Double white dwarfs and AM CVn binaries in the Galactic disc Gijs Nelemans Institute of Astronomy, Cambridge Gijs Nelemans
Outline • Introduction • Models for Galactic binaries • Double white dwarfs – Comparison of results, discussion, observations • AM CVn binaries – Comparison of results, discussion, observations • Best guess LISA signals • What will we learn from LISA? • Conclusions Gijs Nelemans
Introduction • Most stars become white dwarfs and many are part of (close) binary system • After GWR brings them together, stable mass transfer may ensue: AM CVn systems • Close double white dwarfs and AM CVn systems are low-frequency gravitational wave sources • Double white dwarfs are so numerous that they form an unresolved noise for LISA • What do we know and what can we learn about these populations? Gijs Nelemans
Models for Galactic binaries: ingredients • Description of stellar and binary evolution – M , R , L , M core as function M i , t – Recipe for effect of winds, mass transfer, supernova etc on orbit • Initial parameter distributions – M (IMF), m/M , separation a , eccentricity e • Normalization and space distribution – Star formation history – Binary fraction – Galactic distribution Gijs Nelemans
Double white dwarfs • Alternative description common envelope Nelemans, Verbunt, & Yungelson, 2000, A&A, 360, 1011 • IMF Kroupa, Tout & Gilmore, MNRAS, 262, 545 • Galactic model Boissier & Prantzos, 1999, MNRAS, 307, 857 – Inside out disc formation – SFR ( R, t ) ∝ Σ 1 . 5 G R − 1 – Added bulge: mass consistent with dynamics and micro-lensing Gijs Nelemans
New double white dwarf observations, the SPY project • ESO VLT survey of ∼ 1500 white dwarfs for radial velocity variations (PI Napiwotzki) • Current status: – Surveyed 497 white dwarfs – 94 with radial velocity variations → 80 with white dwarf companion → 14 with main sequence companion – 9 double lined systems (see both white dwarfs) – 5 already have period determinations between 0.3 and 1.5 d Gijs Nelemans
Double white dwarfs - comparison with observations • Cooling models important Nelemans, Yungelson, Portegies Zwart & Verbunt, 2001, A&A, 365, 491, with updates Gijs Nelemans
Double white dwarfs - comparison with observations • Alternative common envelope description Alternative Standard Nelemans, Yungelson, Portegies Zwart & Verbunt, 2001, A&A, 365, 491, with updates Gijs Nelemans
Double white dwarfs - comparison ITY97 Han98 NYPV01 HTP02 This talk ∝ M − 2 . 5 P ( M ) MS79 MS79 KTG93 KTG93 P ( m/M ) cnst cnst cnst cnst cnst P (log a ) cnst cnst cnst cnst cnst e − t/ 7 Gyr SFR ( t ) cnst cnst cnst BP99 bin frac. ( f ) 1 1 0.5 1 0.5 7 10 10 8.7 10 10 4 10 10 7.2 10 10 4.1 10 10 � f × SFR d t αλ ( γ ) α T Y = 1 ∼ 0.5 2 (1.75) 1.5 2 (1.75) ν (yr − 1 ) 0.087 0.032 0.048 0.053 0.029 N (10 8 ) 3.5 1.0 2.5 ∼ 3? 2.3 ITY97 = Iben et al 1997, Han98 = Han, 1998, NYPV01 = Nelemans et al 2001, HTP02 = Hurley et al 2002 MS79 = Miller & Scalo 1979, KTG93 = Kroupa et al 1993, BP99 = Boissier & Prantzos 1999 Gijs Nelemans
Double white dwarfs - discussion • Initial distributions – ITY use different m/M and a distribution for close and wide binaries – With MS79 about 1.15 times more double white dwarfs than with KTG93 • Binary evolution – α T Y = 1 corresponds to roughly αλ = 5 – Han uses thermal energy of envelope in common envelope as well – Nelemans et al. alternative to common envelope give 1.7 times mode double white dwarfs – Minimum mass that evolves in age of Galaxy Gijs Nelemans
Double white dwarfs - discussion - BP99 model • 0.75 times lower birthrate (but similar total number) 15 exponential SFR birth rate exponential BP99 + bulge birth rate BP99 10 SFR (Msun/yr) 5 0 5e+09 1e+10 "time ago" (yr) Gijs Nelemans
Double white dwarfs - discussion - BP99 model • More binaries in centre than with exponential disc exponential disc 0.6 BP99 + bulge 0.5 0.4 fraction 0.3 0.2 0.1 0 0 5 10 15 20 distance Gijs Nelemans
AM CVn systems • AM CVn system are ultra-compact binaries with He rich secondaries • Three formation channels – From double white dwarfs that come into contact Paczy´ nski 1967 – From Helium star + white dwarf binaries Iben & Tutukov 1991 – From CV’s with evolved donors Podsiadlowski et al 2002/3? • Large uncertainty in formation of AM CVn systems Gijs Nelemans
AM CVn systems - uncertainties • Double white dwarfs: – Direct impact: large accretor size – Mass ratio of double white dwarfs • Helium star + white dwarf – Edge lit detonation • CV’s with evolved donors – Surface hydrogen – Small number at short periods Gijs Nelemans
AM CVn systems - stability mass transfer in double white dwarfs Marsh, Nelemans & Steeghs, submitted Gijs Nelemans
AM CVn systems - stability mass transfer in double white dwarfs Marsh, Nelemans & Steeghs, submitted Gijs Nelemans
AM CVn systems - stability mass transfer in double white dwarfs AM CVn systems - uncertainties • Double white dwarfs: – Direct impact: large accretor size – Mass ratio of double white dwarfs • Helium star + white dwarf – Edge lit detonation • CV’s with evolved donors – Surface hydrogen – Small number at short periods Gijs Nelemans
AM CVn systems - comparison with observations Nelemans, Portegies Zwart, Verbunt, Yungelson, A&A, 368, 939 Gijs Nelemans
AM CVn systems- comparison TY96 NYPV01 HTP02 This talk PHR023 7 10 10 4 10 10 7.2 10 10 4.1 10 10 6.9 10 10 � f × SFR d t αλ ( γ ) α T Y = 1 2 (1.75) 1.5 2 (1.75) ∼ 0.5 ν (yr − 1 ) 0.013 0.005 0.023 0.002 0.0007 N (10 7 ) 14 4.9 5.5 4.0 TY96 = Tutukov & Yungelson 1996, PHR02 = Podsiadlowski et al 2002 • Large differences in birth rate (mass ratio’s) • Total numbers rather similar Gijs Nelemans
New and future AM CVn observations • Three new (possible) AM CVn systems (7 were known) – V407 Vul (RX J1914.4+2456) P = 9.5 min, X-ray source Cropper et al. 1998, Ramsay et al. 2002, Mash & Steeghs 2002 – KUV 01584-0939 P = 10.3 min Warner & Woudt, 2002 – RX J0806.3+1527 P = 5.3 min, X-ray source Israel et al. 2002, Ramsay et al. 2002 Gijs Nelemans
Removing uncertainties with observations? • Stability of mass transfer in double white dwarfs – From observed possible short period systems • Overall numbers – Distances with HST – Survey for emission line systems • New systems from X-ray surveys? • Chemical abundances from accretion disc spectral modelling Gijs Nelemans
Best guess LISA signals • Population synthesis including all binaries with compact objects • Gravitational waves from (all) compact binaries Nelemans et al. 2001, A&A, 375, 890 • No angular resolution included • Double white dwarf noise background • Many resolved binaries • Quite a few with measurable frequency change Gijs Nelemans
Best guess LISA signals • resolved systems: 12000 (wd, wd), 10000 AM CVn Nelemans, 2002, LISA symposium proceedings Gijs Nelemans
Best guess LISA signals • resolved systems Type birth rate resolved systems with frequency (yr − 1 ) change 2.9 × 10 − 2 (wd, wd) 12163 560 1.8 × 10 − 3 AM CVn 10117 49 1.4 × 10 − 4 (ns, wd) 21 3 3.2 × 10 − 5 (ns, ns) 1 0 3.8 × 10 − 5 (bh, wd) 1 0 1.0 × 10 − 5 (bh, ns) 0 0 total 22303 614 Gijs Nelemans
What can we learn from LISA? • Probe parameter space difficult to reach in other ways • Sensitive to (rare) short period systems • Overall Galactic distribution • Systems with changing frequency: mass limits • Including angular resolution could increase number of detected (neutron star) binaries considerably Gijs Nelemans
What can we learn from LISA? Gijs Nelemans
Conclusions • Models not independent of observations • Improved statistics expected to improve models • LISA will resolve many thousands of Galactic white dwarf binaries and some neutron star binaries • These will check parts of parameter space that are inaccessible to other observations • Next step is to include LISA’s angular resolution Gijs Nelemans
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