Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The Ultraviolet to Near-Infrared Optical Properties of Polycyclic Aromatic Hydrocarbons: A Semiempirical Model
Interstellar polycyclic aromatic hydrocarbon (PAH) infrared emissionfeatures represent an important and unique diagnostic tool of thechemical and physical conditions throughout the universe. However, onechallenge facing the widely accepted PAH emission model has been thedetection of infrared features in regions of low UV flux. Using recentlypublished laboratory near-infrared (NIR) PAH ion absorption datameasured in our laboratory, we build on previous models for PAH ionabsorption in the UV-visible to extrapolate a new model thatincorporates PAH ion absorption in the NIR. This model provides a basisfor comparing the relative energy absorption of PAH ions in theUV-visible and NIR regions for a wide variety of stellar types. Thismodel demonstrates that the radiation from late-type stars can pump themid-IR PAH features.
| Variations of the Mid-Infrared Emission Spectrum in Reflection Nebulae
Using spatial-spectral data cubes of reflection nebulae obtained byISOCAM, we have observed a shift in the central wavelength of the 7.7μm band within several reflection nebulae. This band, composed ofcomponents at 7.85 and 7.65 μm, shows a centroid shift from 7.75μm near the edge of the nebulae to 7.65 μm toward the center ofthe nebulae as the shorter wavelength component becomes relativelystronger. The behavior of the 7.7 μm band center can be explained byassuming either that anions are the origin of the 7.85 μm band andcations the 7.65 μm band, or that the band center wavelength dependson the chemical nature of the polycyclic aromatic hydrocarbons (PAHs).The ratio of the 11.3/7.7 μm bands also changes with distance fromthe central star, first rising from the center toward the edge of thenebula, then falling at the largest distances from the star, consistentwith the 11.3/7.7 μm band ratio being controlled by the PAHionization state.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and United Kingdom) and with the participation of ISAS andNASA.
| 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
| 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.
| 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).
| Does the 3.3 Micron Polycyclic Aromatic Hydrocarbon Emission Feature Require Ultraviolet Excitation?
Unidentified infrared bands (UIBs) have been observed in virtually everydusty astrophysical environment investigated. The UIB carrier must beabundant and ubiquitous. Strong evidence points to polycyclic aromatichydrocarbons as likely candidates, but the identification is notcomplete. Additional diagnostics are needed to further constrain the UIBcarrier, such as probing excitation sources ranging from UV-strong toUV-weak to determine the ``band gap'' of the UIB carrier. Observationsand models suggest that the UIBs can be found in sources with weak UVfields. To that end, we present new results of observing the 3.3 μmspectral region in six stars embedded in reflection nebulae and in sixVega-like stars. These objects have effective temperatures ranging from3500 to 12,000 K. Their environments include dust that should berelatively unprocessed (reflection nebulae) and dust that has mostlikely undergone significant processing (Vega-like) by the embeddedillumination source. Together with data from the literature, we have asample of 27 sight lines. Our analysis suggests that neither thestrength of the UV field impinging on the dust nor the effectivetemperature of the star is the determining factor in whether the 3.3μm UIB emission is present in an object. We found three detections ofthe 3.3 μm emission band, one in a Vega-type object, one in a HerbigAe/Be object, and one in a reflection nebula, and all with disks. Therole of disk geometry is likely to be important in revealing orobscuring the photodissociation regions from which the UIB emissionarises.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Do the Infrared Emission Features Need Ultraviolet Excitation? The Polycyclic Aromatic Hydrocarbon Model in UV-poor Reflection Nebulae
One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6,and 11.3 μm interstellar infrared (IR) emission bands with polycyclicaromatic hydrocarbon (PAH) molecules has been the recent detection ofthese bands in regions with little ultraviolet (UV) illumination, sincesmall, neutral PAH molecules have little or no absorption at visiblewavelengths and therefore require UV photons for excitation. We showhere that our ``astronomical'' PAH model, incorporating the experimentalresult that the visual absorption edge shifts to longer wavelength uponionization and/or as the PAH size increases, can closely reproduce theobserved IR emission bands of vdB 133, a UV-poor reflection nebula. Wealso show that single-photon heating of ``astronomical'' PAHs inreflection nebulae near stars as cool as Teff=3000K canresult in observable emission at 6.2, 7.7, 8.6, and 11.3 μm.Illustrative mid-IR emission spectra are calculated for reflectionnebulae illuminated by cool stars with Teff=3500, 4500, and5000 K. These will allow comparison with future Space Infrared TelescopeFacility observations of vdB 135 (Teff=3600K), vdB 47(Teff=4500K), and vdB 101 (Teff=5000K). Thedependence on the effective temperature of the exciting star of theobserved 12 μm IRAS emission (relative to the total far-IR emission)is consistent with the PAH model for3000K<=Teff<=30,000K.
| Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gasdust shells and/ordiskshells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gasdustshells and/or diskshells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by freefree transitions of electrons and bythermal emission from dust grains in circumstellar gasdust shells(disks).
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| Sodium enrichment of stellar atmospheres. I. Non-variable supergiants and bright giants
48 supergiants and bright giants have been observed in order toinvestigate the sodium enrichment of their atmospheres and itsconnection with stellar gravity. We present the equivalent widths of the6154 Å and 6160 Å Na I lines measured from the programspectra, the results of effective temperature determinations, the NLTEsodium abundances, and the derived relation between the sodiumoverabundance and surface gravity.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
We present a quantitative model for the infrared emission from dust inthe diffuse interstellar medium. The model consists of a mixture ofamorphous silicate grains and carbonaceous grains, each with a wide sizedistribution ranging from molecules containing tens of atoms to largegrains >~1 μm in diameter. We assume that the carbonaceous grainshave properties like polycyclic aromatic hydrocarbons (PAHs) at verysmall sizes and graphitic properties for radii a>~50 Å. On thebasis of recent laboratory studies and guided by astronomicalobservations, we propose ``astronomical'' absorption cross sections foruse in modeling neutral and ionized PAHs from the far-ultraviolet to thefar-infrared. We also propose modifications to the far-infraredemissivity of ``astronomical silicate.'' We calculate energydistribution functions for small grains undergoing ``temperaturespikes'' caused by stochastic absorption of starlight photons usingrealistic heat capacities and optical properties. Using a grain-sizedistribution consistent with the observed interstellar extinction, weare able to reproduce the near-IR to submillimeter emission spectrum ofthe diffuse interstellar medium, including the PAH emission features at3.3, 6.2, 7.7, 8.6, and 11.3 μm. The model is compared with theobserved emission at high Galactic latitudes as well as in the Galacticplane, as measured by the COBE/DIRBE, COBE/FIRAS, IRTS/MIRS, andIRTS/NIRS instruments. The model has 60×10-6 of C(relative to H) locked up in PAHs, with 45×10-6 of C ina component peaking at ~6 Å (NC~100 carbon atoms) toaccount for the PAH emission features and with 15×10-6of C in a component peaking at ~50 Å to account for the 60 μmflux. The total infrared emission is in excellent agreement withCOBE/DIRBE observations at high Galactic latitudes, just as the albedofor our grain model is in accord with observations of the diffuseGalactic light. The aromatic absorption features at 3.3 and 6.2 μmpredicted by our dust model are consistent with observations. Wecalculate infrared emission spectra for our dust model heated by a rangeof starlight intensities, from 0.3 to 104 times the localinterstellar radiation field, and we tabulate the intensities integratedover the SIRTF/IRAC and MIPS bands. We also provide dust opacitiestabulated from the extreme-ultraviolet to submillimeter wavelengths.
| The 3.3 Micron and Extended Red Emissions in Interstellar Clouds: Further Evidence for Carbon Nanoparticles
The excitation of photoluminescence (PL) and infrared emission fromcarbon nanoparticles by the absorption of photons from the interstellarradiation field is discussed using a model based on energy conservation.It is shown that the partitioning of energy between PL and internalexcitation can be used to constrain the type and size of particlesresponsible for the extended red emission (ERE) and to relate theseparticles to sources of the aromatic infrared (AIR) emission features.This analysis demonstrates that the ERE (λ>~600 nm) is likelyemitted by particles containing ~50 carbon atoms and that such particlesare not strong emitters at 3.3 μm. It is found that only particlesthat do not have high PL emission efficiency can radiate significantlyat 3.3 μm, and, as a result, ERE and 3.3 μm emission will notgenerally be cospatial. This effect is greatest in low-excitationobjects. Application of this model to ERE and AIR emission in the RedRectangle and elsewhere provides an estimate of PL efficiency that isconsistent with that from polycyclic aromatic hydrocarbon andhydrogenated amorphous carbon materials. This efficiency is typically0.1 but can be much less in regions where the hardness of the UVradiation field is reduced. An analysis of the photochemical processesinvolved in this conversion shows that ERE will not be produced inobjects where Teff<=104 K. Extended emission atlonger wavelength is discussed, and it is suggested that this occursfrom carbon nanoparticles with more than 50 atoms. In particular,long-wavelength bands at 1.15 and 1.5 μm detected by Gordon andcoworkers are consistent with PL emission from carbon nanoparticleshaving ~70 and ~120 carbon atoms, respectively. These quantitativeresults provide further support for the presence of carbon nanoparticlesin the interstellar medium and their assignment as the source of theextended red and AIR emissions as well as the 217.5 nm absorption band.
| Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
We present a method for calculating the infrared emission from apopulation of dust grains heated by starlight, including very smallgrains for which stochastic heating by starlight photons results inhigh- temperature transients. Because state-to-state transition ratesare generally unavailable for complex mol ecules, we consider modelpolycyclic aromatic hydrocarbon (PAH), graphitic, and silicate grainswith realistic vibrational mode spectra and realistic radiativeproperties. The vibrational density of states is used in astatistical-mechanical description of the emission process. Unlikeprevious treatments, our approach fully incorporates multiphoton heatingeffects, important for large grains or strong radiation fields. Wediscuss how the ``temperature'' of the grain is related to itsvibrational energy. By comparing with an ``exact'' statisticalcalculation of the emission process, we determine the conditions underwhich the ``thermal'' and the ``continuous cooling'' approximations canbe used to calculate the emission spec trum. We present results for theinfrared emission spectra of PAH grains of various sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7, and 11.3μm features depend on grain size, starlight spectrum and intensity,and grain charging conditions. We show results for grains in the ``coldneutral medium'' and ``warm ionized medium'' and representativeconditions in photodissociation regions. Our model results are comparedto observed ratios of emission features for the Milky Way and othergalaxies and for the M17 and NGC 7023 photodissociation regions.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Infrared Properties of Molecular Cirrus. II. Cloud-to-Cloud Variations in Graphite and Polycyclic Aromatic Hydrocarbon Content
We have conducted a photometric survey of translucent molecular cloudsin the four IRAS wave bands. We find notable cloud-to-cloud variationsin mid-IR emission, with the ratio I12/I25 varyingby up to 1 order of magnitude and often above unity. Because the cloudsin our sample are nearby and translucent, the cloud heating is wellconstrained, so that the observed differences in infrared emission mustrepresent differences in grain content. We show from first principlesthat a dust model containing only two components, Mathis-Rumpl-Nordsieck(MRN) grains in thermal equilibrium and very small grains undergoingstochastic heating, can never produceI12/I25>=1 under the conditions that prevail inthe cloudy interstellar medium. We fit the clouds in this sample with athree-component model which contains continuum emission from both MRNgrains and very small graphite grains, plus emission features from amixture of polycyclic aromatic hydrocarbon (PAH). The cloud-to-cloudvariations in mid-IR emission require significant differences in theircomposition of small grains and PAH molecules. Models which describe theformation and evolution of these particles in the interstellar mediummust explain the presence of these variations in a cloud sample which islocated in the solar neighborhood.
| Infrared Space Observatory Mid-Infrared Spectra of Reflection Nebulae
We present 5-15 μm imaging spectroscopy of the reflection nebulae vdB17 (NGC 1333), vdB 59 (NGC 2068), vdB 101, vdB 111, vdB 133, and vdB135, obtained with the infrared camera and circular variable filterwheel on the Infrared Space Observatory (ISO). These nebulae areilluminated by stars with Teff=3,600-19,000 K, implying UV(λ<400 nm) to total stellar flux ratios ofF(λ<400nm)/Ftotal=0.01-0.87. We detectthe infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7μm, broad emission features at 6-9 μm and 11-13 μm, and 5-15μm continuum emission, from the interstellar medium in vdB 17, vdB59, and vdB 133(F(λ<400nm)/Ftotal=0.22-0.87), and placeupper limits on the emission from the interstellar medium in vdB 101,vdB 111, and vdB 135(F(λ<400nm)/Ftotal=0.01-0.20). Our goalis to test predictions of models attributing the IEFs to polycyclicaromatic hydrocarbons (PAHs). Interstellar models predict PAHs changefrom singly ionized to neutral as the UV intensity, G0,decreases. The ratio of PAH emission at 6-10 μm to PAH emission at10-14 μm is expected to be 10 times higher in ionized than in neutralPAHs. We observe no spectroscopic differences with varyingTeff. We analyze the spectra of vdB 17 and vdB 59 as afunction of distance from the star to see how the spectra depend onG0 within each source. The only quantitative difference wefind is a broadening of the 7.7 μm IEF at G0=20-60 withinvdB 17. We observe only a 40% change in the 6-10 μm to 10-14 μmflux ratio over G0=20-6x104. Based on observationswith ISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, the Netherlands and theUnited Kingdom) and with the participation of ISAS and NASA.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Radii and Effective Temperatures for G, K, and M Giants and Supergiants
Interferometrically determined angular diameters obtained at the PalomarTestbed Interferometer (PTI) for 69 giant and supergiant stars arepresented. Spectral types of the 59 giant stars generally lie between G6and M6, although a B7 giant is included; the nine bright giants andsupergiants have spectral types between F5 and M5. Comparison of theresults to those from the IR Optical Telescope Array interferometerindicate no statistically significant difference between the two datasets. The use of Hipparcos parallaxes allows us to measure linear sizesdirectly for these stars, which range in size from 10 to 260 solarradii. In conjunction with previous results as reported by Dyck et al.,the total of 113 giant stars provides empirically determineddependencies of effective temperature and linear radius upon spectraltype and V-K color.
| Empirical calibration of the lambda 4000 Å break
Empirical fitting functions, describing the behaviour of the lambda 4000Ä break, D4000, in terms of effective temperature,metallicity and surface gravity, are presented. For this purpose, thebreak has been measured in 392 stars from the Lick/IDS Library. We havefollowed a very detailed error treatment in the reduction and fittingprocedures, allowing for a reliable estimation of the breakuncertainties. This calibration can be easily incorporated into stellarpopulation models to provide accurate predictions of the break amplitudefor, relatively old, composite systems. Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| High resolution spectroscopy over lambda lambda 8500-8750 Å for GAIA. I. Mapping the MKK classification system
We present an Echelle+CCD high resolution spectroscopic atlas (0.25Ä/pix dispersion, 0.43 Ä FWHM resolution and 20 000 resolvingpower) mapping the MKK classification system over the interval lambdalambda 8500-8750 Ä. The wavelength interval is remarkably free fromtelluric lines and it is centered on the near-IR triplet of Ca II, thehead of hydrogen Paschen series and several strong metallic lines. Thespectra of 131 stars of types between O4 and M8 and luminosity classes Ithrough V are included in the atlas. Special care was put in maintainingthe highest instrumental homogeneity over the whole set of data. Thecapability to derive accurate MKK spectral types from high resolutionobservations over the interval lambda lambda 8500-8750 Ä isdiscussed. The observations have been performed as part of an evaluationstudy of possible spectroscopic performances for the astrometric missionGAIA planned by ESA. Tables~3 and 4 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/ Abstract.html}\fnmsep\thanks{ Thespectra of the stars listed in Table~2 are also available in electronicform at the CDS or via the personal HomePagehttp://ulisse.pd.astro.it/Astro/Atlases/}\fnmsep\thanks{ Figures 3--28are only available in electronic form at http://www.edpsciences.com
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| Observational constraints on the ERE interpretation
Empirical relationships on the properties of the Extended Red Emission(ERE) are presented. They are based on published observational data andon new results obtained on reflection nebulae illuminated by cold stars.The plot of the width versus the central wavelength of the ERE band isin agreement with laboratory properties of the materials commonlyproposed as the ERE carriers. But this is not the case for the plot ofthe ERE band width versus the effective temperature of the nebulailluminating star. Partly based on observations made at Observatoire deHaute Provence du CNRS and at European Southern Observatory (ESO), LaSilla (Chile)}
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Mid-Infrared Emission Features in the ISM: Feature-to-Feature Flux Ratios
Using a limited but representative sample of sources in the interstellarmedium of our Galaxy with published spectra from the Infrared SpaceObservatory, we analyze the flux ratios between the major mid-IRemission features (EFs) centered around 6.2, 7.7, 8.6, and 11.3 mu m,respectively. In a flux ratio-to-flux ratio plot of EF(6.2 mu m)/EF(7.7mu m) as a function of EF(11.3 mu m)/EF(7.7 mu m), the sample sourcesform roughly a Lamda -shaped locus that appears to trace, on an overallbasis, the hardness of a local heating radiation field. But some drivingparameters other than the radiation field may also be required for afull interpretation of this trend. On the other hand, the flux ratio ofEF(8.6 mu m)/EF(7.7 mu m) shows little variation over the samplesources, except for two H II regions that have much higher values forthis ratio because of an "EF(8.6 mu m) anomaly," a phenomenon clearlyassociated with environments of an intense far-UV radiation field. Iffurther confirmed on a larger database, these trends should providecrucial information on how the EF carriers collectively respond to achanging environment.
| Do the Infrared Emission Features Need Ultraviolet Excitation?
We present the results of imaging spectroscopy of the reflection nebulavdB 133, obtained with the infrared camera and circular variable filterwheel on the Infrared Space Observatory. Our observations reveal theinfrared emission features (IEFs), at 6.2, 7.7, 8.7, 11.3, and 12.7 mu{m} , and associated 5-15 mu m continuum emission. The stellar systemilluminating vdB 133 has the lowest ratio of ultraviolet (shortward of0.4 mu m) to total flux of any stars demonstrated to date to excite theIEFs and associated continuum emission from adjacent interstellar dust,as opposed to circumstellar dust. The low fraction of UV flux from thissystem poses a problem for existing models for the emission mechanismand emitting material, which all require substantial UV radiation forthe excitation of the IEFs and associated continuum.
| Micrometer Measures of Double Stars
Micrometer measures of 795 double stars made with the 26 inch (0.66 m)refractor of the US Naval Observatory from 1984 to 1990 are presented.
| Detection of H_2 fluorescent emission from the DR 21 bipolar outflow
Near-infrared emission lines of molecular hydrogen (H_2) measured in theK window (2.01 to 2.43 μm) and covering energy levels from 6000 to 24000 K show indication of different excitation conditions in the easternand western lobes of shock-excited H_2 in the DR 21 bipolar outflow. Weuse H_2 excitation diagrams to demonstrate that neither J- nor C-typeshocks can explain the observed line ratios. The higher H_2 line ratiosmeasured for the eastern lobe are a clear indication of enhancedexcitation for the high-excitation levels of the H_2 molecule, which maybe caused by shock-produced Lyalpha resonance pumping or by direct UVexcitation of H_2 from the central H ii region. This is consistent withthe eastern lobe bordering the central H ii region and thereforeproducing higher far-ultraviolet (FUV) fluxes. We show that the observedH_2 emission can be interpreted by a simple two-component emission modelconsisting of a bow C-type shock which produces the low-excitation H_2emission and an FUV radiation field which produces the high-excitationemission through H_2 fluorescence. The H_2 line ratios are best fittedby a photodissociation region (PDR) model with parameters FUV field inthe range 10^2<=G_0<=10^3 and pre-shock density n_0>=3x10^3cm^-3. Using the PDR models investigated here, we suggest that in DR 21the ortho-to-para ratio is 1.8 in the fluorescent emission component.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cigno |
| Ascensione retta: | 20h30m59.20s |
| Declinazione: | +36°56'09.0" |
| Magnitudine apparente: | 6.19 |
| Distanza: | 485.437 parsec |
| Moto proprio RA: | -1.3 |
| Moto proprio Dec: | -1.9 |
| B-T magnitude: | 7.403 |
| V-T magnitude: | 6.321 |
Cataloghi e designazioni:
|
NEWS@SKY (Science&Space News)
