ASTR 1102-0022008 Fall Semester

Joel E. Tohline, Alumni ProfessorOffice: 247 Nicholson Hall

[Slides from Lecture17]

Interacting Binary Stars

Interacting Binary Stars

Definition: A binary is said to be interacting if mass is transferred or exchanged between the components.

Note:The mass exchange dramatically alters the observational properties of the binary and the evolution of the components.

Mass transfer can affect the evolution of close binary star systems.

Kepler’s 3rd Modified by Newton:Pyr

2 = aAU3/Msuns

Interacting Binary Stars

Why some binaries interact and others don’t?

The following processes are inevitable in a binary:1)The binary separation decreases because of gravitational radiation and other angular momentum losses.2)The component stars will evolve and change size (for example becoming a red giant)

Conclusion:Long period (wide) binaries may never become interacting while short period (close) binaries are much more likely to interact at some stage.

Figu

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9-20

Interacting Binary StarsClassification based on

Roche Lobe Filling

Figu

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9-21

Mass transfer can affect the evolution of close binary star systems.

Semidetached binary where the large red-giant blocks the light from the more luminous, but smaller main-sequence star.

B8V

The Algol Paradox: the less massive star is more evolved!

K0III

Mass transfer can affect the evolution of close binary star systems.

Semidetached binary where mass transfer has produced an accretion disk. The light curve is shallow when the cooler star

and disk are eclipsed by the larger star.

B8V A7Vis eclipsed by the

Large

small star+disk

Mass transfer can affect the evolution of close binary star systems.

Overcontact binary in which both stars overfill their Roche lobes. The short period indicates that the two stars are quite close.

Accretion onto White Dwarfs

• Nova• Type Ia Supernova

Cataclysmic VariablesCataclysmic Variables

DONOR

ACCRETOR

Chandrasekhar Mass

Novae

Figure 21-14

Accretion onto a White Dwarf

Figure 21-15

Peak Luminosityaround 105 L

Figure 20-20a,d

Figure 20-22

Supernovae

Figure 20-20a,d

Figure 20-22

Supernovae

Figure 20-20a,d

Figure 20-22

SupernovaePeak Luminosity4x109 L

Supernovae Type Ia

• SN Ia are extremely luminous and can be seen in very distant galaxies.

• Their peak luminosities are very nearly the same (Lpeak ~ 4x109 L or M= -19). They are excellent Standard Candles.

• They are used to measure distances to galaxies far away.

Sloan Digital Sky Survey

Already rising… At the peak

Sloan Digital Sky Survey

Accretion onto Neutron Stars

• Millisecond pulsar (§21-7)• Black Widow pulsar• Pulsating X-ray sources (§21-8)• X-ray bursters (§21-9)

Different types of close binaries Different types of close binaries depending on the nature of the depending on the nature of the

compact objectcompact objectDonor is a “normal star” either Main-Sequence or Giant.

If the accretor is a

WD: cataclysmic variables including novae

NS: X-ray binaries including X-ray pulsars

BH: Soft X-ray transients, miniquasars

Low-Mass X-ray BinaryLow-Mass X-ray Binary

donorAccretion disk Accretion disk corona

Neutron star

Old pulsars stop pulsing when Old pulsars stop pulsing when they slow down, but some are they slow down, but some are

“reborn” in binary stars“reborn” in binary stars

While they are accreting, theyemit X-ray pulsesand are knownas “X-ray pulsars”

Old pulsars stop pulsing when Old pulsars stop pulsing when they slow down, but some are they slow down, but some are

“reborn” in binary stars“reborn” in binary starsWhen the NS is spinning fast enough, it is “reborn” as a millisecond pulsar, accretion stops and thecompanion is blasted by the pulsar radiation.The side facing the pulsar is hot and evaporating.

Millisecond pulsar

Black Widow Pulsar

Figure 21-11

The fastest pulsars were probably created by mass transfer in close binary systems.

Astronomers have cataloged at least 50 super fast pulsars, called millisecond pulsars, that have been “sped up” by mass from a

companion star that hits the neutron star and speeds it up.

EXAMPLE: PSR 1957+20, the “Black Widow”

Pulsating X-ray Source

Figure 21-13

Figure 21-12

X-ray Bursters

Figure 21-16

ON A WHITE DWARF: NOVA

ON A NEUTRON STAR: X-RAY BURST

Accretion onto Black Holes

• Primary method of identifying stellar-mass black holes (§22-3)

• Gamma-ray bursts (GRBs) (§22-4)

A non-rotating black hole has only a A non-rotating black hole has only a “center” and a “surface”“center” and a “surface”

• The black hole is surrounded by an event horizon which is the sphere for which light cannot escape

• The radius of the event horizon is the Schwarzschild radius (RSch= 2GM/c2) or

RSch= 3 km (M/M)• The center of the black hole is

a point of infinite density and zero volume, called the central singularity.

Maximum Neutron Star Mass

R(km)

M/M

3.0

6.0

9.0

2.01.0 3.0

Black Hole Binaries

• If the spectroscopy is of sufficient quality to enable a determination of the mass of the accretor and this mass exceeds 3 M;

• If the X-ray binary has never shown X-ray; bursts

• Then we consider such a binary as a “confirmed” Black Hole Binary

• We know at least 20 Black Hole Binaries.

Cygnus X-1

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2-10

BH X-Ray Binary Cyg X-1BH X-Ray Binary Cyg X-1

donor

Cygnus X-1Fi

gure

22-

11

Beamed Radiation

Figu

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2-12

GRBsFi

gure

22-

13a

GRBsFi

gure

22-

13b