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IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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) 1 Science drivers A-F stars are an ideal laboratory for studying physical processes IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd radiative diffusion, differential gravitational settling, grain accretion, convection, rotation, magnetic fields, non-radial pulsations that show their most extreme manifestations in these stars. These processes are based on fundamental parameters Mass M, radius R, luminosity L, abundances Effective temperature Teff, surface gravity log g, mean density r From the measurement of these fundamental parameters and theoretical evolutionary tracks, one can put into test models Stellar interiors, evolutionary stages Magnetic field topology, pulsation excitation (when coupled with complementary observational data) 2 Outline IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd How can optical interferometry help to better understand the A stars ? Principle Instruments Results Prospects 3 Interest of High Angular Resolution ◦ photosphere convection cells ◦ dark spots ◦ active areas ◦ chromosphere jets Clues for better understanding: ◦ internal structure ◦ pulsation modes 0.5º = 1 800" IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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: ◦ activity ◦ magnetism Extreme Ultraviolet Imaging Telescope (EIT), Sept. 1999 4 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Telescope angular resolution Telescope size D (m) Resolution l/D (millisecond of arc) 8m (VLT + perfect AO) 18 (l = 0.6 µm) 42 m (E-ELT + perfect AO) ~ 3 (l = 0.6 µm) 200 m (with perfect AO) ~ 0.6 (l = 0.6 µm) Need for optical interferometry Smallest detail « seen » on the stellar surface Main-sequence A stars 5 Interferometry principle IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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). VLTI This technique is fully mature in radioastronomy and produces high angular resolution images. ALMA R Scu (imaged by ALMA) 6 Optical interferometric arrays Array Apertures Maximal Number / diameter baseline b (m) l (µm) Resolution (mas) KECK 2 / 10 m 85 IR ~ 20 VLTI 4/8m 4 / 1.8 m 130 200 NearMid-IR 2 – 15 1.5 – 2.5 CHARA 6/1m 330 Visible Near-IR 0.3 – 0.5 0.9 – 1.5 NPOI 6 / 0.12 430 Visible 0.25 – 0.4 CHARA VLTI NPOI KECK Generally we have not enough apertures to obtain images 7 Interferometric observables IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 V ei = TF{Object} (b/l) Photocenter p p p We can deduce angular diameter, binary orbit, environment extent, etc. 8 How to measure angular diameters? The VLTI V IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd bb combination b b We record fringes for different telescope separations b. We compute the visibility V and the phase for each fringe pattern We fit the curve V = f(b) with an angular diameter model. V 9 Distance d Bolometric flux fbol Distance d Radius R Hipparcos Interferometry Angular diameter LD Spectro-photometry IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Fundamental parameters’ determination Luminosity L Mass M Age Gravity log g Effective temperature Teff 10 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 Teff and 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. Significant improvement of measurement accuracy First statistical studies Application to A-star sub-classes ◦ Exoplanet host stars provide R for planetary system modeling ◦ Chemically Peculiar (CP) stars provide Teff that is not affected by the abnormal surfaces 11 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 12 [Kervella et al. A&A, 413, 251(2004)] Statistical studies 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 25 Average accuracy of 1.5 % 20 15 10 5 0 A F G [Boyajian et al. ApJ, 746, 101 (2012)] d < 22 pc Accuracy on R< 3% [Cunha et al. A&AR, 14, 217 (2007)] 13 Statistical studies Comparison of interferometric diameters with SED ones [Boyajian et al. ApJ, 746, 101 (2012)] [Boyajian et al. ApJ, in prep (2013)] Effective temperature law A stars Relationships useful in extending our knowledge to a larger number of stars, at distances too far to accurately resolve their sizes. 14 Surface-Brightness relationship IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd In the recent distance determination to the Large Magellanic Cloud (the best anchor point of the cosmic distance scale) with an accuracy of 2%, the main uncertainty comes from SurfaceBrightness relationships [Pietrzynski et al. 2013, Nature] + Di Benedetto (2005) + Boyajian (2012) + Brown (1974) + Challouf (in prep) Bright early-type stars (O-A-B) for distances in Local Group Late-type stars (F-G) for LMC distance 15 Interest of multi-technique studies Interferometry + Spectroscopy: IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd R, L, Teff accurate determination requires interferometric angular diameters, accurate parallaxes and accurate bolometric flux. Interferometry + Asteroseismology: [Creevey et al., ApJ, 659, 616 (2007)] Accurate R allows accurate masses M to be derived 16 Case of the roAp stars Small rotational speed (< 100 km/s) IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Abundance inhomogeneities (with a contrast up to 1000) < 1 mas Strong magnetic field (up to ~30 kG) large-scale organized roAp: intersection of MainSequence and instability strip Pulsations (period of a few minutes) Optical interferometric allows to have a direct (and unbiased) measurement of the linear radius of these tiny stars. 17 Example of 10Aql IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd V² LD = 0.275 0.006 mas R = 2.317 0.070 R [CHARA/VEGA] + Bolometric flux and parallax + Large frequency separation Dn + Evolutionary track (CESAM2K) B/l M = 1.95 0.05 M [Perraut et al. A&A, submitted] 18 Test of roAp excitation models [Cunha et al. A&A, in prep] « Interferometric » fundamental parameters for 4 roAp (aCir, bCrB, gEqu, 10 Aql) Stellar interior models Excited g Equ 0 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Not excited Prediction of excitation modes Comparison with observed modes a Cir Observed modes 10 Aql Predicted modes derived for interferometric parameters ( ) are in agreement with observed ones but for aCir. 19 Go beyond diameter measurement IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Optical interferometry is clearly a powerful means to derive accurate fundamental parameters through angular diameter measurement but this technique can also be used to study the environments of A stars: Study debris disks around VEGA-like Search for companion(s) Fomalhaut and coupled with spectrometry for kinematic studies Study of wind and mass loss Deneb A supergiants, (magnetic) Herbig AeBe Study of limb-darkening Imaging of stellar surfaces (rotation) 20 Debris disks IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 bLeo b (m) [Akeson et al. ApJ, 691, 1896 (2009)] 21 Supergiant wind IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd The line-formation region is extended (∼1.5–1.75 R*) since the visibility decreases in the Ha line. There is a significant asymmetry in the line forming region since the phase is not null in the line. [Chesneau et al. A&A, 521, A5 (2010)] Ha line Visibility Phase Deneb l l 22 Rotation Optical interferometry can image the surface of fast rotators. IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Altair [Monnier et al. Science, 317, 342 (2008)] 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. 23 Conclusion IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 limbdarkening to be derived and besides surfaces of fast rotators to be imaged. 24 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Prospects Position of 109 stars with an accuracy of ~20 µas Go towards an Angular Diameter Anthology [Boyajian et al. ApJ, in prep (2013)] Needs to go to higher sensitivity and to smaller targets Increase accuracy for putting into test stellar models Go towards surface imaging across spectral lines 25 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Breakthrough for CP stars HD24712 [Lüftiger et al., A&A, 509, A71 (2010)] [See poster of D. Shulyak et al. in this conference] 26 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Thanks for your attention The CHARA Array 27 IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd An example of simulation : a2CVn • Spectra of a2CVn (CrIIl4824 line) -0.5 Dl 0.5 • Doppler maps (Kochukhov,2002) -0.5 Dl 0.5 Predicted interferometric phases Interferometric phases provide 2D geometrical constraints 28 Abundance study with CHARA/VEGA IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd Line for various stellar phases 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 CHARA/VEGA + a2CVn models Imaging may be difficult due to the small number of telescopes 29 Limb-darkening effect IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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-disk diameter (mas) Ha line l [ten Brummelaar et al. MROI meeting (2011)]30 Determination of fundamental parameters of (Chemically Peculiar) A stars through optical interferometry IAU Conference « Putting A Stars into Context” Moscow, 2013 June 3rd 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 31 [Kervella et al. A&A, 413, 251(2004)]