Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Determining the Physical Properties of the B Stars. II. Calibration of Synthetic Photometry
We present a new calibration of optical (UBV, Strömgren uvbyβ,and Geneva) and near-IR (Johnson RIJHK and Two Micron All Sky Survey)photometry for B and early A stars derived from Kurucz ATLAS9 modelatmospheres. Our sample of stars consists of 45 normal nearby B andearly A stars that have high-quality, low-resolution IUE spectra andaccurate Hipparcos parallaxes. The calibration is unique because itrelies only on the UV spectral energy distributions, the absolute fluxcalibration of the V filter, and the Hipparcos distances to determinethe appropriate model atmospheres for the program stars. These modelsare then used to calibrate the synthetic photometry. We compare ourresults with previous well-accepted results and provide a thoroughdiscussion of the random errors and systematic effects affecting thecalibration. In particular, we demonstrate the influence of vsini onsurface gravities derived from fitting model atmospheres. Finally, wediscuss some of our intended applications of this new calibration.
| Deep optical observations of the supernova remnants G 126.2+1.6, G 59.8+1.2 and G 54.4-0.3
Optical CCD imaging and spectroscopic observations of three supernovaremnants are presented. Optical emission from G 54.4-0.3 and G 59.8+1.2is detected for the first time, while the first flux calibrated CCDimages of the supernova remnant G 126.2+1.6 were performed in theoptical emission lines of Hα+[N II], [O III] and [S II]. A mixtureof filamentary and diffuse structures is observed in G 54.4-0.3 and G59.8+1.2, mainly in Hα+[N II], while the deep optical images of G126.2+1.6 reveal several new filamentary and diffuse structures insidethe extent of the remnant as defined by its known radio emission. In allcases, the radio emission is found to be well correlated with theoptical filaments. [O III] emission was not detected at G 54.4-0.3 and G59.8+1.2 while in G 126.2+1.6, significant morphological differencesbetween the low and medium ionization images are present suggestingincomplete shock structures. Deep long-slit spectra were taken atdifferent positions of the remnants. Both the flux calibrated images andthe long-slit spectra clearly show that the emission originates fromshock-heated gas, while some spectra of G 126.2+1.6 are characterized bylarge [O III]/Hβ ratios. This remnant's [O III] flux suggests shockvelocities into the interstellar "clouds" between 100 and 120 kms-1, while the [O III] absence in the other two remnantsindicates slower shock velocities. For all remnants, the [SII]λλ 6716/6731 ratio indicates electron densities below600 cm-3 with particularly low densities for G 54.4-0.3(below 50 cm-3). Finally, the Hα emission has beenmeasured to be between 3.0 to 15.2 × 10-17 ergs-1 cm-2 arcsec-2, 3.2 ×10-17 erg s-1 cm-2 arcsec-2and between 6.5 to 16.8 × 10-17 erg s-1cm-2 arcsec-2 for G 54.4-0.3, G 59.8+1.2 and G126.2+1.6, respectively.
| The faint supernova remnant G 116.5+1.1 and the detection of a new candidate remnant
The extended supernova remnant G 116.5+1.1 was observed in the opticalemission lines of Hα+[N II], [S II] and [O III}]; deep long slitspectra were also obtained. The morphology of the remnant's observedemission is mainly diffuse and patchy in contrast to the knownfilamentary emission seen along the western limb. The bulk of thedetected emission in the region appears unrelated to the remnant butthere is one area of emission in the south-east which is characterizedby a [S II]/Hα ratio of ~0.5, implying a possible relation to G116.5+1.1. If this is actually the case, it would imply a more extendedremnant than previously realized. Emission in the [O III] 5007 Åline image is not detected, excluding moderate or fast velocity shocksrunning into ionized interstellar clouds. Our current estimate of thedistance to G 116.5+1.1 of ~3 kpc is in agreement with earlier estimatesand implies a very extended remnant (69 pc × 45 pc). Observationsfurther to the north-east of G 116.5+1.1 revealed a network offilamentary structures prominent in Hα+[N II] and [S II] butfailed to detect [O III] line emission. Long slit spectra in a number ofpositions provide strong evidence that this newly detected emissionarises from shock heated gas. Typical Hα fluxes lie in the rangeof 9 to 17 × 10-17 erg s-1 cm-2arcsec-2, while low electron densities are implied by theintensities of the sulfur lines. Weak emission from the mediumionization line at 5007 Å is detected in only one spectrum. Cooldust emission at 60 and 100 microns may be correlated with the opticalemission in a limited number of positions. Surpisingly, radio emissionis not detected in published surveys suggesting that the new candidateremnant may belong to the class of “radio quiet” supernovaremnants.
| Deep Fading of the New Herbig Be Star MisV1147
We report on the discovery of a new variable star, MisV1147, by theMISAO Project and its unusual photometric variations. The light curve ofthe object is characterized by two distinct states, that is, arelatively calm, bright state and an active state during whichrepetitive fadings with large amplitudes of 2 mag were observed.The fading events were associated with optical reddening. The object isidentified with a catalogued Hα emission star, which has beenconfirmed by our spectroscopic observations. The Hα emission lineis narrow and symmetric both in the bright and faint phases. The objecthas infrared colors analogous to T Tau stars or Herbig Ae/Be stars. Inconjunction with the detection of a NaD absorption line and its positionclose to Cep OB1, we conclude that MisV1147 is a Herbig Be star with itsspectral type of B2. Its optical-infrared spectral energy distributionis typical for Herbig Ae/Be stars, which supports this classification.The overall properties of the light curve are reminiscent of a UX Orisub-group, showing Algol-type minima. The most noteworthy feature ofMisV1147 is that it has a spectral type B2, whereas known UX Ori starshave spectral types later than B8. The object, furthermore, has one ofthe largest amplitudes of variations among known Herbig Ae/Be stars.Hence the object breaks the very strong correlation between thevariation amplitude and the spectral type. This implies that dust clumpsin circumstellar matter can significantly obscure a central object evenin the early-type system.
| Ground-based photometric observations of Jupiter's inner satellites Thebe, Amalthea, and Metis at small phase angles
We present the results of photometric measurements of the inner joviansatellites Thebe, Amalthea and Metis based on extensive opticalobservations taken from October 1999 to January 2002. The observationswere made in the phase angle range from 8.1° to 0.3°. TheTwo-Channel Focal Reducer of the Max-Planck Institute for Aeronomyattached to the 2-m RCC telescope at Terskol Observatory (Pik Terskol,Northern Caucasus) was used in coronagraph mode. The observations wereperformed at a wavelength of 0.887 μm. Mean observationaluncertainties corresponding to 1σ rms errors were 3% for theleading and trailing sides of Amalthea, 7 and 9% for the leading andtrailing sides of Thebe and 9% for the leading side of Metis aftertaking into account the longitude brightness variations. Photometricdata calibrated on an absolute scale were used to evaluate thenear-opposition behavior of satellite brightness. All three satellitesexhibit significant opposition brightening, but the strength of thiseffect, measured as the ratios of intensities atα1=1.6° and α2=6.7° does notvary significantly among these satellites. In order to measure theopposition surge parameters the empirical law proposed by Karkoschka andHapke's model were used. The parameters of the satellite oppositioneffects are presented and discussed. The values of geometric albedoscalculated with best-fit Hapke parameters are 0.096, 0.157, and 0.24 forThebe, Amalthea, and Metis respectively. We found that the averageleading/trailing ratios of surface reflectance at the measured phaseangles are 1.53+/-0.05, 1.25+/-0.04, 1.04+/-0.08 for Amalthea, Thebe,and Metis.Until June 30, 2004: Max-Planck-Institut für Aeronomie.
| 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.
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable 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/qcat?J/A+A/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| The faint supernova remnant G 34.7-0.4 (W44)
Flux calibrated images of the known supernova remnant G 34.7-0.4 inbasic optical emission lines are presented. The low ionization imagesshow a relatively flat flux distribution. The diffuse and patchymorphology of the detected optical emission may indicate the presence ofturbulent magnetic fields. Typical observed Hnii fluxes are 8× 10-17 erg s-1 cm-2arcsec-2, while the Sii fluxes are lower around 4 ×10-17 erg s-1 cm-2 arcsec-2.Emission in the medium ionization line of [ion {O}{iii}] 5007 Å isnot detected within our sensitivity limits, probably due to the heavyextinction towards the remnant. The long-slit spectra reveal strong Siiand [ion {N}{ii}] emission relative to Ha and moderate [ion {O}i] 6300Å emission. Shock velocities in the range of 110-150 kms-1 and low electron densities are estimated. Archival MSXinfrared data show emission in the south and west areas of the remnantmatching rather well the optical and radio emission.
| Optical line emission from the supernova remnant G 73.9+0.9
Flux calibrated images of the field around the known supernova remnant G73.9+0.9 in the Hiα+[N Ii], [S Ii], [O Ii], and [O Iii] emissionlines are presented. The low ionization images are characterized bydiffuse emission both within and outside the extent of the remnant. Theflux calibrated images revealed a few small scale structures in the eastareas of G 73.9+0.9. The long-slit spectra identify the emission fromone of them as emission from shock heated gas. This patchy structure is 3 arcmin long and emits Hα flux at a level of 33× 10-17 erg s-1 cm-2arcsec-2. The bright diffuse arc-like structure in the centerof the field seems to be associated to G 73.9+0.9 given its spectralsignature and positional relation with the non-thermal radio emission. A8 arcmin long filamentary structure with an absolute Hα fluxof 9 × 10-17 erg s-1 cm-2arcsec-2 is detected in the [O Iii] emission line to thesouth of G 73.9+0.9 but is probably unrelated. The deep long-slitspectra suggest complete recombination zones, shock velocities below 90km s-1, low electron densities (<50 cm-3) andnon-negligible magnetic field strengths.
| Quantitative Stellar Spectral Classification. II. Early Type Stars
The method developed by Stock & Stock (1999) for stars of spectraltypes A to K to derive absolute magnitudes and intrinsic colors from theequivalent widths of absorption lines in stellar spectra is extended toB-type stars. Spectra of this type of stars for which the Hipparcoscatalogue gives parallaxes with an error of less than 20% were observedwith the CIDA one-meter reflector equipped with a Richardsonspectrograph with a Thompson 576×384 CCD detector. The dispersionis 1.753 Å/pixel using a 600 lines/mm grating in the first order.In order to cover the spectral range 3850 Å to 5750 Å thegrating had to be used in two different positions, with an overlap inthe region from 4800 Å to 4900 Å . A total of 116 stars wasobserved, but not all with both grating positions. A total of 12measurable absorption lines were identified in the spectra and theirequivalent widths were measured. These were related to the absolutemagnitudes derived from the Hipparcos catalogue and to the intrinsiccolors (deduced from the MK spectral types) using linear and secondorder polynomials and two or three lines as independent variables. Thebest solutions were obtained with polynomials of three lines,reproducing the absolute magnitudes with an average residual of about0.40 magnitudes and the intrinsic colors with an average residual of0.016 magnitudes.
| Metallicity Determinations from Ultraviolet-Visual Spectrophotometry. I. The Test Sample
New visual spectrophotometric observations of non-supergiant solarneighborhood stars are combined with IUE Newly Extracted Spectra (INES)energy distributions in order to derive their overall metallicities,[M/H]. This fundamental parameter, together with effective temperatureand apparent angular diameter, is obtained by applying the flux-fittingmethod while surface gravity is derived from the comparison withevolutionary tracks in the theoretical H-R diagram. Trigonometricparallaxes for the stars of the sample are taken from the HipparcosCatalogue. The quality of the flux calibration is discussed by analyzinga test sample via comparison with external photometry. The validity ofthe method in providing accurate metallicities is tested on a selectedsample of G-type stars with well-determined atmospheric parameters fromrecent high-resolution spectral analysis. The extension of the overallprocedure to the determination of the chemical composition of all theINES non-supergiant G-type stars with accurate parallaxes is planned inorder to investigate their atmospheric temperature structure. Based onobservations collected at the INAOE ``G. Haro'' Observatory, Cananea(Mexico).
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Magnetic survey of bright northern main sequence stars
The first results of a systematic search for magnetic fields in thebrightest upper main sequence (MS) stars are presented. The main goal isto survey the stars with about the same detection limit and to improveexisting statistics of their magnetism. The target list contains 57upper MS stars and represents well B0.5-F9 stars. High-resolution Zeemanspectra were obtained for 30 stars of the list. The accuracy of themagnetic field measurements ranges from 20 to 300 G depending mainly onspectral class. In the majority of studied stars we did not detectmagnetic fields. In some stars we suspect the presence of a weakmagnetic field. These are the best candidates for more extensivestudies. A particular case is the star chi Dra where we probablydetected the global magnetic field. The longitudinal field strength isB_l= -54+/-12 G. Further observations of this star are needed to confirmthe detection and to ascertain if the magnetic field is variable withthe period of rotation. Based on observations collected at the 1 mtelescope of the Special Astrophysical Observatory (Nizhnij Arkhyz,Russia).
| Deep optical observations of G 65.3+5.7
We present the first CCD mosaic of the supernova remnant {G 65.3+5.7} inthe optical emission lines of [O Ii] and [O Iii]. The new images revealseveral diffuse and filamentary structures both inside and outside theextent of the remnant as defined by its X-ray and radio emission. Themedium ionization line of [O Iii] 5007 Å provides the sharpestview to the system, while the remnant appears less filamentary in theemission line of [O Ii]. There are significant morphological differencesbetween the two images strongly suggesting the presence of incompleteshock structures. Deep long-slit spectra were taken at several differentpositions of G 65.3+5.7. All spectra originate from shock heated gas,while the majority of them is characterized by large [O Iii]/hbetaratios. The sulfur line ratios indicate electron densities below ~200cm-3, while estimates of the shock velocities lie in therange of 90-140 km s-1. Finally, the observed variations ofthe ^ha/^h$beta ratios may reflect the presence of intrinsic absorptionaffecting the optical spectra.
| First optical light from the supernova remnant G 17.4-2.3
Deep optical CCD images of the supernova remnant G 17.4-2.3 wereobtained and faint emission has been discovered therein. The images,taken in the emission lines of Hα+[N Ii], [S ii] and [O Iii],reveal filamentary structures in the east, south-east area, whilediffuse emission in the south and central regions of the remnant is alsopresent. The radio emission in the same area is found to be wellcorrelated with the brightest optical filament. The flux calibratedimages suggest that the optical filamentary emission originates fromshock-heated gas ([S ii]/Hα) > 0.4), while the diffuse emissionseems to originate from an H Ii region ([S ii]/Hα) < 0.3).Furthermore, deep long-slit spectra were taken at the bright [O Iii]filament and clearly show that the emission originates from shock heatedgas. The [O Iii] flux suggests shock velocities into the interstellar``clouds'' greater than 100 km s-1, while the [S Ii] lambda6716/6731 ratio indicates electron densities ~240 cm-3.Finally, the Hα emission has been measured to be between 7 to 20× 10-17 erg s-1 cm-2arcsec-2.
| A new candidate supernova remnant in Cygnus
Deep optical CCD imaging and spectroscopic observations of four newnebular structures have been performed for the first time. Filamentaryand diffuse emission is detected in this field located to the north-eastof the CTB 80 supernova remnant (SNR). Two longfilaments are discovered to the north of LBN 156, while a 23 arcmin longfilament, emitting strongly in the [O Ii]i line, is present to the eastof LBN 156. A complex and compact network of filaments is located closeto the center of our field. Finally, the last new source of lineemission detected in this field is mainly diffuse and patchy and itsmorphology displays a semi-circular shape. The long-slit spectra ofthese structures indicate emission from shock-heated gas and theobserved variations in the [O Ii]i fluxes most likely reflectdifferences in the shock velocities. Weak radio emission at 4850 MHzseems correlated with almost all of the new structures. It is proposedthat all these structures, with the possible exception of the brightestone, are part of a single supernova remnant. Detailed radio observationsshould allow the determination of the nature of the radio emission andprovide a crucial test of our suggested intepretation.
| Fundamental parameters and new variables of the galactic open cluster NGC 7128
CCD photometry in Johnson UBV and Strömgren uvby systems andmedium-resolution spectroscopy of the galactic open cluster NGC 7128 arepresented. Spectral types of the brightest 12 stars in the cluster fieldwere determined based on equivalent widths of the Hα and the Hei6678-Å line. The spectroscopic observations also revealed twoobvious and one probable Be-type stars showing Hα emission. Theanalysis of the photometric diagrams gave a colour excess ofE(B-V)=1.03+/-0.06mag, a distance modulus DM=13.0+/-0.2mag and an ageabove 10Myr. Time-resolved photometric observations obtained on onenight resulted in the detection of short time-scale light variations ofseven new and three already known variable stars in the cluster field.
| The physical structure of the planetary nebula NGC 6781
The planetary nebula NGC 6781 was imaged in major optical emissionlines. These lines allow us to construct maps of the projected, twodimensional Balmer decrement, electron density, electron temperature,ionization and abundance structure. The average electron density,determined from the [S Ii] lines, is ~500 cm-3, while theelectron temperature distribution, determined from the [N Ii] lines, isflat at ~10 000 K. The Balmer decrement map shows that there arevariations in extinction between the north and south areas of theplanetary nebula. The higher extinction observed to the north of thecentral star is probably caused by dust spatially associated with COemission at blue-shifted velocities. The [N Ii] image reveals the knownoptical halo, at a flux level of ~0.2% of the strong shell emission inthe east, but now the angular extent of 216 arcsec x 190 arcsec is muchlarger than previous measurements. The halo is also present in [O Iii],where we measure an extent of 190 arcsec x 162 arcsec. The ionizationmaps indicate substantial ionization along the caps of the ellipsoid aswell as in the halo. The maps also show a sharp decrease in ionizationalong the outer edge of the shell in the west and the east, south-east.The typical log abundances measured for He, N, O and S are 10.97, 8.14,8.72 and 6.90, respectively.
| The supernova remnants G 67.7+1.8, G 31.5-0.6 and G 49.2-0.7
Optical CCD imaging and spectroscopic observations of three supernovaremnants have been performed for the first time. Filamentary and diffuseemission is discovered from the supernova remnant G 67.7+1.8 located ~82arcmin to the south of CTB 80's pulsar. The Hα and sulfur emissionare almost equally strong at a level of ~ 20 10-17 ergs-1 cm-2 arcsec-2 suggestingshock-heated emission. Electron densities less than 240 cm-3are estimated, while the weak [ion {O}{iii}] emission suggests shockvelocities in the range of 60-80 km s-1. Emission can also beseen in the ROSAT All Sky Survey data which indicate an extended hardX-ray source. Emission from G 31.5-0.6 is detected only in theHα+[ion {N}{ii}] image at a typical flux level of 3510-17 erg s-1 cm-2 arcsec-2.The morphology of the observed radiation is diffuse and partiallycorrelated with the non-thermal radio emission. Deep long-slit spectradetect sulfur line emission which is not strong enough to identify it asemission from shocked gas. Finally, optical emission from G 49.2-0.7 isobscured by several dark nebulae which probably give rise to significantX-ray attenuation. The Hα+[ion {N}{ii}] flux is typically ~4010-17 erg s-1 cm-2 arcsec-2while the [ion {S}{ii}] flux is very weak, not allowing itsidentification as shock-heated. However, a small area of ~3 arcmin× 1 arcmin emits strong sulfur flux relative to Hα ([ion{S}{ii}]/Hα ~ 0.6). This area is located in the south-east of G49.2-0.7, close to the outer boundaries of the X-ray and radio emission.However, deep optical spectra would be required to firmly establish thenature of this emission and its association to G 49.2-0.7.
| BCD spectrophotometry of stars with the B[e] phenomenon. I. Fundamental parameters
Low resolution spectra in the lambda lambda3500 -4600 Å wavelengthrange of 23 stars with the B[e] phenomenon are presented. Spectralclassification of 15 program stars was performed using the BCDspectrophotometric system, based on the study of the Balmerdiscontinuity, which is independent of interstellar and circumstellardust extinctions and of circumstellar gas emissions and/or absorptions.From calibrations of the (lambda1 ,D) BCD parameters wedetermined the (Teff,log g, Mbol) of the studiedstars. For stars where this method could not be applied, we tried toestimate the temperature of the central star by using the Balmer and Hei emission lines and/or their visible energy distribution. The colourtemperature and the temperatures obtained from the study of Balmer andHe i lines are consistent with each other. The new results are comparedwith those obtained previously by other authors and discussed for eachstar individually. For some stars, differences between the effectivetemperatures derived using the BCD classification system and thoseobtained elsewhere, based on photometric or spectroscopic analysis,imply spectral-type classification disagreements ranging from 2-3 up to6 B sub-spectral types. The fundamental parameters of AS 119,CD-24°5721, Hen2-91, HD 316375 and BD-11°4747 were determinedfor the first time. A simple method was introduced to calculate total(interstellar+circumstellar) dust extinction towards the studied stars.For HD 53179, which is a double stellar system, and for HD 45677 and HD50138, which are suspected to be binaries, we predicted thecharacteristics of the components that are consistent with the observed(lambda1 ,D) parameters. However, the possible binarity of HD45677 and HD 50138 still needs to be confirmed spectroscopically. Dataobtained in CASLEO operated under agreement between the CONICET and theUniversities of La Plata, Córdoba and San Juan, Argentina.Partially based on observations done at ESO La Silla, Chili.
| 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 proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Chemical composition of HD 179821 (IRAS 19114+0002)
A LTE analysis of medium resolution spectra of {HD 179821} has beenmade. Derived atmospheric parameters of {HD 179821} are T_eff=5660 K,log g=-1.0 and [Fe/H]=-0.5. The position of the star in the H-R diagram,its high radial velocity (Vr=100 km s-1), its farinfrared excess similar to PNe and its chemical composition suggest that{HD 179821} is a low mass post-AGB carbon-poor supergiant and not apopulation I massive red supergiant. The underabundance of carbon and ofthe s-process element zirconium shows that HD 179821 has not gonethrough the third dredge-up. We emphasize the similarity of the chemicalpattern of the post-AGB star HD 179821 and the C-poor halo PN DDDM-1.Based on observations obtained at the Observatoire de Haute-Provence,France
| Optical CCD imaging of the supernova remnant CTA 1
Deep Hα+[NII], [NII], [SII] and [OIII] CCD images have beenobtained from the high galactic latitude supernova remnant CTA 1. Thefilamentary morphology in the light of the [OIII] emission is nowclearly seen in the full field of the remnant. Contrary to the [OIII]morphology, the remnant appears less sharply defined in the light of theHα+[NII], [NII] and [SII]. The morphological differences betweenthe [OIII] and Hα+[NII] emission lines and the presence of anumber of filaments with projected lengths ranging from 0.5 to severalpc suggest an inhomogeneous insterstellar medium. The presence ofincomplete recombination zones is supported by the variations seen inthe [OIII]/H_beta ratio which ranges from 5-20. A low dispersionspectrum taken in the south of CTA 1 indicates surface brightness valuesof Hα, [NII] and [SII] ~13, 14 and 18 in units of 10-17erg s-1 cm-2 arcsec-2} respectively,while the sulfur line ratio suggests an electron density of ~200cm-3. Rough upper limits to the preshock cloud density are ofthe order of {^s}im 1 nuclei cm-3. The strong [OIII] emissionis indicative of shock velocities higher than 100 km s-1while in certain areas of the remnant where the Hα emissiondominates lower velocities are possible. It is the first time thatsignificant optical emission is detected in the northwest area of theremnant, where diffuse radio emission is mainly seen. The small scalefilaments detected there and their flux suggest that we observe shockedmaterial, most likely associated to CTA 1. Three more distinctfilamentary structures are detected in the south-west area, outside ofthe main emission shell. Their lengths range from 1\farcm 5 to 6\farcm5. The spatial locations of the newly detected emission features suggestan angular shock radius of 59 arcmin.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| Spectrophotometry: Revised Standards and Techniques
The telluric features redward of 6700 Å have been removed from theaccurate spectrophotometric standards of Hamuy et al. to permit morereliable relative and absolute spectrophotometry to be obtained from CCDspectra. Smooth fluxes from 3300 to 10500 Å are best determined bydividing the raw spectra of all objects taken in a night by the rawspectrum of a ``smooth'' spectrum star before deriving the instrumentalresponse function using the revised standard star fluxes. In this waythe telluric features and any large instrumental variation withwavelength are removed from the raw data, leaving smooth spectra thatneed only small corrections to place them on an absolute flux scale.These small corrections with wavelength are well described by alow-order polynomial and result in very smooth flux-calibrated spectra.
| Faint Emission Lines and Temperature Fluctuations in M8
We present echelle spectroscopy in the 3500-10300 Å range of theHourglass Nebula, which is embedded in the Galactic H II region M8. Thedata were obtained using the 2.1 m telescope at ObservatorioAstronómico Nacional in San Pedro Mártir, Baja California.We have measured the intensities of 274 emission lines, in particular 88permitted lines of C^+, N^0, N^+, O^0, O^+, Ne^0, S^0, S^+, Si^0, Si^+,and Si^++, some of them produced by recombination only and others mainlyby fluorescence. We have determined electron temperatures and densitiesusing different line intensity ratios. We derive the He^+, C^++, O^+,and O^++ ionic abundances as well as-for the first time in a nebularobject-the total O abundance from recombination lines; these nebularvalues are independent of the temperature structure of the nebula. Wehave also derived abundances from collisionally excited lines for alarge number of ions and elements; these abundances do depend on thetemperature structure. Accurate t^2 values have been derived bycomparing the C^++, O^+, and O^++ ionic abundances obtained making useof both collisionally excited lines and recombination lines. Acomparison of the solar, Orion Nebula, and M8 chemical abundances ismade.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Pegaso |
| Ascensione retta: | 22h41m27.70s |
| Declinazione: | +10°49'53.0" |
| Magnitudine apparente: | 3.4 |
| Distanza: | 63.939 parsec |
| Moto proprio RA: | 0 |
| Moto proprio Dec: | 0 |
| B-T magnitude: | 3.297 |
| V-T magnitude: | 3.378 |
Cataloghi e designazioni:
|
INHABITED SKY
