Double white dwarfs and AM CVn binaries in the Galactic disc Gijs - - PowerPoint PPT Presentation

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

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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?

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Models for Galactic binaries: ingredients

• Description of stellar and binary evolution

– M, R, L, Mcore as function Mi, 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

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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) ∝ Σ1.5

G R−1

– Added bulge: mass consistent with dynamics and micro-lensing

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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

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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 P(M) ∝ M −2.5 MS79 MS79 KTG93 KTG93 P(m/M) cnst cnst cnst cnst cnst P(log a) cnst cnst cnst cnst cnst SFR(t) cnst cnst e−t/7Gyr cnst BP99 bin frac. (f) 1 1 0.5 1 0.5 f ×

• SFR dt

7 1010 8.7 1010 4 1010 7.2 1010 4.1 1010 αλ (γ) αT 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 (108) 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

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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

– αT 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

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Double white dwarfs - discussion - BP99 model

• 0.75 times lower birthrate (but similar total number)

5e+09 1e+10 "time ago" (yr) 5 10 15 SFR (Msun/yr) exponential SFR birth rate exponential BP99 + bulge birth rate BP99

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Double white dwarfs - discussion - BP99 model

• More binaries in centre than with exponential disc

5 10 15 20 distance 0.1 0.2 0.3 0.4 0.5 0.6 fraction exponential disc BP99 + bulge

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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

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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

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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

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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 f ×

• SFR dt

7 1010 4 1010 7.2 1010 4.1 1010 6.9 1010 αλ (γ) αT 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 (107) 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

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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

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Best guess LISA signals

• Population synthesis including all binaries with compact
• bjects
• 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

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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 (wd, wd) 2.9×10−2 12163 560 AM CVn 1.8×10−3 10117 49 (ns, wd) 1.4×10−4 21 3 (ns, ns) 3.2×10−5 1 (bh, wd) 3.8×10−5 1 (bh, ns) 1.0×10−5 total 22303 614

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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

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What can we learn from LISA?

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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