Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| Spectrally Dispersed K-Band Interferometric Observations of Herbig Ae/Be Sources: Inner Disk Temperature Profiles
We use spectrally dispersed near-IR interferometry data to constrain thetemperature profiles of sub-AU-sized regions of 11 Herbig Ae/Be sources.We find that a single-temperature ring does not reproduce the data well.Rather, models incorporating radial temperature gradients are preferred.These gradients may arise in a dusty disk, or may reflect separate gasand dust components with different temperatures and spatialdistributions. Comparison of our models with broadband spectral energydistributions suggests the latter explanation. The data support the viewthat the near-IR emission of Herbig Ae/Be sources arises from hotcircumstellar dust and gas in sub-AU-sized disk regions. Intriguingly,our derived temperature gradients appear systematically steeper fordisks around higher mass stars. It is not clear, however, whether thisreflects trends in relative dust/gas contributions or gradients withinindividual components.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| 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).
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| 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).
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Resolved Inner Disks around Herbig Ae/Be Stars
We have observed 14 Herbig Ae/Be (HAEBE) sources with the long-baselinenear-IR Palomar Testbed Interferometer. All except two sources areresolved at 2.2 μm, with angular sizes generally <~5 mas. Wedetermine the size scales and orientations of the 2.2 μm emissionusing various models: uniform disks, Gaussians, uniform rings, flataccretion disks with inner holes, and flared disks with puffed-up innerrims. Although it is difficult to distinguish different radialdistributions, we are able to place firm constraints on the inclinationsof most sources; seven objects display significantly inclinedmorphologies. The inner disk inclinations derived from our near-IR dataare generally compatible with the outer disk geometries inferred frommillimeter interferometric observations, implying that HAEBE disks arenot significantly warped. Using the derived inner disk sizes andinclinations, we compute the spectral energy distributions (SEDs) fortwo simple physical disk models and compare these with observed SEDscompiled from the literature and new near-IR photometry. Whilegeometrically flat accretion disk models are consistent with the datafor the earliest spectral types in our sample (MWC 297, V1685 Cyg, andMWC 1080), the later type sources are explained better through modelsincorporating puffed-up inner disk walls. The different inner diskgeometries may indicate different accretion mechanisms for early- andlate-type HAEBE stars.
| 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.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Grain growth in the inner regions of Herbig Ae/Be star disks
We present new mid-infrared spectroscopy of the emission from warmcircumstellar dust grains in Herbig Ae/Be stars. Our surveysignificantly extends the sample that was studied by Bouwman et al.(2001). We find a correlation between the strength of the silicatefeature and its shape. We interpret this as evidence for the removal ofsmall (0.1 mu m) grains from the disk surface while large (1-2 mu m)grains persist. If the evolution of the grain size distribution isdominated by gravitational settling, large grains are expected todisappear first, on a timescale which is much shorter than the typicalage of our programme stars. Our observations thus suggest a continuousreplenishment of micron sized grains at the disk surface. If the grainreplenishment is due to the dredge-up of dust from the disk interior,the mineralogy we observe is representative of the bulk composition ofdust in these stars.Based on observations obtained at the European Southern Observatory(ESO), La Silla, and on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Probing the circumstellar structure of Herbig Ae/Be stars
We present Hα spectropolarimetry observations of a sample of 23Herbig Ae/Be stars. A change in the linear polarization across Hαis detected in a large fraction of the objects, which indicates that theregions around Herbig stars are flattened (disc-like) on small scales. Asecond outcome of our study is that the spectropolarimetric signaturesfor the Ae stars differ from those of the Herbig Be stars, withcharacteristics changing from depolarization across Hα in theHerbig Be stars, to line polarizations in the Ae group. The frequency ofdepolarizations detected in the Herbig Be stars (seven out of 12) isparticularly interesting as, by analogy with classical Be stars, it maybe the best evidence to date that the higher-mass Herbig stars aresurrounded by flattened structures. For the Herbig Ae stars, nine out of11 show a line polarization effect that can be understood in terms of acompact Hα emission that is itself polarized by a rotatingdisc-like circumstellar medium. The spectropolarimetric differencebetween the Herbig Be and Ae stars may be the first indication thatthere is a transition in the Hertzsprung-Russell diagram from magneticaccretion at spectral type A to disc accretion at spectral type B.Alternatively, the interior polarized line emission apparent in the Aestars may be masked in the Herbig Be stars owing to their higher levelsof Hα emission.
| Optical positions of 55 radio stars from astrolabe observations from the Yunnan Observatory
The observations by the photoelectric astrolabe at Yunnan Observatoryrelative to the Hipparcos Catalogue and the optical positions of 55radio stars were obtained from observations between 1991 and 2000. Theyall resulted from processing the photon counts obtained by means of theastrolabe after the automation of the instrument. There are 46 stars incommon with the Hipparcos Catalogue. Tables \ref{t1} and \ref{t2} arealso available in electronic form at the CDS via anonymous ftpcdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/1062.
| 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
| GasDust Shells around Some Early-Type Stars with an IR Excess (of Emission)
The results of an investigation of IR (IRAS) observations of 58OBAF stars of different luminosity classes, whichare mainly members of various associations, are presented. The colorindices of these stars are determined and two-color diagrams areconstructed. The emission excesses at 12 and 25 mm (E 12 and E 25) arealso compared with the absorption A1640 of UV radiation. It is concludedthat 24 stars (of the 58 investigated) are disk systems of the Vegatype, to which Vega = N 53 also belongs. Eight known stars of the Vegatype are also given in the figures for comparison. The remaining 34stars may have gasdust shells and/or shelldisks. The IRemission excesses of the 34 investigated stars and 11 comparison stars(eight of them are Be-Ae stars) are evidently due both to thermalemission from grains and to the emission from freefreetransitions of electrons in the gasdust shells of these stars.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form at the 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/375/670
| 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.
| Analysis of correlations between polarimetric and photometric characteristics of young stars. A new approach to the problem after eleven years' study
We present the results of the investigation of correlations between thepolarimetric and photometric characteristics of a sample (496 objects)of young Herbig Ae/Be (HAEBE) stars and T Tauri (TT) stars. It is shownthat, for 85% of the sample stars there is a general relation betweenthe degree of optical polarization and the infrared colour index(V-L)_obs and the colour excess E(V-L) due to the contribution of acircumstellar dust shell. Polarimetric data were also compared with thevalue of vsin i to search for a possible correlation between thepolarization and an inclination of circumstellar disks. Polarimetricdata as well as IR excesses are considered and compared for differentsubgroups of young stars namely: HAEBE and TT stars with Algol-likeminima of brightness (26 objects), Vega-type stars and post HAEBE stars(114 objects) and young solar-type stars (58 objects). For statisticalpurposes the data for young stars were compared with those collected fordifferent groups of evolved objects such as: classical Be stars (~300objects), Mira Ceti stars (39 objects), early-type supergiants from theSerkowski et al. (\cite{serk}) catalogue (120 objects) and main sequence(MS) stars within 50 pc from the Sun from the Leroy (\cite{leroy})catalogue (68 objects). The value of polarization is discussed incontext with the stages of evolution of circumstellar shells which wereestablished by comparison of spectral energy distribution in the far IR(using the IRAS data). It is shown that most young stars havestatistically larger value of polarization in comparison with the starswhich are on a stage of evolution close to MS. We are able to contendthat the changes in polarimetric behaviour of young stars are connectedwith evolution of their circumstellar shells. Appendices 1 to 5 are onlyavailable in electronic form at http://www.edpsciences.org
| Herbig Ae/Be stars with the IRAS low-resolution spectra.
Not Available
| Hα spectropolarimetry of B[e] and Herbig Be stars
We present the results of medium-resolution (Delta v ~ 60 km s^-1)spectropolarimetric observations across H alpha of a sample of B[e] andHerbig Be objects. A change in linear polarization across H alpha isdetected in a large fraction of the objects, with characteristicsranging from simple depolarization in a couple of Herbig Be stars, tomore complex behaviour in the probable post-main-sequence B[e] stars. Halpha in the spectra of HD 37806 and 50138 each consist of adouble-peaked polarized line and a superposed unpolarized singleemission peak, suggesting two distinct line-forming regions. Multipleobservations of HD 45677 allow for the separation of electron and dustscattering effects for the first time: the difference between derivedintrinsic polarization angles of the two components indicates that thedust scattering region is clumpy. Two unexpected results are thenon-detections of H alpha polarization changes in omicron Ori, wheredepolarization has previously been detected, and in MWC 297, whichexhibits source elongation at radio wavelengths. In omicron Ori timevariability is probably responsible such that the electron scatteringdisc of this star was much weakened at the time of observation. Twohypotheses are advanced that might explain the MWC 297 result. Thegeneral findings are that roughly half of the observed Herbig Be starsshow polarization changes across Hα, implying immediately thattheir ionized envelopes are not spherically symmetric. This pattern, ifconfirmed by observations of a larger sample, could indicate that thenon-detection rate is simply a consequence of sampling randomly orientedcircumstellar discs able to scatter starlight within a few stellarradii. The deduced alignment of the disc of HD 53367 with the localinterstellar magnetic field suggests an orderly star formation processin which the star `remembers' the larger scale magnetic field direction.The stars classified as B[e] stars all show startling polarizationchanges across Hα. The details in each case are different, but thewidely accepted concept of dense Hα-emitting equatorial discsaround these objects is supported.
| Radio star catalogue observed in San Juan (RSSJ95)
Using the data observed in San Juan with the photoelectric AstrolabeMark II of the Beijing Astronomical Observatory from February, 1992 toMarch, 1997, the radio star catalogue in San Juan(RSSJ95) has beencompiled. There are 69 radio stars in this catalogue. The positions ofthe radio stars are for the epoch of observation and the equinox J2000.0and a system close to that of the system FK5. The mean precisions are+/-2.2 ms and +/-0.035'' in right ascensions and declinations,respectively. The magnitudes of stars are from 0.9 to 10.7. Thedeclinations are from -2fdg 5 to -60(deg) . The mean epoch is 1995.1.Finally, the comparison results between the Hipparcos catalogue andRSSJ95 are given.
| The Incidence of lambda Boötis Stars via an Extension of the MK Spectral Classification System to Very Young A-Type Stars
In this paper we introduce an extension to the MK system of spectralclassification that allows the precise classification ofpre-main-sequence (PMS) A-type stars, including most Herbig Ae stars.This classification scheme characterizes the star by a standard MK type,the presence and strength of emission and/or shell lines, and thestrength of the Balmer decrement. It can be used to summarize temporalspectral changes in PMS A-type stars, and to search for peculiar types.We have used this scheme to classify 38 Herbig Ae stars as well as 22PMS stars in the young open clusters NGC 2264, NGC 7160, and IC 348. Wehave also used this extended system to search for lambda Boötisstars among PMS A-type stars. We have found one definite lambdaBoötis star among the Ae stars, and one marginal lambda Boötisstar in NGC 2264, yielding statistics not significantly different fromthose of the lambda Boötis stars in the field. This, in addition toother considerations from previous studies, leads us to conclude thatthe lambda Boötis mechanism is operable from very early ages (PMS)to well into the main-sequence life (a few times 10^8 yr) of only 2%-3%of A-type stars.
| Polarimetry of southern peculiar early-type stars
We present and discuss optical polarimetry of a large group (60 objects)of peculiar early-type stars (Herbig Ae/Be stars and candidate members,B[e] stars, extreme emission line objects etc.). Most were taken from``A new catalogue of members and candidate members of Herbig Ae/Bestellar group'' \cite[The et al. (1994)]{the2}. For 40 out of the 60objects polarization was measured for the first time. On the basis ofthe data we draw conclusions about the circumstellar shell configurationand orientation for several objects. Polarimetric variability ondifferent time-scales (minutes \ldots hours \ldots days) wasinvestigated, as well as comparison with earlier measurements toinvestigate variability over longer time-scales. No circularpolarization was detected to a 5sigma level in most of the investigatedstars. Polarimetric data obtained here as well as data available fromthe literature were analyzed from the point of investigating theevolutionary status of the objects. The relationship between thepolarimetric and photometric data was also investigated. Tables 6 to 22are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsarc.u-strasbg.fr/Abstract.html
| An ultraviolet, optical and infrared study of Herbig Ae/Be stars
We have selected a list of 45 Herbig Ae/Be-type candidates on the baseof their IRAS colors and their spectral types. We propose the presenceof a broad infrared excess as a defining criterion for these stars,rather than the detection of circumstellar nebulosity. In this way, ourselection also includes more evolved young stars, that are no longerembedded in their star-forming region. A few objects in our sample arewell-known Herbig Ae/Be stars, others are new. New optical andnear-infrared photometric observations, as well as ultraviolet ones, arepresented. The position of the objects in several color-color diagrams,as well as their de-reddened energy distributions, permit a reliableclassification. Three objects probably are binaries with a coolsecondary, 9 appear to be related to the Vega-type stars and 33 objectscan be classified as genuine Herbig Ae/Be stars. The majority of theHerbig Ae/Be stars have a dusty environment consisting of a distinct hotand cool component. These isolated Herbig Ae/Be stars suggest anevolution from embedded Herbig Ae/Be stars to beta Pictoris-likemain-sequence stars, an evolution in which planet formation may play animportant role. Based on observations obtained at the European SouthernObservatory, La Silla, Chile; and at the Swiss Telescope, La Silla,Chile; and at the Swiss Telescope, Hochalpine ForschungsstationJungfraujoch, Switzerland, and with the International UltravioletExplorer (IUE)
| 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.
| San Juan radio star catalogue and comparison with HIPPARCOS catalogue.
Not Available
| Emission-Line Stars in the Vilnius Photometric System
Various types of emission-line stars, observed in the Vilniusphotometric system, are analyzed. They include Be-type stars, HerbigAe/Be stars, T Tauri-type stars and K- and M-type dwarfs with activechromospheres. It is shown that all stars of these types, except forlate-type dwarfs, in their active stages can be identified by theirinterstellar reddening-free parameters. For emission-line stars of B andA types interstellar reddening determination is also possible.
| Photoelectric Photometry of Herbig Ae/be and Related Stars in the Vilnius Photometric System
A catalog of photoelectric photometry of 62 Herbig Ae/Be and relatedstars in the Vilnius system is presented. It contains stars down to V =12 mag located mainly in the northern hemisphere and observed in theperiod of 1994--1996.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 05h41m02.29s |
| Declinazione: | -02°43'00.7" |
| Magnitudine apparente: | 7.927 |
| Distanza: | 884.956 parsec |
| Moto proprio RA: | 2.8 |
| Moto proprio Dec: | -1.1 |
| B-T magnitude: | 7.966 |
| V-T magnitude: | 7.931 |
Cataloghi e designazioni:
|
INHABITED SKY
