Stellar Spectroscopy during Exoplanet Transits Revealing structures across stellar surfaces Dainis Dravins 1, Hans-Günter Ludwig 2, Erik Dahlén 1, Hiva Pazira 1 1 Lund Observatory, Sweden, 2 Landessternwarte Königstuhl, Heidelberg, Germany KVA

Very quiet solar disk (GONG/Teide) AN ”IDEAL” STAR ?

Granulation near the limb (towards the top) at 488 nm; Swedish 1-m solar telescope, La Palma A REAL STAR !

MODELING STELLAR SURFACES – White dwarf vs. Red giant Snapshots of emergent intensity during granular evolution on a 12,000 K white dwarf (left) and a 3,800 K red giant. Horizontal areas differ by dozen orders of magnitude: 7x7 km 2 for the white dwarf, and 23x23 R Sun 2 for the giant. (H.-G. Ludwi,g, Heidelberg)

What future stellar astrophysics ? Stellar abundances accurate to ±0.01 dex ? Differential chemistry in stars with or without planets? Accurate stellar oscillations ? Stellar sizes seem not consistent if deduced from naïve models. Stellar differential rotation ? Stellar gas dynamics are deduced from subtle signatures of line profiles. Magnetic and chromospheric activity ? How do magnetic fields affect stellar convection ? Exoplanet signatures ? Exoplanet properties are deduced differentially to the stellar spectrum. Precision stellar physics requires 3-D modeling But simulations are complex. How can one verify such models ?

How to verify or falsify 3-D models ?

Spatially averaged line profiles from 20 timesteps, and temporal averages. = 620 nm  = 3 eV 5 line strengths GIANT STAR T eff = 5000 K log g [cgs] = 2.5 (approx. K0 III) Stellar disk center; µ = cos  = 1.0 Line profiles from 3-D Hydrodynamic simulations Model predictions insensitive to modest spatial smearing (Models by Hans-Günter Ludwig, Landessternwarte Heidelberg)

Spatially and temporally averaged line profiles. = 620 nm  = 1, 3, 5 eV 5 line strengths GIANT STAR T eff = 5000 K log g [cgs] = 2.5 (approx. K0 III) Stellar disk center; µ = cos  = 1.0 Line profiles from 3-D Hydrodynamic simulations Model predictions insensitive to modest spatial smearing (Models by Hans-Günter Ludwig, Landessternwarte Heidelberg)

SUN Profiles from CO 5 BOLD solar model; Five line strengths; excitation potentials  = 1, 3, 5 eV. Left: Solar disk center µ = cos  = 1. Right: Off-center disk position µ = cos  = Line profiles from 3-D Hydrodynamic simulations Model predictions insensitive to modest spatial smearing (Models by Hans-Günter Ludwig, Landessternwarte Heidelberg)

Simulated intensities approaching the solar limb Mats Carlsson, Oslo; in Å.Nordlund, R.F.Stein, M.Asplund: Solar Surface Convection, Liv.Rev.Solar Phys. 6, 2

Spectral lines, spatially and temporally averaged from 3-D models, change their strengths, widths, asymmetries and convective wavelength shifts across stellar disks, revealing details of atmospheric structure. These line profiles from disk center (µ = cos  = 1) towards the limb are from a CO 5 BOLD model of a main-sequence star; solar metallicity, T eff = 6800 K. (Models by Hans-Günter Ludwig, Landessternwarte Heidelberg) Spectral line profiles across stellar disks

 Hiva Pazira, MSc thesis, Lund Observatory (2012) Spatially resolving stellar surfaces

Stellar Spectroscopy during Exoplanet Transits * Exoplanets successively hide segments of stellar disk * Differential spectroscopy provides spectra of those surface segments that were hidden behind the planet * 3-D hydrodynamics studied in center-to-limb variations of line shapes, asymmetries and wavelength shifts * With sufficient S/N, also spectra of surface features such as starspots may become attainable

Exoplanet transit geometry G.Torres, J.Winn, M.J.Holman: Improved Parameters for Extrasolar Transiting Planets, ApJ 677, 1324

HD Promising star for spatially resolved spectroscopy Spectral type: G0 V, Apparent magnitude m V = 7.65 T eff = 6100 K, log g [cgs] = 4.50, [Fe/H] = 0 V rot  4.5 km/s; slow rotator, comparable to Sun sin i = 1 if the star rotates in same plane as transiting planet Sufficiently similar to Sun for same spectral identifications. Somewhat hotter, lines somewhat weaker, less blending. Large planet: Bloated hot Jupiter, R = 1.38 R Jup. More vigorous convection for line differences to be detectable?

Detailed similarities between the spectrum of the G0 V star HD and the well-studied solar spectrum enable numerous line identifications Spectrum of HD very similar to solar

Challenge of extremely high S/N * Retrieving good spectra from behind exoplanet covering  1% of star, requires S/N  10,000 !! * Transit time short: Largest telescopes required * High wavelength stability desired but loses light * High spectral resolution desired but loses light * Single spectral lines insufficient: Need averages * Single spectral lines insufficient: Need averages

HD extensively observed Several observatory archives searched (ESO/VLT/UVES, ESO/3.6m/HARPS, Keck/HIRES, SUBARU/HDS, etc.) Data used here originate from one ESO/VLT/UVES program: Program ID: 077.C-0379(A), “A POWERFUL NEW METHOD TO PROBE THE ATMOSPHERE OF EXOPLANET HD209458B” PI/COI: SNELLEN/ COLLIER CAMERON/ HORNE Observing date: UVES spectrometer slitwidth 0.50 arcsec Spectral resolution  60,000 Maximum S/N = 480

26 Fe I ‘similar’ photospheric lines in HD Their average ‘synthesizes’ a representative profile (ESO/VLT/UVES/REDL) Averaging many ‘similar’ photospheric lines

14 successive exposures during [part of] exoplanet transit Line profile is average of 26 Fe I photospheric lines Changes in representative profile during transit

Line profile changes during exoplanet transit. Red: Ratios of line profiles relative to the profile outside transit. This simulation sequence from a CO 5 BOLD model predicts the behavior of an Fe I line ( 620 nm,  = 3 eV) during the first half of a transit across the stellar equator by a bloated Jupiter-size exoplanet moving in a prograde orbit, covering 2% of a main-sequence star with solar metallicity, T eff = 6300 K, rotating with V sin i = 5 km/s. Simulated line changes during exoplanet transit

Observed line changes during exoplanet transit

Retrieving spatially resolved stellar line profiles

Reconstructed line profiles: H-beta

Reconstructed line profiles: H-alpha

Stellar Spectroscopy during Exoplanet Transits * Now: Marginally feasible with, e.g., VLT * Immediate future: LBT * Near future: VLT * Future: E-ELT ? * Future: E-ELT ? Anytime soon: More exoplanets transiting bright stars Anytime soon: More exoplanets transiting bright stars