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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 analogsbased on high-resolution X-ray spectroscopy and photometry withXMM-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 beenobserved. We use the X-ray spectra to derive coronal element abundancesof C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measuredistribution (EMD). We find that the abundances change from an inversefirst ionization potential (FIP) distribution in stars with ages around0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr andbeyond. This transformation is coincident with a steep decline ofnonthermal radio emission. The results are in qualitative agreement witha simple model in which the stream of electrons in magnetic fieldssuppresses diffusion of low-FIP ions from the chromosphere into thecorona. The coronal emission measure distributions show shapescharacterized by power laws on each side of the EMD peak. The lattershifts from temperatures of about 10 MK in the most rapidly rotating,young stars to temperatures around 4 MK in the oldest target consideredhere. The power-law index on the cooler side of the EMD exceeds expectedslopes for static loops, with typical values being 1.5-3. We interpretthis slope with a model in which the coronal emission is due to asuperposition of stochastically occurring flares, with an occurrencerate that is distributed in radiated energy E as a power law,dN/dE~E-α, as previously found for solar and stellarflares. We obtain the relevant power-law index α from the slope ofthe high-temperature tail of the EMD. Our EMDs indicate α~2.2-2.8,in excellent agreement with values previously derived from light curvesof magnetically active stars. Modulation with timescales reminiscent offlares is found in the light curves of all our targets. Several strongflares are also observed. We use our α-values to simulate lightcurves and compare them with the observed light curves. We thus derivethe range of flare energies required to explain the light-curvemodulation. More active stars require a larger range of flare energiesthan less active stars within the framework of this simplistic model. Inan overall scenario, we propose that flaring activity plays a largerrole in more active stars. In this model, the higher flare rate isresponsible both for the higher average coronal temperature and the highcoronal X-ray luminosity, two parameters that are indeed found to becorrelated.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| On the sizes of stellar X-ray coronae
Spatial information from stellar X-ray coronae cannot be assesseddirectly, but scaling laws from the solar corona make it possible toestimate sizes of stellar coronae from the physical parameterstemperature and density. While coronal plasma temperatures have longbeen available, we concentrate on the newly available densitymeasurements from line fluxes of X-ray lines measured for a large sampleof stellar coronae with the Chandra and XMM-Newton gratings. We compileda set of 64 grating spectra of 42 stellar coronae. Line counts of strongH-like and He-like ions and Fe XXI lines were measured with the CORAsingle-purpose line fitting tool by \cite{newi02}. Densities areestimated from He-like f/i flux ratios of O VII and Ne IX representingthe cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5-11 from the O VII triplet and between logne ≈ 10.5-12 from the Ne IX triplet, but we caution thatthe latter triplet may be biased by contamination from Fe XIX and Fe XXIlines. We find that low-activity stars (as parameterized by thecharacteristic temperature derived from H- and He-like line flux ratios)tend to show densities derived from O VII of no more than a few times1010 cm-3, whereas no definitive trend is foundfor the more active stars. Investigating the densities of the hotterplasma with various Fe XXI line ratios, we found that none of thespectra consistently indicates the presence of very high densities. Weargue that our measurements are compatible with the low-density limitfor the respective ratios (≈ 5× 1012cm-3). These upper limits are in line with constant pressurein the emitting active regions. We focus on the commonly used \cite{rtv}scaling law to derive loop lengths from temperatures and densitiesassuming loop-like structures as identical building blocks. We derivethe emitting volumes from direct measurements of ion-specific emissionmeasures and densities. Available volumes are calculated from theloop-lengths and stellar radii, and are compared with the emittingvolumes to infer filling factors. For all stages of activity we findsimilar filling factors up to 0.1.Appendix A is only available in electronic form athttp://www.edpsciences.org
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Coronal X-ray Spectroscopy of Solar Analogs
Not Available
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars
We present results of X-ray spectral and timing analyses of solar-like(spectral types F5-K8) and intermediate-type (B4-F4) Pleiads observed ina 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities(LX) up to a factor 10 fainter than previous studies usingthe ROSAT PSPC. All eight solar-like members have`quasi-steady'LX>~ 1029erg s-1,consistent with the known rotation-activity relation and four exhibitflares. Using a hydrodynamic modelling technique, we derive loophalf-lengths for the two strongest flares, on H II 1032 and H II 1100.Near the beginning of its flare, the light curve of H II 1100 shows afeature with a profile suggestive of a total occultation of the flaringloop. Eclipse by a substellar companion in a close orbit is possible butwould seem an extraordinarily fortuitous event; absorption by afast-moving cloud of cool material requires NH at least twoorders of magnitude greater than any solar or stellar prominence. Anoccultation may have been mimicked by the coincidence of two flares,though the first, with its decay time being shorter than its rise timeand suggestive of , would be unusual.Spectral modelling of the quasi-steady emission shows a rising trend incoronal temperature from F and slowly rotating G stars to K stars tofast-rotating G stars, and a preference for low coronal metallicity.These features are consistent with those of nearby solar-like stars,although none of the three stars showing `saturated' emission bears thesignificant component at 2 keV seen in the saturated coronae of AB Dorand 47 Cas. Of five intermediate-type stars, two are undetected(LX < 4 × 1027erg s-1) andthree show X-ray emission with a spectrum and LX consistentwith origin from an active solar-like companion.
| A Far Ultraviolet Spectroscopic Explorer Survey of Coronal Forbidden Lines in Late-Type Stars
We present a survey of coronal forbidden lines detected in FarUltraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Twostrong coronal features, Fe XVIII λ974 and Fe XIX λ1118,are observed in 10 of the 26 stars in our sample. Various other coronalforbidden lines, observed in solar flares, also were sought but notdetected. The Fe XVIII feature, formed at logT=6.8 K, appears to be freeof blends, whereas the Fe XIX line can be corrupted by a C I multiplet.FUSE observations of these forbidden iron lines at spectral resolutionλ/Δλ~15,000 provides the opportunity to studydynamics of hot coronal plasmas. We find that the velocity centroid ofthe Fe XVIII feature deviates little from the stellar rest frame,confirming that the hot coronal plasma is confined. The observed linewidths generally are consistent with thermal broadening at the hightemperatures of formation and show little indication of additionalturbulent broadening. The fastest rotating stars, 31 Com, α AurAb, and AB Dor, show evidence for excess broadening beyond the thermalcomponent and the photospheric vsini. The anomalously large widths inthese fast-rotating targets may be evidence for enhanced rotationalbroadening, consistent with emission from coronal regions extending anadditional ΔR~0.4-1.3R* above the stellar photosphere,or represent the turbulent broadening caused by flows along magneticloop structures. For the stars in which Fe XVIII is detected, there isan excellent correlation between the observed Röntgensatellit(ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux.As a result, Fe XVIII is a powerful new diagnostic of coronal thermalconditions and dynamics that can be utilized to study high-temperatureplasma processes in late-type stars. In particular, FUSE provides theopportunity to obtain observations of important transition region linesin the far-UV, as well as simultaneous measurements of soft X-raycoronal emission, using the Fe XVIII coronal forbidden line.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F andlater and derived ``overall'' broadening functions in selectedwavelength regions utilizing a Least Squares Deconvolution (LSD)procedure. Precision values of the projected rotational velocity v \siniwere derived from the first zero of the Fourier transformed profiles andthe shapes of the profiles were analyzed for effects of differentialrotation. The broadening profiles of 70 stars rotating faster than v\sini = 45 km s-1 show no indications of multiplicity nor ofspottedness. In those profiles we used the ratio of the first two zerosof the Fourier transform q_2/q_1 to search for deviations from rigidrotation. In the vast majority the profiles were found to be consistentwith rigid rotation. Five stars were found to have flat profilesprobably due to cool polar caps, in three stars cuspy profiles werefound. Two out of those three cases may be due to extremely rapidrotation seen pole on, only in one case (v \sini = 52 km s-1)is solar-like differential rotation the most plausible explanation forthe observed profile. These results indicate that the strength ofdifferential rotation diminishes in stars rotating as rapidly as v \sini>~ 50 km s-1.Table A.1 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/813Based on observations collected at the European Southern Observatory, LaSilla, 69.D-0015(B).
| Are stellar coronae optically thin in X-rays?. A systematic investigation of opacity effects
The relevance of resonant scattering in the solar corona has always beendiscussed controversially. Ratios of emission lines from identical ionsbut different oscillator strengths have been used in order to estimatedamping of resonance lines due to possible resonant scattering, i.e.,absorption by photo-excitation and re-emission out of the line of sight.The analysis of stellar spectra in analogy to previous works for the Sunis possible now with XMM-Newton and Chandra grating spectra and requiresthis issue to be considered again. In this work we present a sample of45 X-ray spectra obtained for 26 stellar coronae with the RGS on boardXMM-Newton and the LETGS and HETGS on board Chandra. We use ratios ofthe Fe XVII lines at 15.27 Å and 16.78 Å lines to theresonance line at 15.03 Å as well as the He-like f/r ratio of OVII and Ne IX to measure optical depth effects and compare them withratios obtained from optically thin plasma atomic databases such asMEKAL, Chianti, and APEC. From the Fe XVII line ratios we find noconvincing proof for resonance line scattering. Optical depths arebasically identical for all kinds of stellar coronae and we concludethat identical optical depths are more probable when effects fromresonant scattering are generally negligible. The 15.27/15.03Åratio shows a regular trend suggesting blending of the 15.27Åline by a cooler Fe line, possibly Fe XVI. The He-like f/r ratiosfor O and Ne show no indication for significant damping of the resonancelines. We mainly attribute deviations from the atomic databases tostill uncertain emissivities which do not agree well with laboratorymeasurements and which come out with differing results when accountingfor one or the other side effect. We attribute the discrepancies in thesolar data to geometrical effects from observing individual emittingregions in the solar corona but only overall emission for stellarcoronae including photons eventually scattered into the line of sight.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Stellar Radio Astronomy: Probing Stellar Atmospheres from Protostars to Giants
Radio astronomy has provided evidence for the presence of ionizedatmospheres around almost all classes of nondegenerate stars.Magnetically confined coronae dominate in the cool half of theHertzsprung-Russell diagram. Their radio emission is predominantly ofnonthermal origin and has been identified as gyrosynchrotron radiationfrom mildly relativistic electrons, apart from some coherent emissionmechanisms. Ionized winds are found in hot stars and in red giants. Theyare detected through their thermal, optically thick radiation, butsynchrotron emission has been found in many systems as well. The latteris emitted presumably by shock-accelerated electrons in weak magneticfields in the outer wind regions. Radio emission is also frequentlydetected in pre-main sequence stars and protostars and has recently beendiscovered in brown dwarfs. This review summarizes the radio view of theatmospheres of nondegenerate stars, focusing on energy release physicsin cool coronal stars, wind phenomenology in hot stars and cool giants,and emission observed from young and forming stars.
| Flare Heating in Stellar Coronae
An open question in the field of solar and stellar astrophysics is thesource of heating that causes stellar coronae to reach temperatures ofmillions of degrees. One possibility is that the coronae are heated by alarge number of small flares. On the Sun, flares with energies as low asthose of microflares are distributed with energy as a power law of theform (dN/dE)~E-α, with α~1.8, and α appearsto increase to values of 2.2-2.7 for flares of lower energy. If theslope exceeds the critical value of 2, then in principle the entirecoronal energy input can be ascribed to flares that are increasinglyless energetic but are more numerous. Previous analyses of flares inlight curves of active stars have shown that this index generally isgreater than 2, although it may be as low as 1.6 when strong flaresalone are considered. Here we investigate the contribution of very weakflares, covering the milliflare energy range, to the coronal luminosityof low-mass active stars. We analyze Extreme Ultraviolet Explorer/DeepSurvey events data from FK Aqr, V1054 Oph, and AD Leo and conclude thatin all these cases, the coronal emission is dominated by flares to suchan extent that in some cases, the entire emission can be ascribed toflare heating. We have developed a new method to directly model for thefirst time stochastically produced flare emission, includingundetectable flares, and their effects on the observed photon arrivaltimes. We find that αFKAqr=2.60+/-0.34,αV1054Oph=2.74+/-0.35, andαADLeo=2.03-2.32, and the flare component accounts fora large fraction (generally greater than 50%) of the total flux.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| The Flux Deficits in Star Spots
The bolometric flux deficits of the photospheres of spotted stars arederived for the first time in the framework of zonal spottedness modelsfor red dwarfs computed at the Crimean Astrophysical Observatory. Theresulting flux deficits are compared to the estimated radiative lossesfrom the chromospheres and coronas measured during quasi-simultaneousobservations. A linear correlation is found between the logarithms ofthese quantities, with the Sun fitting these relations. Radiative lossesfrom the outer stellar atmospheres in quiescence and during individualsporadic flares are significantly lower than the bolometric deficits ofthe spotted photospheres of active stars. This suggests that the fluxdeficit due to spots leads to global reconstruction of the atmospheresof red dwarfs, analogous to the local atmospheric reconstruction thatoccurs during solar and stellar flares. This process may be realized viathe superposition of a large number of weak impulsive flares and otherdynamic events, which develop on these stars and heat their coronas(i.e., in this view, microflaring is favored as the principal coronalheating mechanism for these stars). A brief analysis of the long-termvariations in the chromospheric and photospheric radiation of F-K starsfrom the HK project and of the Sun suggests that such dynamicalreconstruction of the outer atmosphere by energy associated with theflux deficit of the spotted photosphere occurs at times of increasedsurface activity in all F-M stars.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Extreme Ultraviolet Astronomy
Astronomical studies in the extreme ultraviolet (EUV) band of thespectrum were dismissed during the early years of space astronomy asimpossible, primarily because of the mistaken view that radiation inthis band would be absorbed by the interstellar medium. Observations inthe 1980s from sounding rockets and limited duration orbital spacecraftbegan to show the potential of this field and led to the deployment oftwo spacecraft devoted to EUV astronomy: the UK Wide Field Camera andthe Extreme Ultraviolet Explorer. The instrumentation in these missions,although quite limited in comparison with instrumentation in otherfields of space astronomy, provided unique and far-reaching results.These included new information on solar system topics, stellarchromospheres and corona, white dwarf astrophysics, cataclysmicvariables, the interstellar medium, galaxies, and clusters of galaxies.We summarize these findings herein.
| Extreme-Ultraviolet Flare Activity in Late-Type Stars
Extreme Ultraviolet Explorer Deep Survey observations of cool stars(spectral type F to M) have been used to investigate the distribution ofcoronal flare rates in energy and its relation to activity indicatorsand rotation parameters. Cumulative and differential flare ratedistributions were constructed and fitted with different methods. Powerlaws are found to approximately describe the distributions. A trendtoward flatter distributions for later type stars is suggested in oursample. Assuming that the power laws continue below the detection limit,we have estimated that the superposition of flares with radiatedenergies of about 1029-1031 ergs could explain theobserved radiative power loss of these coronae, while the detectedflares are contributing only ~10%. Although the power-law index is notcorrelated with rotation parameters (rotation period, projectedrotational velocity, Rossby number) and only marginally with the X-rayluminosity, the flare occurrence rate is correlated with all of them.The occurrence rate of flares with energies larger than 1032ergs is found to be proportional to the average total stellar X-rayluminosity. Thus, energetic flares occur more often in X-ray brightstars than in X-ray faint stars. The normalized occurrence rate offlares with energies larger than 1032 ergs increases withincreasing LX/Lbol and stays constant forsaturated stars. A similar saturation is found below a critical Rossbynumber. The findings are discussed in terms of simple statistical flaremodels in an attempt to explain the previously observed trend for higheraverage coronal temperatures in more active stars. It is concluded thatflares can contribute a significant amount of energy to coronal heatingin active stars.
| UV observations of B to F-type stars.
Not Available
| The Brera Multiscale Wavelet ROSAT HRI Source Catalog. II. Application to the HRI and First Results
The wavelet detection algorithm (WDA) described in the accompanyingpaper by Lazzati et al. is suited to a fast and efficient analysis ofimages taken with the High-Resolution Imager (HRI) instrument on boardthe ROSAT satellite. An extensive testing is carried out on thedetection pipeline: HRI fields with different exposure times aresimulated and analyzed in the same fashion as the real data. Positionsare recovered with errors of a few arcseconds, whereas fluxes are withina factor of 2 from their input values in more than 90% of the cases inthe deepest images. Unlike the ``sliding-box'' detection algorithms, theWDA also provides a reliable description of the source extension,allowing for a complete search of, e.g., supernova remnants or clustersof galaxies in the HRI fields. A completeness analysis on simulatedfields shows that for the deepest exposures considered (~120 ks) alimiting flux of ~3x10^-15 ergs s^-1 cm^-2 can be reached over theentire field of view. We test the algorithm on real HRI fields selectedfor their crowding and/or the presence of extended or bright sources(e.g., clusters of galaxies and stars, supernova remnants). We show thatour algorithm compares favorably with other X-ray detection algorithms,such as XIMAGE and EXSAS. Analysis with the WDA of the large set of HRIdata will allow us to survey ~400 deg^2 down to a limiting flux of~10^-13 ergs s^-1 cm^-2, and ~0.3 deg^2 down to ~3x10^-15 ergs s^-1cm^-2. A complete catalog will result from our analysis, consisting ofthe Brera Multiscale Wavelet Bright Source Catalog (BMW-BSC), withsources detected with a significance of >~4.5 σ, and the FaintSource Catalog (BMW-FSC), with sources at >~3.5 σ. Aconservative estimate based on the extragalactic log N-log S indicatesthat at least 16,000 sources will be revealed in the complete analysisof the entire HRI data set.
| Implications from Extreme-Ultraviolet Observations for Coronal Heating of Active Stars
Extreme Ultraviolet Explorer (EUVE) data of two active solar analogs, 47Cas and EK Dra, were used to investigate flare statistics and thedistribution of the flare occurrence rate in energy. The EUVE satelliteobserved each star for almost 7 days. Simultaneous spectral data fromits spectrometers were used to derive temperature and abundancecharacteristics of their coronae. The emission models were derived fromdifferential emission measure distributions by fitting optically thinthermal models to the spectra. The Deep Survey instrument photon listswere analyzed by applying different time binnings. A total of 28 flareswere identified for further analysis. The timing study providedestimates for the total radiative energy loss of each flare. Thedifferential distribution of flares in total X-ray energy is found to bea power law (dN/dE~E^-alpha, with alpha~2.2+/-0.2) valid in the energyrange between 3x10^33 and 6x10^34 ergs. The power-law index is largerthan that for typical solar flares but is similar to indices foundrecently for small-scale solar events. If the power law continues toenergies of moderate solar flares, then the total energy emitted by theensemble of all flares may suffice to explain all of the observedflaring and ``quiescent'' X-ray emissions of the two stars. Aconsiderable portion, if not all, of the energy required to heat theircoronae could thus be provided by flares.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| The ROSAT all-sky survey catalogue of optically bright main-sequence stars and subgiant stars
We present X-ray data for all main-sequence and subgiant stars ofspectral types A, F, G, and K and luminosity classes IV and V listed inthe Bright Star Catalogue that have been detected as X-ray sources inthe ROSAT all-sky survey; several stars without luminosity class arealso included. The catalogue contains 980 entries yielding an averagedetection rate of 32 percent. In addition to count rates, sourcedetection parameters, hardness ratios, and X-ray fluxes we also listX-ray luminosities derived from Hipparcos parallaxes. The catalogue isalso available in electronic form via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| An Optical Atlas of Extreme Ultraviolet Explorer (EUVE) Sources
The Extreme Ultraviolet Explorer (EUVE) has been detecting EUV sourcessince its launch in June 1992. Positions of 540 sources have been madeavailable to the community by the EUVE team. We have extracted 7' X 7'images centered on these 540 EUVE sources from the Space TelescopeScience Institute digitized sky archives. We present these images asmosaic finder charts to aid observers trying to identify EUVE sources,or to characterize known sources. (SECTION: Atlases)
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| The Stromvil System: an Effective Combination of Two Medium-Band Photometric Systems
It is shown that the addition to the Stromgren four-color photometricsystem of three passbands at 374, 516 and 656 nm from the Vilniusphotometric system makes the combined system more universal. This newsystem, called the Stromvil system, makes it possible to classify starsof all spectral types, even in the presence of interstellar reddening.This property of the system is especially important in CCD photometry,allowing the photometric classification of very faint stars. Apreliminary calibration of the system in terms of spectral andluminosity classes, temperatures and surface gravities is available. Alist of preliminary standards for the Stromvil system in the regions ofCygnus, Aquila and near the North Celestial Pole is given.
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