Stellar Atmosphere Models

One of the required inputs for SPECTRUM is a stellar atmosphere model. Three of these have been provided in the distribution, an ATLAS9 model for the sun, one for Vega, and one for the parameters $T_{\rm eff} = 6720$K, $\log g = 4.40$ and [M/H] = 0.0 (the file 67244k2p00.mod. These model atmospheres were computed with the ATLAS9 model atmosphere program written by Kurucz (1993). More such models are available on Robert Kurucz's website: http://kurucz.harvard.edu under the link ``Grids of Model Atmospheres''. Another excellent site where individual models may be downloaded is http://wwwuser.oat.ts.astro.it/castelli/grids.html. Before we get into extracting a particular model atmosphere for use in SPECTRUM let us take a closer look at the model atmosphere for the sun. The first few lines of this model atmosphere, found in the file sun.mod are reproduced below:

5777.0  4.43770  0.00  64
2.62383508E-03   3398.7 7.188E+01 6.618E+09 7.359E-04 1.133E-01 1.500E+05
2.62384162E-03   3871.5 7.188E+01 1.301E+10 7.359E-04 8.267E-02 1.500E+05
3.17880896E-03   3901.4 8.709E+01 1.566E+10 8.502E-04 7.839E-02 1.500E+05
4.02509434E-03   3937.8 1.103E+02 1.966E+10 1.019E-03 7.581E-02 1.500E+05
4.97406188E-03   3969.0 1.363E+02 2.409E+10 1.202E-03 7.460E-02 1.500E+05

The model continues for 59 more lines; in total, this particular model atmosphere is divided up into 64 layers, each layer characterized by a different temperature, pressure, electron density, etc. The first line in this model is the header which gives the effective temperature ( $T_{\rm eff} = 5777.0$K), the $\log g$ (4.43770; logarithm of the surface gravity in units cm/s$^2$), the overall metallicity in logarithmic terms compared with the sun ([M/H]; 0.00), and the number of layers. SPECTRUM will use the value for [M/H] to scale the abundances of the metals (everything except for H and He) in the atomic and molecular data file stdatom.dat. SPECTRUM can handle model atmospheres with up to 100 layers. If you have an application that requires a larger number of layers, please contact me. The single line header ``5777.0 4.43770 0.00 64'' is in the format of the default SPECTRUM header.

The next 64 layers in this atmosphere model contain data needed by SPECTRUM for calculating the synthetic spectrum. The first layer represents the surface. The first column is the mass depth. The second column is the temperature, in Kelvin, of the layer, the third the gas pressure, the fourth the electron density, the fifth the Rosseland mean absorption coefficient, the sixth the radiation pressure and the seventh the microturbulent velocity in meters/second. The newer Kurucz/Castelli models have three additional columns which give the amount of flux transported by convection, the convective velocity and the sound velocity.

The atmosphere models from the Kurucz and Castelli websites (or as directly produced by the ATLAS9 and ATLAS12 programs) have more elaborate headers than the SPECTRUM default header. You can choose either to edit the atmosphere model to remove these headers and replace them with the single line header detailed above, or use the ``t'' switch which alerts SPECTRUM to expect an ATLAS9 or 12 header. The ATLAS9 and 12 headers contain not only effective temperature, $\log g$ and [M/H] information, but also the specific elemental abundances used by ATLAS9 or 12 to compute the stellar atmosphere. SPECTRUM, in the ``t'' mode, picks up $T_{\rm eff}$, $\log g$ and [M/H] from these headers, but does not, at least as presently constituted, use the elemental abundances in these headers. Rather, the elemental abundances in the stdatom.dat file are used (and scaled with the [M/H] value). An option may be added in the future to make it possible to read this information from the stellar atmosphere header. It turns out, however, that for certain applications of SPECTRUM it is very useful to have this information in a separate file.

Even though SPECTRUM was written with the Kurucz atmospheres in mind, this does not mean that other model atmospheres cannot be used. They simply must be cast into the format of the Kurucz models.

The Kurucz atmosphere models are distributed in large files in which many models are concatenated (what I refer to in later sections of this document as a ``supermodel''). These supermodels may be opened with an ascii editor and a single model extracted by cutting and pasting to another file. However, in Chapter 4, a program called selectmod is described which can be used to extract a model from a supermodel.

Richard Gray 2008-09-17