Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
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.
| Pre-main sequence stars in open clusters. I. The DAY-I catalogue
Context: We present the basic ideas and first results from the projectwe are carrying out at present, the search for and characterisation ofpre-main sequence (PMS) stars among the members of Galactic youngclusters. The observations of 10 southern clusters, nine of them locatedin the Carina-Sagittarius spiral arm of the Milky Way are presented. Aims: We aim at listing candidate PMS member stars in young clusters.The catalogued stars will serve as a basis for future spectroscopicstudies of individual objects to determine the properties of stellarformation in the last phases before the main sequence stage. Propertiessuch as the presence of residual envelopes or disks, age spread amongPMS members, and the possible presence of several episodes of starformation in the clusters, are to be addressed. Methods: Multicolourphotometry in the UBVR_CIC system has been obtained for 10southern young clusters in the fourth Galactic quadrant, located betweenGalactic longitudes l = 238° and l = 310°. For six clusters inthe sample, the observations presented here provide the first publishedstudy based on CCD photometry. A quantitative comparison is performedwith post-MS isochrones, and PMS isochrones from three differentevolutionary models are used in the photometric membership analysis forpossible PMS stars. Results: The observations produce photometricindices in the Johnson-Cousins photometric systems for a total of 26 962stars. The matching of our pixel coordinates with corresponding fieldsin the 2MASS data base provides astrometric calibration for allcataloged stars and JHK 2MASS photometric indices for 60% of them.Post-MS cluster ages range from 4 to 60 Myr, whereas the photometricmembership analysis assigns PMS membership to a total of 842 stars,covering an age range between 1 and 10 Myr. This information on the PMScandidate members has been collected into a catalogue, named DAY-I,which contains 16 entries for 842 stars in the field of 10 southernclusters.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1397
| 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.
| An X-Ray and Near-Infrared Study of Young Stars in the Carina Nebula
We present a multiwavelength study of the central region of the CarinaNebula, including Trumpler 16 and part of Trumpler 14. Our analysis ofthe Chandra X-Ray Observatory archival data led to the identification of454 X-ray sources. These sources were then cross-identified with opticalphotometric and spectroscopic information available from the literatureand with newly obtained near-infrared (JHKs) imagingobservations. A total of 38 known OB stars are found to be X-rayemitters. All the O stars and early-B stars follow the nominal relationbetween the X-ray and bolometric luminosities,LX~10-7Lbol. A few mid- to late-B starsare found to be associated with X-ray emission, likely attributable to TTauri companions. We discovered 16 OB star candidates that suffer alarge extinction in the optical wave bands. Some 300 sources have theX-ray and infrared characteristics of late-type pre-main-sequence stars.Our sample represents the most comprehensive census of the young stellarpopulation in the Carina Nebula so far and should be useful for thestudy of the star formation history of this massive starburst region. Wealso report the finding of a compact (2'×4')group of 10 relatively bright X-ray sources, all of which are detectedin the near-infrared wavelengths and are highly reddened. The group isspatially coincident with the dark V-shaped dust lane bisecting theCarina Nebula and may be part of an embedded association. Thedistribution of the young stellar groups surrounding the H II regionassociated with Trumpler 16 is consistent with the collect-and-collapsescenario of triggered star formation.
| CCD photometric search for peculiar stars in open clusters. VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, Ruprecht 44, Ruprecht 115, and Ruprecht 120
Context: We continue our survey of magnetic chemically peculiar (CP2)stars in galactic open clusters to shed more light on their origin andevolution. Aims: To study the group of CP2 stars, it is essential tofind these objects in different galactic environments and at a widerange of evolutionary stages. The knowledge of open cluster ages andmetallicities can help for finding a correlation between theseparameters and the (non-)presence of peculiarities, which has to betaken into account in stellar evolution models. Methods: Theintermediate band Δ a photometric system samples the depth of the5200 Å flux depression by comparing the flux at the centre withthe adjacent regions with bandwidths of 110 Å to 230 Å. Itis capable of detecting magnetic CP2 and CP4 stars with high efficiency,but also the groups of (metal-weak) λ Bootis and classicalBe/shell stars can be successfully investigated. In addition, it allowsthe age, reddening, and distance modulus to be determined withappropriate accuracy by fitting isochrones. Results: From the 1677observed members of the eight open clusters, one Ae and twenty-five CP2stars were identified. Furthermore nineteen deviating stars aredesignated as questionable for several reasons. The estimated age,reddening, and distance for the programme clusters were compared withpublished values of the literature and discussed in this context. Conclusions: .The current paper shows that CP2 stars are continuouslypresent in very young (7 Myr) to intermediate age (500 Myr) openclusters at distances greater than 2 kpc from the Sun.Based on observations at CASLEO, CTIO (Proposal 2003A-0057), and OSN.The Observatorio de Sierra Nevada is operated by the Consejo Superior deInvestigaciones Científicas through the Instituto deAstrofísica de Andalucía (Granada, Spain). Photometricdata are only avaialable in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/591 Full Fig. [seefull textsee full textsee full textsee full textsee full text], Tables[see full textsee full textsee full textsee full textsee full text] and[see full textsee full textsee full textsee full textsee full text] areonly available in electronic form at http://www.aanda.org
| 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.
| 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.
| A census of the Carina Nebula - I. Cumulative energy input from massive stars
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
| 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.
| Physical parameters and wind properties of galactic early B supergiants
We present optical studies of the physical and wind properties, plus CNOchemical abundances, of 25 O9.5-B3 Galactic supergiants. We employnon-LTE, line blanketed, extended model atmospheres, which provide amodest downward revision in the effective temperature scale of early Bsupergiants of up to 1-2 kK relative to previous non-blanketed results.The so-called "bistability jump" at B1 (Teff 21 kK)from Lamers et al. is rather a more gradual trend (with large scatter)from v&infy;/vesc3.4 for B0-0.5 supergiantsabove 24 kK to v&infy;/vesc 2.5 for B0.7-1supergiants with 20 kK ≤ Teff ≤ 24 kK, andv&infy;/vesc 1.9 for B1.5-3 supergiants below20 kK. This, in part, explains the break in observed UV spectralcharacteristics between B0.5 and B0.7 subtypes as discussed by Walbornet al. We compare derived (homogeneous) wind densities with recentresults for Magellanic Cloud B supergiants and generally confirmtheoretical expectations for stronger winds amongst Galacticsupergiants. However, winds are substantially weaker than predictionsfrom current radiatively driven wind theory, especially at mid-Bsubtypes, a problem which is exacerbated if winds are already clumped inthe Hα line forming region. In general, CNO elemental abundancesreveal strongly processed material at the surface of Galactic Bsupergiants, with mean N/C and N/O abundances 10 and 5 times higher thanthe Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating thelowest degree of processing in our sample, and HD 152236 (B1.5Ia+) the highest.
| The oscillation modes of the beta Cephei star in HD 92024 in the open cluster NGC 3293 .
The primary star of the binary HD 92024 is a prominent beta Cepheivariable; three oscillation frequencies are known, it is an eclipsingbinary component and has cluster membership (NGC 3293). Towardsasteroseismic inferences of the star's three known oscillationfrequencies, we use photometry and spectroscopy to find that f_1=5.6400;c d-1, f_2=7.1624 ;c d-1 and f_3=6.6584 ;cd-1 are modes of degree l=2, 4 and 2 respectively, all withm≠0. We also find that by combining pulsational information fromseveral spectral lines, we suppress the noise and improve themode-identification process. In conclusion, HD 92024 is a key object tobe subjected to a seismic analysis.
| 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.
| High-Mass Cloud Cores in the η Carinae Giant Molecular Cloud
We carried out an unbiased survey for massive dense cores in the giantmolecular cloud associated with η Carinae with the NANTEN telescopein the 12CO, 13CO, and C18O J=1-0emission lines. We identified 15 C18O cores, whose typicalline width ΔVcomp, radius r, mass M, column densityN(H2), and average number density n(H2) were 3.3km s-1, 2.2 pc, 2.6×103 Msolar,1.3×1022 cm-2, and 1.2×103cm-3, respectively. Two of the 15 cores are associated withIRAS point sources whose luminosities are larger than 104Lsolar, which indicates that massive star formation isoccurring within these cores. Five cores, including the two with IRASsources, are associated with MSX point sources. We detectedH13CO+ (J=1-0) emission toward fourC18O cores, two of which are associated with IRAS and MSXpoint sources; another one is associated only with an MSX point source,and the other is associated with neither IRAS nor MSX point sources. Thecore with neither IRAS nor MSX point sources shows the presence of abipolar molecular outflow in 12CO (J=2-1), which indicatesthat star formation is also occurring in the core, and the other threeof the four H13CO+ detections show winglikeemission. In total, 6 C18O cores out of 15 (=40%) haveexperienced star formation, and at least 2 of 15 (=13%) are massivestar-forming cores in the η Car GMC. We found that massive starformation occurs preferentially in cores with larger N(H2),M, and n(H2) and a smaller ratio of Mvir/M. Wealso found that the cores in the η Car GMC are characterized bylarge ΔV and Mvir/M on average compared to the cores inother GMCs observed with the same telescope. These properties of thecores may account for the fact that as much as 60%-87% of the cores donot show any signs of massive star formation. We investigated the originof a large amount of turbulence in the η Car GMC. We found thatturbulence injection from stellar winds, molecular outflows, andsupernova remnants that originated from stars formed within the GMC arenot enough to explain the existing turbulence. We propose thepossibility that the large turbulence was preexisting when the GMC wasformed and is now dissipating. Mechanisms such as multiple supernovaexplosions in the Carina flare supershell may have contributed to form aGMC with a large amount of turbulence.
| 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.
| 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
| Revisiting the area of the open cluster Stock 16
Deep CCD UBV(RI)C photometry has been carried out in the areaof the open cluster Stock 16. The different photometric diagramsindicate the presence of three superimposed stellar groups, all composedof early type stars. The first of them, the cluster Stock 16, suffersfrom slightly variable reddening with a mean color excess = 0.51 . The second group, reddened by = 0.84 , includes the Wolf-Rayet star WR 50 (aWC7+OB), and the third one, with a mean < EB-V> = 1.18, also includes another Wolf-Rayet star, WR 51 (a WN4+OB?). Our resultsconfirm the distance and reddening already stated for Stock 16 butinclude new members. Concerning the other two highly reddened groups, weshow that the first one - located at a distance of 3600 pc - is probablyrelated to an anonymous OB association behind Cen OB1 also at 3600 pcalready mentioned in the literature; the other seems to be a distant OBassociation - at more than 9000 pc - which may belong to the Scutum-Cruxspiral arm. In these two groups we find evidence that the absorptionlaw, AV/EB-V = R amounts to 4.0 approximately. Theages of the different star populations in the zone range from 5 ×106 yr to 6.4 × 106 yr for Stock 16, 8× 106 for the second most reddened group and 5 ×106 for the probable association in Scutum-Crux. Adetermination of the IMF slope of Stock 16 was also carried out forstars in the mass range 1.8 < M < 14 Mȯ, givinga normal value x = 1.3.Based on observations collected at the University of Toronto SouthernObservatory (UTSO), Las Campanas, Chile.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/471
| The β Cephei variable in the eclipsing binary HD 92024. I. Determination of the orbit
HD 92024 is a member of the open cluster NGC3293; it is the only eclipsing binary system with a β Cepheicomponent known in the southern hemisphere. This paper presents thefirst spectroscopic analysis of the single-lined binary and provides anorbital analysis based on recent high-resolution spectroscopy andphotometry. A procedure to derive orbital elements with minimalinfluence of the pulsational line-profile variations on the radialvelocities is applied, and we find the stellar masses to be 15.0+3.0-4.0Mȯ and 3.0±0.5 Mȯ respectively, and radii of8.4±0.8 Rȯ, 2.1±0.4 Rȯ. The temperatures are 25500±500 K and 12 500±1000 K. The surface gravity values oflog g= 3.77±0.08 and 4.27±0.18 indicate that the binaryconsists of an evolved early B-type primary and a late-B main sequencecompanion. The resulting distance 2.8±0.3 kpc and systemicvelocity -16 km s-1 confirm cluster membership. Theoretical stellarmodels indicate an age of 10-13 Myr, somewhat higher than theliterature value for the brightest members of NGC 3293.Based on observations made with ESO Telescopes at the La SillaObservatory, Chile, under programmes 66.D-0297, 67.D-0023, 68.D-028, andwith the Strömgren Automatic Telescope, La Silla.
| High-resolution spectroscopy of the eclipsing beta Cephei star HD92024
We present spectroscopic data from a recent orbital analysis of theeclipsing SB-1 type binary system HD92024 which has a beta Cepheicomponent. Fully reduced spectra are given together with technicaldetails from the analysis, published elsewhere. In addition, a high-S/Ntime-averaged spectrum with line identifications is provided ingraphical and digital form. The spectra show pronounced pulsationalvariability, which is essential for application in asteroseismology butdisturbing for the dynamical analysis. Orbital elements derived from thespectra with a procedure devised to minimise influence from theseline-profile variations are found to be more precise than orbitalparameters resulting from classical methods. A line-variabilityindicator is described and then used to guide a selection of spectralline-regions rich in information on the stellar beta Cepheioscillations. Two lines are examined as examples.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| Ambipolar-Diffusion Timescale, Star Formation Timescale, and the Ages of Molecular Clouds: Is There a Discrepancy?
We reexamine critically the estimates of the duration of differentphases of star formation and the lifetimes of molecular clouds based onthe ages of T Tauri stars, age spreads of stars in clusters, andstatistics of prestellar cores. We show that all available observationaldata are consistent with lifetimes of molecular clouds comparable to~107 yr, as well as with the predictions of the theory ofself-initiated, ambipolar-diffusion-controlled star formation. Weconclude that there exists no observational support for either ``young''molecular clouds or ``rapid'' star formation.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
Каталоги и обозначения:
|
