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Ýlgili Makaleler
Chemical Properties of Star-Forming Dwarf Galaxies
Recent studies of the near-infrared (NIR) properties of dwarf irregulargalaxies (dIs) and blue compact dwarfs (BCDs) have provided improvedestimates for the NIR luminosity of old stellar populations in thesegalaxies. These can be used to derive gas fractions, and thereby toevaluate how BCDs have evolved with respect to dIs. Oxygen abundanceshave been derived for four BCDs in the Virgo Cluster from a run atGemini-North in 2003. Combining these new abundances with publishedvalues, we study the correlations among the metallicity, Ksluminosity, gas mass, baryonic mass, and gas fraction. Within errors,the two types of dwarfs appear to share a common relation between theoxygen abundance and the luminosity in Ks. The correlationbetween metallicity and the gas fraction is the same for BCDs as fordIs, indicating that BCD evolution has been similar to that of dIs.Since dIs appear to have evolved as isolated systems, the BCD bursts areunlikely to be a consequence of gas infall or merging.
| 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.
| The SAURON project - VIII. OASIS/CFHT integral-field spectroscopy of elliptical and lenticular galaxy centres
We present high spatial resolution integral-field spectroscopy of 28elliptical (E) and lenticular (S0) galaxies from the SAURONrepresentative survey obtained with the OASIS spectrograph during itsoperation at the Canada-France-Hawaii Telescope. These seeing-limitedobservations explore the central 8 × 10 arcsec2(typically 1 kpc diameter) regions of these galaxies using a spatialsampling four times higher than SAURON (0.27-arcsec versus 0.94-arcsecspatial elements), resulting in almost a factor of 2 improvement in themedian point spread function. These data allow accurate study of thecentral regions to complement the large-scale view provided by SAURON.Here we present the stellar and gas kinematics, stellar absorption-linestrengths and nebular emission-line strengths for this sample. We alsocharacterize the stellar velocity maps using the `kinemetry' technique,and derive maps of the luminosity-weighted stellar age, metallicity andabundance ratio via stellar population models. We give a brief review ofthe structures found in our maps, linking also to larger-scalestructures measured with SAURON. We present two previously unreportedkinematically decoupled components (KDCs) in the centres of NGC3032 andNGC4382. We compare the intrinsic size and luminosity-weighted stellarage of all the visible KDCs in the full SAURON sample, and find twotypes of components: kiloparsec-scale KDCs, which are older than 8 Gyr,and are found in galaxies with little net rotation; and compact KDCs,which have intrinsic diameters of less than a few hundred parsec, show arange of stellar ages from 0.5 to 15 Gyr (with 5/6 younger than 5 Gyr),are found exclusively in fast-rotating galaxies, and are close tocounter-rotating around the same axis as their host. Of the sevengalaxies in the SAURON sample with integrated luminosity-weighted agesless than 5 Gyr, five show such compact KDCs, suggesting a link betweencounter-rotation and recent star formation. We show that this may be dueto a combination of small sample size at young ages, and anobservational bias, since young KDCs are easier to detect than theirolder and/or corotating counterparts.Based on observations obtained at the Canada-France-Hawaii Telescopewhich is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: mcdermid@strw.leidenuniv.nl
| Hot DB White Dwarfs from the Sloan Digital Sky Survey
We present ugriz photometry and optical spectroscopy for 28 DB and DOwhite dwarfs with temperatures between 28,000 and 45,000 K. About 10 ofthese are particularly well observed; the remainder are candidates.These are the hottest DB stars yet found, and they populate the ``DBgap'' between the hotter DO stars and the familiar DB stars cooler than30,000 K. Nevertheless, after carefully matching the survey volumes wefind that the ratio of DA stars to DB stars is a factor of 2.5 larger at30,000 than at 20,000 K, suggesting that the ``DB gap'' is indeeddeficient and that some kind of atmospheric transformation takes placein roughly 10% of DA stars as they cool from 30,000 to 20,000 K.Some observations reported here were obtained at the MMT Observatory, ajoint facility of the University of Arizona and the SmithsonianInstitution.
| Star Formation in Satellite Galaxies
We present narrowband observations of the Hα emission in a sampleof 31 satellites orbiting isolated giant spiral galaxies. The samplestudied spans the range -19 mag
| The first brown dwarf with a disk in Chamaeleon II
In this letter we characterize the candidates for young stars and browndwarfs detected in a survey with ISOCAM in the Chamaeleon II dark cloud.Photometric data coming from a wide-field optical imaging survey,combined with IR measurements from the 2MASS catalogue and from theSpitzer satellite, allow us to study the nature of the candidates. Usingintermediate-band filters we can provide a first estimate of thetemperatures for objects cooler than about 3800 K. On the basis ofspectroscopic and photometric data, we confirm that ISO-CHA II 13 is ayoung substellar object with a mass of about 0.05~Mȯ. Itis thus the first object in Cha II whose substellar nature has beenspectroscopically confirmed. By having a temperature of about 2880 K anddisplaying a substantial infrared excess, it joins the list of youngbrown dwarfs observed to have a surrounding disk.
| The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.
| Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy
Aims.We test the hypothesis that the pulsations in sdB stars arecorrelated with the surface abundances of iron-group elements. Anycorrelation might explain why, when given two spectroscopically similarstars, one will pulsate while the other will not. Methods: .Wehave obtained high-resolution ultraviolet spectra two pulsating andthree non-pulsating sdB stars using the Space Telescope ImagingSpectrograph onboard the Hubble Space Telescope. We determinedabundances for 25 elements including the iron group and even heavierelements such as tin and lead using LTE curve-of-growth and spectrumsynthesis techniques. Results: .We find no clear correlationbetween pulsations and metal abundances, and we comment on the resultingimplications, including whether it is possible to determine thedifference between a pulsating and a non-pulsating sdBspectroscopically. In addition to the main goal of our observations, wehave also investigated the effect of supersolar metallicity onfundamental parameter determination, possible trends with ironabundance, and the hypothesis that weak winds may be selectivelyremoving elements from the stellar envelopes. These effects providechallenges to stellar atmosphere modelling and diffusion models for sdBstars.
| COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses. II. SDSS J0924+0219: the redshift of the lensing galaxy, the quasar spectral variability and the Einstein rings
Aims.To provide the observational constraints required to use thegravitationally lensed quasar SDSS J0924+0219 for the determination ofH0 from the time delay method. We measure here the redshiftof the lensing galaxy, we show the spectral variability of the source,and we resolve the lensed host galaxy of the source. Methods.Wepresent our VLT/FORS1 deep spectroscopic observations of the lensedquasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images ofthe same object. The two-epoch spectra, obtained in the Multi ObjectSpectroscopy (MOS) mode, allow for very accurate flux calibration andspatial deconvolution. This strategy provides spectra for the lensinggalaxy and for the quasar images A and B, free of any mutual lightcontamination. We deconvolve the HST images as well, which reveal adouble Einstein ring. The mass distributions in the lens, reconstructedin several ways, are compared. Results.We determine the redshiftof the lensing galaxy in SDSS J0924+0219: z_lens = 0.394±0.001.Only slight spectral variability is seen in the continuum of quasarimages A and B, while the C III] , Mg II and Fe II emission linesdisplay obvious changes. The flux ratio between the quasar images A andB is the same in the emission lines and in the continuum. One of theEinstein rings found using deconvolution corresponds to the lensedquasar host galaxy at z=1.524 and a second bluer one, is the imageeither of a star-forming region in the host galaxy, or of anotherunrelated lower redshift object. A broad range of lens models give asatisfactory fit to the data. However, they predict very different timedelays, making SDSS J0924+0219 an object of particular interest forphotometric monitoring. In addition, the lens models reconstructed usingexclusively the constraints from the Einstein rings, or usingexclusively the astrometry of the quasar images, are not compatible.This suggests that multipole-like structures play an important role inSDSS J0924+0219.
| Is there a correlation between metal abundance and pulsation in sdB stars?
Like several other classes of pulsating stars (e.g. the PG 1159 stars),pulsating subdwarf B stars coexist with non-pulsators in the same regionof the Teff-log g diagram. And as with other pulsators, thereason for this is unclear. The pulsations in sdBs are believed to bedriven by the heavy element ionisation (of especially iron) at the baseof the stellar photosphere. It has been suggested that perhaps theamount of iron-group elements at the stellar surface may be adiscriminating factor. We have analysed high-resolution UV echellespectra of 5 sdBs obtained using HST/STIS to test this hypothesis andfind that there is no clear and consistent difference between theabundances of pulsators and non-pulsators. This poses the question: whatis the physical reason for pulsators and non-pulsators to bespectroscopically similar?
| Subluminous O Stars
We report results of spectral analyses of sdO stars selected from theSupernova Ia Progenitor Survey, the Hamburg Quasar Survey and the SloanDigital Sky Survey and based on state-of-the-art NLTE model atmospheres.By combining the sdO with the sdB samples we discuss trends of theatmospheric parameters in order to search for evidence for possibleevolutionary linkage. The He-sdO stars are found to cluster near Teff =45 000 K, log g = 5.5, whereas the number of sdO stars in this area isvery small. The ``cooler'' sdO stars seem to form an extension to thesdB sequences. A couple of sdO stars are obviously evolved from theextended horizontal branch and reach temperatures as high as 80 000 K.We conjecture that the He-sdO stars and sdO/sdB stars have a differentevolutionary origin. This is corroborated by the much lower binaryfrequency of the former. Strong enrichments of iron group elements arediscovered for hydrogen-rich sdO stars as well as for sdB stars fromhigh resolution UV spectra and have severe implications for thetemperature scale. We finally highlight the discovery of ahyper-velocity He-sdO star with a radial velocity of 708 kms unbound tothe Galaxy.
| Discovery of magnetic fields in hot subdwarfs
We present polarisation measurements of sdB and sdO stars using FORS1 onthe VLT. The observations were made as part of a project to determinewhether magnetic fields in two super-metal-rich stars can explain theirextreme abundance peculiarities. Field strengths of up to ~1.5 kG rangehave been measured at varying levels of significance in each of our sixtargets, however no clear evidence was found between apparently normalsubdwarfs and the metal-rich objects. The origin of the magnetic fieldsis unknown. We also discuss the implications of our measurements formagnetic flux conservation in late stages of stellar evolution.
| Spectroscopy of the type Ia supernova SN 2002er: Days -11 to +215
We present an extensive set of optical spectroscopy of the nearby typeIa supernova, SN 2002er, with 24 epochs spanning -11 to +34 days. Itsspectral evolution is fairly typical of a type Ia supernova although itsuffers high extinction. Nevertheless, there are differences in thespectral evolution when compared to coeval spectra of other normal typeIa supernova with comparable decline-rate parameters. Modelling of thephotospheric phase spectra using a homogeneous abundance distribution inthe atmosphere provides a fair match to the observations, but only bypushing the adopted distance and risetime close to the observationallimits. Future improvements here will require models with a morerealistic stratified abundance distribution. From simple modelling of anebular spectrum obtained at +215 d, we infer a 56Ni mass of0.69 M_ȯ, consistent with that derived from the light curve.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| Recombination Line versus Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae(PNs) have been found to give abundances that are much larger in somecases than abundances from collisionally excited forbidden lines (CELs).The origins of this abundance discrepancy are highly debated. We presentnew spectroscopic observations of O II and C II recombination lines forsix planetary nebulae. With these data we compare the abundances derivedfrom the optical recombination lines with those determined fromcollisionally excited lines. Combining our new data with publishedresults on RLs in other PNs, we examine the discrepancy in abundancesderived from RLs and CELs. We find that there is a wide range in themeasured abundance discrepancyΔ(O+2)=logO+2(RL)-logO+2(CEL),ranging from approximately 0.1 dex (within the 1 σ measurementerrors) up to 1.4 dex. This tends to rule out errors in therecombination coefficients as a source of the discrepancy. Most RLsyield similar abundances, with the notable exception of O II multipletV15, known to arise primarily from dielectronic recombination, whichgives abundances averaging 0.6 dex higher than other O II RLs. Wecompare Δ(O+2) against a variety of physical propertiesof the PNs to look for clues as to the mechanism responsible for theabundance discrepancy. The strongest correlations are found with thenebula diameter and the Balmer surface brightness; high surfacebrightness, compact PNs show small values of Δ(O+2),while large low surface brightness PNs show the largest discrepancies.An inverse correlation of Δ(O+2) with nebular densityis also seen. A marginal correlation of Δ(O+2) is foundwith expansion velocity. No correlations are seen with electrontemperature, He+2/He+, central star effectivetemperature and luminosity, stellar mass-loss rate, or nebularmorphology. Similar results are found for carbon in comparing C II RLabundances with ultraviolet measurements of C III].
| Lyman Break Galaxies at z~5: Rest-Frame Ultraviolet Spectra
We report initial results for spectroscopic observations of candidatesof Lyman break galaxies (LBGs) at z~5 in a region centered on the HubbleDeep Field-North by using the Faint Object Camera and Spectrographattached to the Subaru Telescope. Eight objects withIC<=25.0 mag, including one active galactic nucleus, areconfirmed to be at 4.5
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| Detection of a companion to the pulsating sdB Feige 48
We present the discovery of a binary companion to the pulsating sdBFeige 48. Using HST/STIS and archival FUSE spectra, we measure a periodof 0.376±0.003 d and a velocity semi-amplitude of 28.0±0.2km s-1. This implies that the companion star must either beof very low mass, or the orbit is at low inclination. Combining 2MASSfluxes, the lack of a reflection effect, results from asteroseismologyand a measurement of the rotation velocity of Feige 48, we show that theorbital inclination must be ≤11.4o and that the unseencompanion is a white dwarf with mass ≥0.46 Mȯ. Theimplications of this discovery, and of binarity amongst sdB pulsators,is then discussed in the context of recent theoretical work on sdBformation. In particular we suggest that radial velocity studies focuson sdB pulsators with no known companion, and that asteroseismologicalstudies of sdBs investigate a larger mass range than previouslyconsidered in order to test formation models.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
We analyze interstellar absorption observed toward two subdwarf O stars,JL 9 and LS 1274, using spectra taken by the Far UltravioletSpectroscopic Explorer (FUSE). Column densities are measured for manyatomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, FeII, and H2), but our main focus is on measuring the D/Hratios for these extended lines of sight, as D/H is an importantdiagnostic for both cosmology and Galactic chemical evolution. We findD/H=(1.00+/-0.37)×10-5 toward JL 9 andD/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σuncertainties). With distances of 590+/-160 and 580+/-100 pc,respectively, these two lines of sight are currently among the longestGalactic lines of sight with measured D/H. With the addition of thesemeasurements, we see a significant tendency for longer Galactic lines ofsight to yield low D/H values, consistent with previous inferences aboutthe deuterium abundance from D/O and D/N measurements. Short lines ofsight with H I column densities of logN(HI)<19.2 suggest that thegas-phase D/H value within the Local Bubble is(D/H)LBg=(1.56+/-0.04)×10-5. However, thefour longest Galactic lines of sight with measured D/H, which haved>500 pc and logN(HI)>20.5, suggest a significantly lower valuefor the true local disk gas-phase D/H value,(D/H)LDg=(0.85+/-0.09)×10-5. Oneinterpretation of these results is that D is preferentially depletedonto dust grains relative to H and that longer lines of sight thatextend beyond the Local Bubble sample more depleted material. In thisscenario, the higher Local Bubble D/H ratio is actually a betterestimate than (D/H)LDg for the true local disk D/H,(D/H)LD. However, if (D/H)LDg is different from(D/H)LBg simply because of variable astration and incompleteinterstellar medium mixing, then (D/H)LD=(D/H)LDg.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by JohnsHopkins University under NASA contract NAS5-32985.
| Planetary nebulae near the Galactic center: Identifications
We surveyed the central 4 × 4 degrees of the Galactic center forplanetary nebulae in the light of [S III] λ9532 and found 94 PNethat were not previously known, plus 3 that were previously identifiedas possible candidates. For 63 of these 97 objects, we obtained spectrathat are consistent with highly reddened PN while the other 34 could notbe recovered spectroscopically and remain unverified. Of the 94candidates, 54 and 57 were detected via radio at 3 and 6 cm,respectively. An additional 20 PNe candidates were found duringfollow-up Hα imaging but have not yet been verifiedspectroscopically. Based on the properties of IRAS sources in thisregion of the Galaxy, and on the total luminosity of the Galactic bulge,the expected number of PNe is ˜250, only 50% more than the 160 PNecandidates now known. Thus, surveys for PNe in the bulge areapproximately two-thirds complete with the remainder likely hiddenbehind dust.Observations made with the Burrell Schmidt of the Warner and SwaseyObservatory, Case Western Reserve University.
| The esdM6.5 Star LSR J0822+1700: A New Ultracool Extreme Subdwarf
Spectroscopic observations reveal that the recently discovered highproper motion star LSR J0822+1700 is an ultracool extreme subdwarf, withspectral type esdM6.5. This is only the second known extreme subdwarfwith a spectral type later than esdM6.0, the other one being the starAPMPM J0559-2903 (esdM7.0). Our new spectra of LSR J0822+1700, of APMPMJ0559-2903, and also of the very metal-poor esdM6.0 star LHS 1826 allshow the defining signature of cool esdM objects: very deep absorptionbands due to molecular CaH flanked by very weak absorption due to TiO.Our new spectrum reveals than the molecular bands in LHS 1826 are alittle weaker than previously reported, and we reclassify the star asesdM5.5. We show that all three stars have low metallicities([m/H]<=-1.5) and kinematics consistent with halo membership. Theseobjects represent the low-mass end of Population II stars in thevicinity of the Sun. The new ultracool extreme subdwarf LSR J0822+1700appears to be significantly more metal-poor than APMPM J0559-2903, asevidenced by its much weaker TiO absorption. An interesting feature inthe spectrum of LSR J0822+1700 is the existence of detectable absorptionlines of Rb I and Cs I, usually associated with much cooler subtypes indwarf stars (L dwarfs). We suggest that Rb I and Cs I are relativelystrong in LSR J0822+1700 because of a critical combination of very lowtemperature and very low metallicity.Based on observations made at the MDM Observatory, maintained andoperated by the University of Michigan, Dartmouth College, the OhioState University, and Columbia University.
| Studying the populations of our Galaxy using the kinematics of sdB stars
We have analysed the kinematics of a sample of 114 hot subdwarf stars.For 2/3 of the stars, new proper motions, spectroscopic and photometricdata are presented. The vast majority of the stars show a kinematicbehaviour that is similar to that of Thick Disk stars. Some stars havevelocities rather fitting to solar, i.e. Thin Disk, kinematics. About˜15 objects have orbital velocities which differ considerably fromthose of Disk stars. These are members of the Galactic Halo. Weinvestigated the velocity dispersions and calculated the orbits. Moststars feature orbits with disk character (eccentricity of less than0.5), a few reach far above the Galactic plane and have very eccentricorbits (eccentricity of more than 0.7). The intermediate eccentricityrange is poorly populated. This seems to indicate that the (Thick) Diskand the Halo are kinematically disjunct. Plotting a histogram of theorbit data points along z leads to the z-distance probabilitydistribution of the star; doing this for the whole sample leads to thez-distance probability distribution of the sample. The logarithmichistogram shows two slopes, each representing the scale height of apopulation. The disk component has a scale height of 0.9 (±0.1)kpc, which is consistent with earlier results and is similar to that ofthe Thick Disk. The other slope represents a component with a scaleheight ˜7 kpc, a much flatter gradient than for the diskcomponent. This shows that the vast majority of the sdBs are disk stars,but a Halo minority is present, too. The kinematic history andpopulation membership of the sdB stars on the whole is different fromthat of the cooler HBA stars, which are predominantly or evenexclusively Halo objects. This leads to the question, whether the HalosdB stars are of similar origin as the HBA stars, or whether theirkinematical behaviour possibly represents another origin, such asinfalling stellar aggregates or inner disk events.Based on data obtained at the 1.54 m telescope of the European SouthernObservatory, runs 58.D-0478, 60.D-0306, 62.L-0220, 64.L-0077,64.H-0023(A), 65.H-0250, 65.H-0341(A) and 68.D-0192(A).
| Spectroscopy of Low Surface Brightness Galaxies with the Hobby-Eberly Telescope
We have obtained low-resolution spectra of 19 red and blue low surfacebrightness (LSB) galaxies, using the Marcario Low-ResolutionSpectrograph on the 9.2 m Hobby-Eberly Telescope. These galaxies form avery heterogeneous class, whose spectra qualitatively resemble those ofhigh surface brightness galaxies covering the full range of spectra seenin galaxies of Hubble types from E to Irr. We use a combination ofemission-line (EW(Hα), [N II] λ6584/Hα) andabsorption-line (Mg b, Hβ, ) based diagnostics toinvestigate the star formation and chemical enrichment histories ofthese galaxies. These are diverse, with some galaxies having lowmetallicity and very young mean stellar ages and other galaxies showingold, supersolar-metallicity stellar populations. In contrast with someprevious studies, which found a strong trend of decreasing metallicitywith decreasing central surface brightness, we find a population ofgalaxies with low surface brightness and near-solar metallicity.Correlations between several of the gas-phase and stellar population ageand metallicity indicators are used to place constraints on plausibleevolutionary scenarios for LSB galaxies. The redshift range spanned bythese galaxies is broad, with radial velocities from 3400 kms-1 to more than 65,000 km s-1. A subset of thesample galaxies have published H I redshifts and gas masses based onobservations with the Arecibo 305 m single-dish radio telescope, whichplace these galaxies far off of the mean Tully-Fisher relation. Our newoptical redshifts do not agree with the published H I redshifts forthese galaxies. Most of the discrepancies can be explained by beamconfusion in the Arecibo observations, causing erroneous H I detectionsfor some of the galaxies.
| A Survey for Pulsating Hot B Subdwarfs in the Northern Hemisphere
We present the final results of a survey to search for pulsating hot Bsubdwarf (sdB) stars. We picked our candidates on the basis of theirvalues of Teff and logg, as estimated from availablequantitative optical spectroscopy. With the help of a three-channelphotometer, we gathered ``white-light'' light curves using a uniformsampling time of 10 s for a total of 74 stars. Our survey had a yield of~5.4%, leading to the discovery of detectable luminosity variations infour objects: PG 1047+003, KPD 2109+4401, KPD 1930+2752, and PG0014+067. For the light curves of the other 70 stars, we derive upperlimits on the amplitudes of possible periodic signals (in the periodwindow 20-1000 s) that range from ~0.05% of the mean brightness of thestar at an apparent magnitude of y~12 to ~0.20% at y~15. We use oursurvey results as well as those of others to discuss the location andextent of the empirical instability ``strip'' for pulsating sdB stars inthe logg-Teff diagram. We also address the question of thecoexistence of pulsators and nonvariable stars in the same region ofthat diagram. Based in part on observations gathered at theCanada-France-Hawaii Telescope, operated by the National ResearchCouncil of Canada, the Centre National de la Recherche Scientifique deFrance, and by the University of Hawaii.
| Evidence for Cluster Evolution from an Improved Measurement of the Velocity Dispersion and Morphological Fraction of Cluster 1324+3011 at z=0.76
We have carried out additional spectroscopic observations in the fieldof cluster Cl 1324+3011 at z=0.76. Combined with the spectroscopyrecently presented by Postman, Lubin, & Oke, we now havespectroscopically confirmed 47 cluster members. With this significantnumber of redshifts, we measure accurately the cluster velocitydispersion to be 1016+126-93 km s-1.The distribution of velocity offsets is consistent with a Gaussian,indicating no substantial velocity substructure. As previously noted forother optically selected clusters at redshifts of z>~0.5, acomparison between the X-ray luminosity (LX) and the velocitydispersion (σ) of Cl 1324+3011 implies that this cluster isunderluminous in X-rays by a factor of ~3-40 when compared with theLX-σ relation for local and moderate-redshift clusters.We also examine the morphologies of those cluster members that haveavailable high angular resolution imaging with the Hubble SpaceTelescope (HST). There are 22 spectroscopically confirmed clustermembers within the HST field of view. Twelve of these are visuallyclassified as early-type (elliptical or S0) galaxies, implying anearly-type fraction of 0.55+0.17-0.14 in thiscluster. This fraction is a factor of ~1.5 lower than that observed innearby rich clusters. Confirming previous cluster studies, the resultsfor cluster Cl 1324+3011, combined with morphological studies of othermassive clusters at redshifts of 0<=z<~1, suggest that the galaxypopulation in massive clusters is strongly evolving with redshift. Thisevolution implies that early-type galaxies are forming out of the excessof late-type (spiral, irregular, and peculiar) galaxies over the ~7 Gyrtimescale.
| Echelle observations of chromospheric activity in post-common-envelope binaries
We present high-resolution echelle spectroscopy that confirms, via theidentification of numerous chromospheric emission lines, the extremelyhigh levels of chromospheric activity displayed by the secondary starsin post-common-envelope binaries (PCEBs). Furthermore, the EG UMa dataset includes the first ever high-resolution spectroscopic observationsof flare activity in a PCEB system. The nature of the EG UMa spectra isinvestigated and a two-flare model that explains all of the observedemission-line features is identified. V-band photometry of EG UMaincludes a further flare event, but contains no evidence of pulsationsfrom the white dwarf primary star in this system. The extensivespectral-line emission observed in EG UMa is also observed in HZ 9. Thecriteria used to identify flare events in the EG UMa data is employed toclassify the HZ 9 spectrum as an observation of extremely high level,quiescent, chromospheric activity, and not arising from a flare event.PG 1026+002 and RE J1629+780 both display H Balmer and He ID3 line emission. The observed activity of the secondary starin PG 1026+002 appears, however, to be at a lower level than thatexhibited by the secondary stars in EG UMa, HZ 9 and RE J1629+780. It isnot clear why PG 1026+002 should be exhibiting a lower level ofactivity, nor is it possible to deduce whether this is a transientphenomenon. The only spectral features attributable to the secondarystar in GD 123 are weak Hα line emission and Na I doublet lineabsorption at 8183, 8195 Å. The He I D3 line is amagnetic activity indicator and its presence in all of the PCEB spectra,with exception of GD 123, is direct evidence for the maintenance ofmagnetic fields in all of these late-type, fully convective, dwarfsecondary stars.
| Deep Lyalpha imaging of two z = 2.04 GRB host galaxy fields
We report on the results of deep narrow-band Lyalpha and broad-band Uand I imaging of the fields of two Gamma-Ray bursts at redshift z=2.04(GRB 000301C and GRB 000926). We find that the host galaxy of GRB 000926is an extended (more than 2 arcsec), strong Lyalpha emitter with arest-frame equivalent width of 71+20-15 Å.The galaxy consists of two main components and several fainter knots.GRB 000926 occurred in the western component, whereas most of theLyalpha luminosity (about 65%) originates in the eastern component.Using archival HST images of the host galaxy we measure the spectralslopes (flambda ~ lambda beta ) of the twocomponents to beta = -2.4+/-0.3 (east) and -1.4+/-0.2 (west). Thisimplies that both components contain at most small amounts of dust,consistent with the observed strong Lyalpha emission. The westerncomponent has a slightly redder V-I colour than the eastern component,suggesting the presence of at least some dust. We do not detect the hostgalaxy of GRB 000301C in neither Lyalpha emission nor in U and Ibroad-band images. The strongest limit comes from combining the narrowand U-band imaging where we infer a limit of U(AB)>27.7 (2sigma limitper arcsec2). The upper limits on the Lyalpha flux implies aLyalpha equivalent width upper limit of ~ 150 Å. We find elevenand eight other galaxies with excess emission in the narrow filter inthe fields of GRB 000301C and GRB 000926 respectively. These galaxiesare candidate Lyalpha emitting galaxies in the environment of the hostgalaxies. Based on these detections we conclude that GRB 000926 occurredin one of the strongest centres of star formation within several Mpc,whereas GRB 000301C occurred in an intrinsically very faint galaxy farfrom being the strongest centre of star formation in its galacticenvironment. Under the hypothesis that GRBs trace star formation, thewide range of GRB host galaxy luminosities implies a very steep faintend slope of the high redshift galaxy luminosity function. Based onobservations made with the Nordic Optical Telescope, operated on theisland of La Palma jointly by Denmark, Finland, Iceland, Norway, andSweden. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. STScI is operated by the association of Universities forResearch in Astronomy, Inc. under the NASA contract NAS5-26555.
| The u'g'r'i'z' Standard-Star System
We present the 158 standard stars that define the u'g'r'i'z' photometricsystem. These stars form the basis for the photometric calibration ofthe Sloan Digital Sky Survey. The defining instrument system andfilters, the observing process, the reduction techniques, and thesoftware used to create the stellar network are all described. Webriefly discuss the history of the star selection process, thederivation of a set of transformation equations for theUBVRCIC system, and plans for future work.
| Spectrophotometric Standards from the Far-Ultraviolet to the Near-Infrared: STIS and NICMOS Fluxes
With photometric repeatability in the 0.2%-0.4% range and wavelengthcoverage from 1150 to 10,200 Å, the Space Telescope ImagingSpectrograph (STIS) low-dispersion spectrophotometry provides excellentabsolute flux standards. STIS observations of 19 stars are used toupdate the database of Hubble Space Telescope standard stars withabsolute fluxes that are based on the white dwarf pure hydrogen scale.Uncertainties in the STIS absolute flux calibration range from 4% in thefar-UV to 2% at longer wavelengths. The relative and absolute fluxes ofour composite standards in the IR are verified by the Near InfraredCamera and Multi-Object Spectrograph (NICMOS) observations and byground-based IR photometry.
| A spectrophotometric atlas of Narrow-Line Seyfert 1 galaxies
We have compiled a list of 83 objects classified as Narrow-Line Seyfert1 galaxies (NLS1s) or known to have a broad Balmer component narrowerthan 2 000 km s-1. Of these, 19 turned out to have beenspectroscopically misidentified in previous studies; only 64 of theselected objects are genuine NLS1s. We have spectroscopically observed59 of them and tried to characterize their Narrow and Broad-Line Regions(NLR and BLR) by fitting the emission-lines with Gaussian and/orLorentzian profiles. In most cases, the broad Balmer components are wellfitted by a single Lorentzian profile, confirming previous claims thatLorentzian rather than Gaussian profiles are better suited to reproducethe shape of the NLS1s broad emission lines. This has consequencesconcerning their FWHMs and line ratios: when the broad Balmer componentsare fitted with a Lorentzian, most narrow line regions have line ratiostypical of Seyfert 2s while, when a Gaussian profile is used for fittingthe broad Balmer components, the line ratios are widely scattered in theusual diagnostic diagrams (Veilleux & Osterbrock \cite{vei87});moreover, the FWHM of the best fitting Lorentzian is systematicallysmaller than the FWHM of the Gaussian. We find that, in general, the [OIII] lines have a relatively narrow Gaussian profile ( ~ 200-500 kms-1 FWHM) with often, in addition, a second broad ( ~ 500-1800 km s-1 FWHM), blueshifted Gaussian component. We do notconfirm that the [O III] lines are weak in NLS1s. As previouslysuggested, there is a continuous transition of all properties betweenNLS1s and classical Broad-Line Seyfert 1 Galaxies (BLS1s) and the limitof 2000 km s-1 used to separate the two species is arbitrary;R4570, the ratio of the Fe II to the Hβ fluxes, could bea physically more meaningful parameter to distinguish them.
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