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|Coronagraphic Search for Extrasolar Planets around ɛ Eri and Vega|
We present the results of a coronagraphic imaging search for extrasolarplanets around the young main-sequence stars ε Eri and Vega.Concentrating the stellar light into the core of the point-spreadfunction by the adaptive optic system and blocking the core by theocculting mask in the coronagraph, we have achieved the highestsensitivity for point sources in close vicinity of the both centralstars. Nonetheless, we had no secure detection of a point source aroundthe stars. The observations give the upper limits on the masses of theplanets to (4-6)MJ and (5-10)MJ at a fewarcseconds from ε Eri and Vega, respectively. Diffuse structuresare also not detected around both stars.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
|Detecting a rotation in the ɛ Eridani debris disc|
The evidence for a rotation of the ɛ Eridani debris disc isexamined. Data at 850-μm wavelength were previously obtained usingthe Submillimetre Common User Bolometer Array (SCUBA) over periods of1997-1998 and 2000-2002. By χ2 fitting after shift androtation operations, images from these two epochs were compared torecover proper motion and orbital motion of the disc. The sameprocedures were then performed on simulated images to estimate theaccuracy of the results.Minima in the χ2 plots indicate a motion of the disc ofapproximately 0.6 arcsec per year in the direction of the star's propermotion. This underestimates the true value of 1 arcsec per year,implying that some of the structure in the disc region is not associatedwith ɛ Eridani, originating instead from background galaxies.From the χ2 fitting for orbital motion, acounterclockwise rotation rate of per year is deduced. Comparisons withsimulated data in which the disc is not rotating show that noise andbackground galaxies result in approximately Gaussian fluctuations with astandard deviation of per year. Thus, counterclockwise rotation of discfeatures is supported at approximately a 2σ level, after a 4-yrtime difference. This rate is faster than the Keplerian rate of per yearfor features at ~65 au from the star, suggesting their motion istracking a planet inside the dust ring.Future observations with SCUBA-2 can rule out no rotation of theɛ Eridani dust clumps with ~4σ confidence. Assuming a rateof about per year, the rotation of the features after a 10-yr periodcould be shown to be >=1° per year at the 3σ level.
|Spitzer Observations of Nearby M Dwarfs|
We present Spitzer IRAC and MIPS observations for a sample of eight Mdwarfs: six dMe, one dM, and one sdMe star. All of our targets are foundto have SEDs that are fitted within the error bars by a purelyphotospheric spectrum out to 24 μm. We find no evidence for IRexcess. None of our targets are detected in the MIPS 70 and 160 μmbands. The estimated ages for all are >10 Myr, suggesting that enoughdisk dissipation has occurred within the inner several AU of the star.For four of these, Mullan et al. had reported IRAS detections at 12μm, although the reported fluxes were below the 5 σ IRASdetection limit (~0.2 Jy). Mullan et al. also pointed out that V-Kcolors in dMe stars are larger than those in dM stars, possibly becauseof the presence of a chromosphere. Here we suggest that metallicityeffects provide a better explanation of the V-K data. For reasons ofobservational selection, our targets are not the most active flare starsknown, but being dMe stars indicates the presence of a chromosphere.Scaling from Houdebine's model of the AU Mic chromosphere, we havecomputed the free-free IR excesses for a range of densities. Our Spitzer24 μm data show that the chromospheres in two of our targets are lessdense than in AU Mic by a factor of 10 or more. This is consistent withthe fact that our sample includes the less active flare stars. Ourmodels also indicate that the chromospheric contribution to the observedAU Mic emission at submillimeter wavelengths is only about 2%.
|HR 1613: A Slowly Rotating A Dwarf Spectroscopic Binary with Solar Abundances|
From two sets of radial velocities we have obtained the orbital elementsof HR 1613. This single-lined binary has an orbital period of 8.11128days and a nearly circular orbit. The primary has an A9 V spectral typeand a vsini value of 11 km s-1, while the unseen secondary islikely a K or M dwarf. Spectral classifications and spectrum synthesisanalysis indicate that the abundances of the primary are normal. Wereject the possibility that the primary of HR 1613 is seen nearlypole-on and instead argue that its rotational inclination is at least20°, resulting in an equatorial rotational velocity of 30 kms-1 or less. Slowly rotating A stars almost always havespectrum peculiarities, being classified as either Ap or Am stars, butHR 1613, with its essentially solar abundances, appears to be anexception.
|Spectral Irradiance Calibration in the Infrared. XVI. Improved Accuracy in the Infrared Spectra of the Secondary and Tertiary Standard Calibration Stars|
We have increased the spectral resolution and accuracy of the absoluteinfrared spectral flux for a subset of stellar standards created byCohen and his collaborators in previous papers in this series. Wecombined the moderate-resolution (λ/Δλ~400)spectroscopy obtained on infrared standard stars by the Short WavelengthSpectrometer on the Infrared Space Observatory with high-qualityphotometry that is tied to our recent absolute calibration from theMidcourse Space Experiment. New spectra were created for 33 stars, 9 ofwhich are Cohen et al. secondary standards and another 20 of which aretertiary standards for which Cohen adopted spectral templates. Theeffective temperatures and angular diameters for the stars are derivedfrom the absolute spectra and compare favorably with independentmeasures of these quantities in the literature. The present spectradisplay systematic differences with those of Cohen and colleagues inthat they have 4%-7% lower fluxes in the 1-4 μm spectral region. Ourspectra remove the A-K star calibration bias recently noted in thecalibration of the Spitzer Space Telescope camera.
|A Spitzer IRAC Search for Substellar Companions of the Debris Disk Star ɛ Eridani|
We have used the Infrared Array Camera (IRAC) on board the Spitzer SpaceTelescope to search for low-mass companions of the nearby debris diskstar ɛ Eri. The star was observed in two epochs 39 days apart,with different focal plane rotation to allow the subtraction of theinstrumental point-spread function, achieving a maximum sensitivity of0.01 MJy sr-1 at 3.6 and 4.5 μm, and 0.05 MJysr-1 at 5.8 and 8.0 μm. This sensitivity is not sufficientto directly detect scattered or thermal radiation from the ɛ Eridebris disk. It is, however, sufficient to allow the detection of Jovianplanets with mass as low as 1MJ in the IRAC 4.5 μm band.In this band, we detected over 460 sources within the 5.70 arcmin fieldof view of our images. To test if any of these sources could be alow-mass companion to ɛ Eri, we have compared their colors andmagnitudes with models and photometry of low-mass objects. Of thesources detected in at least two IRAC bands, none fall into the range ofmid-IR color and luminosity expected for cool, 1 Gyr substellar andplanetary mass companions of ɛ Eri, as determined by both modelsand observations of field M, L, and T dwarfs. We identify three newsources that have detections at 4.5 μm only, the lower limit placedon their [3.6]-[4.5] color consistent with models of planetary massobjects. Their nature cannot be established with the currently availabledata, and a new observation at a later epoch will be needed to measuretheir proper motion in order to determine if they are physicallyassociated to ɛ Eri.
|Anomalous extinction behaviour towards the Type Ia SN 2003cg|
We present optical and near-infrared photometry and spectroscopy of theType Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. Theobservations cover a period between -8.5 and +414d post-maximum. SN2003cg is a normal but highly reddened Type Ia event. Its B magnitude atmaximum Bmax = 15.94 +/- 0.04 andΔm15(B)obs = 1.12 +/- 0.04[Δm15(B)intrinsic = 1.25 +/- 0.05]. AllowingRV to become a free parameter within the Cardelli et al.extinction law, simultaneous matches to a range of colour curves ofnormal SNe Ia yielded E(B - V) = 1.33 +/- 0.11, and RV = 1.80+/- 0.19. While the value obtained for RV is small, suchvalues have been invoked in the past, and may imply a grain size whichis small compared with the average value for the local interstellarmedium.
|The first year of SN 2004dj in NGC 2403|
New BV RI photometry and optical spectroscopy of the Type IIp supernova2004dj in NGC 2403, obtained during the first year since discovery, arepresented. The progenitor cluster, Sandage 96, is also detected onpre-explosion frames. The light curve indicates that the explosionoccurred about 30d before discovery, and the plateau phase lasted about+110 +/- 20 d after that. The plateau-phase spectra have been modelledwith the SYNOW spectral synthesis code using H, NaI, TiII, ScII, FeIIand BaI lines. The SN distance is inferred from the expandingphotosphere method and the standard candle method applicable for SNeIIp.They resulted in distances that are consistent with each other as wellas earlier Cepheid and Tully-Fisher distances. The average distance, D =3.47 +/- 0.29 Mpc is proposed for SN 2004dj and NGC 2403. The nickelmass produced by the explosion is estimated as ~0.02 +/- 0.01Msolar. The spectral energy distribution of the progenitorcluster is reanalysed by fitting population synthesis models to ourobserved BV RI data supplemented by U and JHK magnitudes from theliterature. The χ2 minimization revealed a possible`young' solution with cluster age Tcl = 8 Myr, and an `old'solution with Tcl = 20-30 Myr. The `young' solution wouldimply a progenitor mass M > 20 Msolar, which is higherthan the previously detected progenitor masses for Type II SNe.Based on observations obtained at David Dunlap Observatory (Canada), F.L. Whipple Observatory (USA), Konkoly Observatory and Szeged Observatory(Hungary).E-mail: firstname.lastname@example.org
|Images of the Vega Dust Ring at 350 and 450 μm: New Clues to the Trappingof Multiple-Sized Dust Particles in Planetary Resonances|
We have used the SHARC II camera at Caltech Submillimeter Observatory tomake 350 and 450 μm images of the Vega dust disk at spatialresolutions (FWHM) of 9.7" and 11.1", respectively. The images show aringlike morphology (radius ~100 AU) with inhomogeneous structure thatis qualitatively different from that previously reported at 850 μmand longer wavelengths. We attribute the 350/450 μm emission to agrain population whose characteristic size (~1 mm) is intermediatebetween that of the centimeter-sized grains responsible for emissionlongward of 850 μm and the much smaller grains (<~18 μm) in theextensive halo, visible at 70 μm, discussed by Su et al. We havecombined our submillimeter images with Spitzer data at 70 μm toproduce two-dimensional maps of line-of-sight optical depth (relativecolumn density). These ``tau maps'' suggest that the millimeter-sizedgrains are located preferentially in three symmetrically locatedconcentrations. If so, then this structure could be understood in termsof the Wyatt model in which planetesimals are trapped in the mean motionresonances of a Neptune-mass planet at 65 AU, provided allowance is madefor the spatial distribution of dust grains to differ from that of theparent planetesimals. The peaks of the tau maps are, in fact, locatednear the expected positions corresponding to the 4:3 resonance. If thisidentification is confirmed by future observations, it would resolve anambiguity with regard to the location of the planet.
|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.
|Discovery of a λ Bootis Like Abundance Pattern in the Pulsating Algol-Type System RZ Cassiopeiae|
The abundances of eight elements (C through Fe) were determined in theprimary star of the short-period Algol-type binary RZ Cas relative tothe A3 V spectroscopic standard star α PsA. Definiteunder-abundances (by -0.45dex or more) of Mg, Si, Ti, Cr, and Fe wereestablished. On the other hand, light elements C and O show solarabundances, and Ca is slightly under-abundant ( -0.3dex). Thisabundance pattern demonstrates that RZ Cas shows a characteristic ofmild λ Boo-type stars. This finding raises a question on thephysical relations between the metallicity (abundance anomaly), thepulsational characteristics, and the mass accretion processes.
|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.
|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.
|Circumstellar material in the Vega inner system revealed by CHARA/FLUOR|
Context: .Only a handful of debris disks have been imaged up to now. Dueto the need for high dynamic range and high angular resolution, verylittle is known about the inner planetary region, where small amounts ofwarm dust are expected to be found. Aims: .We investigate theclose neighbourhood of Vega with the help of infrared stellarinterferometry and estimate the integrated K-band flux originating fromthe central 8 AU of the debris disk. Methods: .We performedprecise visibility measurements at both short (~30 m) and long (~150 m)baselines with the FLUOR beam-combiner installed at the CHARA Array (MtWilson, California) in order to separately resolve the emissions fromthe extended debris disk (short baselines) and from the stellarphotosphere (long baselines). Results: .After revising Vega'sK-band angular diameter (θ_UD = 3.202 ± 0.005 mas), weshow that a significant deficit in squared visibility (ΔV2 = 1.88 ± 0.34%) is detected at short baselines withrespect to the best-fit uniform disk stellar model. This deficit can beeither attributed to the presence of a low-mass stellar companion aroundVega, or as the signature of the thermal and scattered emissions fromthe debris disk. We show that the presence of a close companion ishighly unlikely, as well as other possible perturbations (stellarmorphology, calibration), and deduce that we have most probably detectedthe presence of dust in the close neighbourhood of Vega. The resultingflux ratio between the stellar photosphere and the debris disk amountsto 1.29 ± 0.19% within the FLUOR field-of-view (~7.8 AU).Finally, we complement our K-band study with archival photometric andinterferometric data in order to evaluate the main physical propertiesof the inner dust disk. The inferred properties suggest that the Vegasystem could be currently undergoing major dynamical perturbations.
|First sky validation of an optical polarimetric interferometer|
Aims.We present the first lab and sky validation of spectro-polarimetricequipment put at the combined focus of an optical long-baselineinterferometer. We tested the polarimetric mode designed for the visibleGI2T Interferometer to offer spectropolarimetric diagnosis at themilliarcsecond scale. Methods.We first checked the wholeinstrumental polarization in the lab with a fringe simulator, and thenwe observed α Cep and α Lyr as stellar calibrators ofdifferent declinations to tabulate the polarization effects throughoutthe GI2T declination range. Results.The difference between bothlinear polarizations is within the error bars and the visibilitiesrecorded in natural light (i.e. without the polarimeter) for calibrationpurposes are the same order of magnitude as the polarized ones. Wefollowed the α Cep visibility for 2 h after the transit andα Lyr for 1.5 h and detected no decrease with hour angle due tothe fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetricCoudé trains of the GI2T is well-compensated by the fieldrotators, so the instrumental polarization is controlled over arelatively wide hour angle range (±2 h around the transit atleast). Such a polarimetric mode opens new opportunities especially forstudies of circumstellar environments and significantly enhances boththe potential of an optical array and its ability for accuratecalibration.
|Multi wavelength study of the gravitational lens system RXS J1131-1231. II. Lens model and source reconstruction|
Aims.High angular resolution images of the complex gravitational lenssystem RXS J1131-1231 (a quadruply imagedAGN with a bright Einstein ring) obtained with the Advanced Camera forSurveys and NICMOS instruments onboard the Hubble Space Telescope areanalysed to determine the lens model and to reconstruct the host galaxy.Methods. The lens model is constrained by the relative astrometricpositions of the lens and point-like images, and by the extended lensedstructures. The non-parametric light distribution is recovered in thesource plane by means of back ray-tracing. Results.1- Preciseastrometry and photometry of the four QSO lensed images (A-D) and of thelensing galaxy (G) are obtained. They are found in agreement with anindependent study presented in a companion paper. The position andcolours of the X object seen in projection close to the lens are foundto be only compatible with a satellite galaxy associated with the lens.2- The Singular Isothermal Ellipsoid plus external shear provides a goodfit of the astrometry of images A-D. The positions of extendedsubstructures are also well reproduced. However an octupole (m=4) mustbe added to the lens potential in order to reproduce the observed lensposition, as well as the I_B/IC point-like image flux ratio.The ellipticity and orientation of the mass quadrupole are found similarto those of the light distribution, fitted by a Sérsic profile.The lens (z=0.295) is found to be a massive elliptical in a richenvironment and showing possible evolution with respect to z=0. 3- Thehost galaxy (z=0.658) is found to be a substantially magnified (M9) luminous Seyfert 1 spiral galaxy. The angular resolution issufficient to see regions where stars are intensively forming.Interaction with a closeby companion is also observed. 4- Finally, inthe case of RXS J1131-1231, extended lensedstructures do not help much in constraining the lens model.
|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.
|A Mid-Infrared Study of the Class 0 Cluster in LDN 1448|
We present ground-based mid-infrared observations of Class 0 protostarsin LDN 1448. Of the five known protostars in this cloud, we detectedtwo, L1448N:A and L1448C, at 12.5, 17.9, 20.8, and 24.5 μm, and athird, L1448 IRS 2, at 24.5 μm. We present high-resolution images ofthe detected sources and photometry or upper limits for all five Class 0sources in this cloud. With these data we are able to augment existingspectral energy distributions for all five objects and place them on anevolutionary status diagram.
|A search for magnetic fields in the variable HgMn star α Andromedae|
Context: .The chemically peculiar HgMn stars are a class of Bp starswhich have historically been found to be both non-magnetic andnon-variable. Remarkably, it has recently been demonstrated that thebright, well-studied HgMn star α And exhibits clear Hg ii lineprofile variations indicative of a non-uniform surface distribution ofthis element. Aims: .With this work, we have conducted anextensive search for magnetic fields in the photosphere of αAnd. Methods: .We have acquired new circular polarisation spectrawith the MuSiCoS and ESPaDOnS spectropolarimeters. We have also obtainedFORS1 circular polarisation spectra from the ESO Archive, and consideredall previously published magnetic data. This extensive dataset has beenused to systematically test for the presence of magnetic fields in thephotosphere of α And. We have also examined the high-resolutionspectra for line profile variability. Results: .The polarimetricand magnetic data provide no convincing evidence for photosphericmagnetic fields. The highest-S/N phase- and velocity-resolved Stokes Vprofiles, obtained with ESPaDOnS, allow us to place a 3σ upperlimit of about 100 G on the possible presence of any undetected puredipolar, quadrupolar or octupolar surface magnetic fields (and just 50 Gfor fields with significant obliquity). We also consider and dismiss thepossible existence of more complex fossil and dynamo-generated fields,and discuss the implications of these results for explaining thenon-uniform surface distribution of Hg. The very high-quality ESPaDOnSspectra have allowed us to confidently detect variability of Hg iiλ 6149, λ 5425 and λ 5677. The profile variabilityof the Hg ii lines is strong, and similar to that of the Hg ii λ3984 line. On the other hand, variability of other lines (e.g. Mn, Fe)is much weaker, and appears to be attributable to orbital modulation,continuum normalisation differences and weak, variable fringing.
|Astrophysics: A whirling dervish|
Vega is a fundamental reference star for astronomers. But it seems thatour perceptions of it have been misconceived - rather than spinningslowly, the star is a rapid rotator seen pole-on.
|Angular Differential Imaging: A Powerful High-Contrast Imaging Technique|
Angular differential imaging is a high-contrast imaging technique thatreduces quasistatic speckle noise and facilitates the detection ofnearby companions. A sequence of images is acquired with analtitude/azimuth telescope while the instrument field derotator isswitched off. This keeps the instrument and telescope optics aligned andallows the field of view to rotate with respect to the instrument. Foreach image, a reference point-spread function (PSF) is constructed fromother appropriately selected images of the same sequence and subtractedto remove quasistatic PSF structure. All residual images are thenrotated to align the field and are combined. Observed performances arereported for Gemini North data. It is shown that quasistatic PSF noisecan be reduced by a factor ~5 for each image subtraction. Thecombination of all residuals then provides an additional gain of theorder of the square root of the total number of acquired images. A totalspeckle noise attenuation of 20-50 is obtained for a 1 hr long observingsequence compared to a single 30 s exposure. A PSF noise attenuation of100 was achieved for a 2 hr long sequence of images of Vega, reaching a5 σ contrast of 20 mag for separations greater than 8". For a 30minute long sequence, ADI achieves signal-to-noise ratios 30 timesbetter than a classical observation technique. The ADI technique can beused with currently available instruments to search for~1MJup exoplanets with orbits of radii between 50 and 300 AUaround nearby young stars. The possibility of combining the techniquewith other high-contrast imaging methods is briefly discussed.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 (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
|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.
|A Method for Determining the Physical Properties of the Coldest Known Brown Dwarfs|
We present a method for measuring the physical parameters of the coldestT-type brown dwarfs using low-resolution near-infrared spectra. Bycomparing H2O and H2-sensitive spectral ratiosbetween empirical data and theoretical atmosphere models, andcalibrating these ratios to measurements for the well-characterized 2-5Gyr companion brown dwarf Gliese 570D, we derive estimates of theeffective temperatures and surface gravities for 13 mid- and late-typefield T dwarfs. We also deduce the first quantitative estimate ofsubsolar metallicity for the peculiar T dwarf 2MASS 0937+2931. Derivedtemperatures are consistent with prior estimates based on parallax andbolometric luminosity measurements, and examination of possiblesystematic effects indicate that the results are robust. Two recentlydiscovered late-type T dwarfs, 2MASS 0939-2448 and 2MASS 1114-2618, bothappear to be >~50 K cooler than the latest type T dwarf, 2MASS0415-0935, and are potentially the coldest and least luminous browndwarfs currently known. We find that, in general, higher surface gravityT dwarfs have lower effective temperatures and luminosities for a givenspectral type, explaining previously observed scatter in theTeff/spectral type relation for these objects. Masses, radii,and ages are estimated for the T dwarfs in our sample using theevolutionary models of Burrows et al.; we also determine masses andradii independently for eight T dwarfs with measured luminosities. Thesetwo determinations are largely consistent, lending support to thevalidity of evolutionary models at late ages. Our method is well suitedto large samples of faint brown dwarfs and can ultimately be used todirectly measure the substellar mass function and formation history inthe Galaxy.
|Adaptive-Optics-Assisted Near-Infrared Spectroscopy of SVS 13 and Its Jet|
We present long-slit H- and K-band spectroscopy of the low-mass outflowsource SVS 13, obtained with the adaptive-optics-assistedimager-spectrometer NACO on the VLT. With a spatial resolution of<0.25" and a pixel scale of 0.027", we precisely establish therelative offsets of H2, [Fe II], CO, H I, and Na I componentsfrom the source continuum. The H2 and [Fe II] peaks areclearly associated with the jet, while the CO, H I, and Na I peaks arespatially unresolved and coincident with the source, as is expected foremission associated with accretion processes. The H2 profilealong the slit is resolved into multiple components, which increase insize, although they decrease in intensity, with distance from thesource. This trend might be consistent with thermal expansion of packetsof gas ejected during periods of increased accretion activity. Indeed,for the brightest component nearest the source, proper-motionmeasurements indicate a tangential velocity of 0.028" yr-1.It therefore seems unlikely that this emission peak is associated with astationary zone of warm gas at the base of the jet. However, the samecannot be said for the [Fe II] peak, for which we see no evidence formotion downwind, even though radial velocity measurements indicate thatthe emission is associated with higher jet velocities. We postulate thatthe [Fe II] could be associated with a collimation shock at the base ofthe jet.Based on Observations collected at the European Southern Observatory,Paranal, Chile (ESO programs 074.C-0408).
|Wavefront outer scale deduced from interferometric dispersed fringes|
In addition to site characterization, measurements of criticalatmospheric parameters are required to design and to optimize futureadaptive optic systems and long-baseline interferometers. It is possibleto estimate seeing conditions by processing data obtained with existingHigh Angular Resolution instruments. We report the results of jointobservations with the GI2T interferometer and the GSM site-testingmonitor performed over a period of several nights.We compared estimates of the wavefront outer scale done at variousbaselines as well as the seeing (Fried's parameter). We processedinterferometric data by calculating power spectra of dispersed fringeimages. Deduced measurements of the optical path difference lead to theestimates of the outer scale. We found that the outer scale valuesobtained from the GI2T data are mostly in the 5-30m range, in goodagreement with GSM measurements.
|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.
|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.
|A Recalibration of Optical Photometry: Tycho-2, Strömgren, and Johnson Systems|
I use high-quality Hubble Space Telescope (HST) spectrophotometry toanalyze the calibration of three popular optical photometry systems:Tycho-2 BT and VT, Strömgren uvby, and Johnson UBV. For Tycho-2, Irevisit the analysis of an earlier paper to include the newrecalibration of grating/aperture corrections, vignetting, and chargetransfer inefficiency effects produced by the Space Telescope ImagingSpectrograph group and to consider the consequences of both random andsystematic uncertainties. The new results reaffirm the good quality ofboth the Tycho-2 photometry and the HST spectrophotometry but yield aslightly different value for ZPBT-VT of0.033+/-0.001 (random) +/-0.005 (systematic) mag. For the Strömgrenv, b, and y filters I find that the published sensitivity curves areconsistent with the available photometry and spectrophotometry, and Iderive new values for the associated ZPb-y andZPm1. The same conclusion is drawn for the JohnsonB and V filters and the associated ZPB-V. The situation isdifferent for the Strömgren u and the Johnson U filters. There Ifind that the published sensitivity curves yield results that areinconsistent with the available photometry and spectrophotometry, likelycaused by an incorrect treatment of atmospheric effects on theshort-wavelength end. I reanalyze the data to produce new averagesensitivity curves for those two filters and new values forZPc1 and ZPU-B. The new computation ofsynthetic U-B and B-V colors uses a single B sensitivity curve, whicheliminates the previous unphysical existence of different definitionsfor each color. Finally, I find that if one uses values from theliterature for which uncertainties are not given, reasonable estimatesfor these are 1%-2% for Strömgren b-y, m1, andc1 and 2%-3% for Johnson B-V and U-B. The use of the resultsin this paper should lead to a significant reduction of systematicerrors when comparing synthetic photometry models with real colors andindices.
|The Supernova Legacy Survey: measurement of ΩM, ΩΛ and w from the first year data set|
We present distance measurements to 71 high redshift type Ia supernovaediscovered during the first year of the 5-year Supernova Legacy Survey(SNLS). These events were detected and their multi-color light-curvesmeasured using the MegaPrime/MegaCam instrument at theCanada-France-Hawaii Telescope (CFHT), by repeatedly imaging fourone-square degree fields in four bands, as part of the CFHT LegacySurvey (CFHTLS). Follow-up spectroscopy was performed at the VLT, Geminiand Keck telescopes to confirm the nature of the supernovae and tomeasure their redshift. With this data set, we have built a Hubblediagram extending to z=1, with all distance measurements involving atleast two bands. Systematic uncertainties are evaluated making use ofthe multi-band photometry obtained at CFHT. Cosmological fits to thisfirst year SNLS Hubble diagram give the following results: Ω_M=0.263 ± 0.042 (stat) ± 0.032 (sys) for a flat ΛCDMmodel; and w = -1.023 ± 0.090 (stat) ± 0.054 (sys) for aflat cosmology with constant equation of state w when combined with theconstraint from the recent Sloan Digital Sky Survey measurement ofbaryon acoustic oscillations.
|Trigonometric parallaxes from the Southern Hemisphere|
We present an outline of the main motivations, goals, procedures, andfirst scientifiic results of a dedicated astrometric program aimed atdetermining trigonometric parallaxes for new nearby (D < 25) stars inthe Southern sky. This program is being carried out at the Cerro TololoInter-American Observatory (CTIO), Chile.
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