Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
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.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula
The mid-infrared emission from a photodissociation region (PDR) viewededge-on in the Orion Nebula is examined through 8.7-20.6 μm imagesand 8-13 μm spectra. The polycyclic aromatic hydrocarbon (PAH)emission is located between the edges of H II regions and layers of [CI] emission, agreeing with PDR theory. Using a simple model, the spatialvariations in the emission from PAHs detected at 8.6, 11.2, and 12.7μm are demonstrated to be directly proportional to the materialcolumn density and the intensity of the UV field. For a homogeneous,neutral cloud illuminated by a bright OB star, PDR theory predicts thatthe ultraviolet (UV) radiation is attenuated exponentially(e-1.8Av). The predicted UV attenuation isconfirmed by observations of broad PAH emission features found at 8.6,11.2, and 12.7 μm. The PAH emission is found in cool regions havinggreater optical depths relative to regions where mid-infrared emissionfrom ionized gas is observed. Through modeling we determine a gasdensity of 9.7×104 cm-3. On large and smallsize scales, the relative strengths of the 8.6, 11.2, and 12.7 μm PAHfeatures at the bar of the Orion Nebula indicate that there is not asimple transition from ionized to neutral PAHs across the PDR.
| 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.
| Recent astrophysical results from the VLTI.
Not Available
| 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.
| La supernova de 1572, une diversite d'interpretations.
Not Available
| Le message de la lumiere.
Not Available
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| A New Version of Reimers' Law of Mass Loss Based on a Physical Approach
We present a new semiempirical relation for the mass loss of coolstellar winds, which so far has frequently been described by ``Reimers'law.'' Originally, this relation was based solely on dimensional scalingarguments without any physical interpretation. In our approach, the windis assumed to result from the spillover of the extended chromosphere,possibly associated with the action of waves, especially Alfvénwaves, which are used as guidance in the derivation of the new formula.We obtain a relation akin to the original Reimers law, but whichincludes two new factors. They reflect how the chromospheric heightdepends on gravity and how the mechanical energy flux depends, mainly,on the effective temperature. The new relation is tested and sensitivelycalibrated by modeling the blue end of the horizontal branch of globularclusters. The most significant difference from mass-loss rates predictedby the Reimers relation is an increase by up to a factor of 3 forluminous late-type (super)giants, in good agreement with observations.
| H2 Pure Rotational Lines in the Orion Bar
Photodissociation regions (PDRs), where UV radiation dominates theenergetics and chemistry of the neutral gas, contain most of the mass inthe dense interstellar medium of our Galaxy. Observations ofH2 rotational and rovibrational lines reveal that PDRscontain unexpectedly large amounts of very warm (400-700 K) moleculargas. Theoretical models have difficulty explaining the existence of somuch warm gas. Possible problems include errors in the heating andcooling functions or in the formation rate for H2. To date,observations of H2 rotational lines smear out the structureof the PDR. Only by resolving the hottest layers of H2 canone test the predictions and assumptions of current models. Using theTexas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission inthe H2 v=0-0 S(1) and S(2) lines toward the Orion Bar PDR at2'' resolution. We also observed H2 v=0-0 S(4) atselected points toward the front of the PDR. Our maps cover a12'' by 40'' region of the bar where H2rovibrational lines are bright. The distributions of H2 0-0S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail.The high spatial resolution (0.002 pc) of our observations allows us toprobe the distribution of warm gas in the Orion Bar to a distanceapproaching the scale length for FUV photon absorption. We use these newobservational results to set parameters for the PDR models described ina companion paper in preparation by Draine et al. The best-fit model canaccount for the separation of the H2 emission from theionization front and the intensities of the ground-state rotationallines, as well as the 1-0 S(1) and 2-1 S(1) lines. This model requiressignificant adjustments to the commonly used values for the dust UVattenuation cross section and the photoelectric heating rate.
| Near-Infrared Water Lines in V838 Monocerotis
V838 Monocerotis had an intriguing, nova-like outburst in 2002 Januarythat has subsequently led to several studies of the object. It is nowrecognized that the outburst of V838 Mon and its evolution are differentfrom those of a classical nova or other classes of well-known eruptivevariables. V838 Mon, along with two other objects that have analogousproperties, appears to make up a new class of eruptive variables. Thereare limited infrared studies of V838 Mon. Here we present near-infraredH-band (1.5-1.75 μm) spectra of V838 Mon from late 2002 to the end of2004. The principal new result from our work is the detection of severalrotation-vibration lines of water in the H-band spectra. The observedwater lines have been modeled to first establish that they are indeeddue to water. Subsequently the temperature and column densities of theabsorbing material, from where the water absorption features originate,are derived. From our analysis, we find that the water features arisefrom a cool, ~750-900 K, region around V838 Mon that appears to begradually cooling with time.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| 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.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra ofeight giant and supergiant stars reveal that high-temperature(3×105 K) atmospheres are common in luminous cool starsand extend across the color-magnitude diagram from α Car (F0 II)to the cool giant α Tau (K5 III). Emission present in thesespectra includes chromospheric H Lyβ, Fe II, C I, and transitionregion lines of C III, O VI, Si III, and Si IV. Emission lines of FeXVIII and Fe XIX signaling temperatures of ~107 K and coronalmaterial are found in the most active stars, β Cet and 31 Com. Ashort-term flux variation, perhaps a flare, was detected in β Cetduring our observation. Stellar surface fluxes of the emission of C IIIand O VI are correlated and decrease rapidly toward the cooler stars,reminiscent of the decay of magnetically heated atmospheres. Profiles ofthe C III λ977 lines suggest that mass outflow is underway atT~80,000 K and the winds are warm. Indications of outflow at highertemperatures (3×105 K) are revealed by O VI asymmetriesand the line widths themselves. High-temperature species are absent inthe M supergiant α Ori. Narrow fluorescent lines of Fe II appearin the spectra of many giants and supergiants, apparently pumped by HLyα, and formed in extended atmospheres. Instrumentalcharacteristics that affect cool star spectra are discussed.
| 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.
| The Orion Nebula in the Mid-Infrared
We present two wide-field (~5'×3&farcm;5),diffraction-limited (λ/D~=0.5" at 10 μm), broadband 10 and 20μm images of the Orion Nebula, plus six 7-13 μm narrowband(λ/Δλ~=1) images of the BN/KL complex taken at the3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images,centered on the Trapezium and BN/KL regions, are mosaics of35''×35'' frames obtained with standardchopping and nodding techniques and reconstructed using a newrestoration method developed for this project. They show the filamentarystructure of the dust emission from the walls of the H II region andreveal a new remarkable group of arclike structures ~1' to the south ofthe Trapezium. The morphology of the Ney-Allen Nebula, produced bywind-wind interaction in the vicinity of the Trapezium stars, suggests acomplex kinematical structure at the center of the cluster. We findindications that one of the most massive members of the cluster, theB0.5 V star θ1 Ori D, is surrounded by aphotoevaporated circumstellar disk. Among the four historic Trapezium OBstars, this is the only one without a binary companion, suggesting thatstellar multiplicity and the presence of massive circumstellar disks maybe mutually exclusive. In what concerns the BN/KL complex, we findevidence for extended optically thin silicate emission on top of thedeep 10 μm absorption feature. Assuming a simple two-component model,we map with ~=0.5" spatial resolution the foreground optical depth,color temperature, and mid-IR luminosity of the embedded sources. Weresolve a conspicuous point source at the location of the IRc2-A knot,approximately 0.5" north of the deeply embedded H II region ``I.'' Weanalyze the spectral profile of the 10 μm silicate absorption featureand find indication for grain crystallization in the harsh nebularenvironment. In the OMC-1 South region, we detect several point sourcesand discuss their association with the mass-loss phenomenology observedat optical and millimeter wavelengths. Finally, we list the position andphotometry of 177 point sources, the large majority of which aredetected for the first time in the mid-IR. Twenty-two of them lack acounterpart at shorter wavelengths and are therefore candidates fordeeply embedded protostars. The comparison of photometric data obtainedat two different epochs reveals that source variability at 10 μm ispresent up to a level of ~1 mag on a timescale of ~2 yr. With thepossible exception of a pair of OB stars, all point sources detected atshorter wavelengths display 10 μm emission well above thephotospheric level, which we attribute to disk circumstellar emission.The recent model of Robberto et al. provides the simplest explanationfor the observed mid-IR excess.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure
Studies of the winds from single K and early M evolved stars indicatethat these flows typically reach a significant fraction of theirterminal velocity within the first couple of stellar radii. The mostdetailed spatially resolved information of the extended atmospheres ofthese spectral types comes from the ζ Aur eclipsing binaries.However, the wind acceleration inferred for the evolved primaries inthese systems appears significantly slower than for stars of similarspectral type. Since there are no successful theories for mass loss fromK and early M evolved stars, it is important to place strong empiricalconstraints on potential models and determine whether this difference inacceleration is real or an artifact of the analyses. We have undertakena radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V)using the Very Large Array to test the wind density model of Baade etal. that is based on Hubble Space Telescope (HST) Goddard HighResolution Spectrograph ultraviolet spectra. ζ Aur was monitored atcentimeter wavelengths over a complete orbital cycle, and fluxvariations during the orbit are found to be of similar magnitude tovariations at similar orbital phases in the adjacent orbit. Duringeclipse, the flux does not decrease, showing that the radio emissionoriginates from a volume substantially larger thanR3K~(150Rsolar)3 surroundingthe B star. Using the one-dimensional density model of the K4 Ibprimary's wind derived from HST spectral line profile modeling andelectron temperature estimates from previous optical and new HSTstudies, we find that the predicted radio fluxes are consistent withthose observed. Three-dimensional hydrodynamic simulations indicate thatthe accretion flow perturbations near the B star do not contributesignificantly to the total radio flux from the system, consistent withthe radio eclipse observations. Our radio observations confirm the slowwind acceleration for the evolved K4 Ib component. ζ Aur's velocitystructure does not appear to be typical of single stars with similarspectral types. This highlights the need for more comprehensivemultiwavelength studies for both single stars, which have been sadlyneglected, and other ζ Aur systems to determine if its windproperties are typical.
| 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.
| Imaging Saturn's rings with CAMIRAS: thermal inertia of B and C rings
Thermal inertias Γ of Saturn's B and C ring particles have beenderived from infrared observations using the CAMIRAS camera mounted onthe Canada-France-Hawaii Telescope. They are respectivelyΓ_B=5+18-2 {J m-2 K-1s-1/2} and Γ_C=6+12-4 {Jm-2 K-1 s-1/2}. Such low values mightbe characteristic of a frosty and porous regolith fractured by cracks orof very porous particle aggregates. Particles have to be slowly spinningto explain the observed ring temperatures. A large azimuthal asymmetrywith an amplitude about 1 K is detected on the West ansa of the B ring.It cannot be explained by a model that considers the ring as a slab oflow thermal inertia rapidly warming up to the sunlight after its eclipseinto the planetary shadow.
| A giant planet around the massive giant star HD 13189
Most extrasolar planet discoveries using radial velocity measurementshave been for solar-like G-stars. In order to understand better the rolestellar mass for the formation of planets we must learn more about thefrequency of planetary companions around both high- and low-mass stars.Radial velocity searches for planets around high mass main-sequencestars are difficult due to the paucity of lines and often rapid rotationof these early-type stars. On the other hand, evolved stars that havemoved off the main sequence offer us the possibility of searching forplanets around higher mass stars by means of precise radial velocitymeasurements. Here we present radial velocity measurements for the starHD 13189 using measurements taken at the Thüringer LandessternwarteTautenburg, the Harlan J. Smith Telescope at McDonald Observatory, andthe Hobby-Eberly Telescope. We classify the spectral type of this staras K2 with luminosity class II. The radial velocity measurements showlong-period variations with a period of 472 days and an amplitude of 173m s-1. The Ca II S-index is consistent with an inactive starand this shows no variations with the radial velocity period. We alsoinvestigated possible changes in the line shapes by measuring spectralline bisectors. These show no variations with the radial velocityperiod. We interpret the 472-day period as being caused by a sub-stellarcompanion. Based on the estimated absolute magnitude and a comparison toevolutionary tracks we estimate the mass of the progenitor star between2 and 7 M_ȯ which results in a projected mass of the companion of msin i = 8 20 M_J. HD 13189 may be the most massive star known to possessan extrasolar planet. This suggests that the formation of giant planetscan also occur around early-type stars. HD 13189 also shows significantshort term radial velocity variability on time scales of days that ismost likely due to stellar oscillations. This behavior is typical for Kgiant stars.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| Aldebaran's angular diameter: How well do we know it?
The bright, well-known K5 giant Aldebaran,α Tau, is probably the star with the largestnumber of direct angular diameter determinations, achieved over a longtime by several authors using various techniques. In spite of thiswealth of data, or perhaps as a direct result of it, there is not a verygood agreement on a single angular diameter value. This is particularlyunsettling if one considers that Aldebaran is also used as a primarycalibrator for some angular resolution methods, notably for optical andinfrared long baseline interferometry. Directly connected to Aldebaran'sangular diameter and its uncertainties is its effective temperature,which also has been used for several empirical calibrations. Among theproposed explanations for the elusiveness of an accurate determinationof the angular diameter of Aldebaran are the possibility of temporalvariations as well as a possible dependence of the angular diameter onthe wavelength. We present here a few, very accurate new determinationsobtained by means of lunar occultations and long baselineinterferometry. We derive an average value of 19.96±0.03milliarcsec for the uniform disk diameter. The correspondinglimb-darkened value is 20.58±0.03 milliarcsec, or 44.2±0.9Rȯ. We discuss this result, in connection with previousdeterminations and with possible problems that may affect suchmeasurements.Based on observations collected at TIRGO (Gornergrat, Switzerland). TIRGO is operated by CNR - CAISMI Arcetri, Italy.
| Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component
We present results of an investigation of the X-ray properties, agedistribution, and kinematical characteristics of a high-galacticlatitude sample of late-type field stars selected from the ROSAT All-SkySurvey (RASS). The sample comprises 254 RASS sources with opticalcounterparts of spectral types F to M distributed over six study areaslocated at |b| 20 °, and Dec ≥ -9 °. A detailed studywas carried out for the subsample of ~200 G, K, and M stars. Lithiumabundances were determined for 179 G-M stars. Radial velocities weremeasured for most of the 141 G and K type stars of the sample. Combinedwith proper motions these data were used to study the age distributionand the kinematical properties of the sample. Based on the lithiumabundances half of the G-K stars were found to be younger than theHyades (660 Myr). About 25% are comparable in age to the Pleiades (100Myr). A small subsample of 10 stars is younger than the Pleiades. Theyare therefore most likely pre-main sequence stars. Kinematically the PMSand Pleiades-type stars appear to form a group with space velocitiesclose to the Castor moving group but clearly distinct from the LocalAssociation.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, operated by the Max-Planck-Institut fürAstronomie, Heidelberg, jointly with the Spanish National Commission forAstronomy, and at the European Southern Observatory, La Silla, Chile.Tables A2-A4 are only available in electronic form athttp://www.edpsciences.org
| 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
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
