Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
A mid-IR study of the circumstellar environment of Herbig Be stars
Context. The study of the formation of massive stars is complicatedbecause of the short times scales, large distances, and obscuring natalclouds. There are observational and theoretical indications that thecircumstellar environment of Herbig Be (HBe) stars is substantiallydifferent from that of their lower mass counterparts, the T Tauri andHerbig Ae stars. Aims: We map the spatial distribution andmineralogy of the warm circumstellar dust of a sample of HBe stars. Wecompare our results to a sample of less massive Herbig Ae stars. Methods: We used literature photometry to obtain optical extinctions andstellar parameters of the targets. We obtained N-band imaging andlong-slit spectroscopic data with the VISIR instrument at the VLT and weanalyzed these data. We performed photometry of the images and extractedspatial information. We corrected the spectra for extinction andperformed mineralogical fits. We fitted Gaussian profiles tocharacterize the spatial extent of the spectra along the VISIR slit. Results: We find that the mid-infrared (IR) emission of the HBe starsis typically characterized by a circumstellar disk that efficientlyreprocesses a substantial portion of the stellar flux. The mid-IR fluxlevels, the spatial compactness, and the dust composition are quitesimilar to those of the Herbig Ae stars. We find upper limits to thefull-width-at-half-maximum (FWHM) size of the mid-IR emission of ~500AU. The main differences with the lower mass stars are the lower overallIR excess with a greater variety in shapes, the weaker PAH reprocessingpower, and the lack of a silica-forsterite relation. The discrepanciesbetween VISIR and IRAS photometry, the far-IR contributions and thelarge PAH sizes of HBe stars are attributed to natal clouds. Conclusions: Our results suggest that the Herbig Be disks are flatterthan those around lower mass stars and they are likely truncated fromthe outside by photoevaporation.Based on observations collected at the European Southern Observatory,Chile. Under program IDs: 078.C-0750B, 078.C-0750C, 079.C-0207A,079.C-0207B and 080.C-0410A.Tables 4 and 5 are available in electronicform at http://www.aanda.org
| A mid-infrared imaging catalogue of post-asymptotic giant branch stars
Post-asymptotic giant branch (post-AGB) stars are key objects for thestudy of the dramatic morphological changes of low- to intermediate-massstars on their evolution from the AGB towards the planetary nebulastage. There is growing evidence that binary interaction processes mayvery well have a determining role in the shaping process of manyobjects, but so far direct evidence is still weak. We aim at asystematic study of the dust distribution around a large sample ofpost-AGB stars as a probe of the symmetry breaking in the nebulae aroundthese systems. We used imaging in the mid-infrared to study the innerpart of these evolved stars to probe direct emission from dustystructures in the core of post-AGB stars in order to better understandtheir shaping mechanisms. We imaged a sample of 93 evolved stars andnebulae in the mid-infrared using VLT spectrometer and imager for themid-infrared (VISIR)/VLT, T-Recs/Gemini-South and Michelle/Gemini-North.We found that all the proto-planetary nebulae we resolved show a cleardeparture from spherical symmetry. 59 out of the 93 observed targetsappear to be non-resolved. The resolved targets can be divided into twocategories. (i) The nebulae with a dense central core, that are eitherbipolar and multipolar and (ii) the nebulae with no central core, withan elliptical morphology. The dense central torus observed likely hostsbinary systems which triggered fast outflows that shaped the nebulae.Based on observations made at the Very Large Telescope at ParanalObservatory under the programme 081D.0630.
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) 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/514/A2
| 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.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| A new approach to IRAS observations of Be stars
The IRAS associations for 193 Be stars are identified in this paper.From the infrared colors, the IRAS low-resolution spectra (LRS) and thespectral types, some physical properties and environment of the samplesare discussed. It can be concluded that not only free-free emission orfree-bound emission from the circumstellar ionized gas can beresponsible for the large IR excesses of Be stars as suggestedpreviously, but also, for some Be stars, thermal radiation from thecircumstellar dust and/or nebula around the star can produce large IRexcess as well. It is also found that the far IR excess of Be starsincreases with wavelengths.
| Circumstellar dust shells of hot post-AGB stars
Parameters of the circumstellar dust shells of 15 hot post-AGB starshave been derived using a radiative transfer code (DUSTY). Combining theoptical, near and far-infrared (ISO, IRAS) data of the stars, we havereconstructed their spectral energy distributions (SEDs) and estimatedthe dust temperatures, mass loss rates, angular radii of the innerboundary of the dust envelopes and the distances to these stars. Themass loss rates (10-6-10-5 Mȯyr-1) are intermediate between stars at the tip of the AGBand the PN phase. We have also studied the ISO spectra of 7 of thesestars. Amorphous and crystalline silicate features were observed in IRAS14331-6435 (Hen3-1013), IRAS 18062+2410 (SAO 85766) and IRAS 22023+5249(LSIII +5224) indicating oxygen-rich circumstellar dust shells. Thepresence of unidentified infrared (UIR) band at 7.7 μ, SiC emissionat 11.5 μ and the ``26 μ" and ``main 30 μ" features in the ISOspectrum of IRAS 17311-4924 (Hen3-1428) suggest that the central starmay be carbon-rich. The ISO spectrum of IRAS 17423-1755 (Hen3-1475)shows a broad absorption feature at 3.1 μ due toC2H2 and/or HCN which is usually detected in thecircumstellar shells of carbon-rich stars.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| 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.
| UV(IUE) spectra of hot post-AGB candidates
Analysis of the low resolution UV(IUE) spectra (1150 to 3200 Å) of15 hot post-AGB candidates is presented. The UV(IUE) spectra of 10 starssuggest partial obscuration of the hot stars due to circumstellar dust.The reddened continua of these 10 stars were used to model and estimatethe circumstellar extinction. The circumstellar extinction law was foundto be linear in lambda -1 in the case of IRAS 13266-5551(CPD-55 5588), IRAS 14331-6435 (Hen3-1013), IRAS 16206-5956 (SAO243756), IRAS 17074-1845 (Hen3-1347), IRAS 17311-4924 (Hen3-1428), IRAS18023-3409 (LSS 4634), IRAS 18062+2410 (SAO 85766), IRAS 18371-3159 (LSE63), IRAS 22023+5249 (LSIII +5224) and IRAS 22495+5134 (LSIII +5142).There seems to be no significant circumstellar extinction in the case ofIRAS 17203-1534, IRAS 17460-3114 (SAO 209306) and IRAS 18379-1707 (LSS5112). The UV(IUE) spectrum of IRAS 12584-4837 (Hen3-847) shows severalemission lines including that of HeII. It may be a massive youngOB-supergiant or a low mass star in the post-AGB phase of evolution.IRAS 16206-5956 (SAO 243756) and IRAS 18062+2410 (SAO 85766) showvariability in the UV which in addition to stellar pulsations may beattributed to a dusty torus in motion around the hot central stars. TheUV spectrum of the bipolar PPN, IRAS 17423-1755 (Hen3-1475) indicatesthat the central B-type star is obscured by a dusty disk. The stars wereplaced on the log g-log Teff diagram showing the post-AGBevolutionary tracks of Schönberner. Terminal wind velocities of thestars were estimated from the CIV and NV stellar wind features. Thepresence of stellar wind in some of these stars indicates ongoingpost-AGB massloss.Based on observations obtained with the International UltravioletExplorer (IUE), retrieved from the Multimission Archive at STScI.Complete Figs. \ref{fig3} and \ref{fig4} and Appendix A are onlyavailable in electronic form at http://www.edpsciences.org
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Mid-IR observations of small stellar clusters surrounding Herbig AeBe stars
We present TIMMI2 observations at 10 mu m of 12 fields centered on knownHerbig AeBe (HAeBe) stars. We detected sources (in addition to the HAeBestar) in five fields, three of which are associated with early B stars.Using complementary near-IR and optical data, we estimate that thedetected sources are young, embedded (AV, ~ 5-24 mag),intermediate-mass objects ( ~ 1.5-3.5 Msun). In four objectsthe 10 mu m emission can be accounted for by a circumstellar disk, whilein 2 cases (possibly 3) there is evidence that the objects are still inan earlier stage of evolution (Class I). These results confirm theassociation of the early B Herbig stars with rather rich clusters, aswell as their very young age. If our estimates of the stellar parametersare confirmed, this would be the largest sample so far of embedded,intermediate-mass objects available for further studies.Based on observations collected at the European Southern Observatory, LaSilla, Chile, ESO proposal numbers 65.I-0097, 66.C-0064, 66.C-0120,67.C-0041, part of the data were acquired using the ESO Service Mode.
| The evolutionary stage of five southern Galactic unclassified B[e] stars
The spectra of stars with the B[e] phenomenon are dominated by featuresthat are related to physical conditions of circumstellar material aroundthese objects and are not intrinsic to the stars. Because of this, theyform a very heterogeneous group. This group contains objects withdifferent evolutionary stages. Lamers et al. (1998) have suggested anew designation with five sub-groups, which indicate the evolutionarystage. They are: supergiants, pre-main sequence or Herbig Ae/Be, compactplanetary nebulae, symbiotic and unclassified. The unclassified grouphas many objects that need a better study to resolve their evolutionarystatus. Forbidden lines can be a useful tool to solve this problem. They can give informations about chemical composition, ionization anddensity of the circunstellar medium and probably the evolutionary phaseof these objects. We analize spectra of some galactic objects, obtainedwith the FEROS and B&c} spectrographs at the 1.52m telescope in ESO (LaSilla-Chile), with a special focus on the forbidden lines. We havestudied the spectra of five B[e] stars of uncertain evolutionary stage. We find that one of them is a pre-WN star, the other four are supergiantB[e] stars.
| What Are the Hot R Coronae Borealis Stars?
We investigate the evolutionary status of four stars: V348 Sgr, DY Cen,and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have incommon random deep declines in visual brightness, which arecharacteristic of R Coronae Borealis (RCB) stars. RCB stars aretypically cool hydrogen-deficient supergiants. The four stars studied inthis paper are hotter (Teff=15-20 kK) than the majority ofRCB stars (Teff=5000-7000 K). Although these are commonlygrouped together as the hot RCB stars they do not necessarily share acommon evolutionary history. We present new observational data and anextensive collection of archival and previously published data that isreassessed to ensure internal consistency. We find temporal variationsof various properties on different timescales that will eventually helpus to uncover the evolutionary history of these objects. DY Cen and MVSgr have typical RCB helium abundances, which exclude any currentlyknown post-asymptotic giant branch (post-AGB) evolutionary models.Moreover, their carbon and nitrogen abundances present us with furtherproblems for their interpretation. V348 Sgr and HV 2671 are in generalagreement with a born-again post-AGB evolution, and their abundances aresimilar to Wolf-Rayet central stars of planetary nebulae (PNs). Thethree Galactic stars in the sample have circumstellar nebulae, whichproduce forbidden line radiation (for HV 2671 we have no information).V348 Sgr and DY Cen have low-density, low-expansion velocity nebulae(resolved in the case of V348 Sgr), while MV Sgr has a higher density,higher expansion velocity nebula. All three stars, on the other hand,have split emission lines, which indicate the presence of an equatorialbulge but not of a Keplerian disk. In addition, the historical lightcurves for the three Galactic hot RCB stars show evidence for asignificant fading in their maximum-light brightnesses of ~1 mag overthe last 70 yr. From this we deduce that their effective temperaturesincreased by a few thousand degrees. If V348 Sgr is a born-again star,as we presume, this means that the star is returning from the born-againAGB phase to the phase of a central star of PN. Spectroscopically, nodramatic change is observed over the last 50 years for V348 Sgr and MVSgr. However, there is some evidence that the winds of V348 Sgr and DYCen have increased in strength in the last decade. HV 2671, located inthe LMC, has not been analyzed in detail but at 5 Å resolution isalmost identical to V348 Sgr. Through the bolometric correction derivedfor V348 Sgr and the known distance, we can estimate the absolute Vmagnitude of HV 2671 (MV=-3.0 mag) and its bolometricluminosity (~6000 Lsolar).
| Research on the Properties of a Group of Southern B-Type Emission-Line Stars
Not Available
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are 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/380/609
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| The 75th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 916 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| An improved classification of B[e]-type stars
We review the classification criteria for the B[e]-type stars (B typestars with forbidden emission lines in their optical spectrum) and weexpress these in terms of physical characteristics of the stars and thecircumstellar (CS) matter. We show on the basis of observations thatthese criteria can be met in different kinds of stars of different massand different evolutionary stages. We propose that the name "B[e]phen"is more appropriate than the name "B[e] stars". We propose thedefinition of five classes of stars which show the B[e]phen:B[e]gin{itemize} [(a)] B[e] supergiants or "sgB[e] stars" [(b)] pre-mainsequence B[e]-type stars or "HAeB[e] stars" [(c)] compact planetarynebulae B[e]-type stars or "B[e]ppn\ stars" [(d)] symbiotic B[e]-typestars or "SymB[e] stars" [(e)] unclassified B[e]-type stars or "unclB[e]stars" The primary and secondary classification criteria for each ofthese groups are defined. We also present lists of objects for eachgroup, except for the SymB[e] stars. It is possible that some starssatisfy the criteria for more than one of the classes sgB[e], HAeB[e],cPNB[e] and SymB[e]. In that case the evolutionary phase of the star isunclear and the star should be assigned to class unclB[e].
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| A new catalogue of members and candidate members of the Herbig Ae/Be (HAEBE) stellar group
A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and relatedobjects is certainly needed, for both well-seasoned researchers and, inparticular, for new investigators starting to study the many interestingastrophysical properties of these very young objects. We present a briefdiscussion of the current observational characteristics that distinguishthis class from their main sequence counterparts. The HAEBE and relatedstars are listed in five tables, containing 287 objects. Table 1contains all Ae and Be stars which historically are recognized as trueHAEBE stars or potential candidate members. Table 2 gives the stars ofspectral type Fe, and emission line stars with very uncertain or unknownspectral type. In Table 3 are given all known Extreme Emission LineObjects (EELOs), of which most have not been identified to belong to anyspecific group. Table 4a and b list other Bep or B[e] stars with strongIR-excess and unknown spectral type. Table 5 contains the non-emissionline possible young objects. Furthermore, Table 6 contains 35 starsrejected from former published lists of HAEBE stars. In these tables weare including coordinates, spectral types, visual magnitudes, ranges inphotometric variability and references of several key publicationsrelated to each object. Relevant remarks, such as the presence of anebula in the vicinity of an object, are also given.
| Near-infrared photometry of a sample of IRAS point sources
This paper presents the J, H, K, L, M photometry of 516 sourcespertaining to a sample of 787 sources which has been extracted from theIRAS Point Source Catalog in order to study the late stages of stellarevolution and the concomitant phenomena of mass loss. Three differentclssifications of these sources based on broad-band photometry and IRASlow-resolution spectra are given, and the distributions of the sourcesin terms of these classifications are presented. A subsample of peculiarsources, believed to have recently undergone a helium flash, has beenisolated using the K - L, (12-micron) color diagram. Some objectsmeriting further study are also mentioned.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
| 1612 MHz OH survey of IRAS point sources. I - Observations made at Dwingeloo, Effelsberg and Parkes
The data from a large sky survey are presented including a northernpilot study and a detailed southern study in which detections are biasedtoward the most evolved sources and distant sources. Both areinvestigated at the 1612-MHz transition of OH to take advantage of thestrongest line for AGB stars with optically thick dust shells. The IRASsources are chosen by considering their IR colors related to fluxes at12, 25, and 60 microns. Observations are reported for 2703 IRAS pointsources at the 1612-MHz transition, and 738 OH/IR stars are detected.The survey identifies 597 of the sources as previously unidentified, and95 percent of the OH profiles observed have twin-peak masercharacteristics which are related to emission from expandingcircumstellar shells. The other 5 percent of the sources are concludedto be transition objects between OH/IR stars and planetary nebulae.
| Early-type emission-line stars with large infrared excesses
A catalog is presented of early-type emission-line stars obtained bycross-identification between Wackerling's catalog and the IRAS catalogof point sources. A study of the distribution in space of the starsshows that these stars belong to the extreme Population I; thus thepresent compilation provides a rather complete sample for further studyof the evolution of pre- and post-main sequence stars of medium and highmasses.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| Be stars with peculiar infrared excess and their classification.
Not Available
| IRAS catalogues and atlases - Atlas of low-resolution spectra
Plots of all 5425 spectra in the IRAS catalogue of low-resolutionspectra are presented. The catalogue contains the average spectra ofmost IRAS poiont sources with 12 micron flux densities above 10 Jy.
| A Catalogue of Be-Stars
Not Available
| Observations of southern emission-line stars
A catalog of 1929 stars showing H-alpha emission on photographic platesis presented which covers the entire southern sky south of declination-25 deg to a red limiting magnitude of about 11.0. The catalog providesprevious designations of known emission-line stars equatorial (1900) andgalactic coordinates, visual and photographic magnitudes, H-alphaemission parameters, spectral types, and notes on unusual spectralfeatures. The objects listed include 16 M stars, 25 S stars, 37 carbonstars, 20 symbiotic stars, 40 confirmed or suspected T Tauri stars, 16novae, 14 planetary nebulae, 11 P Cygni stars, 9 Bep stars, 87 confirmedor suspected Wolf-Rayet stars, and 26 'peculiar' stars. Two new Tassociations are discovered, one in Lupus and one in Chamaeleon. Objectswith variations in continuum or H-alpha intensity are noted, and thedistribution by spectral type is analyzed. It is found that the skydistribution of these emission-line stars shows significantconcentrations in the region of the small Sagittarius cloud and in theCarina region.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Centaurus |
| Ascensió Recta: | 13h01m17.80s |
| Declinació: | -48°53'18.8" |
| Magnitud Aparent: | 10.585 |
| Moviment propi RA: | -7.8 |
| Moviment propi Dec: | 6.2 |
| B-T magnitude: | 10.669 |
| V-T magnitude: | 10.592 |
Catàlegs i designacions:
|
