Cataclysmic Variables

Cataclysmic Variables

Dr. Edward M. Sion
Department of Astronomy & Astrophysics
Villanova University

Contents:

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Cataclysmic Variables

Cataclysmic Variables (CVs) are a diverse class of short-period semi-detached binaries consisting of an accreting white dwarf primary star and (typically) a low-mass main-sequence secondary star as the mass donor. The orbital periods of CVs typically range from approximately 0.6 day (14 hr) to 0.06 day (90 min). Related systems containing accreting white dwarfs are Symbiotic Variables (red supergiant donors) and supersoft X-Ray binaries having more massive main sequence mass donors.

These binaries are quite small by astronomical standards: the binary separation is 1.1 (Porb/3 hr)2/3 (M1 + M2)1/3 times the Sun's radius of 0.7 X 106 km (where Porb is the binary orbital period in hours and M1 + M2 is the total mass of the binary in solar masses).

There is an interval in the distribution of orbital periods in which virtually no CVs with orbital periods between 2 hr and 3 hr are found (the so-called "period gap").

The orbital evolution of these binaries, and hence the mass-transfer rate (Mdot) from the secondary to the white dwarf is driven by magnetic braking of the secondary for long-period systems (Porb > 3 hr) and gravitational radiation for short-period systems (Porb < 3 hr).

The luminosity of CVs (like all compact binaries) is dominated by accretion:

L = GMdotMwd/Rwd ~ 2.2 (Mdot/10–9 Msun yr–1 (Mwd/Msun) (Rwd/104 km)–1 Lsun

where Mdot is the mass accretion rate in solar masses per year, Mwd is the white dwarf mass in solar masses, Rwd is the white dwarf radius in km, and Lsun is the Sun's luminosity of 4 X 1033 erg s–1.

CVs radiate primarily in the ultraviolet through X-ray bandpasses, hence they are studied extensively with space-based telescopes such as the Hubble Space Telescope (HST), the Hopkins Ultraviolet Telescope (HUT), the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrograph (ORFEUS), the Far-Ultraviolet Explorer (FUSE), the Roentgen Satellite (ROSAT), the Rossi X-Ray Timing Explorer (XTE), the Advanced Satellite for Cosmology and Astrophysics (ASCA), NASA's Chandra X-Ray Observatory, the X-Ray Multi-Mirror Mission (XMM) satellite, and the Extreme Ultraviolet Explorer (EUVE).

With the nearest systems at distances of ~ 100 parsecs (320 light years) from Earth, the space density of CVs is moderately large (a few X 10–5 parsec–3) and the total number in the Galaxy is huge (~ 106).

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Classical Novae

CVs are the progenitors of Classical Novae, which derive their luminosity from explosive nuclear burning on the surface of the white dwarf. Given the observed frequency of classical nova outbursts and the above CV space density, the recurrence time for a given system is ~ 104 to 105yr. This interval is comparable to or longer than the extent of human history, but short compared to the lifetime of a CV, so any given CV has been observed to experience at most one classical nova outburst. However, CVs experience many classical nova outbursts during their lifetimes.

The sequence of events leading up to a classical nova explosion is illustrated below. Figures and text are adapted from a 1997 Space Telescope Science Institute (ST ScI) press release (click here for more information). These were created with support from NASA contract NAS5-26555 to ST ScI, operated by the Association of Universities for Research in Astronomy, Inc., and reproduced with the permission of AURA/ST ScI.

The white dwarf captures matter lost through the inner Lagrange point of the secondary. To conserve angular momentum, this material cannot accrete directly onto the white dwarf, but forms an accretion disk around the compact star.

As it loses angular momentum, the material in the disk slowly drifts inward and accretes onto the surface of the white dwarf.

An envelope or "ocean" of hydrogen-rich material builds up on the white dwarf surface. The intense heat and pressure at the base of this envelope leads to a thermonuclear explosion as the the hydrogen is burned to helium. The explosion, which releases ~ 1046 ergs, blows off the outer layers of the envelope.

The nova outburst lasts for tens to hundreds of days. Eventually, as in these HST images of Nova Cygni 1972, the ejected envelope is visible as a limb-brightened shell expanding away from the binary (the central point sources in this images) at speeds of a few hundred to a few thousand km s–1.

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Cataclysmic Variable Sub-types

CVs are classified into various subgroups based primarily on the strength of the white dwarf's magnetic field. The major CV "flavors" include:

Nominally "non-magnetic" systems (dwarf novae and novalike variables).

Magnetic systems with field strengths in excess of about 106 gauss. Magnetic CVs are further subdivided into:

Intermediate Polars or DQ Her stars with magnetic field strengths ~ 106 to 107 gauss.

Polars or AM Her stars with magnetic field strengths ~ 107 to 109 gauss.

For perspective, the Earth's magnetic field strength is ~ 1 gauss and the strongest magnetic fields on the Sun are ~ 104 gauss.

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Non-magnetic Cataclysmic Variables

There are two important structures in a non-magnetic CV:

The accretion disk, where about half of the gravitational potential energy of the accreting material is released.

The boundary layer between the accretion disk and the surface of the white dwarf, where the kinetic energy of the flow is thermalized and radiated.

Because the effective temperature of the accretion disk ranges from ~ 5000 K at its outer edge to ~ few X 104 K at its inner edge, it radiates over a broad energy range from the optical through the far-UV.

Because of the small size and high luminosity of the boundary layer, its temperature is ~ 105 K (10 eV), so it radiates primarily in the EUV and soft X-ray bandpasses.

A single dwarf nova explosion is equivalent to 1 million trillion megatons of TNT!

In a number of dwarf novae during quiescence and in nova-like variables in low states of little or no accrection, the underlying white dwarf accretor dominates the system light in the far ultraviolet. This circumstance offers the opportunity to study how the deposition of mass, energy, and angular momentum during the accretion process affects the physical properties, structure, and evolution of the white dwarf.

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Intermediate Polars (DQ Her stars)

In intermediate polars, the accretion disk is disrupted at small radii by the white dwarf magnetosphere; the accreting material then leaves the disk and follow the magnetic field lines down to the white dwarf surface in the vicinity of the magnetic poles.

As the accreting material rains down onto the white dwarf surface, it passes through a strong shock where its free-fall kinetic energy is converted into thermal energy. The shock temperature is ~ 108 K (10 keV), so the post-shock plasma is a strong source of hard X-rays.

The X-ray, UV, and optical radiation is pulsed at the spin period of the white dwarf (Pspin) and the beat period between the spin and orbital periods is Pbeat = (1/Pspin – 1/Porb)–1.

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

Polars (AM Her stars)

In ,polars, the white dwarf magnetic field is so strong that:

The white dwarf is spin-synchronized with the binary (Pspin = Porb).

No disk forms – accretion takes place directly into the white dwarf magnetosphere.

During high optical brightness states, a brilliant accretion column dominates all other sources of light in the system. It radiates primarily by cyclotron and thermal bremstrahlung emission.

Like intermediate polars, polars are strong hard X-ray sources, but the X-ray, UV, and optical radiation is pulsed at the binary orbital period.

The photoionizing flux in polars and intermediate polars is brutal: all the exposed plasma in the binary – the inner face of the secondary, the accretion stream, the magnetosphere, and the surface of the white dwarf – are irradiated with a hard photoionizing flux of luminosity ~ 1033 erg s–1 or ~ 2 X 1010 megatons s–1.

Because an accretion disk cannot form, the magnetic white dwarf can be studied directly.

[ Cataclysmic Variables (CVs) ] [ Classical Novae ] [ CV Sub-types ] [ Non-magnetic CVs ]
[ Intermediate Polars (DQ Her stars) ] [ Polars (AM Her stars) ] [ Acknowledgements ]

I am deeply grateful to Dr. Chris Mauche of the Lawrence Livermore National Laboratory for kindly granting me permission to adapt much material from his LLNL website to this one. I am greatly indebted to Mr. Russell Kightly (Russell Kightly Media, www.rkm.com.au) and Mr. Mark Garlick (www.space-art.co.uk) for kindly allowing me to use their images of magnetic CVs for this site, and to Dr. Rex Saffer of Villanova University for site construction and maintenance.