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|The Puzzle of the Metallic Line Stars|
In the puzzle of the metallic line (Am) stars, there still seem to bemissing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak. TheAm stars have no ultraviolet emission lines. They are binaries that,with very few exceptions, have rotational velocities vsini lower than100 km s-1. Of the main-sequence A stars, 20% to 30% are Amstars. Here we rediscuss previous suggestions that tried to explain thepeculiar line strengths in the Am star spectra. In particular, wecompare the well-studied properties of Hyades A and Am stars in order toidentify reasons that can or cannot explain the differences. We findthat accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres.Charge-exchange reactions also seem to be important.
|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.
|Discovery of an Optically Thick, Edge-on Disk around the Herbig Ae Star PDS 144N|
We have discovered an optically thick, edge-on circumstellar disk arounda Herbig Ae star in the binary system PDS 144, providing the firstintermediate-mass analog of HK Tau and similar T Tauri stars. Thissystem consists of a V~13 mag primary and a fainter companion, with thespectra of both stars showing evidence for circumstellar disks andaccretion; both stars were classified as Herbig Ae by the Pico dos DiasSurvey. In Lick adaptive optics polarimetry, we resolved extendedpolarized light scattered from dust around the northern star. Follow-upKeck adaptive optics and mid-infrared observations show that this staris entirely hidden by an optically thick disk at all wavelengths from1.2 to 11.7 μm. The disk major axis subtends ~0.8" on the sky,corresponding to ~800 AU at a distance of 1000 pc. Bright ``wings''extend 0.3" above and below the disk ansae, due most likely toscattering from the edges of an outflow cavity in a circumstellarenvelope. We discuss the morphology of the disk and the spectral energydistributions of the two PDS 144 stars, present preliminary disk models,and identify a number of open questions regarding this fascinatingsystem.Some of the data presented here were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
|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.
|Dust in Resonant Extrasolar Kuiper Belts: Grain Size and Wavelength Dependence of Disk Structure|
This paper considers the distribution of dust that originates in thebreakup of planetesimals that are trapped in resonance with a planet. Itis shown that there are three distinct grain populations with differentspatial distributions: (I) large grains have the same clumpy resonantdistribution as the planetesimals; (II) moderate-sized grains are nolonger in resonance and have an axisymmetric distribution; and (III)small grains are blown out of the system by radiation pressure and sohave a density distribution that falls off as τ~1/r. Population IIIcan be further divided into two subclasses: (IIIa) grains produced frompopulation I that exhibit trailing spiral structure that emanates fromthe resonant clumps and (IIIb) grains produced from population II thathave an axisymmetric distribution. Since observations in differentwavebands are sensitive to different dust sizes, multiwavelength imagingof debris disks can be used to test models that explain thesubmillimeter structure of debris disks as due to resonant trapping ofplanetesimals. For example, a collisional cascade without blowout grainswould appear clumpy in the submillimeter (which samples population I)and smooth at mid- to far-IR wavelengths (which sample population II).The wavelength of transition from clumpy to smooth structure isindicative of the mass of the perturbing planet. The size distributionof Vega's disk is modeled showing that the large quantities ofpopulation III grains detected recently by Spitzer must originate in thedestruction of the grains seen in the submillimeter images. Thus, athigh resolution and sensitivity the far- and mid-IR structure of Vega'sdisk is predicted to include spiral structure emanating from thesubmillimeter clumps.
|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.
|Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer MIPS Survey|
We have searched for infrared excesses around a well-defined sample of69 FGK main-sequence field stars. These stars were selected withoutregard to their age, metallicity, or any previous detection of IRexcess; they have a median age of ~4 Gyr. We have detected 70 μmexcesses around seven stars at the 3 σ confidence level. Thisextra emission is produced by cool material (<100 K) located beyond10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Only one star,HD 69830, shows excess emission at 24 μm, corresponding to dust withtemperatures >~300 K located inside of 1 AU. While debris disks withLdust/L*>=10-3 are rare around oldFGK stars, we find that the disk frequency increases from 2%+/-2% forLdust/L*>=10-4 to 12%+/-5% forLdust/L*>=10-5. This trend in thedisk luminosity distribution is consistent with the estimated dust inour solar system being within an order of magnitude greater or less thanthe typical level around similar nearby stars. Although there is nocorrelation of IR excess with metallicity or spectral type, there is aweak correlation with stellar age, with stars younger than a gigayearmore likely to have excess emission.
|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.
|Computed Hβ indices from ATLAS9 model atmospheres|
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
|The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an elongated polar envelope|
Context: .Be stars show evidence of mass loss and circumstellarenvelopes (CSE) from UV resonance lines, near-IR excesses, and thepresence of episodic hydrogen emission lines. The geometry of theseenvelopes is still uncertain, although it is often assumed that they areformed by a disk around the stellar equator and a hot polar wind.Aims: .We probe the close environment of the fast rotating Be starAchernar at angular scales of a few milliarcseconds (mas) in theinfrared, in order to constrain the geometry of a possible polar CSE. Methods: .We obtained long-baseline interferometric observations ofAchernar with the VINCI/VLTI beam combiner in the H and K bands, usingvarious telescope configurations and baseline lengths with a wideazimuthal coverage. Results: .The observed visibility measurementsalong the polar direction are significantly lower than the visibilityfunction of the photosphere of the star alone, in particular at lowspatial frequencies. This points to the presence of an asymmetricdiffuse CSE elongated along the polar direction of the star. To ourdata, we fit a simple model consisting of two components: a 2Delliptical Gaussian superimposed on a uniform ellipse representing thedistorted photosphere of the fast rotating star. Conclusions: .Weclearly detected a CSE elongated along the polar axis of the star, aswell as rotational flattening of the stellar photosphere. For theuniform-ellipse photosphere we derive a major axis of θ_eq = 2.13± 0.05 mas and a minor axis of θ_pol = 1.51 ± 0.02mas. The relative near-IR flux measured for the CSE compared to thestellar photosphere is f = 4.7 ± 0.3%. Its angular dimensions areloosely constrained by the available data at ρ_eq = 2.7 ± 1.3mas and ρ_pol = 17.6 ± 4.9 mas. This CSE could be linked tofree-free emission from the radiative pressure driven wind originatingfrom the hot polar caps of the star.
|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.
|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.
|Integrated optics for astronomical interferometry. VI. Coupling the light of the VLTI in K band|
Aims. Our objective is to prove that integrated optics (IO) is not onlya good concept for astronomical interferometry but also a workingtechnique with high performance. Methods: . We used thecommissioning data obtained with the dedicated K-band integrated opticstwo-telescope beam combiner that now replaces the fiber coupler MONA inthe VLTI/VINCI instrument. We characterize the behavior of this IOdevice and compare its properties to other single mode beam combinerlike the previously used MONA fiber coupler. Results: . The IOcombiner provides a high optical throughput, a contrast of 89% with anight-to-night stability of a few percent. Even if a dispersive phase ispresent, we show that it does not bias the measured Fourier visibilityestimate. An upper limit of 5×10-3 for the cross-talkbetween linear polarization states has been measured. We take advantageof the intrinsic contrast stability to test a new astronomical procedurefor calibrating diameters of simple stars by simultaneously fitting theinstrumental contrast and the apparent stellar diameters. This methodreaches an accuracy with diameter errors on the order of previous onesbut without the need of an already known calibrator. Conclusions:. These results are an important step for IO, since they prove itsmaturity in an astronomical band where the technology has been speciallydeveloped for astronomical convenience. It paves the way to incomingimaging interferometer projects.
|The Prospects for Finding Brown Dwarfs in Eclipsing Binary Systems and Measuring Brown Dwarf Properties|
We present the results of a simulation to investigate the prospects ofmeasuring mass, age, radius, metallicity, and luminosity data for browndwarfs in fully eclipsing binary systems around dwarf spectral typesfrom late K to early M that could be identified by ultra-wide-fieldtransit surveys such as SuperWASP. These surveys will monitorapproximately a million K and M dwarfs with |b|>20deg(where blending of sources is not a significant problem) at a levelsufficient to detect transits of low-luminosity companions. We look atthe current observational evidence for such systems and suggest that ~1%of late K and early-to-mid M dwarfs could have a very close (~0.02 AU)brown dwarf companion. With this assumption, and using SuperWASP as anexample, our simulation predicts that ~400 brown dwarfs in fullyeclipsing binary systems could be discovered. All of these eclipsingbinaries could yield accurate brown dwarf mass and radius measurementsfrom radial velocity and follow-up light curve measurements. Byinferring the brown dwarf effective temperature distribution, assuming auniform age spread and an α=0.5 companion brown dwarf massfunction, the simulation estimates that brown dwarf brightness couldalso be measurable (at the 10% level) for ~60 of these binary systemsfrom near-infrared follow-up light curves of the secondary eclipse. Weconsider irradiation of these brown dwarfs by their primary stars andconclude that it would be below the 10% level for ~70% of them. Thismeans that in these cases, the measured brown dwarf brightnesses shouldessentially be the same as those of free-floating counterparts. Thepredicted age distribution of the primaries is dominated by youngsystems, and ~20 binaries could be younger than 1 Gyr. Irradiation willbe below the 10% level for ~80% of these. We suggest that many of theseyoung binary systems will be members of ``kinematic moving groups,''allowing their ages to be accurately constrained.
|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.
|Recent astrophysical results from the VLTI.|
|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.
|Predicting accurate stellar angular diameters by the near-infrared surface brightness technique|
I report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids.
|Variability of Stars in the Pulkovo Spectrophotometric Catalog|
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
|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.
|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.
|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.
|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.
|An Excess Due to Small Grains around the Nearby K0 V Star HD 69830: Asteroid or Cometary Debris?|
Spitzer photometry and spectroscopy of the star HD 69830 reveal anexcess of emission relative to the stellar photosphere between 8 and 35μm dominated by strong features attributable to crystalline silicateswith an emitting surface area more than 1000 times that of our zodiacalcloud. The spectrum closely resembles that of the comet C/1995 O1(Hale-Bopp). Since no excess is detected at 70 μm, the emittingmaterial must be quite warm, be confined within a few AU of the star,and originate in grains with low, long-wavelength emissivity, i.e.,grains much smaller than 70μm/2π~10 μm. The strongmineralogical features are evidence for even smaller, possiblysubmicron-sized grains. This small grain size is in direct contrast tothe 10-100 μm grains that dominate the relatively featureless spectraof our zodiacal dust cloud and most other main-sequence stars withexcesses. The upper limit at 70 μm also implies that any Kuiper Beltanalog must be either very cold or less massive than ~5 times our ownKuiper Belt. With collisional and Poynting-Robertson drag times of lessthan 1000 yr for small grains, the emitting material must either (1) becreated through continual grinding down of material in a dense asteroidbelt, or (2) originate in cometary debris arising from either a single``supercomet'' or a very large number of individual comets arriving froma distant reservoir. In the case of a cometary origin for the emission,the mass requirements for continuous generation by many individualcomets are unreasonable, and we favor the capture of a single supercomet into a 0.5-1 AU orbit, where it can evolve a large number of smallgrains over a 2 Myr period.
|UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra|
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
|Numerical Modeling of Dusty Debris Disks|
Infrared and submillimeter observations of nearby Vega-like stars haverevealed a number of clumpy, asymmetric dust debris disks. Previousstudies using semianalytical and numerical methods have suggestedplanetary companions of various mass as the likely cause of mostexamples of disk asymmetry. In this paper, we modify an N-bodysymplectic gravitational integrator to include radiation terms andconduct medium-resolution parameter searches to identify likelyplanetary candidates in observed Vega-like systems. We also presenthigh-resolution models of Vega and ɛ Eridani, comparing ourresults to those of previous authors, and a new model for Fomalhaut.
|Discovery of a Candidate Protoplanetary Disk around the Embedded Source IRc9 in Orion|
We report the detection of spatially extended mid-infrared emissionaround the luminous embedded star IRc9 in OMC-1, as seen in 8.8, 11.7,and 18.3 μm images obtained with the Thermal-Region Camera andSpectrograph on Gemini South. The extended emission is asymmetric, andthe morphology is reminiscent of warm dust disks around other youngstars. The putative disk has a radius of roughly 1.5" (700 AU) and alikely dust mass of almost 10 M⊕. The infrared spectralenergy distribution of IRc9 indicates a total luminosity of ~100Lsolar, implying that it will become an early A-type starwhen it reaches the main sequence. Thus, the candidate disk around IRc9may be a young analog of the planetary debris disks around Vega-likestars and the disks of Herbig Ae stars, and may provide a laboratory inwhich to study the earliest phases of planet formation. A disk aroundIRc9 may also add weight to the hypothesis that an enhanced T Tauri-likewind from this star has influenced the molecular outflow from the OMC-1core.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).
|Planets and Infrared Excesses: Preliminary Results from a Spitzer MIPS Survey of Solar-Type Stars|
As part of a large Spitzer MIPS Guaranteed Time Observation program, wehave searched for infrared excesses due to debris disks toward 26 FGKfield stars known from radial velocity (RV) studies to have one or moreplanets. While none of these stars show excesses at 24 μm, we havedetected 70 μm excesses around six stars at the 3 σ confidencelevel. The excesses are produced by cool material (<100 K) locatedbeyond 10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Theseplanet-bearing stars are, by selection for RV studies, typically olderthan 1 Gyr, and the stars identified here with excesses have a medianage of 4 Gyr. We find a preliminary correlation of both the frequencyand the magnitude of dust emission with the presence of known planets.These are the first stars outside the solar system identified as havingboth well-confirmed planetary systems and well-confirmed IR excesses.
|Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution|
We present an H-band image of the light scattered from circumstellardust around the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ803, HD 197481), obtained with the Keck adaptive optics system. Weresolve the disk both vertically and radially, tracing it over 17-60 AUfrom the star. Our high angular resolution (0.04 or 0.4 AU perresolution element) observations thus offer the opportunity to probe themorphology of the AU Mic debris disk on solar system scales. We identifysubstructures (dust clumps and gaps) in the disk that may point to theexistence of orbiting planets. No planets are seen in our H-band imagedown to a limiting mass of 1MJ at >20 AU, although theexistence of smaller planets cannot be excluded from the current data.Modeling of the disk surface brightness distribution at H band and Rband, and of the optical to submillimeter spectral energy distribution,allows us to constrain the disk geometry and the dust grain properties.We confirm the nearly edge-on orientation of the disk inferred fromprevious observations and deduce that the disk may be clear inward of1-10 AU. We find evidence for a lack of small grains at <50 AU,either as a result of grain growth or because of destruction byPoynting-Robertson and/or corpuscular drag. A change in the power-lawindex of the surface brightness profile is observed near 33 AU, similarto a feature known in the profile of the β Pic circumstellar debrisdisk. By comparing the timescales for interparticle collisions andPoynting-Robertson drag between the two systems, we argue that thebreaks are linked to one or both of these processes.
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