White dwarf populations - Lorentz Center · Outline ‣Introduction: ‣ Population synthesis ‣ Basic ingredients ‣ Observational inputs ‣ White dwarf populations ‣ Different

White dwarf populations

Gijs NelemansRadboud University Nijmegen

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

Introduction: White dwarf progenitors

‣ White dwarfs: remnants of low-mass stars (M < ~8 Msun)

‣ Difference massive stars and low-mass stars

‣ Most of the action at radiative vs convective envelope phases

Introduction: Binary evolution

‣ Two stars, most massive evolves first

‣ Multiple moments for possible interaction

‣ (Non-conservative) mass transfer or merger

‣ Many possibilities

MS

BH/NS/WD/sdB

BH/NS/WD/sdB

BH/NS/WD

MS

MS

WD/sdB

WD

Mass transfer

Mass transfer

Mass transfer

Mass transfer

Mer

ger

Binary population synthesis

• Recipes for stellar and binary evolution (rapid)

• Model for initial distributions (M,m/M,P)

• Model for the star formation history

Portegies Zwart & Verbunt, 1996Nelemans et al. 2001

Nelemans et al. 2004 based on Boissier & Prantzos 1999

Binary population synthesis

• Common envelope, stellar wind...

• Model for initial distributions?

‣ Galactic model and reddening

‣ Schlegel et al dust map

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?

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)

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!)

Merger types

MS

MS

Giants He*

Giants

WD

He*

WD

HeWD + HerzprungGap CO WD + CO WD

HeWD + HeWD

“White dwarf” populations

‣ WD + Main sequence

‣ WD + Giant

‣ WD + WD

‣ sdB stars

‣ WD + NS/BH

White dwarf – Main sequence

‣ Many systems

‣ Detached easily found

‣ Interacting: Cataclysmic variable

‣ Merger ~1/1000 yr

‣ Products

‣ (weird) giant?

White dwarf - Giant

‣ Symbiotic stars

‣ Merger ~1/250 yr

‣ Product:

‣ (weird) giant?

‣ White dwarf?

Double white dwarfs

‣ Theoretically predicted 1970s

‣ First one found in 1980s

‣ Typical formation

‣ ... i.e. low-mass WD Nelemans et al. 2001

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

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?

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

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 ~ 1037 – 1038 erg/s in X-ray (Mv < 0), d max > 1 Mpc!

‣ (recurrent) novae: VERY strong bias towards high MWD

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

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.

Non-mergers: AM CVn stars

‣ Short period variables

‣ Periods between 5.4 and 65 min

‣ He dominated spectra

‣ Different formation channels

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

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

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?

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

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?..)

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!

GAIA

ESA mission, launch 2012Astrometry, photometryand radial velocitiesof ~1 billion stars...

Map structure Galaxy

Marsh & Nelemans in prep

European Galactic plane surveys EGAPS

Homogeneoussurvey of the plane |b|<5in u g r i H HeIusing WFC @ INTand OmegaCam@ VST

PIs Drew/Groot/

Gaensicke

•

‣ Many others: PannSTARRS, SkyMapper, PTF, LSST

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

Green: qLMXBRed: LMXBBlue UCXBLightblue: qUCXB

Galactic Bulge Survey

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....

Direct impact, He Novae, .Ia SN

‣Early phase: direct impact

‣ Algol like geometry

‣ Matches light curves 2 observed systems

‣ He accretion onto CO WD → He Novae

‣ Rare but powerful

‣ Different physics

‣ .Ia faint thermonuclear SN

Mass transfer rate

1e-8 1e-7 1e-6 1e-5

Ignition

mass

1 0.1 1e-2 1e-3 1e-4

Bildsten, Shen etc.

Marsh & Steeghs 2002, Wood 2009

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

??

Gravitational waves from double white dwarfs

Unresolveddouble WDbackground

Above and athigh f systemsresolved: ~10,000 of both doubleWD and AM CVns

Nelemans et al. 2004

(too many AM CVn systems)