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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:

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Presentation on theme: "Asteroseismology of evolved stars: from hot B subdwarfs to white dwarfs Valerie Van Grootel Université de Liège, FNRS Research Associate Main collaborators:"— Presentation transcript:

1 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)

2 Valerie Van Grootel - AG2014 Splinter B, Bamberg 2 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 HR Diagram of pulsating stars

3 Valerie Van Grootel - AG2014 Splinter B, Bamberg 3 Hot B subdwarfs Asteroseismology Focus: Origin of hot B subdwarfs

4 Valerie Van Grootel - AG2014 Splinter B, Bamberg 4 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: Core Envelope 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

5 Valerie Van Grootel - AG2014 Splinter B, Bamberg 5 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   2 main scenarios:

6 Valerie Van Grootel - AG2014 Splinter B, Bamberg Common envelope evolution (close binary sdB systems) 6 CEE: sdB + MS star or white dwarf

7 Valerie Van Grootel - AG2014 Splinter B, Bamberg Stable Roche lobe overflow (wide binary sdB systems) 7 RLOF: sdB + MS star (later than F-G)

8 Valerie Van Grootel - AG2014 Splinter B, Bamberg Single sdBs: single star evolution or He-white dwarfs mergers 8 Envelope ejection at tip of RGB mergers or

9 Valerie Van Grootel - AG2014 Splinter B, Bamberg 9 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

10 Valerie Van Grootel - AG2014 Splinter B, Bamberg 10 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 Figure from Vuckovic et al. (2007)

11 Valerie Van Grootel - AG2014 Splinter B, Bamberg 11 Available samples of sdBs with known masses I. The asteroseismic sample 15 sdB stars modeled by asteroseismology (we took the most recent value in case of several analyses)

12 Valerie Van Grootel - AG2014 Splinter B, Bamberg 12 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 11 in binaries (including 4 pulsators)

13 Valerie Van Grootel - AG2014 Splinter B, Bamberg 13 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 (especially between singles and binaries)

14 Valerie Van Grootel - AG2014 Splinter B, Bamberg 14 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 sub­samples (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.

15 Valerie Van Grootel - AG2014 Splinter B, Bamberg 15 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: all are slow rotators (Vsin i < 10 km s-1)

16 Valerie Van Grootel - AG2014 Splinter B, Bamberg 16 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 contraction phase (Castellani&Castellani 1993; D’Cruz et al. 1996)

17 Valerie Van Grootel - AG2014 Splinter B, Bamberg Extreme mass loss on RGB 17 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) At least 2 BDs (Geier et al. 2011, 2012, Schaffenroth et al. 2014)

18 Valerie Van Grootel - AG2014 Splinter B, Bamberg A consistent scenario 18 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 Figure from Kempton 2011, Nature, 480, 460

19 Valerie Van Grootel - AG2014 Splinter B, Bamberg 19 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 (accurate spectroscopic and photometric observations, stellar models, etc.)

20 Valerie Van Grootel - AG2014 Splinter B, Bamberg 20 White Dwarfs Asteroseismology Focus: instability strip of DA white dwarf pulsators (i.e. ZZ Ceti pulsators)

21 Valerie Van Grootel - AG2014 Splinter B, Bamberg 21 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 DA (H-rich atmosphere): ~80%; DB (no/little H atmosphere): ~20% of WDs

22 Valerie Van Grootel - AG2014 Splinter B, Bamberg 22 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 Pulsating DA white dwarfs

23 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  Figure from Fontaine & Brassard (2008)

24 Valerie Van Grootel - AG2014 Splinter B, Bamberg 24 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 theoretically reproduce

25 Valerie Van Grootel - AG2014 Splinter B, Bamberg 25 Evolutionary DA White Dwarf Models The theoretical instability strip

26 Valerie Van Grootel - AG2014 Splinter B, Bamberg 26 Evolutionary DA models Core Envelope 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) with ML2 version (a  1,b  2,c  16);   1 (ie l  Hp)

27 Valerie Van Grootel - AG2014 Splinter B, Bamberg 27 Sup erficial 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 Evolutionary DA models

28 Valerie Van Grootel - AG2014 Splinter B, Bamberg 28 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 Core Envelope C core He mantle H envelope log q  log (1-M(r)/M * ) -- 0 -4.0 -2.0 Core Envelope 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 Evolutionary DA models

29 Valerie Van Grootel - AG2014 Splinter B, Bamberg 29 Evolutionary DA White Dwarf Models Time-Dependent Convection (TDC) Approach The theoretical instability strip

30 Valerie Van Grootel - AG2014 Splinter B, Bamberg 30 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 eff regime For a standard 0.6M s DA model: (FC is the usual assumption to study the theoretical instability strip)   1500 s   100 s

31 Valerie Van Grootel - AG2014 Splinter B, Bamberg 31 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 mixing-length theory (MLT), with the adopted perturbation of the mixing- length: if    conv (instantaneous adaption): if    conv (frozen convection):

32 Valerie Van Grootel - AG2014 Splinter B, Bamberg 32 Evolutionary DA White Dwarf Models Time-Dependent Convection (TDC) Approach Results The theoretical instability strip

33 Valerie Van Grootel - AG2014 Splinter B, Bamberg 33 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), which means the mode is no longer reflected back by star’s atmosphere

34 Valerie Van Grootel - AG2014 Splinter B, Bamberg 34 non variable (<10mmag); pulsator 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 0.15 Ms

35 Valerie Van Grootel - AG2014 Splinter B, Bamberg Theoretical instability strip (g-modes l=1) 35 Structure models: ML2/   1 TDC blue edge Red edge non variable (<10mmag);pulsator 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 0.15 Ms

36 Valerie Van Grootel - AG2014 Splinter B, Bamberg 36 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 0.15 Ms

37 Valerie Van Grootel - AG2014 Splinter B, Bamberg 37 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 Adiabatic properties are sensitive to exact interior structure

38 Valerie Van Grootel - AG2014 Splinter B, Bamberg 38 Conclusion and prospects

39 Valerie Van Grootel - AG2014 Splinter B, Bamberg 39 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)

40 Valerie Van Grootel - AG2014 Splinter B, Bamberg 40 Supp slides

41 Valerie Van Grootel - AG2014 Splinter B, Bamberg What does it imply ? 41 (the majority of) sdB stars are post-RGB stars, and even post He-flash stars The star has removed all but a small fraction of its envelope and has reached the minimum mass to trigger He-flash at tip of RGB, as a classic RGB-tip flasher ? (classic way for HB stars) an alternative (old and somewhat forgotten) idea: Hot He-flashers (Castellani&Castellani 1993; D’Cruz et al. 1996) i.e., stars that experience a delayed He-flash during contraction, at higher T eff, after leaving the RGB before tip (H-burning shell stops due to strong mass loss on RGB) D’Cruz et al. (1996) showed that such stars populate the EHB, with similar (core) masses -> It’s rather unlikely that the 2 events occur at the same time !

42 Valerie Van Grootel - AG2014 Splinter B, Bamberg Another hint: Horizontal branch/EHB morphology There is a gap between EHB and classic blue HB (BHB) 42 This suggests something “different” for the formation of EHB and classic HB stars Green et al. (2008) Size of dots related to He abundance


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