תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy
High spatial resolution spectroscopy at 8-13 μm with T-ReCS onGemini-S has revealed striking variations in the mid-infrared emissionand absorption in the nucleus of the Circinus galaxy (hereafterCircinus) on subarcsecond scales. The core of Circinus is compact andobscured by a substantial column of cool silicate dust. Weak extendedemission to the east and west coincides with the coronal line region andarises from featureless dust grains which are probably heated by lineemission in the coronal emission zone. The extended emission on the eastside of the nucleus displays a much deeper silicate absorption than thaton the west, indicating significant columns of cool material along theline of sight and corresponding to an additional extinction ofAV~ 25 mag. Emission bands from aromatic hydrocarbons are notsubject to this additional extinction, are relatively weak in the coreand in the coronal line region, and are much more spatially extendedthan the continuum dust emission; they presumably arise in thecircumnuclear star-forming regions. These data are interpreted in termsof an inclined disc-like structure around the nucleus extending overtens of parsecs and possibly related to the inner disc found fromobservations of water masers by Greenhill et al..
| Distribution of Dust Clouds around the Central Engine of NGC 1068
We studied the distribution of dust clouds around the central engine ofNGC 1068 based on shifted and added 8.8-12.3 μm (MIR) multifilterimages and 3.0-3.9 μm (L-band) spectra obtained with the SubaruTelescope. In a region of 100 pc (1.4") around the central peak, wesuccessfully constructed maps of color temperatures and emissivities ofthe MIR and L-band continua as well as the 9.7 and 3.4 μm dustfeatures with spatial resolutions of 26 pc (0.37") in the MIR and 22 pc(0.3") in the L band. Our main results are the following: (1) The colortemperature of the MIR continuum scatters around the thermal equilibriumtemperature with the central engine as the heat source, while that ofthe L-band continuum is higher and independent of distance from thecentral engine. (2) The peak of the 9.7 μm silicate absorptionfeature is shifted to a longer wavelength at some locations. (3) Theratio of the optical depths of the dust features is different from theGalactic values and shows a complicated spatial distribution. (4) Thereis a pie-shaped warm dust cloud as an enhancement in the emissivity ofthe MIR continuum extending about 50 pc to the north from the centralengine. We speculate that material falls into the central engine throughthis cloud.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
In the light of the recent results of the stellar interferometry, weexamine the nature of the extra molecular layer outside the photosphereof red supergiant stars, so far studied mostly with the use of theinfrared spectra. Although the visibility data are more direct probes ofthe spatial structure of the outer atmosphere, it is essential that theyare analyzed in combination with the spectral data. In the case of theM2 supergiant μ Cephei, several sets of data, both spectra andvisibilities, strongly suggested the presence of an extra molecularlayer (which we referred to as ``MOLsphere'' for simplicity), and thebasic parameters of the MOLsphere are estimated to be excitationtemperature Tex~1600 K, column densities of CO andH2O molecules Ncol~3.0×1020cm-2, and located at about one stellar radius above thephotosphere or Rin~2.0R*. The result showsreasonable agreement with the one based on the infrared spectra alone,and the model inferred from the spectra is now fully supported with therecent visibility data. In the case of the M2 supergiant αOrionis, the infrared spectra and visibilities show a consistent picturein that its MOLsphere is closer to the photosphere(Rin~1.3R*) with higher gas temperature(Tex~2250 K) and lower gas column density(Ncol~1020 cm-2), compared with that ofμ Cep. Some controversy on the interpretation of the mid-infrareddata of α Orionis can be reconciled. Given that the presence ofthe extra molecular layer is reasonably well established, the majorunsolved problem is how to understand the origin of such a rather warmand dense layer in the outer atmosphere.
| Water Vapor on Supergiants: The 12 μm TEXES Spectra of μ Cephei
Several recent papers have argued for warm, semidetached, molecularlayers surrounding red giant and supergiant stars, a concept known as aMOLsphere. Spectroscopic and interferometric analyses have oftencorroborated this general picture. Here we present high-resolutionspectroscopic data of pure rotational lines of water vapor at 12 μmfor the supergiant μ Cep. This star has often been used to test theconcept of molecular layers around supergiants. Given the prediction ofan isothermal, optically thick water vapor layer in local thermodynamicequilibrium around the star (MOLsphere), we expected the 12 μm linesto be in emission or at least in absorption but filled in by emissionfrom the molecular layer around the star. Our data, however, show thecontrary; we find definite absorption. Thus, our data do not easily fitinto the suggested isothermal MOLsphere scenario. The 12 μm lines,therefore, put new, strong constraints on the MOLsphere concept and onthe nature of water seen in signatures across the spectra of early Msupergiants. We also find that the absorption is even stronger than thatcalculated from a standard, spherically symmetric model photospherewithout any surrounding layers. A cool model photosphere, representingcool outer layers, is, however, able to reproduce the lines, but thismodel does not account for water vapor emission at 6 μm. Thus, aunified model for water vapor on μ Cep appears to be lacking. It doesseem necessary to model the underlying photospheres of these supergiantsin their whole complexity. The strong water vapor lines clearly revealinadequacies of classical model atmospheres.
| Dust Processing in Disks around T Tauri Stars
The 8-14 μm emission spectra of 12 T Tauri stars in the Taurus/Aurigadark clouds and in the TW Hydrae association obtained with the InfraredSpectrograph (IRS) on board Spitzer are analyzed. Assuming that the 10μm features originate from silicate grains in the optically thinsurface layers of T Tauri disks, the 8-14 μm dust emissivity for eachobject is derived from its Spitzer spectrum. The emissivities are fitwith the opacities of laboratory analogs of cosmic dust. The fitsinclude small nonspherical grains of amorphous silicates (pyroxene andolivine), crystalline silicates (forsterite and pyroxene), and quartz,together with large fluffy amorphous silicate grains. A wide range inthe fraction of crystalline silicate grains, as well as large silicategrains among these stars, are found. The dust in the transitional-diskobjects CoKu Tau/4, GM Aur, and DM Tau has the simplest form ofsilicates, with almost no hint of crystalline components and modestamounts of large grains. This indicates that the dust grains in theseobjects have been modified little from their origin in the interstellarmedium. Other stars show various amounts of crystalline silicates,similar to the wide dispersion of the degree of crystallinity reportedfor Herbig Ae/Be stars of mass <2.5 Msolar. Late spectraltype, low-mass stars can have significant fractions of crystallinesilicate grains. Higher quartz mass fractions often accompany lowamorphous olivine to amorphous pyroxene ratios. Lower contrast of the 10μm feature accompanies greater crystallinity.
| The First Spatially Resolved Mid-Infrared Spectra of NGC 1068 Obtained at Diffraction-limited Resolution with the Keck I Telescope Long Wavelength Spectrometer
We present spatially resolved mid-infrared (mid-IR) spectra of NGC 1068with a diffraction-limited resolution of 0.25" using the Long WavelengthSpectrometer (LWS) at the Keck I telescope. The mid-IR image of NGC 1068is extended along the north-south direction. Previous imaging studieshave shown that the extended regions are located inside the ionizationcones, indicating that the mid-IR emission arises perhaps from the innerregions of the narrow-line clouds instead of the proposed dusty torusitself. The spatially resolved mid-IR spectra were obtained at twodifferent slit position angles, +8.0d and -13.0d across the elongatedregions in the mid-IR. From these spectra, we found only weak silicateabsorption toward the northern extended regions but strong absorption inthe nucleus and the southern extended regions. This is consistent with amodel of a slightly inclined cold obscuring torus that covers much ofthe southern regions but is behind the northern extension. While adetailed analysis of the spectra requires a radiative transfer model,the lack of silicate emission from the northern extended regions promptsus to consider a dual dust population model as one of the possibleexplanations in which a different dust population exists in theionization cones compared to that in the dusty torus. Dust inside theionization cones may lack small silicate grains, giving rise to only afeatureless continuum in the northern extended regions, while dust inthe dusty torus has plenty of small silicate grains to produce thestrong silicate absorption lines toward the nucleus and the southernextended regions.
| Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium
We have analyzed the 9.7 and ``18'' μm interstellar silicateabsorption features along the line of sight toward four heavilyextincted galactic WC-type Wolf-Rayet (WR) stars. We construct twointerstellar extinction curves from 1.25 to 25 μm using near-IRextinction measurements from the literature, along with the silicateprofiles of WR 98a (representing the local ISM) and GCS 3 (representingthe Galactic center). We have investigated the mineralogy of theinterstellar silicates by comparing extinction profiles for amorphoussilicates with olivine and pyroxene stoichiometry to the 9.7 and ``18''μm absorption features in the WR 98a spectrum. In this analysis, wehave considered solid and porous spheres and a continuous distributionof ellipsoids. While it is not possible to simultaneously provide aperfect match to both profiles, we find that the best match requires amixture of these two types of compounds. We also consider iron oxides,aluminosilicates, and silicate carbide (SiC) as grain components. Ironoxides cannot be accommodated in the observed spectrum, while the amountof Si in SiC is limited to <4%. Finally, we discuss the cosmicelemental abundance constraints on the silicate mineralogy, grain shape,and porosity.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| The Circumstellar Environments of NML Cygni and the Cool Hypergiants
We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H2Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H2O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant μ Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.
| 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.
| Amorphous alumina in the extended atmosphere of α Orionis
In this paper we study the extended atmosphere of the late-typesupergiant α Orionis. Infrared spectroscopy of red supergiantsreveals strong molecular bands, some of which do not originate in thephotosphere but in a cooler layer of molecular material above it.Lately, these layers have been spatially resolved by near and mid-IRinterferometry. In this paper, we try to reconcile the IRinterferometric and ISO-SWS spectroscopic results on α Orioniswith a thorough modelling of the photosphere, molecular layer(s) anddust shell. From the ISO and near-IR interferometric observations, wefind that α Orionis has only a very low density water layer closeabove the photosphere. However, mid-IR interferometric observations anda narrow-slit N-band spectrum suggest much larger extra-photosphericopacity close to the photosphere at those wavelengths, even when takinginto account the detached dust shell. We argue that this cannot be dueto the water layer, and that another source of mid-IR opacity must bepresent. We show that this opacity source is probably neither molecularnor chromospheric. Rather, we present amorphous alumina (Al2O3) as thebest candidate and discuss this hypothesis in the framework ofdust-condensation scenarios.
| Mid-infrared images of the massive star forming region W75 N
An infrared study that includes ground-based mid-infrared images between8.7 and 18.7 μm and IRAC images at 3.6, 4.5, 5.8 and 8.0 μm of theW75 N massive star forming region is presented. The 12.5 μm imageshows the presence of four mid-infrared sources in the region W75 N(B),three of which have bright near-infrared counterparts, IRS 1, IRS 2 andIRS 3, all with significant excess emission at λ > 2.0 ~μm.IRS 2 has a steep energy distribution and the computed infraredluminosity is consistent with the presence of a young B3 star. Theobserved IRAC colors of IRS 3 indicate that this source is a Class IIintermediate mass young star, consistent with its infrared energydistribution and luminosity. The fourth, newly discovered, mid-infraredsource appears coincident with the ultracompact HII region VLA 3, and islocated within the millimeter core MM 1. We derived a luminosity of 750~L_ȯ and a visual extinction AV ≃ 90 forthis source. From the IRAC images, we detected 75 sources in an area of120'' × 120 '' centered in W75 N. At least 25 of these sources areassociated with the molecular cloud and form a young stellar cluster asshown in the IRAC two-color and the H-Ks versus K_s-[3.6]diagrams.
| 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.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the ρ Ophiuchi Cloud Core
Results of a comprehensive, new, ground-based mid-infrared imagingsurvey of the young stellar population of the ρ Ophiuchi cloud arepresented. Data were acquired at the Palomar 5 m and at the Keck 10 mtelescopes with the MIRLIN and LWS instruments, at 0.5" and 0.25"resolutions, respectively. Of 172 survey objects, 85 were detected.Among the 22 multiple systems observed, 15 were resolved and theirindividual component fluxes determined. A plot of the frequencydistribution of the detected objects with SED spectral slope shows thatYSOs spend ~4×105 yr in the flat-spectrum phase,clearing out their remnant infall envelopes. Mid-infrared variability isfound among a significant fraction of the surveyed objects and is foundto occur for all SED classes with optically thick disks. Large-amplitudenear-infrared variability, also found for all SED classes with opticallythick disks, seems to occur with somewhat higher frequency at theearlier evolutionary stages. Although a general trend of mid-infraredexcess and near-infrared veiling exists progressing through SED classes,with Class I objects generally exhibiting rK>=1,flat-spectrum objects with rK>=0.58, and Class III objectswith rK=0, Class II objects exhibit the widest range ofrK values, ranging from 0<=rK<=4.5. However,the highly variable value of veiling that a single source can exhibit inany of the SED classes in which active disk accretion can take place isstriking and is direct observational evidence for highly time-variableaccretion activity in disks. Finally, by comparing mid-infrared versusnear-infrared excesses in a subsample with well-determined effectivetemperatures and extinction values, disk-clearing mechanisms areexplored. The results are consistent with disk clearing proceeding fromthe inside out.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
| Study of molecular layers in the atmosphere of the supergiant star μ Cep by interferometry in the K band
Infrared interferometry of supergiant and Mira stars has recently beenreinterpreted as revealing the presence of deep molecular layers.Empirical models for a photosphere surrounded by a simple molecularlayer or envelope have led to a consistent interpretation of previouslyinconsistent data. The stellar photospheres are found to be smaller thanpreviously understood, and the molecular layer is much higher and denserthan predicted by hydrostatic equilibrium. However, the analysis wasbased on spatial observations with medium-band optical filters, whichmixed the visibilities of different spatial structures. This paperreports spatial interferometry with narrow spectral bands, isolatingnear-continuum and strong molecular features, obtained for thesupergiant μ Cep. The measurements confirm strong variation ofapparent diameter across the K-band. A layer model shows that a stellarphotosphere of angular diameter 14.11±0.60 mas is surrounded by amolecular layer of diameter 18.56±0.26 mas, with an opticalthickness varying from nearly zero at 2.15 μm to >1 at 2.39 μm.Although μ Cep and α Ori have a similar spectral type,interferometry shows that they differ in their radiative properties.Comparison with previous broad-band measurements shows the importance ofnarrow spectral bands. The molecular layer or envelope appears to be acommon feature of cool supergiants.
| Star formation in globules in IC 1396
We present a large-scale study of the IC 1396 regionusing new deep NIR and optical images, complemented by 2MASS data. Forten globules in IC 1396 we determine (H-K, J-H) colour-colour diagramsand identify the young stellar population. Five of these globulescontain a rich population of reddened objects, most of them probablyyoung stellar objects. Two new HH objects (HH 865 andHH 864) could be identified by means of [SII]emission, one of them a parsec-scale flow. Using star counts based on2MASS data we create an extinction map of the whole region. This map isused to identify 25 globules and to estimate their mass. The globulemasses show a significant increase with the distance from the excitingO6.5V star HD 206267. We explain this correlation bythe enhanced radiation pressure close to this star, leading toevaporation of the nearby clouds and hence smaller globule masses. Wesee evidence that the radiation from HD 206267 has a major impact on thestar formation activity in these globules.All Appendices 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
| 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.
| Variable Star Network: World Center for Transient Object Astronomy and Variable Stars
Variable Star Network (VSNET) is a global professional-amateur networkof researchers in variable stars and related objects, particularly intransient objects, such as cataclysmic variables, black-hole binaries,supernovae, and gamma-ray bursts. The VSNET has been playing apioneering role in establishing the field of transient object astronomy,by effectively incorporating modern advances in observational astronomyand global electronic networks, as well as collaborative progress intheoretical astronomy and astronomical computing. The VSNET is now oneof the best-featured global networks in this field of astronomy. Wereview the historical progress, design concept, associated technology,and a wealth of scientific achievements powered by VSNET.
| 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.
| 10-μm absorption spectra of silicates for two new diffuse interstellar medium sightlines
We present low resolution 10-μm spectra of the newly identifieddiffuse interstellar medium (ISM) sightlines towards StRS 217 and StRS371 and a new spectrum of Cyg OB2 no. 12 obtained at the UK InfraredTelescope (UKIRT) with the Michelle imager/spectrometer. As previously,the μ Cephei emissivity better represents the profile of diffusemedium silicates than does the broader Trapezium emissivity which ischaracteristic of dust in molecular clouds. These spectra together withseven spectra published by Roche & Aitken comprise the majority ofthe silicates data set for the diffuse ISM. The observed sample includessources in the first galactic quadrant at heliocentric distances ofabout 1.0-4.2 kpc. We find the silicate optical depth,csil=AV/(19.2 +/- 0.6) and the ratio ofcsil to heliocentric distance, d, is in the range 0.1<~csil/d<~ 0.7 kpc-1 for StRS 217 the ratiocould be as high as csil/d<~ 1.2 kpc-1.However, this ratio is dominated by uncertainties in the distanceestimates and does not indicate the degree of clumping in the diffuseinterstellar medium.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 28SiO v = 1 and v = 2, J = 1-0 maser variability in evolved stars. Eleven years of short spaced monitoring
This paper presents and discusses the final data set of a long-term andshort-spaced monitoring of 21 SiO maser sources, mostly evolved stars,carried out in two SiO maser lines at 43 GHz with the ObservatorioAstronómico Nacional 13.7 m telescope at the CentroAstronómico de Yebes (Guadalajara, Spain). In most objects, morethan 80 spectra per transition over a period of 11 years have beenrecorded. The new data presented here, previously unpublished, representnearly 50% of the total SiO data collected in the project. In addition,the availability of optical light curves from the AAVSO for most of theobjects during the whole period of the SiO monitoring, ground-basednear-IR data for four sources overlapping with 3 to 5 observed SiOperiods, and DIRBE near-IR data covering a significant portion of an SiOperiod in 10 sources, make this data set a unique reference forcomparing optical, NIR and SiO variability in order to elucidate thephysical mechanisms that pump SiO masers in evolved stars. The basis forthe conclusions obtained in this work comes from a numerical time seriesanalysis of the suitable SiO, optical and NIR light curves in regularvariables to obtain precise values of the periods and phase lags betweenthe different curves. This analysis shows evidence that in regularvariable evolved stars the three types of emission have the same periodand that the SiO maxima happen in phase with NIR maxima and with a phaselag typically between 0.05 and 0.20 with respect to optical maxima. Weconclude that in these objects the observational evidence presented inthis work favors the radiative pumping of SiO masers against thecollisional pumping.Figures 1-4 and 11-21 are only available in electronic form athttp://www.edpsciences.org
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Galactic environment and the 10-μm silicate feature of young stellar objects
Disc and sphere dust models are used to fit 8-13 μm flux spectra of19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The13 non-photospheric low-mass YSOs in quiescent environments and the fiveHerbig AeBe stars have mean disc temperature indices of 0.4, indicatingthat the emission arises from optically thin layers above a flaredoptically thick disc; 10 out of 14 of the low-mass YSO and four out offive of the Herbig AeBe features contain an optically thin silicateemission component. The radius of the peak 10-μm emission for nineout of the 13 low-mass YSOs is 10-130 au, and three out of the fiveHerbig AeBe stars are 10-30 au in size. In contrast, the five YSOs fromdisrupted molecular clouds that have been shaped by expanding supernovaremnants have temperature indices of between 0.3 and 0.8; four out ofthe five are optically thick and three out of the five have radii <~2au. The photosphere-like continuum of Taurus-Elias 18 could be fittedonly with truncated optically thick models, implying the presence of avoid between the >500 K and cold (<~100 K) foreground dust.Silicates surrounding low-mass YSOs in quiescent molecular clouds aresimilar to those in the Trapezium region of the Orion Nebula except whenAV<~ 2 mag. In the low-AV case and in low-massYSOs in disrupted molecular clouds the silicates are similar tocircumstellar dust around the evolved star μ Cephei.
| 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.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קפיאוס |
| התרוממות ימנית: | 21h43m30.40s |
| סירוב: | +58°46'48.0" |
| גודל גלוי: | 4.08 |
| מרחק: | 1612.903 פארסק |
| תנועה נכונה: | 5.1 |
| תנועה נכונה: | -3.8 |
| B-T magnitude: | 6.993 |
| V-T magnitude: | 4.267 |
קטלוגים וכינוים:
|
