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Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.

Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.

Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.

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.

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.

Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in HD 105
We report infrared spectroscopic observations of HD 105, a nearby (~40pc) and relatively young (~30 Myr) G0 star with excess infraredcontinuum emission, which has been modeled as arising from an opticallythin circumstellar dust disk with an inner hole of size >~13 AU. Wehave used the high spectral resolution mode of the Infrared Spectrometer(IRS) on the Spitzer Space Telescope to search for gas emission linesfrom the disk. The observations reported here provide upper limits tothe fluxes of H2 S(0) 28 μm, H2 S(1) 17 μm,H2 S(2) 12 μm, [Fe II] 26 μm, [Si II] 35 μm, and [SI] 25 μm infrared emission lines. The H2 line upper limitsplace direct constraints on the mass of warm molecular gas in the disk:M(H2)<4.6, 3.8×10-2, and3.0×10-3 MJ at T=50, 100, and 200 K,respectively. We also compare the line flux upper limits to predictionsfrom detailed thermal/chemical models of various gas distributions inthe disk. These comparisons indicate that if the gas distribution has aninner hole with radius ri,gas, the surface density at thatinner radius is limited to values ranging from <~3 g cm-2at ri,gas=0.5 AU to 0.1 g cm-2 atri,gas=5-20 AU. These values are considerably below the valuefor a minimum mass solar nebula, and suggest that less than 1 Jupitermass (MJ) of gas (at any temperature) exists in the 1-40 AUplanet-forming region. Therefore, it is unlikely that there issufficient gas for gas giant planet formation to occur in HD 105 at thistime.

8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.

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).

Dusty Debris Disks as Signposts of Planets: Implications for Spitzer Space Telescope
Submillimeter and near-infrared images of cool dusty debris disks andrings suggest the existence of unseen planets. At dusty but nonimagedstars, semimajor axes of associated planets can be estimated from thedust temperature. For some young stars these semimajor axes are greaterthan 1" as seen from Earth. Such stars are excellent targets forsensitive near-infrared imaging searches for warm planets. To probe thefull extent of the dust and hence of potential planetary orbits, Spitzerobservations should include measurements with the 160 μm filter.

Are Giant Planets Forming around HR 4796A?
We have obtained Far Ultraviolet Spectroscopic Explorer and Hubble SpaceTelescope STIS spectra of HR 4796A, a nearby 8 Myr old main-sequencestar that possesses a dusty circumstellar disk whose inclination hasbeen constrained from high-resolution near-infrared observations to be~17° from edge-on. We searched for circumstellar absorption in theground states of C II λ1036.3, O I λ1039.2, Zn IIλ2026.1, Lyman series H2, and CO (A-X) and failed todetect any of these species. We place upper limits on the columndensities and infer upper limits on the gas masses assuming that the gasis in hydrostatic equilibrium, is well mixed, and has a temperatureTgas~65 K. Our measurements suggest that this systempossesses very little molecular gas. Therefore, we infer an upper limitfor the gas-to-dust ratio (<=4.0) assuming that the gas is atomic. Wemeasure less gas in this system than is required to form the envelope ofJupiter.

Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gas—dust shells and/ordisk—shells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gas—dustshells and/or disk—shells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by free—free transitions of electrons and bythermal emission from dust grains in circumstellar gas—dust shells(disks).

Astrochemistry : The molecular universe
Helen J Fraser, Martin R S McCoustra and David A Williams present asimple guide to astrochemistry. Molecules play a fundamental role inmany regions of our universe. The science where chemistry and astronomyoverlap is known as astrochemistry, a branch of astronomy that has risenin importance over recent years. In this article we review thesignificance of chemistry in several astronomical years. IN this articlewe review the significance of chemistry in several astronomicalenvironments including the early universe, interstellar clouds,starforming regions and protoplanetary disks. We discuss theoreticalmodels, laboratory experiments and observational data, and presentseveral recent and exciting results that challenge our perception of the``molecular universe''.

Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897

Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphere
Within the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105

Substantial reservoirs of molecular hydrogen in the debris disks around young stars
Circumstellar accretion disks transfer matter from molecular clouds toyoung stars and to the sites of planet formation. The disks observedaround pre-main-sequence stars have properties consistent with thoseexpected for the pre-solar nebula from which our own Solar System formed4.5Gyr ago. But the `debris' disks that encircle more than 15% of nearbymain-sequence stars appear to have very small amounts of gas, based onobservations of the tracer molecule carbon monoxide: these observationshave yielded gas/dust ratios much less than 0.1, whereas theinterstellar value is about 100 (ref. 9). Here we report observations ofthe lowest rotational transitions of molecular hydrogen (H2)that reveal large quantities of gas in the debris disks around the starsβ Pictoris, 49 Ceti and HD135344. The gas masses calculated fromthe data are several hundreds to a thousand times greater than thoseestimated from the CO observations, and yield gas/dust ratios of thesame order as the interstellar value.

Dusty Circumstellar Disks
Dusty circumstellar disks in orbit around main-sequence stars werediscovered in 1983 by the infrared astronomical satellite. It was thefirst time material that was not another star had been seen in orbitaround a main-sequence star other than our Sun. Since that time,analyses of data from the infrared astronomical satellite, the infraredspace observatory, and ground-based telescopes have enabled astronomersto paint a picture of dusty disks around numerous main-sequence andpost-main-sequence stars. This review describes, primarily in anevolutionary framework, the properties of some dusty disks orbiting,first, pre-main-sequence stars, then main-sequence andpost-main-sequence stars, and ending with white dwarfs.

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.

Gas—Dust Shells around Some Early-Type Stars with an IR Excess (of Emission)
The results of an investigation of IR (IRAS) observations of 58O—B—A—F stars of different luminosity classes, whichare mainly members of various associations, are presented. The colorindices of these stars are determined and two-color diagrams areconstructed. The emission excesses at 12 and 25 mm (E 12 and E 25) arealso compared with the absorption A1640 of UV radiation. It is concludedthat 24 stars (of the 58 investigated) are disk systems of the Vegatype, to which Vega = N 53 also belongs. Eight known stars of the Vegatype are also given in the figures for comparison. The remaining 34stars may have gas—dust shells and/or shell—disks. The IRemission excesses of the 34 investigated stars and 11 comparison stars(eight of them are Be-Ae stars) are evidently due both to thermalemission from grains and to the emission from free—freetransitions of electrons in the gas—dust shells of these stars.

Mid-Infrared Imaging of Candidate Vega-like Systems
We have conducted deep mid-infrared imaging of a relatively nearbysample of candidate Vega-like stars using the OSCIR instrument on theCerro Tololo Inter-American Observatory 4 m and Keck II 10 m telescopes.Our discovery of a spatially resolved disk around HR 4796A has alreadybeen reported in 1998 by Jayawardhana et al. Here we present imagingobservations of the other members of the sample, including the discoverythat only the primary in the HD 35187 binary system appears to harbor asubstantial circumstellar disk and the possible detection of extendeddisk emission around 49 Ceti. We derive global properties of the dustdisks, place constraints on their sizes, and discuss several interestingcases in detail. Although our targets are believed to be main-sequencestars, we note that several have large infrared excesses compared withprototype Vega-like systems and may therefore be somewhat younger. Thedisk size constraints we derive, in many cases, imply emission fromrelatively large (>~10 μm) particles at mid-infrared wavelengths.

NICMOS Coronagraphic Observations of 55 Cancri
We present new near-infrared (1.1 μm) observations of thecircumstellar environment of the planet-bearing star 55 Cancri. Withthese Hubble Space Telescope (HST) images we are unable to confirm theobservation of bright scattered radiation at longer NIR wavelengthspreviously reported by Trilling and coworkers. NICMOS coronagraphicimages with detection sensitivities to ~100 μJy arcsec-2at 1.1 μm in the region 28-60 AU from the star fail to reveal anysignificant excess flux in point-spread function (PSF) subtracted imagestaken in two HST orbits. These new observations place flux densities inthe 19-28 AU zone at a factor of 10 or more below the reportedground-based observations. Applying a suite of a dozen well-matchedcoronagraphic reference PSFs, including one obtained in the same orbitsas the observations of 55 Cnc, yielded consistently null results indetecting a disk. We also searched for and failed to find a suggestedflux-excess anisotropy in the ratio of ~1.7:1 in the circumstellarbackground along and orthogonal to the plane of the putative disk. Wesuggest that, if such a disk does exist, then the total 1.1 μmspectral flux density in an annular zone 28-42 AU from the star must beno more than ~0.4 mJy, at least 10 times smaller than suggested byTrilling and Brown, upon which their very large estimate for the totaldust mass (0.4 M⊕) was based. Based on the far-infraredand submillimeter flux of this system and observations of scatteredlight and thermal emission from other debris disks, we also expect theintensity of the scattered light to be at least an order of magnitudebelow our upper limits.

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 ( 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( 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.

Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521

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 ( or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110

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.

A Single Circumbinary Disk in the HD 98800 Quadruple System
We present subarcsecond thermal infrared imaging of HD 98800, a youngquadruple system composed of a pair of low-mass spectroscopic binariesseparated by 0.8" (38 AU), each with a K-dwarf primary. Images atwavelengths ranging from 5 to 24.5 μm show unequivocally that theoptically fainter binary, HD 98800B, is the sole source of acomparatively large infrared excess on which a silicate emission featureis superposed. The excess is detected only at wavelengths of 7.9 μmand longer, peaks at 25 μm, and has a best-fit blackbody temperatureof 150 K, indicating that most of the dust lies at distances greaterthan the orbital separation of the spectroscopic binary. We estimate theradial extent of the dust with a disk model that approximates radiationfrom the spectroscopic binary as a single source of equivalentluminosity. Given the data, the most likely values of disk properties inthe ranges considered are Rin=5.0+/-2.5 AU, ΔR=13+/-8AU, λ0=2+4-1.5 μm,γ=0+/-2.5, and σtotal=16+/-3 AU2,where Rin is the inner radius, ΔR is the radial extentof the disk, λ0 is the effective grain size, γis the radial power-law exponent of the optical depth τ, andσtotal is the total cross section of the grains. Therange of implied disk masses is 0.001-0.1 times that of the Moon. Theseresults show that, for a wide range of possible disk properties, acircumbinary disk is far more likely than a narrow ring.

Polarimetric observations of some stars with an infrared (emission) excess.
Not Available

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.

The Circumstellar Disk of HD 141569 Imaged with NICMOS
Coronagraphic imaging with the Near-Infrared Camera and MultiobjectSpectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4")radius, circumstellar disk around the Herbig Ae/Be star HD 141569. Areflected light image at 1.1 μm shows the disk oriented at a positionangle of 356^deg+/-5^deg and inclined to our line of sight by51^deg+/-3^deg the intrinsic scattering function of the dust in the diskmakes the side inclined toward us, the eastern side, brighter. The diskflux density peaks 185 AU (1.85") from the star and falls off to bothlarger and smaller radii. A region of depleted material, or a gap, inthe disk is centered 250 AU from the star. The dynamical effect of oneor more planets may be necessary to explain this morphology.

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Observation and Astrometry data

Right ascension:01h34m37.80s
Apparent magnitude:5.63
Distance:61.275 parsecs
Proper motion RA:96
Proper motion Dec:-2.5
B-T magnitude:5.689
V-T magnitude:5.615

Catalogs and designations:
Proper Names   (Edit)
Flamsteed49 Cet
HD 1989HD 9672
TYCHO-2 2000TYC 5852-2148-1
USNO-A2.0USNO-A2 0675-00563389
BSC 1991HR 451

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