arXiv subjects

Marc Audard

Publications and source records attributed to Marc Audard.

83 records · Page 5Linked to original sources

arXiv2004

X-rays of Stellar Coronae with Chandra and XMM-Newton: Flares and Elemental Composition in Stellar Atmospheres (Invited review)

Observations of magnetically active stars with Chandra and XMM-Newton have deepened our knowledge of the physics of the atmospheres in late-type stars. In this review paper, I discuss two topics that have profited significantly from Chandra and XMM-Newton. Of particular interest, studies of the elemental composition of stellar coronae have taken advantage of the high spectral resolution available with the grating instruments on board these satellites. I summarize the status of our knowledge and discuss the elemental composition in a variety of stellar coronae. I also focus on the topic of flares in stellar coronae, and review the contributions made by XMM-Newton and Chandra to X-ray and multi-wavelength studies.

astro-ph↗

arXiv2004

A Multiwavelength Study of RZ Cassiopeiae: The XMM-Newton/VLA Campaign

XMM-Newton and the VLA simultaneously observed the eclipsing Algol-type binary RZ Cassiopeiae in August 2003. The secondary eclipse (K3 IV companion behind the A3 V primary) was placed at the center of the 15-hour radio campaign, while the X-ray satellite monitored a full 1.2-day orbital period. We present results of the X-ray and radio campaigns. The X-ray light curve shows significant modulation probably related to rotational modulation and active region evolution, and even small flares. However, the X-ray eclipse is not deep, implying that the coronal X-ray emitting material is spatially extended. The Reflection Grating Spectrometer (RGS) spectrum shows a variety of bright emission lines from Fe, Ne, O, N. A strong [C/N] depletion probably reflects the surface composition of the secondary which fills its Roche lobe and loses material onto the primary. The O~\textsc{vii} He-like triplet reflects a low forbidden-to-intercombination ratio; while it generally suggests high electron densities, the ratio is here modified by photoexcitation by the strong UV flux of the primary A3 V star. The radio light curve shows no similarity to the X-ray light curve. The eclipse timings are different, and the radio flux increased while the X-ray flux decreased. The radio spectral slope is shallow ($α= 0 - 1$).

astro-ph↗

arXiv2004

High-Energy Processes in Young Stars: Chandra X-ray Spectroscopy of HDE 283572, RY Tau, and LkCa 21

Weak-lined T Tauri stars (WTTS) represent the important stage of stellar evolution between the accretion phase and the zero-age main sequence. At this stage, the star decouples from its accretion disk, and spins up to a higher rotation rate than in the preceding classical T Tauri phase. Consequently, dynamo processes can be expected to become even stronger at this stage. High energy processes can have effects on the remaining circumstellar material, possibly including protoplanets and planetesimals, and these effects may account for certain observable properties of asteroids in the current solar system. Chandra observed for 100 ks the WTTS HDE 283572 which probes the PMS stage of massive A-type stars. We present first results of the analysis of its high-resolution X-ray spectrum obtained with the High-Energy Transmission Grating Spectrometer. A wide range of Fe lines of high ionization states are observed, indicating a continuous emission measure distribution. No significant signal is detected longward of the O \textsc{viii} Ly$α$ line because of the high photoelectric absorption. We also report on the preliminary analysis of the zeroth order spectra of RY Tau and LkCa21. In particular, we show evidence of an emission line in RY Tau at 6.4 keV that we identify as fluorescent emission by neutral Fe caused by a strong X-ray flare which illuminated some structure in (or surrounding) the CTTS. A comparison of X-ray spectra of classical T Tau stars, other WTTS, and young main-sequence stars is made.

astro-ph↗

arXiv2004

Resolving X-ray sources from B-stars spectroscopically: The example of mu Lep

We present high-resolution X-ray observations of the chemically peculiar late-type B-star: mu Lep. However, we find spectroscopic and astrometric evidence, which show that the X-rays are not traced back to the B-star itself, but rather to a previously unresolved companion: mu Lep-B, whose X-ray spectrum resembles that of a coronally active source. We discuss the possibility that mu Lep-B is a pre-main-sequence companion, most likely of the non-accreting magnetically-active type.

astro-ph↗

arXiv2003

Separating the X-Ray Emissions of UV Ceti A and B with Chandra

We present the first spatially separated X-ray observation of the active UV Ceti binary with \textit{Chandra}, providing important new information on the relative X-ray activity levels of its nearly identical dM5.5e components. The two components had similar X-ray luminosities during low-level periods and emission measure loci derived from Low Energy Transmission Grating spectra reveal similar coronal temperatures ($3 - 6$ MK). However, the B component showed a higher degree of variability, resulting in a higher average X-ray luminosity than that of UV Cet A. We discuss possible causes of this enhanced activity in UV Ceti B.

astro-ph↗

arXiv2002

Are Coronae of Magnetically Active Stars Heated by Flares? II. EUV and X-Ray Flare Statistics and the Differential Emission Measure Distribution

(Abridged) We investigate the EUV and X-ray flare rate distribution in radiated energy of the late-type active star AD Leo using long EUVE and BeppoSAX observations. We compare the observed light curves with light curves synthesized from model flares that are distributed in energy according to a power law with selectable index alpha (dN/dE ~ E^{-alpha}). Two methods are applied, the first comparing flux distributions of the binned data, and the second using the distributions of photon arrival time differences in the unbinned data. We find acceptable alpha values between 2.0-2.5 for the EUVE DS and the BeppoSAX LECS data. The BeppoSAX MECS data indicate a somewhat shallower energy distribution than the LECS data, which is attributed to the harder range of sensitivity of the MECS detector and the increasing peak temperatures of flares with increasing total (radiative) energy. The results suggest that flares can play an important role in the energy release of this active corona. We discuss caveats related to time variability, total energy, and multiple power-law distributions. Studying the limiting case of a corona that is entirely heated by a population of flares, we derive an expression for the time-averaged coronal differential emission measure distribution (DEM) that can be used as a diagnostic for the flare energy distribution. The shape of the analytical DEM agrees with previously published DEMs from observations of active stars.

astro-ph↗

arXiv2002

X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri

Using the XMM-Newton X-ray observatory to monitor the nearest star to the Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically active star ever observed. Correlated X-ray and optical variability provide strong support for coronal energy and mass supply by a nearly continuous sequence of rapid explosive energy releases. Variable emission line fluxes were observed in the He-like triplets of OVII and NeIX during a giant flare. They give direct X-ray evidence for density variations, implying densities between 2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the volume of the line-emitting plasma. We discuss the data in the context of the chromospheric evaporation scenario.

astro-ph↗

arXiv2002

Flare Heating in Stellar Coronae

We investigate the contribution of very weak flares to the coronal luminosity of low-mass active stars. We analyze EUVE/DS events data from FK Aqr, V1054 Oph, and AD Leo and conclude that in all these cases the coronal emission is dominated by flares to such an extent that in some cases the entire emission may be ascribed to flare heating. We have developed a new method to directly model for the first time stochastically produced flare emission, including undetectable flares, and their effects on the observed photon arrival times. We find that the index of the power-law distribution of flare energies (dN/dE ~ E^{-alpha}) is 2.6+-0.34, 2.74+-0.35, and 2.03-2.32 for FK Aqr, V1054 Oph, and AD Leo respectively. We also find that the flare component accounts for a large fraction (generally >50 percent) of the total flux.

astro-ph↗

arXiv2002

Elemental Abundances in Stellar Coronae with XMM-Newton

The XMM-Newton Reflection Grating Spectrometer Team has obtained observations ofa large number of coronal sources of various activity levels, ages, and spectraltypes. In particular, X-ray bright RS CVn binary systems display saturated coronal emission with spectral lines characteristic of hot (10-30 MK) plasma. Furthermore, we have obtained XMM-Newton data from young solar analogs both within and outside the X-ray saturation regime. We have simultaneously analyzed the EPIC MOS and RGS data from these objects and have obtained coronal abundances of various elements (e.g., C, N, O, Ne, Mg, Si, Fe). We show that there is evidence for a transition from an Inverse First Ionization Potential (FIP) effect in most active stars to a ``normal'' solar-like FIP effect in less active stars. We discuss this result with regard to photospheric abundances.

astro-ph↗

arXiv2000

High-resolution X-ray spectroscopy and imaging of supernova remnant N132D

The observation of the supernova remnant N132D by the scientific instruments on board the XMM-Newton satellite is presented. The X-rays from N132D are dispersed into a detailed line-rich spectrum using the Reflection Grating Spectrometers. Spectral lines of C, N, O, Ne, Mg, Si, S, and Fe are identified. Images of the remnant, in narrow wavelength bands, produced by the European Photon Imaging Cameras reveal a complex spatial structure of the ionic distribution. While K-shell Fe emission seems to originate near the centre, all of the other ions are observed along the shell. A high O VII / O VIII emission ratio is detected on the northeastern edge of the remnant. This can be a sign of hot ionising conditions, or it can reflect relatively cool gas. Spectral fitting of the CCD spectrum suggests high temperatures in this region, but a detailed analysis of the atomic processes involved in producing the O VII spectral lines leads to the conclusion that the intensities of these lines alone cannot provide a conclusive distinction between the two scenarios.

astro-ph↗

arXiv2000

EUV Flare Activity in Late-Type Stars

\textit{Extreme Ultraviolet Explorer} Deep Survey observations of cool stars (spectral type F to M) have been used to investigate the distribution of coronal flare rates in energy and its relation to activity indicators and rotation parameters. Cumulative and differential flare rate distributions were constructed and fitted with different methods. Power laws are found to approximately describe the distributions. A trend toward flatter distributions for later-type stars is suggested in our sample. Assuming that the power laws continue below the detection limit, we have estimated that the superposition of flares with radiated energies of about $10^{29}-10^{31}$ergs could explain the observed radiative power loss of these coronae, while the detected flares are contributing only $\approx 10$%. While the power-law index is not correlated with rotation parameters (rotation period, projected rotational velocity, Rossby number) and only marginally with the X-ray luminosity, the flare occurrence rate is correlated with all of them. The occurrence rate of flares with energies larger than $10^{32}$ergs is found to be proportional to the average total stellar X-ray luminosity. Thus, energetic flares occur more often in X-ray bright stars than in X-ray faint stars. The normalized occurrence rate of flares with energies larger than $10^{32}$ergs increases with increasing \lxlbol and stays constant for saturated stars. A similar saturation is found below a critical Rossby number. The findings are discussed in terms of simple statistical flare models in an attempt to explain the previously observed trend for higher average coronal temperatures in more active stars. It is concluded that flares can contribute a significant amount of energy to coronal heating in active stars.

astro-ph↗