Pg. 1

Wavelength (A)

O5V 47000KB0V 35000K

B5V 15000K

A0V 9500K

H0 photoionization cross section

H+

H0, H2

Frac

tion

in fo

rm H

+ 1.0

0.0

Strömgren Spheres Pg. 2

Ionization Zones for Heavier Elements

O6 star, T* = 40,000

B0 star, T* = 30,000

H0

He0

He+

Frequency

Photoionization cross sections

Planetary Nebula

Pg. 3

excitation

Pg. 4

initial

Temperature Balance

Heating(Te) = Cooling(Te)• Sets Te ~ 104K in most cases.

Pg. 5

Including collision de-excitation (denser gas)

Pg. 6

MeasuringTemperature

Pg. 7

Pg. 8

Pg. 9

n

Pg. 10

Homework Assignment 2 – Due Tuesday Sept 20An Atypical HII Region

The Textbook Nebula is a perfectly spherically symmetric ball of gas of absolutely uniform density which is photoionized by a hot star located precisely at its center. There is too much gas for the star to completely ionize, so a sharply-bounded Stromgren sphere is formed with highly ionized gas in the center and neutral gas surrounding it on all sides.

Surprisingly (?), there is no dust in this nebula or along the line of sight to it. We even know the distance to this nebula: 437 pc.

From the following observations, find the mass of ionized hydrogen. Express this in the conventional units of solar masses. Show the values you used for each term in the eqn..

Warning… Pay close attention to units. Cgs units are used here.

Emission Line ( in Ǻ)

Flux (erg cm-2 s-1)

[OIII] 4363 1.07E-10

H 4861 1.00E-8

[OIII] 4959 1.06E-8

[OIII] 5007 3.42E-8

[SII] 6716 3.74E-9

[SII] 6731 3.56E-9

Te (K) 5,000 10000 20,000

H (cm3s-1) 5.37 E-14 3.02E-14 1.61E-14

H is the hydrogen recombination coefficient taking into account the fraction of recombinations that lead to the production of an H photon (the n=4 to n=2 transition).

Measured flux received at Earth from whole visible surface of nebula. (Flux j)

Not the Textbook Nebula

Not the Textbook Nebula Not the Textbook Nebula

Not the Textbook Nebula Not the Textbook Nebula

The Textbook Nebula

HHβeHβHβvol HeHβ mass/mαNhνdvolαNNhν)L(Hβ

Find Te from [F(4959)+F(5007)]/(F4363), then interpolate to get .

In cgs units: h = 6.63E-27 erg s1 pc = 3.09E18 cm; 1 Ǻ = 1.0E-8 cm

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2