Cataclysmic Variables and Novae

In a cataclysmic variable a white dwarf pulls gas from a close companion, and the accreting material can erupt, in disk instabilities as dwarf-nova outbursts or in a thermonuclear flash that briefly makes the star thousands of times brighter as a classical nova.

Definition

Cataclysmic variables are close binaries in which a white dwarf accretes matter from a Roche-lobe-filling companion, and novae are the eruptions, driven by accretion-disk instabilities or by thermonuclear runaway on the white dwarf, that make such systems brighten dramatically.

Scope

The topic covers the structure of cataclysmic variable binaries, the accretion disk that forms around the white dwarf, the disk-instability outbursts of dwarf novae, the thermonuclear runaway on the white dwarf surface that produces classical and recurrent novae, and the connection of these systems to type Ia supernova progenitors.

Core questions

• What is a cataclysmic variable made of?
• Why do dwarf novae undergo recurrent outbursts?
• What causes a classical nova eruption?
• How are these systems linked to type Ia supernovae?

Key concepts

• accretion disk
• Roche-lobe overflow
• dwarf nova
• disk instability
• classical nova
• thermonuclear runaway
• recurrent nova

Key theories

Accretion and disk-instability outbursts

Gas transferred from the companion forms a hot accretion disk around the white dwarf; an instability in the disk, in which the gas switches between low and high viscosity states, causes the recurrent brightenings seen in dwarf novae.

Thermonuclear runaway and classical novae

Hydrogen-rich material accumulating on the white dwarf is compressed and heated until it ignites in a thermonuclear runaway, ejecting a shell of processed gas and brightening the system enormously; if such accretion grows the white dwarf toward the Chandrasekhar limit it may become a type Ia supernova progenitor.

Mechanisms

The companion overflows its Roche lobe and feeds gas through an accretion disk onto the white dwarf, releasing gravitational energy. A thermal-viscous instability in the disk dumps stored matter onto the white dwarf in dwarf-nova outbursts, while the steady buildup of hydrogen on the white dwarf's surface eventually ignites in a runaway that powers a classical nova eruption.

Clinical relevance

Cataclysmic variables are nearby laboratories for accretion physics and disk instabilities that also operate around neutron stars and black holes; novae contribute to galactic nucleosynthesis of certain isotopes, and the accreting white dwarfs are leading candidate progenitors of the type Ia supernovae used in cosmology.

History

Kraft established in the 1960s that cataclysmic variables are close binaries containing a white dwarf, the accretion-disk picture and disk-instability model of dwarf novae developed in the 1970s and 1980s, and the thermonuclear-runaway theory of classical novae was worked out by Starrfield and collaborators.

Debates

The single-degenerate channel for type Ia supernovae

Whether accreting white dwarfs in cataclysmic-variable-like systems grow to the Chandrasekhar mass and explode as type Ia supernovae, or whether most type Ia events instead come from white-dwarf mergers, remains an open question with implications for using them as cosmological tools.

Key figures

• Robert Kraft
• Brian Warner
• Sumner Starrfield
• Jorge Sahade

Seminal works

• warner1995
• starrfield2016

Frequently asked questions

What is the difference between a dwarf nova and a classical nova?

A dwarf nova brightens modestly and repeatedly because of instabilities in the accretion disk dumping matter onto the white dwarf, whereas a classical nova is a much larger, rarer eruption caused by a thermonuclear explosion of accumulated hydrogen on the white dwarf's surface.

Does a nova destroy the star?

No, a classical nova ejects only the surface layer that underwent the thermonuclear runaway; the white dwarf and its companion survive, so accretion resumes and the system can erupt again, with some recurrent novae doing so within decades.