Last update: 14 August 2008
by rfc

Tied column densities:

This is a historic capability which may, or may not, still work. It has not been tried for a while, and is not included in the test procedures when updates occur.

If you want to tie column densities you can through feeding in the parameters from an abundance and photoionization model, and there is provision for only one of these. You need to set up a file which contains the element abundances and ionizations for the elements of interest. This file has to be in the working directory, and be called 'vp_abund.dat'. It contains something like:
H:3.18E10 C:1.18E7 N:3.74E6 O:2.15E7 Mg:1.061E6 Al:8.5E4 Si:1.00E6 Fe:8.3E5 Ni:4.80E4

H .999
C 1.0e-3 .7 .2 .1
N .1 .7 .1 .1 .0000001
O .7 .001 .000001
Mg .1 .7 .1 .1
Al .0001 .7 .2 .1
Si .0001 .7 .2 .1
Fe .0001 .7 .2 .1
Ni .0001 .7 .2 .1

.. the first group is the abundances (max 7 per line), followed by a blank line, and then the ionization fractions, one element per line as element, neutral fraction, 1st stage fraction, 2nd..... (see example above). Blank values are set to 1E-8.

The the input lines will look like:

C II 1.5568E+01e 4.383865c 11.32n 0.00 1.00E+00 0
SiII 1.4496E+01E 4.383865C 11.32N 0.00 1.00E+00 0

.. and at every stage the SiII/CII ratio will be (Si abundance * SiII ion fraction)/(C abundance * CII ion fraction). You don't actually have to get it right for the input guesses for the capitalized tied ions -- the program sorts it out -- but please put a vaguely plausible number in since the read routine expects a number there. If you have a list of elements which are dust depleted to a similar level, and some which are not depleted, you can work out the depletion by tieing the column densities for the depleted and undepleted ones separately and letting the fit go ahead. e.g. for OI and CII in one group, and FeII, SiII in another:

C II 1.5568E+01e 4.383865c 11.32n 0.00 1.00E+00 0
SiII 1.4496E+01f 4.383865C 11.32N 0.00 1.00E+00 0
O I 1.5829E+01E 4.383865C 11.32N 0.00 1.00E+00 0
FeII 1.4415E+01F 4.383865C 11.32N 0.00 1.00E+00 0

Note that ALL systems will have the abundances tied via the SAME model -- we've never wanted to do more than one at a time.