目录
图像
上传图像
DSS Images Other Images
相关文章
H I Spectra and Column Densities toward HVC and IVC Probes
We show 21 cm line profiles in the direction of stars and extragalacticobjects, lying projected on high- and intermediate-velocity clouds (HVCsand IVCs). About half of these are from new data obtained with theEffelsberg 100 m telescope, about a quarter are extracted from theLeiden-Dwingeloo Survey (LDS), and the remaining quarter were observedwith other single-dish telescopes. H I column densities were determinedfor each HVC/IVC. Paper I of this series uses these in combination withoptical and ultraviolet high-resolution measurements to deriveabundances. Here an analysis is given of the difference and ratio of N(HI) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratioN(H I-9')/N(H I-35') lies in the range 0.2-2.5. For low-velocity gasthis ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, butit appears that the correction for stray radiation is slightly off). Thesmaller range for the low-velocity gas may be caused by confusion in theline of sight, so that a low ratio in one component can be compensatedby a high ratio in another-for 11 low-velocity clouds fitted by onecomponent the distribution of ratios has a larger dispersion. Comparisonwith higher angular resolution data is possible for 16 sight lines.Eight sight lines with H I data at 1'-2' resolution show a range of0.75-1.25 for N(H I-2')/N(H I-9'), while in eight other sight lines N(HI-Lyα)/N(H I-9') ranges from 0.74 to 0.98.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| Line identification in the Ca II K spectral region of sharp-lined B-type stars
Previous Ca Ii K observations of the B-type star HD 83206 have revealedputative high-velocity interstellar clouds (HVCs) at Local Standard ofRest (LSR) velocities of -80 and -110 kms-1. Similar resultswere also found for the sightline towards HD 135485. In this article, weshow that these absorption lines are in fact due to stellar S Iifeatures. As the Ca Ii K absorption line in B-type stars is often usedto assess the presence and distance of HVCs, we also present a very highquality spectrum of HD 83206 in the Ca Ii K region ( ~ +/- 4 Å or+/- 300 kms-1), so that in the future confusion betweenstellar lines and HVC features may be avoided.
| The atmospheric parameters and chemical composition of early B-type giants in h and chi Persei
Atmospheric parameters and surface chemical compositions are presentedfor eight stars, classified as B1 or B2 but with a range of luminosityclasses, in the northern double cluster h and chi Persei. Echellespectroscopy (covering the wavelength region 3900 to 4700 Ä) andgrating spectroscopy (of the Balmer, Hγ and Hβ lines) wereanalysed using non-LTE synthetic spectra based on LTE line-blanketedatmosphere structures. High microturbulences are found in our sample,and this quantity must be included in the computation of the non-LTElevel populations; its effect is generally to decrease the derived metalabundances by typically 0.1 dex but by up to 0.4 dex. Our absoluteabundances are in reasonable agreement with those previously found formain sequence B-type stars, while we find some evidence for smallabundance variations (particularly for nitrogen) within our sample. Onestar (BD+56 678) appears to be a spectrum variable and at two epochsshows a highly enriched nitrogen spectrum. Our atmospheric parametersimply that two stars have previously been mis-identified as mainsequence objects and a distance modulus, at the higher end of the valuespreviously deduced. The observational HR diagram is consistent withstellar evolutionary models that explicitly include the effects ofrotation.
| High-resolution optical spectroscopy of the sharp-lined B-type star HD83206
Very-high-resolution (R~160000) spectroscopic observations are presentedfor the early B-type star, HD83206. Because it has very sharp metallines, this star affords an opportunity to test theories of modelatmospheres and line formation. Non-LTE model atmosphere calculationshave been used to estimate the atmospheric parameters and absolute metalabundances (C, N, O, Mg and Si); an LTE analysis was also undertaken toinvestigate the validity of this simpler approach and to estimate aniron abundance. For the non-LTE calculations, there is excellentagreement with observations of the Balmer lines H? and H?and the lines of Siii and Siiii for atmospheric parameters ofTeff~=21700+/-600K and logg~=4.00+/-0.15dex. The agreement isless convincing for the LTE calculations, and a higher gravity isdeduced. Careful comparison of the metal line profiles with non-LTEcalculations implies that the projected rotational and microturbulentvelocities have maximum values of ~=5 and ~=2kms-1,respectively. The latter value is smaller than has often been adopted inLTE model atmosphere analyses of main-sequence stars. Non-LTE absolutemetal abundances are estimated, and a comparison with those for normalB-type stars (deduced using similar non-LTE techniques) shows nosignificant differences. A comparison of the abundances deduced usingnon-LTE and LTE calculations implies systematic differences of0.1-0.2dex, showing the importance of using a non-LTE approach whenaccurate absolute abundances are required. Its location in theHertzsprung-Russell diagram and normal metal abundance lead us toconclude that HD83206 is probably a main-sequence B-type star. As such,it is among the sharpest-lined young B-type star discovered to date.
| Optical observations of three Galactic halo stars: evidence for cloudlets in intermediate- and high-velocity interstellar clouds
Very high resolution (R ~ 160000) absorption line measurements of theinterstellar Ca Ii K and Na I D lines and medium resolution (R ~ 50000)measurements of the interstellar Ti Ii (3384 Ä) line towards threeGalactic halo stars are presented. The data have signal-to-noise ratiosof ~ 90-240. The sightlines studied clearly show severalintermediate-high velocity interstellar clouds with local standard ofrest velocities in the range ~ - 40 to -110 \km. Two different methodshave been used to analyze these data. Line profile fitting allows us tomodel multiple components for the different high-velocity cloudsseparated by only a few km. The close proximity of the lines in velocityspace indicates that the cloudlets are related. Conversion of theabsorption profiles to apparent optical depth profiles also allows us toexamine the column densities and their ratios as a function of velocity.Variations in N(ion {Ca}{ii})/N(ion {Na}i), N(ion {Ca}{ii})/N(ion{Ti}{ii}) and N(ion {Ti}{ii})/N(ion {Na}i) with cloud velocity arediscussed.
| BVR Photometry of Northern Hemisphere Luminous Stars. III. LS and HD Stars
CCD-based BVR photometry of 48 previously unobserved stars drawn fromvolumes V and VI of the Luminous Stars in the Northern Milky Way,Luminous Stars in the Southern Milky Way, and HD catalogs is reported.
| 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
| LS 4825: A Blue Supergiant on the Far Side of the Galaxy
We present high-resolution spectroscopic observations of LS 4825, a V =12 B-type star in the Galactic center direction. On the basis of itsstellar and interstellar spectra, we infer that it is likely to be ayoung supergiant at a distance of 21 +/- 5 kpc, and hence lying on thefar side of the Galaxy. Adopting this hypothesis, a differentialabundance analysis shows LS 4825 to have a chemical composition that isconsistent with local B-type supergiants. These observations thereforerepresent the first detailed investigation of a star on the far side ofthe Galactic center. We trace multiple interstellar components in Ca IIK and Na I D spectra, with velocities -206 <= vlsr <= +93 km s-1.We consider the likely origin of this gas and find that some componentsappear to trace matter lying close to the Galactic center. We discussthe possible use of such sight lines in furthering our understandingboth of the nature of gas around the Galactic center and of theabundance gradient of the Galaxy.
| Optical and HI studies of high- and intermediate-velocity gas towards Complex A
We present high-resolution optical and 21-cm Hi spectra of sevenearly-type stellar sightlines towards the high-velocity cloud Complex A,at distances of up to 4.6 kpc from the Galactic plane. We do notoptically detect the -160 kms^-1 interstellar gas associated withComplex A, and so are unable to establish limits on its distance.However, we do detect gas associated with the Low Latitude IntermediateVelocity Arch, placing it at z<0.9kpc, and also establish distancelimits (z<=1.2-3.3kpc) on several other intermediate- andhigh-velocity clouds in this region that have not been cataloguedpreviously.
| Radial velocity measurements. IV - Ground-based accompaniment to the HIPPARCOS observation program
The paper presents 396 radial velocities of stars distributed in 19fields of 4 x 4 degrees. The study employs the Fehrenbach objectiveprism method and the same measuring technique used in a previous paper(Fehrenbach et al., 1987).
| Four-colour and H-beta photometry of blue stars selected from a balloon-ultraviolet survey and other sources
New uvby and/or H-beta photometry is obtained at the Chiranhigh-altitude outstation for 105 stars. Certain of the program stars areselected from a comparison of the SCAP 2000 balloon-ultraviolet skysurvey of the Laboratoire d'Astronomie Spatiale with the correspondingblue and red prints of the Palomar Observatory sky survey. Only a smallportion of these stars turn out to be B stars, whereas, the rest of thestars, which are selected from a variety of other sources, are mostly Bstars and if of normal luminosity are in many cases many Population Iscale heights from the galactic plane. It is shown that theidentification of B stars is practicable only at a good observing siteand that if H-beta photometry is also obtained, sdOB, sdB, and hothorizontal branch B stars can be distinguished from normal stars bytheir position in the beta/c(0) diagram.
| 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
| Four-colour photometry of B stars north of B = + 45 deg and comparison with the south
Four-color photometry of 33 Henry Draper B stars north of b = + 45 degis presented. Most are little-reddened B or intermediate-A stars. A newAm star is discovered. The new measures essentially complete uvbyphotometry of all HD B stars within 45 deg of either galactic pole. Thenorthern and southern cones of HD B stars are compared, and selectioneffects deduced. Far from the galactic plane, it appears that B starsmay be equally distributed north and south of the plane; closer to thesun, an asymmetry associated with Gould's Belt is evident.
| Search and photographic photometry of ultraviolet excess objects in the region of the 5C1 radio survey
A sample of ultraviolet excess objects in a 12.56 square degree field ispresented. In addition to starlike or and nearly starlike objects asample of more diffuse ones is listed, containing many Markarjan-typegalaxies. UBV-photometry and first spectroscopic results arecommunicated; the number magnitude relation and the projectednumber-density of the objects are discussed.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
