Frequently asked questions for the Astrophysical Web Page

Q1: I want the opacity for Log T = 4.0 and Log R = -7.75. When I enter these numbers, the Web Page gives me an error message. Why can't I get the opacity for this T-R point.

A: The opacity tables on this Web Page are precalculated on a fixed grid of temperatures and R's. There are no interpolation routines provided with the Web Page to obtain an opacity off of the standard T-R grid. If you need to have the opacity for an R value not on this table (but at one of the standard temperature points), you can convert the R value to a density and obtain the result from the main Opacity Web Page.

Q2: I notice that some of the opacities in the tables are set to a value of 9.999, which do not seem physical. What do these values mean ?

A: There are several limitations in the equation of state (EOS) model used in the opacity calculations. The calculations are done assuming Local Thermodynamic Equilibrium (LTE), which is not a good assumption at very low densities. Some of the R values for low temperatures correspond to densities of 10-12 gm/cc or less and are not included in these tables. The missing high R values (corresponding to high densities) can not be calculated because our EOS model (ideal gas with plasma corrections) does not converge at these densities. Rather than leave these points blank, we have filled in all of the missing points with a value of 9.999.

Q3: When I display a GIF plot or a data table for the second time, all that I get is the first plot or table over and over again.

A: The Web Page displays a file when showing plots or tables and uses the same file name for each new data set. Many browsers store the file name and data in their cache memeory and if you request a second plot or table, the browser retrieves it from the cache memeory instead of displaying the new data. You must change your browser's cache preference settings from "once a session" to "every time" to force the browser to choose the new data file for each new plot or table. Clicking the Reload button on your browser will also give you the latest plot or table, but you will have to do that every time if you do not reset the brower preference.

Q4: When I plot the Rosseland opacities, many of the high T-R points are missing from the graphs. Why is this ?

A: As mentioned in question 2 above, we fill in all T-R points that we can not calculate with a value of 9.999. Plotting these points would make the graphs too confusing, so we remove them from the plots, but not from the tables.

Q5: When I calculated a T-R point (converting the R to the correct density) on the main Opacity Web Page, I did not get quite the same value as you have on the Astrophysical Web Page. Why is that ?

A: There are two answers to your question. First of all, the astrophysical data tables were calculated on the CRAY computers, using a slightly different interpolation scheme than on the SUN that runs this Web Page. This can cause small numerical differences. Second, the astrophysical tables were run with the elemental opacities that were current as of March, 2000. As new elemental opacity tables are calculated and added to the main Web Page, users should expect to see differences until the astrophysical tables are updated. At present, we do not expect to update the astrophysical tables until we have a significient number of changes in the elemental data.

Q6: When I display a postscript plot with Ghostview®, I have trouble reading the Legends on the plot or I can not get the landscape mode to display horizontally. What should I do?

A: We have tried Ghostview® on different platforms and have found that it operates quite differently on the different platforms. The Macintosh® version seems the most limited. All that we can suggest is that you play with the settings and/or preferences until you have the best possible display. The other option is to switch to the GIF format.

Q7: I plan to use some of your opacities in a publication. What is the best reference to cite?

A: You can click on the Opacities methods and references for a list of refernces. The latest reference is

N. H. Magee, Jr., J. Abdallah, Jr., R. E. H. Clark, et al., "Atomic Structure Calculations and New Los Alamos Astrophysical Opacities", Astronomical Society of the Pacific Conference Series (Astrophysical Applications of Powerful New Databases, S. J. Adelman and W. L. Wiese eds.) 78, 51 (1995).

You may also refer to the web page:

http:/opacity/tops.html

Please mail questions or comments to:
LANL T-4 Opacities <opacity@t4.lanl.gov>