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Quantitative spectroscopy of BA-type supergiants
Luminous BA-type supergiants have enormous potential for modernastrophysics. They allow topics ranging from non-LTE physics and theevolution of massive stars to the chemical evolution of galaxies andcosmology to be addressed. A hybrid non-LTE technique for thequantitative spectroscopy of these stars is discussed. Thorough testsand first applications of the spectrum synthesis method are presentedfor the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe),HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution andhigh-S/N Echelle spectra. Stellar parameters are derived fromspectroscopic indicators, consistently from multiple non-LTE ionizationequilibria and Stark-broadened hydrogen line profiles, and they areverified by spectrophotometry. The internal accuracy of the methodallows the 1σ-uncertainties to be reduced to 1-2% in T_effand to 0.05-0.10 dex in log g. Elemental abundances are determined forover 20 chemical species, with many of the astrophysically mostinteresting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTEcomputations reduce random errors and remove systematic trends in theanalysis. Inappropriate LTE analyses tend to systematicallyunderestimate iron group abundances and overestimate the light andα-process element abundances by up to factors of two to three onthe mean. This is because of the different responses of these species toradiative and collisional processes in the microscopic picture, which isexplained by fundamental differences of their detailed atomic structure,and not taken into account in LTE. Contrary to common assumptions,significant non-LTE abundance corrections of ~0.3 dex can be found evenfor the weakest lines (Wλ 10 mÅ). Non-LTEabundance uncertainties amount to typically 0.05-0.10 dex (random) and~0.10 dex (systematic 1σ-errors). Near-solar abundances arederived for the heavier elements in the sample stars, and patternsindicative of mixing with nuclear-processed matter for the lightelements. These imply a blue-loop scenario for η Leo because offirst dredge-up abundance ratios, while the other three objects appearto have evolved directly from the main sequence. In the most ambitiouscomputations several ten-thousand spectral lines are accounted for inthe spectrum synthesis, permitting the accurate reproduction of theentire observed spectra from the visual to near-IR. This prerequisitefor the quantitative interpretation of intermediate-resolution spectraopens up BA-type supergiants as versatile tools for extragalacticstellar astronomy beyond the Local Group. The technique presented hereis also well suited to improve quantitative analyses of less extremestars of similar spectral types.
| Lyngå 1, A Small Open Cluster Containing a Red-Supergiant Member
We present CCD {UBV}RI} (Cousins system) photometric observationscomplemented with spectroscopic and polarimetric observations that werecarried out in the open cluster Lyngå 1. Our data indicate thatthe cluster reddening is E(B-V)= 0.45+/-0.03, the ratio A[V]/E(B-V)= Rsuggests that the extinction law may be slightly anomalous ( R ≈3.5) and that the cluster distance modulus is V0 - M[V] =11.40+/-0.2. The age of Lyngå 1 is between 100 and 125 Myraccording to a fitting of theoretical isochrones, and the slope of itsmass spectrum is x ≈ 1.7. The brightest red star in the field isa cluster member of spectral type K2 II-Ib.
| Quantitative Spectroscopy of Supergiants
Blue supergiants of spectral types B and A are the visually brighteststars in spiral and irregular galaxies, with their most luminous members(at M_V=-10) outshining entire dwarf galaxies. This characteristicallows us to use them as probes to study the Local Universe in greatdetail. In principle, already the existing large telescopes andinstrumentation facilitate quantitative spectroscopy of these objects asfar as the Virgo and Fornax clusters of galaxies. Beyond theirchallenging stellar atmospheres and opportunities for testingsophisticated non-LTE physics they offer numerous applications to modernastrophysics. Quantitative spectroscopy of supergiants can contribute toimprove our understanding of massive star evolution. Galactic abundancegradients and abundance patterns, as can be obtained from studies oflarge ensembles of supergiants in our own and other galaxies, willfoster the understanding of galactochemical evolution. Finally, they arepromising independent indicators for calibrating the extragalacticdistance scale, by application of the wind momentum-luminosity and theflux-weighted gravity-luminosity relationships. In view of this largepotential, the objective of this thesis is to improve the status ofquantitative spectroscopy of BA-type supergiants and to provide firstapplications on a sample of Galactic and extragalactic targets, withinthe Local Group and beyond.
| CCD UBVI Photometry and Polarimetry in the Open Cluster Trumpler 21
CCD UBVI photometry was carried out in the field of the open clusterTrumpler 21. Our data locate the cluster at a distance of 1380 pc in alow absorption zone with E(B-V) = 0.25. The age of the cluster isestimated to be 25-30 × 106 yr based on thesuperposition of isochrones computed with mass loss and overshooting.The cluster mass spectrum has a slope of 1.44 similar to the typicalvalue 1.35 for field stars. UBVRI polarimetry of its brightest membersled us to the finding that diffuse interstellar material is responsiblefor both polarization percentage and its direction.
| H I Shells behind the Coalsack
We report the discovery of two new large H I shells in the direction ofthe Coalsack Nebula. Both shells were observed with the Parkes RadioTelescope as part of the Southern Galactic Plane Survey. The largestshell, GSH 304-00-12, is at a distance of ~1.2 kpc and has derivedphysical dimensions of 280×200 pc. The second shell, GSH305+01-24, is at a distance of ~2.2 kpc and has derived dimensions of280×440 pc. We present a simple numerical model to show that GSH305+01-24 most likely formed from stellar winds in the Centaurus OB1stellar association. There is associated radio, infrared, and Hαcontinuum emission. Both shells are situated in the Sagittarius-Carinaarm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 isat the near side of the arm and opens into the interarm region. We findno evidence of closure at the near side of the shell and thereforedescribe the geometry as conical. Emission from the near side of theshell may be lost in absorption by the Coalsack Nebula.
| Non-LTE line formation for N: Abundances and stellar parameters. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line formation calculations forneutral and singly-ionized nitrogen is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, nitrogen abundances are determined on thebasis of line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations excellently for a given nitrogen abundance in eachobject. Moreover, the ionization equilibrium of \ion{N}{i/ii} proves tobe a sensitive temperature indicator for late B-type and early A-typesupergiants - even at low metallicities - due to the apparent nitrogenoverabundance in these objects. All supergiants within our sample showan enrichment of nitrogen on the order of ~ 0.3-0.6 dex, indicating themixing of CN-cycled material into atmospheric layers, with the sum ofthe CNO abundances staying close to solar. This finding is in accordancewith recent stellar evolution models accounting for mass-loss androtation. For Vega, an underabundance of nitrogen by 0.25 dex is found,in good agreement with the similar underabundance of other lightelements. The dependence of the non-LTE effects on the atmosphericparameters is discussed with special emphasis on the supergiants where astrong radiation field at low particle densities favours deviations fromLTE. Non-LTE effects systematically strengthen the \ion{N}{i/ii} lines.For some N I lines in supergiants non-LTE abundance corrections inexcess of 1 dex are found and they react sensitively to modifications ofthe collisional excitation data. The influence of microturbulence on thestatistical-equilibrium calculations is also investigated: theline-strengths of the strong N I features show some sensitivity due tomodifications of the line-formation depths and the departurecoefficients, while the - in this parameter range - weak N Ii linesremain unaffected. Based on observations collected at the EuropeanSouthern Observatory, Chile (ESO Ndeg 62.H-0176).
| Non-LTE line-formation for neutral and singly-ionized carbon. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line-formation calculations forneutral and singly-ionized carbon is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, carbon abundances are determined on the basisof line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations well for a single carbon abundance in each object. Fortwo supergiants, eta Leo and HD 111613, lines of both species aresimultaneously present in the spectra, giving consistent C I and C Iiabundances (within the error bars). However, the uncertainties of theabundances are large, on the order of ~ 0.3 dex(statistical+systematical error), thus the ionization equilibrium of\ion{C}{i/ii} is of restricted use for the determination of stellarparameters. All supergiants within our sample show a depletion of carbonon the order of 0.2-0.5 dex, indicating the mixing of CN-cycled materialinto the atmospheric layers, with the sum of the CNO abundancesremaining close to solar. This finding is in accordance with recentstellar evolution models accounting for mass-loss and rotation. ForVega, an underabundance of carbon by 0.3 dex is found, in excellentagreement with the similar underabundance of other light elements. Thedependence of the non-LTE effects on the atmospheric parameters isdiscussed and the influence of systematic errors is estimated. Specialemphasis is given to the supergiants where a strong radiation field atlow particle densities favours deviations from LTE. Non-LTE effectssystematically strengthen the \ion{C}{i/ii} lines. For the C I lines inthe infrared, a strong sensitivity to modifications in thephotoionization and collisional excitation data is found. An increasingdiscrepancy between our model predictions and the observations for the CIi doublet lambda lambda 6578-82 is perceived with rising luminosity,while the other C Ii doublet and quartet lines remain consistent.Furthermore, the influence of microturbulence on thestatistical-equilibrium calculations is investigated. Based onobservations collected at the European Southern Observatory, Chile (ESONdeg 62.H-0176).
| The open cluster Havlen-Moffat No. 1 revisited
A deep CCD UBVRI photometric survey combined with UBVRI polarimetricobservations of 21 bright stars was carried out in the region of theopen cluster Havlen-Moffat No. 1. Our data reveal that the extinctionlaw in this cluster is variable and that six cluster stars show veryhigh polarisation values (>4%), probably because of the presence of anearby small dust cloud. The cluster is at a distance of d = 3300 pc, itis 2-4 Myr old and the initial mass function of its most massive stars(M > 3 Msun ) has a flat slope of x~ 0.7. As an additionalresult, it was possible to reconcile the absolute magnitudes of the twoWN7-type members using the R-values valid in the regions where they arelocated. Based on observations collected at the University of TorontoSouthern Observatory, Las Campanas, Chile, and the ComplejoAstronómico El Leoncito, (CASLEO), Argentina, operated underagreement between the Consejo Nacional de InvestigacionesCientíficas y Técnicas de la República Argentinaand the National Universities of La Plata, Córdoba and San Juan.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/371/908.
| 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
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Polarimetry of the LMC X-Ray Binary A0538-66 during an ``On/Off'' Transition in 1991
We present polarimetric and spectroscopic observations of the X-raybinary A0538-66 obtained in 1991 near periastron passage during cycles291, 294, and 310. The system was in an active phase during cycle 291(phi=0.98), with X-ray outbursts being observed in the previous twocycles. By cycle 294 (phi=0.00-0.12) the system had entered alow-activity phase, and by cycle 310 (phi=0.06) the system appears to befully ``off.'' We address the implications of these new observations andinterpret them within the context of the model put forward by Clayton etal. in which the polarization arises from scattering of the B2 IIIestar's light from circumstellar disk material as well as additionalmaterial in the orbital plane. The rapid change in position angle nearperiastron passage, observed in the previous polarimetric studies andascribed to additional scattering material in the Clayton et al. model,is not seen in our postoutburst observation. We suggest that the absenceof additional scatterers may be associated with the transition from the``on'' to the ``off'' state that took place in 1991.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| UBV polarimetry of 361 A- and F-type stars in selected areas
We present simultaneous UBV linear polarization measurements for 361 A-and F-type stars with accurate colour excess and distance determination.These stars are distributed in 35 Kapteyn's Selected Areas, covering thethird and fourth quadrants of the galactic plane (|b| <= 30degr ).The obtained polarization and the known colour excess are compared. Ananalysis of the polarization distribution as a function of the stellardistance is also performed. Based on observations collected at theEuropean Southern Observatory (ESO), La Silla, Chile.
| On the Variability of Early A-Type Supergiants
An examination of the Hipparcos photometry of 26 bright early A-typesupergiants shows that they are all variable.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| 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.
| Stromgren and H-Beta Photometry of Associations and Open Clusters - Part Three - CENTAURUS-OB1 and CRUX-OB1
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.269..289K&db_key=AST
| A reassessment of some polarization standard stars
There have been recent reports suggesting that several polarizedstandard stars exhibit variability. In the investigation here, weconcentrate on the work of Bastien et al. (AJ, 495, 900 (1988)) whoclaim that the majority of their measurements reveal instability of thecataloged standards (11 stars out of 13). We criticize their statisticalassessments and comment on the shortcomings in their data and analyses.Firstly, it is demonstrated that their test based on comparisons ofinternal and external variances does not give rise to criteria forassessing variability. Secondly, their normality tests based on kurtosisand skewness are also shown to be fallacious and that in any case theylack rigor. Problems with the homogeneity of the observations arediscussed with respect to dead-time corrections, instrumentalpolarization and modulation efficiency, all of which may give rise tovariable systematic errors within the pooled data. For four stars whichhave sufficient homogeneous measurements, tests previously used oncatalogs of unpolarized stars have been adopted to investigate the dataand only one star (HD 111613) provides firm evidence for variability.
| Narrow band multicolor photometry of reddened and unreddened early-type stars
Photometric measurements and interstellar extinction curves of reddenedand unreddened early-type Southern Hemisphere stars are presented. It isshown that a number of shallow features exist in the extinction curvesrelative to a straight-line approximation.
| Observational tests of convective core overshooting in stars of intermediate to high mass in the Galaxy
This study presents the results of 14 tests for the presence ofconvective overshooting in large convecting stellar cores for stars withmasses of 4-17 solar masses which are members of detached close binarysystems and of open clusters in the Galaxy. A large body of theoreticaland observational data is scrutinized and subjected to averaging inorder to minimize accidental and systematic errors. A conservative upperlimit of d/HP less than 0.4 is found from at least four tests, as wellas a tighter upper limit of d/HP less than 0.2 from one good test thatis subject to only mild restrictions and is based on the maximumobserved effective temperature of evolved blue supergiants. It isconcluded that any current uncertainty about the distance scale forthese stars is unimportant in conducting the present tests forconvective core overshooting. The correct effective temperature scalefor the B0.5-B2 stars is almost certainly close to one of the proposedhot scales.
| The calibration of the Stromgren photometric system for A, F and early G supergiants. I - The observational data
An empirical calibration of the Stromgren uvby-beta photometric systemfor the A, F, and early G supergiants is being derived. This paperexplains the observational program and the photometric reductiontechniques used and presents a catalog of new Stromgren photometry forover 600 A, F, and G supergiants.
| Getting to the bottom of the lagoon - Dust, magnetism, and star formation
A V-band polarimetric survey of stars associated with the Lagoon Nebula(M8; NGC 6523) and the embedded cluster NGC 6530 has been conducted. Thedata have been combined with existing photometric and spectroscopicobservations in order to study the dust in the immediate vicinity and toinvestigate the alignment of magnetic field lines with identifiablesymmetry axes. Using the cluster method, it has been possible toseparate multiple stars from single stars and thereby derive acccuratevalues for the ratio of total to selective extinction and the distancemodulus. The value of R for the associated dust is 4.6 + or - 0.3.Anomalous reddening is a characteristic of the region, not just Herschel36. Electric vectors show a high degree of alignment perpendicular tothe direction of the extension of the large-scale distributions ofionized gas and massive stars. The overall collapse of molecular gasappears to have been asymmetrical, proceeding most rapidly along adirection parallel to magnetic field lines. A lot of the dust mixed withthe ionized gas has been destroyed, so the large-scale uniformity ofreddening laws and polarization position angles indicates that much ofthe dust responsible for extinction and polarization is associated withneutral gas.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| Polarization variablity among Wolf-Rayet stars. IV - A complete lack of circular polarization in the optical continuum
Quasi-simultaneous blue and red, broadband optical monitoring in linearand circular polarization and in intensity has been carried out over aninterval of three weeks for several Wolf-Rayet stars that showrelatively large Delta P variations in linear polarization. Nosignificant varying Delta V component of circular polarization isdetected in any of these stars. The lower upper limit Delta V/Delta Pimplies that the intrinsic linearly polarized light which does varycannot be produced by electrons gyrating in a magnetic field, unlessthey are ultrarelativistic - a rather unlikely situation. The low meancircular polarization typically observed is probably interstellar inorigin. Lack of periodicity in the observed variations of linearpolarization implies that even weak magnetic field loops are unlikely tobe involved in confining pockets of wind plasma. The observed linearpolarization variations are related mainly to electron scattering.
| The spectral energy distribution of early-type stars. II - The extinction law towards O-type stars
Photometric measurements through different pass-bands are used todetermine the color-excess E(B-V) for O-type stars in the UV and IRspectral regions. The results are used to examine the extinctioncharacter of the stars. It is found that, in the UV, each O-type starhas its own extinction character. In general, the visual and NIRextinction in the direction of O-type stars are normal.
| Stromgren and H-beta photometry of stars earlier than G0 in the Southern Coalsack direction
Four-color and H-beta photometry have been obtained for 236 starsearlier than G0 in order to study the color excess distribution in theSouthern Coalsack direction. The beta histogram of the stars reveals arelative absence of stars in the range of A4 to A9. The (b-y)distribution for B-type stars suggests the existence of three differenttypes of stars located at different distances.
| Physical parameters of stars in the Scorpio-Centaurus OB association
Walraven photometry is presented of established and probable members ofthe Scorpio-Centaurus OB association. For each star, effectivetemperature and surface gravity are derived using Kurucz (1979)atmosphere models. From the Straizys and Kuriliene (1981) tables,absolute magnitudes are calculated. Distance moduli and visualextinctions are determined for all stars. From a comparison of theHR-diagrams of the stars in each subgroup with theoretical isochrones,the ages of the three subgroups are derived. The distances to the threesubgroups are shown to be different; there is a general trend (alsowithin each subgroup) for the distances to be larger at higher galacticlongitudes. The visual extinction in the youngest subgroupUpper-Scorpius, is well correlated with the IRAS 100-micron map. Thedistance toward the Ophiuchus dark clouds is found to be 125 pc, basedon the photometric distances to the stars. Most of the early-type starsin Upper-Scorpius are located at the far side of the dark clouds.
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Osservazione e dati astrometrici
| Costellazione: | Croce del Sud |
| Ascensione retta: | 12h51m17.80s |
| Declinazione: | -60°19'47.0" |
| Magnitudine apparente: | 5.72 |
| Distanza: | 917.431 parsec |
| Moto proprio RA: | -5.7 |
| Moto proprio Dec: | -0.2 |
| B-T magnitude: | 6.129 |
| V-T magnitude: | 5.771 |
Cataloghi e designazioni:
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