Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| The Structure of the Homunculus. I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of η Carinae
High-resolution long-slit spectra obtained with the Phoenix spectrographon Gemini South provide our most accurate probe of the three-dimensionalstructure of the Homunculus Nebula around η Carinae. The newnear-infrared spectra dramatically confirm the double-shell structureinferred previously from thermal dust emission, resolving the nebulainto a very thin outer shell seen in H2 and a warmer, thickerinner layer seen in [Fe II]. The remarkably thin and uniform H2 skin hints that the most important mass loss during the 19thcentury eruption had a very short duration of <~5 yr. H2emission traces the majority of the more than 10 Msolar ofmaterial in the nebula and has an average density of ordernH>~106.5 cm-3. This emission, inturn, yields our first definitive picture of the exact shape of thenebula, plus a distance of 2350+/-50 pc and an inclination angle of~41°. The distribution of the H2 emission provides thefirst measure of the latitude dependence of the speed, mass loss, andkinetic energy associated with η Car's 19th century explosion.Almost 75% of the total mass and more than 90% of the kinetic energy inthe ejecta were released at high latitudes between 45° and the polaraxis. This rules out a model for the bipolar shape in which an otherwisespherical explosion was pinched at the waist by a circumstellar torus.Instead, most of the mass appears to have been directed poleward by theexplosion itself. H2 emission also provides our firstreliable picture of the critical innermost waist of the Homunculus,yielding clues to the observed morphology of the core and the moreextended equatorial debris.Based on observations obtained at the Gemini Observatory, which isoperated by AURA, Inc., under a cooperative agreement with the NSF onbehalf of the Gemini partnership: the National Science Foundation (USA),the Particle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Dust scattering in the Miras R Car and RR Sco resolved by optical interferometric polarimetry
We present optical interferometric polarimetry measurements of theMira-like variables R Car and RR Sco, using the Sydney UniversityStellar Interferometer. By making visibility measurements in twoperpendicular polarizations, the relatively low-surface brightness lightscattered by atmospheric dust could be spatially separated from thebright Mira photospheric flux. This is the first reported successful useof long-baseline optical interferometric polarimetry. Observations wereable to place constraints on the distribution of circumstellar materialin R Car and RR Sco. The inner radius of dust formation for both starswas found to be less than 3 stellar radii: much closer than the expectedinnermost stable location for commonly assumed astrophysical `dirtysilicate' dust in these systems (silicate dust with a significant ironcontent). A model with the dust distributed over a shell which isgeometrically thin compared to the stellar radius was preferred over anoutflow. We propose dust components whose chemistry and opacityproperties enable survival at these extreme inner radii.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| 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
| The systemic velocity of Eta Carinae*
High-resolution spectra of molecular hydrogen in the Homunculus nebulaallow for the first direct measurement of the systemic velocity of ηCarinae. Near-infrared long-slit data for H2 1-0 S(1)λ21218 obtained with the Phoenix spectrometer on the Gemini Southtelescope give Vsys=-8.1 +/- 1 km s-1(heliocentric), or VLSR=-19.7 +/- 1 km s-1, fromthe average of the near and far sides of the Homunculus. Thismeasurement considerably improves the precision for the value of -7 +/-10 km s-1 inferred from neighbouring O-type stars in theCarina nebula. New near-infrared spectra also provide a high-resolutionline profile of [FeII]λ16435 emission from gas condensationsknown as the Weigelt objects without contamination from the centralstar, revealing a line shape with complex kinematic structure.Previously, uncertainty in the kinematics of the Weigelt knots wasdominated by the adopted systemic velocity of η Car.
| The physical properties of normal A stars
Designating a star as of A-type is a result of spectral classification.After separating the peculiar stars from those deemed to be normal usingthe results of a century of stellar astrophysical wisdom, I define thephysical properties of the "normal" stars. The hotter A stars haveatmospheres almost in radiative equilibrium. In the A stars convectivemotions can be found which increase in strength as the temperaturedecreases.
| First Look at the Fomalhaut Debris Disk with the Spitzer Space Telescope
We present Spitzer Space Telescope early release observations ofFomalhaut, a nearby A-type star with dusty circumstellar debris. Thedisk is spatially resolved at 24, 70, and 160 μm using the MultibandImaging Photometer for Spitzer (MIPS). While the disk orientation andouter radius are comparable to values measured in the submillimeter, thedisk inner radius cannot be precisely defined: the central hole in thesubmillimeter ring is at least partially filled with emission from warmdust, seen in Spitzer Infrared Spectrograph (IRS) 17.5-34 μm spectraand MIPS 24 μm images. The disk surface brightness becomesincreasingly asymmetric toward shorter wavelengths, with thesouth-southeast ansa always brighter than the north-northwest one. Thisasymmetry may reflect perturbations on the disk by an unseen interiorplanet.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| 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.
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/413/1037
| 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.
| High resolution spectroscopy of circumstellar material around A stars
We have analysed a time series of high resolution spectra (R=217 000) ofthe CaII K line of 9 stars which are candidates for the presence ofcircumstellar material from our previous studies. We have searched forvariable narrow absorption components similar to those extensivelystudied in the case of beta Pictoris. Our data show long-termvariations in the spectra of HR 2550 and HR 3685 which can be attributedto the dynamics of circumstellar gas. About one third of the samplestars show variable line absorption but only beta Pictoris seems toexhibit uniquely strong variations on short (nightly) timescales. Inorder to examine possible interstellar contributions we compared ourresults with interstellar data from the literature. The column densitiesof our absorption features are up to three orders of magnitude higherthan those found for the Local Interstellar Cloud.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| 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 Ionization of Nearby Interstellar Gas
We present new calculations of the photoionization of interstellarmatter within ~5 pc of the Sun (which we refer to as the ``complex oflocal interstellar clouds,'' or CLIC) by directly observed radiationsources, including nearby hot stars and the diffuse emission of the softX-ray background (SXRB). In addition, we model the important, unobservedextreme-ultraviolet (EUV) emission both from the hot gas responsible forthe SXRB and from a possible evaporative boundary between the CLIC andthe hot gas. We carry out radiative transfer calculations and show thatthese radiation sources can provide the ionization and heating of thecloud required to match a variety of observations. The ionizationpredicted in our models shows good agreement with pickup ion results,interstellar absorption-line data toward ɛ CMa, and EUV opacitymeasurements of nearby white dwarf stars. Including the radiation fromthe conductive boundary improves agreement with data on the temperatureand electron density in the cloud. The presence of dust in the cloud, orat least depleted abundances, is necessary to maintain theheating/cooling balance and reach the observed temperature. Using thecolumn density observations as inputs, we derive the gas-phaseabundances of C, N, O, Mg, Si, S, and Fe. Based on these inferreddepletions, it appears that silicate and iron dust exists in the CLIC,while carbonaceous dust has been destroyed. In addition, we findevidence that the Ne abundance in the CLIC is larger than solar.
| 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.
| An Empirical Procedure to Estimate Distances to Stellar Clusters
A most desirable feature of a standard candle to estimate astronomicaldistances is robustness against changes in metallicity and age. It isargued that the radii of main-sequence stars with spectral types fromsolar to A0 show predictable changes with metallicity and detectablechanges with evolution. Such stars populate the solar neighborhood andtherefore benefit from measurements of angular diameters. Also, reliabledeterminations of their masses and radii are available from observationsof eclipsing binaries. Three empirical relationships are defined andsuggested for estimating distances to dwarfs from only BVK photometry.Comparison with Hipparcos trigonometric parallaxes shows that the methodprovides errors of about 15% for a particular star, which can be reducedto roughly 1.5% when applied to young clusters (age <~1-2 Gyr) with~100 stars of the appropriate spectral types. If reddening is unknown,main-sequence stars with effective temperatures close to 8000 K canconstrain it, although an estimate of R≡A(V)/E(B-V) isrequired.
| 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
| Incidence and survival of remnant disks around main-sequence stars
We present photometric ISO 60 and 170 μm measurements, complementedby some IRAS data at 60 μm, of a sample of 84 nearby main-sequencestars of spectral class A, F, G and K in order to determine theincidence of dust disks around such main-sequence stars. Fifty starswere detected at 60 μm. 36 of these emit a flux expected from theirphotosphere while 14 emit significantly more. The excess emission weattribute to a circumstellar disk like the ones around Vega and betaPictoris. Thirty four stars were not detected at all; the expectedphotospheric flux, however, is so close to the detection limit that thestars cannot have an excess stronger than the photospheric flux densityat 60 μm. Of the stars younger than 400 Myr one in two has a disk;for the older stars this is true for only one in ten. We conclude thatmost stars arrive on the main sequence surrounded by a disk; this diskthen decays in about 400 Myr. Because (i) the dust particles disappearand must be replenished on a much shorter time scale and (ii) thecollision of planetesimals is a good source of new dust, we suggest thatthe rapid decay of the disks is caused by the destruction and escape ofplanetesimals. We suggest that the dissipation of the disk is related tothe heavy bombardment phase in our Solar System. Whether all starsarrive on the main sequence surrounded by a disk cannot be established:some very young stars do not have a disk. And not all stars destroytheir disk in a similar way: some stars as old as the Sun still havesignificant disks. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA. Tables 2, 3 and 4 are also available in electronicform at the CDS via anonymous ftp cdsarc.u-strasbg.fr (130.79.128.5)} orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/365/545
| 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 excitation and kinematical properties of H2 and [Feii] in the HH 46/47 bipolar outflow
Long-slit spectra of the molecular outflow Herbig-Haro (HH) 46/47 havebeen taken in the J and K near-infrared bands. The observedH2 line emission confirms the existence of a bright andextended redshifted counter-jet outflow south-west of HH 46. In contrastwith the optical appearance of this object, we show that this outflowseems to be composed of two different emission regions characterized bydistinct heliocentric velocities. This implies an acceleration of thecounter-jet. The observed [Feii] emission suggests an average extinctionof 7-9 visual magnitudes for the region associated with the counter-jet.Through position-velocity diagrams, we show the existence of differentmorphologies for the H2 and [Feii] emission regions in thenorthern part of the HH 46/47 outflow. We have detected for the firsttime high-velocity (-250kms-1) [Feii] emission in the regionbridging HH 46 to HH 47A. The two strong peaks detected can beidentified with the optical positions B8 and HH 47B. The H2excitation diagrams for the counter-jet shock suggest an excitationtemperature for the gas of Tex~2600K. The lack of emissionfrom the higher energy H2 lines, such as the 4-3 S(3)transition, suggests a thermal excitation scenario for the origin of theobserved emission. Comparison of the H2 line ratios withvarious shock models yielded useful constraints about the geometry andtype of these shocks. Planar shocks can be ruled out whereas curved orbow shocks (both J- and C-type) can be parametrized to fit our data.
| The intermediate-band approach to the surface-brightness method for Cepheid radii and distance determination
The surface-brightness parameter Fν is calibrated in termsof the Strömgren intermediate-band colour b-y. The relationFν-(b-y)o valid for Cepheids is calibratedusing accurate near-infrared radii and distances for selected Cepheids.We have obtained uvby photometry for non-Cepheid giant and supergiantstars with known angular diameters and compared the slope and zero-pointof their Fν-(b-y)o relation with the Cepheidcalibration. We found that the two calibrations are significantlydifferent. The theoretical models lie in between the two calibrations.It is remarked that Fν-colour relations derived fromnon-Cepheids and involving blue colours (e.g. B-V or b-y) are notapplicable to Cepheids, while those involving redder colours (e.g. V-R,V-K or V-J) also produce good radii for Cepheids. Selected Cepheids ascalibrators lead to the accurate relationFν=3.898(+/-0.003)-0.378(+/-0.006)(b-y)o, whichallowed the calculation of radii and distances for a sample of 59Galactic Cepheids. The uncertainties in the zero-point and slope of theabove relation are similar to those obtained from near-infrared colours,and determine the accuracies in radii and distance calculations. Whileinfrared light and colour curves for Cepheids may be superior inprecision, the intermediate-band b-y colour allows the recovery of meanradii with an accuracy comparable to those obtained from the infraredsolutions. The derived distances are consistent within the uncertaintieswith those predicted by a widely accepted period-luminosityrelationship. Likewise, the resulting period-radius relation from theintermediate-band approach is in better agreement with infrared versionsthan with optical versions of this law. It is highlighted that theintermediate-band calibration of the surface-brightness method in thiswork is of comparable accuracy to the near-infrared calibrations. Thepresent results stress the virtues of uvby in determining the physicalparameters of supergiant stars of intermediate temperature.
| In-Flight Calibration of the ROSAT HRI Ultraviolet Sensitivity
Comparing measured and estimated count rates of a few selected samplestars, we confirm the validity and provide the in-flight calibration ofthe ROSAT HRI UV/visible effective area model in Zombeck et al. Thecount rate estimates for Betelgeuse derived with this model are inagreement with the measured HRI upper limit. This result is alsoconfirmed in an erratum by Berghöfer et al. aimed at revising theirprevious calculation, which was overestimated by more than 2 orders ofmagnitude. Adopting this ROSAT HRI UV/visible effective area model andmeasured UV/visible spectra of a set of sample stars covering the rangeof Teff 3000-40,000 K, we have built the calibration curvesto estimate UV/visible contamination count rates for any star of knownTeff, mv, and NH.
| The Disk of β Pictoris in the Light of Polarimetric Data
We model the linear polarization of the radiation of β Picscattered by dust particles in the circumstellar disk. The observedspatial distribution and the wavelength dependence of the polarizationtogether with the colors of the β Pic disk require that particlesin a wide size range be present in the disk, with the grains smallerthan a few microns in size being somewhat depleted but still ofimportance for the polarization and colors. The inferred sizedistribution is consistent with the production and loss mechanisms: thesources-presumably collisions and evaporation of largebodies-continuously produce dust with a power-law size distribution withthe exponent ~3.5 over a broad range of sizes, but the particles smallerthan a few microns are blown away by the radiation pressure, whichshortens the time they spend in the disk and decreases their numberdensities. Compact (or slightly porous) silicates are found to givebetter agreement with the observations, although other materials arestill not ruled out and a high fluffiness of the large particles ispossible. The observed asymmetry in the polarization of two wings can beexplained if more small grains (by 20%-30%) are present on the northeastside of the disk. We show that such an asymmetry in the sizedistributions in two wings might be caused by an influence of theinterstellar medium; a required amount of small grains could be producedby destructive collisions of interstellar grains with the circumstellardust particles.
| Empirical Luminosities and Radii of Early-Type Stars after Hipparcos
Using Hipparcos parallaxes we derive empirical luminosities and radii ofthe early-type stars for which the effective temperatures are known fromdirectly measured angular diameters and total absolute fluxes. Theempirical luminosities allow a direct comparison of the position ofthese stars in the fundamental HR diagram with evolutionary tracks. Thecomparison shows an overall agreement with the Y=0.30 and Z=0.02 trackscomputed with OPAL opacities and moderate amount of overshooting fromthe convective core. In addition, we present evidence that systematicerrors of the masses read off the evolutionary tracks are below 10%.Consequently, the surface gravities obtained from these "evolutionary"masses and the empirical radii are very nearly model-independent.Spectrographic and photometric observations of these stars can thereforebe used for verifying model atmospheres and calibrating photometric logg indices.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
