M. Güdel

Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)

We report on the results of the Sun in Time multiwavelength program (X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of providing the spectral irradiance evolution of solar-type stars to be used in the study and modeling of planetary atmospheres. The results from the Sun in Time program suggest that the coronal X-ray-EUV emissions of the young main-sequence Sun were ~100-1000 times stronger than those of the present Sun. Similarly, the transition region and chromospheric FUV-UV emissions of the young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than at present. When we consider the integrated high-energy emission from 1 to 1200 A, the resulting relationship indicates that about 2.5 Gyr ago the solar high-energy flux was about 2.5 times the present value and about 3.5 Gyr ago was about 6 times the present value (when life supposedly arose on Earth). The strong radiation emissions inferred should have had major influences on the thermal structure, photochemistry, and photoionization of planetary atmospheres and have played an important role in the development of primitive life in the solar system. Some examples of the application of the Sun in Time results on exoplanets and on early solar system planets are discussed.

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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X-ray astronomy of stellar coronae

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

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Stellar Radio Astronomy: Probing Stellar Atmospheres from Protostars to Giants

-->. 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 Spectroscopy of Stars

(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.

High resolution X-ray spectroscopy of zeta Puppis with the XMM-Newton reflection grating spectrometer

Abstract.X-ray emission from stars in the cool half of the Hertzsprung-Russell diagram is generally attributed to the presence of a magnetic corona that contains plasma at temperatures exceeding 1 million K. Coronae are ubiquitous among these stars, yet many fundamental mechanisms operating in their magnetic fields still elude an interpretation through a detailed physical description. Stellar X-ray astronomy is therefore contributing toward a deeper understanding of the generation of magnetic fields in magnetohydrodynamic dynamos, the release of energy in tenuous astrophysical plasmas through various plasma-physical processes, and the interactions of high-energy radiation with the stellar environment. Stellar X-ray emission also provides important diagnostics to study the structure and evolution of stellar magnetic fields from the first days of a protostellar life to the latest stages of stellar evolution among giants and supergiants. The discipline of stellar coronal X-ray astronomy has now reached a level of sophistication that makes tests of advanced theories in stellar physics possible. This development is based on the rapidly advancing instrumental possibilities that today allow us to obtain images with sub-arcsecond resolution and spectra with resolving powers exceeding 1000. High-resolution X-ray spectroscopy has, in fact, opened new windows into astrophysical sources, and has played a fundamental role in coronal research.The present article reviews the development and current status of various topics in the X-ray astronomy of stellar coronae, focusing on observational results and on theoretical aspects relevant to our understanding of coronal magnetic structure and evolution.

EXTREME-ULTRAVIOLET FLARE ACTIVITY IN LATE-TYPE STARS

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.

First light measurements with the XMM-Newton reflection grating spectrometers: Evidence for an inverse first ionisation potential effect and anomalous Ne abundance in the Coronae of HR 1099

▪ Abstract Radio astronomy has provided evidence for the presence of ionized atmospheres around almost all classes of nondegenerate stars. Magnetically confined coronae dominate in the cool half of the Hertzsprung-Russell diagram. Their radio emission is predominantly of nonthermal origin and has been identified as gyrosynchrotron radiation from mildly relativistic electrons, apart from some coherent emission mechanisms. Ionized winds are found in hot stars and in red giants. They are detected through their thermal, optically thick radiation, but synchrotron emission has been found in many systems as well. The latter is emitted presumably by shock-accelerated electrons in weak magnetic fields in the outer wind regions. Radio emission is also frequently detected in pre–main sequence stars and protostars and has recently been discovered in brown dwarfs. This review summarizes the radio view of the atmospheres of nondegenerate stars, focusing on energy release physics in cool coronal stars, wind phenomenolog...