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Karine ROUSSELET-PERRAUT Institut de Planétologie et d’Astrophysique de Grenoble

Determination of fundamental parameters of (Chemically Peculiar) A stars through optical interferometry. Karine ROUSSELET-PERRAUT Institut de Planétologie et d’Astrophysique de Grenoble (with contributions of Denis Mourard , Margarida Cunha, Nicolas Nardetto ).

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Karine ROUSSELET-PERRAUT Institut de Planétologie et d’Astrophysique de Grenoble

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  1. Determination of fundamental parameters of (Chemically Peculiar) A stars through optical interferometry KarineROUSSELET-PERRAUT Institut de Planétologie et d’Astrophysique de Grenoble (with contributions of Denis Mourard, Margarida Cunha, Nicolas Nardetto) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  2. Science drivers A-F stars are an ideallaboratoryfor studyingphysicalprocesses • radiative diffusion, differentialgravitationalsettling, grain accretion, convection, rotation, magneticfields, non-radial pulsations thatshow their most extreme manifestations in these stars. Theseprocesses are based on fundamentalparameters • Mass M, radius R, luminosity L, abundances • Effective temperatureTeff, surface gravity log g, meandensityr From the measurement of thesefundamentalparameters and theoreticalevolutionarytracks, one can put into test models • Stellarinteriors, evolutionary stages • Magneticfieldtopology, pulsation excitation (whencoupledwithcomplementaryobservational data) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  3. Outline • How canopticalinterferometryhelp to betterunderstandthe A stars ? • Principle • Instruments • Results • Prospects IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  4. Interest of High AngularResolution • High Angular Resolution (HAR) is fundamental for understanding star formation and evolution as well as physical process at play within stellar objects. • Sun is the best known star since we manage to "resolve" its surface, i.e. to see details on its surface like: • photosphere convection cells • dark spots • active areas • chromosphere jets • Clues for better understanding: • internal structure • pulsation modes • activity • magnetism IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 0.5º = 1 800" Extreme Ultraviolet Imaging Telescope (EIT), Sept. 1999

  5. Main-sequence A stars Telescopeangularresolution IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Need for optical interferometry

  6. Interferometryprinciple Interferometry is a imaging technique with (small) diluted apertures, which allows a giant mirror to be synthetized. The longer the distance b between the apertures, the higher the angular resolution (given by l/b). This technique is fully mature in radioastronomy and produces high angular resolution images. VLTI IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd ALMA R Scu (imaged by ALMA)

  7. Optical interferometricarrays VLTI CHARA NPOI KECK Generally we have not enough apertures to obtain images

  8. p p Interferometric observables In optical range we generally observe interference fringe patterns between the different apertures. The visibility V and the phase  of these fringe patterns are related to the Fourier transform of the object brightness (Van-Cittert theorem) V Photocenterp V eij= TF{Object} (b/l) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Wecandeduceangulardiameter, binaryorbit, environmentextent, etc.

  9. The VLTI The VLTI b b The VLTI b How to measureangulardiameters? V combination b • We record fringes for different telescope separations b. • We compute the visibility V and the phase j for each fringe pattern • We fit the curve V = f(b) with an angular diameter model. IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd V

  10. Fundamentalparameters’ determination AngulardiameterLD Distance d Radius R Interferometry Hipparcos Luminosity L Bolometric flux fbol Distance d IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Spectro-photometry Mass M Age Gravity log g Effective temperatureTeff

  11. Application to A stars In addition to their many other peculiarities, the ages of A stars are poorly known. As main sequence stars, they evolve in Teffand in L/R, which affords an opportunity to establish their ages through interferometric means. Observational data can be compared to models on an H-R diagram, which will then indicate their ages and masses. • Significantimprovementof measurementaccuracy • First statisticalstudies • Application to A-star sub-classes • Exoplanet host stars • provide R for planetary system modeling • ChemicallyPeculiar (CP) stars • provideTeffthatis not affected by the abnormal surfaces IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  12. Improvement of accuracy The recent improvement of the accuracy on interferometric observables has led to an angular diameter precision of typically 1-2% and consequently to an improved determination of stellar fundamental parameters, which in turn allows to test stellar models in an independent way. Procyon « classical » error box « interferometric » error box IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd [Kervella et al. A&A, 413, 251(2004)]

  13. Statisticalstudies Several surveys of main-sequence stars have been led with optical interferometry but they mainly consists of G and K targets. Survey of 44 stars by T. Boyajian with CHARA Averageaccuracy of 1.5 % d < 22 pc Accuracy on R< 3% [Cunha et al. A&AR, 14, 217 (2007)] [Boyajian et al. ApJ, 746, 101 (2012)]

  14. Statisticalstudies [Boyajian et al. ApJ, 746, 101 (2012)] [Boyajian et al. ApJ, in prep (2013)] Comparison of interferometric diameters with SED ones Effective temperature law A stars Relationshipsuseful in extendingourknowledge to a largernumber of stars, at distances too far to accuratelyresolvetheir sizes.

  15. Surface-Brightnessrelationship In the recentdistancedeterminationto the Large Magellanic Cloud (the best anchor point of the cosmic distance scale) with an accuracy of 2%, the main uncertainty comes from Surface-Brightness relationships [Pietrzynski et al. 2013, Nature] + Di Benedetto (2005) + Boyajian (2012) + Brown (1974) + Challouf (in prep) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Bright early-type stars (O-A-B) for distances in Local Group Late-typestars (F-G) for LMC distance

  16. Interest of multi-techniquestudies • Interferometry + Spectroscopy: R, L, Teffaccuratedeterminationrequiresinterferometricangulardiameters, accurateparallaxes and accuratebolometric flux. • Interferometry + Asteroseismology: [Creevey et al., ApJ, 659, 616 (2007)] IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Accurate R allowsaccurate masses M to bederived

  17. Case of the roAp stars Small rotational speed (< 100 km/s) Abundanceinhomogeneities (with a contrast up to 1000) < 1 mas Strongmagneticfield (up to ~30 kG) large-scaleorganized IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Pulsations (period of a few minutes) roAp: intersection of Main-Sequence and instabilitystrip Optical interferometricallows to have a direct (and unbiased) measurement of the linear radius of thesetiny stars.

  18. Example of 10Aql qLD = 0.275 0.006 mas [CHARA/VEGA] R = 2.317  0.070 R V² + Bolometric flux and parallax + Large frequencyseparationDn + Evolutionarytrack (CESAM2K)  B/l M = 1.95  0.05 M IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd [Perraut et al. A&A, submitted]

  19. Test of roAp excitation models [Cunha et al. A&A, in prep] g Equ Excited • « Interferometric » fundamentalparameters for 4 roAp(aCir, bCrB, gEqu, 10 Aql) • Stellarinteriormodels • Prediction of excitation modes • Comparisonwithobserved modes 0 Not excited Observed modes 10 Aql  Cir IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Predicted modes derived for interferometricparameters ( ) are in agreement withobservedones but for Cir.

  20. Go beyonddiametermeasurement Optical interferometryisclearly a powerfulmeans to deriveaccuratefundamentalparametersthroughangulardiametermeasurement but this technique canalsobeused to study the environments of A stars: • Studydebrisdisksaround VEGA-like • Search for companion(s) and coupledwithspectrometry for kinematicstudies • Studyof wind and mass loss • A supergiants, (magnetic) HerbigAeBe • Study of limb-darkening Imaging of stellar surfaces (rotation) Fomalhaut IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Deneb

  21. Debrisdisks The short-baseline visibilities are lower than expected for the stellar photosphere alone. The visibility offset of a few percent is interpreted as a near-infrared excess arising from dust grains which must be located within several AU of the central star. V² IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Leo b (m) [Akeson et al. ApJ, 691, 1896 (2009)]

  22. Supergiantwind The line-formation region is extended(∼1.5–1.75 R*) since the visibilitydecreases in the H line. There is a significant asymmetry in the line forming regionsince the phase is not null in the line. [Chesneau et al. A&A, 521, A5 (2010)] Ha line IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Visibility Phase Deneb l l

  23. Rotation • Optical interferometry can image the surface of fast rotators. • The image clearly reveals the strong effect of gravity darkening on the highly-distorted stellar photosphere. • Standard models for a uniformly rotating star cannot explain the results, requiring differential rotation, alternative gravity darkening laws, or both. [Monnier et al. Science, 317, 342 (2008)] Altair IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  24. Conclusion • Optical interferometry is a powerful means for deriving accurate fundamental parameters of A stars through accurate angular diameter determinations. • With long-baseline arrays an angular resolution of about 0.3 mas is now reachable. • There is a huge potential of combining interferometric (radius and derived effective temperature) and asteroseismic (large frequency separation) data to improve the determination of the mass of pulsating stars. • Coupled with spectroscopy, optical interferometry can allow kinematics studies of A stars’ environments. • Going beyong angular diameter measurements allows limb-darkening to be derived and besides surfaces of fast rotators to be imaged. IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  25. Prospects Position of 109 stars with an accuracy of ~20 µas  Go towardsan AngularDiameterAnthology Needs to • go to highersensitivityand to smallertargets • Increaseaccuracy for putting into test stellarmodels • Go towardssurface imagingacross spectral lines [Boyajian et al. ApJ, in prep (2013)] IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  26. Breakthrough for CP stars HD24712 [Lüftiger et al., A&A, 509, A71 (2010)] IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd [See poster of D. Shulyak et al. in thisconference]

  27. Thanks for your attention The CHARA Array IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd

  28. Spectra of 2CVn (CrII4824 line) • Doppler maps (Kochukhov,2002)  -0.5 0.5   -0.5 0.5 An example of simulation : 2CVn IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Predicted interferometric phases Interferometric phases provide 2D geometrical constraints Rousselet-Perraut et al., A&A, 422, 193

  29. Line for various stellar phases Abundance study with CHARA/VEGA • Resolved target • Large visibility effects • Observations in the visibility lobes CHARA Abundance spots of a fraction of stellar diameter can be detected in the 2nd and 3rd visibility lobes BUT Imaging may be difficult due to the small number of telescopes IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd CHARA/VEGA + 2CVn models

  30. Limb-darkeningeffect Limb darkening in an absorption line is expected to be less than it would be for the continuum at the wavelength of the line because the line is formed all the way out to the stellar surface. Sirius Uniform-diskdiameter (mas) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Ha line l [tenBrummelaar et al. MROI meeting (2011)]

  31. Determination of fundamental parameters of (Chemically Peculiar) A stars through optical interferometry Karine ROUSSELET-PERRAUT (IPAG, France) The recent improvement of the accuracy on interferometric observables has led to an angular diameter precision of typically 1-2% and consequently to an improved determination of stellar fundamental parameters, which in turn allows to test stellar models in an independent way. Procyon « classical » error box « interferometric » error box IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd [Kervella et al. A&A, 413, 251(2004)]

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