目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| 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
| 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.
| EXPORT: Near-IR observations of Vega-type and pre-main sequence stars
We present near-IR JHK photometric data of a sample of 58 main-sequence,mainly Vega-type, and pre-main sequence stars. The data were takenduring four observing runs in the period May 1998 to January 1999 andform part of a coordinated effort with simultaneous optical spectroscopyand photo-polarimetry. The near-IR colors of the MS stars correspond inmost cases to photospheric colors, although noticeable reddening ispresent towards a few objects, and these stars show no brightnessvariability within the observational errors. On the other hand, the PMSstars show near-IR excesses and variability consistent with previousdata. Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110
| 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
| HD 98800: A Unique Stellar System of Post-T Tauri Stars
HD 98800 is a system of four stars, and it has a large infrared excessthat is thought to be due to a dust disk within the system. In thispaper we present new astrometric observations made with Hipparcos, aswell as photometry from Hubble Space Telescope WFPC2 images. Combiningthese observations and reanalyzing previous work allow us to estimatethe age and masses of the stars in the system. Uncertainty in these agesand masses results from uncertainty in the temperatures of the stars andany reddening they may have. We find that HD 98800 is most probablyabout 10 Myr old, although it may be as young as 5 Myr or as old as 20Myr. The stars in HD 98800 appear to have metallicities that are aboutsolar. An age of 10 Myr means that HD 98800 is a member of the post-TTauri class of objects, and we argue that the stars in HD 98800 can helpus understand why post-T Tauris have been so elusive. HD 98800 may haveformed in the Centaurus star-forming region, but it is extraordinary inbeing so young and yet so far from where it was born.
| 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.
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
