6.2 Mass transfer and accretion in binary systems

Binary systems can get wild when stars start swapping material. As one star expands beyond its Roche lobe, it begins dumping mass onto its companion. This cosmic dance reshapes both stars and their orbits, leading to fascinating outcomes.

Mass transfer happens in various ways, from gentle stellar winds to dramatic common envelope phases. The accreting star often forms a disk of hot gas, which can trigger outbursts and jets. These processes leave telltale signs we can observe across the electromagnetic spectrum.

Binary System Mass Transfer and Accretion

Mass transfer in binary systems

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• Exchange of material between two stars in close orbit occurs when one star expands beyond its Roche lobe
• Donor star loses mass and shrinks potentially becoming a white dwarf, neutron star, or black hole (Sirius B)
• Accretor star gains mass and grows larger possibly becoming a blue straggler or rejuvenated star (47 Tucanae)
• Orbital dynamics change altering period and redistributing angular momentum
• Roche lobe overflow happens when star expands beyond critical equipotential surface
• Material flows through L1 Lagrange point in close binary systems (Algol)

Modes of mass transfer

• Roche lobe overflow: material flows through L1 Lagrange point in close binaries (Cygnus X-1)
• Stellar winds: powerful winds captured by companion's gravity in wide binaries (WR 140)
• Tidal stripping: outer layers removed by tidal forces in very close systems (AM CVn stars)
• Common envelope evolution: stars share single envelope leading to rapid orbital decay (V1309 Scorpii)

Accretion disks in binaries

• Forms around accretor as material from donor conserves angular momentum
• Structure includes inner and outer regions with hot spot where material impacts disk
• Viscosity causes inward spiral generating heat through friction
• Thermal and viscous instabilities can trigger outbursts (dwarf novae)
• Magnetic fields shape disk structure influence accretion rate and create jets in some systems (SS 433)

Signatures of mass transfer

• X-ray emission from hot gas in accretion disk indicates compact object accretors (Scorpius X-1)
• Optical variability includes eclipses in edge-on systems and ellipsoidal variations from tidal distortion
• Doppler shifts in spectral lines reveal orbital motion
• Accretion disk produces broad emission lines and short-timescale flickering
• Outbursts and novae cause sudden brightness increases from disk instabilities or thermonuclear runaways (U Geminorum)
• Jets and outflows generate radio emission from relativistic particles and spectral signatures of high-velocity material (SS 433)