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Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars
Context: .The X-ray emission from B-type main-sequence stars is alongstanding mystery in stellar coronal research. Since there is notheory at hand that explains intrinsic X-ray emission fromintermediate-mass main-sequence stars, the observations have often beeninterpreted in terms of (unknown) late-type magnetically activecompanion stars. Aims: .Resolving the hypothesized companionsrequires high spatial resolution observations in the infrared and inX-rays. We use Chandra imaging observations to spatially resolve asample of main-sequence B-type stars with recently discovered companionsat arcsecond separation. Methods: .Our strategy is to search forX-ray emission at the position of both the B-type primary and the faintcompanion. Results: .We find that all spatially resolvedcompanions are X-ray emitters, but seven out of eleven intermediate-massstars are also X-ray sources. If this emission is interpreted in termsof additional sub-arcsecond or spectroscopic companions, this implies ahigh multiplicity of B-type stars. Firm results on B star multiplicitypending, the alternative, that B stars produce intrinsic X-rays, cannotbe discarded. An appropriate scenario would be a magnetically confinedwind, as suggested for the X-ray emission of the magnetic Ap star IQAur. However, the only Ap star in the Chandra sample is not detected inX-rays, and therefore does not support this picture.
| An empirical temperature calibration for the Δ a photometric system . I. The B-type stars
We establish an empirical effective temperature calibration of mainsequence, luminosity class V to III B-type stars for the Δ aphotometric system which was originally developed to detect magneticchemically peculiar objects of the upper main sequence (early B-type toearly F-type) at 5200 Å. However, this system provides the index(g_1-y) which shows an excellent correlation with (B-V) as well as (b-y)and can be used as an indicator of the effective temperature. This issupplemented by a very accurate color-magnitude diagram, y or V versus(g_1-y), which can be used, for example, to determine the reddening,distance and age of an open cluster. This makes the Δ aphotometric system an excellent tool to investigate theHertzsprung-Russell-Diagram (HRD) in more detail. Using thereddening-free parameters and already established calibrations withinthe Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, apolynomial fit of third degree for the averaged effective temperaturesto the individual (g_1-y)0 values was derived. For thispurpose, data from the literature as well as new observations were takenresulting in 225 suitable bright normal B-type objects. The statisticalmean of the error for this sample is 238 K which is sufficient toinvestigate the HRD of distant galactic open clusters as well asextragalactic aggregates in the future.
| Resolving X-Ray Sources from B Stars Spectroscopically: The Example of μ Leporis
We present high-resolution X-ray observations of the chemically peculiarlate-type B star μ Lep. However, we find spectroscopic andastrometric evidence, which shows that the X-rays are not traced back tothe B star itself but rather to a previously unresolved companion, μLep-B, whose X-ray spectrum resembles that of a coronally active source.We discuss the possibility that μ Lep-B is a pre-main-sequencecompanion, most likely of the nonaccreting magnetically active type.
| Late B-type stars and their candidate companions resolved with Chandra
We present the first results from a series of Chandra observationscarried out with the aim to examine the origin of X-ray emission inmain-sequence late B-type stars. X-ray detections of late-B and earlyA-type stars have remained a mystery as none of the two major theoriesfor stellar X-ray emission applies in this spectral range: while O- andearly B-type stars drive strong winds that are subject to instabilities,late-type stars produce X-rays as a result of magnetic dynamo action.Since any dynamo works only in the presence of a convective zone,early-type stars are not magnetically active. We use high spatialresolution X-ray observations to enlighten the prevalent speculationthat previously unknown late-type or low-mass companion stars are thesites of the X-ray emission, instead of the B-type primaries. Here wepresent the results for HD 1685, HD 113703, HD 123445, HD 133880, and HD169978. Adaptive optics observations have recently revealed at least onefaint object near each of these B-type stars (at separation of1-6''). Four of the new infrared objects show infrared colorsand magnitudes typical for low-mass pre-main sequence stars, and arelikely true companions to the ~ 10-50 Myr old B-type stars. Thesemultiple systems are now resolved for the first time in X-ray light. Weuncover that four of the new companions are X-ray emitters, and thefifth one is likely to be a weak X-ray source below the detection limit.Three of the B-type primaries are X-ray dark down to the detection limitof Lx ~ 1028 erg/s. But we do detect X-rayemission from the position of HD 1685 A and HD 169978 A. The latter oneindeed is a spectroscopic binary. The characteristics of all X-raysources are compatible with those of typical young late-type stars: hardX-ray spectrum (kT > 0.5 keV) and high X-ray luminosity (logLx ~ 29...30 erg/s). Spectroscopic observations in theinfrared will solve the question whether the one remaining X-raydetected B-star in our sample, HD 1685 A, also has an even closercompanion or whether this is an intrinsic X-ray emitter.This work made use of observations obtained at the European SouthernObservatory, La Silla, Chile (ESO program No. 67.C-0261(B)).
| 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
| Search for low-mass PMS companions around X-ray selected late B stars
We have observed 49 X-ray-detected bright late B-type dwarfs to searchfor close low-mass pre-main sequence (PMS) companions using the EuropeanSouthern Observatory's ADONIS (Adaptive Optics Near Infrared System)instrument. We announce the discovery of 21 new companions in 9binaries, 5 triple, 4 quadruple system and 1 system consisting of fivestars. The detected new companions have K magnitudes between 6.5m and17.3m and angular separations ranging from 0.12 arcsec to 14.1 arcsec(18-2358 AU). Based on observations obtained at the European SouthernObservatory, La Silla, Chile (ESO programme No.~62.I-0477, and Swiss70~cm photometric telescope).
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| 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
| 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
| The δ Scuti star θ Tucanae. II. UVBY colour variations and pulsational/orbital properties.
On the basis of almost 1500 uvby photometric observations of θTucanae collected in 1993, we discuss the colour variation of thismulti-periodic δ Scuti star and debate some of its binary andpulsational properties. The frequencies f_1_=0.281 and f_2_=0.142c/dembody orbital motion in a 7.04d day ellipsoidal configurationcharacterised by a light curve with unequal maxima and minima and acolour index that becomes bluer during the minimum phases. The firstresults of an analysis of the light curve points towards a mass ratio qof the order of 0.10-0.15. We present dependable numerical values forcolour phase differences and amplitude ratios A_b-y_/A_y_,φ_b-y_-φ_y_, A_v-y_/A_y_, φ_v-y_-φ_y_ andA_u-y_/A_y_, φ_u-y_-φ_y_ which can be used for pulsation-modeidentification. The principal pulsation frequency f_7_=20.28c/d isreconcilable with a radial mode.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Complex behaviour of the δ Scuti star θ Tucanae. I. Frequencies in the light variation.
On the basis of more than 2300 new Stroemgren y and Johnson Vphotometric observations collected during 246 hours spread over 42nights at three sites in 1993, we present a frequency analysis of thelight variation of θ Tuc. 10 frequencies were found in the rangeof 15.8 to 20.28cycles/day displaying an extreme regularity. Frequenciesare situated in groups and these groups are equally spaced. Some kind ofsplitting is definitely involved. The frequencies are constant inamplitude on a short time scale. Two frequencies, 0.282 and0.142cycles/day (3.56 and 7.04days period) were found to be responsiblefor the mean light level variation of θ Tuc. Although θTuc is known as a single star, the length and shape of the mean lightlevel variation and the unusual behaviour of θ Tuc in ultravioletand infrared suggest that the δ Scuti star θ Tuc is aprimary in a binary system with a late F type companion.
| Photoelectric Observations of \theta Tucanae and AB Doradus
Not Available
| A catalog of far-ultraviolet point sources detected with the fast FAUST Telescope on ATLAS-1
We list the photometric measurements of point sources made by the FarUltraviolet Space Telescope (FAUST) when it flew on the ATLAS-1 spaceshuttle mission. The list contains 4698 Galactic and extragalacticobjects detected in 22 wide-field images of the sky. At the locationssurveyed, this catalog reaches a limiting magnitude approximately afactor of 10 fainter than the previous UV all-sky survey, TDl. Thecatalog limit is approximately 1 x 10-14 ergs A sq cm/s,although it is not complete to this level. We list for each object theposition, FUV flux, the error in flux, and where possible anidentification from catalogs of nearby stars and galaxies. Thesecatalogs include the Michigan HD (MHD) and HD, SAO, the HIPPARCOS InputCatalog, the Position and Proper Motion Catalog, the TD1 Catalog, theMcCook and Sion Catalog of white dwarfs, and the RC3 Catalog ofGalaxies. We identify 2239 FAUST sources with objects in the stellarcatalogs and 172 with galaxies in the RC3 catalog. We estimate thenumber of sources with incorrect identifications to be less than 2%.
| 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.
| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| Close binaries observed polarimetrically
Not Available
| Observations of 2.3-micron CO bands in 47 TUC giants
The 2.3-micron CO bands of four 47 Tuc red giants have been observedwith the AAT Fabry-Perot Infrared Grating Spectrometer with the goal ofderiving carbon abundances as well as C-12/C-13 ratios. All four starshave (V-K) values between 3.42 and 3.52 (i.e., Teff = 3950 + or - 70 K),yet show substantial differences in their DDO C(41-42) colors, implyinglarge variations in CN band strengths. Their CO colors also reveal ananticorrelation with CN band strength. The C-13)O lines at 2.345 micronsare seen in the present spectra of all four stars, although they areslightly stronger in the CN-strong/CO-weak stars. Simultaneous fits for(C/A) and C-12/C-13 in the observed spectra indicate that (C/A) isindeed lowest in the CN-strong stars.
| Interstellar polarization from observations of A and F stars in high and intermediate galactic latitudes, and from stars in the Mathewson and Ford polarization catalogue
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986A&AS...64..487K&db_key=AST
| First Stellar Catalog in Rio-Grande
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985RMxAA..10..377M&db_key=AST
| The local system of early type stars - Spatial extent and kinematics
Published uvby and H-beta photometric data and proper motions arecompiled and analyzed to characterize the structure and kinematics ofthe bright early-type O-A0 stars in the solar vicinity, with a focus onthe Gould belt. The selection and calibration techniques are explained,and the data are presented in extensive tables and graphs and discussedin detail. The Gould belt stars of age less than 20 Myr are shown togive belt inclination 19 deg to the Galactic plane and node-lineorientation in the direction of Galactic rotation, while the symmetricaldistribution about the Galactic plane and kinematic properties (purecircular differential rotation) of the belt stars over 60 Myr oldresemble those of fainter nonbelt stars of all ages. The unresolveddiscrepancy between the expansion observed in the youngest nearby starsand the predictions of simple models of expansion from a point isattributed to the inhomogeneous distribution of interstellar matter.
| The A0 stars
A photometric grid, standardized on MK spectral standards, has been usedto compare spectral types and luminosity classes obtainedphotometrically with those in two extensive spectral surveys coveringthe entire sky. Major discrepancies include the spectroscopicclassification of B9.5, which may indicate an otherwise unrecognizedspectral peculiarity, a different A0/A1 spectral type boundary in thetwo samples involved, the well-known misclassification of weak heliumstars, and an appreciable percentage of stars which are called dwarfsspectroscopically but are of higher photometric luminosity. The spacemotion vectors of these stars for which radial velocities are available,and excluding the minimum of 25 percent that are spectroscopic binarieswithout orbital elements, show structure in their distribution in the(U, V)-plane, with members of the Local Association and the Hyades andSirius superclusters forming obvious concentrations. The members of theLocal Association in the samples are mainly old (more than 200 millionyears) mode A stars, although a few much younger stars are included. Themembers of the Hyades and Sirius superclusters contain many bluestragglers, including several peculiar stars of the Hg, Mn, and Sivarieties.
| Four-colour photometry of B stars north of B = + 45 deg and comparison with the south
Four-color photometry of 33 Henry Draper B stars north of b = + 45 degis presented. Most are little-reddened B or intermediate-A stars. A newAm star is discovered. The new measures essentially complete uvbyphotometry of all HD B stars within 45 deg of either galactic pole. Thenorthern and southern cones of HD B stars are compared, and selectioneffects deduced. Far from the galactic plane, it appears that B starsmay be equally distributed north and south of the plane; closer to thesun, an asymmetry associated with Gould's Belt is evident.
| On the stability of observed frequencies in Delta Scuti stars - A reanalysis of Theta TUC
The claim that some Delta Scuti stars change their frequencies on timescales as short as 24 hr is examined. It is suggested that thehypothesis that these Delta Scuti stars have stable frequencies isviable. Stobie and Shobbrook's (1976) data on Theta Tuc is reanalyzedalong with 954 new observations obtained during 70 hr on 21 nights in1979. The frequency of highest amplitude is shown to be present at aconstant amplitude over the 7-yr time span of the entire data set. Apossible set of frequencies is fitted to the Theta Tuc data. Other DeltaScuti stars which have been claimed to have variable frequencies arediscussed. Some of these stars are now known to have stable frequencies.A reanalysis of Stobie, Pickup and Shobbrook's data (1977) and Gupta'sdata (1973) on 21 Mon gives the same result found by Stobie, Pickup, andShobbrook: 21 Mon appears to have changed frequencies in the 2-yrinterval between the data sets. It is pointed out that claims ofchanging frequencies for Delta Scuti stars should only be made withcaution and a large amount of data.
| Space velocities and ages of nearby early-type stars
Photometric distances and space velocities have been calculated for 458B0-A0 stars with apparent magnitudes not exceeding 6.5. UsingStromgren's ubvy-beta photometry the effective temperature and theposition in bolometric magnitude over the zero-age main sequence of thestars were derived. These quantities were used to obtain age and massfor 423 of the stars by interpolation in the models of stellar evolutionfor the chemical composition (X Z) = (0.7, 0.03). A relation forderiving interstellar reddening for normal stars in the intermediategroup is given.
| Is star formation bimodal ? II. The nearest early-type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977PASP...89..187E&db_key=AST
| Spectral classification from the ultraviolet line features of S2/68 spectra. II - Late B-type stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977A&AS...30...71C&db_key=AST
| Frequency analysis of the del SCT star, the Tuc.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976MNRAS.174..401S&db_key=AST
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Osservazione e dati astrometrici
| Costellazione: | Tucano |
| Ascensione retta: | 00h20m39.00s |
| Declinazione: | -69°37'30.0" |
| Magnitudine apparente: | 5.51 |
| Distanza: | 93.897 parsec |
| Moto proprio RA: | -3 |
| Moto proprio Dec: | -0.8 |
| B-T magnitude: | 5.44 |
| V-T magnitude: | 5.491 |
Cataloghi e designazioni:
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