Contents
Images
Upload your image
DSS Images Other Images
Related articles
Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| A search for magnetic fields in the variable HgMn star α Andromedae
Context: .The chemically peculiar HgMn stars are a class of Bp starswhich have historically been found to be both non-magnetic andnon-variable. Remarkably, it has recently been demonstrated that thebright, well-studied HgMn star α And exhibits clear Hg ii lineprofile variations indicative of a non-uniform surface distribution ofthis element. Aims: .With this work, we have conducted anextensive search for magnetic fields in the photosphere of αAnd. Methods: .We have acquired new circular polarisation spectrawith the MuSiCoS and ESPaDOnS spectropolarimeters. We have also obtainedFORS1 circular polarisation spectra from the ESO Archive, and consideredall previously published magnetic data. This extensive dataset has beenused to systematically test for the presence of magnetic fields in thephotosphere of α And. We have also examined the high-resolutionspectra for line profile variability. Results: .The polarimetricand magnetic data provide no convincing evidence for photosphericmagnetic fields. The highest-S/N phase- and velocity-resolved Stokes Vprofiles, obtained with ESPaDOnS, allow us to place a 3σ upperlimit of about 100 G on the possible presence of any undetected puredipolar, quadrupolar or octupolar surface magnetic fields (and just 50 Gfor fields with significant obliquity). We also consider and dismiss thepossible existence of more complex fossil and dynamo-generated fields,and discuss the implications of these results for explaining thenon-uniform surface distribution of Hg. The very high-quality ESPaDOnSspectra have allowed us to confidently detect variability of Hg iiλ 6149, λ 5425 and λ 5677. The profile variabilityof the Hg ii lines is strong, and similar to that of the Hg ii λ3984 line. On the other hand, variability of other lines (e.g. Mn, Fe)is much weaker, and appears to be attributable to orbital modulation,continuum normalisation differences and weak, variable fringing.
| Observations of Hα, iron, and oxygen lines in B, Be, and shell stars
We carried out a spectroscopic survey of several B, Be, and shell starsin optical and near-infrared regions. Line profiles of the Hα lineand of selected Fe II and O I lines are presented.
| Elemental abundance analyses with DAO spectrograms. XXIX. The mercury-manganese stars 53 Tau, β Tau, γ Crv, and υ Her
We performed elemental abundances analyses of the mercury-manganese(HgMn) stars 53 Tau, β Tau, and γ Crv consistent withprevious studies of similar stars in this series. The derived v sin ivalues for the last two stars are 59 and 32 km s-1,respectively, both of which are greater than those of HgMn starspreviously studied. For 53 Tau this analysis using selected wavelengthregions replaces one performed with coadded photographic spectra. Astricter upper limit is placed on any Hg II λ3984 line. Theabundances of β Tau are usually greater than those of γ Crv.But the later star has a Hg II λ3984 line which the former lacks.Our analysis of υ Her, which was performed with spectrogramsobtained with CCD detectors, is extended by using additionalspectrograms to increase the range of wavelengths covered longward ofλ4800. Minor changes in the derived abundances result.
| An Improved Infrared Passband System for Ground-based Photometry: Realization
We describe new simulations and field trials of the new infraredpassband system developed and discussed by Young, Milone, & Stagg,who discussed and illustrated the state of infrared photometry andsuggested ways in which it could be improved. In particular, theypresented a new set of passbands that minimize the dependence of thephotometry on the water vapor bands of the atmospheric windows, whichdefined the edges of many previous infrared passbands, especially whenused at sites and under conditions for which they were not designed. Inthis paper, we present numerical simulations for three atmosphericmodels, demonstrate a measure of the signal-to-noise ratio in the newpassbands for these models, and present observational data obtained at arelatively low-elevation site. The latter demonstrate the utility ofthis system for most astronomical sites where photometry can beperformed, and permit the transformation of observations to this system.Publications of the Rothney Astrophysical Observatory, No. 74.
| The Star of Bethlehem: a type Ia/Ic supernova in the Andromeda galaxy?
Not Available
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| 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.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| 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
| Spectroscopic binary orbits from photoelectric radial velocities. Paper 177: 85 Pegasi
Not Available
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Orbital solutions for SB2 systems with a HgMn component
From a new set of spectroscopic observations we determined orbitalparameters of six SB2 systems with one or both components being HgMnstars.We slightly refined the orbital periods for HD 32964,HD 173524, HD 174933 andHD 216494. Our results for HD 358are in agreement with the previous literature studies. RegardingHD 33647, our orbital period is shorter than previousdeterminations. HD 173524 is a triple system. Fromthe variations of the γ-velocity deduced from our and literaturedata, we refined the orbital period and we estimated the eccentricity ofthe third companion, for which we get e ≈ 0.13 and Porb =36 ± 3 years. HD 191110 and HD216494 seem to be the only synchronous SB2 within our sample.Following the hypothesis of rotational axes perpendicular to the orbitalplane, we estimated the angle of the orbital plane to the line of sight.We identified in our spectra the signature of the Hβ ofthe third component of HD 216494 and we highlight aslight variation of the γ-velocity due to the presence of thiscomponent.Based on observations collected at the Stellar Station ``M. G.Fracastoro'' of the Catania Astrophysical Observatory, Italy.Appendix A is only available in electronic form athttp://www.edpsciences.org
| Orbit of periodic comet 153P/Ikeya-Zhang
The orbit of Comet C/2002C1 (Ikeya-Zhang) has a similarity to that ofComet C/1661C1 (Hevelius), and the numerical integration of the motionof C/2002C1 backward shows a possible linkage of those two comets. Thus,153P/Ikeya-Zhang was designated a periodic comet. Historical records ofcomets in 877 and 1273 are also identified with Comet 153P/Ikeya-Zhang.The integrated orbital elements during 77 and 2362, and historicalrecords of the comet are also presented and discussed.
| Infrared L'band (lambda-cen- ~3.9 mu-m) observations with TIFR near-infrared camera (TIRCAM).
TIRCAM is based on the SBRC InSb Focal Plane Array (58 x 62 pixels)sensitive between 1- 5 μm. TIRCAM system is described in Ojha et al.(2002). TIRCAM had its first light observations during March-April 2001from Gurusikhar 1.2m PRL telescope at Mt. Abu. After having thesuccessful first run in 2001, the TIRCAM was used for the L bandobservations of a few scientific targets in combination with freshtelescope mirrors in Nov. 2002 &Jan. 2003 (the aluminising operationof the primary &secondary mirrors of 1.2m telescope was carried outin mid 2002). Several at 3.9 μm. We could detect the stars upto ~ 7mag in L band from the Gurusikhar site. The TIRCAM L mag of detectedstars in Trapezium cluster were compared with the L band (3.5 μm)data of Muench et al. (2002). These measurements are in good agreementwith each other considering the two different center wavelengths. Weplan to explore TIRCAM's performance in the broad L (3.5 μm) &M(4.5 μm) bands from Hanle site in near future.
| 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.
| 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.
| Vertical distribution of chromium in the atmospheres of HgMn stars
We use multiplet 30 Cr II lines in the wings of Hbeta totest the hypothesis of an anomalous concentration of Cr in the upperlayers of the atmospheres of a sample of 10 HgMn stars. These lines areat different distances from the H_beta line center and are thereforeformed at different depths in the stellar atmosphere. Except for HD49606, all HgMn stars show an increase in Cr abundance with height inthe stellar atmosphere. A similar vertical distribution of Cr, but lesspronounced, has been previously found in Am stars. In contrast, novariation of Cr abundance with the depth has been found for the normallate B-type star HD 196426 and the weak magnetic late B-type star HD168733. It is possible that in HgMn stars the vertical stratificationparameter, a, depends on T_eff, with the strongest vertical gradientbeing found in the hotter stars. No correlation was found between a andthe average stellar abundance log varepsilon (Cr/H).Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programmes Nos. 62.L-0348, 65.I-0644,67.D-0579).
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| The latitude and epoch for the formation of the southern Greek constellations
Not Available
| JHK Standards for Small Telescopes
The AAVSO Futures meeting, held in Madison, WI, in May 2001, proposedthat the AAVSO support near-infrared research with small telescopes. Aphotometer, the SSP-4, has been developed to provide J- and H-bandcapability for a reasonable cost. However, proper calibrated photometryrequires a set of standard stars. This paper describes such a set ofstars, suitable for small telescopes, and with accurate coordinates,proper motions, and high-quality magnitudes.
| The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae
The variability of the Hg II λ3984 line in the primary of thebinary star α And was discovered through the examination ofhigh-dispersion spectra with signal-to-noise ratios greater than 500.This first definitively identified spectrum variation in anymercury-manganese star is not due to the orbital motion of thecompanion. Rather, the variation is produced by the combination of the2.38236 day period of rotation of the primary that we determined and anonuniform surface distribution of mercury that is concentrated in itsequatorial region. If the surface mercury distribution exhibitslong-term stability, then it is likely that a weak magnetic fieldoperates in its atmosphere, but if changes are observed in the lineprofile over a period of a few years, then these would constitute directevidence for diffusion.
| 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.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| 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.
| Optical Interferometry
The field of optical and infrared (IR) interferometry has seen rapidtechnical and scientific progress over the past few years. A number ofinstruments capable of precise visibility measurements have been built,and closure-phase imaging with multitelescope arrays has beendemonstrated. Astronomical results from these instruments includemeasurements of stellar diameters and their wavelength dependence, limbdarkening, stellar surface structure, and distances of Cepheids and ofNova Cygni 1992. Precise stellar masses have been obtained frominterferometric observations of spectroscopic binaries, andcircumstellar disks and shells have been resolved. Searches forsubstellar companions and extrasolar planets with interferometricastrometry will begin soon. Nulling interferometry will enable studiesof exozodiacal disks from the ground and the detection andcharacterization of terrestrial extrasolar planets from space. Thesedevelopments are reviewed, as well as progress in some key technologicalareas.
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
