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New γ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103
A growing number of early Be stars exhibit X-ray luminosities that areintermediate between those typical of early type stars and those emittedby most Be/X-ray binaries in quiescence. We report on XMM-Newtonobservations of two such Be stars, SAO 49725 andHD 161103, which were originally discovered in asystematic cross-correlation between the ROSAT all-sky survey andSIMBAD. The new observations confirm the X-ray luminosity excess(LX ~ 1032-33 erg s-1) and the hardnessof their X-ray spectra, which are both unusual for normal early typestars. An iron Kα complex is clearly detected in HD161103 in which the H-like, He-like, and fluorescentcomponents are resolved, while strong evidences also exist for thepresence of similar features in SAO 49725. X-rayspectra can be equally well-fitted by a thermal plasma (mekal) with T ~108 K and solar abundances or by a power law + iron linemodel with photon index ~1.5-1.8, both with a soft thermal componentwith T ~ 107 K. The intensity of the fluorescence 6.4 keVline reflects the presence of large amounts of cold material close tothe X-ray sources and strongly argues against accretion onto a companionneutron star in a large orbit. On the other hand, the probable thermalorigin of the X-ray emission, as supported by the ionised iron lines,disagrees with those observed in all known Be/X-ray binaries, in which anon-thermal component is always required. Remarkably, the X-ray featuresare similar to those of white dwarves in several cataclysmic variables.There is no evidence of high frequency pulsations in neither of the twosystems. However, a large oscillation in the light curve of HD161103 with P ~ 3200 s is readily detected. The X-ray lightcurve of SAO 49725 exhibits clear variability by ~80%on time scales as short as ~1000 s. New optical observations provideupdated spectral types (B0.5 III-Ve) and disclose a dense, large, andapparently stable circumstellar disc for both stars. The nature of theexcess X-ray emission is discussed in light of the models proposed forγ Cas, magnetic disc-star interaction, oraccretion onto a compact companion object - whether neutron star orwhite dwarf. These two new objects, added to similar cases discovered inXMM-Newton surveys, point to the emergence of a new class ofγ Cas analogs.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Magnetic survey of bright northern main sequence stars
The first results of a systematic search for magnetic fields in thebrightest upper main sequence (MS) stars are presented. The main goal isto survey the stars with about the same detection limit and to improveexisting statistics of their magnetism. The target list contains 57upper MS stars and represents well B0.5-F9 stars. High-resolution Zeemanspectra were obtained for 30 stars of the list. The accuracy of themagnetic field measurements ranges from 20 to 300 G depending mainly onspectral class. In the majority of studied stars we did not detectmagnetic fields. In some stars we suspect the presence of a weakmagnetic field. These are the best candidates for more extensivestudies. A particular case is the star chi Dra where we probablydetected the global magnetic field. The longitudinal field strength isB_l= -54+/-12 G. Further observations of this star are needed to confirmthe detection and to ascertain if the magnetic field is variable withthe period of rotation. Based on observations collected at the 1 mtelescope of the Special Astrophysical Observatory (Nizhnij Arkhyz,Russia).
| Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org
| Search for low-mass PMS companions around X-ray selected late B stars
We have observed 49 X-ray-detected bright late B-type dwarfs to searchfor close low-mass pre-main sequence (PMS) companions using the EuropeanSouthern Observatory's ADONIS (Adaptive Optics Near Infrared System)instrument. We announce the discovery of 21 new companions in 9binaries, 5 triple, 4 quadruple system and 1 system consisting of fivestars. The detected new companions have K magnitudes between 6.5m and17.3m and angular separations ranging from 0.12 arcsec to 14.1 arcsec(18-2358 AU). Based on observations obtained at the European SouthernObservatory, La Silla, Chile (ESO programme No.~62.I-0477, and Swiss70~cm photometric telescope).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Extreme Ultraviolet Astronomy
Astronomical studies in the extreme ultraviolet (EUV) band of thespectrum were dismissed during the early years of space astronomy asimpossible, primarily because of the mistaken view that radiation inthis band would be absorbed by the interstellar medium. Observations inthe 1980s from sounding rockets and limited duration orbital spacecraftbegan to show the potential of this field and led to the deployment oftwo spacecraft devoted to EUV astronomy: the UK Wide Field Camera andthe Extreme Ultraviolet Explorer. The instrumentation in these missions,although quite limited in comparison with instrumentation in otherfields of space astronomy, provided unique and far-reaching results.These included new information on solar system topics, stellarchromospheres and corona, white dwarf astrophysics, cataclysmicvariables, the interstellar medium, galaxies, and clusters of galaxies.We summarize these findings herein.
| What Is the Nature of the Spectroscopic Companion of the Early B Star λ Scorpii?
The B star λ Sco is known to be a spectroscopic binary system.The companion, which is in a short periodic orbit with the B star, is sofar unknown. X-ray observations with ROSAT by Berghöfer et al. in1997 have shown a super-soft X-ray excess, which is unusual for B starsof spectral type B1.5 IV. Here we present an analysis of our longExtreme Ultraviolet Explorer (EUVE) observation of λ Sco. Basedon these data and all available X-ray observations of this star, weconstrain the physical parameters of the companion. As long as no otherexplanation is available for the EUV/soft X-ray excess, thespectroscopic companion of λ Sco is most likely an ultramassivewhite dwarf. The primary B star is thus the most massive star known tohave a white dwarf companion. Such a stellar system can have evolvedonly by mass transfer. Stellar evolution scenarios predict the existenceof such binary systems, which are expected to be precursors of theultrasoft X-ray sources and which finally explode in a supernova TypeIa. The EUV light curve of λ Sco shows significant short-termvariations on a 20% level. A period-folding search carried out tofurther investigate the EUV light curve of λ Sco does not provideclear evidence for any periodicity present in the data. It is worthwhileto mention that the analysis of variance periodogram shows a 2 σfeature at 4.7 cycles days-1, which is close to the mainpulsation frequency of the β Cep-type B star. Further observationshave to confirm the existence of such a periodicity in the EUV lightcurve. Furthermore, when folded with the orbital period, the EUV lightcurve of λ Sco shows two broader dips of 30% intensity loss atphases φ=0.56 and 0.11. At these two phases, the stars pass eachother in the line of sight. We discuss these features in the EUV lightcurve of λ Sco in terms of orbital dependent changes in theabsorption column of the primary's wind along the line of sight towardthe white dwarf companion.
| Spectroscopic confirmation of a white dwarf companion to the B star 16 Dra
Using an Extreme Ultraviolet Explorer (EUVE) spectrum, we confirm theidentification of a white dwarf companion to the B9.5V star 16 Dra(HD150100), and constrain its surface temperature to lie between 29,000K and 35,000 K. This is the third B star + white dwarf non-interactingSirius-type binary to be confirmed, after y Pup (HR2875, HD59635) andtheta Hya (HR3665, HD79469). 16 Dra and its white dwarf companion aremembers of a larger resolved proper motion system including the B9V star17 Dra A (HD150117). The white dwarf must have evolved from a progenitormore massive than this star, M_MS~3.7M_sun. White dwarf companions to Bstars are important since they set an observational limit on the maximummass for white dwarf progenitors, and can potentially be used toinvestigate the high mass ends of the initial-final mass relation andthe white dwarf mass-radius relation.
| An analysis of the Johnson et al. Catalina UBVRI photometry for second order extinction effects.
Not Available
| Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections
We present the results of high-resolution spectroscopic, low-resolutionspectrophotometric and spectropolarimetric and broad-band multicolourobservations of four B-type stars (HD 4881, 5839, 224648 and 179218) andtwo A-type stars (HD 32509 and 184761) with strong far-infrared (IR)excesses. The excess in HD 184761, which is located at a distance of 65pc from the Sun, was recognized for the first time. Double-peakedHα emission line profiles are found in HD 4881 and HD 5839, whileHD 184761, HD 224648 and HD 32509 display no emission in Hα. Theremarkable variations observed in the Hα profile of HD 179218 arealso observed in some classical Be and Herbig Ae/Be stars. An intrinsiccomponent of polarization is clearly present in HD 179218, only aninterstellar component is detected in HD 4881 and HD 224648, and HD184761 was found to be unpolarized. Improved effective temperatures forall six objects were derived. Parallaxes measured by the Hipparcossatellite were used to determine positions of the stars in the HRdiagram. HD 4881 and HD 5839 are an order of magnitude more luminousthan main-sequence stars of similar temperatures and are most likelynewly discovered classical Be stars. Study of the high-resolution IRASmaps and modelling of the spectral energy distributions of HD 4881, HD5839 and HD 224648 suggest that the observed large IR excesses arecaused by radiation from circumstellar dust rather than free--freeradiation or infrared cirrus, so they may be higher mass counterparts ofbeta Pictoris stars. HD 32509, HD 224648 and HD 184761, which have verysmall near-IR excesses, are probably young main-sequence stars. HD179218, which exhibits the largest near- and far-IR excess in thesample, is an isolated pre-main-sequence Herbig Be star.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Barnard's Merope Nebula (IC 349): an Interstellar Interloper
Barnard's Merope Nebula (IC 349) is the optically brightest portion ofthe diffuse nebulosity that envelops the Pleaides but is notmorphologically similar to those nebulae. Knowledge of its true spacemotion can help clarify whether the object has a kinematic associationand possibly a common origin with the Pleiades. Here we report a meanradial velocity result obtained in 1996 from spectra where v_hel=-44.4km s^-1 and sigma_v=5.42 km s^-1 (N=5). The radial velocity result ispresented along with recent values for the object's proper motion,yielding its space motion vector. Galactic space velocity components (U,V, W)=(50.6+/-5.3, -10.3+/-6.7, 11.3+/-6.4) km s^-1, referred to theLSR, were calculated for the object. In addition, the region wasobserved in the near-infrared to determine if a protostellar object ispresent within the dusty envelope of the nebula; to an equivalentluminosity upper limit of L=0.23+/-0.05 L_solar, none was observed.These results suggest that IC 349 is kinematically unrelated to thePleiades and that it does not harbor a protostellar object in its dustyinterior.
| Theta Hya: spectroscopic identification of a second B star+white dwarf binary
We report the identification, in an Extreme Ultraviolet Explorer (EUVE)spectrum, of a hot white dwarf companion to the 3rd magnitude late-Bstar theta Hya (HR3665, HD79469). This is the second B star+white dwarfbinary to be conclusively identified; Vennes et al. (1997), and Burleigh& Barstow (1998) had previously reported the spectroscopic discoveryof a hot white dwarf companion to the B5V star y Pup (HR2875). Sincethese two degenerate stars must have evolved from main sequenceprogenitors more massive than their B star companions, they can be usedto place observational lower limits on the maximum mass for white dwarfprogenitors, and to investigate the upper end of the initial-final massrelation. Assuming a pure hydrogen composition, we constrain thetemperature of the white dwarf companion to theta Hya to lie between25,000K and 31,000K. We also predict that a third bright B star, 16 Dra(B9.5V), might also be hiding an unresolved hot white dwarf companion.
| HR 2875 - Spectroscopic discovery of the first B star + white dwarf binary
We report the discovery, in an Extreme Ultraviolet Explorer (EUVE)shortwavelength spectrum, of an unresolved hot white dwarf companion tothe 5th magnitude B5Vp star HR 2875. This is the first time that anoninteracting white dwarf + B star binary has been discovered;previously, the earliest type of star known with a white dwarf companionwas Sirius (A1V). As the white dwarf must have evolved from amain-sequence progenitor with a mass greater than that of a B5V star(6.0 solar masses or greater), this places a lower limit on the maximummass for white dwarf progenitors, with important implications for ourknowledge of the initial-final mass relation. Assuming a pure-hydrogenatmospheric composition, we constrain the temperature of the white dwarfto be between 39,000 and 49,000 K. We also argue that this degeneratestar is likely to have a mass significantly greater than the mean massfor white dwarf stars (0.55 solar mass). Finally, we suggest that otherbright B stars (e.g., Theta Hya) detected in the extreme ultravioletsurveys of the ROSAT Wide Field Camera and EUVE may also be hiding hotwhite dwarf companions.
| Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions
We present an analysis of optical, ultraviolet, and X-ray spectralproperties of a sample of 13 hot hydrogen-rich (DA) white dwarfs, eachpaired with a luminous unresolved companion. Using low-dispersionInternational Ultraviolet Explorer spectra, ROSAT photometry, andExtreme-Ultraviolet Explorer photometry and spectroscopy, we estimatethe effective temperature, mass, and distance of the white dwarfs.Additionally, we examine the question of their atmospheric composition.We establish orbital properties for most binaries by means ofhigh-dispersion optical spectroscopy obtained with the Hamilton echellespectrograph at Lick Observatory; the same data help uncover evidence ofactivity in some of the secondary stars that is also notable in ROSATX-ray measurements. In particular, we find high-amplitude (>20 kms-1) velocity variations in only two stars (HD 33959C and HR 8210),low-amplitude variations in four additional objects (HD 18131, HR 1608,theta Hya, and BD +27 deg1888), and no variations (<2 km s-1) in theremainder. We have observed Ca H and K in emission in four (BD +08deg102, HD 18131, HR 1608, and EUVE J0702+129) of the six objects thatwere also detected in the 0.52-2.01 keV ROSAT PSPC band, while thesource of the hard X-ray emission in HD 33959C remains unknown; otherinvestigators have noted some evidence of activity in the remaining0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs arealso investigated; EUV spectroscopy shows the effect of a low heavyelement abundance in the atmosphere of the white dwarf in HD 33959C andof a high heavy element abundance in HD 223816; measurements of allother objects are apparently consistent with emission from pure-hydrogenatmospheres. However, current data do not constrain well the white dwarfparameters, and, to remedy the situation, we propose to obtainspectroscopy of the complete H Lyman line series.
| Delta a and Stroemgren photometry of stars in the Renson-catalogue of AP and AM stars
We have observed 131 stars of \cite[Renson's (1991)]{re91} catalogue ofAp and Am stars both in the Stroemgren & Maitzen's (1976) Delta asystem as a contribution to the photometric studies of the lambda 5200broad band flux depression feature in chemically peculiar stars. Withfew exceptions the probability grouping of Renson for membership in theCP2 group of peculiar stars is nicely reflected by peculiar values ofDelta a. Comparison with already available Delta a values yieldsslightly larger values due to a minor shift in the filter g_1 samplingthe depression. As found by \cite[Maitzen & Vogt (1983)]{ma83} theGeneva system peculiarity parameters correlate well with Delta a. Thisstudy demonstrates the advantageous performance of a photoelectricphotometer with a rapidly rotating filter wheel moving in a stop and gomode. Tables 3 and 4 are also available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A spectroscopic study of some suspected chemically peculiar stars
Time-resolved spectra in the 410 - 700 nm range have been obtained toinvestigate 10 stars which are classified as suspected chemicallypeculiar stars in The General Catalogue of Ap and Am stars by Renson etal. (1991) or in the latest literature. Among the studied stars, HD20629presents metal abundances which are significantly different from mainsequence star values and it should be classified as Silicon star. Fromthe Hei587.6 nm line, we conclude that HD35575 is a helium weak star.Because of the found radial velocity variability and the derivedabundances, the periodic photometric variable HD38602 could be anellipsoidal variable.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| An Optical Atlas of Extreme Ultraviolet Explorer (EUVE) Sources
The Extreme Ultraviolet Explorer (EUVE) has been detecting EUV sourcessince its launch in June 1992. Positions of 540 sources have been madeavailable to the community by the EUVE team. We have extracted 7' X 7'images centered on these 540 EUVE sources from the Space TelescopeScience Institute digitized sky archives. We present these images asmosaic finder charts to aid observers trying to identify EUVE sources,or to characterize known sources. (SECTION: Atlases)
| A search for hidden white dwarfs in the ROSAT extreme ultraviolet survey
The ROSAT Wide Field Camera survey has provided us with evidence for theexistence of a previously unidentified sample of hot white dwarfs (WDs)in non- interacting binary systems, through the detection of extremeultraviolet (EUV) and soft X-ray emission. These stars are hidden atoptical wavelengths because of their close proximity to much moreluminous main- sequence (MS) companions (spectral type K or earlier).However, for companions of spectral type ~A5 or later, the white dwarfsare easily visible at far-UV wavelengths, and can be identified inspectra taken by the International Ultraviolet Explorer (IUE). Eleven WDbinary systems have previously been found in this way from ROSAT,Extreme Ultraviolet Explorer (EUVE) and IUE observations. In this paperwe report the discovery of three more such systems through ourprogrammes in recent episodes of IUE. The new binaries are HD 2133, REJ0357+283 (the existence of which was predicted by Jeffries, Burleigh& Robb in 1996), and BD+27 deg1888. In addition, we haveindependently identified a fourth new WD+MS binary, RE J1027+322, whichhas also been reported in the literature by Genova et al., bringing thetotal number of such systems discovered as a result of the EUV surveysto 15. We also discuss here six stars which were observed as part of theprogramme, but for which no white dwarf companion was found. Four ofthese are coronally active. Finally, we present an analysis of theWD+K0IV binary HD 18131, which includes the ROSAT PSPC X-ray data.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | נחש מים |
| התרוממות ימנית: | 09h14m21.90s |
| סירוב: | +02°18'51.0" |
| גודל גלוי: | 3.88 |
| מרחק: | 39.463 פארסק |
| תנועה נכונה: | 127.2 |
| תנועה נכונה: | -309.3 |
| B-T magnitude: | 3.803 |
| V-T magnitude: | 3.853 |
קטלוגים וכינוים:
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INHABITED SKY
