FIP) distribution in stars with ages around 0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr and beyond. This transformation is coincident with a steep decline of non-thermal radio emission. The results are in qualitative agreement with a simple model in which the stream of electrons in magnetic fields suppresses diffusion of low-FIP ions from the chromosphere into the corona. The coronal emission measure distributions show shapes characterized by power-laws on each side of the EMD peak. The latter shifts from temperatures of about 10 MK in the most rapidly rotating, young stars to temperatures around 4 MK in the oldest target considered here. The power-law index on the cooler side of the EMD exceeds expected slopes for static loops, with typical values being 1.5-3. We interpret this slope with a model in which the coronal emission is due to a superposition of stochastically occurring flares, with an occurrence rate that is distributed in radiated energy E as a power-law, dN/dE ∝ E −α , as previously found for solar and stellar flares. We obtain the relevant power-law index α from the slope of the high-temperature tail of the EMD. Our EMDs indicate α ≈ 2.2 − 2.8, in excellent agreement with values previously derived from light curves of magnetically active stars. Modulation with time scales reminiscent of flares is found in the light curves of all our targets. Several strong flares are also observed. We use our α values to simulate light curves and compare them with the observed light curves. We thus derive the range of flare energies required to explain the light-curve modulation. More active stars require a larger range of flare energies than less active stars within the framework of this simplistic model. In an overall scenario, we propose that flaring activity plays a larger role in more active stars. In this model, the higher flare rate is responsible both for the higher average coronal temperature and the high coronal X-ray luminosity, two parameters that are indeed found to be correlated.

Context. T Tau stars display different X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) to study differences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (L X ) vs. mass (M) relation, L X ∝ M 1.69 ± 0.11 , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L * ) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates L X = const.L * , although the constant is different for the two subsamples: const. = 10 −3.73 ± 0.05 for CTTS and const. = 10 −3.39 ± 0.06 for WTTS. Given a similar L * distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of ≈2 compared to WTTS. The average electron temperatures T av are correlated with L X in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed L X for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the L X vs. T av dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.

Context. Differences have been reported between the X-ray emission of accreting and non-accreting stars. Some observations have suggested that accretion shocks could be responsible for part of the X-ray emission in classical T Tauri stars (CTTS). Aims. We present high-resolution X-ray spectroscopy for nine pre-main sequence stars in order to test the proposed spectroscopic differences between accreting and non-accreting pre-main sequence stars. Methods. We used X-ray spectroscopy from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. We interpret the spectra using optically thin thermal models with variable abundances, together with an absorption column density. For BP Tau and AB Aur we derive electron densities from the O vii triplets. Results. Using the O vii/O viii count ratios as a diagnostic for cool plasma, we find that CTTS display a soft excess (with equivalent electron temperatures of ≈2.5−3 MK) when compared with WTTS or zero-age main-sequence stars. Although the O vii triplet in BP Tau is consistent with a high electron density (3.4 × 10 11 cm −3 ), we find low density for the accreting Herbig star AB Aur (n e < 10 10 cm −3 ). The element abundances of accreting and non-accreting stars are similar. The Ne abundance is found to be high (4−6 times the Fe abundance) in all K and M-type stars. In contrast, for the three G-type stars (SU Aur, HD 283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4−0.8 times solar photospheric values) compared to later-type stars. Conclusions. Adding the results from our sample to former high-resolution studies of T Tauri stars, we find a soft excess in all accreting stars, but in none of the non-accretors. On the other hand, high electron density and high Ne/Fe abundance ratios do not seem to be present in all accreting pre-main sequence stars.

Abstract. Spatial information from stellar X-ray coronae cannot be assessed directly, but scaling laws from the solar corona make it possible to estimate sizes of stellar coronae from the physical parameters temperature and density. While coronal plasma temperatures have long been available, we concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of stellar coronae with the Chandra and XMM-Newton gratings. We compiled a set of 64 grating spectra of 42 stellar coronae. Line counts of strong H-like and He-like ions and Fe  lines were measured with the CORA single-purpose line fitting tool by Ness & Wichmann (2002). Densities are estimated from He-like f /i flux ratios of O  and Ne  representing the cooler (1-6 MK) plasma components. The densities scatter between log n e ≈ 9.5−11 from the O  triplet and between log n e ≈ 10.5−12 from the Ne  triplet, but we caution that the latter triplet may be biased by contamination from Fe  and Fe  lines. We find that low-activity stars (as parameterized by the characteristic temperature derived from H-and He-like line flux ratios) tend to show densities derived from O  of no more than a few times 10 10 cm −3 , whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various Fe  line ratios, we found that none of the spectra consistently indicates the presence of very high densities. We argue that our measurements are compatible with the low-density limit for the respective ratios (≈5 × 10 12 cm −3 ). These upper limits are in line with constant pressure in the emitting active regions. We focus on the commonly used Rosner et al. (1978) scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive the emitting volumes from direct measurements of ionspecific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii, and are compared with the emitting volumes to infer filling factors. For all stages of activity we find similar filling factors up to 0.1.