Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Lithium abundances and rotational behavior for bright giant stars
Aims.We study the links possibly existing between the lithium content ofbright giant stars and their rotational velocity. Methods: .Weperformed a spectral analysis of 145 bright giant stars (luminosityclass II) spanning the spectral range from F3 to K5. All these starshave homogeneous rotational velocity measurements available in theliterature. Results: .For all the stars of the sample, we provideconsistent lithium abundances (A_Li), effective temperatures (T_eff),projected rotational velocity (v sin i), mean metallicity ([Fe/H]),stellar mass, and an indication of the stellar multiplicity. The gradualdecrease in lithium abundance with T_eff is confirmed for bright giantstars, and it points to a dilution factor that is at least assignificant as in giant stars. From the F to K spectral types, the A_Lispans at least three orders of magnitude, reflecting the effects ofstellar mass and evolution on dilution. Conclusions: .We find thatthe behavior of A_Li as a function of v sin i in bright giant starspresents the same trend as is observed in giants and subgiants: starswith high A_Li are moderate or fast rotators, while stars with low A_Lishow a wide range of v sin i values.
| 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.
| An Analysis of Fundamental Waffle Mode in Early AEOS Adaptive Optics Images
Adaptive optics (AO) systems have significantly improved astronomicalimaging capabilities over the last decade and are revolutionizing thekinds of science possible with 4-5 m class ground-based telescopes. Athorough understanding of AO system performance at the telescope canenable new frontiers of science as observations push AO systems to theirperformance limits. We look at recent advances with wave-frontreconstruction (WFR) on the Advanced Electro-Optical System (AEOS) 3.6 mtelescope to show how progress made in improving WFR can be measureddirectly in improved science images. We describe how a ``waffle mode''wave-front error (which is not sensed by a Fried geometry Shack-Hartmannwave-front sensor) affects the AO point-spread function. We modeldetails of AEOS AO to simulate a PSF that matches the actual AO PSF inthe I band and show that while the older observed AEOS PSF containedseveral times more waffle error than expected, improved WFR techniquesnoticeably improve AEOS AO performance. We estimate the impact of theseimproved WFRs on H-band imaging at AEOS, chosen based on theoptimization of the Lyot Project near-infrared coronagraph at thisbandpass.Based on observations made at the Maui Space Surveillance System,operated by Detachment 15 of the US Air Force Research Laboratory'sDirected Energy Directorate.
| Shapes of Spectral Line Bisectors for Cool Stars
The shape of the line bisector for the prototype spectral line Fe Iλ6253 was measured for an array of 54 stars on the cool half ofthe HR diagram. These bisectors are given in tables along with theirerrors. The classic C shape is shown by only a rather restricted rangein effective temperature and luminosity. The detailed change in bisectorshape with effective temperature and luminosity is documented moreprecisely than in previous work. The most blueward point on the bisectorchanges its height systematically with luminosity and can be used as aluminosity or gravity discriminant. The wide range of bisector shapescontains significant information about the velocity fields in theatmospheres of these stars, but extracting that information may requireextensive modeling.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| MWC 930 - a new luminous blue variable candidate
We present the results of optical high-resolution and near-infraredlow-resolution spectroscopy and multicolour optical and near-infraredphotometry of the emission-line star MWC 930. The spectrum is rich withFeII emissions, most of which have P Cyg-type profiles. The emissionlines are strong and narrow, indicating a powerful stellar wind with alow terminal velocity (v&infy;~ 140kms-1). Thephotospheric absorption lines are broad and show splitting, which mightbe due to the object's binarity. MWC 930 is most probably located in theNorma spiral arm at a distance of D= 3-4kpc. This strong and slow windas well as the star's luminosity (logL/Lsolar~ 5.5) and theinfrared excess shape suggest that MWC 930 is an unusual B-typesupergiant, most likely undergoing the luminous blue variableevolutionary phase.
| 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.
| 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.
| 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.
| 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.
| 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}
| 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.
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| Near-Infrared Molecular Hydrogen Emission from the Central Regions of Galaxies: Regulated Physical Conditions in the Interstellar Medium
The central regions of many interacting and early-type spiral galaxiesare actively forming stars. This process affects the physical andchemical properties of the local interstellar medium, as well as theevolution of the galaxies. We observed near-infrared H2emission lines: v=1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from thecentral ~1 kpc regions of the archetypical starburst galaxies M82 andNGC 253 and the less dramatic but still vigorously star-forming galaxiesNGC 6946 and IC 342. Like the far-infrared continuum luminosity, thenear-infrared H2 emission luminosity can directly trace theamount of star formation activity because the H2 emissionlines arise from the interaction between hot and young stars and nearbyneutral clouds. The observed H2 line ratios show that boththermal excitation and nonthermal excitation are responsible for theemission lines but that the great majority of the near-infraredH2 line emission in these galaxies arises from energy statesexcited by ultraviolet fluorescence. The derived physical conditions,e.g., far-ultraviolet radiation field and gas density, from [C II] and[O I] lines and far-infrared continuum observations when used as inputsto photodissociation models also explain the luminosity of the observedH2 1-0 S(1) line. The ratio of the H2 1-0 S(1)line to far-IR continuum luminosity is remarkably constant over a broadrange of galaxy luminosities:LH2/LFIR~=10-5, in normallate-type galaxies (including the Galactic center), in nearby starburstgalaxies, and in luminous IR galaxies (LIRGs:LFIR>1011 Lsolar). Examining thisconstant ratio in the context of photodissociation region models, weconclude that it implies that the strength of the incident UV field ontypical molecular clouds follows the gas density at the cloud surface.
| Dynamical Mass Estimates for Five Young Massive Stellar Clusters
We have obtained high-dispersion spectra for four massive star clustersin the dwarf irregular galaxies NGC 4214 and NGC 4449, using the HIRESspectrograph on the Keck I telescope. Combining the velocity dispersionsof the clusters with structural parameters and photometry from imagestaken with the Hubble Space Telescope, we estimate mass-to-light (M/L)ratios and compare these with simple stellar population models in orderto constrain the stellar mass functions (MFs) of the clusters. For allclusters we find M/L values that are similar to or slightly higher thanfor a Kroupa MF and thereby rule out any MF that is deficient inlow-mass stars compared with a Kroupa-type MF. The four clusters havevirial masses ranging between 2.1×105 and1.5×106Msolar, half-light radii between 3.0and 5.2 pc, estimated core densities in the range 2×103to 2×105Msolarpc-3, and agesbetween 200 and 800 Myr. We also present new high-dispersionnear-infrared spectroscopy for a luminous young (~15 Myr) cluster in thenearby spiral galaxy NGC 6946, which we have previously observed withHIRES. The new measurements in the infrared agree well with previousestimates of the velocity dispersion for this cluster, yielding a massof about 1.7×106Msolar. Including animproved estimate of the reddening toward this cluster, the new datayield an M/L in excellent agreement with a Kroupa-type MF also for thiscluster. The properties of the clusters studied here are all consistentwith the clusters being young versions of the old globular clustersfound around all major galaxies.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. Also based on observations with the HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| Interferometric observations of the supergiant stars α Orionis and α Herculis with FLUOR at IOTA
We report the observations in the K band of the red supergiant starα Orionis and of the bright giant star α Herculis with theFLUOR beamcombiner at the IOTA interferometer. The high quality of thedata allows us to estimate limb-darkening and derive precise diametersin the K band which combined with bolometric fluxes yield effectivetemperatures. In the case of Betelgeuse, data collected at high spatialfrequency although sparse are compatible with circular symmetry andthere is no clear evidence for departure from circular symmetry. We havecombined the K band data with interferometric measurements in the L bandand at 11.15 μm. The full set of data can be explained if a 2055 Klayer with optical depths τK=0.060±0.003,τL=0.026±0.002 and τ11.15 μm=2.33±0.23 is added 0.33 R* above the photosphereproviding a first consistent view of the star in this range ofwavelengths. This layer provides a consistent explanation for at leastthree otherwise puzzling observations: the wavelength variation ofapparent diameter, the dramatic difference in limb darkening between thetwo supergiant stars, and the previously noted reduced effectivetemperature of supergiants with respect to giants of the same spectraltype. Each of these may be simply understood as an artifact due to notaccounting for the presence of the upper layer in the data analysis.This consistent picture can be considered strong support for thepresence of a sphere of warm water vapor, proposed by \cite{tsuji2000}when interpreting the spectra of strong molecular lines.Based on observations collected at the IOTA interferometer, WhippleObservatory, Mount Hopkins, Arizona.
| Properties of galactic B[e] supergiants. III. MWC 300
We present the results of optical and near-IR spectroscopic and mid-IRimaging observations of the emission-line star MWC 300. Its propertiesand evolutionary state are still under debate (a B[e] supergiant or aHerbig Be star). For the first time we detected radial velocityvariations of the photospheric lines and found a correlation betweenthem and those of the He I λ5876 Åline. Most of the pureemission lines had stable positions for nearly 20 years. New estimatesof the object's luminosity (log L/Lȯ=5.1±0.1),distance (D=1.8±0.2 kpc), and systemic velocity (+26±2 kms-1) were derived. We found that both the circumstellarextinction in the disk-like dusty envelope and the interstellarextinction play a significant role in the attenuation of the stellarbrightness. Our 2D modeling of the observed spectral energy distributionin the wavelength range from 0.3 μm to 1.3 mm suggests that the staris viewed through a gaseous-and-dusty flared disk with an opening angleof 30 °, an inclination angle of 10 °, an equatorial opticaldepth τV=3.0, and a total mass of 0.08Mȯ. We argue that MWC 300 is most likely a binarysystem, because of the similarities of its observed parameters withthose of recognized B[e] binaries.Partially based on observations collected at the Canada-France-Hawaiitelescope (CFHT), operated by the National Research Council of Canada,the Centre National de la Recherche Scientifique, and Univeristy ofHawaii.
| Five Star-forming Cores in the Galactic Ring Survey: A Mid-Infrared Study
We have imaged five dense molecular cores, selected from the GalacticRing Survey (GRS), in the mid-infrared with the MIRAC3 instrument. Weobtained high spatial resolution (~1") images through narrowband filtersat 12.5 and 20.6 μm. Four of the five cores show multiple compactsources, extended structure, or both. Lower resolution observations bythe Infrared Space Observatory (ISO) and the Midcourse Space Experiment(MSX) suggest that the fifth core is also surrounded by extendedemission on large scales (>~2'). The extended mid-infrared structureis well-correlated with the radio continuum morphology in each of thefive cores. This similarity suggests that the hot dust traced by themid-infrared is located within the H II region, traced by the radiocontinuum, and not merely in a surrounding photodissociation region ormolecular cloud. If a single exciting source is assumed for each core,estimates of the zero-age main-sequence spectral types based on theinfrared luminosities are typically 1-2 spectral types earlier thanthose based on the radio free-free emission. However, allowing formultiple exciting sources and apportioning the far-infrared and radiofluxes to the component sources according to the mid-infrared fluxdistribution produces better agreement between the derived spectraltypes, with an average difference of less than half a spectral type.
| Resolved Mid-Infrared Emission in the Narrow-Line Region of NGC 4151
We present subarcsecond-resolution mid-infrared images of NGC 4151 at10.8 and 18.2 μm. These images were taken with the University ofFlorida mid-IR camera/spectrometer OSCIR at the Gemini North 8 mtelescope. We resolve emission at both 10.8 and 18.2 μm, extending~3.5" across at a P.A. of ~60°. This coincides with the narrow-lineregion of NGC 4151, as observed in [O III] by the Hubble SpaceTelescope. The most likely explanation for this extended mid-IR emissionis dust in the narrow-line region heated by a central engine. We find noextended emission associated with the proposed torus and place an upperlimit on its mid-IR size of <~35 pc.
| Angular Diameters of Stars from the Mark III Optical Interferometer
Observations of 85 stars were obtained at wavelengths between 451 and800 nm with the Mark III Stellar Interferometer on Mount Wilson, nearPasadena, California. Angular diameters were determined by fitting auniform-disk model to the visibility amplitude versus projected baselinelength. Half the angular diameters determined at 800 nm have formalerrors smaller than 1%. Limb-darkened angular diameters, effectivetemperatures, and surface brightnesses were determined for these stars,and relationships between these parameters are presented. Scatter inthese relationships is larger than would be expected from themeasurement uncertainties. We argue that this scatter is not due to anunderestimate of the angular diameter errors; whether it is due tophotometric errors or is intrinsic to the relationship is unresolved.The agreement with other observations of the same stars at the samewavelengths is good; the width of the difference distribution iscomparable to that estimated from the error bars, but the wings of thedistribution are larger than Gaussian. Comparison with infraredmeasurements is more problematic; in disagreement with models, coolerstars appear systematically smaller in the near-infrared than expected,warmer stars larger.
| The Albedo Distribution of Jovian Trojan Asteroids
We present radiometrically derived V-band geometric albedos andeffective radii for 32 Jovian Trojan asteroids, using near-simultaneousmid-infrared and visible observations. We sampled the large end of thegroup's size distribution, down to a radius of 25 km, using 14 objectsin the L4 swarm and 18 in the L5 swarm. We find that the albedodistribution is much narrower than previously derived from IRASmeasurements. The Trojans, for the most part, have very similar albedos.The actual mean and standard deviation of the distribution depend on theaverage Trojan beaming parameter η. The ``standard'' value of 0.756,which was used for the IRAS analysis, yields a mean albedo of0.056+/-0.003 and a standard deviation of 0.009. However, a value ofη=0.94, which we found represented our data better, yields0.041+/-0.002 and a standard deviation of just 0.007. The thermalbehavior of the Trojans seems to follow the ``slow rotator'' model, andthe thermal inertia itself can be no greater than about half the Moon'svalue. The Kolmogorov-Smirnov test was used to compare the Trojans'albedo distribution with that of cometary nuclei, dead-comet candidates,and outer solar system objects. We find that the Trojan distribution issimilar only to the cometary ones, and only if the Trojans' η~1.Observations of the binary (617) Patroclus reveal that its albedo israther typical among the distribution. We have also discovered that(4709) Ennomos has an extremely elevated albedo, about 0.15. This objectmay have a very unusual thermal behavior or have recently suffered alarge impact that excavated the surface down to a layer of highlyreflective, pristine ice.
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| Local Heating in the Galactic Center Western Arc
We present arcsecond resolution, 10.8, 11.7, and 18.2 μm images ofthe central few parsecs of the Galaxy. These images show for the firsttime the clumpiness of the dust in the western arc. The 11.7 and 18.2μm images of part of the western arc were used to evaluate the dusttemperature and optical depth distribution of this region. We seeseveral mid-infrared emission peaks that coincide with dust temperaturepeaks and regions of low optical depth, and we infer the presence ofembedded sources in the western arc. Minimum luminosity estimates fortwo of these sources (5×104 Lsolar and2×104 Lsolar) suggest that the circumnuclearring is being locally heated by relatively young stars.
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K/'-band observations of five Mira stars with the IOTAinterferometer. The interferograms were obtained with the FLUOR fiberoptics beam combiner, which provides high-accuracy visibilitymeasurements in spite of time-variable atmospheric conditions. For theM-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB wederived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas,and 7.9 mas (/+/-0.3 mas), respectively. Simultaneous photometricobservations yielded the bolometric fluxes. The derived angularRosseland radii and the bolometric fluxes allowed the determination ofeffective temperatures. For instance, the effective temperature of R Aqlwas determined to be 2970/+/-110 K. A linear Rosseland radius for R Aqlof 250+100-60Rsolar was derived fromthe angular Rosseland radius of 5.5+/-0.2mas and the HIPPARCOS parallaxof 4.73+/-1.19mas. The observations were compared with theoretical Mirastar models of Bessel et al. [A&A 307 (1996) 481] and Hofmann et al.[A&A 339 (1998) 846]. The effective temperatures of the M-type Mirasand the linear radius of R Aql indicate fundamental mode pulsation.
| JHK Standards for Small Telescopes
The AAVSO Futures meeting, held in Madison, WI, in May 2001, proposedthat the AAVSO support near-infrared research with small telescopes. Aphotometer, the SSP-4, has been developed to provide J- and H-bandcapability for a reasonable cost. However, proper calibrated photometryrequires a set of standard stars. This paper describes such a set ofstars, suitable for small telescopes, and with accurate coordinates,proper motions, and high-quality magnitudes.
| Is SGR 1900+14 Associated with the 4 B.C.E. ``Po Star''?
Based on the public observed data of SGR 1900+14 and the relevantancient records, the possible association between SGR 1900+14 and the 4B.C.E. ``po star'' is suggested, according to the agreement in theirvisual position, distance, and age-the principle of four-dimensionalidentification. The 4 B.C.E. po star is consistent with being ahistorical hypernova.
| High-Resolution Mid-Infrared Morphology of Cygnus A
We present subarcsecond-resolution mid-infrared images at 10.8 and 18.2μm of Cyg A. These images were obtained with the University ofFlorida mid-IR camera/spectrometer OSCIR at the Keck II 10 m telescope.Our data show extended mid-IR emission primarily to the east of thenucleus with a possible western extension detected after imagedeconvolution. This extended emission is closely aligned with thebiconical structure observed at optical and near-IR wavelengths by theHubble Space Telescope. This emission is consistent with dust heatedfrom the central engine of Cyg A. We also marginally detect large-scalelow-level emission extending more than 1.5 kpc from the nucleus, whichmay be caused by in situ star formation, line emission, and/orpolycyclic aromatic hydrocarbon contamination within the bandpass of ourwide N-band filter.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Орао |
| Ректацензија: | 19h46m15.60s |
| Deклинација: | +10°36'48.0" |
| Apparent магнитуда: | 2.72 |
| Даљина: | 141.243 parsecs |
| Proper motion RA: | 15.9 |
| Proper motion Dec: | -2.6 |
| B-T magnitude: | 4.718 |
| V-T magnitude: | 2.861 |
Каталог и designations:
|
