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Spectroscopic and physical parameters of Galactic O-type stars. I. Effects of rotation and spectral resolving power in the spectral classification of dwarfs and giants
Context. The modern-era spectral classification of O-stars relies oneither the Walborn or the Conti-Mathys scheme. Since both of theseapproaches have been developed using low-quality photographic data,their application to high-quality digital data might not bestraightforward and be hampered by problems and complications that havenot yet been appreciated. Aims: We investigate the correspondencebetween photographic and digital classification of low luminosity O-typestars (dwarfs and giants) of solar metallicity. Methods: Usinghigh-resolution spectra obtained with the ESO/MPG 2.2 m telescope in LaSilla and following the premises of the Walborn and Conti classificationschemes, we determined the spectral types and luminosity classes of 19Galactic O-type stars and compared them to those attributed by Walbornand Mathys based on low-quality data. Results: Our analysisreveals that the morphological spectral types assigned usinghigh-resolution data are systematically later (by up to 1.5 subtypes)then those attributed by Walborn. By means of line-profile simulations,we show that part of this discrepancy is more likely caused by thecombined effect of stellar rotation and high spectral resolution on thedepth of helium lines used as spectral type indicators. In addition, wedemonstrate that at least for narrow-lined stars the "rotational effect"does not disappear when the high-resolution spectra are degraded to theresolution of the Walborn standards. We also find evidence of asystematic difference between our high-resolution quantitative spectraltypes and those assigned by Mathys. Conclusions: Rotation andspectral resolution are important third parameters in the spectralclassification of O-type stars. To obtain reliable spectral classeswithin the Walborn approach, the unknown and the standard spectra mustbe compared at the same resolution and vsini . Owing to resolutioneffects, the Conti approach might also need to be updated.Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile, under programmeID 072.D-0196.Appendices A and B are only available in electronic format http://www.aanda.org
| Exploring the origin of magnetic fields in massive stars: a survey of O-type stars in clusters and in the field
Context. Although the effects of magnetic fields in massive stars havebeen found to be substantial by recent models and observations, themagnetic fields of only a small number of massive O-type stars have sofar been investigated. Additional observations are of the utmostimportance to constraining the conditions that are conducive to magneticfields and to determine the first trends about their occurrence rate andfield strength distribution. Aims: To investigate statisticallywhether magnetic fields in massive stars are ubiquitous or appear onlyin stars with a specific spectral classification, certain ages, or in aspecial environment, we acquired 41 new spectropolarimetric observationsfor 36 stars. Among the observed sample, roughly half of the stars areprobable members of clusters at different ages, whereas the remainingstars are field stars not known to belong to any cluster or association. Methods: Spectropolarimetric observations were obtained duringthree different nights using the low-resolution spectropolarimetric modeof FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-mAntu telescope of the VLT. To assess the membership in open clusters andassociations, we used astrometric catalogues with the highest qualitykinematic and photometric data currently available. Results: Afield at a significance level of 3? was detected in ten O-typestars. The strongest longitudinal magnetic fields were measured in twoOf?p stars: ? Bz ? = -381 ± 122 G for CPD-282561 and ? Bz ? = -297 ± 62 G for HD 148937,the latter of which had previously been detected by ourselves asmagnetic. The observations of HD 148937 obtained on three differentnights indicate that the magnetic field is slightly variable. Our newmeasurements support our previous conclusion that large-scale organizedmagnetic fields with polar field strengths in excess of 1 kG are notwidespread among O-type stars. Among the stars with a detected magneticfield, only one star, HD 156154, belongs to an open cluster at highmembership probability. According to previous kinematic studies, fourmagnetic O-type stars in the sample are well-known candidate runawaystars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programme 085.D-0667(A)).
| Detection of a magnetic field on HD108: clues to extreme magnetic braking and the Of?p phenomenon
We report the detection of a magnetic field on the Of?p starHD108. Spectropolarimetric observations conducted in 2007, 2008 and2009, respectively, with NARVAL@Télescope Bernard Lyot (TBL) andEchelle SpectroPolarimetric Device for the Observation of Stars atCanada-France-Hawaii Telescope (ESPaDOnS@CFHT) reveal a clear Zeemansignature in the average Stokes V profile, stable on time-scales of daysto months and slowly increasing in amplitude on time-scales of years. Wespeculate that this time-scale is the same as that on which H?emission is varying and is equal to the rotation period of the star. Thecorresponding longitudinal magnetic field, measured during each of thethree seasons, increases slowly from 100 to 150 G, implying that thepolar strength of the putatively dipolar large-scale magnetic field ofHD108 is at least 0.5 kG and most likely of the order of 1-2 kG.The stellar and wind properties are derived through a quantitativespectroscopic analysis with the code CMFGEN. The effective temperatureis difficult to constrain because of the unusually strong HeI??4471, 5876 lines. Values in the range of 33000-37000Kare preferred. A mass-loss rate of about10-7Msolaryr-1 (with a clumping factorf = 0.01) and a wind terminal velocity of 2000 km s-1 arederived. The wind confinement parameter ?* is larger than100, implying that the wind of HD108 is magnetically confined.Stochastic short-term variability is observed in the wind-sensitivelines but not in the photospheric lines, excluding the presence ofpulsations. Material infall in the confined wind is the most likelyorigin for lines formed in the inner wind. Wind clumping also probablycauses part of the H? variability. The projected rotationalvelocity of HD108 is lower than 50 km s-1, consistent withthe spectroscopic and photometric variation time-scales of a fewdecades. Overall, HD108 is very similar to the magnetic O star HD191612except for an even slower rotation.Based on observations obtained at the Télescope Bernard Lyot(TBL) and at the Canada-France-Hawaii Telescope (CFHT). CFHT is operatedby the National Research Council of Canada, the Institut National desSciences de l'Univers of the Centre National de la RechercheScientifique of France (INSU/CNRS), and the University of Hawaii, whileTBL is operated by CNRS/INSU.E-mail: martins@graal.univ-montp2.fr
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| Early Results from the Galactic O-Star Spectroscopic Survey: C III Emission Lines in Of Spectra
On the basis of an extensive new spectroscopic survey of Galactic Ostars, we introduce the Ofc category, which consists of normal spectrawith C III ??4647-4650-4652 emission lines of comparableintensity to those of the Of defining lines N III??4634-4640-4642. The former feature is strongly peaked tospectral type O5, at all luminosity classes, but preferentially in someassociations or clusters and not others. The relationships of thisphenomenon to the selective C III ?5696 emission throughout thenormal Of domain, and to the peculiar, variable Of?p category, for whichstrong C III ??4647-4650-4652 emission is a definingcharacteristic, are discussed. Magnetic fields have recently beendetected on two members of the latter category. We also present two newextreme Of?p stars, NGC 1624-2 and CPD -28° 2561, bringing thenumber known in the Galaxy to five. Modeling of the behavior of thesespectral features can be expected to better define the physicalparameters of both normal and peculiar objects, as well as the atomicphysics involved.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| 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.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| UBV photometry of OB+ stars in the southern Milky Way
One thousand two hundred and twenty six new observations are combinedwith previously published results of the author to yield an internalyconsistent set of magnitudes and colors on the international UBV systemfor 666 stars classified as OB+ in the Stephenson-Sanduleak OB starsurvey. The U - B, B - V diagram indicates that these stars consistprimarily of O-type stars and early B-type supergiants, reddened by upto E(B - V) = 2.1 mag.
| Spectroscopic binary frequency among CNO stars
Radial velocities of 35 stars with carbon, nitrogen, and oxygen (CNO)anomalies in the OB spectral range have been measured. Using these newdata, it has been confirmed that the short-period spectroscopic binaryfrequency among the OBN stars is substantially larger than among the OBCstars. After allowance for the undiscovered systems, practically allstars with nitrogen anomalies belong to short-period spectroscopicbinaries, while the frequency among the OBC stars is statisticallysimilar to the frequency of normal supergiants. Among the stars withmoderate nitrogen anomalies, a frequency of short-period binaries whichranges between 50 percent (N-enhanced) and 83 percent (N-deficient) wasfound.
| The distribution of luminous stars at (l,b) = (245 deg, 0 deg) in Puppis
Munch (1951, 1954) drew attention to the presence of a concentration offaint OB stars along the galactic equator at l = 245 deg in Puppis. Thedistribution of OB stars in this field has subsequently been studied bya number of workers. It has now been found that the distribution of OBstars in Puppis is not well established. The present investigation has,therefore, the objective to reexamine all available spectroscopic andphotometric data on these OB stars in a systematic fashion, utilizingthe method of spectroscopic parallaxes rather than H-beta photometry orZAMS fitting. Attention is given to interstellar extinction, stellardistribution perpendicular to the Galactic plane, and the stellardensity distribution.
| A photoelectric UBV catalogue of 610 stars in Puppis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1983MNRAS.205..241R&db_key=AST
| UBV photometry for southern OB stars
New UBV photometry of 1227 OB stars in the southern Milky Way ispresented. For 1113 of these stars, MK spectral types have been reportedpreviously in a comprehensive survey to B = 10.0 mag.
| A study of galactic structure in a region of Puppis
Galactic structure and rotation in the region of the Puppis OB 1 and OB2 associations are investigated on the basis of spectrophotometric andkinematic observations. Data was obtained from measurements of platestaken at the ESO objective prism astrograph at a spectral dispersion of110 A/mm. Spectral types, B magnitudes, distance moduli andgalactocentric distances are presented for 102 stars, including 74 oftypes O5 through B3 and the remainder of types B5 and B6, and radialvelocities determined by Fehrenbach's method as well as derived circularvelocities are presented for 63 OB stars. The results demonstrate thereality of the Pup OB 1 association, which is found at a distancemodulus of 12.20 with a mean radial velocity of 37 km/sec. The physicalreality of the Pup OB 2 association, however, appears doubtful, due tothe spread of the more distant group of OB stars (distance modulus13.45, mean radial velocity 57 km/sec) over a distance of more than 3kpc; this more distant group, as well as the H II region S 311, areassigned to an extension of the Perseus arm. The galactic rotation curvederived is found to exhibit a clear rise for galactocentric distancesgreater than 12.5 kpc.
| Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...38..309H&db_key=AST
| MK spectral classifications for southern OB stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977ApJS...35..111G&db_key=AST
| Finding list and spectral classifications for southern luminous stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..225M&db_key=AST
| UBV, H-beta photometry of OB stars in groups - PUP OB2
Evidence is presented in support of the physical reality of theassociation Pup OB2 at an H-beta-derived distance of 4.3 kpc.Radial-velocity measures for several OB stars in the vicinity of theassociation verify the H-beta distance scale. The extreme sensitivity ofUBV 'minimum distances' to systematic errors in photometry isreemphasized. The faint blue stars identified in recent photographicinvestigations are probably less luminous members of Pup OB2, whichitself is quite young (2 million yr) and dense. Pup OB2 is on theoutskirts of the local arm of the Galaxy. A possible relationshipbetween Pup OB2 and an apparent velocity bubble in the H I distributionis suggested.
| A catalogue of galactic O stars. The ionization of the low density interstellar medium by runaway stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974RMxAA...1..211C&db_key=AST
| The space distribution of the O stars in the solar neighborhood.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973AJ.....78.1067W&db_key=AST
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