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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.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Large dust grains in the inner region of circumstellar disks
Context. Simple geometrical ring models account well for near-infraredinterferometric observations of dusty disks surrounding pre-mainsequence stars of intermediate mass. Such models demonstrate that thedust distribution in these disks has an inner hole and puffed-up inneredge consistent with theoretical expectations. Aims. In thispaper, we reanalyze the available interferometric observations of sixintermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC758, V1295 Aql and AB Aur) in the framework of a more detailed physicalmodel of the inner region of the dusty disk. Our aim is to verifywhether the model will allow us to constrain the disk and dustproperties. Methods. Observed visibilities from the literature arecompared with theoretical visibilities from our model. With theassumption that silicates are the most refractory dust species, ourmodel computes self-consistently the shape and emission of the inneredge of the dusty disk (and hence its visibilities for giveninterferometer configurations). The only free parameters in our modelare the inner disk orientation and the size of the dust grains. Results.In all objects with the exception of AB Aur, ourself-consistent models reproduce both the interferometric results andthe near-infrared spectral energy distribution. In four cases, grainslarger than ~1.2 μm, and possibly much larger are either required byor consistent with the observations. The inclination of the inner diskis found to be always larger than ~30°, and in at least two objectsmuch larger.
| Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAMPlateau de Bure interferometer in the isotopologues of CO, and incontinuum at 3 and 1.3 mm. These observations allow us to trace thestructure of the circumstellar material of AB Aur inregions where optical and IR imaging is impossible because of theemission from the star. These images reveal that the environment of ABAur is widely different from the proto-planetary disks that surround TTauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 inseveral aspects. Instead of being centrally peaked, the continuumemission is dominated by a bright, asymmetric (spiral-like) feature atabout 140 AU from the central star. Little emission is associated withthe star itself. The molecular emission shows that AB Aur is surroundedby a very extended flattened structure ("disk"), which is rotatingaround the star. Bright molecular emission is also found towards thecontinuum "spiral". The large-scale molecular structure suggests the ABAur disk is inclined between 23 and 43 degrees, but the strong asymmetryof the continuum and molecular emission prevents an accuratedetermination of the inclination of the inner parts. Analysis of theemission in terms of a Keplerian disk provides a reasonable fit to thedata, but fails to give a consistent picture because the inclinationsdetermined from 12CO J~= 2~→ 1, 13CO J~=2~→ 1, 13CO J~= 1~→ 0 and C18O J~=1~→ 0 do not agree. The mass predicted for the central star in suchKeplerian models is in the range 0.9 to 1.2 Mȯ, muchsmaller than the expected 2.2 Mȯ from the spectral typeof AB Aur. Better and more consistent fits to the 13CO J~=2~→ 1, 13CO J~= 1~→ 0 data are obtained by relaxingthe Keplerian hypothesis. We find significant non-Keplerian motion, witha best fit exponent for the rotation velocity law of 0.41 ± 0.01,but no evidence for radial motion. The disk has an inner hole about 70AU in radius. The disk is warm and shows no evidence of depletion of CO.The dust properties suggest that the dust is less evolved than intypical T Tauri disks. Both the spiral-like feature and the departurefrom purely Keplerian motion indicates the AB Aur disk is not inquasi-equilibrium. Disk self-gravity is insufficient to create theperturbation. This behavior may be related either to an early phase ofstar formation in which the Keplerian regime is not yet fullyestablished and/or to a disturbance of yet unknown origin. An alternate,but unproven, possibility is that of a low mass companion located about40 AU from AB Aur.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Discovery of a Nearly Edge-on Disk around HD 32297
We report the discovery of a nearly edge-on disk about the A0 star HD32297 seen in light scattered by the disk grains revealed in NICMOSPSF-subtracted coronagraphic images. The disk extends to a distance ofat least 400 AU (3.3") along its major axis with a 1.1 μm fluxdensity of 4.81 +/- 0.57 mJy beyond a radius of 0.3" from thecoronagraphically occulted star. The fraction of 1.1 μm starlightscattered by the disk, 0.0033 +/- 0.0004, is comparable to itsfractional excess emission at 25 + 60 μm of ~0.0027 as measured fromIRAS data. The disk appears to be inclined 10.5d +/- 2.5d from anedge-on viewing geometry, with its major axis oriented 236.5d +/- 1°eastward of north. The disk exhibits unequal brightness on opposingsides and a break in the surface brightness profile along the NE-sidedisk major axis. Such asymmetries might implicate the existence of oneor more (unseen) planetary mass companions.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
| Spectroscopic behaviour of the unusual Ae star HD 190073
The results of high-resolution spectroscopy of the peculiar Ae star HD190073 obtained within the framework of a cooperative observingprogramme in 1994-2002 are presented. The temporal behaviour of theHα, Hβ, Hγ, Hδ, He i λ 5876 Å, Na iD and other circumstellar line profiles are investigated in detail.Special attention has been paid to the analysis of the deepmulticomponent blueshifted Ca ii H and K absorption lines. It has beenfound that the fine structure of their profiles is variable ontimescales from months to decades. The analysis of the circumstellarspectrum of HD 190073, rich in shell-like and emission lines with narrowabsorption cores, allows us to conclude that all absorption lines andcores are likely to be of photospheric origin. The emission lines arevariable in time and demonstrate signs of a stellar wind as well as adense equatorial circumstellar disk. As a preliminary hypothesis, wepropose that a global magnetic field of a specific topology can beresponsible for the formation of stable latitudinal stratification ofthe outflowing gas resulting in appearance of the complex structure ofthe Ca ii H and K line profiles. We emphasise that a measurement of thestellar magnetic field and an investigation of its detailedconfiguration would be an important step in understanding the nature ofHD 190073.
| 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.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| 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.
| The Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disksizes using the 85 m baseline Keck Interferometer. Targets were chosento span the maximum range of stellar properties to probe the disk sizedependence on luminosity and effective temperature. For most targets,the measured near-infrared sizes (ranging from 0.2 to 4 AU) support asimple disk model possessing a central optically thin (dust-free)cavity, ringed by hot dust emitting at the expected sublimationtemperatures (Ts~1000-1500 K). Furthermore, we find a tightcorrelation of disk size with source luminosity R~L1/2 for Aeand late Be systems (valid over more than two decades in luminosity),confirming earlier suggestions based on lower quality data.Interestingly, the inferred dust-free inner cavities of the highestluminosity sources (Herbig B0-B3 stars) are undersized compared topredictions of the ``optically thin cavity'' model, likely because ofoptically thick gas within the inner AU.
| The 90-110 μm dust feature in low to intermediate mass protostars: Calcite?
We present ISO spectra between 60 and 180 μm of 32 protostars of lowto intermediate mass. About half of the spectra present a dust featurebetween ~90 and ~110 μm. We describe the observationalcharacteristics of this feature, which seems to be due to one singlecarrier. In Class 0 sources the feature peaks around 100 μm, while inAeBe stars it peaks around 95 μm. The feature peak position seems tomostly depend on the temperature of the dust of the source, suggestingreprocessing of the dust. We present arguments for the identification ofthe observed feature as due to calcite, and estimate that about 10% to30% of elemental Ca is locked up in it. Therefore, calcite seems to beformed relatively easily around protostars despite the observation thaton Earth it needs aqueous solutions. This rises the question of whetherconditions simulating liquid water are common around forming stars andwhat creates them.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) with the participation of ISAS andNASA.
| Chemical analysis of 24 dusty (pre-)main-sequence stars
We have analysed the chemical photospheric composition of 24 HerbigAe/Be and Vega-type stars in search for the λ Bootis phenomenon.We present the results of the elemental abundances of the sample stars.Some of the stars were never before studied spectroscopically at opticalwavelengths. We have determined the projected rotational velocities ofour sample stars. Furthermore, we discuss stars that depict a(selective) depletion pattern in detail. HD 4881 andHD 139614 seem to display an overall deficiency.AB Aur and possibly HD 126367 havesubsolar values for the iron abundance, but are almost solar in silicon.HD 100546 is the only clear λ Bootis star inour sample.Appendix is only available in electronic form athttp://www.edpsciences.org
| 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.
| Long-Term Spectral Variability of the Herbig Ae-Star HD 179218
The results of high-resolution long-term spectral monitoring of HerbigAe star HD 179218 in the region of emission H line and the sodiumresonance doublet Na I D are presented. The received data show theexistence of short-term variability (about 10 days) and long-termwave-like variability of equivalent width EW, intensity of circumstellar(CS) emission I and parameters of H emission profile. The analysis ofthese data allows us to suppose the existence of the global variabilityof parameters of accretion disk.
| High-Resolution 4.7 Micron Keck/NIRSPEC Spectroscopy of the CO Emission from the Disks Surrounding Herbig Ae Stars
We explore the high-resolution (λ/Δλ=25,000Δv=12 km s-1) M-band (4.7-5.1 μm) spectra of severaldisk-dominated Herbig Ae (HAe) systems: AB Aur, MWC 758, MWC 480, HD163296, and VV Ser. All five objects show 12CO v=1-0 emissionlines up to J=42, but there is little or no evidence of moderate-J,v=2-1 transitions despite their similar excitation energies. AB Aurshows 13CO emission as well. The line/continuum ratios andintensity profiles are well correlated with inclination, and they tracecollisionally driven emission from the inner disk(Rth<~0.5-1 AU) as well as resonance fluorescence to muchlarger radii (Rhν<~50-100 AU for J<~10). Thetemperature, density, and radiation field profiles required to fit theCO emission are in good agreement with models of HAe disks derived fromtheir spectral energy distributions. High-resolution and high dynamicrange infrared spectroscopy of CO, and future observations of lessabundant species, thus provide direct access to the physicochemicalproperties and surface structure of disks in regions where planetformation likely occurs.
| Modeling the Continuum Emission from the Circumstellar Environment of Herbig Ae/Be Stars
This paper discusses a model for the continuum emission of the HerbigAe/Be stars in the light of an updated set of observational dataspanning 5 orders of magnitude in wavelength and including thelow-resolution spectra obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatory(ISO). The model is used to reproduce the continuum emission of the 36Herbig Ae/Be stars included in the list by Thé and coworkers andobserved by ISO. The circumstellar matter responsible for the observedspectral energy distributions has been investigated by comparing the setof the observations with the model spectra computed for differentpossible distributions of circumstellar matter. Cases have beenconsidered with the circumstellar regions partially evacuated along thepolar axis by the action of the stellar wind, a phenomenon that is quitecommon in these pre-main-sequence objects. The inclusion of the polarcavities indirectly allows geometries in which a small-scale disklikestructure around the central star is present. The possible coexistenceof two different density profiles, in the inner and the outer region ofthe envelope, respectively, has been also considered. The comparison ofthe computed models with the observed spectral energy distributionsselects the parameter values in such a way that the larger dust grainsare preferentially associated with the later spectral types. We findthat 17 objects are reasonably fitted, eight of which with a purelyspherical model and the remaining nine with the inclusion of the polarcavities. For 10 further objects the fit is worse, and for the remainingnine, almost all associated to IR companions, our model is clearlyinappropriate. A linear relationship is suggested between the logarithmof the initial density n0 and the exponent p of the power lawn(r)=n0(R*/r)p adopted for thecircumstellar density distribution.
| Active phenomena in the circumstellar environmentof the Herbig Ae star HD 31648
We present the results of observations of the Herbig Ae star HD 31648including high-resolution spectroscopy near the Hα, He I λ5876 Å and DNa I lines, multi-color photometry and polarimetry.These observations have revealed many features of the observationalappearance of HD 31648 similar to those of the ``classical'' Herbig Aestars. Among them are spectral manifestations of both the mass loss andthe disk accretion onto the star and certain types of line profilevariations. The totality of the observed properties of HD 31648 providesevidence in favor of complicated structure in the stellar wind zone,exhibiting latitudinal stratification and containing loinhomogeneitiesin the form of outflowing streams and density condensations. Structuralvariations of the stellar wind have been found to correlate with changesin the accretion process in the envelope. The circumstellar environmentof HD 31648 is supposed to contain dust particles of different formsmanifesting themselves, in particular, in the variable multi-componentintrinsic polarization.
| 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.
| ISO Observations of the Infrared Continuum of the Herbig Ae/Be Stars
We present a study of the whole sample of the Herbig Ae/Be starsobserved with the spectrometers on board of the Infrared SpaceObservatory (ISO/ESA). These objects have been studied not only by meansof their infrared continuum emission but also with respect to all theavailable photometric data, collected from the optical region to theradio wavelengths. The global spectral energy distributions (SEDs) havebeen compared with the results of radiative transfer calculations, thathave been made in the framework of a circumstellar (CS) model. Theresults of the selection of the best models allow us to infer on therelationship between the geometry of the CS matter distribution and theevolutionary stage of these objects.
| 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.
| The complete far infrared spectroscopic survey of Herbig AeBe stars obtained by ISO-LWS
The ISO-LWS archive has been systematically searched in order to obtaina complete far IR spectrophotometric survey of Herbig AeBe (HAEBE)stars. The investigated sample is constituted by 15 objects which,together with the 11 HAEBE we have published in two previous papers,represents about 25% of all the known HAEBE. This catalogue constituesan essential data-base in view of far IR forthcoming space missions(e.g. Herschel Space Observatory), whose scientific programs are now inthe planning phase. The new sources are analysed following the sameapproach as in our previous papers and both differences and similaritiesare discussed in a coherent framework. The [OI] 63 mu m and the [CII]158 mu m lines are observed in many of the investigated sources, whilethe [OI] 145 mu m remains often undetected, due to its relativefaintness. Molecular lines, in form of CO high-J rotational transitionsare detected in only three cases and appear associated to local densitypeaks. A new class of ISO-LWS spectra of HAEBE emerges, constituted byobjects without any detected gas feature either in emission or inabsorption. Not unexpectedly, these HAEBE are isolated from molecularclouds and, as such, lack of the cold circumstellar material probed byfar IR ionic and molecular transitions. By comparing line intensityratios with model predictions we find that photodissociation caused bythe stellar photons and active in a clumpy medium is likely theprevalent excitation mechanism for the far IR lines. Finally, anevolutionary trend is found according to which the contribution of thefar IR line emission to the total emitted energy is less and lessimportant with time.
| ISO LWS Spectra of T Tauri and Herbig AeBe stars
We present an analysis of ISO-LWS spectra of eight T Tauri and HerbigAeBe young stellar objects. Some of the objects are in the embeddedphase of star-formation, whereas others have cleared their environs butare still surrounded by a circumstellar disk. Fine-structure lines of[OI] and [CII] are most likely excited by far-ultraviolet photons in thecircumstellar environment rather than high-velocity outflows, based oncomparisons of observed line strengths with predictions ofphoton-dominated and shock chemistry models. A subset of our stars andtheir ISO spectra are adequately explained by models constructed byChiang & Goldreich (\cite{cg97}) and Chiang et al. (\cite{c01}) ofisolated, passively heated, flared circumstellar disks. For thesesources, the bulk of the LWS flux at wavelengths longward of 55 mumarises from the disk interior which is heated diffusively by reprocessedradiation from the disk surface. At 45 mum , water ice emission bandsappear in spectra of two of the coolest stars, and are thought to arisefrom icy grains irradiated by central starlight in optically thin disksurface layers. Based in part on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands, and the United Kingdom) andwith participation of ISAS and NASA.
| Close frequency pairs in Delta Scuti stars
The majority of the well-studied delta Scuti stars shows frequency pairsin the power spectra with frequency separations less than 0.06cd-1 (0.7 mu Hz) as well as amplitude variability. We examinethe interpretation in terms of separate excited stellar pulsation modes,single modes with variable amplitudes, and observational problems. Thevariable-phase technique, which examines the phase jumps near the timesof minimum amplitude of an assumed single frequency, is applied to theextensive data of the star BI CMi, which shows some of the most extremebehavior. The following results are found for the 5 features in thepower spectrum which could be explained as single modes with variableamplitudes or as double modes: for three features it can be shown thatthese are indeed pairs of separate pulsation modes beating with eachother: at times of minimum amplitude the phase jumps are observed andboth the observed amplitude and phase variations can be predictedcorrectly by assuming two separate modes of nearly equal frequencies.Artifacts caused by observational error, insufficient frequencyresolution or variable amplitudes can be ruled out. A fourth pair has aprobable origin in two excited modes, while a 5th case is inconclusivedue to long time scales of variability and small amplitudes. Theexistence of close frequency needs to be taken into account in planningthe lengths of earth-based as well as space campaigns so that sufficientfrequency resolution is obtained. Possible reasons for the existence ofclose frequencies in delta Scuti stars are considered. They include thedense frequency spacing caused by the presence of mixed modes,rotational splitting as well as near-coincidence of the frequencies ofmodes with different l values (the so-called Small Spacing).
| 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
| H2 and CO Emission from Disks around T Tauri and Herbig Ae Pre-Main-Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas
We present ISO Short-Wavelength Spectrometer observations ofH2 pure-rotational line emission from the disks around low-and intermediate-mass pre-main-sequence stars as well as from youngstars thought to be surrounded by debris disks. The pre-main-sequencesources have been selected to be isolated from molecular clouds and tohave circumstellar disks revealed by millimeter interferometry. Wedetect ``warm'' (T~100-200 K) H2 gas around many sources,including tentatively the debris-disk objects. The mass of this warm gasranges from ~10-4 Msolar up to8×10-3 Msolar and can constitute anonnegligible fraction of the total disk mass. Complementary single-dish12CO 3-2, 13CO 3-2, and 12CO 6-5observations have been obtained as well. These transitions probe coolergas at T~20-80 K. Most objects show a double-peaked CO emission profilecharacteristic of a disk in Keplerian rotation, consistent withinterferometer data on the lower J lines. The ratios of the12CO 3-2/13CO 3-2 integrated fluxes indicate that12CO 3-2 is optically thick but that 13CO 3-2 isoptically thin or at most moderately thick. The 13CO 3-2lines have been used to estimate the cold gas mass. If aH2/CO conversion factor of 1×104 is adopted,the derived cold gas masses are factors of 10-200 lower than thosededuced from 1.3 millimeter dust emission assuming a gas/dust ratio of100, in accordance with previous studies. These findings confirm that COis not a good tracer of the total gas content in disks since it can bephotodissociated in the outer layers and frozen onto grains in the colddense part of disks, but that it is a robust tracer of the disk velocityfield. In contrast, H2 can shield itself fromphotodissociation even in low-mass ``optically thin'' debris disks andcan therefore survive longer. The warm gas is typically 1%-10% of thetotal mass deduced from millimeter continuum emission, but it canincrease up to 100% or more for the debris-disk objects. Thus, residualmolecular gas may persist into the debris-disk phase. No significantevolution in the H2, CO, or dust masses is found for starswith ages in the range of 106-107 yr, although adecrease is found for the older debris-disk star β Pictoris. Thelarge amount of warm gas derived from H2 raises the questionof the heating mechanism(s). Radiation from the central star as well asthe general interstellar radiation field heat an extended surface layerof the disk, but existing models fail to explain the amount of warm gasquantitatively. The existence of a gap in the disk can increase the areaof material influenced by radiation. Prospects for future observationswith ground- and space-borne observations are discussed. Based in parton observations with ISO, an ESA project with instruments funded by ESAmember states (especially the PI countries: France, Germany,Netherlands, and the United Kingdom) and with participation of ISAS andNASA.
| 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.
| Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with Infrared Space Observatory LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passivecircumstellar disks constructed by Chiang & Goldreich. New featuresinclude (1) an account for a range of particle sizes, (2) employment oflaboratory-based optical constants of representative grain materials,and (3) numerical solution of the equations of radiative and hydrostaticequilibrium within the original two-layer (disk surface plus diskinterior) approximation. We systematically explore how the spectralenergy distribution (SED) of a face-on disk depends on grain sizedistributions, disk geometries and surface densities, and stellarphotospheric temperatures. Observed SEDs of three Herbig Ae and two TTauri stars, including spectra from the Long Wavelength Spectrometer(LWS) aboard the Infrared Space Observatory (ISO), are fitted with ourmodels. Silicate emission bands from optically thin, superheated disksurface layers appear in nearly all systems. Water ice emission bandsappear in LWS spectra of two of the coolest stars. Infrared excesses inseveral sources are consistent with significant vertical settling ofphotospheric grains. While this work furnishes further evidence thatpassive reprocessing of starlight by flared disks adequately explainsthe origin of infrared-to-millimeter wavelength excesses of young stars,we emphasize by explicit calculations how the SED alone does not providesufficient information to constrain particle sizes and disk massesuniquely.
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