Atomic and Molecular data file

Another required input for SPECTRUM is an atomic and molecular data file. One such file, stdatom.dat, which includes solar atomic abundances from Grevesse & Sauval (1998), is included in the distribution. The format of that file can be seen from the following sample lines from the beginning

code   Abund   Amass   I1/D0   I2/rdmass     I3         I4     maxcharge
  1    -0.0360 1.008  13.5984   0.000       0.000      0.000      0
  2    -1.1060 4.003  24.5874  54.4178      0.000      0.000      1
  3   -10.88   6.941   5.3917  75.6400    122.4543     0.000      1
  4   -10.89   9.012   9.3227  18.2111    153.8966   217.7187     1
  5    -9.25  10.811   8.2980  25.1548     37.9306   259.3752     1
  6    -3.49  12.011  11.2603  24.3833     47.8878    64.4939     3

and from the end

606    -7.50  24.02    6.15     6.0055      1.00      00.00       0
607    -7.50  26.01    7.66     6.4627      1.00      00.00       0
608    -7.50  28.01   11.108    6.8604      1.00      00.00       0
813    -7.50  42.981   5.27    10.0436      1.00      00.00       0
814    -7.50  44.06    8.26    10.1897      1.00      00.00       0
822    -7.50  63.879   6.87    11.9921      1.00      00.00       0
840    -7.50 107.223   7.85    13.6118      1.00      00.00       0

The first column is the atomic or molecular code, described in the previous section. For instance, ``6'' refers to carbon, and ``606'' to the molecule C$_2$. The second column contains, for the atoms, the logarithmic solar abundance in terms of number densities relative to the total number density: $\log(A/N_{\rm total})$. This is the way that the abundance scale is defined in ATLAS9 and 12, but it is not the standard way of representing elemental abundances. See §  for more details. For the molecules, this column is set equal to -7.50 for obscure reasons. The third column contains the mean atomic mass for both atoms and molecules. The fourth column contains for atoms the first ionization energy in electron volts (eV); for molecules this column contains the dissociation energy, also in eV. The fifth column contains for atoms the second ionization energy and for molecules the ``reduced mass'' ( $m_1 m_2/(m_1 + m_2)$). The sixth column contains, for the atoms, the third ionization energy, and for the molecules a ``fudge'' factor by which the ``$gf$'' value for each spectral line is multiplied. This gives the user a very rough and blunt instrument for modifying molecular band strengths, for instance, after their appearance in the solar spectrum. The seventh column gives, for the atoms the fourth ionization energy and is identically zero for the molecules. Finally, the eighth column gives the maximum charge (the highest ionization state) supported by SPECTRUM. For instance, a ``1'' means that SPECTRUM can compute spectral lines only for neutral and singly ionized species (I and II), a ``3'' means that SPECTRUM has support for ionization states I, II, III and IV. For molecules this entry is 0. All naturally occurring elements are supported (hydrogen - uranium) by SPECTRUM and 15 diatomic molecules. Adding support for another molecule is not as simple as adding another line to stdatom.dat. It also involves code changes, addition of the molecular partition function, etc.

If you want to compute a spectrum with non-standard abundances, this can be accomplished by modifying the relevant abundances in stdatom.dat. However, a number of points must be kept in mind.

First, the overall metallicity in the header for the stellar atmosphere model (see section above) is used to scale the abundances for the metals in stdatom.dat. Thus, if [M/H] = -1.0, SPECTRUM subtracts 1.0 from the logarithmic abundances in stdatom.dat for lithium through uranium. Since the opacity due to the iron peak elements is so important in establishing the structure of the stellar atmosphere, it is advisable to use a stellar atmosphere model with [M/H] as close as possible to the [Fe/H] of the star you are analyzing.^3.1

Let us suppose that you want to compute a stellar atmosphere model with all but a handful of metals having an abundance one tenth that of the sun. Assuming that iron is not included in that handful, select the appropriate stellar atmosphere model with [M/H] = -1.0 and then individually adjust the abundances in stdatom.dat for the handful. For instance, in the sun (and in stdatom.dat), the logarithmic abundance of calcium (code = 20) is -5.68. Let us suppose that you actually want to use an abundance of -6.38. If the [M/H] of the model is $-1.0$, then the [M/H] scaling changes the abundance from stdatom.dat to -6.68, so this means that to get an abundance of -6.38 you will need to adjust the abundance in stdatom.dat to -5.38. Note that this is equivalent to [Ca/Fe] = +0.30 if [M/H] = [Fe/H].

Another example: Let us suppose that you want to compute a spectrum with iron and all other metals (except for, say, calcium) with abundances 0.15 dex less than found in the sun. Let us say that [Ca/Fe] = +0.30. Select a Kurucz atmosphere model with [M/H] = -0.20 (ATLAS9 models with [M/H] = -0.15 are not available, so -0.20 is the closest). Adjust the abundances of all the metals (except for calcium) in stdatom.dat upwards by 0.05 dex, and calcium upwards by 0.35 dex.

If this all seems to be complicated enough to drive one to tears, v2.76 now has a facility to input a ``fixed abundance'' file that can be used to specify individual fixed elemental abundances which will not be adjusted by the [M/H] scaling factor. Suppose that we want to compute a spectrum with a logarithmic calcium abundance = -6.38, regardless of the value of [M/H] of the model. Create a file (name it anything you like) with the contents:

TOTAL
20 -6.38

and run SPECTRUM with the ``x'' switch (see § ). SPECTRUM will prompt you for a ``fixed abundance file''. Supply it with the name of the file you just created. This file can contain abundances for any element except for hydrogen and helium. You may specify abundances on the basis of total number densities (use the keyword TOTAL on the top line of the file), or in terms of the number density of hydrogen (use the HYDROGEN keyword on the top line) - see § . The abundances of any number of elements may be specified in the fixed abundance file, each element on a separate line. Note that the element code (in this case 20, signifying calcium) must be entered as an integer, i.e. with no decimal points. Specifying 20.0 instead of 20 will result in unpredictable results!