Download document 8901482

Ay20 Basic Astronomy and the Galaxy Interstellar Medium and Star Forma;on h=p://www.astro.caltech.edu/~lah/ay20 Syllabus Interstellar Medium • 
• 
• 
• 
low density collisions between par;cles infrequent interac;ons between radia;on and ma=er rare thermal equilibrium not valid •  hea;ng – 
– 
– 
– 
stellar photons (uv) high energy photons (x-­‐rays) and par;cles (cosmic rays) shocks (from supernovae, novae, young star jets, winds) gas/grain interac;on •  cooling – 
– 
– 
– 
for gas, collisional excita;on followed by radia;ve de-­‐excita;on atoms and molecules for dust, thermal ``blackbody” emission op;cally thin path for photons to escape surrounding gas/dust Gas •  99% of the ISM is gas, with the rest dust •  Atomic neutral (mostly HI) and ionized (HII) •  Molecular (mostly H2 but use CO as tracer) •  “Phases” of the ISM range from hot, diffuse  cold, dense (it’s all about the density and temperature) Interstellar Molecule Soup •  \ Optical
image!
NASA, Hubble Heritage Team!
Molecular gas
(CO J=3-2)
image!
Caltech Submillimeter
Observatory!
Dust • 
• 
• 
• 
only 1% of the ISM, but high impact vehicle for forma;on of molecules role in hea;ng/cooling of ISM ex;nc;on = absorp;on and sca=ering of starlight (wavelength dependent – more at smaller λ) Ex#nc#on Law •  The “molecular clouds” within which stars form contain both gas and dust, that are well-­‐mixed. •  The dust absorbs, heats up, and re-­‐radiates these photons, and it also sca=ers them out of the line of sight. •  There is a general rela;onship between ex;nc;on and wavelength. •  Usually quote in terms of AV or “ex;nc;on” at op;cal wavelengths. But depending on how determined, may be reported as e.g. AJ or AK. σ π λ λ λ <a> is 0.05 microns but there is a range of par;cle sizes with n(a) ~ a-­‐3.5 “very small grains” Sites of Star Formation
Perseus-­‐Taurus-­‐Orion op;cal photograph
mid-­‐infrared -­‐ IRAS
Local molecular clouds (same size scale)
Perseus
Ophiuchus
Taurus
Pipe
10 pc
10 pc
Orion
Alves Lombardi Lada
This Cloud/Core is not yet Forming Stars This One Is Current star formation rate in
our galaxy is about 3 MSun yr -1
In the ONC alone it is almost
10-3 MSun yr -1 , and factors of
a few more if all of the greater
Orion region is included.
Alves & Bouy
radius decreases
density increases
for Planetary Systems HL Tau at op;cal and millimeter wavelengths HL Tau at op;cal and millimeter wavelengths Cartoon Evolu#on of an Individual Young Star and its Circumstellar Material 10"
105 yr"
Disk/wind!
Lstar"
104 yr"
Planet building!
1"
107 yr"
109 yr"
Planetary system!
100 AU"
Main
s
eque
8,000"
5,000"
nce"
Cloud collapse!
2,000"
Tstar (K)"
Beckwith & Sargent
Isochrones = Lines of Constant Age R. Jeffries

σ(log t) = 1.5 σ(log L) CMD Mg Log L/L HRD Higher mass stars evolve away from MS before lower mass stars reach it R. Jeffries
CMD MV Log L/L HRD ZAMS ZAMS