Search results for " Stars: Coronae"
showing 10 items of 23 documents
On X-ray Optical Depth in the Coronae of Active Stars
2007
We have investigated the optical thickness of the coronal plasma through the analysis of high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra. In particular, we probed for the presence of significant resonant scattering in the strong Lyman series lines arising from hydrogen-like oxygen and neon ions. The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac show significant optical depth. For these active coronae, the Lya/Lyb ratios are significantly depleted as compared with theoretical predictions and with the same ratios observed in similar active stars. Interpreting these decremen…
XMM-Newton survey of two upper Scorpius regions
2006
We study X-ray emission from young stars by analyzing deep XMM-Newton observations of two regions of the Upper Scorpius association, having an age of 5 Myr. Based on near infrared and optical photometry we identify 22 Upper Scorpius photometric members among the 224 detected X-ray sources. We derive coronal properties of Upper Scorpius stars by performing X-ray spectral and timing analysis. The study of four strong and isolated stellar flares allows us to derive the length of the flaring loops. Among the 22 Upper Scorpius stars, 13 are identified as Upper Scorpius photometric members for the first time. The sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while the na…
Geometry Diagnostics of a Stellar Flare from Fluorescent X-Rays
2008
We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum of the single G-type giant HR 9024 during a large flare. In analogy to solar X-ray observations, we interpret the observed Fe K$\alpha$ line as being produced by illumination of the photosphere by ionizing coronal X-rays, in which case, for a given Fe photospheric abundance, its intensity depends on the height of the X-ray source. The HETGS observations, together with 3D Monte Carlo calculations to model the fluorescence emission, are used to obtain a direct geometric constraint on the scale height of the flaring coronal plasma. We compute the Fe fluorescent emission induced by the emission of a single flaring coro…
Simultaneous Kepler/K2 and XMM‐Newton observations of superflares in the Pleiades
2019
Proceeding of a contributed talk given at the meeting: "TIME-DOMAIN ASTRONOMY: A HIGH ENERGY VIEW" held at ESAC, Madrid, 13 - 15 JUNE 2018 Together with coronal mass ejection, flares are the most energetic stellar magnetic events, ignited by a sudden release of magnetic energy, which triggers a cascade of interconnected phenomena, each resulting in emission in different bands. For this reason, flares are intrinsic multiwavelength phenomena. In particular, optical and soft X-ray emission probes two different events occurring during flares: the heating of plasma in the upper photosphere at the footpoints of the magnetic loops and the heating and cooling of the plasma confined in the loops in …
X-ray flares on the UV Ceti-type star CC Eridani: a "peculiar" time-evolution of spectral parameters
2007
Context: Weak flares are supposed to be an important heating agent of the outer layers of stellar atmospheres. However, due to instrumental limitations, only large X-ray flares have been studied in detail until now. Aims: We used an XMM-Newton observation of the very active BY-Dra type binary star CC Eri in order to investigate the properties of two flares that are weaker than those typically studied in the literature. Methods: We performed time-resolved spectroscopy of the data taken with the EPIC-PN CCD camera. A multi-temperature model was used to fit the spectra. We inferred the size of the flaring loops using the density-temperature diagram. The loop scaling laws were applied for deriv…
X-ray Flares in Orion Low Mass Stars
2007
Context. X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims. Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M_sun) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods. We adopt a maximum likelihood piece-wise representation of the observed X-r…
High‐Resolution X‐Ray Spectroscopy of the Post–T Tauri Star PZ Telescopii
2004
We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating P_(rot)=0.94 d post T Tauri (~20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods we have derived the coronal emission measure distribution, em(T), and chemical abundances. The em(T) peaks at log T = 6.9 and exhibits a significant emission measure at temperatures log T > 7. The coronal abundances are generally ~0.5 times the solar photospheric values that are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abu…
The Sun as an X-ray star: Active region evolution, rotational modulation, and implications for stellar X-ray variability
2004
We study the contribution of an active region and its core to the luminosity and the spectrum of the Sun in the X-ray band and to the relevant solar emission measure vs. temperature distribution, EM(T). We also study the relevant changes in the course of four solar rotations, and the solar rotational modulation due to this active region, the only one present at that time. To this end, we have used a large sample of full-disk Yohkoh/SXT observations taken between July and October 1996, covering most of the active region evolution. From the Yohkoh/SXT data we have synthesized the X-ray spectra of the whole solar corona, and the focal plane data as they would be collected with Rosat/PSPC, XMM-…
On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS
2003
In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over different time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ra…
Modeling an X-ray Flare on Proxima Centauri: evidence of two flaring loop components and of two heating mechanisms at work
2003
We model in detail a flare observed on Proxima Centauri with the EPIC-PN on board XMM-Newton at high statistics and high time resolution and coverage. Time-dependent hydrodynamic loop modeling is used to describe the rise and peak of the light curve, and a large fraction of the decay, including its change of slope and a secondary maximum, over a duration of more than 2 hours. The light curve, the emission measure and the temperature derived from the data allow us to constrain the loop morphology and the heating function and to show that this flare can be described with two components: a major one triggered by an intense heat pulse injected in a single flaring loop with half-length ~1.0 10^{…