The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons
E. L. O. Bakes, A. G. G. M. Tielens;
ApJ, 1994, 427, 822
ABSTRACT:We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon
grains which extends into the molecular domain.
We have considered a wide
range of physical conditions for the interstellar gas (1 less than G(sub 0)
less than 10(exp 5), with G(sub 0) being the intensity of the incident far-UV
field in units of the Habing interstellar radiation field; 2.5 x 10( exp -3)
less than n(sub e) less than 75/cu cm, with n(sub e) being the electron
density; 10 less than T less than 10,000 K, with T being the gas temperature).
The
results show that about half of the heating is due to grains less than 1500 C atoms
(less than 15 A).
The other half originates in somewhat larger grains
(1500-4.5 x 10(exp 5) C atoms; 15 less than 100 A).
While grains larger than this do
absorb about half of the available far-UV photons, they do not contribute
appreciably to the gas heating.
This strong dependence of gas heating on size
results from the decrease in yield and from the increased grain charge (hence
larger Coulomb losses) with increasing grain size.
We have determined the
net photoelectric heating rate and evaluated a simple analytical
expression for the heating efficiency, dependent only on G(sub 0), T, and n(sub
e).
This expression is accurate to 3% over the whole parameter range and is valid
up to gas temperatures of 10(exp 4) K, at which point the dominant gas-dust
heat exchange mechanism becomes the recombination of electrons with
grains rather than photoelectric ejection.
The calculated heating
efficiency for neutral grains is in good agreement with that derived from
observations of the diffuse interstellar clouds.
Our results also agree well with
the Far Infrared Absolute Spectrometer (FIRAS) observations on the
Cosmic Background Explorer Satellite.
Finally, our photoelectric heating
efficiency is compared to previous studies.
KEYWORDS: cosmic dust, gas heating, interstellar gas, interstellar matter, molecular interactions, photoelectric effect, polycyclic aromatic hydrocarbons, charge distribution, cooling flows (astrophysics), cosmic background explorer satellite, gas temperature, ion recombination, ionization potentials
PERSOKEY:dust, general ism, pah, ,
CODE: bakes94