Contents
Images
Upload your image
DSS Images Other Images
Related articles
Transit Detectability of Ring Systems around Extrasolar Giant Planets
We investigate whether rings around extrasolar planets could be detectedfrom those planets' transit light curves. To this end, we develop abasic theoretical framework for calculating and interpreting the lightcurves of ringed planet transits on the basis of the existing frameworkused for stellar occultations, a technique that has been effective fordiscovering and probing ring systems in the solar system. We find thatthe detectability of large Saturn-like ring systems is largest duringingress and egress and that a reasonable photometric precision of~(1-3)×10-4 with 15 minute time resolution should besufficient to discover such ring systems. For some ring particle sizes,diffraction around individual particles leads to a detectable level offorward-scattering that can be used to measure modal ring particlediameters. An initial census of large ring systems can be carried outusing high-precision follow-up observations of detected transits and bythe upcoming NASA Kepler mission. The distribution of ring systems as afunction of stellar age and as a function of planetary semimajor axiswill provide empirical evidence to help constrain how rings form and howlong rings last.
| Latitudinal transport by barotropic waves in Titan's stratosphere. II. Results from a coupled dynamics-microphysics-photochemistry GCM
We present a 2D general circulation model of Titan's atmosphere,coupling axisymmetric dynamics with haze microphysics, a simplifiedphotochemistry and eddy mixing. We develop a parameterization oflatitudinal eddy mixing by barotropic waves based on a shallow-water,longitude-latitude model. The parameterization acts locally and in realtime both on passive tracers and momentum. The mixing coefficient variesexponentially with a measure of the barotropic instability of the meanzonal flow. The coupled GCM approximately reproduces the Voyagertemperature measurements and the latitudinal contrasts in thedistributions of HCN and C2H2, as well as the mainfeatures of the zonal wind retrieved from the 1989 stellar occultation.Wind velocities are consistent with the observed reversal time of theNorth-South albedo asymmetry of 5 terrestrial years. Model resultssupport the hypothesis of a non-uniform distribution of infrared opacityas the cause of the Voyager temperature asymmetry. Transport by the meanmeridional circulation, combined with polar vortex isolation may be atthe origin of the latitudinal contrasts of trace species, with eddymixing remaining restricted to low latitudes most of the Titan year. Weinterpret the contrasts as a signature of non-axisymmetric motions.
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| Saturn's F Ring: Kinematics and Particle Sizes from Stellar Occultation Studies
The occultation of GSC5249-01240 by Saturn's rings was observed in aspectrally resolved mode using the Faint Object Spectrograph on theHubble Space Telescope. By combining these data with other occultationdata, we have determined the inclination of the F ring to be0.0065+/-0.0014 deg. Our inclined F ring orbit model explains an abruptdecrease in flux at the west ansa of the ring that was observed duringthe November 1995 ring-plane crossing as well as the nondetection of1995 S5 during one set of observations in November 1995. The F ring'sequivalent depth is found to have no significant dependence onwavelength between 0.27 and 0.74 μm, indicating the presence of apopulation of ring particles larger in size than ~10 μm. Thiscontrasts with the results from a previous analysis of Voyager images atvarious phase angles, in which the particles were determined to bepredominantly submicron in size. The difference may be due to temporalor longitudinal variability within the ring. .
| The Structure of Titan's Stratosphere from the 28 Sgr Occultation
A dozen lightcurves obtrained during the ground-based observations ofthe occulation of 28 Sgr by Titan (3 July 1989) are reanalyzed. Profilesof density and temperature between altitude levels z of 290 and 500 km(pressures p from 110 to 1.4 μbar) are derived. A mean number-density scale height of 50.5+/-1.4 km is found with no significantdifference between immersion and emrsion. Two in-version layers areobserved at 425 and 450-455 km, respectively (p ~7 μbar and p ~4μbar), with an increase in temperature of about 10 K in less thanΔz=10 km. These layers are visible both at immersion and atemersion, at latitudes ranging from 46 degS to 20 degN, and are thusglobal features of the stratosphere. The profiles of temperaturegradients exhibit a clear cutoff at the adiabatic lapse rate, indicatingthat fluctuations lead to marginal convective instabilities. Althoughray crossing can also cause an apparent cut-off of the temperaturegradients, we estimate it probably does not play an important role inthe observed cutoff, at least for the larger structures under study. Thevertical power spectra of fluctuations show a general power lawbehavior, with an exponent close to -3, between vertical wavelengths of~5 and 50 km. The finite stellar diameter and ray crossings can distortthe real spectra, and we can only conclude that the original powerspectra have slopes between -2 and -3. The horizontal structure of theatmosphere exhibits typical aspect (horizontal-to-vertical) ratios of15-45, with a tail in the distribution with values as high as 100-200for some structures. Finally, the horizontal spectrum of fluxtuations isa power law with an exponent close to -4 (between horizontal wavelengthsof ~25 and 250 km), if we assume it is separable from the verticalspectrum.
| The effective temperature scale of giant stars (F0-K5). I. The effective temperature determination by means of the IRFM
We have applied the InfraRed Flux Method (IRFM) to a sample ofapproximately 500 giant stars in order to derive their effectivetemperatures with an internal mean accuracy of about 1.5% and a maximumuncertainty in the zero point of the order of 0.9%. For the applicationof the IRFM, we have used a homogeneous grid of theoretical modelatmosphere flux distributions developed by \cite[Kurucz (1993)]{K93}.The atmospheric parameters of the stars roughly cover the ranges: 3500 K<= T_eff <= 8000 K; -3.0 <= [Fe/H] <= +0.5; 0.5 <= log(g) <= 3.5. The monochromatic infrared fluxes at the continuum arebased on recent photometry with errors that satisfy the accuracyrequirements of the work. We have derived the bolometric correction ofgiant stars by using a new calibration which takes the effect ofmetallicity into account. Direct spectroscopic determinations ofmetallicity have been adopted where available, although estimates basedon photometric calibrations have been considered for some stars lackingspectroscopic ones. The adopted infrared absolute flux calibration,based on direct optical measurements of stellar angular diameters, putsthe effective temperatures determined in this work in the same scale asthose obtained by direct methods. We have derived up to fourtemperatures, TJ, TH, TK and T_{L'},for each star using the monochromatic fluxes at different infraredwavelengths in the photometric bands J, H, K and L'. They show goodconsistency over 4000 K, and there is no appreciable trend withwavelength, metallicity and/or temperature. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of error and their effect on final temperatures. Wealso provide a comparison of the results with previous work.
| Catalogs of temperatures and [Fe/H] averages for evolved G and K stars
A catalog of mean values of [Fe/H] for evolved G and K stars isdescribed. The zero point for the catalog entries has been establishedby using differential analyses. Literature sources for those entries areincluded in the catalog. The mean values are given with rms errors andnumbers of degrees of freedom, and a simple example of the use of thesestatistical data is given. For a number of the stars with entries in thecatalog, temperatures have been determined. A separate catalogcontaining those data is briefly described. Catalog only available atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Stellar Occultation Observations of Saturn's North-Polar Temperature Structure
We have observed a stellar occultation of GSC5249-01240 by Saturn'snorth polar region on November 20, 1995 from NASA's Infrared TelescopeFacility (IRTF). This is the first recorded occultation by the polarregion of a giant planet. The occulted region extends 88 km in verticalheight and 660 km in horizontal length, over a region from 82.5 deg to85 deg in planetocentric latitude and from 20 deg to 30 deg inplanetocentric longitude. Based on isothermal model fits to the lightcurve, we find an equivalent isothermal temperature of 130 +/- 10 K at apressure level of 1.6 +/- 0.1 mubar, which corresponds to a half-lightlatitude of 83.2 +/- 0.2 deg and longitude of 24.1 +/- 0.5 deg. Usingnumerical inversion procedures, we have retrieved the temperatureprofile of the occulted region, which suggests an increase intemperature (with radius) of 14.5 K between 6 and 10 mubar. We also findtemperature fluctuations of 1 to 5 K along the path probed by theoccultation; if the observed temperature gradients of these fluctuationsapply to the vertical direction only, then this region issuper-adiabatic. More likely, these thermal gradients are due to acombination of diffractive scintillations and horizontal temperaturevariations. Given that isothermal model fits and numerical inversionscannot separate individual contributions to observed temperaturegradients, such as from vertical variations, horizontal variations, andscintillations, this occultation requires further study.
| Broad-band JHK(L') photometry of a sample of giants with 0.5 > [Fe/H] > -3
We present the results of a three-year campaign of broad-band photometryin the near-infrared J, H, K and L' bands for a sample of approximately250 giant stars carried out at the Observatorio del Teide (Tenerife,Spain). Transformations of the Telescopio Carlos Sanchez systeminto/from several currently used infrared systems are extended to theredward part of the colour axis. The linearity of our photometric systemin the range -3 mag [Fe/H] >-3. Data of comparable quality previouslypublished have been added to the sample in order to increase thereliability of the relations to be obtained. We also provide mean IRcolours for giant stars according to spectral type.ables 1, 2 and 3 are only available in electronic form via the CDS(anonymous ftp 130.79.128.5 or http://cdsweb.u-strasbg.fr/Abstract.html
| Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants
I report on the implementation of the empirical surface brightnesstechnique using the near-infrared Johnson broadband { (V-K)} colour assuitable sampling observable aimed at providing accurate effectivetemperatures of 537 dwarfs and giants of A-F-G-K spectral-type selectedfor a flux calibration of the Infrared Space Observatory (ISO). Thesurface brightness-colour correlation is carefully calibrated using aset of high-precision angular diameters measured by moderninterferometry techniques. The stellar sizes predicted by thiscorrelation are then combined with the bolometric flux measurementsavailable for a subset of 327 ISO standard stars in order to determineone-dimensional { (T, V-K)} temperature scales of dwarfs and giants. Theresulting very tight relationships show an intrinsic scatter induced byobservational photometry and bolometric flux measurements well below thetarget accuracy of +/- 1 % required for temperature determinations ofthe ISO standards. Major improvements related to the actual directcalibration are the high-precision broadband { K} magnitudes obtainedfor this purpose and the use of Hipparcos parallaxes for dereddeningphotometric data. The temperature scale of F-G-K dwarfs shows thesmallest random errors closely consistent with those affecting theobservational photometry alone, indicating a negligible contributionfrom the component due to the bolometric flux measurements despite thewide range in metallicity for these stars. A more detailed analysisusing a subset of selected dwarfs with large metallicity gradientsstrongly supports the actual bolometric fluxes as being practicallyunaffected by the metallicity of field stars, in contrast with recentresults claiming somewhat significant effects. The temperature scale ofF-G-K giants is affected by random errors much larger than those ofdwarfs, indicating that most of the relevant component of the scattercomes from the bolometric flux measurements. Since the giants have smallmetallicities, only gravity effects become likely responsible for theincreased level of scatter. The empirical stellar temperatures withsmall model-dependent corrections are compared with the semiempiricaldata by the Infrared Flux Method (IRFM) using the large sample of 327comparison stars. One major achievement is that all empirical andsemiempirical temperature estimates of F-G-K giants and dwarfs are foundto be closely consistent between each other to within +/- 1 %. However,there is also evidence for somewhat significant differential effects.These include an average systematic shift of (2.33 +/- 0.13) % affectingthe A-type stars, the semiempirical estimates being too low by thisamount, and an additional component of scatter as significant as +/- 1 %affecting all the comparison stars. The systematic effect confirms theresults from other investigations and indicates that previousdiscrepancies in applying the IRFM to A-type stars are not yet removedby using new LTE line-blanketed model atmospheres along with the updatedabsolute flux calibration, whereas the additional random component isfound to disappear in a broadband version of the IRFM using an infraredreference flux derived from wide rather than narrow band photometricdata. Table 1 and 2 are only available in the electronic form of thispaper
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Giants with infrared excess.
We have correlated optical and infrared catalogs in order to extract alarge sample of luminosity class III stars with known infrared fluxdensities. For a non-negligible fraction of G and K giants, afar-infrared excess emission was found, starting beyond 25μm. Anexplanation in terms of present-day mass loss thus becomes unlikely,since the dust should then be warmer and the excess emission less far inthe infrared. We believe that the far-infrared excesses of theseobjects, most likely first-ascent giants, are related to the Vegaphenomenon. The dusty disks around these stars, gradually cooled downduring their main-sequence phase, could be reheated once the star leavesthe main sequence and enters the luminous post-main-sequence phase. Thefairly large sample we constructed enables us to derive an estimationfor the occurrence of excesses. This fraction of G or K giants withfar-infrared excess appears to be distinctly smaller than amongmain-sequence stars. Since the higher radiation field of giants couldlead to a larger evaporation rate of the circumstellar debris, this factdoes not conflict with our hypothesis.
| The occultation of SAO 78505 by Jupiter
On 13 December 1989, Jupiter occulted a star with visual magnitude 8.7and an estimated K magnitude of 7. We observed the event from Kitt Peak,Arizona, using a 64 x 64 infrared camera at a wavelength of 2.16microns. The resulting data on refractive defocusing of the stellarsignal give information on the temperature of the jovian atmosphere atpressures approximately 2 to 10 microbar, at near-equatorial latitudes.These are the first new stellar-occulation data for the high jovianatmosphere since the widely observed occulation of beta Sco A and C in1971. Because of improvements in instrumental capability, our data arecomparable to the best beta Sco A data though the star is six magnitudesfainter. We derive a mean atmospheric temperature of 176 +/- 12 K on alevel surface corresponding to an equatorial radius of 71,880 km and apressure of 1.8 microbar at a jovi- centric latitude of 8 deg. Thisresult complements the beta Sco results by providing improved precisionat low jovicentric latitudes where the fainter star beta Sco C was usedin 1971.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Saturn's Mesospheric Temperature from 28 Sgr Occultations
Not Available
| Photometric surveys of suspected small-amplitude red variables. 3: an AAVSO photometric photometry survey
We have carried out a survey of the photometric (V) variability of 61'known' or suspected small-amplitude red variables, mostly M giants.Approximately two-thirds appear to be variable; several suspectedvariable comparison stars have also been identified. The incidence andaverage amplitude of variability increase rapidly from spectral type M0III to M6 III.
| The occultation of 28 Sagittarii by Titan
A report is presented of the occultation of 28 Sagittarii by Titan on1989 July 3. Visual, video and photoelectric observations, somedepicting a central flash event, are reported. The opticalcharacteristics of Titan's upper atmosphere (300-700km altitude) arederived. It is postulated that the distribution of light in the centralflash pattern exhibits dispersion as a function of wavelength, and thatone unfiltered CCD video recording contains features indicative ofabsorption by methane. An upper limit for the diameter of 28 Sgr isdetermined.
| The occultation of 28 SGR by Saturn - Saturn pole position and astrometry
Saturn's ring plane-defined pole position is presently derived from thegeometry of Saturn's July 3, 1989 occultation of 28 Sgr, as indicated bythe timings of 12 circular edges in the Saturn C-ring as well as theedges of the Encke gap and the outer edge of the Keeler gap. The edgetimings are used to solve for the position angle and opening angle ofthe apparent ring ellipses; the internal consistency of the data set andthe redundancy of stations indicates an absolute error of the order of 5km. The pole position thus obtained is consistent with the pole and ringradius scale derived from Voyager occultation observations.
| An Occultation by Saturn's Rings on 1991 October 2-3 October 2-3 Observed with the Hubble Space Telescope
An occultation of the star GSC 6323-01396 (V = 11.9) by Saturn's ringswas observed with the High-Speed Photometer on the Hubble SpaceTelescope (HST) on 1991 October 2-3. This occultation occurred whenSaturn was near a stationary point, so the apparent motion of Saturnrelative to the star was dominated by the HST orbital motion (8 km/s).Data were recorded simultaneously at effective wavelengths of 3200 and7500 A, with an integration time of 0.15 s. Fifteen segments ofoccultation data, totaling 6.8 h, were recorded in 13 successive orbitsduring the 20.0 h interval from UTC 1991 October 2, 19:35 until UTC 1991October 3, 15:35. Occultations by 43 different features throughout theclassical rings were unambiguously identified in the light curve, with asecond occultation by 24 of them occurring due to spacecraft orbitalparallax during this extremely slow event. Occultation times forfeatures currently presumed circular were measured and employed in ageometrical model for the rings. This model, relating the observedoccultation times to feature radii and longitudes, is presented here andis used in a least-squares fit for the pole direction and radius scaleof Saturn's ring system.
| The occultation of 28 SGR by Titan
We present a comprehensive analysis of data obtained during the 1989July 3 occultation of 28 Sgr by Titan. The data set includes 23lightcurves from 15 separate stations, spanning wavelengths from 0.36 to0.89 micron. A detailed model of the structure of Titan's atmosphere inthe altitude range 250 to 450 km is developed, giving the distributionof temperature, pressure, haze optical depth, and zonal wind velocity asa function of altitude and latitude. Haze layers detected in Titan'sstratosphere are about one scale height higher than inferred fromVoyager data, and show a wavelength dependence indicative of particlesizes on the order of 0.1 micron. A marked north-south dichotomy in hazedensity is observed with a transition to lower density south of about-20 deg latitude. Zonal wind speeds are inferred from global distortionsfrom spherical symmetry and are of the order of 100 m/s with significantincrease toward higher latitudes. Titan's high atmosphere showssubstantial axial symmetry; the position angle of the symmetry axis isequal to the position angle of Saturn's spin axis to within about 1 deg.
| Saturn 1989
Observations of Saturn in 1989 by BAA members and correspondents arepresented.
| The Eclipse of 28-SAGITTARII by Titan
Not Available
| A critical appraisal of published values of (Fe/H) for K II-IV stars
'Primary' (Fe/H) averages are presented for 373 evolved K stars ofluminosity classes II-IV and (Fe/H) values beween -0.9 and +0.21 dex.The data define a 'consensus' zero point with a precision of + or -0.018 dex and have rms errors per datum which are typically 0.08-0.16dex. The primary data base makes recalibration possible for the large(Fe/H) catalogs of Hansen and Kjaergaard (1971) and Brown et al. (1989).A set of (Fe/H) standard stars and a new DDO calibration are given whichhave rms of 0.07 dex or less for the standard star data. For normal Kgiants, CN-based values of (Fe/H) turn out to be more precise than manyhigh-dispersion results. Some zero-point errors in the latter are alsofound and new examples of continuum-placement problems appear. Thushigh-dispersion results are not invariably superior to photometricmetallicities. A review of high-dispersion and related work onsupermetallicity in K III-IV star is also given.
| Saturn Pole Position and Ring Radius Scale from 28 Sgr Occultation
Not Available
| Results for Titan's atmosphere from its occultation of 28 Sagittarii
The occultation of the bright K giant star 28 Sgr by Titan was observedfrom three stations in the Mediterranean area, and the resulting dataset is examined here. Average mesospheric temperatures of about 180 Kare derived, with evidence for lateral and vertical atmosphericinhomogeneities on scales ranging from about 10-1000 km. The results areconsistent with published models of Titan's atmosphere.
| Planetary Satellites - an Occult View of Titan
Not Available
| Photometric observations of the occultation of 28 SGR by Titan
Titan's dense atmosphere can be studied by analyzing the starlightintensity variations observed during its recent occultation of a brightstar. An overall light curve is presented for the event, a prominentfeature is the central brightening, with a more intense burst near itscenter. The overall size of the atmosphere of Titan implied by thelength of the event indicates that it was detectable up to a height ofat least 600 km, or one-fourth of Titan's radius.
| Saturn's Rings and Atmosphere: Results from the 28 Sgr Occultation
Not Available
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Sagittarius |
| Right ascension: | 18h46m20.60s |
| Declination: | -22°23'32.0" |
| Apparent magnitude: | 5.37 |
| Distance: | 392.157 parsecs |
| Proper motion RA: | 30.7 |
| Proper motion Dec: | 2.7 |
| B-T magnitude: | 7.479 |
| V-T magnitude: | 5.56 |
Catalogs and designations:
|
INHABITED SKY
