Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Atmospheric Scintillation at Dome C, Antarctica: Implications for Photometryand Astrometry
We present low-resolution turbulence profiles of the atmosphere aboveDome C, Antarctica, measured with the MASS instrument during 25 nightsin 2004 March-May. Except for the lowest layer, Dome C has significantlyless turbulence than Cerro Tololo and Cerro Pachón. Inparticular, the integrated turbulence at 16 km is always less than themedian values at the two Chilean sites. From these profiles we evaluatethe photometric noise produced by scintillation, and the atmosphericcontribution to the error budget in narrow-angle differentialastrometry. In comparison with the two midlatitude sites in Chile, DomeC offers a potential gain of about 3.6 in both photometric precision(for long integrations) and narrow-angle astrometry precision. Thesegain estimates are preliminary, being computed with average wind-speedprofiles, but the validity of our approach is confirmed by independentdata. Although the data from Dome C cover a fairly limited time frame,they lend strong support to expectations that Dome C will offersignificant advantages for photometric and astrometric studies.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| Deuterium Depletion and Magnesium Enhancement in the Local Disk
The local disk deuterium is known to be depleted in comparison to thelocal bubble. We used Hubble Space Telescope (HST) spectra to obtaincolumn densities of Si, Mg and Fe. We compared normalized columndensities of these elements in the directions with high and lowdeuterium abundances.We show, that the lines of sight that are depleted in deuterium, areenhanced in magnesium. This observation implicates that astration isresponsible for both deuterium depletion and magnesium enhancement.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Lithium abundances of the local thin disc stars
Lithium abundances are presented for a sample of 181 nearby F and Gdwarfs with accurate Hipparcos parallaxes. The stars are on circularorbits about the Galactic centre and, hence, are identified as belongingto the thin disc. This sample is combined with two published surveys toprovide a catalogue of lithium abundances, metallicities ([Fe/H]),masses, and ages for 451 F-G dwarfs, almost all belonging to the thindisc. The lithium abundances are compared and contrasted with publishedlithium abundances for F and G stars in local open clusters. The fieldstars span a larger range in [Fe/H] than the clusters for which [Fe/H]~=0.0 +/- 0.2. The initial (i.e. interstellar) lithium abundance of thesolar neighbourhood, as derived from stars for which astration oflithium is believed to be unimportant, is traced from logɛ(Li) =2.2 at [Fe/H]=-1 to logɛ(Li) = 3.2 at +0.1. This form for theevolution is dependent on the assumption that astration of lithium isnegligible for the stars defining the relation. An argument is advancedthat this latter assumption may not be entirely correct, and, theevolution of lithium with [Fe/H] may be flatter than previouslysupposed. A sharp Hyades-like Li dip is not seen among the field starsand appears to be replaced by a large spread among lithium abundances ofstars more massive than the lower mass limit of the dip. Astration oflithium by stars of masses too low to participate in the Li dip isdiscussed. These stars show little to no spread in lithium abundance ata given [Fe/H] and mass.
| The Structure of the Local Interstellar Medium. III. Temperature and Turbulence
We present 50 individual measurements of the gas temperature andturbulent velocity in the local interstellar medium (LISM) within 100pc. By comparing the absorption line widths of many ions with differentatomic masses, we can satisfactorily discriminate between the twodominant broadening mechanisms, thermal broadening and macroscopicnonthermal, or turbulent, broadening. We find that the successful use ofthis technique requires a measurement of a light ion, such as D I, andan ion at least as heavy as Mg II. However, observations of more linesprovide an important consistency check and can also improve theprecision and accuracy of the measurement. Temperature and turbulentvelocity measurements are vital to understanding the physical propertiesof the gas in our local environment and can provide insight into thethree-dimensional morphological structure of the LISM. The weighted meangas temperature in the LISM warm clouds is 6680 K and the dispersionabout the mean is 1490 K. The weighted mean turbulent velocity is 2.24km s-1 and the dispersion about the mean is 1.03 kms-1. The ratio of the mean thermal pressure to the meanturbulent pressure is PT/Pξ~26. Turbulentpressure in LISM clouds cannot explain the difference in the apparentpressure imbalance between warm LISM clouds and the surrounding hot gasof the Local Bubble. Pressure equilibrium among the warm clouds may bethe source of a moderately negative correlation between temperature andturbulent velocity in these clouds. However, significant variations intemperature and turbulent velocity are observed. The turbulent motionsin the warm partially ionized clouds of the LISM are definitelysubsonic, and the weighted mean turbulent Mach number for clouds in theLISM is 0.19 with a dispersion of 0.11. These measurements provideimportant constraints on models of the evolution and origin of warmpartially ionized clouds in our local environment.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 AR-09525.01A. Theseobservations are associated with program 9525.
| 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.
| The Structure of the Local Interstellar Medium. II. Observations of D I, C II, N I, O I, Al II, and Si II toward Stars within 100 Parsecs
Moderate- and high-resolution measurements(λ/Δλ>~40,000) of interstellar resonance lines ofD I, C II, N I, O I, Al II, and Si II (hereafter called light ions) arepresented for all available observed targets located within 100 pc thatalso have high-resolution observations of interstellar Fe II or Mg II(heavy ions) lines. All spectra were obtained with the Goddard HighResolution Spectrograph or the Space Telescope Imaging Spectrographinstrument aboard the Hubble Space Telescope. Currently, there are 41sight lines to targets within 100 pc with observations that include aheavy ion at high resolution and at least one light ion at moderate orhigh resolution. We present new measurements of light ions along 33 ofthese sight lines and collect from the literature results for theremaining sight lines that have already been analyzed. For all of thenew observations we provide measurements of the central velocity,Doppler width parameter, and column density for each absorptioncomponent. We greatly increase the number of sight lines with usefullocal interstellar medium (LISM) absorption-line measurements of lightions by using knowledge of the kinematic structure along a line of sightobtained from high-resolution observations of intrinsically narrowabsorption lines, such as Fe II and Mg II. We successfully fit theabsorption lines with this technique, even with moderate-resolutionspectra. Because high-resolution observations of heavy ions are criticalfor understanding the kinematic structure of local absorbers along theline of sight, we include 18 new measurements of Fe II and Mg II in anAppendix. We present a statistical analysis of the LISM absorptionmeasurements, which provides an overview of some physicalcharacteristics of warm clouds in the LISM, including temperature andturbulent velocity. This complete collection and reduction of allavailable LISM absorption measurements provides an important databasefor studying the structure of nearby warm clouds, including ionization,abundances, and depletions. Subsequent papers will present models forthe morphology and physical properties of individual structures (clouds)in the LISM.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 AR-09525.01A. Theseobservations are associated with program 9525.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| The Nature of Overluminous F Stars Observed in a Radial-Velocity Survey
We have conducted a radial-velocity survey of bright (V<9),overluminous, F stars within 80 pc. The criterion of overluminosity isΔMc0=Mc0-MV>=0.5,where MV is the absolute magnitude based on the Hipparcosparallax and Mc0 is the absolute magnitude fromuvby photometry. For F stars, overluminosity at a level ofΔMc0>0.15 has been argued to beindicative of the presence of a bright companion, so many of theoverluminous ``single'' F stars are expected to be, in fact, binarieswith comparably bright components. Therefore, the main goals of thesurvey have been to gain insight into the nature of overluminous F starsand to test specifically the hypothesis of the duplicity of overluminousF stars by searching for previously unrecognized binary stars amongthem. Other goals of this project included the determination, whereverpossible, of the orbits of the binaries discovered as a result of thesurvey and comparing the properties of the binaries with those of thevisually single stars with constant radial velocity. The program listcomprised 111 stars that appear in the Hipparcos catalog as single, plusseven members of multiple systems whose components have individualentries in the catalog.Of the total of 118 stars selected, the radial velocities of 77 havebeen measured with the Cambridge CORAVEL in the present survey. Amongthe remainder, six have not allowed us to determine their radialvelocities because of the weakness or lack of a ``dip'' in theirradial-velocity traces, and one was optically inaccessible to thetelescope. All the rest proved to be sufficiently well known alreadyeither as binary or as constant-radial-velocity (CRV) stars. The surveydiscovered 25 new binary systems, not counting two that were notgenerally known as such but were already under observation by R. F. G.and two that were discovered by Hipparcos to show eclipses but had neverbeen recognized as SBs. With the previously known binaries, there is atotal of 61 definite plus three probable binaries out of a total of 111stars upon which we can adjudicate-a binary fraction of ~58%, justdouble the ~29% found in a sample of randomly selected F stars byNordström et al. (1997). Of the binaries, 52 are SB2 and 12 areSB1. Orbits (some of them still rather preliminary) have been determinedfor the first time for 28 binaries, 21 of which are SB2.Comparison of the CRV stars (a total of 47) with the SB2 binaries hasshown that the two groups have different kinematics. The differenceimplies that on average the CRV stars are 3 to 4 Gyr older than theSB2s, although several of the CRVs appear to be very young. At the sametime the SB2 stars turn out to have about the same kinematics, hence thesame average age, as both binary and normal single F stars from acontrol sample drawn from the Hipparcos catalog. The much older age ofthe bulk of the overluminous CRV F stars may be indicative of anomalousstellar evolution that results in anomalously long main-sequencelifetimes for those stars. The SB1 stars appear to be closer to the CRVsthan to the SB2s and are probably also very old. The properties of the``no-dip'' stars suggest very young age; some of the stars are probablypre-main-sequence (PMS). A few PMS candidates have been identified alsoamong the CRV stars. Thus, overluminous F stars comprising our sampleappear to consist of three distinct groups: mostly young SB2 binarieswith comparably bright components, mostly very old CRV and SB1 stars,and very young no-dip stars.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Elemental Abundances and Mass Densities of Dust and Gas in the Local Interstellar Cloud
Observationally derived gas-phase abundances and appropriate assumptionsfor the total elemental abundances of dust and gas determine theelemental composition of dust and the elemental depletion from gas inthe interstellar medium (ISM). In addition to the elemental abundances,the total mass ratio of hydrogen atoms to dust grains per spatial volumeis a measure of the interaction between dust and gas in the ISM. Recentremote astronomical observations and in situ measurements provide theopportunity of estimating the elemental abundances and the hydrogengas-to-dust mass ratio of the Local Interstellar Cloud (LIC), in whichthe Sun is currently embedded. We show that the composition of dust inthe LIC is similar to that of cometary dust in the solar system,although the nitrogen abundance remains uncertain. Depletions ofelements from the LIC gas are consistent with measurements of warmneutral clouds in the Galactic disk, except for Mg and Si, which areheavily depleted in the LIC. Remote astronomical observations and insitu measurements give essentially the same value for the gas-to-dustmass ratio of the LIC, which is comparable to the average value of thediffuse ISM in the Galaxy. This indicates the association of dust withgas in the LIC, which is also inferred from the depletion pattern in theLIC. Neither the depletions of elements nor the gas-to-dust mass ratioshow evidence for severe grain destruction that would result from shockswith velocity ~1.5×107 cm s-1 as suggestedby a model that postulates the LIC to be a fragment of the expandingLoop I superbubble shell. Our results rather favor an alternative modelthat describes the origin of the LIC as being one of H I cloudletsexpelled from the interaction zone between the Loop I superbubble andthe Local Bubble, which encloses the LIC and similar clouds in the solarneighborhood.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars
We obtained high-resolution spectroscopic observations of 34 γDoradus candidates. From the red-wavelength spectra, we determinedspectral classes, radial velocities, and projected rotationalvelocities. The spectra of seven late A or early F stars show metalliclines that have composite profiles consisting of a narrow component nearthe center of a broad line, indicating that they may be shell stars orbinaries. Several stars, including HD 152896, HD 173977, HD 175337, andHD 195068/9, show large line profile asymmetries. Two stars, HD 11443(=α Trianguli) and HD 149420, are ellipsoidal variables and notγ Doradus stars. The percentage of binary systems in our samplemay be as high as 74%.
| Lithium and rotation in F and G dwarfs and subgiants
Lithium abundances have been determined in 127 F and G Pop I stars basedon new measurements of the equivalent width of the lambda 6707 ÅLi I line from their high resolution CCD spectra. Distances and absolutemagnitudes of these stars have been obtained from the HipparcosCatalogue and their masses and ages derived, enabling us to investigatethe behaviour of lithium as a function of these parameters. Based ontheir location on the HR diagram superposed on theoretical evolutionarytracks, the sample of the stars has been chosen to ensure that they havemore or less completed their Li depletion on the main sequence. A largespread in the Li abundances is found at any given effective temperatureespecially in the already spun down late F and early G stars. Thisspread persists even if the ``Li-dip'' stars that have evolved from themain sequence temperature interval 6500-6800 K are excluded. Stars inthe mass range up to 2 M/Msun when divided into threemetallicity groups show a linear correlation between Li abundance andmass, albeit with a large dispersion around it which is not fullyaccounted for by age either. The large depletions and the observedspread in Li are in contrast to the predictions of the standard stellarmodel calculations and suggest that they are aided by non-standardprocesses depending upon variables besides mass, age and metallicity.The present study was undertaken to examine, in particular, the effectsof rotation on the depletion of Li. No one-to-one correlation is foundbetween the Li abundance and the present projected rotational velocity.Instead the observed abundances seem to be dictated by the rotationalhistory of the star. However, it is noted that even this interpretationis subject to the inherent limitation in the measurement of the observedLi EQW for large rotational velocities.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/251
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Selection criteria for targets of asteroseismic campaigns
Various dedicated satellite projects are underway or in advanced stagesof planning to perform high-precision, long duration time seriesphotometry of stars, with the purpose of using the frequencies ofstellar oscillations to put new constraints on the internal structure ofstars. It is known (cf. \cite{Bro+94}) that the effectiveness ofoscillation frequencies in constraining stellar model parameters issignificantly higher if classical parameters such as effectivetemperature and luminosity are known with high precision. In order tooptimize asteroseismic campaigns it is therefore useful to selecttargets from among candidates for which good spectroscopic andastrometric data already exists. This paper presents selection criteria,as well as redeterminations of stellar luminosity and reddening forstars satisfying these criteria.
| The Structure of the Local Interstellar Medium. I. High-Resolution Observations of Fe II, Mg II, and Ca II toward Stars within 100 Parsecs
High-resolution absorption measurements(λ/Δλ>~100,000) of the resonance lines of Fe II,Mg II, and Ca II are presented for all available observed targets within100 pc. The Fe II and Mg II spectra were obtained with the Goddard HighResolution Spectrograph (GHRS) and the Space Telescope ImagingSpectrograph (STIS) instruments aboard the Hubble Space Telescope (HST).Of the 63 observations of targets within 100 pc, we present newmeasurements for 24 lines of sight. We also compiled all publishedabsorption measurements based on Ca II spectra obtained by variousground-based instruments. For each observation we provide measurementsof the central velocity, Doppler parameter, and column density for eachabsorption component. These three ions provide the best opportunity tomeasure the component velocity structure. Because these are the heaviestions observed in absorption through the warm local interstellar medium(LISM), the narrow line widths minimize significant blending ofcomponents and allow for accurate measurements of the central velocity.We present a statistical analysis of the LISM absorption measurements,which provides an overview of some physical characteristics of warmclouds in the LISM, such as, temperature, turbulent velocity, ionizationdegree, and depletion. The complete collection and reduction of all LISMabsorption measurements provides an important database for studying thestructure of nearby warm clouds. Subsequent papers will present modelsfor the morphology and physical properties of individual structures(clouds) in the LISM.
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Microstructure of the Local Interstellar Cloud and the Identification of the Hyades Cloud
We analyze high-resolution UV spectra of the Mg II h and k lines for 18members of the Hyades to study inhomogeneity along these proximate linesof sight. The observations were taken by the Space Telescope ImagingSpectrograph (STIS) instrument on board the Hubble Space Telescope(HST). Three distinct velocity components are observed. All 18 lines ofsight show absorption by the Local Interstellar Cloud (LIC); 10 starsshow absorption by an additional cloud, which we name the Hyades Cloud;and one star exhibits a third absorption component. The LIC absorptionis observed at a lower radial velocity than predicted by the LICvelocity vector derived by Lallement & Bertin in 1992 and Lallementet al. in 1995[vpredicted(LIC)-vobserved(LIC)=2.9+/-0.7 kms-1], which may indicate a compression or deceleration at theleading edge of the LIC. We propose an extension of the Hyades Cloudboundary based on previous HST observations of other stars in thegeneral vicinity of the Hyades, as well as ground-based Ca IIobservations. We present our fits of the interstellar parameters foreach absorption component. The availability of 18 similar lines of sightprovides an excellent opportunity to study the inhomogeneity of thewarm, partially ionized local interstellar medium (LISM). We find thatthese structures are roughly homogeneous. The measured Mg II columndensities do not vary by more than a factor of 2 for angular separationsof <~8°, which at the outer edge of the LIC correspond tophysical separations of <~0.6 pc.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| The Three-dimensional Structure of the Warm Local Interstellar Medium. II. The Colorado Model of the Local Interstellar Cloud
In this second paper in a series on the structure of the localinterstellar medium (LISM), we construct a three-dimensional model ofthe local interstellar cloud (LIC) based on Hubble Space Telescope(HST), Extreme Ultraviolet Explorer (EUVE), and ground-based Ca IIspectra. Starting with hydrogen column densities derived from deuteriumcolumn densities measured with the Goddard High Resolution Spectrographinstrument on HST for 16 lines of sight to nearby stars, we derive amodel consisting of the sum of nine spherical harmonics that best fitthe data. We then rederive the model by including the lines of sight tothree hot white dwarfs observed by EUVE and 13 lines of sight with Ca IIcolumn densities at the projected LIC velocity. The LIC model is clearlynot a long thin filamentary structure like optical images of someinterstellar clouds (e.g., reflection nebulae in the Pleiades), butneither is it spherical in shape. As seen from the north Galactic pole,the LIC is egg-shaped with an axis of symmetry that points in thedirection l~315deg. Since the direction of the center of theScorpius-Centaurus association is l=320deg, the shape of theLIC could be determined by the flow of hot gas from Sco-Cen. The modelshows that the Sun is located just inside the LIC in the direction ofthe Galactic center and toward the north Galactic pole. The absence ofMg II absorption at the LIC velocity toward α Cen indicates thatthe distance to the edge of the LIC in this direction is <=0.05 pcand the Sun should cross the boundary between the LIC and the Galactic(G) cloud in less than 3000 yr. We estimate that the volume of the LICis about 93 pc3 and its mass is about 0.32 Msolar.The physical parameters and hydrogen column density of the LIC areroughly consistent with theoretical models of the warm interstellarmedium that assume pressure and ionization equilibrium. However, theempirical hydrogen ionization of the LIC is much higher and the gastemperature lower than the theoretical models predict. Therefore, theionization is likely due to shock activity from a nearby supernova thathas not yet reached equilibrium. The higher ionization increases the gascooling, which can explain why the gas is 2400 K cooler than theionization equilibrium models predict. Computed and observedtemperatures are in agreement for a model with the observed LIC electrondensity. Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| The Three-dimensional Structure of the Warm Local Interstellar Medium. I. Methodology
In this first in a series of papers, we develop a methodology forconstructing three-dimensional models of the local interstellar cloud(LIC) and adjacent warm clouds in the local interstellar medium (LISM).Our models are based on the column density of neutral hydrogen gas(NHI) inferred primarily from measurements of the deuteriumcolumn density toward nearby stars obtained from the analysis of HubbleSpace Telescope ultraviolet spectra. We also use values ofNHI inferred from spectra of hot white dwarfs and B-typestars obtained by the Extreme Ultraviolet Explorer satellite. These verydifferent methods give consistent results for the three white dwarfstars in common. We assume that along each line of sight allinterstellar gas moving with a speed consistent with the LIC velocityvector has a constant density, NHI=0.10 cm-3, andextends from the heliosphere to an edge determined by the value ofNHI moving at this speed. A number of stars have velocitiesand/or depletions that indicate absorption by other warm clouds in theirlines of sight. On this basis α Cen A and B and probably alsoɛ Ind lie inside the Galactic center (G) cloud, HZ 43 and 31 Comlie inside what we call the north Galactic pole cloud, and β Cet islocated inside what we call the south Galactic pole cloud. We show thelocations of these clouds in Galactic coordinates. The Sun is locatedvery close to the edge of the LIC toward the Galactic center and thenorth Galactic pole. The absence of Mg II absorption at the LIC velocitytoward α Cen indicates that the distance to the edge of the LIC inthis direction is <=0.05 pc and the Sun should leave the LIC and perhapsenter the G cloud in less than 3000 yr. Comparison of LIC and totalvalues of NHI toward pairs of stars with separations between0.9d and 20 deg reveals a pattern of good agreement so long as bothstars lie within 60 pc of the Sun. Thus the LIC and perhaps also othernearby warm clouds have shapes that are smooth on these angular scales.In our second paper we will therefore fit the shape of the LIC with aset of smooth basis functions (spherical harmonics).
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
|
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: | Triangulum |
| Ascensió Recta: | 01h53m04.90s |
| Declinació: | +29°34'44.0" |
| Magnitud Aparent: | 3.41 |
| Distancia: | 19.658 parsecs |
| Moviment propi RA: | 12 |
| Moviment propi Dec: | -234.9 |
| B-T magnitude: | 3.984 |
| V-T magnitude: | 3.457 |
Catàlegs i designacions:
|
