Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| New planetary nebulae in the Galactic bulge region with l > 0°- II
The presentation of new results from an [OIII] 5007-Å survey in asearch for planetary nebulae (PNe) in the Galactic bulge is continued. Atotal of 60 objects, including 19 new PNe, have been detected in theremaining 34 per cent of the survey area, while 41 objects are alreadyknown. Deep Hα+[NII] CCD images as well as low-resolution spectrahave been acquired for these objects. Their spectral signatures suggestthat the detected emission originates from photoionized nebulae. Inaddition, absolute line fluxes have been measured and the electrondensities are given. Accurate optical positions and optical diametersare also determined.
| 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.
| Stellar abundance gradients in galactic discs - I. Method and spectral line gradients
We describe the technique of absorption-line imaging of galaxy discsusing the Taurus Tunable Filter on the Anglo-Australian Telescope anddemonstrate its sensitivity to the behaviour of spectral featuresassociated with Mg and Fe. Radial profiles of Mg2 and Fe5270line strengths are presented for a sample of eight face-on spiralgalaxies spanning a range of Hubble types. Signatures of phenomenaincluding merger-induced star formation, HII rings and galactic bars arealso reported. This study demonstrates the capacity of tunable filtersto measure Mg and Fe line strengths across the face of spiral galaxies,which can ultimately reveal clues about the star formation history andchemical evolution.
| 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.
| 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.
| 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 supernova remnant G 6.4-0.1 and its environment
Flux calibrated CCD images, in the Hα+[N II], S II, and [O III]emission lines, of a wide field around the supernova remnant G 6.4-0.1are presented. The low ionization images identify a front of enhanced SII/Hα+[N II] ratio along the east-west direction. This front isvery well correlated with the filamentary radio emission of the remnantas well as with molecular CO emission and may indicate the interactionof the primary blast wave with molecular clouds present in the vicinityof the remnant. We estimate a total Hα flux, corrected forinterstellar extinction, of 2 × 10-8 erg s-1cm-2, and a total S II flux of 1.1 × 10-8erg s-1 cm-2. The Hα+[N II] and S II imagesprovide evidence for the presence of emission from shock heated gas tothe south-west and to the east of the bulk of the known opticalemission, implying that the primary shock wave is able to driveradiative shocks into the interstellar clouds. The image in the mediumionization line of [O III] 5007 Å does not reveal any filamentarystructures. On the contrary, the emission is diffuse and very weak,close to our detection limit of 5 × 10-17 ergs-1 cm-2 arcsec-2 (3 σ), andappears to be mainly present in the south-east to north-west areas ofthe remnant. The long-slit spectra indicate significant extinction inall positions observed, while the measured variations are within the 3σ error. The [O III] emission in the spectra, whenever present, isweaker than the ^hbeta flux suggesting shock velocities around 70 kms-1 or less all around the remnant in accordance with the [OIII] imagery. Thus, the low shock velocities are a common characteristicof G 6.4-0.1 and not just of the areas where the spectra were acquired.The average sulfur line ratio suggests postshock electron densitiesbelow 120 cm-3 at the 3 σ limit.
| 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
| New planetary nebulae in the Galactic bulge region with l > 0°- I. Discovery method and first results
We present the first results of an [OIII] 5007 Å interferencefilter survey for planetary nebulae (PNe) in the Galactic bulge.Covering (at first) the 66 per cent of the survey area, we detected atotal of 90 objects, including 25 new PNe, 57 known PNe and eight knownPNe candidates. Deep Hα+[NII] CCD images have been obtained aswell as low-resolution spectra for the newly discovered PNe. Theirspectral signature suggests that the detected emission originates from aphotoionized nebula. In addition, absolute line fluxes have beenmeasured and the electron densities are given. Accurate opticalpositions and optical diameters have also been determined.
| Weak Emission-Line Central Stars of Planetary Nebulae
We present new spectroscopic data of 14 candidates for the weakemission-line type of central stars of planetary nebulae ([WELS]). Witha medium resolution of about 2 Å, we confirmed the main spectralcharacteristics of these stars but found that two objects of our sampleare in fact early-type [WC] ([WCE]) stars. The λ4650 and C IVλλ 5801, 5812 features are resolved, and they were used ina tentative classification. Based on the C IV lines the [WELS] stars ofour sample can be divided qualitatively into two main groups: clearemission and very weak or no emission. We compared mean equivalentwidths of carbon and oxygen lines of the [WELS] stars with those of the[WCE] and [WCL] types. The results are not conclusive but point in favorof an increase in temperature from [WCL] to [WCE], followed perhaps by adecrease from [WCE] to [WELS].Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are 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/411/447
| Abundance analysis of late B stars. Evidence for diffusion and against weak stellar winds
Based on high S/N spectra obtained at La Silla, Chile, and the SpecialAstrophysical Observatory, Russia, the abundances of He, C, O, Ne, Mg,Si, Ca, Fe, Sr, and Ba in 27 optically bright B5-B9 main-sequence starswere determined. NLTE effects were taken into account. A variety ofabundance patterns is present in late B stars. Accurate surfaceabundances of the diffusion indicators O, Mg, Ca, Sr and Ba suggest thatelement stratification due to diffusion is common in the program stars.Models of stellar atmospheres which include meridional mixing canexplain the observed anomalies. Although the program stars representonly a volume-limited sample of the solar neighbourhood this result isimportant for the cosmochemical evolution of the Galaxy: the surfaceabundances of the stars investigated do not necessarily reflect thechemical composition of the interstellar cloud they originated from.Furthermore, five program stars show narrow absorption lines in Ca II Kwhich can be attributed to circumstellar gas. Neon serves as a traceelement for the occurrence of weak stellar winds. Neon overabundances ofsome stars derived under the assumption of LTE suggest that such windshave been detected. In sharp contrast, the more realistic treatment ofNLTE leads to solar neon abundances and thus reveals that weak stellarwinds are absent in the program stars.Based on observations collected at the European Southern Observatory, LaSilla, Chile and at the Special Astrophysical Observatory, NizhnijArkhyz, Russia.
| 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.
| Chemical Abundances of NGC 5461 and NGC 5471 Derived from Echelle Spectrophotometry
We present high resolution spectroscopic data of the two giantextragalactic H II regions NGC 5461 and NGC 5471 in M101, which havebeen obtained with the 2.1-m telescope of the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California. We measured theintensities of several H and He recombination lines, and of forbiddenlines of a large number of ions. We calculate the physical conditions inthe two nebulae with a large number of diagnostics and determine theirchemical abundances by applying ionization correction factors (icf's) to the observed ionic abundances. For NGC 5461, theicf's are based on a tailored photoionization model of the region(Luridiana & Peimbert 2001), while for NGC 5471 they are computedfrom those predicted by a photoionization model of NGC 2363 (Luridiana,Peimbert, & Leitherer 1999), a region which is similar to NGC 5471in the ionization structure. For both regions, the icf's arecompared to those computed following the prescriptions by Mathis &Rosa (1991). Such comparison shows large discrepancies for severalelements, including nitrogen, neon, and chlorine.
| A study of the behaviour of the NaI/KI column density ratio in the interstellar medium using the Na ultraviolet doublet
Here we make a new study of the behaviour of the NaI/KI column densityratio in the interstellar medium, using a sample of new observations of28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and1997, and previously published observations (obtained by some of theauthors) of 21 stars. The sightlines cover a range of distances anddirections, including into the Galactic halo. We make use of newobservations of the NaI ultraviolet (UV) doublet for some 18 stars. Thisdoublet is much weaker than the NaI D doublet and so is less susceptibleto saturation effects, and it is well known that it can be used toobtain more accurate NaI column densities with a smaller error range. Wefind an average N(NaI)/N(KI) ratio from the NaI UV data of about 90,which is rather higher than that found previously by Hobbs and Lequeux.The Na UV-KI ratio shows a small increase in value with increasingcolumn density, while we also find a sample of low N(NaI)/N(KI) ratioclouds generally seen towards distant objects on high-latitudesightlines that reach into the halo, so that the ratio decreases moresharply at lower column densities. As the values of the ratio for thesehalo clouds (10-20) bracket the cosmic Na/K abundance ratio, we suggestthat these ratios result from a harder radiation field in the lowerhalo, such that the ionized fractions of NaI and KI become similar.Clearly caution needs to be applied in using any kind of `standardvalue' for the NaI/KI column density ratio.
| 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.
| New Spectroscopic Data and Classification of Southern [WC] Stars
New spectroscopic data of a sample of 10 southern [WC] stars arepresented and described. Equivalent widths of carbon line transitions (CII λ4267, C III λ5696, and C IV λλ5801,5812) are used to revise the classification of the seven late-type [WCL]stars of the sample. For them, our spectra allowed us to employ aquantitative system. We have confirmed the previous classification forthree of the objects, but for three others, we have encountereddifferent subtypes. Because of the weakness of the oxygen lines, it wasnot possible to suggest subtypes for the three [WCE] stars. Based onobservations made with the 1.52 m telescope at the European SouthernObservatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| New optical filamentary structures in Pegasus
Deep Hα N II CCD images have been obtained in the area of thePegasus Constellation. The resulting mosaic covers an extent of ~ 7.5deg × 7.5 and filamentary and diffuse emission was discovered.Several long filaments (up to ~ 1 deg ) are found within the field,while diffuse emission is present mainly in the central and northernareas. The filaments show variations in intensity along their extentsuggesting inhomogeneous interstellar clouds. Faint soft X-ray emissionwas also detected in the ROSAT All-Sky Survey. It is mainly concentratedin the central areas of our field and overlaps the optical emission. Thelow ionization images of [S II] of selected areas mainly show faintdiffuse emission, while in the medium ionization images of [O III]diffuse and faint filamentary structures are present. Spectrophotometricobservations were performed on the brightest filaments and indicateemission from photoionized or shock-heated gas. The sulfur line ratiosindicate electron densities below ~ 600 cm-3, while theabsolute Hα emission lies in the range of 1.1 - 8.8 ×10-17 erg s-1 cm-2 arcsec-2.The detected optical line emission could be part of a single or multiplesupernova explosions.
| 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.
| Optical observations of the supernova remnant G 69.4+1.2
We performed deep optical observations of the area of the new supernovaremnant G 69.4+1.2 in the emission lines of [O Iii], Hα+[N Ii] and[S Ii]. The low ionization images reveal diffuse and filamentaryemission in the central and south, south-west areas of our field.Estimates of the [S Ii]/Hα ratio suggest that the detectedemission in these areas originates from shock heated gas, while thestrong extended source in the north must be an H Ii region. The mediumionization image of [O Iii] shows a single filament close to the fieldcenter. Emission from [O Iii] is not detected elsewhere in the field butonly in the north from LBN 069.96+01.35. Deep long-slit spectra taken atthe position of the [O Iii] filament suggest shock velocities ~120 kms-1, while in other areas velocities around 50 kms-1 are expected. The sulfur lines ratio indicates electrondensities less than 120 cm-3. The absolute Hα flux is~5 × 10-17 erg s-1 cm-2arcsec-2. The optical emission is very well correlated withthe radio emission, especially in the south west. The soft X-rayemission detected in the ROSAT All-Sky survey shows a satisfactorydegree of correlation with the optical data in the south-west suggestingtheir association.
| 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.
| Imaging and spectroscopy of the faint remnant G 114.3+0.3
We present the first calibrated CCD images of the faint supernovaremnant {G 114.3+0.3} in the emission lines of [O Ii], [O Iii],Hα+NII and [S Ii]. The deep low ionization CCD imagesreveal diffuse emission in the south and central areas of the remnant.These are correlated with areas of intense radio emission, whileestimates of the [S Ii]/Hα ratio suggest that thedetected emission originates from shock heated gas. In the mediumionization image of [O Ii]i we discovered a thin filament in the southmatching very well the outer radio contours. This filament is notcontinuous over its total extent but shows variations in the intensity,mainly in the south-west, suggesting inhomogeneous interstellar clouds.Deep long-slit spectra were also taken along the [O Iii] filamentclearly identifying the observed emission as emission from shock heatedgas. The Hα emission is a few times 10-17erg s-1 cm-2 arcsec-2, while thevariations seen in the [O Iii] flux suggest shock velocities into theinterstellar clouds around or below 100 km s-1. The sulfurline ratio approaches the low density limit, implying electron densitiesless than ~500 cm-3.
| 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.
| 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.
| Search for low-mass PMS companions around X-ray selected late B stars
We have observed 49 X-ray-detected bright late B-type dwarfs to searchfor close low-mass pre-main sequence (PMS) companions using the EuropeanSouthern Observatory's ADONIS (Adaptive Optics Near Infrared System)instrument. We announce the discovery of 21 new companions in 9binaries, 5 triple, 4 quadruple system and 1 system consisting of fivestars. The detected new companions have K magnitudes between 6.5m and17.3m and angular separations ranging from 0.12 arcsec to 14.1 arcsec(18-2358 AU). Based on observations obtained at the European SouthernObservatory, La Silla, Chile (ESO programme No.~62.I-0477, and Swiss70~cm photometric telescope).
| 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 HB Narrowband Comet Filters: Standard Stars and Calibrations
We present results concerning the development and calibration of a newset of narrowband comet filters, designated the HB filter set, which wasdesigned and manufactured to replace aging IHW filters. Information isalso presented about the design and manufacturing of the filters,including the reasoning that was used for deciding the final wavelengthsand bandpasses. The new filters are designed to measure five differentgas species (OH, NH, CN, C2, C3), two ions(CO+, H2O+), and four continuum points.An improved understanding of extended wings from emission bands in cometspectra, gained since the development of the IHW filters, wasincorporated into the new design, so that contamination from undesiredspecies is significantly reduced compared to previous filters. Inaddition, advances in manufacturing techniques lead to squarertransmission profiles, higher peak transmission and UV filters withlonger lifetimes. We performed the necessary calibrations so that dataobtained with the filters can be converted to absolute fluxes, allowingfor, among other things, accurate subtraction of the continuum from thegas species. Flux standards and solar analogs were selected andobserved, and the data were used to establish a magnitude system for theHB filters. The star measurements were also used to evaluate which solaranalogs were best representatives of the Sun and to explore how the fluxstandards differed in the UV with respect to their spectral type. Newprocedures were developed to account for the non-linear extinction inthe OH filter, so that proper extrapolations to zero airmass can beperformed, and a new formalism, which can account for mutualcontaminations in two (or more) filters, was developed for reducingcomet observations. The relevant equations and reduction coefficientsare given, along with detailed instructions on how to apply them. Wealso performed a series of tests involving factors that can affecteither the filter transmission profiles or the distribution of theemission lines in the gas species to determine how these effectspropagate through to the calibration coefficients. The results indicatethat there are only two factors that are a concern at a level of morethan a few percent: f-ratios smaller than f/4, and a few individualfilters whose transmission profiles are significantly different from thefilters used in the calibrations.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Virgo |
| Ascensió Recta: | 14h45m30.20s |
| Declinació: | +00°43'02.0" |
| Magnitud Aparent: | 5.69 |
| Distancia: | 188.324 parsecs |
| Moviment propi RA: | -40.3 |
| Moviment propi Dec: | -7.1 |
| B-T magnitude: | 5.659 |
| V-T magnitude: | 5.663 |
Catàlegs i designacions:
|
