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Models for Massive Stellar Populations with Rotation
We present and discuss evolutionary synthesis models for massive stellarpopulations generated with the Starburst99 code in combination with anew set of stellar evolution models accounting for rotation. The newstellar evolution models were compiled from several data releases of theGeneva group and cover heavy-element abundances ranging from twice solarto one-fifth solar. The evolution models were computed for rotationvelocities on the zero-age main sequence of 0 and 300 km s-1and with the latest revision of stellar mass-loss rates. Since the masscoverage is incomplete, in particular at nonsolar chemical composition,our parameter study is still preliminary and must be viewed asexploratory. Stellar population properties computed with Starburst99 andthe new evolution models show some marked differences in comparison withmodels obtained using earlier tracks. Since individual stars now tend tobe more luminous and bluer when on the blue side of theHertzsprung-Russell diagram, the populations mirror this trend. Forinstance, increases by factors of 2 or more are found for thelight-to-mass ratios at ultraviolet to near-infrared wavelengths, aswell as for the output of hydrogen-ionizing photons. If these resultsare confirmed once the evolution models have matured, recalibrations ofcertain star formation and initial mass function indicators will berequired.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.
| Rotational Velocities for B0-B3 Stars in Seven Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
We present the results of a study aimed at assessing the differences inthe distribution of rotation speeds N(vsini) among young (1-15 Myr) Bstars spanning a range of masses 6 Msolar>1 Msolar pc-3)ensembles that will survive as rich, bound stellar clusters for ageswell in excess of 108 yr. Our results demonstrate (1) thatindependent of environment, the rotation rates for stars in this massrange do not change by more than 0.1 dex over ages t~1 to ~15 Myr; and(2) that stars formed in high-density regions lack the cohort of slowrotators that dominate the low-density regions and young field stars. Wesuggest that the differences in N(vsini) between low- and high-densityregions may reflect a combination of initial conditions andenvironmental effects: (1) the higher turbulent speeds that characterizemolecular gas in high-density, cluster-forming regions; and (2) thestronger UV radiation fields and high stellar densities thatcharacterize such regions. Higher turbulent speeds may lead to highertime-averaged accretion rates during the stellar assembly phase. In thecontext of stellar angular momentum regulation via ``disk-locking,''higher accretion rates lead to both higher initial angular momenta andevolution-driven increases in surface rotation rates as stars contractfrom the birth line to the zero-age main sequence (ZAMS). Stronger UVradiation fields and higher densities may lead to shorter disk lifetimesin cluster-forming regions. If so, B stars formed in dense clusters aremore likely to be ``released'' from their disks early during theirpre-main-sequence lifetimes and evolve into rapid rotators as theyconserve angular momentum and spin up in response to contraction. Bycontrast, the majority of their brethren in low-density,association-forming regions can retain their disks for much or all oftheir pre-main-sequence lifetimes, are ``locked'' by their disks torotate at constant angular speed, and lose angular momentum as theycontract toward the ZAMS, and thus arrive on the ZAMS as relativelyslowly rotating stars.
| Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters
Aims.We propose the Wind of Fast Rotating Massive Stars scenario toexplain the origin of the abundance anomalies observed in globularclusters. Methods: We compute and present models of fast rotating starswith initial masses between 20 and 120 M_ȯ for an initialmetallicity Z = 0.0005 ([Fe/H]≃-1.5). We discuss thenucleosynthesis in the H-burning core of these objects and present thechemical composition of their ejecta. We consider the impact ofuncertainties in the relevant nuclear reaction rates. Results: Fastrotating stars reach critical velocity at the beginning of theirevolution and remain near the critical limit during the rest of the mainsequence and part of the He-burning phase. As a consequence they loselarge amounts of material through a mechanical wind which probably leadsto the formation of a slow outflowing disk. The material in this slowwind is enriched in H-burning products and presents abundance patternssimilar to the chemical anomalies observed in globular cluster stars. Inparticular, the C, N, O, Na and Li variations are well reproduced by ourmodel. However the rate of the 24Mg(p,γ) has to beincreased by a factor 1000 around 50 × 106 K in orderto reproduce the amplitude of the observed Mg-Al anticorrelation. Wediscuss how the long-lived low-mass stars currently observed in globularclusters could have formed out of the slow wind material ejected bymassive stars.
| Statistical properties of a sample of periodically variable B-type supergiants. Evidence for opacity-driven gravity-mode oscillations
Aims.We have studied a sample of 28 periodically variable B-typesupergiants selected from the HIPPARCOS mission and 12 comparison starscovering the whole B-type spectral range. Our goal is to test if theirvariability is compatible with opacity-driven non-radialoscillations. Methods: .We have used the NLTE atmosphere codeFASTWIND to derive the atmospheric and wind parameters of the completesample through line profile fitting. We applied the method to selectedH, He, and Si line profiles, measured with the high resolution CESspectrograph attached to the ESO CAT telescope in La Silla, Chile.Results: .From the location of the stars in the (log T_eff, log g)diagram, we suggest that variability of our sample supergiants is indeeddue to the gravity modes resulting from the opacity mechanism. We findnine of the comparison stars to be periodically variable as well, andsuggest them to be new α Cyg variables. We find marginal evidenceof a correlation between the amplitude of the photometric variabilityand the wind density. We investigate the wind momentum-luminosityrelation for the whole range of B spectral type supergiants, and findthat the later types (>B5) perfectly follow the relation for Asupergiants. Additionally, we provide a new spectral type - T_effcalibration for B supergiants. Conclusions: .Our results imply thepossibility of probing internal structure models of massive stars ofspectral type B through seismic tuning of gravity modes.Figures of the spectral line fits and discussion of the individualobjects, Appendices A, B and Table 6 are only available in electronicform at http://www.aanda.org
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable 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/qcat?J/A+A/463/789
| Crux - our southern heritage.
Not Available
| Faint open clusters with 2MASS: BH 63, Lyngå 2, Lyngå 12 and King 20
Context: .Structural and dynamical parameters of faint open clusters areprobed with quality 2MASS-photometry and analytical procedures developedfor bright clusters. Aims: .We derive fundamental parameters ofthe faint open clusters Lyngå 2, BH 63, Lyngå 12 and King20, the last three of which have no prior determinations. We also focuson the structure and dynamical state of these clusters. Methods:.J, H and Ks 2MASS photometry with errors smaller than 0.2mag are used to build CMDs, radial density profiles, colour-colourdiagrams, luminosity and mass functions. Colour-magnitude filters areused to isolate probable member stars. Field-star decontamination isapplied to Lyngå 2, Lyngå 12 and King 20. Results:.Reddening values are in the range 0.22≤E(B-V)≤1.9, with BH 63 themost reddened object. Ages of Lyngå 2, King 20, Lyngå 12 andBH 63 are ≈90, ≈200, ≈560 and ≈700 Myr, respectively. Theradial density distributions of Lyngå 12 and King 20 arewell-represented by King profiles. Lyngå 2 and BH 63 are verysmall with core and limiting radii of ≈0.12 pc and ≈1.5 pc. Yet,they fit in the small-radii tail of the open cluster size distribution.Lyngå 12 and King 20 have R_core≈0.43 pc and R_lim≈3.9 pc.Lyngå 2 and Lyngå 12 are inside the Solar circle. Totalstellar masses (extrapolating the MFs to stars with 0.08 M_ȯ) rangefrom ≈340 M_ȯ (BH 63) to ≈2300 M_ȯ (Lyngå 12).Observed masses are ~1/4 of these values. In all clusters the core massfunction is flatter than the halo's. Conclusions: .Faint openclusters can be probed with 2MASS when associated with colour-magnitudefilters and field-star decontamination. BH 63 appears to be in anadvanced dynamical state, both in the core and halo. To a lesser degreethe same applies to King 20. Marginal evidence of dynamical evolution ispresent in the cores of Lyngå 2 and Lyngå 12.
| The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611
An analysis is presented of VLT-FLAMES spectroscopy for three Galacticclusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmospherecalculations have been used to estimate effective temperatures (fromeither the helium spectrum or the silicon ionization equilibrium) andgravities (from the hydrogen spectrum). Projected rotational velocitieshave been deduced from the helium spectrum (for fast and moderaterotators) or the metal line spectrum (for slow rotators). The origin ofthe low gravity estimates for apparently near main sequence objects isdiscussed and is related to the stellar rotational velocity. Theatmospheric parameters have been used to estimate cluster distances(which are generally in good agreement with previous determinations) andthese have been used to estimate stellar luminosities and evolutionarymasses. The observed Hertzsprung-Russell diagrams are compared withtheoretical predictions and some discrepancies including differences inthe main sequence luminosities are discussed. Cluster ages have beendeduced and evidence for non-coeval star formation is found for allthree of the clusters. Projected rotational velocities for targets inthe older clusters, NGC 3293 and NGC 4755, have been found to besystematically larger than those for the field, confirming recentresults in other similar age clusters. The distribution of projectedrotational velocities are consistent with a Gaussian distribution ofintrinsic rotational velocities. For the relatively unevolved targets inthe older clusters, NGC 3293 and NGC 4755, the peak of the velocitydistribution would be 250 km s-1 with afull-width-half-maximum of approximately 180 km s-1. For NGC6611, the sample size is relatively small but implies a lower meanrotational velocity. This may be evidence for the spin-down effect dueto angular momentum loss through stellar winds, although our results areconsistent with those found for very young high mass stars. For allthree clusters we deduce present day mass functions with Γ-valuesin the range of -1.5 to -1.8, which are similar to other young stellarclusters in the Milky Way.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Aims.We derive accurate parameters related to the CMD, structure anddynamical state of M 52 and NGC 3960, whose fields are affected bydifferential reddening. Previous works estimated their ages in theranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods: .J, Hand Ks 2MASS photometry with errors <0.2 mag is used tobuild CMDs, radial density profiles, luminosity and mass functions, andcorrect for differential reddening. Field-star decontamination isapplied to uncover the cluster's intrinsic CMD morphology, andcolour-magnitude filters are used to isolate stars with high probabilityof being cluster members. Results: .The differential-reddeningcorrected radial density profile of M 52 follows King's law with coreand limiting radii of R_core =0.91 ± 0.14 pc and R_lim =8.0± 1.0 pc. NGC 3960 presents an excess of the stellar density overKing's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ±0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 areR_tidal=13.1 ± 2.2 pc and R_tidal=10.7 ± 3.7 pc. Clusterages of M 52 and NGC 3960 derived with Padova isochrones are constrainedto 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF(χ_core=0.89 ± 0.12) is flatter than the halo's(χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up inMS/evolved stars in M 52 is ~1200 M_ȯ, and the total mass(extrapolated to 0.08M_ȯ) is ~3800 M_ȯ. The total mass in NGC3960 is ~1300 M_ȯ. Conclusions: .Compared to open clusters indifferent dynamical states studied with similar methods, the core andoverall parameters of M 52 are consistent with an open cluster moremassive than 1000 M_ȯ and ~60 Myr old, with some mass segregationin the inner region. The core of NGC 3960 is in an advanced dynamicalstate with strong mass segregation in the core/halo region, while thesomewhat flat overall MF (χ≈ 1.07) suggests low-mass starevaporation. The excess stellar density in the core may suggestpost-core collapse. The dynamical evolution of NGC 3960 may have beenaccelerated by the tidal Galactic field, since it lies ≈0.5 kpcinside the Solar circle.
| Detection of K_s-excess stars in the 14 Myr open cluster NGC 4755
Aims.We derive the structure, distribution of MS and PMS stars anddynamical state of the young open cluster NGC 4755. We explore thepossibility that, at the cluster age, some MS and PMS stars stillpresent infrared excesses related to dust envelopes and proto-planetarydiscs. Methods: .J, H and Ks 2MASS photometry is usedto build CMD and colour-colour diagrams, radial density profiles,luminosity and mass functions. Field-star decontamination is applied touncover the cluster's intrinsic CMD morphology and detect candidate PMSstars. Proper motions from UCAC2 are used to determine clustermembership. Results: .The radial density profile follows King'slaw with a core radius Rcore=0.7 ± 0.1 pc and alimiting radius Rlim=6.9 ± 0.1 pc. The cluster agederived from Padova isochrones is 14 ± 2 Myr. Field-stardecontamination reveals a low-MS limit at ≈1.4 M_ȯ. The core MF(χ=0.94 ± 0.16) is flatter than the halo's (χ=1.58± 0.11). NGC 4755 contains 285 candidate PMS stars of age 1{-}15 Myr, and a few evolved stars. The mass locked up in PMS, MS andevolved stars amounts to 1150 M_ȯ. Proper motions show thatK_s-excess MS and PMS stars are cluster members. K_s-excess fractions inPMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5%respectively, consistent with the cluster age. The core is deficient inPMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halobut they are scarce in the core. Conclusions: .Compared to openclusters in different dynamical states studied with similar methods, NGC4755 fits relations involving structural and dynamical parameters in theexpected locus for its age and mass. On the other hand, the flatter coreMF probably originates from primordial processes related to parentmolecular cloud fragmentation and mass segregation over 14 Myr. Starformation in NGC 4755 began ≈14 Myr ago and proceeded for about thesame length of time. Detection of K_s-excess emission in member MS starssuggests that some circumstellar dust discs survived for 10^7 yr,occurring both in some MS and PMS stars for the age and spread observedin NGC 4755.
| On the difference between nuclear and contraction ages
Context: .Ages derived from low mass stars still contracting onto themain sequence often differ from ages derived from the high mass onesthat have already evolved away from it. Aims: .We investigate thegeneral claim of disagreement between these two independent agedeterminations by presenting UBVRI photometry for the young galacticopen clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning theage range ~10-150 Myr Methods: .We derived reddenings, distances,and nuclear ages by fitting ZAMS and isochrones to color-magnitudes andcolor-color diagrams. To derive contraction ages, we used four differentpre-main sequence models, with an empirically calibratedcolor-temperature relation to match the Pleiades cluster sequence.Results: .When exclusively using the V vs. V-I color-magnitude diagramand empirically calibrated isochrones, there is consistency betweennuclear and contraction ages for the studied clusters. Although thecontraction ages seem systematically underestimated, in none of thecases do they deviate by more than one standard deviation from thenuclear ages.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| Quantitative spectroscopy of BA-type supergiants
Luminous BA-type supergiants have enormous potential for modernastrophysics. They allow topics ranging from non-LTE physics and theevolution of massive stars to the chemical evolution of galaxies andcosmology to be addressed. A hybrid non-LTE technique for thequantitative spectroscopy of these stars is discussed. Thorough testsand first applications of the spectrum synthesis method are presentedfor the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe),HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution andhigh-S/N Echelle spectra. Stellar parameters are derived fromspectroscopic indicators, consistently from multiple non-LTE ionizationequilibria and Stark-broadened hydrogen line profiles, and they areverified by spectrophotometry. The internal accuracy of the methodallows the 1σ-uncertainties to be reduced to 1-2% in T_effand to 0.05-0.10 dex in log g. Elemental abundances are determined forover 20 chemical species, with many of the astrophysically mostinteresting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTEcomputations reduce random errors and remove systematic trends in theanalysis. Inappropriate LTE analyses tend to systematicallyunderestimate iron group abundances and overestimate the light andα-process element abundances by up to factors of two to three onthe mean. This is because of the different responses of these species toradiative and collisional processes in the microscopic picture, which isexplained by fundamental differences of their detailed atomic structure,and not taken into account in LTE. Contrary to common assumptions,significant non-LTE abundance corrections of ~0.3 dex can be found evenfor the weakest lines (Wλ 10 mÅ). Non-LTEabundance uncertainties amount to typically 0.05-0.10 dex (random) and~0.10 dex (systematic 1σ-errors). Near-solar abundances arederived for the heavier elements in the sample stars, and patternsindicative of mixing with nuclear-processed matter for the lightelements. These imply a blue-loop scenario for η Leo because offirst dredge-up abundance ratios, while the other three objects appearto have evolved directly from the main sequence. In the most ambitiouscomputations several ten-thousand spectral lines are accounted for inthe spectrum synthesis, permitting the accurate reproduction of theentire observed spectra from the visual to near-IR. This prerequisitefor the quantitative interpretation of intermediate-resolution spectraopens up BA-type supergiants as versatile tools for extragalacticstellar astronomy beyond the Local Group. The technique presented hereis also well suited to improve quantitative analyses of less extremestars of similar spectral types.
| The VLT-FLAMES survey of massive stars.
Not Available
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| NGC 146: a young open cluster with a Herbig Be star and intermediate mass pre-main sequence stars
We present UBV CCD photometry and low-resolution spectra of stars in thefield of the young open cluster NGC 146. UBV photometry of 434 starswere used to estimate the E(B-V) reddening of 0.55 ± 0.04 mag andBV photometry of 976 stars were used to estimate a distance modulus of(m-M)0 = 12.7 ± 0.2 mag, corresponding to a distanceof 3470+335-305 pc. We estimated 10-16 Myr as theturn-off age for the upper main sequence of the cluster using isochronesand synthetic colour magnitude diagrams. We identified two B type starswith Hα in emission and located on the MS using slit-less spectra.A higher resolution spectrum of the brighter Be star indicated thepresence of a number of emission lines, with some lines showing thesignature of gas infall. This star was found to be located in the regionof Herbig Ae/Be stars in the (J-H) vs. (H-K) colour-colour diagram.Thus, we identify this star as a Herbig Be star. On the other hand, 54stars were found to show near infrared excess, of which 17 were found tobe located in the region of Herbig Ae/Be stars and 18 stars were foundto be located in the region of Be stars in the NIR colour-colourdiagram. Thus NGC 146 is a young cluster with a large number ofintermediate mass pre-main sequence stars. The turn-on age of thecluster is found to be ~3 Myr. Though NGC 146 shows an older turn off,the bulk of stars in this cluster seems to belong to the youngerpopulation of 3 Myr.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| Influence of the Coriolis force on the instability of slowly pulsating B stars
This paper explores the effect of rotation on the κ-mechanisminstability of slowly pulsating B stars. A new non-adiabatic code, whichadopts the so-called traditional approximation to treat the Coriolisforce, is used to investigate the influence exerted by rotation over thestability of stellar models covering the mass range2.5Msolar<=M*<= 13.0Msolar. Theprincipal finding is that, for all modes considered apart from theprograde sectoral (PS) class, rotation shifts the κ-mechanisminstability toward higher luminosities and effective temperatures; theseshifts are accompanied by broadenings in the extent of instabilitystrips. Such behaviour is traced to the shortening of mode periods underthe action of the Coriolis force. Instability strips associated with PSmodes behave rather differently, being shifted to marginally lowerluminosities and effective temperatures under the influence of rotation.The implications of these results are discussed in the context of theobservational scarcity of pulsation in B-type stars having significantrotation; various scenarios are explored to explain the apparentdichotomy between theory and observations. Furthermore, the possiblesignificance of the findings to Be stars is briefly examined.
| Pulsating Stars in the ASAS-3 Database. I. beta Cephei Stars
We present results of an analysis of the ASAS-3 data for short-periodvariables from the recently published catalog of over 38000 stars. Usingthe data available in the literature we verify the results of theautomatic classification related to \beta Cep pulsators. In particular,we find that 14 stars in the catalog can be classified unambiguously asnew beta Cep stars. By means of periodogram analysis we derive thefrequencies and amplitudes of the excited modes. The main modes in thenew beta Cep stars have large semi-amplitudes, between 35 and 80 mmag.Up to four modes were found in some stars. Two (maybe three) new betaCep stars are members of southern young open clusters: ASAS164409-4719.1 belongs to NGC 6200, ASAS 164630-4701.2 is a member ofHogg 22, and ASAS 164939-4431.7 could be a member of NGC 6216.We also analyze the photometry of four known beta Cep stars in theASAS-3 catalog, namely IL Vel, NSV 24078, V1449 Aql and SY Equ. Finally,we discuss the distribution of beta Cep stars in the Galaxy.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
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