目录
图像
上传图像
DSS Images Other Images
相关文章
A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for17 different elements sampled over 243 sight lines in the local part ofour Galaxy reveals that the depletions into solid form (dust grains) areextremely well characterized by trends that employ only three kinds ofparameters. One is an index that describes the overall level ofdepletion applicable to the gas in any particular sight line, and theother two represent linear coefficients that describe how to derive eachelement's depletion from this sight-line parameter. The information fromthis study reveals the relative proportions of different elements thatare incorporated into dust at different stages of grain growth. Anextremely simple scheme is proposed for deriving the dust contents andmetallicities of absorption-line systems that are seen in the spectra ofdistant quasars or the optical afterglows of gamma-ray bursts. Contraryto presently accepted thinking, the elements sulfur and krypton appearto show measurable changes in their depletions as the general levels ofdepletions of other elements increase, although more data are needed toascertain whether or not these findings are truly compelling. Nitrogenappears to show no such increase. The incorporation of oxygen into solidform in the densest gas regions far exceeds the amounts that can takethe form of silicates or metallic oxides; this conclusion is based ondifferential measurements of depletion and thus is unaffected byuncertainties in the solar abundance reference scale.Based in large part on published observations from (1) the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555, (2) theFar Ultraviolet Spectroscopic Explorer (FUSE) mission operated by JohnsHopkins University, supported by NASA contract NAS5-32985, and (3) TheCopernicus satellite, supported by NASA grant NAGW-77 to PrincetonUniversity.
| Runaway and Hypervelocity Stars in the Galactic Halo: Binary Rejuvenation and Triple Disruption
Young stars observed in the distant Galactic halo are usually thought tohave formed elsewhere, either in the Galactic disk or perhaps theGalactic center (GC), and subsequently ejected at high velocities totheir current position. However, some of these stars have apparentlifetimes shorter than the required flight time from the Galacticdisk/GC. We suggest that such stars have evolved in close runaway orhypervelocity binaries. Stellar evolution of such binaries can drivethem into mass transfer configurations and even mergers. Such evolutioncould then rejuvenate them (e.g., blue stragglers) and extend theirlifetime after their ejection. The extended lifetimes of such starscould then be reconciled with their flight times to the Galactic halo.We study the possibilities of binary runaway and hypervelocity stars(HVSs) and show that such binaries could have been ejected in tripledisruptions and other dynamical interactions with stars or with massiveblack holes (MBHs). We show that currently observed "too young" star inthe halo could have been ejected from the Galactic disk or the GC and beobservable in their current position if they were ejected as binaries.Specifically, it is shown that the HVS HE 0437-5439 could besuch a rejuvenated star. Other suggestions for its ejection from theLarge Magellanic Cloud are found to be highly unlikely. Moreover, it isshown that its observed metallicity is most consistent with a Galacticorigin and a GC origin cannot currently be ruled out. In addition, wesuggest that triple disruptions by the MBH in the GC could also capturebinaries in close orbits near the MBH, some of which may later evolve tobecome more massive rejuvenated stars.
| 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.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| 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.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| 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.
| The Spacelab-1 Very Wide Field Survey of UV-excess objects. IV. The performance of the instrument in combination with optical photometry as a means of identifying stars with peculiar properties.
UV (195 nm) and Stroemgren uvby photometry of a 110 square degree fieldat high southern galactic latitudes are analyzed through a comparison ofi) UV magnitudes for 57 stars of various types common to the publishedTD1 catalogue and the Very Wide Field Camera (VWFC); and ii) observedand theoretical two-colour diagrams. The higher sensitivity of the VWFC(=~0.5magnitude) and its more complete survey are exemplified by thedetection and UV measurement of a series of objects with moderateUV-excess in addition to detection of some very blue objects of variousnature down to fainter than 12th magnitude in the optical domain. Adeeper survey with a VWFC-type instrument could provide a completesample for studies of the group properties of faint blue stars. Duringthe uvby reductions it was found that the usual procedure of plottingresiduals as functions of declination, hour angle and airmass can be apowerful and diagnostic test of photometer rigidity.
| 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.
| 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
| A high-resolution optical and radio study of Milky Way halo gas
Optical interstellar absorption lines of Ti II and Ca II and the 21 cmemission line of H I were observed at high-resolution (6 and 1 km/s,respectively) and high detection sensitivity along 25 lines of sight inthe Galactic halo. The sample includes 16 distant halo stars matchedwith one or more nearly aligned foreground stars as well as local starsalong five extragalactic sight lines. The data show substantialinterstellar material, at both low and intermediate velocities, between250 and 1000 pc beyond the Galactic plane. As much as one-third of thetotal gas observed in Ca II absorption may be beyond 1 kpc, and thegaseous Ti II may lie in an even thicker layer. The directly determinedgaseous Ti abundance above the Galactic plane exceeds that in the disk,on the average, by a factor of 4 to 6 and, for individual cloudcomponents, is further enhanced at higher LSR velocity. Thirty threediscrete high-latitude clouds are detected in Ca II absorption, and 17discrete clouds, including three high-velocity clouds, are identified inH I emission. The kinematics of the high-latitude gas observed in Ti IIand Ca II absorption is characterized by significant peculiar velocitieswith respect to a model corotating halo.
| Infrared Observations of Possible Hot Post-Asymptotic Giant Branch Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...272..243C
| 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.
| 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.
| 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 kinematics of Milky Way halo gas. I - Observations of low-ionization species
Ultraviolet interstellar line day observed with the IUE toward 70 halostars and four extragalactic sight lines are analyzed in a study of thelarge-scale kinematic properties of the Milky Way halo gas. The motionsof the low-ionization gas is focused on. Large systematic velocities arefound, and a pronounced asymmetry in the absorption characteristics ofhalo gas toward the Galactic poles is indicated. In the north,substantial amounts of material are falling toward the disk atvelocities up to about 120 km/s in the most extreme case. Toward thesouth, low-ionization material shows no extreme or systematic motions.
| IUE-IRAS studies of the infrared cirrus
The 60 and 100 micron cirrus emission around 256 lines of sight in theIRAS all-sky survey was measured, and the flux averages were used tostudy the distribution, variations, and correlations of the IRASinfrared cirrus fluxes with various interstellar parameters. It wasfound that the 60 and 100 micron fluxes correlate with the depletion ofSi and show a trend with the depletion of Fe for 51 lines of sighttoward the Galactic halo. No correlation was found with the abundancesof Si, Mn, Fe, S, or Zn or with abundance ratios for the full sample of256 stars. An abundance ratio of about 3 x 10 to the 7th by numberrelative to H was derived from 60 and 100 micron flux ratios and the Hcolumn along the line of sight; this ratio appears to decrease by afactor of 10 into the halo.
| Galactic interstellar abundance surveys with IUE. III - Silicon, manganese, iron, sulfur, and zinc
This paper continues a survey of intestellar densities, abundances, andcloud structure in the Galaxy using the IUE satellite. A statisticaldata set of 223 O3-B2.5 stars is constructed, including 53 stars in theGalactic halo. It is found that S II lines in B stars, of luminosityclasses IV and V, have possible contamination from stellar S II,particular for stars with v sin i less than 200 km/s. The meanlogarithmic depletions are -1.00, -1.19. -0.63, and -0.23 (Si, Mn,Fe,S,Zn). Depletions of Si, Mn, and Fe correlate with the mean hydrogendensity n-bar along the line of sight, with a turnover for n-bar greaterthan 1/cm. Sulfur depletions correlate with n-bar along the line ofsight. The slight Zn depletion correlation also appears to bestatistically insignificant. No correlation of depletion is found withthe physical density derived from H2 rotational states in 21 lines ofsight. Depletion variations in the disk are consistent with a Galacticabundance gradient or with enhanced mean depletions in the anticenterregion.
| PHL 346, a Beta Cephei star situated at more than 5 KPC from the Galactic plane?
The high-latitude early-B star PHL 346 is a pulsating variable with aperiod of 0.152 days. This period, the fact that color and brightnessvary in phase, and the atmospheric parameters of PHL 346 suggest that itis a ? Cephei variable. The star would then have a mass of about 10solar masses and an age of about 107years. PHL 346 is faint(mv = 11.44) and is located at a high galactic latitude of-58°, so that it appears excluded that it originated in the galacticplane only 107yr ago. The pulsation of PHL 346 thus lendssupport to the idea that star formation far from the plane of the galaxycan occur. It is argued, more generally, that the study of thevariability of high-latitude B stars provides a strong test for thenature of these stars.
| Galactic interstellar abundance surveys with IUE. II - The equivalent widths and column densities
This paper continues a survey of interstellar densities, abundances, andcloud structure in the Galaxy, using the International UltravioletExplorer (IUE) satellite. Equivalent widths of 18 ultraviolet resonancetransitions are presented and column densities for Si II, Mn II, Fe II,S II, and Zn II toward 261 early-type stars are derived. Theseequivalent widths and column densities agree within the stated errors ofearlier Copernicus, BUSS, or IUE surveys of Mn II, Fe II, S II, and ZnII for 45 stars in common. The column densities are derived fromsingle-component curves of growth with a common b-value based on that ofFe II and Si II.
| Interstellar CA II absorption toward early-type stars at high Galactic latitudes
New high resolution AAT and INT observations of the Ca II 3933 Åinterstellar line towards nine early-type stars at high galacticlatitudes are presented. These are combined with the authors' earlierAAT data to investigate the distribution of Ca II in the galactic halo.A comparison of these results with Ca II equivalent widths forextragalactic sightlines implies that there is a significant amount ofthis ion out to z ≅ 1 kpc, but not much beyond z ≅ 2 kpc. Nocoincident high velocity Ca II absorption is detected in the spectra ofstars near known high velocity clouds (HVC). Although this may indicatethat the HVCs are either at greater distances than the stars or haveangular extents smaller than the separations of the stellar and HVCsightlines, it is also possible that they are nearby but contain toolittle Ca II to produce an observable absorption line.
| 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.
| The extent of the local H I halo
Forty-five high-latitude, OB stars have been observed in the Ly alphaand 21 cm lines of HI in an effort to map out the vertical distributionand extent of the local HI halo. The 25 stars for which a reliable HIcolumn density can be obtained from Ly alpha lie between 60 and 3100 pcfrom the plane. The principal result is that the total column density ofHI at an absolute value of z greater than 1 kpc is, on the average, 5 +or - 3 x 10 to the 19th power/sq cm, or 15 percent of thetotalHI. At relatively low z the data toward some starssuggest a low effective scale height and fairly high average foregrounddensity, while toward others the effective scale height is large and theaverage density is low. This can be understood as the result ofirregularities in the interstellar medium. A model with half of the HImass in clouds having radii of a few pc and a Gaussian verticaldistribution with sigma2 = 135 pc, and half of the mass in anexponential component with a scale height of 500 pc, gives asatisfactory fit to the data. The technique of comparing Ly alpha and 21cm column densities is also used to discuss the problem of estimatingthe distance to several possibly subluminous stars.
| Galactic interstellar abundance surveys with IUE. I - Neutral hydrogen
A survey is initiated of interstellar densities, abundances, and cloudstructure in the Galaxy, using the International Ultraviolet Explorer.From high-resolution (0.1-A) spectra, interstellar column densities arederived toward 244 early-type stars by fitting the damping wings of theLy-alpha absorption line. Published B-V photometry is used to deriveE(B-V) color excesses, stellar distances r, and the mean hydrogendensities, n-bar = N(H I)/r. The data set includes stars out to r = 8.5kpc and E(B-V) = 0.86, with 68 stars in the galactic halo (absolute bgreater than 20 deg). The statistical sample consists of 205 stars oftype O3-B2.5 within 5 kpc (53 stars in the halo). The column densities,log N(H I), range from less than 19.44 per square centimeter for alphaVir to 21.81 for HD 37061. A statistical analysis of the E(B-V)distribution suggests variations in the line-of-sight cloud density andthe mean reddening per cloud.
| 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.
|
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:
|
