Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://mavr.sao.ru/hq/ssl/IRAS04296/node8.html
Дата изменения: Unknown Дата индексирования: Tue Oct 2 08:03:06 2012 Кодировка: Поисковые слова: http news.cosmoport.com 2001 09 19 2.htm |
Details of the computer code used for solution of the radiative transfer in dusty envelopes can be found in Szczerba et al. (1997). In brief: the frequency-dependent radiative transfer equation is solved for a dust under assumption of spherically-symmetric geometry for its distribution taking into account particle size distribution and quantum heating effects for the very small dust particles.
The modelled source is certainly C-rich (see Omont et al. 1995 and Kwok et al. 1995). Therefore, for modelling of its spectral energy distribution (SED) we assumed that dust is composed of: policyclic aromatic hydrocarbons (PAH) for dust sizes a between 5 and 10Å (see Szczerba et al. (1997) for details concerning PAH properties), amorphous carbon grains (of AC type from Rouleau & Martin 1991) for a>50Å and dust with an opacity obtained from averaging of the absorption efficiences for PAH and AC grains according to the formula:
The m feature was approximated by a gaussian with
parameters determined from modelling of IRAS07134+1005
(centre wavelength equal to
m, and width of
m) which has the strongest
feature among the known
m sources. In the case of
m band we used the addition of two half-gausians
with the same strength and different width. Initial fit was done
to IRAS 22272+5435 and its parameters were: width for short
wavelength side
m, width for
long wavelength side
m and central
wavelength
m (see Szczerba et al. 1997). For
modelling of IRAS04296 we have reduced the strength of this
feature by 50%. Superposition of the 21 and
m
features was added to the absorption properties of amorphous carbon
in order to construct an empirical opacity function (EOF).
In Fig.7 the best fit obtained from the solution
of the radiative transfer problem including quantum heating
effects for the PAH grains is shown together with observational
data which will be described in detail elsewhere. Note, however,
that we present also two sets of photometry (from B to M band)
corrected for interstellar extinction (open symbols)
according to the average extinction law of Cardelli et
al. (1989), assuming that total extinction at V is 1.0 or 2.0
magnitudes and plotting only the smallest and largest value of
corrected fluxes at given band.
This estimate of the total extinction range can be inferred from the analysis of data presented by Burstein & Heiles (1982).
parameter | value |
![]() |
6500K |
log (![]() ![]() |
3.92 |
d | 5.4kpc |
![]() |
0.5pc |
![]() |
12kms-1 |
![]() |
6.410-4pc |
![]() ![]() |
870K |
![]() |
![]() |
![]() ![]() |
4.010-7![]() |
![]() |
7.0610-3pc |
![]() ![]() |
270K |
![]() |
![]() |
![]() ![]() |
1.7010-5![]() |
![]() ![]() |
2.1910-4![]() |
a- | 5Å |
a+ | 0.25 ![]() |
p | 3.5 |
![]() |
575yr |
![]() |
0.0071![]() |
The best fit to the spectral energy distribution of IRAS04296
is shown by heavy solid line (see Table2 for details
concerning parameters of the model). Our modelling procedure was
such that we tried to get fits to SED which fall in between the
extinction corrected fluxes. In this way, we have taken into
account not only the effect of the circumstellar extinction but
also of interstellar extinction. The thin long-dashed line
represents the input energy distribution of the central star for
logg=0.5 and =6500K according to model
atmosphere calculations of Kurucz (private communication). The
heavy short-dashed line shows the fit which was obtained with
the same assumptions but changing the effective temperature of
the star to 6000K. As one can immediately see in the IR range
of the spectrum the quality of the fits are very similar.
However, in the optical and ultraviolet (UV) part of the
spectrum the fit assuming
=6000K is not able to
explain extinction corrected data. In consequence, we are quite
convinced that our estimation of
for IRAS04296 close
to 6500K is reasonable and, what is even more important,
agrees pretty well with the spectroscopic estimation (6300K).
Note that spectral type of this source was found to be G0 Ia
from the low resolution spectrum (Hrivnak 1995) which implies an
effective temperature of around 5500K for the star if we asume
that the same relationship applies for post-AGB supergiants as
for ``normal'' ones (see Schmidt-Kaler 1982). For such a low
temperature we were not able to fit even the reddenned data in
the UV.
The thin solid line in the wavelength range from about 18 to
m represents the model continuum level found after
solution of radiative transfer equation for dust without using
the EOF parameters as in Tab.2 while keeping the dust
temperature (or probability distribution of dust temperature)
the same as for the case of dust with EOF. Taking into account
the estimated continuum level and assuming that 21
m feature
extends from 18 to 22
m we estimate the energy emitted in 21
m band as about 5.7 % of the total IR flux (251
for
's from 5 to 300
m assuming a distance to the source
of 1 kpc). With the dotted line for wavelengths longer than 18
m we present the fit which was obtained using an opacity
function with the EOF for only 21
m component. It is clear
that such fit is not able to explain IRAS photometry at 25
m.
Our recent ISO observations show that this source is also a
30
m emitter. In the forthcoming paper (Szczerba et al. 1998, in
preparation) we will discuss this finding in detail.