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Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Chandra X-Ray Observations of the Young Open Cluster NGC 2516
We present a comprehensive study of the Chandra X-ray observations ofthe young open cluster NGC 2516. We have analyzed eight individualChandra observations. We have combined the data to achieve the greatestsensitivity, reaching down to logfX=-14.56 (ergss-1 cm -2), or logLX=28.69 (ergss-1) at the distance of NGC 2516. Out of 284 X-ray sourcesdetected, 155 are identified with photometric cluster members, with verylittle ambiguity. We have studied the X-ray luminosity functions for thevarious spectral types, correcting for nonmember contamination those forlater type stars. We find strong X-ray emission from a couple of B starsthat cannot be due to companions. Among A stars, chemically peculiarstars show a detection fraction much larger than normal A stars. Theluminosity functions decrease monotonically from F to M stars, and thosefor K stars are significantly lower than similar stars in the Pleiades,a cluster only slightly younger than NGC 2516. We discuss possiblecauses of this discrepancy, including differences in age and stellarrotation.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| The Open Cluster NGC 2516. I. Optical Photometry
We present UBVI CCD photometry of the intermediate-age open cluster NGC2516. From this new photometry we derive the cluster parameters[V0-MV=7.77+/-0.11 mag, E(B-V)=0.112+/-0.024 mag],age [log(age)=8.2+/-0.1], and a photometric abundance([Fe/H]=-0.10+/-0.04) from a comparison with theoretical isochrones ofthe Padua group. We find that the X-ray bright stars[logLX(ergs s-1)>~29.0] are systematicallybluer in B-V and U-B. It was necessary to modify the photometricmembership criterion used by Sung & Bessell due to the UV excess ofX-ray bright stars and due to the abrupt increase in the slope of the(MV, B-V) ZAMS relation at B-V>~1.4. A minimum binaryfrequency 40% (+/-5%) was estimated from the distribution of distancemoduli of unevolved photometric members. The photometric abundanceobtained in this study is similar to that of the Pleiades and thereforethe lower metallicity is not the cause of strong X-ray emission ofF-type stars in NGC 2516. We believe that this results from them beingclose binaries, as suggested by their different distribution of distancemoduli compared to other cluster members and surmise that the tidalforces between the two stars must increase their X-ray activity.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| High and intermediate-resolution spectroscopy of Be stars 4481 lines
We present an atlas of Hγ , He i lambda 4471 and Mg ii lambda 4481line profiles obtained in a 10 year observation period of 116 Be stars,which enabled many of them to be observed at quite different emissionepochs. From the best fit of the observed He i lambda 4471 line profileswith non-LTE, uniform (Teff,log g) and full limb-darkenedmodel line profiles, we determined the V sin i of the program stars. Toaccount, to some degree, for the line formation peculiarities related tothe rapid rotation-induced non-uniform distributions of temperature andgravity on the stellar surface, the fit was achieved by considering(Teff,log g) as free parameters. This method produced V sin iestimations that correlate with the rotational velocities determined bySlettebak (1982) within a dispersion sigma <= 30 km s-1and without any systematic deviation. They can be considered as given inthe new Slettebak's et al. (1975) system. Only 13 program stars havediscrepant V sin i values. In some objects, this discrepancy could beattributed to binary effects. Using the newly determined V sin iparameters, we found that the ratio of true rotational velocitiesV/Vc of the program Be stars has a very low dispersion aroundthe mean value. Assuming then that all the stars are rigid rotators withthe same ratio V(/lineω)/Vc, we looked for the value of/line ω that better represents the distribution of V sini/Vc for randomly oriented rotational axes. We obtained/lineω = 0.795. This value enabled us to determine the probableinclination angle of the stellar rotation axis of the program stars. Inthe observed line profiles of Hγ , He i lambda 4471, Mg ii lambda4481 and Fe ii lambda 4351 we measured several parameters related to theabsorption and/or emission components, such as: equivalent width,residual emission and/or absorption intensity, FWHM, emission peakseparations, etc. The parameters related to the Hγ line emissionprofiles were used to investigate the structure of the nearbyenvironment of the central star. From the characteristics of thecorrelations between these quantities and the inferred inclinationangle, we concluded that in most of cases the Hγ line emissionforming regions may not be strongly flattened. Using a simplerepresentation of the radiation flux emitted by the star+envelopesystem, we derived first order estimates of physical parameterscharacterizing the Hγ line emission formation region. Thus, weobtained that the total extent of the Hγ region is Rf=~ 2.5 +/- 1.0 R* and that the density distribution in theselayers can be mimicked with a power law rho ~ R-alpha , wherealpha =2.5+2.2-0.6. The same approach enabled usto estimate the optical depth of the Hγ line emission formationregion. From its dependence with the aspect angle, we concluded thatthese regions are caracterized by a modest flattening and that the rho(equator)/rho (pole) density contrast of the circumstellar envelope nearthe star should be two orders of magnitude lower than predicted bymodels based on a priori disc-shaped circumstellar envelopes. We foundthat the separation between the emission peaks, Deltap, andthe full width at half maximum, Delta 1/2, of the Hγline emission are not only sensitive to kinematic effects, but to lineoptical depth as well. This finding agrees with previous theoreticalpredictions and confirms that Huang's (1972) relation overestimates theextent of the Hγ line emission formation region. Data obtained atCASLEO operated under agreement between the CONICET and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina, at ESOLa Silla, Chile and at OHP, France.}\fnmsep\thanks{Tables 2 to 7 andFigs. 1 and 2 are only available in full in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/861}} \subtitle{Anatlas of Hγ , He {\fontsize {10pt}{12pt}\selectfont I} 4471 and Mg{\fontsize {10pt}{12pt}\selectfont II
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| 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
| On the Variability of O4-B5 Luminosity Class III-V Stars
We investigate the Hipparcos Satellite photometry of O4-B5 luminosityclass III-V stars. Some for which further study is desirable areidentified. These stars in general are more variable than cooler stars
| Radial Velocities, Binarity, and Kinematic Membership in the Open Cluster NGC 2516
We present echelle spectroscopic observations for 36 bright (V<9.6)stars in the open cluster NGC 2516, including several blue stragglercandidates and four red giants. Radial velocities are derived bycross-correlations using high signal-to-noise ratio standard spectra astemplates. From 22 cluster members a mean cluster velocity of+22.0+/-0.2 km s-1 was derived. Membership probabilities ofthe observed stars are computed on the basis of their distance to thecluster center and kinematic criteria. We report the discovery of threedouble-lined spectroscopic binaries and several probable binaries amongmain-sequence stars. A binary frequency of more than 26% is found amongthe high-mass main-sequence stars. The blue straggler HD 66341 is aslowly rotating cluster member with constant velocity, while HD 66194 isa fast-rotating Be star with probable variations in radial velocity.Other blue straggler candidates, such as HD 65663, 65950, 66066, and65987, must be considered turnoff stars. The observations presented herewere obtained at the Complejo Astronómico El Leoncito (CASLEO),which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina (CONICET) and the national universities of LaPlata, Córdoba, and San Juan.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Rosat HRI observations of the open cluster NGC 2516
We present ROSAT HRI observations of the NGC 2516, a young southern opencluster with low metallicity. We detect 73 X-ray sources alreadydetected with the PSPC and 12 new X-ray sources. The higher HRI spatialresolution allows us to improve the position of the previous knownsources, and to better determine the optical counterparts of X-raysources with respect to the PSPC data. We find 28 sources with nocatalogued counterparts, most with faint objects within their X-raypositional error circle. Multiband CCD photometry of these possiblecounterparts shows that about half of them are consistent with being dKand dM cluster members. Assuming that they are true NGC 2516 members,the derived X-ray luminosity is consistent with that observed in dK anddM stars in more metal-rich open clusters of similar age, suggestingthat the activity level at which X-ray saturation occurs is insensitiveto a change of a factor two in stellar metallicity.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| An X-ray survey of the young open cluster NGC 2516
We present an analysis of a 60-ks ROSAT X-ray observation of theGalactic open cluster NGC 2516, which has an age of about 110 Myr and aless than solar metallicity. 159 X-ray sources (0.5-2.0 keV) are foundin the central portion of the Position Sensitive Proportional Counter(PSPC) together with six soft X-ray sources (0.1-0.4 keV). From theliterature, we have constructed a uniform catalogue of photometricallyselected cluster candidates. 65 of the X-ray sources are identified withphotometric members of NGC 2516, and 25 X-ray sources are identifiedwith probable cluster non-members or stars with no photometricmeasurements with which to assess cluster membership. The X-rayluminosity threshold is approximately 10^29 erg s^-1 and X-ray upperlimits are determined for a further 136 possible cluster members. X-rayemission is observed across the Hertzsprung-Russell diagram, fromspectral types of B2Ve to the early K stars which define the faintnesslimit of our optical catalogue. At least 73 X-ray sources have noplausible counterpart brighter than V=15. Some may be foreground orbackground stars, 10-15 are probably extragalactic, but the majority arelikely to be lower mass stars in the cluster. Three of the soft X-raysources are probable hot white dwarfs in binary systems with late-typestars. One of these may be in the cluster, the other two are foregroundobjects. X-ray emission from the hottest star in the cluster isattributed to a shocked stellar wind. 20 per cent of late B- and A-typestars are detected, which is consistent with the X-ray emission beingthe result of unresolved late-type companions. At least four out of thesix magnetic, chemically peculiar stars are X-ray sources. We concludethat this is probably intrinsic, although an explanation involvingbinary companions cannot be entirely ruled out. Many F, G and K starsare detected, presumably as a result of dynamo-generated coronalactivity. The peak level of X-ray activity is reached among the late Gstars, which have an X-ray to bolometric flux ratio of 10^-3. Thissuggests that these stars have not yet spun down below the empiricalX-ray saturation rotation speed of 10-20 km s^-1. Interpretation of thelate-type star X-ray luminosity functions is hampered by the fact thatthere are likely to be contaminating field stars among the X-ray upperlimits. The U-B, B-V colour-colour diagram for X-ray-selected clustermembers reveals an ultraviolet excess among the F and G stars of NGC2516, which is best explained by a less than solar metallicity,[Fe/H]=-0.32+/-0.06. Comparison with younger and older clusters showsthat the late-type stars in NGC 2516 do not simply obey the widelyaccepted rotation-activity-age paradigm. We suggest a modification tothe paradigm which explains the observations in terms of the differentconvection zone properties that late-type stars of differingmetallicities have at the same colour or mass.
| ICCD Speckle Observations of Binary Stars.XVIII.An Investigation of Be =
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2112M&db_key=AST
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| ROSAT survey of stellar X-ray sources in the young open cluster NGC 2516.
The ROSAT PSPC detector was used to obtain a deep pointing with itscenter on the young open cluster NGC 2516 and total integration time of9284 sec. Altogether 64 X-ray sources were detected showing strongconcentration towards the optical cluster centre. This suggests thatmost of the sources are actually associated with NGC 2516. 42 of thesesources could be identified with optical stars in the field of thecluster; after correction for a mean offset between optical and nominalROSAT positions of about 13", optical star and X-ray source positionsare found to agree, on the average, within about 9", while maximumobserved positional differences amount to 27". X-ray sources identifiedwith optical cluster stars include HR 3147 (=HD 66194; B2.5IVe), thevisually brightest B star in the cluster, three close visual late B-typebinaries (h 4027 A,B, h 4031 A,B, and I 1104 A,B) with projectedseparations ranging between about 4x10^16^cm and 6x10^16^cm, as well asseveral (six) chemically peculiar Bp/Ap(Si) and Ap(SrCrEu) stars.According to their known spectral or photometric properties, opticalcounterparts for most identified cluster X-ray sources are main-sequencestars between spectral types B9 and K0. No indication for a pulsedcomponent of the X-ray flux pointing to a compact companion could bedetected from X-ray photon arrival times for the prominent bluestraggler star HR 3147, while one of the visual binary counterparts ofX-ray cluster sources (I 1104 A/B) possibly exhibits a pulsed component.The fact that about half of the chemically peculiar Bp/Ap stars known inthe cluster, all of them of the (magnetic) CP2 variety, are found to becounterparts to ROSAT X-ray sources, can be taken to suggest thatstellar magnetic fields play a role in generating X-ray fluxes fromearly-type main-sequence stars. Luminosities of X-ray sources detectedin the cluster are rather uniform, ranging betweenL_x_(0.1-2.5keV)=0.4x10^30^ and 4.5x10^30^erg/s. 80 percent of thesources have relatively hard spectra, X-ray photons being recorded onlyin PSPC high-energy channels above 0.5keV. For only very few (six)bright X-ray sources in the field of NGC 2516, at least 30% of observedX-ray quanta fall into the low-energy channels between 0.16 and 0.5keV;four of these soft-spectrum sources could be identified with opticallate-type stars in front of the cluster, including the nearby solar-typestar HR 3138 (=HD 65907, MK: G0V).
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| High-resolution spectroscopy of southern and equatorial Be stars: flux excess at λ4471 A.
We present a catalogue of high-resolution He I 4471, Mg II 4481 andHγ line profiles observed simultaneously for 37 southern andequatorial Be stars. In this paper we focus on the He I 4471 line, whichis studied using non-LTE model line profiles. A systematic difference isfound between the observed and the theoretical He I 4471 line equivalentwidths. This difference strongly correlates with the Hγ emissionand we interpret it as being due mainly to the veiling effect producedby the continuum emission excess originating in the circumstellarenvelope. To determine the continuum flux excess we use two methods. Onemethod enables us to obtain simultaneously the V sin i parameter. Theflux excess we determined is overestimated by about 9% on average,because in the line profile analysis we neglected the gravity darkeningeffect due to stellar rotation. We find that 14 program stars presenttwo possible solutions for V sin i. The second solution is, on average,14% smaller and produces a small residual or emission-like differencebetween the observed He I 4471 line profile and the model profile.
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| Hα emission and infrared excess in Be stars: probing the circumstellar disc.
We present near-simultaneous (within one week) high-resolution Hαspectra and near-infrared photometry of 69 B and Be stars. We confirmthat, for the Be stars, the infrared excess and Hα emission arecorrelated, albeit with a rather large scatter. Given thenear-simultaneity of our observations, this scatter probably is notrelated to intrinsic variability, but rather reflects a genuinestar-to-star variation in physical conditions. In order to deriveinformation about the density and velocity structure of Be star discs,we calculate theoretical infrared excesses, Hα equivalent widthsand Hα line profiles for two different models. We find that themodel used by Poeckert & Marlborough (1978a) to fit the spectrum ofγ Cas, is not capable of reproducing the observed correlationbetween the infrared excess and the excess Hα equivalent width,producing - for given infrared excess - Hα emission lines that areweaker than observed. The model of Waters (1986), used successfully tofit the infrared excess continuum emission of Be stars, also fails,producing Hα emission lines that are too strong. We discuss theseresults in terms of the density structure of the wind.
| Photometry of F-K type bright giants and supergiants. 3: The luminosity, reddening, and heavy element abundance of GK stars
The reddening, luminosity, and heavy element abundance of 250 brightgiants and supergiants of type GK are discussed on the basis of 4 color,DDO, RI, and Geneva photometry Bright giants of type GK with age greaterthan 5 x 108 yr, and of type G0/3 with age greater than 2 x108 yr, are very scarce in the solar neighborhood. The medianspace motion vectors of the bright giants and supergiants with welldetermined space motions are (U, V, W) = (+10.6, -13.2, -7.7) +/- (12.3,8.8, 8.8) km/s. The M1 index for 4 color photometry, whencorrected for luminosity (gravity) effects, is sensitive to heavyelement abundance of the GK stars but may not be reliable for those oftype G0/3. The available spectroscopic determinations of (Fe/H) givemixed results with the two largest samples being internally consistentbut with a large zero-point difference. There is a similar zero-pointshift in the peak frequency of the photometrically determined values ofP(Fe/H) for the F type and for the GK type stars. There is littleevidence for an appreciable galactic, radial gradient in the P(Fe/H)values. A previously noted correlation of the heavy element abundanceindex, delta M1, with the amplitude defect in the B lightcurves of Cepheids, FB, and interpretation of the(PLFB) relation as a (PL(Fe/H)) relation needs furtherinvestigation in light of the apparent sensitivity of M1 toFe/H for the F stars (the domain of the Cepheids with P less than 10 d)and lack of this sensitivity for the G0/3 star (domain of the Cepheidswith P greater than 10 d).
| On the rotation properties of Be stars and their envelopes.
We present the results of low resolution spectroscopy (4A) obtained atCTIO for a sample of 42 Be, 4 B and 3 O stars covering theλ3700-7000A spectral range. Equivalent widths of the Balmer lineswere measured up to H_10_. We also give the V/R state of the Hαemission line. In addition, the peak separation and the width at thebase of the Hβ and Hγ emission lines are tabulated for atotal of 243 medium resolution (1A) spectra of 51 southern Be starstaken at the Manuel Foster Observatory, Chile. We analyzed our Hαequivalent widths (Wα) together with published data for a total of122 Be and 2 Oe stars. An upper limit of Wα increasing with theprojected rotational velocity vsini is shown by the data. The Wαvalues depend strongly on spectral type, being lower for later-type Bestars. This could indicate that the maximum strength of emission dependsmainly on the spectral type and rotation velocity. The relation betweenWα(max) and vsini, together with the observed excess in number oflow vsini Be stars and the reported anti-correlation between photometricperiod and vsini give evidence for a considerable range of the truerotation velocities of Be stars: definitely there are intrinsically slowrotators among them. However, our results could also be interpreted interms of anisotropic Hα emission coming from the disk. Theanalysis of the net equivalent widths and peak separations of Hαdouble emission line profiles strongly favor a disk type emittingenvelope with a r^-j^ rotation law with j=1.4+/-0.2 which do not dependon the spectral subtype. This result is interpreted as evidence ofradial motions in the envelopes of Be stars. The mean radial extensionsof the regions which emit Balmer lines were determined to range between30% (H_10_) and 60% (Hβ) of the Hα emitting envelope in thecase j=1. The mean electron density within the envelope has been foundto vary only by a factor of 4. We compare our results with earlierinvestigations.
| The distribution of interstellar dust in the solar neighborhood
We surveyed the IRAS data base at the positions of the 1808 O6-B9.5stars in The Bright Star Catalog for extended objects with excessemission at 60 microns, indicating the presence of interstellar dust atthe location of the star. Within 400 pc the filling factor of theinterstellar medium, for dust clouds with a density greater than 0.5/cucm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the densitydistribution function appears to follow a power law index - 1.25. Whenthe dust clouds are mapped onto the galactic plane, the sun appears tobe located in a low-density region of the interstellar medium of widthabout 60 pc extending at least 500 pc in the direction of longitudes 80deg - 260 deg, a feature we call the 'local trough'.
| Effects of rotation on the colours and line indices of stars. VI - The reality of the blue straggler phenomenon
The effect of rotation on the observed colors of stars has beenconsidered as a possible cause for the blue straggler phenomenon inclusters listed by Mermilliod (1982). It appears that this phenomenon isdefinitely not real in the case of the late B and early A spectral typeblue stragglers that are intrinsic slow rotators. Among clusterscontaining the early B type blue stragglers it is found that theanomalous position of the stragglers in NGC 6633, NGC 4675, and NGC 2516cannot be accounted for by rotation effects alone.
| First giant branch and asymptotic giant branch stars in nearby aggregates
The properties of the brightest red stars in several aggregates in theGalaxy are compared with theoretical models. 22 asymptotic giant branch(AGB) stars are identified, four of which are in the thermally pulsingAGB (TPAGB) phase and four of which are TPAGB carbon stars. Also, fourcases of RGB stars are identified which have accreted substantial massfrom the carbon-rich TPAGB precursor of a current white dwarf companion.There is general agreement between the observed and theoretical slopesof the RGB and early AGB branches, and quantitative differences betweenthe positioning of observed sequences can be understood in terms ofdifferences in metallicity and mass predicted by the theory.
| Spectral energy distributions of Be stars. III - Envelope models derived from new measurements for 17 stars
Spectrophotometric observations of 17 bright southern and equatorialearly Be stars are reported and analyzed. Data obtained at 320-850 nmwith spectral resolution 1 nm using a spectrum scanner on the 61-cmCassegrain telescope at ESO in 1978 and 1982 are presented in extensivetables and graphs and characterized in detail, and the principal stellarparameters are calculated by fitting the data with theoretical energydistributions. It is found that the optical depths of Be-star envelopesincrease from small values in the Paschen continuum to near unity at theshort-wave side of the Balmer discontinuity; the envelope electrontemperature and density are estimated as 9500 K and 10 to the 12th/cucm, respectively.
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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: | שדרית |
| התרוממות ימנית: | 07h58m50.50s |
| סירוב: | -60°49'28.0" |
| גודל גלוי: | 5.81 |
| מרחק: | 315.457 פארסק |
| תנועה נכונה: | -4.1 |
| תנועה נכונה: | 10.8 |
| B-T magnitude: | 5.658 |
| V-T magnitude: | 5.75 |
קטלוגים וכינוים:
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