Keywords: weeds

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# Identifying lines in a spectrum

Figure 1: Example of a spectrum obtained with the IRAM-30m telescope towards the IRAS 16293 protostar. One of the methanol transition has been labeled with lid.

Fig. 1 shows a part of a spectral survey3. The spectrum on the figure shows several well detected lines that we want to identify. This can be done with `lid` which allows us to select a line on the spectrum 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 `use in` command, as explained above) for lines within 10 channels of the selected region(s) of the spectrum4. For each line found, the command marks the rest frequency of the line on the spectrum, and a table displaying the line frequency, upper level energy, Einstein coefficient and quantum numbers and is printed:

    [fontsize=\scriptsize]
    LAS90> lid
    I-SELECT,  1 lines found in the frequency range 96737.8185473 to 96740.9437312 MHz
      # Species                 Freq[MHz] Err[MHz] Eup[K]  Gup  Aij[s-1]    Upper level -- Lower level    Origin
      1 CH3OH, vt=0,1           96739.362   0.005    12.5    5  2.56e-06     2 -1  2  0 -- 1 -1  1  0     cdms

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=\scriptsize]
    LAS90> lid
    I-SELECT,  4 lines found in the frequency range 96739.7756444 to 96742.9008283 MHz
      # Species                 Freq[MHz] Err[MHz] Eup[K]  Gup  Aij[s-1]    Upper level -- Lower level    Origin
      1 n-C3H7CN                96741.208   0.009   158.0   50  7.60e-06       12 310 1 -- 11 2 9 1       cdms
      2 gGG'g-1,3-Propanediol   96741.245   0.030    40.1   31  1.58e-05       15 213 1 -- 14 212 0       cdms
      3 C4H, v7=1               96741.314   0.092   214.2   44  3.74e-06         10 111 -- 9-110          cdms
      4 CH3OH, vt=0,1           96741.375   0.005     7.0    5  3.41e-06     2  0  2 +0 -- 1  0  1 +0     cdms

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 allows to restrict searches to a given species.

It should be noted that `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: Using spectral line indexes Up: weeds Previous: Selecting a spectral database