David Garcia-Alvarez

The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin-up

The zero-age main-sequence star AB Dor and the K dwarf component of the V471 Tau close binary have essentially identical rotation rates and spectral types. An analysis of their high resolution {\it Chandra} X-ray spectra reveals remarkably similar coronal characteristics in terms of both temperature structure and element abundances. Both stars show depletions of low FIP elements by factors of

X-Ray Flaring on the dMe Star, Ross 154

\sim 3

Detailed Diagnostics of an X-Ray Flare in the Single Giant HR 9024

, with higher FIP elements showing more mild depletions. No evidence for enhancements of very low FIP (

Coronae of Young Fast Rotators

<7

Geometry Diagnostics of a Stellar Flare from Fluorescent X-Rays

eV) elements, such as Na, Al and Ca, as compared to other low FIP elements was found. The abundance anomaly pattern for AB Dor and V471 Tau is similar to, though less extreme than, the abundance anomalies exhibited by active RS-CVn-type binaries. While we find statistically significant structure in the underlying differential emission measure distributions of these stars over narrow temperature intervals, this structure is strongly dependent on the lines used in the analysis and is probably spurious. Based on their X-ray similarities, we conclude that the exact evolutionary state of a star has little effect on coronal characteristics, and that the parameters that dominate coronal structure and composition are simply the rotation rate and spectral type.The zero-age main-sequence star AB Dor and the K dwarf component of the V471 Tau close binary have essentially identical rotation rates and spectral types. An analysis of their high-resolution Chandra X-ray spectra reveals remarkably similar coronal characteristics in terms of both temperature structure and element abundances. Both stars show depletions of low first ionization potential (FIP) elements by factors of ~3, with higher FIP elements showing more mild depletions. No evidence for enhancements of very low FIP (<7 eV) elements, such as Na, Al, and Ca, as compared to other low-FIP elements, was found. The abundance anomaly pattern for AB Dor and V471 Tau is similar to, although less extreme than, the abundance anomalies exhibited by active RS CVn-type binaries. While we find statistically significant structure in the underlying differential emission measure distributions of these stars over narrow temperature intervals, this structure is strongly dependent on the lines used in the analysis and is probably spurious. On the basis of their X-ray similarities, we conclude that the exact evolutionary state of a star has little effect on coronal characteristics and that the parameters that dominate coronal structure and composition are simply the rotation rate and spectral type.

Evidence of the FIP Effect in the Coronae of Late-Type Giants

We present results from two Chandra imaging observations of Ross 154, a nearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a very large flare occurred (the equivalent of a solar X3400 event, with -->LX = 1.8 ? 1030 ergs s?1) in which the count rate increased by a factor of over 100. The early phase of the flare shows evidence for the Neupert effect, followed by a further rise and then a two-component exponential decay. A large flare was also observed at the end of a later 48 ks HRC-I observation. Emission from the nonflaring phases of both observations was analyzed for evidence of low-level flaring. From these temporal studies we find that microflaring probably accounts for most of the quiescent emission and that, unlike for the Sun and the handful of other stars that have been studied, the distribution of flare intensities does not appear to follow a power law with a single index. Analysis of the ACIS spectra, which was complicated by exclusion of the heavily piled-up source core, suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5 compared to the commonly adopted solar abundance ratio and that the Ne/O ratio and overall coronal metallicity during the flare appear to be enhanced relative to quiescent abundances. Based on the temperatures and emission measures derived from the spectral fits, we estimate the length scales and plasma densities in the flaring volume and also track the evolution of the flare in color-intensity space. Lastly, we searched for a stellar wind charge exchange X-ray halo around the star but without success; because of the relationship between mass-loss rate and the halo surface brightness, not even an upper limit on the stellar mass-loss rate can be determined.

The FIP effect on late-type stellar coronae: from dwarfs to giants

We analyze a 96 ks Chandra HETGS observation of the single G-type giant HR 9024. The high flux allows us to examine spectral line and continuum diagnostics at high temporal resolution, to derive plasma parameters. A time-dependent one-dimensional hydrodynamic model of a loop with half-length L = 5 × 1011 cm (~R/2) and cross-sectional radius r = 4.3 × 1010 cm, with a heat pulse of 15 ks and 2 × 1011 ergs cm-2 s-1 deposited at the loop footpoints, satisfactorily reproduces the observed evolution of temperature and emission measure, derived from the analysis of the strong continuum emission. For the first time we can compare predictions from the hydrodynamic model with single spectral features, other than with global spectral properties. We find that the model closely matches the observed line emission, especially for the hot (~108 K) plasma emission of the Fe XXV complex at ~1.85 A. The model loop has L/R ~ 1/2 and aspect ratio r/L ~ 0.1, as typically derived for flares observed in active stellar coronae, suggesting that the underlying physics is the same for these very dynamic and extreme phenomena in stellar coronae independently of stellar parameters and evolutionary stage.

Stellar Coronae in Saturated and Supersaturated late-type Stars

AB Dor, Speedy Mic, and Rst 137B are in their early post-T Tauri evolutionary phase (<100 Myr), at the age of fastest rotation in the life of late-type stars. They straddle the coronal saturation-supersaturation boundary first defined by young stars in open clusters. High-resolution Chandra X-ray spectra have been analyzed to study their coronal properties as a function of coronal activity parameters Rossby number, LX/Lbol, and a coronal temperature index. Differences between stars suggest that as supersaturation is reached the DEM slope below the temperature of peak DEM becomes shallower, while the DEM drop-off above this temperature becomes more pronounced. A larger sample comprising our three targets and 22 active stars studied in the recent literature reveals a general increase of plasma at T 107 K toward the saturated-supersaturated boundary but a decline beyond this among supersaturated stars. The coronal Fe abundances of the stellar sample are inversely correlated with LX/Lbol, declining slowly with rising LX/Lbol, but with a much more sharp decline at LX/Lbol 3 × 10−4. For dwarfs the Fe abundance is also well correlated with Rossby number. The coronal O/Fe ratios for dwarfs show a clear increase with decreasing Rossby number, apparently reaching saturation at [O/Fe] = 0.5 at the coronal supersaturation boundary.

Coronal Structure in Young Fast Rotators

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 Ka 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 three-dimensional 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 coronal loop that well reproduces the observed X-ray temporal and spectral properties according to a detailed hydrodynamic modeling. The predicted Fe fluorescent emission is in good agreement with the observed value within observational uncertainties, pointing to a scale height . Comparison of the HR 9024 flare with that recently observed on II Peg by Swift indicates the 0.3R∗ latter is consistent with excitation by X-ray photoionization. Subject headings: hydrodynamics — plasmas — stars: coronae — X-rays: stars