The default mode of operation of SPECTRUM calculates the normalized integrated-intensity spectrum using the abundances in the atomic and molecular data file, stdatom.dat. In this default mode, SPECTRUM prints to the screen the wavelength currently being computed along with two other numbers. The actual synthetic spectrum is printed to an ascii file. The two numbers printed to the screen refer to the number of spectral lines under computation at the current wavelength and the number of spectral lines in a ``look-ahead'' list. See § 3.8 for more details. The mode of operation of SPECTRUM can be changed by using one or more of the following ``switches''.

Spectrum Switches:

Prompts the user to supply the name of a custom atomic and molecular data file. This is useful if the user needs to compute a spectrum with a non-standard set of abundances. Without this switch, SPECTRUM will read in the default file stdatom.dat. The custom file should have exactly the same format as the supplied file stdatom.dat. While stdatom.dat must be in your working directory, this switch gives the user the opportunity to specify the path to the custom data file. The full path must be given. See § 3.3 for details on the information contained in stdatom.dat.
This switch will cause SPECTRUM to output the synthetic spectrum in a binary format. The advantage of this is that the size of the output file will be about 5 times smaller than the standard ascii output. The disadvantage is that it is much more difficult to work with than the normal ascii output file. This mode cannot be used in under Windows, and if used under Linux/UNIX the output file permissions must be altered with chmod to make it readable by the SPECTRUM auxiliary programs BSMOOTH2 and VSINI. This switch was introduced ``in the old days'' when disk space was at a premium. I don't recommend using this switch at present.
Ignores all spectral lines and outputs only the continuum.
This switch causes SPECTRUM to compute the disk integrated absolute flux (in erg/cm$^2$/s/Å as measured at the stellar surface).
An experimental mode that allows the user to input a ``velocity gradient'' file to study the effects of non-zero velocities on the line profiles. The format is an ascii file with a single column of velocities, one line corresponding to each layer in the stellar atmosphere. This switch should only be used in conjunction with the specific intensity modes (switches m and M) and only for center of disk calculations.
Isotope mode. SPECTRUM fully supports the calculation of both atomic and molecular isotopic spectra, but this requires the use of a special format for the linelist file. In addition, in this mode SPECTRUM will prompt the user for an isotope file. The default isotope file is isotope.iso. See § 3.6 for more details.
Both switches invoke the specific intensity mode. This allows the user to calculate, for instance, the center-of-disk solar spectrum, or the emergent specific intensity spectrum at any other point on the disk. The ``m'' switch causes SPECTRUM to output the normalized specific intensity (i.e. the continuum is normalized to 1.0); the ``M'' switch outputs the true, non-normalized specific intensity. This mode could be used to build up a spectrum for a star with a non-uniform surface, such as a rapidly rotating star with $T_{\rm eff}$ and $\log g$ which vary from point to point on the surface, or a spotted star, or even the emergent spectrum from a binary system.
Silent mode. In this mode, SPECTRUM prints out only a few lines of information and then runs silently. It will not even prompt the user for the various inputs, and thus in this mode SPECTRUM must be used with a response file. See § 3.9 for more details. The advantage of running in this mode is that SPECTRUM will run much faster if it doesn't have to print wavelength information to the screen.
This switch will cause SPECTRUM to print out number densities of selected species (such as atoms, ions and molecules) at each depth in the atmosphere.
This switch signals SPECTRUM that the input stellar atmosphere model is in the ATLAS9/12 format. See § 3.2 for more details.
An experimental mode that allows the user to compute a spectrum with a microturbulent velocity that varies with depth. The program will read the microturbulent velocity from the seventh column of the stellar atmosphere model.

Switches are used on the command line. For example, if we want SPECTRUM to operate in the isotope mode, to read in a stellar atmosphere model in the ATLAS9 format, and to output the absolute flux, we would use the command line:
spectrum itf
Or, if we wish to operate SPECTRUM in the silent mode with a response file (spec.rsp):
spectrum n < spec.rsp

Notice that the ``switch string'' should be separated from the spectrum command by at least one space, and that the string itself should not contain any spaces. For instance, the following is illegal and will not produce the desired result:
spectrum ni tf < spec.rsp

Using a Response File with SPECTRUM

As mentioned in the above paragraph, SPECTRUM may be run from the command line using a response file. This file contains the responses to all the prompts SPECTRUM prints to the screen. Thus, the file vega.rsp (included in the distribution) may be used to run the test example in § 2.3 simply by using the command line:
spectrum n < vega.rsp
The contents of vega.rsp are as follows:

This assumes that the linelist file luke.lst is in the working directory. If it is not, that line should include the full path. Note that the last line of the response file must end in a carriage return. By using response files, shell batch files can be set up that will run multiple SPECTRUM sessions successively.

Richard Gray 2008-09-17