ASTR377: A six week marathon through the firmament by Orsola De Marco Office: E7A 316 Phone: Week 6, May 24-27, 2009

Overview of the course

RGB: R ≤ 100; Capture radius ≤ 400 R o AGB: R ≤ 500; Capture R ≤ 5000 R o R Stellar evolution with a companion

The Roche Lobe The Roche Lobe is a volume of space around a star in a binary system where the material is bound to that star. Outside of this volume the material will flow to the other star. When stars expand they can fill their Roche lobe and transfer mass to a companion. Edouard Roche French

Roche Lobe overflow Spiral-in The common envelope interaction MeMe M2M2 McMc M 1 =M c + M e

Short-period binary Merged star The common envelope interaction

Double Degenerates & Supernovae Type Ia Close binary subdwarf Close-binary subdwarf-B Close-binary central stars of PN Cataclysmic Variables WD-main sequence binaries Low Mass X-ray binaries … also high mass binaries A bunch of “unexplainable” binaries (post common envelope binaries) If at least one of the two stars in a close binary was once larger than the binary orbit, we wonder what happened!

Physical description of the problem  E bin =  CE  E orb

The common envelope interaction efficiency  CE

Companion's orbit AGB star 6 AU How do we study it: common envelope simulations The determination of  CE De Marco et al 2003

Density greyscale Orbital Plane Iso-Density Surface of the Two Cores Density greyscale Perpendicular Plane Common envelope simulations Top-of-the-AGB giant and 0.1 Mo companion

1 Main Sequence Mass = 1.5 M o How do we study it: common envelope simulations Results of two simulations An additional energy source is in place: primary’s thermal energy. Interpretation of results needs to be interlaced with analytical work as well as stellar structure calculation (De Marco, Passy, Moe, Mac Low in preparation)

Types of evolved binaries Depending on the masses, separation and evolutionary phase of the stars, you will get a different observational phenomenon. There are therefore quite a few classes of evolved close binaries. A few examples: Novae, dwarf novae and type Ia SNe.

Cataclysmic variables A system goes through a common envelope resulting in a WD and a main sequence companion with small orbital separation. The system reduces its separation further due to magnetic breaking and the radiation of gravity waves. The WD accretes mass from the main sequence companion (which has filled its Roche lobe). The material accumulates in a disk which every so often (months) flushes itself onto the WD producing a small outburst. Eventually material piles onto the surface of a WD and ignites resulting in a Nova. Systems like this one might be one channel to SNe Type Ia because mass might be accumulating onto the WD till the Chandrasekhar limit is reached.

Accretion The luminosity achieved by a system (WD) accreting mass can be calculated by determining the potential energy of the accreted mass.