תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| CaII K interstellar observations towards early-type disc and halo stars, abundances and distances of intermediate- and high-velocity clouds
We present CaII K (λair = 3933.661Å)interstellar observations towards 20 early-type stars, to place lowerdistance limits to intermediate- and high-velocity clouds (IHVCs) intheir lines of sight. The spectra are also employed to estimate the Caabundance in the low-velocity gas towards these objects, when combinedwith Leiden-Dwingeloo 21-cm HI survey data of spatial resolution . Nineof the stars, which lie towards IHVC complexes H, K and gp, wereobserved with the intermediate dispersion spectrograph on the IsaacNewton Telescope at a resolution R = λ/Δλ of 9000(~33kms-1) and signal-to-noise ratio (S/N) per pixel of75-140. A further nine objects were observed with the Utrecht EchelleSpectrograph on the William Herschel Telescope at R = 40000(~7.5kms-1) and S/N per pixel of 10-25. Finally, two objectswere observed in both CaII K and NaI D lines using the 2D COUDÉspectrograph on the McDonald 2.7-m telescope at R = 35000 (~8.5kms-1). The abundance of CaII K {log10(A) =log10[N(CaII K)]-log10[N(HI)]} plotted against HIcolumn density for the objects in the current sample with heights abovethe Galactic plane (z) exceeding 1000pc is found to obey the Wakker& Mathis (2000) relation. Also, the reduced column density of CaII Kas function of z is consistent with the larger sample taken from Smokeret al. (2003). Higher S/N observations than those previously takentowards HVC complex H stars HD13256 and HILT 190 reinforce the assertionthat this lies at a distance exceeding 4000pc. No obvious absorption isdetected in observations of ALS10407 and HD357657 towards IVC complexgp. The latter star has a spectroscopically estimated distance of~2040pc, although this was derived assuming the star lies on the mainsequence and without any reddening correction being applied. Finally, noCaII K absorption is detected towards two stars along the line of sightto complex K, namely PG1610+529 and PG1710+490. The latter is at adistance of ~700pc, hence placing a lower distance limit to thiscomplex, where previously only an upper distance limit of 6800pc wasavailable.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. II. Kinematics and Full Sample Analysis
This paper continues the analysis of faint high-latitude B stars fromMartin. Here we analyze the kinematics of the stars and combine themwith the abundance information from the first paper to classify eachone. The sample contains 31 Population I runaways, 15 old evolved stars(including 5 blue horizontal-branch [BHB] stars, 3 post-HB stars, 1pulsating helium dwarf, and 6 stars of ambiguous classification), 1 Fdwarf, and 2 stars that do not easily fit in one of the othercategories. No star in the sample unambiguously shows thecharacteristics of a young massive star formed in situ in the halo. Thetwo unclassified stars are probably extreme Population I runaways. Thelow binary frequency and rotational velocity distribution of thePopulation I runaways imply that most were ejected from dense starclusters by the dynamic ejection scenario. However, we remain puzzled bythe lack of runaway Be stars. We also confirm that PB 166 and HIP 41979are both nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory, operated by the University of Texas at Austin.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. I. Composition and Spectral Features
The existence of faint blue stars far above the Galactic plane that havespectra that are similar to nearby Population I B stars presents severalinteresting questions. Among them are the following: Can a Population IB star travel from the disk to a position many kiloparsecs above theplane in a relatively short main-sequence lifetime? Is it possible thatsingle massive star formation is occurring far from the Galactic plane?Are these objects something else masquerading as main-sequence B stars?This paper (the first of two) analyzes the abundances of a sample ofthese stars and reveals several that are chemically similar to nearbyPopulation I B stars, whereas others clearly have abundance patternsmore like those expected in blue horizontal-branch (BHB) orpost-asymptotic giant branch stars. Several of those with old evolvedstar abundances also have interesting features of note in their spectra.We also consider why this sample does not have any classical Be starsand identify at least two nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory operated by the University of Texas at Austin.
| Early-type stars in the Galactic halo from the Palomar-Green survey-III. Completion of a magnitude range limited sample
High-resolution (R~ 40000) echelle spectroscopic observations of 13high-latitude early-type stars are presented. These stars comprise thefinal part of a complete magnitude range limited sample based onlow-resolution spectroscopy of targets drawn from the Palomar-Greensurvey. The magnitude range under consideration is 13<=BPG<= 14.6, corresponding to an approximate distancelimit for main-sequence B-type objects of 5 <=d<= 40 kpc. Threestars are found to be apparently normal, young stars, based on theirpositions on the (Teff, log g) diagram, normal abundancepatterns and relatively large projected rotational velocities. A furtherstar, PG 1209+263, was found to belong to the chemically peculiar (CP)silicon star class of objects. The remainder are evolved subluminousstars lying on post-horizontal branch (post-HB) tracks, with theexception of PG 2120+062, which appears to be in a post-asymptotic giantbranch evolutionary stage. For the young stars in the sample, we havederived distance and age estimates through comparison of the atmosphericparameters with recent theoretical evolutionary models. We discussformation scenarios by comparing times-of-flight and evolutionarytime-scales. It is found that all stars could have formed in theGalactic disc and been ejected from there soon after their birth, withthe exception of PG 1209+263. The adopted proper motion is found to be acrucial factor in the kinematical analysis. We also present some numberdensities for young B-type halo stars, which indicate that they areextremely scarce objects.
| 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.
| Strömgren uvby photometry of B-type stars from the Palomar-Green Survey
We present Strömgren uvby photometry for a sample of 31 highGalactic latitude stars selected from the Palomar-Green Survey. The datainclude photometric magnitudes accurate to <= 0.01 mag in most cases,plus colours and the reddening free [c_1] and [u-b] indices, whichpossess a precision of better than 0.02 and 0.04 mag, respectively. Thelatter should be suitable for the reliable determination of stellarphotometric temperatures.
| 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.
| The Mount Wilson Halo Mapping Project 1975-1985 I: The UBV(RI)_M_W Photometric System Compared with Other Standard Systems: The Adopted Trigonometric HR Diagram in (R-I)_M_W and (V-I)_M_W
Photometry of stars is described that defines a UBV(RI)_M_W photometricsystem, established between 1975 and 1980 using the Mount Wilson 60-inchand 100-inch reflectors. The MW natural system has been reduced to theUBV(RI)_C Cape-Cousins system by linear equations for stars hotter thanM0. Strong non-linear color and magnitude equations exit in R and Ibetween the two systems for stars cooler than M0. The non-linearity isdue to the change of the effective wavelengths of the red passbandscaused by effect of the TiO b ands combines with the substantialdifference between red response of the extended S20 cathodes used hereand the GaAs cells used by Cousins (1976, 1980a,b), Bessell (1979,1990), Landolt (1983, 1992), and others for the Cape system. We report(R-I)_M_W and (V-I)_M_W colors based on a linear extension of the _M_Wnatural system to the red for cool stars with (V-R)_Cape greater than0.8, (V-I)_Cape greater than 1.8 (spectral types later than M0), aszero-pointed to the Cape system for bluer stars. Color equations arederived between the MW, the Cape, the Kron-Eggen, and Johnsonphotometric systems, and also the Hubble Space Telescope (V-I) colors asreduced to the (V-I)_C Cape system by Holtzman et al. (1995). TheKron-Eggen and the Cape-Cousins R and I systems differ in their zeropoints of color and magnitudes, showing that the Kron-Eggen and theCousins systems are not the same. The appellation of a "Kron-Cousins"photometric system used in much of the current literature is incorrect.Preparatory to determining photometric parallaxes of local higher propermotion M dwarfs in Paper IV, trigonometric HR diagrams are determined inV, (V-R)_M_W and (V-I)_M_W on the Mount Wilson system using 160available parallax stars with parallaxes larger than 0.099 arcsec. Afirst appendix illustrates the difference in the effective wavelengthsof the Cape-Cousins, Kron-Eggen, and Johnson systems for cool stars,based on the observed slope coefficients of the color equations thatlink them. A second appendix contains explicit color equations based onstars in common between Bessell's (1990) definition of the Cape systemfor the reddest stars and the Kron-Eggen and the MW photometric systems.(SECTION: Stars)
| A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..103..433B&db_key=AST
| Vacuum ultraviolet imagery of the Virgo cluster region
The results are reported of an experiment using the UV imager aboard anattitude-controlled S520 type sounding rocket. The total UV fluxes ofgalaxies in the Virgo Cluster as well as the flux level of the diffuseUV background around the cluster were measured. The data on NGC 4486 andNGC 4472 confirm the variation in the degree of the 'turnup' below 200nm in the energy spectrum of the total light of elliptical galaxies. Attwo-color diagram of galaxies of visual/near-UV/vacuum UV indicates thatcolors of spiral galaxies are distributed within a strip andwell-correlated with the morphological type, while elliptical galaxiesare located differently from spiral galaxies.
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| The Palomar-Green catalog of ultraviolet-excess stellar objects
The Palomar-Green Catalog of Ultraviolet Excess Stellar Objects ispresented, with data consisting of positions on the sky accurate toabout 8 arcsec in each coordinate, photographic B-magnitudes accurate to0.29 mag, spectral types, some cross references, and photoelectricbroadband, multichannel, and Stromgren colors when available. Extensivediscussion is given on magnitude calibration using a combination ofphotoelectric measurements and star count modeling, and on thestatistical completeness of the sample based on internal and externaltests. Of the 1874 objects in the catalog, 1715 comprise a statisticallycomplete sample covering 10,714 square degrees from 266 fields taken onthe Palomar 18-inch (46-cm) Schmidt telescope. Limiting magnitudes varyfrom field to field, distributed around B = 16.1 mag, ranging from 15.49to 16.67. The dominant population is that of the hot, hydrogenatmosphere subdwarfs, the sdB stars, which comprise nearly 40 percent ofthe sample. At 16th magnitude, the hot subdwarf (sdB, sdO) number countsare increasing by about a factor of 2 per mag, the hot white dwarfs by 4per mag, and quasars by 8 per mag. The result is a very different mix ofspectral types expected at 18th mag than is found at 16th mag. Thesuggestion is made that the sdB stars are part of the old disk ratherthan a halo population.
| A catalog of ultraviolet interstellar extinction excesses for 1415 stars
Ultraviolet interstellar extinction excesses are presented for 1415stars with spectral types B7 and earlier. The excesses with respect to Vare derived from Astronomical Netherlands Satellite (ANS) 5-channel UVphotometry at central wavelengths of approximately 1550, 1800, 2500, and3300 A. A measure of the excess extinction in the 2200-A extinction bumpis also given. The data are valuable for investigating the systematicsof peculiar interstellar extinction and for studying the character of UVinterstellar extinction in the general direction of stars for which theextinction-curve shape is unknown.
| Ultraviolet photometry with the Astronomical Netherlands Satellite /ANS/ - Faint blue stars in the halo
Blue stars at high galactic latitudes have been observed with the UVtelescope on board ANS. In this paper a subset of the collected datapertaining to the cooler stars is discussed. Most of them have energydistributions in general agreement with the visual spectral type. Onestar is exceptionally blue, and of seven possible horizontal-branchstars, two have UV energy distributions distinct from main-sequencestars in the sense that they have an excess at 1550 A and a large Balmerjump.
| A two-color photometric system for the near infrared
A new two-color system for photoelectic photometry has been devised toaid interpretation of near-infrared photographs taken on the IV-Nemulsion. The system should also lend itself to applications usingsilicon diode detectors. This paper describes the instrumentation,establishment of standard stars, and transformation to other systems. Inaddition, photoelectric sequences are given for some fields near thegalactic plane.
| The Nature of Faint Blue Stars in the Halo. II
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974ApJS...28..157G&db_key=AST
| On the detectability of Bp-type abundance anomalies in early main-sequence stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&A....34....1B
| 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
| The redshift-distance relation. V. Galaxy colors as functions of galactic latitude and redshift : observed colors compared with predicted distributions for various world models.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973ApJ...183..711S&db_key=AST
| A Photometric Classification of the Feige Blue Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1970PASP...82.1305G
| Hγ Observations for Fourteen of Feige's Faint Blue Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1968PASP...80..238W
| The Nature of the Faint Blue Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1966ApJ...144..496G
| The Colors of Some High-Latitude Blue Stars.
Not Available
| An Atlas of Indentification Charts for 113 Blue Stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1959ApJ...129..600F
| A Search for Underluminous Hot Stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1958ApJ...128..267F
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | בתולה |
| התרוממות ימנית: | 13h35m11.98s |
| סירוב: | +13°27'44.0" |
| גודל גלוי: | 11.77 |
| תנועה נכונה: | -6.9 |
| תנועה נכונה: | -8.2 |
| B-T magnitude: | 11.724 |
| V-T magnitude: | 11.767 |
קטלוגים וכינוים:
|
