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Ýlgili Makaleler
High-resolution spectroscopy of the blue compact dwarf galaxy Haro 15 - II. Chemodynamics
We present a detailed study of the physical properties of the nebularmaterial in four star-forming knots of the blue compact dwarf galaxyHaro 15. Using long-slit and echelle spectroscopy obtained at LasCampanas Observatory, we study the physical conditions (electron densityand temperatures), ionic and total chemical abundances of several atoms,reddening and ionization structure, for the global flux and for thedifferent kinematical components. The latter was derived by comparingthe oxygen and sulphur ionic ratios to their corresponding observedemission-line ratios (the ? and ?' plots) in different regionsof the galaxy. Applying the direct method or empirical relationships forabundance determination, we perform a comparative analysis between theseregions. The similarities found in the ionization structure of thedifferent kinematical components imply that the effective temperaturesof the ionizing radiation fields are very similar in spite of some smalldifferences in the ionization state of the different elements.Therefore, the different gaseous kinematical components identified ineach star-forming knot are probably ionized by the same star cluster.However, the difference in the ionizing structure of the two knots withknot A showing a lower effective temperature than knot B suggests adifferent evolutionary stage for them consistent with the presence of anolder and more evolved stellar population in the first.
| Optical and Near-infrared Monitoring of the Black Hole X-Ray Binary GX 339-4 during 2002-2010
We present the optical/infrared (O/IR) light curve of the black holeX-ray binary GX 339-4 collected at the SMARTS 1.3 m telescope from 2002to 2010. During this time the source has undergone numerous statetransitions including hard-to-soft state transitions when we see largechanges in the near-IR flux accompanied by modest changes in opticalflux, and three rebrightening events in 2003, 2005, and 2007 after GX339-4 transitioned from the soft state to the hard. All but one outburstshow similar behavior in the X-ray hardness-intensity diagram. We showthat the O/IR colors follow two distinct tracks that reflect either thehard or soft X-ray state of the source. Thus, either of these two X-raystates can be inferred from O/IR observations alone. From thesecorrelations we have constructed spectral energy distributions of thesoft and hard states. During the hard state, the near-IR data have thesame spectral slope as simultaneous radio data when GX 339-4 was in abright optical state, implying that the near-IR is dominated by anon-thermal source, most likely originating from jets. Non-thermalemission dominates the near-IR bands during the hard state at all butthe faintest optical states, and the fraction of non-thermal emissionincreases with increasing optical brightness. The spectral slope of theoptical bands indicate that a heated thermal source is present duringboth the soft and hard X-ray states, even when GX 339-4 is at itsfaintest optical state. We have conducted a timing analysis of the lightcurve for the hard and soft states and find no evidence of acharacteristic timescale within the range of 4-230 days.
| The Spectroscopic Diversity of Type Ia Supernovae
We present 2603 spectra of 462 nearby Type Ia supernovae (SNe Ia),including 2065 previously unpublished spectra, obtained during 1993-2008through the Center for Astrophysics Supernova Program. There are onaverage eight spectra for each of the 313 SNe Ia with at least twospectra. Most of the spectra were obtained with the FAST spectrograph atthe Fred Lawrence Whipple Observatory 1.5 m telescope and reduced in aconsistent manner, making this data set well suited for studies of SN Iaspectroscopic diversity. Using additional data from the literature, westudy the spectroscopic and photometric properties of SNe Ia as afunction of spectroscopic class using the classification schemes ofBranch et al. and Wang et al. The width-luminosity relation appears tobe steeper for SNe Ia with broader lines, although the result is notstatistically significant with the present sample. Based on theevolution of the characteristic Si II ?6355 line, we proposeimproved methods for measuring velocity gradients, revealing a largerrange than previously suspected, from ~0 to ~400 km s-1day-1 considering the instantaneous velocity declinerate at maximum light. We find a weaker and less significant correlationbetween Si II velocity and intrinsic B - V color at maximum lightthan reported by Foley et al., owing to a more comprehensive treatmentof uncertainties and host galaxy dust. We study the extent of nuclearburning and the presence of unburnt carbon in the outermost layers ofthe ejecta and report new detections of C II ?6580 in 23early-time SN Ia spectra. The frequency of C II detections is not higherin SNe Ia with bluer colors or narrower light curves, in conflict withthe recent results of Thomas et al. Based on nebular spectra of 27 SNeIa, we find no relation between the FWHM of the iron emission feature at~4700 Å and ?m 15(B) after removing the twolow-luminosity SN 1986G and SN 1991bg, suggesting that the peakluminosity is not strongly dependent on the kinetic energy of theexplosion for most SNe Ia. Finally, we confirm the correlation ofvelocity shifts in some nebular lines with the intrinsic B - Vcolor of SNe Ia at maximum light, although several outliers suggest apossible non-monotonic behavior for the largest blueshifts.Based in part on observations obtained at the F. L. Whipple Observatory,which is operated by the Smithsonian Astrophysical Observatory, the MMTObservatory, a joint facility of the Smithsonian Institution and theUniversity of Arizona, and with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
We present improved atmospheric parameters of nearby white dwarfs lyingwithin 20 pc of the Sun. The aim of the current study is to obtain thebest statistical model of the least-biased sample of the white dwarfpopulation. A homogeneous analysis of the local population is performedcombining detailed spectroscopic and photometric analyses based onimproved model atmosphere calculations for various spectral typesincluding DA, DB, DC, DQ, and DZ stars. The spectroscopic technique isapplied to all stars in our sample for which optical spectra areavailable. Photometric energy distributions, when available, are alsocombined to trigonometric parallax measurements to derive effectivetemperatures, stellar radii, as well as atmospheric compositions. Arevised catalog of white dwarfs in the solar neighborhood is presented.We provide, for the first time, a comprehensive analysis of the massdistribution and the chemical distribution of white dwarf stars in avolume-limited sample.
| The Properties of the 500 K Dwarf UGPS J072227.51-054031.2 and a Study of the Far-red Flux of Cold Brown Dwarfs
We present i and z photometry for 25 T dwarfs and 1 L dwarf. Combinedwith published photometry, the data show that the i - z, z - Y, and z -J colors of T dwarfs are very red, and continue to increase through tothe late-type T dwarfs, with a hint of a saturation for the latest typeswith T eff ? 600 K. We present new 0.7-1.0 ?m and2.8-4.2 ?m spectra for the very late type T dwarf UGPSJ072227.51-054031.2, as well as improved astrometry for this dwarf.Examination of the spectral energy distribution using new and publisheddata, with Saumon & Marley models, shows that the dwarf has Teff = 505 ± 10 K, a mass of 3-11 M Jupiter,and an age between 60 Myr and 1 Gyr. This young age is consistent withthe thin disk kinematics of the dwarf. The mass range overlaps with thatusually considered to be planetary, despite this being an unbound objectdiscovered in the field near the Sun. This apparently young rapidrotator is also undergoing vigorous atmospheric mixing, as determined bythe IRAC and WISE 4.5 ?m photometry and the Saumon & Marleymodels. The optical spectrum for this 500 K object shows clearlydetected lines of the neutral alkalis Cs and Rb, which are emitted fromdeep atmospheric layers with temperatures of 900-1200 K.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theScience and Technology Facilities Council (United Kingdom), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), Ministério da Ciência e Tecnologia(Brazil), and Ministerio de Ciencia, Tecnología eInnovación Productiva (Argentina); also based on data collectedat Subaru Telescope, which is operated by the National AstronomicalObservatory of Japan; and also based on observations made at the UKInfrared Telescope, which operated by the Joint Astronomy Centre onbehalf of the Science and Technology Facilities Council of the UK.
| Metallicity in the GRB 100316D/SN 2010bh Host Complex
The recent long-duration GRB 100316D, associated with supernova SN2010bh and detected by Swift, is one of the nearest gamma-ray burst(GRB)-supernovae (SNe) ever observed (z = 0.059). This provides us witha unique opportunity to study the explosion environment on ~kpc scale inrelation to the host galaxy complex. Here we present spatially resolvedspectrophotometry of the host galaxy, focusing on both the explosionsite and the brightest star-forming regions. Using these data, weextract the spatial profiles of the relevant emission features(H?, H?, [O III]?5007, and [N II]?6584) and usethese profiles to examine variations in metallicity and star formationrate (SFR) as a function of position in the host galaxy. We concludethat GRB 100316D/SN2010bh occurred in a low-metallicity host galaxy, andthat the GRB-SN explosion site corresponds to the region with the lowestmetallicity and highest SFR sampled by our observations.This paper includes data gathered with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| A Comprehensive Spectroscopic Analysis of DB White Dwarfs
We present a detailed analysis of 108 helium-line (DB) white dwarfsbased on model atmosphere fits to high signal-to-noise opticalspectroscopy. We derive a mean mass of 0.67 M sun for oursample, with a dispersion of only 0.09 M sun. White dwarfsalso showing hydrogen lines, the DBA stars, comprise 44% of our sample,and their mass distribution appears similar to that of DB stars. As inour previous investigation, we find no evidence for the existence oflow-mass (M < 0.5 M sun) DB white dwarfs. We derive aluminosity function based on a subset of DB white dwarfs identified inthe Palomar-Green Survey. We show that 20% of all white dwarfs in thetemperature range of interest are DB stars, although the fraction dropsto half this value above T eff ~ 20,000 K. We also show thatthe persistence of DB stars with no hydrogen features at lowtemperatures is difficult to reconcile with a scenario involvingaccretion from the interstellar medium, often invoked to account for theobserved hydrogen abundances in DBA stars. We present evidence for theexistence of two different evolutionary channels that produce DB whitedwarfs: the standard model where DA stars are transformed into DB starsthrough the convective dilution of a thin hydrogen layer and a secondchannel where DB stars retain a helium atmosphere throughout theirevolution. We finally demonstrate that the instability strip ofpulsating V777 Her white dwarfs contains no non-variables, if thehydrogen content of these stars is properly accounted for.
| 1 ?m Excess Sources in the UKIDSS. I. Three T Dwarfs in the Sloan Digital Sky Survey Southern Equatorial Stripe
We report the discovery of two field brown dwarfs, ULAS J0128-0041 andULAS J0321+0051, and the rediscovery of ULAS J0226+0051 (IfA 0230-Z1),in the Sloan Digital Sky Survey (SDSS) southern equatorial stripe. Theyare found in the course of our follow-up observation program of 1 ?mexcess sources in the United Kingdom Infrared Telescope Infrared DeepSky Survey. The Gemini Multi-Object Spectrographs spectra at red opticalwavelengths (6500-10500 Å) are presented, which reveal that theyare early-T dwarfs. The classification is also supported by theiroptical to near-infrared colors. It is noted that ULAS J0321+0051 is oneof the faintest currently known T dwarfs. The estimated distances to thethree objects are 50-110 pc, thus they are among the most distant fieldT dwarfs known. The dense temporal coverage of the target fieldsachieved by the SDSS-II Supernova Survey allows us to perform a simpletime-series analysis of the dwarfs. We create stacked images of eachyear from 2002-2007 and find significant proper motions of 150-290 masyr-1 or transverse velocities of 40-100 km s-1 forULAS J0128-0041 and ULAS J0226+0051. We also find that there are nodetectable, long-term (a-few-year) brightness variations above a fewtimes 0.1 mag for the two brown dwarfs.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theScience and Technology Facilities Council (United Kingdom), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), Ministério da Ciência e Tecnologia(Brazil), and Ministerio de Ciencia, Tecnología eInnovación Productiva (Argentina).
| Galaxy-wide Shocks in Late-merger Stage Luminous Infrared Galaxies
We present an integral field spectroscopic study of two nearby luminousinfrared galaxies (LIRGs), IC 1623 and NGC 3256, which exhibit evidenceof widespread shock excitation induced by ongoing merger activity. Weshow the importance of carefully separating excitation due to shocksversus excitation by H II regions and the usefulness of integral fieldunit data in interpreting the complex processes in LIRGs. Our analysisfocuses primarily on the emission line gas, which is extensive in bothsystems and is a result of the abundant ongoing star formation as wellas widespread LINER-like excitation from shocks. We use emission lineratio maps, line kinematics, line-ratio diagnostics, and new models asmethods for distinguishing and analyzing shocked gas in these systems.We discuss how our results inform the merger sequence associated withlocal U/LIRGs and the impact that widespread shock excitation has on theinterpretation of emission line spectra and derived quantities of bothlocal and high-redshift galaxies.
| The Optical Afterglow and z = 0.92 Early-type Host Galaxy of the Short GRB 100117A
We present the discovery of the optical afterglow and early-type hostgalaxy of the short-duration GRB 100117A. The faint afterglow isdetected 8.3 hr after the burst with r AB = 25.46 ±0.20 mag. Follow-up optical and near-infrared observations uncover acoincident compact red galaxy, identified as an early-type galaxy at aspectroscopic redshift of z ? 0.915 with a mass of ~3 ×1010 M sun, an age of ~1 Gyr, and a luminosity ofL B ~= 0.5 L *. From a possible weak detection of[O II]?3727 emission at z = 0.915 we infer an upper bound on thestar formation rate of ~0.1 M sun yr-1, leading toa specific star formation rate of lsim0.004 Gyr-1. Thus, GRB100117A is only the second short burst to date with a secure early-typehost (the other being GRB 050724 at z = 0.257) and it has one of thehighest short gamma-ray burst (GRB) redshifts. The offset between thehost center and the burst position, 470 ± 310 pc, is the smallestto date. Combined with the old stellar population age, this indicatesthat the burst likely originated from a progenitor with no significantkick velocity. However, from the brightness of the optical afterglow weinfer a relatively low density of n ? 3 × 10-4epsilon-3 e,-1epsilon-1.75 B,-1 cm-3. The combination of an optically faintafterglow and host suggests that previous such events may have beenmissed, thereby potentially biasing the known short GRB host populationagainst z >~ 1 early-type hosts.
| Optical IFU Observations of the Brightest Cluster Galaxy NGC 4696: The Case for a Minor Merger and Shock-excited Filaments
We present deep optical integral-field spectroscopic observations of thenearby (z ~ 0.01) brightest cluster galaxy NGC 4696 in the core of theCentaurus Cluster, made with the Wide Field Spectrograph on theAustralian National University 2.3 m telescope at Siding SpringObservatory. We investigate the morphology, kinematics, and excitationof the emission-line filaments and discuss these in the context of amodel of a minor merger. We suggest that the emission-line filaments inthis object have their origin in the accretion of a gas-rich galaxy andthat they are excited by v ~ 100-200 km s-1 shocks driveninto the cool filament gas by the ram pressure of the transonic passageof the merging system through the hot halo gas of NGC 4696.
| Ancient Planetary Systems are Orbiting a Large Fraction of White Dwarf Stars
Infrared studies have revealed debris likely related to planet formationin orbit around ~30% of youthful, intermediate mass, main-sequencestars. We present evidence, based on atmospheric pollution by variouselements heavier than helium, that a comparable fraction of the whitedwarf descendants of such main-sequence stars are orbited by planetarysystems. These systems have survived, at least in part, through allstages of stellar evolution that precede the white dwarf. During thetime interval (~200 million years) that a typical polluted white dwarfin our sample has been cooling it has accreted from its planetary systemthe mass of one of the largest asteroids in our solar system (e.g.,Vesta or Ceres). Usually, this accreted mass will be only a fraction ofthe total mass of rocky material that orbits these white dwarfs; forplausible planetary system configurations we estimate that this totalmass is likely to be at least equal to that of the Sun's asteroid belt,and perhaps much larger. We report abundances of a suite of eightelements detected in the little studied star G241-6 that we find to beamong the most heavily polluted of all moderately bright white dwarfs.
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopicsubtypes, and stellar population subcomponents of the white dwarfswithin 20 pc of the Sun. We find no convincing evidence of halo whitedwarfs in the total 20 pc sample of 129 white dwarfs nor is thereconvincing evidence of genuine thick disk subcomponent members within 20parsecs. Virtually, the entire 20 pc sample likely belongs to the thindisk. The total DA to non-DA ratio of the 20 pc sample is 1.6, amanifestation of deepening envelope convection which transforms DA starswith sufficiently thin H surface layers into non-DAs. The addition offive new stars to the 20 pc sample yields a revised local space densityof white dwarfs of (4.9 ± 0.5) × 10–3pc–3 and a corresponding mass density of (3.3 ±0.3) × 10–3 M sunpc–3. We find that at least 15% of the white dwarfswithin 20 parsecs of the Sun (the DAZ and DZ stars) have photosphericmetals that possibly originate from accretion of circumstellar material(debris disks) around them. If this interpretation is correct, thissuggests the possibility that the same percentage have planets orasteroid-like bodies orbiting them.
| Spectroscopic Confirmation of Two Massive Red-Sequence-Selected Galaxy Clusters at z ~ 1.2 in the SpARCS-North Cluster Survey
The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is adeep z'-band imaging survey covering the Spitzer Wide-Area InfraredExtragalactic Survey (SWIRE) Legacy fields designed to create the firstlarge homogeneously selected sample of massive clusters at z > 1using an infrared adaptation of the cluster red-sequence method. Wepresent an overview of the northern component of the survey which hasbeen observed with Canada-France-Hawaii Telescope (CFHT)/MegaCam andcovers 28.3 deg2. The southern component of the survey wasobserved with Cerro Tololo Inter-American Observatory (CTIO)/MOSAICII,covers 13.6 deg2, and is summarized in a companion paper byWilson et al. We also present spectroscopic confirmation of two richcluster candidates at z ~ 1.2. Based on Nod-and-Shuffle spectroscopyfrom GMOS-N on Gemini, there are 17 and 28 confirmed cluster members inSpARCS J163435+402151 and SpARCS J163852+403843 which have spectroscopicredshifts of 1.1798 and 1.1963, respectively. The clusters have velocitydispersions of 490 ± 140 km s-1 and 650 ± 160km s-1, respectively, which imply masses (M 200)of (1.0 ± 0.9) × 1014 M sun and (2.4± 1.8) × 1014 M sun. Confirmation ofthese candidates as bonafide massive clusters demonstrates thattwo-filter imaging is an effective, yet observationally efficient,method for selecting clusters at z > 1.
| GRB 070714B—Discovery of the Highest Spectroscopically Confirmed Short Burst Redshift
We detect the optical afterglow and host galaxy of GRB 070714B. Ourobservations of the afterglow show an initial plateau in the light curvefor approximately the first 5-25 minutes, and then steepening to apower-law decay with index ? = 0.86 ± 0.10 for the periodbetween 1 and 24 hr postburst. This is consistent with the X-ray lightcurve which shows an initial plateau followed by a similar subsequentdecay. At late time, we detect a host galaxy at the location of theoptical transient. Gemini Nod & Shuffle spectroscopic observationsof the host show a single emission line at 7167 Å which, based ona griz JHK photometric redshift, we conclude is the 3727 Å [O II]line. We therefore find a redshift of z = 0.923. This redshift, as wellas a subsequent probable spectroscopic redshift determination of GRB070429B at z = 0.904 by two other groups significantly exceeds theprevious highest spectroscopically confirmed short burst redshift of z =0.546 for GRB 051221. This dramatically moves back the time at which weknow short bursts were being formed and suggests that the presentevidence for an old progenitor population may be observationally biased.
| Electron Temperatures of Planetary Nebulae Determined from the He I Discontinuities
We have used the He I discontinuities at 3421 Å to determine theelectron temperatures, designated T e(He I), for a sample offive Galactic planetary nebulae (PNe). We compared T e(He I)with the electron temperatures derived from the hydrogen Balmer jump at3646 Å, designated T e(H I), and found that Te(He I) are generally lower than T e(H I). Thereare two possible interpretations, (1) the presence of substantialHe2+ zone, or (2) the presence of hydrogen-deficient coldclumps within diffuse nebulae. A series of photoionization models wereconstructed to test the two scenarios. We found that the observed Te(He I)/T e(H I) discrepancies are beyond thepredictions of chemically homogeneous models. Our modeling shows thatthe presence of a small amount of hydrogen-deficient inclusions seems tobe able to reproduce the observed intensities of He I discontinuities.We stress the value of He I discontinuities in investigating nebularphysical conditions. Albeit with some observational and technicallimitations, He I discontinuities should be considered in futuremodeling work.
| On the Age of the Widest Very Low Mass Binary
We have recently identified the widest very low mass binary (2M0126AB),consisting of an M6.5V and an M8V dwarf with a separation of ~5100 AU,which is twice as large as that of the second widest known system and anorder of magnitude larger than those of all other previously known widevery low mass binaries. If this binary belongs to the field population,its constituents would have masses of ~0.09 M sun, at thelower end of the stellar regime. However, in the discovery paper, wepointed out that its proper motion and position in the sky are bothconsistent with being a member of the young (30 Myr) Tucana/Horologiumassociation, raising the possibility that the binary is a pair of ~0.02M sun brown dwarfs. We obtained optical spectroscopy at theGemini South Observatory in order to constrain the age of the pair andclarify its nature. The absence of lithium absorption at 671 nm, modestH? emission, and the strength of the gravity-sensitive Na doubletat 818 nm all point toward an age of at least 200 Myr, ruling out thepossibility that the binary is a member of Tucana/Horologium. We furtherestimate that the binary is younger than 2 Gyr based on its expectedlifetime in the galactic disk.
| The Baryon Content of Dark Matter Halos: Empirical Constraints from Mg II Absorbers
We study the extent and covering fraction of cool baryons aroundgalaxies of different luminosity and mass, based on a survey of Mg II??2796, 2803 absorption features near known galaxies. Theinitial sample consists of 13 galaxy and absorber pairs and 10 galaxiesthat do not produce Mg II absorption lines to within sensitive upperlimits. The redshifts of the galaxy and absorber pairs range fromz=0.2067 to 0.892 with a median of z=0.3818. We find that galaxies atlarger impact parameters produce on average weaker Mg II absorbers. Thisanticorrelation is substantially improved when accounting for theintrinsic luminosities of individual galaxies. In addition, there existsa distinct boundary at ?=Rgas, beyond which no Mg IIabsorbers are found. A maximum likelihood analysis shows that theobservations are best described by an isothermal density profile and ascaling relationRgas=91(LB/LB*)0.35+/-0.05h-1 kpc [or 69 h-1 kpc at W(2796)=0.3 Å]with a mean covering factor of =80%-86%. Together withthe scaling relation between halo mass and galaxy luminosity inferredfrom halo occupation studies, this scaling of Rgas indicatesthat gas radius is a fixed fraction of the dark matter halo radius. Wecompare our results with previous studies and discuss the implicationsof our analysis for constraining the baryon content of galactic halosand for discriminating between competing scenarios for understanding thenature of the extended gas.
| Abundance variations in the globular cluster M71 (NGC 6838)
Context: Abundance variations in moderately metal-rich globular clusterscan give clues about the formation and chemical enrichment of globularclusters. Aims: CN, CH, Na, Mg and Al indices in spectra of 89stars of the template metal-rich globular cluster M71 are measured andimplications on internal mixing are discussed. Methods: Starsfrom the turn-off up to the Red Giant Branch (0.87 < log g < 4.65)observed with the GMOS multi-object spectrograph at the Gemini-Northtelescope are analyzed. Radial velocities, colours, effectivetemperatures, gravities and spectral indices are determined for thesample. Results: Previous findings related to the CN bimodalityand CN-CH anticorrelation in stars of M71 are confirmed. We also find aCN-Na correlation, and Al-Na, as well as an Mg2-Al anticorrelation. Conclusions: A combination of convective mixing and a primordialpollution by AGB or massive stars in the early stages of globularcluster formation is required to explain the observations.Tables 1, 3-5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/486/941
| Ultraviolet Spectroscopy of HD 44179
We have reanalyzed the ultraviolet spectrum of HD 44179, the centralstar(s) of the Red Rectangle Nebula, providing improved estimates of thecolumn density, rotational, and vibrational temperatures of the 4thPositive A-X system of CO in absorption. The flux shortward of 2200Å is a complex blend of CO features with no discernible stellarphotosphere, making the identification of other molecular speciesdifficult, and the direct derivation of the dust extinction curveimpossible. We confirm that the spin-forbidden CO(a-X) Cameron bands arelikely produced by either collisional excitation or a chemical reaction,not photoexcitation, but with a higher internal vibrational excitationthan previously determined. We also detect the spin-forbidden COa'-X, d-X, and e-X absorption features. The hot CO(A-X) bandsexhibit a blueshift of ~300 km s-1, likely occurring close tothe white dwarf star(s) suspected as the original source of theultraviolet flux in the system, and forming the base of the outflow ofmaterial in the Red Rectangle. The OH ``comet-band'' system near 3000Å is also analyzed, and estimates of its rovibrationaltemperatures determined. The source of the molecules studied in thissystem is still unknown, but may be a combination of gaseous materialassociated with the star(s), or processed material from the surroundingdust torus.
| a New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of whitedwarfs lying within 20 pc of the Sun. A new revised catalog of localwhite dwarfs containing 122 entries (126 individual degenerate stars) ispresented. This list contains 27 white dwarfs not included in a previouslist from 2002, as well as new and recently published trigonometricparallaxes. In several cases new members of the local white dwarfpopulation have come to light through accurate photometric distanceestimates. In addition, a suspected new double degenerate system (WD0423+120) has been identified. The 20 pc sample is currently estimatedto be 80% complete. Using a variety of recent spectroscopic,photometric, and trigonometric distance determinations, we re-compute aspace density of 4.8 ± 0.5 × 10 3 pc3 corresponding to a mass density of 3.2 ± 0.3 ×10 3 M sun pc 3 from the completeportion of the sample within 13 pc. We find an overall mean mass for thelocal white dwarfs of 0.665 M sun, a value larger than mostother non-volume-limited estimates. Although the sample is small, wefind no evidence of a correlation between mass and temperature in whichwhite dwarfs below 13,000 K are systematically more massive than thoseabove this temperature. Within 20 pc 25% of the white dwarfs are inbinary systems (including double degenerate systems). Approximately 6%are double degenerates and 6.5% are Sirius-like systems. The fraction ofmagnetic white dwarfs in the local population is found to be 13%.
| Discovery of new nearby L and late-M dwarfs at low Galactic latitude from the DENIS data base
We report on new nearby L and late-M dwarfs (dphot <= 30pc) discovered in our search for nearby ultracool dwarfs (I-J >= 3.0,later than M8.0) at low Galactic latitude (|b| < 15°) over 4800deg2 in the Deep Near Infrared Survey of the Southern Sky(DENIS) data base. We used late-M (>= M8.0), L and T dwarfs withaccurate trigonometric parallaxes to calibrate the MJ versusI-J colour-luminosity relation. The resulting photometric distances havestandard errors of ~15 per cent, which we used to select candidatesdphot <= 30 pc. We measured proper motions frommulti-epoch images found in the public archives ALADIN, DSS, 2MASS andDENIS, with at least three distinct epochs and time baselines of 10-21yr. We then used a maximum reduced proper motion cut-off to select 28candidates as ultracool dwarfs (M8.0-L8.0) and to reject one as adistant red star. No T dwarf candidates were found in this search, whichrequired an object to be detected in all three DENIS bands. Ourlow-resolution optical spectra confirmed that 26 were indeed ultracooldwarfs, with spectral types from M8.0 to L5.5. Two contaminants and onerejected by the maximum reduced proper motion cut-off were all reddenedF-K main sequence stars. 20 of these 26 ultracool dwarfs are new nearbyultracool dwarf members, three L dwarfs within 15 pc with one L3.5 atonly ~10 pc. We determine a stellar density of dwarfs pc-3mag-1 over 11.1 <= MJ <= 13.1 based on thissample of M8-L3.5 ultracool dwarfs. Our ultracool dwarf density value isin good agreement with the measurement by Cruz et al. of the ultracooldwarf density at high Galactic latitude.
| The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
We determine the ratio of helium- to hydrogen-atmosphere white dwarfstars as a function of effective temperature from a model atmosphereanalysis of the infrared photometric data from the Two Micron All SkySurvey combined with available visual magnitudes. Our study surpassesany previous analysis of this kind, both in terms of the accuracy of theTeff determinations and the size of the sample. We observethat the ratio of helium- to hydrogen-atmosphere white dwarfs increasesgradually from a constant value of ~0.25 between Teff=15,000and 10,000 K to a value twice as large in the range 10,000K>Teff>8000 K, suggesting that convective mixing, whichoccurs when the bottom of the hydrogen convection zone reaches theunderlying convective helium envelope, is responsible for this gradualtransition. The comparison of our results with an approximate model usedto describe the outcome of this convective mixing process implieshydrogen mass layers in the rangeMH/Mtot=10-10 to 10-8 forabout 15% of the DA stars that survived the DA-to-DB transition nearTeff~30,000 K, the remainder having presumably more massivelayers above MH/Mtot~10-6.
| CPD -20 1123 (Albus 1) Is a Bright He-B Subdwarf
Based on photometric and astrometric data it has been proposed thatAlbus 1 (also known as CPD -20 1123) might be a hot white dwarf similarto G191-B2B or, alternatively, a hot subdwarf. We obtained a series ofoptical spectra showing that CPD -20 1123 is a bright He-B subdwarf. Weanalyzed the H I Balmer and He I line spectra and measuredTeff = 19,800 +/- 400 K, logg=4.55+/-0.10, andlogN(He)/N(H)=0.15+/-0.15. This peculiar object belongs to a family ofevolved helium-rich stars that may be the products of double-degeneratemergers, or, alternatively, the products of post horizontal- orgiant-branch evolution.
| Albus 1: A Very Bright White Dwarf Candidate
We have serendipitously discovered a previously unknown, bright source(BT=11.75+/-0.07 mag) with a very blueVT-Ks color, which we have named Albus 1. Aphotometric and astrometric study using Virtual Observatory tools hasshown that it possesses an appreciable proper motion and magnitudes andcolors very similar to those of the well-known white dwarf G191-B2B. Weconsider Albus 1 as a DA-type white dwarf located at about 40 pc. If itsnature is confirmed, Albus 1 would be the sixth brightest isolated whitedwarf in the sky, which would make it an excellent spectrophotometricstandard.
| High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey. II. DB and DBA stars
Context: We present a detailed spectroscopic analysis of the stars withhelium-dominated spectra in the ESO Supernova Ia Progenitor Survey(SPY). Aims: Atmospheric parameters, masses, and abundances of tracehydrogen are determined and discussed in the context of spectralevolution of white dwarfs. Methods: The spectra are compared withtheoretical model atmospheres using a χ2 fittingtechnique, leading to determinations of effective temperature, surfacegravity, and hydrogen abundance. Results: Our final sample contains 71objects, of which 6 are new detections and 14 are reclassified from DBto DBA because of the presence of H lines. One is a cool DO with weak Heii lines, 2 are composite DB+dM. 55% of the DB sample show hydrogen andare thus DBA, a significantly higher fraction than found before. Conclusions: The large incidence of DBA, and the derived total hydrogenmasses are compatible with the scenario that DBs “reappear”around 30 000 K from the DB gap by mixing and diluting a thin hydrogenlayer of the order of 10-15 Mȯ. This hydrogenmass is then during the evolution continuously increased by interstellaraccretion. There are indications that the accretion rate increasessmoothly with age or decreasing temperature, a trend which continuouseven below the current low temperature limit (Dufour 2006). A remainingmystery is the low accretion rate of H compared to that of Ca observedin the DBZA, but a stellar wind extending down to the lowesttemperatures with decreasing strength might be part of the solution.Based on data obtained at the Paranal Observatory of the EuropeanSouthern Observatory for programmes 165.H-0588 and 167.D-0407. AppendixA is only available in electronic form at http://www.aanda.org
| Measuring the Balmer Jump and the Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (logg) inlate-type stars using broadband (e.g., UBV or SDSS) or intermediate-band(uvby) photometric systems, especially when the stars can cover a rangeof metallicities and reddenings. However, simple spectroscopicobservational and data reduction techniques can yield accurate valuesfor logg through comparison of the Balmer jumps of low-resolutionspectra with recent grids of synthetic flux spectra.
| Multicolour CCD measurements of visual double and multiple stars. III
Context: Recent CCD observations were performed in the period 1998-2004for a large sample of visual double and multiple stars selected from theHipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Aims: Accurate astrometric and photometric data allowing us tocharacterise the individual components are provided. These data arecompared to Hipparcos data or to data from an older epoch to assess thenature of the observed systems. Methods: We simultaneously apply aMoffat-Lorentz profile with a similar shape to all detected componentsand adjust the profile parameters from which we obtain the relativeastrometric position (epoch, position angle, angular separation) as wellas differential multi-colour photometry (filters (B)VRI). Results: Wethus acquired recent data for 71 visual systems of which 6 are orbitalbinaries, 27 are nearby, and 30 are multiple systems. In three of thesecases, the systems remained unresolved. 23 new components were detectedand measured. Two new visual double stars of intermediate separationwere also found. The estimated accuracies in relative position are0.04° and 0.01 arcsec respectively, while those in differentialphotometry are of the order of 0.01-0.02 mag in general. Conclusions:.The nature of the association of 55 systems is evaluated. New basicbinary properties are derived for 20 bound systems. Component coloursand masses are provided for two orbital binaries.Based on observations collected at the National AstronomicalObservatory, Rozhen, and the Astronomical Observatory, Belogradchik,both operated by the Institute of Astronomy, Bulgarian Academy ofSciences. Also based on data obtained by the Hipparcos astrometrysatellite. Appendix A is only available in electronic form athttp://www.aanda.org Tables 4-6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/641
| Deep GMOS spectroscopy of extremely red galaxies in GOODS-South: ellipticals, mergers and red spirals at 1 < z < 2
We have performed a deep (35.5-h exposure) spectroscopic survey ofextremely red (I - K > 4) galaxies (ERGs) on the Great ObservatoriesOrigins Deep Survey (GOODS)-South Field, using the Gemini multi-objectspectrograph on the 8-m Gemini South Telescope. We present here spectraand redshifts for 16 ERGs at 0.87 < z < 2.02, to a limit ofKs ~= 20.2. In the spectra of 10 of these galaxies we findemission lines of [OII] 3727Å with fluxes corresponding to a meanstar formation rate (uncorrected for dust) of1.6Msolaryr-1. For three ERGs we detect noemission lines and the remaining three lie at z > 1.5 where this linewould be outside our wavelength range. Absorption features are seen inmost ERG spectra.We examine the morphologies of these ERGs on Hubble Space TelescopeAdvanced Camera for Surveys (HST-ACS) images and fit radii and Sersicindices. We find three broad classes: (i) spheroidals; (ii) mergers at avariety of stages (some are spheroidal-disc pairs, some have tidaltails), often with a high surface brightness and (iii) red spirals(which may have star forming regions in their outer discs).We perform an age-dating analysis by fitting the spectra and nine-bandphotometry (BVIZJHK, plus 3.6/4.5-?m fluxes from Spitzer) of the ERGswith two-component models, consisting of passively evolving, old stellarpopulations combined with a younger, continuously star formingcomponent, for which the age and dust extinction are allowed to varyfrom 10 to 800Myr and E(B - V) = 0.0 to 0.5mag. For only one ERG is thebest-fitting model purely passive, for the others the best fit isobtained by including a star forming component, which typically forms afew (0.26-13.5) per cent of the stellar mass, and is subject to dustreddening averaging E(B - V) ~= 0.35. The ages of the star formingcomponents tend to be youngest (10-40Myr) in the merging ERGs, and older(200-800Myr) in spiral ERGs, with mixed ages for the spheroidals.The best-fitting mean ages for the old stellar populations range from0.6 to 4.5Gyr, averaging 2.1Gyr, with masses from 3 ×1010 to 2 × 1011Msolar. The meanstellar formation redshifts of ERGs are spread from ~0.5Gyr before theepoch of observation out to z ~ 5. We propose that most ERGs aregalaxies, or mergers of galaxies, formed some Gyr earlier, in an early(z ~ 5) phase of massive galaxy formation, which since then haveexperienced a wide variety of merger and star formation (SF) histories,accounting for the wide range of observed stellar ages.Finally, we examine the clustering of the ERGs on this field, as afunction of the photometric redshifts estimated by Caputi et al. Thecomoving correlation radius is r0 ~= 13h-1Mpc forthe full sample and, dividing by redshift, is constant or increasingwith redshift, thus favouring comoving (? = -1.2) evolution overa stable (? = 0) clustering model.
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| Takýmyýldýz: | Kartal |
| Sað Açýklýk: | 19h20m34.92s |
| Yükselim: | -07°40'00.2" |
| Görünürdeki Parlaklýk: | 13.158 |
| Uzaklýk: | 11.226 parsek |
| özdevim Sað Açýklýk: | -61.2 |
| özdevim Yükselim: | -162.8 |
| B-T magnitude: | 12.422 |
| V-T magnitude: | 13.098 |
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