3. Line lists
In Papers I and II, atomic transitions and their gf-values were from Abbott (1982) and Kurucz & Peytremann (1975). In this work, these data have been replaced by the data on a CD-ROM distributed by Kurucz & Bell (1995). This data is manipulated as described below. In particular, in contrast to the previous stellar wind and supernova codes, the present code's improved treatment of line formation is greatly facilitated by creating two line lists containing identical data but differently ordered.
The first step in using the Kurucz-Bell data is to infer their atomic model for each ion. Thus, a file (I) is constructed in which, for each ion, the energy levels featuring in the transition array and their J-values are tabulated. A second file (II) can then be constructed in which, for each ion, its transitions' log gf values are tabulated, with the transitions being identified only by u and l, the indices in file I of the upper and lower energy levels. These two files provide compact storage of the data and allow rapid construction of working line lists for specific models.
Because of the difference in the excitation formula (Sect. 2) between normal and metastable levels, it is useful to identify metastable levels in these files. This is done by replacing by in file I and l by in file II.
From these basic files, a list of transitions relevant for a particular model is created as follows:
1) All lines of an element are excluded if it is absent from the adopted mixture.
2) All lines of an element's -th ion are excluded if , an appropriate limiting potential.
3) Of the surviving transitions, a further culling eliminates all those with Sobolev optical depths everywhere , an appropriately small cut-off value.
Having thus excluded all inconsequential transitions, those that survive are organized into two line lists as follows:
I) Line list A: lines ordered according to frequency with .
II) Line list B: lines grouped according to ion and then, for each ion, according to the index u of the transitions' upper levels.
Because of the expansion of the SN's envelope, the co-moving frequency of a photon in free flight decreases with time. Line list A serves therefore as usual to identify efficiently the next transition with which a photon (or photon packet) might interact. On the other hand, after a packet has been absorbed by the transition , the possible radiative decays are the set of permitted transitions with the same upper level. But these transitions are contiguous in line list B, and so an appropriate random selection of the re-emitted packet's frequency can be simply and efficiently made.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999