Coronal evolution of solar-like stars: X-ray spectroscopy of stars in star- forming regions and the solar neighborhood

Solar-like stars are strong X-ray emitters in both their pre-main sequence (PMS) and main-sequence (MS) phases. In analogy to the Sun, X-rays are thought to originate in a corona. However, in the case of pre-main sequence stars, accretion processes might influence the X-ray properties of the stars.

In this thesis, results from X-ray spectroscopy of main-sequence solar analogs, pre-main sequence solar-like stars and a Herbig Ae/Be star are presented and discussed. All X-ray spectra have been obtained by the Reflection Grating Spectrometers (RGS) and the European Photon Imaging Cameras (EPIC) on board the XMM-Newton satellite.

In the first part of the thesis, high-resolution (RGS) X-ray spectra of a sample of six main-sequence G-type stars with ages between [approximate] 0.1 Gyr and [approximate] 1.6 Gyr have been analyzed. Using individual spectral lines, the Emission Measure Distributions (EMD) and the coronal abundances have been derived. As a solar analog evolves, its rotation rate decreases and its internal magnetic dynamo weakens, resulting in a decrease of magnetic activity and a decrease of the star's X-ray luminosity. The mean coronal temperatures derived from the EMDs decrease from [approximate] 10 MK for the youngest stars to [approximate] 4 MK for the oldest star in our stellar sample. These results have been interpreted with a model in which the coronal emission is produced by a superposition of stochastically occurring flares; more active stars are found to require a larger range of flare energies than less active stars. Abundances change from an inverse First Ionization Potential (FIP) effect, where abundances with high FIP are enhanced with respect to abundances with low FIP, to a solar-like FIP effect at ages >= 0.3 Gyr.

The analysis has then been extended to pre-main sequence stars in the Taurus- Auriga complex. The results presented here are part of a large survey, the "XMM-Newton Extended Survey of the Taurus Molecular Cloud" (XEST). High- and medium-resolution spectroscopy has been used to study the differences in the X- ray output of low-mass accreting stars (classical T Tauri Stars, CTTS) and non- accreting stars (weak-line T Tauri stars, WTTS). For nine PMS stars, high- resolution RGS spectra were obtained. For two accreting stars, BP Tau and AB Aur, the electron densities were derived from the O VII triplets. Using the O VII/O VIII flux ratio to quantify the cool plasma, a soft excess has been discovered; this feature seems to be a general feature in the spectra of CTTS, but is not present in WTTS and zero-age main-sequence stars. In BP Tau, the O VII triplet suggests high electron densities for this cool plasma ( n e = 3.4 × 10 11 cm -3 ), a signature that at least part of the X-ray emission might originate in accretion shocks. However, low electron densities ( n e < 10 10 cm -3 ) are inferred for the accreting Herbig star AB Aur. No significant, systematic differences are present between the abundances of accreting and non- accreting stars. Rather, the Ne/Fe coronal abundance ratio is a function of spectral type. It systematically increases from G to early M stars.

In a second approach, results from the medium-resolution EPIC spectra have been used to study systematics in the X-ray properties of CTTS and WTTS. (Abstract shortened by UMI.)