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Modeling a spectrum

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.

Figure 3: Observed (black histogram) and predicted spectrum (in red) displayed with the modshow command.
\includegraphics[angle=-90,width=14cm]{weeds-f3}

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.


next up previous
Next: For more information Up: weeds Previous: Using spectral line indexes
Gildas manager 2011-04-01