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Ýlgili Makaleler
Long-Term Optical Light Variations of the Peculiar Massive Runaway Star HD 108
The results of seventeen-year-long observations of three-colourBVR-photoelectric photometry of the peculiar Ofp star HD 108 arereported. New observations show a steady decrease of optical lightduring last 10 years.
| Identification of the Infrared Counterpart to a Newly Discovered X-Ray Source in the Galactic Center
We present first results of a campaign to find and identify new compactobjects in the Galactic center. Selecting candidates from a combinationof Chandra and 2MASS survey data, we search for accretion disksignatures via infrared spectroscopy. We have found the infraredcounterpart to the Chandra source CXO J174536.1-285638, the spectrum ofwhich has strong Brγ and He I emission. The presence of C III, NIII, and He II indicate a binary system. We suspect that the system issome form of high-mass binary system, either a high-mass X-ray binary ora colliding-wind binary.
| Chlorine in the Galactic Interstellar Medium: Revised f-Values with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph
Cl I is the atomic species most directly coupled to molecular hydrogendue to its chemistry. Its weakest lines are thereby probably the besttracer of optically thick H2 components in diffuse clouds. Wereport on the empirical determination of the oscillator strengths forfour Cl I absorption lines predicted to be weak and often detectedtoward moderately reddened sight lines observed with the Far UltravioletSpectroscopic Explorer (FUSE). We compared our oscillator strengthestimates with the oscillator strength calculations listed in Morton. Wefind that our empirical oscillator strength values for the Cl I 1004,1079, 1090, and 1094 Å lines differ from the theoreticalpredictions by factors of ~3.1, 1.2, 2.4, and 0.42, respectively. Webriefly discuss the value of Cl I as tracer of molecular gas for ourstar sample.Based on observations made with the NASA/ESA Hubble Space Telescope(HST), obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| New light on the peculiar star HD 108
HD 108 is a peculiar massive star of type Of?p, which present recurrentline profile variations. Archival data confirm the very long (50-60 yr)period of this star. Recent data show that the minimum state is not yetreached.
| Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit
Massive X-ray binaries are usually classified by the properties of thedonor star in classical, supergiant and Be X-ray binaries, the maindifference being the mass transfer mechanism between the two components.The massive X-ray binary 4U 2206+54 does not fit inany of these groups, and deserves a detailed study to understand how thetransfer of matter and the accretion on to the compact object takeplace. To this end we study an IUE spectrum of the donor and obtain awind terminal velocity (v_&infy;) of ~350 km s-1, which isabnormally slow for its spectral type. We also analyse here more than 9years of available RXTE/ASM data. We study the long-term X-rayvariability of the source and find it to be similar to that observed inthe wind-fed supergiant system Vela X-1, reinforcingthe idea that 4U 2206+54 is also a wind-fed system.We find a quasi-period decreasing from ~270 to ~130 d, noticed inprevious works but never studied in detail. We discuss possiblescenarios for its origin and conclude that long-term quasi-periodicvariations in the mass-loss rate of the primary are probably drivingsuch variability in the measured X-ray flux. We obtain an improvedorbital period of P_orb=9.5591±0.0007 d with maximum X-ray fluxat MJD 51856.6±0.1. Our study of the orbital X-ray variability inthe context of wind accretion suggests a moderate eccentricity around0.15 for this binary system. Moreover, the low value of v_&infy; solvesthe long-standing problem of the relatively high X-ray luminosity forthe unevolved nature of the donor, BD +53°2790,which is probably an O9.5 V star. We note that changes in v_&infy;and/or the mass-loss rate of the primary alone cannot explain thedifferent patterns displayed by the orbital X-ray variability. Wefinally emphasize that 4U 2206+54, together withLS 5039, could be part of a new population ofwind-fed HMXBs with main sequence donors, the natural progenitors ofsupergiant X-ray binaries.
| Isolated, Massive Supergiants near the Galactic Center
We have carried out a pilot project to assess the feasibility of usingradio, infrared, and X-ray emission to identify young, massive starslocated between 1 and 25 pc from the Galactic center. We first comparedcatalogs compiled from the VLA, Chandra, and 2MASS. We identified twomassive, young stars: the previously identified star that is associatedwith the radio H II region H2 and a newly identified star that we referto as CXOGC J174516.1-290315. The infrared spectra of both stars exhibitvery strong Brγ and He I lines and resemble those of massivesupergiants that have evolved off of the main sequence but not yetreached the Wolf-Rayet phase. We estimate that each star has abolometric luminosity >~106 Lsolar. These twostars are also associated with bright mid-infrared sources from the MSXsurvey, although the origin of this emission is uncertain. Likewise, thedetection of these two sources in X-rays is surprising because stars atsimilar evolutionary states are not uniformly bright X-ray sources.Therefore, we suggest that both stars are in binary systems that containeither OB stars whose winds collide with those of the luminoussupergiants or compact objects that are accreting from the winds of thesupergiants. We also identify X-ray emission from a nitrogen-typeWolf-Rayet star and place upper limits on the X-ray luminosities ofthree more evolved, massive stars that previously have been identifiedbetween 1 and 25 pc from Sgr A*. Finally, we briefly discuss theimplications that future searches for young stars will have for ourunderstanding of the recent history of star formation near the Galacticcenter.
| A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitudelight curve with a period near 540 days has recently been reported fromHipparcos photometry. This period satisfies all of the spectroscopysince at least 1982, including extensive new observations during 2003and 2004, and it has predicted the next transition during 2004September-October. Measurements of the Hα equivalent width reveala sharp emission peak in the phase diagram, in contrast to theapparently sinusoidal light curve. The He II absorption-line strength isessentially constant, while He I varies strongly, possibly filled in byemission in the O6 state, thus producing the apparent spectral typevariations. The O8 state appears to be the ``normal'' one. Twointermediate O7 observations have been obtained, which fall at theexpected phases, but these are the only modern observations of thetransitions so far. The period is too long for rotation or pulsation;although there is no direct evidence as yet for a companion, a model inwhich tidally induced oscillations drive an enhanced wind nearperiastron of an eccentric orbit appears promising. Further observationsduring the now predictable transitions may provide a critical test.Ultraviolet and X-ray observations during both states will likely alsoprove illuminating.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| HD 108: The mystery deepens with XMM-Newton observations
In 2001, using a large spectroscopic dataset from an extensivemonitoring campaign, we discovered that the peculiar Of star HD 108displayed extreme line variations. This strange behaviour could beattributed to a variety of models, and an investigation of the highenergy properties of HD 108 was needed to test the predictions fromthese models. Our dedicated XMM-Newton observation of HD 108 shows thatits spectrum is well represented by a two temperature thermal plasmamodel with kT10.2 keV and kT21.4 keV. Inaddition, we find that the star does not display any significantshort-term changes during the XMM-Newton exposure. Compared to previousEinstein and ROSAT detections, it also appears that HD 108 does notpresent long-term flux variations either. While the line variationscontinue to modify HD 108's spectrum in the optical domain, the X-rayemission of the star appears thus surprisingly stable: no simple modelis for the moment able to explain such an unexpected behaviour. Thanksto its high sensitivity, the XMM-Newton observatory has also enabled theserendipitous discovery of 57 new X-ray sources in the field of HD 108.Their properties are also discussed in this paper.Based on observations collected at the Observatoire de Haute-Provence(France) and with XMM-Newton, an ESA Science Mission with instrumentsand contributions directly funded by ESA Member States and the USA(NASA).
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Observational Data on Galactic Supernova Remnants: II. The Supernova Remnants Within l = 90^o - 270^o
We have collected all the available data on Galactic supernova remnantsgiven in the literature. The data of Galactic supernova remnants locatedin the Galactic longitude interval l=90^o-270^o in all spectral bandsare represented in this work. We have adopted distance values for theSNRs by examining these data. The data of various types on neutron starsconnected to these supernova remnants are also represented. Remarks ofsome authors and by ourselves regarding the data and some properties ofboth the supernova remnants and the point sources are given.
| The Remarkable Alternating Spectra of the Of?p Star HD 191612
The spectrum of HD 191612 has been found to display large, recurrentvariations between two highly reproducible, peculiar states; at leastfour transformations have occurred since 1950. In one state, thespectral type is O6-O7, with C III λ4650 emission comparable to NIII λ4640 (the definition of the Of?p category) and P Cygniprofiles at He II λ4686 and Hα. In the other state, thespectral type is O8, with the C III emission absent, very strong N IIIλ4097 absorption, broad He II λ4686 absorption with narrowcentral emission (a profile that may be unprecedented in this line amongknown O-type spectra), and a broad asymmetrical absorption at Hα.One observing sequence over several consecutive nights shows no spectralvariations, practically ruling out a short-period, interacting binary asthe origin of the phenomenon; moreover, no significant radial velocityvariations have been found. Although the sporadic observational recordprior to the discovery of the variations in early 2001 precludesdefinite conclusions, it is possible that a given state is maintainedfor a decade or longer, but one transformation occurred within 13months, and the data obtained during 2002 suggest an event with ashorter timescale.The Of?p category currently contains only five members: three in theGalaxy and two in the Small Magellanic Cloud. The other two Galacticmembers also display bizarre and unexplained phenomena; in the case ofHD 108, they are strikingly similar to those described here. Because oftheir relatively high X-ray luminosities, all three Galactic objectshave been suggested to have collapsed companions. If the spectralvariations of HD 108 and HD 191612 are due to binary interactions, theyare likely multiyear, eccentric systems like WR 140 and η Carinae.The axisymmetric shell ejections of HD 148937 could have a similarorigin. Alternatively, these stars may be rapid rotators or in anunstable evolutionary transitional stage. Further intensivespectroscopic monitoring is required to reveal their nature.
| Spectropolarimetry of O supergiants
We present medium-resolution spectropolarimetry at high signal-to-noiseratio of the Hα emission line of 20 O-type supergiants. Five stars(25 per cent) of the sample show a statistically significant change inpolarization through the line. We combine our Hα data with newK-band spectropolarimetry and archival low-resolution opticalspectropolarimetry to determine the polarigenic mechanism in the starsthat show a line effect. We show that the line polarization change inthe binary systems is caused by the classical `dilution' mechanism, inwhich the Hα emission is essentially unpolarized and the continuumpolarization is caused by intrabinary scattering. We find that the lineeffect in HD 108 is also well modelled by pure dilution, but suggestthat the continuum polarization is the result of stochastic windclumping. A similar description applies to the continuum polarization ofHD 188001, although the line effect cannot be reproduced by puredilution. We use low-resolution spectropolarimetry to determine theinterstellar polarization vector to λ Cephei, and confirm thatthe intrinsic polarization of the object is very low (<0.1 per cent,corresponding to an equator:pole density ratio of <1.25). The linepolarization of this star is modelled using the TORUS three-dimensionalradiative-transfer code. We show that the line effect is a consequenceof symmetry breaking caused by the rapid rotation of the system (>200km s-1), and that the system is similar polarimetrically tothe O4 supergiant ζ Puppis. Finally, we note that the precision ofcurrent photo- and spectro-polarimetric observations is insufficient totest structured wind models, which predict a continuum polarization of~0.1 per cent.
| Empirical calibration of the near-infrared Ca II triplet - III. Fitting functions
Using a near-infrared stellar library of 706 stars with a wide coverageof atmospheric parameters, we study the behaviour of the CaII tripletstrength in terms of effective temperature, surface gravity andmetallicity. Empirical fitting functions for recently definedline-strength indices, namely CaT*, CaT and PaT, are provided. Thesefunctions can be easily implemented into stellar population models toprovide accurate predictions for integrated CaII strengths. We alsopresent a thorough study of the various error sources and their relationto the residuals of the derived fitting functions. Finally, the derivedfunctional forms and the behaviour of the predicted CaII are comparedwith those of previous works in the field.
| The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation
We used the Space Telescope Imaging Spectrograph (STIS) with itssmallest entrance aperture (0.03" wide slit) and highest resolutionechelle gratings (E140H and E230H) to record the interstellar absorptionfeatures for 10 different multiplets of neutral carbon at a resolvingpower of λ/Δλ=200,000 in the UV spectra of 21early-type stars. Our objective was to measure the amount of C I in eachof its three fine-structure levels of the ground electronic state, sothat we could determine the thermal pressures in the absorbing gas andhow much they vary in different regions. Our observations areprincipally along directions out to several kiloparsecs in the Galacticplane near longitudes l=120deg and 300°, with the moredistant stars penetrating nearby portions of the Perseus andSagittarius-Carina arms of the Galaxy. We devised a special analysistechnique to decipher the overlapping absorption features in thedifferent multiplets, each with different arrangements of the closelyspaced transitions. In order to derive internally consistent results forall multiplets, we found that we had to modify the relative transitionf-values in a way that made generally weak transitions stronger thanamounts indicated in the current literature. We compared our measuredrelative populations of the excited fine-structure levels to thoseexpected from equilibria calculated with collisional rate constants forvarious densities, temperatures, and compositions. The median thermalpressure for our entire sample was p/k=2240 cm-3 K, orslightly higher if the representative temperatures of the material aremuch above or below a most favorable temperature of 40 K for theexcitation of the first excited level at a given pressure. For gas thatis moving outside the range of radial velocities permitted bydifferential Galactic rotation between us and the targets, about 15% ofthe C I indicates a thermal pressure p/k>5000 cm-3 K. Forgas within the allowed velocities, this fraction is only 1.5%. Thiscontrast reveals a relationship between pressure enhancements and thekinematics of the gas. Regardless of velocity, we usually can registerthe presence of a very small proportion of the gas that seems to be atp/k>~105 cm-3 K. We interpret these ubiquitouswisps of high-pressure material to arise either from small-scale densityenhancements created by converging flows in a turbulent medium or fromwarm turbulent boundary layers on the surfaces of dense clouds movingthrough an intercloud medium. For turbulent compression, our C Iexcitations indicate that the barytropic indexγeff>~0.90 must apply if the unperturbed gas startsout with representative densities and temperatures n=10 cm-3and T=100 K. This value for γeff is larger than thatexpected for interstellar material that remains in thermal equilibriumafter it is compressed from the same initial n and T. However, ifregions of enhanced pressure reach a size smaller than ~0.01 pc, thedynamical time starts to become shorter than the cooling time, andγeff should start to approach the adiabatic valuecp/cv=5/3. Some of the excited C I may arise fromthe target stars' H II regions or by the effects of optical pumping fromthe submillimeter line radiation emitted by them. We argue that thesecontributions are small, and our comparisons of the velocities ofstrongly excited C I to those of excited Si II seem to support thisoutlook. For six stars in the survey, absorption features frominterstellar excited O I could be detected at velocities slightlyshifted from the persistent features of telluric origin. These O I* andO I** features were especially strong in the spectra of HD 93843 and HD210839, the same stars that show exceptionally large C I excitations. Inappendices of this paper, we present evidence that (1) the wavelengthresolving power of STIS in the E14OH mode is indeed about 200,000, and(2) the telluric O I* and O I** features exhibit some evidence formacroscopic motions, since their broadenings are in excess of thatexpected for thermal Doppler broadening at an exospheric temperatureT=1000 K. Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| What is the real nature of HD 108?
Since the beginning of the past century, the nature of HD 108 has been asubject of intense debate. One after another, astronomers explored itsvariability and attributed it either to binarity, or to changes in thestellar wind of a single star. In this article, we analyse a 30 yearcampaign of spectroscopic observations of this star with specialemphasis on the last 15 years during which photographic plates have beenreplaced by CCD detectors. Our investigation of the radial velocities ofHD 108 yields no significant short- or long-term period and does notconfirm the published periodicities either. Though the radial velocityof HD 108 appears clearly variable, the variations cannot be explainedby the orbital motion in a spectroscopic binary. However, our datareveal spectacular changes in the H I Balmer lines and some He Iprofiles over the years. These lines continuously evolved from P Cygniprofiles to ``pure'' absorption lines. A similar behaviour has alreadybeen observed in the past, suggesting that these changes are recurrent.HD 108 seems to share several characteristics of Oe stars and we discussdifferent hypotheses for the origin of the observed long-termvariations. As we are now in a transition period, a continuousmonitoring of HD 108 should be considered for the next few years. Basedon observations collected at the Observatoire de Haute-Provence(France).
| A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We report on a comprehensive search for Wolf-Rayet (W-R) stars in theSMC using interference filter imaging. Photometry of over 1.6 millionstellar images on multiple, overlapping fields covering 9.6deg2 found the previously known W-R stars at very highsignificance levels, two known Of-type stars, plus additionalcandidates, which we examined with slit spectroscopy. We discovered twonew Wolf-Rayet stars, both of type ``WN3+abs,'' bringing the totalnumber in the SMC to 11. We discuss their spectra, as well asreclassifying the previously known ones with our new data. Our surveyalso revealed four newly found Of-type stars, including one of the O5f?ptype, which is one of the earliest type stars known in the SMC. Anothernewly identified Of star is AV 398 (O8.5 If), a star often used inextinction studies under the assumption that it is of early B type. Werecover S18 (AV 154), a B[e] star whose spectrum currently lacks He IIλ4686 emission but which must have had strong emission a yearearlier; we compare this star to S Dor, suggesting that it is indeed aluminous blue variable. We also find a previously unknown symbiotic starwhose spectrum is nearly identical to the Galactic symbiotic AG Dra.More important, perhaps, than any of these discoveries is thedemonstration that there is not a significant number of W-R starswaiting to be discovered in the SMC. The number of W-R stars is a factorof 3 times lower in the SMC (per unit luminosity) than in the LMC. Thisstrongly suggests that at the low metallicity that characterizes the SMConly the most massive stars can evolve to W-R type.
| XID: Cross-Association of ROSAT/Bright Source Catalog X-Ray Sources with USNO A-2 Optical Point Sources
We quantitatively cross-associate the 18,811 ROSAT Bright Source Catalog(RASS/BSC) X-ray sources with optical sources in the USNO A-2 catalog,calculating the probability of unique association (Pid)between each candidate within 75" of the X-ray source position, on thebasis of optical magnitude and proximity. We present catalogs ofRASS/BSC sources for which Pid>98%, Pid>90%,and Pid>50%, which contain 2705, 5492, and 11,301 uniqueUSNO A-2 optical counterparts respectively down to the stated level ofsignificance. Together with identifications of objects not cataloged inUSNO A-2 due to their high surface brightness (M31, M32, ...) andoptical pairs, we produced a total of 11,803 associations to aprobability of Pid>50%. We include in this catalog a listof objects in the SIMBAD database within 10" of the USNO A-2 position,as an aid to identification and source classification. This is the firstRASS/BSC counterpart catalog which provides a probability of associationbetween each X-ray source and counterpart, quantifying the certainty ofeach individual association. The catalog is more useful than previouscatalogs which either rely on plausibility arguments for association ordo not aid in selecting a counterpart between multiple off-band sourcesin the field. Sources of high probability of association can beseparated out, to produce high-quality lists of classes (Seyfert 1/2s,QSOs, RS CVns) desired for targeted study, or for discovering newexamples of known classes (or new classes altogether) through thespectroscopic classification of securely identified but unclassifiedUSNO A-2 counterparts. Low Pid associations can be used forstatistical studies and follow-on investigation-for example, performingfollow-up spectroscopy of the many low-mass stars to search forsignatures of coronal emission, or to investigate the relationshipbetween X-ray emission and classes of sources not previouslywell-studied for their X-ray emissions (such as pulsating variablestars). We find that a fraction ~65.8% of RASS/BSC sources have anidentifiable optical counterpart, down to the magnitude limit of theUSNO A-2 catalog which could be identified by their spatial proximityand high optical brightness.
| Long-term spectroscopic and photoelectric variability of the peculiar runaway O7I fp star HD 108
The results of fifteen-year-long observations of optical spectra andthree-color BVR photoelectric photometry of the runaway star HD 108 arepresented. We detected periodic variations in the radial velocities ofabsorption and emission lines and optical variability on a long timescale (several years). Our search for periodicities in theradial-velocity variations of all available spectroscopic observationsrevealed the period P = 1627.6 days, which is most likely a result ofthe binary nature of the star. Long-term optical light variations areaccompanied by color variations in the star. The motion of the star incolor-magnitude diagrams may be due mainly to a change of the Paschencontinuum in the stellar envelope. The enhanced absorption in thecontinuum and the reddening of the star in 1996-1997 are assumed to beproduced by the commenced envelope ejection. The detected long-termphotometric variability may be a direct result of the influence of themassive unseen secondary component on the primary optical component.
| 1-m spectroscopy of normal OB stars
We have obtained spectra of 70 normal OB stars in the near-IR I(1-μm) band. The strongest features are those due to lines of thehydrogen Paschen series and neutral and ionized helium, which are, forthe most part, in absorption. The information content in this spectralrange is sufficient for only a rough classification of hot stars into`early O', `late O' and `B' types. Curiously, the leading He i tripletline, He i λ1.0830 μm, is usually not detectable, although ina few stars it is in emission; its behaviour generally correlates withthe leading helium singlet line, He i λ 2.058 μ m. These twofeatures appear to be present in emission only in stars with extremes ofmass loss or wind extension.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| Evolutionary Oddities in Old Disk Population Clusters
With a luminosity zero point fixed by the kinematics of old disksuperclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups(Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colorsof evolved old disk stars, especially red horizontal branch (RHB), earlyasymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch[AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71,M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) TheRHB stars in the old disk all have M_V = +0.7 +/- 0.1 (M_K = -1.3 +/-0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stableperiods and light curves are old disk stars on AGB(2). (3) AGB(1)objects include CH stars and semiregular (SRa) variables. (4) Thepopulous and overabundant cluster NGC 6791 may be the only disk clusterwith sdOB stars, populating the lower portion of the bifurcated extendedhorizontal branch that is usual in most ``blue tailed'' and high-densityhalo clusters. (5) Post-red giant branch (RGB) stars in old diskclusters show a B - V (b - y) defect when compared with RGB stars,possibly because of a change in the character of the atmospheres. (6) Ifthe bulk of the LARVs are pulsating in the fundamental mode, R Vir (P =145 days) is possibly a first-overtone pulsator. (7) The overabundantold disk clusters are within the solar circle, with Liller 1 being atthe Galactic center. (8) Several probable RHB stars at the southGalactic pole are identified. (9) The period-age relation, combined withthe known spatial distribution of Galactic LARVs, leads to a relationbetween age and scale height of distribution that monotonicallyincreases with age, leaving no obvious reason for a bifurcation of thepopulation.
| The polarimetric nature of HD 108
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1998MNRAS.295..423F&db_key=AST
| 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.
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| A Lower Limit to the Distance of High-Velocity Cloud Complex H
We derive a lower limit for the distance of the high-velocity cloud(HVC) complex H, which is a structure covering 480 square degrees on thesky and is centered on l = 131 deg, b = 1 deg. Considering theuncertainties in the derivation of stellar distances, we find that thedistance to the HVC is certainly larger than 3.4 kpc, probably largerthan 5 kpc, and possibly larger than 6.5 kpc. This distance limit isbased on the result that we do not find absorption associated with theHVC in IUE spectra of 17 OB stars. The three most distant of these starswere observed by us; we used the IUE archives to analyze the spectra ofthe other 14 stars. We do not have conclusive evidence that heavyelements are present in this HVC. This would require a detection ofabsorption in the spectrum of an extragalactic background source.However, the nondetections can still be considered secure, as the columndensity detection limits for the Mg II lambda lambda 2796, 2802, C IIlambda 1334, and O I lambda 1302 lines are a factor of 30-4100 below thecolumn density values expected for normal interstellar medium gas phaseabundances. Our lower limit to the distance is used to discuss possibleorigins of HVC complex H. It seems unlikely that it is associated with asuperbubble at large Galactocentric radii, an infalling dwarf galaxy, orthe outer arm. It might be an unusual Galactic fountain cloud or anintergalactic cloud.
| Accurate Two-dimensional Classification of Stellar Spectra with Artificial Neural Networks
We present a solution to the long-standing problem of automaticallyclassifying stellar spectra of all temperature and luminosity classeswith the accuracy shown by expert human classifiers. We use the 15Angstroms resolution near-infrared spectral classification systemdescribed by Torres-Dodgen & Weaver in 1993. Using the spectrum withno manual intervention except wavelength registration, artificial neuralnetworks (ANNs) can classify these spectra with Morgan-Keenan types withan accuracy comparable to that obtained by human experts using 2Angstroms resolution blue spectra, which is about 0.5 types (subclasses)in temperature and about 0.25 classes in luminosity. Accuratetemperature classification requires a hierarchy of ANNs, whileluminosity classification is most successful with a single ANN. Wepropose an architecture for a fully automatic classification system.
| A Survey for H alpha Emission in Massive Binaries: The Search for Colliding Wind Candidates
I report the results of the first all-sky survey of H alpha emission inthe spectra of O-type binaries. The survey includes 26 systems, of which10 have emission that extends clearly above the continuum. This is thefirst report of emission for four of these. An additional three systemsshow small distortions in the H alpha profile that may result from weakemission. I compare the distribution of emission systems in H-R diagramsfor both binary and single stars, using a survey of single O-type starsdone by Conti (1974). Emission in main-sequence systems is extremelyrare and is completely absent in my sample of binary stars. Among binarystars, 78% of the systems containing giants show some emission, while nosingle giants in Conti's sample do. In the case of supergiants, 78% ofsingle stars show emission, while all supergiant binaries show strongemission. H alpha emission may come from a variety sources, but the factthat binaries have a higher incidence and strength of emission inpost--main-sequence stages may indicate that wind interactions are acommon source of emission in massive binaries. To ascertain whether ornot colliding winds have been observed, it will be necessary to studythe H alpha line profile throughout several orbits of each candidatecolliding wind system and look for recurring orbital-phase--relatedvariations. Such a study is underway.
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
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| Takýmyýldýz: | Zincirli Prenses |
| Sað Açýklýk: | 00h06m03.39s |
| Yükselim: | +63°40'46.8" |
| Görünürdeki Parlaklýk: | 7.384 |
| Uzaklýk: | 12500 parsek |
| özdevim Sað Açýklýk: | -5.2 |
| özdevim Yükselim: | -1.3 |
| B-T magnitude: | 7.52 |
| V-T magnitude: | 7.396 |
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