Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
CaII K interstellar observations towards early-type disc and halo stars - distances to intermediate- and high-velocity clouds
We compare existing high spectral resolution(R=λ/Δλ~ 40000) CaII K observations(λair= 3933.66 Å) towards 88 mainly B-typestars, and new observations taken using the Intermediate dispersionSpectrograph and Imaging System (ISIS) on the William Herschel Telescopeat R~ 10000 towards three stars taken from the Palomar-Green Survey,with 21-cm HI emission-line profiles, in order to search for opticalabsorption towards known intermediate- and high-velocity cloudcomplexes. Given certain assumptions, limits to the gas phase abundanceof CaII are estimated for the cloud components. We use the data toderive the following distances from the Galactic plane (z). (i)Tentative lower z-height limits of 2800 and 4100 pc towards complex Cusing lack of absorption in the spectra of HD341617 and PG0855+294,respectively. (ii) A weak lower z-height of 1400 pc towards complexWA-WB using lack of absorption in EC09470-1433 and a weak lower limit of2470 pc using lack of absorption in EC09452-1403. (iii) An upperz-height of 2470 pc towards a southern intermediate-velocity cloud (IVC)with vLSR=-55 km s-1 using PG2351+198. (iv)Detection of a possible IVC in CaII absorption at vLSR=+52 kms-1 using EC20104-2944. No associated HI in emission isdetected. At this position, normal Galactic rotation predicts velocitiesof up to ~+25 km s-1. The detection puts an upper z-height of1860 pc to the cloud. (v) Tentative HI and CaII K detections towards anIVC at ~+70 km s-1 in the direction of high-velocity cloud(HVC) complex WE, sightline EC06387-8045, indicating that the IVC may beat a z-height lower than 1770 pc. (vi) Detection of CaII K absorption inthe spectrum of PG0855+294 in the direction of IV20, indicating thatthis IVC has a z-height smaller than 4100 pc. (vii) A weak lowerz-height of 4300 pc towards a small HVC with vLSR=+115 kms-1 at l, b= 200°, + 52°, using lack of absorption inthe CaII K spectrum of PG0955+291.
| CaII K interstellar observations towards early-type disc and halo stars
We present high-resolution (R=λ/Δλ~ 40000) CaII Kinterstellar observations (λair= 3933.66Å)towards 88 mainly B-type stars, of which 74 are taken from theEdinburgh-Cape or Palomar-Green surveys, and 81 have |b| > 25°.The majority of the data come from previously existing spectroscopy,although also included are 18 new observations of stars with echellespectra taken with UVES on the Very Large Telescope UT2 (Kueyen). Some49 of the sample stars have distance estimates above the Galactic plane(|z|) >= 1 kpc, and are thus good probes of the halo interstellarmedium. Of the 362 interstellar Ca K components that we detect, 75 (21per cent) have absolute values of their LSR velocity values exceeding 40km s-1. In terms of the deviation velocity for the sightlineswith distance estimates, 46/273 (17 per cent) of components havevelocity values exceeding those predicted by standard Galactic rotationby more than 40 km s-1. Combining this data set with previousobservations, we find that the median value of the reduced equivalentwidth (REW) of stars with |z| >= 1 kpc (EW×sin|b|) is ~115mÅ (n= 80), similar to that observed in extragalactic sightlinesby Bowen. Using data of all z distances, the REW at infinity is found tobe ~130 mÅ, with the scaleheight (l) of the CaII K column densitydistribution being ~800 pc (n= 196) and reduced column density atinfinity of log[N(CaII K) cm-2]~12.24. This implies that ~30per cent of CaII K absorption occurs at distances exceeding ~1 kpc. Fornine sightlines with distance exceeding 1 kpc and with a companionobject within 5°, we find that all but two have values of CaIIreduced equivalent width the same to within ~20 per cent, when the REWof the nearest object is extrapolated to the distance of the further ofthe pair, and assuming l= 800 pc. For 29 of our sightlines with |z|>= 1 kpc and a HI detection from the Leiden-Dwingeloo survey(beamsize of 0.5°), we find log(N(CaII K)/N(HI)) ranging from -7.4to -8.4. Values of the CaII K abundance relative to neutral hydrogen(log[N(CaIIK)cm-2]-log[N(HI)cm-2]) are found to bemore than ~0.5dex higher in stars with distances exceeding ~100 pc, whencompared with the (log[N(CaII K) cm-2]-log[N(Htot) cm-2]) values found in nearbysightlines such as those in Wakker & Mathis (2000). Finally, stellarCaII K equivalent widths of the sample are determined for 26 objects.
| Highly Ionized Gas in the Galactic Halo: A FUSE Survey of O VI Absorption toward 22 Halo Stars
Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactichalo stars are studied to determine the amount of O VI in the Galactichalo between ~0.3 and ~10 kpc from the Galactic midplane. Strong O VIλ1031.93 absorption was detected toward 21 stars, and a reliable3 σ upper limit was obtained toward HD 97991. The weaker member ofthe O VI doublet at 1037.62 Å could be studied toward only sixstars because of stellar and interstellar blending problems. Themeasured logarithmic total column densities vary from 13.65 to 14.57with =14.17+/-0.28 (1 σ). The observed columns arereasonably consistent with a patchy exponential O VI distribution with amidplane density of 1.7×10-8 cm-3 and scaleheight between 2.3 and 4 kpc. We do not see clear signs of stronghigh-velocity components in O VI absorption along the Galactic sightlines, which indicates the general absence of high-velocity O VI within2-5 kpc of the Galactic midplane. This result is in marked contrast tothe findings of Sembach et al., who reported high-velocity O VIabsorption toward ~60% of the complete halo sight lines observed byFUSE. The line centroid velocities of the O VI absorption do not reflectGalactic rotation well. The O VI velocity dispersions range from 33 to78 km s-1, with an average of =45+/-11 kms-1 (1 σ). These values are much higher than the valueof ~18 km s-1 expected from thermal broadening for gas atT~3×105 K, the temperature at which O VI is expected toreach its peak abundance in collisional ionization equilibrium.Turbulence, inflow, and outflow must have an effect on the shape of theO VI profiles. Kinematical comparisons of O VI with Ar I reveal thateight of 21 sight lines are closely aligned in LSR velocity(|ΔVLSR|<=5 km s-1), while nine of 21exhibit significant velocity differences(|ΔVLSR|>=15 km s-1). This dual behaviormay indicate the presence of two different types of O VI-bearingenvironments toward the Galactic sight lines. The correlation betweenthe H I and O VI intermediate-velocity absorption is poor. We couldidentify the known H I intermediate-velocity components in the Ar Iabsorption but not in the O VI absorption in most cases. Comparison of OVI with other highly ionized species suggests that the high ions areproduced primarily by cooling hot gas in the Galactic fountain flow andthat turbulent mixing also has a significant contribution. The role ofturbulent mixing varies from negligible to dominant. It is mostimportant toward sight lines that sample supernova remnants like Loops Iand IV. The average N(C IV)/N(O VI) ratios for the nearby halo (thiswork) and complete halo (Savage et al.) are similar (~0.6), but thedispersion is larger in the sample of nearby halo sight lines. We areable to show that the O VI enhancement toward the Galactic center regionthat was observed in the FUSE survey of complete halo sight lines(Savage et al.) is likely associated with processes occurring near theGalactic center by comparing the observations toward the nearby HD177566 sight line to those toward extragalactic targets.
| A New Measurement of the Average Far-Ultraviolet Extinction Curve
We have measured the extinction curve in the far-ultraviolet wavelengthregion of (900-1200 Å) using spectra obtained with the BerkeleyEUV/FUV spectrometer during the ORFEUS I and the ORFEUS II (Orbiting andRetrievable Far and Extreme Ultraviolet Spectrometer) missions in 1993and 1996. From the complete sample of early-type stars observed duringthese missions, we have selected pairs of stars with the same spectraltype but different reddenings to measure the differential FUVextinction. We model the effects of molecular hydrogen absorption andexclude affected regions of the spectrum to determine the extinctionfrom dust alone. We minimize errors from inaccuracies in the catalogedspectral types of the stars by making our own determinations of spectraltypes based on their IUE spectra. We find substantial scatter in thecurves of individual star pairs and present a detailed examination ofthe uncertainties and their effects on each extinction curve. We findthat, given the potentially large uncertainties inherent in using thepair method at FUV wavelengths, a careful analysis of measurementuncertainties is critical to assessing the true dust extinction. Wepresent a new measurement of the average far-ultraviolet extinctioncurve to the Lyman limit; our new measurement is consistent with anextrapolation of the standard extinction curve of Savage & Mathis.
| IUE Absorption-Line Observations of the Moderately and Highly Ionized Interstellar Medium toward 164 Early-Type Stars
We present measurements of Galactic interstellar Al III, Si IV, and C IVabsorption recorded in high-resolution archival ultraviolet spectra of164 hot early-type stars observed by the International UltravioletExplorer (IUE) satellite. The objects studied were drawn from the listof hot stars scheduled to be observed with the Far UltravioletSpectroscopic Explorer (FUSE) satellite as part of observing programsdesigned to investigate absorption by O VI in the Galactic disk andhalo. Multiple IUE echelle-mode integrations have been combined toproduce a single ultraviolet (1150-1900 Å) spectrum of each starwith a spectral resolution of ~25 km s-1 (FWHM). Selectedabsorption-line profiles are presented for each star along with plots ofthe apparent column density per unit velocity for each line of the AlIII, Si IV, and C IV doublets. We report absorption-line equivalentwidths, absorption velocities, and integrated column densities based onthe apparent optical depth method of examining interstellar absorptionlines. We also determine column densities and Doppler parameters fromsingle-component curve-of-growth analyses. The scientific analysis ofthese observations will be undertaken after the FUSE satellite producessimilar measurements for absorption by interstellar O IV, Fe III, S III,and other ions. Based on archival data from observations obtained withthe International Ultraviolet Explorer (IUE) satellite sponsored byNASA, SERC, and ESA.
| Rotational velocities of B-type stars from the Edinburgh-Cape survey
The projected rotational velocity distribution for a sample of 34 highGalactic latitude B-type stars from the Edinburgh-Cape Faint Blue ObjectSurvey is presented to investigate the evolutionary status of the groupas a whole. Statistical analyses of the distribution show it to besimilar to that expected if the sample contained mainly normalPopulation i early B-type stars, although a contamination of up to 20 %by evolved stars cannot be ruled out. This implies that a large fractionof the sample consists of normal Population i B-type stars similar tothose found in the Galactic disk. Possible mechanisms explaining thepresence of these stars in the halo are briefly discussed.
| High-resolution spectroscopic observations of B-type stars from the Edinburgh-Cape survey
High-resolution spectroscopy has been obtained for 25 high-latitudestars identified from the Edinburgh-Cape faint blue object survey ashaving B-type spectra. Five objects are found to be subluminous(subdwarf or horizontal branch), chemically peculiar, or later thanB-type. We present model atmosphere analyses for the other 20 objects,and conclude that 17 stars exhibit stellar properties typical of youngB-type dwarfs. Photospheric abundances determined for a subset of starswere also found to be consistent with a Population I composition.Furthermore, we believe EC 05229-6058 to be an evolved object currentlyon the post-asymptotic giant branch phase, whilst EC 20411-2704 and11074-2912 are consistent with being zero-age horizontal branch andpost-blue horizontal branch objects respectively. A kinematic analysisof the normal stars implies that all could have formed in, and have beensubsequently ejected from, the Galactic disc.
| Coronal Gas in the Halo. II. ORFEUS Observations of Galactic Halo Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...465..296H&db_key=AST
| 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.
| Interstellar titanium in the galactic halo
We present observations of Ti II lambda 3384 absorption toward 15distant stars in the Galactic halo and the Magellanic Clouds. These newdata extend existing surveys of the distribution of Ti(+) to largerdistances from the plane of the Galaxy than sampled previously, allowingthe scale height of titaniumn to be determined for the first time. Wefind hTi(+ = 1.5 +/- 0.2 kpc, a value which although greaterthan those of other tracers of neutral gas, is not as large as had beensuspected. We interpret the extended distribution of Ti(+) as anindication that its severe depletion in interstellar clouds in the diskis reduced at the lower densities prevailing in the halo. The data areconsistent with a simple power-law dependence of the Ti abundance on theambient density, with exponent k approximately = -1. If the model iscorrect, it implies that refractory elements like Ti are fully returnedto the gas phase at distances beyond approximately 1 kpc from the planeof the Galaxy.
| Evidence for an extended neutral Galactic halo
Singly ionized titanium is the dominant ionization stage in H I regionsand provides an excellent tracer of the neutral interstellar medium.Here we present new interstellar Ti II absorption line observationstoward six halo stars located at absolute value of z greater than 1 kpc.Although the gas distribution is probably patchy, significant neutralgas does exist beyond 1 kpc from the Galactic plane and the projectedcolumn density, N(Ti II) sin (b), continues to increase out to the mostdistant star observed, at z = 8.7: kpc. Along this line of sight, the TiII absorption is the strongest yet detected and also has the highestgas-phase abundance of titanium seen in interstellar gas. Ti IIabsorption observed toward halo stars may arise from a new component ofthe halo interstellar medium which contains low column density neutralgas over a wide velocity range and which may extend beyond several H Iscale heights from the plane. The scale height of this component has yetto be determined, but could be well beyond several kpc, coincident withor even farther than the highly ionized gas observed in C IV and Si IV.Observations toward a large sample of extragalactic sources are neededto determine the actual scale height of neutral halo gas.
| Ultraviolet FeIII lines in the spectra of high galactic latitude early-type stars
Using high resolution spectral data from the International UltravioletExplorer satellite, we present qualitative and quantitative comparisonsof blends of Fe III absorption lines in the region1890A<=λ<=1930A for a sample of fifteen high latitudeB-type and standard stars. Standard and halo stars were matched ineffective temperature and surface gravity using Stroemgren [c_1_] andHβ photometry, and LTE model atmosphere codes were used toinvestigate whether they had similar iron abundances. We conclude thatwhile most of the halo stars have Population I iron abundances and maybe young objects, one star, HD 177566, has significant iron and silicondepletions and is most probably an old, evolved star. In view of thecoincidence of the atmospheric parameters of this star with those ofyoung B-type stars, we consider a post-AGB evolutionary status to belikely for HD 177566.
| The distribution of neutral hydrogen in the interstellar medium. 1: The data
We compile, from the existing literature, the largest sample to date(842 data points) of hydrogen column density measurements, N(H I), ofthe gas in the interstellar medium. We include only results obtainedfrom absorption measurements toward individual stars (594 in our sample)in an effort to construct a three-dimensional picture of theinterstellar gas. We derive hydrogen column densities toward a fractionof the stars in the sample from published column density measurements ofmetal ions. A three-dimensional physical model derived from this dataset will be presented in a companion paper. The observed stars spandistances from a few parsecs to a few thousand parsecs, and more thanhalf of the sample serves to describe the local interstellar mediumwithin a few hundred parsecs of the Sun. Hydrogen column densities rangefrom 1017 to 1022/sq cm. We describe here thevarious observational methods used to estimate the hydrogen columndensities and present the table with the stellar and hydrogen columndensity data. The provided table is intended as a global reference work,not to introduce new results.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| A Search for Beta-Cephei Type Variability in a Sample of Intermediate Galactic Latitude to High Galactic Latitude B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.267.1103H&db_key=AST
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
| First results from the Edinburgh-Cape faint blue object survey - Normal stars at high galactic latitudes
A simple analysis using low-dispersion Reticon spectroscopy andStromgren photometry is presented for a sample of 20 apparently normalearly-type stars detected in the Edinburgh-Cape faint blue object surveyof high galactic latitudes. Four stars are not normal, showing highgravity or helium abundance anomalies; 12 stars appear to be at moderatedistances from the galactic plane and four stars have derivedz-distances greater than about 5 kpc. The sample was selected from 33survey fields completed to B = 16.5 mag and indicates that the totalgalactic population of 'very high-z' B stars is only of the order100-1000. The Dyson and Hartquist (1983) model for the formation of suchobjects by cloudlet-cloudlet collisions within high-velocity cloudscannot therefore be ruled out on the basis of star formation rates.
| The distribution of interstellar AL III away from the Galactic plane
IUE spectra are analyzed to study the density distribution ofinterstellar Al III away from the Galactic plane. In most cases, themeasured values of the relative line strengths are consistent with onlymodest levels of line saturation. Al III is found to have an exponentialscale height and 1 sigma errors of 1.02(+0.36, -0.24) kpc. For the sameset of 70 stars, the scale height and 1 sigma errors for H I are0.67(+0.21, -0.16) kpc. The Al III scale height is similar to the valueobtained for free electrons from pulsar dispersion measures. The ionizedgas traced by Al III is somewhat more extended than the neutral gastraced by H I but less extended than the very highly ionized gas tracedby Si IV, C IV, and N V.
| Estimation of the equivalent width of the interstellar CA II K absorption line
Literature values for the equivalent width of the interstellarabsorption line of Ca II K (3933 A) are used to constrain a simpleempirical model for the strength of this line as a function of distancefrom the galactic plane. The best-fit model has an integrated Ca II Kequivalent width in the direction perpendicular to the galactic plane ofapproximately 200 mA and a 'scale height' of approximately 1100 pc. Sucha model is useful for estimating (and correcting for) the contributionof the interstellar Ca II K feature to the total observed Ca II K linestrength in the spectra of metal-deficient stars in the galactic halo.
| The density distribution of refractory elements away from the Galactic plane
The density distributions of the three refractory elements Ti II, Ca II,and Fe II away from the Galactic plane are compared with thedistribution of hydrogen and dust by examining plots of N s in b versusz. It is found that Ti II and Ca II are considerably more extended in zthan the H I and dust and that Fe II has an intermediate extension.Although the results are strongly influenced by sample bias, theindicated exponential scale heights for the data sample are h(Ti II) notless than 2 kpc, h(Ca II) = 1 kpc, h(Fe II) = 0.5 kpc, H(H I) = 0.3 kpc,and h(E/B-V) = 0.1 kpc. Furthermore, it is demonstrated that Ti II andCa II are much more smoothly distributed in space than the hydrogen ordust. The large scale heights for Ti II and Ca II and their smoothdistributions are most easily understood as the effect of a mixturealong the line of sight of two H I phases namely, a diffuse cloud phase,in which nearly all of the Ti and Ca are tied up in dust, and anintercloud medium, where refractory elements are less depleted. It isfound that Ti II and Ca II mostly trace the smoothly distributedintercloud medium. The smoothness of the distributions of Ti II and CaII makes them candidates for use as distance indicators.
| Radial velocities and spectral types for a sample of faint blue stars.
Not Available
| Non-radial pulsations in the extreme helium star HD 160641
Mean periods of 0.35 + or - 0.01, 0.71 + or - 0.05, 1.12 + or - 0.13 and1.77 + or - 0.34 day are identified from separately analyzed photometricobservations of the extreme helium star HD 160641. Radial velocitieswere obtained simultaneously with photometry on three nights in 1982.Optical variations are not accompanied by detectable color changes, andradial velocities are not characteristic of radial pulsation. HD 160641is therefore regarded as a nonradial pulsator. Radial velocity and lightvariations are tentatively interpreted as l = 4 mode pulsation,corresponding to a Wesselink radius of 7.9 + or - 1.0 solar radii. Theconsequent luminosity of log (L/L-solar) = 4.7 + or - 0.3 would beconsistent with Schoenberner's (1977) evolution model for a 1 solar massextreme helium star.
| 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 CA II and NA I line profiles towards halo OB stars
High resolution Ca II and Na I interstellar line profiles towardsseveral halo OB stars are presented. The profiles have been correctedwhere necessary for the presence of stellar features, and analyzed usingmulticomponent models to derive information on the radial velocities,internal velocity dispersions, and column densities of individualinterstellar clouds. A method is described for estimating peculiarvelocities for the clouds, and a significant trend of decreasing R =N(Na I)/N(Ca II) with increasing peculiar velocity is found. The ratio,R, is also shown to be generally smaller in the halo than in the plane,implying that many of the halo clouds may have peculiar velocities. Thedata indicate that there are both few clouds and a small Ca II densitybeyond the absolute z value of 2 kpc.
| A study of interstellar absorption at high galactic latitudes. I - Highly ionized gas
IUE interstellar absorption line data for C IV, Si IV, N V and Fe II, ina sample of 24 distant, early-type stars at high galactic latitudes, areanalyzed with other published results in order to study the distributionand physical conditions of highly ionized gas in the halo of the Galaxy.The C IV and Si IV lines, which are detected in front of approximatelytwo-thirds of the stars observed, are significantly stronger that thoseobserved in the spectra of disk stars of similar spectral types, andtheir velocities show no correlation with the velocities of thebackground stars. The observed variation of the column densities of C(3+) and Si (3+) with distance z from the plane is consistent with thesuggested existence of a Galactic transition layer between cool gas inthe disk and outer halo regions at temperatures greater than thoseobservable with IUE. Attention is drawn to the fact that the observed C(3+)/Si (3+) ratio may be a characteristic signature of hot gas ingalactic halos.
| 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.
| On the apparent normality of O and B stars far from the galactic plane
Results of a search for subluminosity or other peculiarities in thespectra of apparently normal OB stars located at large distances fromthe galactic plane are presented. Photographic spectrograms werecompared for stars of type B5 and earlier located within 0.5 kpc of andfurther than 1.5 kpc from the galactic plane, and for three subdwarfstars. Values of the projected axial rotation, surface gravity, absolutemagnitude and the strengths of certain He I and Balmer lines determinedfor both groups of OB stars are found to be similar, and distinct fromthe spectral characteristics of the subdwarfs. The results suggest that,unless the normality of the distant OB stars is illusory, either OBstars can be formed outside the galactic plane, or a mechanism mustexist to eject them from the plane with large velocities, unless theirmain sequence lifetimes are substantially longer than currentlybelieved.
| On the origin of intermediate-latitude OB stars
An attempt is made to trace the origin of early-type stars observed atappreciable distances from the galactic plane. Because uncertainties inthe proper motions make space motions and hence dynamical lifetimesrather inaccurate, a theory of oscillations normal to the plane has beenused to compute radial velocities for 138 intermediate-latitude OBstars. These theoretical values are then compared with the observedradial velocities, and it is found that the low-velocity stars wereprobably ejected from the plane some time after formation, while thehigh-velocity stars were ejected very soon after formation. Velocitiesof ejection perpendicular to the plane are computed and show a narrowdistribution with a mean absolute value of 7 km/s together with a spreadof velocities from about 40 to over 200 km/s. The data are in reasonableagreement with a 'sling' effect and 'runaway' origin for the stars inthe sample.
| H-beta photometry of southern early-type stars and galactic structure away from the plane
H-beta photoelectric photometry is reported for 165 early-type stars atintermediate and high galactic latitudes. The data are combined withearlier UBV and spectroscopic results to determine the stellar spacedistribution. Stars of type B2 and earlier, at distances of up to 1 kpcfrom the galactic plane, appear to follow spiral structure in the plane.The available material, particularly the derived color excesses, is usedto select a number of blue stars which may be subluminous.
| Photometry and spectral classification of early-type stars away from the galactic plane.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970MNRAS.150...23H&db_key=AST
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Capricornio |
| Ascensión Recta: | 20h32m14.70s |
| Declinación: | -24°04'03.7" |
| Magnitud Aparente: | 9.208 |
| Movimiento Propio en Ascensión Recta: | 1.1 |
| Movimiento Propio en Declinación: | -12.1 |
| B-T magnitude: | 9.011 |
| V-T magnitude: | 9.192 |
Catálogos y designaciones:
|
