Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| PSF-fitting techniques for crowded field 3D spectroscopy
From a historical point of view, it was only through the advent of theCCD as a linear, high dynamic range panoramic detector that it becamepossible to overcome the source confusion problem for stellarphotometry, e.g., in star clusters or nearby galaxies. The ability ofaccurately sampling the point-spread-function (PSF) in two dimensionsand to use it as a template for fitting severely overlapping stellarimages is of fundamental importance for crowded-field photometry, andhas thus become the foundation for the determination of accuratecolor-magnitude diagrams of globular clusters and the study of resolvedstellar populations in nearby galaxies. Analogous to CCDs, theintroduction of integral field spectrographs has opened a new avenue forcrowded-field 3D spectroscopy, which benefits in the same way fromPSF-fitting techniques as CCD photometry does. This paper presents firstexperience with sampling the PSF in 3D spectroscopy, reviews the effectsof atmospheric refraction, discusses background subtraction problems,and presents several science applications as obtained from observationswith the PMAS instrument at Calar Alto Observatory.
| An Atlas of Spectrophotometric Landolt Standard Stars
We present CCD observations of 102 Landolt standard stars obtained withthe Ritchey-Chrétien spectrograph on the Cerro TololoInter-American Observatory 1.5 m telescope. Using stellar atmospheremodels, we have extended the flux points to our six spectrophotometricsecondary standards, in both the blue and the red, allowing us toproduce flux-calibrated spectra that span a wavelength range from 3050Å to 1.1 μm. Mean differences between UBVRI spectrophotometrycomputed using Bessell's standard passbands and Landolt's publishedphotometry were determined to be 1% or less. Observers in bothhemispheres will find these spectra useful for flux-calibrating spectra,and through the use of accurately constructed instrumental passbands,will be able to compute accurate corrections to bring instrumentalmagnitudes to any desired standard photometric system (S-corrections).In addition, by combining empirical and modeled spectra of the Sun,Sirius, and Vega, we calculate and compare synthetic photometry toobserved photometry taken from the literature for these three stars.
| New giant HII regions in the southern sky
We present results of a search for giant HII regions in southerngalaxies. Using high-resolution spectra, obtained with the MagellanInamori Kyocera Echelle (MIKE) at the Las Campanas Magellan IItelescope, we were able to resolve the emission-line profiles anddetermine the intrinsic velocity dispersion of the ionized gas. Out offour observed regions, selected from previous CCD narrow-bandphotometry, we detected three HII regions showing supersonic velocitydispersion, characteristic of giant HII regions, and their location indiagnostic diagrams suggests that a powerful starburst is the source ofionization energy.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| Heliocentric Dependence of Sodium Emission of Comet Hale-Bopp (C/1995O1)
A sequential spectroscopic observation of comet C/1995 O1 (Hale-Bopp)was performed from 1996 September to 1997 May. After the first positivedetection in early 1997 February, the development of the neutral sodiumemission was monitored during about 4 months around the perihelionpassage. The ratio of the sodium D-line emission relative to thecontinuum emission strength varied with the days from perihelion. Thesodium D-line emission was weak relative to the continuum near theperihelion because of the Swings effect. We corrected the Swings effect,the phase angle dependence of continuum emission, and the differences ingeocentric distance. We found that the corrected sodium-to-continuumratio varied with the dependence of r-k; k=5-6, r denotes aheliocentric distance of the comet. Our result supports the hypothesisof thermal desorption of sodium atoms from dust grains.
| 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.
| A Population of Massive Globular Clusters in NGC 5128
We present velocity dispersion measurements of 14 globular clusters inNGC 5128 (Centarus A) obtained with the MIKE echelle spectrograph on the6.5 m Magellan Clay telescope. These clusters are among the mostluminous globular clusters in NGC 5128 and have velocity dispersionscomparable to the most massive clusters known in the Local Group,ranging from 10 to 30 km s-1. We describe in detail ourcross-correlation measurements, as well as simulations to quantify theuncertainties. These 14 globular clusters are the brightest NGC 5128globular clusters with surface photometry and structural parametersmeasured from the Hubble Space Telescope. We have used thesemeasurements to derive masses and mass-to-light ratios for all of theseclusters and establish that the fundamental plane relations for globularclusters extend to an order-of-magnitude higher mass than in the LocalGroup. The mean mass-to-light ratio for the NGC 5128 clusters is ~3+/-1,higher than measurements for all but the most massive Local Groupclusters. These massive clusters begin to bridge the mass gap betweenthe most massive star clusters and the lowest mass galaxies. We findthat the properties of NGC 5128 globular clusters overlap quite wellwith the central properties of nucleated dwarf galaxies and ultracompactdwarf galaxies. As six of these clusters also show evidence forextratidal light, we hypothesize that at least some of these massiveclusters are the nuclei of tidally stripped dwarfs.
| Spectrophotometry of Planetary Nebulae in the Bulge of M31
We introduce crowded-field integral field (3D) spectrophotometry as auseful technique for the study of resolved stellar populations in nearbygalaxies. The spectroscopy of individual extragalactic stars, which isnow feasible with efficient instruments and large telescopes, isconfronted with the observational challenge of accurately subtractingthe bright, spatially and wavelength-dependent nonuniform background ofthe underlying galaxy. As a methodological test, we present a pilotstudy with selected extragalactic planetary nebulae (XPNe) in the bulgeof M31, demonstrating how 3D spectroscopy is able to improve the limitedaccuracy of background subtraction that one would normally obtain withclassical slit spectroscopy. It is shown that because of the absence ofslit effects, 3D spectroscopy is a most suitable technique forspectrophometry. We present spectra and line intensities for five XPNein M31, obtained with the MPFS instrument at the Russian 6 m BolshoiTeleskop Azimutal'nij, INTEGRAL at the William Herschel Telescope , andPMAS at the Calar Alto 3.5 m telescope. The results for two of ourtargets, for which data are available in the literature, are comparedwith previously published emission-line intensities. The three remainingPNe have been observed spectroscopically for the first time. One objectis shown to be a previously misidentified supernova remnant. Ourmonochromatic Hα maps are compared with direct Fabry-Pérotand narrowband filter images of the bulge of M31, verifying the presenceof filamentary emission of the interstellar medium in the vicinity ofour objects. We present an example of a flux-calibrated andcontinuum-subtracted filament spectrum and demonstrate how theinterstellar medium component introduces systematic errors in themeasurement of faint diagnostic PN emission lines when conventionalobserving techniques are employed. It is shown how these errors can beeliminated with 3D spectroscopy, using the full two-dimensional spatialinformation and point-spread function (PSF) fitting techniques. Using 3Dspectra of bright standard stars, we demonstrate that the PSF is sampledwith high accuracy, providing a centroiding precision at themilliarcsecond level. Crowded-field 3D spectrophotometry and the use ofPSF fitting techniques is suggested as the method of choice for a numberof similar observational problems, including luminous stars in nearbygalaxies, supernovae, QSO host galaxies, gravitationally lensed QSOs,and others.
| Star Formation History and Extinction in the Central Kiloparsec of M82-like Starbursts
We report on the star formation histories and extinction in the centralkiloparsec region of a sample of starburst galaxies that have similarfar-infrared (FIR), 10 μm, and K-band luminosities as those of thearchetype starburst M82. Our study is based on new optical spectra andpreviously published K-band photometric data, both sampling the samearea around the nucleus. Model starburst spectra were synthesized as acombination of stellar populations of distinct ages formed over theHubble time and were fitted to the observed optical spectra and K-bandflux. The model is able to reproduce simultaneously the equivalentwidths of emission and absorption lines, the continuum fluxes between3500 and 7000 Å, and the K-band and FIR flux. A good fit requiresa minimum of three populations: (1) a young population of age <=8Myr, with its corresponding nebular emission, (2) an intermediate-agepopulation (age <500 Myr), and (3) an old population that forms partof the underlying disk or/and bulge population. The birthrate parameter,which is defined as the ratio of the current star formation rate to theaverage past rate, is found to be in the range 1-12. The contribution ofthe old population to the K-band luminosity depends on the birthrateparameter and remains above 60% in the majority of the sample galaxies.Even in the blue band, the intermediate-age and old populationscontribute more than 40% of the total flux in all the cases. Arelatively high contribution from the old stars to the K-band nuclearflux is also apparent from the strength of the 4000 Å break andthe Ca II K line. The extinction of the old population is found to bearound half that of the young population. The contribution to thecontinuum from the relatively old stars has the effect of diluting theemission equivalent widths below the values expected for young bursts.The mean dilution factors are found to be 5 and 3 for the Hα andHβ lines, respectively.
| The heterogeneous class of lambda Bootis stars
We demonstrate that it is arduous to define the lambda Boo stars as aclass of objects exhibiting uniform abundance peculiarities which wouldbe generated by a mechanism altering the structure of their atmosphericlayers. We collected the stars classified as lambda Boo up to now anddiscuss their properties, in particular the important percentage ofconfirmed binaries producing composite spectra (including our adaptiveoptics observations) and of misclassified objects. The unexplained RVvariables (and thus suspected binaries), the known SB for which we lackinformation on the companion, the stars with an UV flux inconsistentwith their classification, and the fast rotating stars for which noaccurate abundance analysis can be performed, are also reviewed.Partly based on observations collected at the CFH Telescope (Hawaii) andat TBL of the Pic du Midi Observatory (France).Table \ref{tab5} is only available in electronic form athttp://www.edpsciences.org
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Abundances in Comet 19P/Borrelly at the Time of the Spacecraft Encounters
A spectroscopic observation of comet 19P/Borrelly was carried out on2001 September 23, using a low-dispersion spectrograph in the wavelengthrange 3800-7600Å. It was performed about 20hours after anencounter of the spacecraft Deep Space 1 (DS1) with the comet. Wedetected species such as CN, C2, [OI], andNH2, and estimated the gas-production rates of CN,C2, and NH2 relative to that of H2Obased on a Haser model. The production-rate ratios and the gas-to-dustratio were: Q(CN)/Q(H2O) = 0.094%,Q(C2)/Q(H2O) = 0.096%,Q(NH2)/Q(H2O) = 0.13%, andlog[Q(H2O)/Afρ] = 26.0. Our calculated relativeproduction rates of C2 and NH2 to CN were similarto to CN were similar to those of previous apparitions. Therefore, thecircumstances of the comet when DS1 encountered may be the same as thoseduring the last several apparitions, and many ground-based observationsperformed during previous apparitions will be useful to reveal thenature of the comet during the in situ observations by DS1.
| Physical Structure of the Proto-Planetary Nebula CRL 618. I. Optical Long-Slit Spectroscopy and Imaging
In this paper we present optical long-slit spectroscopy and imaging ofthe proto-planetary nebula CRL 618. The optical lobes of CRL 618 consistof shock-excited gas, which emits many recombination and forbiddenlines, and dust, which scatters light from the innermost regions. Fromthe analysis of the scattered Hα emission, we derive a nebularinclination of i=24deg+/-6deg. The spectrum of theinnermost part of the east lobe (visible as a bright, compact nebulosityclose to the star in the Hα Hubble Space Telescope image) isremarkably different from that of the shocked lobes but similar to thatof the inner H II region, suggesting that this region represents theoutermost parts of the latter. We find a nonlinear radial variation ofthe gas velocity along the lobes. The largest projected LSR velocities(with respect to the systemic velocity), ~80 km s-1, aremeasured at the tips of the lobes, where the direct images show thepresence of compact bow-shaped structures. The velocity of the shocks inCRL 618 is in the range ~75-200 km s-1, as derived fromdiagnostic line ratios and line profiles. We report a brightening(weakening) of [O III] λ5007 ([O I] λ6300) over the last~10 years that may indicate a recent increase in the speed of theexciting shocks. From the analysis of the spatial variation of thenebular extinction, we find a large density contrast between thematerial inside the lobes and beyond them: the optical lobes seem to be``cavities'' excavated in the asymptotic giant branch (AGB) envelope byinteraction with a more tenuous post-AGB wind. The electron density,with a mean value ne~5×103-104cm-3, shows significant fluctuations but no systematicdecrease along the lobes, in agreement with most line emission arisingin a thin shell of shocked material (the lobe walls) rather than in thepost-AGB wind filling the interior of the lobes. The masses of atomicand ionized gas, respectively, in the east (west) lobe are>1.3×10-4 (>7×10-5) and~6×10-5 (~4×10-5) Msolar.The shocks in CRL 618 are in a radiative regime and may lead in thefuture to the evolution of the optically emitting lobes into a fast,bipolar molecular outflow. The time required by the dense, shocked gasto cool down significantly is <~2 yr, which is substantially lowerthan the kinematical age of the lobes (<~180 yr). This resultsuggests that a fast wind is currently active in CRL 618 and keepsshocking the circumstellar material.
| 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).
| High-Resolution Optical Observations of Extreme-Ultraviolet-selected Active Late-type Stars
We present high-resolution optical spectra of EUV-selected late-typestars. We have detected seven new spectroscopic binaries, including apair of F stars and an RS Canum Venaticorum candidate. Many of the starsin our sample show remarkable Hα emission with equivalent widthsup to 8 Å. Several sources show strong He I 5876 Å emission.We use cross-correlation techniques to estimate line-of-sight rotationalvelocities and find that they range from less than 5 to over 80 kms-1. Several of the stars show a strong Li 6707 Å line,which we use to determine their Li abundances. The EUV-to-bolometricluminosity, a measure of the coronal heating efficiency, is as high as10-3. The present sample extends the EUV-selected late-typestars to lower Rossby numbers and makes the coronal saturation boundarybetter pronounced. Our results indicate that there is a drop in the EUVemission from the corona in late M dwarfs possibly because of a changein the nature of the magnetic dynamo in fully convective stars.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Electron temperature fluctuations in 30 Doradus
We present an observational study of the spatial variation of theelectron temperature in the 30 Doradus Nebula. Weused the [ion {O}{iii}] (lambda 4959 + lambda 5007)/lambda 4363 ratio toestimate the electron temperature at 135 positions in the nebula acrossthree different directions. We analysed long-slit spectrophotometricdata of high signal-to-noise in the range of 4100 to 5030 Åobtained with the Cassegrain spectrograph attached to the 1.60 mtelescope of the Laboratório Nacional de Astrofísica,Brazil. No large-scale electron temperature gradient was detected in 30Doradus. The electron temperature estimates obtained are fairlyhomogeneous with a mean value of 10 270 +/- 140 (3sigma ) K. Thecompatibility between the present estimates with optical and radiotemperature determinations found in the literature for other positionsor for the entire nebula corroborates this conclusion. Temperaturefluctuations of small amplitude were observed with a variance relativeto the mean of ts2= 0.0025 or equivalently with adispersion of only 5%. The areas with lower surface brightness seem topresent slightly higher electron temperatures. This would indicate thatthe bright arcs of 30 Doradus, which correspond to the densest regions,would have lower electron temperatures than the most diffuse areas.
| Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org
| 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
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
| Optical long-slit spectroscopy and imaging of OH 231.8+4.2
We present optical long-slit spectra and complementary broad and narrowband images of the bipolar proto-planetary nebula OH231.8+4.2. Absolute J2000 coordinates have been calculated forour maps from the position of nearby stars. Our maps of the opticalcontinuum show the spatial distribution of the starlight scattered bydust grains. This component is found to be highly elongated along thenebular axis, with a structure very similar to that of the molecularemission. Flux variations with time of the red continuum emission aredetected. Our long-slit spectroscopy of Hα and other atomic linesreveals wide spectral profiles and, in general, a complex spatial andspectral emission distribution. The emission arises from two broad lobesand is shifted toward shorter (north lobe) and longer wavelengths (southlobe), indicating that the gas is flowing outwards at high velocity. Theclumpiness of the emission nebula is remarkable. A simple model has beenused to describe the complex structure and kinematics of this source.Our model suggests that, in addition to the two extended, hollow lobesidentified in the Hα images, a smaller, bubble-like expandingstructure should lie inside the south lobe. A comparison of the emissionline spectrum with predictions of theoretical shock models confirms thatthe optical lines have been shock excited. We have estimated theelectron density of the lobes and deduced the total ionized mass of thenebula, obtaining a low value of ~5\times10-4M\odot. We interpret the shaping and evolution of OH231.8+4.2 in the wind interaction scenario for planetarynebulae formation. The peculiar structure and kinematics of themolecular outflow and the ionized envelope are explained in terms of ashock regime transition: the well collimated molecular outflow and thedifferent components of the optical nebula would consist ofcircumstellar material swept-up by a unique shock in a radiative, andnon-radiative regime, respectively. Finally, we briefly discuss thecontroversial evolutionary status of OH 231.8+4.2.
| 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.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Observations of BN and AN stars: New Be stars
From a survey of spectra of Bn/An stars, we have detected seven new Bestars: HR 1056 (A0Vn), 1544 (A1Vn), 2191 (A0Vnn), 2300 (B8Vn), 3134(B9.5 Vn), 3878 (B0.5 IIIn) and 4552 (B9IIIpSi). H_alpha profiles ofthese stars are presented. Measured values of equivalent widths, fullwidths at half intensity maximum, and the peak-separations of theH_alpha emission profiles are also tabulated in this paper. We have alsocomputed the radii of emission disks of the newly detected Be stars.
| How many lambda Bootis stars are binaries?
In the attempt to shed new light on the lambda Boo phenomenon weanalyzed the astrometric, photometric and spectroscopic characteristicsof stars out of a list of recently selected lambda Boo candidates. Weshow that the class is still ill-defined and discuss the possibilitythat some, if not most stars presently classified as lambda Boo, are infact binary pairs and that peculiar abundances may not correspond toactual values if the average values of the atmospheric parameters{Teff} and log g are assumed and the effect of veiling is nottaken into account. Partly based on data from the ESA Hipparcosastrometry satellite.
| The chemical composition of the young, Inter-Cloud population
High-resolution AAT spectroscopy and lower resolution spectrophotometryare presented for three early B-type stars that are members of theyoung, Inter-Cloud population between the Magellanic Clouds. Thesespectra have been analyzed using LTE model-atmosphere techniques, toderive the stellar atmospheric parameters and photospheric chemicalcompositions. The latter should reflect that of the present-dayinterstellar medium (ISM) within the Inter-Cloud Region (ICR). From adifferential analysis, the three ICR stars appear to have a mean metalabundance of ~ 1.1 dex lower than their Population i Galactic analogues,and 0.5 dex lower than the SMC star (AV 304). Hence, the ICR gas doesnot reflect the present-day composition of either the SMC (or LMC) ISM.Age (and distance) estimates were obtained using the theoreticalisochrones of Bertelli et al. (\cite{ber94}); these imply that theyoung, Inter-Cloud population has an age dispersion of at least 10-40Myr, and provide evidence for a distance gradient across the ICR. Wediscuss our results within the context of recent numerical simulationsof the gravitational interactions between the Galaxy-LMC-SMC, thatpredict that the ICR was tidally disrupted from the SMC some 200 Myrago. If the SMC was chemically homogeneous, a comparison of the ICRabundance determinations with the SMC age-metallicity relationship wouldthen imply that the formation of the ICR must have occurred ~ 8.5 Gyrago. Alternatively and more plausible, we postulate that the ICR gasformed from a mixture of SMC gas and an unenriched component. This isconsistent with model-predictions that both a halo and disc componentshould have contributed to the material within the ICR during the tidaldisruption.
| CCD images and long-slit spectroscopy of the ring nebula around theta MUS
We present the first digital CCD images and long-slit spectroscopy ofthe optical ring nebula around the Wolf-Rayet star theta Mus. The CCDimages obtained through narrow-band filters centred at [Oiii] and Hαshow that the nebula has a filamentary structure, similar to supernovaremnants, mainly seen in [Oiii]. A spatial detachment between [Oiii] andHα images suggests excitation stratification, or multiple rings. Ananalysis of the physical conditions in the nebula was performed by meansof long-slit CCD spectra. The spectral images show that the nebula is oflow density and medium excitation. By means of quotients ofrecombination and collisional spectral line fluxes we determine that theprincipal excitation mechanism is photoionization. We have determinedthe electronic temperature and density, and chemical abundances for theoxygen at different sites within the nebula. Nebular chemical abundancesare found to be similar to the Galactic ISM, indicating that the nebulais mainly composed of swept up material.
| The High-Z Supernova Search: Measuring Cosmic Deceleration and Global Curvature of the Universe Using Type IA Supernovae
The High-Z Supernova Search is an international collaboration todiscover and monitor Type Ia supernovae (SNe Ia) at z > 0.2 with theaim of measuring cosmic deceleration and global curvature. Ourcollaboration has pursued a basic understanding of supernovae in thenearby universe, discovering and observing a large sample of objects anddeveloping methods to measure accurate distances with SNe Ia. This paperdescribes the extension of this program to z >= 0.2, outlining oursearch techniques and follow-up program. We have devised high-throughputfilters that provide accurate two-color rest frame B and V light curvesof SNe Ia, enabling us to produce precise, extinction-correctedluminosity distances in the range 0.25 < z < 0.55. Sources ofsystematic error from K-corrections, extinction, selection effects, andevolution are investigated, and their effects estimated. We presentphotometric and spectral observations of SN 1995K, our program's firstsupernova (SN), and use the data to obtain a precise measurement of theluminosity distance to the z = 0.479 host galaxy. This object, whencombined with a nearby sample of SNe, yields an estimate for the matterdensity of the universe of Omega_M=-0.2^+1.0_-0.8 if Omega_Lambda = 0.For a spatially flat universe composed of normal matter and acosmological constant, we find Omega_M=0.4^+0.5_-0.4,Omega_Lambda=0.6^+0.4_-0.5. We demonstrate that with a sample of ~30objects, we should be able to determine relative luminosity distancesover the range 0 < z < 0.5 with sufficient precision to measureOmega_M with an uncertainty of +/-0.2.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 04h50m36.70s |
| Declinazione: | +08°54'01.0" |
| Magnitudine apparente: | 4.36 |
| Distanza: | 59.382 parsec |
| Moto proprio RA: | -0.7 |
| Moto proprio Dec: | -31.3 |
| B-T magnitude: | 4.369 |
| V-T magnitude: | 4.343 |
Cataloghi e designazioni:
|
INHABITED SKY
