OBAMA OBAMA OBAMA Santiago, 7 Nov 2012 Science Seminar
Multiplicity of Spectroscopic Binaries among Massive Stars: Origin and Implications on “Stergers” Hans Zinnecker SOFIA Science Center, NASA-Ames “Blue Stragglers” Workshop, Santiago Science Seminar
Slide 1 (why I am here) Blue tragglers, in one model, are low-mass binary stellar mergers What about the chances for high-mass binary stellar mergers? Why would we be interested in massive binary stellar mergers? Science Seminar
Slide 2 (census of massive close binaries) Most massive born as very close binaries (almost all massive binaries short-period) Examples: M17 cluster, Orion Trapezium (hier. quadruple system, high multiplicity) ------------------------------------------------------- Recent O-star observational surveys (Chini et al. 2012, Sana et al. 2012) Stellar and dynamical evolution imply close/spectroscopic binaries will merge Science Seminar
The multiplicity of high-mass stars based on work by R. Chini et al M17 cluster image .. (Chini, priv. comm.) Celebrating the binary nature of young stars ….. Are almost all massive stars born as twins?
Stellar multiplicity as a function of mass • the binary fraction decreases from 90% (80 Mʘ) to 20% (3 Mʘ) • O stars like to be born as twins of similar mass
System parameters O6.5 V
Hugues Sana, priv. comm. Hughes Sana (priv. commun.)
Slide 3 (Chini et al., Sana et al. stats) O-star spectroscopic binary statistics (Galactic field stars and open clusters) Binary frequency as a function of mass Orbital period and mass ratio distribution Science Seminar
Slide 4 (exotic phenomena from HMIB) HMIB: High mass interacting binaries HMXB, msec pulsars, gamma-ray bursts Here: how mergers may help to explain long-duration gamma-ray bursts (GRB) Science Seminar
Slide 5 (theory of gamma-ray bursts) GRB collapsar model (CC supernova, BH) disk around stellar mass BH + relativistic jet GRB progenitor: rapidly rotating massive star of low-metallicity no wind, no spin-down Rapid rotation from coalescence of the two components of a massive close binary system Rapid rotation leads to fully mixed star and thus homogeneous stellar evolution Science Seminar
Slide 5b (theory of gamma-ray bursts) Note: Complete mixing and homogeneous evolution expected more often for higher mass stars, lower metallicity, and faster rotation (threshold) cf. Langer 2012 (Ann. Rev. Astron.Astrophys.) Science Seminar
Slide 6 (homogeneous stellar evolution) Binary merger (or stellar collision) rapid rot. Rapid rotation complete chemical mixing possibly stellar magnetic dynamo, rot. braking? danger to lose rapid rotation and GRB condition supernova explosion, type Ic (no hydrogen, WR) Rapidly rotating stellar core BH disk + rel. jet Science Seminar
Slide 7 (GRB production rate … RARE objects) Observed long-duration GRB rate: 1 event/day seen from Earth ( ~10^4 rarer than SN type II) Tentative explanation: beaming angle (few deg) and postulating stellar merger rate 1 per Myr per Hubble Ultradeep Field high-z dwarf galaxy (dense massive cluster birthrate is ca. 1 Myr-1) Case A binary evolution (orb. separation ~0.2AU) followed by binary merger due to tidal interaction Science Seminar
Slide 8 (origin of massive close binaries/SB2) Massive stars born by disk accretion with very high accretion rate; leads to puffed-up protostar (aka bloatar, size ~ few AU, like red supergiant) After accretion subsides, bloatar shrinks to become a hot O-star with size ~0.1AU; spin-up due to conservation of angular mom. Break-up due to dynamical instability: FISSION? (remains to be proven by numerical simulations) Science Seminar
Slide 9 (some key references) Chini et al. 2012, MNRAS (OB-star binary frequency) Sana et al. 2012, Science (orb. periods, mass ratios) Langer 2012, ARAA (massive star evol. with rotation) Yoon, Langer, Norman 2006 AA (chem. homog. evol.) Zinnecker 2007, ASPC 367 “interacting massive binaries” (GRB binary origin/merger in metal-poor clusters, R136) Hypothesis can be tested by high-spatial res. ELT obs. end Science Seminar
HANS ZINNECKER BASED ON WORK BY R. CHINI ET AL. ARE ALMOST ALL MASSIVE STARS BORN AS TWINS? Cf. MNRAS 2012 Science Seminar
Multiplicity of galactic O stars (Hartkopf 2010, Mason 2009) Category cluster/association field runaway Visual Multiplicity Total No. systems 249 56 42 Double / multiple 108 14 11 Duplicity fraction 43%25%26% Spectroscopic properties Total No. stars 272 56 42 Double / multiple 66 6 8 Suspected SB? 60 12 4 Duplicity fraction 57% 46% 29% Duplicity (without ?) 30% 15% 19% Fraction with any companion Total (without ?) 66% 41% 37%
Spectroscopic monitoring of southern high-mass stars • O stars: complete sample for V < 8 (but contains stars up to V ~ 10) • B stars: volume limited sample (d < 125 pc, V < 8) complementary sample (V < 8) for similar numbers in all spectral type bins • 4300 multi-epoch spectra for 250 southern O and ~ 470 B stars (3 < Mʘ < 80) • ~700 spectra from the ESO archive Where do the remaining 3600 spectra come from?
Armazones 2005 2008 2006 present Lat. = 24°35'53"S Long. = 70°11'47"W Alt. = 2817 m
Hexapod-Telescope 1.5 m active M1 BESO Fiber- Echelle-Spectrograph Δλ = 3700 Å – 8600 Å resolution λ/Δλ ~ 50.000 Fiber aperture = 3.4” This is where the remaining 3600 spectra come from!
Spectroscopic monitoring of high-mass stars O7 V Drawbacks • blind survey as far as time coverage is concerned • integration time adapted to primary component (detectable mass ratio > 0.5)
Multiplicity of galactic O stars (Hartkopf 2010, Mason 2009) Category cluster/association field runaway Visual Multiplicity Total No. systems 249 56 42 Double / multiple 108 14 11 Duplicity fraction 43%25%26% Spectroscopic properties Total No. stars 272 56 42 Double / multiple 66 6 8 Suspected SB? 60 12 4 Duplicity fraction 57% 46% 29% Duplicity (without ?) 30% 15% 19% New results 74% 48% 75%
Consequences for OB star formation in binary systems The OB binary star formation process seems to be a function of mass. The high frequency of close pairs with components of similar mass argues in favor of a multiplicity originating from the formation process rather than from a tidal capture. Many tight binaries (orbital periods less than a week) are being detected. The period distribution and mass ratio distribution are not yet well established. The high binary frequency of runaway stars points to the importance of ejection from young star clusters (as opposed to supernovae in binary systems, A. Blaauw) The small number of field O stars (6) suggests that all O stars are born in clusters. The high multiplicity for O stars is in agreement with the suggestion that most of the progenitors of high-mass X-ray binaries and double neutron stars must arise from massive interacting binary systems (van den Heuvel).
System parameters • determination of RV periods • preliminary results: P = 1.1 – 90 days • complementary observations started • disentangling composite spectra • just started (H. Lehmann, Tautenburg) • orbit parameters, mass ratios
System parameters P = 8.65 days M2 / M1 = 0.69
Eclipsing O star binaries • photometric monitoring of 250 O stars • (~ 20 – 30 epochs per star) • preliminary results: • 211 stars analyzed until now • 37 known variables • 34 (16%) new EBs • expectations: • ~ 40 new O-type EBs • revision of absolute O star magnitudes • improvement of O star masses two robotic 15 cm refractors