The Coronal Temperatures of Low-Mass Main-Sequence Stars

Aims. We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. Methods. We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. Results. We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, Fx, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to Fx by Tcor=0.11 Fx^0.26, where Tcor is in MK and Fx is in erg/s/cm^2.Comment: 5 pages, 3 figures, 1 table; accepted for publication in A&

What Makes a Planet Habitable?

Antes de que la vida pueda surgir y desarrollarse en una superficie planetaria, son necesarias muchas condiciones de naturaleza astrofísica. La radiación y las partículas provenientes de la estrella central, el campo magnético del planeta, la acumulación o disipación de la atmósfera en un planeta joven, y varios otros factores deben actuar conjuntamente y de manera equilibrada para dar a la vida la oportunidad de florecer. En este artículo describimos estos prerrequisitos cruciales para la habitabilidad y exponemos el estado actual de nuestro conocimiento

High-energy radiation and particles in the environments of young stellar objects

High-energy radiation and particles profoundly affect circumstellar disk gas and solids. We discuss stellar high-energy sources and summarize their effects on circumstellar disk

Magnetic activity, high-energy radiation and variability: from young solar analogs to low-mass objects

Magnetic activity on cool stars expresses itself in a bewildering variety of radiative and particle output originating from magnetic regions between the photosphere and the corona. Given its origin in evolving magnetic fields, most of this output is variable in time. Radiation in the ultraviolet, the extreme ultraviolet, and the X-ray ranges are important for heating and ionizing upper planetary atmospheres and thus driving atmospheric evaporation. Additionally, stellar winds interact with the upper atmospheres and may lead to further erosion. The stellar high-energy output is therefore a prime factor in determining habitability of planets. We summarize our knowledge of magnetic activity in young solar analogs and lower-mass stars and show how the stellar output changes on evolutionary timescale

The Faint Young Sun Paradox: An Observational Test of an Alternative Solar Model

We report the results of deep observations at radio (3.6 cm) wavelengths of the nearby solar‐type star π^01 Ursa Majoris with the Very Large Array (VLA) intended to test an alternative theory of solar luminosity evolution. The standard model predicts a solar luminosity only 75% of the present value and surface temperatures below freezing on Earth and Mars at 4 Ga, seemingly in conflict with geologic evidence for liquid water on these planets. An alternative model invokes a compensatory mass loss through a declining solar wind that results in a more consistent early luminosity. The free‐free emission from an enhanced wind around nearby young Sun‐like stars should be detectable at microwave frequencies. Our observations of π^01 UMa, a 300 million year‐old solar‐mass star, place an upper limit on the mass loss rate of 4–5 × 10^(−11) M_⊙ yr^(−1). Total mass loss from such a star over 4 Gyr would be less than 6%. If this star is indeed an analog of the early Sun, it casts doubt on the alternative model as a solution to the faint young Sun paradox, particularly for Mars

The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star

Aims. We aim to describe the pre-main sequence and main-sequence evolution of X-ray and extreme-ultaviolet radiation of a solar mass star based on its rotational evolution starting with a realistic range of initial rotation rates. Methods. We derive evolutionary tracks of X-ray radiation based on a rotational evolution model for solar mass stars and the rotation-activity relation. We compare these tracks to X-ray luminosity distributions of stars in clusters with different ages. Results. We find agreement between the evolutionary tracks derived from rotation and the X-ray luminosity distributions from observations. Depending on the initial rotation rate, a star might remain at the X-ray saturation level for very different time periods, approximately from 10 Myr to 300 Myr for slow and fast rotators, respectively. Conclusions. Rotational evolution with a spread of initial conditions leads to a particularly wide distribution of possible X-ray luminosities in the age range of 20 to 500 Myrs, before rotational convergence and therefore X-ray luminosity convergence sets in. This age range is crucial for the evolution of young planetary atmospheres and may thus lead to very different planetary evolution histories.Comment: 4 pages, 4 figures, accepted for publication in A&

X-ray spectroscopy of stars

Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR star

First Detection of Thermal Radio Emission from Solar-Type Stars with the Karl G. Jansky Very Large Array

We present the first detections of thermal radio emission from the atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These stars all resemble the Sun in age and level of magnetic activity, as indicated by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We observed these stars with the Karl G. Jansky Very Large Array with sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz. {\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6 for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux densities correspond to stellar disk-averaged brightness temperatures of roughly 10,000 K, similar to the solar brightness temperature at the same frequency. We explain this emission as optically- thick thermal free-free emission from the chromosphere, with possible contributions from coronal gyroresonance emission above active regions and coronal free-free emission. These and similar quality data on other nearby solar-type stars, when combined with ALMA observations, will enable the construction of temperature profiles of their chromospheres and lower transition regions.Comment: 9 pages, 2 figures. Accepted for publication in Ap

X-ray astronomy of stellar coronae

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 evolutio