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β Crt (Al Sharasif)



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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).

An Archive of IUE Low-Dispersion Spectra of the White Dwarf Stars
We have produced an archive of the ultraviolet low-dispersion spectrafor the full set of white dwarf stars observed with the InternationalUltraviolet Explorer (IUE) satellite over the course of its 18 yrmission. This archive contains the spectra of 322 individual degeneratestars which have been processed to optimize the signal-to-noise for eachstar. In particular, all spectra have been corrected for residualtemporal and thermal effects and placed on the Hubble Space TelescopeFaint Object Spectrograph absolute flux scale using procedures describedby Massa & Fitzpatrick. Wherever possible, multiple observations ofindividual stars have been co-added to further enhance signal-to-noiseand have been combined into a single spectrum including the full 1150 to3150 Å wavelength region observed by IUE. The contents of thisspectral archive are described and the details of data reductionprocedures are provided, along with the url for access to the electronicfiles of the processed spectra.

Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars
We report the results of a high-resolution spectroscopic survey for COfundamental emission from T Tauri stars. CO fundamental emission isfrequently detected, with the likely origin of the emission in thecircumstellar disk. An initial assessment of the line profiles indicatesthat the emission region includes the equivalent of the terrestrialplanet region of our solar system, a result that suggests the utility ofCO fundamental emission as a probe of disks at planet formationdistances. Since fundamental emission is detected frequently from bothclose binary and apparently single stars, it appears that both lowcolumn density regions, such as disk gaps, and temperature inversionregions in disk atmospheres can produce significant emission. Theestimated excitation temperature of the emitting gas is unexpectedlywarm for the disk radii that they appear to probe. Thus, the surfacegaseous component of inner disks may be significantly warmer than thesurface dust component. We also detect CO emission from a transitional TTauri star. Because fundamental emission from CO and its isotopes issensitive to a wide range of gas masses, including masses<

3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447

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 ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897

Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphere
Within the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105

A comparison of DA white dwarf temperatures and gravities from Lyman and Balmer line studies
We present measurements of the effective temperatures and surfacegravities for a sample of hot DA white dwarfs, using the Lyman line dataavailable from the HUT, ORFEUS and FUSE FUV space missions. Comparingthe results with those from the standard Balmer line technique, we findthat there is a general good overall agreement between the two methods.However, significant differences are found for a number of stars, butnot always of a consistent nature in that sometimes the Balmertemperature exceeds that derived from the Lyman lines and in otherinstances it is lower. We conclude that, with the latest modelatmosphere calculations, these discrepancies probably do not arise froman inadequate theoretical treatment of the Lyman lines but rather fromsystematic effects in the observation and data reduction processes,which dominate the statistical errors in these spectra. If thesesystematic data reduction effects can be adequately controlled, theLyman line temperature and gravity measurements are consistent withthose obtained from the Balmer lines when allowance is made forreasonable observational uncertainties.

The low-mass white dwarf companion to β Crateris
We present FUSE H Lyman series spectroscopy of the hot white dwarfcompanion to the 4th magnitude A1III star βCrt, which shows that ithas an unusually low mass, MWD=0.43Msolar, and hasalmost certainly evolved through binary interaction. This system couldbe a long-sought remnant of Algol-type evolution, although radialvelocity measurements appear to show that the pair are not close.Instead, micro-variations in the proper motion of βCrt as measuredby Hipparcos suggest that the period could be as high as ~10yr. However,a low-mass white dwarf in a system with a period >~3yr is difficultto explain by conventional models for binary evolution. We speculate onalternative models for the evolution of this system which involve aneccentric binary or multiple components.

The properties of field elliptical galaxies at intermediate redshift - II. Photometry and spectroscopy of an HST-selected sample
A sample of field early-type galaxies (E/S0) at intermediate redshift(z~0.1-0.6) is selected, based on morphology and colours from HST-WFPC2parallel images. Photometric structural parameters (effective radiusRe and effective surface brightness SBe) arederived through the F606W and F814W filters, using luminosity profilefitting and two-dimensional fitting techniques. The combined parameterthat enters the Fundamental Plane(logRe-βSBe, with β~0.32) is shown tosuffer from significantly smaller uncertainties (rms 0.03) than theindividual structural parameters (e.g. ~15 per cent rms on the effectiveradius). High signal-to-noise ratio, intermediate-resolution spectra,taken at the ESO 3.6-m telescope, yield redshifts for 35 galaxies andcentral velocity dispersions for 22 galaxies. Central velocitydispersions are derived using a library of stellar templates covering awide range of spectral types, in order to study the effects of templatemismatches. The average random error on the central velocity dispersionis found to be 8 per cent, and the average systematic error caused bytemplate mismatch is found to be 5 per cent. The errors in the velocitydispersion measurement and the effects of template mismatches arestudied by means of extensive Monte Carlo simulations. In addition, weinvestigate whether the determination of the velocity dispersion issensitive to the spectral range used, finding that the value of velocitydispersion is unchanged when the spectral regions that include theabsorption features Ca H and K and NaD are masked out during the fit.

Resolving Sirius-like binaries with the Hubble Space Telescope
We present initial results from a Hubble Space Telescope ultravioletimaging survey of stars known to have hot white dwarf companions whichare unresolved from the ground. The hot companions, discovered throughtheir EUV or UV emission, are hidden by the overwhelming brightnesses ofthe primary stars at visible wavelengths. Out of 17 targets observed, wehave resolved eight of them with the Wide Field Planetary Camera 2,using various ultraviolet filters. Most of the implied orbital periodsfor the resolved systems are hundreds to thousands of years, but in atleast three cases (56 Persei, ζ Cygni and RE J1925-566) it shouldbe possible to detect the orbital motions within the next few years, andthey may eventually yield new dynamically determined masses for thewhite dwarf components. The 56 Persei and 14 Aurigae systems are foundto be quadruple and quintuple, respectively, including the known opticalcomponents as well as the newly resolved white dwarf companions. Themild barium star ζ Cygni, known to have an 18-year spectroscopicperiod, is marginally resolved. All of these newly resolved Sirius-typebinaries will be useful in determining gravitational redshifts andmasses of the white dwarf components.

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 ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521

The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222

Another Search for Maia Variable Stars
We have used the Hipparcos epoch photometry database, andautocorrelation analysis, to search for the elusive Maiavariables-short-period B7-A3 near-main-sequence pulsating variablestars. Of several hundred stars considered, and several dozen starsstudied in detail, only a handful are possible variables: three arepossible shallow eclipsing variables; three have possible periods in therange 0.25-0.5 day, but their amplitudes are so low that they areprobably nonvariable. The most promising are HD 29573, with a period of1.6 days (but possibly a rotating variable), and γ CrB, with aperiod of 0.9 day-a period also found spectroscopically by Lehmann andcoworkers. Sirius shows variations which are probably instrumental. Twopreviously suspected Maia stars-Maia and γ UMi-are photometricallyconstant. The Maia variables-if they exist-are very rare and veryelusive.

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).

Spectroscopic confirmation of a white dwarf companion to the B star 16 Dra
Using an Extreme Ultraviolet Explorer (EUVE) spectrum, we confirm theidentification of a white dwarf companion to the B9.5V star 16 Dra(HD150100), and constrain its surface temperature to lie between 29,000K and 35,000 K. This is the third B star + white dwarf non-interactingSirius-type binary to be confirmed, after y Pup (HR2875, HD59635) andtheta Hya (HR3665, HD79469). 16 Dra and its white dwarf companion aremembers of a larger resolved proper motion system including the B9V star17 Dra A (HD150117). The white dwarf must have evolved from a progenitormore massive than this star, M_MS~3.7M_sun. White dwarf companions to Bstars are important since they set an observational limit on the maximummass for white dwarf progenitors, and can potentially be used toinvestigate the high mass ends of the initial-final mass relation andthe white dwarf mass-radius relation.

The eclipsing binary V578 Mon in the Rosette nebula: age and distance to NGC 2244 using Fourier disentangled component spectra
V578 Mon is a member of the young stellar cluster NGC 2244 which isembedded in the Rosette nebula. It is a double-lined binary, consistingof two early-B type components which eclipse each other partially duringtheir 2.40848-day orbit. A novel technique of Fourier disentangling isapplied to disentangle the spectrum of the binary into the spectra ofits components, allowing a direct spectroscopic temperaturedetermination for each component. Together with an analysis of the lightcurve, the spectral disentangling process allows us to derive theorbital and fundamental stellar parameters. The hotter star (T_eff = 30000 K) has a mass of 14.5 Msun and a radius of 5.2 Rsun. The cooler one (T_eff = 26 400 K) has a mass of 10.3 Msun and a radius of 4.3 Rsun . Both stars are well insidetheir critical Roche lobe and move in an eccentric orbit (e=0.087) whichhas a semi-major axis of 22 Rsun and is seen at aninclination of 72.6degr . The rotation of both stars is synchronizedwith the orbital motion. The systemic velocity of 34.9 km s-1,confirms that the binary belongs to NGC 2244. The age of the binary,and hence of NGC 2244, is (2.3±0.2) 106 years. Fromthe fundamental stellar parameters and the interstellar absorption, thedistance to the cluster is derived to be 1.39 ±0.1 kpc, i.e.slightly lower than that found from photometric studies. Its age andspace velocity indicate that NGC 2244 is formed in the galactic plane.The high accuracy obtained (e.g. probable errors of 0.5% for the masses,1m -1.5% for the radii, despite the lack of total eclipses,and 1.5% for the temperatures) is a direct consequence of the spectraldisentangling technique, which as a by-product delivers very accurateradial velocities for all orbital phases. Based on observations obtainedat the European Southern Observatory (ESO), La Silla, Chile Table 5 isonly available in electronic form at the CDS via anonymous ftp to130.79.128.5 or at http://cdsweb.u-strasbg.fr

Radial velocities of HIPPARCOS southern B8-F2 type stars
Radial velocities have been determined for a sample of B8-F2 type starsobserved by the Hipparcos satellite. Observations were obtained withinthe framework of an ESO key-program. Radial velocities have beenmeasured using a cross-correlation method, the templates being a grid ofsynthetic spectra. The obtained precision depends on effectivetemperature and projected rotational velocity of the star as well as ona possible asymmetry of the correlation peak generally due to secondarycomponents. New spectroscopic binaries have been detected from theseasymmetries and the variability of the measured radial velocity.Simulations of binary and triple systems have been performed. Forbinaries our results have been compared with Hipparcos binary data.Adding the variable radial velocities, the minimum binary fraction hasbeen found 60% for physical systems. Radial velocities have beendetermined for 581 B8-F2 stars, 159 being new. Taking into accountpublished radial velocities, 39% south A-type stars with V magnitudelower than 7.5 have a radial velocity. Based on observations obtained atthe European Southern Observatory (ESO, La Silla, Chile) and on datafrom the ESA Hipparcos astrometry satellite.}\fnmsep \thanks{Tables 7, 8and 9 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

A search for hidden white dwarfs in theROSATEUV survey - II. Discovery of a distant DA+F6/7V binary system in a direction of low-density neutral hydrogen
The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet(EUV) has provided us with evidence for the existence of a previouslyunidentified sample of hot white dwarfs in unresolved, detached binarysystems. These stars are invisible at optical wavelengths due to theclose proximity of their much more luminous companions (spectral type Kor earlier). However, for companions of spectral type ~A5 or later thewhite dwarfs are easily visible at far-ultraviolet wavelengths, and canbe identified in spectra taken by IUE. 16 such systems have beendiscovered in this way through ROSATEUVEIUE observations, including fouridentified by us in Paper I. In the present paper we report the resultsof our continuing search during the final year of IUE operations. Onenew system, RE J0500-364 (DA+F6/7V), has been identified. This starappears to lie at a distance of ~500-1000 pc, making it one of the mostdistant white dwarfs, if not the most distant, to be detected in the EUVsurveys. The very low line-of-sight neutral hydrogen volume density tothis object could place a lower limit on the length of the beta CMainterstellar tunnel of diffuse gas, which stretches away from the LocalBubble in a similar direction to RE J0500-364. In this paper we alsoanalyse a number of the stars observed where no white dwarf companionwas found. Some of these objects show evidence for chromospheric andcoronal activity. Finally, we present an analysis of the previouslyknown WD+active F6V binary HD 27483 (Bohm-Vitense 1993), and show that,at T~22000K, the white dwarf may be contributing significantly to theobserved EUV flux. If so, it is one of the coolest such stars to bedetected in the EUV surveys.

Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions
We present an analysis of optical, ultraviolet, and X-ray spectralproperties of a sample of 13 hot hydrogen-rich (DA) white dwarfs, eachpaired with a luminous unresolved companion. Using low-dispersionInternational Ultraviolet Explorer spectra, ROSAT photometry, andExtreme-Ultraviolet Explorer photometry and spectroscopy, we estimatethe effective temperature, mass, and distance of the white dwarfs.Additionally, we examine the question of their atmospheric composition.We establish orbital properties for most binaries by means ofhigh-dispersion optical spectroscopy obtained with the Hamilton echellespectrograph at Lick Observatory; the same data help uncover evidence ofactivity in some of the secondary stars that is also notable in ROSATX-ray measurements. In particular, we find high-amplitude (>20 kms-1) velocity variations in only two stars (HD 33959C and HR 8210),low-amplitude variations in four additional objects (HD 18131, HR 1608,theta Hya, and BD +27 deg1888), and no variations (<2 km s-1) in theremainder. We have observed Ca H and K in emission in four (BD +08deg102, HD 18131, HR 1608, and EUVE J0702+129) of the six objects thatwere also detected in the 0.52-2.01 keV ROSAT PSPC band, while thesource of the hard X-ray emission in HD 33959C remains unknown; otherinvestigators have noted some evidence of activity in the remaining0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs arealso investigated; EUV spectroscopy shows the effect of a low heavyelement abundance in the atmosphere of the white dwarf in HD 33959C andof a high heavy element abundance in HD 223816; measurements of allother objects are apparently consistent with emission from pure-hydrogenatmospheres. However, current data do not constrain well the white dwarfparameters, and, to remedy the situation, we propose to obtainspectroscopy of the complete H Lyman line series.

The ROSAT all-sky survey catalogue of optically bright late-type giants and supergiants
We present X-ray data for all late-type (A, F, G, K, M) giants andsupergiants (luminosity classes I to III-IV) listed in the Bright StarCatalogue that have been detected in the ROSAT all-sky survey.Altogether, our catalogue contains 450 entries of X-ray emitting evolvedlate-type stars, which corresponds to an average detection rate of about11.7 percent. The selection of the sample stars, the data analysis, thecriteria for an accepted match between star and X-ray source, and thedetermination of X-ray fluxes are described. Catalogue only available atCDS via anonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

The absolute magnitude of the early type MK standards from HIPPARCOS parallaxes
We analyse the standards of the MK system with the help of Hipparcosparallaxes, using only stars for which the error of the absolutemagnitude is <= 0.3 mag. We find that the main sequence is a wideband and that, although in general giants and dwarfs have differentabsolute magnitudes, the separation between luminosity classes V and IIIis not clear. Furthermore, there are a number of exceptions to thestrict relation between luminosity class and absolute magnitude. Weanalyse similarly the system of standards defined by Garrison & Gray(1994) separating low and high rotational velocity standards. We findsimilar effects as in the original MK system. We propose a revision ofthe MK standards, to eliminate the most deviant cases. Based on datafrom the ESA Hipparcos astrometry satellite

The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.

An Optical Atlas of Extreme Ultraviolet Explorer (EUVE) Sources
The Extreme Ultraviolet Explorer (EUVE) has been detecting EUV sourcessince its launch in June 1992. Positions of 540 sources have been madeavailable to the community by the EUVE team. We have extracted 7' X 7'images centered on these 540 EUVE sources from the Space TelescopeScience Institute digitized sky archives. We present these images asmosaic finder charts to aid observers trying to identify EUVE sources,or to characterize known sources. (SECTION: Atlases)

The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)

The chemical composition and binarity of beta Crateris
A detailed abundance analysis of the bright A-type star betaCrt ispresented. It is found that most of the abundances are essentiallysolar, indicating that the star is not a hot Am star. The only prominentanomalies are an underabundance of Al and overabundances of S, Sr andBa. The barium overabundance is the most prominent and has importantimplications for the past history of this supposed binary system. Thepresence of the ROSAT-discovered white dwarf companion could indicatethat the A-type star may have accreted material from the white dwarfprogenitor. There are indications in the literature that the system is arelatively short-period binary. However, the radial velocitymeasurements presented in this paper show no apparent velocity changes,and do not support this hypothesis. It may be that the system is a widelong-period binary, or not even physically associated. Additional radialvelocity measurements are required to investigate the orbit of thissystem.

A Minisurvey of Interstellar Titanium from the Southern Hemisphere
We describe the results of a minisurvey of interstellar Ti II and Ca IIabsorption toward 42 early-type stars observed from the southernhemisphere at a spectral resolution of 4.5 km s-1. Results are alsopresented for the Na I ultraviolet line (3302 Angstroms) detected towardnine of these targets. We examine the dependence of the integratedcolumn densities of N(Ti II), N(Ca II), and N(Na I) on distance,reddening, neutral hydrogen column density, and their Galactic elementalabundance. Our findings support the proposition that Ti II and Ca IIabsorption originates in the same regions of the pervasive, warm, andneutral intercloud gas of the interstellar medium. We have observed acorrelation of decreasing Ti and Ca abundance with increasingline-of-sight gas density. The Ti II/Ca II abundance ratio has beenfound to be essentially constant under all the interstellar densityconditions we have sampled. Thus, we conclude that the generalabsorption properties of titanium (and calcium) are similar throughoutthe entire disk of our Galaxy.

An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.

Systematic Errors in the FK5 Catalog as Derived from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..850S&db_key=AST

Is beta Crateris a Sirius-like system?
New radial velocity (RV) measurements were obtained for the star betaCrt, which has been suspected to be a Sirius-like binary. Although apreliminary period analysis indicated a period as short as 8 days, nosignificant change of the RV over 8 nights could be detected. While thisresult is similar to what Smalley et al. (1997) found, our value of themean RV is much smaller. We, therefore, conclude that the RV of beta Crtis indeed variable, maybe but not necessarily due to binarity, but sofar no period can be determined. --- Based on observations obtained atthe 2.0m Ritchey-Chretien-Coudé Telescope of the NationalAstronomical Observatory, Rozhen, Bulgaria.

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Observation and Astrometry data

Right ascension:11h11m39.50s
Apparent magnitude:4.48
Distance:81.566 parsecs
Proper motion RA:0
Proper motion Dec:0
B-T magnitude:4.486
V-T magnitude:4.452

Catalogs and designations:
Proper NamesAl Sharasif
Bayerβ Crt
Flamsteed11 Crt
HD 1989HD 97277
TYCHO-2 2000TYC 6649-1274-1
USNO-A2.0USNO-A2 0600-13951291
BSC 1991HR 4343
HIPHIP 54682

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