Stellar Spectroscopy during Exoplanet Stellar Spectroscopy during Exoplanet Transits Transits

Dissecting fine structure across stellar surfaces

Dainis Dravins*, Hans-Günter Ludwig, Erik Dahlén, Hiva Pazira

*Lund Observatory, Sweden, www.astro.lu.se/~dainis

KVA

STELLAR SURFACESSTELLAR SURFACESSimulations feasible for widely different stars

But … any precise physical conclusion depends on the reliability of modeling

(metallicity, magnetic activity, gravitational redshift, center-to-limb wavelength changes)

How does one verify/falsify 3-D simulations(except for the spatially resolved Sun) ?

High-resolution spectroscopy acrossspatially resolved stellar disks !

Granulation on a 12,000 K white dwarf (top) and a 3,800 K red giant. Areas differ by enormous factors: 7x7 km2 for the white dwarf, and 23x23 RSun

2 for the giant. (H.-G. Ludwig, Heidelberg)

Hiva Pazira, MSc thesis, Lund Observatory (2012)

Spatially resolving stellar Spatially resolving stellar surfacessurfaces

Stellar Spectroscopy during Exoplanet Stellar Spectroscopy during Exoplanet TransitsTransits

* Exoplanets successively hide segments of stellar * Exoplanets successively hide segments of stellar diskdisk

* Differential spectroscopy provides spectra of those * Differential spectroscopy provides spectra of those surface segments that were hidden behind the surface segments that were hidden behind the planetplanet

* 3-D hydrodynamics studied in center-to-limb * 3-D hydrodynamics studied in center-to-limb variations of line shapes, asymmetries and variations of line shapes, asymmetries and wavelength shiftswavelength shifts

* With sufficient S/N, also spectra of surface features * With sufficient S/N, also spectra of surface features such as starspots may become attainablesuch as starspots may become attainable

Hiva Pazira, MSc thesis, Lund Observatory (2012)

Spectral line hidden by exoplanetSpectral line hidden by exoplanet(Rapidly rotating solar model; noise and limited spectral

resolution)

Spatially averagedline profiles from20 timesteps, andtemporal averages.

= 620 nm = 3 eV5 line strengths

GIANT STAR

Teff= 5000 Klog g [cgs] = 2.5(approx. K0 III)

Stellar disk center;µ = cos = 1.0

Line profiles from 3-D Hydrodynamic Line profiles from 3-D Hydrodynamic simulationssimulations

Model predictions insensitive to modest spatial smearingModel predictions insensitive to modest spatial smearing

(Models by Hans-Günter Ludwig, Landessternwarte Heidelberg)

(Adapted from calculations by Hans-Günter Ludwig, Landessternwarte Heidelberg)

Profiles from CO5BOLD solar model; Five line strengths; three excitation potentials.

Left: Solar disk center. Right: Disk position µ = cos = 0.59.

Synthetic line profiles across stellar Synthetic line profiles across stellar disksdisks

Bisectors of the same

spectral line in

different stars

Adapted from

Dravins & Nordlund, A&A 228, 203

From left: Procyon (F5 IV-V),Beta Hyi (G2 IV),

Alpha Cen A (G2 V),Alpha Cen B (K1 V). Velocity [m/s]

In stars with “corrugated”

surfaces, convective blueshifts increase

towards the stellar limb

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 CO5BOLD model of a main-sequence star; solar metallicity, Teff = 6800 K.(Hans-Günter Ludwig)

Spectral line profiles across stellar Spectral line profiles across stellar disksdisks

Line profile changes during exoplanet transit. Red: Ratios of line profiles relative to the profile outside transit. This simulation sequence from a CO5BOLD 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, Teff = 6300 K, rotating with V sin i = 5 km/s.

Simulated line changes during exoplanet Simulated line changes during exoplanet transittransit

Apparent radial velocity during transit (Rossiter-McLaughlin effect). Wavelengths (here Gaussian fits to synthetic line profiles) are shorter than laboratory values due to convective blueshift. Curves before and after mid-transit (µ = 0.21, 0.59, 0.87) are not exact mirror images due to intrinsic stellar line asymmetries. This simulation from a CO5BOLD model predicts the behavior of an Fe I line ( 620 nm, = 3 eV) during 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, Teff = 6300 K, rotating with V sin i = 5 km/s.

Simulated Rossiter-McLaughlin effectSimulated Rossiter-McLaughlin effect

Observations with current facilitiesObservations with current facilities

For a few bright host stars, already current facilities, such as UVES @ VLT, permit reconstructions of stellar surface spectra, also for single stronger lines. Signatures of weaker photospheric lines require averaging over many similar ones. Improved possibilities will come once ongoing exoplanet searches find more transiting planets with bright host stars.

Observations with current facilitiesObservations with current facilities

For a few bright host stars, already current facilities, such as UVES @ VLT, permit reconstructions of stellar surface spectra, also for single stronger lines. Signatures of weaker photospheric lines require averaging over many similar ones. Improved possibilities will come once ongoing exoplanet searches find more transiting planets with bright host stars.

Observations with current facilitiesObservations with current facilities

For a few bright host stars, already current facilities, such as UVES @ VLT, permit reconstructions of stellar surface spectra, also for single stronger lines. Signatures of weaker photospheric lines require averaging over many similar ones. Improved possibilities will come once ongoing exoplanet searches find more transiting planets with bright host stars.

Observations with current facilitiesObservations with current facilities

For a few bright host stars, already current facilities, such as UVES @ VLT, permit reconstructions of stellar surface spectra, also for single stronger lines. Signatures of weaker photospheric lines require averaging over many similar ones. Improved possibilities will come once ongoing exoplanet searches find more transiting planets with bright host stars.

Stellar Spectroscopy during Exoplanet Stellar Spectroscopy during Exoplanet TransitsTransits

* Now: Marginally feasible with, e.g., UVES @ VLT* Now: Marginally feasible with, e.g., UVES @ VLT

* Immediate future: PEPSI @ LBT* Immediate future: PEPSI @ LBT

* Near future: ESPRESSO @ VLT* Near future: ESPRESSO @ VLT

* Future: HIRES @ E-ELT ?* Future: HIRES @ E-ELT ?

Anytime soon: More exoplanets transiting bright stars Anytime soon: More exoplanets transiting bright stars

Exoplanet transit geometriesExoplanet transit geometries

G.Torres, J.Winn, M.J.Holman: Improved Parameters for Extrasolar Transiting Planets, ApJ 677, 1324, 2008

Work plan – HD 209458Work plan – HD 209458HD 209458 selected as the most promising star for

resolving spectral differences across stellar surface.

Spectral type: G0 V (F9V, G0V)Teff = 6100 K (6082, 6099, 6118 ± 25 K)log g [cgs] = 4.50 (± 0.04)[Fe/H] = 0 (- 0.06, + 0.03 ± 0.02)Vrot = 4.5 (± 0.5 km/s); slow rotator, comparable to Sunsin 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 RJup.

More vigorous convection for line differences to be detectable?

Synthetic spectra for 5900 and 6250 K, log gcgs = 4.5, [Fe/H]= 0.

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