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Fast transition between classical and weak lined T Tauri stars due to external UV dissipation
The discovery of optical jets immersed in the strong UV radiation fieldof the Rosette Nebula sheds new light on, but meanwhile poses challengesto, the study of externally irradiated jets. The jet systems in theRosette are found to have a high state of ionization and show uniquefeatures. In this paper, we investigate the evolutionary status of thejet-driving sources for young solar-like stars. To our surprise, thesejet sources indicate unexpected near infrared properties with no excessemission. They are bathed in harsh external UV radiation such thatevaporation leads to a fast dissipation of their circumstellar material.This could represent a transient phase of evolution of young solar-likestars between classical and weak lined T Tauri stars. Naked T Tauristars formed in this way have indistinguishable evolutionary ages fromthose of classical T Tauri stars resulting from the same episode of starformation. However, it would be hard for such sources to be identifiedif they are not driving an irradiated jet in a photoionized medium.
| On the Likelihood of Supernova Enrichment of Protoplanetary Disks
We estimate the likelihood of direct injection of supernova ejecta intoprotoplanetary disks using a model in which the number of stars withdisks decreases linearly with time, and clusters expand linearly withtime such that their surface density is independent of stellar number.The similarity of disk dissipation and main-sequence lifetimes impliesthat the typical supernova progenitor is very massive, ~75-100Msolar. Such massive stars are found only in clusters with>~104 members. Moreover, there is only a small regionaround a supernova within which disks can survive the blast yet beenriched to the level observed in the solar system. These two factorslimit the overall likelihood of supernova enrichment of a protoplanetarydisk to <~1%. If the presence of short-lived radionucleides inmeteorites is to be explained in this way, however, the solar systemmost likely formed in one of the largest clusters in the Galaxy, morethan 2 orders of magnitude greater than Orion, where multiple supernovaeimpacted many disks in a short period of time.
| 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.
| Spitzer/IRAC-MIPS Survey of NGC 2244: Protostellar Disk Survival in the Vicinity of Hot Stars
We present the results from a survey of NGC 2244 from 3.6 to 24 μmwith the Spitzer Space Telescope. The 24 μm-8 μm-3.6 μm colorcomposite image of the region shows that the central cavity surroundingthe multiple O and B stars of NGC 2244 contains a large amount of cooldust visible only at 24 μm. Our survey gives a detailed look at disksurvivability within the hot-star-dominated environment in this cavity.Using mid-infrared two-color diagrams ([3.6]-[4.5] vs. [5.8]-[8.0]), weidentified 337 class II and 25 class I objects out of 1084 objectsdetected in all four of these bands with photometric uncertainty betterthan 10%. Including the 24 μm data, we found 213 class II and 20class I sources out of 279 stars also detected at this latter band. Thecenter of the class II density contours is in very good agreement withthe center of the cluster detected in the 2MASS images. We studied thedistribution of the class II sources relative to the O stars and foundthat the effect of high-mass stars on the circumstellar disks issignificant only in their immediate vicinity.
| Resolving the Nature of the Rosette HH 1 Jet Facing Strong UV Dissipation
The Rosette HH 1 jet is a collimated flow immersed in the strong UVradiation field of the Rosette Nebula. We investigate the physicalproperties of the Rosette HH 1 jet using high-quality narrowband imagesand high-dispersion spectroscopy. The new images show that the axis ofthe jet is not precisely aligned with the star near the base of the jet.The high resolution of the spectra allows us to accurately determine thecontributions from the H II region, jet, and star. The approaching andreceding sides of the expanding shell of the Rosette Nebula are atheliocentric velocities of 13 and 40 km s-1, while the jetreaches a maximum velocity offset at a heliocentric velocity of -30 kms-1. The [S II] doublet ratios indicate an electron densityof ~1000 cm-3 in the jet and <=100 cm-3 in theH II region. With a careful subtraction of the nebular and jetcomponents, we find the stellar Hα line is dominated by a broadabsorption profile with little or no emission component, indicating alack of substantial circumstellar material. The circumstellar materialhas most likely been photoevaporated by the strong UV radiation field inthe Rosette Nebula. The evaporation timescale is 103-104 yr. The Rosette HH 1 jet source provides evidence for anaccelerated evolution from a CTTS to a WTTS due to the strong UVradiation field; therefore, both CTTSs and WTTSs can be spatially mixedin regions with massive star formation.
| 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.
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| New magnetic chemically peculiar stars
Spectropolarimetric observations of 96 chemically peculiar (CP)main-sequence stars have been carried out at the 6-m telescope at theSpecial Astrophysical Observatory of the Russian Academy of Sciences(SAO RAS) with the aim of searching for the presence of stellar magneticfields. The stars selected for investigation were CP stars known to havestrong anomalies in the wavelength region of the continuum fluxdepression around λ 5200Å. This selection was conductedwith the aid of low-resolution spectral observations, made with the SAORAS 1-m telescope, and of published differential photometric data.Magnetic fields have been successfully detected in 72 stars of whichonly three stars were previously known to have magnetic fields. For twostars, the longitudinal component of the magnetic field Beexceeds 5 kG: HD178892 - 7.4 kG, and HD258686 - 6.7 kG. We failed toreliably detect the magnetic field in the other 24 CP stars. These starsare mostly fast rotators, a feature which hampers accurate measurementsof Be. It is demonstrated in this paper that selectingcandidate magnetic stars by considering their photometric indices Z orΔa, or alternatively, by inspecting low-resolution spectra aroundthe λ5200Å flux depression, considerably increases thedetection rate.This paper is based on data obtained at the 6-m telescope of the RussianAcademy of Sciences.E-mail: dkudr@sao.ru
| Spitzer MIPS 24 μm Detection of Photoevaporating Protoplanetary Disks
We present 24 μm images of three protoplanetary disks beingphotoevaporated around high-mass O-type stars. These objects have``cometary'' structure where the dust pulled away from the disk by thephotoevaporating flow is forced away from the O star by photon pressureon the dust and heating and ionization of the gas. Models of the 24 and8 μm brightness profiles agree with this hypothesis. These modelsshow that the mass-loss rate needed to sustain such a configuration isin agreement with or somewhat less than the theoretical predictions forthe photoevaporation process.
| 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 XMM-Newton view of Plaskett's star and its surroundings
XMM-Newton data of Plaskett's star (HD 47129) are used in order toanalyse its X-ray spectrum and variability and hence to derive furtherconstraints on the wind interaction in this early-type binary (O6 I +O7.5 I) system.Conventional models fail to provide a consistent fit of the EuropeanPhoton Imaging Camera (EPIC) and Reflexion Grating Spectrometer (RGS)spectra. The lines seen in the RGS spectrum have a temperature ofmaximum emissivity between 0.18 and 1.4 keV. The EPIC and RGS spectraare best fitted by a non-equilibrium model consisting of abremsstrahlung continuum at 2.2 +/- 0.1 keV and a number of independentemission lines. Our tests also suggest that an overabundance in nitrogenby a factor of ~6 might be indicated to best represent the RGS spectrum.On the other hand, a short-term variability study of the light curves ofthe system indicates that the X-ray flux of Plaskett's star did notdisplay any significant variability during our observation. This resultholds for all time-scales investigated here (from a few minutes to aboutone hour). Combining our XMM-Newton data with ROSAT archivalobservations, we find, however, a significant variability on the orbitaltime-scale. If this behaviour is indeed phase locked, it suggests aminimum in the X-ray flux when the primary star is in front. This mightbe attributed to an occultation of the colliding wind region by the bodyof the primary.Finally, 71 other X-ray sources have been detected in the field aroundPlaskett's star and most of them have a near-infrared (near-IR)counterpart with colours that are consistent with those of slightlyreddened main-sequence objects. Actually, a sizeable fraction of theX-ray sources in the EPIC images could be either foreground orbackground sources with no direct connection to HD 47129.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.E-mail: linder@astro.ulg.ac.be (NL), rauw@astro.ulg.ac.be (GR) ‡Research Associate FNRS, Belgium.
| 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.
| 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.
| Turbulent Gas Flows in the Rosette and G216-2.5 Molecular Clouds: Assessing Turbulent Fragmentation Descriptions of Star Formation
The role of turbulent fragmentation in regulating the efficiency of starformation in interstellar clouds is examined from new wide-field imagingof 12CO and 13CO J=1-0 emission from the Rosetteand G216-2.5 molecular clouds. The Rosette molecular cloud is a typicalstar-forming giant molecular cloud, and G215-2.5 is a massive molecularcloud with no OB stars and very little low-mass star formation. Theproperties of the turbulent gas flow are derived from the set ofeigenvectors and eigenimages generated by principal component analysis(PCA) of the spectroscopic data cubes. While the two clouds representquite divergent states of star formation activity, the velocitystructure functions for both clouds are similar. The sonic scale,λS, defined as the spatial scale at which turbulentvelocity fluctuations are equivalent to the local sound speed, and theturbulent Mach number evaluated at 1 pc, M1pc, are derivedfor an ensemble of clouds including the Rosette and G216-2.5 regionsthat span a large range in star formation activity. We find no evidencefor the positive correlations between these quantities and the starformation efficiency that are predicted by turbulent fragmentationmodels. A correlation does exist between the star formation efficiencyand the sonic scale for a subset of clouds withLFIR/M(H2)>1 that are generating young stellarclusters. Turbulent fragmentation must play a limited and nonexclusiverole in determining the yield of stellar masses within interstellarclouds.
| 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.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| The Pre-Main-Sequence Population of L988
L988 is a large (~0.5d×0.7d) dark cloud complex at about 600 pcthat contains several bright pre-main-sequence objects (such as V1331Cyg and LkHα 321), but this paper deals in detail only with asmall region on its eastern edge, near the HAeBe star LkHα 324.That star and its distant companion LkHα 324SE lie at the apex ofa V-shaped area apparently excavated from the edge of L988, and are thebrightest members of a small cluster containing about 50Hα-emission stars. A median age of about 0.6 Myr (with largedispersion) is inferred from its color-magnitude diagram, constructedfrom VRI photometry to V=22. Keck HIRES spectra show that LkHα324SE is probably also an HAeBe. Its image is nonstellar, and within 3"to the northwest are three condensations having complex [S II] and [O I]profiles and radial velocities up to -200 km s-1. Theyprobably originate in an outflow from LkHα 324SE. A bright Ap starwith strong Si II lines is embedded in the heavy obscuration 8' to thewest. It illuminates a small reflection nebulosity, has several faintHα emitters nearby, and shares the radial velocity of L988, soclearly it was formed in that cloud. It demonstrates again that suchchemical peculiarities can be established very early in young stars ofmoderate mass.
| The maximum stellar mass, star-cluster formation and composite stellar populations
We demonstrate that the mass of the most massive star in a clustercorrelates non-trivially with the cluster mass. A simple algorithm,according to which a cluster is filled up with stars that are chosenrandomly from the standard initial mass function (IMF) but sorted withincreasing mass, yields an excellent description of the observationaldata. Algorithms based on random sampling from the IMF without sortedadding are ruled out with a confidence larger than 0.9999. A physicalexplanation of this would be that a cluster forms by more-massive starsbeing consecutively added until the resulting feedback energy sufficesto revert cloud contraction and stops further star formation. This hasimportant implications for composite populations. For example,104 clusters of mass 102Msolar will notproduce the same IMF as one cluster with a mass of106Msolar. It also supports the notion that theintegrated galaxial stellar IMF (IGIMF) should be steeper than thestellar IMF and that it should vary with the star formation rate of agalaxy.
| 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.
| Multiseeded Multimode Formation of Embedded Clusters in the Rosette Molecular Complex: Structured Star Formation toward the Southeastern Boundary
The Rosette Molecular Complex contains embedded clusters with diverseproperties and origins. We have previously explored the shell mode offormation in the north (regions A and B) and the massive concentrationsin the ridge (region C). Here we explore star formation toward the southof the complex, region D, based on data from the spatially complete TwoMicron All Sky Survey. We find that stars are forming prolificallythroughout this region in a highly structured mode, with both clustersand loose aggregates detected. The most prominent cluster (region D1)lies in the north-center. This cluster is over 20 pc to the south of theMonoceros Ridge, the interface of the emerging young OB cluster NGC 2244with its ambient molecular clouds. In addition, there are severalbranches stemming from AFGL 961 in region C and extending to thesoutheastern boundary of the cloud. We invoke a tree model to interpretthis pattern, corresponding to probable tracks of abrupt turbulentexcitation and subsequent decay. Alternatively, we discussgravoturbulent collapse scenarios based on numerical simulations.Relative stellar ages and gas flow directions will differentiate betweenthese mechanisms.
| 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.
| Confirmation of a Stellar Microjet in the Rosette H II Region (NGC 2244)
A possible microjet from a low-mass but young star, which has alreadyshed its cocoon, could be rendered observable by the Lyman photon fluxin the interior of the Rosette Nebula. Outside this environment it maynot have been observable at optical wavelengths. The kinematics of thisproposed monopolar microjet from an F8 Ve star have been investigated byspatially resolved, long-slit, spectral observations with the ManchesterEchelle Spectrometer on the San Pedro Martir telescope (Mexico). Theflow is shown to be approaching to give a radial velocity differencefrom the host nebula of -56 km s-1. An outflow velocity of,at the most, a few hundreds of km s-1 is therefore indicated.If the flow velocity is taken as 200 km s-1, which is foundin other microjets, then this jet's inclination to the sky is ~16°.The mass in the outflowing ionized gas is estimated from the surfacebrightness of the Hα emission as ~6×1027 g togive an estimated mass-loss rate of 10-8 Msolaryr-1, which, along with the detection of the outflowvelocity, confirms its microjet identification even though an uncertainfilling factor was used in these calculations. The hottest cluster star,which is also in the neighborhood of the microjet, is found alone toemit marginally sufficient Lyman photons to account for the ionizationof the jet, although direct observations of the local electron densityfrom optical line ratios are required to confirm this pointconclusively.
| The Newly Hatched Rich Massive Cluster in the Ridge of the Rosette Molecular Cloud
We explore loose congregations of medium- to high-mass protoclustersidentified to the southeast of NGC 2244. On using data from thespatially complete Two Micron All Sky Survey, the true extent of theburst of cluster formation along the ridge of the Rosette MolecularCloud is revealed. Here we investigate the properties and finestructures of the most prominent cluster embedded in the densest rim ofthe cloud. This protocluster is resolved into two compact subclustersaligned along the major axis of the entire complex, in line with NGC2244. The subclusters are found to have a physical scale of around 1 pc,typical of known embedded clusters. The K-band luminosity function alsosuggests a young age. However, near-infrared excess emission is found inapproximately one-sixth of the reddened objects. This is stillcommensurate with an age estimate of <1 Myr, provided that themassive stars have rapidly stripped the circumstellar material fromtheir neighbors. The well-known massive young binary associated withAFGL 961, however, is situated to the south of the major components ofthe cluster, where the stellar density is comparatively low. This isinconsistent with mass segregation and signifies a different formationprocess for these high-mass protostellar objects.
| Abundances from disentangled component spectra: the eclipsing binary V578 Mon
Chemical abundances of the early-B type components of the binary V578Mon are derived from disentangled component spectra. This is a pilotstudy showing that, even with moderately high line-broadening, metalabundances can be derived from disentangled spectra with a precision of0.1 dex, differential to sharp-lined single stars of the same spectraltype. This binary is well-suited to such an assessment because of itsyouth as a member of the Rosette Nebula cluster NGC 2244, strengtheningthe expectation of an unevolved ZAMS chemical composition. The method isuseful for studying rotationally driven mixing in main-sequence stars,since fundamental stellar parameters are known with higher accuracy in(eclipsing) binaries. This paper also evaluates of the bias that mightbe present in disentangled spectra.
| 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.
| A Survey of N IV and O IV Features near 3400 Å in O2-O5 Spectra
We have conducted a survey of little-known N IV and O IV multiplets near3400 Å in an extensive sample of well-classified, very earlyO-type spectra. The initial motivation was to search for additionaluseful classification criteria for these types, but an unexpected resultis the high sensitivity of these features to evolutionary CNOprocessing. We have found a useful discriminant between O2 and latertypes in the relative strengths of the O IV multiplets, one of which issubject to selective emission in the hottest spectra; the overallstrengths of these lines also decrease between spectral types O4 and O5.More remarkable, however, are the variations in the N/O ratios amongboth individual stars and clusters. For instance, several O4 If+ spectrahave very large ratios, while main-sequence stars in the Carina Nebulagenerally have smaller values than others of the same spectral types inother regions. These effects correspond to different degrees of mixingof processed material as a function of evolutionary age and initialrotational velocities; the second effect provides significant furtherevidence that very massive stars mix while still on the main sequence.Thus, further analysis of these features will likely provide valuablediagnostics of important evolutionary parameters.
| A Parsec-Scale Outflow in the Rosette Molecular Cloud?
We report on new observations of a purported parsec-scale outflow in thehostile environment at the boundary of the Rosette Nebula, a well-knownH II region driven by several O stars in the open cluster NGC 2244, andthe Rosette Molecular Cloud (RMC). Several emission features were firstreported by Ybarra & Phelps in 2004 , but with only optical [S II]imaging, their nature could not then be established. Our newnear-infrared (NIR) H2 imaging observations reveal bulletedNIR emission between a Two Micron All Sky Survey (2MASS) NIR excesspoint source and the optical [S II] emission for RMC C of Ybarra &Phelps. Analysis of the 2MASS colors indicates that the point source islikely to be a Class I young stellar object (YSO). Together theseresults imply that RMC C is in fact part of a true YSO outflow, nowdesignated HH 871. The presence of additional [S II] emission,apparently connected to the HH 871 outflow but separated by ~1.2 pc,assuming a distance of 1600 pc for the RMC, also lends support to theYbarra & Phelps hypothesis that a parsec-scale outflow is present inthis harsh ionizing environment.
| Is the Emerging Massive Cluster NGC 2244 a Twin Cluster?
We present in this paper the first near-infrared study of the young opencluster NGC 2244, which is well known for its partially embedded naturein the Rosette Nebula. Based on the spatially complete Two Micron AllSky Survey, the young OB cluster indicates apparent substructures. It issurprisingly resolved into a compact core that matches well thecongregation of massive OB stars in the optical, a satellite cluster ata distance of 6.6 pc west of the center of NGC 2244 and probably a majorstellar aggregate resembling an arc in structure right below the core.This infrared study provides various new updates on the nature of theyoung open cluster, including its central position, physical scale, andstellar population. A disk fraction of ~20.5%+/-2.8% is achieved for itsmembers with masses above 0.8 Msolar. NGC 2244 is thus aunique example for the study of embedded clusters.
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