Multiplicity of contact binary stars

Theodor PribullaAstronomical Institute of the Slovak Academy of Sciences,

Tatranská Lomnica, Slovak Republic

Slavek M. RucinskiDavid Dunlop Observatory, University of Toronto,

Richmond Hill, Canada

Overview

1. Motivations and goals formation of close binaries

2. Contact binary stars

3. Assessment of capability of different observational techniques and present status of available data

4. Compilation of observational evidence for the presence of distant companions

Contact binary stars• Contact binary – system of two stars embedded in common envelope

almost uniform temperature distribution on the surface

• Solar-type components, early K, through G,F to mid A sp. types (few early-type systems exist)

• Orbital periods 0.22 to about 1 day

• Relatively easy to discover due to (i) short period (ii) continuous photometric variation (iii) all are SB2 since secondary is over-luminous, >700 systems in GCVS

• Lowest angular momentum between systems with non-degenerate components, for q < 0.05 AM can be stored in rotation of primary hard to form without third or multiple companions

• Many are members of triple or quadruple systems, e.g., BV Dra + BW Dra visual binary composed of two contact binaries

Detection of multiple components

1. Direct detection and astrometry

2. Spectroscopic methods

3. Photometric detection

4. Other indicators

Direct detection and astrometry• Direct resolving of the components: AO imaging at 3.6m CFHT undertaken

by SMR in 1998, H and K bands (1.65m and 2.2 m), FWHM of corrected images is 0.143 arcsec (diffraction limited), 7 new detections out of 45 targets with visual component at < 5 arcsec (GZ And, AH Aur, CK Boo, SW Lac, V508 Oph, U Peg and RZ Tau), third component in CK Boo clearly visible at = 0.12 arcsec, H and K indicate M-dwarf companions, new observations planned in September 2005

• Hipparcos astrometry: Hipparcos Double & multiple annex: (i) astrometric solution type (C,X,G) (ii) double or multiple star solution (A,B,C and S), (iii) Hipparcos parallax error, acceleration term (ER Ori, V2388 Oph), stochastic solutions with large “cosmic error” (CL Cet, BE Scl, AK Her, V401 Cyg, CW Sge) – sensitive to relatively short-period triples with P3 < 10 years

• TYCHO 2 proper motions: combined error of proper motion as indicator of multiplicity, dependence of proper-motion error on VT , sensitive on relatively long-period triples with P3 > 10 years

HIPPARCOS parallax errors

TYCHO2 proper motion errors

Percentage of known visual binaries, Hipparcos S,G and X

Distribution of projected separations of components in visual binaries containing contact binaries, visual component to EM Cep probably not physical, average mutual distance of stars in solar neighborhood 1 pc (log(d) = 5.3)

Spectroscopic detection [DDO]• Additional components to sp. lines or BFs: physical multiple usually if

V0 V3, (+) good estimate of β=L3/(L1+L2) at given wavelength, RV of 3rd component easily measured; (-) usually low precision of V0 , hierarchy unsure, few data points for V0 available, RVs of close pair affected by third peak, β must be reasonable (0.02 < β < 2), bias: does not depend on P3, and distance, just on β, 3rd components with low v.sin i most simply detected

• Systemic RV changes: (+) can detect shorter-period orbits, L3 can be small if m3 substantial (non-stellar components detectable); (-) inconclusive since V0 non-homogeneous usually, few data points; bias: sensitive to massive 3rd components on short-period orbits

• Composite spectra - detection of M dwarf features in averaged spectra, detection by spectral disentangling, (+) sensitive to lower β

VW LMi – quadruple (Vmax = 8.10, sp. F3V, P12 = 0.4775 days)

New spectroscopic multiples or SB comp. to known visual pairs from DDO program:quadruples: VW LMi (SB2+SB2), TV UMi (SB2+SB2), ET Boo (SB2+SB2)V899 Her (SB2+SB1), VZ Lib (SB2+SB1), HT Vir (SB2+SB1), EE Cet (SB2+SB2),SW Lyn (SB2+SB1), V410 Aur (SB2+ ?)triples: V401 Cyg, AG Vir

AG Vir (Vmax = 8.50, sp. A7V, P = 0.6425 days), active contact binary

TV UMi (Vmax = 9.09, sp. F8V, P = 0.41555 days), low-amplitude contact binary

ET Boo – known visual binary with P = 67 years, P12 = 0.6450 days (detached close binary), P34=31.5185 days (detached binary on highly eccentric orbit)

LITE• Light-time effect (LITE): periodic change of the apparent period due

to changes of distance to eclipsing binary (EB), result of finite speed of light, if enough times of minima the elements of EB around the mass center can be determined, m3 only if i3 known, otherwise only f(m), the orbital elements similar to sp. elements (V0 is derivative of distance); (+) sensitive in low-mass (and faint) companions with long P3, chance of detection accumulates with time and No. of minima, can be applied to pulsating stars (-) inconclusive due to other possible causes of cyclic changes (Applegate, spot cycles)

• XY Leo (K2V, V = 9.45 – 9.93, =15.861.80 mas, P = 0.2841 days), member of quadruple system with RS CVn-like binary (P = 0.8050 days)

• Both pairs 0.1” apart but perfectly detected by LITE with P = 19.6 0.5 years

•Feasible LITE solutions found in 18 out of 129 contact binaries having at least 15 CCD or pe minima in Krakow database (provided by Prof. Kreiner)•For the first time LITE interpretation in TY Boo, TZ Boo, CK Boo, GW Cep, UX Eri, V566 Oph and BB Peg•SW Lac possible quadruple system with double LITE solution

Percentage of feasible LITE solutions strongly depends on the number of available minima (rank) and time span of observations, 12 solutions for 20 best observed systems, indicate very high multiplicity

Other indicators and possible detection techniques

Period-colour relation for contact binaries, SPBE, (B-V)0 = 0.04 P-2.25

X-ray/bolometric flux ratio

Precession of the orbit in V685 Cen, i = 5 in 27 years, l3 = 0.2 – 0.6, P3 year, U several hundred years, Mayer, Pribulla & Chochol (2004)

•Tidally invoked precession of orbit: observed as changes of the LC amplitude, occurs in systems with low P3/P12 , in detached binaries can result in apsidal motion, U and P3 can be estimated from the rate of the inclination change (7 systems known IU Aur, SV Gem, SS Lac, AY Mus, RW Per, V907 Sco and V685 Cen)•No case between contact binaries...

Other techniques• Spectral energy distribution (SED)

• Infrared or UV excess, detection of pre-MS or WD components

• Period-colour-absolute magnitude diagram for contact binaries

• Lunar occultations: only XY Leo observed with negative result

• Third light (l3) in the LC solution – tight correlations with other photometric elements – multicolour LCs from UV to IR

Individual systems• CK Boo – third component detected by AO observations with CFHT and

indicated by LITE, Krzesinski (1991) detected small UV excess WD ?

• EE Cet – known visual binary, while southern component is contact binary, southern is also SB2 (according to DDO observations) with P ~ 0.5 days

• SW Lac – probable quadruple system indicated by LITE, lines of third component detected by Hendry & Mochnacki (1998)

• V752 Mon – contact binary is fainter comp. of the visual binary, primary is rapidly rotating (v.sin i ~ 220 km/s) A-type star obliterating CB in BFs

• AW UMa – known common proper-motion pair, spectroscopy by Pribulla et al. (1999) indicates possible massive body on short-period orbit

• CK Boo and BB Peg indicated by LITE + M-dwarf features (Prof. Kerkwijk), V2082 Cyg bye X-ray excess + M dwarf features,

Final statistics• For statistical investigation only systems brighter than Vmax = 10 mag taken

into account

• Primary source of systems is CCBS (Pribulla et al., 2002), present electronic version 391 systems

• Systems with suspicion from independent techniques taken as detections, results of Prof. Kerkwijk from averaged spectra taken into account

• 58 out of 138 (425%) objects seem to be multiple, on Northern sky 47 out of 84 (568%) objects are multiple, on Southern sky only 11 out of 54 (206%) were detected

• Taking all suspicions for Northern sky we see 68 9% incidence

• Part of true multiples are beyond the reach of any current technique, detected incidence supports an idea that all close binaries originate in multiple systems

Future prospects• Derived incidence needs further confirmation by thorough observations of individual

systems and reliable accounting for the observational bias

• Most multiples (44 out of 58 positive detections) were found by visual, AO or speckle observations most powerful method, detection must be supported at least by common proper motion or reasonable brightness and colour difference, even most distant visual pairs are physical

• Spectroscopic detection either in BFs or in averaged spectra follows

• LITE and other techniques require more data or longer time span than available at present

• Necessary observations: (i) spectroscopy and photometry of southern close binaries (ii) speckle and AO confirmation of LITE and Hipparcos astrometric indications (iii) lunar occultations (iv) dedicated search for M or L dwarf features in close binaries using TiO and VO bands (iv) simultaneous solutions (LITE, astrometry, RVs)

THE END