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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.
| 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.
| 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.
| Polarization measurements of Vega-like stars
Optical linear polarization measurements are presented for about 30Vega-like stars. These are then compared with the polarization observedfor normal field stars. A significant fraction of the Vega-like starsare found to show polarization much in excess of that expected to be dueto interstellar matter along the line of sight to the star. The excesspolarization must be intrinsic to the star, caused by circumstellarscattering material that is distributed in a flattened disk. Acorrelation between infrared excess and optical polarization is foundfor the Vega-like stars.
| Spectroscopic, photometric, and polarimetric study of the Herbig AE candidate HD 36112
We describe observations of the Herbig Ae candidate HD 36112 includinghigh-resolution spectroscopy of photospheric and circumstellar lines,multi-colour photometry and polarimetry. These observations reveal thatmany features of the observational appearance of HD 36112 are similar tothose of `classical' Herbig Ae stars. Among these are: the far IRexcess, manifestations of the high-temperature zone in the innerenvelope, distinctive types of the Hα profile variations, andvariable multi-component polarization and brightness parameters. Thefundamental characteristics of the star have been determined. Inparticular, HD 36112 proves to be an unreddened A8 v of age t ~ 5/10x10(6) years. This means, that the star is still at the PMS phase ofevolution, but close to the ZAMS. The anomalous abundances of barium andsilicon discovered in HD 36112 allow to suggest that chemical anomaliescan arise already at the early stages of stellar evolution. The totalityof observed properties of HD 36112 give evidence of complicatedstructure of the gaseous-dusty envelope around this object, includingthe variable stellar wind with an extended acceleration zone, as well aslarge and small-grained dust, located in the orthogonal planes.
| 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.
| 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.
| Infrared and submillimetre studies of Vega-excess stars
We report new infrared and submillimetre observations of a sample of 24candidate Vega-excess stars, and derive CO masses, dust masses, gas todust ratios and the strengths of various emission lines. Most of thesestars have dustier discs than the class archetypes (Vega, Fomalhaut,beta Pic, etc.), yet, like the archetypes, all the stars observed in COshow the gas content of their discs to be depleted compared withmolecular cloud values. We discuss how the extra dust content mightimply that these stars are less evolved than the archetypes, and useother infrared and submillimetre characteristics to support thiscontention.
| Infrared excesses in A-type stars
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| Inhibition of Giant Planet Formation by Rapid Gas Depletion around Young Stars
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| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
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