David P. Huenemoerder

Evidence for Accretion: High-Resolution X-Ray Spectroscopy of the Classical T Tauri Star TW Hydrae

We present high-resolution X-ray spectra of the X-ray-bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 A. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne IX and O VII indicate densities near log ne = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ks observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not seem strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming that accretion powers the X-ray emission, our results for log ne suggest an accretion rate of ~10-8 M☉ yr-1.

The Chandra High-Energy Transmission Grating: Design, Fabrication, Ground Calibration, and 5 Years in Flight

United States. National Aeronautics and Space Administration. George C. Marshall Space Flight Center (Contract NAS8-38249)

X-Ray Spectroscopy of II Pegasi: Coronal Temperature Structure, Abundances, and Variability

We have obtained high-resolution X-ray spectra of the coronally active binary II Pegasi (HD 224085), covering the wavelength range of 1.5-25 A. For the first half of our 44 ks observation, the source was in a quiescent state with constant X-ray flux, after which it flared, reaching twice the quiescent flux in 12 ks, then decreased. We analyze the emission-line spectrum and continuum during quiescent and flaring states. The differential emission measure derived from lines fluxes shows a hot corona with a continuous distribution in temperature. During the nonflare state, the distribution peaks near log T = 7.2, and when flaring, it peaks near 7.6. High-temperature lines are enhanced slightly during the flare, but most of the change occurs in the continuum. Coronal abundance anomalies are apparent, with iron very deficient relative to oxygen and significantly weaker than expected from photospheric measurements, while neon is enhanced relative to oxygen. We find no evidence of appreciable resonant scattering optical depth in line ratios of iron and oxygen. The flare light curve is consistent with solar two-ribbon flare models but with a very long reconnection time constant of about 65 ks. We infer loop lengths of about 0.05 to about 0.25 stellar radii in the flare, if the flare emission originated from a single, low-density loop.

Enhanced Noble Gases in the Coronae of Active Stars

We have analyzed Chandra High-Energy Transmission Grating spectra of the active RS CVn‐type binary V711 Tauri (HR 1099; HD 22468) in order to examine the chemical composition of its coronae. Observed fluxes and flux upper limits for spectral lines from a range of charge states of iron, covering species Fe xvi‐Fe xxv, have been used to determine the emission measure distribution as a function of temperature, while the observed Fe line-to-continuum ratio has been used to examine the absolute iron abundance, Fe/H. Abundances of elements O, Ne, Mg, Si, S, and Ar relative to both Fe and H have been estimated by comparison of observed line fluxes with predictions based on the emission measure distribution. We confirm results of earlier studies finding the coronae of V711 Tau to be metal-poor and derive an iron abundance of . We find the noble Fe/H p 7.0 5 0.1 gas elements Ne and Ar to be enhanced relative to the local cosmic value and enhanced by an order of magnitude relative to Fe. Very mild enhancements of O and Mg relative to Fe are also discerned. By examination of coronal abundances of Ne relative to Fe culled from the literature, in addition to Ne lines seen in hitherto unpublished Chandra spectra, we conclude that large Ne abundance enhancements are a common feature of active stellar coronae.

Modeling the Ne IX Triplet Spectral Region of Capella with the Chandra and XMM-Newton Gratings

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (ne < 2 × 1010 cm-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

X-Ray Variation Statistics and Wind Clumping in Vela X-1

We investigate the structure of the wind in the neutron star X-ray binary system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant flaring, with some flares reaching fluxes of more than 3.0 Crab between 20‐60 keV for several 100 seconds, while the average flux is around 250 mCrab. We analyzed all archival INTEGRAL data, calculating the brightness distribution in the 20‐60 keV band, which, as we show, closely follows a log-normal distribution. Orbital resolved analysis shows that the structure is strongly variable, explainable by shocks and a fluctuating accretion wake. Analysis of RXTE ASM data suggests a strong orbital change of NH. Accreted clump masses derived from the INTEGRAL data are on the order of 5 10 19 ‐10 21 g. We show that the lightcurve can be described with a model of multiplicative random numbers. In the course of the simulation we calculate the power spectral density of the system in the 20‐100 keV energy band and show that it follows a red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and turbulence can explain the measured mass distribution. As the recently discovered class of supergiant fast X-ray transients (SFXT) seems to show the same parameters for the wind, the link between persistent HMXB like Vela X-1 and SFXT is further strengthened.

The close classical T Tauri binary V4046 Sgr: complex magnetic fields and distributed mass accretion

We report here the first results of a multi-wavelength campaign focusing on magnetospheric accretion processes within the close binary system V4046 Sgr, hosting two partly convective classical T Tauri stars of masses ≃0.9 M_⊙ and age ≃12 Myr. In this paper, we present time-resolved spectropolarimetric observations collected in 2009 September with ESPaDOnS at the Canada–France–Hawaii Telescope (CFHT) and covering a full span of 7 d or ≃2.5 orbital/rotational cycles of V4046 Sgr. Small circularly polarized Zeeman signatures are detected in the photospheric absorption lines but not in the accretion-powered emission lines of V4046 Sgr, thereby demonstrating that both system components host large-scale magnetic fields weaker and more complex than those of younger, fully convective classical T Tauri stars (cTTSs) of only a few Myr and similar masses. Applying our tomographic imaging tools to the collected data set, we reconstruct maps of the large-scale magnetic field, photospheric brightness and accretion-powered emission at the surfaces of both stars of V4046 Sgr. We find that these fields include significant toroidal components, and that their poloidal components are mostly non-axisymmetric with a dipolar component of 50–100 G strongly tilted with respect to the rotation axis; given the similarity with fields of partly convective main-sequence stars of similar masses and rotation periods, we conclude that these fields are most likely generated by dynamo processes. We also find that both stars in the system show cool spots close to the pole and extended regions of low-contrast, accretion-powered emission; it suggests that mass accretion is likely distributed rather than confined in well-defined high-contrast accretion spots, in agreement with the derived magnetic field complexity.

A COORDINATED X-RAY AND OPTICAL CAMPAIGN OF THE NEAREST MASSIVE ECLIPSING BINARY, δ ORIONIS Aa. IV. A MULTIWAVELENGTH, NON-LTE SPECTROSCOPIC ANALYSIS

T.S. is grateful for financial support from the Leibniz Graduate School for Quantitative Spectroscopy in Astrophysics, a joint project of the Leibniz Institute for Astrophysics Potsdam (AIP) and the institute of Physics and Astronomy of the University of Potsdam. L.M.O. acknowledges support from DLR grant 50 OR 1302. M.F.C., J.S.N., and W.L.W. are grateful for support via Chandra grants GO3-14015A and GO3-14015E. A.F.J.M. acknowledges financial aid from NSERC (Canada) and FRQNT (Quebec). J.M.A. acknowledges support from (a) the Spanish Government Ministerio de Economia y Competitividad (MINECO) through grants AYA2010-15 081 and AYA2010-17 631 and (b) the Consejeria de Educacion of the Junta de Andalucia through grant P08-TIC-4075. Caballero N.D.R. gratefully acknowledges his Centre du Recherche en Astrophysique du Quebec (CRAQ) fellowship. Y.N. acknowledges support from the Fonds National de la Recherche Scientifique (Belgium), the Communaute Francaise de Belgique, the PRODEX XMM and Integral contracts, and the “Action de Recherche Concertee” (CFWB-Academie Wallonie Europe). J.L.H. acknowledges support from NASA award NNX13AF40G and NSF award AST-0807477. I.N. is supported by the Spanish Mineco under grant AYA2012-39364-C02-01/02, and the European Union.

On the Weak-wind Problem in Massive Stars: X-Ray Spectra Reveal a Massive Hot Wind in μ Columbae

United States. National Aeronautics and Space Administration (Chandra X-ray Observatory (U.S.) Award GO1-12017B)

TGCat *: THE CHANDRA TRANSMISSION GRATING DATA CATALOG AND ARCHIVE

The Chandra Transmission Grating Data Archive and Catalog (TGCat) provides easy access to analysis-ready products, specifically, high-resolution X-ray count spectra and their corresponding calibrations. The web interface makes it easy to find observations of a particular object, type of object, or type of observation; to quickly assess the quality and potential usefulness of the spectra from pre-computed summary plots; or to customize a view with an interactive plotter, optionally combining spectra over multiple orders or observations. Data and responses can be downloaded as a package or as individual files, and the query results themselves can be retrieved as ASCII or Virtual Observatory tables. Portable reprocessing scripts used to create the archive and which use the Chandra X-ray Center’s (CXC’s) software and other publicly available software are also available, facilitating standard or customized reprocessing from Level 1 CXC archival data to spectra and responses with minimal user interaction.