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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 very luminous, highly extinguished, very fast nova - V1721 Aquilae
Studies indicate that fast novae are primarily located within the planeof the Milky Way and slow novae are found within its bulge. Because ofhigh interstellar extinction along the line of sight many novae lyingclose to the plane are missed and only the brightest seen. One novalying very close to the Galactic plane is V1721 Aquilae, which wasdiscovered in outburst on 2008 September 22.5 UT. Examination of spectraobtained 2.69 days after outburst revealed very high expansionvelocities (FWHM of the H? emission ?6450 km s-1).In this paper we have used available pre- and post-outburst photometryand post-outburst spectroscopy to conclude that the object is a veryfast, luminous, and highly extinguished (AV = 11.6 ±0.2) nova system with an average ejection velocity of ? 3400 kms-1. Pre-outburst near-IR colours from the 2MASS point sourcecatalogue indicate that at quiescence the object is similar to manyquiescent classical novae and appears to have a main sequence/sub-giantsecondary rather than a giant counterpart. Based on the speed of declineof the nova and its emission line profiles we hypothesise that the axisratio of the nova ejecta is ~1.4 and that its inclination is such thatthe central binary accretion disc is face-on to the observer. As such,the accretion disc's blue contribution to the system's near-IR quiescentcolours may be significant. Simple models of the nova ejecta have beenconstructed using the morphological modelling code XS5, and the resultssupport the above hypothesis. Precise spectral classification of thisobject has been exceptionally difficult owing to low signal-to-noiselevels and high extinction, which has eliminated all evidence of anyHe/N or Fe II emission within the spectra. We suggest two possibilitiesfor the nature of V1721 Aql: that it is a U Sco type RN with a sub-giantsecondary or, less likely, that it is a highly energetic bright and fastclassical nova with a main sequence secondary. Future monitoring of theobject for possible RN episodes may be worthwhile, as would archivalsearches for previous outbursts.
| Candidate subdwarfs and white dwarfs from the 2MASS, Tycho-2, XPM and UCAC3 catalogues
Photometry from the Two-Micron All-Sky Survey (2MASS), United StatesNaval Observatory CCD Astrograph Catalog (UCAC3) and SuperCosmoscatalogues, together with proper motions from the Tycho-2, KharkivProper Motions (XPM) and UCAC3 catalogues, is used to select all-skysamples of 28 candidate white dwarfs, 1826 evolved and 7641 unevolvedsubdwarfs for R from 9-17 mag. The samples are separated frommain-sequence stars with an admixture of less than 10 per cent, owing toan analysis of the distribution of the stars in colour index versusreduced proper-motion diagrams for various latitudes using related MonteCarlo simulations. It is shown that the XPM and UCAC3 catalogues havethe same level of proper-motion accuracy. Most of the selected starshave at least six-band photometry. This allows us to eliminate someadmixtures and reveal some binaries. Empirical calibrations of absolutemagnitude versus colour index and reduced proper motion for Hipparcosstars give us distances and a three-dimensional (3D) distribution forall the selected stars. It is shown that the subdwarf samples are almostcomplete for the Tycho-2 stars, i.e. to 11 mag or 150 pc from the Sun.For fainter stars from the XPM and UCAC3 catalogues, the subdwarfsamples are complete only to 20-60 per cent because of the selectionmethod and incompleteness of the catalogues. Some conclusions can bemade, however, especially for Tycho-2 stars with known radial velocitiesand metallicities. The subdwarfs show some concentration in the GalacticCentre hemisphere, with voids due to extinction in the Gould belt andthe Galactic plane. Some as yet unexplained overdensities of evolvedsubdwarfs are seen in several parts of the sky. For 176 stars withradial velocities, the 3D motion and Galactic orbits are calculated. For57 stars with Fe/H we find relations of the metallicity with colourindex, asymmetric drift velocity and orbital eccentricity. All the dataare consistent with the suggestion that most unevolved subdwarfs belongto the low-metallicity halo with large asymmetric drift, whereas evolvedsubdwarfs have various metallicities and velocities and include bothdisc and halo stars. The lower limit of the local mass density ofunevolved subdwarfs, estimated as 2 × 10-5M&sun; pc-3, appears twice as high as traditionalestimates. The selected stars are listed in the new catalogue ofcandidate subdwarfs and white dwarfs from the 2MASS, Tycho-2, XPM andUCAC3 catalogues (hereafter SDWD catalogue) for future spectroscopicconfirmation of the subluminous status of these stars, because themajority of them are now classified for the first time.
| The Abundance Scatter in M33 from H II Regions: Is There Any Evidence for Azimuthal Metallicity Variations?
Optical spectra of 25 H II regions in the inner 2 kpc of the M33 diskhave been obtained with the Gemini Multi-Object Spectrograph at theGemini North telescope. The oxygen abundance gradient measured from thedetection of the [O III] ?4363 auroral line displays a scatter ofapproximately 0.06 dex, a much smaller value than recently reported byRosolowsky & Simon in this galaxy. The analysis of the abundancesfor a large sample of H II regions derived from the R 23strong-line indicator confirms that the scatter is small over the fulldisk of M33, consistent with the measuring uncertainties, and comparableto what is observed in other spiral galaxies. No evidence is thereforefound for significant azimuthal variations in the present-daymetallicity of the interstellar medium in this galaxy on spatial scalesfrom ~100 pc to a few kpc. A considerable fraction of M33 H II regionswith auroral line detections show spectral features revealing sources ofhard ionizing radiation (such as He II emission and large [Ne III], [OIII] line fluxes). Since R 23 is shown to severelyunderestimate the oxygen abundances in such cases, care must be taken inchemical abundance studies of extragalactic H II regions based on thisstrong-line indicator.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).
| 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.
| Investigation of the stellar population and determination of interstellar extinction toward the Chandra Bulge Field based on RTT-150 data
We have investigated the stellar population of the Chandra Bulge Field(CBF) 35' × 35' in area using the Russian-Turkish RTT-150telescope with the goal of constructing an interstellar extinction mapand determining the extinction law. The optical extinction has beendetermined from the positions of red clump giants (a group of red giantswith the same luminosity and color) on the color-magnitude diagram indifferent parts of the field. This has allowed an interstellarextinction map of the field under consideration to be constructed with aresolution of 1' × 1'. Based on the results of our analysis, wehave also shown that the extinction law in the investigated fielddiffers significantly from the standard one, most likely because thedust properties in the Galactic bulge differ from those in the Galacticdisk. The derived extinction law confirms the measurements in the outerparts of the Galactic bulge.
| Discoveries from a Near-infrared Proper Motion Survey Using Multi-epoch Two Micron All-Sky Survey Data
We have conducted a 4030 deg2 near-infrared proper motionsurvey using multi-epoch data from the Two Micron All-Sky Survey(2MASS). We find 2778 proper motion candidates, 647 of which are notlisted in SIMBAD. After comparison to Digitized Sky Survey images, wefind that 107 of our proper motion candidates lack counterparts at B, R,and I bands and are thus 2MASS-only detections. We present results ofspectroscopic follow-up of 188 targets that include the infrared-onlysources along with selected optical-counterpart sources with faintreduced proper motions or interesting colors. We also establish a set ofnear-infrared spectroscopic standards with which to anchor near-infraredclassifications for our objects. Among the discoveries are six youngfield brown dwarfs, five "red L" dwarfs, three L-type subdwarfs, twelveM-type subdwarfs, eight "blue L" dwarfs, and several T dwarfs. Wefurther refine the definitions of these exotic classes to aid futureidentification of similar objects. We examine their kinematics and findthat both the "blue L" and "red L" dwarfs appear to be drawn from arelatively old population. This survey provides a glimpse of the kindsof research that will be possible through time-domain infrared projectssuch as the UKIDSS Large Area Survey, various VISTA surveys, and WISE,and also through z- or y-band enabled, multi-epoch surveys such asPan-STARRS and LSST.Some of the spectroscopic data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership amongthe California Institute of Technology, the University of California,and the National Aeronautics and Space Administration. The Observatorywas made possible by the generous financial support of the W. M. KeckFoundation. Other spectroscopic data were collected at the SubaruTelescope, which is operated by the National Astronomical Observatory ofJapan.
| The Einstein Cross: Constraint on Dark Matter from Stellar Dynamics and Gravitational Lensing
We present two-dimensional line-of-sight stellar kinematics of the lensgalaxy in the Einstein Cross, obtained with the GEMINI 8 m telescope,using the GMOS integral-field spectrograph. The stellar kinematicsextend to a radius of 4'' (with 0farcs2 spaxels), covering abouttwo-thirds of the effective (or half-light) radius Re ~= 6''of this early-type spiral galaxy at redshift zl ~= 0.04, ofwhich the bulge is lensing a background quasar at redshift zs~= 1.7. The velocity map shows regular rotation up to ~100 kms-1 around the minor axis of the bulge, consistent withaxisymmetry. The velocity dispersion map shows a weak gradientincreasing toward a central (R < 1'') value of ?0 =170 ± 9 km s-1. We deproject the observed surfacebrightness from Hubble Space Telescope imaging to obtain a realisticluminosity density of the lens galaxy, which in turn is used to buildaxisymmetric dynamical models that fit the observed kinematic maps. Wealso construct a gravitational lens model that accurately fits thepositions and relative fluxes of the four quasar images. We combinethese independent constraints from stellar dynamics and gravitationallensing to study the total mass distribution in the inner parts of thelens galaxy. We find that the resulting luminous and total massdistribution are nearly identical around the Einstein radiusRE = 0farcs89, with a slope that is close to isothermal, butwhich becomes shallower toward the center if indeed mass follows light.The dynamical model fits to the observed kinematic maps result in atotal mass-to-light ratio Upsilondyn = 3.7 ± 0.5Upsilonsun,I (in the I band). This is consistent with theEinstein mass ME = 1.54 × 1010 Msun divided by the (projected) luminosity withinRE , which yields a total mass-to-light ratio of UpsilonE = 3.4 Upsilonsun,I , with an error of at most a fewpercent. We estimate from stellar population model fits to colors of thelens galaxy a stellar mass-to-light ratio Upsilonsstarf from2.8 to 4.1 Upsilonsun,I . Although a constant dark matterfraction of 20% is not excluded, dark matter may play no significantrole in the bulge of this ~L sstarf early-type spiral galaxy.
| When galaxies collide: understanding the broad absorption-line radio galaxy 4C +72.26
We present a range of new observations of the `broad absorption-lineradio galaxy' 4C +72.26 (z ~ 3.5), including sensitive rest-frameultraviolet integral field spectroscopy using the Gemini/GMOS-Ninstrument and Subaru/CISCO K-band imaging and spectroscopy. We showthat 4C +72.26 is a system of two vigorously star-forming galaxiessuperimposed along the line of sight separated by ~1300 +/- 200 kms-1 in velocity, with each demonstrating spectroscopicallyresolved absorption lines. The most active star-forming galaxy alsohosts the accreting supermassive black hole which powers the extendedradio source. We conclude that the star formation is unlikely to havebeen induced by a shock caused by the passage of the radio jet, andinstead propose that a collision is a more probable trigger for the starformation. Despite the massive starburst, the ultraviolet-mid-infraredspectral energy distribution suggests that the pre-existing stellarpopulation comprises ~1012Msolar of stellar mass,with the current burst only contributing a further ~2 per cent,suggesting that 4C +72.26 has already assembled most of its finalstellar mass.
| 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.
| Spectroscopic study of a few Herbig Ae/Be stars in young open clusters
We present the spectroscopic study of 5 Herbig Ae/Be stars in young openclusters. These are identified from a survey of emission-line stars inyoung open clusters. The Herbig Ae/Be stars are found to show a linearcorrelation in H_? equivalent width versus reddening corrected(H-K_s) colour plot, with a clear offset from the distribution ofClassical Be stars. The candidates are found to show near infraredexcess which was revealed through de-reddened (J-H) versus (H-K_s)colour-colour diagram and spectral energy distribution. Fromoptical/near-IR photometry and spectroscopy, we suggest that Bochum 6-1,IC 1590-1 and NGC 6823-1 are Herbig Be while IC 1590-2 and NGC 7380-4are Herbig Ae candidates. Bochum 6-1 is an interesting Herbig B[e] starwith a high H_? equivalent width of -206 Å, which is thehighest among the surveyed stars. We combined the optical and near-IRphotometry to estimate the duration of star formation in the clusters,Bochum 6, IC 1590, NGC 6823 and NGC 7380. We found ongoing starformation in all these clusters, with an appreciable number of pre-mainsequence stars. The age of these Herbig Ae/Be stars, estimated usingpre-main sequence isochrones, were found to range between 0.25-3 Myr. IC1590 is found to be an interesting young cluster (˜ 4 Myr) with 3emission stars, each belonging to Herbig Ae, Herbig Be and Classical Berespectively. All the four clusters studied here were found to beforming stars for the last 10 Myr.
| The SPLASH Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, II, III, VII, X, and XIV Dwarf Spheroidal Galaxies
We present new Keck/DEIMOS spectroscopic observations of hundreds ofindividual stars along the sightline to the first three of the Andromeda(M31) dwarf spheroidal (dSph) galaxies to be discovered, And I, II, andIII, and combine them with recent spectroscopic studies by our team ofthree additional M31 dSphs, And VII, X, and XIV, as a part of the SPLASHSurvey (Spectroscopic and Photometric Landscape of Andromeda's StellarHalo). Member stars of each dSph are isolated from foreground Milky Waydwarf stars and M31 field contamination using a variety of photometricand spectroscopic diagnostics. Our final spectroscopic sample of memberstars in each dSph, for which we measure accurate radial velocities witha median uncertainty (random plus systematic errors) of 4-5 kms-1, includes 80 red giants in And I, 95 in And II, 43 in AndIII, 18 in And VII, 22 in And X, and 38 in And XIV. The sample ofconfirmed members in the six dSphs is used to derive each system's meanradial velocity, intrinsic central velocity dispersion, mean abundance,abundance spread, and dynamical mass. This combined data set presents uswith a unique opportunity to perform the first systematic comparison ofthe global properties (e.g., metallicities, sizes, and dark mattermasses) of one-third of Andromeda's total known dSph population withMilky Way counterparts of the same luminosity. Our overall comparisonsindicate that the family of dSphs in these two hosts have bothsimilarities and differences. For example, we find that theluminosity-metallicity relation is very similar between L ~105 and 107 L sun, suggesting that thechemical evolution histories of each group of dSphs are similar. Thelowest luminosity M31 dSphs appear to deviate from the relation,possibly suggesting tidal stripping. Previous observations have notedthat the sizes of M31's brightest dSphs are systematically larger thanMilky Way satellites of similar luminosity. At lower luminositiesbetween L = 104 and 106 L sun, we findthat the sizes of dSphs in the two hosts significantly overlap and thatfour of the faintest M31 dSphs are smaller than Milky Way counterparts.The first dynamical mass measurements of six M31 dSphs over a largerange in luminosity indicate similar mass-to-light ratios compared toMilky Way dSphs among the brighter satellites, and smaller mass-to-lightratios among the fainter satellites. Combined with their similar orlarger sizes at these luminosities, these results hint that the M31dSphs are systematically less dense than Milky Way dSphs. Theimplications of these similarities and differences for generalunderstanding of galaxy formation and evolution are summarized.
| 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.
| 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.
| New study of the quasi-molecular Lyman-? satellites due to H-H+ collisions
Context: Quasi-molecular line satellites in the red wings of Lymanseries of atomic hydrogen have been identified in the spectra ofhydrogen-rich white dwarfs. These features are produced by radiativecollisions. Aims: Structures observed about 995 Åin theLyman-? wing of hot white dwarfs have been shown to be caused byquasi-molecular absorption of H2+ molecules. Methods:Improvements to previous theoretical calculations of the Lyman-?line profiles can be achieved by using a unified theory that takes intoaccount the dependence of the dipole moments on internuclear distanceduring the collision. Results: For the first time, we havecomputed the transition dipole moments. We measure a significantincrease in the region of the formation of the satellites, which altersthe general shape of the profile. Conclusions: A large increasein the strength of the two main satellites at 992 and 996 Å leadsto a deeper broad absorption in the synthetic spectra, which shouldimprove the comparison with observation as previous predictedLyman-? satellites were too weak.
| IC10: the history of the nearest starburst galaxy through its Planetary Nebula and HII region populations
We report the results of spectroscopic observations, obtained with theGemini North Multi-Object Spectrograph, of nine planetary nebulae (PNe)and 15 HII regions located in the 5.5 × 5.5arcmin2inner region of the nearby starburst galaxy IC10. Twelve new candidatesPNe have been discovered during our pre-imaging phase. Nine of them havebeen spectroscopically confirmed. The direct availability of theelectron temperature diagnostics in several nebulae allowed an accuratedetermination of the metallicity map of IC10 at two epochs: thepresent-time from HII regions and the old/intermediate-age from PNe. Wefound a non-homogeneous distribution of metals at both epochs, butsimilar average abundances were found for the two populations. Thederived age-metallicity relation shows a little global enrichmentinterpreted as the loss of metals by supernova winds and to differentialgas outflows. Finally, we analysed the production of oxygen - throughthe third dredge-up - in the chemical abundance patterns of the PNpopulations belonging to several dwarf irregular galaxies. We found thatthe third dredge-up of oxygen is a metallicity dependent phenomenonoccurring mainly for 12 + log(O/H) <= 7.7 and substantially absent inIC10 PNe.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership.E-mail: laura@arcetri.astro.it
| The Discovery of a Large Ly?+He II Nebula at z ? 1.67: A Candidate Low Metallicity Region?
We have discovered a ?45 kpc Ly? nebula (or Ly? "blob")at z ? 1.67 which exhibits strong, spatially extended He II emissionand very weak C IV and C III] emission. This is the first spatiallyextended Ly?+He II emitter observed and the lowest redshiftLy? blob yet found. Strong Ly? and He II?1640emission in the absence of metal lines has been proposed as a uniqueobservational signature of primordial galaxy formation (e.g., fromgravitational cooling radiation or Population III star formation), butno convincing examples of spatially extended Ly?+He II emittershave surfaced either in Ly?-emitting galaxy surveys at highredshifts (z > 4) or in studies of Ly? nebulae at lowerredshifts. From comparisons with photoionization models, we find thatthe observed line ratios in this nebula are consistent with lowmetallicity gas (Z lsim 10-2-10-3 Zsun), but that this conclusion depends on the unknownionization parameter of the system. The large He II equivalent width(?37 ± 10 Å) and the large He II/Ly? ratio (0.12± 0.04) suggest that the cloud is being illuminated by a hardionizing continuum, either an active galactic nucleus (AGN) or very lowmetallicity stars, or perhaps powered by gravitational coolingradiation. Thus far there is no obvious sign of a powerful AGN in ornear the system, so in order to power the nebula while remaining hiddenfrom view even in the mid-infrared, the AGN would need to be heavilyobscured. Despite the strong Ly?+He II emission, it is not yetclear what is the dominant power source for this nebula. The systemtherefore serves as an instructive example of how the complexities oftrue astrophysical sources will complicate matters when attempting touse a strong Ly?+He II signature as a unique tracer of primordialgalaxy formation.
| A Spectroscopic Census of the M82 Stellar Cluster Population
We present a spectroscopic study of the stellar cluster population ofM82, the archetype starburst galaxy, based primarily on new Gemini-Northmulti-object spectroscopy of 49 star clusters. These observationsconstitute the largest to date spectroscopic data set of extragalacticyoung clusters, giving virtually continuous coverage across the galaxy;we use these data to deduce information about the clusters as well asthe M82 post-starburst disk and nuclear starburst environments.Spectroscopic age dating places clusters in the nucleus and disk between(7, 15) and (30, 270) Myr, with distribution peaks at ~10 and 140 Myr,respectively. We find cluster radial velocities (RVs) in the range vR in (-160, 220)km s -1 (with respect to thegalaxy center), and line-of-sight Na I D interstellar absorption linevelocities v Na ID R in (-75, 200) kms-1, in many cases entirely decoupled from the clusters. Asthe disk cluster RVs lie on the flat part of the galaxy rotation curve,we conclude that they comprise a regularly orbiting system. Ourobservations suggest that the largest part of the population was createdas a result of the close encounter with M81 ~220 Myr ago. Clusters inthe nucleus are found in solid body rotation on the bar. The possibledetection of Wolf-Rayet features in their spectra indicates that clusterformation continues in the central starburst zone. We also report thepotential discovery of two old populous clusters in the halo of M82,aged gsim8 Gyr. Using these measurements and simple dynamicalconsiderations, we derive a toy model for the invisible physicalstructure of the galaxy, and confirm the existence of two dominantspiral arms.
| Nearby Young Stars Selected by Proper Motion. I. Four New Members of the β Pictoris Moving Group From The Tycho-2 Catalog
We describe a procedure to identify stars from nearby moving groups andassociations out of catalogs of stars with large proper motions. We showthat from the mean motion vector of a known or suspected moving group,one can identify additional members of the group based on proper motiondata and photometry in the optical and infrared, with minimalcontamination from background field stars. We demonstrate this techniqueby conducting a search for low-mass members of the β Pictorismoving group in the Tycho-2 catalog. All known members of the movinggroup are easily recovered, and a list of 51 possible candidates isgenerated. Moving group membership is evaluated for 33 candidates basedon X-ray flux from ROSAT, Hα line emission, and radial velocitymeasurement from high-resolution infrared spectra obtained at InfraredTelescope Facility. We confirm three of the candidates to be new membersof the group: TYC 1186-706-1, TYC 7443-1102-1, and TYC 2211-1309-1 whichare late-K and early-M dwarfs 45-60 pc from the Sun. We also identify acommon proper motion companion to the known β Pictoris Moving Groupmember TYC 7443-1102-1, at a 26farcs3 separation; the new companion isassociated with the X-ray source 1RXS J195602.8 – 320720. We arguethat the present technique could be applied to other large proper motioncatalogs to identify most of the elusive, low-mass members of knownnearby moving groups and associations.Based on data obtained in part with the 2.4 m Hiltner telescope of theMDM observatory. Based on data obtained in part with the CTIO 1.5 mtelescope, operated by SMARTS, the Small and Medium Aperture TelescopeSystem consortium, under contract with the Associated Universities forResearch in Astronomy (AURA).
| Discovery in IC10 of the farthest known symbiotic star
We report the discovery of the first known symbiotic star in IC10, astarburst galaxy belonging to the Local Group, at a distance of ~750kpc. The symbiotic star was identified during a survey of emission-lineobjects. It shines at V = 24.62 +/- 0.04,V - RC = 2.77 +/-0.05 and RC - IC = 2.39 +/- 0.02, and suffers fromEB- V = 0.85 +/- 0.05 reddening. The spectrum of the coolcomponent well matches that of solar neighbourhood M8III giants. Theobserved emission lines belong to Balmer series, [SII], [NII] and[OIII]. They suggest a low electronic density, negligible optical deptheffects and 35000 < Teff < 90000 K for the ionizingsource. The spectrum of the new symbiotic star in IC10 is an almostperfect copy of that of Hen 2-147, a well-known Galactic symbiotic starand Mira.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. We also retrieved UBVRI images of the `Survey of LocalGroup Galaxies Currently Forming Stars', Massey et al. (2007).E-mail: denise@ov.ufrj.br
| Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich?
We present the discovery of two nearby L dwarfs from our 2MASSproper-motion search, which uses multiepoch 2MASS observations covering~4700 deg2 of sky. 2MASS J18212815+1414010 and 2MASSJ21481628+4003593 were overlooked by earlier surveys due to their faintoptical magnitudes and their proximity to the Galactic plane(10deg<=|b|<=15°). Assuming that both dwarfs aresingle, we derive spectrophotometric distances of ~10 pc, thusincreasing the number of known L dwarfs within 10 pc to 10. In thenear-infrared, 2MASS J21481628+4003593 shows a triangular H-bandspectrum, strong CO absorption, and a markedly red J-Ks color(2.38+/-0.06) for its L6 optical spectral type. 2MASS J18212815+1414010also shows a triangular H-band spectrum and a slightly redJ-Ks color (1.78+/-0.05) for its L4.5 optical spectral type.Both objects show strong silicate absorption at 9-11 μm.Cumulatively, these features imply an unusually dusty photosphere forboth of these objects. We examine several scenarios to explain theunderlying cause for their enhanced dust content and find that ametal-rich atmosphere or a low surface gravity are consistent with theseresults. 2MASS J18212815+1414010 may be young (and therefore have a lowsurface gravity) based on its low tangential velocity of 10 kms-1. On the other hand, 2MASS J21481628+4003593 has a hightangential velocity of 62 km s-1 and is therefore likely old.Hence, high metallicity and low surface gravity may lead to similareffects.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan. Some of the datapresented herein were obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| Clusters of Galaxies in the First Half of the Universe from the IRAC Shallow Survey
We have identified 335 galaxy cluster and group candidates, 106 of whichare at z>1, using a 4.5 μm-selected sample of objects from a 7.25deg2 region in the Spitzer Infrared Array Camera (IRAC)Shallow Survey. Clusters were identified as three-dimensionaloverdensities using a wavelet algorithm, based on photometric redshiftprobability distributions derived from IRAC and NOAO Deep Wide-FieldSurvey data. We estimate only ~10% of the detections are spurious. Todate 12 of the z>1 candidates have been confirmed spectroscopically,at redshifts from 1.06 to 1.41. Velocity dispersions of ~750 kms-1 for two of these argue for total cluster masses wellabove 1014 Msolar, as does the mass estimated fromthe rest-frame near-infrared stellar luminosity. Although not selectedto contain a red sequence, some evidence for red sequences is present inthe spectroscopically confirmed clusters, and brighter galaxies aresystematically redder than the mean galaxy color in clusters at allredshifts. The mean I-[3.6] color for cluster galaxies up to z~1 is wellmatched by a passively evolving model in which stars are formed in a 0.1Gyr burst starting at redshift zf=3. At z>1, a wider rangeof formation histories is needed, but higher formation redshifts (i.e.,zf>3) are favored for most clusters.
| 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.
| Redshift Distribution of Extragalactic 24 ?m Sources
We present the redshift distribution of a complete, unbiased sample of24 ?m sources down to f?(24 ?m)=300 ?Jy (5?). The sample consists of 591 sources detected in the Boötesfield of the NOAO Deep Wide-Field Survey. We have obtained opticalspectroscopic redshifts for 421 sources (71%). These have a redshiftdistribution peaking at z~0.3, with a possible additional peak at z~0.9,and objects detected out to z=4.5. The spectra of the remaining 170(29%) exhibit no strong emission lines from which to determine aredshift. We develop an algorithm to estimate the redshift distributionof these sources, based on the assumption that they have emission linesbut that these lines are not observable due to the limited wavelengthcoverage of our spectroscopic observations. The redshift distributionderived from all 591 sources exhibits an additional peak of extremelyluminous (L8-1000?m>3×1012Lsolar) objects at z~2, consisting primarily of sourceswithout observable emission lines. We use optical line diagnostics andIRAC colors to estimate that 55% of the sources within this peak areAGN-dominated. We compare our results to published models of theevolution of infrared-luminous galaxies. The models which best reproduceour observations predict a large population of star-formation-dominatedULIRGs at z>1.5 rather than the AGN-dominated sources we observe.Based on observations made with the Spitzer Space Telescope, operatedby the Jet Propulsion Laboratory under NASA contract 1407.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| The Host Galaxy and the Extended Emission-Line Region of the Radio Galaxy 3C 79
We present extensive ground-based spectroscopy and HST imaging of 3C 79,an FR II radio galaxy associated with a luminous extended emission-lineregion (EELR). Surface brightness modeling of an emission-line-free HSTR-band image reveals that the host galaxy is a massive elliptical with acompact companion 0.8" away and 4 mag fainter. The host galaxy spectrumis best described by an intermediate-age (1.3 Gyr) stellar population(4% by mass), superimposed on a 10 Gyr old population and a power law(αλ=-1.8) the stellar populations are consistentwith supersolar metallicities, with the best fit given by the 2.5Zsolar models. We derive a dynamical mass of4×1011 Msolar within the effective radiusfrom the velocity dispersion. The EELR spectra clearly indicate that theEELR is photoionized by the hidden central engine. Photoionizationmodeling shows evidence that the gas metallicity in both the EELR andthe nuclear narrow-line region is mildly subsolar (0.3-0.7Zsolar), significantly lower than the supersolarmetallicities deduced from typical active galactic nuclei in the SloanDigital Sky Survey. The more luminous filaments in the EELR exhibit avelocity field consistent with a common disk rotation. Fainter clouds,however, show high approaching velocities that are uncoupled from thisapparent disk rotation. The striking similarities between this EELR andthe EELRs around steep-spectrum radio-loud quasars provide furtherevidence for the orientation-dependent unification schemes. Themetal-poor gas is almost certainly not native to the massive hostgalaxy. We suggest that the close companion galaxy could be the tidallystripped bulge of a late-type galaxy that is merging with the hostgalaxy. The interstellar medium of such a galaxy is probably the sourcefor the low-metallicity gas in 3C 79.Based in part on observations obtained at the Gemini Observatory, whichis operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (United States),the Particle Physics and Astronomy Research Council (United Kingdom),the National Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil) and CONICET (Argentina).Gemini Program ID: GN-2006B-C-3. Some of the data presented herein wereobtained at the W. M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation. Based also in part onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| 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%.
| Gemini Spectroscopy and HST Imaging of the Stellar Cluster Population in Region B of M82
We present new spectroscopic observations of the stellar clusterpopulation of region B in the prototype starburst galaxy M82 obtainedwith the Gemini North 8.1 m telescope. By coupling the spectroscopy withUBVI photometry acquired with the Advanced Camera for Surveys (ACS) onthe Hubble Space Telescope (HST), we derive ages, extinctions, andradial velocities for seven young massive clusters (YMCs) in region B.We find the clusters to have ages between 80 and 200 Myr and velocitiesin the range 230-350 km s-1, while the extinctionsAV vary between ~1 and 2.5 mag. We also find evidence ofdifferential extinction across the faces of some clusters, which hindersthe photometric determination of ages and extinctions in these cases.The cluster radial velocities indicate that the clusters are located atdifferent depths within the disk and are on regular disk orbits. Ourresults overall contradict the findings of previous studies, in whichregion B was thought to be a bound region populated by intermediate-ageclusters that formed in an independent, offset starburst episode thatcommenced 600 Myr-1 Gyr ago. Our findings instead suggest that region Bis optically bright because of low-extinction patches, and that thisallows us to view the cluster population of the inner M82 disk, whichprobably formed as a result of the last encounter with M81. This studyforms part of a series of papers whose aim is to study the clusterpopulation of M82 using deep optical spectroscopy and multibandphotometry.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), CNPq (Brazil), and CONICET (Argentina).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program 10853.
| Discovery of a Wide Companion near the Deuterium-burning Mass Limit in the Upper Scorpius Association
We present the discovery of a companion near the deuterium-burning masslimit located at a very wide distance, at an angular separation of4.6''+/-0.1'' (projected distance of ~ 670 AU)from UScoCTIO 108, a brown dwarf of the very young Upper Scorpiusassociation. Optical and near-infrared photometry and spectroscopyconfirm the cool nature of both objects, with spectral types of M7 andM9.5, respectively, and that they are bona fide members of theassociation, showing low gravity and features of youth. Their masses,estimated from the comparison of their bolometric luminosities andtheoretical models for the age range of the association, are 60+/-20 and14+2-8 MJup, respectively. Theexistence of this object around a brown dwarf at this wide orbitsuggests that the companion is unlikely to have formed in a disk basedon current planet formation models. Because this system is rather weaklybound, they probably did not form through dynamical ejection of stellarembryos.
| 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.
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Osservazione e dati astrometrici
| Costellazione: | Volpetta |
| Ascensione retta: | 20h34m21.88s |
| Declinazione: | +25°03'49.8" |
| Magnitudine apparente: | 11.566 |
| Distanza: | 14.782 parsec |
| Moto proprio RA: | -402.6 |
| Moto proprio Dec: | -563.3 |
| B-T magnitude: | 11.405 |
| V-T magnitude: | 11.553 |
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INHABITED SKY
