Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Surface density of extremely red objects with R - J ≥ 5
We present the results of a wide-field survey for extremely red objects(EROs), based on J and R band imaging. The survey covers 2.89deg2, consisting of 50 single fields, each approximately 210arcmin2 in size. This survey provides a sample of 60 extendedobjects with a colour of R-J ≥ 5 and J band magnitudes more than10σ above background (18^mag≤ J ≤ 20^mag). Thirty five ofthese objects have been detected in both J and R-band. We derive asurface density for such EROs of 5.7±0.1×10-3arcmin-2, which is about seven times lower than for galaxieswith R-K≥6. Stellar population models suggest that these EROs aremassive galaxies with an old stellar population in a redshift range of1.4≤ z ≤ 3.0. For this scenario, we estimate the co-movingvolume density to be (9.57 ± 1.24) ×10-6Mpc-3.
| A photometric survey of stars with circumstellar material
We present the result of a follow-up Strömgren photometric surveyof sixteen southern bright stars with circumstellar material, in orderto detect possible weak photometric variations. We found new variationsof the β~ Pictoris brightness from 1999 to 2002 with a weaklong-term variation of ~-0.8× 10-3 mag per year, overabout 3 years. These variations look similar to those seen from 1975 to1981 and from 1995 to 1998 (Nitschelm et al. 2000, A&AS, 145, 275).They can be due to differential occultation by dust inhomogeneitiestransiting the star through the years. We detected new periodicvariations for HD 256 (HR 10) with periods ranging from 0.35 day to 6.69days during several months. These variations may also be interpreted interms of variable obscuration due to structures in the circumstellardisk suspected to be surrounding this star.
| A Search for Sodium Absorption from Comets around HD 209458
We monitored the planet-bearing solar-type star HD 209458 for sodiumabsorption in the region of the stellar Na I D1 line that would beindicative of cometary activity in the system. We observed the starusing the Hobby-Eberly Telescope High Resolution Spectrograph with ahigh signal-to-noise ratio (S/N) and high spectral resolution for sixnights over the course of 2 years, from 2001 July to 2003 July. Frommodeling we determine a 20% likelihood of a detection, based on apredicted number of comets similar to that of the solar system. We findthat our analytical method is able to recover a signal and that our S/Nis sufficient to detect this feature in the spectral regions on eitherside of the core of the D1 line, where it is most likely to appear. Nosignificant absorption was detected for any of the nights based on a 3σ detection limit. We derive upper limits on the column density ofsodium of <~6×109 cm-2 for a signal inthe region around the line core and <~2×1010cm-2 for a signal in the core of the photospheric D1 line.These numbers are consistent with the sodium released in a singleperiodic comet in our own system, although a higher S/N may be necessaryto uncover a signal in the core of the D1 line. Implications forcometary activity in the HD 209458 system are discussed.
| Classification of 2.4-45.2 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer
The Infrared Space Observatory observed over 900 objects with the ShortWavelength Spectrometer in full-grating scan mode (2.4-45.2 μm). Wehave developed a comprehensive system of spectral classification usingthese data. Sources are assigned to groups based on the overall shape ofthe spectral energy distribution (SED). The groups include naked stars,dusty stars, warm dust shells, cool dust shells, very red sources, andsources with emission lines but no detected continuum. These groups arefurther divided into subgroups based on spectral features that shape theSED such as silicate or carbon-rich dust emission, silicate absorption,ice absorption, and fine-structure or recombination lines. Caveatsregarding the data and data reduction, as well as biases intrinsic tothe database, are discussed. We also examine how the subgroups relate tothe evolution of sources to and from the main sequence and how thisclassification scheme relates to previous systems. Based on observationswith the Infrared Space Observatory (ISO), a European Space Agency (ESA)project with instruments funded by ESA Member States (especially thePrinciple Investigator countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of the Institute of Space andAstronautical Science and the National Aeronautics and SpaceAdministration (NASA).
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphere
Within the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105
| The Radio-to-Submillimeter Flux Density Ratio of Galaxies as a Measure of Redshift
We re-examine the technique of determining the redshifts of galaxiesfrom the ratio of the submillimeter-to-radio continuum flux densitiesbased on a recently published catalog of 850µm sources. We derivedthe expected variation of this ratio as a function of redshiftincorporating the expected average luminosity and the spectral energydistribution (SED) of dust emission and the radio continuum. We findthat the existing data for most of the high redshift (z gtsim 1) sourcescorrespond to our new calculation. Amongst the well-identified sources,there is none with an index significantly higher than predicted. Sourceswhich have an index lower than predicted are either within the errorzone or much lower, the latter presumably having AGN-dominatedradio-continuum emission. We find the median redshift to be ~ 2 which isconsistent with that deduced by previous work (~ 3) within the error. Wealso discuss the various systematic effects that can affect the accuracyof the redshift estimate. We examine other methods of redshiftestimation, like photometric ratio in the submillimeter and locating thepeak of the SED in the rest system of the objects. We conclude thatwhile the various methods are helpful in identifying high-redshiftobjects and making a crude estimate of the redshift, they are not, atpresent, accurate enough for a detailed study of redshift distributionof the submillimeter galaxies.
| Dusty Circumstellar Disks
Dusty circumstellar disks in orbit around main-sequence stars werediscovered in 1983 by the infrared astronomical satellite. It was thefirst time material that was not another star had been seen in orbitaround a main-sequence star other than our Sun. Since that time,analyses of data from the infrared astronomical satellite, the infraredspace observatory, and ground-based telescopes have enabled astronomersto paint a picture of dusty disks around numerous main-sequence andpost-main-sequence stars. This review describes, primarily in anevolutionary framework, the properties of some dusty disks orbiting,first, pre-main-sequence stars, then main-sequence andpost-main-sequence stars, and ending with white dwarfs.
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable 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/379/564
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available 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/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| 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
| Falling Evaporating Bodies around Herbig stars. A theoretical study
Transient spectral absorption events monitored now for years towards thestar beta Pictoris have been interpreted as resultingfrom the transit across the line of sight of evaporating star-grazingkilometer-sized bodies (Falling Evaporating Bodies, or FEBs). SeveralHerbig Ae/Be stars of various ages have been observed to exhibit somehowsimilar absorption events that have been attributed to similar FEBevents. We investigate here this question from a modeling point of view.Adapting the FEB simulation code we had developed earlier specificallyfor beta Pic, to the case of typical Herbig Ae/Bestars, we try to derive in which conditions FEB-like objects maygenerate detectable transient absorption events. We compare theseconditions with those found in the case of beta Pic.A major difference with beta Pic is that Herbig Ae/Bestars have strong stellar winds (10-9-10-7 M_sunyr-1). Those winds appear to have a drastic interaction withthe gaseous material escaped from the FEBs. With the presence of suchstellar winds, the spectral signatures of FEBs are not detectable,unless their mass loss rate is huge. This translates into very largebodies (~100 km size), instead of ~15 km for beta PicFEBs. This appears unrealistic in terms of amount of planetesimal massneeded in the disks surrounding these stars. We discuss then thevalidity of the FEB hypothesis for specific example stars. It turns outthat for the younger (a few 106 yr old) Herbig Ae/Be starslike AB Aur, with well identified winds gtrsim10-8 M_sun yr-1, the variable features sometimesobserved are not likely to be due to FEBs, unless produced in wind freecavities. For older (gtrsim 107 yr old) stars Herbig Ae/Belike HD 100546, the FEB scenario could still explainthe spectral events observed, but either the wind must to be weaker than~m10-10 M_sun yr-1 (which cannot be excluded sofar), or the FEBs approach the star in wind free cavities.
| EXPORT: Near-IR observations of Vega-type and pre-main sequence stars
We present near-IR JHK photometric data of a sample of 58 main-sequence,mainly Vega-type, and pre-main sequence stars. The data were takenduring four observing runs in the period May 1998 to January 1999 andform part of a coordinated effort with simultaneous optical spectroscopyand photo-polarimetry. The near-IR colors of the MS stars correspond inmost cases to photospheric colors, although noticeable reddening ispresent towards a few objects, and these stars show no brightnessvariability within the observational errors. On the other hand, the PMSstars show near-IR excesses and variability consistent with previousdata. Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110
| Subaru First-Light Deep Photometry of Galaxies in A 851 Field
We report on the results of optical and near-infrared photometry of thecluster of galaxies A 851 (=CL 0939+4713) carried out as one of thefirst-light observations of the 8.2 m Subaru telescope on Mauna Kea.Images as sharp as 0.''3 FWHM in K' and 0.''45 in R were obtained inthese observations. The 3 sigma limiting magnitudes for points sourcesin 0.''3 seeing with a 0.''6 software aperture were shown to go down toR=28.1, J=25.1, and K'=24.0, respectively, for about 1 hour integration.Subaru photometry of the galaxies in this cluster has shown in itscolor-magnitude diagrams a well-defined sequence of the early-typegalaxy population that is consistent with the track predicted for asingle-burst passive-evolution model of galaxies at z = 0.4. Weattempted a morphological classification of galaxies in the SubaruR-band image using the C_in method, and found that discriminationbetween ellipticals and spirals can be achieved fairly consistently withthe types assigned by MORPHs on the HST F702W image down to R<23. Aweak lensing analysis made on the Subaru R-band image yielded areconstructed surface mass-density distribution that shows a significantmaximum corresponding to the peak of the smoothed luminositydistribution of cluster galaxies. We found no significant excess offaint and/or small galaxies in the putative cluster area around the z=2quasar and no difference in the colors for galaxies in this area fromthose in the main cluster either. Two extremely red objects (ERO) withR-K>6 are newly identified in the field. The colors of the reddest,disk-shaped galaxy R1 are found compatible with those of an unreddenedE/S0 galaxy at z = 1.6.
| A three-year Strömgren photometric survey of suspected beta Pictoris-like stars
We carried out a Strömgren photometric survey of thirteen southernbright stars, including beta Pictoris itself, during three years,d'Astrophysique de Paris, in order to detect possible weak photometricvariations. beta Pictoris presents a small long-term variation with achange of brightness by -2.1 10-3 mag per year, over abouttwo years from beginning of 1996, a situation relatively similar to theone about 18 years ago. Among the other stars, only HD 38392 presentsweak photometric variations with a period of 21.4 days, probably relatedto the star rotation period. The negative result concerning photometricvariations of all other stars suggests that those stars are actuallyreally stable and strengthens the reality of the variations discoveredin the case of beta Pictoris and HD 38392. Based on observationsobtained at the Danish 50 cm telescope (SAT) at ESO, La Silla, Chile.
| A-shell stars in the Geneva system
Among the various kinds of A stars having a peculiar spectrum, we findthe A-shell stars. Many questions are still open concerning these stars,including their evolutionary status. In the present study we have useddata from the Hipparcos catalogue to examine this point. We have foundthat the majority of A-shell stars are well above the main sequence. Nodifferences could be established between A-shell stars in luminosityclasses III and I and those in luminosity class V as regardsvariability, duplicity, or the importance of the shell feature.
| Dust emission from quasars and quasar host galaxies
We test emission models of circumnuclear dust tori around quasars, atlow and high redshifts, by using a large collection of photometric datafor an unbiased sample of 120 optically selected objects withmillimetric and submillimetric fluxes, including new unpublished data.Under the assumption that the dust is heated by a point-like source witha power-law primary spectrum, as defined by the observedoptical-ultraviolet continuum, we infer the basic model parameters, suchas dust masses, temperature distributions and torus sizes, bynumerically solving the radiative transfer equation in the dustdistribution. In addition to the substantiated statistics, an essentialimprovement over previous analyses comes from the use ofoptical-ultraviolet data to constrain the primary illuminatingcontinuum, which is needed to estimate dust temperatures and sizes. Thedependences of the best-fitting parameters on luminosity and redshiftare studied and the contribution of dust in the host galaxy to theobserved fluxes is briefly mentioned. This analysis constrains theproperties of the enriched interstellar medium in the galaxies hostingthe quasars. The dust abundance does not display appreciable trends as afunction of redshift, from z ~= 1 to almost 5, and shows that at theseearly epochs dust and metals are at least as abundant as, and often moreabundant than, they are in local Galactic counterparts. Thisevolutionary pattern is remarkably at variance with respect to what isexpected for disc galaxies, like the Milky Way, slowly building metalsduring the whole Hubble time. It rather points in favour of a much moreactive phase of star formation at early epochs, probably related to theformation of massive spheroidal galaxies.
| Radial velocities of HIPPARCOS southern B8-F2 type stars
Radial velocities have been determined for a sample of B8-F2 type starsobserved by the Hipparcos satellite. Observations were obtained withinthe framework of an ESO key-program. Radial velocities have beenmeasured using a cross-correlation method, the templates being a grid ofsynthetic spectra. The obtained precision depends on effectivetemperature and projected rotational velocity of the star as well as ona possible asymmetry of the correlation peak generally due to secondarycomponents. New spectroscopic binaries have been detected from theseasymmetries and the variability of the measured radial velocity.Simulations of binary and triple systems have been performed. Forbinaries our results have been compared with Hipparcos binary data.Adding the variable radial velocities, the minimum binary fraction hasbeen found 60% for physical systems. Radial velocities have beendetermined for 581 B8-F2 stars, 159 being new. Taking into accountpublished radial velocities, 39% south A-type stars with V magnitudelower than 7.5 have a radial velocity. Based on observations obtained atthe European Southern Observatory (ESO, La Silla, Chile) and on datafrom the ESA Hipparcos astrometry satellite.}\fnmsep \thanks{Tables 7, 8and 9 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Observations of BN and AN stars: New Be stars
From a survey of spectra of Bn/An stars, we have detected seven new Bestars: HR 1056 (A0Vn), 1544 (A1Vn), 2191 (A0Vnn), 2300 (B8Vn), 3134(B9.5 Vn), 3878 (B0.5 IIIn) and 4552 (B9IIIpSi). H_alpha profiles ofthese stars are presented. Measured values of equivalent widths, fullwidths at half intensity maximum, and the peak-separations of theH_alpha emission profiles are also tabulated in this paper. We have alsocomputed the radii of emission disks of the newly detected Be stars.
| A search for circumstellar gas around normal A stars and Lambda Bootis stars
We have searched for interstellar or circumstellar absorption lines inthe center of Ca II K towards bright A-type stars that are mostly within80 pc of the Sun. Narrow absorption features are found in about 30 % ofthe 28 normal main-sequence A stars and 18 metal-deficient lambda Bootisstars studied. We have determined surface gravities and projectedrotational velocities. Most of the stars with detectable Ca K featureshave comparatively low gravities and high projected rotationalvelocities. This correlation with stellar properties implies that mostof the narrow absorption features are of circumstellar rather thaninterstellar origin. The preference of low gravity and rapid rotationfurthermore suggests that most of the gas shells around A stars developin the pre-main-sequence phase of evolution, and disappear largelybefore the star arrives at the ZAMS. Among the normal A stars studied,about 50 % are known to have dust disks. Unlike A stars withcircumstellar gas, these dusty stars do not prefer low log g and high vsin i. This results in an apparent lack of correlation between gas anddust, and indicates that normal A stars with gas shells and those withdust disks are not in the same evolutionary stage. We conjecture thatdust disks tend to develop after most of the gas has disappeared. Basedon observations collected at the European Southern Observatory, LaSilla, Chile
| β Pictoris, a young planetary system? A review
β Pictoris is a bright southern hemisphere star observed in 1983 bythe IRAS satellite as presenting a large and unexpected IR excess. Thisexcess was called the Vega-like phenomenon and quickly identified as dueto circumstellar dust. Subsequently in 1984, using stellar coronography,dust was also directly seen as an edge-on disk extended to severalhundreds of AUs. Since then, β Pictoris has been continuouslyobserved. We present here a review of our present understanding of theβ Pictoris circumstellar environment which still appears unique inthe solar neighborhood. The circumstellar dust disk is predominantlymade of relatively large particles (one micron or more) extendingoutward to more than 1000 AU and presenting a clearer (dust free)central region away to about 35 AU from the star. The gas is detectedthrough stable and variable spectroscopic signatures revealing apermanent gas disk with sporadic inflows and also a few outflows. Theseare partially interpreted in terms of evaporation of kilometer-sizedbodies very close to the star. Evaporation or destruction throughcollisions of kilometer-sized bodies seems to be needed also to explainboth the dust as well as the very presence of the CO molecule detectedin the circumstellar gas. Several indirect arguments along with theobservation of a very peculiar photometric variation of the starsuggests that even giant planets may have already formed in the βPictoris system. β Pictoris is thus possibly the missing linkbetween young stellar objects presenting proto-planetary circumstellardisks and much more evolved systems in which planets (at least giantones) are already formed. β Pictoris is probably a unique placewhere we may now observe planetary formation as well as other phenomenathat have taken place in the first 108 years of a youngstellar system.
| Infrared Photometry of beta Pictoris Type Systems
We obtained small-aperture (4"-5" diameter) infrared (2-20 μm)photometry of 10 early-type main-sequence stars with infrared excessesfrom circumstellar dust. These systems possibly exemplify the betaPictoris phenomenon. We observed them with either the NASA MarshallSpace Flight Center bolometer array camera (``Big Mac'') or the InfraredTelescope Facility 2-30 μm single-channel bolometer system.Measurements were obtained in the KLMNQ filters and the narrowband(Deltalambda ~ 1 μm) 10 μm ``silicate'' filters. We fitted Kuruczphotospheric models to the photometric data to determine excess-emissionspectra. We report the nondetection of small-aperture circumstellar dustemission from HR 10 and 21 LMi. We confirmed previous nondetections ofnear-infrared or 10 μm excess emission from 68 Oph, alpha PsA, and HR4796A. We did not detect prominent silicate emission from any of thesources. The spectra of gamma Oph, sigma Her, HR 2174A, beta UMa, andzeta Lep show weak 10 μm excesses. We fitted simple models to thesedata, together with IRAS excess fluxes, to determine plausibledistributions of temperature and density of circumstellar dust grains.Significant quantities of these grains around HR 2174A, zeta Lep, andbeta UMa are at temperatures similar to terrestrial material in thesolar system.
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Beta Pic-like circumstellar disk gas surrounding HR 10 and HD 85905
We present high spectral resolution observations of the absorption linesof Ca II and Na I associated with the circumstellar gas disk surroundingthe two A-type shell stars HR 10 and HD85905. Data taken over two four-night periods in January andNovember 1997 reveal substantial changes in the circumstellar absorptionline profiles between successive observations of both stars. Suchvariable features have both blue and red-shifted velocities up to 50 kms(-1) away from the central absorbing component, and are similar tothose routinely observed in the beta Pictoris system. The sporadicpresence of the circumstellar absorption components observed towardsboth HR 10 and HD 85905 may be explained by the infalling evaporatingcomet model developed for the beta Pictoris system by Beust {et al./}(\cite{beust90}). We note that variable circumstellar absorptionfeatures have also been detected in rapidly rotating A-type stars, suchthat they may be suffering irregular mass-loss that could give rise tosimilar circumstellar disks and shells.
| Les disques autour des etoiles de la sequence principale.
Not Available
| Hot Inner Disks that Appear and Disappear around Rapidly Rotating A-Type Dwarfs
At any one time, approximately one-quarter of the most rapidly rotatingnormal A-type dwarfs (V sin i >= 200 km s-1) show shell lines of TiII in the near-ultraviolet. Our observations during 22 years show thatthe lines appear and disappear on timescales of decades but do notdisplay significant changes within 1 year. This implies that they arenot remnants of the star formation but rather are probably caused bysporadic mass-loss events. A working hypothesis is that all A-type starsthat are rotating near their limits have these shells, but for onlyone-quarter of the time. Because these lines do not appear in stars withsmaller sin i, the shells must be disks. These are hot inner disks thatmay or may not be related to the cool outer disks seen by Smith andTerrile around beta Pic or through infrared excesses around Vega andother A-type dwarfs. The similar, limited line widths indicate that thedisks are ~7 R* above the stellar surfaces.
| Beta Pictoris light variations. I. The planetary hypothesis
The beta Pic disk is probably a young planetary system in theclearing-out phase and similar to ours 4 billion years ago. Theunderstanding of that system may shed light on the origin and evolutionof our own planetary system. A very important question is related to thepresence of large bodies, from kilometer size to planets. It is shownthat many indirect arguments seem to indicate that even large planetsmust be already formed within the system. Because it is seen nearlyedge-on, photometric observations of the star were carried on in orderto detect some signatures of inhomogeneities within the dust disk. Thisedge-on geometry is also very favorable to detect an eventualoccultation by an object orbiting the star. An exceptional andsignificant photometric event was observed on Julian Day 2444918 (Nov10, 1981), when the lightcurve shows a brightening during about 10 dayswith a central dip during less than one day. We discuss several possibleexplanations. The two most likely ones are: (1) occultation by a planetthat is located in the dust disk, with a dust-free area around theplanet, (2) the passage of a large cloud of dust with a highly forwardpeaked scattering in front of the star. In this paper we model thepredicted lightcurve for the occultation by a planet in a dust ring. Themodel takes into account the partial occultation phase and thelimb-darkening effect. Even fine details of the light curve can beexplained by this model. We find that the planet is of about Jupitersize and orbits the star at a distance of about 5 AU. We discuss thestrong and the weak points of this model. The model of the forwardscattering dust cloud is studied in a separate paper.
| HST-GHRS observations of candidate β Pictoris-like circumstellar gaseous disks.
We present HST-GHRS observations of four stars in search of βPictoris-like circumstellar gaseous disks. We detected gas around HR 10,HR 2174 and 51 Oph at large distances from the stars; this gas iscircumstellar since the absorption lines from very excited levelsrequire densities incompatible with those found in the diffuseinterstellar medium. The shape of the lines and the Mgii doublet ratiogive evidence that clumpy gas is continuously falling onto these stars.Possible interpretations are discussed.
| Nebuleuse Helix: la fin d'un systeme solaire.
Not Available
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Ballena |
| Ascensión Recta: | 00h07m18.20s |
| Declinación: | -17°23'11.0" |
| Magnitud Aparente: | 6.19 |
| Distancia: | 160.256 parsecs |
| Movimiento Propio en Ascensión Recta: | -11.3 |
| Movimiento Propio en Declinación: | 17.1 |
| B-T magnitude: | 6.345 |
| V-T magnitude: | 6.218 |
Catálogos y designaciones:
|
