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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.
| 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.
| 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.
| On optical studies of high-velocity clouds
Lists of distant objects that can be used to study physical conditionsin, and distances of, 21 cm (Oort) high-velocity clouds are presented.Recent published observations are used to compile positions, velocities,and distances of the clouds.
| 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.
| IUE observations of variability and differences in the UV spectra of double quasar 0957+561 A, B
The continuum flux from 2500-2800 A and the emission flux in the Lymanalpha line were observed to change substantially in component A of thedouble quasar but not in component B. The equivalent width of the lineincreased by 50 percent in A but not in B, which was probably not due toinstrument effects. It was expected that these changes should recur in Bafter an appropriate time delay. The continuum flux ratio B/A started toincrease, reached a maximum, and then recovered its intrinsic value. Theobserved changes were clearly seen to be confined to component A,probably the result of the lensed quasar being intrinsically variable,the change having been propagated along sight-line A but not alongsightline B. This is supported by the fact that the UV radiation is morein step with visible radiation than with radio emission. Currenttheoretical estimates of the time delay are uncertain because ofuncertainties in the mass distribution of the lensing galaxy.
| UV spectra of the twin QSOs 0957+561 A, B
The similarity of all the observed features of the twin QSOs 0957 + 561A and B has led to the proposal that they constitute a single object, adouble image of which is formed by an intervening massive galaxy whichacts as a gravitational lens. In the present paper, the first UVobservations of Q0957 + 561 A, B are presented and are discussed interms of the lens hypothesis. The discussion shows that the two lightpaths are almost extinction free, either because the intervening galaxyis dust-free or the light paths are through its dust-free halo, and thata ratio of unity for the UV and radio flux ratios is in keeping with oneof the most fundamental predictions of the gravitational lens theory,and therefore provides strong evidence in favor of the lens hypothesis.
| International Ultraviolet Explorer (IUE) observations of the twin QSO'S 0957 + 561 A, B
The ultraviolet spectra of the twin quasars 0957 + 561 A,B were obtainedwith International Ultraviolet Explorer. These quasars are suspected tobe the images of a single object formed by an intervening galaxy actingas a gravitational lens. The ultraviolet data give an intensity ratiofor the two objects which is the same, within the errors, as thatderived at radio frequencies. This shows that differential reddeningbetween the two light paths is negligible and also given support to thegravitational lens hypothesis.
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