Download ASTR2050 Spring 2005 •

ASTR2050 Spring 2005
Lecture 10am 22 March 2005
Please turn in your homework now!
In this class we will cover “Close Binaries”:
• Orbits and evolution in close binary systems
• Novae and “Type I” Supernovae
• X-ray binaries and the binary pulsar
• SS433: A truly spectacular oddball
1
2
Orbits and Evolution in Close Binary Systems
Consider two heavy orbiting bodies, plus a very light one.
Equipotential surfaces: Saddles (L1,L2, L3) and Peaks (L4,L5)
For each heavy body, there is a region of space where all
gravitational forces point towards that body: Roche Lobe
See also Kutner Figure 12.1
3
Classifying Binary Stars by Orbit
Detached: “Normal” binary stars,
each with atmosphere within its
Roche Limit.
Semi-Detached: One star fills its
Roche Limit, so it can transfer
mass to its partner.
Contact: Both stars fill their
Roche Limits. Mass transfer
happens in both directions.
Kutner Figure 12.3
4
Mass transfer affects stellar evolution!
(a)
Star expands into a red
giant (b) and evolves
into a white dwarf or
(d) neutron star (d) which
then takes mass from
its partner (e,f).
(b)
Material “orbits” the
(e) compact star in an
“accretion disk”.
(c)
(f)
5
Example: How did
Sirius A & B happen?
Novae and “Type I” Supernovae
White Dwarf Stars in Close Binary Systems
T Pyxidis
Hydrogen gas flows from large
partner to the white dwarf.
It heats up by radiating energy
as it falls. (More on this soon!)
One Parsec
Kutner Figure 12.5
Eventually, enough piles on to
the white dwarf so that nuclear
fusion is ignited: “Nova”
This repeats with periods on
the order of hundreds of years.
Can it go on forever?? No!
6
Mass exceeds1.4MSun “Type I” Supernova
Different process than massive star core collapse (“Type II”)
“Light Curve”
6.1 days
77 days
56Fe
56Ni
56Co
Radioactive
decay heats
the envelope.
Note: Type I Supernova make excellent “standard candles”!
7
X-Ray Binaries
Close Binary Systems with a Neutron Star or Black Hole
Transferred mass m falls into a deep well and makes X-rays:
!
" !
"
GMm
GMm
GMm R is small so
E= −
− −
=
this is big.
!
R
R
Luminosity:
!
" Large values possible even for
dE GM dm
L=
=
modest mass infall rate dm/dt .
dt
R
dt
(See homework.)
How do these form? Interesting questions in stellar evolution!
8
Example: Her X-1=HZ Her
(X-Ray Source)
(Variable star)
The X-ray luminosity is ≈1000× Sun in Optical!
It pulses like a
neutron star (and
has consistent mass):
1.24 sec
Time in seconds
It eclipses like a
binary star (and
also in optical):
It also has a
period of 35 days (Precession?)
(Kutner Fig.12.7a)
9
1.7 days
The Mass Function and Black Holes
How can you determine the mass of an unseen companion?
Assume you observe the “normal” star spectroscopically!
Gm1m2
m2v22
2!r2
m
r
=
m
r
and
and
=
v
=
1
1
2
2
2
2
(r1 + r2)
r2
P
!
"
m2
Gm31
P 3
2
r1 + r2 = r2
+ 1 so
= r2v2 = v2
2
m1
(m1 + m2)
2!
!
"
3
3
P
m1 sin i
i
3
v
=
2
r
Earth
2!G
(m1 + m2)2
r1 +
"
! m2 1
+
r
2 m1
r2 =
i = “inclination
angle”
10
“Mass Function” (for m1)
(See Homework)
Radial Velocity (km/sec)
B1913+16: “The” Binary Pulsar
Gravitational Radiation
The orbit is
“decaying”
Time (Periods)
Orbit Precession
4.2deg/year
General
Relativity
11
SS433: A Truly Spectacular Oddball
See Studio Exercise on Friday
Doppler??
H!(3 → 2)
12