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Ýlgili Makaleler
Mid-Infrared Observations of V838 Mon
We present 8-13 µm narrow-band photometry of the peculiarstar V838 Mon in 2002 September. Our spectrum is consistent with asimple power-law spectrum, although a spectrum with excess emissionbetween 9 and 12 µm provides a better fit.
| 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.
| A New Stellar Library in the K Band
Stellar population synthesis models are crucial for the understanding ofthe large amount of data which is being gathered for galaxies at low andhigh redshift, and provide the only way to compare the ``real world''with the theoretical framework. The best models require extensiveempirical stellar spectral libraries, which at present are starting tobe quite complete in the optical range. However the situation isdifferent in the near-infrared, which observational windows have been,until recently, poorly exploited. This is specially due to the lack ofappropriate instrumentation. We present the preliminary results of anongoing observational program aimed to overcome this problem and toprovide a stellar library in the K band with the required coverage ofphysical stellar parameters: effective temperature, gravity, metallicityand non-solar abundance ratios. In particular, the CO feature at 2.3μ m is a very promising spectroscopic line-strength index that willhelp to face outstanding problems in galaxy formation and evolution. Theavailability of this library will be essential to interpret the stellarcontent of composite stellar populations with EMIR. In addition, thislibrary will be also useful for other purposes, like the study of highlyreddened objects, and the spectral classification of late type stars.
| 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.
| Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in HD 105
We report infrared spectroscopic observations of HD 105, a nearby (~40pc) and relatively young (~30 Myr) G0 star with excess infraredcontinuum emission, which has been modeled as arising from an opticallythin circumstellar dust disk with an inner hole of size >~13 AU. Wehave used the high spectral resolution mode of the Infrared Spectrometer(IRS) on the Spitzer Space Telescope to search for gas emission linesfrom the disk. The observations reported here provide upper limits tothe fluxes of H2 S(0) 28 μm, H2 S(1) 17 μm,H2 S(2) 12 μm, [Fe II] 26 μm, [Si II] 35 μm, and [SI] 25 μm infrared emission lines. The H2 line upper limitsplace direct constraints on the mass of warm molecular gas in the disk:M(H2)<4.6, 3.8×10-2, and3.0×10-3 MJ at T=50, 100, and 200 K,respectively. We also compare the line flux upper limits to predictionsfrom detailed thermal/chemical models of various gas distributions inthe disk. These comparisons indicate that if the gas distribution has aninner hole with radius ri,gas, the surface density at thatinner radius is limited to values ranging from <~3 g cm-2at ri,gas=0.5 AU to 0.1 g cm-2 atri,gas=5-20 AU. These values are considerably below the valuefor a minimum mass solar nebula, and suggest that less than 1 Jupitermass (MJ) of gas (at any temperature) exists in the 1-40 AUplanet-forming region. Therefore, it is unlikely that there issufficient gas for gas giant planet formation to occur in HD 105 at thistime.
| The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
| X-Rays from Hybrid Stars
The late-type giants and supergiants of the ``hybrid chromosphere''class display signatures of cool (T<~2×104 K) windstogether with hot emission lines from species like C IV(T~105 K). A survey of such stars by Reimers et al. usingROSAT reported numerous X-ray detections (T~106 K),strengthening the (then heretical) idea that hot coronae and cool windscan coexist in luminous giants. However, several of the candidatesources were offset from the predicted stellar coordinates, calling intoquestion the identifications. In an effort to secure better knowledge ofthe X-ray luminosities of the hybrids, the ROSAT fields from the Reimerset al. survey were reexamined, exploiting the USNO-A2.0 astrometriccatalog to register the pointings to a few arcseconds accuracy. On thebasis of positional mismatches, at least two of the previously reporteddetections of key hybrid stars-γ Dra (K5 III) and β Aqr (G0Ib)-must be rejected. The new X-ray upper limits for these stars,combined with the remaining candidate detections (and nondetections)from the original survey, place the hybrids into the same ``X-raydeficient'' category as the ``noncoronal'' red giants like Arcturus(α Boo: K1.5 III) and Aldebaran (α Tau: K5 III). A few ofthe hybrid X-ray sources are exceptional, however. The archetype αTrA (K2 II-III), in particular, is securely detected in terms ofpositional coincidence, but its anomalous, contradictory coronalproperties suggest that an unseen companion-a young hyperactive Gdwarf-might dominate the X-ray emission.
| Far-Infrared and Millimeter Continuum Studies of K Giants: α Bootis and α Tauri
We have imaged two normal, noncoronal, infrared-bright K giants, αTau and α Boo, in the 1.4 and 2.8 mm continua using BIMA. Thesestars have been used as important absolute calibrators for severalinfrared satellites. Our goals are (1) to establish whether these starsradiate as simple photospheres or possess long-wavelength chromospheresand (2) to make a connection between millimeter-wave and far-infrared(FIR) absolute flux calibrations. To accomplish these goals we alsopresent Infrared Space Observatory Long Wavelength Spectrometermeasurements of both these K giants. The FIR and millimeter continuumradiation is produced in the vicinity of the temperature minimum inα Tau and α Boo. We find that current photospheric modelspredict fluxes in reasonable agreement with those observed forwavelengths that sample the upper photosphere, namely, <=125 μm inα Tau and α Boo. We clearly detect chromospheric radiationfrom both stars by 2.8 mm (1.4 mm in the case of α Boo). Onlyadditional observations can determine precisely where beyond 125 μmthe purely radiative models fail. Until then, purely radiative modelsfor these stars can only be used with confidence for calibrationpurposes below 125 μm.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z = 0.02
We present new synthetic broad-band photometric colors for late-typegiants based on synthetic spectra calculated with the PHOENIX modelatmosphere code. The grid covers effective temperatures T_eff=3000dots5000 K, gravities log g=-0.5dots{+3.5}, and metallicities[M/H]=+0.5dots{-4.0}. We show that individual broad-band photometriccolors are strongly affected by model parameters such as molecularopacities, gravity, microturbulent velocity, and stellar mass. Ourexploratory 3D modeling of a prototypical late-type giant shows thatconvection has a noticeable effect on the photometric colors too, as italters significantly both the vertical and horizontal thermal structuresin the outer atmosphere. The differences between colors calculated withfull 3D hydrodynamical and 1D model atmospheres are significant (e.g.,Δ(V-K)0.2 mag), translating into offsets in effectivetemperature of up to 70 K. For a sample of 74 late-type giants in theSolar neighborhood, with interferometric effective temperatures andbroad-band photometry available in the literature, we compare observedcolors with a new PHOENIX grid of synthetic photometric colors, as wellas with photometric colors calculated with the MARCS and ATLAS modelatmosphere codes. We find good agreement of the new synthetic colorswith observations and published T_eff-color and color-color relations,especially in the T_eff-(V-K), T_eff-(J-K) and (J-K)-(V-K) planes.Deviations from the observed trends in the T_eff-color planes aregenerally within ±100 K for T_eff=3500 to 4800 K. Syntheticcolors calculated with different stellar atmosphere models agree to±100 K, within a large range of effective temperatures andgravities. The comparison of the observed and synthetic spectra oflate-type giants shows that discrepancies result from the differencesboth in the strengths of various spectral lines/bands (especially thoseof molecular bands, such as TiO, H2O, CO) and the continuum level.Finally, we derive several new T_eff-log g-color relations for late-typegiants at solar-metallicity (valid for T_eff=3500 to 4800 K), based bothon the observed effective temperatures and colors of the nearby giants,and synthetic colors produced with PHOENIX, MARCS and ATLAS modelatmospheres.
| 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
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| Infrared Irradiance Calibration
Infrared astronomical measurements are calibrated against referencesources, usually primary standard stars that are, in turn, calibratedeither by direct or indirect means. A direct calibration compares thestar with a certified source, typically a blackbody. Indirect methodsextrapolate a direct measurement of the flux at one wavelength to theflux at another. Historically, α Lyr (Vega) has been used as theprimary standard as it is bright, easily accessible from the northernhemisphere, and is well calibrated in the visual. Until recently, thedirect absolute infrared calibrations of α Lyr and those derivedfrom the absolute solar flux scaled to the observed spectral energydistributions of solar type stars increasingly diverged with wavelengthfrom those obtained using a model atmosphere to extrapolate the absolutevisual flux of Vega into the infrared. The exception is the directcalibration by the 1996/97 Midcourse Space Experiment of the absolutefluxes for a number of the commonly used infrared standard stars,including Vega.
| The European Large Area ISO Survey - VIII. 90-μm final analysis and source counts
We present a re-analysis of the European Large Area Infrared SpaceObservatory (ISO) Survey (ELAIS) 90-μm observations carried out withISOPHOT, an instrument on board the ISO of the European Space Agency.With more than 12 deg2, the ELAIS survey is the largest areacovered by ISO in a single programme and is about one order of magnitudedeeper than the IRAS 100-μm survey. The data analysis is presentedand was mainly performed with the PHOT interactive analysis software butusing the pairwise method of Stickel et al. for signal processing fromedited raw data to signal per chopper plateau. The ELAIS 90-μmcatalogue contains 237 reliable sources with fluxes larger than 70 mJyand is available in the electronic version of this article. Numbercounts are presented and show an excess above the no-evolution modelprediction. This confirms the strong evolution detected at shorter (15μm) and longer (170 μm) wavelengths in other ISO surveys. TheELAIS counts are in agreement with previous works at 90 μm and inparticular with the deeper counts extracted from the Lockman holeobservations. Comparison with recent evolutionary models show that themodels of Franceschini et al. and Guiderdoni et al. (which includes aheavily extinguished population of galaxies) give the best fit to thedata. Deeper observations are nevertheless required to discriminatebetter between the model predictions in the far-infrared, and arescheduled with the Spitzer Space Telescope, which has already startedoperating, and will also be performed by ASTRO-F.
| 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.
| Excess mid-infrared emission in cataclysmic variables
We present a search for excess mid-infrared emission due tocircumbinary (CB) material in the orbital plane of cataclysmic variables(CVs). Our motivation stems from the fact that the strong brakingexerted by a CB disc on the binary system could explain several puzzlesin our current understanding of CV evolution. Since theoreticalestimates predict that the emission from a CB disc can dominate thespectral energy distribution (SED) of the system at λ > 5μm, we obtained simultaneous visible to mid-infrared (mid-IR) SEDsfor eight systems. We report detections of SS Cyg at 11.7 μm and AEAqr at 17.6 μm, both in excess of the contribution from the secondarystar. In AE Aqr, the IR likely originates from synchrotron-emittingclouds propelled by the white dwarf. In SS Cyg, we argue that theobserved mid-IR variability is difficult to reconcile with simple modelsof CB discs and we consider free-free emission from a wind. In the othersystems, our mid-IR upper limits place strong constraints on the maximumtemperature of a putative CB disc. The results show that if any sizeableCB discs are present in these systems, they must be self-shadowed orperhaps dust-free, with the peak thermal emission shifted to far-IRwavelengths.
| Seasonal variation of Titan's stratospheric ethylene (C2H4) observed
All previous observations of seasonal change on Titan have been ofphysical phenomena such as clouds and haze. We present here the firstobservational evidence of chemical change in Titan's atmosphere. Imagestaken during 1999-2002 (late southern spring on Titan) with the W.M.Keck I 10-meter telescope at 8-13 μm show a significant accumulationof ethylene (C2H4) in the south polar stratosphereas well as north-south stratospheric temperature variation (colder atpoles). Our observations restrict this newly discovered south polarethylene accumulation to latitudes south of 60° S. The only otherobservations of the spatial distribution of C2H4were those of Voyager I, which found a significant north polaraccumulation in early northern spring. We see no build-up in the north,although the highest northern latitudes are obstructed from view in thecurrent season. Our observations constrain any unobserved north polaraccumulation of C2H4 to north of 50° Nlatitude. Comparison of the Voyager I results with our new results showseasonal chemical change has occurred in Titan's atmosphere.
| MARCS: Model Stellar Atmospheres and Their Application to the Photometric Calibration of the Spitzer Space Telescope Infrared Spectrograph (IRS)
We describe state-of-the-art MARCS-code model atmospheres generated fora group of A dwarf, G dwarf, and late-G to mid-K giant standard stars,selected to photometrically calibrate the Spitzer Space TelescopeInfrared Spectrograph (IRS) and compare the synthetic spectra toobservations of HR 6688, HR 6705, and HR 7891. The general calibrationprocesses and uncertainties are briefly described, and the differencesbetween various templated composite spectra of the standards areaddressed. In particular, a contrast between up-to-date modelatmospheres and previously published composite and synthetic spectra isillustrated for wavelength ranges around 8 μm (where the SiOΔv=1 band occurs for the cooler standards) and λ>=20μm, where the use of the Engelke function will lead to increasinglylarge discrepancies as a result of the neglect of gravity in cool stars.At this point, radiometric requirements are being met, absolute fluxcalibration uncertainties (1 σ) are ~20% in the Short-High andLong-High data and ~15% in the Short-Low and Long Low data, andorder-to-order flux uncertainties are ~10% or less. Iteration betweenthe MARCS model atmosphere inputs and the data processing will improvethe S/N ratios and calibration accuracies.
| 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).
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| Adaptive Optics Nulling Interferometric Constraints on the Mid-Infrared Exozodiacal Dust Emission around Vega
We present the results of mid-infrared nulling interferometricobservations of the main-sequence star α Lyr (Vega) using the 6.5m MMT with its adaptive secondary mirror. From the observations at 10.6μm, we find that there is no resolved emission from the circumstellarenvironment (at separations greater than 0.8 AU) above 2.1% (3 σlimit) of the level of the stellar photospheric emission. Thus, we areable to place an upper limit on the density of dust in the inner systemof 650 times that of our own solar system's zodiacal cloud. This limitis roughly 2.8 times better than those determined with photometricexcess observations such as those by IRAS. Comparison with far-infraredobservations by IRAS shows that the density of warm dust in the innersystem (<30 AU) is significantly lower than cold dust at largerseparations. We consider two scenarios for grain removal, thesublimation of ice grains and the presence of a planetary mass''sweeper.'' We find that if sublimation of ice grains is the onlyremoval process, a large fraction (>80%) of the material in the outersystem is ice.The results presented here made use of the MMT Observatory, a facilityjointly operated by the University of Arizona and the SmithsonianInstitution.
| Spectral Irradiance Calibration in the Infrared. XV. Absolute Calibration of Standard Stars by Experiments on the Midcourse Space Experiment
Calibration experiments were conducted with the SPIRIT III infraredinstrument on the Midcourse Space Experiment (MSX) against a number ofinfrared standard stars and five emissive reference spheres that wereejected at various times during the mission. The physical properties ofthe 2 cm diameter spheres, such as size and emissivity, were preciselymeasured in the laboratory. The energy balance equation between thetotal flux absorbed and that emitted by the sphere is solved to obtainthe time-dependent temperature of the sphere under the assumption thatthe sphere radiates as a blackbody with the measuredwavelength-dependent emissivity. The estimated uncertainties in themodeling of the sphere are about 1 K in the thermal component and 3% forthe geometric contribution. MSX also measured over 150 mean fluxes foreight standard infrared calibration stars during the 10 month mission.The measurements were scaled to the absolute fluxes that Cohen et al.adopt for α CMa (Sirius). The measured spectral energydistributions of the calibration stars relative to Sirius are within theuncertainties that Cohen et al. assign to the absolute fluxes from thesestars, with a few exceptions. However, the MSX measurement uncertaintiesare generally much smaller, and the mission-averaged fluxes revealstatistically significant deviations from the Cohen et al. values. Ofthe calibration stars, only β Peg was found to be variable. MSXalso measured excess fluxes for α Lyr (Vega) in the 12.1, 14.7,and 21.3 μm spectral bands; the excesses in the latter two bands areconsistent with the published thermal model for the dust ring aroundthis star. The absolute calibration of the fluxes of the stellarstandards based on the average of the measurements of the spheres overall MSX bands and the five experiments agrees with those predicted towithin the 1.4% MSX measurement uncertainties. The zero-magnitudeabsolute fluxes proposed by Cohen et al. are validated if the flux fromSirius is increased by 1%.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| On the analysis of band 3 of the ISO-SWS calibration sources
We analyse ISO-SWS 01 (R 1500) 12-27.5 μm (band 3) spectra ofthe 10 standard calibration stars with the highest flux using syntheticspectra generated from (MARCS) atmosphere models. The comparison betweenthe observed and synthetic spectra reveals the quality of (1) theatmospheric model construction and subsequent synthetic spectracomputation and of (2) the (OLP 10.1) calibration and data reduction ofthe spectrometer at these wavelengths.The models represent the general features of the observations, but thesynthetic spectrum computation is hampered by the lack of comprehensivemolecular and atomic line lists. We also suspect some problems with thetemperature distribution in the outer layers of the model andinaccuracies in the extrapolation of the collision-induced absorptioncoefficients of H2 pairs. We detect baseline ripples andfringes in the observed spectra, that survive the calibration anddetailed reduction process. Photometric calibration uncertainties areestimated by means of the scaling factors between the synthetic andobserved spectra.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited
We describe a general yet simple method to analyse the propagation ofnuclear reaction rate uncertainties in a stellar nucleosynthesis andmixing context. The method combines post-processing nucleosynthesis andmixing calculations with a Monte Carlo scheme. With this approach wereanalyse the dependence of theoretical oxygen isotopic ratiopredictions in first dredge-up red giant branch stars in a systematicway. Such predictions are important to the interpretation of pre-solarAl2O3 grains from meteorites. The reaction rateswith uncertainties were taken from the NACRE compilation of Angulo etal. We include seven reaction rates in our systematic analysis ofstellar models with initial masses from 1 to 3 Msolar. Wefind that the uncertainty of the 18O(p,α)15N reaction rate typically causes an error in thetheoretical 16O/18O ratio of ~= +20/ - 5 per cent.The error of the 16O/17O prediction is 10-40 percent depending on the stellar mass, and is persistently dominated by thecomparatively small uncertainty of the 16O(p,γ)17F reaction. With the new estimates on reaction rateuncertainties by the NACRE compilation, the p-capture reactions17O(p, α)14N and 17O(p,γ)18F have virtually no impact on theoreticalpredictions for stellar mass <=1.5 Msolar. However, theuncertainty in 17O(p, α)14N has an effectcomparable to or greater than that of 16O(p,γ)17F for masses >1.5 Msolar, where coremixing and subsequent envelope mixing interact. In these cases wherecore mixing complicates post-dredge-up surface abundances, uncertaintyin other reactions have a secondary but noticeable effect on surfaceabundances.
| Infrared stellar populations in the central parts of the Milky Way galaxy
Near- and mid-IR survey data from DENIS and ISOGAL are used toinvestigate the structure and formation history of the inner 10°(1.4 kpc) of the Milky Way galaxy. Synthetic bolometric corrections andextinction coefficients in the near- and mid-infrared (mid-IR) arederived for stars of different spectral types, to allow thetransformation of theoretical isochrones into observablecolour-magnitude diagrams. The observed IR colour-magnitude diagrams areused to derive the extinction, metallicity and age for individual stars.The inner galaxy is dominated by an old population (>~7 Gyr). Inaddition, an intermediate-age population (~200 Myr-7 Gyr) is detected,which is consistent with the presence of a few hundred asymptotic giantbranch stars with heavy mass loss. Furthermore, young stars (<~200Myr) are found across the inner bulge. The metallicities of thesestellar population components are discussed. These results can beinterpreted in terms of an early epoch of intense star formation andchemical enrichment that shaped the bulk of the bulge and nucleus, and amore continuous star formation history that gradually shaped the discfrom the accretion of subsolar metallicity gas from the halo. A possibleincrease in star formation ~200 Myr ago might have been triggered by aminor merger. Ever since the formation of the first stars, mechanismshave been at play that mix the populations from the nucleus, bulge anddisc. Luminosity functions across the inner Galactic plane indicate thepresence of an inclined (bar) structure at >~1 kpc from the GalacticCentre, near the inner Lindblad resonance. The innermost part of thebulge, within ~1 kpc from the Galactic Centre, seems azimuthallysymmetric.
| A Uniform Database of 2.4-45.4 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer
We present a complete set of all valid SWS full-scan 2.4-45.4 μmspectra processed and renormalized in as uniform a manner as possible.The processing produces a single spectrum for each observation from the288 individual spectral segments, which are the most processed formavailable from the ISO archive. The spectra, and the programs used tocreate them, are available to the community on-line.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| High-Precision Stellar Radial Velocities in the Galactic Center
We present radial velocities for 85 cool stars projected onto thecentral parsec of the Galaxy. The majority of these velocities haverelative errors of ~1 km s-1, or a factor of ~30-100 smallerthan those previously obtained with proper-motion or other radialvelocity measurements for a similar stellar sample. The error in atypical individual stellar velocity, including all sources ofuncertainty, is 1.7 km s-1. Two similar data sets wereobtained 1 month apart, and the total error in the relative velocitiesis 0.80 km s-1 in the case where an object is common to bothdata sets. The data are used to characterize the velocity distributionof the old population in the Galactic center. We find that the starshave a Gaussian velocity distribution with a mean heliocentric velocityof -10.1+/-11.0 km s-1 (blueshifted) and a standard deviationof 100.9+/-7.7 km s-1 the mean velocity of the sample isconsistent with no bulk line-of-sight motion with respect to the localstandard of rest. At the 1 σ level, the data are consistent with asymmetric velocity distribution about any arbitrary axis in the plane ofthe sky. We find evidence for a flattening in the distribution oflate-type stars within a radius of ~0.4 pc and infer a volume densitydistribution of r-1/4 in this region. Finally, we establish afirst epoch of radial velocity measurements that can be compared withsubsequent epochs to measure small accelerations (1 km s-1yr-1), corresponding to the magnitude expected over a timespan of several years for stars nearest to Sgr A*.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Buried Alive in the Coronal Graveyard
We have used the High Resolution Camera (HRC-I) of the Chandra X-RayObservatory to search for coronal (T~106 K) emission from thearchetype ``noncoronal'' red giants Arcturus (α Bootis=HD 124897,K1 III) and Aldebaran (α Tauri=HD 29139, K5 III). Our programfollows up previous detections of ultraviolet coronal proxies such as CIV λ1548 (T~1×105 K) and O VI λ1031(T~3×105 K). The deep (~19 ks) HRC-I pointings obtaineda tentative 3 σ detection of Arcturus, withfX(0.2-2keV)=1.0+1.8-0.8×10-15ergs cm-2 s-1 (95% confidence limits [CLs]), butfailed to record Aldebaran, with an upper limit of<~1.5×10-15 ergs cm-2 s-1(also at 95% CL). The corresponding LX/Lbol ratiosare a factor of ten thousand less than the Sun, a low-activity coronaldwarf. At the same time, Hubble Space Telescope Imaging Spectrographfar-ultraviolet spectra suggest the presence of a ``cool absorber,''probably near the base of the red giant chromosphere, imprintingdiscrete low-excitation absorptions on top of highly ionized featuressuch as Si IV λ1393. The hot emission zones thus are at leastpartially buried under a large column of chromospheric material, whichwould severely attenuate any soft X-rays that might be emitted. Thesubmerged hot structures presumably are magnetic because of their hightemperatures and broad C IV profiles (FWHM~130 km s-1).Perhaps these structures are analogous to small-scale ephemeral bipolarregions seen ubiquitously on the Sun throughout the sunspot cycle andthought to be of direct convective origin. If small-scale magneticfields indeed are present in the lower atmospheres of red giants such asArcturus and Aldebaran, they might play a role in initiating the coolwinds of such stars, perhaps through a mechanism similar to solarspicules.
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