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Resim Yükleyin
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Ýlgili Makaleler
Physical and Chemical Properties of Massive Clumps in the AFGL 333 Cloud
We have mapped two massive clumps, clump A and B, of the AFGL 333 cloudin the N2H+ J=1-0, CCSJN=43-32, HC3N J=5-4, andNH3 (J,K)=(1,1) and (2,2) lines with the Nobeyama RadioObservatory 45 m telescope. Intense N2H+ emissioncomes from the two clumps, and its overall distribution is similar tothat of C18O J=1-0. On the other hand, the CCS andHC3N emissions are more intense toward clump B than towardclump A. Thus, the column density ratios of [CCS]/[N2H+] and [HC3N]/[N2H+]tend to be higher toward clump B than toward clump A, indicating thatclump B is younger than clump A. This is supported by a fact that the2MASS sources with H-K>2 and the Spitzer 24 μm sources are mostlyassociated with clump A, whereas only a few such sources exist in clumpB. Clump B involves a few dense cores that are dark in the mid-infraredmaps. Such cores are thought to be good candidates of starless cores forhigh- or intermediate-mass stars.
| Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| 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.
| A Sino-German λ6 cm polarization survey of the Galactic plane. I. Survey strategy and results for the first survey region
Aims.Polarization measurements of the Galactic plane at λ6 cmprobe the interstellar medium (ISM) to larger distances compared tomeasurements at longer wavelengths, enabling us to investigateproperties of the Galactic magnetic fields and electron density. Methods: We are conducting a new λ6 cm continuum and polarizationsurvey of the Galactic plane covering 10°≤ l≤230° and|b|≤5°. Missing large-scale structures in the U and Q maps arerestored based on extrapolated polarization K-band maps from the WMAPsatellite. The λ6 cm data are analyzed together with maps inother bands. Results: We discuss some results for the first surveyregion, 7°×10° in size, centered at (l,b)=(125.5 °,0°). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1,were detected. They cause significant rotation of backgroundpolarization angles but little depolarization. G124.9+0.1 was identifiedas a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with asize of about 46 pc, has neither a counterpart in enhanced Hαemission nor in total intensity. A model combining foreground andbackground polarization modulated by the Faraday screen was developed.Using this model, we estimated the strength of the ordered magneticfield along the line of sight to be 3.9 μG for G124.9+0.1, andexceeding 6.4 μG for G125.6-1.8. We obtained an estimate of 2.5 and6.3 mK kpc-1 for the average polarized and total synchrotronemissivity towards G124.9+0.1. The synchrotron emission beyond thePerseus arm is quite weak. A spectral curvature previously reported forSNR G126.2+1.6 is ruled out by our new data, which prove a straightspectrum. Conclusions: .The new λ6 cm survey will play animportant role in improving the understanding of the properties of themagneto-ionic ISM. The magnetic fields in HII regions can be measured.Faraday screens with very low electron densities but large rotationmeasures were detected indicating strong and regular magnetic fields inthe ISM. Information about the local synchrotron emissivity can beobtained.
| The Fragmenting Superbubble Associated with the H II Region W4
New observations at high latitudes above the H II region W4 show thatthe structure formerly identified as a chimney candidate, an opening tothe Galactic halo, is instead a superbubble in the process offragmenting and possibly evolving into a chimney. Data at high Galacticlatitudes (b>5deg) above the W3/W4 star-forming region at1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U(linear polarization) reveal an egg-shaped structure with morphologicalcorrelations between our data and the Hα data of Dennison,Topasna, and Simonetti. Polarized intensity images show depolarizationextending from W4 up the walls of the superbubble, providing strongevidence that the radio continuum is generated by thermal emissioncoincident with the Hα emission regions. We conclude that theparts of the H II region hitherto known as W4 and the newly revealedthermal emission are all ionized by the open cluster OCl 352. At anassumed distance of 2.35 kpc, the ovoid structure is 164 pc wide andextends 246 pc above the midplane of the Galaxy. The shell's emissiondecreases in total intensity and polarized intensity in variouslocations, appearing to have a break at its top and another on one side.Using a geometric analysis of the depolarization in the shell's walls,we estimate that a magnetic field line-of-sight component of 3-5 μGexists in the shell. We explore the connection between W4 and theGalactic halo, considering whether sufficient radiation can escape fromthe fragmenting superbubble to ionize the kiloparsec-scale Hα loopdiscovered by Reynolds, Sterling and Haffner.
| The WENSS & Dwingeloo surveys and the Galactic magnetic field
Aims.We investigate the structure of the Galactic magnetic field in the2nd Galactic quadrant using radio continuum polarization data from the325 MHz WENSS (WEsterbork Northern Sky Survey) survey in combinationwith earlier single-dish observations. Methods: .We studygradients in polarization angle along Galactic longitude and latitude inthe region 130° ⪉ l ⪉ 173° and -5° ⪉ b ⪉31°. These gradients were determined with a new method that wedeveloped to efficiently and reliably fit linear gradients to periodicdata like polarization angles. Since the WENSS data were obtained with asynthesis array they suffer from the "missing short spacing" problem. Wehave tried to repair this by adding an estimate of the large-scaleemission based on the single-dish (Dwingeloo) data obtained by Brouw andSpoelstra. Combining all available data we derive a rotation measure mapof the area, from which we estimate all 3 components of the magneticfield vector. Results: .In the part of WENSS where large-scalestructure in polarized intensity is relatively unimportant, we find thatthe magnetic field is predominantly perpendicular to the line-of-sight,and parallel to the Galactic plane. The magnetic field components alongthe line-of-sight and along Galactic latitude have comparable values,and the strength of these components is much smaller than the strengthof the total magnetic field. Our observations also cover part of theso-called "fan" region, an area of strong polarized intensity, wherelarge-scale structure is missing from our WENSS data. We tentativelyshow that Faraday rotation occurring in front of the Perseus arm iscausing both the WENSS RM and the RM towards the fan region observed inprevious single-dish surveys, and we suggest that the fan is formed bylocal emission that originates in front of the emission we see inWENSS.
| Spectropolarimetric Study on Circumstellar Structure of Microquasar LS I+61°303
We present an optical linear spectropolarimetry study of the microquasarLS I +61°303. The continuum emission is mildly polarized (up to1.3%) and shows almost no temporal change. We found a distinct change ofpolarization across the Hα emission line, indicating the existenceof a polarization component intrinsic to the microquasar. We estimatedthe interstellar polarization component from polarization of theHα line and derived the intrinsic polarization component. Thewavelength dependence of the intrinsic component is well explained byThomson scattering in an equatorial disk of a Be-type mass donor. Theposition angle (PA) of the intrinsic polarization is ˜25°, andrepresents the rotational axis of the Be disk. This PA is nearlyperpendicular to the PA of a radio jet found during quiescent phases.Assuming an orthogonal disk-jet geometry around the compact star, therotational axis of the accretion disk is almost perpendicular to that ofthe Be disk. Moreover, according to the orbital parameters of themicroquasar, the compact star is likely to get across the Be disk aroundtheir periastron passage. We discuss the peculiar circumstellarstructure of this microquasar inferred based on our observation, andsuggest a possible connection with its high-energy activities.
| Atomic Carbon in the AFGL 333 Cloud
We have mapped the W3 giant molecular cloud in theC03P1-3P0 ([C I]492 GHz) and 12CO J=3-2 emission lines with the Mount FujiSubmillimeter-wave Telescope. The [C I] distribution is extended overthe molecular cloud, having peaks at three star forming clouds, W3 Main,W3(OH), and AFGL 333. The [C I] emission is found to be strong in theAFGL 333 cloud, where the 12CO J=3-2 emission is relativelyweak. In order to characterize the physical and chemical states of theAFGL 333 cloud, we have also observed the CO J=1-0 isotopomer lines andthe CCS and N2H+ lines with the Nobeyama RadioObservatory 45 m Telescope. The [C0]/[CO] and[CCS]/[N2H+] abundance ratios are found to behigher in the AFGL 333 cloud than in the W3(OH) cloud, suggesting thatthe AFGL 333 cloud is younger than the W3(OH) cloud. In the AFGL 333cloud we have found two massive cores without any sign of active starformation. They are highly gravitationally bound and are regarded asgood candidates for a massive prestellar core.
| 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.
| Early-type stars in the young open cluster IC 1805. II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558
Aims.We address the issue of the multiplicity of the three brightestearly-type stars of the young open cluster IC 1805, namely HD 15570, HD15629 and HD 15558. Methods: .For the three stars, we measured theradial velocity by fitting Gaussian curves to line profiles in theoptical domain. In the case of the massive binary HD 15558, we also useda spectral disentangling method to separate the spectra of the primaryand of the secondary in order to derive the radial velocities of the twocomponents. These measurements were used to compute orbital solutionsfor HD 15558. Results: .For HD 15570 and HD 15629, the radialvelocities do not present any significant trend attributable to a binarymotion on time scales of a few days, nor from one year to the next. Inthe case of HD 15558 we obtained an improved SB1 orbital solution with aperiod of about 442 days, and we report for the first time on thedetection of the spectral signature of its secondary star. We derivespectral types O5.5III(f) and O7V for the primary and the secondary ofHD 15558. We tentatively compute a first SB2 orbital solution althoughthe radial velocities from the secondary star should be considered withcaution. The mass ratio is rather high, i.e. about 3, and leads to veryextreme minimum masses, in particular for the primary object. Minimummasses of the order of 150 ± 50 and 50 ± 15 M_ȯ arefound respectively for the primary and the secondary. Conclusions:.We propose that HD 15558 could be a triple system. This scenario couldhelp to reconcile the very large minimum mass derived for the primaryobject with its spectral type. In addition, considering new andpreviously published results, we find that the binary frequency amongO-stars in IC 1805 has a lower limit of 20%, and that previouslypublished values (80%) are probably overestimated.
| Stellar Rotation: A Clue to the Origin of High-Mass Stars?
We present the results of a study aimed at assessing whether low- andhigh-mass stars form similarly. Our approach is (1) to examine theobserved projected rotational velocities among a large sample of newlyformed stars spanning a range in mass between 0.2 and 50Msolar and (2) to search for evidence of a discontinuity inrotational properties that might indicate a difference in the stellarformation process at some characteristic mass. Our database includesrecently published values of vsini for young intermediate- and low-massstars in Orion, as well as new observations of O stars located in youngclusters and OB associations. We find that the median of the quantityvobs/vc (observed rotational speed divided byequatorial breakup velocity) is typically about 0.15 and shows noevidence of a discontinuity over the full range of stellar masses, whilethe quantity Jsini/M (derived angular momentum per unit mass) exhibits aslow, monotonic rise (J/M~M0.3) with increasing mass with noevidence of a discontinuity. We suggest that these observations are mostsimply interpreted as indicative of a single stellar formation andangular momentum regulation mechanism, one that results in rotationrates well below breakup and angular momenta per unit mass that differsystematically by no more than a factor of 3-4 over a mass rangespanning a factor of 250.
| Observations of 14 young open star clusters with the HEGRA system of Cherenkov telescopes
Context: .A sample of 14 young open star clusters has been observed inthe TeV energy regime with the stereoscopic system of the HEGRA (HighEnergy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002,resulting in more than 300 h of observation time. Aims: .Youngopen star clusters may contribute to the acceleration of cosmic rays.The detection of γ-rays (from decaying π^0s produced inhadronic interactions) from these objects could be evidence for such acontribution. The results of our observations are compared to availableγ-ray data and to a simple hadronic model in the framework ofshock front acceleration of cosmic rays in the stellar winds of thecluster members to test the potential of the presently available data onyoung open star clusters to constrain this type of model. Methods:.The stereoscopic system of HEGRA Cherenkov telescopes makes use of theatmospheric imaging technique. Air showers initiated by primaryGamma-Rays are recorded as elliptical images in the telescope cameras.The images from the different telescopes are then superimposed toreconstruct the parameters of the primary particle. This technique(stereoscopy) was pioneered by the HEGRA experiment. Results: .Nosignificant excess has been found in the analysed data set of young openstar clusters. The derived upper limit on the TeV gamma-ray flux fromBerkeley 87 and the available EGRET data from the same direction do notallow us to fully constrain the simple hadronic model used here. Thecomparison of the upper limits derived for all 14 objects with the fluxdetected from TeV J2032+4130 (under the assumption of an association ofthe TeV-signal with the compact stellar association Cyg OB2) suggeststhat γ-ray emission from young open star clusters as an objectclass cannot be ruled out.
| Rotating elephant trunks
Aims.We investigate the structure and velocity of cold molecularpillars, "elephant trunks", in expanding H II regions. Methods:.The trunks are seen in silhouette against the bright background in ourHα images. All trunks are filamentary, and show signs of beingtwisted. Four such trunks in NGC 7822, IC 1805, the Rosette Nebula, andDWB 44 were selected, and then mapped mainly in 12CO and13CO. We determine the mass and density of the trunks. Mostof the mass is concentrated in a head facing the central cluster, and insub-filaments forming the body of the trunk that is connected toV-shaped filaments to the outer expanding shell. Results: .Wediscovered that all four trunks rotate as rigid bodies (to a firstapproximation) about their major axes, and that at least two trunks arestretching along their major axes, meaning that the massive heads arelagging behind in the general expansion of the H II regions. Therotational periods are of the order of a few million years - similar tothe age of the clusters. Rotation, then, is responsible for the twistedappearance of many elephant trunks, since they are rooted in the outershells. The trunks carry surprisingly large amounts of angular momentum,3× 1048{-}2× 1050 kg m2s-1, with corresponding rotational energies of up to 1037 J. However, we estimate the total magnetic energies tobe even larger. The trunks continuously reshape, and the formation oftwined, and in many cases helical, sub-filaments can be understood as aconsequence of electromagnetic and inertia forces inside the trunks. Atheory based on the concept of magnetically twisted trunks is developedfurther, where the initial angular momentum is a consequence of thetwisting of parent filaments containing mass condensations. Our resultsalso suggest a new process of removing angular momentum from parentmolecular clouds.
| 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.
| Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool
An advanced approach to the kinematic distance method is developed. Themethod is applicable to second- and third-quadrant Galactic objects withknown velocities. It is based on fitting a model of the density andvelocity features in an isothermal H I disk to observed H I data. Thevelocity field of the gas is modeled with a power law for basic circularrotation, underlying noncircular motions from a two-armed density wavepattern. With a reasonable number of adjustable parameters andconstraints the model reproduces observations toward many Galacticobjects, and accurate distances are found from the modeled velocityfield. High-resolution H I spectral line data from the Canadian GalacticPlane Survey (CGPS; Taylor et al.) are used to discriminate clouds fromthe intercloud medium (the ``stratum'') for which the model is intended.The ability of the model to reproduce these data is demonstrated in one-[Tb(v)] and two- [Tb(l, v)] dimensional fits.Distances to 22 H II regions and SNRs calculated by the fitted velocityfield compare extremely well with other kinematically independentdistances.
| 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.
| Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.
| Binary and Multiple O-Type Stars in the Cassiopeia OB6 Association
We present the results of time-resolved spectroscopy of 13 O-type starsin the Cas OB6 stellar association. We conducted a survey for radialvelocity variability in search of binary systems, which are expected tobe plentiful in young OB associations. Here we report the discovery oftwo new single-lined binaries, and we present new orbital elements forthree double-lined binaries (including one in the multiple-star systemHD 17505). One of the double-lined systems is the eclipsing binarysystem DN Cas, and we present a preliminary light-curve analysis thatyields the system inclination, masses, and radii. We compare the spectraof the single stars and the individual components of the binary starswith model synthetic spectra to estimate the stellar effectivetemperatures, gravities, and projected rotational velocities. We alsomake fits of the spectral energy distributions to derive E(B-V),R=AV/E(B-V), and angular diameter. A distance of 1.9 kpcyields radii that are consistent with evolutionary models. We find that7 of 14 systems with spectroscopic data are probable binaries,consistent with the high binary frequency found for other massive starsin clusters and associations.
| An additional Hα emission component in LS I +61°303: Further evidence for the Hα emission related to the neutron star
A long-term program has been established since 1992 to monitor more thana dozen of observable X-ray binaries. We present the long-termspectroscopic results of the Be/X-ray binary, LS I +61°303. TheHα emission line, usually observed as a double-peaked profile witha dominant red peak, varies noticeably during our program. While theblue peak of the Hα emission line decreases gradually, the redpeak increases considerably in 2002 October observational run andremains relatively unchanged in 2004 September run. We attribute thesevariations to the asymmetric perturbation in the Be envelope. Finally,we report on the discovery of a two-component structure in the Hαemission line when close to the apastron. In addition to the originaldouble-peaked Hα emission component, an additional emissioncomponent emerges at the center part of the emission line, which couldoriginate in the accretion disc around the neutron star.
| O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic Odwarfs to track the conditions under which weak winds (i.e. mass lossrates lower than 10-8 Mȯ yr-1)appear. The sample is composed of low and high luminosity dwarfsincluding Vz stars and stars known to display qualitatively weak winds.Atmosphere models including non-LTE treatment, spherical expansion andline blanketing are computed with the code CMFGEN (Hillier & Miller1998, ApJ, 496, 407). Both UV and Hα lines are used to derive windproperties while optical H and He lines give the stellar parameters. Wefind that the stars of our sample are usually 1 to 4 Myr old. Mass lossrates of all stars are found to be lower than expected from thehydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). Forstars with log {L}/{Lȯ} 5.2, the reduction is byless than a factor 5 and is mainly due to the inclusion of clumping inthe models. For stars with log {L}/{Lȯ} 5.2 thereduction can be as high as a factor 100. The inclusion of X-rayemission (possibly due to magnetic mechanisms) in models with lowdensity is crucial to derive accurate mass loss rates from UV lines,while it is found to be unimportant for high density winds. The modifiedwind momentum - luminosity relation shows a significant change of slopearound this transition luminosity. Terminal velocities of low luminositystars are also found to be low. Both mass loss rates and terminalvelocities of low L stars are consistent with a reduced line forceparameter α. However, the physical reason for such a reduction isstill not clear although the finding of weak winds in Galactic starsexcludes the role of a reduced metallicity. There may be a link betweenan early evolutionary state and a weak wind, but this has to beconfirmed by further studies of Vz stars. X-rays, through the change inthe ionisation structure they imply, may be at the origin of a reductionof the radiative acceleration, leading to lower mass loss rates. Abetter understanding of the origin of X-rays is of crucial importancefor the study of the physics of weak winds.
| 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.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions
We analyse the optical spectrum of the very massive binary system WR 20a(WN6ha + WN6ha). The most prominent emission lines, Hα and He iiλ 4686, display strong phase-locked profile variability. From thevariations of their equivalent widths and from a tomographic analysis,we find that part of the line emission probably arises in a windinteraction region between the stars. Our analysis of the opticalspectrum of WR 20a indicates a reddening of AV ≃ 6.0mag and a distance of 7.9 kpc, suggesting that the star actuallybelongs to the open cluster Westerlund 2. The location of the system at 1.1 pc from the cluster core could indicate that WR 20a was gentlyejected from the core via dynamical interactions. Using a non-LTE modelatmosphere code, we derive the fundamental parameters of each component:Teff = 43 000 ± 2000 K,log{Lbol/Lȯ} ≃ 6.0, dot{M} = 8.5× 10-6 Mȯ yr-1 (assuming aclumped wind with a volume filling factor f = 0.1). Nitrogen is enhancedin the atmospheres of the components of WR 20a, while carbon isdefinitely depleted. Finally, the position of the binary components inthe Hertzsprung-Russell diagram suggests that they are core hydrogenburning stars in a pre-LBV stage and their current atmospheric chemicalcomposition probably results from rotational mixing that might beenhanced in a close binary compared to a single star of same age.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass(mmax) in an ensemble of N stars, as a function of theinitial mass function (IMF) upper mass cutoff (mup) and N. Westatistically demonstrate that the upper IMF of the local massive starcensus observed thus far in the Milky Way and Magellanic Clouds clearlyexhibits a universal upper mass cutoff around 120-200 Msolarfor a Salpeter IMF, although the result is more ambiguous for a steeperIMF.
| Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanicalfeedback from massive stars trigger star formation, because there arenumerous examples of superbubbles showing secondary star formation attheir edges. However, although these systems show an age sequence, theydo not provide strong evidence of a causal relationship. The W3/W4Galactic star-forming complex suggests a three-generation hierarchy: thesupergiant shell structures correspond to the oldest generation; thesetriggered the formation of IC 1795 in W3, the progenitor of a molecularsuperbubble that in turn triggered the current star-forming episodes inthe embedded regions W3-North, W3-Main, and W3-OH. We present UBVphotometry and spectroscopic classifications for IC 1795, which show anage of 3-5 Myr. This age is intermediate between the reported 6-20 Myrage of the supergiant shell system and the extremely young ages(104-105 yr) for the embedded knots of theultracompact H II regions, W3-North, W3-Main, and W3-OH. Thus, an agesequence is indeed confirmed for the entire W3/W4 hierarchical system.This therefore provides some of the first convincing evidence thatsuperbubble action and mechanical feedback are indeed a triggeringmechanism for star formation.
| High-resolution spectroscopic observations of the D'-type symbiotic stars HD 330036 and AS 201
We present high-resolution spectroscopic analyses of two starsclassified as D'-type symbiotic stars, HD 330036 and AS 201. These twostars display both rapid rotation and enhancements of the s-processelements that are synthesized via slow neutron captures during stellarevolution along the asymptotic giant branch (AGB). Both characterisiticsof rapid rotation and s-process overabundances have also been discoveredrecently in another D'-type symbiotic, S190. The stellar parametersderived here for HD 330036 are Teff=6200 K and log g=2.4,while AS 201 has Teff=6000 K and log g=2.3. Resultingluminosity and distance estimates are 650 Lȯ and 2.3 kpcfor HD 330036, and 700 Lȯ and 4.3 kpc for AS 201. BothHD 330036 and AS 201 have evolved away from the main sequence and areapproaching the base of the red-giant branch. These stars havenear-solar abundances of iron and calcium, but substanial enhancements(by about +0.9 dex for HD 330036 and +1.7 dex for AS 201) of thes-process element barium. The observed barium overabundances in thecurrent cool-star members of these two binary systems probably resultedfrom mass-transfer when the current white dwarf was an AGB star. Therapid rotation found in the cool stars may also be due to mass-transfer,with the mass-gaining stars being spun up with the transfer of angularmomentum from the AGB winds. As only a few (six) D'-type symbiotics areknown, the fact that the 3 studied to date at high-spectral resolutionall display rapid rotation and s-process elemental overabundances mayindicate that these two traits are signatures of these rare binarysystems.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under the agreement with theCNPq-Observatório Nacional (Brazil).
| Radio observations of X-ray binaries .
This paper describes how observations at radio wavelengths havecontributed to shape our current picture of X-ray binaries as accretionpowered sources in the Galaxy. A significant part of the progress duringthe past twenty years has been the result of modern radiointerferometry. This path will certainly be continued with theconstruction of several improved and much more sensitive arrays. Here, Ireview the generalities of several observational facts which appearrelevant to me for our current understanding of these accreting compactobjects. Attention is also focused on new observational issues likely toprovide important advances in a nearby future. These include the searchfor signatures of interaction between relativistic jets from X-raybinaries and the interstellar medium as well as the confirmation thatthese systems can be sources of high energy gamma -rays.
| On the class of Oe stars\fnmsep
We present high-quality spectra of the majority of stars that have beenclassified as Oe and find that their published spectral types aregenerally too early, most likely due to infilling of He I lines. As amatter of fact, all stars classified as Oe actually fall inside therange O9-B0 with the important exception of HD 155806 (O7.5 III) andperhaps HD 39680 (difficult to classify, but likely O8.5 V).Observations of a sample of objects with published spectral types in theO9-B0 range previously classified as peculiar or emission-line starsfail to reveal any new Oe star with spectral type earlier than O9.5.Most objects classified as peculiar in ``classical'' literature showsigns of binarity in our spectra, but no spectral anomalies. We concludethat there is likely a real decline in the fraction of Be stars forspectral types earlier than B0, not due to observational bias. The fewOe stars with spectral types earlier than O9.5 deserve detailedinvestigation in order to provide constraints on the physical reasons ofthe Be phenomenon.Based on observations obtained at the Isaac Newton Telescope (La Palma,Spain).
| The Stellar Birthline and Birth-Rate Function in IC 1805: A Feasibility Study
HR diagrams for the youngest, optically-visible clusters potentiallyencode (a) the range of mass accretion rates, dM/dt, that characterizedthe stellar assembly phase; and (b) the rate of star formation as afunction of mass, N(M,t). In this contribution, we make use of publishedphotometry and spectroscopy for the young cluster IC 1805 to explore thefactors that influence our ability to deduce dM/dt and N(M,t), and tosuggest observations that could in principle provide more robustconstraints on these quantities.
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