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Using spectral line indexes

Once we have identified a line from some species in our spectral survey, we may want to search for other lines of that species at different frequencies. For this, Weeds allows to create a line index, which work in a similar way as CLASS scan indexes. Scan indexes are created by the CLASS find command and they are listed by the list command. A given scan from the current index can then be loaded and plotted using the get and plot commands. Likewise, line indexes are created with the lfind command, listed with the llist command, and a given line may be loaded and plotted using the lget and lplot commands.

For example, let's assume that we have identified a methanol line on the spectrum shown on Fig. 1, and that we want to see if other methanol lines are present in other scans. The methanol line index is built with:

[fontsize=\footnotesize]
LAS90> lfind "CH3OH, vt=0,1" 80e3 120e3 
I-LFIND, 137 lines found.

and then listed with:

[fontsize=\footnotesize]
LAS90> llist
----------------------------------------------------------------------------------------
| Index | Species          |  Freq. (GHz) | Eup (K) | Upper level    -- Lower level    |
----------------------------------------------------------------------------------------
|     1 | CH3OH, vt=0,1    |    80.993241 |   102.8 | 7  2  6 -0     -- 8  1  7 -0     |
|     2 | CH3OH, vt=0,1    |    81.147462 |  1130.8 | 21 +8 13  1    -- 21 +6 15  1    |
|     3 | CH3OH, vt=0,1    |    81.318424 |   381.8 | 17 -2 16  0    -- 17 +2 15  0    |
|     4 | CH3OH, vt=0,1    |    81.653080 |   493.1 | 18 +4 14  0    -- 19 +3 16  0    |
(...)
|   137 | CH3OH, vt=0,1    |   119.271820 |   481.5 | 12 +0 12  1    -- 11 -3  8  1    |
----------------------------------------------------------------------------------------

The command print all methanol lines comprised between 80 and 120 GHz, ordered by increasing rest frequencies. It also builds an internal index containing all these lines. Note that the species name must be typed exactly as it appear in the database and between double quotes, i.e. "CH3OH, vt=0,1" in this case.

Like for scans, each line is associated with an entry number, that we can use to load and to plot the line. Since, by default, lines are ordered by frequency, subsequent entry numbers will correspond to lines with increasing frequencies. However, if we search for lines of a given species, it usually a good idea to look for lines with the lowest upper energy levels, because we expect them to be brighter than higher lying lines. For this, we can re-order the lines by increasing upper level energies using the /order e option:

[fontsize=\footnotesize]
LAS90> lfind "CH3OH, vt=0,1" 80e3 120e3 /order e 
I-LFIND, 137 lines found.
LAS90> llist
----------------------------------------------------------------------------------------
| Index | Species          |  Freq. (GHz) | Eup (K) | Upper level    -- Lower level    |
----------------------------------------------------------------------------------------
|     1 | CH3OH, vt=0,1    |    96.741375 |     7.0 | 2  0  2 +0     -- 1  0  1 +0     |
|     2 | CH3OH, vt=0,1    |    96.739362 |    12.6 | 2 -1  2  0     -- 1 -1  1  0     |
|     3 | CH3OH, vt=0,1    |   108.893963 |    13.1 | 0 +0  0  0     -- 1 -1  1  0     |
|     4 | CH3OH, vt=0,1    |    96.744550 |    20.1 | 2 +0  2  0     -- 1 +0  1  0     |
(...)
|   137 | CH3OH, vt=0,1    |    89.801138 |  1665.5 | 25+10 15  1    -- 25+11 14  1    |
----------------------------------------------------------------------------------------

Since we have already identified the two first lines on the index, let's have a look at the third one:

[fontsize=\footnotesize]
LAS90> lget 3
I-SCANFIND, Found scan #7182 
I-LGET, Found line frequency in the current scan index
----------------------------------------------------------------------------------------
| Index | Species          |  Freq. (GHz) | Eup (K) | Upper level    -- Lower level    |
----------------------------------------------------------------------------------------
|     3 | CH3OH, vt=0,1    |   108.893963 |    13.1 | 0 +0  0  0     -- 1 -1  1  0     |
----------------------------------------------------------------------------------------

The lget command does several things. It first searches in the scan index for the first scan that covers the reset frequency of the line3. If it finds one, the scan is loaded in the CLASS buffers, and the scan frequency is then modified so that the velocity axis is centered at the line rest frequency. We can now plot the line:

[fontsize=\footnotesize]
LAS90> lplot

which gives the spectrum shown on Fig. 2.

Figure 2: Line displayed with lplot command. The blue vertical line show the rest frequency of the line. The upper x-axis show the velocity offset from the line rest frequency.
\includegraphics[angle=-90,width=14cm]{weeds-f2}

The lget and lplot commands allow to quickly ``navigate'' in a spectral survey to look for the different lines of a given species. lget f will load the first scan of the index, then lget n will get the next one. lget p will get the previous one, etc. If you get lost at some point, you can always type llist to display the line index again.


next up previous
Next: Modeling a spectrum Up: weeds Previous: Identifying lines on a
Gildas manager 2011-04-01