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Stellar Spectroscopy during Exoplanet Transits
Dissecting fine structure across stellar surfaces
Dainis Dravins*, Hans-Günter Ludwig, Erik Dahlén, Hiva Pazira
*Lund
Observatory, Sweden, www.astro.lu.se/~dainis
STELLAR SURFACES
Simulations 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 across
spatially 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 RSun2 for the giant. (H.-G. Ludwig, Heidelberg)
Spatially resolving stellar surfaces

Hiva Pazira, MSc thesis, Lund Observatory (2012)

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
Spectral line hidden by exoplanet
(Rapidly rotating solar model; noise and limited spectral resolution)
Hiva Pazira, MSc thesis, Lund Observatory (2012)
Line profiles from 3-D Hydrodynamic simulations
Model predictions insensitive to modest spatial smearing
Spatially averaged
line profiles from
20 timesteps, and
temporal averages.
 = 620 nm
 = 3 eV
5 line strengths
GIANT STAR
Teff= 5000 K
log g [cgs] = 2.5
(approx. K0 III)
Stellar disk center;
µ = cos  = 1.0
(Models by Hans-Günter Ludwig, Landessternwarte Heidelberg)
(Adapted from calculations by Hans-Günter Ludwig, Landessternwarte Heidelberg)
Synthetic line profiles across stellar disks
Profiles from CO5BOLD solar model; Five line strengths; three excitation potentials.
Left: Solar disk center. Right: Disk position µ = cos = 0.59.
Bisectors of
the same
spectral line
in different
stars
Adapted from
Dravins & Nordlund,
A&A 228, 203
In stars with
“corrugated”
surfaces,
convective
blueshifts
increase
towards the
stellar limb
From left:
Procyon (F5 IV-V),
Beta Hyi (G2 IV),
Alpha Cen A (G2 V),
Alpha Cen B (K1 V).
Velocity [m/s]
Spectral line profiles across stellar disks
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)
Simulated line changes during exoplanet transit
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 Rossiter-McLaughlin effect
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 Vsin i = 5 km/s.
Observations 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 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 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 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 Transits
* Now: Marginally feasible with, e.g., UVES @ VLT
* Immediate future: PEPSI @ LBT
* Near future: ESPRESSO @ VLT
* Future: HIRES @ E-ELT ?
Anytime soon: More exoplanets transiting bright stars 
Exoplanet transit geometries
G.Torres, J.Winn, M.J.Holman: Improved Parameters for Extrasolar Transiting Planets, ApJ 677, 1324, 2008
Work plan – HD 209458
HD 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 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 RJup.
More vigorous convection for line differences to be detectable?
Synthetic spectra for 5900 and 6250 K, log gcgs = 4.5, [Fe/H]= 0.
…