Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, from COBE Observations
E. Dwek, R. G. Arendt, D. J. Fixsen, T. J. Sodroski, N. Odegard, J. L. Weiland, W. T. Reach, M. G. Hauser, T. Kelsall, S. H. Moseley, R. F. Silverberg, R. A. Shafer, J. Ballester, D. Bazell, R. Isaacman;
ApJ, 1997, 475, 565
ABSTRACT:Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5--240 mu m spectrum of high-latitude dust.
Combined with a
spectrum obtained by the FIRAS instrument, these data represent the most
comprehensive wavelength coverage of dust in the diffuse interstellar medium,
spanning the 3.5--1000 mu m wavelength regime.
At wavelengths shorter than ~60
mu m the spectrum shows an excess of emission over that expected from dust
heated by the local interstellar radiation field and radiating at an
equilibrium temperature.
The DIRBE data thus extend the observations of this
excess, first detected by the IRAS satellite at 25 and 12 mu m, to shorter
wavelengths.
The excess emission arises from very small dust particles undergoing
temperature fluctuations.
However, the 3.5--4.9 mu m intensity ratio cannot be
reproduced by very small silicate or graphite grains.
The DIRBE data strongly
suggest that the 3.5--12 mu m emission is produced by carriers of the ubiquitous
3.3, 6.2, 7.7, 8.6, and 11.3 mu m solid state emission features that have been
detected in a wide variety of astrophysical objects.
The carriers of these
features have been widely identified with polycyclic aromatic hydrocarbons
(PAHs).
Our dust model consists of a mixture of PAH molecules and bare astronomical
silicate and graphite grains with optical properties given by Draine &
Lee.
We obtain a very good fit to the DIRBE spectrum, deriving the size
distribution, abundances relative to the total hydrogen column density, and
relative contribution of each dust component to the observed IR emission.
At
wavelengths above 140 mu m the model is dominated by emission from T ~ 17--20 K
graphite and 15--18 K silicate grains.
The model provides a good fit to the FIRAS
spectrum in the 140--500 mu m wavelength regime but leaves an excess Galactic
emission component at 500--1000 mu m.
The nature of this component is still
unresolved.
We find that (C/H) is equal to (7.3 +/- 2.2) x 10-5 for PAHs and equal to (2.5
+/- 0.8) x 10-4 for graphite grains, requiring about 20% of the cosmic
abundance of carbon to be locked up in PAHs, and about 70% in graphite grains [we
adopt (C/H)&sun; = 3.6 x 10-4].
The model also requires all of the available
magnesium, silicon, and iron to be locked up in silicates.
The power emitted by
PAHs is 1.6 x 10-31 W per H atom, by graphite grains 3.0 x 10-31 W per H atom, and
by silicates 1.4 x 10-31 W per H atom, adding up to a total infrared
intensity of 6.0 x 10-31 W per H atom, or ~2 L&sun; M^{-1}_{solar} .
The [C II] 158 mu
m line emission detected by the FIRAS provides important information on
the gas phase abundance of carbon in the diffuse ISM.
The 158 mu m line
arises predominantly from the cold neutral medium (CNM) and shows that for
typical CNM densities and temperatures C+/H = (0.5--1.0) x 10-4, which is
~14%--28% of the cosmic carbon abundance.
The remaining carbon abundance in the
CNM, which must be locked up in dust, is about equal to that required to
provide the observed IR emission, consistent with notion that most (>~75%)
of this emission arises from the neutral component of the diffuse
ISM.
The model provides a good fit to the general interstellar extinction
curve.
However, at UV wavelengths it predicts a larger extinction.
The excess
extinction may be the result of the UV properties adopted for the PAHs.
If real, the
excess UV extinction may be accounted for by changes in the relative
abundances of PAHs and carriers of the 2200 Angstroms extinction bump.
KEYWORDS: ism: dust, extinction, infrared: ism: lines and bands, ism: abundances, ism: clouds
PERSOKEY:dust, dirbe, fir, mir, cirrus, extinction, size distribution, ,
CODE: dwek97