CHEOPS Science Workshop Venice 3-4 June 2014 · Valérie Van Grootel – CHEOPS workshop 2014 2 Why...

Characterizing planet-hosting stars

Valerie Van Grootel (University of Liege)

CHEOPS Science Workshop Venice 3-4 June 2014

How to get the stellar mass, radius, and age

Science Team WG A2: Target Characterization (S. Sousa, F. Bouchy, and associates)

Valérie Van Grootel – CHEOPS workshop 2014 2

Why is stellar characterization so important ?

Rp α R*

Mp α M*2/3

+ the age of the star is the best proxy for the age of its planets

(Sun: 4.57 Gyr, Earth: 4.54 Gyr)

CHEOPS = ultra-high precision photometry We want to be both precise and accurate on stellar target

⇒ combination of techniques

Radial velocities Transits

Valérie Van Grootel – CHEOPS workshop 2014

The targets of CHEOPS

3

Main sequence nearby bright solar-like stars: F, G, and K-types

Direct techniques: interferometry (R*), eclipsing binaries (M*, R*) Indirect techniques: GAIA parallaxes (R*), stellar evolution modeling (M*, R*, age), asteroseismology (M*, R*, age), transit light curve (ρ*)

Note (M. Gillon): CHEOPS won’t provide accurate ρ* from transits (shape and e≠0)

6<V<12


Interferometry

Available now: -  CHARA (Mount Wilson USA, 330-m baseline) -  VLTI (Chile, 150-m baseline)

By the launch of CHEOPS: MROI (New Mexico, 400-m baseline)

To get R* to ~ 1-2% (depending on size, distance and magnitude of the star)


GAIA parallaxes

To get R* to ~ 1-2%

(normally not affected by GAIA’s stray light issues)

Independently from interferometry ⇒ increase the accuracy on stellar radius

Method: Π: parallax Av: interstellar extinction BC: bolometric correction Teff: effective temperature (see Sousa et al. poster)


Stellar evolution modeling

To get R*, M* and age

5800620066007000

1.1

0.9

0.7

0.5

0.3 1.27M![Fe/H]= 0.22

1.40M![Fe/H]= 0.14

1.41M![Fe/H]= !0.08

4.85 Gyr

5.20 Gyr3.35 Gyr

3.85 Gyr

2.60 Gyr

2.95 GyrYG

!ov = 0.2!MLT = 1.8

Te! (K)

log(

""/

"!

)

WASP-68WASP-73WASP-88

Delrez, Van Grootel et al. (2014)

-  Teff, Z (from spectroscopy); Z~[Fe/H] (better if other abundances)

-  log g (spectroscopy), and/or ρ* (transits), and/or L* (parallax)

Inputs:

Stellar evolution codes: CLES (Liege), MESA (open source), Padova,…


Stellar evolution modeling


-  General method, always applicable -  Only require spectroscopic information, generally available (see

S. Sousa et al. poster for specific issues on spectroscopy) -  Get R*, M* and age

Pros:

Cons: -  Not very precise: Providing ~50 K on Teff, 0.05 dex on [M/H] and 1% on L* (GAIA):

R* to ~ 1-2% M* to ~ 5-10% Age to 2-3 Gyr

Main uncertainties: from stellar interiors (helium initial abundance, efficiency of convection, importance of mixing processes)

Will be improved in the coming years from bulk asteroseismic results from CoRoT and Kepler ?


Asteroseismology


Principle: Use stellar oscillations to constrain stellar interiors and to get structural parameters

For solar-like stars, main seismic indicators: the large separation Δν (α ρ*), frequency at maximum power

νmax (α g/Teff0.5), small separations δν (α age)


Asteroseismology

For CHEOPS targets:

•  Low-quality seismic data: only Δν~ρ* to 5% ρ* + R* (~1-2% by parallaxes/interferometry)

M* to 8-10% •  Mid-quality seismic data: 1-month TESS, ground-based (ESPRESSO) - Δν~ρ* to 0.5% (V=6) + R* (~1-2% by parallaxes/interferometry)

M* to ~3-4%

- Δν~ρ* to 2% (V=9) + R* (~1-2% by parallaxes/interferometry) " " " M* to ~5%

- Age still difficult to do better than 2-3 Gyr •  High-quality seismic data: not for CHEOPS (waiting for PLATO ) R* ~ 1-2%, M* ~ 2-4%, age ~ 10% (!)


About Eclipsing Binary Law

Enoch et al. (2010) proposed a M*(ρ*,Fe/H,Teff) relation

based on 19 eclipsing binaries (38 stars) of Torres et al. (2010) (M* and R* model-independent)

(X=log(Teff)-4.1)

Applied to Exoplanet-hosting stars (e.g. Gillon et al.): Precision of 3-4% on M* (and then R* from ρ*)



Are eclipsing binaries representative stars?

No: they are in binaries, generally close (Porb < 5 d) ⇒ More active stars than “normal” ⇒ Efficiency of envelope convection (M*<1.1 Msun) is lower ⇒ Stellar Radius is larger (and density lower) than “normal”

Sun and wide binaries

Close binaries

efficiency of convection

Ste

llar a

ctiv

ity p

roxy

Fernandes et al. (2012)



The M*(ρ*,Fe/H,Teff) relation is not unique

5200540056005800600062006400

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

YG

!MLT = 1.8[Fe/H]= 0.22

Te! (K)

log(

"!/

""

)

1.27M", !em = 0.21.18M", !em = 0.0

same ρ*,Fe/H,Teff, but not same M* (only the core extra-mixing is varied, in very reasonable proportion)

*



The M*(ρ*,Fe/H,Teff) relation is not unique

same ρ*,Fe/H,Teff for a while, but not same M* (only the initial He abundance is varied, in very reasonable proportion)

5600580060006200

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

!em = 0.2!MLT = 1.8[Fe/H]= 0.22

Te! (K)

log(

"!/

""

)

1.265M", YG1.333M", Y"


Conclusion

Work to do in years to come:

 Explore the validity of Eclipsing Binary Law for any star   Fully exploit high-quality seismic data from CoRoT and Kepler to improve accuracy on stellar mass and age from stellar models (constrain He, efficiency of convection, mixing processes)

•  Stellar radius: accurate and precise up to 1-2% (GAIA, interferometry)

•  Stellar mass: •  Brightest targets with TESS seismic data: M* ~ 3-4% (asteroseismology) •  More generally (9<V<12): M* ~ 5-10% (ρ* + R*, stellar evolution modeling)

•  Stellar age: not better than 2-3 Gyr (stellar evolution modeling, empirical methods)

CHEOPS Science Workshop Venice 3-4 June 2014 · Valérie Van Grootel – CHEOPS workshop 2014 2 Why...

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Transcript of CHEOPS Science Workshop Venice 3-4 June 2014 · Valérie Van Grootel – CHEOPS workshop 2014 2 Why...