Interacting Binary Stars

Algol-Type Binaries

Geary E. AlbrightSMP March 2, 2013

Algol - The Demon Star

Algol derives from Arabic ra's al-ghūl : head (ra's) of the ogre (al-ghūl) (see "ghoul") which was given from its position in the constellation Perseus, representing the head of Gorgon Medusa.

The Evolution of Binary-Star Systems

If the stars in a binary-star system are widely

separated, the evolution proceeds as if they were

single stars.

In closer systems, gas can transfer from one

star to another, changing the evolutionary

paths. These are interacting binary systems.

The Evolution of Binary-Star Systems

There are different types of binary-star systems, depending on how close the stars are.

In a detached binary, each star has its own Roche lobe:

The Evolution of Binary-Star Systems

Particles inside the lobe “belong” to the central star.

The Lagrangian points are where the gravitational forces are equal.

Evolution of binary systems:

Matter inside a star’s Roche surface is gravitationally bound to the star, but…

Matter can be transferred from one star to the other through the inner Lagrangian point.

Two ways in which matter can be transferred through L1;

1. Stellar wind (slow)

2. If the star expands past its Roche surface (rapid)

Gravitational field of the stars combined with the rotation of the system define the “Roche surface.”

The Evolution of Binary-Star Systems

In a semidetached binary, such as the Algol system, one star can transfer mass to the other:

Accretion disks:

Because stars rotate, matter that leaves the star has angular momentum…

Conservation of angular momentum creates an accretion disk.

Tidal forces and friction cause two things to happen;

1. Heats the disk

2. Dissipates the angular momentum and allows the gas to fall to the star

If the accreting star happens to be a white dwarf…

One of two things can happen…

Nova or supernova….

The Evolution of Binary-Star Systems

As the stars evolve, the type of binary system can evolve as well. This is the Algol system.

It is thought to have begun as a detached binary:

Novae:

A star that appears for a while and then fades away…

It’s not a new star, but an old star flaring up.

Hydrogen is accreted from the binary partner onto the white dwarf.

Very hot, dense layer of hydrogen accumulates on the white dwarf surface.

This layer grows denser and hotter until…

BAM!Hydrogen fuses in a sudden explosion that blows the surface off the star.

Nova Cygni 1975

~ 100,000 more luminous than the sun.

Explosion lasts only minutes to hours, the brightness fades in ~ 1-3 months.

Type Ia, Ib, and II Supernovae:

Type II:

Type I:

Contain hydrogen lines

No hydrogen lines

Produced by the collapse of a massive star

Leaves behind a neutron star or a black hole

Type Ia Type Ib, Ic

Produced by a collapsing massive star which lost its envelope to a binary companion

Produced when a WD accretes enough matter to exceed the Chandresekhar limit

WD completely blown apart… no NS or BH. (The WD contains usable fuel….)

Type Ib = Type II in which the massive star lost its atmosphere…

What are Algol binaries?

Algol is the most famous binary system in sky.

• Periods of 1 to 300+ days (this research on concentrates on the Short Period Algol Systems, P < 5 days)

• Primary star is a hot B or A type Main Sequence Star (prime of life), 10,000 -15,000 K

• Secondary star is a cool Giant or Subgiant star F-K III of IV (star that is dying) 4000 - 6000 K

Algol Systems

Eclipsing Systems• Primary eclipse is when the cooler dimmer

secondary star blocks the hot primary star.

• Secondary eclipse is when the hotter brighter primary star blocks the cooler secondary star.

• This was first suggested in 1783 by John Goodricke.

Algol System

Algol Light Curve

http://www.astro.uiuc.edu/~kaler/sow/algol.html

The Algol Paradox

Yet in Algol, the low mass star is a subgiant K (cool) star is in the process of dying. It’s companion is a higher mass B star that has a shorter lifespan, but is NOT dying.

How can this happen??

Stellar evolution theory tells us that larger mass stars have shorter life spans than lower mass stars…

The “Algol paradox”

The less massive star became a giant while the more massive star remained on the main-sequence!?!

This would correspond to the Algol system

2.5

1

M

Mass transfer explains this paradox!

Q: How can we explain the Algol paradox?

The Evolution of Binary-Star Systems

As the blue giant star entered its red giant phase, it expanded to the point where mass transfer occurred

When enough mass is accreted onto the smaller star that it becomes a blue giant, leaving the other star as a red subgiant

Why Study Algols?

• Some of the best astrophysical laboratories for studying gas streams and mass transfer in binary systems

• Can study variations in mass transfer rate

• Mass transfer is an important phenomenon in many other systems, Cataclysmic Variables, Novae, Type Ia Supernova, X-Ray Binaries, etc.

• Algols are the brightest systems with accretion disks surrounding them.

Phases of the Binary Orbit

• To study the distribution of gas in the disks surrounding these systems we use the technique of Doppler Tomography

•Produces a two-dimensional map of the gas in the system, but in Velocity Space not the Cartesian Space that we are used to…

•Why velocity space?

•How do you create a map in velocity space?

Doppler Tomography

Cartesian Vs. Velocity Space

Doing Doppler Tomography

• Observe an Algol binary, getting ideally 100 high resolution spectral observations evenly spaced around the orbit (wavelength vs. intensity)

•Using the Doppler formula (blackboard) convert the wavelength scale to a velocity scale… Why?

•The gas in the accretion disks surrounding in the Short Period Algols do not have the needed Keplerian velocity (blackboard) to be stable features.

Kitt Peak National Observatory Tucson, AZ

Visible Spectra

Absorption Spectrum of the Sun

Emission Line Spectrum

Spectra of Algol Systems Near Primary Eclipse

Why velocity space?

• Because the gas hits the star before it reaches a stable orbital speed, there is no way to tell where the gas is in the system.

• Therefore the only way to map it is with the velocities from the Doppler Shift and phases from the timing of the observations.

• Make a contour map using velocity and phase information.

Doppler Tomograms of U Sge

Movie Of Mass Transfer in Algol

What did we find?

• Two morphologies seem to be found in the Short Period Systems

• Disk-Like Morphology - Transient disk is found in the tomograms. U Sge, SW Cyg, TT Hya

•Stream-Like Morphology - Dominant gas seems to be along the path of the gas stream. U CrB

•What causes each of these? Why do systems switch from one type to another? U Sge and U CrB

Mass transfer rate? Ram pressure? Other?

QUESTIONS?