Molecular Clouds, Jeans

1
Molecular Clouds, Jeans Mass and T-Tauri Stars
2
Mass 40 35 20 H I H2 H I H II
3
about 35 of the mass of the interstellar medium
4
Molecular Clouds

• Traced by radio emission from molecules,
  especially CO and H2
• Radial distribution peaks at 3.5 7.5 kpc
• about 1 cloud every 1 2 kpc within ring
• density 104 atoms cm-3 (variable) T 10
  100 K
• 10 pc in diameter (Orion 30 pc)
• Half or more of hydrogen interior to suns orbit
  is H2
• Contain dust. Dust shields molecules from
  destruction by starlight esp. uv.

5
(No Transcript)
6
Molecular Clouds (continued)

• Live 106 107 years
• Definitely regions of active star formation
• M 104 106 solar masses. Taurus a few
  thousand solar masses Orion 200,000
• Dust 0.1 to 1 of the mass
• See many molecules including organic
  molecules.
• Star formation may propagate as a front
• Origin uncertain. May be formed by compression
  of ISM around a large OB association.
  Propagates?

7
(No Transcript)
8
In 2003 an amino acid, glycine, was detected in a
molecular cloud
9
123 different molecules have now been detected in
space. Most are organic. HC11N is the heaviest.
http//www.cv.nrao.edu/awootten/allmols.html
http//dsnra.jpl.nasa.gov/IMS/
10
Nearest Example of a Giant Molecular Cloud Orion

• Size 30 pc (diameter)
• distance 500 pc
• M 200,000 solar masses
• Age 12 My
• Evidence for thousands of embedded young
  stars. Best seen in infrared.

11
Trapezium
12
(HST 1997)
The Trifid Nebula, 3000 parsecs away in the
constellation Sagitarius, is also a molecular
cloud where new stars are being born. Here the
bright emission of the central stars is eroding
the surroundings of several nearby stars about 8
light years away. Note the nebula is quite dusty.
The stalk has survived because at its tip there
is still gas that is dense enough to resist being
boiled away by the nearby bright stars.
13
Star birth in the EagleNebula, 7000 light
years away in the constellation Serpens. This is
a column of cool molecular hydrogen and dust that
is an incubator for new stars. Each finger- like
protrusion is larger than our solar system. This
pillar of creation is being slowly eroded
away by the ultraviolet light of nearby young
stars.
14
(No Transcript)
15
0.5 m
1 m 10-6 m 10-4cm
large variation in grain sizes but all very small
16
1000 Angstroms
10-5 cm 0.1 micron
17
(No Transcript)
18
These infrared line features are
characteristic of ices and silicates.
19
Light is most effectively scattered by dust that
has a size comparable with the wavelength of the
light
20
(No Transcript)
21
(No Transcript)
22
Star Formation
23
The Jeans Mass
Ignore factor of 2 in the Virial Theorem
24
(No Transcript)
25
(No Transcript)
26
Mass 40 35 20
H I H2 H I H II
27
(No Transcript)
28
How long does the collapse take?
29
(No Transcript)
30
Power of observing in the infrared
31
T-Tauri discovered by John Hind in 1852 as a 10th
magnitude star. A faint nebula was subsequently
discovered nearby (Hinds nebula). Both the
star and nebula had variable brightness. The
nebula was a reflection nebula, shining from
the reflected light of T-Tauri. By 1861 the
nebula disappeared from view and by 1890 T-Tauri
itself had faded to 14th magnitude, about the
limit of telescopes then. A faint nebula at the
site of T-Tauri itself was observed at that
time, Over the next 10 20 years,
T-Tauri brightened back to 10th magnitude and
its local nebula became invisible against the
glare. T-Tauri has remained at about 10th
magnitude since (but varies).
T-Tauri in Taurus close to the Pleaides
32
T-Tauri Stars

• Short lived phase in life of stars under 2
  solar masses. Heavier stars evolve quicker
  and start burning by the time the star is
  visible.
• Accretion disks and jets are common features
• Emission and absorption lines
• Powered by gravitational contraction, not
  nuclear burning
• May be forming planetary systems
• High lithium abundance
• Embedded in dense, dusty regions
• Can be highly variable

33
When the star first becomes visible it may still
be surrounded by the gas and dust from which it
formed. Often jets are seen.
Because of rotational support matter hangs up in
the equatorial plane forming an accretion
disk. Matter first rains down on the poles, but
then later reverses direction in a strong outflow
called a jet.
34
T-Tauri Stars
Young binary T-Tauri stars clearing out the disk
around them.
UY Aur
Canadian-French-Hawii Telescope HST composite.
Infrared, 4 arc sec. Flows (300 km s-1) to upper
left and lower right are jets. Greenish region
is an accretion disk (size 150 AU)
35
XZ-Tauri is a T-Tauri star in orbit (3) with a
red proto-star. There is evidence for a disk.
Bubbles of emerging gas leaving the young binary
T-Tauri like system XZ Tauri about 500 ly away
36
30 west of the brightest point in Hinds nebula
is a disk-jet system, Herbig-Haro 30. At the
center of this is probably another T-Tauri like
star.
37
Protostars start off with very large radii
because they begin as contracting clouds of gas.
They additionally have high luminosities
because they are fully convective (more
about this later) and able to transport the
energy released by gravitational
contraction efficiently to their surface. Most
of the time is spent close to the main sequence.
38
(No Transcript)
39
Stellar Interiors - Kinds of Pressure
40
p mv
n(p) is the number of particles with momentum p
per cm3
It is convenient to think of a surface, though
there need not be one. Each particle striking the
surface and bouncing off gives a small
kick. This provides a net force pushing on the
wall. Recall that force is defined as the rate at
which momentum changes.
41
nb units
42
(No Transcript)
43
DEGENERACY PRESSURE
Pressure due entirely to the Uncertainty
PrincipleSuppose one packs as many electrons
into a volume, V, as are allowed. ?x p h
Each electron or pair of electrons occupies a
cell of size (?x)3, but ?x h/p
This is commonly called the Fermi Momentum
44
DEGENERACY PRESSURE
The contribution of electrons, when present, is
much larger than from neutrons or protons
because of the 1/m
45
At around 107 gm cm-3 the electrons will move
close to the speed of light.
46
(No Transcript)
47
Once the electrons move near the speed of light,
the pressure does not increase as rapidly with
density as before.
48
(No Transcript)
49
(No Transcript)
50
(No Transcript)
51
Radiation T4
sun
Ideal rT
Degenerate electrons ?5/3
?4/3