目录
图像
上传图像
DSS Images Other Images
相关文章
HESS very-high-energy gamma-ray sources without identified counterparts
Context: The detection of gamma rays in the very-high-energy (VHE)energy range (100 GeV-100 TeV) provides a direct view of the parentpopulation of ultra-relativistic particles found in astrophysicalsources. For this reason, VHE gamma rays are useful for understandingthe underlying astrophysical processes in non-thermal sources. Aims: Weinvestigate unidentified VHE gamma-ray sources that have been discoveredwith HESS in the most sensitive blind survey of the Galactic plane atVHE energies conducted so far. Methods: The HESS array of imagingatmospheric Cherenkov telescopes (IACTs) has a high sensitivity comparedwith previous instruments (~0.01 Crab in 25 h observation time for a5σ point-source detection), and with its large field of view, iswell suited for scan-based observations. The on-going HESS survey of theinner Galaxy has revealed a large number of new VHE sources, and foreach we attempt to associate the VHE emission with multi-wavelength datain the radio through X-ray wavebands. Results: For each of the eightunidentified VHE sources considered here, we present the energy spectraand sky maps of the sources and their environment. The VHE morphology iscompared with available multi-wavelength data (mainly radio and X-rays).No plausible counterparts are found.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Long-term monitoring of active stars. VIII. UBV(RI)_(c) photometry collected in February 1992
As a part of an extensive program focusing on the global properties andevolution of active stars, high-precision UBV(RI)_c photometry of 31selected stars, collected at the European Southern Observatory over the14-29 February 1992 interval, is 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 the inferred photometric parallaxes arecompared, whenever possible, with the values measured by the Hipparcossatellite. Flare events were detected for the star HD16157 = CC Eri, EXO 055609-3804.4 TY Coland HD 119285 = V851 Cen. Optical variability wasdiscovered for the Pop II binary HD 89499. Theseobservations contribute to the establishment of a time-extendedphotometric database which can give important clues on topics such asthe stability of spotted areas, differential rotation, solar-like cyclesand the correlation between inhomogeneities at different atmosphericlevels. based on data collected at the European Southern Observatory, LaSilla, Chile.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. III. Photometric properties.
We present high-precision UBV(RI)_c_ photometry for a sample of activestars detected serendipitously by the EXOSAT satellite. Fourteenobjects, out of 47 likely optical counterparts, turned out to be clearlyvariable, with periods in the range 1-8 days, including a newlydiscovered eclipsing binary. For most of them the optical variability isconsistent with the presence of photospheric cool spots. We have usedour multicolour photometry to estimate spectral classifications anddistances. The derived distances indicate that some of the observedstars are previously unidentified nearby (d<=25pc) M dwarfs. Whencombined with the results of high resolution spectroscopy, ourphotometric observations allow us to investigate the stellar content ofthe selected X-ray sample. Young stars and RS CVn-type binaries appearto constitute a large fraction of the selected sample.
| The 72nd Name-List of Variable Stars
Not Available
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images II. Lithium abundances
We present high-resolution spectra in the Li I 6708 A region, andmulticolor photometry, for a sample of active stars detectedserendipitously by the EXOSAT satellite. We analyse the spectral datawith a spectral synthesis code and derive Li abundances, rotation ratesand metallicities. We use our multicolor photometry to infer thespectral classification and distance of the stars in the sample. Thederived Li abundances are compared with those of various samplesincluding randomly selected field stars, RS CVn binaries, and clustersand associations of different ages. We show that Li per se is notsufficient to determine unambiguously the nature and evolutionary stateof the sample stars, but can provide useful constraints if used inconjunction with other diagnostics like binarity, spectral class,rotation rate and X-ray luminosity. We show in particular that at leastone third of the EXOSAT sample is constituted by young stars, with agescomparable to or younger than the Pleiades (~7x10^7^ yr). Another thirdconsists of RS CVn binaries, while the classification of the remainingobjects is uncertain. They could be either young stars or evolved activebinaries. We argue that a similar composition also applies to the sampleof X-ray stellar sources detected serendipitously by the EinsteinObservatory.
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. I - Time variability and spectral classification of eight southern stars
As part of a larger program to study the optical properties ofserendipitous Exosat sources, optical photometry and low-resolutionspectroscopy are presented for eight southern stars which have beenidentified as optical counterparts of Exosat sources. X-ray fluxvariability has been detected for three of them. In all three cases theoptical counterparts show RS CVn-type variability. Of the remaining fivestars, two are found to be variable in the optical. The high X-rayluminosities inferred from the derived spectroscopic parallaxesindicates that these variable sources are all very active systems,possibly RS CVn binaries. The three nonvariable sources are more likelynormal main-sequences stars, two of them with a rather high level ofcoronal emission.
| Estimation of spectral classifications for bright southern stars with interesting Stromgren indices
This paper investigates the degree of success with which uvby photometrycan be applied to predict spectral classifications for 947 A, F, and Gstars brighter than an apparent magnitude of 8.3 and with four-colorindices indicating some kind of interesting, unusual, or peculiarspectrum. One or several possible spectral classifications are estimatedfor each star from photometry alone, double stars are distinguished, andthe estimates are compared with published classifications. The resultsshow that the framework provided by uvby photometry can be extended toinclude most G and K stars, reddened stars, peculiar stars, and certaintypes of double star.
| A survey of stars with composite spectra.
Not Available
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
