today in astronomy 328: binary stars - george mason...
TRANSCRIPT
Lecture 3 1
Today in Astronomy 328: binary stars
Binary-star systems.Direct measurements of stellar mass and radius in eclipsing binary-star systems.
At right: two young binary star systems in the Taurus star-forming region, CoKu Tau 1 (top) and HK Tau/c (bottom), by Deborah Padgett and Karl Stapelfeldt with the HST (STScI/NASA).
Lecture 3 2
Stellar mass and radius
Radius of isolated stars: Stars are so distant compared to their size that normal telescopes cannot make images of their surfaces or measurements of their sizes; this requires stellar interferometry.Mass: measure speeds, sizes and orientations of orbits in gravitationally-bound multiple star systems, most helpfully in binary star systems.
Observations of certain binary star systems can also help in the determination of radius and temperature.
There are enough nearby stars to do this for the full range of stellar types.
Lecture 3 3
Binaries
Resolved visual binaries: see stars separately, measure orbital axes and radial velocities directly. There aren’t very many of these. (Example: Sirius A and B.)Astrometric binaries: only brighter member seen, with periodic wobble in the track of its proper motion.Spectroscopic binaries: unresolved (relatively close) binaries told apart by periodically oscillating Doppler shifts in spectral lines. Periods = days to years.
Spectrum binaries: orbital periods longer than period of known observations.Eclipsing binaries: orbits seen nearly edge on, so that the stars actually eclipse one another. (Most useful.)
Sirius A and B in X rays (NASA/CfA/CXO)
Lecture 3 4
Measurements of stellar radial velocities with the Doppler effect
Radial velocity : the component of velocity along the line of sight.Doppler effect: shift in wavelength of light due to motion of its source with respect to the observer.
Positive (negative) radial velocity leads to longer (shorter) wavelength than the rest wavelength.To measure small radial velocities, a light source with a very narrow range of wavelengths, like a spectral line, must be used.
restobserved 0
rest 0
rvc
λ λ λ λλ λ
− −= =
rv
Lecture 3 5
Determination of binary-star masses using Kepler’s Laws
#1: all binary stellar orbits are coplanar ellipses, each with one focus at the center of mass.
The stars and the center of mass are collinear, of course.Most binary orbits turn out to have very low eccentricity (are nearly circular).
#2: the position vector from the center of mass to either star sweeps out equal areas in equal times. #3: the square of the period is proportional to the cube of the sum of the orbit semimajor axes, and inversely proportional to the sum of the stellar masses:
PG m m
a a22
1 21 2
34=
++
πb g b g
Lecture 3 6
Binary stellar orbit simulations
There are some useful computer simulations that you can run, written in Java by Prof. Terry Herter (Cornell U.) and his students, athttp://instruct1.cit.cornell.edu/courses/astro101/java/simulations.htmCheck out the simulations of:
Binary orbits with spectraEclipsing binary with light curve
Lecture 3 7
Eclipsing binary stellar system
Flux
Time
vr1v
2v
PrimarySecondary Minima
P
Lecture 3 8
Stellar masses determined for binary systems
If orbital major axes (relative to center of mass) or radial velocities known, so is the ratio of masses:
If furthermore the period and sum of major axis lengths known, Kepler’s third law can the used with this relation to solve for the two masses separately.
21 2 2
2 1 1 1
r
r
vm a vm a v v
= = =
Lecture 3 9
Stellar masses determined for binary systems (continued)
If only the radial velocity amplitudes v1 and v2 are known, the sum of masses is (from Kepler’s third law)
You’ll prove this in Homework #2.
If orientation of the orbit with respect to the line of sight isknown, this allows separate determination of the masses; that’s why eclipsing binaries are so important (if the system eclipses, we must be viewing the orbital plane very close to edge on: sin i is very close to 1).
31 2
1 2 .2 sin
P v vm mG iπ
+⎛ ⎞+ = ⎜ ⎟⎝ ⎠
Lecture 3 10
Stellar radii determined for totally-eclipsing binary systems
Duration of eclipses and shape of light curve can be used to determine sizes (radii) of stars:
Relative depth of primary and secondary brightness minima of eclipses can be used to determine the ratio of effective temperatures of the stars.
( )
( )
1 22 1
1 23 1
2
2
sv vR t t
v vR t t
+= −
+= −
TimeFl
ux1t 2t 3t 4t
Lecture 3 11
Example
An eclipsing binary is observed to have a period of 8.6 years. The two components have radial velocity amplitudes of 11.0 and 1.04 km/s and sinusoidal variation of radial velocity with time. The eclipse minima are flat-bottomed and 164 days long. It takes 11.7 hours from first contact to reach the eclipse minimum.
What is the orbital inclination?What are the orbital radii?What are the masses of the stars?What are the radii of the stars?
Lecture 3 12
Example (continued)
8.6 years
1.04km/s
11km/s
vr
Flux
Time
Closeup ofprimary minimum
11.7hours
164days
Lecture 3 13
Example (continued)
AnswersSince it eclipses, the orbits must be observed nearly edge on; since the radial velocities are sinusoidal the orbits must be nearly circular.Orbital radii:
11 10.61.04
s
s
vmm v
= = =
141.42 10 cm2
=9.5 AU
s sPr vπ
= = ×
131.34 10 cm2
=0.90 AU
10.4 AU
Pr v
r
π= = ×
=
Lecture 3 14
Example (continued)
Masses:
Stellar radii (note: solar radius = ):
3
2 15.2
10.61.3 , 13.9
s
s s
s
rm m MP
m mm M m M
+ = =
+ =⇒ = =
(Kepler’s third law)
(previous result)
( )3 12369
sv vR t t
R
+= −
=
( )
( )( )
2 1
-1
10
26.02 km s 11.7 hr
7.6 10 cm 1.1
ss
v vR t t
R
+= −
=
= × =
6 96 1010. × cm
Lecture 3 15
Data on eclipsing binary stars
Latest big compendium of eclipsing binary data is by O. Malkov. See following slides.This, and vast amounts of other data, can be found on line at the NASA Astrophysics Data Center:
http://adc.gsfc.nasa.gov/adc.html
Why do the graphs appear as they do? That’s what we’ll try to figure out, as we study stellar structure during the next few lectures.
Lecture 3 16
Luminosities of eclipsing binary stars (Malkov 1993)
0.1 1 10Mass (solar masses)
10-410-310-210-1100101102103104105106
Lum
inos
ity (s
olar
lum
inos
ities
)
Lecture 3 17
Radii of eclipsing binary stars (Malkov 1993)
0.1 1 10Mass (solar masses)
0.1
1
10
Radi
us (s
olar
radi
i)
Lecture 3 18
Effective temperatures of eclipsing binary stars (Malkov 1993)
0.1 1 10Mass (solar masses)
2
3
4567
104
2
3
45
Effe
ctiv
e te
mpe
ratu
re (K
)
Lecture 3 19
Hertzsprung-Russell (H-R) diagram for eclipsing binary stars (Malkov 1993)
40000 30000 20000 10000 0Effective temperature (K)
10-3
10-2
10-1
100
101
102
103
104
105
106
Lum
inos
ity (s
olar
lum
inos
ities
)
Lecture 3 20
H-R diagram for binaries and other nearby stars
Stars within 25 parsecs of the Sun (Gliese and Jahreiss1991)Nearest and Brightest stars (Allen 1973)Pleiades X-ray sources (Stauffer et al. 1994)Binaries with measured temperature and luminosity (Malkov1993)
1 105 1 104 1 1031 10 5
1 10 4
1 10 3
0.01
0.1
1
10
100
1 103
1 104
1 105
1 106
Effective temperature (K)
Lum
inos
ity (s
olar
lum
inos
ities
)