Marc Audard

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

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.

X-ray Emission from T Tauri Stars and the Role of Accretion: Inferences from the XMM-Newton Extended Survey of the Taurus Molecular Cloud

Context. T Tau stars display di erent X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) to study di erences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (LX) vs. mass (M) relation, LX/ M 1:69 0:11 , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L ) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates LX= constL , although the constant is di erent for the two subsamples: const= 10 3:73 0:05 for CTTS and const= 10 3:39 0:06 for WTTS. Given a similar L distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of 2 compared to WTTS. The average electron temperatures Tav are correlated with LX in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed LX for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the LX vs. Tav dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.

Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages

We investigate the long-term evolution of X-ray coronae of solar analogs based on high-resolution X-ray spectroscopy and photometry with XMM-Newton. Six nearby main-sequence G stars with ages between ≈0.1 and ≈1.6 Gyr and rotation periods between ≈1 and 12.4 days have been observed. We use the X-ray spectra to derive coronal element abundances of C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measure distribution (EMD). We find that the abundances change from an inverse first ionization potential (FIP) distribution in stars with ages around 0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr and beyond. This transformation is coincident with a steep decline of nonthermal radio emission. The results are in qualitative agreement with a simple model in which the stream of electrons in magnetic fields suppresses diffusion of low-FIP ions from the chromosphere into the corona. The coronal emission measure distributions show shapes characterized by power laws on each side of the EMD peak. The latter shifts from temperatures of about 10 MK in the most rapidly rotating, young stars to temperatures around 4 MK in the oldest target considered here. The power-law index on the cooler side of the EMD exceeds expected slopes for static loops, with typical values being 1.5-3. We interpret this slope with a model in which the coronal emission is due to a superposition of stochastically occurring flares, with an occurrence rate that is distributed in radiated energy E as a power law, dN/dE ∝ E-α, as previously found for solar and stellar flares. We obtain the relevant power-law index α from the slope of the high-temperature tail of the EMD. Our EMDs indicate α ≈ 2.2-2.8, in excellent agreement with values previously derived from light curves of magnetically active stars. Modulation with timescales reminiscent of flares is found in the light curves of all our targets. Several strong flares are also observed. We use our α-values to simulate light curves and compare them with the observed light curves. We thus derive the range of flare energies required to explain the light-curve modulation. More active stars require a larger range of flare energies than less active stars within the framework of this simplistic model. In an overall scenario, we propose that flaring activity plays a larger role in more active stars. In this model, the higher flare rate is responsible both for the higher average coronal temperature and the high coronal X-ray luminosity, two parameters that are indeed found to be correlated.

ARE CORONAE OF MAGNETICALLY ACTIVE STARS HEATED BY FLARES? II. EXTREME ULTRAVIOLET AND X-RAY FLARE STATISTICS AND THE DIFFERENTIAL EMISSION MEASURE DISTRIBUTION

We investigate the EUV and X-ray flare rate distribution in radiated energy of the late-type active star AD Leo. Occurrence rates of solar flares have previously been found to be distributed in energy according to a power law, dN/dE ∝ E-α, with a power-law index α in the range 1.5-2.6. If α ≥ 2, then an extrapolation of the flare distribution to low flare energies may be sufficient to heat the complete observable X-ray/EUV corona. We have obtained long observations of AD Leo with the EUVE and BeppoSAX satellites. Numerous flares have been detected, ranging over almost 2 orders of magnitude in their radiated energy. 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 α. 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 (for EUVE). Subsets of the light curves are tested individually, and the quiescent flux has optionally been treated as a superposition of flares from the same flare distribution. We find acceptable α values between 2.0 and 2.5 for the EUVE DS and the BeppoSAX LECS data. Some variation is found depending on whether or not a strong and long-lasting flare occurring in the EUVE data is included. The BeppoSAX MECS data indicate a somewhat shallower energy distribution (smaller α) than the simultaneously observed 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.

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

We report results from a comprehensive study of the nearby M dwarf Proxima Centauri with the XMM-Newton satellite, using simultaneously its X-ray detectors and the Optical Monitor with its U band filter. We find strongly variable coronal X-ray emission, with flares ranging over a factor of 100 in peak flux. The low-level emission is found to be continuously variable on at least three time scales (a slow decay of several hours, modulation on a time scale of 1 hr, and weak flares with time scales of a few minutes). Several weak flares are characteristically preceded by an optical burst, compatible with predictions from standard solar flare models. We propose that the U band bursts are proxies for the elusive stellar non-thermal hard X-ray bursts suggested from solar observations. In the course of the observation, a very large X-ray flare started and was observed essentially in its entirety. Its peak luminosity reached 3.9 × 10 28 erg s −1 (0.15-10 keV), and the total X-ray energy released in the same band is derived to be 1.5 × 10 32 ergs. This flare has for the first time allowed to measure significant density variations across several phases of the flare from X-ray spectroscopy of the O  He-like triplet; we find peak densities reaching up to 4 × 10 11 cm −3 for plasma of about 1-5 MK. Abundance ratios show little variability in time, with a tendency of elements with a high first ionization potential to be overabundant relative to solar photospheric values. Using Fe  lines with different oscillator strengths, we do not find significant effects due to opacity during the flare, indicating that large opacity increases are not the rule even in extreme flares. We model the large flare in terms of an analytic 2-Ribbon flare model and find that the flaring loop system should have large characteristic sizes (≈1R∗) within the framework of this simplistic model. These results are supported by full hydrodynamic simulations. Comparing the large flare to flares of similar size occurring much more frequently on more active stars, we propose that the X-ray properties of active stars are a consequence of superimposed flares such as the example analyzed in this paper. Since larger flares produce hotter plasma, such a model also explains why, during episodes of low-level emission, more active stars show hotter plasma than less active stars.

A study of coronal abundances in RS CVn binaries

XMM-Newton has been performing comprehensive studies of X-ray luminous RS CVn binary systems in its calibration and guaranteed time programs. We present results from ongoing investigations in the context of a systematic study of coronal emission from RS CVns. We concentrate here on coronal abundances and investigate the abundance pattern in RS CVn binaries as a function of activity and average temperature. We find a transition from an Inverse First Ionization Potential (FIP) effect towards an absence of a clear trend (no FIP) in intermediately active RS CVn systems. This scheme fits well into the long-term evolution from an IFIP to a FIP effect found in solar analogs. We further study variations in the elemental abundances during a large flare.

Million-Degree Plasma Pervading the Extended Orion Nebula

Most stars form as members of large associations within dense, very cold (10 to 100 kelvin) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon must be widespread throughout our Galaxy.

Discovery of a bipolar X-ray jet from the T Tauri star DG Tauri

Aims. We have obtained and analyzed Chandra ACIS-S observations of the strongly accreting classical T Tauri star DG Tau. Our principal goals are to map the immediate environment of the star to characterize possible extended X-rays formed in the jet, and to re-visit the anomalous, doubly absorbed X-ray spectrum of DG Tau itself. Methods. We combine our new ACIS-S data with a data set obtained previously. The data are superimposed to obtain flux and hardness images. Separate X-ray spectra are extracted for DG Tau and areas outside its point spread function. Results. We detect a prominent X-ray jet at a position angle of PA ≈ 225 deg (tentatively suggested by Gudel et al. 2005, ApJ, 626, L53), coincident with the optical jet axis. We also identify a counter jet at PA = 45 deg. The X-ray jets are detected out to a distance of ≈5 �� from the star, their sources being extended at the ACIS-S resolution. The jet spectra are soft, with a best-fit electron temperature of 3.4 MK. We find evidence for excess absorption of the counter jet. The spectrum of the DG Tau point source shows two components with largely different temperatures and absorption column densities. Conclusions. The similar temperatures and small absorbing gas columns of the jet sources and the soft component of the “stellar” source suggest that these sources are related, produced either by shocks or by magnetic heating in the jets. Cooling estimates suggest that the pressure in the hot gas contributes to jet expansion. The hard “stellar” component, on the other hand, is associated with a stellar corona or magnetosphere. The excessive photoelectric absorption of this component suggests the presence of dust-depleted accretion streams above coronal magnetic fields.

The XMM-Newton view of stellar coronae: High-resolution X-ray spectroscopy of Capella

We present the high-resolution RGS X-ray spectrum of the stellar binary Capella observed by the XMM- Newton satellite. A multi-thermal approach has rst been applied to t the data and derive elemental abundances. Using the latter, the emission measure distribution has been reconstructed using a Chebychev polynomial t. Its shape is found to display a sharp peak around 7 MK, consistent with previous EUVE and ASCA results. A smaller but signicant amount of emission measure is required around 1.8 MK in order to explain the O vii He-like triplet and the C vi Ly line. We have applied the temperature diagnostics of dielectronic recombination satellite lines to the He-like O vii triplet to constrain the cool plasma temperature, and have obtained a lower limit consistent with the global reconstruction of the emission measure distribution. We have used line ratios from the forbidden, intercombination, and resonance lines of the O vii triplet to derive an average density for the cool coronal plasma (ne < 11 0 10 cm 3 ). Implications for the coronal structure of Capella are discussed.