תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Ejection velocities of high Galactic latitude runaway stars
We estimate the distribution of ejection velocities for the knownpopulation of high Galactic latitude runaway stars. The initial sampleis a collection of 174 early-type stars selected from the literature.The stars are first classified according to their evolutionary status inorder to obtain a homogeneous sample of 96 genuine main-sequence stars.Their present velocities and flight times are then estimated usingproper motion data from various astrometric catalogues (includingTycho-2, UCAC2 and USNO-B) and the ejection velocities are computed bytracing their orbits back in time, based on a Galactic potential. Thepotential used is constructed from a mass density model chosen to fitthe most recent observational constraints.We find evidence for two different populations of runaway stars: a'high' velocity population, with a maximum ejection velocity of about400-500 km s-1, and a 'low' velocity population, with amaximum ejection velocity of about 300 km s-1. We argue thatthe observed limit of 500 km s-1 and the bimodality of theobserved ejection velocity distribution are natural consequences of theso-called Binary Ejection Mechanism. We discuss the connection betweenthe 'high' velocity population and the so-called hypervelocity stars,showing how previously studied hypervelocity stars are consistent withthe results obtained.We also find that some stars that were once thought to be best explainedas being formed in the halo are compatible with a runaway hypothesisonce proper motions are included in the analysis. However, three starsin the selected sample appear to be inconsistent with ejection from theGalactic disc. Possible scenarios are discussed, including a possibleformation in the Galactic halo.
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| UBVRI Photometric Standard Stars Around the Celestial Equator: Updates and Additions
New broadband UBVRI photoelectric observations on theJohnson-Kron-Cousins photometric system have been made of 202 starsaround the sky, and centered at the celestial equator. These starsconstitute both an update of and additions to a previously publishedlist of equatorial photometric standard stars. The list is capable ofproviding, for both celestial hemispheres, an internally consistenthomogeneous broadband standard photometric system around the sky. Whenthese new measurements are included with those previously published byLandolt (1992), the entire list of standard stars in this paperencompasses the magnitude range 8.90 < V < 16.30, and the colorindex range -0.35 < (B - V) < +2.30.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Young Stars far from the Galactic Plane: Runaways from Clusters
Quite recently, a significant number of OB stars far from the galacticplane have been found, situated at z-distances ranging from severalhundreds of pc to several kpc. The short lifetimes of these stars poseproblems for their interpretation in terms of the standard picture ofstar formation. Different mechanisms have been put forward to explainthe existence of these stars, either within the conventional view, orpostulating star formation in the galactic halo itself. These mechanismsrange from arguing that they are misidentified evolved or abnormalstars, to postulating powerful ejection mechanisms for young disk stars;in situ formation also admits several variants. We have collected fromthe literature a list of young stars far from the plane, for which theevidence of youth seems convincing. We discuss two possible formationmechanisms for these stars: ejection from the plane as the result ofdynamical evolution of small clusters (Poveda et al. 1967) and in situformation, via induced shocks created by spiral density waves (Martos etal. 1999). We compute galactic orbits for these stars, and identify thestars that could be explained by one or the other mechanism. We findthat about 90 percent of the stars can be accounted for by the clusterejection mechanism, that is, they can be regarded as runaway stars inthe galactic halo.
| Faint Blue Objects at High Galactic Latitude. VIII. Performance Characteristics of the US Survey
The US survey has cataloged 3987 objects in seven high Galactic latitudefields according to their optical colors, magnitudes, and morphologiesusing photographic techniques. This paper analyzes the effectiveness ofthe survey at producing finding lists for complete samples of hot starsand quasars that exhibit blue and/or ultraviolet excess (B-UVX) relativeto the colors of halo F and G subdwarf stars. A table of 599spectroscopic identifications summarizes the spectroscopic coverage ofthe US objects that has been accomplished to date. In addition, some ofthe survey plates have been reexamined for objects missed during theoriginal selection, and the literature has been searched for all otherspectroscopically identified blue stars and quasars with z<2.2 thathave been selected by other surveys within the US survey areas. Theseresults are used to estimate empirically both the accuracy of the USsurvey selection boundaries (in color, morphology, and brightness) andthe completeness of the resulting samples of B-UVX US objects withinthose boundaries. In particular, it is shown that the reliability of theUS color classifications is high and that the previously derived USmorphological boundary for the complete selection of unresolved quasarsis accurate. The contribution of color and morphological classificationerrors to B-UVX sample incompleteness is therefore correspondinglysmall. The empirical tests indicate high levels of completeness(95+1-2%) for the samples of US quasars and hotstars isolated within the stated survey selection limits. Errata andimprovements to some of the published catalog data are presented inAppendices.
| 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.
| Relative Fluxes by Spectrophotometry with RUBIKON
By means of the RUBIKON multi-channel detector system with integratedgrism spectrograph, attached to the Bochum 61cm telescope, five primarystandars stars have been intercompared and an additional secondarystandard star (HD 37027 in the center field of the LMC) has been set upin the wavelength range of 326-777 nm.
| A new, complete sample of faint B stars in the Galactic halo
A new sample of 31 faint B and A0 stars is reported, 30 of whichcomprise a complete sample within the limits (U-V) less than 0 and Bbetween 10.0 and 18.0. The sample is based on low- andintermediate-resolution spectrophotometry of color-excess objectsselected in the U.S. survey. Atmospheric parameters for the stars arederived through the use of synthetic colors, Balmer-line strengths, andmodel-atmosphere fitting. The atmospheric parameters and preliminarymetallicity estimates indicate that most of the stars are distributedalong the blue horizontal branch, with low metallicities (Fe/H = 1.0)and with both the first and second Newell gaps present. However, nine ofthe B/A0 stars can be identified as candidate mainsequence stars, basedon evidence of high metallicities (Fe/H = 0) and/or derived effectivetemperatures and surface gravities which place them close to themain-sequence relation. The completeness characteristics of the sampleare discussed, and its surface density is compared to that of otherrecently isolated B-star samples. The sample exhibits a shallow integralnumber-count slope. This new sample will help provide increasedstatistical coverage of the B-star population in the Galactic halothrough its relatively faint magnitude-completeness limits and itsrelatively red color-completeness limit.
| A Large-Scale Spectroscopic Survey of Early-Type Stars at High Galactic Latitudes
We present new model atmosphere analyses of optical spectroscopy of alarge sample of B-type stellar candidates. Of a total of 298 objects,the largest sample of its kind to date, 205 were drawn from the PalomarGreen Survey of high Galactic latitude ultraviolet-excess stellarobjects and comprise a complete magnitude-limited sample. Effectivetemperatures, surface gravities, and helium abundances for the hotsubdwarf (high-gravity) component of the sample are derived from adetailed line profile analysis of the hydrogen and helium absorptionlines in intermediate-resolution (3-5 A FWHM) optical spectra. Aseparate analysis of the lower gravity component is made using a newlycalculated grid of synthetic spectra. Additional estimates of theeffective temperatures are made from wide- and intermediate-bandphotometry taken from the literature. We are currently undertaking twofollow-up programs. (1) Detailed abundance analyses of high-resolutionechelle spectra of the lower gravity component of the survey usingmodern model atmosphere and synthetic spectrum techniques willdifferentiate between massive Population I main-sequence B stars andlow-mass, lower luminosity Population II blue horizontal branch starsand post-asymptotic giant branch stars. (2) The derived atmosphericparameters for the higher gravity component, the field extendedhorizontal branch stars, will be combined with radial velocitymeasurements to determine their spatial and kinematic distributions,which will distinguish between competing evolutionary scenarios for thishot, evolved stellar population.
| Quantitative spectral classification of galactic disc K-M stars from spectrophotometric measurements
New spectral observations for 47 southern galactic red supergiantsobtained with the new RUBIKON spectrophotometer (developed at theAstronomisches Institut der Ruhr-Universitat Bochum) at the Bochum 61-cmtelescope on La Silla are presented. The spectra range from 4800 to 7700A and their resolution is 10 A. The mean error of absolute fluxes is0.028 mag and that of relative fluxes 0.021 mag. The spectra will beavailable at the Strasbourg Stellar Database (CDS). Together with datataken from recently published spectral catalogues, the new observationshave been used to define spectral indices as measures of the strengthsof the following features: Fe i+TiOalpha_1, Mgb+TiOalpha_0,NaD+TiOgamma'_1, TiOgamma'_0 and TiOgamma_1 systems. The indices havebeen checked against errors introduced by reductions, interstellarreddening and different resolutions of different spectral catalogues,and have been found to be very insensitive to all these effects.Therefore, different catalogues may be combined without any loss ofaccuracy and homogeneity. The mean error of a single index has beenfound to be 0.011 mag. For stars from K4 to M7, a strong temperaturedependence is found for all indices. For the Fe i+TiO and especially theMgb+TiO features, a strong dependence on luminosity has also beenobserved. These indices therefore have been combined to form aluminosity index, while the others together form a spectral index. Thecombined indices have been calibrated in terms of MK data using thestepwise linear regression technique, and may be used for quantitativetwo- dimensional spectral classification of late K- and M-type stars.The mean error of the classification is 0.6 of spectral subtype and 0.8of luminosity class, which is much higher than would be expected fromthe uncertainty of the indices alone (which, e.g., for an M4 giantcorrespond to an uncertainty of 0.1 of spectral subtype and 0.3 ofluminosity class). This may be explained by the uncertainty of theoriginal MK classifications and the variability of some programme stars.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Spectral emission of three AGN candidates
Not Available
| UBV(RI)c photometry of equatorial standard stars - A direct comparison between the northern and southern systems
UBV(RI)c photometry of 212 stars from Landolt's list of equatorialstandards is presented. The observations are tied to the system definedby Cousin's E-region standards. A comparison of the present results withLandolt's reveals reasonably good agreement for (V-R)c and (V-I)c, butmarked systematic differences for (B-V) and (U-B). The UBV systems ofCousins and Landolt are evidently not the same and both probably differfrom Johnson's original system.
| The runaway nature of distant early-type stars in the galactic halo
The kinematics of a sample of 32 distant halo B-stars with masses in therange 3-21 solar masses are investigated using the current 'runawaystar' hypotheses to establish if they belong to this group of stars.Whether postmass transfer secondaries in evolved massive close binariescan be normal low mass B-stars is discussed, but support for this ideais not found. Thus it appears that the lower mass stars could not havebeen ejected out of the disk as a result of supernova explosions inmassive close binaries, while the higher mass objects do not in generalreveal a binary nature as expected from this theory. The results ofrecent N-body simulations of the purely dynamical ejection of runawaystars from young galactic star clusters are in excellent agreement withthe observations. Thus it appears that these halo stars are the mostdistant subgroup of OB runaways produced by cluster ejection yetobserved.
| The stellar content of two OB associations in the LMC - LH 117 (NGC 2122) and LH 118
Photometry for stars in two adjacent OB associations in the LMC, numbers117 and 118 in the list of Lucke and Hodge (1970), is presented. Eachassociation contains about 50 stars with mass larger than 10 solar,including two newly confirmed red supergiants. Most of the stars inthese associations are in the first half of their lives on the mainsequence. However, the two red supergiants and a B2 I stars are of lowermass than several of the unevolved, more massive members of theassociations, and must have formed six to 10 million yr earlier thanmost of the association members. The initial mass function of theassociations has a slope of - 1.8, similar to the one observed formassive stars near the sun.
| Spectrophotometry of stars of intermediate brightness
Spectrophotometric observations of 15 stars in a magnitude range m ofbetween 8.0 and 10.9, located mostly near the equator and covering allthe range of right ascensions, are presented. The energy distributionshave been obtained in Hayes' (1970) system as revised by Hayes andLatham (1975), and in the augmented system presented by Taylor (1984).Comments about some of the standard stars are presented as well as acomparison of the results obtained with both systems.
| B, V, R, I light curves of the very short-period binary AD CANCRI
The very short-period eclipsing binary system AD Cnc was observed onthree nights with the facilities at Kitt Peak National Observatory.Standard magnitudes have been derived for AD Cnc and for the check andcomparison stars. The observations covering the eclipse portions of thelight curve yielded five epochs of minimum light. An improved ephemerisis presented. The light curves, defined by 735 observations with boththe B and R filter, 733 with the V filter, and 730 with the I filter,are fairly symmetric. The magnitude differences in the maxima areslightly more than the probable errors of the observations. Thedifference in the depths of the eclipse curves is small with the depthof the primary exlipse curve averaaging 0.4 m.
| UBV Photoelectric Photometry Catalogue (1986). III Errors and Problems on DM and HD Stars
Not Available
| A search for far-infrared (IRAS) emission from early-type stars at high Galactic latitudes
Recently Lamers et al. (1986) have found that HR 4049 (B supergiant at z= 1300 pc) is surrounded by a dust cloud which produces a large IRexcess, from which it is inferred that the star may be a much lessluminous low mass object close to the Galactic plane. Here, the resultsof a search for strong FIR (IRAS) emission from a sample of early-typehigh-Galactic-latitude stars, estimated to be at z-distances of up to9000 pc from optical spectral analyses, are reported. No IR emission wasdetected for any of the stars investigated, which would not be expectedif they were nearby objects with dust clouds. It is therefore concludedthat they are normal stars at the large distances previously estimated.
| Interstellar absorption lines in the directions of extragalactic objects. II - Analysis of 25 sight lines
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986MNRAS.220..927M&db_key=AST
| Observations of early-type stars at high galactic latitudes
Using equivalent widths and line profiles measured from high resolutionAAT spectra in conjunction with model atmosphere calculations, it isshown that a sample of eleven high galactic latitude early-type starsare normal, distant objects. Stellar velocities in the z-directiondetermined from the spectra, coupled with the galactic gravitationalfield g(z), allow the times the stars would have taken to reach theirpresent positions if they were ejected from the disc to be derived. Acomparison of these times with the evolutionary ages indicate that atleast four of the stars have been formed in situ, possibly from galacticfountain material.
| The galactic reddening law - The evidence from uvby-beta photometry of B stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&A...142..189T&db_key=AST
| UBVRI photometric standard stars around the celestial equator
It is pointed out that accurate, internally consistent, and readilyaccessible standard star photometric sequences are necessary for thecalibration of the intensity and color data which astronomers obtain atthe telescope. The photometric results provided in connection with thepresent study represent the first part of an effort which is concernedwith the presentation of UBVRI photoelectric photometric standard starsin the magnitude range from 7 to 17 over as broad a range in color aspossible. All of the photometric observations were made with a 31034type photomultiplier used in a pulse counting mode. Some 15 to 25standard stars chosen from Cousins' lists (1973, 1976) in the E-regionswere observed with an 0.4-m telescope each night along with the programstars. UBVRI standard stars were observed periodically throughout thenight. Observations with a 0.9-m telescope were also conducted. TheUBVRI photoelectric observations take into account 223 stars.
| Atmospheric parameters and chemical compositions of eighteen halo OB stars
Stellar equivalent widths and He line profiles, measured from 5 and 10A/mm IPCS spectra obtained at the AAT, are presented for 18 halo OBstars. Effective temperatures and gravities have been estimated usingStromgren and H-beta photometry in conjunction with these data. Alsoderived are the abundances relative to hydrogen of helium, carbon,nitrogen, oxygen and calcium. From the normal chemical compositions andatmospheric parameters obtained, it is concluded that the stars are notsubluminous and show no evidence of any other peculiarities. Usingconservative and nonconservative evolutionary tracks, masses, ages anddistances are deduced for the stars, implying that several of them havebeen ejected from the galactic plane with velocities in excess of 100km/s.
| A search for faint blue stars in high galactic latitudes. II - Fourteen PSS fields at declinations + 6 deg and 0 deg near the South Galactic Pole
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&AS...39...39B&db_key=AST
| Photometry of faint blue stars - III. Hbeta photometry of some southern stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1977MNRAS.178..369K
| Photometry of faint blue stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1975MNRAS.173..625K
| The Evolutionary Status of the Blue Halo Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973ApJS...26...37N&db_key=AST
| Photoelectric UBV photometry of late-type stars in two regions at high galactic latitude
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973AJ.....78...44D&db_key=AST
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
INHABITED SKY
