Subdwarf B stars from He white dwarf mergers Haili Hu Subdwarf B stars from various formation channels Haili Hu March 4, 2016March 4, 2016March 4, 2016 subdwarf B stars Extreme Horizontal Branch (Heber 86, Dorman et al 93) He-burning core (~0.5 M ) Very thin, inert H-envelope ( ~2 M : Non-degenerate He-ignition Hu et al 2007 Haili Hu, Stellar Mergers Lorentz Workshop March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop ULTRACAM / VLT lightcurves (May 18, 2005) P = 2.4 hr Vuckovic et al 2007 PG / NY Vir: a post-CE, eclipsing, pulsating sdB binary pre-CE phase of NY Vir At onset of mass transfer: or: Model assumptions: Z = 0.02 Reimers mass loss ( = 0.4) M 2 = 0.12 M P i (d) M ZAMS (M ) M core = M core,tip M core = M core,min March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Hu et al 2007 Results: The CE phase -formalism (Webbink 1984) : -formalism (Nelemans et al 2000, 2005) : where M giant, R giant, M remnant and E bind,env follow from evolution calculations, a f and M 2 from orbital light curve solution ~1.5 0 < < 1 March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Results: The CE parameter for NY Vir M Assuming M remnant = M sdB and M 2 = 0.12 M March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Hu et al 2007 Results: The CE parameter for NY Vir M Assuming M remnant = M sdB and M 2 = 0.12 M March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Hu et al 2007 March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop NY Vir From orbital lightcurve solution (Vuckovic et al 2007) and asteroseismology (Charpinet et al 2008): M-dwarf 0.12 M sdB 0.47 M March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Helium flash vs non-degenerate ignition Hu et al 2008 March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Helium flash vs non-degenerate ignition Comparison between two models with same log g and T eff Hu et al 2008 March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop M2M2 (i)Allow unstable frequencies of non-degenerate model to be matched to any frequency of post-flash model (ii) Assume mode identification: allow matching only to modes with same l - value (iii) Assume log g & T eff known: i.e. allow matching only to modes with ~same log g & T eff (iv) Allow matching only to unstable modes Helium flash vs non-degenerate ignition March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop He-flash vs non-degenerate models M zams = 1.50 M At RGB tip: M core = M March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop He-flash vs non-degenerate models M zams = 3.00 M At RGB tip: M core = M Thus seismic difference between post-flash (M zams ~2 M ) sdB stars Also true for sdB stars from He WD mergers? March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Helium flash vs non-degenerate ignition How to model a He WD merger Approach so far: Accretion of He on a He white dwarf (Iben 1990, Saio & Jeffery 2000, Han et al 2002)Accretion of He on a He white dwarf (Iben 1990, Saio & Jeffery 2000, Han et al 2002) Maybe ok for following subsequent evolution NOT ok for comparing seismic properties Besides global structure parameters (M *, R *, T eff ), we need detailed information on core composition, chemical stratifications, mass of H- envelope, March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Approach (future work) SPH simulation of WD merger product (or use entropy sorting algorithm?) (Rosswog) Import merger product into 1D stellar evolution code (Glebbeek) Evolve product further Compute pulsational properties (frequencies, excitation, mode degree) Compare observables: log g, T eff, f, l March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Approach (future work) SPH simulation of WD merger product (or use entropy sorting algorithm?) (Rosswog) Import merger product into 1D stellar evolution code (Glebbeek) Evolve product further Compute pulsational properties (frequencies, excitation, mode degree) Compare observables: log g, T eff, f, l March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop Difficulties/Problems How much H is left on merger remnant? i.e. how much H is burned during merger? How to deal with He flash? Numerical problem in the Eggleton code And many other issues discussed these weeks March 4, 2016March 4, 2016March 4, 2016Haili Hu, Stellar Mergers Lorentz Workshop