Dr. Reed L. RiddleClose binaries, stellar interactions and novaeGuest lectureAstronomy 20November 2, 2004• Look at the forces acting on one body orbiting another- more pull on closer side• Subtract center of mass force- object is “squashed” and “stretched”Gravitational Tides• Gravitational tides are a differential force• Force of gravity:• Differential of gravitational force over dr:• Tidal force is:- proportional to mass of the primary- inversely proportional to distance cubed• Eventually, tidal force circularizes orbits and synchronizes rotation- constant of angular momentum- minimum energy stateGravitational TidesGravitational Tides• What if an object gets too close? • Stars close enough to each other will have large gravitational influence• Lagrange points- points of local gravitational stability• Roche Lobe- equipotential surface -- Equal Potential- balance of gravitational force between two stars• Mass transfer- stellar winds (miniscule)- Roche Lobe overflow (maybe)- much mass is lost to the system• Stellar interactions- Algol paradox- spectroscopic observations of “streams”!2 !10 1 2!3.0!2.5!2.0!1.5!1.0x/a"/[G(M1+M2)/a]L1L2L3M1M2Lagrange Points• Points of gravitational stability in two body system• Five points- L1, L2, L3 are unstable to perturbations- L4, L5 can be stable• Mass transfer can occurthrough the L1 point• Large energy releasefrom material that istransferred betweentwo starsLagrange PointsRoche Lobe• Named for Edouard Roche,who calculate the Rochelimit for planets• Equipotential surfaces ofgravitational force in a two body system• Material that crosses the Roche lobe can be transferred to the otherstar, into orbit in thesystem, or ejectedMass Transfer• Transfer depends on - distance between stars- mass fraction of the stars- dynamics of the binary system- types of stars• Stars are sloppy eaters- mass outside Roche lobe can leave system- mass transferred through L1 can leave system• As mass is transferred, stars move closer until M1 = M2• Algols - Algol systems comprise two ‘normal’ main sequence or subgiant stars in a semi-detached system.• W Serpens Stars - These are active Algol systems.• RS Canum Venaticorum Stars - Chromospherically active binaries of F and later spectral type components.• BY Draconis Stars - Similar to RS Canum Venaticorum systems.• W Ursae Majoris Contact Systems - Short period (0.2 to 0.8 day) contact binaries exhibiting very high levels of magnetic activity.• Barium and S-Star Binaries - Long period binaries in which the originally more massive component evolved into a white dwarf, transferring some of its nuclear-processed gas to a giant type K or M companion.Types of Close Binaries• Zeta Aurigae Systems - Long period binaries comprising interacting type G or K supergiant and hot (~type B) companion. Although not originally interacting systems, they become so when the more massive star evolves to become a supergiant.• VV Cephei Systems - Similar to Zeta Aurigae binaries except a type M supergiant replaces the G or K type.• Symbiotic Binaries - Long period (~200 to 1500 days) interacting binaries comprising a cool type M giant (sometimes a pulsating Mira- type variable) and a hot accreting companion such as a white dwarf, sub-dwarf, or low-mass main sequence star. Where the cool star fills its Roche lobe, the system becomes a symbiotic Algol.• Post-Common Envelope Binaries - Usually comprising a hot white dwarf or subdwarf, and a cooler secondary, which have passed through the common envelope phase. The binary nuclei of planetary nebulae are examples.Types of Close BinariesTypes of Close Binaries• Massive binaries- O and B stars- large, fast stellar winds- short lifespans• Colliding winds- X-ray observations- spectroscopic observations of “streams”- shock front between stars• “Exciting” evolution- intense interactions- stars large enough to supernova• X-Ray Binaries - Binaries with a neutron star or, more rarely, a black hole primary. Accretion onto the primary produces strong x-ray emissions.• A classical nova is a rapid brightening of a star, which then fades over a few weeks• Novae are thought to result from the explosion of an accreted gas layer on the surface of a white dwarf: - Surface layer builds up until temperature and density at base ignite nuclear fusion. - Layer is blown off in the explosion, which produces a nova plus an expanding shell.• Process repeats on a time scale of thousands of years.Novae• Analogous process on neutron star surfaces is called a Type 1 X-ray burst.Secondary starPrimary starAccretion diskL1HotspotMassstreamOrbit of secondary about center of mass ( )ASTR 3730: Fall 2003Simulation of a nova explosionrwhite dwarf surfaceDepending upon the accretion rate, accumulating gas can either build up without exploding, explode periodically as a nova, or burn