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Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| 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.
| 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.
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| The chemical composition of δ Scuti*
We present chemical abundances in the photosphere of δ Scuti(δ Sct) - a prototype of the class of pulsating variables -determined from the analysis of a spectrum obtained by using the 2-mtelescope at the Peak Terskol Observatory and a high-resolutionspectrometer with R= 52000, a signal-to-noise ratio 250 and fromInternational Ultraviolet Explorer spectra. The abundance pattern ofδ Sct consists of 49 chemical elements. The abundances of 19elements have not been investigated previously. The abundances of Pr andNd obtained from the lines of the second and third spectra are equal.The abundances of heavy elements indicate the overabundances withrespect to the solar system values up to 1 dex. The abundance pattern ofδ Sct is similar to the abundance patterns of Am-Fm or δ Deltype stars.A splitting of the cores of all clean lines is observed for the spectraof δ Sct and HD 57749. This can signify evidence of non-radialpulsations in these stars.
| 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.
| Caroline Herschel as observer
Not Available
| 8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Radial Distribution of Dust Grains around HR 4796A
We present high dynamic range images of circumstellar dust around HR4796A that were obtained with MIRLIN at the Keck II telescope atλ=7.9, 10.3, 12.5, and 24.5 μm. We also present a newcontinuum measurement at 350 μm obtained at the Caltech SubmillimeterObservatory. Emission is resolved in Keck images at 12.5 and 24.5 μmwith point-spread function FWHM values of 0.37" and 0.55", respectively,and confirms the presence of an outer ring centered at 70 AU. Unresolvedexcess infrared emission is also detected at the stellar position andmust originate well within 13 AU of the star. A model of dust emissionfitted to flux densities at 12.5, 20.8, and 24.5 μm indicates thatdust grains are located 4+3-2 AU from the starwith effective size 28+/-6 μm and an associated temperature of260+/-40 K. We simulate all extant data with a simple model ofexozodiacal dust and an outer exo-Kuiper ring. A two-component outerring is necessary to fit both Keck thermal infrared and Hubble SpaceTelescope scattered-light images. Bayesian parameter estimates yield atotal cross-sectional area of 0.055 AU2 for grains roughly 4AU from the star, and an outer-dust disk composed of a narrowlarge-grain ring embedded within a wider ring of smaller grains. Thenarrow ring is 14+/-1 AU wide with inner radius 66+/-1 AU and totalcross-sectional area 245 AU2. The outer ring is 80+/-15 AUwide with inner radius 45+/-5 AU and total cross-sectional area 90AU2. Dust grains in the narrow ring are about 10 times largerand have lower albedos than those in the wider ring. These propertiesare consistent with a picture in which radiation pressure dominates thedispersal of an exo-Kuiper belt.
| Spectral Types for Four OGLE-III Transit Candidates: Could These Be Planets?
We present spectral types for OGLE (Optical Gravitational LensingExperiment) transiting planet candidates OGLE-TR-134 through 137 basedon low-resolution spectra taken at Kitt Peak. Our main objective is toaid those planning radial velocity monitoring of transit candidates. Weobtain spectral types with an accuracy of 2 spectral subtypes, alongwith tentative luminosity classifications. Combining the spectral typeswith light-curve fits to the OGLE transit photometry, and with TwoMicron All Sky Survey counterparts in two cases, we conclude thatOGLE-TR-135 and 137 are not planetary transits, while OGLE-TR-134 and136 are good candidates and should be observed with precision radialvelocity monitoring to determine whether the companions are of planetarymass. OGLE-TR-135 is ruled out chiefly because a discrepancy between thestellar parameters obtained from the transit fit and those inferred fromthe spectra indicates that the system is a blend. OGLE-TR-137 is ruledout because the depth of the transit combined with the spectral type ofthe star indicates that the transiting object is stellar. OGLE-TR-134and 136, if unblended main-sequence stars, are each orbited by atransiting object with radius below 1.4 RJ. The caveats arethat our luminosity classification suggests that OGLE-TR-134 could be agiant (and therefore a blend), while OGLE-TR-136 shows a (much smaller)discrepancy of the same form as OGLE-TR-135, which may indicate that thesystem is a blend. However, since our luminosity classifications areuncertain at best, and the OGLE-TR-136 discrepancy can be explained ifthe primary is a slightly anomalous main-sequence star, the stars remaingood candidates.
| Extrasolar planets and brown dwarfs around A-F type stars. I. Performances of radial velocity measurements, first analyses of variations
We present the performances of a radial velocity measurement method thatwe developed for A-F type stars. These perfomances are evaluated throughan extensive set of simulations, together with actual radial velocityobservations of such stars using the ELODIE and HARPS spectrographs. Wereport the case of stars constant in radial velocity, the example of abinary detection on HD 48097 (an A2V star, with v sin{i} equal to 90 kms^-1) and a confirmation of the existence of a 3.9 M_Jup planet orbitingaround HD 120136 (Tau Boo). The instability strip problem is alsodiscussed. We show that with this method, it is in principle possible todetect planets and brown dwarfs around A-F type stars, thus allowingfurther study of the impact of stellar masses on planetary systemformation over a wider range of stellar masses than is currently done.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| On the Spectroscopic Nature of HD 221866
On the basis of a new classification-resolution spectrum, we find thatHD 221866, a member of the newly discovered γ Doradusvariable-star class, is an Am (metallic-line A-type) star. Observationswith the Cambridge radial-velocity spectrometer reveal that the star isa double-lined spectroscopic binary with a period of 135 days and a massratio of 1.11+/-0.03. We have determined the basic physical parametersof both components through spectral analysis of theclassification-resolution spectrum and a medium-resolution spectrum fromthe KPNO coudé-feed spectrograph in conjunction with fluxes fromvisible spectrophotometry and the TD-1 satellite. We confirm the resultsof Fekel et al. that the primary is the Am star, whereas the secondaryappears to be a normal early F-type dwarf. We reanalyze the time-seriesphotometric data for HD 221866 and confirm the existence of two periodsfound by Henry & Fekel.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| The physical properties of normal A stars
Designating a star as of A-type is a result of spectral classification.After separating the peculiar stars from those deemed to be normal usingthe results of a century of stellar astrophysical wisdom, I define thephysical properties of the "normal" stars. The hotter A stars haveatmospheres almost in radiative equilibrium. In the A stars convectivemotions can be found which increase in strength as the temperaturedecreases.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Dusty Debris Disks as Signposts of Planets: Implications for Spitzer Space Telescope
Submillimeter and near-infrared images of cool dusty debris disks andrings suggest the existence of unseen planets. At dusty but nonimagedstars, semimajor axes of associated planets can be estimated from thedust temperature. For some young stars these semimajor axes are greaterthan 1" as seen from Earth. Such stars are excellent targets forsensitive near-infrared imaging searches for warm planets. To probe thefull extent of the dust and hence of potential planetary orbits, Spitzerobservations should include measurements with the 160 μm filter.
| The Structure of the Local Interstellar Medium. II. Observations of D I, C II, N I, O I, Al II, and Si II toward Stars within 100 Parsecs
Moderate- and high-resolution measurements(λ/Δλ>~40,000) of interstellar resonance lines ofD I, C II, N I, O I, Al II, and Si II (hereafter called light ions) arepresented for all available observed targets located within 100 pc thatalso have high-resolution observations of interstellar Fe II or Mg II(heavy ions) lines. All spectra were obtained with the Goddard HighResolution Spectrograph or the Space Telescope Imaging Spectrographinstrument aboard the Hubble Space Telescope. Currently, there are 41sight lines to targets within 100 pc with observations that include aheavy ion at high resolution and at least one light ion at moderate orhigh resolution. We present new measurements of light ions along 33 ofthese sight lines and collect from the literature results for theremaining sight lines that have already been analyzed. For all of thenew observations we provide measurements of the central velocity,Doppler width parameter, and column density for each absorptioncomponent. We greatly increase the number of sight lines with usefullocal interstellar medium (LISM) absorption-line measurements of lightions by using knowledge of the kinematic structure along a line of sightobtained from high-resolution observations of intrinsically narrowabsorption lines, such as Fe II and Mg II. We successfully fit theabsorption lines with this technique, even with moderate-resolutionspectra. Because high-resolution observations of heavy ions are criticalfor understanding the kinematic structure of local absorbers along theline of sight, we include 18 new measurements of Fe II and Mg II in anAppendix. We present a statistical analysis of the LISM absorptionmeasurements, which provides an overview of some physicalcharacteristics of warm clouds in the LISM, including temperature andturbulent velocity. This complete collection and reduction of allavailable LISM absorption measurements provides an important databasefor studying the structure of nearby warm clouds, including ionization,abundances, and depletions. Subsequent papers will present models forthe morphology and physical properties of individual structures (clouds)in the LISM.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| On the Flux of Extrasolar Dust in Earth's Atmosphere
Micron-size extrasolar dust particles have been convincingly detected bysatellites. Larger extrasolar meteoroids (5-35 μm) have most likelybeen detected by ground-based radar at Arecibo and New Zealand. Wepresent estimates of the minimum detectable particle sizes and thecollecting areas for both radar systems. We show that particles largerthan ~10 μm can propagate for tens of parsecs through theinterstellar medium, opening up the possibility that ground-based radarsystems can detect AGB stars, young stellar objects such as T Tauristars, and debris disks around Vega-like stars. We provide analyticaland numerical estimates of the ejection velocity in the case of a debrisdisk interacting with a Jupiter-mass planet. We give rough estimates ofthe flux of large micrometeoroids from all three classes of sources.Current radar systems are unlikely to detect significant numbers ofmeteors from debris disks such as β Pictoris. However, we suggestimprovements to radar systems that should allow for the detection ofmultiple examples of all three classes.
| Ten Micron Observations of Nearby Young Stars
We present new 10 μm photometry of 21 nearby young stars obtained atthe Palomar 5 m and at the Keck I 10 m telescopes as part of a programto search for dust in the habitable zone of young stars. Thirteen of thestars are in the F-K spectral type range (``solar analogs''), four haveB or A spectral types, and four have spectral type M. We confirmexisting IRAS 12 μm and ground-based 10 μm photometry for 10 ofthe stars and present new insight into this spectral regime for therest. Excess emission at 10 μm is not found in any of the young solaranalogs, except for a possible 2.4 σ detection in the G5 V star HD88638. The G2 V star HD 107146, which does not display a 10 μmexcess, is identified as a new Vega-like candidate, based on our 10μm photospheric detection, combined with previously unidentified 60and 100 μm IRAS excesses. Among the early-type stars, a 10 μmexcess is detected only in HD 109573A (HR 4796A), confirming priorobservations; among the M dwarfs, excesses are confirmed in AA Tau, CD-40°8434, and Hen 3-600A. A previously suggested N-band excess inthe M3 dwarf CD -33°7795 is shown to be consistent with photosphericemission. We calculate infrared to stellar bolometric luminosity ratiosfor all stars exhibiting mid-infrared excesses and infer the total massof orbiting dust in the cases of optically thin disks. For a derivedmedian photometric precision of +/-0.11 mag, we place an upper limit ofMdust~2×10-5 M⊕ on the dustmass (assuming a dust temperature of 300 K) around solar analogs notseen in excess at 10 μm. Our calculations for the nearby K1 V star HD17925 show that it may have the least massive debris disk known outsideour solar system (Mdust>~7×10-6M⊕). Our limited data confirm the expected tendency ofdecreasing fractional dust excessfd=LIR/L* with increasing stellar age.However, we argue that estimates of fd suffer from adegeneracy between the temperature and the amount of circumstellar dustMdust, and we propose a relation of Mdust as afunction of age instead.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Chemical analysis of 24 dusty (pre-)main-sequence stars
We have analysed the chemical photospheric composition of 24 HerbigAe/Be and Vega-type stars in search for the λ Bootis phenomenon.We present the results of the elemental abundances of the sample stars.Some of the stars were never before studied spectroscopically at opticalwavelengths. We have determined the projected rotational velocities ofour sample stars. Furthermore, we discuss stars that depict a(selective) depletion pattern in detail. HD 4881 andHD 139614 seem to display an overall deficiency.AB Aur and possibly HD 126367 havesubsolar values for the iron abundance, but are almost solar in silicon.HD 100546 is the only clear λ Bootis star inour sample.Appendix is only available in electronic form athttp://www.edpsciences.org
| VLTI near-IR interferometric observations of Vega-like stars. Radius and age of α PsA, β Leo, β Pic, ɛ Eri and τ Cet
We report in this paper the direct interferometric measurement of theangular diameter of five nearby Vega-like stars: α PsA, βLeo, β Pic, ɛ Eri and τ Cet. The near-infrared (K and Hbands) observations were conducted at the VLTI during the commissioningperiod with the VINCI instrument and three different baselines rangingfrom 66 m to 140 m. The five stellar photospheres are resolved and wederive their angular diameters with a 1 to 2% accuracy, except forβ Pic (14%). We discuss the detectability and the influence of apossible small amount of warm circumstellar dust on our measurements. Inaddition, we have used the stellar evolution code CESAM (Morel\cite{m97}) to compare the computed fundamental parameters to theobserved values (linear diameter, luminosity, temperature and chemicalabundance). As a result of the simulation, the age of the stars isinferred and found to be in good agreement with previous estimates fromvarious other methods.
| The angular sizes of dwarf stars and subgiants. Surface brightness relations calibrated by interferometry
The availability of a number of new interferometric measurements of MainSequence and subgiant stars makes it possible to calibrate the surfacebrightness relations of these stars using exclusively direct angulardiameter measurements. These empirical laws make it possible to predictthe limb darkened angular diameters θLD of dwarfs andsubgiants using their dereddened Johnson magnitudes, or their effectivetemperature. The smallest intrinsic dispersions of σ ≤1% inθLD are obtained for the relations based on the K andL magnitudes, for instance log θLD = 0.0502 (B-L) +0.5133 - 0.2 L or log θLD = 0.0755 (V-K) + 0.5170 -0.2 K. Our calibrations are valid between the spectral types A0 and M2for dwarf stars (with a possible extension to later types when using theeffective temperature), and between A0 and K0 for subgiants. Suchrelations are particularly useful for estimating the angular sizes ofcalibrators for long-baseline interferometry from readily availablebroadband photometry.Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
| A search for circumstellar dust disks with ADONIS
We present results of a coronographic imaging search for circumstellardust disks with the Adaptive Optics Near Infrared System (ADONIS) at theESO 3.6 m telescope in La Silla (Chile). 22 candidate stars, known to beorbited by a planet or to show infrared excess radiation, were examinedfor circumstellar material. In the PSF-subtracted images no clear diskwas found. We further determine the detection sensitivities and outlinehow remaining atmospheric fluctuations still can hamper adaptive opticsobservations.Based on observations collected at the European Southern Observatory, LaSilla, Chile (65.I-0098, 67.C-0240, 68.C-0070).
| S4N: A spectroscopic survey of stars in the solar neighborhood. The Nearest 15 pc
We report the results of a high-resolution spectroscopic survey of allthe stars more luminous than M_V = 6.5 mag within 14.5 pc from the Sun.The Hipparcos catalog's completeness limits guarantee that our survey iscomprehensive and free from some of the selection effects in othersamples of nearby stars. The resulting spectroscopic database, which wehave made publicly available, includes spectra for 118 stars obtainedwith a resolving power of R ≃ 50 000, continuous spectral coveragebetween 362-921 nm, and typical signal-to-noise ratios in therange 150-600. We derive stellar parameters and perform a preliminaryabundance and kinematic analysis of the F-G-K stars in the sample. Theinferred metallicity ([Fe/H]) distribution is centered at about -0.1dex, and shows a standard deviation of 0.2 dex. A comparison with largersamples of Hipparcos stars, some of which have been part of previousabundance studies, suggests that our limited sample is representative ofa larger volume of the local thin disk. We identify a number ofmetal-rich K-type stars which appear to be very old, confirming theclaims for the existence of such stars in the solar neighborhood. Withatmospheric effective temperatures and gravities derived independentlyof the spectra, we find that our classical LTE model-atmosphere analysisof metal-rich (and mainly K-type) stars provides discrepant abundancesfrom neutral and ionized lines of several metals. This ionizationimbalance could be a sign of departures from LTE or inhomogeneousstructure, which are ignored in the interpretation of the spectra.Alternatively, but seemingly unlikely, the mismatch could be explainedby systematic errors in the scale of effective temperatures. Based ontransitions of majority species, we discuss abundances of 16 chemicalelements. In agreement with earlier studies we find that the abundanceratios to iron of Si, Sc, Ti, Co, and Zn become smaller as the ironabundance increases until approaching the solar values, but the trendsreverse for higher iron abundances. At any given metallicity, stars witha low galactic rotational velocity tend to have high abundances of Mg,Si, Ca, Sc, Ti, Co, Zn, and Eu, but low abundances of Ba, Ce, and Nd.The Sun appears deficient by roughly 0.1 dex in O, Si, Ca, Sc, Ti, Y,Ce, Nd, and Eu, compared to its immediate neighbors with similar ironabundances.Based on observations made with the 2.7 m telescope at the McDonaldObservatory of the University of Texas at Austin (Texas), and the 1.52 mtelescope at the European Southern Observatory (La Silla, Chile) underthe agreement with the CNPq/Observatorio Nacional (Brazil).Tables 3-5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/420/183
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/413/1037
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