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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.
| First Two-Micron Imaging Polarimetry of β Pictoris
High-resolution K-band imaging polarimetry of the β Pic dust diskhas been conducted with adaptive optics and a coronagraph using theSubaru 8.2 m telescope. Polarization of ~10% is detected out to r~120 AUwith a centrosymmetric vector pattern around the central star,confirming that the disk is seen as an infrared reflection nebula. Wehave modeled our near-infrared and previous optical polarization resultsin terms of dust scattering in the disk and have found that both thedegrees of polarization and the radial intensity profiles are wellreproduced. We argue that the observed characteristics of the disk dustare consistent with the presence of ice-filled fluffy aggregatesconsisting of submicron grains in the β Pic system. There is a gaparound 100 AU in both the intensity and polarization profiles, whichsuggests a paucity of planetesimals in this region. The radial intensityprofile also shows ripple-like structures, which are indicative of thepresence of multiple planetesimal belts, as in the case of the M-typeVega-like star AU Mic.
| The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039
We report the discovery of a debris system associated with the ~30 Myrold G3/5V star HD 12039 using Spitzer Space Telescope observations from3.6-160 μm. An observed infrared excess(LIR/L*=1×10-4) above theexpected photosphere for λ>~14 μm is fit by thermallyemitting material with a color temperature of T~110 K, warmer than themajority of debris disks identified to date around Sun-like stars. Theobject is not detected at 70 μm with a 3 σ upper limit 6 timesthe expected photospheric flux. The spectrum of the infrared excess canbe explained by warm, optically thin material comprised ofblackbody-like grains of size >~7 μm that reside in a beltorbiting the star at 4-6 AU. An alternate model dominated by smallergrains, near the blowout size a~0.5 μm, located at 30-40 AU is alsopossible but requires the dust to have been produced recently, sincesuch small grains will be expelled from the system by radiation pressurein approximately a few times 102 yr.
| Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
| Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star
The slow but persistent collisions between asteroids in our Solar Systemgenerate a tenuous cloud of dust known as the zodiacal light (because ofthe light the dust reflects). In the young Solar System, such collisionswere more common and the dust production rate should have been manytimes larger. Yet copious dust in the zodiacal region around stars muchyounger than the Sun has rarely been found. Dust is known to orbitaround several hundred main-sequence stars, but this dust is cold andcomes from a Kuiper-belt analogous region out beyond the orbit ofNeptune. Despite many searches, only a few main-sequence stars revealwarm (> 120K) dust analogous to zodiacal dust near the Earth. Signsof planet formation (in the form of collisions between bodies) in theregions of stars corresponding to the orbits of the terrestrial planetsin our Solar System have therefore been elusive. Here we report anexceptionally large amount of warm, small, silicate dust particlesaround the solar-type star BD+20 307 (HIP8920, SAO75016). Thecomposition and quantity of dust could be explained by recent frequentor huge collisions between asteroids or other `planetesimals' whoseorbits are being perturbed by a nearby planet.
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| Identification of Main-Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: β Pictoris Analogs?
Spitzer IRAC 3.6-8 μm photometry obtained as part of the GLIMPSEsurvey has revealed mid-infrared excesses for 33 field stars with knownspectral types in a 1.2 deg2 field centered on the southernGalactic H II region RCW 49. These stars comprise a subset of 184 starswith known spectral classification, most of which were preselected tohave unusually red IR colors. We propose that the mid-IR excesses arecaused by circumstellar dust disks that are either very late remnants ofstellar formation or debris disks generated by planet formation. Ofthese 33 stars, 29 appear to be main-sequence stars on the basis ofoptical spectral classifications. Five of the 29 main-sequence stars areO or B stars with excesses that can be plausibly explained by thermalbremsstrahlung emission, and four are post-main-sequence stars. The loneO star is an O4 V((f)) at a spectrophotometric distance of3233+540-535 pc and may be the earliest member ofthe Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18have spectral energy distributions that are consistent with hot dustydebris disks, a possible signature of planet formation. Modeling theexcesses as blackbodies demonstrates that the blackbody components havefractional bolometric disk-to-star luminosity ratios,LIR/L*, ranging from 10-3 to10-2 with temperatures ranging from 220 to 820 K. Theinferred temperatures are more consistent with asteroid belts than withthe cooler temperatures expected for Kuiper belts. Mid-IR excesses arefound in all spectral types from late B to early K.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Radial Distribution of Dust Grains around HR 4796A
We present high dynamic range images of circumstellar dust around HR4796A that were obtained with MIRLIN at the Keck II telescope atλ=7.9, 10.3, 12.5, and 24.5 μm. We also present a newcontinuum measurement at 350 μm obtained at the Caltech SubmillimeterObservatory. Emission is resolved in Keck images at 12.5 and 24.5 μmwith point-spread function FWHM values of 0.37" and 0.55", respectively,and confirms the presence of an outer ring centered at 70 AU. Unresolvedexcess infrared emission is also detected at the stellar position andmust originate well within 13 AU of the star. A model of dust emissionfitted to flux densities at 12.5, 20.8, and 24.5 μm indicates thatdust grains are located 4+3-2 AU from the starwith effective size 28+/-6 μm and an associated temperature of260+/-40 K. We simulate all extant data with a simple model ofexozodiacal dust and an outer exo-Kuiper ring. A two-component outerring is necessary to fit both Keck thermal infrared and Hubble SpaceTelescope scattered-light images. Bayesian parameter estimates yield atotal cross-sectional area of 0.055 AU2 for grains roughly 4AU from the star, and an outer-dust disk composed of a narrowlarge-grain ring embedded within a wider ring of smaller grains. Thenarrow ring is 14+/-1 AU wide with inner radius 66+/-1 AU and totalcross-sectional area 245 AU2. The outer ring is 80+/-15 AUwide with inner radius 45+/-5 AU and total cross-sectional area 90AU2. Dust grains in the narrow ring are about 10 times largerand have lower albedos than those in the wider ring. These propertiesare consistent with a picture in which radiation pressure dominates thedispersal of an exo-Kuiper belt.
| Mid-IR observations of circumstellar disks. II. Vega-type stars and a post-main sequence object
We present spectral energy distributions and new N-band photometry andspectroscopy for a sample of six main sequence stars and one post-MSobject using the ESO TIMMI2 camera at La Silla observatory (Chile). Allobjects are thought to possess circumstellar material and for themajority of the targets this is their first N-band spectroscopicobservation. The emission spectra (observed in three cases), modelledwith a mixture of silicates consisting of different grain sizes andcomposition, confirm the suspected presence of disks around thesetargets. The most important discovery is that HD 113766, a youngVega-type star, is host to highly processed dust which is probablysecond generation. It is the first time a Vega-type star with suchhighly evolved dust has been observed. Silicate emission of basicallyunevolved dust is seen in case of the post-MS object HD 41511 and theVega-type star HD 172555. In addition, to study the cold dust, weobserved a subsample at 1200 μm with the bolometer array SIMBA at theSEST in La Silla but we only got upper limits for those five objects.This shows that these Vega-type stars have a smaller amount of dust thantheir precursors, the T Tauri and Herbig Ae/Be stars.Based on observations collected at the European Southern Observatory, LaSilla, Chile (70.C-0468, 71.C-0001).
| New Debris-Disk Candidates: 24 Micron Stellar Excesses at 100 Million years
Sixty-three members of the 100 Myr old open cluster M47 (NGC 2422) havebeen detected at 24 μm with the Spitzer Space Telescope. The Be starV 378 Pup shows an excess both in the near-infrared and at 24 μm(K-[24]=2.4 mag), probably due to free-free emission from the gaseousenvelope. Seven other early-type stars show smaller excesses,K-[24]=0.6-0.9. Among late-type stars, two show large excesses: P922, aK1 V star with K-[24]=1.08+/-0.11, and P1121, an F9 V star withK-[24]=3.72+/-0.02. P1121 is the first known main-sequence star showingan excess comparable to that of β Pic, which may indicate thepresence of an exceptionally massive debris disk. It is possible that amajor planetesimal collision has occurred in this system, consistentwith the few hundred Myr timescales estimated for the clearing of thesolar system.
| Testing the Reality of Strong Magnetic Fields on T Tauri Stars: The Naked T Tauri Star Hubble 4
High-resolution optical and infrared (IR) echelle spectra of the naked(diskless) T Tauri star Hubble 4 are presented. The K-band IR spectrainclude four Zeeman-sensitive Ti I lines along with several magneticallyinsensitive CO lines. Detailed spectrum synthesis combined with modernatmospheric models is used to fit the optical spectra of Hubble 4 inorder to determine its key stellar parameters: Teff=4158+/-56K; logg=3.61+/-0.50 [M/H]=-0.08+/-0.05 vsini=14.6+/-1.7 kms-1. These stellar parameters are used to synthesize K-bandspectra to compare with the observations. The magnetically sensitive TiI lines are all significantly broadened relative to the lines producedin the nonmagnetic model, while the magnetically insensitive CO linesare well matched by the basic nonmagnetic model. Models with magneticfields are synthesized and fitted to the Ti I lines. The best-fit modelsindicate a distribution of magnetic field strengths on the stellarsurface characterized by a mean magnetic field strength of 2.51+/-0.18kG. The mean field is a factor of 2.0 greater than the maximum fieldstrength predicted by pressure equipartition arguments. To confirm thereality of such strong fields, we attempt to refit the observed profilesusing a two-component magnetic model in which the field strength isconfined to the equipartition value representing plagelike regions inone component and the field is allowed to vary in a cooler componentrepresenting spots. It is shown that such a model is inconsistent withthe optical spectrum of the TiO bandhead at 7055 Å.
| On the Flux of Extrasolar Dust in Earth's Atmosphere
Micron-size extrasolar dust particles have been convincingly detected bysatellites. Larger extrasolar meteoroids (5-35 μm) have most likelybeen detected by ground-based radar at Arecibo and New Zealand. Wepresent estimates of the minimum detectable particle sizes and thecollecting areas for both radar systems. We show that particles largerthan ~10 μm can propagate for tens of parsecs through theinterstellar medium, opening up the possibility that ground-based radarsystems can detect AGB stars, young stellar objects such as T Tauristars, and debris disks around Vega-like stars. We provide analyticaland numerical estimates of the ejection velocity in the case of a debrisdisk interacting with a Jupiter-mass planet. We give rough estimates ofthe flux of large micrometeoroids from all three classes of sources.Current radar systems are unlikely to detect significant numbers ofmeteors from debris disks such as β Pictoris. However, we suggestimprovements to radar systems that should allow for the detection ofmultiple examples of all three classes.
| Ten Micron Observations of Nearby Young Stars
We present new 10 μm photometry of 21 nearby young stars obtained atthe Palomar 5 m and at the Keck I 10 m telescopes as part of a programto search for dust in the habitable zone of young stars. Thirteen of thestars are in the F-K spectral type range (``solar analogs''), four haveB or A spectral types, and four have spectral type M. We confirmexisting IRAS 12 μm and ground-based 10 μm photometry for 10 ofthe stars and present new insight into this spectral regime for therest. Excess emission at 10 μm is not found in any of the young solaranalogs, except for a possible 2.4 σ detection in the G5 V star HD88638. The G2 V star HD 107146, which does not display a 10 μmexcess, is identified as a new Vega-like candidate, based on our 10μm photospheric detection, combined with previously unidentified 60and 100 μm IRAS excesses. Among the early-type stars, a 10 μmexcess is detected only in HD 109573A (HR 4796A), confirming priorobservations; among the M dwarfs, excesses are confirmed in AA Tau, CD-40°8434, and Hen 3-600A. A previously suggested N-band excess inthe M3 dwarf CD -33°7795 is shown to be consistent with photosphericemission. We calculate infrared to stellar bolometric luminosity ratiosfor all stars exhibiting mid-infrared excesses and infer the total massof orbiting dust in the cases of optically thin disks. For a derivedmedian photometric precision of +/-0.11 mag, we place an upper limit ofMdust~2×10-5 M⊕ on the dustmass (assuming a dust temperature of 300 K) around solar analogs notseen in excess at 10 μm. Our calculations for the nearby K1 V star HD17925 show that it may have the least massive debris disk known outsideour solar system (Mdust>~7×10-6M⊕). Our limited data confirm the expected tendency ofdecreasing fractional dust excessfd=LIR/L* with increasing stellar age.However, we argue that estimates of fd suffer from adegeneracy between the temperature and the amount of circumstellar dustMdust, and we propose a relation of Mdust as afunction of age instead.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Age Dependence of the Vega Phenomenon: Observations
We study the time dependency of Vega-like excesses using infraredstudies obtained with the imaging photopolarimeter ISOPHOT on board theInfrared Space Observatory. We review the different studies published onthis issue and critically check and revise ages and fractionalluminosities in the different samples. The conclusions of our studydiffer significantly from those obtained by other authors (e.g., Hollandand coworkers; Spangler and coworkers), who suggested that there is aglobal power law governing the amount of dust seen in debris disks as afunction of time. Our investigations lead us to conclude that (1) forstars at most ages, a large spread in fractional luminosity occurs, but(2) there are few very young stars with intermediate or small excesses;(3) the maximum excess seen in stars of a given age is aboutfd~10-3, independent of time; and (4) Vega-likeexcess is more common in young stars than in old stars.
| A Tidally Disrupted Asteroid around the White Dwarf G29-38
The infrared excess around the white dwarf G29-38 can be explained byemission from an opaque flat ring of dust with an inner radius of 0.14Rsolar and an outer radius of less than 1 Rsolar.This ring lies within the Roche region of the white dwarf where anasteroid could have been tidally destroyed, producing a systemreminiscent of Saturn's rings. Accretion onto the white dwarf from thiscircumstellar dust can explain the observed calcium abundance in theatmosphere of G29-38. Either as a bombardment by a series of asteroidsor because of one large disruption, the total amount of matter accretedonto the white dwarf may have been ~4×1024 g,comparable to the total mass of asteroids in the solar system, or,equivalently, about 1% of the mass in the asteroid belt around themain-sequence star ζ Lep.
| Stellar and circumstellar activity of the Be star omega CMa. I. Line and continuum emission in 1996-2002
Echelle spectroscopy and mostly unaided-eye photometry of the southernBe star omega CMa were obtained in the period1996-2003. The monitoring is bracketed by two brightenings by 0.4m-0.5m.The results of a literature search suggest that such phases occur aboutonce a decade and have various commonalities. Along with thesephotometric events goes enhanced line emission. This is due to anincreased total mass of the disk as well as to a change in its densityprofile. The models by Poeckert & Marlborough(\cite{1978ApJS...38..229P}, \cite{1979ApJ...233..259P}) imply that theenhanced continuum flux originates from the inner disk. Higher-orderBalmer line emission is correlated with brightness. The increase inHα is retarded by some months, possibly indicating a time delayin filling up and ionizing the outer disk. In the (U-B) vs. (B-V) colourdiagram and the D54 vs. D34 Balmer decrementdiagram the path from the ground to the bright state is distinct fromthe return path. This could result from the bulk of the disk matterbeing in the outer (inner) disk during the photometric ground (high)state, while the two transitions between the two states are both due tochanges progressing radially outward. Some mu Cen-like outbursts(Rivinius et al. \cite{1998A&A...333..125R}) seem to occur in allphases. It is conceivable that the build-up of the inner disk is causedby more frequent or more effective outbursts. During the photometricbright state various other phenomena gain in prominence and suggest thisto be a phase of increased activity. Of particular interest, butpossibly only apparently related to this phase, are absorptioncomponents at redshifts well beyond the range covered by the combinationof rotation and nonradial pulsation.Based on observations collected at the European Southern Observatory atLa Silla, Chile, ESO (proposal nos. 55.D-0502, 56.D-0381, 58.D-0697,62.H-0319, 64.H-0548).
| The field brown dwarf LP 944-20 and the Castor moving group
A reliable age estimation for the field brown dwarf LP944-20 is accomplished through the analysis of its kinematicproperties. The space velocities of this star strongly suggest itsmembership in the so-called Castor moving group. LP 944-20 can besensibly assumed to have the group's age, which is estimated to be ~320 Myr, and metal content, which is found to be roughly solar. Withthese new constrains and the available photometry and lithium abundance,current brown dwarf models are put to a test. Using the IR magnitudesand the lithium diagnostics, the models are able to provide a reasonabledescription of the brown dwarf's properties (to within a few sigma) butyield an age which is roughly 50% larger than our estimate. Possiblereasons for this discrepancy are discussed.
| Dust and Gas Around Young Stars
Asteroids, planets, and comets are believed to form within circumstellardisks of gas and dust around young stars (ages < 100 Myr). We haveobtained 10 - 20 μ m images, using the Keck I telescope and FUSEspectra of Herbig Ae and main sequence stars to characterize the dustand gas in these systems. We report the following results: (1) For zetaLep, we find that the majority of dust is located within 6 AU from thestar. Since the Poytning-Robertson drag lifetime of grains around thisstar is 104 years, significantly shorter than the 300 Myryear age of the system, we infer the presence of parent bodies with atotal mass 200 times that of the main asteroid belt in our solar system.(2) For the binary system sigma Herculis, which possess a Vega-likeinfrared excess, we observe circumstellar C 2*,N 2* and N 2**, blueshifted by asmuch as 40 km/sec. We propose that there is a radiatively driven wind,generated by sigma Her's high luminosity. In this model, the material inthe wind is created through collisions between parent bodies at 20 AUfrom the star, the approximate distance at which blackbodies are inradiative equilibrium with the star and at which 3-body orbits becomeunstable. (3) For the pre main sequence star AB Aur, we measure a dustmass 6x10-9 Mȯ , significantly less than the10-4 Mȯ inferred from millimeter photometry,suggesting the presence of a cold optically thick disk. We find thatmodels which include dust envelopes fit the data somewhat better thanmodels which incorporate flared circumstellar disks.
| 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 (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| A 25 micron search for Vega-like disks around main-sequence stars with ISO
We present an ISO 25 mu m photometric survey of a sample of 81 nearbymain-sequence stars in order to determine the incidence of ``warm'' dustdisks. All stars were detected by ISO. We used an empirical relation toestimate the photospheric flux of the stars at 25 mu m. We find 5 stars(6%) with excess above the photospheric flux which we attribute to aVega-like disk. These stars show disk temperatures not warmer than 120K. Our study indicates that warm disks are relatively rare. Not a singlestar in our sample older than 400 Myr has a warm disk. We find an upperlimit of Mdisk = 2x 10-5 Moplus forthe mass of the disks which we did not detect. ISO is an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the United Kingdom) andwith the participation of ISAS and NASA.
| 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 (130.79.125.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105
| A Possible Massive Asteroid Belt around ζ Leporis
We have used the Keck I telescope to image at 11.7 and 17.9 μm thedust emission around ζ Leporis, a main-sequence A-type star at 21.5pc from the Sun with an infrared excess. The excess is at mostmarginally resolved at 17.9 μm. The dust distance from the star isprobably <=6 AU, although some dust may extend to 9 AU. The mass ofobserved dust is ~1022 g. Since the lifetime of dustparticles is about 104 yr because of the Poynting-Robertsoneffect, we robustly estimate at least 4×1026 g mustreside in parent bodies, which may be asteroids if the system is in asteady state and has an age of ~300 Myr. This mass is approximately 200times that contained within the main asteroid belt in our solar system.
| Ages of A-Type Vega-like Stars from uvbyβ Photometry
We have estimated the ages of a sample of A-type Vega-like stars byusing Strömgren uvbyβ photometric data and theoreticalevolutionary tracks. We find that 13% of these A stars have beenreported as Vega-like stars in the literature and that the ages of thissubset run the gamut from very young (50 Myr) to old (1 Gyr), with noobvious age difference compared to those of field A stars. We clearlyshow that the fractional IR luminosity decreases with the ages ofVega-like 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.
| Stellar encounters with the solar system
We continue our search, based on Hipparcos data, for stars which haveencountered or will encounter the solar system(García-Sánchez et al. \cite{Garcia}). Hipparcos parallaxand proper motion data are combined with ground-based radial velocitymeasurements to obtain the trajectories of stars relative to the solarsystem. We have integrated all trajectories using three different modelsof the galactic potential: a local potential model, a global potentialmodel, and a perturbative potential model. The agreement between themodels is generally very good. The time period over which our search forclose passages is valid is about +/-10 Myr. Based on the Hipparcos data,we find a frequency of stellar encounters within one parsec of the Sunof 2.3 +/- 0.2 per Myr. However, we also find that the Hipparcos data isobservationally incomplete. By comparing the Hipparcos observations withthe stellar luminosity function for star systems within 50 pc of theSun, we estimate that only about one-fifth of the stars or star systemswere detected by Hipparcos. Correcting for this incompleteness, weobtain a value of 11.7 +/- 1.3 stellar encounters per Myr within one pcof the Sun. We examine the ability of two future missions, FAME andGAIA, to extend the search for past and future stellar encounters withthe Sun.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
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