Contents
Images
Upload your image
DSS Images Other Images
Related articles
A Classification of H II Regions Based on Oxygen and Helium Lines: The Cases of TOL 2146-391 and TOL 0357-3915
We present long-slit spectrophotometry of two H II galaxies: TOL2146-391 and TOL 0357-3915. We performed a detailed analysisthat involves abundance determinations relaxing the assumption ofhomogeneous temperature. The temperature inhomogeneity values, t2, were obtained through two methods: (1) comparingabundances from oxygen recombination lines to abundances fromcollisionally excited lines and (2) by using the line intensity ratiosof a set of He I lines together with the HELIO10 program. We find thatthe HELIO10 program is a good alternative to obtain a t 2value in photoionized regions where recombination lines of heavyelements are not available. We have plotted 27 high- and low-metallicityH II regions in an oxygen degree of ionization versus t 2diagram; we find areas populated by H II regions and areas void of them;the physical characteristics of each area are discussed. In addition, anaverage t 2 value can be determined for the objects in eacharea. We propose to use this langt 2rang value for the caseswhere a direct measurement of t 2 cannot be determined.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 69.C-0203(A).
| Constraints on fast ejecta in the Crab supernova remnant from optical spectral lines
The low kinetic energy and mass of the Crab supernova remnant challengeour understanding of core-collapse supernova explosions. A possibilityis that the Crab nebula is surrounded by a shell of fast ejectacontaining the ‘missing’ kinetic energy and mass. The onlydirect evidence for such a fast shell comes from an absorption featurein the Crab pulsar spectrum as a result of C IV?1550. Thevelocities inferred from the C IV line absorption extend to at least˜2500 km s-1, which is about twice as fast as theexpansion of main shell of the remnant in our direction. We havesearched for additional evidence of fast-moving ejecta in the opticalspectra obtained with the FORS1 instrument at the European SouthernObservatory (ESO) 8.2-m Very Large Telescope (VLT) and with theAndalucia Faint Object Spectrograph and Camera (ALFOSC) at the 2.56-mNordic Optical Telescope (NOT), with the focus on absorption in CaII??3934,3968, and emission components in [O III]??4959,5007. The data are compared with the CIV?1550 absorption, and with theoretical expectations derivedfrom shell models with ballistic gas motion, and a power-law densityprofile of the fast ejecta. Along the line of sight to the pulsar, wefind that no gas in the nebula moves faster towards us than ?1400 kms-1. We identify this gas as part of the known main shell ofthe remnant. This velocity agrees with previous results showing that theCrab nebula is moving slowly in this direction. It is slower than thevelocity of 1680 km s-1 used in the models of Sollerman etal. as a minimum velocity of the presumed fast shell of supernova ejectato account for the C IV line absorption. We find faster moving gaswithin 3-10 arcsec north and south of the pulsar, where the fastest gasmoving towards us, as traced by [O III], has a velocity of 1650-1700 kms-1. The fastest [O III] emitting gas along the line of sightto the pulsar, on the rear side of the nebula, has a velocity of?+1800 km s-1, which is higher than the velocitypreviously recorded for that direction. However, neither the [O III] norCa II lines display any signatures of fast shell ejecta at thevelocities inferred from the C IV line absorption. To fully rule out thepossibility that a chimney-like structure directed towards us could beresponsible for the C IV line absorption, we need deep observationstaken with 8-10-m class telescopes with good spectral resolution. Weshow that a spectral resolution better than ˜200 km s-1is needed to draw any conclusions on emission lines from gas movingtowards us, along the line of sight of the pulsar, faster than ?1700km s-1. To probe the fast shell ejecta, new observations fromthe Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS)should be substantially more powerful than the previous HST SpaceTelescope Imaging Spectrograph (STIS) data to fully explore the CIV?1550 absorption-line profile.
| Analysis of Two Small Magellanic Cloud H II Regions Considering Thermal Inhomogeneities: Implications for the Determinations of Extragalactic Chemical Abundances
We present long-slit spectrophotometry considering the presence ofthermal inhomogeneities (t 2) of two H II regions in theSmall Magellanic Cloud (SMC): NGC 456 and NGC 460. Physical conditionsand chemical abundances were determined for three positions in NGC 456and one position in NGC 460, first under the assumption of uniformtemperature and then allowing for the possibility of thermalinhomogeneities. We determined t 2 values based on threedifferent methods: (1) by comparing the temperature derived using oxygenforbidden lines with the temperature derived using helium recombinationlines (RLs), (2) by comparing the abundances derived from oxygenforbidden lines with those derived from oxygen RLs, and (3) by comparingthe abundances derived from ultraviolet carbon forbidden lines withthose derived from optical carbon RLs. The first two methods averaged t2 = 0.067 ± 0.013 for NGC 456 and t 2 =0.036 ± 0.027 for NGC 460. These values of t 2 implythat when gaseous abundances are determined with collisionally excitedlines they are underestimated by a factor of nearly two. From theseobjects and others in the literature, we find that in order to accountfor thermal inhomogeneities and dust depletion, the O/H ratio inlow-metallicity H II regions should be corrected by 0.25-0.45 dexdepending on the thermal structure of the nebula or by 0.35 dex if suchinformation is not available.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 69.C-0203(A).
| A deep spectroscopic study of the filamentary nebulosity in NGC 4696, the brightest cluster galaxy in the Centaurus cluster
We present results of deep integral-field spectroscopy observationsusing high-resolution optical (4150-7200 Å) VLT VIsibleMultiObject Spectrograph spectra of NGC 4696, the dominant galaxy in theCentaurus cluster (Abell 3526). After the Virgo cluster, this is thesecond nearest (z= 0.0104) example of a cool-core cluster. NGC 4696 issurrounded by a vast, luminous H? emission-line nebula(LH?= 2.2 × 1040 erg s-1).We explore the origin and excitation of the emission-line filaments andfind their origin consistent with being drawn out, under rising radiobubbles, into the intracluster medium as in other similar systems.Contrary to previous observations, we do not observe evidence for shockexcitation of the outer filaments. Our optical spectra are consistentwith the recent particle heating excitation mechanism of Ferland et al.
| Structure and Feedback in 30 Doradus. I. Observations
We have completed a new optical imaging and spectrophotometric survey ofa 140 × 80 pc2 region of 30 Doradus centered on R136,covering key optical diagnostic emission lines including H?,H?, H?, [O III] ??4363, 4959, 5007, [N II]??6548, 6584, [S II] ??6717, 6731 [S III]?6312, and in some locations [S III] ?9069. We presentmaps of fluxes and intensity ratios for these lines, and catalogs ofisolated ionizing stars, elephant-trunk pillars, and edge-on ionizationfronts. The final science-quality spectroscopic data products areavailable to the public. Our analysis of the new data finds that, whilestellar winds and supernovae undoubtedly produce shocks and areresponsible for shaping the nebula, there are no global spectralsignatures to indicate that shocks are currently an important source ofionization. We conclude that the considerable region covered by oursurvey is well described by photoionization from the central clusterwhere the ionizing continuum is dominated by the most massive O stars.We show that if 30 Dor were viewed at a cosmological distance, itsintegrated light would be dominated by its extensive regions of lowersurface brightness rather than by the bright, eye-catching arcs.
| Cosmic-ray Acceleration Efficiency versus Temperature Equilibration: The Case of SNR 0509-67.5
We study the 0509-67.5 supernova remnant in the Large Magellanic Cloudwith the VLT/FORS2 spectrograph. We detect a broad component in theH? emission with an FWHM of 2680 ± 70 km s-1 and3900 ± 800 km s-1 for the southwest (SW) and northeast(NE) shocks, respectively. For the SW, the proton temperature appears tobe too low for the shock velocity, which we attribute to a cosmic-raypressure behind the shock front of at least 20% of the total pressure.For the NE, the post-shock proton temperature and the shock velocity arecompatible, only if the plasma behind the shock front has a degree ofthermal equilibrium of over 20%, which is at odds with current modelsfor temperature equilibration behind fast shocks, which do notaccelerate cosmic rays. If we assume the electron temperature to be lessthan 10% of the proton temperature, we find a post-shock cosmic-raypressure of at least 7%.
| Abundances and Density Structure of the Inner Circumstellar Ring Around SN 1987A
We present optical spectroscopic data of the inner circumstellar ringaround supernova (SN) 1987A from the Anglo-Australian Telescope and theVery Large Telescope (VLT) between ~1400 and ~5000 days post-explosion.We also assembled the available optical and near-infrared line fluxesfrom the literature between ~300 and ~2000 days. These line light curveswere fitted with a photoionization model to determine the densitystructure and the elemental abundances for the inner ring. We founddensities ranging from 1 × 103 to 3 ×104 atoms cm-3 and a total mass of the ionized gasof ~5.8 × 10-2 M sun within the inner ring.Abundances inferred from the optical and near-infrared data were alsocomplemented with estimates of Lundqvist & Fransson based onultraviolet lines. This way we found an He/H ratio (by number of atoms)of 0.17 ± 0.06 which is roughly 30% lower than previouslyestimated and twice the solar and the Large Magellanic Cloud (LMC)value. We found an N/O ratio of 1.5 ± 0.7, and the total(C+N+O)/(H+He) abundance about 1.6 times its LMC value or roughly 0.6times the most recent solar value. An iron abundance of 0.20 ±0.11 times solar was found which is within the range of the estimatesfor the LMC. We also present late time (~5000-7500 days) line lightcurves of [O III], [Ne III], [Ne IV], [Ar III], [Ar IV], and [Fe VII]from observations with the VLT. We compared these with model fluxes andfound that an additional 102 atoms cm-3 componentwas required to explain the data of the highest ionization lines. Suchlow-density gas is expected in the H II-region interior to the innerring which likely extends also to larger radii at higher latitudes (outof the ring plane). At epochs later than ~5000 days, our modelsunderproduce the emission of most of these lines as expected due to thecontribution from the interaction of the SN ejecta with the ring.
| Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association
Aims: We search for new ultracool dwarfs and report here ourobservations of 78 ultracool dwarf candidates that have beenphotometrically selected using the DENIS survey point source catalogue.We analyze low-resolution optical spectroscopic observations to estimatespectral types of all candidates. Methods: We derive spectraltypes for each object using measurements of the PC3 spectral index asdefined in Martín et al. They range from M6 to L3. The H?emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalentwidths are measured in the spectra to identify young stellar objects.Spectroscopic indices of TiO, VO, CrH, and FeH molecular features arealso reported. Results: A rule-of-thumb criterion for selectingyoung very low-mass objects using the NaI doublet equivalent width isgiven. It is used to confirm seven new members of the Upper Sco OBassociation and two new members of the R Cr-A star-forming region. Fourof our field objects are also classified as very young, but are notmembers of any known nearby young association. The frequency oflower-gravity young objects in our field ultracool sample is 8.5%. Ourresults provide the first spectroscopic classification for 42 ultracooldwarfs in the solar vicinity with spectrophotometric distances in therange 17 pc to 65 pc (3 of them being new L dwarfs within 20 pc).
| A new population of planetary nebulae discovered in the Large Magellanic Cloud - III. The luminosity function
Our previous identification and spectroscopic confirmation of 431 faint,new planetary nebulae (PNe) in the central 25 deg2 region ofthe Large Magellanic Cloud (LMC) permit us to now examine the shape ofthe LMC planetary nebula luminosity function (PNLF) through anunprecedented 10 mag range. The majority of our newly discovered andpreviously known PNe were observed using the 2dF, multi-object fibrespectroscopy system on the 3.9-m Anglo-Australian Telescope and theFLAMES multi-object spectrograph on the 8-m Very Large Telescope. Wepresent reliable [OIII]5007Å and H? flux estimates based oncalibrations to well-established PN fluxes from previous surveys andspectroscopic standard stars. The bright cut-off (M*) of the PNLF isfound by fitting a cumulative function to the bright end of the PNLFover a 3.4 mag range. This cut-off is used to estimate a new distancemodulus of 18.46 to the LMC, in close agreement with previous PNLFstudies and the best estimates by other indicators. The bright-endcut-off is robust to small samples of bright PNe since significantlyincreased PN samples do not change this fiducial. We then fit atruncated exponential curve directly to the bright end of the functionover a 6 mag range and test the curve's ability to indicate the positionof M*. Because of the significant increase in the number of LMC PNe, theshape of the PNLF is now examined in greater detail than has previouslybeen possible. Newly discovered features include a small increase in thenumber of PNe over the brightest 4 mag followed by a steep rise over 2mag, a peak at 6 mag below the bright cut-off and an almost lineardrop-off to the faint end. Dips at the bright end of the PNLF areexamined in relation to the overall shape of the PNLF and theexponential increase in the number of PNe. Through cumulative functions,the new LMC PNLF is compared to those from the Small Magellanic Cloudand a new deep local Galactic sample revealing the effects ofincompleteness. The new [OIII]5007Å LMC PNLF is then compared toour new H? LMC PNLF using calibrated and measured fluxes for thesame objects, revealing the effects of metallicity on the[OIII]5007Å line.
| Stellar populations of Virgo cluster early-type dwarf galaxies with and without discs: a dichotomy in age?
Using Very Large Telescope/FORS2 spectroscopy, we have studied theproperties of the central (inner 320pc) stellar populations of a sampleof 26 nucleated early-type dwarf (dE) galaxies in the Virgo cluster witha magnitude range of -18.59 <= Mr <= -15.39 mag. Withthe addition of the data of the MAGPOP-IPP team (Michielsen et al.),extending the sample to 38 dEs, we find that these galaxies do notexhibit the same average stellar population characteristics fordifferent morphological subclasses. The nucleated galaxies withoutdiscs, which are, on average, fainter than dEs with discs anddistributed in regions of higher local density, are older and moremetal-poor (mean ages 7.5 +/- 1.89 and 3.1 +/- 0.83Gyr, mean [Z/H] =-0.54 +/- 0.14 and -0.31 +/- 0.10dex, respectively). The ?-elementabundance ratio appears consistent with the solar value for bothmorphological types. Besides a well-defined relation between metallicityand luminosity, we also find a clear anticorrelation between age andluminosity. More specifically, there appears to be a bimodality:brighter galaxies (Mr <= -16.5 mag), including the discgalaxies, exhibit significantly younger ages than fainter dEs(Mr >= -17.0 mag). The magnitude overlap between these twosubgroups appears to be resolved when considering, in addition, the weakcorrelation between age and local density, such that older galaxies at agiven magnitude are located at higher densities. Thus, there seems to beno significant difference between the stellar populations of dEs withand without discs when compared at the same magnitude and density.Thereby, we revisit the question whether both could belong to the sameintrinsic population, with discs surviving only in lower-densityregions. However, it appears less likely that fainter and brighter dEshave experienced the same evolutionary history, as the well-establishedtrend of decreasing average stellar age when going from the mostluminous ellipticals towards low-luminosity ellipticals and bright dEsis broken here. The older and more metal-poor dEs could have had anearly termination of star formation activity, possibly being`primordial' galaxies in the sense that they have formed along with theprotocluster or experienced very early infall. By contrast, the youngerand relatively metal-rich brighter dEs, most of which have discs, mighthave undergone structural transformation of infalling disc galaxies.Based on observations collected at the European Organization forAstronomical Research in the Southern Hemisphere, Chile (programme078.B-0178).E-mail: sjy@x-astro.net
| NGC 300 X-1 is a Wolf-Rayet/black hole binary
We present Very Large Telescope/FORS2 time-series spectroscopy of theWolf-Rayet (WR) star #41 in the Sculptor group galaxy NGC 300. Weconfirm a physical association with NGC 300 X-1, since radial velocityvariations of the HeII ?4686 line indicate an orbital period of32.3 +/- 0.2h which agrees at the 2? level with the X-ray periodfrom Carpano et al. We measure a radial velocity semi-amplitude of 267+/- 8kms-1, from which a mass function of 2.6 +/-0.3Msolar is obtained. A revised spectroscopic mass for theWN-type companion of 26+7-5Msolaryields a black hole mass of 20 +/- 4Msolar for a preferredinclination of 60°-75°. If the WR star provides half of themeasured visual continuum flux, a reduced WR (black hole) mass of15+4-2.5Msolar(14.5+3-2.5Msolar) would be inferred.As such, #41/NGC 300 X-1 represents only the second extragalactic WRplus black hole binary system, after IC 10 X-1. In addition, the compactobject responsible for NGC 300 X-1 is the second highest stellar-massblack hole known to date, exceeded only by IC 10 X-1.Based on observations made with European Southern Observatory (ESO)telescopes at the Paranal Observatory under programme ID 384.D-0093(A).E-mail: Paul.crowther@sheffield.ac.uk
| Subaru FOCAS Spectroscopic Observations of High-Redshift Supernovae
We present spectra of high-redshift supernovae (SNe) that were takenwith the Subaru low-resolution optical spectrograph, FOCAS. These SNewere found in SN surveys with Suprime-Cam on Subaru, the CFH12k cameraon the Canada-France-Hawaii Telescope, and the Advanced Camera forSurveys on the Hubble Space Telescope. These SN surveys specificallytargeted z > 1 Type Ia supernovae (SNe Ia). From the spectra of 39candidates, we obtained redshifts for 32 candidates andspectroscopically identified 7 active candidates as probable SNe Ia,including one at z = 1.35, which is the most distant SN Ia to bespectroscopically confirmed with a ground-based telescope. An additional4 candidates were identified as likely SNe Ia from thespectrophotometric properties of their host galaxies. Seven candidatesare not SNe Ia, either being SNe of another type or active galacticnuclei. When SNe Ia were observed within one week of the maximum light,we found that we could spectroscopically identify most of them up to z =1.1. Beyond this redshift, very few candidates were spectroscopicallyidentified as SNe Ia. The current generation of super red-sensitive,fringe-free CCDs will push this redshift limit higher.
| Extremely faint high proper motion objects from SDSS stripe 82. Optical classification spectroscopy of about 40 new objects
Aims: By pushing the magnitude limit of high proper motion surveysbeyond the limit of photographic Schmidt plates, we aim to discovernearby and very fast low-luminosity objects of different classes: coolwhite dwarfs (CWDs), cool subdwarfs (sd), and very low-mass stars andbrown dwarfs at the very faint end of the main sequence (MS). Methods: The deep multi-epoch Sloan Digital Sky Survey data in a 275square degree area along the celestial equator (SDSS stripe 82) allow usto search for extremely faint (i>21) objects with proper motionsgreater than 0.14 arcsec/yr. A reduced proper motion diagram H_z/(i-z)clearly reveals three sequences (MS, sd, CWD) where our faintestcandidates are representative of the still poorly known bottom of eachsequence. We classify 38 newly detected objects with low-resolutionoptical spectroscopy using FORS1 @ ESO VLT. Together with our targets weobserve six known L dwarfs in stripe 82, four (ultra)cool sd and one CWDas comparison objects. Distances and tangential velocities are estimatedusing known spectral type/absolute magnitude relations. Results:All 22 previously known L dwarfs (and a few of the T dwarfs) in stripe82 have been detected in our high proper motion survey. However, 11 ofthe known L dwarfs have smaller proper motions (0.01sdM7) subdwarfs. Some M subdwarf candidates havebeen classified based on spectral indices with large uncertainties. Conclusions: We failed to detect new nearby (d<50 pc) L dwarfs,probably because the SDSS stripe 82 area was already well-investigatedbefore. With our survey we have demonstrated a higher efficiency infinding Galactic halo CWDs than previous searches. The space density ofhalo CWDs is according to our results about 1.5-3.0 ×10-5 pc-3.Based on observations with VLT/FORS1 at the European SouthernObservatory (ESO program 078.D-0595).
| The eclipsing LMC star OGLE05155332-6925581: a clue for double periodic variables
We investigate the nature of OGLE05155332-6925581, one of the brightestmembers of the enigmatic group of double periodic variables (DPVs)recently found in the Magellanic Clouds. The modelling of archivalorbital light curves (LCs), along with the analysis of the radialvelocities (RVs) suggest that this object is a semidetached binary withthe less massive star transferring matter to the more massive and lessevolved star, in an Algol-like configuration. We find evidence foradditional orbital variability and H? emission, likely caused byan accretion disc around the primary star. As in the case of ?Lyrthe circumprimary disc seems to be more luminous than the primary, butwe do not detect orbital period changes. We find that the LC follows aloop in the colour-magnitude diagram during the long cycle; the systemis redder when brighter and the rising phase is bluer than duringdecline. Infrared excess is also present. The source of the long-termperiodicity is not eclipsed, indicating its circumbinary origin. Strongasymmetries, discrete absorption components (DACs) and a ? shiftare new and essential observational properties in the infrared HI lines.The DACs strength and RV follow a saw-teeth pattern during the orbitalcycle. We suggest that the system experiences supercycles of massoutflow feeding a circumbinary disc. Mass exchange and mass loss couldproduce comparable but opposite effects in the orbital period on a longtime-scale, resulting in a quasi-constancy of this parameter.Based on observations carried out at ESO and CTIO telescopes: ESOproposal 076.D-0126(A) and NOAO proposal CHILE03B-01.E-mail: rmennick@astro-udec.cl
| Medium-resolution spectroscopy of FORJ0332-3557: probing the interstellar medium and stellar populations of a lensed Lyman-break galaxy at z = 3.77
We recently reported the discovery of FORJ0332-3557, a lensedLyman-break galaxy at z = 3.77 in a remarkable example of stronggalaxy-galaxy gravitational lensing. We present here a medium-resolutionrest-frame ultraviolet (UV) spectrum of the source, which appears to besimilar to the well-known Lyman-break galaxy MS1512-cB58 at z = 2.73.The spectral energy distribution is consistent with a stellar populationof less than 30Ma, with an extinction of Av = 0.5 mag and anextinction-corrected star formation rate SFRUV of200-300h-170Msolara-1. TheLy? line exhibits a damped profile in absorption produced by acolumn density of about NHI = (2.5 +/- 1.0) ×1021cm-2, superimposed on an emission line shiftedboth spatially (0.5 arcsec with respect to the UV continuum source) andin velocity space (+830kms-1 with respect to thelow-ionization absorption lines from its interstellar medium), a clearsignature of outflows with an expansion velocity of about270kms-1. A strong emission line from HeII ?164.04 nmindicates the presence of Wolf-Rayet stars and reinforces theinterpretation of a very young starburst. The metallic lines indicatesubsolar abundances of elements Si, Al and C in the ionized gas phase.Based on observations made at the ESO VLT under programmes 74.A-0536 and78.A-0240.E-mail: remi.cabanac@ast.obs-mip.fr (RAC); david.valls-gabaud@obspm.fr(DV-G); clidman@eso.org (CL)
| A Combined EIS-NVSS Survey Of Radio Sources (CENSORS) - III. Spectroscopic observations
The Combined EIS-NVSS Survey Of Radio Sources (CENSORS) is a 1.4-GHzradio survey selected from the NRAO VLA Sky Survey (NVSS) and completeto a flux density of 7.2mJy. It targets the ESO Imaging Survey (EIS)Patch D, which is a 3 × 2-deg2 field centred on(J2000). This paper presents the results of spectroscopic observationsof 143 of the 150 CENSORS sources. The primary motivation for theseobservations is to achieve sufficient spectroscopic completeness so thatthe sample may be used to investigate the evolution of radio sources.The observations result in secure spectroscopic redshifts for 63 percent of the sample and likely redshifts (based on a single emissionline, for example) for a further 8 per cent. Following theidentification of the quasars and star-forming galaxies in the CENSORSsample, estimated redshifts are calculated for the remainder of thesample via the K - z relation for radio galaxies.Comparison of the redshift distribution of the CENSORS radio sources todistributions predicted by the various radio luminosity functionevolution models of Dunlop & Peacock, results in no good match. Thisdemonstrates that this sample can be used to expand upon previous workin that field.
| Near infrared photometric and optical spectroscopic study of 22 low mass star clusters embedded in nebulae
Aims:Among the star clusters in the Galaxy, those embedded in nebulaerepresent the youngest group, which has only recently been explored. Theanalysis of a sample of 22 candidate embedded stellar systems inreflection nebulae and/or HII environments is presented. Methods:We employed optical spectroscopic observations of stars in thedirections of the clusters carried out at CASLEO (Argentina) togetherwith near infrared photometry from the 2MASS catalogue. Our analysis isbased on source surface density, colour-colour diagrams and ontheoretical pre-main sequence isochrones. We take into account the fieldstar contamination by carrying out a statistical subtraction. Results: The studied objects have the characteristics of low masssystems. We derive their fundamental parameters. Most of the clusterages are younger than 2 Myr. The studied embedded stellar systems inreflection nebulae and/or HII region complexes do not have stars ofspectral types earlier than B. The total stellar masses locked in theclusters are in the range 20-220 M&sun;. They are found to begravitationally unstable and are expected to dissolve in a timescale ofa few Myr.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba and San Juan, Argentina.
| Integrated spectral properties of 22 small angular diameter galactic open clusters
Aims.Flux-calibrated integrated spectra of a sample of 22 Galactic openclusters of small angular diameter are presented. With one exception(ESO 429-SC2), all objects have Galactic longitudes in the range208° < l < 33°. The spectra cover the range ≈3600-6800Å, with a resolution of ≈14 Å. The properties of thepresent cluster sample are compared with those of well-studied clusterslocated in two 90° sectors, centred at l = 257° and l =347°. The dissolution rate of Galactic open clusters in these twosectors is examined. Methods: Using the equivalent widths of the Balmerlines and comparing line intensities and continuum distribution of thecluster spectra with those of template cluster spectra with knownproperties, we derive both foreground reddening values and ages. Thus,we provide information independent of that determined throughcolour-magnitude diagrams. Results: The derived E(B-V) values for thewhole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, whilethe ages range from ~3 Myr (NGC 6604 and BH 151) to ~3.5 Gyr (Ruprecht2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2,BH 151 and Hogg 9) the foreground E(B-V) colour excesses and ages aredetermined for the first time. The results obtained for the remainingclusters show, in general terms, good agreement with previousphotometric results. Conclusions: The age and reddening distributionsof the present sample match those of known clusters in the two selectedGalactic sectors. The present results would favour a major dissolutionrate of star clusters in these two sectors. Two new solar-metallicitytemplates are defined corresponding to the age groups of (4-5) Myr and30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piattiet al. templates of 20 Myr and (3-4) Gyr are here redefined.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina (CONICET) and the National Universities of LaPlata, Córdoba and San Juan, Argentina. Tables [see full text]-[see full text] and Appendix are only availablein electronic form at http://www.aanda.org
| The XMM-SSC survey of hard-spectrum XMM-Newton sources - I. Optically bright sources
We present optical and X-ray data for a sample of serendipitousXMM-Newton sources that are selected to have 0.5-2 versus 2-4.5 keVX-ray hardness ratios which are harder than the X-ray background. Thesources have 2-4.5 keV X-ray flux >=10-14 ergcm-2 s-1, and in this paper we examine a subsampleof 42 optically bright (r < 21) sources; this subsample is 100 percent spectroscopically identified. All but one of the opticalcounterparts are extragalactic, and we argue that the single exception,a Galactic M star, is probably a coincidental association rather thanthe correct identification of the X-ray source. The X-ray spectra of allthe sources are consistent with heavily absorbed power laws (21.8 1044 erg s-1, and two of thesesources have optical spectra which are dominated by narrow emissionlines, that is, are type 2 QSOs. Only a small fraction of the sources(7/42) show broad optical emission lines, and all of these haveNH < 1023 cm-2. This implies thatratios of X-ray absorption to optical/ultraviolet extinction equivalentto >100 times the Galactic gas-to-dust ratio are rare in AGNabsorbers (at most a few per cent of the population), and may berestricted to broad absorption line QSOs. Seven objects appear to havean additional soft X-ray component in addition to the heavily absorbedpower law; all seven are narrow emission-line objects with z < 0.3and 2-10 keV intrinsic luminosities <1043 ergs-1. We consider the implications of our results in the lightof the AGN unified scheme. We find that the soft components innarrow-line objects are consistent with the unified scheme provided that>4 per cent of broad-line AGN (BLAGN) have ionized absorbers thatattenuate their soft X-ray flux by >50 per cent. In at least one ofthe X-ray-absorbed BLAGN in our sample the X-ray spectrum requires anionized absorber, consistent with this picture.
| Late-Type Near-Contact Eclipsing Binary [HH97] FS Aur-79
The secondary photometric standard star number 79 for the FS Aur field(Henden & Honeycutt 1997), designated as [HH97] FS Aur-79 (GSC1874-399), is a short-period (0.2508 days) eclipsing binary whose lightcurve is a combination of the β Lyr and BY Dra type variables. Highsignal-to-noise ratio multicolor photometry was obtained using the USNaval Observatory 1 m telescope. These light curves show asymmetry atquadrature phases (the O'Connell effect), which can be modeled with thepresence of starspots. A low-resolution spectrum obtained with the 3.5 mWisconsin-Indiana-Yale-NOAO telescope at orbital phase 0.76 isconsistent with a spectral type of dK7e and dM3e. A radial velocitycurve for the primary star was constructed using 24 high-resolutionspectra from the 9.2 m Hobby-Eberly Telescope. Spectra show Hα andHβ in emission confirming chromospheric activity and possibly thepresence of circumstellar material. Binary star models thatsimultaneously fit the U, B, V, R, and radial velocity curves are thosewith a primary star of mass 0.59+/-0.02 Msolar, temperature4100+/-25 K, and mean radius 0.67 Rsolar, just filling itsRoche lobe, and a secondary star of mass 0.31+/-0.09 Msolar,temperature 3425+/-25 K, and mean radius 0.48 Rsolar, justwithin its Roche lobe. An inclination angle of83deg+/-2deg with a center-of-mass separation of1.62 Rsolar is also derived. Starspots, expected for arotation period of less than 1 day, had to be included in the modelingto fit the O'Connell effect.
| NGC 5011C: An Overlooked Dwarf Galaxy in the Centaurus A Group
A critical study of the properties of groups of galaxies can be doneonly once a complete census of group members is available. Despiteextensive surveys, even nearby groups can lead to surprises. Indeed, wereport the discovery of a previously unnoticed member of the Centaurus AGroup, NGC 5011C. While the galaxy is a well-known stellar system listedwith a NGC number, its true identity has remained hidden because ofcoordinate confusion and wrong redshifts in the literature. NGC 5011Cattracted our attention since, at a putative distance of 45.3 Mpc, itwould be a peculiar object with a very low surface brightness typical ofa dwarf galaxy and, at the same time, a size typical of an early-typespiral or S0 galaxy. To confirm or reject this peculiarity, ourimmediate objective was to have the first reliable measurement of itsrecession velocity. The observations were carried out with EFOSC2 at the3.6 m European Southern Observatory (ESO) telescope, and the spectrawere obtained with the instrument in long-slit mode. The redshifts ofboth NGC 5011C and its neighbor NGC 5011B were computed bycross-correlating their spectra with that of a radial velocity standardstar. We found that NGC 5011C indeed has a low redshift ofvodot = 647 ± 96 km s-1 and thus is anearby dwarf galaxy rather than a member of the distant Centauruscluster, as believed for the past 23 years. Rough distance estimatesbased on photometric parameters also favor this scenario. As aby-product of our study we update the redshift for NGC 5011B tovodot = 3227 ± 50 km s-1. Applyingpopulation synthesis techniques, we find that NGC 5011B has aluminosity-weighted age of 4 ± 1 Gyr and a solar metallicity, andthat the luminosity-weighted age and metallicity of NGC 5011C are 0.9± 0.1 Gyr and 1/5 solar. Finally, we estimate a stellar mass ofNGC 5011C comparable to that of dwarf spheroidal galaxies in the LocalGroup.Based on observations made with European Southern Observatory telescopesat the La Silla Observatory.
| Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs
We present spectra of 59 nearby star candidates, M dwarfs and whitedwarfs, previously identified using high proper motion catalogues andthe DENIS database. We review the existing spectral classificationschemes and spectroscopic parallax calibrations in the near-infrared Jband and derive spectral types and distances of the nearby candidates.Forty-two stars have spectroscopic distances smaller than 25 pc, threeof them being white dwarfs. Two targets lie within 10 pc, one M8 star at10.0 pc (APMPM J0103-3738), and one M4 star at 8.3 pc (L 225-57). Onestar, LHS 73, is found to be among the few subdwarfs lying within 20 pc.Furthermore, together with LHS 72, it probably belongs to the closestpair of subdwarfs we know.
| A new population of planetary nebulae discovered in the Large Magellanic Cloud - II. Complete PN catalogue
This paper presents accurate homogeneous positions, velocities and otherpertinent properties for 460 newly discovered and 169 previously knownplanetary nebulae (PNe) in the central 25-deg2 bar region ofthe Large Magellanic Cloud (LMC). Candidate emission sources werediscovered using a deep, high-resolution H? map of the LMCobtained by median stacking a dozen 2 h H? exposures taken withthe UK Schmidt Telescope (UKST). Our spectroscopic follow-up of morethan 2000 compact (i.e. <20 arcsec) H? emission candidatesuncovered has tripled the number of PNe in this area. All of the 169previously known PNe within this region have also been independentlyrecovered and included in this paper to create a homogeneous data set.Of the newly discovered PNe, we classify 291 as `true', 54 as `likely'and 115 as `possible' based on the strength of photometric andspectroscopic evidence. Radial velocities have been measured using bothweighted averaging of emission lines and cross-correlation techniquesagainst high-quality templates. Based on the median comparison of thetwo systems, we define a measurement error of +/-4 km s-1. Anew velocity map of the central 25 deg2 of the LMC, based onresults from the combined new and previously known PNe, is presented,indicating an averaged heliocentric velocity differential of 65 kms-1 perpendicular to the line of nodes for the entire PNpopulation across our survey area. Averaged velocities of our PNe andmolecular hydrogen (from the literature) across 37 × 37arcmin2 subareas are compared. The PNe are found to have ahigher vertical velocity dispersion than the HI disc to a maximum of 10times the spread of the HI disc, in keeping with the findings ofMeatheringham et al. In addition, moving out from the main bar, we findthat the PN population follows a plane which is somewhat warped inrelation to the HI disc. We estimate the total PN population of theentire LMC system, based on our Requiv H? limitingmagnitude of ~22, to be 956 +/- 141.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Blue and yellow long-period variables in the direction of the Small Magellanic Cloud
Aims.We investigate the nature of a sample of 17 long-term periodicvariables in the direction of the Small Magellanic Cloud. Methods:.Based on new spectroscopic data, we determined spectral types, radialvelocities, absolute magnitudes, and colors for these stars. We presenta refined discussion of their OGLE light curves along with an analysisof their 2MASS photometry. Results: .Most stars turned out to beB-A giants members of the Small Magellanic Cloud. We find a newinteracting eclipsing binary with a period of 184 days and two newearly-type ellipsoidal variables. One of our objects is the ROSAT sourceRX J0058.2-7231. We analyzed 11-years of data forthis Be X-ray binary finding that their photometric period varies by ~4%quasiperiodically, on a time scale of ~1200 days. We find evidence ofmultiple photometric periods in 2 Ae-type and 1 late-Be type stars. Thecase of OGLE00445466-7328029 is especiallyinteresting, as this late-type Be star shows a beating phenomenonprimarily caused by two closely-spaced frequencies, 0.05733 c/d (17.44d) and 0.06347 c/d (15.76 d). Four other emission-line objects showstable long-term periodicities and probably correspond to Be-starbinaries. Transient photometric periods are only found in 4 non-emissionearly-type stars.
| Spectroscopy of blue and yellow long-period variables in the Small Magellanic Cloud
We present spectroscopy for 17 bright variable stars possible members ofthe Small Magellanic Cloud. These stars are OGLE variables whosephotometric periodicity is poorly understood. We have determinedspectral types and radial velocities for the stars of our sample. Mosttargets turned out to be emission-line objects of spectral type B-F andluminosity class giant or supergiant. From the analysis of the OGLElight curves, we find a new interacting eclipsing binary with a periodof 184 days, and two new early-type ellipsoidal variables. The case ofOGLE00445466-7328029 is specially interesting; this late-type Be starshows a beating phenomenon primarily caused by two closely spacedfrequencies, 0.05733 c/d (17.44 d) and 0.06347 c/d (15.76 d). We discussthis case in particular.
| Uncovering Planetary Nebulae in Early-Type Galaxies using the Rutgers Fabry-Pérot
We report on observations of four early-type galaxies performed with theRutgers Fabry-Pérot in order to search for planetary nebulae(PNe) in these systems. The aim is to use the PNe as kinematic tracersof the galaxy potential. We describe our data reduction and analysisprocedure and show that the proper calibration of our detectionstatistic is crucial in getting down to our limiting magnitude ofm5007=26.1. In the case of the two Leo galaxies, we findmoderately sized samples: 54 PNe in NGC 3379 and 50 PNe in NGC 3384; NGC4636 (two PNe) and NGC 1549 (six PNe) are included for completeness. Wepresent our samples in tabular form, as well as the spectrum for eachPN. We constructed simple nonparametric spherical mass models for NGC3379 using a Monte Carlo Markov chain method to explore the space oflikely mass models. We find a remarkably constant mass-to-light ratiowithin five half-light radii with an overall B-band mass-to-light ratioof ~5. A simple mass-to-light ratio estimate for NGC 3384 yieldsΥB~11, but it is likely an overestimate.
| uvby-β photometry of high-velocity and metal-poor stars. XI. Ages of halo and old disk stars
New uvby-β data are provided for 442 high-velocity and metal-poorstars; 90 of these stars have been observed previously by us, and 352are new. When combined with our previous two photometric catalogues, thedata base is now made up of 1533 high-velocity and metal-poor stars, allwith uvby-β photometry and complete kinematic data, such as propermotions and radial velocities taken from the literature. Hipparcos, plusa new photometric calibration for Mv also based on theHipparcos parallaxes, provide distances for nearly all of these stars;our previous photometric calibrations give values for E(b-y) and [Fe/H].The [Fe/H], V(rot) diagram allows us to separate these stars intodifferent Galactic stellar population groups, such as old-thin-disk,thick-disk, and halo. The X histogram, where X is our stellar-populationdiscriminator combining V(rot) and [Fe/H], and contour plots for the[Fe/H], V(rot) diagram both indicate two probable components to thethick disk. These population groups and Galactic components are studiedin the (b-y)0, Mv diagram, compared to theisochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), toderive stellar ages. The two thick-disk groups have the meancharacteristics: ([Fe/H], V(rot), Age, σW') ≈ (-0.7dex, 120 km s-1, 12.5 Gyr, 62.0 km s-1), and≈(-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups,-2.31 ≤ [Fe/H] ≤ -1.31, show a mean age of 13.0 ± 0.2(mean error) Gyr, giving a mean difference from the WMAP results for theage of the Universe of 0.7 ± 0.3 Gyr. These results for the agesand components of the thick disk and for the age of the Galactic halofield stars are discussed in terms of various models and ideas for theformation of galaxies and their stellar populations.
| Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the LocalGroup irregular galaxy NGC 6822. The data consist of VLT FORSobservations in the 3450-7500 Å range. We have obtained electrontemperatures and densities using different line intensity ratios. Wehave derived the He, C, and O abundances relative to H based onrecombination lines; the abundance ratios among these elements arealmost independent of the temperature structure of the nebulae. We havealso determined the N, O, Ne, S, Cl, and Ar abundances based oncollisionally excited lines; the ratios of these abundances relative tothat of H depend strongly on the temperature structure of the nebulae.The chemical composition of NGC 6822 V is compared with those of theSun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC.The value of O/H derived from recombination lines is in good agreementwith that derived by Venn and coworkers from two A-type supergiants inNGC 6822.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 69.C-0203(A).
| Lithium and Lithium Depletion in Halo Stars on Extreme Orbits
We have determined Li abundances in 55 dwarfs and subgiants that aremetal-poor (-3.6<[Fe/H]<-0.7) and have extreme orbital kinematics.Our purpose is to examine the Li abundance in the Li plateau stars andits decrease in low-temperature, low-mass stars. For the stars in oursample we have determined chemical profiles given in 2002 by Stephens& Boesgaard. The Li observations are primarily from the echellespectrograph on the 10 m Keck I telescope, with HIRES covering 4700-6800Å with a spectral resolution of ~48,000. The spectra have highsignal-to-noise ratios, from 70 to 700 pixel-1, with a medianof 140. The Li I resonance doublet was detected in 42 of the 55 stars.Temperatures were found spectroscopically by Stephens & Boesgaard.Abundances or upper limits were determined for all stars, with typicalerrors of 0.06 dex. Corrections for the deviations from nonlocalthermodynamical equilibrium for Li in the stellar atmospheres have beenmade, which range from -0.04 to +0.11 dex. Our 14 dwarf and turnoffstars on the Li plateau with temperatures greater than 5700 K and[Fe/H]<-1.5 give A(Li)=logN(Li)/N(H)+12.00 of 2.215+/-0.110,consistent with earlier results. We find a dependence of the Liabundance on metallicity as measured by [Fe/H] and the Fe-peak elementsCr and Ni, with a slope of ~0.18. We have examined the possible trendsof A(Li) with the chemical abundances of other elements and find similardependences of A(Li) with the α-elements Mg, Ca, and Ti. Theseslopes are slightly steeper at ~0.20, resulting from an excess in[α/Fe] with decreasing [Fe/H]. For the n-capture, rare-earthelement Ba, we find a relation between A(Li) and [Ba/H] that has ashallower slope of ~0.13 over a range of 2.6 dex in [Ba/H], the Liabundance spans only a factor of 2. We have also examined the possibletrends of A(Li) with the characteristics of the orbits of our halostars. We find no trends in A(Li) with kinematic or dynamic properties.For the stars with temperatures below the Li plateau, there are severalinteresting results. The group of metal-poor stars possess, on average,more Li at a given temperature than metal-rich stars. When we divide thecool stars into smaller subsets with similar metallicities, we findtrends of A(Li) with temperature for the different metallicity groups.The decrease in A(Li) sets in at hotter temperatures for the highermetallicity stars than for the lower metallicity stars. The increased Lidepletion in cooler stars could be a result of the increased action ofconvection, since cooler stars have deeper convection zones. This wouldalso make it easier for additional mixing mechanisms, such as thoseinduced by rotation, to have a greater effect in cooler stars. Since themodel depth of the convection zone is almost independent of metallicityat a given effective temperature, the apparent metallicity dependence ofthe Li depletion in our data may be pointing to subtle but poorlyunderstood mixing effects in low-mass halo dwarfs. Predictions for Lidepletion from standard and nonstandard models seem to underestimate thedegree of depletion inferred from the observations of the cool stars.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Colombe |
| Right ascension: | 05h56m24.74s |
| Declination: | -27°51'32.4" |
| Apparent magnitude: | 12.378 |
| Proper motion RA: | 236.3 |
| Proper motion Dec: | -163.8 |
| B-T magnitude: | 12.298 |
| V-T magnitude: | 12.372 |
Catalogs and designations:
|
