Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
3He in the Milky Way Interstellar Medium: Ionization Structure
The cosmic abundance of the 3He isotope has importantimplications for many fields of astrophysics. We are using the 8.665 GHzhyperfine transition of 3He+ to determine the3He/H abundance in Milky Way H II regions and planetarynebulae. This is one in a series of papers in which we discuss issuesinvolved in deriving accurate 3He/H abundance ratios from theavailable measurements. Here we describe the ionization correction weuse to convert the 3He+/H+ abundance,y+3, to the 3He/H abundance,y3. In principle the nebular ionization structure cansignificantly influence the y3 derived for individualsources. We find that in general there is insufficient informationavailable to make a detailed ionization correction. Here we make asimple correction and assess its validity. The correction is based onradio recombination line measurements of H+ and4He+, together with simple core-halo sourcemodels. We use these models to establish criteria that allow us toidentify sources that can be accurately corrected for ionization andthose that cannot. We argue that this effect cannot be very large formost of the sources in our observational sample. For a wide range ofmodels of nebular ionization structure we find that the ionizationcorrection factor varies from 1 to 1.8. Although larger corrections arepossible, there would have to be a conspiracy between the density andionization structure for us to underestimate the ionization correctionby a substantial amount.
| The Calibration of the O/H Abundance Indicators for Extragalactic H II Regions based on O II Recombination Lines
Based on O II recombination lines we present a new calibration (called OII_{RL}) of Pagel's O_{23} indicator to determine the O/H abundanceratio in extragalactic H II regions and emission line galaxies. The OII_{RL} calibration produces O/H abundances about a factor of two higherthan those derived from the T(4363) method with t^2 = 0.00. The OII_{RL} calibration has implications for the study of differentproperties of emission line galaxies such as their metallicity, starformation rate, and initial mass function. The O II_{RL} calibrationalso affects the abundance determinations based on other O/H indicators,that include collisionally excited lines, like those known as O_3N_2,N_2, S_{23], Ar_3O_3, and S_3O_3. We argue that the controversy betweenthe T(4363) method and the photoionization models method to derive O/Hvalues is mainly due to temperature variations inside the observed H IIregions.
| The chemical composition of the Galactic H II regions M8 and M17. A revision based on deep VLT echelle spectrophotometry
We present new echelle spectrophotometry of the Galactic H II regions M8and M17. The data have been taken with the VLT UVES echelle spectrographin the 3100 to 10400 Å range. We have measured the intensities of375 and 260 emission lines in M8 and M17 respectively, increasingsignificatively the number of emission lines measured in previousspectrophotometric studies of these nebulae. Most of the detected linesare permitted lines. Electron temperatures and densities have beendetermined using different diagnostics. We have derived He^+,C++, O^+ and O^{++} ionic abundances from pure recombinationlines. We have also derived abundances from collisionally excited linesfor a large number of ions of different elements. Highly consistentestimations of t^2 have been obtained by using different independentindicators; the values are moderate and very similar to those obtainedin other Galactic H II regions. We report the detection of deuteriumBalmer emission lines, up to Dɛ, in M8 and show that theirintensities are consistent with continuum fluorescence as their mainexcitation mechanism.
| The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry
We present echelle spectrophotometry of the blue compact dwarf galaxyNGC 5253 obtained with the VLT UVES. We have measured the intensities ofa large number of permitted and forbidden emission lines in four zonesof the central part of the galaxy. We detect faint C II and O IIrecombination lines, the first time that these are unambiguouslydetected in a dwarf starburst galaxy. The physical conditions of theionized gas have been derived using a large number of different lineintensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fehave been determined following standard methods. C++ andO++ abundances have been derived from pure recombinationlines and are larger than those obtained from collisionally excitedlines (from 0.30 to 0.40 dex for C++ and from 0.19 to 0.28dex for O++). This result is consistent with a temperaturefluctuation parameter (t2) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized Nenrichment in certain zones of NGC 5253 and detect a possible slight Heoverabundance in the same zones. The enrichment pattern agrees with thatexpected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. Theamount of enriched material needed to produce the observed overabundanceis consistent with the mass lost by the number of W-R stars estimated inthe starbursts. We discuss the possible origin of the difference betweenabundances derived from recombination and collisionally excited lines(the so-called abundance discrepancy problem) in H II regions, findingthat a recent hypothesis based on the delayed enrichment by SN ejectainclusions seems not to explain the observed features.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 70.C-0008(A).
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| 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.
| Radio Recombination Lines in Galactic H II Regions
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.
| Stark parameters of neutral helium 318.8 nm line
Aims.Our aims is to measure with good accuracy the Stark parameters(width and shift) of the He I 318.8 nm (2s ^3S-4p ^3P^o) line and tocalibrate Stark width and shift dependencies of this line as a functionof the electron density. In this way, it is possible to obtain analternative method of diagnosing the plasmas of astrophysical origin andto check the quality of theoretical models that predict theseparameters. Methods.We made a spectroscopic and interferometricanalysis of a pulsed plasma. The electron temperature was obtained withan intensity ratio of He II lines and ranges from 19 000 to 23 000 K.Electron density was determined by interferometry ranging from 1.25×1022 m-3 to 6.22×1022m-3.Results.These experimental results are new forthese plasma conditions. When joined with previous bibliographic data,they allowed us to obtain an empirical calibration expression for theStark width and the shift of this line in a broad range of electrondensities. Comparisons with different theoretical models are alsoincluded.
| Deep photometric studies in the Third Quadrant: NGC 2467
The region of NGC 2467, placed in the ``Puppis Window'' of the thirdquadrant, contains a number of young open clusters and nebular entitiesreported at different distances. We present a new analysis of the fieldof NGC 2467 based on deep images, in the UBVRI filters, obtained withthe 2.2-m MPG/ESO telescope at La Silla Observatory, Chile. We analyzethe stellar content and establish new distances to the clusters. Wesuggest the presence of a population of young stellar objects, includingthe detection of HH-like features. Also, we discover tens of backgroundgalaxies seen through the galactic plane.
| 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.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| 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.
| 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.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| 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.
| Faint C and O recombination lines in H II regions and large telescopes: The case of S311
We present preliminary results on the analysis of very deep echelle UVES(8m VLT) spectra of the Galactic H II region S311 (NGC 2467) locatedoutside the Solar circle. The data cover from 3100 to 10450 Å witha resolution R ~ 8800. We have detected and measured more than 300emission lines, some of them are faint recombination lines ofheavy-element ions. We have derived the O abundances from OI and OIIrecombination lines and the C abundance from CII lines and an IonizationCorrection Factor (ICF). This kind of observations will permit us toderive the Galactic abundance gradient of C and O from nebularrecombination lines, which are almost independent on the realtemperature structure of the nebula.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| Deep echelle spectrophotometry of S 311, a Galactic HII region located outside the solar circle
We present echelle spectrophotometry of the Galactic HII region S 311.The data have been taken with the Very Large TelescopeUltraviolet-Visual Echelle Spectrograph in the 3100-10400 Årange.We have measured the intensities of 263 emission lines; 178 arepermitted lines of H0, D0 (deuterium),He0, C0, C+, N0,N+, O0, O+, S+,Si0, Si+, Ar0 and Fe0; someof them are produced by recombination and others mainly by fluorescence.Physical conditions have been derived using different continuum- andline-intensity ratios. We have derived He+, C++and O++ ionic abundances from pure recombination lines aswell as abundances from collisionally excited lines for a large numberof ions of different elements. We have obtained consistent estimationsof t2 applying different methods. We have found that thetemperature fluctuations paradigm is consistent with theTe(HeI) versus Te(HI) relation for HII regions, incontrast with what has been found for planetary nebulae. We report thedetection of deuterium Balmer lines up to Dδ in the blue wings ofthe hydrogen lines, whose excitation mechanism seems to be continuumfluorescence.
| 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.
| Low-Resolution Spectroscopy and uvby-β Photometry of Selected Stars in Haffner 18
We present MK spectral types of a sample of selected (bright) starsassociated with the galactic cluster Haffner 18 in Puppis. They wereobtained from low-resolution (λ / Δ λ = 1720)spectra by H α slit spectroscopy. Some of the selected stars werealso observed photoelectrically in the uvby- β system. Ourspectroscopic data show that over 1/4 of the estimated 50 star membersof the cluster are O and early B stars. We give MK spectral types anddiscuss the individual stars observed. Finally, based on thesespectroscopic results, on our photometry, and on published data webriefly reinspect the physical parameters of the cluster.
| Carbon, Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,nitrogen, and oxygen yields adopted, have been computed to reproduce theGalactic O/H values obtained from H II regions. All the models fit theoxygen gradient, but only two models also fit the carbon gradient, thosebased on carbon yields that increase with metallicity owing to stellarwinds in massive stars (MSs) and decrease with metallicity owing tostellar winds in low- and intermediate-mass stars (LIMSs). Thesuccessful models also fit the C/O versus O/H evolution history of thesolar vicinity obtained from stellar observations. We also compare thepresent-day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Feversus Fe/H evolution histories of the solar vicinity predicted by ourtwo best models with those derived from H II regions and from stellarobservations. While our two best models fit the C/H and O/H gradients,as well as the C/O versus O/H history, only model 1 fits well the N/Hgradient and the N/O values for metal-poor stars but fails to fit theN/H values for metal-rich stars. Therefore, we conclude that our twobest models solve the C enrichment problem but that further work needsto be done on the N enrichment problem. By adding the C and O productionsince the Sun was formed predicted by models 1 and 2 to the observedsolar values, we find an excellent agreement with the O/H and C/H valuesof the solar vicinity derived from H II region O and C recombinationlines. Our results are based on an initial mass function (IMF) steeperthan Salpeter's a Salpeter-like IMF predicts C/H, N/H, and O/H ratioshigher than observed. One of the most important results of this paper isthat the fraction of carbon due to MSs and LIMSs in the interstellarmedium is strongly dependent on time and on the galactocentric distance;at present about half of the carbon in the interstellar medium of thesolar vicinity has been produced by MSs and half by LIMSs.
| Carbon and Oxygen Galactic Gradients: Observational Values from H II Region Recombination Lines
We present results of deep echelle spectrophotometry of eight Galactic HII regions located at Galactocentric distances between 6.3 and 10.4 kpc.The data have been taken with the Very Large Telescope UltravioletEchelle Spectrograph in the 3100-10360 Å range. We have derivedC++ and O++ abundances from recombination linesfor all the objects as well as O+ abundances from this kindof line for three of the nebulae. The intensity of recombination linesis almost independent of the assumed electron temperature as well as ofthe possible presence of spatial temperature variations or fluctuationsinside the nebulae. These data allow the determination of the gas-phaseC and O abundance gradients of the Galactic disk, of paramountimportance for chemical evolution models. This is the first time the Cgradient is derived from such a large number of H II regions distributedin such a wide range of Galactocentric distances. Abundance gradientsare found of the form Δlog(O/H)=-0.044+/-0.010 dexkpc-1, Δlog(C/H)=-0.103+/-0.018 dex kpc-1,and Δlog(C/O)=-0.058+/-0.018 dex kpc-1.Based on observations collected at the European Southern Observatory,Chile; proposals ESO 68.C-0149(A) and ESO 70.C-0008(A).
| A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry
We present Very Large Telescope (VLT) UVES echelle spectrophotometry ofthe Orion nebula in the 3100-10400 Årange. We have measured theintensity of 555 emission lines, many of them corresponding to permittedlines of different heavy-element ions. This is the largest set ofspectral emission lines ever obtained for a Galactic or extragalacticHII region. We have derived He+, C2+,O+, O2+ and Ne2+ abundances from purerecombination lines. This is the first time that O+ andNe2+ abundances have been obtained from these kinds of linesin the nebula. We have also derived abundances from collisionallyexcited lines for a large number of ions of different elements. In allcases, ionic abundances obtained from recombination lines are largerthan those derived from collisionally excited lines. We have obtainedremarkably consistent independent estimations of the temperaturefluctuation parameter, t2, from different methods, which arealso similar to other estimates from the literature. This resultstrongly suggests that moderate temperature fluctuations (t2between 0.02 and 0.03) are present in the Orion nebula. We have comparedthe chemical composition of the nebula with those of the Sun and otherrepresentative objects. The heavy-element abundances in the Orion nebulaare only slightly higher than the solar ones, a difference that can beexplained by the chemical evolution of the solar neighbourhood.
| The Effective Temperatures of Hot Stars. II. The Early-O Types
We derived the stellar parameters of a sample of Galactic early-O typestars by analyzing their UV and far-UV spectra from the Far UltravioletSpectroscopic Explorer (905-1187 Å), the International UltravioletExplorer, the Hubble Space Telescope STIS, and the Orbiting andRetrievable Far and Extreme Ultraviolet Spectrometer (1200-2000Å). The data have been modeled with spherical, hydrodynamic,line-blanketed, non-LTE synthetic spectra computed with the WM-BASICcode. We obtain effective temperatures ranging fromTeff=41,000 to 39,000 K for the O3-O4 dwarf stars andTeff=37,500 K for the only supergiant of the sample (O4If+). Our values are lower than those from previous empiricalcalibrations for early-O types by up to 20%. The derived luminosities ofthe dwarf stars are also lower by 6%-12% however, the luminosity of thesupergiant is in agreement with previous calibrations within the errorbars. Our results extend the trend found for later O types in a previouswork by Bianchi & Garcia.Based on observations with the NASA-CNES-CSA FUSE, which is operated byThe Johns Hopkins University under NASA contract NAS5-32985, on IUEobservations from the MAST and INES archives and on MAST archival datafrom the HST and the ORFEUS mission.
| Determination of temperature of the ionizing stars of H II regions
The determination of temperature (T_eff) of the ionizing stars of H Iiregions was considered. In this work we used photoionization models forH Ii regions ionized by a single star to show that the index R=log ([OIi]lambda lambda 3726+3729/[O Iii]lambda 5007) can be used to estimateT_eff. The relation R vs. T_eff proved to be rather independent of thechemical abundances, but strongly dependent on the ionization parameterof the nebula. In order to check the reliability of using R fortemperature determination, we compared the values of T_eff obtained viathe index R for a sample of H Ii regions with data available in theliterature with independent estimations.
| On the oxygen abundance in our Galaxy
The compilation of published spectra of Galactic H II regions withavailable diagnostic [OIII]lambda 4363 line information has been carriedout. Our list contains 71 individual measurements of 13 H II regions inthe range of galactocentric distances from 6.6 to 14.8 kpc. The oxygenabundances in all the H II regions were recomputed in the same way,using the classic T_e-method. The oxygen abundance at the solargalactocentric distance traced by those H II regions is in agreementwith the oxygen abundance in the interstellar medium in the solarvicinity derived with high precision from the interstellar absorptionlines towards stars. The derived radial oxygen abundance distributionwas compared with that for H II regions from the Shaver et al.(\cite{Shaver83}) sample which is the basis of many models for thechemical evolution of our Galaxy. It was found that the original Shaveret al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygenabundances in H II regions from the Shaver et al. sample have beenredetermined with the recently suggested P-method. The radialdistribution of oxygen abundances from the Shaver et al. sampleredetermined with the P-method is in agreement with our radialdistribution of (O/H)T_e abundances.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
