Photoionization and Recombination

Photoionization X + hnà X+ + e-

Radiative recombination X+ + e- à X + hn

2/23/19

1

HII region Planetary Nebula

H and He Recombination Rates

Use in HII regions

Hydrogen:aA ~ 4.17x10-13 (T/104K)-0.721-0.018ln(T/104K)

aB ~ 2.59x10-13 (T/104K)-0.833-0.035ln(T/104K)

Helium:aB ~ 2.72x10-13 (T/104K)-0.789

Photoionization Cross-sectionPh

otoi

oniza

tion

Cros

s-se

ctio

n 10

-18

cm2

s ~ E-3

s0 = 6.304 x 10-18 cm2

Ionizing Properties of Massive Stars