Stephen L. Skinner

The X-Ray Sun in Time: A Study of the Long-Term Evolution of Coronae of Solar-Type Stars

We have used the ASCA and ROSAT X-ray satellites to probe the coronae of a sample of nine solar-like G stars. These stars are all ostensibly single with ages ranging from 70 Myr to 9 Gyr and have X-ray luminosities ranging from 1 to 500 times that of the quiet Sun. Specifically, we investigate the dependence of the coronal temperature and emission measure structure of these stars on age and rotation period. In the younger stars, a considerable portion of the volume emission measure resides at very high temperatures, reaching up to ~20-30 MK in EK Dra. Such temperatures are comparable to temperatures that are achieved on the Sun during short flaring episodes. In two-temperature fits to ROSAT data, the higher temperature decays rapidly within the first few 100 Myr; the decay may be described by an inverse power law, Thot age-0.3. We also find a power-law dependence between the total X-ray luminosity and the higher temperature LX

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)

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The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry

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Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages

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ASCA Detection of a Superhot 100 Million K X‐Ray Flare on the Weak‐lined T Tauri Star V773 Tauri

-->. We interpret this as evidence of a decrease in the efficiency of high-temperature coronal heating as a solar-like star ages and its rotation slows down. A reconstruction of the coronal differential emission measure (DEM) distribution in three of the stars using ASCA data indicates a bimodal distribution in temperature, with the hotter plasma at 12-30 MK and the cooler plasma below 10 MK. We infer, for the first time, a consistent evolution of the DEM structure in a solar-type star. The emission measure of the hotter component rapidly decreases with age and becomes unimportant at ages beyond ~500 Myr. The emitted X-ray emission of the young Sun thus rapidly softened, which had important implications for the young planetary atmospheres. We suggest that the high-temperature component is the result of superimposed but temporally unresolved flaring events and support this picture by reconstructing the time-integrated (average) emission measure distribution of a typical solar X-ray flare. Radio observations of active stars fit well into this picture and suggest that the presence of nonthermal electrons in coronae is linked to the presence of hot ( > 10 MK) plasma, very much the same situation as in solar flares. We find, however, that radio emission saturates, if at all, at smaller rotation periods than does X-ray emission.

X‐Ray Emission from Colliding Wind Shocks in the Wolf‐Rayet Binary WR 140

(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) surveys the most populated ~5 square degrees of the Taurus star formation region, 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. 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. 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.

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

We report on the properties of pre-main-sequence objects in the Taurus molecular clouds as observed in seven mid- and far-infrared bands with the Spitzer Space Telescope. There are 215 previously identified members of the Taurus star-forming region in our ~44 deg^2 map; these members exhibit a range of Spitzer colors that we take to define young stars still surrounded by circumstellar dust (noting that ~20% of the bona fide Taurus members exhibit no detectable dust excesses). We looked for new objects in the survey field with similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet imaging, and found 148 new candidate members of Taurus. We have obtained follow-up spectroscopy for about half the candidate sample, thus far confirming 34 new members, three probable new members, and 10 possible new members, an increase of 15%–20% in Taurus members. Of the objects for which we have spectroscopy, seven are now confirmed extragalactic objects, and one is a background Be star. The remaining 93 candidate objects await additional analysis and/or data to be confirmed or rejected as Taurus members. Most of the new members are Class II M stars and are located along the same cloud filaments as the previously identified Taurus members. Among non-members with Spitzer colors similar to young, dusty stars are evolved Be stars, planetary nebulae, carbon stars, galaxies, and active galactic nuclei.

Million-Degree Plasma Pervading the Extended Orion Nebula

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 and ≈1.6 Gyr and rotation periods between ≈1 and 12.4 days 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 nonthermal 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 timescales 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.

Discovery of a bipolar X-ray jet from the T Tauri star DG Tauri

We present results of a ≈ 40 ks ASCA observation of the active weak-lined T Tauri star V773 Tau (HD 283447) and the surrounding Barnard 209 dark cloud, obtained in 1995 February. During this observation, V773 Tau exhibited a dramatic X-ray flare, with the X-ray count rate increasing rapidly by a factor of ~20, then decreasing exponentially with an e-folding timescale of ≈ 2.3 hr. The peak flare luminosity was at least ~1033 ergs s-1 (0.7-10 keV; distance = 150 pc), which is among the highest X-ray luminosities observed to date for T Tauri stars. The total energy release was ~1037 ergs. However, the most spectacular aspect of this flare was its temperature, which reached a maximum value of at least 100 million K. Spectral fits near flare maximum give a temperature of ~10 keV, which slowly declined to a value ~6 keV at the end of the observation. These temperature measurements are based on high signal-to-noise ratio spectra, and provide the first unambiguous evidence for superhot flaring plasma at temperatures of ~108 K in T Tauri stars. A simple cooling-loop model gives electron densities that are similar to those of solar flares, but requires loop sizes that are comparable to or larger than the star itself. The flare showed other interesting behavior, including a high (and possibly variable) absorption column density, NH = 4 × 1022 cm-2, and an apparent increase in the global metal abundance during the flare.

Simultaneous Chandra and very large array observations of young stars and protostars in ρ Ophiuchus cloud core A

We analyze four ASCA X-ray observations of the Wolf-Rayet binary system WR 140 obtained between 1993 and 1997 by making use of hydrodynamic colliding wind (CW) shock models. The analysis shows that the CW shock models are able to accurately reproduce the X-ray spectra at different orbital phases using mass-loss and orbital parameters that are within the ranges allowed by the uncertainties. However, some adjustment in the currently accepted values of the semimajor axis and time of periastron passage may eventually be required. Models that allow for different electron and ion temperatures provide better fits to the data. Extra absorption is inferred from CW shock models above that expected from the winds and interstellar medium, the origin of which is not yet known. We also report the serendipitous discovery of hot plasma at temperatures in excess of ~2 keV and X-ray emission lines in spectra extracted from the diffuse Cygnus superbubble background in the vicinity of WR 140.