Contents
Images
Upload your image
DSS Images Other Images
Related articles
MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| Analysis of the circumstellar environment of the B[ e] star HD 45677 (FS Canis Majoris)
Aims.We studied the circumstellar environment of the B[ e] star HD 45677through the analysis of the emission lines from ionized metals.Methods: .We used the statistical approach of the self absorption curvemethod (SAC) to derive physical parameters of the line emittingregion. Results: .The Fe II and Cr II double-peaked emission linestructure is explained by the presence of a thin absorption componentred shifted by ~3 km s-1. This absorption component can beinterpreted geometricaly as being due to infalling materialperpendicularly to the disk seen nearly pole-on, as indicated by theemission line structure. The Cr II and Fe II emission lines have acomplex structure with two (narrow and broad) components, of 45 and 180km s-1 FWHM for the permitted lines and 25 and 100 kms-1 FWHM for the forbidden ones, respectively. From our bestdata set of 1999, we obtained a Boltzmann-type population law whoseexcitation temperature is 3900-600+900 K and3150-300+350 K for the narrow component lower andupper levels respectively. We obtained an excitation temperature of3400-300+350 K for the broad component upperlevels. The forbidden lines are found to be formed in the outer regionswith higher excitation temperatures of 10 500 ± 1000 K and 8000± 1500 K for the narrow and broad components respectively in1999. Our results are consistent with line formation in a rotating disk,around a young star. In the framework of a very simplified geometricalmodel, we argue that the narrow components are principaly emitted by anoptically thin disk seen nearly pole-on, in a region whose minimumradius is estimated to be 4×1012 cm, while the broadones are formed in a disk-linked wind.
| 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.
| Extragalactic binaries as core-collapse supernova progenitors
Binary star systems are likely the progenitors of many core-collapse(Type II, Type Ib/c) supernovae (SNe). We present observationalinvestigations using ground-based and Hubble Space Telescope opticalimaging and radio monitoring of SNe and their environments, which eitherindicate or attempt to constrain the possible binary nature of the SNprogenitors. For example, from radio observations with the Very LargeArray of the Type II-linear SN 1979C in M100 we conclude that theprogenitor was possibly in a massive, highly eccentric binary, similarto the VV Cephei systems. The Type IIb SN 1993J in M81 is presumed tohave a massive progenitor in an interacting binary system, and fromHubble imaging we cannot yet constrain the nature of the presumedmassive, blue companion. We will present additional results for otherType Ib/c and II SNe.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| NLTE Radiative Transfer in the Extended Atmospheres and Winds of Cool Stars
Not Available
| Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine the astrometric positions of 19radio stars in the International Celestial Reference Frame (ICRF). Thepositions of these stars were directly linked to the positions ofdistant quasars through phase-referencing observations. The positions ofthe ICRF quasars are known to 0.25 mas, thus providing an absolutereference at the angular resolution of our radio observations. Averagevalues for the errors in our derived positions for all sources were 13and 16 mas in αcosδ and δ, respectively, withaccuracies approaching 1-2 mas for some of the stars observed.Differences between the ICRF positions of the 38 quasars and thosemeasured from our observations showed no systematic offsets, with meanvalues of -0.3 mas in αcosδ and -1.0 mas in δ.Standard deviations of the quasar position differences of 17 and 11 masin αcosδ and δ, respectively, are consistent with themean position errors determined for the stars. Our measured positionswere combined with previous Very Large Array measurements taken from1978 to 1995 to determine the proper motions of 15 of the stars in ourlist. With mean errors of ~1.6 mas yr-1, the accuracies ofour proper motions approach those derived from Hipparcos and, for a fewof the stars in our program, are better than the Hipparcos values.Comparing the positions of our radio stars with the Hipparcos Catalogue,we find that at the epoch of our observations, the two frames arealigned to within formal errors of approximately 3 mas. This resultconfirms that the Hipparcos frame is inertial at the expected level.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| First far-UV observations of KQ Puppis with FUSE
We report the first far-ultraviolet spectrum of the emission linespectroscopic binary KQ Pup (M2Iab+B0Ve), obtained with FUSE shortlyafter conjunction at orbital phase Phi =0.13. The spectrum presents asharp flux cutoff at 1040 Å; longwards, it is dominated by a largeamount of resonant and low excitation transitions of neutral and singlyionized species, probably mostly of circumsystem origin, and by theLyman absorption bands of H2, whose strength corresponds toan interstellar H2 column density of ~ 2x 1020cm-2, indicating a fraction of molecular hydrogen2N(H2)/[N(HI)+2N(H2)] of ~ 0.4. AnN(HI)/EB-V ratio of 4.3x 1021 is derived from theIUE spectra. The long-term UV monitoring of KQ Pup with IUE, HST andFUSE reveals a large decrease of the far-UV flux since orbital phase0.82 and occupying more than 0.3 of the orbital period. This ``shell''episode is attributed to line absorption and to Lyalpha Rayleighscattering due to an extended dense cool envelope in the line of sightof the B star.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985. The HST spectrum used in thispaper was obtained from the Multimission Archive at the Space TelescopeScience Institute (MAST).
| Optical and Ultraviolet Spectroscopy of SN 1995N: Evidence for Strong Circumstellar Interaction
Optical and ultraviolet observations of the Type IIn supernova SN 1995Nat epochs between 321 and 1799 days after the explosion show threedistinct velocity components. The narrow lines come from circumstellargas and show both low and high ionization. This component has a lowfilling factor and is photoionized by X-rays from the shock. Theintermediate component, which is dominated by newly processed oxygen,originates in a shell with velocity of 2500-5000 km s-1 andmost likely comes from the ejecta. The hydrogen- and helium-dominatedgas has a low ionization, a high density, and velocities that extend outto >~10,000 km s-1. Strong signatures of Lyα-pumpedfluorescence lines of Fe II are seen in the near-infrared andultraviolet. The He/H ratio, ~0.3 by number, and the nitrogenoverabundance provide strong evidence for CNO-burning products. Thefluxes of the broad hydrogen and helium lines decrease considerablyfaster than the oxygen lines. The Hα line profile shows strongevolution, with the red wing decreasing faster than the blue. Possiblescenarios, involving either a clumpy circumstellar medium or anaspherical distribution of the surrounding gas, are discussed based onthe line profiles and physical conditions. Finally, we propose that TypeIIn supernovae have their origin in red supergiants in a superwindphase. Based in part on observations obtained with the Hubble SpaceTelescope, which is operated by AURA, Inc., under NASA contract NAS5-26555 in part on observations collected at the European SouthernObservatory, Paranal, Chile; in part on data from the Lick Observatory,California; and in part on observations from the Keck Observatory,Hawaii.
| Properties of the P Cygni wind found using the Self Absorption Curve method
We have used the optical lines of N II and Fe III to study the wind ofthe luminous blue variable P Cyg. This was performedby applying a version of the Self Absorption Curve (SAC) method,involving few assumptions, to lines whose flux can be measured. A rathersurprising result was obtained; the lines of more excited multipletswithout blue shifted absorption components appear to be optically thick,while the lines of the most excited multiplets may show some indicationsof being optically thicker than the lines of less excited ones.Explanations of such effects are discussed, including possibleinhomogeneities in the wind.
| Exploring the nature of the symbiotic stars with high resolution spectroscopy.
There are some fundamental aspects of the "symbiotic phenomenon" whichare poorly understood. Modern technology now allows to observe faintstars with efficient very high resolution spectrographs, such as theSARG mounted at TNG, which will be crucial for our understanding of thenature and evolutionary stage of symbiotic stars and related sytems. Ofgreat importance will be the determination of luminosity, rotation, massloss, chemical composition of the cool stellar components which is basicfor settling the mass transfer in the system and tracing back its pasthistory. High resolution observations are also needed to derive thevelocity, temperature and abundance gradients in the nebula, and tounveil peculiar excitation processes.
| Speckle Observations of Composite Star Candidates
Speckle observations are reported for 53 stars with composite spectraselected from the Michigan Spectral Catalogue in an effort to improvethe orbits of known binaries and to discover new binaries among thisclass of objects. The data were obtained using the 1.3 and 2.4 mtelescopes at the Michigan-Dartmouth-MIT Observatory and the GeorgiaTech Research Institute speckle imaging system. For 50 stars in thesample for which no indication of duplicity was found in our data, wemay place an upper limit of <=0.2" on the separations at the epochsof observation. Three stars were resolved, including HD 26796, alsodiscovered to be double by Hipparcos. A discussion of the accuracy ofthe measurements based on comparisons with speckle standard stars ispresented.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 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?J/A+A/375/670
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The Ultraviolet Spectrum of VV Cephei Out of Eclipse
The International Ultraviolet Explorer (IUE) satellite observed VVCephei (M2 Iab+B?) over nearly a full 20.3 year orbital period, but allthe observations were obtained outside total eclipse. Egress of thechromospheric eclipse was the only portion of the eclipse observed byIUE. We have examined all 124 high-resolution IUE spectra of VV Cepheiand describe the spectrum and its variations. Contrary to initialexpectations, the spectrum remained complex throughout the entire orbit,and five line components have been identified: (1) emission lines; (2)neutral, low-excitation (e.g., Fe I) lines, which disappear as the hotcomponent emerges from eclipse; (3) broad, high-ionization (e.g., Si IVand Fe III) lines arising from the hot component or nearby circumstellarmaterial; (4) narrow absorption cores from the circumstellar envelope ofthe M supergiant; and (5) ``shell'' absorption lines with variableprofiles which do not absorb below a minimum depth. Mg II h and kemission broadens and strengthens when the system is near periastron andsecondary eclipse. Most of the variation in the other emission featuresappears to be stochastic rather than phase dependent. The continuumshows rapid variations on timescales as short as 2 weeks. At any onetime, the continuum flux between 1400 and 3000 Å is fairlyconstant but varies, apparently stochastically, in the range(2-7)x10-12 ergs s-1 cm-2Å-1 for observations obtained outside of chromosphericeclipse. The dereddened continuum fluxes vary as~λ-0.8, implying that the source is not purelystellar. Radial velocity measurements of the circumstellar and shelllines show that neither follows the radial velocity curve of the twostars. Circumstellar line velocities range from the systemic velocity to~15 km s-1 more negative, consistent with their formation inan extended circumstellar envelope of the M supergiant which is largecompared to the scale of the orbit. The shell lines are redshifted fromthe systemic velocity by ~30 km s-1. The last IUEobservations (near phase 0.9) showed shell lines with greater redshiftsand deeper absorption than around the rest of the cycle. The ultravioletspectrum of VV Cep is compared to spectra of stars showing similarfeatures: the Be shell star 28 Tauri, the B[e] star HD 45677, and the Msupergiant+B binary systems α Scorpii and KQ Puppis. A tabledetailing the appearance and behavior of numerous line multiplets ispresented.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Fe II excitation mechanism in KQ Puppis
We discuss different excitation processes behind the Fe II emissionlines in the IUE spectrum of KQ Puppis (Boss 1985), a VV Cephei type ofspectroscopic binary. Several papers have been published on the subjectsuggesting a number of processes behind the strong Fe II emission lines.We propose that there are two processes operating: selectivephotoexcitation by continuum radiation (PCR) from the B-star companion,and photoexcitation by accidental resonance (PAR) by the H Ly alpharadiation field. We suggest excitation channels for each of the Fe IIemission lines identified in the spectrum.
| Fe+ column density and line opacities of the UV2 multiplet of Fe bt II in laboratory and peculiar stellar sources in laboratory and peculiar stellar sources
The Self-Absorption Curve method has been applied to the UV2 multipleta6D-z6Fo of Fe ii observed in thelaboratory and in emission-line stars. For the laboratory observationsline opacities of this multiplet were obtained using various sets ofatomic data. The best fit is obtained using data by Bergeson et al.(1996), which provide line opacities that are in remarkable agreementwith results by Kastner (1999a), derived using independent methods. Avalue of 7.3x 1012 cm-2 for the Fe+column density is derived for the laboratory source, and lower limits of2.1x 1015 and ~ 2x 1017 cm-2 areobtained for the peculiar stars KQ Pup and RR Tel, respectively. Basedon observations made with the International Ultraviolet retrieved fromthe IUE archive.}
| New forbidden and fluorescent Fe III lines identified in HST spectra of eta Carinae
We discuss the origin of eight emission lines in the spectra of gasblobs close to the central star of eta Carinae. The spectra have beenobtained with the Goddard High Resolution Spectrograph (GHRS) and theSpace Telescope Imaging Spectrograph (STIS) onboard the Hubble SpaceTelescope. Between 2400 and 2500 Å five narrow lines areidentified as new forbidden lines of doubly ionized iron, [Fe III]. Wepresent gA-value data for the corresponding transitions, which combinetwo different metastable configurations of Fe III. An anomalousintensity of the narrow Fe III line (UV 34) at 1914 Å is explainedas fluorescence due to HLyalpha pumping. A level mixing of about 1%increases the f-value of the pumped excitation channel by more than twoorders of magnitude, which makes the pumping efficient and thefluorescence significant. We introduce a new designation forfluorescence lines photoexcited by an accidental resonance, eg. < FeIII> in the case of doubly ionized iron. Based on observations withthe NASA/ESA Hubble Space Telescope, and supported by grant numbersGO-6501 and GO-7302 from the Space Telescope Science Institute. TheSTScI is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| From OAO2 to HST. A quarter of a century of ultraviolet astronomy.
Not Available
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| Early Radio Positions of Stars
Early radio positions for a sample of 100 Hipparcos stars, threeTycho-only stars, and nine radio stars with optical positions referredto the Hipparcos/ICRS frame are analyzed. The optical proper motions areused to compare the optical and radio positions. From an original sampleof 247 radio positions for the above 112 stars, a set of 220 showscoincidence between the optical and radio centers of emission closerthan 500 mas. This set is analyzed for systematic departures between theoptical and radio positions. A smaller subset of 136 early radiopositions for 72 stars show radio-minus-optical offsets smaller than 100mas and are useful for monitoring of the spin of the Hipparcos frame.
| Radio star catalogue observed in San Juan (RSSJ95)
Using the data observed in San Juan with the photoelectric AstrolabeMark II of the Beijing Astronomical Observatory from February, 1992 toMarch, 1997, the radio star catalogue in San Juan(RSSJ95) has beencompiled. There are 69 radio stars in this catalogue. The positions ofthe radio stars are for the epoch of observation and the equinox J2000.0and a system close to that of the system FK5. The mean precisions are+/-2.2 ms and +/-0.035'' in right ascensions and declinations,respectively. The magnitudes of stars are from 0.9 to 10.7. Thedeclinations are from -2fdg 5 to -60(deg) . The mean epoch is 1995.1.Finally, the comparison results between the Hipparcos catalogue andRSSJ95 are given.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| San Juan radio star catalogue and comparison with HIPPARCOS catalogue.
Not Available
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample
We have obtained or retrieved IUE spectra for over 100 middle- andlate-type giant and supergiant stars whose spectra indicate the presenceof a hot component earlier than type F2. The hot companions areclassified accurately by temperature class from their far-UV spectra.The interstellar extinction of each system and the relative luminositiesof the components are derived from analysis of the UV and opticalfluxes, using a grid of UV intrinsic colors for hot dwarfs. We find thatthere is fair agreement in general between current UV spectralclassification and ground-based hot component types, in spite of thedifficulties of assigning the latter. There are a few cases in which thecool component optical classifications disagree considerably with thetemperature classes inferred from our analysis of UV and opticalphotometry. The extinction parameter agrees moderately well with otherdeterminations of B-V color excess. Many systems are worthy of furtherstudy especially to establish their spectroscopic orbits. Further workis planned to estimate luminosities of the cool components from the dataherein; in many cases, these luminosities' accuracies should becomparable to or exceed those of the Hipparcos parallaxes.
| The luminosity index for M stars and the distance to the LMC.
Not Available
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
