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Near-infrared imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| 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.
| Braking the Gas in the β Pictoris Disk
The star β Pictoris hosts the best studied circumstellar disk todate. Nonetheless, a long-standing puzzle has been around since thedetection of metallic gas in the disk: radiation pressure from the starshould blow the gas away, yet the observed motion is consistent withKeplerian rotation. In this work we search for braking mechanisms thatcan resolve this discrepancy. We find that all species affected byradiation force are largely ionized and dynamically coupled into asingle fluid by Coulomb collisions, reducing the radiation force onspecies feeling the strongest acceleration. For a gas of solarcomposition, the effective radiation force still exceeds gravity, whilea gas of enhanced carbon abundance could be self-braking. We alsoexplore two other braking agents: collisions with dust grains andneutral gas. Grains surrounding β Pic are photoelectrically chargedto a positive electrostatic potential. If a significant fraction of thegrains are carbonaceous (10% in the midplane and larger at higheraltitudes), ions can be slowed down to satisfy the observed velocityconstraints. For neutral gas to brake the ion fluid, we find a minimumrequired mass ~0.03 M⊕, consistent with observed upperlimits on the hydrogen column density and substantially reduced relativeto previous estimates. Our results favor a scenario in which metallicgas is generated by grain evaporation in the disk, perhaps duringgrain-grain collisions. We exclude a primordial origin for the gas butcannot rule out its production by falling evaporating bodies near thestar.
| Gas Flow across Gaps in Protoplanetary Disks
We analyze the gas accretion flow through a planet-produced gap in aprotoplanetary disk. We adopt the α-disk model and ignore effectsof planetary migration. We develop a semianalytic, one-dimensional modelthat accounts for the effects of the planet as a mass sink and alsocarry out two-dimensional hydrodynamic simulations of a planet embeddedin a disk. The predictions of the mass flow rate through the gap basedon the semianalytic model generally agree with the hydrodynamicsimulations at the 25% level. Through these models, we are able toexplore steady state disk structures and over large spatial ranges. Thepresence of an accreting ~1MJ planet significantly lowers thedensity of the disk within a region of several times the planet'sorbital radius. The mass flow rate across the gap (and onto the centralstar) is typically 10%-25% of the mass accretion rate outside the orbitof the planet, for planet-to-star mass ratios that range from5×10-5 to 1×10-3.
| Spitzer Observations of Two TW Hydrae Association Brown Dwarfs
We present Spitzer Space Telescope observations of two TW Hydraeassociation brown dwarfs, 2MASSW J1207334-393254 and 2MASSWJ1139511-315921, in the IRAC and MIPS 24 μm bands. On the basis oftheir IRAC colors, we have classified them as classical and weak-line TTauri stars, respectively. For 2MASSW J1207334-393254, we have foundthat a flat-disk model fits the data very well. This brown dwarf showsthe presence of warm (T>~100 K) circumstellar dust close (R<~0.2AU) to it and does not display any signs of cleansing of dust withinseveral AU of the star. In comparison with other TWA members that showexcess in IR, we suggest that there exists a different diskevolution/dust processing mechanism for stellar and substellar objects.The star 2MASSW J1139511-315921 does not show any significant excess inany of the IRAC bands but a small one at 24 μm, which is notsignificant enough to suggest the presence of warm dust around thisstar. It shows signs of dust cleansing in the inner several AU, similarto most of the other TWA members.
| 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.
| Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.
| First Scattered Light Images of Debris Disks around HD 53143 and HD 139664
We present the first scattered light images of debris disks around a Kstar (HD 53143) and an F star (HD 139664) using the coronagraphic modeof the Advanced Camera for Surveys (ACS) on board the Hubble SpaceTelescope (HST). With ages of 0.3-1 Gyr, these are among the oldestoptically detected debris disks. HD 53143, viewed ~45° from edge-on,does not show radial variation in disk structure and has a width >55AU. HD 139664 is seen close to edge-on and has a beltlike morphologywith a dust peak 83 AU from the star and a distinct outer boundary at109 AU. We discuss evidence for significant diversity in the radialarchitecture of debris disks that appears unconnected to stellarspectral type or age. HD 139664 and possibly the solar system belong ina category of narrow belts 20-30 AU wide. HD 53143 represents a class ofwide-disk architecture with a characteristic width >50 AU.
| A Hybrid N-Body-Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planetsfrom an initial ensemble of planetesimals. The algorithm uses acoagulation code to treat the growth of planetesimals into oligarchs andexplicit N-body calculations to follow the evolution of oligarchs intoplanets. To validate the N-body portion of the algorithm, we use abattery of tests in planetary dynamics. Several complete calculations ofterrestrial planet formation with the hybrid code yield good agreementwith previously published calculations. These results demonstrate thatthe hybrid code provides an accurate treatment of the evolution ofplanetesimals into planets.
| Dynamical Evolution of the TW Hydrae Association
Using Galactic dynamics we have determined the age of the low-masspost-T Tauri stars in the TW Hya Association (TWA). To do so we appliedthe method of Ortega and coworkers to five stars of the association withHipparcos-measured distances (TWA 1, TWA 4, TWA 9, TWA 11, and TWA 19).The method is based on the calculation of the past three-dimensionalorbits of the stars. Of these stars, only TWA 9 presents a quitedifferent orbit so that it does not appear to be a dynamical member ofthe TWA. The four remaining stars have their first maximum orbitalconfinement at the age of -8.3+/-0.8 Myr, which is considered thedynamical age of the TWA. This confinement fixes the probablethree-dimensional forming region of the TWA within a mean radius of 14.5pc. This region is related to the older subgroups of the Sco-Cen OBassociation, Lower Centaurus Crux and Upper Centaurus Lupus, both with amean age of about 18 Myr. This dynamical age of the TWA and that of theβ Pic Moving Group, 11 Myr, also discussed here, introduce a moreprecise temporal scale for studies of disk evolution and planetaryformation around some stars of these associations. Using the retracedorbit of the runaway star HIP 82868 we examine the possibility that theformation of the TWA was triggered by a supernova explosion. It is shownthat for the four considered TWA stars, the expansion in volume is afactor of 5 from their origin to the present state. This is mainly dueto the currently more distant star TWA 19.
| Evidence for Companion-induced Secular Changes in the Turbulent Disk of a Be Star in the Large Magellanic Cloud MACHO Database
The light curve of a blue variable in the MACHO LMC database (FTS ID78.5979.72) appeared nearly unvarying for about 4 yr (the quasi-flatsegment) but then rapidly changed to become periodic with noisy minimafor the remaining 4 yr (the periodic segment); there are no antecedentindications of a gradual approach to this change. Lomb periodogramanalyses indicate the presence of two distinct periods of ~61 and 8 daysin both the quasi-flat and the periodic segments. Minima of the periodicsegment cover at least 50% of the orbital period and contain spikes oflight with the 8 day period; maxima do not show this short period. Thesystem typically shows maxima to be redder than minima. The most recentOGLE-III light curve shows only a 30 day periodicity. The variable's Vand R magnitudes and color are those of a Be star, and recent sets ofnear-infrared spectra 4 days apart, secured during the time of theOGLE-III data, show Hα emission near and at a maximum, confirmingits Be star characteristics. The model that best fits the photometricbehavior consists of a thin ringlike circumstellar disk of low mass withfour obscuring sectors orbiting the central B star in unison at the 61day period. The central star peers through the three equispacedseparations between the four sectors producing the 8 day period. Thesesectors could be dusty vortices comprised of particles larger thantypical interstellar dust grains that dim but selectively scatter thecentral star's light, while the remainder of the disk contains hydrogenin emission, making maxima appear redder. A companion star of lower massin an inclined and highly eccentric orbit produces an impulsiveperturbation near its periastron to change the disk's orientation,changing eclipses from partial to complete within ~10 days. The mostrecent change to a 30 day period observed in the OGLE-III data may becaused by obscuring sectors that have coalesced into larger ones andspread out along the disk.
| 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.
| The origins of the substellar companion to GQ Lupi
The recently discovered substellar companion to GQ Lup possiblyrepresents a direct test of current planet formation theories. Weexamine the possible formation scenarios for the companion to GQ Lupassuming it is a ~2 M_Jup object. We determine that GQ Lup B most likelywas scattered into a large, eccentric orbit by an interaction withanother planet in the inner system. If this is the case, severaldirectly observable predictions can be made, including the presence of amore massive, secondary companion that could be detected throughastrometry, radial velocity measurements, or sculpting in GQ Lup'scircumstellar disk. This scenario requires a highly eccentric orbit forthe companion already detected. These predictions can be tested withinthe next decade or so. Additionally, we look at scenarios of formationif the companion is a brown dwarf. One possible formation scenario mayinvolve an interaction between a brown dwarf binary and GQ Lup. We lookfor evidence of any brown dwarfs that have been ejected from the GQ Lupsystem by searching the 2MASS all-sky survey.
| Stellar Coronagraphy: Study and Test of a Hybrid Interfero-Coronagraph
We present a device for high dynamic range imaging of faint structuresin the close angular vicinity of stars. An advantage of this device isthe capacity to explore the field of view both uniformly and as close(or near) to the parent star as allowed by the diffraction limit of thetelescope. The design is adapted from the AchromaticInterfero-Coronagraph (AIC), which suppresses the light scattered bydiffraction from an on-axis unresolved source, but yields two twinimages of the stellar environment, displayed symmetrically in the focalplane (Gay & Rabbia 1996; Baudoz et al. 2000a). Our HybridInterfero-Coronagraph (HIC) avoids the 180° ambiguity and istherefore well suited for the study of extended objects distributed allaround the star. Like the AIC, the on-axis extinction with HIC is basedon destructive interference after amplitude division of the incidentfield from a single telescope. An achromatic π-dephasing and aspatial filtering of the wave front are performed on one arm of theinterferometer. The interferometric process occurs between the wavefront from the star and a spatially filtered wave front. Because of thisspatial filtering, the destructive process no longer remains achromatic.However, the residual chromaticity is compatible with astrophysicalapplications on ground-based telescopes. Numerical simulations show thatHIC performance does not suffer from telescope central obscuration. Anappropriate size of the spatial filtering mask and a suitableapodization allow the nulling of an on-axis unresolved star at the levelof 10-6, with a spectral resolutionR=λ0/Δλ=15 and in the absence ofwave-front phase or intensity errors. Preliminary laboratory tests witha HIC prototype are reported, and as a first step, an extinction of thesource by a factor of 200 was obtained, with a resolution R=6 in thevisible, using a simple setup.
| A planetary system as the origin of structure in Fomalhaut's dust belt
The Sun and >15per cent of nearby stars are surrounded by dusty disksthat must be collisionally replenished by asteroids and comets, as thedust would otherwise be depleted on timescales <107years(ref. 1). Theoretical studies show that the structure of a dusty diskcan be modified by the gravitational influence of planets, but theobservational evidence is incomplete, at least in part because maps ofthe thermal infrared emission from the disks have low linear resolution(35AU in the best case). Optical images provide higher resolution, butthe closest examples (AU Mic and β Pic) are edge-on, preventing thedirect measurement of the azimuthal and radial disk structure that isrequired for fitting theoretical models of planetary perturbations. Herewe report the detection of optical light reflected from the dust grainsorbiting Fomalhaut (HD 216956). The system is inclined 24° away fromedge-on, enabling the measurement of disk structure around its entirecircumference, at a linear resolution of 0.5AU. The dust is distributedin a belt 25AU wide, with a very sharp inner edge at a radial distanceof 133AU, and we measure an offset of 15AU between the belt's geometriccentre and Fomalhaut. Taken together, the sharp inner edge and offsetdemonstrate the presence of planetary-mass objects orbiting Fomalhaut.
| Expansion of the TW Hydrae association and the encounter with Vega
We investigate the paths of several probable members of the youngassociation around the star TW Hydrae (TWA) with accurate distances,proper motions and radial velocities. We find that three of thepreviously identified members, TWA 1, TWA 4 and TWA 11, together withtwo other young nearby stars, HD 139084 and HD 220476, form a rapidlyexpanding association with an expansion age of 4.7 +/- 0.6 Myr. Initialvelocities of member stars with respect to the common centre of massrange from 4 to 10km s-1. A characteristic size of theassociation in the initial configuration is 21 pc, which may be somewhatbiased upwards due to the uncertainties in the observational data. TheLower Centaurus Crux (LCC) OB association passed near TWA, at a distanceof 36 +/- 6 pc, 11 Myr ago. A plausible scenario, which accounts for thedifference between the isochrone age (~=10 Myr) and expansion age (5Myr), is that star formation was stimulated in the TWA progenitor cloudby the near passage of the LCC, but that the newly formed stars were notreleased from the cloud until a subsequent collision with one of theother molecular clouds in the North Ophiuchus region. Vega was insidethe TWA association, and close to its centre of gravity, at the time ofmaximum compression 4.7 Myr ago. If this alignment is a chanceencounter, the powerful particular disc around Vega could have beenenhanced by the passage through the TWA progenitor cloud at 8 kms-1.
| 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.
| Rotation periods for stars of the TW Hydrae association: the evidence for two spatially and rotationally distinct pre-main-sequence populations
We have conducted a photometric study of late-type members of the TWHydrae association (TWA) and measured the rotation periods for 16 starsin 12 systems. For TWA stars listed by Webb et al. and Sterzik et al.(TWA 1-13 led by TW Hya = TWA 1) we find a median period of 4.7 d.However, for stars that we measured in the TWA 14-19 group identified byZuckerman et al., we find a median period of only 0.7 d. The perioddistributions of the two groups cannot be reconciled at the 3σsignificance level. Using photometric arguments supported by theHipparcos distance to HD 102458 (= TWA 19A), we find that TWA 14-19reside at an average distance of d~ 90 pc, spatially at the nearboundary of the Lower Centaurus-Crux (LCC) subgroup of theOphiuchus-Scorpius-Centaurus OB-star association. Proper motions for HD102458, TWA 14, 18 and 19B link these stars to the LCC subgroup. FromHertzsprung-Russell diagram placement, we derive an age of ~17 Myr forthe HD 102458 system that may be the representative age for the TWA14-19 group. Merging various lines of evidence, we conclude that thesestars form a spatially and rotationally distinct population of olderpre-main-sequence (PMS) stars, rather than being an extension of the TWAbeyond those stars associated with TW Hya that have an age of ~10 Myrand reside at d~ 55 pc. Instead, TWA 14-19 likely represent thepopulation of low-mass stars still physically associated with the LCCsubgroup.
| An 850 μm Survey for Dust around Solar-Mass Stars
We present the results of an 850 μm JCMT/SCUBA survey for dust around13 nearby solar-mass stars. The dust mass sensitivity ranged from5×10-3 to 0.16 M⊕. Three sources weredetected in the survey, one of which (HD 107146) has been previouslyreported. One of the other two submillimeter sources, HD 104860, was notdetected by IRAS and is surrounded by a cold, massive dust disk with adust temperature and mass of Tdust=33 K andMdust=0.16 M⊕, respectively. The thirdsource, HD 8907, was detected by IRAS and ISO at 60-87 μm and has adust temperature and mass of Tdust=48 K andMdust=0.036 M⊕, respectively. We find thatthe deduced masses and radii of the dust disks in our sample are roughlyconsistent with models for the collisional evolution of planetesimaldisks with embedded planets. We also searched for residual gas in two ofthe three systems with detected submillimeter excesses and place limitson the mass of gas residing in these systems. When the propertiesmeasured for the detected excess sources are combined with the largerpopulation of submillimeter excess sources from the literature, we findstrong evidence that the mass in small grains declines significantly ona ~200 Myr timescale, approximately inversely with age. However, we alsofind that the characteristic dust radii of the population, obtained fromthe dust temperature of the excess and assuming blackbody grains, isuncorrelated with age. This is in contrast to self-stirred collisionalmodels for debris disk evolution, which predict a trend of radiusincreasing with age tage~R3d. The lackof agreement suggests that processes beyond self-stirring, such as giantplanet formation, play a role in the evolutionary histories ofplanetesimal disks.
| A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association
A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin etal. They find that the temperature and luminosity of 2M1207b areconsistent with a young, ~5MJ planet. The 2M1207 system ispurported to be a member of the TW Hya association (TWA) and situated~70 pc away. Using a revised space motion vector for TWA and improvedproper motion for 2M1207, I use the moving cluster method to estimatethe distance to the 2M1207 system and other TWA members. The deriveddistance for 2M1207 (53+/-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M1207Aand 2M1207b decrease to ~21 and ~3-4MJ, respectively, withthe mass of 2M1207b well below the observed tip of the planetary massfunction and the theoretical deuterium-burning limit. After removingprobable Lower Centaurus Crux (LCC) members from the TWA sample, as wellas the probable nonmember TWA 22, the remaining TWA membership is foundto have distances of 49+/-3 (s.e.m.)+/-12 (1 σ) pc and an internalone-dimensional velocity dispersion of 0.8+0.3-0.2km s-1. There is weak evidence that the TWA is expanding, andthe data are consistent with a lower limit on the expansion age of 10Myr (95% confidence).
| 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.
| Dust Distribution in Gas Disks. II. Self-induced Ring Formation through a Clumping Instability
Debris rings of dust are found around young luminous stars such as HR4796A and HD 141569. Some of these entities have sharp edges and gaps,which have been interpreted as evidence for the presence of shepherdingand embedded planets. Here we show that gaps and sharp edges in thedebris disks of dust can also be spontaneously self-generated if theyare embedded in optically thin regions of gaseous disks. This clumpinginstability arises in regions where an enhancement in the dust densityleads to local gas temperature and pressure increases. Consequently, therelative motion between the gas and the dust is modified. The subsequenthydrodynamic drag on the dust particles leads to further enhancement oftheir concentration. We show that this process is linearly unstable andleads to the formation of ringlike structures within the estimatedlifetime of such young objects. Once the gas is removed (e.g., byphotoevaporation), the structures are ``frozen'' and will persist, evenwhen the gas might not be observable anymore.
| Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun-like Stars
We present the discovery of debris systems around three Sun-like starsbased on observations performed with the Spitzer Space Telescope as partof a Legacy Science Program, ``The Formation and Evolution of PlanetarySystems'' (FEPS). We also confirm the presence of debris around twoother stars. All the stars exhibit infrared emission in excess of theexpected photospheres in the 70 μm band but are consistent withphotospheric emission at <=33 μm. This restricts the maximumtemperature of debris in equilibrium with the stellar radiation toT<70 K. We find that these sources are relatively old in the FEPSsample, in the age range 0.7-3 Gyr. On the basis of models of thespectral energy distributions, we suggest that these debris systemsrepresent materials generated by collisions of planetesimal belts. Wespeculate on the nature of these systems through comparisons to our ownKuiper Belt, and on the possible presence of planet(s) responsible forstirring the system and ultimately releasing dust through collisions. Wefurther report observations of a nearby star HD 13974 (d=11 pc) that areindistinguishable from a bare photosphere at both 24 and 70 μm. Theobservations place strong upper limits on the presence of any cold dustin this nearby system(LIR/L*<10-5.2).
| Exploring Terrestrial Planet Formation in the TW Hydrae Association
Spitzer Space Telescope infrared measurements are presented for 24members of the TW Hya association (TWA). High signal-to-noise ratio 24μm photometry is presented for all these stars, including 20 starsthat were not detected by IRAS. Among these 20 stars, only a singleobject, TWA 7, shows excess emission at 24 μm at the level of only40% above the star's photosphere. TWA 7 also exhibits a strong 70 μmexcess that is a factor of 40 brighter than the stellar photosphere atthis wavelength. At 70 μm, an excess of similar magnitude is detectedfor TWA 13, although no 24 μm excess was detected for this binary.For the 18 stars that failed to show measurable IR excesses, thesensitivity of the current 70 μm observations does not rule outsubstantial cool excesses at levels 10-40 times above their stellarcontinua. Measurements of two T Tauri stars, TW Hya and Hen 6-300,confirm that their spectacular IR spectral energy distributions (SEDs)do not turn over even by 160 μm, consistent with the expectation fortheir active accretion disks. In contrast, the Spitzer data for theluminous planetary debris systems in the TWA, HD 98800B and HR 4796A,are consistent with single-temperature blackbody SEDs and agree withprevious IR, submillimeter, and millimeter measurements. The major newresult of this study is the dramatic bimodal distribution found for theassociation in the form of excess emission at a wavelength of 24 μm,indicating negligible amounts of warm (>~100 K) dust and debrisaround 20 of 24 stars in this group of very young stars. This bimodaldistribution is especially striking given that the four stars in theassociation with strong IR excesses are >~100 times brighter at 24μm than their photospheres. Clearly, two terrestrial planetarysystems, HD 98800B and HR 4796A, exist in some form. In addition, thereare at least two active accreting objects, TW Hya and Hen 6-300, thatmay still be forming planetesimals. The remaining stars may possesssignificant amounts of cold dust, as in TWA 7 and 13, that have yet tobe found.
| Photophoresis and the Pile-up of Dust in Young Circumstellar Disks
A rapidly growing number of observations reveal ever more structure inyoung circumstellar disks that are presumed to be forming planetarysystems. Prominent features observed are ring-shaped dust distributionswith sharp inner edges around stars like the young, main-sequence starHR 4796A. Models aiming to explain the formation of these dust rings bygrain migration incorporate radiation pressure of the central star asone shaping force in radial direction. However, the radiometric effectof photophoresis has been ignored, so far, in this context. This effectis based on a radiation-induced temperature gradient on the surface of aparticle and the consequential nonuniform interaction with surroundinggas. The resulting force is able to effectively influence the motion ofparticles in gaseous environments, but so far photophoresis has beenlimited to applications in the field of aerosol science. Here we presentcalculations that underline the relevance of the photophoretic force forthe dynamics of particles in gas-rich, optically thin circumstellardisks. Depending on the gas pressure, photophoresis can be stronger thanradiation pressure, gas drag, and gravity by orders of magnitude. Thenthe motion of particles ranging in size from 1 μm to 10 m will bedominated by photophoresis. Since the photophoretic force is a functionof the gas density, it provides an efficient mechanism for fast radialmigration of particles to a definite distance from the star where thegas density reaches a value at which photophoresis is in equilibriumwith all other forces at work. By this effect, material is swept outfrom the inner region of the disk and piled-up in a more or lessconfined belt around the star. Thus, the formation of ringlikestructures of the dust distribution can most naturally be explainedwithout any further assumptions. Since photophoretic pile-up also worksfor larger bodies, it might even trigger the formation of Kuiper belts.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Discovery of a Nearly Edge-on Disk around HD 32297
We report the discovery of a nearly edge-on disk about the A0 star HD32297 seen in light scattered by the disk grains revealed in NICMOSPSF-subtracted coronagraphic images. The disk extends to a distance ofat least 400 AU (3.3") along its major axis with a 1.1 μm fluxdensity of 4.81 +/- 0.57 mJy beyond a radius of 0.3" from thecoronagraphically occulted star. The fraction of 1.1 μm starlightscattered by the disk, 0.0033 +/- 0.0004, is comparable to itsfractional excess emission at 25 + 60 μm of ~0.0027 as measured fromIRAS data. The disk appears to be inclined 10.5d +/- 2.5d from anedge-on viewing geometry, with its major axis oriented 236.5d +/- 1°eastward of north. The disk exhibits unequal brightness on opposingsides and a break in the surface brightness profile along the NE-sidedisk major axis. Such asymmetries might implicate the existence of oneor more (unseen) planetary mass companions.
| The Accretion Disk of the Lithium-depleted Young Binary St 34
We present the infrared spectrum of the young binary system St 34obtained with the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope. The IRS spectrum clearly shows excess dust emission,consistent with the suggestion of White & Hillenbrand that St 34 isaccreting from a circumbinary disk. The disk emission of St 34 is low incomparison with the levels observed in typical T Tauri stars; silicatefeatures at ~10 and 20 μm are much weaker than typically seen in TTauri stars; and excess emission is nearly absent at the shortestwavelengths observed (~5 μm). These features of the infrared spectrumsuggest substantial grain growth (to eliminate silicate features) andpossible settling of dust to the disk midplane (to reduce the continuumexcess emission levels), along with a relatively evacuated inner disk,as expected due to gravitational perturbations by the binary system.Although the position of St 34 in the H-R diagram suggests an age of8+/-3 Myr, assuming that it lies at the distance of the Taurus-Aurigamolecular clouds, White & Hillenbrand could not detect any Li Iabsorption, which would indicate a Li depletion age of roughly 25 Myr ormore. We suggest that St 34 is closer than the Taurus clouds by ~ 30-40pc and has an age roughly consistent with Li depletion models. Such anadvanced age would make St 34 the oldest known low-masspre-main-sequence object with a dusty accretion disk. The persistence ofoptically thick dust emission well outside the binary orbit may indicatea failure to make giant planets that could effectively remove dustparticles.
| The Differential Lifetimes of Protostellar Gas and Dust Disks
We construct a protostellar disk model that takes into account thecombined effect of viscous evolution, photoevaporation, and thedifferential radial motion of dust grains and gas. For T Tauri disks,the lifetimes of dust disks that are mainly composed of millimeter-sizedgrains are always shorter than the gas disks' lifetimes and becomesimilar only when the grains are fluffy (density <~0.1 gcm-3). If grain growth during the classical T Tauri phaseproduces plenty of millimeter-sized grains, such grains completelyaccrete onto the star in 107 yr, before photoevaporationbegins to drain the inner gas disk and the star evolves to the weak-lineT Tauri phase. In the weak-line phase, only dust-poor gas disks remainat large radii (>~10 AU), without strong signs of gas accretion or ofmillimeter thermal emission from the dust. For Herbig Ae/Be stars, thestrong photoevaporation clears the inner disks in 106 yr,before the dust grains in the outer disk migrate to the inner region. Inthis case, the grains left behind in the outer gas disk accumulate atthe disk inner edge (at 10-100 AU from the star). The dust grains remainthere even after the entire gas disk has been photoevaporated and form agas-poor dust ring similar to that observed around HR 4796A. Hence,depending on the strength of the stellar ionizing flux, our modelpredicts opposite types of products around young stars. For low-massstars with a low photoevaporation rate, dust-poor gas disks with aninner hole would form, whereas for high-mass stars with a highphotoevaporation rate, gas-poor dust rings would form. This predictionshould be examined by observations of gas and dust around weak-line TTauri stars and evolved Herbig Ae/Be stars.
| A Spitzer Study of Dusty Disks in the Scorpius-Centaurus OB Association
We have obtained Spitzer Space Telescope MIPS observations of 40 F- andG-type common proper motion members of the Scorpius-Centaurus OBassociation with ages between 5 and 20 Myr at 24 and 70 μm. We reportthe detection of 14 objects that possess 24 μm fluxes >=30% largerthan their predicted photospheres, tentatively corresponding to a diskfraction of >=35%, including seven objects that also possess 70 μmexcesses >=100 times larger than their predicted photospheres. The 24μm plus 70 μm excess sources possess high fractional infraredluminosities, LIR/L*=7×10-4 to3×10-3 either they possess optically thin, dusty βPictoris-like disks or compact, opaque HD 98800-like disks.
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Observation and Astrometry data
| Constellation: | Centaure |
| Right ascension: | 12h36m01.20s |
| Declination: | -39°52'12.0" |
| Apparent magnitude: | 5.8 |
| Distance: | 67.069 parsecs |
| Proper motion RA: | -53.3 |
| Proper motion Dec: | -21.2 |
| B-T magnitude: | 5.794 |
| V-T magnitude: | 5.775 |
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