ULTRACAM Observations of a Stripped Red Giant Star

1. ULTRACAM Observations of a Stripped Red Giant Star Pierre Maxted, Keele University, UK Ultracam Team, Andrea Miglio, Aldo Serenelli, Uli Heber

2. 1SWASPJ024743.37-251549.2

3. 1SWASPJ024743.37-251549.2 { { Total eclipse Transit

4. Other Pre-He-WD

5. Pre-He-WD evolution Possible CNO shell-flashes Very sensitive to details of model Driebe et al., 1999

6. Recycled pulsars • Giant/sub-giant stripped by neutron star • Spin-up of neutron star => binary millisecond pulsar • Two clocks … • white dwarf cooling age • pulsar spin-down age Lorrimer, 2008

7. Spin-down age v. cooling age • Cooling age - very sensitive to mass of H-layer on ELMWD • Are LMWD born with thick H-layers? • Models suggest yes, depending on total mass and metalicity • Shell-flashes can remove H-layers – hard to predict. • Pulsar irradiation may also play a role - ``black widow’’ binaries • PSR J0751+1807 • Teff ≈ 4000K – too cool to have a thick hydrogen envelope • white dwarf not massive enough for shell flashes • no evidence for strong irradiation by the pulsar • Spin down age • usually assume braking index n=3 – dipole radiation • Several young pulsars recently found with n≠3

8. Ultracam data

9. Ultracam data

10. Pulsation characteristics • J0247-25A • Metal poor δ Scuti star (SX Phe) • Non-radial p-modes, P ≈ 40 mins • Driving from κ-mechanism in He II ionisation zone • J0247-25B • Probably similar driving mechanism • Mixed modes, P ≈ 6 mins • Envelope – p-modes • Core – g-modes • Non-radial and radial with k ≈10

11. UVES follow-up of J0247-25 • MA = 1.356 ± 0.007M⊙ • MB = 0.186 ± 0.002M⊙ • A-star rotation is 30% sub-synchronous • Vrot sin i = 95 km/s • Vsynch sin i = 128 km/s

12. Asteroseismology

13. Radial v. non-radial modes

14. Mode ID – amplitude ratios • Limb darkening ⇒ amplitude varies with wavelength • Also phase differences • Ultracam very useful for measuring these • Model dependent Jeffery et al., 2004

15. Eclipse mapping

16. Eclipse mapping Janos Nuspl Barna Biro

17. EL CVn – type binaries … plus 12 others currently being followed-up.

18. Visualization of J0247-25

19. Spectra of J0247-25B

20. Spectral energy distribution

21. Conclusions • Studying EL CVn-type binaries will … • improve our understanding of HeWD • test predictions of models for HeWD formation • provide direct constraints on poorly understood aspects of binary star evolution • provide insights into the structure of RGB stars • Asteroseismology of J0247-25B will … • Enable the H-layer mass to be measured • Determine whether diffusion is important • Lead to more reliable cooling ages for HeWD • Test mode identification techniques

22. Why was J0247-25A not spun-up?

23. Very low mass white dwarfs • WD with masses ≾ 0.4M • Cannot form by single star evolution • Helium-rich core (HeWD) • Found in … • millisecond pulsars – Neutron star + HeWD • Kepler “hot white dwarf” binaries with A-/B-stars • cores of globular clusters – stellar collisions • galactic centre and near super-massive black holes • SDSS • Extremely Low Mass White Dwarfs (ELMWDs)

24. Hertzsprung-Russell diagram

25. Kinematics of J0247-25 • Thick-disk star • Age >~ 7 Gyr • Metal-poor: – 1 ~< [Fe/H] ~< – 0.3 • Enhanced α-element abundance • J0247-25 is a blue straggler

26. GMOS/NTT/WHT spectroscopy Spectral type mid-A KA = 39 ± 4 km/s γ = 53 ± 3 km/s

27. Pre-He-WD evolution Normal main-sequence Starts ascent of RGB Driebe et al., 1999

28. Pre-He-WD evolution Mass transfer to companion Hydrogen envelope stripped Driebe et al., 1999

29. Pre-He-WD evolution Shell hydrogen burning (p-p) Luminosity set by core mass Driebe et al., 1999

30. Pre-He-WD evolution Shell burning ends Start of WD cooling track Driebe et al., 1999

31. Pre-He-WD evolution Possible CNO shell-flashes Very sensitive to details of model Driebe et al., 1999

32. Pre-He-WD evolution WD cooling track Driebe et al., 1999