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Chandra Observations of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation
We present and analyze the first high-resolution X-ray images everobtained of the Eagle Nebula star-forming region. On 2001 July 30 theChandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula(M16) that includes the core of the young galactic cluster NGC 6611 andthe dark columns of dust and cold molecular gas in M16 known as the``Pillars of Creation.'' We find a total of 1101 X-ray sources in the17'×17' ACIS-I field of view. Most of theX-ray sources are low-mass pre-main-sequence or high-mass main-sequencestars in this young cluster. A handful of hard X-ray sources in thepillars are spatially coincident with deeply embedded young stellarobjects seen in high-resolution near-infrared images recently obtainedwith the VLT (McCaughrean & Andersen). In this paper, we focus onthe 40 X-ray sources in and around pillars 1-4 at the heart of the EagleNebula. None of the X-ray sources are associated with the evaporatinggaseous globules (EGGs) first observed by Hester and coworkers) in HSTWFPC2 images of M16, implying either that the EGGs do not containprotostars or that the protostars have not yet become X-ray active.Eight X-ray counts are coincident with the Herbig-Haro object HH 216,implying logLX~30.0.
| Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
| 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.
| Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611
An analysis is presented of VLT-FLAMES spectroscopy for three Galacticclusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmospherecalculations have been used to estimate effective temperatures (fromeither the helium spectrum or the silicon ionization equilibrium) andgravities (from the hydrogen spectrum). Projected rotational velocitieshave been deduced from the helium spectrum (for fast and moderaterotators) or the metal line spectrum (for slow rotators). The origin ofthe low gravity estimates for apparently near main sequence objects isdiscussed and is related to the stellar rotational velocity. Theatmospheric parameters have been used to estimate cluster distances(which are generally in good agreement with previous determinations) andthese have been used to estimate stellar luminosities and evolutionarymasses. The observed Hertzsprung-Russell diagrams are compared withtheoretical predictions and some discrepancies including differences inthe main sequence luminosities are discussed. Cluster ages have beendeduced and evidence for non-coeval star formation is found for allthree of the clusters. Projected rotational velocities for targets inthe older clusters, NGC 3293 and NGC 4755, have been found to besystematically larger than those for the field, confirming recentresults in other similar age clusters. The distribution of projectedrotational velocities are consistent with a Gaussian distribution ofintrinsic rotational velocities. For the relatively unevolved targets inthe older clusters, NGC 3293 and NGC 4755, the peak of the velocitydistribution would be 250 km s-1 with afull-width-half-maximum of approximately 180 km s-1. For NGC6611, the sample size is relatively small but implies a lower meanrotational velocity. This may be evidence for the spin-down effect dueto angular momentum loss through stellar winds, although our results areconsistent with those found for very young high mass stars. For allthree clusters we deduce present day mass functions with Γ-valuesin the range of -1.5 to -1.8, which are similar to other young stellarclusters in the Milky Way.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| Chemical Abundances for a Sample of Southern OB Stars
Chemical abundances are presented for carbon, nitrogen, oxygen,magnesium, aluminum, silicon, and sulfur in a sample of 25 OB stars thatare members of open clusters, OB associations, and H II regions havingGalactocentric distances that lie inside the solar Galactic orbit. Theelemental abundances are derived from fits of observed high-resolutionspectra to non-LTE synthetic line profiles. Our effective temperaturescale is based on calibrations of the Strömgren photometric indicescoupled to Hγ line profiles. For stars without Strömgrenindices, the reddening-free Q-parameter, defined from UBV photometry isused, with the Q-parameter calibrated against Teff for starsthat have Strömgren photometry. We also investigated othereffective temperature scales from the literature and conclude thatadoption of these other Teff scales, which typically resultin lower temperatures, produces dependences of the derived abundanceswith Teff. Our non-LTE abundances reveal dispersions that arevery similar to those expected from the analysis uncertainties. Thesolar abundances, some of which have been recently revised, fall withinthe abundance distributions defined by this sample of inner disk OBstars for all seven studied elements.Based on observations collected with the 1.52 m telescope at theEuropean Southern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Visual binaries among high-mass stars. An adaptive optics survey of OB stars in the NGC 6611 cluster
We have searched for visual binaries with projected separations in therange 200-3000 AU (0\farcs1-1\farcs5) among a sample of 96 stars in themassive young NGC 6611 cluster, 60 of them being subsequently identifiedas high probability cluster members of mainly OB spectral type. This isthe first visual binary survey among such a large and homogeneous sampleof high-mass stars. We find an uncorrected binary frequency of 18 +/- 6%over the surveyed separation range. Considering only binaries with massratios q>=0.1, we find that OB stars in NGC 6611 host more companionsthan solar-type field stars. We derive mass ratios for the detectedbinaries from their near-infrared flux ratios and conclude that abouthalf of the detected binaries have qla0 .2, which does not contradictthe assumption that companion masses are randomly drawn from the initialmass function. There is no evidence in our sample that wide-binaryproperties depend upon the mass of the primary star. The high frequencyof massive binaries in a cluster as rich as NGC 6611 and the lack of astrong mass dependence of their properties are difficult to reconcilewith the scenario whereby massive stars form as the result of mergers ofsmaller stars. The canonical protostellar accretion scenario togetherwith cloud fragmentation, on the other hand, can naturally explain mostof the observed binary properties, although the very high stellardensity in the protocluster is likely to require significantmodification to that picture as well.
| Observational Evidence of Supershell Blowout in GS 018-04+44: The Scutum Supershell
Emission in the H I 21 cm line has been mapped for a region of theGalaxy that includes two known supershells, GS 018-04+44 and GS034-06+65. We focus on the GS 018-04+44, hereafter referred to as theScutum Supershell, which is an elongated shell about 5 deg in diameterextending to -7 deg below the Galactic plane. The Scutum shell lies at akinematic distance of ~3300 pc, implying a shell diameter of ~290 pcwith a vertical extension of ~400 pc away from the Galactic plane. TheScutum shell contains 6.2x105 Msolar swept intothe walls. We observe that the top of the shell is missing, and asubstantial column of H I rises from the shell walls tob=-11deg, culminating in a large cloud of neutral hydrogen,3.74x104 Msolar, located ~630 pc from the plane.ROSAT data show X-ray emission that closely anticorrelates with the 21cm emission. This emission probably originates from hot gas within theScutum Supershell. After approximately correcting for the foregroundabsorption, we find that the 1.5 keV X-rays peak at the base of theshell, the 0.75 keV emission peaks in the interior and at the top of theshell, and the 0.25 keV emission extends to high latitudes above theshell. The X-ray luminosity is roughly ~5x1036 ergss-1. The Wisconsin Hα Mapper (WHAM) survey shows thepresence of Hα emission that exhibits a morphology similar to thatof the H I. Spectra indicate the presence of ionized hydrogen atvelocities similar to the H I, placing ionized material at the samekinematic distance as the neutral material. IRAS images in the 60 and100 μm wavebands reveal the presence of dust correlated with theneutral hydrogen. Infrared surface brightness indicates an excess in the100 μm emission, which could indicate a molecular hydrogen componentwith a column density of 2.4x1021 cm-2 in thedensest regions of the high-latitude cloud of neutral hydrogen. IUEultraviolet high dispersion spectra of HD 177989 (l=17.89d, b=-11.88d)and HD 175754 (l=16.40d, b=-9.92d) reveal the presence of very strongabsorption by highly ionized gas at a velocity that associates theabsorption with the ejecta of the Scutum Supershell. In the case of HD177989, the high ion column density ratios suggest an origin in aturbulent mixing layer where hot and cool gases mix in the presence ofshear flows. The Hα and X-ray emission suggest that a multitude ofenergetic phenomena exist in this region, providing the necessaryionizing radiation. Indeed, there are multiple supernova remnants, H IIregions, and hot stars, which could all contribute sizeable amounts ofenergy and ionizing radiation. The combination of these data setsindicates observational evidence of a ``blowout'' phenomena whereby hotmaterial produced within the Scutum Supershell has blown through the topof the shell and been pushed to high latitude. Based on observationsfrom the Green Bank 43m radio telescope, the ROSAT All-Sky Survey, theWHAM All-Sky Survey, IRAS, and IUE.
| Spectrophotometric Observations of Lin 593
An analysis is reported of the low-resolution optical spectra of thevery low excitation emission-line object Lin 593. It is demonstratedthat this object has a hot star that ionizes the surrounding nebula,thus creating a Strömgren sphere. The hot star is the source of theobserved continuum, while the lines arise from the nebula ionized by thephotons from the hot star. From the measurements of line emission andcontinuum, the chemical abundance and the physical parameters of the hotstar as well as those of the ionized nebula were obtained. It is shownthat the chemical abundance of Lin 593 follows the SMC abundance asderived from previous analysis of similar objects, except for oxygen,which presents a low abundance compared to the SMC average abundancepattern. It is also shown that the hot star is a main-sequence star withspectral type in the range O9.5-B0.5 and, finally, that the ionizednebula of Lin 593 forms a H II region for which several parameters wereobtained, characterizing it as a class III-IV, classical-dense, H IIregion.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Dust properties in NGC 6611
An investigation on dust properties in the NW portion of NGC6611 has been conducted polarimetrically to test the existenceof abnormal extinction in that section of the open cluster in anindependent way. As previously suggested for the eta Carinae nebula byTapia et al. (\cite{TRMR88b}) and subsequently confirmed by Marraco etal. (\cite{MVV93}), the canonical relation between {E_{ V-K}/E_{ B-V}}and lambda_max (the wavelength of maximun interstellar polarization) isnot valid for stars belonging to dusty H ii regions, as is the case forthe observed portion of M 16. This may arise mainlyfrom the presence of silicate grains of a slightly larger size than thestandard ISM and also from a considerable increase in mean graphitegrain size, according to previous results from Chini & Wargau(\cite{CW90}). About 50% of the observed stars in NGC6611 present indications of intrinsic polarization in theirmeasurements; a similar percentage was found in IC2944 (Vega et al. \cite{VOM94}), another young open cluster inclose relation with an H ii region. Table 1 only available in electronicform at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Stellar Spectroscopy in NGC 6611: Binary Frequency and New Spectral Types of Several Early Type Stars
Not Available
| The extremely young open cluster NGC 6611: Compiled catalogue, absorption map and the HR diagram
A compiled catalogue of astrometric and photometric data for about 2200stars down to V=16.8() m is presented. The stars are located in an areawith a radius of 22.8 arcmin around the extremely young open cluster NGC6611 with the cluster center at (alpha ,delta )_{2000.0}=18() h18()m40() s,-13(deg) 47.1'. The catalogue is based on new reductions ofpositions, proper motions, and of photometric data that have beenpublished in this region and includes, among others, Hipparcos, AC/GSCand Tautenburg Schmidt plate observations. For the membershipdetermination, spatial and proper motion distributions of stars in theregion were analysed. An extinction map of the area and the clustercolor-magnitude diagram down to M_V=+1() m were constructed. Thedistance of NGC 6611 was determined as 2.14 +/- 0.10 kpc. The completecatalogue is available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| ISOCAM images of the `elephant trunks' in M16
We have used ISOCAM to obtain images of the `elephant trunks' in the M16`Eagle nebula' in the two broadband filters LW2 (5-8.5mu m) and LW3(12-18mu m). These pillars are clearly seen in both our filters, theiremission has a colour temperature in the range 250-320K. We have alsodetected bright extended emission in the LW3 filter, in an area outsidethe `elephant trunks', and with no LW2 counterpart. We report thedetection of a deeply embedded source, presumably connected withM16-E31, which we identify as a YSO. However, to our levels of angularresolution and sensitivity we cannot, in general, correlate discretesources with the EGGs discovered by the HST. Our observations areconsistent with a relatively low level of ongoing low-mass starformation, a fact which may be related to disk disruption taking placeat early times in M16. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Star formation in NGC 6611 with ADONIS and Hubble.
Not Available
| Conditions of search for magnetic stars at early evolution stages.
Not Available
| The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..914V&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Intrinsic colour indices of O- and B- type stars in the Vilnius photometric system.
Not Available
| Uvbyβ photometry of Orion population stars and related objects associated with six star forming regions
Homogeneous uvbyβ photometry of 150 stars associated with the starforming regions Taurus-Auriga, Orion, Canis Mayoris, Ophiuchus, Serpensand Cygnus, is presented here. We also present uvbyβ photometry of10 Herbig Ae/Be stars not belonging to the specific regions studiedhere. Using the (V,b-y), (β,[m1]), ([c1],[m1]) and ([u-b], T_eff_)diagrams we study the photometric properties of the sample. The effectsof duplicity, stellar rotation, and interstellar extinction on thelocation in the magnitude-color and color-color diagrams of our programstars are briefly investigated. We derive reliable photometric spectraltypes and discuss membership to the regions and to the Orion population,as well as peculiarities of the individual stars. We also find new Bstars associated with the CMa R1 star forming region and newintermediate-low mass PMS members associated with theBD+404124/BD+413731 region. The FU-Orionis stars Z CMa, V1515 Cyg andV1057 Cyg have a peculiar position in the reddening free (β,[m1])diagram. With diagrams such as (V-M _v_-5 log r+5) vs. E(b-y), E(c1) vs.E(b-y), E(m1) vs. E(b-y) and E(u-b) vs. E(b-y) we investigate the ISextinction law towards the star-forming regions. We find that the totalto selective extinction ratio to these regions is steeper than normalwith R_y_=5.4 (R_v_=4.0) and the coefficients E(c1)/E(b-y) andE(m1)/E(b-y) used to define the reddening free indices [c1] and [m1]should be numerically smaller than the generally adopted values.
| The galactic distribution of chemical elements as derived from B-stars in open clusters. 2: NGC 6611, S 285, and S 289
The chemical composition of the open cluster NGC 6611 and the Sharplessregions S 285 and S 289 have been analyzed, and stellar parameters(effective temperature, gravity, microturbulence) and chemicalabundances of 4 unevolved B-stars in NGC 6611, 2 B-stars in S 285, and 1B-star in S 289 are given. Line blanketed local thermodynamicequilibrium (LTE) model atmospheres and non-local thermodynamicequilibrium (NLTE) line formation calculations were used for theabundance determinations of C, N, O, Mg, Al, and Si; LTE line formationcalculations were used for the abundance determination of Ne, S, and Fe.In agreement with results obtained from B-type stars in the solarneighborhood and the young open cluster NGC 6231 presented in earlierpapers, all elements except neon show an average underabundance of0.1-0.4 dex with respect to the solar value. Among the cluster starsabundance differences of 0.2-0.7 dex may be observed. First results forthe galactic metallicity gradient show an insignificant value for mostobserved elements, except for neon and sulfur, which is in contrast toresults of analyses of H II regions by Shaver et al. (1983).
| NGC 6611: A cluster caught in the act
We have combined optical CCD photometry and spectroscopy with infraredimaging photometry to study the young cluster NGC 6611. We use thesedata to derive improved values for the reddening law (R = 3.75) and thedistance modulus (m - M = 11.5), and to construct a physicalHertzsprung-Russell (HR) diagram from which we can probe the ages,masses, and evolutionary states of this stellar ensemble. The HR diagramshows a strong population of high-mass stars, the most massive of whichhas a mass of roughly 80 solar mass, similar to what we find in otherGalactic and Magellanic Cloud clusters and associations. The age of themassive stellar population in NGC 6611 is approximately 2 million yr,with an age spread of, perhaps, 1 million yr, although the data are alsoconsistent with there being no discernible age spread among the mostmassive stars. However, the HR diagram does reveal that one star ofsomewhat lower mass (30 solar mass) must have formed approximately 6million yr ago. The upper end of the mass function of NGC 6611 is foundto have a slope of Gamma = -1.1 +/- 0.3, indistinguishable from aSalpeter slope, and similar to what we have found in other Galacticassociations, but shallower than what we have found in the MagellanicClouds. Our most significant result, however, is that we catch thiscluster in the act of forming intermediate-mass (3-8 solar mass) stars.This is the first well-established case where large numbers ofintermediate-mass stars have been seen on their way to the Zero Age MainSequence (ZAMS). That intermediate-mass pre-main-sequence stars areindeed present is evidenced both by their location above the ZAMS in theHR diagram, and in some cases by their spectroscopic and infraredsignatures of (possibly remnant protostellar) circumstellar material.The pre-main-sequence population ranges from as young as 0.25 million yrto at least 1 million yr of age. We find an highly unusual number (27)of emission-line stars, which appear quite similar in their optical andinfrared continuum and optical spectroscopic properties to 'classicalBe/Ae' stars (as opposed to Herbig Be/Ae stars). Our data areinconsistent with the traditional interpretation that these classicalBe/Ae stars are slightly evolved stars undergoing mass loss. Instead, weoffer the conjecture that these may be young stars whose circumstellardisks have become optically thin, and produce Balmer emission lines. Theinfrared data do indicate a number of stars, particularly among theembedded sample, whose colors are consistent with those of stars thoughtto be surrounded by optically thick circumstellar accretion disks. Theidentification of such disks around young massive stars continues to berare, and implies that the disk survival times around intermediate- andhigh-mass stars are much shorter (less than 0.5 Myr) than those of diskssurrounding lower-mass stars.
| Photoelectric photometry of OB stars in the Vilnius system
Not Available
| Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
| Quantitative spectroscopy of hot stars
A review on the quantitative spectroscopy (QS) of hot stars ispresented, with particular attention given to the study of photospheres,optically thin winds, unified model atmospheres, and stars withoptically thick winds. It is concluded that the results presented heredemonstrate the reliability of Qs as a unique source of accurate valuesof the global parameters (effective temperature, surface gravity, andelemental abundances) of hot stars.
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Observation and Astrometry data
| Constellation: | へび座 |
| Right ascension: | 18h18m52.67s |
| Declination: | -13°49'42.6" |
| Apparent magnitude: | 9.44 |
| Proper motion RA: | -1.6 |
| Proper motion Dec: | 1.4 |
| B-T magnitude: | 9.733 |
| V-T magnitude: | 9.465 |
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INHABITED SKY
