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Публикации по объекту
Improvement of Hipparcos Proper Motions in Declination
More than a decade elapsed after the HIPPARCOS ESA mission (ESA 1997)observations have been collected. This first astronomical satellitemission was less than 4 years long so that 1991.25 is the epoch of theHIPPARCOS Catalogue. Many other projects have checked or improvedHIPPARCOS data. Also, a long series of ground - based opticalobservations of some stars included in HIPPARCOS Catalogue, made withPhotographic Zenith Tubes (PZT) are useful for the task of improving theproper motions of these stars. The ARIHIP Catalogue (after ACT, TYCHO -2, FK6, GC+HIP, TYC2+HIP) is a combination of the HIPPARCOS and someground - based data, and the ARIHIP proper motions are more accuratethan the HIPPARCOS ones. Here we present a new step of our procedure ofcalculation; between PZT data we added the HIPPARCOS position withsuitable weight - the point with the coordinates (1991.25, 0ŭ0)in our case. The method was applied to 202 stars observed at RichmondPZTs in the course of a few decades. The result is better proper motionsin declination for these HIPPARCOS stars, and a good agreement withARIHIP proper motions (we found 128 common Richmond and ARIHIP stars tocheck our result). Also, we present the result for other 74 Richmondstars which are not found in ARIHIP.
| On the Spectroscopic Determination of Atmospheric Parameters and O/Fe Abundances of RR Lyrae Stars
In order to study how the conventional spectroscopic method based on theequivalent widths of the Fe I and Fe II lines effectively applies todetermining the atmospheric parameters (Teff, log g,vt, [Fe/H]) of RR Lyrae variables and how accurately theabundances can be established from such constructed model atmospheres,we analyzed 15 high-dispersion spectrograms of RR Lyr, DX Del, DH Peg,and VY Ser taken at several different phases by using the HDSspectrograph of the Subaru Telescope, and examined the consistency ofthe resulting phase-to-phase abundances. Taking oxygen as the targetelement along with Fe, we determined its non-LTE abundance from the O I6155-8 and 7771-5 triplets. It was found that consistent abundances wereobtained for O as well as Fe to a level of 0.1dex, irrespective ofthe pulsation phase, except for the special near-maximumhigh-temperature phase. This suggests that classical model atmospheresare reliably applicable to abundance determinations of RR Lyrae stars inmost cases. While the oxygen abundances derived from O I 7771-5 wellcorrelate with those from O I 6155-8, the former tends to besystematically larger by 0.1-0.2dex than the latter, which may beinterpreted as being due to the depth-dependence of the microturbulenceincreasing with height; i.e., the vt value derived fromdeep-forming Fe lines of weak/medium-strength may not be simply appliedto higher forming strong lines, such as O I 7771-5.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| Low-Ionization Emission-Line Regions around the Nucleus of the Seyfert Galaxy NGC 1068
We present the results of tridimensional spectrophotometric observationsof the central region of the Seyfert galaxy NGC 1068 obtained by usingKyoto tridimensional spectrograph I. A brief description of theinstrument is presented. We have found the existence of low-ionized gasextending out of the ionization cone. It is characterized by higher ([SII ] λ6716+λ6731) / Hα than within the cone. Theratio peaks at 5'' ‑ 6'' east and west of the nucleus and reachesto 0.5-0.6. The ionization of the gas outside the cone by the scatterednuclear continuum is examined. We argue that a substantial fraction ofthe gases outside the cone could be ionized by the scattered nuclearcontinuum, including the diffuse arc structure seen in an HST image. Inaddition, we also attempt discussions according to the kinematic andspectroscopic properties of two possible gaseous components derived bythe line decomposition: a narrow component that seems to follow thegalactic rotation, and a broad one that may be deviated from it.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities(vsini) of a sample of 45 candidate field horizontal-branch (HB) starsspanning a wide range of effective temperatures, from red HB stars withTeff~=5000K to blue HB stars with Teff of 17,000K.Among the cooler blue HB stars (Teff=7500-11500 K), weconfirm prior studies showing that, although a majority of stars rotateat vsini<15kms-1, there exists a subset of ``fastrotators'' with vsini as high as 30-35 km s-1. All but one ofthe red HB stars in our sample have vsini<10kms-1, and noanalogous rotation bimodality is evident. We also identify anarrow-lined hot star (Teff~=16,000K) with enhancedphotospheric metal abundances and helium depletion, similar to theabundance patterns found among hot BHB stars in globular clusters, andfour other stars that may also belong in this category. We discussdetails of the spectral line fitting procedure that we use to deducevsini and explore how measurements of field HB star rotation may shedlight on the issue of HB star rotation in globular clusters.
| The Century Survey Galactic Halo Project. I. Stellar Spectral Analysis
The Century Survey Galactic Halo Project is a photometric andspectroscopic survey from which we select relatively blue stars(V-R<0.30 mag) as probes of the Milky Way halo. The survey stripspans the range of Galactic latitude 35°
| 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
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Distant field blue horizontal branch stars and the mass of the Galaxy - I. Classification of halo A-type stars
This is the first in a series of three papers presenting a newcalculation of the mass of the Galaxy based on radial velocities anddistances measured for a sample of some 100 faint 16 < B < 20field blue horizontal branch (BHB) stars. This study aims to reduce theuncertainty in the measured mass of the Galaxy by increasing the numberof halo objects at Galactocentric distances r > 30 kpc with measuredradial velocities by a factor of 5. Faint A-type stars in the Galactichalo have been identified from ubjr photometry in six UKSchmidt fields. These samples include field BHB stars as well as lessluminous stars of main-sequence surface gravity, which are predominantlyfield blue stragglers. We obtain accurate charge-coupled devicephotometry and spectra to classify these stars. This paper describes ourmethods for separating out clean samples of BHB stars in a way that isefficient in terms of telescope time required. We use the highsignal-to-noise (S/N) ratio spectra of A-type stars of Kinman, Suntzeff& Kraft (published in 1994), and their definitive spectrophotometricΛ classifications, to assess the reliability of two methods, andto quantify the S/N ratio requirements. First we revisit, refine andextend the hydrogen linewidth versus colour relation as a classifier(here called the D0.15-colour method). The second method isnew and compares the shapes of the Balmer lines. With this method (herecalled the scale-width-shape method) there is no need for colours orspectrophotometry. Using the equivalent width of the CaII K line as anadditional filter we find we can reproduce Kinman, Suntzeff &Kraft's Λ classifications with both methods. In a sample of starswith strong Balmer lines, equivalent width Hγ > 13Å[equivalent to the colour range 0 <= (B-V)0<=0.2], halo BHB stars can be separated from halo blue stragglersreliably. For the spectroscopy (i.e. both classification methods) theminimum required continuum S/N ratio is 15 Å-1. For theD0.15-colour method (B-V)0 colours accurate to0.03 mag are needed.
| Tridimensional Spectroscopic Observation of the Interacting System NGC 7592
We performed a tridimensional spectroscopic study of NGC 7592, aninfrared-luminous interacting system of three galaxies, one of whichcontains a Seyfert 2 nucleus. Narrow-band images of Hα and [O III] were obtained using an imaging Fabry-Perot interferometer with atunable filter. An area-spectroscopic observation was also made by aslit scan in a direction perpendicular to the slit. The Seyfert nucleuswas found to be surrounded by a knotty star-forming region of disk- orring-shape. Along the axis of the disk or ring, a highly-ionized bipolargaseous region was identified. From kinematical analyses, it has beensuggested that this system is composed of two galaxies; one apparentnucleus is considered to be a giant H II region belonging to one galaxy.This result is supported by the morphological properties innear-infrared. Further, it has been suggested that one galaxy is anearly-type spiral, whereas the other is a late-type one, and the twogalaxies show a marked difference in the distribution of thestar-forming regions within each. The cause of the difference was arguedin relation with the dynamical perturbation between the two galaxies.
| X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her
We report on ASCA and BeppoSAX X-ray broad band observations of thegalactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and1998, respectively, and on (quasi-)simultaneous ground observations ofits optical counterpart, V934 Her, from the Loiano 1.5-meter telescope.In order to better understand the nature of the source we also analyzepublic archival ROSAT and RXTE data as well as the RXTE ASM light curveof 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The opticalspectra are typical of a M2 III star; this allows us to determine arevised distance to the object of ~ 400 pc. While these spectra do notshow either any spectral change between the two epochs or any peculiarfeature apart from those observed in normal red giants, thespectroscopic measurements carried out in X-rays reveal a complex andlong-term variable spectrum, with a clear soft excess. The X-rayspectral properties of the source are best described by a thermalComptonization spectrum plus a soft energy (<1 keV) excess, which canbe modeled in the form of a blackbody emission with kT_BB ~ 1 keV; thelatter component is not detected at the lowest source flux levels. Theratio between the two components varies substantially with the sourceflux. The X-ray emission from the object appears to become harder as itsluminosity increases: indeed, the RXTE data acquired during an outburstoccurred in October-November 1997 display a hard tail, detected up to100 keV and modeled with a comptonizing cloud which is hotter and lessopaque than that seen in the low intensity state. Apart from erraticshot-noise variability on timescales of tens to thousands of seconds, nosignificant properties (such as pulsations or QPOs) are found from thetiming analysis of the X-ray light curves extracted from theobservations presented here. With the new distance determination, the2-10 keV X-ray luminosity range spanned in the considered observationslies between ~ 2x1032 and ~ 1x1034 ergs-1. All this information, combined with the findings byother authors, allows us to suggest that the scenario which bestdescribes the object consists of a wide binary system in which a neutronstar accretes matter from the wind of a M-type giant star. Implicationsof such a model are discussed. Partly based on observations collected atthe Bologna Astronomical Observatory in Loiano, Italy.
| Kurucz Model Energy Distributions: A Comparison With Real Stars. II. Metal-Deficient Stars
Energy distributions of synthetic spectra for Kurucz model atmospheresare compared with observed energy distributions of metal-deficient starsof the blue horizontal-branch (BHB), F--G--K subdwarf (SD) and G--Kgiant (MDG) types. The best coincidence of the synthetic and observedenergy curves is found for BHB stars. The largest differences are foundin the ultraviolet wavelengths for subdwarfs and cool MDGs. Theinfluence of errors of effective temperatures, gravities andmetallicities is estimated.
| Stellar parameters for Pop II A-type stars from IUE spectra and new-ODF ATLAS9 model atmospheres
Stellar parameters for twenty-seven field horizontal branch A-typestars, a post-AGB star (BD +32 2188), and a possible cool sdB star (BD+00 0145) were obtained by fitting the whole IUE energy distributionstaken from the IUE-INES archive to the ultraviolet energy distributionspredicted by new-ODF ATLAS9 model atmospheres, which include theLyman-alpha H-H+ and H-H quasi-molecular absorptions near1400 Å and 1600 Å. The sample of stars was extensivelystudied by Kinman et al. (2000), who derived stellar parameters for themby using visual observations and also an ultraviolet color index. Theeffective temperatures obtained by fitting the IUE spectra to thenew-ODF models agree with T_eff derived by Kinman et al. (2000) for mostof the stars in the sample. The gravities from UV agree with those fromKinman et al. (2000) for stars hotter than about 8700 K, while they arelower, on average, by 0.3 dex for the cooler stars. The same discrepancyis present when log g from the ultraviolet energy distribution iscompared with log g from the visible energy distribution. The differenceis insensitive to reddening, microturbulent velocity, metallicity, ormixing-length parameter for the treatment of the convection. Figures A.1to A.15 are only available in electronic form athttp://www.edpsciences.org
| Empirical calibration of the near-infrared Ca ii triplet - I. The stellar library and index definition
A new stellar library at the near-IR spectral region developed for theempirical calibration of the Caii triplet and stellar populationsynthesis modelling is presented. The library covers the rangeλλ8348-9020 at 1.5-Å (FWHM) spectral resolution, andconsists of 706 stars spanning a wide range in atmospheric parameters.We have defined a new set of near-IR indices, CaT*, CaT and PaT, whichmostly overcome the limitations of previous definitions, the formerbeing specially suited for the measurement of the Caii triplet strengthcorrected for the contamination from Paschen lines. We also present acomparative study of the new and the previous Ca indices, as well as thecorresponding transformations between the different systems. A thoroughanalysis of the sources of index errors and the procedure to calculatethem is given. Finally, index and error measurements for the wholestellar library are provided together with the final spectra.
| A spectroscopic study of field BHB star candidates
New spectroscopic observations are presented for a sample of thirty-oneblue horizontal branch (BHB) star candidates that are sufficientlynearby to have reliable proper motions. Comments are given on a furthertwenty-five stars that have previously been suggested as BHB starcandidates but which were not included in our sample. Moderatelyhigh-resolution spectra (lambda /Delta lambda ~ 15 000) of twenty fiveof our program stars were taken with the coudé feed spectrographat Kitt Peak. Twelve of the program stars were also observed with theCAT spectrograph at ESO. Six of these program stars were observed fromboth hemispheres. IUE low-resolution spectra are available for most ofour candidates and were used, in addition to other methods, in thedetermination of their Teff and reddening. A compilation ofthe visual photometry for these stars (including new photometry obtainedat Kitt Peak) is also given. Abundances were obtained from these spectrausing models computed by Castelli with an updated version of the ATLAS9code (Kurucz 1993a). All thirty one candidates are halo stars. Of these,twenty eight are classified as BHB stars because: [(1)]they lie close tothe ZAHB (in a similar position to the BHB stars in globular clusters)in the Teff versus log g plot. For all but one of thesestars, far-UV data were available which were consistent with other data(Strömgren photometry, energy distributions, Hγ profiles) forderiving Teff and log g. [(2)]they have a distribution of kms-1i (<=40 km s-1) that is similar to thatfound for the BHB in globular clusters. Peterson et al. (1995) and Cohen& McCarthy (1997) have shown that the BHB stars in the globularclusters M13 and M92 have a higher km s-1i (<= 40 kms-1) than those in M3 and NGC 288 (<=20 kms-1). The mean deprojected rotational velocity (/line{v}) wascalculated for both the two globular clusters and the nearby BHB starsamples. A comparison of these suggests that both globular cluster kms-1i types are present in our nearby sample. No obvious trendis seen between km s-1i and either (B-V)o or [Fe/H].[(3)]they have -0.99>=[Fe/H]>=-2.95 (mean [Fe/H] -1.67; dispersion0.42 dex), which is similar to that found for field halo RR Lyrae andred HB stars. These local halo field stars appear (on average) to bemore metal-poor than the halo globular clusters. The local sample of redgiant stars given by Chiba & Yoshii (1998) contains a greaterfraction of metal-poor stars than either our halo samples or the haloglobular clusters. The stars in our sample that have a Teffthat exceeds about 8 500 K show the He i (lambda 4471) line with astrength that corresponds to the solar helium abundance. [(4)]they showa similar enhancement of the alpha -elements (< [Mg/Fe]right > =+0.43+/-0.04 and also < [Ti/Fe]right > = +0.44+/-0.02) to thatfound for other halo field stars of similar metallicity. Based onobservations obtained at KPNO, operated by the Association ofUniversities for Research in Astronomy, Inc., under contract with theNational Science Foundation, and the European Southern Observatory,Chile. Tables 4 and 5 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr
| Deep Optical Imaging of a Compact Group of Galaxies: Seyfert's Sextet
To investigate the dynamical status of Seyfert's Sextet (SS), we haveobtained a deep optical (VR+I) image of this group. Our image shows thata faint envelope, down to a surface brightnessμoptical(AB)~=27 mag arcsec-2, surrounds themember galaxies. This envelope is irregular in shape. It is likely thatthis shape is attributed either to recent-past or to ongoing galaxyinteractions in SS. If the member galaxies have experienced a number ofmutual interactions over a long timescale, the shape of the envelopeshould be rounder. Therefore, the irregularly shaped morphology suggeststhat SS is in an early phase of dynamical interaction among the membergalaxies. It is interesting to note that the soft X-ray image obtainedwith ROSAT (Pildis, Bregman, & Evrard) is significantly similar inmorphology. We discuss the possible future evolution of SS briefly.
| Kinematical trends among the field horizontal branch stars
Horizontal branch (HB) stars in the field of the Milky Way can be usedas tracers for the study of early stages of the evolution of our galaxy.Since the age of individual HB stars is not known a priori, we havestudied the kinematics of a sample of field HB stars measured withHipparcos to look for signs of age and population nature. Our samplecomprises 14 HBA, 2 HBB and 5 sdB/O stars. We found that the kinematicsof the HBA stars is very different from that of the sdB/O stars(including those from an earlier study). The HBA stars have low orbitalvelocities, some are even on retrograde orbits. Their orbits have largeeccentricities and in many cases reach large distances above thegalactic plane. In contrast, the sdB/O stars show disk-like orbitalcharacteristics. The few HBB stars (with T_eff> 10,000 K) in oursample seem to have kinematics similar to that of the sdB/O stars. Inorder to see if there is a trend among the HB stars in their kinematics,we investigated also RR Lyrae stars measured with Hipparcos. Here wefound a mixed kinematical behaviour, which was already known fromprevious studies. Some RR Lyrae stars have disk-like orbits (most ofthese being metal rich) but the majority has halo-like orbits, verysimilar to those of our HBA stars. Since the atmospheres of most typesof HB stars do not reflect original metallicities any more thekinematics is the only aspect left to study the origin and populationmembership of these stars. Thus, the clear trend found in kinematics ofstars along the HB, which is also a sequence in stellar mass, shows thatthe different kinds of field HB stars arose from stars having differentorigins in age and, e.g., metallicity or mass loss rate. Based in parton HIPPARCOS data
| The First 50 Years at Palomar, 1949-1999 Another View: Instruments, Spectroscopy and Spectrophotometry and the Infrared
We review the research on a wide variety of topics using data obtainedwith the 200-inch Hale telescope. Using state-of-the-art spectrographs,photometers, spectrometers and infrared detectors, the Palomarastronomers investigated the spectra of stars, interstellar matter, AGNsand quasars in great detail. Spectral resolutions ranged from 1000 A forbroad-band photometry to 0.04 A using interferometric techniques.
| Non-LTE analyses of strontium abundances in stars. A stars
Not Available
| Empirical calibration of the lambda 4000 Å break
Empirical fitting functions, describing the behaviour of the lambda 4000Ä break, D4000, in terms of effective temperature,metallicity and surface gravity, are presented. For this purpose, thebreak has been measured in 392 stars from the Lick/IDS Library. We havefollowed a very detailed error treatment in the reduction and fittingprocedures, allowing for a reliable estimation of the breakuncertainties. This calibration can be easily incorporated into stellarpopulation models to provide accurate predictions of the break amplitudefor, relatively old, composite systems. Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Identification of lambda Bootis stars using IUE spectra. II. High resolution data
Stars included in the catalogue of lambda Bootis stars by Paunzen et al.(\cite{Paunzen97}) with high resolution spectra (FWHM: 0.10-0.25 Ä)in the INES Archive of the International Ultraviolet Explorer satellite(IUE) are analyzed here in order to establish membership criteria forthe lambda Bootis group. Line-ratios of carbon to heavier elements (Si,Al, Ca) were adopted as criteria in the SWP range (1150-1980 Ä).For the LWP range (1850-3350 Ä), the intensity of metallic lines(Fe and Mg) was used. These criteria, together with those derived forlow resolution spectra, make the IUE Final Archive a powerful tool tofind new {lambda Bootis} candidates.
| A confirmed location in the Galactic halo for the high-velocity cloud `chain A'
The high-velocity clouds of atomic hydrogen, discovered about 35 yearsago,, have velocities inconsistent with simple Galactic rotation modelsthat generally fit the stars and gas in the Milky Way disk. Theirorigins and role in Galactic evolution remain poorly understood, largelyfor lack of information on their distances. The high-velocity cloudsmight result from gas blown from the Milky Way disk into the halo bysupernovae,, in which case they would enrich the Galaxy with heavyelements as they fall back onto the disk. Alternatively, they mayconsist of metal-poor gas-remnants of the era of galaxy formation,,accreted by the Galaxy and reducing its metal abundance. Or they mightbe truly extragalactic objects in the Local Group of galaxies. Here wereport a firm distance bracket for a large high-velocity cloud, chain A,which places it in the Milky Way halo (2.5 to 7 kiloparsecs above theGalactic plane), rather than at an extragalactic distance, andconstrains its gas mass to between 105 and2×106 solar masses.
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| Spectroscopy of Hot Stars in the Galactic Halo. III. Analysis of a Large Sample of Field Horizontal-Branch and Other A-Type Stars
We present results from an analysis of medium-resolution spectroscopyand UBV photometry for a sample of 1121 A-type stars in the halo (anddisk) of the Galaxy. A previously developed calibration technique isused to assign estimates of effective temperature, surface gravity, andstellar metal abundance, as parameterized by [Fe/H]. Radial velocitiesare reported with an accuracy of ~10 km s^-1. Distance estimates areobtained for the stars with well-determined luminosity classes. Notethat although we refer to ``A-type'' stars, which dominate the presentsample, the present data set includes roughly 100 stars of laterspectral types, as a result of the temperature range we have chosen toexplore in this paper (6000 K<=T_eff<=10,000 K). Included in thehot star sample are 444 stars we classify as field horizontal-branchstars, 416 we classify as main-sequence-gravity A-type (or slightlylater) stars (including stars that are likely members of the bluemetal-poor population, the so-called BMPs), 140 stars we classify aslikely metallic-line (Am) or peculiar (Ap) stars, and 121 stars thatcannot be unambiguously classified based on the present data.Examination of the distributions in metallicity and velocity indicatesthat the field horizontal-branch and main-sequence A-type samples arequite distinct; hence we expect only a modest amount ofcross-contamination between the subsamples. We identify 58 RR Lyraecandidates among the hot star sample, based on incompatibilities intheir photometric and spectroscopic data. There are 19 stars in thesample that have been previously classified as RR Lyrae variables, andone additional star that had been previously suggested as a variable,though not necessarily of the RR Lyrae class. There are 115 stars in thesample that were previously classified as BMPs by Preston, Beers, &Shectman, most of which fall into the main-sequence A-type category, but10 of which are found among the Am/Ap classifications. Furthermore, 53of stars previously considered to be BMPs are shown by our analysis tohave metal abundances [Fe/H]>-1, which calls their identificationwith this population somewhat into question.
| Spectroscopy of Hot Stars in the Galactic Halo. II. The Identification and Classification of Horizontal-Branch and Other A-Type Stars
We discuss a spectroscopic and photometric technique that enables theidentification and classification of field horizontal-branch (FHB) andother A-type stars, even from relatively low signal-to-noise ratiomedium-resolution spectra. This technique makes use of broadband UBVcolors predicted from model atmosphere calculations and Balmer lineprofiles and Ca II K equivalent widths determined from synthetic spectrato estimate the physical parameters T_eff, log g, and [Fe/H] for starsin the effective temperature range 6000-10,000 K. A comparison of ourmethod with high signal-to-noise ratio spectra of standard starsindicates a scatter in the derived parameters of sigma(T_eff)=+/-250 K,sigma(log g)=+/-0.14 dex, and sigma([Fe/H])=+/-0.12 dex. This precisionallows for a separation of low surface gravity FHB and other, generallyhigher surface gravity, A-type (and somewhat later) stars. We alsodevelop a synthetic-template comparison technique, which is veryeffective in the identification of metallic-line and peculiar A-typestars. A detailed investigation of the influence of noise in the spectraon the determination of physical parameters shows that, for spectra withsignal-to-noise ratios in the range 10
| The Size and Age of Sakurai's Planetary Nebula and the Temperature of Its Central Star
We present high-resolution, spatially resolved spectra of the nebulasurrounding Sakurai's object. We find the expansion velocity of thenebula to be 30.9 +/- 0.7 km s^-1, and the nebula extent to be 44" indiameter. We discuss the wide range of reported distances to Sakurai'sobject and the dilemma that these present, since some evidence stronglyfavors a short distance (1.1 kpc) while other data strongly favor alonger distance (5-8 kpc). We also present spectra of Sakurai's starcentered near the lambda4216 CN band that suggests the star had cooledsignificantly between 1996 October and 1997 May, but had not cooledfurther by 1997 July.
| The absolute magnitude of field metal-poor horizontal branch stars
Hipparcos satellite parallaxes for 22 metal-poor field horizontal branchstars with V_0<9 are used to derive their absolute magnitude. Theweighted mean value is M_V=+0.69+/-0.10 for an average metallicity of[Fe/H]=-1.41 a somewhat brighter average magnitude of M_V=+0.60+/-0.12for an average metallicity of [Fe/H]=-1.51 is obtained by eliminating HD17072, which might be on the first ascent of the giant branch ratherthan on the horizontal branch. The present values agree with thedeterminations based on proper motions and application of theBaade-Wesselink method to field RR Lyraes; they are 0.1-0.2 mag fainterthan those based on calibration of cluster distances obtained by usinglocal subdwarfs and on alternative distance calibrators for the LargeMagellanic Cloud (LMC). The possibility that there is a real differencebetween the luminosity of the horizontal branch for clusters and thefield is briefly commented on.
| A search for magnetic stars in late stages of stellar evolution
No abstract submitted
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Геркулес |
| Прямое восхождение: | 17h46m40.63s |
| Склонение: | +25°44'57.0" |
| Видимая звёздная величина: | 6.981 |
| Расстояние: | 172.117 парсек |
| Собственное движение RA: | -35.8 |
| Собственное движение Dec: | -42.9 |
| B-T magnitude: | 7.162 |
| V-T magnitude: | 6.996 |
Каталоги и обозначения:
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