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Analysis of the dust evolution in the circumstellar disks of T Tauri stars
Aims.We present a compositional analysis of 8{-}13 μ m spectra of 32young stellar objects (YSOs). Our sample consists of 5 intermediate-massstars and 27 low-mass stars. Although some previous studies give reasonsfor the similarity between the dust in circumstellar disks of T Tauristars and Herbig Ae/Be stars, a quantitative comparison has been lackingso far. Therefore, we include a discussion of the results of the 10μm spectroscopic survey of van Boekel et al. (2005, A&A, 437,189), who focus on Herbig Ae/Be stars, the higher mass counterparts of TTauri stars, and draw comparisons to this and other studies. Methods: .While the spectra of our 32 objects and first scientificresults have already been published elsewhere we perform a more detailedanalysis of the 10 μ m silicate feature. In our analysis weassume that this emission feature can be represented by a linearsuperposition of the wavelength-dependent opacity κ_abs(λ)describing the optical properties of silicate grains with differentchemical composition, structure, and grain size. Determining an adequatefitting equation is another goal of this study. Using a restrictednumber of fitting parameters, we investigate which silicate species arenecessary for the compositional fitting. Particles, with radii of 0.1μm- and 1.5 μm and consisting of amorphous olivine and pyroxene,forsterite, enstatite, and quartz are considered. Only compact,homogeneous dust grains are used in the presented fitting procedures. Inthis context we show that acceptable fitting results can also beachieved if emission properties of porous silicate grains are consideredinstead. Results: .Our analysis shows - in terms of the propertiesof the circumstellar dust-like crystallinity - T Tauri systems are acontinuation of HAeBe systems at their lower mass end. However, a weakcorrelation between grain growth and stellar luminosity could be found,in contrast to HAeBe systems.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| 8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.
| HD 19993 and HD 29035: New bright A-type emission-line stars
We present the results of photometric and spectroscopic observations oftwo A-type stars (HD 19993 and HD 29035), optical counterparts of theIRAS sources (F03108+3729 and 04331+5211, respectively). Hα lineemission is found in the optical spectra of both objects for the firsttime. Our analysis of the spectral line profiles and the photometricinformation shows that HD 19993 is an A7/8 II star (Teff~7500+/-200 K, log g =2.7+/-0.2, v sin i =35+/-2 km s-1,M=4.8+/- 0.8 Msun ) with a mild metal deficit ([Fe/H] ~-0.3+/-0.1), while HD 29035 is a B9/A0 V star (Teff ~10 000K, log g ~ 4, vsin i ~ 150 km s-1). The presence of an IRexcess and the Hα emission in combination with a very differentlocation in the Hertzsprung-Russell diagram suggest the different natureand evolutionary stages of the objects. We conclude that HD 29035 is acandidate Herbig Ae type star, a possible member of a group of similarobjects. HD 19993 seems to be a post-main-sequence object.
| The pre-main-sequence star IP Persei
We present the results of high- and low-resolution spectroscopic andbroadband multicolour photometric observations of the emission-lineA-type star IP Per. Significant variations of the Balmer line profilesand near-IR brightness are detected. Comparison with the spectra ofother stars and theoretical models allowed us to derive its fundamentalparameters as follows: T_eff =~ 8000 K, log g =~ 4.4, logLbol/Lsun =~ 1.0. They correspond to the MK typeA7 v. We also found that the metallicity of the object's atmosphere isnearly 40 per cent that of the Sun. Our result for the star's gravityimplies that it is located at the zero-age main-sequence. We concludethat IP Per is a pre-main-sequence Herbig Ae star, and belongs to thegroup of UX Ori-type stars showing irregular photometric minima. Arecent result by Kovalchuk & Pugach (\cite{kp97}), that IP Per is anevolved high-luminosity star, is not confirmed. The discrepancy in thelog g determination, which led to the difference in the luminosity,seems to be due to uncertainties in the échelle data reductionfor broad lines and a different estimate for the star's temperature.
| Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections
We present the results of high-resolution spectroscopic, low-resolutionspectrophotometric and spectropolarimetric and broad-band multicolourobservations of four B-type stars (HD 4881, 5839, 224648 and 179218) andtwo A-type stars (HD 32509 and 184761) with strong far-infrared (IR)excesses. The excess in HD 184761, which is located at a distance of 65pc from the Sun, was recognized for the first time. Double-peakedHα emission line profiles are found in HD 4881 and HD 5839, whileHD 184761, HD 224648 and HD 32509 display no emission in Hα. Theremarkable variations observed in the Hα profile of HD 179218 arealso observed in some classical Be and Herbig Ae/Be stars. An intrinsiccomponent of polarization is clearly present in HD 179218, only aninterstellar component is detected in HD 4881 and HD 224648, and HD184761 was found to be unpolarized. Improved effective temperatures forall six objects were derived. Parallaxes measured by the Hipparcossatellite were used to determine positions of the stars in the HRdiagram. HD 4881 and HD 5839 are an order of magnitude more luminousthan main-sequence stars of similar temperatures and are most likelynewly discovered classical Be stars. Study of the high-resolution IRASmaps and modelling of the spectral energy distributions of HD 4881, HD5839 and HD 224648 suggest that the observed large IR excesses arecaused by radiation from circumstellar dust rather than free--freeradiation or infrared cirrus, so they may be higher mass counterparts ofbeta Pictoris stars. HD 32509, HD 224648 and HD 184761, which have verysmall near-IR excesses, are probably young main-sequence stars. HD179218, which exhibits the largest near- and far-IR excess in thesample, is an isolated pre-main-sequence Herbig Be star.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
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Osservazione e dati astrometrici
| Costellazione: | Volpetta |
| Ascensione retta: | 19h34m58.97s |
| Declinazione: | +27°13'31.2" |
| Magnitudine apparente: | 6.738 |
| Distanza: | 64.558 parsec |
| Moto proprio RA: | 59.9 |
| Moto proprio Dec: | 35.2 |
| B-T magnitude: | 7.061 |
| V-T magnitude: | 6.765 |
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