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Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| 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.
| [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.
| HD 34700 is a T Tauri multiple system
We have imaged the young double-lined spectroscopic binary HD 34700 inthe near-infrared and find evidence for two faint, visual, stellarcomponents at 5.2 Arcsec and 9.2 arcsec distance. High-resolutionechelle spectroscopy of both stars shows strong Li I 6708 Åabsorption and Hα emission. The spectral types of the companionsare estimated from the spectra and photometry as M1-M2 and M2-M3. Theirradial velocities are similar to the center-of-mass velocity of thecentral SB2; hence all four stars are most probably physically bound andconstitute a young quadruple stellar system with an inner short-periodbinary. We provide a list of pre-main sequence spectroscopic binarieswith additional components of which HD 34700 is yet another example. Theavailable statistics strengthen the suspicion that dynamical effects inmultiple systems play a key role in the formation of very close binariesearly in their evolution.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Does the 3.3 Micron Polycyclic Aromatic Hydrocarbon Emission Feature Require Ultraviolet Excitation?
Unidentified infrared bands (UIBs) have been observed in virtually everydusty astrophysical environment investigated. The UIB carrier must beabundant and ubiquitous. Strong evidence points to polycyclic aromatichydrocarbons as likely candidates, but the identification is notcomplete. Additional diagnostics are needed to further constrain the UIBcarrier, such as probing excitation sources ranging from UV-strong toUV-weak to determine the ``band gap'' of the UIB carrier. Observationsand models suggest that the UIBs can be found in sources with weak UVfields. To that end, we present new results of observing the 3.3 μmspectral region in six stars embedded in reflection nebulae and in sixVega-like stars. These objects have effective temperatures ranging from3500 to 12,000 K. Their environments include dust that should berelatively unprocessed (reflection nebulae) and dust that has mostlikely undergone significant processing (Vega-like) by the embeddedillumination source. Together with data from the literature, we have asample of 27 sight lines. Our analysis suggests that neither thestrength of the UV field impinging on the dust nor the effectivetemperature of the star is the determining factor in whether the 3.3μm UIB emission is present in an object. We found three detections ofthe 3.3 μm emission band, one in a Vega-type object, one in a HerbigAe/Be object, and one in a reflection nebula, and all with disks. Therole of disk geometry is likely to be important in revealing orobscuring the photodissociation regions from which the UIB emissionarises.
| A Double-lined Spectroscopic Orbit for the Young Star HD 34700
We report high-resolution spectroscopic observations of the young starHD 34700, which confirm it to be a double-lined spectroscopic binary. Wederive an accurate orbital solution with a period of 23.4877+/-0.0013days and an eccentricity of e=0.2501+/-0.0068. The stars are found to beof similar mass (M2/M1=0.987+/-0.014) andluminosity. We derive also the effective temperatures (5900 and 5800 K)and projected rotational velocities (28 and 22 km s-1) of thecomponents. These values of vsini are much higher than expected formain-sequence stars of similar spectral type (G0) and are not due totidal synchronization. We also discuss the indicators of youth availablefor the object. Although there is considerable evidence that the systemis young-strong infrared excess, X-ray emission, Li I λ6708absorption (0.17 Å equivalent width), Hα emission (~0.6Å), rapid rotation-the precise age cannot yet be establishedbecause the distance is unknown.
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.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) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| Correlation between grain growth and disk geometry in Herbig Ae/Be systems
We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Bestars, for which we can determine the infrared spectral energydistribution (SED). We find a clear correlation between the strength ofthe ratio of the near- to mid-infrared excess of these sources, and theslope of the (sub-)mm energy distribution. Based on earliermulti-dimensional modeling of disks around Herbig Ae stars, we interpretthis as a correlation between the geometry of the disk (flared orself-shadowed) and the size of the grains: self-shadowed disks have, onaverage, larger grains than their flared counterparts. These datasuggest that the geometry of a young stellar disk evolves from flared toself-shadowed.
| Detections of Rovibrational H2 Emission from the Disks of T Tauri Stars
We report the detection of quiescent H2 emission in thev=1-->0 S(1) line at 2.12183 μm in the circumstellar environmentof two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution(R~=60,000) spectra, bringing to four, including TW Hya and theweak-lined T Tauri star DoAr 21, the number of T Tauri stars showingsuch emission. The equivalent widths of the H2 emission linelie in the range 0.02-0.10 Å, and in each case the centralvelocity of the emission line is centered at the star's systemicvelocity. The line widths range from 9 to 14 km s-1, inagreement with those expected from gas in Keplerian orbits incircumstellar disks surrounding K-type stars at distances >=10 AUfrom the sources. UV fluorescence and X-ray heating are likely candidatemechanisms responsible for producing the observed emission. We presentmass estimates from the measured line fluxes and show that the estimatedmasses are consistent with those expected from the possible mechanismsresponsible for stimulating the observed emission. The high temperaturesand low densities required for significant emission in the v=1-->0S(1) line suggests that we have detected reservoirs of hot H2gas located in the low-density upper atmospheres of circumstellar disksof these stars.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| HD 34700: A new T Tauri double-lined spectroscopic binary
We find the star HD 34700 to be a double-lined spectroscopic binary. Wealso identify it as a weak-line T Tauri object. The spectra of bothcomponents are very similar and both show the Li I feature at 6708Å. Strong arguments in favour of the binary nature of the star asopposed to other possibilities are offered. It is very likely that thecompanion is also a T Tauri star of similar mass. A projected rotationalvelocity, vsin i, of 25 and 23 km s-1 has been estimated forthe blue and red components. We present a list of the lines identifiedand the radial velocities of the two components on three spectraobtained on consecutive nights.Based on observations obtained at the Haute-Provence Observatory,France.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/L29
| BVRIJHK photometry of post-AGB candidates
BVRIJHK photometric observations are presented for 27 post-AGBcandidates. Almost all objects show a double peaked SED curve in theoptical to far-infrared wavelengths. Seventeen objects were classifiedas post-AGB stars on the basis of their spectral type, location in theIRAS color-color diagram and SED. The physical parameters of theobserved post-AGB stars, the inner radius of the detached shell, themass of the shell and the distance were derived using the simple dustshell model. We compared our observational sequence of post-AGB objectsto the theoretical evolutionary sequence (Schönberner\cite{schonberner1983}; Blöcker \cite{blocker}) in the stellartemperatures versus age diagram. We found that two post-AGB stars, IRAS05040+4820 and 08187-1905, have low stellar temperature with a largedynamical age of the dust shell. They appear to provide the firstobservational evidence that some low-mass stars bypass the planetarynebulae stage because of their slow increase in stellar temperature.Based on observations obtained at Kiso Observatory, Nagano, Japan.Table2 is also 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/385/884
| 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: 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
| Combining NICMOS Imaging and Mid-infrared Measurements of Circumstellar Disks
We present high angular resolution near-infrared imaging with HST/NICMOSand mid-infrared imaging and spectroscopy from the Keck Observatorywhich together elucidate the morphology and composition of circumstellardisks. In particular, we highlight results for HD 141569, TW Hydrae, andHD 34700. HD 141569 is a 5 Myr old AeBe star with a spectrum apparentlydominated by PAH emission. TW Hya is of similar age but with a spectrumdominated by silicates. HD 34700 may not be a young object at all but apost-main sequence mass-loss star. Its disk and/or shell is very brightand extended in near-infrared scattered light (NICMOS). All three starsshow extended dust emission at mid-infrared wavelengths, which, whencombined with their 7 - 13 micron spectra constrain the emissivity andlocation of their constituent dust. This work is supported in part byNASA grant NAG 5-3042 and based on observations with the NASA/ESA HubbleSpace Telescope and the W. M. Keck Observatory.
| Optical, infrared and millimetre-wave properties of Vega-like systems - IV. Observations of a new sample of candidate Vega-like sources
Photometric observations at optical and near-infrared wavelengths arepresented for members of a new sample of candidate Vega-like systems, ormain sequence stars with excess infrared emission due to circumstellardust. The observations are combined with IRAS fluxes to define thespectral energy distributions of the sources. Most of the sources showonly photospheric emission at near-IR wavelengths, indicating a lack ofhot (~1000K) dust. Mid-infrared spectra are presented for four sourcesfrom the sample. One of them, HD 150193, shows strong silicate emission,while another, HD 176363, was not detected. The spectra of two starsfrom our previous sample of Vega-like sources both show UIR-bandemission, attributed to hydrocarbon materials. Detailed comparisons ofthe optical and IRAS positions suggest that in some cases the IRASsource is not physically associated with the visible star. Alternativeassociations are suggested for several of these sources. Fractionalexcess luminosities are derived from the observed spectral energydistributions. The values found are comparable to those measuredpreviously for other Vega-like sources.
| ISOPHOT Observations of Dust Disks around Main Sequence (Vega-Like) Stars
The photometer (ISOPHOT) on the Infrared Space Observatory (ISO) hasproved to be invaluable for investigating the dust around main sequencestars (both prototypes and candidate Vega-like stars). The longwavelength camera (at 60 and 90 μm) has been used to map the areaaround the stars to establish whether the dust disk is extended.Low-resolution spectra between 5.8 and 11.6 μm show whether the dustis composed of silicate grains, and whether molecular features arepresent. The four prototype Vega-like stars (Vega, β Pic,Fomalhaut, ɛ Eri) are studied, as well as eight other stars,which are main sequence stars with cool dust associated with them. Wefind that the spectra of β Pic, 49 Cet, HD98800, and HD135344 showexcess emission from the cool dust around the star, HD144432 andHD139614 show silicate dust emission, HD169142 and HD34700 show emissionfeatures from carbon-rich molecules (possibly PAHs, polycyclic aromatichydrocarbon molecules), and HD142666 shows emission features from bothcarbon-rich molecules and silicate dust. Up to 11.6 μm, the emissionfrom Vega, Fomalhaut, and ɛ Eri is dominated by the stellarphotosphere. At 60 and 90 μm, the extended dust emission is mapped,and the disk resolved in eight cases. The dust mass in the disks isfound to range from around 10-9 to 10-4Msolar. Since several of the stars are younger than the Sun,and the disks have sufficient material of the type found in the SolarSystem, these disks could be in the early stages of planet formation.
| 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.
| 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.
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| 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
| Inhibition of Giant Planet Formation by Rapid Gas Depletion around Young Stars
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