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Analysis of the Pico Dos Dias Survey Herbig Ae/Be Candidates
A large sample of Herbig Ae/Be (HAeBe) candidates, distributed indifferent Galactic regions south to declination +30°, wereidentified by the Pico dos Dias Survey (a search for young stellarobjects based on IRAS colors). Most of the candidates are nearby orassociated with star-forming clouds, but several others are consideredisolated objects. Aiming to verify the young nature of 93 HAeBecandidates, we searched for additional information that could be usefulto confirm if they are pre-main-sequence (PMS) stars or evolved objects,which coincidentally show similar IRAS colors. By adopting a spectralindex that is related to the amount of infrared excess and the shape ofthe spectral energy distribution, we have classified the sampleaccording to three groups, which are analyzed on the basis of (1)circumstellar luminosity; (2) spatial distribution; (3) opticalpolarization; (4) near-infrared colors; (5) stellar parameters (mass,age, effective temperature); and (5) intensity of emission lines. Ouranalysis indicates that only 76% of the studied sample, mainly the groupwith intermediate to low levels of circumstellar emission, can be moreconfidently considered PMS stars. The nature of the remaining stars,which are in the other group that contains the highest levels ofinfrared excess, remains to be confirmed. They share the samecharacteristics of evolved objects, requiring complementary studies inorder to correctly classify them. At least seven objects showcharacteristics typical of post-asymptotic giant branch orproto-planetary nebulae.Based on observations made at the Observatório do Pico dosDias/LNA (Brazil).
| Debris Disks in the Upper Scorpius OB Association
We present MIPS 24 ?m and 70 ?m photometry for 205 members of theUpper Scorpius OB Association. These data are combined with publishedMIPS photometry for 15 additional association members to assess thefrequency of circumstellar disks around 5 Myr old stars with spectraltypes between B0 and M5. Twelve stars have a detectable 70 ?m excess,each of which also has a detectable 24 ?m excess. A total of 54 starsare identified with a 24 ?m excess more than 32% above the stellarphotosphere. The MIPS observations reveal 19 excess sources—8A/F/G stars and 11 K/M stars—that were not previously identifiedwith an 8 ?m or 16 ?m excess. The lack of short-wavelengthemission and the weak 24 ?m excess suggests that these sources aredebris systems or the remnants of optically thick primordial disks withinner holes. Despite the wide range of luminosities of the stars hostingapparent debris systems, the excess characteristics are consistent withall stars having dust at similar orbital radii after factoring invariations in the radiation blowout particle size with spectral type.The results for Upper Sco are compared to similar photometric surveysfrom the literature to re-evaluate the evolution of debris emission.After considering the completeness limits of published surveys and theeffects of stellar evolution on the debris luminosity, we find that themagnitude of the 24 ?m excess around F-type stars increases betweenages of 5 and 17 Myr as found by previous studies, but at lsim2.6?confidence. For B7-A9 and G0-K5 stars, any variations in the observed 24?m excess emission over this age range are significant at less than2? confidence.
| The Alignment of the Polarization of Herbig Ae/Be Stars with the Interstellar Magnetic Field
We present a study of the correlation between the direction of thesymmetry axis of the circumstellar material around intermediate massyoung stellar objects and that of the interstellar magnetic field. Weuse CCD polarimetric data on 100 Herbig Ae/Be stars. A large numberof them show intrinsic polarization, which indicates that theircircumstellar envelopes are not spherical. The interstellar magneticfield direction is estimated from the polarization of field stars. Thereis an alignment between the position angle of the Herbig Ae/Be starpolarization and that of the field stars for the most polarized objects.This may be an evidence that the ambient interstellar magnetic fieldplays a role in shaping the circumstellar material around young stars ofintermediate mass and/or in defining their angular momentum axis.Based on observations made at the Observatório do Pico dos Dias,Brazil, operated by the Laboratório Nacional deAstrofísica.
| PAH Emission from Herbig Ae/Be Stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) starsobtained with the Infrared Spectrograph on Spitzer. All but one of theHerbig stars show emission from PAHs, and seven of the spectra show PAHemission, but no silicate emission at 10 μm. The central wavelengthsof the 6.2, 7.7-8.2, and 11.3 μm emission features decrease withstellar temperature, indicating that the PAHs are less photoprocessed incooler radiation fields. The apparent low level of photoprocessing inHAeBe stars, relative to other PAH emission sources, implies that thePAHs are newly exposed to the UV-optical radiation fields from theirhost stars. HAeBe stars show a variety of PAH emission intensities andionization fractions but a narrow range of PAH spectral classificationsbased on positions of major PAH feature centers. This may indicate that,regardless of their locations relative to the stars, the PAH moleculesare altered by the same physical processes in the protoplanetary disksof intermediate-mass stars. Analysis of the mid-IR SEDs indicates thatour sample likely includes both radially flared and moreflattened/settled disk systems, but we do not see the expectedcorrelation of overall PAH emission with disk geometry. We suggest thatthe strength of PAH emission from HAeBe stars may depend not only on thedegree of radial flaring but also on the abundance of PAHs inilluminated regions of the disks and possibly on the vertical structureof the inner disk as well.
| Stellar parameters and evidence of circumstellar activity for a sample of Herbig Ae/Be stars
Aims.We investigate evidence of accretion in a sample of 15 Herbig Ae/Bestars to determine whether these events originate in a remnant gaseousstructure from the primordial cloud (rich in hydrogen) or in ametal-rich body (like comets in our Solar System). During such analysiswe also determine precise stellar parameters for this sample ofstars. Methods: .The stars were observed using high resolutionspectroscopy (R = 48 000). A synthetic photospheric spectrum wasconstructed and then subtracted from the observed one to obtain thecircumstellar component. An iterative procedure was applied to find thestellar parameters that were used to build the synthetic photosphericspectrum. Results: .Evidence of circumstellar activity were foundin four stars: HD 100546, HD 142666, HD 144432 and HD 145718. Thepresence of redshifted absorption features only in the Balmer linesimplies that the accreting material is hydrogen-rich, excluding thepossibility that the accretion events might have been created bycomet-like bodies. We determined effective temperature, surface gravity,metallicity and the projected rotational velocity for the stars in oursample.
| Circumstellar Activity in Herbig Ae/Be Stars
We presented evidences of accretion of matter in a sample of HerbigAe/Be stars and we determined whether these events originated in aremnant gaseous structure from the primordial cloud (rich in Hydrogen)or in a metal rich body (like comets in our Solar System). During suchanalysis we also determined precise stellar parameters for this sampleof stars. The stars were observed using high resolution spectroscopy (R= 48,000). A synthetic photospheric spectrum was constructed and thensubtracted from the observed one in order to obtain the circumstellarcomponent. An iterative procedure was applied in order to find thestellar parameters that were used to build the synthetic photosphericspectrum. Evidences of accretion were found for three stars: HD100546,HD142666 and HD145718. A chemical analysis of the accretion episodessuggest that the material is Hydrogen rich, what excludes thepossibility that the accretion events were created by comet-like bodies.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Large Proper Motions in the Young Low-Mass Protostellar System IRAS 16293-2422
We compare high angular resolution VLA 3.6-cm continuum observations ofthe protostellar system IRAS 16293-2422 obtained in 1989 and 1994, andshow that the positions of the three VLA sources in IRAS 16293-2422 havechanged significantly between the two epochs. The corresponding angulardisplacements are much larger than the parallax of those sources, andimply transverse velocities (10-30 km s-1) well above the Keplerianrotation speeds expected for those low-mass sources. The proper motionsof two of the components appear to be very similar to one another, andto the proper motions of pre-main sequence stars in the same direction.We argue that they correspond to the overall proper motion of the smallcloud (L1689N) harboring IRAS 16293-2422. The displacement of the thirdsource, however, is larger and in a different direction. That componenthas previously been argued to be a shock between a partially ionizedwind and the ambient medium, so some fast motions are not unexpected. Itis somewhat puzzling, however, that the direction of the motion does notcoincide with the direction of any of the known outflows powered by IRAS16293-2422.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is 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/Abstract.html
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Eclipsing binaries as IRAS sources.
In a systematic search we looked for coincidences in the positions ofeclipsing binaries and IRAS point sources as evidence of a physicalassociation. In a detailed discussion of the available optical andinfrared data combined with model calculations, we show that 50-75% ofthe coincidences between eclipsing binaries and IRAS sources are real.We discuss a subsample of 44 stars in some detail and surveyed thevicinity of them on POSS and SRC/ESO atlases for possible opticalcounterparts and present identification maps. In the cases of BS Sco andV 718 Sco the infrared radiation may come from an accretion disk.
| Search for T Tauri Stars Based on the IRAS Point Source Catalog. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2146T&db_key=AST
| Inhibition of Giant Planet Formation by Rapid Gas Depletion around Young Stars
Not Available
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| A study of dust shells around high latitude supergiants
A sample of 22 stars with infrared excess emission and many of whichhave supergiant-like spectra was surveyed in the submillimeter continuumat 438, 761 and 1100μm using the James Clerk Maxwell Telescope (JCMT,Mauna Kea, Hawaii). Most of the sources are at high galactic latitudesbut stars with similar properties at lower galactic latitudes are alsoincluded. For 16 sources it is believed that they are post AsymptoticGiant Branch stars (post-AGB stars) and 6 of them are detected. The restof the sample consists of 2 binaries, 1 Herbig Ae star, 1 Luminous BlueVariable and 2 sources for which the post-AGB nature is uncertain. Theobservations are combined with optical, near infrared and IRAS data andare fitted with a spherically symmetric and optically thin dust shellmodel. It is found that a large fraction of the excess emissionoriginates from relatively hot dust near the star. Only 2 sources lacksuch a hot dust component. For all stars with a hot dust component itwas found that the emission longward of 60μm cannot be explained withonly one dust shell. Several possible explanations are discussed and itis concluded that the extra excess at far IR and submm wavelengths iscaused by a second colder and thus more distant dust component. For thepost-AGB stars this component is believed to be the remnant AGB shell.The relatively low stellar temperatures (<10,000 K) in combinationwith the relatively large ages of the remnant AGB shell (>10^3^ yr)confirm earlier suggestions that many of these stars are very slowlyevolving towards higher temperatures, as expected for low mass post-AGBstars. Evidence was found that stars may stop losing mass and evolve offthe AGB at temperatures below 5,000 K (as assumed for the Schoenbernertracks) causing a slower evolution towards higher temperatures.
| The mass loss history of high latitude supergiants
A sample of 25 high latitude supergiants with spectral types B-G weresurveyed in the (C-12)O J = 1-0 and J = 2-1 lines using the Swedish ESOSubmillimeter Telescope (SEST). Twenty percent of the sources weredetected. For the analysis an additional six sources were added forwhich CO observations are reported in the literature. The majority ofthe detected sources have post-AGB mass loss rates and expansionvelocities that confirm earlier suggestions that these stars, despitetheir supergiant-like spectra, are in fact low mass post-AGB stars(Trams et al. 1989). The nondetections and their systematic lower 60micron fluxes can be explained assuming that they are low mass post-AGBstars, whose remnant AGB shells are significantly diluted and which areevolving rather slowly to higher temperatures.
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
| Identification of IRAS point sources in Scorpio-Centaurus-Lupus
We present the results of an investigation of 184 sources selected fromthe IRAS Point Source Catalog, situated in a region of 1309 deg-squaredin Scorpio-Centaurus-Lupus. The sources satisfy flux density criteriasuited to the selection of candidate young stellar objects (YSOs),namely, high or moderate quality flux at both 25 and 60 microns, andS(12) is less than S(25). Identifications are assigned on the basis ofIRAS photometry, known associations, and the appearance of proposedoptical associations on the POSS (E, O) prints and/or SERC (J, R) films.Of the 184 sources, 62 are identified with galaxies and 39 areidentified with evolved galactic objects (main sequence stars, AGB, andpost-AGB stars, or planetary nebulae). A total of 28 sources areconfirmed as YSOs, all but two of which lie towards dark clouds. Afurther 11 sources located in the vicinity of OB stars may representcondensations of locally-heated interstellar matter. The 44 remainingsources are potentially YSOs on the basis of IRAS data, but in all casesalternative interpretations (as galaxies or evolved galactic objects)are also possible.
| A search for T Tauri stars based on the IRAS point source catalog.
The results of the first part of a survey for new T Tauri stars, withspectroscopic and photometric observations of about 100 stars, arepresented. A list of candidates has been selected by means of the IRASPoint Source Catalog, based on appropriate far-infrared colors. Coudespectra of the selected objects in the 655-673 nm wavelength range,which includes H-alpha and the resonance Li I line, have been obtained.Thirty-three new T Tauri stars, and a number of other interestingobjects, like early type premain-sequence stars and Li-rich K giants,have been detected. Several new isolated T Tauri were found, includingHen 1, which may be the T Tauri star with the highest galactic latitudeknown, if its nature is confirmed.
| Eclipsing binaries in open clusters. I - Preliminary study
A preliminary list of suspected eclipsing binaries in open clusters ispresented. The period, the mass ratio, the correlation between the massratio and cluster age, and the periodicity diagram are investigated.
| Catalogue of Eclipsing and Spectroscopic Binary Stars in the Regions of Open Clusters
Not Available
| A uvby, beta photometric survey of southern hemisphere
A uvby, beta photometric study of southern hemisphere eclipsing binarystars has been undertaken at Cerro Tololo Inter-American Observatory.The standardized colors and V magnitudes for 288 binaries at quadratureand/or at minimum are presented, along with an indication of theaccuracy of the standardization and photometry. Discussions of theresolving time of the pulse-counting photometers and of the atmosphericextinction at CTIO are included.
| Some Variable Stars in the SRS Catalogue
Not Available
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观测天体数据
目录:
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