Experience shows that in order to identify a species in a spectral survey securely, it is often necessary to make a basic model of the candidate line. For example, you may want to check if the relative intensities of each candidate line 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 equilibrium5. The source is assumed to have, for each species, a given column density, excitation temperature, line FWHM, 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 in our
spectrum (see Fig. 2). The emission can be
modeled using the modsource
command, which takes two arguments:
the name of the 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 provide the partition function for this species:
[fontsize=\scriptsize] LAS90> use in jpl I-USE, jpl (online) selected
Our model file, that we have named iras16293.mdl
, looks like
this:
[fontsize=\scriptsize] ! species Ntot Tex source_size v_off width ! (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=\scriptsize] LAS> modsource iras16293.mdl 30 I-SELECT, 4 lines found in the frequency range 96710.0 to 96770.0 MHz I-MODSOURCE, 4 CH3OH lines found in the frequency range I-MODSOURCE, log10 of the partition function at 10.0 K from jpl is 1.3419 # Species Freq[MHz] Err[MHz] Eup[K] Gup Aij[s-1] Upper level -- Lower level Origin Tau 1 CH3OH 96739.358 0.002 12.5 5 2.56e-06 2-1 0 -- 1-1 0 jpl 7.25e-01 2 CH3OH 96741.371 0.002 7.0 5 3.41e-06 2 0 + 0 -- 1 0 + 0 jpl 1.69e+00 3 CH3OH 96744.545 0.002 20.1 5 3.41e-06 2 0 0 -- 1 0 0 jpl 4.56e-01 4 CH3OH 96755.501 0.002 28.0 5 2.62e-06 2 1 0 -- 1 1 0 jpl 1.59e-01 I-MODEL, Blanking value: -1000.00000 I-RESAMPLE, Frequency resolution: .31252 MHz (observatory), .31252 MHz (rest frame) 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 are found in the frequency range. Note the
/verbose
option, that prints the frequency, 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 the observed and modeled spectrum into
buffers. These buffers can be listed with:
[fontsize=\scriptsize] LAS90> memorize I-MEMORIZE, Current memories: OBS TB_MODEL
The TB_MODEL
buffer contains the modeled brightness
temperature. The buffer can be retrieved with the retrieve
command. The OBS
buffer contains the observed spectrum, which is
saved automatically by modsource
.
The predicted spectrum can be drawn over the observed spectrum using
the modshow
command6. 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.
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Line indexes can be used together with modshow
to examine other
lines between 90 and 100 GHz, and to check whether our model can also
reproduce them. For example:
[fontsize=\scriptsize] LAS90> lfind "CH3OH" 90e3 100e3 /sortby e I-SELECT, 142 lines found in the frequency range 90000.0 to 100000.0 MHz LAS90> lget 4 LAS90> lplot LAS90> modsource iras16293.mdl 30 I-SELECT, 1 lines found in the frequency range 95898.6840807 to 95929.9359193 MHz I-MODSOURCE: 1 CH3OH lines found in the frequency range I-MODEL, Blanking value: -1000.00000 I-RESAMPLE, Frequency resolution: .31252 MHz (observatory), .31252 MHz (rest frame) 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. We must also rebuild the line index after each
modsource
, because the commands builds its own line index,
containing only lines in the frequency range covered by the current
window. Scripts can be easily created to loop over line index and
examine each of the observed and predicted lines.