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Next: Using spectral line indexes Up: weeds Previous: Selecting a spectral database

Identifying lines on a spectrum

Figure 1: Example spectra obtained with the IRAM-30m telescope towards the IRAS 16293 protostar. One of the methanol transition has been labeled with lid.
\includegraphics[angle=-90,width=14cm]{weeds-f1}

Fig. 1 show an example scan from a spectral survey. Spectral surveys typically have many different scans of this kind, each corresponding to a different frequency setting. The scan on the figure show several well detected lines that we want to identify. This can be done with lid which allow us to select a line on the scan with the mouse. Clicking on the left button of the mouse over some part of the spectrum selects a line. Then the command searches in the database (which has been selected previously with the dbselect command, as explained above) for lines within 1 MHz of the selected region(s) of the spectrum. For each line found, the command mark the rest frequency of the line on the scan, and a table displaying the line frequency, upper level energy, Einstein coefficient and quantum numbers and is printed:

[fontsize=\footnotesize]
LAS90> lid
--------------------------------------------------------------------------------------------
| Species          |  Freq. (GHz) | Eup (K) | Aul (s-1) | Upper level    -- Lower level    |
--------------------------------------------------------------------------------------------
| CH3OH, vt=0,1    |    96.739362 |    12.5 |  2.56e-06 | 2 -1  2  0     -- 1 -1  1  0     |
--------------------------------------------------------------------------------------------

Most of the time, several line candidates are found for a given frequency. For example, using lid to search for lines in the catalog around the brightest line in Fig. 1, we obtain:

[fontsize=\footnotesize]
LAS90> lid
--------------------------------------------------------------------------------------------
| Species          |  Freq. (GHz) | Eup (K) | Aul (s-1) | Upper level    -- Lower level    |
--------------------------------------------------------------------------------------------
| n-C3H7CN         |    96.741208 |   158.0 |  7.60e-06 | 12 310 1       -- 11 2 9 1       |
| C4H, v7=1        |    96.741314 |   214.2 |  3.74e-06 | 10 111         -- 9-110          |
| CH3OH, vt=0,1    |    96.741375 |     7.0 |  3.41e-06 | 2  0  2 +0     -- 1  0  1 +0     |
--------------------------------------------------------------------------------------------

Searches in the database may be refined to lines with an upper level energy lower than a given value using the /energy option, or to lines with an Einstein coefficient greater than a given value with the /aeinstein option. The /species option allow to restrict searches to a given species.

It should be noted lid prints the line upper and lower level quantum numbers exactly as they appear in the selected database. We refer the user to the documentation of these databases for the meaning of these quantum numbers.


next up previous
Next: Using spectral line indexes Up: weeds Previous: Selecting a spectral database
Gildas manager 2011-04-01