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Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| An XMM-Newton Study of the Coronae of σ2 Coronae Borealis
We present results of XMM-Newton Guaranteed Time observations of the RSCVn binary σ2 Coronae Borealis. The spectra obtainedwith the Reflection Grating Spectrometers and the European PhotonImaging Camera MOS2 were simultaneously fitted with collisionalionization equilibrium plasma models to determine coronal abundances ofvarious elements. Contrary to the solar first ionization potential (FIP)effect, in which elements with a low FIP are overabundant in the coronacompared to the solar photosphere, and contrary to the ``inverse'' FIPeffect observed in several active RS CVn binaries, coronal abundanceratios in σ2 CrB show a complex pattern, as supportedby similar findings in the Chandra HETGS analysis of σ2CrB with a different methodology by Osten and coworkers in 2003. Low-FIPelements (<10 eV) have abundance ratios relative to Fe that areconsistent with the solar photospheric ratios, whereas high-FIP elementshave abundance ratios that increase with increasing FIP. We find thatthe coronal Fe abundance is consistent with the stellar photosphericvalue, indicating that there is no metal depletion inσ2 CrB. However, we obtain a higher Fe absoluteabundance than Osten and coworkers did. Except for Ar and S, ourabsolute abundances are about 1.5 times larger than those reported byOsten and coworkers. However, a comparison of their model with ourXMM-Newton data (and vice versa) shows that both models work adequatelyin general. We find, therefore, no preference for one methodology overthe other for deriving coronal abundances. Despite the systematicdiscrepancy in absolute abundances, our abundance ratios are very closeto those obtained by Osten and coworkers. Finally, we confirm themeasurement of a low density in O VII (<4×1010cm-3) but could not confirm the higher densities measured inspectral lines formed at higher temperatures that were derived by otherstudies of σ2 CrB due to the lower spectral resolutionof the XMM-Newton grating spectrometers.
| 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.
| High mass X-ray binaries in the LMC: Dependence on the stellar population age and the ``propeller'' effect
We study the population of compact X-ray sources in the Large MagellanicCloud using the archival data of the XMM-Newton observatory. The totalarea of the survey is ≈ 3.8 square degrees with a limitingsensitivity of ≈10-14 erg/s/cm2, correspondingto a luminosity of ≈3× 1033 erg/s at the LMCdistance. Out of ˜460 point sources detected in the 2-8 keV energyband, the vast majority are background CXB sources, observed through theLMC. Based on the properties of the optical and near-infraredcounterparts of the detected sources we identified 9 likely HMXBcandidates and 19 sources, whose nature is uncertain, thus providinglower and upper limits on the luminosity distribution of HMXBs in theobserved part of LMC. When considered globally, the bright end of thisdistribution is consistent within statistical and systematicuncertainties with extrapolation of the universal luminosity function ofHMXBs. However, there seems to be fewer low luminosity sources,log(LX)⪉ 35.5, than predicted. We consider the impact ofthe ``propeller effect'' on the HMXB luminosity distribution and showthat it can qualitatively explain the observed deficit of low luminositysources. We found significant field-to-field variations in the number ofHMXBs across the LMC, which appear to be uncorrelated with the starformation rates inferred by the FIR and Hα emission. Wesuggest that these variations are caused by the dependence of the HMXBnumber on the age of the underlying stellar population. Using theexistence of large coeval stellar aggregates in the LMC, we constrainthe number of HMXBs as a function of time τ elapsed since the starformation event in the range of τ from ˜1-2 Myr to˜10-12 Myr.
| Some Like It Hot: The X-Ray Emission of the Giant Star YY Mensae
We present an analysis of the X-ray emission of the rapidly rotatinggiant star YY Mensae observed by Chandra HETGS and XMM-Newton. Thehigh-resolution spectra display numerous emission lines of highlyionized species; Fe XVII to Fe XXV lines are detected, together withH-like and He-like transitions of lower Z elements. Although no obviousflare was detected, the X-ray luminosity changed by a factor of 2between the XMM-Newton and Chandra observations taken 4 months apart(from logLX~32.2 to 32.5 ergs s-1, respectively).The coronal abundances and the emission measure distribution have beenderived from three different methods using optically thin collisionalionization equilibrium models, which is justified by the absence ofopacity effects in YY Men as measured from line ratios of Fe XVIItransitions. The abundances show a distinct pattern as a function of thefirst ionization potential (FIP), suggestive of an inverse FIP effect asseen in several active RS CVn binaries. The low-FIP elements (<10 eV)are depleted relative to the high-FIP elements; when compared to itsphotospheric abundance, the coronal Fe abundance also appears depleted.We find a high N abundance in YY Men's corona, which we interpret as asignature of material processed in the CNO cycle and dredged up in thegiant phase. The corona is dominated by a very high temperature (20-40MK) plasma, which places YY Men among the magnetically active stars withthe hottest coronae. Lower temperature plasma also coexists, albeit withmuch lower emission measure. Line broadening is reported in some lines,with a particularly strong significance in Ne X Lyα. We interpretsuch broadening as Doppler thermal broadening, although rotationalbroadening due to X-ray-emitting material high above the surface couldbe present as well. We use two different formalisms to discuss the shapeof the emission measure distribution. The first one infers theproperties of coronal loops, whereas the second formalism uses flares asa statistical ensemble. We find that most of the loops in the corona ofYY Men have their maximum temperature equal to or slightly larger thanabout 30 MK. We also find that small flares could contributesignificantly to the coronal heating in YY Men. Although there is noevidence of flare variability in the X-ray light curves, we argue thatYY Men's distance and X-ray brightness do not allow us to detect flareswith peak luminosities LX<=1031 ergss-1 with current detectors.
| The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
The Burst and Transient Source Experiment (BATSE), aboard the ComptonGamma Ray Observatory (CGRO), provided a record of the low-energygamma-ray sky (~20-1000 keV) between 1991 April and 2000 May (9.1 yr).BATSE monitored the high-energy sky using the Earth occultationtechnique (EOT) for point sources whose emission extended for times onthe order of the CGRO orbital period (~92 min) or greater. Using the EOTto extract flux information, a catalog of sources using data from theBATSE Large Area Detectors has been prepared. The first part of thecatalog consists of results from the all-sky monitoring of 58 sources,mostly Galactic, with intrinsic variability on timescales of hours toyears. For these sources, we have included tables of flux and spectraldata, and outburst times for transients. Light curves (or fluxhistories) have been placed on the World Wide Web.We then performed a deep-sampling of these 58 objects, plus a selectionof 121 more objects, combining data from the entire 9.1 yr BATSE dataset. Source types considered were primarily accreting binaries, but asmall number of representative active galaxies, X-ray-emitting stars,and supernova remnants were also included. The sample represents acompilation of sources monitored and/or discovered with BATSE and otherhigh-energy instruments between 1991 and 2000, known sources taken fromthe HEAO 1 A-4 and Macomb & Gehrels catalogs. The deep sampleresults include definite detections of 83 objects and possibledetections of 36 additional objects. The definite detections spannedthree classes of sources: accreting black hole and neutron starbinaries, active galaxies, and supernova remnants. The average fluxesmeasured for the fourth class, the X-ray emitting stars, were below theconfidence limit for definite detection.Flux data for the deep sample are presented in four energy bands: 20-40,40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 yr)for the sample varies from 3.5 to 20 mcrab (5 σ) between 20 and430 keV, depending on systematic error, which in turn is primarilydependent on the sky location.To strengthen the credibility of detection of weaker sources (~5-25mcrab), we generated Earth occultation images, searched for periodicbehavior using FFT and epoch folding methods, and critically evaluatedthe energy-dependent emission in the four flux bands. The deep sampleresults are intended for guidance in performing future all-sky surveysor pointed observations in the hard X-ray and low-energy gamma-ray band,as well as more detailed studies with the BATSE EOT.
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/927
| From the Solar Corona to Clusters of Galaxies: The Radio Astronomy of Bruce Slee
Owen Bruce Slee is one of the pioneers of Australian radio astronomy.During World War II he independently discovered solar radio emission,and, after joining the CSIRO Division of Radiophysics, used a successionof increasingly more sophisticated radio telescopes to examine anamazing variety of celestial objects and phenomena. These ranged fromthe solar corona and other targets in our solar system, to differenttypes of stars and the ISM in our Galaxy, and beyond to distant galaxiesand clusters of galaxies. Although long retired, Slee continues to carryout research, with emphasis on active stars and clusters of galaxies. Aquiet and unassuming man, Slee has spent more than half a century makingan important, wide-ranging contribution to astronomy, and his workdeserves to be more widely known.
| A Giant Outburst at Millimeter Wavelengths in the Orion Nebula
Berkely-Illinois-Maryland Association (BIMA) array observations of theOrion nebula discovered a giant flare from a young star previouslyundetected at millimeter wavelengths. The star briefly became thebrightest compact object in the nebula at 86 GHz. Its flux densityincreased by more than a factor of 5 on a timescale of hours, to a peakof 160 mJy. This is one of the most luminous stellar radio flares everobserved. Remarkably, the Chandra X-Ray Observatory was in the midst ofa deep integration of the Orion nebula at the time of the BIMAdiscovery; the source's X-ray flux increased by a factor of 10approximately 2 days before the radio detection. Follow-up radioobservations with the VLA and BIMA showed that the source decayed on atimescale of days, then flared again several times over the next 70days, although never as brightly as during the discovery. Circularpolarization was detected at 15, 22, and 43 GHz, indicating that theemission mechanism was cyclotron. VLBA observations 9 days after theinitial flare yield a brightness temperatureTb>5×107 K at 15 GHz. Infraredspectroscopy indicates that the source is a K5 V star with faint Brγ emission, suggesting that it is a weak-line T Tauri object.Zeeman splitting measurements in the infrared spectrum find B~2.6+/-1.0kG. The flare is an extreme example of magnetic activity associated witha young stellar object. These data suggest that short observationsobtained with the Atacama Large Millimeter Array will uncover hundredsof flaring young stellar objects in the Orion region.
| Doppler imaging of stellar surface structure. XX. The rapidly-rotating single K2-giant HD 31993 = V1192 Orionis
We present two Doppler images from two consecutive stellar rotations ofthe single K2-giant HD 31993. Each Doppler image is reconstructed fromspectra obtained within a single stellar rotation. With its 25-dayrotational period and a radius of ~18 solar radii, HD 31993 isconsidered a very rapidly rotating star and thus allows the applicationof the Doppler-imaging technique, despite the unusually long period. Allmaps reveal 7 isolated, predominantly low-latitude spots with atemperature difference, photosphere minus spot, of just ~200 K. No polarspot or high-latitude activity above, say, +60° is seen. A largewarm feature is detected at high latitude and is believed to be real.These spots act as tracers for a cross correlation analysis and yield aclear signature of anti-solar differential surface rotation, i.e. thepolar regions rotating faster than the equator, with alpha =0.125+/-0.05 corresponding to a lap time of ~200 days. A detailed parameterstudy is carried out to verify the reality of the HD 31993 maps.
| Long-term monitoring of active stars. X. Photometry collected in 1994
As part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 18 selected stars is presented. UBV(RI)_c observations were collectedat the European Southern Observatory over the intervals 21-28 September1994 and 25 November-05 December 1994. Additional UBV photometryobtained late in 1994 by Catania Astrophysical Observatory AutomaticPhotoelectric Telescope is also presented. Significant evolution of thelight curves, period variations and evidence for long-term variabilityof the global degree of spottedness are found. Some spectralclassifications are revised and photometric parallaxes are compared,whenever possible, with the values measured by the Hipparcos satellite.These observations are finalized to the construction of an extendedphotometric database, which can give important clues on topics such asthe stability of spotted areas, differential stellar rotation,solar-like activity cycles and the correlation between inhomogeneitiesat different atmospheric levels.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables 1, 2 and 4 and the complete data set are only available inelectronic form at the 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/400/659 (Table 3 is alsoavailable at the CDS).
| Chandra and XMM-Newton X-Ray Spectroscopy of the Hot Corona of YY Mensae
We present Chandra and XMM-Newton X-ray spectra of the rapidly rotatingK giant YY Mensae (d = 290 pc). YY Men is a member of a loosely-definedclass of rapidly rotating single cool giant stars (``FK Com stars''),whose outstanding property is a projected equatorial velocity measuredup to 110 km/s, in contrast with the expected maximum of 6 km/s forgiants. One of the leading theories to explain the extreme properties ofFK Com stars suggests that they were formed by coalescence of a contactbinary when one of the components entered into the giant stage. Previousobservations have shown that its corona displays plasma with a very hottemperature of about 3 keV, with probably no or very little plasma attemperatures below 1 keV. The new X-ray observatories now provideexcellent high-resolution X-ray spectra to study the coronal propertiesof this extreme coronal source. The X-ray spectrum is dominated by astrong continuum and by bright emission lines, mostly from H-liketransitions and Fe XXIV lines. Thanks to its wavelength range and itssensitivity, the Reflection Grating Spectrometer onboard XMM-Newtonprovides important information on the presence of cool coronal plasma,mostly seen at wavelengths between 20 and 40 Å. Simultaneous EPICCCD spectra also constrain the high-temperature component of theemission measure distribution. The Chandra High-Energy TransmissionGrating Spectrometer data are ideal to study at very high resolution theshort wavelength range of this hot coronal source. We derive the coronalemission measure distribution, elemental abundances, and densityestimates. We acknowledge support by SAO grant GO2-3016X.
| The Asiago-ESO/RASS QSO Survey. II. The Southern Sample
This is the second paper of a series describing the Asiago-ESO/RASS QSOsurvey, a project aimed at the construction of an all-sky, statisticallywell defined sample of very bright QSOs (BJ<=15). Such asurvey is required to remove the present uncertainties about theproperties of the local QSO population and constitutes an homogeneousdatabase for detailed evolutionary studies of active galactic nuclei(AGNs). We present here the complete southern sample, which comprises243 bright (12.60<=BJ<=15.13) QSO candidates at highgalactic latitudes (|bgal|>=30°). The area covered bythe survey is 5660 deg2. Spectroscopy for the 137 stillunidentified objects has been obtained. The total number of AGNs turnsout to be 111, 63 of which are new identifications. The properties ofthe selection are discussed. The completeness and the success rate forthis survey at the final stage are 63% and 46%, respectively.Based on observations collected at the European Southern Observatory,Chile (ESO P66.A-0277 and ESO P67.A-0537), with the Steward Observatoryand with National Telescope Galileo during AO3 period.
| Chromospherically Active Stars. XXI. The Giant, Single-lined Binaries HD 89546 And HD 113816
We have obtained spectroscopy and photometry of the chromosphericallyactive, single-lined spectroscopic binaries HD 89546 and HD 113816. HD89546 has a circular orbit with a period of 21.3596 days. Its primaryhas a spectral type of G9 III and is somewhat metal-poor with[Fe/H]~-0.5. HD 113816 has an orbit with a period of 23.6546 and a loweccentricity of 0.022. Its mass function is extremely small, 0.0007Msolar, consistent with a very low inclination. The primaryis a slightly metal-poor K2 III. A decade or more of photometricmonitoring with an automatic telescope demonstrates that both systemsdisplay brightness variations due to rotational modulation of thevisibility of photospheric star spots, as well as light-curve changesresulting from the redistribution of star spots by differential rotationand long-term changes in the filling factor of the spots. We determinedrotation periods for each season when the observations were numerousenough. Our mean rotation periods of 21.3 and 24.1 days for HD 89546 andHD 113816, respectively, confirm that the giants in each system aresynchronously rotating. The orbital elements and properties of the giantcomponents of these two systems, including levels of surface magneticactivity, are quite similar. However, the two rotational inclinationsare rather different, 57° for HD 89546 and 13° for HD 113816.Thus the latter giant is seen nearly pole on. We analyzed the lightcurves for similarities and differences that result from viewing thesetwo systems from quite different inclinations.
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| 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.
| Hydroxyl 1.563 Micron Absorption from Starspots on Active Stars
We present results from a study of starspots on active stars using apair of vibrational-rotational absorption lines of the OH molecule near1.563 μm. We detect excess OH absorption due to dark, cool starspotson several active stars of the RS CVn and BY Dra classes. Our resultsfor the single-lined spectroscopic binaries II Pegasi, V1762 Cygni, andλ Andromedae augment those from a previous study that used a lesssensitive detector. In this study, we were able for the first time touse molecular absorption features to measure starspot properties ondouble-lined spectroscopic binaries. Measuring the equivalent widths ofthese OH lines in inactive giant and dwarf stars of spectral types G, K,and M, we find that the total equivalent width of the line pairincreases approximately linearly as effective temperature decreases from5000 to 3000 K. We measure starspot filling factors by fitting thespectra of active stars with linear combinations of comparison starspectra representing the spot and nonspot regions of the star.
| On the Wilson-Bappu relationship in the Mg II k line
An investigation is carried out on the Wilson-Bappu effect in the Mg Iik line at 2796.34 Å. The work is based on a selection of 230 starsobserved by both the IUE and HIPPARCOS satellites, covering a wide rangeof spectral types (F to M) and absolute visual magnitudes (-5.4<=MV <=9.0). A semi-automatic procedure is used to measurethe line widths, which applies also in the presence of strong centralabsorption reversal. The Wilson-Bappu relationship here provided isconsidered to represent an improvement over previous recent results forthe considerably larger data sample used, as well as for a properconsideration of the measurement errors. No evidence has been found fora possible dependence of the WB effect on stellar metallicity andeffective temperature.
| Doppler imaging of stellar surface structure. XVI. A time-series analysis of the moderately-rotating K1-giant sigma Geminorum
We present a simultaneous photometric and spectroscopic imaging analysisof the long-period RS CVn binary sigma Gem, covering 3.6 consecutiverotation cycles with high time resolution. From six overlapping butconsecutive Doppler maps we trace the evolution of individual spotsthroughout the time range covered. All spots group either along a bandat approximately +45 degrees latitude and a width of 30 degrees, orappear centered at the equator. No polar spot is detected. We did notfind a conclusive migration pattern from the cross-correlation maps fromone rotation to the next and attribute this to a masking effect ofshort-term spot changes.
| 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.
| Long-term monitoring of active stars. IX. Photometry collected in 1993
As a part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 31 selected stars is presented. The UBV(RI)_c observations werecollected at the European Southern Observatory over the 31 December1992-18 January 1993 and the 20 November-3 December 1993 intervals.Additional UBV photometry obtained by the ``Phoenix" and by the CataniaAstrophysical Observatory Automatic Photoelectric Telescopes from 1990to 1993 is also presented for some of the program stars. Significantevolution of the light curves, period variations and evidence forlong-term variability of the global degree of spottedness are found.Some spectral classifications are revised and the inferred photometricparallaxes are compared, whenever possible, with the values measured bythe Hipparcos satellite. These observations are finalized to theconstruction of an extended photometric database, which can giveimportant clues on topics such as the stability of spotted areas,differential rotation, solar-like cycles and the correlation betweeninhomogeneities at different atmospheric levels. Based on data collectedat the European Southern Observatory, La Silla, Chile. Tables and thecomplete data set are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/910
| A BATSE Earth Occultation Catalog of 0.03-1.8 MEV Gamma-Ray Source Spectra and Light Curves for Phases 1-3 (1991-1994)
Using the powerful Earth-occultation technique, long-term, nearlycontinuous monitoring of the entire low-energy gamma-ray sky is nowpossible with the advent of BATSE, the Burst and Transient SourceExperiment on board the Compton Gamma Ray Observatory (CGRO). In thispaper, we present a catalog of 34 moderately strong gamma-ray sourcesmeasured by BATSE. It consists of 0.03-1.8 MeV photon spectra averagedover weeks and months, and light curves of the 35-200 keV flux, with 1day resolution, covering the first three phases of the CGRO mission(1991 May through 1994 October). These results have been obtained usingthe JPL Enhanced BATSE Occultation Package (EBOP) developed under theCGRO Guest Investigator Program. The EBOP concept and approach are alsodescribed in some depth. This paper presents highlights extracted from alarge EBOP database which has now been archived at the ComptonObservatory Science Support Center (COSSC). This database contains acomplete record of ~1200 daily source count rates in 14 energy channelsalong with the corresponding Poisson and systematic errors for 64sources, including 30 not described here. An interface to XSPEC isincluded in the archive allowing conversion from count rates to photonfluxes. This paper therefore serves also as a reference and entree intothe archive and provides an index and guide for those investigatorsusing the EBOP system and database for their respective scientificinvestigations.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Evolution of X-ray activity and rotation on G-K giants
The recent availability of stellar parallaxes provided by the Hipparcosstar catalogue (ESA 1997) enables an accurate determination of thepositions of single field giants in a theoretical H-R diagram and areliable estimate of their masses. The present study combines these newastrometric data with previously published X-ray fluxes and rotationalvelocities. The results confirm the existence of a sharp decrease ofX-ray emission at spectral type K1 for 2.5 M_sun < M < 5 M_sungiants. The study shows that the rotational velocity of these starsreaches a minimum at the same location in the H-R diagram. However, notight relationship between X-ray luminosities and projected equatorialvelocities was found among the sample stars. I suggest that theseresults could reflect the importance of differential rotation indetermining the level of coronal emission among >= 2.5Msun G and K giants. The restoration of rigid rotation at thebottom of the red giant branch could prevent the maintenance of largescale magnetic fields, thus explaining the sharp decrease of coronalX-ray emission at spectral type K1.
| Study of FK Comae Berenices. I. Surface images for 1994 and 1995
We present new surface images of FK Com for August 1994 and July 1995.For the 1995 images two different inversion methods, Tikhonovregularization and the Occamian approach, are used to check thedependence on the formal assumptions. The images are found to be verysimilar when the same local line profiles, models, stellar parametersand observations are used as inputs for both methods. The validity ofthe maps and their temperature scales are independently checked withphotometric observations. The maps for both years show active regions ofvery similar substructures and latitudes. It seems that the same spotgroup has survived on the surface of FK Com for the 11 months betweenthe observations; however, during that time it has moved about 0.2 inphase. The movement and evolution of the spot groups seem to cause thephotometrically observed ``flip-flop'' phenomenon, which is noticed tobe repeated with an average period of 6.5 years, similar to some RSCVn-stars. Based on the observations obtained at the Nordic OpticalTelescope, Observatorio Roque de los Muchachos, La Palma, CanaryIslands, Spain.
| Coordinated multiwavelength observations of the RS CVn system CF TUC
The relatively bright eclipsing RS CVn binary CF Tuc was observed inboth optical (photometry and spectroscopy) and microwave ranges in a`multisite, multiwavelength' campaign in 1996. The microwave data,gathered using the Australia Telescope Compact Array, covered slightlymore than one complete orbital cycle at 4.8 and 8.64 GHz in onecontinuous run. There was also limited coverage at 1.38 and 2.38 GHz.High-dispersion spectroscopy was obtained using the McLellan 1-mtelescope and echelle spectrograph at Mt John University Observatory(New Zealand). Supporting photometry came from smaller scale facilitiesassociated with the Royal Astronomical Society of New Zealand (RASNZ).The data show the correlation between microwave signal enhancement andphotometric flux diminution (maculation effect) noted previously insimilar studies. The spectroscopy also points to the phenomena beingrelated to a very enhanced active region on the secondary star locatedclose to the maculation region. Cross-correlation of the microwaveemission data between the 4.8- and 8.64-GHz ranges shows a strongpositive correlation, with a significant lag of the 4.8-GHz behind the8.64-GHz variations. This indicates energized waves propagating throughthe corona of the K4 subgiant at the electron sound speed. The emittingregion may have been eclipsed by the primary star, but such eclipseeffects are of marginal significance for the present analysis. Anattempt is made to put all the evidence into one coherent picture of theactive region.
| Astrophysics in 1998
From Alpha (Orionis and the parameter in mixing-length theory) to Omega(Centauri and the density of the universe), the Greeks had a letter forit. In between, we look at the Sun and planets, some very distantgalaxies and nearby stars, neutrinos, gamma rays, and some of theanomalies that arise in a very large universe being studied by roughlyone astronomer per 10^7 Galactic stars.
| Circumstellar emission and flares on FK Comae Berenices. Analysis and modelling of Balmer and He I D3 line variations
We present results on spectroscopic observations of the fast-rotatingactive giant FK Comae, obtained mainly with theESA-MUSICOS spectrograph at the Isaac Newton Telescope (INT) in 1996 and1997 and also with the Aurélie spectrograph at the Observatoirede Haute Provence (OHP) in 1997. The profiles analysed are those of theBalmer, Hα \ and Hβ , and He I D3 (lambda 5876 Angstroms)lines. We analyse the Balmer line variability and phase behaviour. Weconfirm these lines as highly variable, with excess emission thatoriginates from extended structures and exhibits clear signs ofrotational modulation. We have described the line profiles for twodistinct states of activity, using different modelling approaches.Similar techniques were applied to the He I D3 spectra. A large flareevent lasting several days was detected in both Balmer lines and in theHe I D3 line. The energy released during this flare in Hα \ is ofthe order of 10(37) erg, making it the largest Hα \ flare reportedon a cool star. Our results confirm the extreme complexity of thecircumstellar environment of FK Comae. The activity level of this staris quite variable demanding different approaches to the line profileanalysis. Based on observations obtained at the Isaac Newton Telescopewith the ESA-MUSICOS spectrograph and at the Observatoire de HauteProvence with the Aurélie spectrograph.
| Microarcsecond instability of the celestial reference frame
The fluctuation of the angular positions of reference extragalacticradio and optical sources under the influence of the irregulargravitational field of visible Galactic stars is considered. It is shownthat these angular fluctuations range from a few up to hundreds ofmicroarcseconds. This leads to a small rotation of the celestialreference frame. The non-diagonal coefficients of the rotation matrixare of the order of a microarcsecond. The temporal variation of thesecoefficients due to the proper motion of the foreground stars is of theorder of one microsecond per 20 years. The celestial reference frame cantherefore be considered inertial and homogeneous only to microarcsecondaccuracy. Astrometric catalogues with microarcsecond accuracy will beunstable, and must be re-established every 20 years.
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Observation and Astrometry data
| Constellation: | Table |
| Right ascension: | 04h58m17.94s |
| Declination: | -75°16'38.0" |
| Apparent magnitude: | 8.061 |
| Distance: | 291.545 parsecs |
| Proper motion RA: | -3 |
| Proper motion Dec: | -2.6 |
| B-T magnitude: | 9.405 |
| V-T magnitude: | 8.172 |
Catalogs and designations:
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