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 Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component AnalysisThe Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB. A multiwavelength investigation of the temperature of the cold neutral mediumWe present measurements of the HI spin temperatures (Ts) ofthe cold neutral medium (CNM) towards radio sources that are closelyaligned with stars for which published H2 ortho-paratemperatures (T01) are available from ultraviolet (UV)observations. Our sample consists of 18 radio sources close to 16 nearbystars. The transverse separation of the lines of sight of thecorresponding UV and radio observations varies from 0.1 to 12.0 pc atthe distance of the star. The UV measurements do not have velocityinformation, so we use the velocities of low ionization species (e.g.NaI/KI/CI) observed towards these same stars to make a plausibleidentification of the CNM corresponding to the H2 absorption.We then find that T01 and Ts match withinobservational uncertainties for lines of sight with H2 columndensity above 1015.8cm-2, but deviate from eachother below this threshold. This is consistent with the expectation thatin the CNM Ts tracks the kinetic temperature due tocollisions and that T01 is driven towards the kinetictemperature by proton exchange reactions. Spitzer and Magellan Observations of NGC 2264: A Remarkable Star-forming Core near IRS 2We analyze Spitzer and Magellan observations of a star-forming core nearIRS 2 in the young cluster NGC 2264. The submillimeter source IRAS 12S1, previously believed to be an intermediate-mass Class 0 object isshown to be a dense collection of embedded, low-mass stars. We arguethat this group of stars represents the fragmenting collapse of a dense,turbulent core, using a number of indicators of extreme youth. Withreasonable estimates for the velocity dispersion in the group, weestimate a dynamical lifetime of only a few times 104 yr.Spectral energy distributions of stars in the core are consistent withClass I or Class 0 assignments. We present observations of an extensivesystem of molecular hydrogen emission knots. The luminosity of theobjects in the core region are consistent with roughly solar massprotostars. The Homogeneity of Interstellar Elemental Abundances in the Galactic DiskWe present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA. The Discordance of Mass-Loss Estimates for Galactic O-Type StarsWe have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments. Contact Binaries with Additional Components. I. The Extant DataWe have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems. Can single O stars produce non-thermal radio emission?We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems. Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. Rotational velocities, stellar parameters, and oxygen abundancesWe present the results of a spectroscopic analysis of the Trapeziumcluster stars inside the Orion nebula. The rotational velocities wereobtained using the Fourier analysis method, and we found agreement withvalues derived by the usual method based on linewidth measurements. Therotational velocity derived for θ1 Ori C by thismethod is consistent with the variability of some of its spectralfeatures that have a period of 15.42 days. By means of the fit of H, HeI, and He II observed profiles with Fastwind synthetic profiles, stellarparameters and wind characteristics were derived. This methodology letus estimate the errors associated with these parameters, and we foundthat macroturbulence effects have to be included for a good fit to theHe I-II lines in the spectrum of θ1 Ori C. By means ofa very accurate study, oxygen abundances were derived for the threeB0.5V stars θ1 Ori A, D, and θ2 OriB. Final abundances are consistent with the nebular gas-phase resultspresented in Esteban et al. (2004) and are lower than those given byCunha & Lambert (1994). Our results suggest a lower dust depletionfactor of oxygen than previous estimations for the Orion nebula. Evolution of X-ray emission from young massive star clustersThe evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603. X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264Images of the NGC 2264 star-forming region, which we have acquired withthe XMM-Newton spacecraft, reveal strong X-ray emission from threedeeply embedded (AV>=10 mag) young stellar objects in thevicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz& Grasdalen's infrared source (RNO-EW = IRS 2). Thermal plasmamodels for the brightest source in X-rays, located 11" southwest ofAllen's star, yield a quasi-steady luminosity ofLX=1033 ergs s-1 and an extraordinarilyhigh X-ray temperature of 100 MK. The high temperature is consistentwith the presence of emission lines of Fe XXV and Fe XXVI at photonenergies of 6.7 and 6.9 keV, respectively. An even higher temperature ofnearly ~140 MK was observed during the rise phase of a powerfulimpulsive X-ray flare of another young star in the IRS 2 region.Moderate-resolution near-infrared (1-4 μm) spectra of the embeddedobjects, obtained at the NASA Infrared Telescope Facility, exhibit deepwater ice absorption bands, as well as a variety of emission andabsorption features of H I, CO, and both neutral and ionized metals. Photometric Accretion Signatures Near the Substellar BoundaryMultiepoch imaging of the Orion equatorial region by the Sloan DigitalSky Survey has revealed that significant variability in the bluecontinuum persists into the late-M spectral types, indicating thatmagnetospheric accretion processes occur below the substellar boundaryin the Orion OB1 association. We investigate the strength of theaccretion-related continuum veiling by comparing the reddening-invariantcolors of the most highly variable stars against those of main-sequenceM dwarfs and evolutionary models. A gradual decrease in the g-bandveiling is seen for the cooler and less massive members, as expected fora declining accretion rate with decreasing mass. We also see evidencethat the temperature of the accretion shock decreases in the very lowmass regime, reflecting a reduction in the energy flux carried by theaccretion columns. We find that the near-IR excess attributed tocircumstellar disk thermal emission drops rapidly for spectral typeslater than M4. This is likely due to the decrease in color contrastbetween the disk and the cooler stellar photosphere. Since accretion,which requires a substantial stellar magnetic field and the presence ofa circumstellar disk, is inferred for masses down to 0.05Msolar, we surmise that brown dwarfs and low-mass stars sharea common mode of formation. The T Tauri Star Population of the Young Cluster NGC 2264An Hα emission survey of the young cluster NGC 2264 in the Mon OB1association resulted in the detection of 490 Hα emission stars ina 25'×40' field approximately centeredbetween the O7 V multiple star S Mon and the Cone Nebula. The survey wascarried out with the wide-field grism spectrograph (WFGS) on theUniversity of Hawaii 2.2 m telescope on Mauna Kea. X-ray observationsmade with the European Photon Imaging Camera on board the European SpaceAgency's XMM-Newton satellite observatory will be discussed in asubsequent paper. Optical (BVRCIC) photometry wasobtained for selected fields to supplement similar data from theliterature. Spectra covering the 6000-8000 Å region at aresolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were obtained for 150 Hα and X-rayemission sources with the Gemini Multi-Object Spectrograph.Near-infrared spectra (1-2.5 μm) of a number of T Tauri stars (TTSs),X-ray sources, and LHα25 (W90) were also obtained using SpeX onthe Infrared Telescope Facility. Ages and masses for the Hαemitters were inferred from the isochrones and evolutionary tracks ofD'Antona & Mazzitelli. The median age for the TTS population isabout 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, existsfor individual objects. Several fields in the cluster were observed withthe WFGS on more than one occasion, permitting an examination ofHα variability over long baselines in time. About 90% of theclassical T Tauri stars (CTTSs) showed changes in W(Hα) of atleast 10%, while 57% varied at levels of 50% or more. No evidence wasfound for a significant pool of dormant Hα emitters. Summing themasses of the TTSs and the OB stellar population of NGC 2264, a lowerlimit for the total stellar mass content of the cluster is about 430Msolar. This is less than 1% of the total mass of the atomicand molecular gas believed to be associated with NGC 2264. Evidence forhierarchical structure within the cluster is suggested by the spatialdistribution of TTSs. Four concentrations of Hα emitters areevident: two near S Mon and two near the Cone Nebula. The median age ofthe TTSs in the immediate vicinity of S Mon was found to be greater thanthat of the TTSs near Allen's infrared source (IRS-1), but a significantdispersion is present. From the rotational data of Lamm et al. andMakidon et al., 241 of the TTSs are periodic variables, 150 weak-line TTauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregularvariables (30 WTTSs and 93 CTTSs). A weak-to-moderate positivecorrelation is found between H-K color and Prot for theCTTSs, in the sense that stars having longer periods tend to have largerH-K colors. A similar positive correlation is found betweenLHα and Prot among the CTTSs. Nostatistically significant correlation is found between Protand theoretical age or between Prot and LX. Othertopics discussed include the fraction of Hα emitters that areWTTSs, f(WTTS)=N(WTTS)/N(TTS), for clusters of different ages; therelative detectability of Hα emission using WFGS and narrowbandfilter imaging techniques; and the correlation of W(Li I) withTe, age, H-K color, and W(Hα). To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveysOB-stars have the highest luminosities and strongest stellar winds ofall stars, which enables them to interact strongly with theirsurrounding ISM, thus creating bow shocks. These offer us an idealopportunity to learn more about the ISM. They were first detected andanalysed around runaway OB-stars using the IRAS allsky survey by vanBuren et al. (1995, AJ, 110, 2614). Using the geometry of such bowshocks information concerning the ISM density and its fluctuations canbe gained from such infrared observations. As to help to improve the bowshock models, additional observations at other wavelengths, e.g.Hα, are most welcome. However due to their low velocity these bowshocks have a size of  1°, and could only be observed as awhole with great difficulties. In the light of the new Hα allskysurveys (SHASSA/VTSS) this is no problem any more. We developeddifferent methods to detect bow shocks, e.g. the improved determinationof their symmetry axis with radial distance profiles. Using twoHα-allsky surveys (SHASSA/VTSS), we searched for bow shocks andcompared the different methods. From our sample we conclude, that thecorrelation between the direction of both proper motion and the symmetryaxis determined with radial distance profile is the most promisingdetection method. We found eight bow shocks around HD17505, HD 24430, HD48099, HD 57061, HD92206, HD 135240, HD149757, and HD 158186 from 37 candidatestaken from van Buren et al. (1995, AJ, 110, 2614). Additionally to thetraditional determination of ISM parameters using the standoff distanceof the bow shock, another approach was chosen, using the thickness ofthe bow-shock layer. Both methods lead to the same results, yieldingdensities ( 1 cm-3) and the maximal temperatures (104 K), that fit well to the up-to-date picture of the WarmIonised Medium. A near-infrared stellar spectral library: I. H-band spectra.This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html The Indo-US Library of Coudé Feed Stellar SpectraWe have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http. A Galactic O Star CatalogWe have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services. The Homogeneity of Interstellar Oxygen in the Galactic DiskWe present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985. The N Enrichment and Supernova Ejection of the Runaway Microquasar LS 5039We present an investigation of new optical and ultraviolet spectra ofthe mass donor star in the massive X-ray binary LS 5039. The opticalband spectral line strengths indicate that the atmosphere is N-rich andC-poor, and we classify the stellar spectrum as type ON6.5 V((f)). TheN-strong and C-weak pattern is also found in the stellar wind P Cygnilines of N V λ1240 and C IV λ1550 (narrow absorptioncomponents in the former indicate that the wind terminal velocity isV&infy;=2440+/-190 km s-1). We suggest that the Nenrichment may result from internal mixing if the O star was born as arapid rotator or the O star may have accreted N-rich gas prior to acommon envelope interaction with the progenitor of the supernova. Wereevaluated the orbital elements to find an orbital period ofP=4.4267+/-0.0005 days. We compared the spectral line profiles with newnon-LTE line-blanketed model spectra from Lanz and Hubeny, from which wederive an effective temperature Teff=37.5+/-1.7 kK, gravitylogg=4.0+/-0.1, and projected rotational velocity Vsini=140+/-8 kms-1. We fitted the UV, optical, and IR flux distribution byusing a model spectrum and extinction law with parametersE(B-V)=1.28+/-0.02 and R=3.18+/-0.07. We confirm the covariability ofthe observed X-ray flux and stellar wind mass-loss rate derived from theHα profile, which supports the wind accretion scenario for theX-ray production in LS 5039. Wind accretion models indicate that thecompact companion has mass MX/Msolar=1.4+/-0.4,consistent with its identification as a neutron star. We argue that theO star has mass in the range 20-35 Msolar (based on a lowerlimit for the distance and the lack of eclipses). The observedeccentricity and runaway velocity of the binary can be reconciled onlyif the neutron star received a modest kick velocity due to a slightasymmetry in the supernova explosion (during which more than 5Msolar was ejected).Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555. The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. I. The Initial Mass Function around S MonocerotisThe young open cluster NGC 2264 was observed on 2002 January 6-7 withthe CFH12K mosaic CCD on the 3.6 m Canada-France-Hawaii Telescope toinvestigate the shape of the initial mass function (IMF). In order toselect the pre-main-sequence (PMS) members more completely, ChandraX-Ray Observatory archival data were retrieved and used for theidentification of X-ray-emitting weak-line T Tauri stars. In addition,the (R-I,V-I) diagram was used to exclude background late-type giants,and a statistical approach was applied to estimate the number ofcontaminating foreground main-sequence stars. The shape of the IMF ofthe Chandra field of view is flat between logm=-0.4 and ~-1.0 (m insolar mass units), with a weak signature of a dip near logm=-0.2. Theoverall shape of the IMF is very similar to those obtained for thePleiades or the Trapezium. The fraction of disked PMS stars, as noted byRebull and coworkers, does not show any signature of mass dependency.The age of PMS stars between logm=0 and -1 is 3.1+/-0.5 Myr. Chandra X-Ray Observations of Young Clusters. I. NGC 2264 DataWe present results of a Chandra observation of a field in NGC 2264. Theobservations were taken with the ACIS-I camera with an exposure time of48.1 ks. We present a catalog of 263 sources, which includes X-rayluminosity, optical and infrared photometry, and X-ray variabilityinformation. We found 41 variable sources, 14 of which have a flarelikelight curve, and two of which have a pattern of a steady increase ordecrease over a 10 hr period. The optical and infrared photometry forthe stars identified as X-ray sources are consistent with most of theseobjects being pre-main-sequence stars with ages younger than 3 Myr. On the Hipparcos parallaxes of O starsWe compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected. A radio and mid-infrared survey of northern bright-rimmed cloudsWe have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org A rotational and variability study of a large sample of PMS stars in NGC 2264We present the results of an extensive search for periodic and irregularvariable pre-main sequence (PMS) stars in the young (2-4 Myr) opencluster NGC 2264, based on photometric monitoringusing the Wide Field Imager (WFI) on the 2.2 m telescope on La Silla(Chile). In total, about 10 600 stars with IC magnitudesbetween 9.8 mag and 21 mag have been monitored in our 34 arcmin ×33 arcmin field. Time series data were obtained in the ICband in 44 nights between Dec. 2000 and March 2001; altogether weobtained 88 data points per star. Using two different time seriesanalysis techniques (Scargle periodogram and CLEAN) we found 543periodic variable stars with periods between 0.2 days and 15 days. Also,484 irregular variable stars were identified using aχ2-test. In addition we have carried out nearlysimultaneous observations in V, RC and a narrow-band Hαfilter. The photometric data enable us to reject background andforeground stars from our sample of variable stars according to theirlocation in the IC vs. (RC-IC)colour-magnitude and (RC-Hα) vs.(RC-IC) colour-colour diagrams. We identified 405periodic variable and 184 irregular variable PMS stars as clustermembers using these two different tests. In addition 35 PMS stars forwhich no significant variability were detected could be identified asmembers using an Hα emission index criterion. This yields a totalof 624 PMS stars in NGC 2264, of which only 182 were previously known.Most of the newly found PMS stars are fainter than IC ≃15 mag and of late spectral type (M 2). We find that the periodicvariables, as a group, have a smaller degree of variability and smallerHα index than the irregular variables. This suggests that thesample of periodic variables is biased towards weak-line T Tauri stars(WTTSs) while most of the irregular variables are probably classical TTauri stars (CTTSs). We have quantified this bias and estimated that theexpected fraction of WTTSs among PMS stars in the cluster is 77%. Thisis relatively close to the fraction of WTTSs among the periodicvariables which is 85%. We also estimated the total fraction ofvariables in the cluster using only two well selected concentrations ofPMS stars called NGC 2264 N & S in which we can easily estimate thetotal number of PMS stars. We find that at least 74% of the PMS stars inthe cluster with IC≤ 18.0 mag were found to be variable(either periodic or irregular) by our study. This number shows that oursearch for PMS stars in NGC 2264 through extensive and accuratephotometric monitoring is very efficient in detecting most PMS starsdown to at least IC=18.0 mag.Full Tables 4 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/557 Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketingWe study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations. The Trapezium Stars. Preliminary Results on Detailed Atmosphere ModelingA spectroscopic study of the Trapezium Cluster stars in the Orion Nebulais presented. We determine the stellar parameters of the two B0.5V starstheta(1}_{) Ori A and D. The variable spectral features of theta(1}_{)Ori C are studied, and some consequences on the modeling of the stellaratmosphere are considered. Fast Line-Profile Variability in the Spectra of O StarsA program of the search for and analysis of profile variability in thespectra of bright O supergiants with a time resolution of 5-30 min isdescribed. Preliminary results of the spectroscopic observations of thestars lambda Ori, alpha Cam, and 19 Cep with the 1-m SpecialAstrophysical Observatory telescope in 2001 are presented. Line-profilevariability was detected in the spectra of all the stars studied;variability in the H_alpha and C III lambda5696 lines in the spectrum oflambda Ori has been found for the first time. The variability amplitudeis 4-5% for 19 Cep and 1-2% for alpha Cam and lambda Ori on time scalesfrom several hours to 3 or 4 days, and the width of the variablefeatures reaches 2 Angstroms (100 km/s). We detected cyclical variationsin the He II lambda4686 and C III lambda5696 line profiles in thespectrum of lambda Ori on time scales of 1.3-1.6 days. Rapid profilevariations on time scales of 3.5-7 h were found in the violet parts ofthe H_alpha and He I lambda4715 line profiles in the spectrum of lambdaOri A. High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse CloudsWe present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion ofgrain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I. The Massive Triple Star System HD 16429 AHD 16429 A is a triple star system consisting of a single-linedspectroscopic binary and a widely separated third component, previouslyidentified via speckle interferometry. Here I present the first orbitalelements for the unblended spectroscopic binary as well as estimates ofthe spectral types and relative flux contributions for each visiblecomponent based on a Doppler tomographic reconstruction of theirspectra. There are several stars around HD 16429 A, including the nearbyBe X-ray binary and microquasar LS I +61deg303, which allprobably belong to a subcluster within the Cas OB6 association. Transitions from Autoionized Single-ionized Tin States: A Theoretical Study of the 5s5p (3Po) nl (nl = 5d, 6s) Levels of Sn IILines corresponding to several transitions from autoionized states ofsingle-ionized tin were identified in a laser-produced plasma generatedby 10640 Å irradiation of an Sn target at a flux of2×1010 W cm-2. Spectra were recorded andanalyzed between 2000 and 7000 Å. Theoretical analysis of Sn IIwas extended using relativistic Hartree-Fock calculations andconfiguration interactions in an intermediate-coupling scheme with thesupport of the Cowan code. Our calculations support the experimentalvalue obtained by Schectman et al. in 2000 of the lifetime of5s25d2D3/2 and the absorptionoscillator strength of the resonance transition to this level at 1400.45Å. The parametric description of 5s5p (3Po)nl (nl=5d, 6s) of Sn II levels is improved by taking into account thefar 5p3 configuration mixing effects. The results obtained inthis study will allow a substantial improvement in the interpretation ofthe data of the ultraviolet spectrum of Sn II observed by the GoddardHigh Resolution Spectrograph aboard the Hubble Space Telescope. X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary ImplicationsThe Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105yr, it harbors a number of very young hot stars, which likely are on thezero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectrain the wavelength range of 1.5-25 Å of three of itsX-ray-brightest members (Θ1 Ori A, C, and E) obtainedwith the High Energy Transmission Grating Spectrometer (HETGS) on boardthe Chandra X-Ray Observatory. We measured X-ray emission lines,calculated differential emission measure distributions (DEMs), andfitted broadband models to the spectra. The spectra from all three starsare very rich in emission lines, specifically from highly ionized Fe,which includes emission from Fe XVII to Fe XXV ions. A complete linelist is included. This is a mere effect of high temperatures rather thanan overabundance of Fe, which in fact turns out to be underabundant inall three Trapezium members. Similarly there is a significantunderabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appearclose to solar. The DEM derived from over 80 emission lines in thespectrum of Θ1 Ori C indicates three peaks located at7.9, 25, and 66 MK. The emission measure varies over the 15.4 day windperiod of the star. For the two phases observed, the low-temperatureemission remains stable, while the high-temperature emission showssignificant differences. The line widths seem to show a similarbifurcation, where we resolve some of the soft X-ray lines withvelocities up to 850 km s-1 (all widths are stated ashalf-width at half-maximum), whereas the bulk of the lines remainunresolved with a confidence limit of 110 km s-1. Thebroadband spectra of the other two stars can be fitted with severalcollisionally ionized plasma model components within a temperature rangeof 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK forΘ1 Ori A. The high-temperature emissivity contributesover 70% to the total X-ray flux. None of the lines are resolved forΘ1 Ori A and E with a confidence limit of 160 kms-1. The influence of the strong UV radiation field on theforbidden line in the He-like triplets allows us to set an upper limiton distance of the line-emitting region from the photosphere. The bulkof the X-ray emission cannot be produced by shock instabilities in aradiation-driven wind and are likely the result of magnetic confinementin all three stars. Although confinement models cannot explain all theresults, the resemblance of the unresolved lines and of the DEM withrecent observations of active coronae in II Peg and AR Lac during flaresis quite obvious. Thus we speculate that the X-ray production mechanismin these stars is similar, with the difference that the Orion stars maybe in a state of almost continuous flaring driven by the wind. Weclearly rule out major effects due to X-rays from a possible companion.The fact that all three stars appear to be magnetic and are near zeroage on the main sequence also raises the issue of whether the Orionstars are simply different or whether young massive stars enter the mainsequence carrying significant magnetic fields. The ratiologLX/Lbol using the wind'' component of thespectrum is -7 for the Trapezium stars, consistent with the expectationfrom O stars. This suggests that massive ZAMS stars generate their X-rayluminosities like normal O stars and magnetic confinement provides anadditional source of X-rays.
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