Experience shows that in order to identify a species in a spectral survey securely, it is often necessary to make a basic modeling of the line candidates. For example, you may want to check if the relative intensities of each line candidates imply a reasonable kinetic temperature. You may also want to check that other lines with similar predicted intensity are also detected.
Weeds allows to compute the emission of a source under the assumption of local thermodynamical equilibrium4. The source is assumed to have, for each species, a given column density, excitation temperature, turbulent velocity, systemic velocity and size. Several components (with e.g. different temperatures and/or sizes) can be added. Weeds will compute the emission of these various components - taking into account the line opacity and the beam dilution factor - and it will display them on the observed spectrum. The different source parameters can be adjusted until a good match between the model and the observation is obtained.
Let us go back to the methanol lines that we have identified on our
spectrum (see Fig. 2). The emission can be
modeled using the modsource
command, which takes two arguments:
the name of file containing the source parameters, and the size of the
antenna that we have used for the observations, in meters. To model
the emission, we need to use the JPL database, because the CDMS does
not provides partition functions:
[fontsize=\footnotesize] LAS90> dbselect jpl I-DBSELECT, jpl database selected (online).
Our model file, that we have named iras16293.mdl
, looks like
this:
[fontsize=\footnotesize] # Source model for IRAS16293 # # species Ntot Tex theta v_off delta_v # (cm-2) (K) (``) (km/s) (km/s) # CH3OH 2.0e15 10 10 0 3.0
Lines that start with a #
are comments: they are ignored. The
last line gives the name of the species, its column density, excitation
temperature, the size of the emission in arc seconds, the offset
velocity (with respect to the source velocity in the class file
header), and the line width. The spectrum, as it would be observed
with the IRAM-30m antenna, can be computed with:
[fontsize=\footnotesize] LAS90> modsource iras16293.mdl 30 /verbose I-MODSOURCE: 4 CH3OH lines found in the frequency range I-MODSOURCE: Partition function at 10.0 K is 41.1 --------------------------------------------------------- | Freq. (GHz) | Eup (K) | gu | Aul (s-1) | tau | --------------------------------------------------------- | 96.739358 | 12.5 | 5 | 3.79e-06 | 5.73e-01 | | 96.741371 | 7.0 | 5 | 5.05e-06 | 1.34e+00 | | 96.744545 | 20.1 | 5 | 5.05e-06 | 3.61e-01 | | 96.755501 | 28.0 | 5 | 3.89e-06 | 1.26e-01 | --------------------------------------------------------- I-MODEL, Blanking value: -1000.0000 I-MODSOURCE, Model has been stored in memory
For efficiency reasons, the command computes the spectrum over the
frequency range covered by the current window only. In our case, four
methanol lines have found in the frequency range. Note the
/verbose
option, that prints the frequencies, upper level
energy and statistical weight, Einstein coefficient and computed
opacity at the line center. The modsource
command itself does
not plot anything; it just stores modeled spectrum is stored into
buffers. These buffers can be listed with:
[fontsize=\footnotesize] LAS90> memorize I-MEMORY, Current memories OBS TA_MODEL
The TA_MODEL
buffer contains the predicted antenna
temperature. The buffer can be retrieved with the retrieve
command. The OBS
buffer contains the observed spectra, which is
saved automatically by modsource
.
The predicted spectrum can be drawn over the observed spectrum using
the modshow
command5. This command gives the spectra shown on
Fig. 3. As it can be seen on this figure, our
model is in quite good agreement with the observed spectrum.
![]() |
The lget
command can be used together with modshow
to
examine other lines, and to check whether our model can also reproduce
them. For example, we can examine the third line of the line index
with:
[fontsize=\footnotesize] LAS90> lget 2 LAS90> lplot LAS90> modsource iras16293.mdl 30 I-MODSOURCE: 3 CH3OH, vt=0,1 lines found in the frequency range I-MODEL, Blanking value: -1000.0000 I-MODEL, Blanking value: -1000.0000 I-MODSOURCE, Model has been stored in memory LAS90> modshow
Note that we must use the modsource
command again, because the
command computes a spectrum only over the frequency range covered by
the current scan. Scripts can be easily created to loop over line
index and examine each of the observed and predicted lines.