Where are the Accreting Helium White Dwarfs?? Drawing by T. Piro

Binary evolution (and common sense!) predicts that many accreting white dwarfs in CVs should have pure helium cores. Why have they not been found?We are undertaking a broad theoretical study to reveal how these objects may reveal themselves in CVs: Large radius (i.e. low gravity), and a cold core leads to large hydrogen ignition masses (Shara, Prialnik & Kovetz ’93) Long supersoft phases expected after Classical Novae Possibly in the ZZ Ceti instability strip, allowing for future seismic probing In collaboration with Irit Idan (Technion, Haifa) and Ken Shen (UCSB)

Binary Evolution Expectations de Kool ‘92; de Kool & Ritter ‘93; Iben & Tutukov ’93. Politano ‘96 Binaries tight enough so that the < 2 M  primary fills the Roche lobe on the first ascent of the red giant branch will reveal a degenerate He core of mass M  Some will be driven to short enough orbital periods by the common envelope to reach contact in 10 Gyr If the donor is low enough mass, this will be a stable mass transfer CV!

Resulting CV Expectations Due to mass transfer stability, longest possible orbital period is 3-4 hours (for Main Sequence donors) ~10-30% of CVs below the period gap should be Helium Core WDs de Kool ’92 Howell et al. ‘01 Where are they??

A Helium core white dwarf !!! Will come into contact with the brown dwarf in 1.5 Gyr, initiating stable mass transfer => CV! They are appearing as Pre-CVs!

A Partially Eclipsing White Dwarf Binary!! Steinfadt, Bildsten & Howell ‘08 Orbital period of hours Fantastic system for probing WD limb darkening, and MS properties Radial velocity of MS star found by Pyrzas et al. ‘08!

Helium WD Pre-CVs Binary NameWD Mass (M  ) Donor Mass (M  ) Merger Time (Gyr) References SDSS J ± ± Rebassa-Masergas et al. ‘08 SDSS J ± Rebassa-Masergas et al. ‘08 HR Cam0.41± ±0.004~5Maxted et al. ’98; Schreiber & Gaensicke ‘03 SDSS J Steinfadt et al. ‘08; Pyrzas et al. ‘09 WD Maxted et al ‘07 It is both a selection effect that the Main Sequence companions to these are only low mass (<0.25M  ), as finding the faint WD in the light of a more massive (and brighter) donor is difficult, and due to magnetic braking in more massive donors reducing lifetimes (Schreiber’s talk)...

GW-Driven Mass Transfer Rates We calculated two scenarios, both assuming that all accumulated mass is ejected in CNe for a 0.4 M  WD Scenario 1 (Solid line): Initial orbital period 2 hours (0.2M  donor) Scenario 2 (Dashed line) WD 0137 in the future, initial 0.05 M  Time to sort out how they will appear!

Hydrogen Burning Will Be Unstable Fujimoto ‘76; Nomoto et al ‘07; Townsley & Bildsten 2005; Shen & Bildsten 2008 Accumulated mass Taking the current WD temperature for WD 0137 and allowing it to cool for 1.5 Gyr prior to merger gives a core temperature of T c =6 x 10 6 K at contact Cold cores, low M, and low mass transfer rates => large classical novae ignition masses > M  (Shara et al ’93, but also Irit’s talk next!)

Bulk Property Evolution

Effective Temperature Evolution Townsley and Gaensicke ‘08 The WD core temperature evolution predicts a changing surface temperature, 10,000-12,000 K. We added these (red lines) to prior data and theory (Townsley & Gaensicke ’08) Pulsations may reveal a difference due to lower gravity (Townsley et al ‘04; Arras et al ‘06)

Mode Excitation Arras, priv. commun Conclusions from Arras, Townsley & LB ‘06 Biggest contrast with isolated WDs is the presence of Helium in the atmosphere. Contrast for lower mass is to make the blue edge colder likely <11,000 K Prevalence of pulsators at low effective temperature may be an indicator of He Core...but not definitive

M87 in Virgo Some numbers: 20 Classical Novae (Hydrogen fuel) per year, implying a white dwarf/main sequence contact binary birthrate (Townsley & LB 2005) of one every 400 years. One Type Ia Supernovae every 250 years, or one in 500 WDs explode! Two WDs are made per year in a M  elliptical galaxy. The observed rates for thermonuclear events are: Likely less than one in 100 CNe will come from the Helium core WDs. We don’t yet know if these will be unusual in some way, but current surveys are finding LOTS of CNe!

Kulkarni et al (2007) Kasliwal ‘09 Odd Novae Found.. more to come!

Long Lived Stable Burning Phase The large accumulated masses means that the resulting SS phase after the CNe lasts a very long time (Shara et al ‘93) Dashed line is nuclear luminosity, solid is total luminosity. Since the novae only occur every 0.1 Gyr, this may be how we find them. At ergs/sec for 1000 years, burning 4x10 -5 M  ~ 1000 year SuperSoft phase after the CNe

Supersoft Imposters! Supersoft sources are known to arise in two ways: - Stable burning due to high mass transfer (requires > 6-8 hr) - Prolonged stable burning after a CNe, for 1-10 years However, He WDs have a much longer stable phase post CNe Two very short orbital period systems are: RX J (3.5 hr) 1E (4.1 hr) King et al ’01 showed the challenge to the standard story of thermal timescale mass transfer at these short orbital periods.  Maybe these are He Cores post CNe! Greiner et al ‘00

Conclusions Binary evolution and known pre-CVs tell us that He accretors must be out there, our initial work has highlighted three issues: Cold core, low accretion rates and large radii lead to large ignition masses and prolonged (~1000 yr) stable burning phase => New mode for supersoft sources (all those systems that went off in the last 1000 year.... Expected rate=?) Low effective temperatures and seismology are less direct, but potentially better indicators Clearly need to continue radial velocity studies to directly find the Pre-Cvs of higher mass donors that might be unstable or marginally stable.