Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Star Cluster Complexes and the Host Galaxy in Three H II Galaxies: Mrk 36, UM 408, and UM 461
We present a stellar population study of three H II galaxies (Mrk 36, UM408, and UM 461) based on the analysis of new ground-basedhigh-resolution near-infrared J, H, and Kp broadband andBr? narrowband images obtained with Gemini/NIRI. We identify anddetermine the relative ages and masses of the elementary star clustersand/or star cluster complexes of the starburst regions in each of thesegalaxies by comparing the colors with evolutionary synthesis models thatinclude the contribution of stellar continuum, nebular continuum, andemission lines. We found that the current star cluster formationefficiency in our sample of low-luminosity H II galaxies is ~10%.Therefore, most of the recent star formation is not in massive clusters.Our findings seem to indicate that the star formation mode in our sampleof galaxies is clumpy, and that these complexes are formed by a fewmassive star clusters with masses gsim104 M sun.The age distribution of these star cluster complexes shows that thecurrent burst started recently and likely simultaneously over shorttimescales in their host galaxies, triggered by some internal mechanism.Finally, the fraction of the total cluster mass with respect to the lowsurface brightness (or host galaxy) mass, considering our complete rangein ages, is less than 1%.
| High-contrast Imaging Search for Planets and Brown Dwarfs around the Most Massive Stars in the Solar Neighborhood
There has been a long-standing discussion in the literature as towhether core accretion or disk instability is the dominant mode ofplanet formation. Over the last decade, several lines of evidence havebeen presented showing that core accretion is most likely the dominantmechanism for the close-in population of planets probed by radialvelocity and transits. However, this does not by itself prove that coreaccretion is the dominant mode for the total planet population, sincedisk instability might conceivably produce and retain large numbers ofplanets in the far-out regions of the disk. If this is a relevantscenario, then the outer massive disks of B-stars should be among thebest places for massive planets and brown dwarfs to form and reside. Inthis study, we present high-contrast imaging of 18 nearby massive starsof which 15 are in the B2-A0 spectral-type range and provide excellentsensitivity to wide companions. By comparing our sensitivities to modelpredictions of disk instability based on physical criteria forfragmentation and cooling, and using Monte Carlo simulations for orbitaldistributions, we find that ~85% of such companions should have beendetected in our images on average. Given this high degree ofcompleteness, stringent statistical limits can be set from thenull-detection result, even with the limited sample size. We find that<30% of massive stars form and retain disk instability planets, browndwarfs, and very low mass stars of <100 M jup within 300AU, at 99% confidence. These results, combined with previous findings inthe literature, lead to the conclusion that core accretion is likely thedominant mode of planet formation.Based on data from the Gemini North telescope under programsGN-2008B-Q-59, GN-2009A-DD-6, GN-2010A-Q-86, and GN-2010B-Q-14, on Keckdata under program A200N2, and on Subaru data under program S09B-016.
| The inverse problem of the theory of degenerate dwarfs
Based on the radii and masses of degenerate dwarfs derived fromHIPPARCOS and other observations, we estimate the microscopic parametersof a Chandrasekhar model (the relativistic parameter at the stellarcenter x 0, and the chemical-composition parameterµ e = A/Z, where A is the mass number and Z is thenuclear charge). We have obtained analytical expressions for themacroscopic characteristics (mass, radius, energy) as functions of x0 and µ e . From the calculated dependenceof the energy on these parameters, we have found constraints on therange of variability of x 0, which are in good agreement withthe observed radius distribution of dwarfs. The critical value of x0 at which stability breaks down due to general-relativisitceffects is found more accurately than previously. We propose ageneralized model with an inhomogeneous (coordinate-dependent) chemicalcomposition, with µ e = µ e ( r).
| A selection of hot subluminous stars in the GALEX survey - I. Correlation with the Guide Star Catalog
We assembled a catalogue of bright, hot subdwarf and white dwarf starsextracted from a joint ultraviolet, optical and infrared source list.The selection is secured using colour criteria that correlate well witheffective temperatures Teff? 12 000 K. We built aNUV- V versus V - J diagram for ?60 000 bright sourcesusing the Galaxy Evolution Explorer (GALEX) NUV magnitude(NUV < 14), the associated Guide Star Catalog (GSC2.3.2)photographic quick-V magnitude and the Two-Micron All-Sky Survey J and Hmagnitudes. This distillation process delivered a catalogue of ?700sources with NUV- V < 0.5 comprising ˜160 known hotsubdwarf stars and another ˜60 known white dwarf stars. A reducedproper-motion diagram built using the proper-motion measurementsextracted from the Naval Observatory Merged Astrometric Dataset allowedus to identify an additional ˜120 new hot subdwarf candidates and˜10 hot white dwarf candidates. We present a spectroscopic studyof a subset of 52 subdwarfs, 48 of them analysed here for the firsttime, and with nine objects brighter than V ˜ 12. Our sample ofspectroscopically confirmed hot subdwarfs comprises 10 sdO-type starsand 42 sdB-type stars suitable for pulsation and binary studies. We alsopresent a study of 50 known white dwarfs selected in the GALEX surveyand six new white dwarfs from our catalogue of subluminous candidates.Ultraviolet, optical and infrared synthetic magnitudes employed in theselection and analysis of white dwarf stars are listed in appendix.Based on observations made with ESO telescopes at the La Silla ParanalObservatory under programmes 82.D-0750, 83.D-0540 and085.D-0866.Visiting Astronomer, Kitt Peak National Observatory, NationalOptical Astronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA) under cooperativeagreement with the National Science Foundation.
| Identification of blue high proper motion objects in the Tycho-2 and 2MASS catalogues using Virtual Observatory tools
Aims: With available Virtual Observatory tools, we looked for newbright blue high proper motion objects in the entire sky: white dwarfs,hot subdwarfs, runaway OB stars, and early-type stars in nearby youngmoving groups. Methods: We performed an all-sky cross-matchbetween the optical Tycho-2 and near-infrared 2MASS catalogues withAladin, and selected objects with proper motions ? > 50 masyr-1 and colours VT - Ks < -0.5 magwith TOPCAT. We also collected multi-wavelength photometry, constructedthe spectral energy distributions and estimated effective temperaturesfrom fits to atmospheric models with VOSA for the most interestingtargets. Results: We assembled a sample of 32 bright blue highproper motion objects, including ten sdO/B subdwarfs, nine DA whitedwarfs, five young early-type stars (two of which are runaway stars),two blue horizontal branch stars, one star with poor information, andfive objects reported for the first time in this work. These last fiveobjects have magnitudes BT ? 11.0-11.6 mag, effectivetemperatures Teff ? 24 000-30 000 K, and are located inthe region of known white dwarfs and hot subdwarfs in a reduced propermotion-colour diagram. We confirmed the hot subdwarf nature of one ofthe new objects, Albus 5, with public far-ultraviolet spectroscopic dataobtained with FUSE.
| Hard X-ray Emission Associated with White Dwarfs. III.
Hard X-ray emission associated with white dwarfs (WDs) can be used todiagnose the presence of late-type binary companions, mass accretionfrom companions, or physical processes with unknown origins. Since ourprevious systematic searches for hard X-ray emission associated withWDs, the Galactic WD catalog has been augmented by >10,000 new WDsfrom the Sloan Digital Sky Survey and new X-ray point-source catalogsfrom XMM-Newton and ROSAT have become available. Therefore, we haveextended the search using the updated catalogs, and found 17 new casesof WDs associated with hard X-ray emission. The 32 WDs associated withhard X-ray emission, from the current and previous searches, can bedivided into five categories: (1) binary WD with a coronal companion,(2) binary WD with mass transfer from a companion, (3) single hot WDwith a hard X-ray component peaking near 1 keV in addition to a softphotospheric component, (4) two PG 1159 stars with very faint X-rayemission in the 0.9-2.0 keV band, and (5) two DA WDs whose photosphericemission component has a hard shoulder extending to 0.5-0.9 keV. Theorigin of the hard X-ray emission in the latter three categories is notyet known. Deeper X-ray observations with higher angular and spectralresolutions are needed to help us understand these WDs' hard X-rayemission.
| Ultraviolet-selected Field and Pre-main-sequence Stars Toward Taurus and Upper Scorpius
We have carried out a Galaxy Evolution Explorer (GALEX) Cycle 1 guestinvestigator program covering 56 deg2 near the Taurus Tassociation and 12 deg2 along the northern edge of the UpperScorpius OB association. We combined photometry in the GALEXfar-ultraviolet and near-ultraviolet bands with data from the Two MicronAll Sky Survey to identify candidate young (lsim100 Myr old) stars asthose with an ultraviolet excess relative to older main-sequence stars.Follow-up spectroscopy of a partial sample of these candidates suggestsfive new members of Taurus, with 8-20 expected from additionalobservations, and five new members of Upper Scorpius, with three to sixexpected from additional observations. These candidate new membersappear to represent a distributed, non-clustered population in eitherregion, although our sample statistics are as of yet too poor toconstrain the nature or extent of this population. Rather, our studydemonstrates the ability of GALEX observations to identify young stellarpopulations distributed over a wide area of the sky. We also highlightthe necessity of a better understanding of the Galactic ultravioletsource population to support similar investigations. In particular, wereport a large population of stars with an ultraviolet excess but nooptical indicators of stellar activity or accretion, and briefly argueagainst several interpretations of these sources.
| 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.
| Subaru Weak-Lensing Survey II: Multi-Object Spectroscopy and Cluster Masses
We present the first results of a multi-object spectroscopic campaign tofollow up cluster candidates located via weak lensing. Our main goalsare to search for spatial concentrations of galaxies that are plausibleoptical counterparts of the weak-lensing signals, and to determine thecluster redshifts from those of member galaxies. Around each of 36targeted cluster candidates, we obtained 15-32 galaxy redshifts. For 28of these targets, we confirmed a secure cluster identification, withmore than five spectroscopic galaxies within a velocity of±3000km s-1. This includes three cases where twoclusters at different redshifts are projected along the sameline-of-sight. In 6 of the 8 unconfirmed targets, we found multiplesmall galaxy concentrations at different redshifts, each containing atleast three spectroscopic galaxies. The weak-lensing signal around thosesystems was thus probably created by the projection of groups or smallclusters along the same line-of-sight. In both of the remaining twotargets, a single small galaxy concentration was found. In somecandidate super-cluster systems, we found additional evidence offilaments connecting the main density peak to an additional nearbystructure. For a subsample of our most cleanly measured clusters, weinvestigated the statistical relation between their weak-lensing mass(MNFW, ?SIS) and the velocity dispersion oftheir member galaxies (?v), comparing our sample withoptically and X-ray selected samples from the literature. Ourlensing-selected clusters are consistent with ?v =?SIS, with a similar scatter to that of optically andX-ray selected clusters. We also derived an empirical relation betweenthe cluster mass and the galaxy velocity dispersion,M200E(z) = 11.0 × 1014 ×(?v/1000km s-1)3.0 h-1Modot, which is in reasonable agreement with predictions ofN-body simulations in the ? CDM cosmology.
| Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars
This paper presents a catalogue and the method of determining averagedquadratic effective magnetic fields for 1212main-sequence and giant stars, and 11 white dwarf stars. The catalogueincludes stars that are members of several open clusters. We havecompiled measurements of the longitudinal magnetic field for thosestars, which were scattered in the existing literature. A new parameter,magnetization (MA), has been defined, and we present values of MA forstars of various spectral classes. Our sample includes a subset of 610chemically peculiar early-type stars. We confirm the conclusion of ourprevious study that the number distribution of all chemically peculiarstars versus the averaged magnetic field strength is described by adecreasing exponential function. Relations of this type also hold forstars of all the analysed subclasses of chemical peculiarity.Magnetization tends to correlate with the effective temperature only athigh MA, for He-weak and He-rich stars.
| GRB090423: y-band field calibration.
Not Available
| On the evolutionary status of short-period cataclysmic variables
We present high-speed, three-colour photometry of seven short-period(Porb <= 95 min) eclipsing cataclysmic variables (CVs)from the Sloan Digital Sky Survey. We determine the system parametersvia a parametrized model of the eclipse fitted to the observed lightcurve by ?2 minimization. Three out of seven of thesystems possess brown dwarf donor stars and are believed to have evolvedpast the orbital period minimum. This is in line with the predictionsthat 40-70 per cent of CVs should have evolved past the orbital periodminimum. Therefore, the main result of our study is that the missingpopulation of post-period minimum CVs has finally been identified. Thedonor star masses and radii are, however, inconsistent with modelpredictions; the donor stars are approximately 10 per cent larger thanexpected across the mass range studied here. One explanation for thediscrepancy is the enhanced angular momentum loss (e.g. fromcircumbinary discs); however, the mass-transfer rates, as deduced fromwhite dwarf effective temperatures, are not consistent with enhancedangular momentum loss. We show that it is possible to explain the largedonor radii without invoking enhanced angular momentum loss by acombination of geometrical deformation and the effects of starspots dueto strong rotation and expected magnetic activity. Choosingunambiguously between these different solutions will require independentestimates of the mass-transfer rates in short-period CVs.The white dwarfs in our sample show a strong tendency towards highmasses. We show that this is unlikely to be due to selection effects.The dominance of high-mass white dwarfs in our sample implies thaterosion of the white dwarf during nova outbursts must be negligible, oreven that white dwarfs grow in mass through the nova cycle. Amongst oursample, there are no helium-core white dwarfs, despite predictions that30-80 per cent of short-period CVs should contain helium-core whitedwarfs. We are unable to rule out selection effects as the cause of thisdiscrepancy.
| A Far Ultraviolet Archival Study of Cataclysmic Variables. I. FUSE and HST STIS Spectra of the Exposed White Dwarf in Dwarf Nova Systems
We present a synthetic spectral analysis of Far UltravioletSpectroscopic Explorer (FUSE) and Hubble Space Telescope Space TelescopeImaging Spectrograph (HST STIS) spectra of five dwarf novae above andbelow the period gap during quiescence. We use our synthetic spectralcode, including options for the treatment of the hydrogenquasi-molecular satellite lines (for low-temperature stellaratmospheres), non-LTE (NLTE) approximation (for high-temperature stellaratmospheres), and for one system (RU Peg) we model the interstellarmedium (ISM) molecular and atomic hydrogen lines. In all the systemspresented here the FUV flux continuum is due to the white dwarf (WD).These spectra also exhibit some broad emission lines. In this work weconfirm some of the previous FUV analysis results, but we also presentnew results. For four systems we combine the FUSE and STIS spectra tocover a larger wavelength range and to improve the spectral fit. Thiswork is part of our broader HST archival research program, in which weaim to provide accurate system parameters for cataclysmic variablesabove and below the period gap by combining FUSE and HST FUV spectra.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Precise Estimates of the Synthetic Spectral Distances to DA White Dwarfs
Using recent photometric calibrations, we develop greatly improveddistance estimates for DA white dwarfs using multi-band syntheticphotometry based on spectroscopic temperatures and gravities. Very goodcorrelations are shown to exist between our spectroscopically basedphotometric distance estimates and those derived from trigonometricparallaxes. We investigate the uncertainties involved in our distanceestimates, as well as discuss the circumstances where such techniquesare most likely to fail. We apply our techniques to the large sample ofSloan Digital Sky Survey DA white dwarfs where automated fitting of H IBalmer profiles yields spectrometric temperatures and gravities. Wedetermine simple empirical corrections to these temperatures andgravities with respect to published slit spectroscopy. After applyingthese T eff-log g corrections as well as appropriateinterstellar extinction corrections, where necessary, we derivespectroscopically based photometric distances for 7062 DA stars fromthis sample.
| 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%.
| The Century Survey Galactic Halo Project III: A Complete 4300 DEG2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo
We present a complete spectroscopic survey of 2414 2MASS-selected bluehorizontal branch (BHB) candidates selected over 4300 deg2 ofthe sky. We identify 655 BHB stars in this non-kinematically selectedsample. We calculate the luminosity function of field BHB stars, andfind evidence for very few hot BHB stars in the field. The BHB starslocated at a distance from the Galactic plane |Z| < 4 kpc trace whatis clearly a metal-weak thick disk population, with a mean metallicityof [Fe/H] = -1.7, a rotation velocity gradient of dvrot/d|Z|= -28 ± 3.4 km s-1 in the region |Z| < 6 kpc, and adensity scale height of hZ = 1.26 ± 0.1 kpc. The BHBstars located at 5 < |Z| < 9 kpc are a predominantly inner-halopopulation, with a mean metallicity of [Fe/H] = -2.0 and a mean Galacticrotation of -4 ± 31 km s-1. We infer the density ofhalo and thick disk BHB stars is 104 ± 37 kpc-3 nearthe Sun, and the relative normalization of halo to thick-disk BHB starsis 4 ± 1% near the Sun.
| WD0433+270: an old Hyades stream member or an Fe-core white dwarf?
Context: G39-27/289 is a common proper-motion pair formed by a whitedwarf (WD0433+270) and a main-sequence star (BD+26 730) that apparentlyhas been classified as a member of the Hyades open cluster. Previousstudies of the white dwarf component yielded a cooling time of ~4 Gyr.Although it has not been pointed out before explicitly, this result is 6times greater than the age of the Hyades cluster, giving rise to anapparent conflict between the physics of white dwarfs and clustermain-sequence fitting. Aims: We investigate whether this systembelongs to the Hyades cluster and, accordingly, give a plausibleexplanation for the nature of the white dwarf member. Methods: Wehave performed and analyzed spectroscopic observations to bettercharacterize these objects, and used their kinematic properties toevaluate their membership in the Hyades. Then, different mass-radiusrelations and cooling sequences for different core compositions (He,C/O, O/Ne and Fe) were employed to infer the mass and cooling time ofthe white dwarf. Results: From kinematic and chemical compositionconsiderations, we believe that the system was a former member of theHyades cluster and therefore has an evolutionary link with it. However,the evidence is not conclusive. With regards to the nature of the whitedwarf component, we find that two core compositions - C/O and Fe - arecompatible with the observed effective temperature and radius. Thesecompositions yield very different cooling times of ~4 Gyr and ~1 Gyr,respectively. Conclusions: We distinguish two possible scenarios.If the pair does not belong to the Hyades cluster but only to the Hyadesstream, this would indicate that such a stream contains rather oldobjects and is definitely not coeval with the cluster. This hasinteresting consequences for Galactic dynamics. However, our favoredscenario is that of a white dwarf with a rather exotic Fe core, having acooling time compatible with the Hyades age. This is a tantalizingresult that would have implications for the thermonuclear explosion ofwhite dwarfs and explosion theories of degenerate nuclei.Based on observations obtained at: Calar Alto Observatory,Almería, Spain and McDonald Observatory, Texas, USA.
| A Subaru Weak-Lensing Survey. I. Cluster Candidates and Spectroscopic Verification
We present the results of an ongoing weak-lensing survey conducted withthe Subaru Telescope whose initial goal is to locate and study thedistribution of shear-selected structures, or halos. Using a Suprime-Camimaging survey spanning 21.82 deg2, we present a catalog of100 candidate halos located from lensing-convergence maps. Our sample isreliably drawn from that subset of our survey area (totaling 16.72deg2) uncontaminated by bright stars and edge effects and islimited at a convergence signal-to-noise ratio of 3.69. To validate thesample, detailed spectroscopic measures have been made for 26 candidatesusing the Subaru multiobject spectrograph, FOCAS. All are confirmed asclusters of galaxies, but two arise as the superposition of multipleclusters viewed along the line of sight. Including data available in theliterature and an ongoing Keck spectroscopic campaign, a total of 41halos now have reliable redshifts. For one of our survey fields, theXMM-LSS field, we compare our lensing-selected halo catalog with itsX-ray equivalent. Of 15 halos detected in the XMM-LSS field, 10 matchpublished X-ray-selected clusters, and a further two are newly detectedand spectroscopically confirmed in this work. Although three halos havenot yet been confirmed, the high success rate within the XMM-LSS field(12/15) confirms that weak lensing provides a reliable method forconstructing cluster catalogs, irrespective of the nature of theconstituent galaxies or the intracluster medium.
| A Comparative Study of Optical and Ultraviolet Effective Temperatures for DA White Dwarfs from the IUE Archive
We present a comparative study of effective temperatures determined fromthe hydrogen Balmer lines and from the UV energy distribution for 140 DAwhite dwarfs drawn from the International Ultraviolet Explorer (IUE)archive. Our results indicate that the optical and UV temperatures ofthe majority of stars below Teff~40,000 K and within ~75 pcare in fairly good agreement given the uncertainties. At highertemperatures and/or larger distances, however, significant discrepanciesare observed. Several mechanisms are investigated to account for thesediscrepancies, including the effect of interstellar reddening, thepresence of metals in the photosphere, and the existence of unresolvedbinary white dwarfs. The results of our analysis reveal thatwavelength-dependent extinction is the most natural explanation for theobserved temperature differences. We also attempt to predict thedifferences in optical and UV temperatures expected from unresolveddegenerate binaries by performing an exhaustive simulation of compositemodel spectra. In light of these simulations, we then discuss some knowndouble degenerates and identify new binary candidates by restricting ouranalysis to stars located within 75 pc, where the effect of interstellarreddening is significantly reduced.
| 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.
| A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with theSpitzer Space Telescope. Objects were observed simultaneously at 4.5 and8.0 ?m with sensitivities better than 0.1 mJy. This data set can beused to test models of white dwarf atmospheres in a new wavelengthregime, as well as to search for planetary companions and debris disks.
| The mass and luminosity functions and the formation rate of DA white dwarfs in the Sloan Digital Sky Survey
Aims.The SDSS Data Release 1 includes 1833 DA white dwarfs (WDs) andforms the largest homogeneous sample of WDs. This sample provides thebest opportunity to study the statistical properties of WDs. Methods:We adopt a recently established theoretical model to calculate the massand distance of each WD using the observational data. Then we adopt abin-correction method to correct for selection effects and use the 1/Vweight-factor method to calculate the luminosity function, thecontinuous mass function and the formation rate of these WDs. Results:The SDSS DA WD sample is incomplete and suffers seriously from selectioneffects. After corrections for the selection effects, only 531 WDsremain. From this final sample we derive the most up-to-date luminosityfunction and mass function, in which we find a broad peak of WD massescentered around 0.58 Mȯ. The DA WD space density iscalculated as 8.81×10-5 pc-3 and theformation rate is 2.579×10-13 pc-3yr-1. Conclusions: The statistical properties of the SDSS DAWD sample are generally in good agreement with previous observationaland theoretical studies, and provide us information on the formation andevolution of WDs. However, a larger and more complete all-sky WD sampleis still needed to explain some subtle disagreements and unresolvedissues.Full Table 2 is 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/466/627
| A Photometric and Spectroscopic Study of the Cataclysmic Variable ST LMi during 2005-2006
We present orbit-resolved spectroscopic and photometric observations ofthe polar ST LMi during its recent low and high states. In the low-statespectra, we report the presence of blue and red satellites in theH? emission line; the velocities and visibility of the satellitesvary with phase. This behavior is similar to emission-line profilevariations recently reported in the low state of AM Her, which wereinterpreted as being due to magnetically confined gas motions in largeloops near the secondary. Our low-state spectroscopy of ST LMi isdiscussed in terms of extreme chromospheric activity on the secondarystar. Concurrent photometry indicates that occasional low-levelaccretion may be present, as well as cool regions on the secondary nearthe inner Lagrangian point, L1. Furthermore, we report a new "extremelow state" of the system at V ~ 18.5 mag. Our orbital high-statespectroscopy reveals changes in the emission-line profiles with orbitalphases that are similar to those reported by earlier high-state studies.The complicated emission-line profiles generally consist of two maincomponents. The first has radial velocity variations identical to thatof the major emission H? component seen in the low state. Thesecond is an additional redshifted component appearing at the phases ofmaximum visibility of the accreting column of the white dwarf; it isinterpreted as being due to infall velocities on the accreting magneticpole of the white dwarf. At the opposite phases, an extended blueemission wing appears on the emission-line profiles. We confirm thepresence of a broad absorption feature near 6275 Å, which has beenpreviously identified as a Zeeman ?- absorptioncomponent of H?. This feature appears at just those phases whenthe accretion pole region is most directly visible and most nearlyface-on to the observer.Based on observations obtained with the Mayall 4 m telescope at KittPeak National Observatory, a division of the National Optical AstronomyObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.
| Infrared Photometric Analysis of White Dwarfs from the Two Micron All Sky Survey and the Spitzer Space Telescope
We review the available near- and mid-infrared photometry for whitedwarfs obtained from the Two Micron All Sky Survey (2MASS) and by theSpitzer Space Telescope. Both data sets have recently been used to seekwhite dwarfs with infrared excesses due to the presence of unresolvedcompanions or circumstellar disks, and also to derive the atmosphericparameters of cool white dwarfs. We first attempt to evaluate thereliability of the 2MASS photometry by comparing it with an independentset of published JHK CIT magnitudes for 160 cool white dwarf stars, andalso by comparing the data with the predictions of detailed modelatmosphere calculations. The possibility of using 2MASS to identifyunresolved M dwarf companions or circumstellar disks is then discussed.We also revisit the analysis of 46 binary candidates from Wachter et al.using the synthetic flux method and confirm the large near-infraredexcesses in most objects. We perform a similar analysis by fittingSpitzer 4.5 and 8 ?m photometric observations of white dwarfs withour grid of model atmospheres, and demonstrate the reliability of boththe Spitzer data and the theoretical calculations up to 8 ?m.Finally, we search for massive disks resulting from the merger of twowhite dwarfs in a 2MASS sample composed of 57 massive degenerates, andshow that massive disks are uncommon in such stars.
| Optical Multicolor Photometry of Spectrophotometric Standard Stars
Photoelectric data on the Johnson-Kron-Cousins UBVRI broadbandphotometric system are provided for a set of stars that have been usedas spectrophotometric standard stars for the Hubble Space Telescope.
| Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
| A Search for Kilogauss Magnetic Fields in White Dwarfs and Hot Subdwarf Stars
We present new results of a survey for weak magnetic fields among DAwhite dwarfs, including some brighter hot subdwarf stars. We havedetected variable circular polarization in the H? line of the hotsubdwarf star Feige 34 (spectroscopic type: sdO). From these data, weestimate that the longitudinal magnetic field of this star varies from-1.1+/-3.2 to +9.6+/-2.6 kG, with a mean of about +5 kG and a periodlonger than 2 hr. In this study, we also confirm the magnetic nature ofwhite dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado andcoworkers, and present upper limits of kilogauss longitudinal magneticfields of the five brightest DA white dwarfs. Our data support thefinding of Aznar Cuadrado and coworkers that ~25% of white dwarfs havekilogauss magnetic fields. This frequency also confirms results of earlyestimates obtained using the magnetic field function of white dwarfs(Fabrika & Valyavin).
| Calibration of Synthetic Photometry Using DA White Dwarfs
We have calibrated four major ground-based photometric systems withrespect to the Hubble Space Telescope (HST) absolute flux scale, whichis defined by Vega and four fundamental DA white dwarfs. Thesephotometric systems include the Johnson-Kron-Cousins UBVRI, theStrömgren uvby filters, the Two Micron All Sky SurveyJHKs, and the Sloan Digital Sky Survey ugriz filters.Synthetic magnitudes are calculated from model white dwarf spectrafolded through the published filter response functions; these magnitudesin turn are absolutely calibrated with respect to the HST flux scale.Effective zero-magnitude fluxes and zero-point offsets of each systemare determined. In order to verify the external observationalconsistency, as well as to demonstrate the applicability of thesedefinitions, the synthetic magnitudes are compared with the respectiveobserved magnitudes of larger sets of DA white dwarfs that havewell-determined effective temperatures and surface gravities and span awide range in both of these parameters.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| SDSS J0806+2006 and SDSS J1353+1138: Two New Gravitationally Lensed Quasars from the Sloan Digital Sky Survey
We report the discoveries of two two-image gravitationally lensedquasars selected from the Sloan Digital Sky Survey: SDSS J0806+2006 atzs=1.540 and SDSS J1353+1138 at zs=1.629 withimage separations of ??=1.40" and ??=1.41",respectively. Spectroscopic and optical/near-infrared imaging follow-upobservations show that the quasar images have identical redshifts andpossess extended objects between the images that are likely to be lensgalaxies at zl~=0.6 in SDSS J0806+2006 and zl~=0.3in SDSS J1353+1138. The field of SDSS J0806+2006 contains several nearbygalaxies that may significantly perturb the system, and SDSS J1353+1138has an extra component near its Einstein ring that is probably aforeground star. Simple mass models with reasonable parameters reproducethe quasar positions and fluxes of both systems.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Ursa Major |
| Ascensió Recta: | 11h37m05.11s |
| Declinació: | +29°47'58.1" |
| Magnitud Aparent: | 12.07 |
| Distancia: | 15.319 parsecs |
| Moviment propi RA: | -147.3 |
| Moviment propi Dec: | -10.8 |
| B-T magnitude: | 12.855 |
| V-T magnitude: | 12.135 |
Catàlegs i designacions:
|
