1,858 research outputs found
Circumstellar grain formation
Dust formation around cool giant and supergiant stars is examined in terms of grain formulation. Optical properties of small clusters, molecular physics of cluster nucleation and growth, circumstellar mass flows, and their application to alpha Ori are discussed
The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and
11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH)
molecules, have been recently ascribed to coal- or kerogen-like organic
nanoparticles with a mixed aromatic-aliphatic structure. However, we show in
this Letter that this hypothesis is inconsistent with observations. We estimate
the aliphatic fraction of the UIE carriers based on the observed intensities of
the 3.4 and 6.85 micrometer emission features by attributing them exclusively
to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes,
respectively. We derive the fraction of carbon atoms in aliphatic form to be
<15%. We conclude that the UIE emitters are predominantly aromatic with
aliphatic material at most a minor part of the UIE carriers. The PAH model is
consistent with astronomical observations and PAHs dominate the strong UIE
bands.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter
Structure of Stationary Photodissociation Fronts
The structure of stationary photodissociation fronts is revisited. H_2 self-
shielding is discussed, including the effects of line overlap. We find that
line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV
pumping models, and compare these with simple analytic approximations for the
effects of self-shielding.
The overall fluorescent efficiency of the photodissociation front is obtained
for different ratios of chi/n_H (where chi characterizes the intensity of the
incident UV) and different dust extinction laws. The dust optical depth
tau_{pdr} to the point where 50% of the H is molecular is found to be a simple
function of a dimensionless quantity phi_0 depending on chi/n_H, the rate
coefficient for H_2 formation on grains, and the UV dust opacity. The
fluorescent efficiency of the PDR also depends primarily on phi_0 for chi<3000
and n_H<10^4 cm^{-3}; for stronger radiation fields and higher densities
radiative and collisional depopulation of vibrationally-excited levels
interferes with the radiative cascade. The emission spectrum from the PDR is
essentially independent of the color temperature of the incident UV
for T_{color}>10^4K, but shows some sensitivity to the v-J distribution of
newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the
ortho/para ratio, and the rotational temperature in the =1 and =2 levels
are computed as functions of the temperature and density, for different values
of chi and n_H.
We apply our models to the reflection nebula NGC 2023. We are best able to
reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.Comment: 50 pages, 24 eps figures, uses aaspp4.sty . To appear in Ap.
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