Polarimetry of evolved stars. III. RV Tau and R CrB stars
We present broadband optical polarimetry, and broadband optical andinfrared photometry, of eight RV Tau-type and five R CrB-type stars;much of the photometry and polarimetry was obtained simultaneously. Fornine of the objects polarimetric data is reported for the first time. Wehave estimated and subtracted the interstellar component ofpolarization, allowing us to determine the level of intrinsicpolarization. In some cases this is =~ 1%-2% even when the star is in abright photometric state. We consider this to be evidence for thepresence of permanent clumpy non-spherical dust shells around the RV Tauand R CrB-type stars we observed. Our polarimetric and photometric datalead us to conclude that, for most of our programme stars, neutralextinction must be significant in their circumstellar envelopes. Apartfrom the brightness variations due to pulsations and changes in theeffective temperature of stars, there is clear evidence ofwavelength-independent flux variations - with amplitude from 0fm 5 to1fm 0 - implying the presence of large (a>~ 0.15 mu m) dustparticles. Rapid ( ~ 2 hours) evolution of the infrared fluxdistribution at the level of ~ 0fm 6 in the JHKL bands was detected inthe RV Tau star R Sct.Table 2 is only available in electronic form at the CDS via anomymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/405Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
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Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
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Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
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Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
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Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
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Study of Neglected Variable Stars Classified as Type II Cepheids
Four variable stars classified as Type II Cepheids have been studiedpartly based on new photometry and the Hipparcos photometric data. Thecorrect period is derived for V341 Ara and NW Pav and it is pointed outthat RR Mic and V1711 Sgr do not belong to Type II Cepheids, insteadthey may be semi-regular variables. The cycle length of theirvariability is also determined.
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IRAS Detections of Metal-poor Red Giants
A number of relatively bright metal-poor red giants from the HD and BDcatalogs are found to have been detected by the IRAS satellite. Data forthese stars have been retrieved from the IRAS Point Source Catalog (PSC)and/or the Faint Source Catalog (FSC). The majority of metal-poor giantsin these samples fall along relatively well-defined sequences in plotsof V-[12] versus B-V and V-I; for these stars, the 12 μm fluxdetected is presumed to arise from the photosphere. Only a subset ofstars detected at 12 μm were detected at 25 μm these are displayedin a plot of [12]-[25] versus V-[12]. There are a small number of giantsthat exhibit notable 12 and/or 25 μm excesses relative to the meansequences defined by the bulk of the sample. Those stars with the mostunambiguous evidence for infrared excesses are variable stars, eitherlong-period or semiregular variables or RV Tauri stars. As such, thosestars exhibiting infrared excesses in the metal-poor giant sample arelikely in the asymptotic giant branch (AGB) or post-AGB phase ofevolution. There is no clear evidence for nonvariable first-ascent redgiants having been detected among the infrared-excess stars. In fact,some metal-poor red giants known to exhibit outflows in theirchromospheres do not show infrared excesses. A Population II starascending the red giant branch for the first time appears to have toolow a mass-loss rate to be recognizable as an infrared-excess star inthe IRAS PSC or FSC.
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Search for resonance effects in long period Cepheids.
Light curves of classical Cepheids with period longer than 8 days havebeen Fourier decomposed with the purpose of studying the characteristicsof high order Fourier parameters, and to detect possible effects ofresonances between pulsation modes other than the well known resonanceat P~10d. The possible effects of two expected resonances have beententatively identified: P_0_/P_1_=3/2 at P_0_~24 d and P_0_/P_3_=3 atP_0_~27d. The identification is not completely certain owing to the poornumber of Cepheids. The limitation could be overcome by observingaccurately other relatively faint Cepheids in our Galaxy, and severalCepheids in nearby galaxies.
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Stellar Evolution and Late Type Stars
Not Available
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Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
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Color Excesses on a Uniform Scale for 328 Cepheids
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990ApJS...72..153F&db_key=AST
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Population studies. II - Kinematics as a function of abundance and galactocentric position for (Fe/H) of -0.6 or less
A catalog is presented of some 1200 Galactic objects which have radialvelocities and (Fe/H) abundances of -0.6 or less. These data areanalyzed to yield information on the kinematic properties of the olderpopulations of the Galaxy and on the interdependence between kinematicsand abundance. It is found that the kinematics of the availablekinematically selected stars differ from those of the nonkinematicallyselected objects. No evidence is found for any significant difference inthe kinematic properties of the various halo subgroups, nor for anydependence of kinematics on abundance. While the rotation of the halo issmall at about 37 km/s for (Fe/H) of -1.2 or less, it rises quickly forhigher abundances to a value of about 160 km/s at (Fe/H) = 0.6. Objectsin the abundance range -0.9 to -0.6 appear to belong predominantly to apopulation possessing the kinematic characteristics of a thick disk. Theimplications of these findings for the suggestion that globular clustersbelong to the same population as the noncluster objects, for the originof the thick disk, and for the mass of the Galaxy are discussed.
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Detection of Cepheid variables by the Infrared Astronomical Satellite
Using the IRAS Point-Source Catalog (PSC) as a data base, midand far-IRemission near Cepheids were searched for. Of the more than 700 Cepheidvariables listed in the General Catalogue of Variable Stars, sixty-eightconfirmed detections have been made in the PSC. The vast majority ofCepheids detected are bright, nearby classical Cepheids, whose IR fluxis simply that produced by the stellar photosphere. For those Cepheidsthat show IR excesses, it is speculated that the excesses are caused bythermal emission from dust formed by mass loss. The results arediscussed in the framework of simple mass-loss models for stars withmasses comparable to Cepheid variables.
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Circumstellar material and the light variations of RV Tauri stars
Infrared photometry on the JHKL system is given for 35 stars of the RVTauri and related types. The RV Tauri stars show infrared excessesindicating the presence of circumstellar dust with a temperature ofabout 900 K. These dust shells can be equally prominent in oxygen-richor carbon-rich stars; the high carbon content of the latter appears tocounteract the inhibiting effect of their generally lower metalabundance in promoting dust formation. The wide range of energydistributions of RV Tauri stars is attributed in part to differences inthe radial distribution of dust. The RVb stars (whose mean light variescyclically) are redder than the RVa stars (which have constant meanlight), indicating that their dust shells are denser (especially in theinner regions, which radiate strongly in the 1-3 micron band).Far-infrared data reveal cooler dust around some of the stars of groupa. Three W Virginis stars, two of which were already known to showaffinities to the RV Tauri stars, have dust shells. The dust shells forwhich IRAS infrared spectra are available show the 10-micron silicateemission feature regardless of whether the optical spectra are carbon-or oxygen-rich. Some considerations regarding possible models are given.A comparison with the R Coronae Borealis variables has revealedsimilarities.
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A catalogue of field Type II Cepheids
A catalogue of field Type II Cepheids is presented. The primary listconsists of 152 Cepheids sufficiently far from the galactic plane to bevery probably Type II stars. A second list contains 56 additional starsthat are likely, but less certain, Type II Cepheids, including bothstars estimated to be at large distances from the galactic plane butwith uncertain distances and stars close to the galactic plane believedto be Type II for independent reasons.
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Kinematics of field Type II Cepheid variables
The spectra of 49 field Type II Cepheids have been assembled andcombined with 14 additional stars with previously published radialvelocities in order to investigate the kinematics and spectralcharacteristics of these stars. Many of the stars have low peculiarvelocities consistent with circular rotation around the galaxy, whilesome are high-velocity stars. Outside the nuclear bulge, the velocitiesare correlated with photometric metallicities in the sense thatmetal-rich stars show a nearly normal, disklike rotation velocity and alow velocity dispersion. This correlation shows that the low-velocity,metal-rich stars have an old-disk origin. Fewer Cepheids are found inthe field than expected relative to clusters, suggesting a differencebetween the populations of the halo field and clusters. The kinematicproperties of Cepheids in the nuclear bulge appear to be different inthat both metal-poor and metal-rich Cepheids have a high velocitydispersion and show little systematic galactic rotation.
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Photometric abundances of Type II Cepheid variables
Washington-system colors and V magnitudes of 63 Type II Cepheids areused to investigate the distribution of metal abundances in these starsand correlations of their abundances with position in the Galaxy. Only asmall fraction of Cepheids in the field represent a true halopopulation; most have characteristics of a transition population betweenthe halo and the disk. The origin of the metal-rich stars and theirrelation to the metal-rich RR Lyraes is discussed. The abundancedistribution of all Type II Cepheids shows no strong dependence ondistance from the galactic center.
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BVI Photometry of Southern Cepheids
Not Available
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VBI photometry of some southern cepheid and RR Lyrae variables
Not Available
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Metal-to-H ratios in the Galaxy as indicated by narrow-band photometry of Cepheids.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966AJ.....71..615W&db_key=AST
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Southern Cepheid Photometry.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1961ApJS....6..253I&db_key=AST
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Le type de population des céphéides galactiques; critères de distinction des types et catalogue
Not Available
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INHABITED SKY
