White dwarf populations. Gijs Nelemans Radboud University Nijmegen

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1 White dwarf populations Gijs Nelemans Radboud University Nijmegen

2 Outline Introduction: Population synthesis Basic ingredients Observational inputs White dwarf populations Different types and mergers WD + non-deg star Double white dwarf mergers: many outcomes Non-mergers: AM CVn systems WD + NS/BH mergers? Observations, now and future Conclusion and Outlook

3 Introduction: White dwarf progenitors White dwarfs: remnants of lowmass stars (M < ~8 Msun) Difference massive stars and low-mass stars Most of the action at radiative vs convective envelope phases

4 Introduction: Binary evolution Two stars, most massive evolves first MS Multiple moments for possible interaction Many possibilities Mass transfer BH/NS/WD/sdB Merger (Non-conservative) mass transfer or merger MS MS Mass transfer BH/NS/WD/sdB WD/sdB Mass transfer BH/NS/WD Mass transfer WD

5 Binary population synthesis Recipes for stellar and binary evolution (rapid) Portegies Zwart & Verbunt, 1996 Nelemans et al Model for initial distributions (M,m/M,P) Model for the star formation history Nelemans et al based on Boissier & Prantzos 1999

6 Binary population synthesis Common envelope, stellar wind... Model for initial distributions? Galactic model and reddening Schlegel et al dust map

7 How to treat mass transfer and mergers? Follow mass transfer vs. recipes Single star tracks: what about mass transfer and mergers? Best to use models that follow core evolution separately (dmc/dt ~ L) Speed?

8 Observational input Check stellar evolution Initial distributions Normalisation: Assume 1 binary for each single star Currently 2 binaries/singles formed per year Interesting binaries (M1 > ~0.9 Msun) 1 per 7.5 yr Total number ~6 Billion (current world population ~6.5 Billion)

9 Mergers Many stars merge! 22% of interesting binaries merge Current merger rate: 1 per 25 year Total number 1.3 Billion (current population China) Massive binaries (M1 > 8 Msun) Rate 1 per 2000 yr Total number 22 Million (current population Australia) Leiden? (110,000 BH + BH mergers!)

10 Merger types WD He* Giants MS MS Giants He* WD HeWD + HeWD HeWD + HerzprungGap CO WD + CO WD

11 White dwarf populations WD + Main sequence WD + Giant WD + WD sdb stars WD + NS/BH

12 White dwarf Main sequence Many systems Detached easily found Interacting: Cataclysmic variable Merger ~1/1000 yr Products (weird) giant?

13 White dwarf - Giant Symbiotic stars Merger ~1/250 yr Product: (weird) giant? White dwarf?

14 Double white dwarfs Theoretically predicted 1970s First one found in 1980s Typical formation... i.e. low-mass WD Nelemans et al. 2001

15 Expected population of double white dwarfs Total number: 100 million Birth rate: 1/50 years Merger rate: 1/125 years Including selection effects Compare to observations Reasonable agreement Nelemans et al. 2001a,b, 2005

16 Observations: SPY survey PI Napiwotzki 611 WD with 2 spectra Search for RV variation 34 certain binaries Less than previously thought 7 periods derived Many interesting other objects Also many single He WD Planetary interactions?

17 Formation of single He WD He white dwarf merger RG enhanced wind Ad hoc... Planet/brown dwarf Companion survives Companion just evaporates Soker et al. Nelemans & Tauris 1998

18 Double white dwarf mergers Type Ia supernova progenitors? Rates promising Short as well as long delays Rapid accretion more likely to produce AIC and NS? No real convincing case seen yet (V458 Vul?), few close ones WARNING Should be careful in comparing observations of possible progenitors Double white dwarfs: L ~ 1030 erg/s (Mv = +12), d max ~ 1 kpc! Single degenerate: L ~ erg/s in X-ray (Mv < 0), d max > 1 Mpc! (recurrent) novae: VERY strong bias towards high MWD

19 Double white dwarf mergers Around 90% of mergers will certainly not be SNIa... Some bright and easy(er) to study Many low(er)-mass mergers He + He single He WD, subdwarf B stars... He + CO R CrB stars/extreme He stars CO + CO Accretion induced collapse Single He/sdB connection with planetary companions

20 Non-mergers: mass transfer stability Classical stability q < 2/3 M2 > 0.3 Mass transfer (highly) super Eddington ( merger?) Ang. Momentum problem as direct impact Finite entropy (temperature) white dwarfs are larger and more stable Roelofs & Deloye in prep.

21 Non-mergers: AM CVn stars Short period variables Periods between 5.4 and 65 min He dominated spectra Different formation channels

22 Galactic populations: SDSS Detailed study of AM CVn stars in SDSS Not as many as hoped/predicted (by > factor 10!) Roelofs et al. 2007, MNRAS

23 Mining SDSS for new AM CVn systems Complete spectroscopic survey in colour-selected sample (Roelofs, Groot) ~1500 objects, should contain another ~40 AM CVns Currently several 100 observed (VLT, NOT, WHT...) 5 New systems found Current number of AM CVn systems 28

24 SdB stars and mergers SdB stars are He core burning stars Many sdb stars in close binaries (constrain CE) Not all Mergers? Single star evolution? Planets? Mergers sdb + sdb sdb + Giant sdb + WD Products?

25 WD + NS/BH mergers? WD + NS Ultra-compact X-ray binaries Stable mass transfer Stability: M2 < ~0.5 Msun Most will merge Mergers 1/5000 yr Product? WD + BH mergers 1 per million years Nelemans et al 2004, 2006, Werner et al 2006

26 Future observational work Finish SPY sample... Exploit SDSS (SWARMS Survey, GN, TRM work) New possibilities from massive photometry GAIA (launch 2012) Variability surveys (RATS, OmegaWhite, LSST, Pan-Starrs, SkyMapper?..)

27 Galactic populations: RATS & OmegaWhite RATS (Ramsay) Survey short timescales with wide field camera's on 2m class telescopes Happening NOW OmegaWhite (Groot) Survey short timescales with OmegaCam on VST (hopefully...) Start 2009 (?) Many new systems!

28 GAIA ESA mission, launch 2012 Astrometry, photometry and radial velocities of ~1 billion stars... Map structure Galaxy Marsh & Nelemans in prep

29 European Galactic plane surveys EGAPS Homogeneous survey of the plane b <5 in u g r i H HeI using INT and VST PIs Drew/Groot/ Gaensicke Many others: PannSTARRS, SkyMapper, PTF, LSST

30 Galactic Bulge Survey GBS: Optical and X-ray (Chandra) imaging of Galactic Bulge PI Peter Jonker Hundreds of X-ray sources Many X-ray binaries Model Galactic massive star populations

31 Galactic Bulge Survey Green: qlmxb Red: LMXB Blue UCXB Lightblue: qucxb

32 Conclusions Population synthesis is difficult Need observational checks and inputs But nice tool for sampling possible (non) mergers Many stars and many types merge Most are low-mass stars (MS + MS, MS + Giant, Giant + WD, WD + WD) Observations of merger events and non-mergers increasing Stay tuned...

33 Direct impact, He Novae,.Ia SN Early phase: direct impact Algol like geometry Matches light curves 2 observed systems Marsh & Steeghs 2002, Wood He accretion onto CO WD He Novae Different physics.ia faint thermonuclear SN Bildsten, Shen etc. 1e-8 1e-7 1e-6 1e-5 Mass transfer rate Ignition mass 0.1 1e-2 1e-3 1e-4 Rare but powerful

34 Gravitational waves: LISA: verification sources Five AM CVn systems Parallax from HST FGS VLT resolved spectroscopy reveals structure in disc and sometimes the accreting star can use to constrain masses Together with distances gives estimate gravitational wave signature! LISA Verification sources Roelofs et al. 2007, ApJ??

35 Gravitational waves from double white dwarfs Unresolved double WD background Above and at high f systems resolved: ~10,000 of both double WD and AM CVns Nelemans et al (too many AM CVn systems)