Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| A Large Millimeter Flare on the RS CVn Binary σ Geminorum
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| X-ray luminosity function of faint point sources in the Milky Way
We assessed the contribution to the X-ray (above 2 keV) luminosity ofthe Milky Way by different classes of low-mass binary systems and singlestars. We began by using the RXTE Slew Survey of the sky at|b|>10° to construct an X-ray luminosity function (XLF) of nearbyX-ray sources in the range 1030 ergs-1
| Extragalactic Source Counts in the 20-50 keV Energy Band from the Deep Observation of the Coma Region by INTEGRAL IBIS
We present the analysis of serendipitous sources in a deep, 500 ks, hardX-ray observation of the Coma Cluster region with the IBIS instrument onboard INTEGRAL. In addition to the Coma Cluster, the final 20-50 keVimage contains 12 serendipitous sources with statistical significance>4 σ. We use these data (after correcting for expected numberof false detections) to extend the extragalactic source counts in the20-50 keV energy band down to a limiting flux of1.0×10-11 ergs s-1 cm-2 (~=1mcrab). This is a more than a factor of 10 improvement in sensitivitycompared to the previous results in this energy band obtained with theHEAO 1 A-4 instrument. The derived source counts are consistent with theEuclidean relation N(>f)~f-3/2. A large fraction ofidentified serendipitous sources are low-redshift, z<0.02, activegalactic nuclei (AGNs), mostly of Seyfert 1 type. The surface density ofhard X-ray sources is (1.4+/-0.5)×10-2 deg-2above a flux threshold of 10-11 ergs s-1cm-2. These sources directly account for ~3% of the cosmicX-ray background in the 20-50 keV energy band. Given the low redshiftdepth of our sample, we expect that similar sources at higher redshiftsaccount for a significant fraction of the hard X-ray background. Ourfield covers only 3% of the sky; a systematic analysis of otherextragalactic INTEGRAL observations can produce much larger sourcesamples and is, therefore, critically important.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| 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.
| 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
| 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
| 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.
| The Structure of Stellar Coronae in Active Binary Systems
A survey of 28 stars (22 active binary systems, plus six single stars orwide binaries for comparison) using extreme ultraviolet spectra has beenconducted to establish the structure of stellar coronae in active binarysystems from the emission measure distribution (EMD), electrondensities, and scale sizes. Observations obtained by the ExtremeUltraviolet Explorer satellite (EUVE) during 9 years of operation areincluded for the stars in the sample. EUVE data allow a continuous EMDto be constructed in the range logTe(K)~5.6-7.4, using ironemission lines. These data are complemented with IUE observations tomodel the lower temperature range [logTe(K)~4.0-5.6].Inspection of the EMD shows an outstanding narrow enhancement, or``bump'' peaking around logTe(K)~6.9 in 25 of the stars,defining a fundamental coronal structure. The emission measure per unitstellar area decreases with increasing orbital (or photometric) periodsof the target stars; stars in binaries generally have more material atcoronal temperatures than slowly rotating single stars. High electrondensities (Ne>~1012 cm-3) arederived at ~logTe(K)~7.0 for some targets, implying smallemitting volumes.The observations suggest the magnetic stellar coronae of these stars areconsistent with two basic classes of magnetic loops: solar-like loopswith maximum temperature around logTe(K)~6.3 and lowerelectron densities (Ne>~109-1010.5cm-3), and hotter loops peaking aroundlogTe(K)~6.9 with higher electron densities(Ne>~1012 cm-3). For the most activestars, material exists at much higher temperatures[logTe(K)>=6.9] as well. However, current ab initiostellar loop models cannot reproduce such a configuration. Analysis ofthe light curves of these systems reveals signatures of rotation ofcoronal material, as well as apparent seasonal (i.e., year-to-year)changes in the activity levels.
| Very Long Baseline Interferometry Imaging of the RS Canum Venaticorum Binary Star System HR 5110
We present VLBI images of the RS CVn binary star HR 5110 (=BH CVn; HD118216), obtained from observations made at 8.4 GHz on 1994 May 29/30 insupport of the NASA/Stanford Gravity Probe B project. Our images show anemission region with a core-halo morphology. The core was 0.39+/-0.09mas (FWHM) in size, or 66%+/-20% of the 0.6+/-0.1 mas diameter of thechromospherically active K subgiant star in the binary system. The halowas 1.95+/-0.22 mas (FWHM) in size, or 1.8+/-0.2 times the 1.1+/-0.1 masseparation of the centers of the K and F stars. (The uncertainties givenfor the diameter of the K star and its separation from the F star eachreflect the level of agreement of the two most recent publisheddeterminations.) The core increased significantly in brightness over thecourse of the observations and seems to have been the site of flareactivity that generated an increase in the total flux density of ~200%in 12 hr. The fractional circular polarization simultaneously decreasedfrom ~10% to 2.5%.
| 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.
| Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine the astrometric positions of 19radio stars in the International Celestial Reference Frame (ICRF). Thepositions of these stars were directly linked to the positions ofdistant quasars through phase-referencing observations. The positions ofthe ICRF quasars are known to 0.25 mas, thus providing an absolutereference at the angular resolution of our radio observations. Averagevalues for the errors in our derived positions for all sources were 13and 16 mas in αcosδ and δ, respectively, withaccuracies approaching 1-2 mas for some of the stars observed.Differences between the ICRF positions of the 38 quasars and thosemeasured from our observations showed no systematic offsets, with meanvalues of -0.3 mas in αcosδ and -1.0 mas in δ.Standard deviations of the quasar position differences of 17 and 11 masin αcosδ and δ, respectively, are consistent with themean position errors determined for the stars. Our measured positionswere combined with previous Very Large Array measurements taken from1978 to 1995 to determine the proper motions of 15 of the stars in ourlist. With mean errors of ~1.6 mas yr-1, the accuracies ofour proper motions approach those derived from Hipparcos and, for a fewof the stars in our program, are better than the Hipparcos values.Comparing the positions of our radio stars with the Hipparcos Catalogue,we find that at the epoch of our observations, the two frames arealigned to within formal errors of approximately 3 mas. This resultconfirms that the Hipparcos frame is inertial at the expected level.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| Lithium and rotation in F and G dwarfs and subgiants
Lithium abundances have been determined in 127 F and G Pop I stars basedon new measurements of the equivalent width of the lambda 6707 ÅLi I line from their high resolution CCD spectra. Distances and absolutemagnitudes of these stars have been obtained from the HipparcosCatalogue and their masses and ages derived, enabling us to investigatethe behaviour of lithium as a function of these parameters. Based ontheir location on the HR diagram superposed on theoretical evolutionarytracks, the sample of the stars has been chosen to ensure that they havemore or less completed their Li depletion on the main sequence. A largespread in the Li abundances is found at any given effective temperatureespecially in the already spun down late F and early G stars. Thisspread persists even if the ``Li-dip'' stars that have evolved from themain sequence temperature interval 6500-6800 K are excluded. Stars inthe mass range up to 2 M/Msun when divided into threemetallicity groups show a linear correlation between Li abundance andmass, albeit with a large dispersion around it which is not fullyaccounted for by age either. The large depletions and the observedspread in Li are in contrast to the predictions of the standard stellarmodel calculations and suggest that they are aided by non-standardprocesses depending upon variables besides mass, age and metallicity.The present study was undertaken to examine, in particular, the effectsof rotation on the depletion of Li. No one-to-one correlation is foundbetween the Li abundance and the present projected rotational velocity.Instead the observed abundances seem to be dictated by the rotationalhistory of the star. However, it is noted that even this interpretationis subject to the inherent limitation in the measurement of the observedLi EQW for large rotational velocities.Table 1 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/409/251
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| The HRX-BL Lac sample - Evolution of BL Lac objects
The unification of X-ray and radio selected BL Lacs has been anoutstanding problem in the blazar research in the past years. Recentinvestigations have shown that the gap between the two classes can befilled with intermediate objects and that apparently all differences canbe explained by mutual shifts of the peak frequencies of the synchrotronand inverse Compton component of the emission. We study the consequencesof this scheme using a new sample of X-ray selected BL Lac objectscomprising 104 objects with z<0.9 and a mean redshift bar {z} = 0.34.77 BL Lacs, of which the redshift could be determined for 64 (83%)objects, form a complete sample. The new data could not confirm ourearlier result, drawn from a subsample, that the negative evolutionvanishes below a synchrotron peak frequency log nupeak =16.5. The complete sample shows negative evolution at the 2sigma level(< Ve/Va > = 0.42 +/- 0.04). We concludethat the observed properties of the HRX BL Lac sample show typicalbehaviour for X-ray selected BL Lacs. They support an evolutionarymodel, in which flat-spectrum radio quasars (FSRQ) with high energeticjets evolve towards low frequency peaked (mostly radio-selected) BL Lacobjects and later on to high frequency peaked (mostly X-ray selected) BLLacs.Appendix (Tables 8 and 9, Fig. 8) is only available in electronic format http://www.edpsciences.org
| Automated spectroscopic abundances of A and F-type stars using echelle spectrographs. II. Abundances of 140 A-F stars from ELODIE
Using the method presented in Erspamer & North (\cite{erspamer},hereafter Paper I), detailed abundances of 140 stars are presented. Theuncertainties characteristic of this method are presented and discussed.In particular, we show that for a S/N ratio higher than 200, the methodis applicable to stars with a rotational velocity as high as 200 kms-1. There is no correlation between abundances and Vsin i,except a spurious one for Sr, Sc and Na which we explain by the smallnumber of lines of these elements combined with a locally biasedcontinuum. Metallic giants (Hauck \cite{hauck}) show larger abundancesthan normal giants for at least 8 elements: Al, Ca, Ti, Cr, Mn, Fe, Niand Ba. The anticorrelation for Na, Mg, Si, Ca, Fe and Ni with Vsin isuggested by Varenne & Monier (\cite{varenne99}) is not confirmed.The predictions of the Montréal models (e.g. Richard et al.\cite{richard01}) are not fulfilled in general. However, a correlationbetween left [(Fe)/(H)right ] and log g is found for stars of 1.8 to 2.0M_sun. Various possible causes are discussed, but the physical realityof this correlation seems inescapable.Based on observations collected at the 1.93 m telescope at theObservatoire de Haute-Provence (St-Michel l'Observatoire, France) andCORALIE.Based on observations collected at the Swiss 1.2 m Leonard Eulertelescopes at the European Southern Observatory (La Silla, Chile).Tables 5 and 6 are only 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/398/1121
| Origin of Prolonged X-Ray Flares on Active Late-Type Stars
Soft X-ray data for prolonged flares in subgiants in RS CVn binarysystems and some other active late-type stars (AB Dor, Algol) areanalyzed. During these nonstationary events, a large amount of hotplasma with temperatures exceeding 108 K exists for many hours.Numerical simulations of gas-dynamical processes in the X-raysourcegiant loopscan yield reliable estimates of theplasma parameters and flare-source size. This confirms that suchphenomena exist while considerable energy is supplied to the top part ofa giant loop or system of loops. Refined estimates of the flare energy(up to 1037 erg) and scales contradict the widely accepted idea thatprolonged X-ray flares are associated with the evolution of localmagnetic fields. The energy of the current component of the large-scalemagnetic field arising during the ejection of magnetic field by plasmajets or stellar wind is estimated. Two cases are considered: a globalstellar field and fields connecting regions with oppositely directedunipolar magnetic fields. The inferred energy of the current componentof the magnetic field associated with distortion of the initial MHDconfiguration is close to the total flare energy, suggesting thatlarge-scale magnetic fields play an important role in prolonged flares.The flare process encompasses some portion of a streamer belt and maypropagate along the entire magnetic equator of the star during the mostpowerful prolonged events.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| A highly sensitive search for magnetic fields in B, A and F stars
Circular spectropolarimetric observations of 74 stars were obtained inan attempt to detect magnetic fields via the longitudinal Zeeman effectin their spectral lines. The sample observed includes 22 normal B, A andF stars, four emission-line B and A stars, 25 Am stars, 10 HgMn stars,two lambda Boo stars and 11 magnetic Ap stars. Using the Least-SquaresDeconvolution multi-line analysis approach (Donati et al.\cite{donati97etal}), high precision Stokes I and V mean signatures wereextracted from each spectrum. We find absolutely no evidence formagnetic fields in the normal, Am and HgMn stars, with upper limits onlongitudinal field measurements usually considerably smaller than anypreviously obtained for these objects. We conclude that if any magneticfields exist in the photospheres of these stars, these fields are notordered as in the magnetic Ap stars, nor do they resemble the fields ofactive late-type stars. We also detect for the first time a field in theA2pSr star HD 108945 and make new precise measurements of longitudinalfields in five previously known magnetic Ap stars, but do not detectfields in five other stars classified as Ap SrCrEu. We also report newresults for several binary systems, including a new vsin i for therapidly rotating secondary of the Am-delta Del SB2 HD 110951. Based onobservations obtained using the MuSiCoS spectropolarimeter on theBernard Lyot telescope, l'Observatoire du Pic du Midi, France.
| Search for point sources of gamma radiation above 15 TeV with the HEGRA AIROBICC array
A search for potential point sources of very high energy gamma rays hasbeen carried out on the data taken simultaneously by the HEGRA AIROBICCand Scintillator arrays from August 1994 to March 2000. The list ofsought sources includes supernova remnants, pulsars, AGNs and binarysystems. The energy threshold is around 15 TeV. For the Crab Nebula, amodest excess of 2.5 standard deviations above the cosmic ray backgroundhas been observed. Flux upper limits (at 90% c.l.) of around 1.3 timesthe flux of the Crab Nebula are obtained, in average, for the candidatesources. A different search procedure has been used for an all-skysearch which yields absolute flux upper limits between 4 and 9 crabsdepending on declination, in the band from delta = 0 to delta =60o. The full versions of Tables 1 and 2, including thecoordinates of the sources, are 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/390/39
| Optical positions of 55 radio stars from astrolabe observations from the Yunnan Observatory
The observations by the photoelectric astrolabe at Yunnan Observatoryrelative to the Hipparcos Catalogue and the optical positions of 55radio stars were obtained from observations between 1991 and 2000. Theyall resulted from processing the photon counts obtained by means of theastrolabe after the automation of the instrument. There are 46 stars incommon with the Hipparcos Catalogue. Tables \ref{t1} and \ref{t2} arealso available in electronic form at the CDS via anonymous ftpcdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/1062.
| A complete sample of Seyfert galaxies selected at 0.25 keV
We have used the ROSAT Bright Source Catalogue to extract a completesample of sources selected in the band from 0.1-0.4keV. This0.25keV-selected sample is composed of 54 Seyfert galaxies, 25 BLLacertae objects, 4 clusters and 27 Galactic stars or binaries.Seyfert-type galaxies with `ultrasoft' X-ray spectra can very often beclassed optically as narrow-line Seyfert 1s (NLS1s). Such objects arereadily detected in 0.25keV surveys; the sample reported here contains20 NLS1s, corresponding to a 40 per cent fraction of the Seyferts.Optical spectra of the Seyfert galaxies were gathered for correlativeanalysis, which confirmed the well-known relations between X-ray slopeand optical spectral properties (e.g. [Oiii]/Hβ ratio; Feiistrength, Hβ width). The various intercorrelations are most likelydriven, fundamentally, by the shape of the photoionizing continuum inSeyfert nuclei. We argue that a steep X-ray spectrum is a betterindicator of an `extreme' set of physical properties in Seyfert galaxiesthan is the narrowness of the optical Hβ line. The correlationstudies were also used to isolate a number of Seyfert galaxies withapparently `anomalous' properties. Of particular interest are the sixobjects with relatively weak permitted line emission (Hβ and Feii)and weak optical continua. Such objects are rare in most surveys, buttwo of these (IC 3599 and NGC 5905) are known to be transient activegalactic nuclei in which the X-ray flux has faded by factors ~100. Ifthe other four objects also turn out to be transient, this woulddemonstrate that 0.25keV surveys provide an efficient way of finding aninteresting class of object. Finally, the luminosity function of the0.25keV-selected Seyfert galaxies was determined and broken down intosubsamples to investigate the relative space densities of Seyferts whenseparated on the basis of either X-ray slope or Hβ linewidth.
| 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
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Los Perros de Caza |
| Ascensión Recta: | 13h34m47.80s |
| Declinación: | +37°10'57.0" |
| Magnitud Aparente: | 4.98 |
| Distancia: | 44.524 parsecs |
| Movimiento Propio en Ascensión Recta: | 85 |
| Movimiento Propio en Declinación: | -9.6 |
| B-T magnitude: | 5.413 |
| V-T magnitude: | 4.978 |
Catálogos y designaciones:
|
