arXiv subjects

J. H. M. M. Schmitt

Publications and source records attributed to J. H. M. M. Schmitt.

At least 199 records · Page 11Linked to original sources

arXiv2005

Doppler imaging of Speedy Mic using the VLT, Fast spot evolution on a young K-dwarf star

We study the short-term evolution of starspots on the ultrafast-rotating star HD197890 ("Speedy Mic" = BO Mic, K 0-2V, P_rot = 0.380 d) based on two Doppler images taken about 13 stellar rotations apart. Each image is based on spectra densely sampling a single stellar rotation. The images were reconstructed by our Doppler imaging code CLDI (Clean-like Doppler imaging) from line profiles extracted by spectrum deconvolution. Our Doppler images constructed from two independent wavelength ranges agree well on scales down to 10 degrees on the stellar surface. In conjunction with nearly parallel V-band photometry our observations reveal a significant evolution of the spot pattern during as little as two stellar rotations. We suggest that such a fast spot evolution demands care when constructing Doppler images of highly active stars based on spectral time series extending over several stellar rotations. The fast intrinsic spot evolution on BO Mic impedes the determination of a surface differential rotation; in agreement with earlier results by other authors we determine an upper limit of |alpha| < 0.004 +- 0.002.

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arXiv2005

X-rays from accretion shocks in T Tauri stars: The case of BP Tau

We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O {\sc vii} triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.

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arXiv2005

The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-ray, and H-alpha Observations

We present the first simultaneous, multi-wavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-ray Observatory, and the Kitt Peak 4-m telescope. We detect strongly variable and periodic radio emission (P=3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of L_X/L_{bol}<2e-5, several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect H-alpha emission to a limit of L_{H-alpha}/L_{bol}<2e-7, the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least four orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs) providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than three years. The detected 3-hour period may be due to (i) the orbital motion of a companion at a separation of about five stellar radii, similar to the configuration of RS CVn systems, (ii) an equatorial rotation velocity of about 37 km/s and an anchored, long-lived magnetic field, or (iii) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and H-alpha emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. [abridged]

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arXiv2004

Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star

We have derived the mean broadening profile of the star V102 in the region of the open cluster IC4665 from high resolution spectroscopy. At a projected equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong deviation from classical rotation. We discuss several scenarios, the most plausible being strong differential rotation in latitudinal direction. For this scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8 rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a spectral type of A9 and support photometric measurements classifying IC4665 V102 as a non-member of IC4665. At such early spectral type this is the strongest case of differential rotation observed so far. Together with three similar stars, IC4665 V102 seems to form a new class of objects that exhibit extreme latitudinal shear in a very shallow convective envelope.

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arXiv2004

On the sizes of stellar X-ray coronae

Spatial information from stellar X-ray coronae can be inferred by use of scaling laws from the solar corona. This requires measurement of the physical parameters temperature and density. We concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of 64 grating spectra of 42 stellar coronae obtained with Chandra and XMM-Newton. Line counts of strong H-like and He-like ions and FeXXI lines were measured with the CORA line fitting tool by Ness and Wichmann 2002. Densities are estimated from He-like f/i flux ratios of OVII and NeIX representing only cooler (1-6MK) plasma components. The densities scatter between log n_e = 9.5-11 from the OVII triplet and between log n_e = 10.5-12 from the NeIX triplet, but we caution that the latter triplet may be biased by contamination from FeXIX and FeXXI lines. We find that low-activity stars tend to show densities of no more than a few times 10^10cm^-3, whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various FeXXI line ratios, we found no consistent proof for the presence of very high densities. This result is in line with constant pressure in the emitting active regions. We focus on the commonly used RTV78 scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive emitting volumes from direct measurements of ion-specific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii. The ratios of these volumes depict volume filling factors, which we find similar up to 0.1 for all stages of activity for OVII and increasing for NeIX.

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arXiv2004

Detection and high-resolution spectroscopy of a huge flare on the old M9 dwarf DENIS 104814.7-395606.1

We report a flare on the M9 dwarf DENIS 104814.7-395606.1, whose mass places it directly at the hydrogen burning limit. The event was observed in a spectral sequence during 1.3 hours. Line shifts to bluer wavelengths were detected in H alpha, H beta, and in the Na D lines, indicating mass motions. In addition we detect a flux enhancement on the blue side of the two Balmer lines in the last spectrum of our series. We interpret this as rising gas cloud with a projected velocity of about 100 km/s which may lead to mass ejection. The higher Balmer lines H gamma to H 8 are not seen due to our instrumental setup, but in the last spectrum there is strong evidence for H 9 being in emission.

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arXiv2004

Discovery of X-ray flaring on the magnetic Bp-star sigma Ori E

We report the detection of an X-ray flare on the Bp star sigma Ori E with the ROSAT high resolution imager (HRI). The flare is shown to have likely occurred on the early-type star, rather than on an hypothesized late-type companion. We derive flare parameters such as total energy release, coarse estimates of size and density, and also present arguments for a magnetic origin of the flare. We place our observations in the context of a magnetic character of Bp-type stars and speculate on a common physical basis and connection between Bp and Be stars.

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arXiv2004

X-ray emission from a metal depleted accretion shock onto the classical T Tauri star TW Hya

We present the X-ray spectrum of TW Hya observed at high and intermediate spectral resolution with the Reflection Grating Spectrometer (RGS) and the European Photon Imaging Camera (EPIC) onboard the XMM-Newton satellite. TW Hya is the first classical T Tauri star for which simultaneous X-ray data with both high spectral resolution and high sensitivity were obtained, thus allowing to probe the X-ray emission properties of stars in the early pre-main sequence phase. The high plasma density, the (comparatively) low temperature, and peculiar chemical abundances in the X-ray emitting region on TW Hya are untypical for stellar coronae. An alternative X-ray production mechanism is therefore called for and a natural explanation is an accretion column depleted of grain forming elements. The metal depletion could be either due to the original molecular cloud that formed TW Hya or due to a settling of dust in the circumstellar disk of TW Hya.

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arXiv2004

X-ray emission from Saturn

We report the first unambiguous detection of X-ray emission originating from Saturn with a Chandra observation, duration 65.5 ksec with ACIS-S3. Beyond the pure detection we analyze the spatial distribution of X-rays on the planetary surface, the light curve, and some spectral properties. The detection is based on 162 cts extracted from the ACIS-S3 chip within the optical disk of Saturn. We found no evidence for smaller or larger angular extent. The expected background level is 56 cts, i.e., the count rate is (1.6 +- 0.2) 10^-3 cts/s. The extracted photons are rather concentrated towards the equator of the apparent disk, while both polar caps have a relative photon deficit. The inclination angle of Saturn during the observation was -27 degrees, so that the northern hemisphere was not visible during the complete observation. In addition, it was occulted by the ring system. We found a small but significant photon excess at one edge of the ring system. The light curve shows a small dip twice at identical phases, but rotational modulation cannot be claimed at a significant level. Spectral modeling results in a number of statistically, but not necessarily physically, acceptable models. The X-ray flux level we calculate from the best-fit spectral models is 6.8 10^-15 erg/cm^2/s (in the energy interval 0.1-2keV), which corresponds to an X-ray luminosity of 8.7 10^14 erg/s. A combination of scatter processes of solar X-rays requires a relatively high albedo favoring internal processes, but a definitive explanation remains an open issue.

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arXiv2004

Fe XIII coronal line emission in cool M dwarfs

We report on a search for the Fe xiii forbidden coronal line at 3388.1 Åin a sample of 15 M-type dwarf stars covering the whole spectral class as well as different levels of activity. A clear detection was achieved for LHS 2076 during a major flare and for CN Leo, where the line had been discovered before. For some other stars the situation is not quite clear. For CN Leo we investigated the timing behaviour of the Fe xiii line and report a high level of variability on a timescale of hours which we ascribe to microflare heating.

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arXiv2003

Detection of Saturnian X-ray emission with XMM-Newton

The giant planet Saturn was observed by XMM-Newton in September 2002. We present and analyse these XMM-Newton observations and compare our findings to the Chandra observations of Saturn. Contamination of the XMM-Newton data by optical light is found to be severe in the medium and thin filters, but with the thick filter all optical light is sufficiently blocked and the signal observed in this filter is interpreted as genuine X-ray emission, which is found to qualitatively and quantitatively resemble Saturn's Chandra spectrum very well.

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arXiv2003

Coronal abundances from high-resolution X-ray data: The case of Algol

We discuss the determination of elemental abundances from high resolution X-ray data. We emphasize the need for an accurate determination of the underlying temperature structure and advocate the use of a line ratio method which allows us to utilize, first, the strongest lines observed in the X-ray spectra, and second, lines that span a rather wide temperature range. We point out the need to use continuous emission measure distributions and show via example that modeling in terms of individual temperature components yields errors of more than 50%. We stress the need to derive differential emission measure distributions based on physical assumptions and considerations. We apply our methods to the Chandra LETGS spectrum of Algol and show that nitrogen is considerably enhanced compared to cosmic abundances by a factor of 2 while carbon is depleted by at least a factor of 25. Iron, silicon, and magnesium, are all depleted compared to cosmic abundances, while the noble gas neon has the relatively highest abundance.

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arXiv2003

Spatially resolved X-ray emission of EQ Pegasi

We present an analysis of an XMM-Newton observation of the M dwarf binary EQ Pegasi with a special focus on the the spatial structure of the X-ray emission and the analysis of light curves. Making use of data obtained with EPIC (European Photon Imaging Camera) we were for the first time able to spatially resolve the two components in X-rays and to study the light curves of the individual components of the EQ Peg system. During the observation a series of moderate flares was detected, where it was possible to identify the respective flaring component.

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arXiv2003

NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars

We present a final summary of all ROSAT X-ray observations of nearby stars. All available ROSAT observations with the ROSAT PSPC, HRI and WFC have been matched with the CNS4 catalog of nearby stars and the results gathered in the Nearby X-ray and XUV-emitting Stars data base, available via www from the Home Page of the Hamburger Sternwarte at http://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. New volume-limited samples of F/G-stars (d_lim = 14 pc), K-stars (d_lim = 12 pc), and M-stars (d_lim = 6 pc) are constructed within which detection rates of more than 90 % are obtained; only one star (GJ 1002) remains undetected in a pointed follow-up observation. F/G-stars, K-stars and M-stars have indistinguishable surface X-ray flux distributions, and the lower envelope of the observed distribution at F_X \approx 10^4 erg/cm^2/sec is the X-ray flux level observed in solar coronal holes. Large amplitude variations in X-ray flux are uncommon for solar-like stars, but maybe more common for stars near the bottom of the main sequence; a large amplitude flare is reported for the M star LHS 288. Long term X-ray light curves are presented for Alpha Cen A/B and Gl~86, showing variations on time scales of weeks and demonstrating that Alpha Cen B is a flare star.

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arXiv2003

A spatially resolved limb flare on Algol B observed with XMM-Newton

We report XMM-Newton observations of the eclipsing binary Algol A (B8V) and B (K2III). The XMM-Newton data cover the phase interval 0.35 - 0.58, i.e., specifically the time of optical secondary minimum, when the X-ray dark B-type star occults a major fraction of the X-ray bright K-type star. During the eclipse a flare was observed with complete light curve coverage. The decay part of the flare can be well described with an exponential decay law allowing a rectification of the light curve and a reconstruction of the flaring plasma region. The flare occurred near the limb of Algol B at a height of about 0.1R with plasma densities of a few times 10^11 cm^-3 consistent with spectroscopic density estimates. No eclipse of the quiescent X-ray emission is observed leading us to the conclusion that the overall coronal filling factor of Algol B is small.

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arXiv2003

Are stellar coronae optically thin in X-rays? - A systematic investigation of opacity effects

The relevance of resonant scattering (absorption by photo-excitation and re-emission out of the line of sight) in the solar corona has always been discussed controversially. Ratios of emission lines from identical ions but different oscillator strengths are tracers for damping of resonance lines due to possible resonant scattering. We present an analysis of a large sample of 44 stellar spectra obtained with all gratings on board Chandra and XMM-Newton and systematically measure line ratios of FeXVII lines at 15.27A and 16.78A lines to the resonance line at 15.03A as well as the He-like f/r ratio of OVII and NeIX. The same ratios obtained from optically thin plasma atomic databases such as MEKAL, Chianti, and APEC are found to be discrepant, however, we find no convincing proof for resonance line scattering. Optical depths are basically identical for all kinds of stellar coronae and we conclude that identical optical depths are more probable when effects from resonant scattering are generally negligible. The 15.27/15.03A ratio shows a regular trend suggesting blending of the 15.27A line by a cooler Fe line, possibly FeXVI. The He-like f/r ratios for O and Ne show no indication for significant damping of the resonance lines. We mainly attribute deviations from the atomic databases to still uncertain emissivities which do not agree well with laboratory measurements and which come out with differing results when accounting for one or the other side effect.

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arXiv2003

Field ZAMS stars in the solar neighborhood: Where have they come from ?

In the course of an all-sky survey for young stars in the solar neighborhood we have found a tight kinematic group of ten F-G type zero-age main sequence stars in the field. Here we discuss the origin of these stars. Backtracking the space motions of these stars we argue that likely candidates for the parent association are the Perseus OB3 (Per OB3), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC) associations, and that we are witnessing the ongoing diffusion of (at least one of) these associations into the field.

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arXiv2003

A Systematic Study Of X-Ray Variability In The ROSAT All-Sky Survey

We present a systematic search for variability among the ROSAT All-Sky Survey (RASS) X-ray sources. We generated lightcurves for about 30000 X-ray point sources detected sufficiently high above background. For our variability study different search algorithms were developed in order to recognize flares, periods and trends, respectively. The variable X-ray sources were optically identified with counterparts in the SIMBAD, the USNO-A2.0 and NED data bases, but a significant part of the X-ray sources remains without cataloged optical counterparts. Out of the 1207 sources classified as variable 767 (63.5 %) were identified with stars, 118 (9.8 %) are of extragalactic origin, 10 (0.8 %) are identified with other sources and 312 (25.8 %) could not uniquely be identified with entries in optical catalogs. We give a statistical analysis of the variable X-ray population and present some outstanding examples of X-ray variability detected in the ROSAT all-sky survey. Most prominent among these sources are white dwarfs, apparently single, yet nevertheless showing periodic variability. Many flares from hitherto unrecognised flare stars have been detected as well as long term variability in the BL Lac 1E1757.7+7034.

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