תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The magnetic fields of hot subdwarf stars
Context. Detection of magnetic fields has been reported in several sdOand sdB stars. Recent literature has cast doubts on the reliability ofmost of these detections. The situation concerning the occurrence andfrequency of magnetic fields in hot subdwarfs is at best confused. Aims: We revisit data previously published in the literature, and wepresent new observations to clarify the question of how common magneticfields are in subdwarf stars. Methods: We consider a sample ofabout 40 hot subdwarf stars. About 30 of them have been observed withthe FORS1 and FORS2 instruments of the ESO VLT. Results have beenpublished for only about half of the hot subdwarfs observed with FORS.Here we present new FORS1 field measurements for 17 stars, 14 of whichhave never been observed for magnetic fields before. We also criticallyreview the measurements already published in the literature, and inparticular we try to explain why previous papers based on the same FORS1data have reported contradictory results. Results: All new andre-reduced measurements obtained with FORS1 are shown to be consistentwith non-detection of magnetic fields. We explain previous spuriousfield detections from data obtained with FORS1 as due to a non-optimalmethod of wavelength calibration. Field detections in other surveys arefound to be uncertain or doubtful, and certainly in need ofconfirmation. Conclusions: There is presently no strong evidencefor the occurrence of a magnetic field in any sdB or sdO star, withtypical longitudinal field uncertainties of the order of 2-400 G. Itappears that globally simple fields of more than about 1 or 2 kG instrength occur in at most a few percent of hot subdwarfs. Furtherhigh-precision surveys, both with high-resolution spectropolarimetersand with instruments similar to FORS1 on large telescopes, would be veryvaluable.Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile under observingprogrammes 072.D-0290 and 075.D-0352, or obtained from the ESO/ST-ECFScience Archive Facility.
| The MUCHFUSS project - searching for hot subdwarf binaries with massive unseen companions. Survey, target selection and atmospheric parameters
The project Massive Unseen Companions to Hot Faint Underluminous Starsfrom SDSS (MUCHFUSS) aims at finding sdBs with compact companions likesupermassive white dwarfs (M > 1.0 M&sun;), neutron starsor black holes. The existence of such systems is predicted by binaryevolution theory and recent discoveries indicate that they are likely toexist in our Galaxy. A determination of the orbital parameters issufficient to put a lower limit on the companion mass by calculating thebinary mass function. If this lower limit exceeds the Chandrasekhar massand no sign of a companion is visible in the spectra, the existence of amassive compact companion is proven without the need for any additionalassumptions. We identified about 1100 hot subdwarf stars from the SDSSby colour selection and visual inspection of their spectra. Stars withhigh velocities have been reobserved and individual SDSS spectra havebeen analysed. In total 127 radial velocity variable subdwarfs have beendiscovered. Binaries with high RV shifts and binaries with moderateshifts within short timespans have the highest probability of hostingmassive compact companions. Atmospheric parameters of 69 hot subdwarfsin these binary systems have been determined by means of a quantitativespectral analysis. The atmospheric parameter distribution of theselected sample does not differ from previously studied samples of hotsubdwarfs. The systems are considered the best candidates to search formassive compact companions by follow-up time resolved spectroscopy.Based on observations at the Paranal Observatory of the EuropeanSouthern Observatory for programme number 081.D-0819. Based onobservations at the La Silla Observatory of the European SouthernObservatory for programme number 082.D-0649. Based on observationscollected at the Centro Astronómico Hispano Alemán (CAHA)at Calar Alto, operated jointly by the Max-Planck Institut fürAstronomie and the Instituto de Astrofísica de Andalucía(CSIC). Based on observations with the William Herschel Telescopeoperated by the Isaac Newton Group at the Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias on the island ofLa Palma, Spain.Tables 2-4 and Appendix A are available in electronicform at http://www.aanda.org
| Identification of blue high proper motion objects in the Tycho-2 and 2MASS catalogues using Virtual Observatory tools
Aims: With available Virtual Observatory tools, we looked for newbright blue high proper motion objects in the entire sky: white dwarfs,hot subdwarfs, runaway OB stars, and early-type stars in nearby youngmoving groups. Methods: We performed an all-sky cross-matchbetween the optical Tycho-2 and near-infrared 2MASS catalogues withAladin, and selected objects with proper motions ? > 50 masyr-1 and colours VT - Ks < -0.5 magwith TOPCAT. We also collected multi-wavelength photometry, constructedthe spectral energy distributions and estimated effective temperaturesfrom fits to atmospheric models with VOSA for the most interestingtargets. Results: We assembled a sample of 32 bright blue highproper motion objects, including ten sdO/B subdwarfs, nine DA whitedwarfs, five young early-type stars (two of which are runaway stars),two blue horizontal branch stars, one star with poor information, andfive objects reported for the first time in this work. These last fiveobjects have magnitudes BT ? 11.0-11.6 mag, effectivetemperatures Teff ? 24 000-30 000 K, and are located inthe region of known white dwarfs and hot subdwarfs in a reduced propermotion-colour diagram. We confirmed the hot subdwarf nature of one ofthe new objects, Albus 5, with public far-ultraviolet spectroscopic dataobtained with FUSE.
| Hot subdwarf stars in close-up view. I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions
The origin of hot subdwarf B stars (sdBs) is still unclear. About halfof the known sdBs are in close binary systems for which common envelopeejection is the most likely formation channel. Little is known aboutthis dynamic phase of binary evolution. Since most of the known sdBsystems are single-lined spectroscopic binaries, it is difficult toderive masses and unravel the companions' nature, which is the aim ofthis paper. Due to the tidal influence of the companion in close binarysystems, the rotation of the primary becomes synchronised to its orbitalmotion. In this case it is possible to constrain the mass of thecompanion, if the primary mass, its projected rotational velocity aswell as its surface gravity are known. For the first time we measuredthe projected rotational velocities of a large sdB binary sample fromhigh resolution spectra. We analysed a sample of 51 sdB stars in closebinaries, 40 of which have known orbital parameters comprising half ofall such systems known today. Synchronisation in sdB binaries isdiscussed both from the theoretical and the observational point of view.The masses and the nature of the unseen companions could be constrainedin 31 cases. We found orbital synchronisation most likely to beestablished in binaries with orbital periods shorter than 1.2 d. Only infive cases it was impossible to decide whether the sdB's companion is awhite dwarf or an M dwarf. The companions to seven sdBs could be clearlyidentified as late M stars. One binary may have a brown dwarf companion.The unseen companions of nine sdBs are white dwarfs with typical masses.The mass of one white dwarf companion is very low. In eight cases(including the well known system KPD1930+2752) the companion massexceeds 0.9~M_&sun;, four of which even exceed the Chandrasekhar limitindicating that they may be neutron stars. Even stellar mass black holesare possible for the most massive companions. The distribution of theinclinations of the systems with low mass companions appears to beconsistent with expectations, whereas a lack of high inclinationsbecomes obvious for the massive systems. We show that the formation ofsuch systems can be explained with common envelope evolution and presentan appropriate formation channel including two phases of unstable masstransfer and one supernova explosion. The sample also contains acandidate post-RGB star, which rotates fast despite its long orbitalperiod. The post-RGB stars are expected to spin-up caused by theirongoing contraction. The age of the sdB is another important factor. Ifthe EHB star is too young, the synchronisation process might not befinished yet. Estimating the ages of the target stars from theirpositions on the EHB band, we found PG 2345+318, which is known not tobe synchronised, to lie near the zero-age extreme horizontal branch asare the massive candidates PG 1232-136, PG 1432+159 and PG 1101+249.These star may possibly be too young to have reached synchronisation.The derived large fraction of putative massive sdB binary systems in lowinclination orbits is inconsistent with theoretical predictions. Even ifwe dismiss three candidates because they may be too young and assumethat the other sdB primaries are of low mass, PG 1743+477 and, inparticular, HE 0532-4503 remain as candidates whose companions may havemasses close to or above the Chandrasekhar limit. X-ray observations andaccurate photometry are suggested to clarify their nature. As highinclination systems must also exist, an appropriate survey has alreadybeen launched to find such binaries.Based on observations at the Paranal Observatory of the EuropeanSouthern Observatory for programmes number 165.H-0588(A), 167.D-0407(A),068.D-0483(A), 069.D-0534(A), 070.D-0334(A), 071.D-0380(A),071.D-0383(A) and 382.D-0841(A). Based on observations at the La SillaObservatory of the European Southern Observatory for programmes number073.D-0495(A), 074.B-0455(A) and 077.D-0515(A). Some of the data used inthis work were obtained at the Hobby-Eberly Telescope (HET), which is ajoint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen, for programmes numberUT07-2-004 and UT07-3-005. The HET is named in honor of its principalbenefactors, William P. Hobby and Robert E. Eberly. Based onobservations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC). Some of the data presented here were obtainedat the W.M. Keck Observatory, which is operated as a scientificpartnership among the California Institute of Technology, the Universityof California, and the National Aeronautics and Space Administration.The Observatory was made possible by the generous financial support ofthe W.M. Keck Foundation. Some of the data used in this work wereobtained at the Palomar Observatory, owned and operated by theCalifornia Institute of Technology. Based on observations with theWilliam Herschel Telescope operated by the Isaac Newton Group at theObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias on the island of La Palma, Spain.
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy
Aims.We test the hypothesis that the pulsations in sdB stars arecorrelated with the surface abundances of iron-group elements. Anycorrelation might explain why, when given two spectroscopically similarstars, one will pulsate while the other will not. Methods: .Wehave obtained high-resolution ultraviolet spectra two pulsating andthree non-pulsating sdB stars using the Space Telescope ImagingSpectrograph onboard the Hubble Space Telescope. We determinedabundances for 25 elements including the iron group and even heavierelements such as tin and lead using LTE curve-of-growth and spectrumsynthesis techniques. Results: .We find no clear correlationbetween pulsations and metal abundances, and we comment on the resultingimplications, including whether it is possible to determine thedifference between a pulsating and a non-pulsating sdBspectroscopically. In addition to the main goal of our observations, wehave also investigated the effect of supersolar metallicity onfundamental parameter determination, possible trends with ironabundance, and the hypothesis that weak winds may be selectivelyremoving elements from the stellar envelopes. These effects providechallenges to stellar atmosphere modelling and diffusion models for sdBstars.
| Is there a correlation between metal abundance and pulsation in sdB stars?
Like several other classes of pulsating stars (e.g. the PG 1159 stars),pulsating subdwarf B stars coexist with non-pulsators in the same regionof the Teff-log g diagram. And as with other pulsators, thereason for this is unclear. The pulsations in sdBs are believed to bedriven by the heavy element ionisation (of especially iron) at the baseof the stellar photosphere. It has been suggested that perhaps theamount of iron-group elements at the stellar surface may be adiscriminating factor. We have analysed high-resolution UV echellespectra of 5 sdBs obtained using HST/STIS to test this hypothesis andfind that there is no clear and consistent difference between theabundances of pulsators and non-pulsators. This poses the question: whatis the physical reason for pulsators and non-pulsators to bespectroscopically similar?
| Discovery of Four Radial Velocity Variable Sdb Stars with Eccentric Orbits
Radial velocity curves for 14 bright subdwarf B close binary systemshave been measured using high precision radial velocity measurementsfrom high S/N optical high-resolution spectra. The companions for allsystems are unseen. The periods range from about 0.18 days up to about 6days. The radial velocity semi-amplitudes are found to lie between 15and 130 km s-1. The invisible companions for three of ourprogram stars are undoubtedly white dwarfs. In the other cases theycould be either white dwarfs or main-sequence stars. For two stars thesecondaries could possibly be brown dwarfs. The orbits for most observedsystems are circular. However, small periodic deviations of the measuredradial velocity points from the adopted theoretical sine curve arediscovered for four program stars. These deviations can be perfectlymatched assuming slightly eccentric orbits with small eccentricities ofe = 0.02--0.06. Possibly, this is the first time that non-circularorbits have been detected in sdB binaries.
| Atmospheric Parameters for Subdwarf B Stars: a Consistency Check Between the Balmer Lines and the Far-Ultraviolet Spectrum
The use of Balmer line profiles to determine atmospheric parameters ofsubdwarf B stars is a well-established method that relies on acomparison of spectroscopic observations with synthetic spectragenerated from model atmospheres. This method allows us to determineparameters such as the effective temperature, surface gravity andphotospheric helium abundance. The self-consistency of these parameterscan be investigated by examining the Lyman lines covered by currentfar-ultraviolet observations. We discuss the results of a preliminaryanalysis of a sample of ten subdwarf B stars for which both optical andfar-ultraviolet spectra were secured. At temperatures below 30 000 K wefind good consistency between optical and ultraviolet spectra when someallowance is made for the metal-line blanketing present in theultraviolet region. At higher effective temperatures, however, theconsistency is not as satisfactory. Possible solutions to this puzzleare considered.
| Subluminous O Stars
We report results of spectral analyses of sdO stars selected from theSupernova Ia Progenitor Survey, the Hamburg Quasar Survey and the SloanDigital Sky Survey and based on state-of-the-art NLTE model atmospheres.By combining the sdO with the sdB samples we discuss trends of theatmospheric parameters in order to search for evidence for possibleevolutionary linkage. The He-sdO stars are found to cluster near Teff =45 000 K, log g = 5.5, whereas the number of sdO stars in this area isvery small. The ``cooler'' sdO stars seem to form an extension to thesdB sequences. A couple of sdO stars are obviously evolved from theextended horizontal branch and reach temperatures as high as 80 000 K.We conjecture that the He-sdO stars and sdO/sdB stars have a differentevolutionary origin. This is corroborated by the much lower binaryfrequency of the former. Strong enrichments of iron group elements arediscovered for hydrogen-rich sdO stars as well as for sdB stars fromhigh resolution UV spectra and have severe implications for thetemperature scale. We finally highlight the discovery of ahyper-velocity He-sdO star with a radial velocity of 708 kms unbound tothe Galaxy.
| High resolution spectroscopy of bright subdwarf B stars. I. Radial velocity variables
Radial velocity curves for 15 bright subdwarf B binary systems have beenmeasured using high precision radial velocity measurements from high S/Noptical high-resolution spectra. In addition, two bright sdB stars arediscovered to be radial velocity variable but the period could not yetbe determined. The companions for all systems are unseen. The periodsrange from about 0.18 days up to more than ten days. The radial velocitysemi amplitudes are found to lie between 15 and 130 {km s-1}.Using the mass functions, the masses of the unseen companions have beenconstrained to lower limits of 0.03 up to 0.55 Mȯ, andmost probable values of 0.03 up to 0.81 Mȯ. Theinvisible companions for three of our program stars are undoubtedlywhite dwarfs. In the other cases they could be either white dwarfs ormain sequence stars. For two stars the secondaries could possibly bebrown dwarfs. As expected, the orbits are circular for most of thesystems. However, for one third of the program stars we find slightlyeccentric orbits with small eccentricities of ɛ≈ 0.02 0.06.This is the first time that non-circular orbits have been found in sdBbinaries. No correlation with the orbital period can be found.
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| Detection of a companion to the pulsating sdB Feige 48
We present the discovery of a binary companion to the pulsating sdBFeige 48. Using HST/STIS and archival FUSE spectra, we measure a periodof 0.376±0.003 d and a velocity semi-amplitude of 28.0±0.2km s-1. This implies that the companion star must either beof very low mass, or the orbit is at low inclination. Combining 2MASSfluxes, the lack of a reflection effect, results from asteroseismologyand a measurement of the rotation velocity of Feige 48, we show that theorbital inclination must be ≤11.4o and that the unseencompanion is a white dwarf with mass ≥0.46 Mȯ. Theimplications of this discovery, and of binarity amongst sdB pulsators,is then discussed in the context of recent theoretical work on sdBformation. In particular we suggest that radial velocity studies focuson sdB pulsators with no known companion, and that asteroseismologicalstudies of sdBs investigate a larger mass range than previouslyconsidered in order to test formation models.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| Studying the populations of our Galaxy using the kinematics of sdB stars
We have analysed the kinematics of a sample of 114 hot subdwarf stars.For 2/3 of the stars, new proper motions, spectroscopic and photometricdata are presented. The vast majority of the stars show a kinematicbehaviour that is similar to that of Thick Disk stars. Some stars havevelocities rather fitting to solar, i.e. Thin Disk, kinematics. About˜15 objects have orbital velocities which differ considerably fromthose of Disk stars. These are members of the Galactic Halo. Weinvestigated the velocity dispersions and calculated the orbits. Moststars feature orbits with disk character (eccentricity of less than0.5), a few reach far above the Galactic plane and have very eccentricorbits (eccentricity of more than 0.7). The intermediate eccentricityrange is poorly populated. This seems to indicate that the (Thick) Diskand the Halo are kinematically disjunct. Plotting a histogram of theorbit data points along z leads to the z-distance probabilitydistribution of the star; doing this for the whole sample leads to thez-distance probability distribution of the sample. The logarithmichistogram shows two slopes, each representing the scale height of apopulation. The disk component has a scale height of 0.9 (±0.1)kpc, which is consistent with earlier results and is similar to that ofthe Thick Disk. The other slope represents a component with a scaleheight ˜7 kpc, a much flatter gradient than for the diskcomponent. This shows that the vast majority of the sdBs are disk stars,but a Halo minority is present, too. The kinematic history andpopulation membership of the sdB stars on the whole is different fromthat of the cooler HBA stars, which are predominantly or evenexclusively Halo objects. This leads to the question, whether the HalosdB stars are of similar origin as the HBA stars, or whether theirkinematical behaviour possibly represents another origin, such asinfalling stellar aggregates or inner disk events.Based on data obtained at the 1.54 m telescope of the European SouthernObservatory, runs 58.D-0478, 60.D-0306, 62.L-0220, 64.L-0077,64.H-0023(A), 65.H-0250, 65.H-0341(A) and 68.D-0192(A).
| A Survey for Pulsating Hot B Subdwarfs in the Northern Hemisphere
We present the final results of a survey to search for pulsating hot Bsubdwarf (sdB) stars. We picked our candidates on the basis of theirvalues of Teff and logg, as estimated from availablequantitative optical spectroscopy. With the help of a three-channelphotometer, we gathered ``white-light'' light curves using a uniformsampling time of 10 s for a total of 74 stars. Our survey had a yield of~5.4%, leading to the discovery of detectable luminosity variations infour objects: PG 1047+003, KPD 2109+4401, KPD 1930+2752, and PG0014+067. For the light curves of the other 70 stars, we derive upperlimits on the amplitudes of possible periodic signals (in the periodwindow 20-1000 s) that range from ~0.05% of the mean brightness of thestar at an apparent magnitude of y~12 to ~0.20% at y~15. We use oursurvey results as well as those of others to discuss the location andextent of the empirical instability ``strip'' for pulsating sdB stars inthe logg-Teff diagram. We also address the question of thecoexistence of pulsators and nonvariable stars in the same region ofthat diagram. Based in part on observations gathered at theCanada-France-Hawaii Telescope, operated by the National ResearchCouncil of Canada, the Centre National de la Recherche Scientifique deFrance, and by the University of Hawaii.
| Metal abundances of sdB stars
The surface abundance patterns of 13 sdB stars have been determined fromhigh resolution, high S/N, optical spectra. As typical in early B typestars, the metal lines are few and very weak. The metal abundancepatterns are compared with the atmospheric parameters to search forpossible trends, which are predicted by diffusion theory. However, it isremarkable that some metals (e.g. N, Mg, Fe) have similar abundancesirrespective of the stellar parameters, whereas other metals (e.g. C, O)and the helium abundances vary considerably from star to star. Threepeculiar sdB stars were found which show an enormous enrichment of irongroup elements. An isotopic anomaly of helium has been found in two sdBstars. In these stars 4He is largely replaced by the isotope3He. We discovered additionally four single lined, shortperiod radial velocity variable sdB stars and measured their radialvelocity curves.
| The Montreal-Cambridge-Tololo Survey of Southern Subluminous Blue Stars: The South Galactic Cap
Results for the south Galactic cap region of theMontréal-Cambridge-Tololo survey of blue subluminous stars arepresented. This region overlaps the 840 deg2 region studiedalmost three decades ago by Slettebak & Brundage. We present a list ofequatorial coordinates, photographic photometry, and spectroscopicidentifications, as well as finding charts, for 188 blue objects[(U-B)pg<=-0.6] brighter than Bpg=16.5 in thisarea. Completeness of the survey and comparisons with other similarefforts are discussed.
| Spectrophotometry: Revised Standards and Techniques
The telluric features redward of 6700 Å have been removed from theaccurate spectrophotometric standards of Hamuy et al. to permit morereliable relative and absolute spectrophotometry to be obtained from CCDspectra. Smooth fluxes from 3300 to 10500 Å are best determined bydividing the raw spectra of all objects taken in a night by the rawspectrum of a ``smooth'' spectrum star before deriving the instrumentalresponse function using the revised standard star fluxes. In this waythe telluric features and any large instrumental variation withwavelength are removed from the raw data, leaving smooth spectra thatneed only small corrections to place them on an absolute flux scale.These small corrections with wavelength are well described by alow-order polynomial and result in very smooth flux-calibrated spectra.
| Supplementary southern standards for UBV(RI)c photometry
We present UBV(RI)c photometry for 80 southern red and blue stars foruse as additional standards. The data are tied to the Johnson UBV andCousins (RI)c systems and extend the range of the available stars forcolor equation determination, especially in (U-B) for blue stars and(V-R) and (V-I) for red stars. Comparisons with published data are madeand particularly good agreement is found with Bessell for the red(Gliese) stars.
| Timing the eclipse of HD185510
HD185510 (=V1379 Aql) is an eclipsing double-lined RS CVn binarycontaining a K0 III/IV giant and a hot subluminous companion. AnInternational Ultraviolet Explorer (IUE) ultraviolet light curve hasbeen obtained through the eclipse of the companion with an average timeresolution of one measurement every 19min. This was sufficient toresolve all four times of contact and, with data from previous work onthis system, to derive a solution for the absolute dimensions of thesystem. The atmospheric parameters of both components have beeninvestigated. From its out-of-eclipse flux distribution, the K0 star hasT_eff=4500+/-300K and appears to be metal-deficient. However, thesemeasurements are sensitive to the fractional spot coverage at the timephotometry was obtained. The IUE flux distribution, low-resolutionLyalpha profile and a noisy high-resolution UV spectrum of the hotcompanion have been analysed to obtain T_eff=31500+/-1500K,logg=7.2+/-0.3 and E(B-V)= 0.13+/-0.03. C and Si are deficient by ~1dex; it is not possible to deduce other metal abundances from theexisting data. The spectroscopic gravity is higher than indicated by theeclipse geometry, indicating a weakness in the Lyalpha analysis.Neglecting Lyalpha, an alternative solution of T_eff=31000+/-1500K,logg=605+/-0.2 and E(B-V)= 0.1+/-0.03 is obtained from the orbitalsolution below. The relative radii of the subdwarf and K giant arerelated by the secondary-to-primary ratio r_s/r_p=0. 0058+/-0.0010.These data were used with the eclipse geometry in an attempt to obtainthe orbital inclination. In order to avoid severe contradictions withother diagnostics it was necessary to introduce a non-negligible eclipsedue to the cool star atmosphere. Whilst available data favour i=90+/-5,uncertainties introduced by the atmospheric eclipse meant that i~80 degis also possible. With i=90 deg, the masses of the cool and hot starsare 2.27+/-0.17 and 0.304+/-0.015M_solar respectively. Whilst thehigh-gravity (Lyalpha) solution indicates the identification of the hotstar as a helium white dwarf, the orbital solution favours anidentification with sdOB stars such as SB707. A final resolution of thenature of the hot star is limited partly by data quality but also, to alarge extent, by the intrinsic properties of the system itself.
| Studies of hot B subdwarfs. 9: Cousins BVRI photometry and the binary fraction of hot, hydrogen-rich subdwarfs in the Palomar-Green survey
Cousins BVRI photometry is presented for a sample of 100 hot,hydrogen-rich subdwarf candidates selected principally from thePalomar-Green (PG) and Kitt Peak-Downes colorimetric surveys. Thecombination of these data with existing Stroemgren photometry permitsthe construction of energy distributions in the range lambda lambda3500-7900. With the help of flux-ratio diagrams, we have isolated 31objects with flat energy distributions indicative of a composite nature.Approximate effective temperatures and spectral types are derived forthe primary and secondary stars, respectively. We present evidence,based on the absolute visual magnitudes determined for the primarycomponents, that the secondary stars in most systems are likely to beoverluminous compared to normal main-sequence stars. Because ofselection effects, the spectral types of the secondary stars arerestricted to the range late G to early M. When allowance is made forthese selection effects, the fraction of hot hydrogen-rich subdwarfswith main-sequence companions in the PG survey can be inferred, and isestimated to be approximately 54%-66%.
| Radial velocities and spectral types for a sample of faint blue stars.
Not Available
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| Studies of hot B subdwarfs. III - Carbon, nitrogen, and silicon abundances in three sdB stars
Optical and high-dispersion IUE observations of three hot B subdwarfs(UV 1758 + 36, Ton S-227, and Feige 65) are presented. These data areanalyzed with model atmosphere techniques, and element abundances for C,N, and Si are derived. The abundances are either near (N) or below (C,Si) the solar value; large variations (1) in the extent ofunderabundances of carbon and silicon among the objects, as well as (2)in the abundances (with respect to the solar values) characterizing eachstar are observed. A preliminary interpretation of the observedvariations in these and other hot subdwarfs in terms of radiative forcesdisrupting the downward diffusion of heavy elements is presented.
| Analysis of high-depression IUE spectra of subdwarf B stars
High-dispersion IUE spectra of the subdwarf B stars UV 1758+36 and TonS-227 are presented. These spectra are characterized by the presence ofa large number of photospheric low- and medium-excitation lines fromnumerous ions. The lines of C, N, and Si are used in conjunction withlocal thermodynamic equilibrium (LTE) metal line calculations to derivepreliminary abundances for these elements. These results are comparedand contrasted with those obtained or the hotter OB and O subdwarfs.
| Studies of hot B subdwarfs. I - Stromgren photometry of the Greenstein-Sargent-sample
Stromgren colors for a sample of 25 hot B subdwarfs are presented. Thesestars form a well-defined sequence in a two-color diagram, the existenceof which is shown to be consistent with the evolutionary scenario forsdB stars proposed by Wesemael et al. (1982). A comparison of thephotometric data with appropriate model atmosphere calculations suggeststhat hot B subdwarfs have effective temperatures ranging from 25,000 Kto upward of 40,000 K, and surface gravities generally above log g =5.0.
| The atmosphere of subluminous B stars
The spectra of eight high galactic latitude B stars, suspected to besubluminous, are analyzed for effective temperature, gravity, and heliumto hydrogen ratio. Model atmospheres include (LTE) UV line blanketing(for T/eff/ less than or equal to 30,000 K) and NLTE effects (for T/eff/greater than 30,000 K). Among the eight confirmed subdwarfs, six havetemperatures around 26,000 K. Two objects are considerably hotter(34,000 K and 41,000 K) and belong to the sdOB subclass. Gravity rangesfrom log g = 5.0 to 5.5 for the sdB's, the sdOB's having log g = 5.7 and6.0, respectively. Helium is strongly depleted in the sdB's and sdOB's:from He I 4471 (if present) the number fraction obtained is 0.0016n(He)/n(H) = 0.0016-0.016. From the g, T(eff)-diagram it is concludedthat the subluminous B stars and sdOB stars are (generalized) heliummain sequence stars with approximately 0.5 solar mass. The heliumdeficiency of the envelope is brought about by diffusion.
| Spectral Analysis of High-Dispersion IUE Data of Two Hot Subdwarfs B Stars
Not Available
| Lowell Proper Motion Survey - Summary Catalogue of GD and GR Stars
Not Available
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | לויתן |
| התרוממות ימנית: | 01h43m48.54s |
| סירוב: | -24°05'10.2" |
| גודל גלוי: | 11.375 |
| תנועה נכונה: | 80.2 |
| תנועה נכונה: | -47.8 |
| B-T magnitude: | 11.461 |
| V-T magnitude: | 11.383 |
קטלוגים וכינוים:
|
