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Spectroscopic metallicities of Vega-like stars
Aims. We aim to determine the metallicities of 113 Southern HemisphereVega-like candidate stars in relation to the giant exoplanet host groupand field stars. Methods: We applied two spectroscopic methods ofabundance determinations: equivalent width measurements together withthe ATLAS9 model atmospheres and the WIDTH9 program, and a comparison ofobserved spectra with a grid of synthetic spectra. Results: For theVega-like group, the metallicities are indistinguishable from those offield stars not known to be associated with planets or disks. Thisresult is quite different from the metallicities of giant exoplanet hoststars, which are metal-rich in comparison to field stars.Tables 1-4 are only available in electronic form at http://www.aanda.org
| Dust in the solar system and in extra-solar planetary systems
Among the observed circumstellar dust envelopes a certain population,planetary debris disks, is ascribed to systems with optically thin dustdisks and low gas content. These systems contain planetesimals andpossibly planets and are believed to be systems that are most similar toour solar system in an early evolutionary stage. Planetary debris diskshave been identified in large numbers by a brightness excess in thenear-infrared, mid-infrared and/or submillimetre range of their stellarspectral energy distributions. In some cases, spatially resolvedobservations are possible and reveal complex spatial structures. Actingforces and physical processes are similar to those in the solar systemdust cloud, but the observational approach is obviously quite different:overall spatial distributions for systems of different ages for theplanetary debris disks, as opposed to detailed local information in thecase of the solar system. Comparison with the processes of dustformation and evolution observed in the solar system therefore helpsunderstand the planetary debris disks. In this paper, we review ourpresent knowledge of observations, acting forces, and major physicalinteractions of the dust in the solar system and in similar extra-solarplanetary systems.
| Near-infrared imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| Modeling the gas reservoir of circumstellar disks around young G-type stars
Interpretation of molecular line observations in tenuous circumstellardisks around young G-type stars in terms of a disk mass is difficultwithout a model that describes the chemical structure of these disks.This paper discusses the chemistry in tenuous disks around youngsolar-type stars based on disk models that take into account thepresence of a stellar chromosphere. The example of the disk around a 70Myr old solar-type star shows that the dissociating radiation from thechromosphere is stronger than the interstellar ultraviolet radiationfield up to a distance of ˜400 AU from the star. Similar to otherstudies in this research field, it is found that, due tophotodissociation, the CO-to-H2 ratio is far from thecanonical value of 10-4 for molecular clouds. Moreover, thedust-to-gas mass ratio as well as the dust grain size plays an importantrole for the H2 abundance in these disks.
| Submillimetre observations and modelling of Vega-type stars
We present new submillimetre observations of Vega-excess stars, andconsistent modelling for all known Vega-excess stars with submillimetredata. Our analysis uses dust grain models with realistic opticalproperties, with the aim of determining physical parameters of theunresolved discs from just their spectral energy distributions (SEDs).For the resolved targets, we find that different objects require verydifferent dust grain properties in order to fit the image data and SEDsimultaneously. Fomalhaut and Vega require solid dust grains, while HR4796 and HD 141569 can only be fitted using porous grains. The olderstars tend to have grains which are less porous than the younger stars,which may indicate that collisions in the discs have reprocessed theinitially fluffy grains into a more solid form. ɛ Eri appears to bedeficient in small dust grains compared with our best-fitting model.This may show that it is important to include all the factors that causethe size distribution to depart from a simple power law for grains closeto the radiation pressure blow-out limit. Alternatively, thisdiscrepancy may be due to some external influence on the disc (e.g. aplanet).When the model is applied to the unresolved targets, an estimate of thedisc size can be made. However, the large diversity in dust compositionfor the resolved discs means that we cannot make a reliable assumptionas to the composition of the grains in an unresolved disc, and there iscorresponding uncertainty in the disc size. In addition, the poor fitfor ɛ Eri shows that the model cannot always account for the SEDeven if the disc size is known. These two factors mean that it may notbe possible to determine the size of a disc without actually resolvingit.
| Discovery of Reflection Nebulosity around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars revealreflection nebulosities, five of which were previously unknown. Thenebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, andHD 49662 resemble that of the Pleiades, indicating an interstellarorigin for dust grains. The reflection nebulosity around HD 123160 has adouble-arm morphology, but no disklike feature is seen as close as 2.5"from the star in K-band adaptive optics data. We demonstrate that auniform density dust cloud surrounding HD 23362, HD 23680, and HD 123160can account for the observed 12-100 μm spectral energy distributions.For HD 4881, HD 26676, and HD 49662, an additional emission source, suchas from a circumstellar disk or nonequilibrium grain heating, isrequired to fit the 12-25 μm data. These results indicate that insome cases, particularly for Vega-like stars located beyond the LocalBubble (>100 pc), the dust responsible for excess thermal emissionmay originate from the interstellar medium rather than from a planetarydebris system.
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/379/564
| SCUBA photometry of candidate Vega-like sources
New SCUBA measurements at millimetre wavelengths are presented for asample of Vega-like stars. Six stars were detected, while sensitiveupper limits were obtained for a further 11 sources. Most of the sampleselected from a recent catalogue of Vega-like stars have infraredexcesses similar to those of the prototype Vega-like stars α Lyrand α PsA. Their IR-submm spectral indices are steep, indicatingthat the submm emission from the discs is dominated by grains which aresmaller than the wavelength of observation and that only small grainsexist in those dusty discs. HD 98800 has an IR-submillimetre spectralindex of less than two, which suggests that grains have grown to morethan 0.3mm in size. Hipparcos parallax data for HD 42137 and HD 123160suggest that these two stars are giants rather than dwarfs, similar tothe situation previously found for HD 233517. Dust masses, or upperlimits, were derived for the sample; these indicate that most of thesources do not have as much dust as Herbig Ae/Be or T Tauri stars, butare likely to have dust masses comparable to those of the prototypeVega-like stars.
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| Orbiting Molecular Reservoirs around Evolved Red Giant Stars
We report molecular emission from the circumstellar envelopes of twocarbon-rich stars with oxygen-rich envelopes, EU And and BM Gem. We finda narrow (FWHM~5 km s^-1) CO (2-1) emission line from EU And and an evennarrower (FWHM ~ 1 km s^-1) ^13CO emission line from BM Gem. We alsoplace upper limits to the emission of HCN, SiO, SO, HCO^+, and CS fromBM Gem. We argue that the narrow CO emission lines are signatures oflong-lived reservoirs of orbiting gas and that standard models for COemission from red giant winds are not appropriate for these two stars.By including the Red Rectangle and AC Her, narrow CO emissioncharacteristic of gravitationally bound gas has been detected from fourpost-main-sequence systems, and we can begin to characterize theseapparently similar environments. Some common characteristics are thefollowing: (1) Their diameters are typically between ~100 and ~1000 AU.(2) The masses of CO are near 10^27 g. (3) Unlike the envelopes aroundmass-losing carbon stars where M_CO/M_dust~ 2, the circumstellarorbiting reservoirs often appear to have M_CO
| High-resolution spectroscopy of Vega-like stars - II. Age indicators, activity and circumstellar gas
We have completed a high-resolution optical study of 14 stars classifiedas Vega-like, having an IR excess attributable to dust emission. Surfacelithium abundances were measured for the four G- and K-type stars of thesample, to test the suggestion that these Vega-like stars areintermediate in evolutionary state between pre-main-sequence objects andestablished main-sequence stars. Abundances ranged from a very highvalue in the G5e star HD 143006, implying a very low age of only 1Myr,to below the limit of measurement for the K2V star HD 23362, which weconclude to be already well established on the main sequence. Theemission-line characteristics of all the stars in our sample werestudied to compare with those seen in the classical pre-main-sequenceHerbig Ae/Be (HAeBe) stars and T Tauri stars. Activity levels rangedfrom stars showing little or no activity, such as HD 23362, to thoseexhibiting extensive activity, such as the A9/F0Ve star HD 144432, whichshowed distinctive P Cyg profiles in its spectrum, and HD 143006, whichis young enough to be included in the T Tauri class of stars. The A2/3estar HD 35187 shows evidence of variability in its Hα and Heilambda5876 lines, with four other A-type stars in our sample alsoshowing evidence of Hei lambda5876 activity in the form of emission orabsorption. We interpret the excess absorption and/or emission in thelambda5876 line as providing direct evidence for ongoing accretionactivity on these systems. We find that the emission characteristics ofthe Hα, Nai D, Hei and Caii K lines are not significantly differentfrom those of HAeBe stars and T Tauri stars. Some of our sample havealso been previously classified as pre-main-sequence or candidatepre-main-sequence stars, which would seem to suggest that there is nodistinct boundary between Vega-like stars and the HAeBe and T Tauristars. The surface gravities of the A- and F-type stars in our sampleimply that they have already reached the main sequence, consistent withthe short time-scales to reach the main sequence predicted for stars oftheir mass and the fact that they are not located close to starformation regions. On the other hand, the ages derived for the threeemission-line G- and K-type stars in our sample imply that they haveprobably not yet reached the main sequence. It is likely that theseemission-line Vega-like stars represent the intermediate stage betweenclassical pre-main-sequence stars with `dusty' IR excesses and starsthat are well established on the main sequence. We also studied oursample for evidence of optical circumstellar gas absorption features. Ofthe 14 stars, seven show evidence for narrow absorption lines in theirspectra. Most of these appear to be of interstellar origin. One of thesestars, HD 144432, has a narrow absorption component in the absorptiontrough of its Nai D P Cyg profile, at a heliocentric velocity of-91kms^-1, which excludes an interstellar origin and therefore morelikely originates in its circumstellar environment. We also detectnarrow absorption lines in the spectrum of HD 158643 (51 Oph) arisingfrom excited-state Feii lines, which can only come from thecircumstellar environment of the star.
| Optical, infrared and millimetre-wave properties of Vega-like systems - III. Models with thermally spiking grains
Vega-like stars are main-sequence stars that exhibit excess IR emissiondue to circumstellar dust grains which are probably distributed indiscs. We have recently published an observational data base for a largesample of candidate Vega-like systems, comprising optical, near-IR andmm/submm-wave photometry, and mid-IR spectra. In a previous paper wepresented radiative transfer models of eight sources from our samplethat had low fractional excess luminosities. Here we present models of afurther eight sources, all with large fractional excess luminositiesdominated by excess emission at near-IR wavelengths. It was found thatno single distribution of dust grains at thermal equilibrium in a disccould simultaneously match the excess emission at near-IR and longerwavelengths. We attempted to model the near-IR emission as due tothermally spiking small grains, which can temporarily attain the hightemperatures required to produce excess near-IR emission. A near-IRspectrum of SAO 186777 shows the 3.3-μm UIR emission band, confirmingour earlier detection of UIR emission at longer wavelengths, andsuggesting that small carbonaceous particles are responsible for some ofthe near-IR emission. The thermally spiking models were only partiallysuccessful and many of the sources required the presence of grainsemitting in thermal equilibrium at ~ 1000-1500 K. These grains musteither be located very close to the stars (<1 au), or else be poweredby accretion luminosity. Calculations of the optical depths of the modeldiscs suggest the discs are optically thick at visual wavelengths;optically thick modelling of these sources is desirable. The discs areoptically thin at mm wavelengths, allowing us to confirm the presence oflarge grains in the discs. The stars presented in this paper may well beyounger than the prototype Vega-like stars.
| High-resolution spectroscopy of Vega-like stars - I. Effective temperatures, gravities and photospheric abundances
Vega-like stars are young main-sequence stars exhibiting an excessemission of infrared radiation. Modelling this excess depends not onlyon the parameters assigned to the grains, but on those assigned to thestars themselves. In an effort to update and improve the informationavailable on this class of star, we have analysed 13 stars classed asVega-like, having an infrared excess attributable to dust emission,along with two spectral standards which have also been found to showexcess emission from dust. In this, the first of two papers, we derivestellar properties (spectral type, effective temperature and log g) andphotospheric abundances. The spectral types derived revealed that one ofthe sample was a luminosity class III giant, ruling it out of theVega-like class, and two others underwent a significantreclassification. The remainder had their type confirmed. All but twoprogramme stars have been found to be emission-line stars - theiremission-line properties are discussed in Paper II. Attention hasrecently been drawn to the possible link between Vega-like stars and thephotospheric metal-depleted class of A-type stars, the lambda Bootisstars. These latter stars are hypothesized to have obtained theirunderabundances by the accretion of depleted circumstellar gas on to thephotosphere of the star. Since Vega-like stars are expected to havediscs of dust, it might be expected that accretion may cause this samephenomenon. We have analysed four A-type stars in our sample and twoA-type standards, deriving photospheric abundances for up to 10elements. No pattern of underabundance similar to lambda Bootis starswas found, although a depletion of silicon was found in two stars (up to0.86 dex below solar) and of magnesium in one star (0.56 dex lower). Thedepletion could be attributable to the accretion of those elements on tograins in the circumstellar environment of these stars.
| Optical, infrared and millimetre-wave properties of Vega-like systems - II. Radiative transfer modelling
Vega-like stars are main-sequence stars that exhibit excess infraredemission due to dust grains which are believed to be distributed incircumstellar discs. We have recently published a new observationaldatabase for a large sample of candidate Vega-like systems, comprisingoptical, near-infrared and mm/submm-wave photometry, and mid-infraredspectra. This paper presents radiative transfer models of eight of oursources, calculated using an optically thin code. For some of thesources the observations are unable to constrain the modelssuccessfully, while for those with larger fractional excessluminosities, we are able to determine the parameters of the dust discs.Dust masses for the well-constrained models lie in the range(1-70)x10^-7 M_solar - considerably less than the derived masses forpre-main-sequence discs. The grain size distributions favour smallergrains, as is generally found for interstellar and Solar system dust.Large (~1 mm) grains are found to be abundant around SAO 179815 (HD98800), but not around SAO 112630 and SAO 140789. The disc densities arefound to fall off steeply with distance from the central stars. Thederived inner radii of the discs vary from approximately one to severalhundred au.
| Optical, infrared and millimetre-wave properties of Vega-like systems.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996MNRAS.279..915S&db_key=AST
| A survey for infrared excesses among high galactic latitude SAO stars
This project involves extending the previous analysis of infraredexcesses among a volume-limited sample of 134 nearby A-K main-sequencestars to a magnitude-limited sample of stars, culled from the SAOCatalog, with excesses determined from the IRAS Point Source Catalogflux density ratios. This new sample includes 5706 B-M type stars, 379of which have infrared excesses. The objective involved use of astatistically complete survey of objects in a standard catalog in orderto assess the frequency with which different physical processes canaffect the infrared output of stars. These processes include, but arenot limited to, orbiting cold particle clouds and the onset of rapidmass loss. It is concluded that cold disks are consistent with theinfrared excesses found among A-G dwarfs and G-K giants in the sample.
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