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HI and Molecules. 0. Observing Neutral Hydrogen: The 21-cm (radio) line. Forbidden hyperfine-structure transition: A = 2.95x10 -15 s -1. Equal magnetic fields repel => Higher energy. Opposite magnetic fields attract => Lower energy. Magnetic field due to proton spin. 21 cm line.
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0 Observing Neutral Hydrogen:The 21-cm (radio) line Forbidden hyperfine-structure transition: A = 2.95x10-15 s-1 Equal magnetic fields repel => Higher energy Opposite magnetic fields attract => Lower energy Magnetic field due to proton spin 21 cm line Magnetic field due to electron spin
0 Observations of the 21-cm Line G a l a c t i c p l a n e All-sky map of emission in the 21-cm line
0 Molecular Clouds • Molecules are easily destroyed (“dissociated”) by ultraviolet photons from hot stars. They can only survive within dense, dusty clouds, where UV radiation is completely absorbed. “Molecular Clouds”: UV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there. Largest molecular clouds are called “Giant Molecular Clouds”: Molecules survive Cold, dense molecular cloud core Diameter ≈ 15 – 60 pc Temperature ≈ 10 K HI Cloud Total mass ≈ 100 – 1 million solar masses
0 Molecules in Space • CO = Carbon Monoxide • OH = Hydroxyl • Radio emission from rotational (and vibrational) transitions • H2 = Molecular Hydrogen Ultraviolet absorption and emission: Difficult to observe! But: Where there’s H2, there’s also CO. Use CO as a tracer for H2 in the ISM!
