Asteroseismology of evolved stars: from hot B subdwarfs to white dwarfs

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Asteroseismology of evolved stars:

from hot B subdwarfs to white dwarfs

Valerie Van Grootel

Université de Liège, FNRS Research Associate

Main collaborators:

AG2014 – Splinter B

24 September 2014

S. Charpinet

(IRAP Toulouse)

G. Fontaine

(U. Montréal)

S.K. Randall

(ESO)

M.A. Dupret

(U. Liège)

E.M. Green

(U. Arizona)

P. Brassard

(U. Montréal)

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Evolved stars Asteroseismology

•

_{Hot B subdwarf (sdB) stars}

extreme horizontal branch

(EHB) stars

•

_{White dwarf pulsators:}

• _{GW Vir (atm. He/C/O)}

• _{DBV (atm. He)}

• _{DQV (atm. C-rich)}

• _{DAV (atm. H) = 80% of WDs}

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Valerie Van Grootel - AG2014 Splinter B, Bamberg

Hot B subdwarfs Asteroseismology

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Hot B subdwarfs

Hot (T

_eff

 20 000 - 40 000 K) and compact stars (log g  5.2 - 6.2)

belonging to the Extreme Horizontal Branch (EHB)

• _{He-burning to C+O (I), radiative He mantle (II) and very thin H-rich envelope (III)}

• _{Lifetime of ~ 10}8 _{ans (100 Myr) on EHB, then evolve to white dwarfs without AGB phase} • ~50% of sdBs reside in binary systems, generally in close orbits (P_orb  10 d)

> Short periods (P ~ 80 - 600 s), A  1%, p-modes (envelope)

> Long periods (P ~ 45 min - 3 h), A  0.1%, g-modes (mantle). Space observations!

> K-mechanism for pulsation driving (Fe accumulation in envelope)

2 classes of pulsators:

C ore Env elop e He/C/O core He mantle H-rich envelope log q log (1-M(r)/M_*)

M

_*

~ 0.5 M

_s

M

_env

< 0.005 M

_s

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The red giant lose its envelope at tip of RGB, when He-burning ignites (He-flash)

The formation of sdB stars

1.

Single star evolution:

enhanced mass loss at tip of RGB, at He-burning ignition (He-flash) mechanism quite unclear (cf later)

2.

The merger scenario:

Two low mass He white dwarfs merge to form a He core burning sdB star

•

_{For sdB in binaries (~50%)}

How such stars form is a long standing problem

in the red giant phase:

Common envelope ejection (CEE), stable mass transfer by Roche lobe overflow (RLOF)

•

_{For single sdB stars (~50%)}

Remains the stripped core of the former red giant, which is the sdB star, with a close stellar companion



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Common envelope evolution (close binary sdB systems)

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Stable Roche lobe overflow (wide binary sdB systems)

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Single sdBs: single star evolution or He-white dwarfs mergers

Envelope ejection at tip of RGB

mergers

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The formation of sdB stars

CE (common envelope) RLOF (Roche Lobe overflow)

mergers

Weighted mean distribution

for binary evolution:

(including selection effects)

0.30  M

_*

/Ms  0.70

peak ~ 0.46 Ms (CE, RLOF)

high masses (mergers)

Figures from Han et al. (2003)

•

Single star evolution

(“almost impossible”)

:

Mass range in

0.40 - 0.43  M

_*

/Ms  0.52

(Dorman et al. 1993)

•

_{Binary star evolution:}

_{numerical simulations on binary population synthesis}

(Han et al. 2002, 2003)

This is the theoretical mass distribution we want to test

by eclipsing/reflecting binaries and by asteroseismology

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Search the stellar model(s) whose theoretical periods best fit all the observed

ones, in order to minimize

• _{Optimization codes (based on Genetic Algorithms) to find the minima of S}2

• External constraints: T_eff, log g from spectroscopy

• _Results:_{global parameters (mass, radius), internal structure (envelope & core mass,…)}

The method for sdB asteroseismology

> Example: PG 1336-018, pulsating sdB + dM eclipsing binary (a unique case!)

 _{Light curve modeling (Vuckovic et al. 2007):} M  0.466  0.006 M_s, R  0.15  0.01 R_s,

and log g  5.77  0.06

 _{Seismic analysis (Van Grootel et al. 2013):} M  0.471  0.006 M_s, R  0.1474  0.0009 R_s, and log g  5.775  0.007

 Our asteroseismic method is sound and free of significant systematic effects

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Available samples of sdBs with known masses

I. The asteroseismic sample

15 sdB stars modeled by asteroseismology

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Available samples

II. The extended sample

(sdB + WD or dM star)

Need uncertainties to build a mass distribution

 7 sdB stars retained in this subsample

Light curve modeling + spectroscopy  mass of the sdB component

Extended sample:

15+7  22 sdB stars with accurate mass estimates

•

_{11 (apparently) single stars}

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Building the mass distributions

Extended sample:

(white) Mean mass: 0.470 Ms Median mass: 0.471 Ms Range of 68.3% of stars: 0.439-0.501 Ms

Binning the distribution in the form of an

histogram

(bin width    0.024 Ms)

Asteroseismic sample:

(shaded) Mean mass: 0.470 Ms Median mass: 0.470 Ms Range of 68.3% of stars: 0.441-0.499 Ms

No detectable significant differences between distributions

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Comparison with theoretical distributions



_{A word of caution: still small number}

statistics (need ~30 stars for a

significant sample)



_{Distribution strongly peaked near}

0.47 Ms



_{No differences between sub samples}

(eg, binaries vs single sdB stars)



_{It seems to have a deficit of high}

mass sdB stars, i.e. from the merger

channel. Especially, the single sdBs

distribution ≠ merger distribution.

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Comparison with theoretical distributions

(the majority of) sdB stars are post-RGB stars

The single sdBs distribution ≠ merger channel distribution

Han et al. 2003

merger channel

Single sdB stars can not be explained

only in terms of binary evolution via

merger channel

Moreover, Geier & Heber (2012): 105 single or in wide binaries sdB stars:

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sdB stars are (in majority) post-RGB stars, and

even post He-flash stars

So: the red giant has expelled almost all its envelope and

has reached the minimum mass for He-flash

•

_{At the He-flash: rather unlikely that these 2 events occur simultaneously}

MS mass of sdB progenitors:

0.7 – 1.8 M

_s

(e.g. Castellani et al. 2000)

•

_Alternative:

_{“hot flash”,}

_{after having left the RGB (before tip), in the}

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Extreme mass loss on RGB

•

_{For binary stars: ok, thanks to stellar companion}

•

_{For single stars, it’s more difficult:}

-

_{Internal rotation => mixing of He => enhanced mass loss on RGB}

(Sweigart 1997)

-

_{Dynamical interactions:}

_{Substellar companions}

_{(Soker 1998)}

If this scenario holds true, the red giant has experienced extreme mass loss

on RGB (Red Giant Branch)

What could cause extreme mass loss on RGB ?

Indeed, planets and brown dwarfs are discovered around sdB stars:

In short orbits:

•

_{5 planets}

_{(Charpinet et al. 2011, Silvotti et al. 2014)}

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A consistent scenario



_{Former close-in giant planets/BDs were deeply engulfed in the red giant envelope}



_{The planets’ volatile layers were removed and only the dense cores survived and}

migrated where they are now seen



_{The star probably left RGB when envelope was too thin to sustain H-burning shell}

and experienced a delayed He-flash (or, less likely, He-flash at tip of RGB)



_{Planets/BDs are responsible of strong mass loss and kinetic energy loss of the}

star along the RGB



_{As a bonus: this scenario explains why “single” sdB stars are all slow rotators}

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Conclusions



_{No significant differences between distributions of various samples}

(asteroseismic, light curve modeling, single, binaries, etc.)



_{Single star evolution scenario does exist; importance of the merger}

scenario? (single stars with presumably fast rotation)



_{A consistent scenario to form “single” sdB stars: delayed}

He-flasher + strong mass loss on RGB due to planets/brown dwarfs?

But:



_{Currently only 22 objects: 11 single stars and 11 in binaries}



Among  2000 known sdB, ~100 pulsators are now known



_{Both light curve modeling and asteroseismology are a challenge}

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White Dwarfs Asteroseismology

Focus: instability strip of DA white dwarf pulsators

(i.e. ZZ Ceti pulsators)

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White dwarfs

4 types of g-mode pulsators along

the cooling sequence:

• _{GW Vir stars}_{(He/C/O atmospheres)}

T_eff ~ 120,000 K, discovered in 1979

• _{V777 Her stars}_{(He-atmosphere), 1982}

T_eff ~ 25,000 K

• _{Hot DQ stars}_{(C-rich/He atmosphere)}

T_eff ~ 20,000 K, discovered in 2007

• _{ZZ Ceti stars}_{(H-atmosphere, DA)}

T_eff ~ 12,000 K, discovered in 1968 Most numerous (~200 known including

SDSS+Kepler)

Late stages of evolution of ~97% of stars in the Universe

From Saio (2012), LIAC40 proceedings

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Excitation mechanism of ZZ Ceti stars

• _{Don Winget (1981):}

H recombination around T_eff~12,000 K

 envelope opacity increase

 strangle the flow of radiation

 _{modes instabilities}

• _{Pulsations are destabilized at the}

base of the convection zone

(details: e.g. Van Grootel et al. 2012)

“convective driving”

log q log (1-M(r)/M_*)

Pulsations are driven when the convection zone is sufficiently deep and developed

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Valerie Van Grootel - AG2014 Splinter B, Bamberg 23

Pulsating DA white dwarfs

Empirical ZZ Ceti instability strip (classic view)

• _{Typical mass range: 0.5-1.1 Ms} • _{Multiperiodic pulsators, observed}

period range: 100-1500 s (g-modes)

• _{Reliable atmospheric parameters:}

work of Bergeron et al., Gianninas et al., here with ML2/α=0.6

• _{(most probably) a}_pure_strip

• log g/T_eff correlation (with a more

pronounced slope for red edge): the lower log g, the lower edge T_eff Observed pulsator ; O non-variable DA white dwarf



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Pulsating DA white dwarfs

Empirical ZZ Ceti instability strip (2014 view)

Hermes et al. (2012, 2013a,b):

5 Extremely-Low-Mass pulsators

non variable (<10mmag); pulsator

Hermes et al. (2013c):

1 ultra-massive pulsator

1.2 Ms 0.20 Ms

0.15 Ms

This is the observed

instability strip we want to

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• _{Evolutionary DA White Dwarf Models}

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Evolutionary DA models

C ore Env elop e C/O core He mantle H envelope log q log (1-M(r)/M_*) - 0 -4.0 -2.0

“onion-like” stratification

Base of the atmosphere

•

_{A standard DA white dwarf structure model (C/O core)}

•

Evolutionary tracks computed for 0.4M

_s

to 1.2M

_s

(0.1M

_s

step)

•

from T

_eff

35,000 K to 2,000 K (~150 models)

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Superficial

convection

zone

Detailed modeling of the

superficial layers

Our evolutionary models have the same T stratification as the complete (1D) model atmospheres

”feedback” of the convection on the global atmosphere structure

Base of the atmosphere

• _{Standard grey atmosphere}

• _{Detailed atmosphere}

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•

_{Extremely Low Mass (ELM) DA white dwarf:}

H envelope on top of He core

ELM white dwarfs come from stars that never experienced any He-flash, because of extreme mass loss on RGB (from binary interactions or due to high Z)

•

_{2 kinds of evolutionary tracks computed here:}

I. Standard C core models, but for 0.125M_s and 0.15-0.4M_s (steps 0.05M_s) II. Pure He core/H envelope models, for the same masses, thick envelopes

C ore Env elop e C core He mantle H envelope log q log (1-M(r)/M_*) - 0 -4.0 -2.0 C ore Env elop e He core H envelope log q log (1-M(r)/M_*) - 0 -2.0

Instability strip location in T

_eff

-log g plane insensitive to detailed core

composition and envelope thickness

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• _{Evolutionary DA White Dwarf Models}

• _{Time-Dependent Convection (TDC) Approach}

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The Time-Dependent Convection approach

•T_eff ~ 12,000 K: convective turnover timescale _conv  (pulsation periods)

 convection adapts quasi-instantaneously to the pulsations

•T_eff ~ 11,000 K: _conv≈  NEED full Time-Dependent Convection (TDC)

• Frozen convection (FC), i.e. _conv : NEVER justified in the ZZ Ceti T_effregime

For a standard 0.6M

_s

DA model:

(FC is the usual assumption to study the theoretical instability strip)

  1500 s   100 s

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The Time-Dependent Convection theory

•_{Full development in}_{Grigahcène et al.(2005)}_{, following the theory of}_{M. Gabriel (1974,1996)}_,

based on ideas of Unno et al. (1967)

•_{The Liege nonadiabatic pulsation code MAD}_{(Dupret 2002)}_{is the only one to implement}

convenient TDC treatment

•_{The timescales of pulsations and convection are both taken into account} •_{Perturbation of the convective flux taken into account here:}

•_{Built within the length theory (MLT), with the adopted perturbation of the}

mixing-length:

if   _conv(instantaneous adaption): if   _conv(frozen convection):

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• _{Evolutionary DA White Dwarf Models}

• _{Time-Dependent Convection (TDC) Approach}

• _Results

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Results: computing the theoretical instability strip

•

_{We applied the MAD code to all evolutionary sequences}

•“normal” CO-core DA models, 0.4 – 1.2M_s, log q(H)=-4.0 • ELM, C-core models: 0.125-0.4 M_s,log q(H)=-4.0

• ELM, He-core models: 0.125-0.4 M_s, log q(H)=-2.0 •0.17Ms, He-core models, “thin” envelope log q(H)=-3.7

•

We computed the degree l1 in the range 10-7000 s (p- and g-modes)

with ML2/  1, detailed atmospheric modeling, and TDC treatment

•

_{For the red edge (long-standing problem):}

based on the idea of Hansen, Winget & Kawaler (1985): red edge arises when



_th

~ P

_crit

α (l(l+1))

-0.5

(

_th

: thermal timescale at the base of the convection zone),

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Empirical ZZ Ceti instability strip (2014 view)

Spectroscopic estimates:

•

_{ELM white dwarfs: D.}

Koester models (Brown et

al. 2012)

•

_{UHM white dwarf:}

Gianninas et al. (2011)

•

_{Standard ZZ Ceti: P.}

Bergeron et al.

But all

ML2/α=0.6

(must be consistent)

+ standard ZZCeti: spectroscopic observations gathered during several cycles of pulsations

1.2 Ms 0.20 Ms

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Theoretical instability strip (g-modes l=1)

•

_{Structure models:}

ML2/  1

TDC blue edge

Red edge

Model atmospheres:

ML2/  0.6

•

_{Narrower strip at low masses}

(larger slope for the red edge)



Convective efficiency

increases with depth?

(consistent with hydrodynamical simulations; Ludwig et al. 1994, Tremblay & Ludwig 2011)

NB: evolutionary and atmospheric MLT calibrations are dependent

1.2 Ms 0.20 Ms

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Is the whole ZZ Ceti instability strip pure?

Theoretical instability strip (g-modes l=1)

•

_{Need only small fine-tuning}

•

_{J2228 is a little bit tricky}

•

_{Consistency between ML}

calibrations atmospheres 

structure models

•

_{Spectra must cover a few}

pulsational cycles !

YES

BUT

•

_{Are all ELM pure H (DA)}

white dwarfs or with traces of

He ?

1.2 Ms 0.20 Ms

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SDSS J1112+1117

T_eff~ 9400±490 K, logg ~ 5.99±0.12 He-core model, log q(H)=-2.0

Qualitative fit to the observed periods of the ELM pulsators

SDSS J1840+6423

T_eff~ 9140±170 K, logg ~ 6.16±0.06 He-core model, log q(H)=-2.0

SDSS J1518+0658

T_eff~ 9810±320 K, logg ~ 6.66±0.06 He-core model, log q(H)=4.0 and -2.0

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Conclusion and Prospects

•

_{Is the ZZ Ceti instability strip pure? Traces of He in ELM white dwarfs?}

•

_{Instability strip with structure models including 3D atmospheres?}

•

_{Asteroseismology of ELM/standard/massive ZZ Ceti pulsators}

1.

internal structure & fundamental parameters

2.

age

3.

understanding of matter under extreme conditions

Conclusions:

Prospects:

•

_{Excellent agreement between theoretical and observed instability strip:}

-Blue edge, TDC approach

-_{Red edge, by energy leakage through the atmosphere}

•

_{ELM pulsators are low mass equivalent to standard ZZ Ceti pulsators}

excited by convective driving

 such pulsators exist from 0.15 to 1.2 M

_s

(log g = 5 – 9 !)

•

_{Is ML2/α=1.0 the good flavor for convection inside white dwarfs?}

Related to spectroscopic calibration (here ML2/α=0.6) and 3D hydrodynamical simulations (Tremblay et al. 2011,2012, 2014)