Lars Bildsten Kavli Institute for Theoretical Physics University of California Santa Barbara

1. Putting Helium Layers onto Massive White Dwarfs Lars Bildsten Kavli Institute for Theoretical Physics University of California Santa Barbara

2. The newfound prevalence of double white dwarfs with one being a low-mass (M<0.20M) helium white dwarf has re-motivated our exploration of possible explosive outcomes, as has the discovery of the first Helium burning star in a tight binary. Recent Collaborators: Jared Brooks (UCSB), David Kaplan (UWM), Kevin Moore (UCSB=>UCSC), Bill Paxton (KITP), Ken Shen (UCB), Justin Steinfadt (UCSB=>ATT), & Dean Townsley (U Alabama)

3. Disclaimer from my 2010 Leiden Talk “I was asked by the organizers to give a pedagogical talk…. so my referencing of all the vast literature is not thorough. Please don’t be offended if I don’t reference YOUR most excellent paper.” Trust me, I read your paper and I loved it!!

4. Double White Dwarfs + WD/SdB Stars Galore SDSS revealed a large population of Helium WDs • These stay bright due to a stably burning H envelope (Panei et al ’07) • Many found to be with WDs in tight orbits (Badenes, Brown, Kilic, Mullally, Steinfadt. . • Many will reach contact in 10 Gyr • Note lack of binaries with Mt>1.4 Kaplan, L.B. & Steinfadt ‘12

5. Helium Burning Stars + WD Binaries • A long discussed channel has been helium burning donors. These must come into contact in <100 Myr so that the Helium star is still burning and has a large radius. • Early work found one (KPD 1930+2752; Geier et al. ’07) but at Porb=136 minutes, too long. • Now have one good system CD-30 11223 (Vennes et al. 2013; Geier et al. 2013) with Porb=70 minutes and M1=0.74, M2=0.47. Will come into contact prior to burnout of the He burning star!

6. Gravitational Waves Drive the Objects into Contact: Direct Impact destabilizes many of the Double WDs Marsh, Nelemans & Steeghs ‘04

7. Outcomes for Unstable Mass Transfer • One distinct possibility is formation of an R Cor Bor Star, with a stably burning He shell on the WD core • Another option is that the He detonates and creates a thermonuclear transient. . . • I am not discussing these cases here . . . but, yes, I read your paper!!

8. Three Pieces when Stably Accreting • Resulting accretion rate sets He shell masses • He Burning stars made from Common Envelope evolution are large and fill RL at 20-40 minutes • Low mass He WDs that fill RL “sooner” than expected • Unstable helium shell burning outcomes • Defines a He Shell mass above which dynamical burning can occur. • Outcome from dynamical burning, potentially detonations that, themselves, can shock the C/O WD = > Type Ia ?

9. MESA is open source: anyone (over 600 users!) can download the source code, compile it, and run it for their own research or education purposes. Bill Paxton, Father of MESA

10. Second “Instrument Paper” just Appeared http://mesa.sourceforge.net Third MESA Summer School at UCSB August 11-15, 2014

11. Stable and Unstable Helium Burning Iben & Tutukov 1989

12. Mass Transfer for Roche Lobe Filling Low Mass Helium WDs Kaplan, Bildsten & Steinfadt ‘12 MWD=0.8

13. Mass Transfer for RL Filling He Burning Stars Brooks et al, in preparation

14. Mass Transfer Rates and Helium Mass for Ignition Helium Donor Mass Evolution in time Evolution in time • The higher accretion rate for the Helium WD donors lead to many flashes • He burning star donors (dashed line) have a much lower accretion rate and lead to thicker shell accumulation. Iben & Tutukov ‘89 He Ignition Mass MESA calculations of accumulated masses.

15. The radial expansion of the convective region allows the pressure at the base to drop. For low shell masses, this quenches burning. For a massive shell, however, the heating timescale set by nuclear reactions: Path to Dynamical Helium Shells will become comparable to lthe dynamical time, Allowing for a more dynamic outcome than just mass loss in a wind or RL overflow that would be a nova.. (V445 Puppis?) Shen & LB ‘09

16. MESA Calculations for He Accretion from a WD Donor • Series of weak He flashes: basically He Novae • Mass loss occurs due to Roche Lobe overflow during the Helium flashes • Final flash has a minimum heating time of 10 seconds (Bildsten et al. ‘07; Shen & LB ‘09) and becomes dynamical.

17. Onset of Rapid Burning => ? ? • From bottom to top, three different accretion rate, 1.0x10-7 M/year. 6x10-8 M/year. 3x10-8 M/year. • Lower accretion rates far more likely to become dynamical..

18. Burns Incompletely and Moves Slow (0.028 M He on a 1.0 MWD) Propagated at 9000 km/sec. Made mostly 44Ti, 48Cr, little 52Fe and 56Ni Townsley, Moore and LB, 2012 Distance (108 cm) Distance (108 cm) Distance (108 cm) Distance (108 cm)

19. Open Questions • He burning star channel gives He masses likely large enough to trigger detonations. Does this ignite the C/O or remain a .Ia like outcome (especially if there is an O/Ne WD)? • What about rotation? I don’t think it’s a big issue, as B fields drive accreted material into solid body rotation long before ignition; but see Yoon and Langer • More to follow on the diversity of outcomes of the He Detonations. .

20. MESA He Flash Calculation Accretion onto a 1.0 M at 3.7x10-8 M/year. Accumulated He was 0.093M whereas convective shell has 0.022 M