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A photometric and spectroscopic study of the hottest pulsating extreme helium star, V2076 Oph (HD 160641)
We present results from a three-site photometric and high-resolutionspectroscopic campaign on the hottest known extreme helium star V2076Oph (HD 160641). A core programme of intensive observations covered twoweeks and a much lower sampling rate extended over another two months.Despite the fact that the data seem to indicate periodicity near half aday (though the light curves are clearly not formed by a singleperiodicity), conventional Fourier analysis of the data fails to revealcoherent frequencies. Furthermore, we are unable to recover frequencieswhich were apparently clear in an earlier campaign on the star. Evidenceof monotonic pulsation amplitude changes is seen at the higherfrequencies from a wavelet analysis, but more data are needed beforethis study can be extended to lower frequencies. The application oflinear stochastic differential equation (LSDE) methods indicates thatthe observed light variations could be a result of random variationsgiving the appearance of intermittent periodicity. High-resolutionspectroscopic observations were obtained during the campaign andadditional observations were made three years later. Complex lineprofile variations were observed. It is proposed that the differentbehaviour of the emission line studied may indicate it is associatedwith a stellar wind or resident circumstellar material. The frequenciesthat are extracted from the velocity data do not conform to thoseidentified in the current or previous photometric campaigns.
| A common puzzle for Extreme Helium stars and evolved delta Scuti stars.
The Extreme Helium stars are hot luminous stars of about one solar massthat are believed to be shell He-burning. We modeled the pulsations ofthe hottest ExHe star V2076 Oph, for which the 2000 multisite campaignof Wright et al. detected as many as eight pulsation modes with periodsof 0.4 to 2.5 days. Our shallower envelope-only models predict suchperiods for ℓ=0 and 1. However, if the radiative core is included inthe models, a large number of closely-spaced modes are predicted thatare not observed, since a large number of g-type nodes are present inthe eigenfunctions in the deeper interior. This problem occurs also formodels of evolved shell H-burning delta Scuti stars such as 4 CVn.
| New Runaway O-stars Based on Data from HIPPARCOS
12 new runaway O-stars are identified using an analysis of their propermotions based on data from HIPPARCOS. The peculiar tangential and totaltransverse velocities of these stars are determined. A list of theobserved runaway stars is given.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905
| Merged binary white dwarf evolution: rapidly accreting carbon-oxygen white dwarfs and the progeny of extreme helium stars
We have examined the evolution of merged low-mass double white dwarfsthat become luminous helium stars. We have approximated the mergingprocess by the rapid accretion of matter, consisting mostly of helium,on to a carbon-oxygen (CO) white dwarf. After a certain mass isaccumulated, a helium shell flash occurs, the radius and luminosityincrease and the star becomes a yellow giant. Mass accretion is stoppedartificially when the total mass reaches a pre-determined value. Whenthe mass above the helium-burning shell becomes small enough, the starevolves blueward almost horizontally in the Hertzsprung-Russell diagram.The theoretical models for the merger of a 0.6-Msolar COwhite dwarf with a 0.3-Msolar He white dwarf agree very wellwith the observed locations of extreme helium stars in thelogTeff-logg diagram, with their observed rates of bluewardevolution, and with luminosities and masses obtained from theirpulsations. Together with predicted merger rates for CO+He white dwarfpairs, the evolutionary time-scales are roughly consistent with theobserved numbers of extreme helium stars. Predicted surface carbon andoxygen abundances can be consistent with the observed values if carbonand oxygen produced in the helium shell during a previous asymptoticgiant branch phase are assumed to exist in the helium zone of theinitial CO white dwarfs. These results establish the CO+He white dwarfmerger as the best, if not only, viable model for the creation ofextreme helium stars and, by association, the majority of R CoronaeBorealis stars.
| The automatic photometric telescope at the South African Astronomical Observatory.
Not Available
| Secular contraction in extreme helium stars and the future of V4334 Sgr
Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated asthe precursors of extreme helium stars (EHes). The question arises towhether Sakurai's Object will evolve down the [WC]-PG1159 evolutiontrack, or the RCB-EHe-HesdO track. From a number of IUE observations, wehave measured the secular contraction rates and pulsation masses ofseveral EHes. These are inconsistent with the predicted contractionrates for helium-shell burning giants produced by a final helium-shellflash (the [WC]-PG1159 track). Although there may be some similaritiesbetween Sakurai's Object and some RCBs, if the RCB-EHe conection isvalid then these must be superficial rather than structural.
| Variability in the extreme helium star LSS 5121
We report a photometric and spectroscopic study of the hot extremehelium star LSS 5121. We found photometricvariability, but no period was evident in its periodogram. This isconsistent with the previous proposal, based on spectral linevariations, that LSS 5121 is a non-radial pulsator similar to other hotextreme helium stars. Based on observations made at the University ofTexas McDonald Observatory, Fort Davis, Texas and the Jacobus KapteynTelescope operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofisica de Canarias.
| Cyclic and secular variation in the temperatures and radii of extreme helium stars
The ultraviolet properties of 17 extreme helium stars have been examinedusing 150 IUE spectra. Combining short-wave and long-wave image pairsand using a grid of hydrogen-deficient model atmospheres and aχ2 minimization procedure, 70 measurements of effectivetemperature (Teff), angular diameters (θ) andinterstellar extinction (EB_V) were obtained. In most cases,these were in good agreement with previous measurements, but there aresome ambiguities in the case of the hotter stars, where the solutionsfor Teff and EB_V become degenerate, and in thecase of the cooler stars with large EB_V, where the totalflux is no longer dominated by the ultraviolet. The behaviour of 12helium stars was examined over an interval exceeding 10yr. The surfacesof four stars (HD 168476, HD 160641, BD -9°4395 and BD -1°3438)were found to be heating at rates between 20 and 120Kyr-1, inremarkable agreement with theoretical predictions. This result providesthe first direct evidence that extreme helium stars are heliumshell-burning stars of up to ~0.9Msolar contracting towardsthe white dwarf sequence. Low-luminosity helium stars do not show adetectable contraction, also in agreement with theory, although one, BD+10°2179, may be expanding. The short-term behaviour of threevariable helium stars (PV Tel variables: HD 168476, BD +1°4381, LSIV-1°2) was examined over a short interval in 1995. All three showedchanges in Teff and θ on periods consistent withprevious observations. Near-simultaneous radial velocity (v)measurements were used to establish the total change in radius, withsome reservations concerning the adopted periods. Subsequently,measurements of the stellar radii and distances could be derived. WithTeff and surface gravities established previously, stellarluminosities and masses were thus obtained directly from observation. Inthe case of HD 168476, the mass is 0.94 ± 0.68 M\odot.Assuming a similar gravity for LSIV -1°2 based on its neutral heliumline profiles, its mass becomes 0.79 ± 0.46 M\odot.The θ amplitude for BD +1°4381 appears to be overestimated bythe IUE measurements and leads to a nonsensical result. These firstdirect measurements of luminous extreme helium star masses agree wellwith previous estimates from stellar structure and pulsation theory.
| Non-adiabatic linear pulsation models for low-mass helium stars
Non-adiabatic linear pulsation models have been calculated for low-massstars with effective temperatures between 16000 and 35000K, and withsurface gravities in the range 30.3, although the position of the red edge is sensitive to X. Wehave found that non-radial pulsations are also excited in the sameinstability region as radial pulsations. The implications of theseresults for individual low-mass helium stars are discussed. It isconcluded that Z-bump driven pulsations (radial and/or non-radial) maybe excited in some helium-rich subdwarf B stars, representing a possiblemajor extension to the class of variable stars represented by theprototype V652 Her.
| A photometric study of the hydrogen-deficient stars BD+1^ deg4381 (FQ Aqr) and BD-1^ deg3438 (NO Ser)
Photometric monitoring during the period 1993-1997 has been carried outin the UBV system for the cool extreme helium stars BD+1 deg4381 (FQAqr) and BD-1 deg3438 (NO Ser). Periodograms of the data for both starsfor each season of observation show the apparent presence of severalperiods but, if real, none seems to persist for more than one season.Analysis of the full data set for each star reveals no significantperiodicities, although models predict that highly non-adiabaticpulsational variations should occur in hydrogen-deficient stars at thesetemperatures (~ 10 000 K).
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| HIPPARCOS observations of hydrogen-deficient carbon stars
Parallax measurements for 21 hydrogen-deficient carbon stars have beenmade by the Hipparcos satellite. These stars include most of thebrighter R Coronae Borealis (RCB) variables, other coolhydrogen-deficient carbon (HdC) stars, and several higher-temperatureextreme helium (eHe) stars. Most of these stars have either negative orstatistically insignificant parallaxes, indicating that they lie beyondthe detection capability of Hipparcos. Although the distances to thegalactic hydrogen-deficient carbon stars remain unknown, at least theHipparcos observations do confirm that these objects must have highluminosity like the LMC RCB stars, for which M_bol = -4 to -5. Basedupon Hipparcos proper motions, we derive UVW velocities for the RCB andHdC stars, assuming M_bol = -3 and -5. The UW-velocity dispersion of theRCB/HdC stars is similar to that already reported for the eHe stars,further supporting that these groups of stars have predominantly bulgedistributions. However, UW Cen may be a second example of a halo RCBstar currently seen transitting the galactic plane.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. II. Photometry
Abundant {HIPPARCOS photometry over 3 years of 141 O and Wolf-Rayetstars, including 8 massive X-ray binaries, provides a magnificentvariety of light curves at the sigma ~ 1-5% level. Among the mostinteresting results, we mention: optical outbursts in HD 102567 (MXRB),coinciding with periastron passages; drastic changes in the light curveshape of HD 153919 (MXRB); previously unknown long-term variability ofHD 39680 (O6V:[n]pe var) and WR 46 (WN3p); unusual flaring of HDE 308399(O9V); ellipsoidal variations of HD 64315, HD 115071 and HD 160641;rotationally modulated variations in HD 66811=zeta Pup (O4Inf) and HD210839=lambda Cep (O6I(n)fp); dust formation episode in WR 121 (WC9). Ina statistical sense, the incidence of variability is slightly higheramong the WR stars, which might be explained by the higher percentage ofknown binary systems. Among the presumably single WR stars, thecandidate runaways appear to be more variable then the rest. Based ondata from the ESA Hipparcos astrometry satellite
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Spectral analysis of the extreme helium star LSS3184
LSS3184 is a hydrogen-deficient, early B-type giant, recently found topulsate with a period of 2.5 hours. Its photospheric parameters havebeen derived from optical high-resolution spectra by the method of fineanalysis. The principal results are T_eff=23300 K, log g=3.35,{n_H}/{n_He}simle0.00015, {n_C}/{n_He}=0.003, {n_N}/{n_He}=0.0005, and{n_O}/{n_He}=0.0003. Hydrogen is extremely deficient. The effectivetemperature is consistent with broad-band visual and ultravioletspectrophotometry and an extinction E_B-V ~ 0.27. Its previous evolutionis reflected in the chemistry of the atmosphere, which contains enrichednitrogen from CNO-cycle hydrogen burning, and carbon from 3alpha heliumburning. Thus LSS3184 is a true extreme helium star with a compositionsimilar to \bd{-9}{4395}. With T_eff, log g, and pulsation propertiesvery similar to the C-poor and N-rich helium star V652Her, evolutionarymechanisms which can result in very different surface compositions forthe two stars must be examined. Based on observations obtained at theEuropean Southern Observatory, La Silla, Chile, and with the IUEsatellite retrieved from the IUE archive at the World Data Centre,Rutherford Appleton Laboratory, UK
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| On the Origin of Hydrogen-deficient Supergiants and Their Relation to R Coronae Borealis Stars and Non-DA White Dwarfs
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...456..750I
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| A linear analysis of the radial pulsations of H-deficient stars
This paper presents results of a linear analysis of the radialpulsations (oscillations) of H-deficient stars, in which recentlypublished OPAL opacity data were used. The classical kappa mechanismassociated with the Fe opacity peak at T~2x10^5 K extends theinstability region downward (like a `finger') around log T_eff~4.3 inthe Hertzsprung-Russell (HR) diagram. The pulsations of V652 Her and LSS3184, relatively less luminous helium stars, are attributed to theclassical kappa mechanism. The strange modes caused by the opacity peakmake envelopes with T_eff as high as 10^5K overstable if the luminosityis sufficiently high. Pulsating luminous H-deficient stars are locatedin the strange-mode instability region in the HR diagram. Their periodsare more or less consistent with theoretical ones of overstablelow-order radial modes for M~1 to ~0.7 M_solar. The pulsation period ofthe hydrogen-deficient binary upsilon Sgr is found to be consistent withmodels having a mass of ~3 M_solar, although its radius is larger thanthose predicted by evolutionary models for pure helium stars. The massis high enough to be a progenitor of Type Ib supernovae.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| Far-ultraviolet stellar photometry: Fields in Sagittarius and Scorpius
Far-ultraviolet photometry for 741 objects in a field in Sagittariuscentered near M8 and 541 objects in a field centered near sigma Scorpiiis presented. These data were extracted from electographic imagesobtained with two cameras during a shuttle flight in 1991 April/May. Thecameras provided band passes with lambdaeff = 1375 A andlambdaeff = 1781 A. Synthetic colors show that these bandsare sensitive to effective temperature for hot stars. Our measurementswere placed on a quantitative far-ultraviolet magnitude scale byconvolving the spectra of stars observed by IUE with our cameras'spectral response functions. Fifty-eight percent of the ultravioletobjects were identified with visible stars using the SIMBAD databasewhile another 40% of the objects are blends of early type stars tooclose together to separate with our resolution. Our photometry iscompared with that from the TD-1, OAO 2, and ANS satellites and the S201(Apollo 16) far-ultraviolet camera and found to agree at the level of afew tenths of a magnitude. Unlike previous studies, almost half of theidentified visual counterparts to the ultraviolet objects are early Bstars. A plot of distance modulus against ultraviolet color excessreveals a significant population of stars with strong ultravioletexcess.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| High-resolution optical spectroscopy of the hot R CrB star V348 Sagittarii
High resolution optical spectra of the unique hot R CrB star V348 Sgrare presented and more than 500 spectral lines of 14 elements areidentified. The spectrum is very complex and four types of line profilescan be distinguished: a) pure absorption lines, b) P-Cygni lines, c)pure emission lines and d) P-Cygni (HeI) or emission (CII) profiles withcentral absorption reversal. The spectrum is interpreted as that of anexpanding atmosphere of small optical thickness with wind velocitiesbetween 150 and 200 km/s. Hydrogen is definitely present in theexpanding atmosphere. The absorption line spectrum is very similar tothat of the extreme helium stars DY Cen and LSE 78.
| The Diffuse Ionized Interstellar Medium: Structures Resulting from Ionization by O Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...417..579M&db_key=AST
| Mid-ultraviolet and optical photometry of helium stars
Mid-ultraviolet and optical photometric analysis of helium stars arepresented. A linear relation exists between the effective temperaturederived from model atmospheres and (1965-V)0 index. The effectivetemperatures derived from (1965-V)0 index are somewhat higher than thatof MK spectral type estimates especially for late B-type helium objects.
| UBV photometry of OB+ stars in the southern Milky Way
One thousand two hundred and twenty six new observations are combinedwith previously published results of the author to yield an internalyconsistent set of magnitudes and colors on the international UBV systemfor 666 stars classified as OB+ in the Stephenson-Sanduleak OB starsurvey. The U - B, B - V diagram indicates that these stars consistprimarily of O-type stars and early B-type supergiants, reddened by upto E(B - V) = 2.1 mag.
| DY CEN and the hot R Coronae Borealis stars
The first observations of the short-term photometric behavior of the hotR CrB star DY Cen along with spectroscopic data are presented. Analysisof the latter yields nH less than 0.9 nHe and T(eff) = (14,000 + or -1500) K, while the star has been found to exhibit low-amplitude,short-period (3.8-5.5 d) photometric variations superimposed on one ofmuch longer duration (longer than 25 d). Provided DY Cen is extremelyhydrogen-deficient, and these variations may be interpreted as radialpulsations, then L of about 13,000-18,000 solar luminosities and M ofabout 0.71-0.78 solar mass are implied. The long-term variation cannotbe interpreted within the framework of the period-temperature law forhydrogen-deficient stars. The classification criteria and evolutionarystatus of the hot R CrB stars are reviewed and, in this context, it isshown that DY Cen displays marked differences compared to the othermembers, indicating a different evolutionary status for this object. Thestar's physical properties are intermediate between the Extreme Heliumstars and the classical R CrB stars and, hence, may represent a uniquetransition object between these groups.
| The UV spectrum of the WC 11 star CPD -56 deg 8032
UV observations of the WC 11 central star of the compact planetarynebula CPD -56 deg 8032 show that the continuum flux is variable. Thisvariability appears to be caused by a change in the circumstellarextinction properties. The UV energy distribution can be fitted withhelium star models with T(eff) about 25000 K, after correcting for twotypes of reddening: (1) E(B-V) about 0.4 using Seaton's (1979) reddeningcurve applicable to diffuse interstellar medium; (2) E(B-V) about 0.25to 0.30 using the reddening curve defined by the laboratory extinctionproperties of amorphous carbon grains of Bussolletti et al. (1987). Thestellar wind mass loss is estimated from the P-Cygni profile of C II1760.6 from the high resolutions IUE spectrum. Similarities with otherWC 11 stars are discussed.
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