scispace - formally typeset
Search or ask a question

Showing papers in "Astronomy & Astrophysics Supplement Series in 1996"


Journal ArticleDOI
TL;DR: The SExtractor ( Source Extractor) as mentioned in this paper is an automated software that optimally detects, deblends, measures and classifies sources from astronomical images, which is particularly suited to the analysis of large extragalactic surveys.
Abstract: We present the automated techniques we have developed for new software that optimally detects, deblends, measures and classifies sources from astronomical images: SExtractor ( Source Extractor ). We show that a very reliable star/galaxy separation can be achieved on most images using a neural network trained with simulated images. Salient features of SExtractor include its ability to work on very large images, with minimal human intervention, and to deal with a wide variety of object shapes and magnitudes. It is therefore particularly suited to the analysis of large extragalactic surveys.

10,983 citations


Journal ArticleDOI
TL;DR: The ELODIE spectrograph of the Observatoire de Haute-Provence (OHP) as mentioned in this paper was designed as an updated version of the cross-correlation spectrometer CORAVEL to perform very accurate radial velocity measurements.
Abstract: The bre{fed echelle spectrograph of Observatoire de Haute{Provence, ELODIE, is presented. This instrument has been in operation since the end of 1993 on the 1.93 m telescope. ELODIE is designed as an updated version of the cross{correlation spectrometer CORAVEL, to perform very accurate radial velocity measurements such as needed in the search, by Doppler shift, for brown{dwarfs or giant planets orbiting around nearby stars. In one single exposure a spectrum at a resolution of 42000 (=) ranging from 3906 A to 6811 Ai s recorded on a 10241024 CCD. This performance is achieved by using a tan = 4 echelle grating and a combination of a prism and a grism as cross{disperser. An automatic on{line data treatment reduces all the ELODIE echelle spectra and computes cross{correlation functions. The instrument design and the data reduction algorithms are described in this paper. The eciency and accuracy of the instrument and its long term instrumental stability allow us to measure radial velocities with an accuracy better than 15 m s 1 for stars up to 9th magnitude in less than 30 minutes exposure time. Observations of 16th magnitude stars are also possible to measure velocities at about 1 km s 1 accuracy. For classic spectroscopic studies (S=N>100) 9th magnitude stars can be observed in one hour exposure time.

1,285 citations


Journal ArticleDOI
TL;DR: In this article, a nonlinear least squares algorithm is used to solve for any subset of allowed parameters, which include atomic data (loggf and van der Waals damping constants), model atmosphere specications (Te,l og g), elemental abundances, and radial, turbulent, and rotational velocities.
Abstract: We describe a new software package that may be used to determine stellar and atomic parameters by matching observed spectra with synthetic spectra generated from parameterized atmospheres. A nonlinear least squares algorithm is used to solve for any subset of allowed parameters, which include atomic data (loggf and van der Waals damping constants), model atmosphere specications (Te ,l og g), elemental abundances, and radial, turbulent, and rotational velocities. LTE synthesis software handles discontiguous spectral intervals and complex atomic blends. As a demonstration, we t 26 Fe I lines in the NSO Solar Atlas (Kurucz et al. 1984), determining various solar and atomic parameters.

896 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that the mathematical operation of outer matrix multiplication provides the missing link between radio interferometry and optical polarimetry, and propose a unified framework of Stokes parameters and Wolf coherency matrix, the Jones and Mueller calculi from optics, and the techniques of Radio Interferometry based on multiplying correlators.
Abstract: The measurement of polarized radiation uses entirely different methods at optical and radio wavelengths. As a result, the algebraic analysis of polarimeter performance differs and, in the case of radio interferometry, is unnecessarily complicated. We demonstrate that the mathematical operation of outer matrix multiplication provides the missing link between the two approaches. Within one coherent framework, we then unite the concepts of Stokes parameters and Wolf coherency matrix, the Jones and Mueller calculi from optics, and the techniques of radio interferometry based on multiplying correlators. We relate the polarization performance of a complete radio interferometer to the (matrix) polarization properties of its successive signal processing stages, providing a clear view of how a radio polarimeter works. Our treatment also clarifies the nature of and the relations between the various types of transformations used in optical polarimetry. We develop the analysis from the radio interferometrist's point of view, but include enough background for a wider audience. In a companion paper, we discuss in more detail the application to the calibration of radio interferometer systems; in a third paper we investigate the IAU (1973) radio definition of the Stokes parameters and its precise translation into mathematical form.

344 citations


Journal ArticleDOI
TL;DR: In this paper, a large grid of stellar evolutionary models with the initial chemical composition (Z = 0:0001; Y = 0 :23) is presented, and the corresponding theoretical isochrones are presented.
Abstract: We present a large grid of stellar evolutionary models with the initial chemical composition (Z = 0:0001;Y =0 :23). These tracks are conceived to extend the grid of stellar models described in the previous papers of this series, and are computed with the new radiative opacities by Iglesias et al. (1992) and convective overshoot. The tracks span the range of initial masses from 0:6 M to 100 M, and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specically, low- and intermediate-mass stars are followed till the beginning of the thermally pulsing regime of the asymptotic red giant branch phase (TP-AGB), while massive stars are followed till the core C-ignition. With respect to previous papers of this series, these models incorporate a number of small modications in the input physics, particularly on the equation of state, which now incorporates the eect of Coulomb interactions between charged particles. The eect of these modications is discussed. The corresponding theoretical isochrones are presented.

322 citations


Journal ArticleDOI
TL;DR: In this paper, a catalogue of supernova remnants in the southern Galaxy within the area 245 l 355,jbj < 1:5 has been produced from observations made at 0.843 GHz with a resolution of 43 00 using the Molonglo Observatory Synthesis Telescope (MOST).
Abstract: A catalogue of supernova remnants in the southern Galaxy within the area 245 l 355,jbj< 1:5 has been produced from observations made at 0.843 GHz with a resolution of 43 00 using the Molonglo Observatory Synthesis Telescope (MOST). The catalogue, presented here, provides greyscale images and contour maps of known and newly-discovered remnants. The increased resolution and sensitivity of these observations has resulted in better statistics for southern remnants.

318 citations


Journal ArticleDOI
TL;DR: In this paper, the surface photometry of a galaxy with an exponential light profile for both bulge and disk and, when necessary, with a Freeman bar was modeled and compared with one-dimensional decomposition techniques, where the luminosity profile of the galaxy is used.
Abstract: In this Paper I present a new two-dimensional decomposition technique, which models the surface photometry of a galaxy with an exponential light profile for both bulge and disk and, when necessary, with a Freeman bar. The new technique was tested for systematic errors on both artificial and real data and compared with widely used one-dimensional decomposition techniques, where the luminosity profile of the galaxy is used. The comparisons indicate that a decomposition of the two-dimensional image of the galaxy with an exponential light profile for both bulge and disk yields the most reproducible and representative bulge and disk parameters. An extensive error analysis was made to determine the reliability of the model parameters. If the model with an exponential bulge profile is a reasonable description of a galaxy, the maximum errors in the derived model parameters are of order 20%. The uncertainties in the model parameters will increase, if the exponential bulge function is replaced by other often used bulge functions as the de Vaucouleurs law. All decomposition methods were applied to the optical and near-infrared data set presented by de Jong & van der Kruit (1994), which comprises 86 galaxies in six passbands.

289 citations


Journal ArticleDOI
TL;DR: In this article, the authors applied the InfraRed Flux Method (IRFM) to a sample of 475 dwarfs and subdwarfs in order to derive their eective temperatures with a mean accuracy of about 1.5%.
Abstract: We have applied the InfraRed Flux Method (IRFM) to a sample of 475 dwarfs and subdwarfs in order to derive their eective temperatures with a mean accuracy of about 1.5%. We have used the new homogeneous grid of theoretical model atmosphere flux distributions developed by Kurucz (1991, 1993) for the application of the IRFM. The atmospheric parameters of the stars cover, roughly, the ranges: 3500 K Te 8000 K; 3.5 (Fe/H) +0:5; 3.5 log(g) 5. The monocromatic infrared fluxes at the continuum, and the bolometric fluxes are derived using recent results, which satisfy the accuracy requeriments of the work. Photometric calibrations have been revised and applied to estimate metallicities, although direct spectroscopic determinations were preferred when available. The adopted infrared absolute flux calibration, based on direct optical measurements of angular stellar diameters, sets the eective temperatures determined using the IRFM on the same scale than those obtained by direct methods. We derive three temperatures, TJ , TH and TK , for each star using the monochromatic fluxes at dierent infrared wavelengths in the photometric bands J, H ,a nd K. They show good consistency over 4000 K, and no trend with wavelength may be appreciated. We provide a detailed description of the steps followed for the application of the IRFM, as well as the sources of the errors associated to the dierent inputs of the method, and their transmission into the nal temperatures. We also provide comparison with previous works.

269 citations


Journal ArticleDOI
TL;DR: In this paper, fractional abundances in dense interstellar clouds for selected atomic and molecular species using three different homogeneous, pseudo-time-dependent models discussed by Bettens, Lee, & Herbst (1995): the new standard model, the new neutral-neutral model, and model 4.
Abstract: In this paper we present calculated fractional abundances in dense interstellar clouds for selected atomic and molecular species using three different homogeneous, pseudo-time-dependent models discussed by Bettens, Lee, & Herbst (1995): the new standard model, the new neutral-neutral model, and model 4. We have run each model with 3 different hydrogen densities – 103 , 104 , and 105 cm-3 – and two temperatures – 10 K and 50 K. “Low metal” elemental abundances have been used for all three models; the new standard model has also been run with “high metal” abundances.

261 citations


Journal ArticleDOI
TL;DR: In this article, a CCD optical imaging survey of the ionized gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalog to represent a broad variety of X-ray, radio, infrared and kinematical properties.
Abstract: We present results of a CCD optical imaging survey of the ionized gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalog to represent a broad variety of X-ray, radio, infrared and kinematical properties. For each galaxy we have used broad-band R images and narrow-band images centered at the and emission lines to derive the luminosity and distribution of the ionized gas. We found that a large fraction of E (72%) and S0 (85%) galaxies in our sample contain ionized gas. The gas morphology appears to be rather smooth for most galaxies; however of the sample galaxies show a very extended filamentary structure. According to the morphology and size of the gas distribution, the galaxies have been classified into three broad groups, named small disk (SD), regular extended (RE) and filamentary structure (F). The mean diameter of the emitting region ranges between 1 and 10 kpc; the derived mass of the ionized gas ranges between 103 and 105 solar masses. A significant correlation between and X-ray luminosities is found for those galaxies (27% of the sample) for which we have detected ionized gas and are also listed as X-ray sources. However, there are relatively strong X-ray emitting galaxies for which we have not detected emission and objects which show emission-lines but are not listed either in the EINSTEIN or in the ROSAT databases. The distribution of datapoint and upper limits in this diagram suggests that galaxies with warm gas are also X-ray emitters, while there are X-ray emitters without measurable emission. Similar characteristics are present in the correlation between the infrared luminosity in the 12 band and ; correlations with other infrared wavelengths are weaker. A strong correlation was also found between the luminosity and the luminosity in the B band inside the region occupied by the line-emitting gas. We use these correlations to discuss the possible mechanisms responsible for the gas ionization and excitation, analyzing in particular the role of the post-AGB stars and the thermal conduction from the X-ray halo in providing the necessary source of ionization.

218 citations


Journal ArticleDOI
TL;DR: In this article, the authors present new B and R CCD surface photometry for a sample of 4 Blue Compact Dwarf Galaxies (BCDs) and 2 higher luminosity starburst galaxies.
Abstract: We present new B and R CCD surface photometry for a sample of 4 Blue Compact Dwarf Galaxies (BCDs) and 2 higher luminosity starburst galaxies. We find that the BCD surface brightness profiles (SBPs) can be generally well fitted by the sum of the light of 3 distinct components: 1) an exponential component at large radii, with or without a central flattening which describes the underlying older stellar population; 2) a plateau component at intermediate radii and 3) a gaussian component at small radii. The last 2 components describe the ongoing starburst superimposed on the older stellar component. We apply the 3–component decomposition scheme to 8 more BCDs from the literature and, for the total sample of 14 galaxies, derive structural properties such as central surface brightness and scale length, and perform a deprojection to obtain the luminosity density distribution for each component. We also derive color profiles and find that BCDs show generally a reddening outwards, followed by a blueing beyond a transition radius.

Journal ArticleDOI
TL;DR: In this article, the authors present high resolution images and spectra toward 21 H 2 O maser sources in the vicinity of ultracompact (UC) HII regions.
Abstract: In this paper we present high resolution images and spectra toward 21 H 2 O maser sources in the vicinity of ultracompact (UC) HII regions. This survey provides the basis for future studies with milli–arcsecond resolution, utilizing very long baseline interferometry (VLBI) techniques. Emission from the masing transition of interstellar H 2 O is observed in the close vicinity of UC HII regions with a median angular distance of and a median linear projected distance of pc from the continuum peak. We find that for UC HII regions with cometary morphology the water maser emission is located in front of the cometary arc whereas for non-cometary UC HII regions the water masers are often observed projected onto the contours of the ionized gas. Due to the large median distance of the water masers from the I-front of the UC HII region, it is unlikely, that the water masers are formed in the shocked layer of warm molecular gas in the interface between the ionized gas of the UC HII region and surrounding molecular gas which is predicted by the Bow Shock theory of UC HII regions. A comparison with maps in the NH 3 inversion transitions shows that in at least 7 cases, the water masers are associated with hot ( T K), dense ( n (Hcm -3 ) molecular clumps. For the UC HII regions G5.89-0.38 and G45.07+0.13 we find spatial and velocity correspondence between water masers and outflowing molecular gas. It is thus likely, that for these sources the H 2 O masers are taking part in the bipolar outflow.

Journal ArticleDOI
TL;DR: In this article, the authors presented an atlas of very high resolution H α line profiles of 63 pre-main sequence stars, divided among 43 T Tauri stars, 18 Herbig Ae/Be stars, and 2 FU Orionis objects.
Abstract: We present an atlas of very high resolution ( R ~ 50000) H α line profiles of 63 pre-main sequence stars, divided among 43 T Tauri stars, 18 Herbig Ae/Be stars, and 2 FU Orionis objects. H α emission is the most common and prominent spectroscopic feature of pre-main sequence stars, and although it is optically very thick it is still the most frequently modelled emission line in young stars. In T Tauri stars the principal models involve magnetically driven winds, and more recently the role of infalling magnetospheric material has been explored. For Herbig Ae/Be stars a variety of models have been proposed, current emphasis is directed towards obscuration by clumpy circumstellar disk structures. In order to provide constraints on such models, we have made a statistical analysis of the 63 high resolution profiles. We here ignore the considerable variability of the H α emission, which is discussed in detail in a second paper. Most of our observed lines show complex profiles due to an interplay between emission and absorption features, and we suggest a two-dimensional classification scheme to describe these line profiles, based on the relative height of a secondary peak to the primary peak, as well as whether the absorption is blue- or red-shifted. Among T Tauri stars, 25% have symmetric profiles, 49% have blueshifted absorption dips, and 5% have P Cygni profiles; the remaining 21% show a variety of redshifted absorptions. For Herbig Ae/Be stars symmetric lines are quite rare (11%), indeed almost all of these stars have deep and prominent central absorptions. We have measured the extent of the line wings for all of our stars at the I max /40 level, and find that almost all have very extended wings, with typical extents of ± 350 km/s, but in high spectra the wings can be traced to lower intensities, and velocities as high as ± 900 km/s have been observed. Pronounced asymmetries of these extended wings are found for many stars, suggesting the possibility that the highest velocity material could be non-uniformly distributed. The equivalent widths of the H α emission in our sample of stars span two orders of magnitude, with a distribution that increases with decreasing equivalent width.

Journal ArticleDOI
TL;DR: For the detailed statistical analysis of the X-ray emission of hot stars, this paper selected all stars of spectral type O and B listed in the Yale Bright Star Catalogue and searched for them in the ROSAT All-Sky Survey.
Abstract: For the detailed statistical analysis of the X-ray emission of hot stars we selected all stars of spectral type O and B listed in the Yale Bright Star Catalogue and searched for them in the ROSAT All-Sky Survey. In this paper we describe the selection and preparation of the data and present a compilation of the derived X-ray data for a complete sample of bright OB stars.

Journal ArticleDOI
TL;DR: The MPE near infrared imaging spectrometer 3D as mentioned in this paper is based on a 2562 NICMOS-3 Rockwell array and can simultaneously obtain 256 H - or K-band spectra at R = 1100 or 2100 from a square 16×16 pixel field on the sky.
Abstract: The new MPE near infrared imaging spectrometer 3D represents a new generation of astronomical instrumentation. It is based on a 2562 NICMOS-3 Rockwell array and can simultaneously obtain 256 H - or K -band spectra at R = 1100 or 2100 from a square 16×16 pixel field on the sky. Typical pixel scales are 0.3″/pixel or 0.5″/pixel. 3D is a combination of a novel image slicer and a liquid nitrogen cooled long slit spectrometer. It includes high definition on-axis lens optics, a high efficiency directly ruled KRS-5 grism as well as a cold closed-loop piezo-driven tilt mirror allowing full spectral sampling. The instrument efficiency including detector is 15%. Combining the advantages of imaging and spectroscopy increases the observing efficiency on key astronomical objects (e.g. galactic nuclei) by such a large factor over existing grating or Fabry-Perot spectrometers that subarcsecond near-IR spectroscopy of faint Seyferts, starbursts, quasars, or distant galaxy clusters becomes feasible for the first time with 4m-class telescopes. As a portable instrument 3D has already been successfully deployed on several 2 and 4m-class telescopes.

Journal ArticleDOI
TL;DR: In this article, it was shown that high-resolution images of extra-solar planets can be obtained directly in the recombined focal plane, if there are enough ground-based arrays.
Abstract: In the recent years, interferometric arrays of optical telescopes have reached sizes of the order of 100 m, but they have yet to produce high-resolution images. The analysis of image formation now shows that such images are theoretically obtainable directly in the recombined focal plane, if there are enough telescopes. Resolved images of extra-solar planets are in principle obtainable with 10 km ground-based arrays.

Journal ArticleDOI
J. P. Hamaker1, J. D. Bregman1
TL;DR: In this paper, a new theory of radio-interferometric polarimetry and its application to the calibration of interferometer arrays are presented, and the definition of the Stokes parameters adopted by Commission 40 of the IAU (1974) and the way this definition works out in the mathematical equations.
Abstract: In two companion papers (Paper I, Hamaker et al. 1996; Paper II, Sault et al. 1996), a new theory of radio-interferometric polarimetry and its application to the calibration of interferometer arrays are presented. To complete our study of radio polarimetry, we examine here the definition of the Stokes parameters adopted by Commission 40 of the IAU (1974) and the way this definition works out in the mathematical equations. Using the formalism of Paper I, we give a simplified derivation of the frequently-cited `black-box' formula originally derived by Morris et al. (1964). We show that their original version is in error in the sign of Stokes V , the correct sign being that given by Weiler (1973) and Thompson et al. (1986).

Journal ArticleDOI
TL;DR: In this paper, an approach to imaging and deconvolvolving interferometric mosaics of dirty images has been proposed, and the advantages and limitations of this technique have been discussed.
Abstract: In radio astronomy, mosaicing is the practice of combining multiple pointings of a telescope to produce an image significantly larger than the telescope's primary beam. We consider an approach to imaging and deconvolving interferometric mosaic observations. This approach directly deconvolves a linear mosaic of dirty images. We consider the advantages and limitations of this technique. Projection issues for mosaicing are also discussed. We present an example from a 320-pointing, spectral-line, mosaiced observation of the Small Magellanic Cloud.

Journal ArticleDOI
TL;DR: In this paper, a mathematical formalism to describe the polarimetric response of a radio interferometer was presented, and the determination of these parameters both by a traditional radio-interferometry instrumental approach as well as by using optical polarimetry principles was considered.
Abstract: In a companion paper, a mathematical formalism to describe the polarimetric response of a radio interferometer was presented. Some of the instrumental parameters, however, are either unknown or poorly known. Here we consider the determination of these parameters both by a traditional radio-interferometry instrumental approach as well as by using optical polarimetry principles. In doing so, we establish links between the two fields. We show that some degrees of freedom cannot be solved for with various calibration or self-calibration schemes. These degrees of freedom are identified with instrumental parameters and physical source properties. The number of unsolvable degrees of freedom is reduced for a long synthesis with alt-az antennas. We also consider the effect of errors in the assumed instrumental parameters on the resultant calibrated data. The polarimetric calibration procedure for some telescopes is reviewed in the context of this analysis.

Journal ArticleDOI
TL;DR: In this paper, an atlas of time series of ultraviolet spectra is presented for 10 bright O stars, with the spectra obtained with the International Ultraviolet Explorer during seven observing campaigns lasting several days over a period of 6 years.
Abstract: An atlas of time series of ultraviolet spectra is presented for 10 bright O stars. The spectra were obtained with the International Ultraviolet Explorer during seven observing campaigns lasting several days over a period of 6 years. The UV P Cygni lines in 9 out of the 10 studied stars exhibit a characteristic pattern of variability in the form of discrete absorption components (DACs) migrating through the absorption troughs on a timescale of a day to a week. This pattern is significantly different for each star, but remains relatively constant during the time span of our observations for a given star. A quantitative evaluation of the statistical significance of the variability is given. The winds of a number of stars appear to vary over the full range of wind velocities: from 0 km s-1 up to velocities exceeding the terminal velocity of the wind as measured by the asymptotic velocity reached by DACs. The amplitude of variability reaches a maximum at about 0.75 in the unsaturated resonance lines of stars showing DACs. In saturated resonance lines we find distinct changes in the steep blue edge. This edge variability is also found, although with smaller amplitude, in unsaturated resonance lines. The subordinate line of N IV at 1718 A in ξ Per shows weak absorption enhancements at low velocities in the blue-shifted absorption that are clearly associated with the DACs in the UV resonance lines. We interpret these three manifestations of variation as reflecting a single phenomenon. The DACs are the most conspicuous form of the variability. The changes at the edge can often be interpreted as DACs, but superposed on a saturated underlying wind profile; in many cases, however, at the same time two or more absorption events in different stages of their evolution can be identified in the unsaturated profiles, hampering a detailed interpretation of the edge variability. The low velocity absorption enhancements in the subordinate lines are the precursors of DACs when they are formed close to the star. The constancy of the pattern of variability over the years and the (quasi-)periodic recurrence of DACs strongly suggest that rotation of the star is an essential ingredient for controlling wind variability. The observation of low-velocity variations in subordinate lines, which are supposedly formed at the base of the stellar wind, indicate an origin of wind variability close to or at the photosphere of the star.

Journal ArticleDOI
TL;DR: A 330-360 GHz spectral survey of the hot molecular core associated with the 'cometary' ultracompact HII region G 34.3+/-0.15 observed with the James Clerk Maxwell Telescope has detected 338 spectral lines from at least 35 distinct chemical species plus 19 isotopomers as mentioned in this paper.
Abstract: A 330--360 GHz spectral survey of the hot molecular core associated with the 'cometary' ultracompact HII region G 34.3+/-0.15 observed with the James Clerk Maxwell Telescope has detected 338 spectral lines from at least 35 distinct chemical species plus 19 isotopomers. 70 lines remain unidentified. Chemical abundance and rotation temperature have been determined by rotation diagram analysis for 12 species, and lower limits to abundance found for 38 others.

Journal ArticleDOI
TL;DR: In this article, an atlas of high-S=N, high-resolution (v 6k m s 1 ) data of Be star emission and shell proles is presented.
Abstract: We present an atlas of high-S=N , high-resolution (v 6k m s 1 ) data of Be star emission and shell proles. We have collected proles of H and of Fe ii ,m ostly of the5317 transition. These lines have been selected to provide measures for the overall emission strength and for the velocity eld in these disks. We have collected data for 77 southern and equatorial programme stars, covering the period 1982-1993. This is the most comprehensive overview of prole shapes in Be disks. We propose a three-dimensional scheme in which most observed proles can be classied. The parameters are i) inclination, ii) optical depth, and iii) the pattern of the velocity eld. A search for short-term variability (timescales between ve days and a few minutes) in six stars ended with negative result. Shortest observed timescale for variability is a few days for well-developed disks in binary systems (HR 1910, HR 2142).

Journal ArticleDOI
TL;DR: In this paper, the authors presented detailed JHK surface photometry with ellipse ts of 13 galaxies selected from previous optical observations as likely candidates for having a secondary bar or a triaxial bulge within the primary bar.
Abstract: We present detailed JHK surface photometry with ellipse ts of 13 galaxies selected from previous optical observations as likely candidates for having a secondary bar or a triaxial bulge within the primary bar. We have found 7 double-barred galaxies, 3 double-barred galaxies with an additional intermediate structure with twisted isophotes, and 3 galaxies with a bar and central twisted isophotes. A global analysis of the structural parameter characteristics in the I -a nd K-bands is presented. Various numerical models of galaxies with bars within bars are also analysed using the ellipse tting technique and compared to the observations. A thorough review of the possible hypotheses able to explain this phenomenon is given with emphasis on the most likely ones.

Journal ArticleDOI
Abstract: GAIA is a preliminary concept for an astrometric mission, recently recommended within the context of ESA's `Horizon 2000 Plus' long-term scientific programme. In its present form, the experiment is estimated to lead to positions, proper motions, and parallaxes of some 50 million objects, down to about mag, with an accuracy better than 10 microarcsec, along with multi-colour multi-epoch photometry of each object. The scientific case for such a mission is dramatic: distances and kinematical motions for tens of millions of objects, throughout our Galaxy, would be obtained—the expected accuracy is such that direct (trigonometric) distance estimates to the galactic centre would be accurate to 10%, with transverse motions accurate to about 1 km s-1 at 20 kpc. As `by-products', the global measurements would yield unprecedented information on the space-time metric (γ to a precision of about 1 part in 106 or better, close to values which might distinguish currently competing theories of gravity), angular diameters of hundreds of stars, and a vast body of information on double and multiple systems. Perhaps the most dramatic of these subsidiary goals would be the possibility of screening some stars within 100 pc for periodic photocentric motions, which would provide the most powerful and systematic method of detecting possible planetary companions proposed to date.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the infrared (IR) photometric system for the single channel photometers at ESO, which have been used from 1983 until 1994, and they also extend the set of NIR standard stars by Bouchet et al. towards fainter objects.
Abstract: We describe the infrared (IR) photometric system for the single channel photometers at ESO, which have been used from 1983 until 1994. In addition to the broadband near infrared (NIR, 1 5 m) photometric system presented in 1991 by Bouchet et al. and Bersanelli et al., we describe a narrow-band NIR photometric system and a mid infrared (MIR, 7 20 m) photometric system. We also extend the set of NIR standard stars by Bouchet et al. towards fainter objects (K'9). The photometric data of the standard stars in these systems were extracted from the complete IR photometric data archive of ESO, covering 10 years. The zeropoints of the NIR photometry are set by assuming that HR 3314 has a V -magnitude of 3.89, and that V K= 0.05, J K= 0.01, H K= 0.01, K L 0 =0.00, K M=0.00. The zeropoints of the MIR photometry are set by assuming that the colours of Hyi (HR 0098) and CenA (HR 5459) are equal to the colours of the Sun. We adopt the absolute calibration of M egessier (1995) for the NIR and we argue that this calibration can be extrapolated to 20 m, using the MIR calibrations by Rieke et al. (1985) and Cohen et al. (1992). The denition of the zeropoints is consistent with the absolute calibration. We obtained accurate (' 0.02 mag.) NIR photometry of about 240 standard stars and MIR photometry of about 40 standard stars (' 0.04 mag). Comparison of our NIR photometric system with other well established systems shows that there are some small colour dependencies and zeropoint osets which are always smaller than about 0.02 mag. except for the L 0 band.

Journal ArticleDOI
Abstract: We describe a procedure that we have implemented to use epoch folding to estimate pulse period, shape, amplitude and phase (with uncertainties) for coherent oscillations in time series data. We improve on traditional techniques by fitting the as a function of test period with a response function which takes into account both data sampling and the oscillation pulse shape. It is shown that the epoch folding procedure makes optimum usage of the full pulse shape information, or equivalently all its Fourier components, in the period determination. An analytic expression for the period error is also given for this general, non-sinusoidal, case. The error estimate, which is equivalent to that for a least-squares fit of a set of harmonically related Fourier components, is verified by Monte Carlo simulations.

Journal ArticleDOI
TL;DR: In this article, the authors search for evidences for the presence of unseen companions in some eclipsing binary systems using their O-C diagrams, assuming that light-time effect (LITE) is responsible for the periodic components on the O -C diagrams sets of orbital parameters.
Abstract: The aim of this paper is to search for evidences for the presence of unseen companions in some eclipsing binary systems using their O–C diagrams. A total of 8507 times of minima (6890 visual and 1617 photographic or photoelectric ones) of 18 systems have been collected from the remarkable database of late Dieter Lichtenknecker (Lichtenknecker 1988) and from other more recent papers. Assuming that light-time effect (LITE) is responsible for the periodic components on the O–C diagrams sets of orbital parameters were derived using Kopal's (1978) method. In some cases (AB And, TV Cas, XX Cep, AK Her) the resulting orbital parameters and masses of the hypothetical third components are physically acceptable. Other systems show good evidence for multiplicity, but the available data sets are not suited to compute reliable solutions (W Del, U Peg, AT Peg, ST Per). For RT And, XZ And, OO Aql, Y Cam, RS CVn, CQ Cep, U CrB and MR Cyg) an unambigous identification of third components is not possible. In the cases of TW Cas and SW Lac we could not find any LITE solution at all.

Journal ArticleDOI
TL;DR: In this article, the authors present VR cI c Johnson-Cousins magnitudes of comparison stars for twelve BL Lacertae objects belonging to the largest sample of blazars which is monitored at the Perugia University Observatory.
Abstract: W e presentVR cI c Johnson-Cousins magnitudes of eld comparison stars for twelve BL Lacertae objects belonging to the largest sample of blazars which is monitored at the Perugia University Observatory. The sequences reported here can be used in the long{term CCD photometric monitoring of BL Lac objects aimed to understand the characteristics of their variability. Finding charts for all of these comparison sequences are also included in this paper.

Journal ArticleDOI
TL;DR: In this article, the relativistic effects have been taken into account in the computations carried out with the help of two independent computer programs, SUPERSTRUCTURE (SST) and RELATIVISTIC HARTREE-FOCK (HFR).
Abstract: Radiative transition probabilities have been calculated for the magnetic dipole (M1) and electric quadrupole (E2) transitions connecting the 63 metastable levels in the 3d 6 4s, 3d 7 and 3d 5 4s 2 configurations in Fe II. The most important configuration interaction (CI) and relativistic effects have been taken into account in the computations carried out with the help of two independent computer programs, SUPERSTRUCTURE (SST) and RELATIVISTIC HARTREE-FOCK (HFR). The results obtained in the present work are compared with previous theoretical studies and with some astrophysical observations. The new data presented here are probably the most reliable to date.

Journal ArticleDOI
TL;DR: In this paper, the authors present results of the spectroscopic and photometric follow-up observations of the ROSAT all- sky survey in the direction of the Orion cloud complex.
Abstract: We present results of the spectroscopic and photometric follow-up observations of the ROSAT all- sky survey in the direction of the Orion cloud complex. The main goal of these observations is the search for X-ray emitting pre-main sequence stars. 820 X-ray sources were detected with high condence in about 450 square degrees. The mean density of X-ray sources in this region is a factor of about two higher than that of the whole RASS. 5% of the RASS sources in this region are identied with previously known and likely pre-main sequence stars. We have investigated spectroscopically 181 new RASS sources widely distributed over the entire cloud complex. On the basis of the presence of strong Li I 6707 absorption, spectral type later than F0 and chromospheric emission, 112 new weak-line T Tauri stars could be found. We present coordinates, X-ray count-rates and nding charts of the new PMS. Optical UBV(RI)KC, near-infrared JHKLM and uvby- photometry for the new WTTS is also provided. In addition 24 dKe-dMe stars were also found on the basis of the RASS data.