Graduate student and astronomy writer

Isochrone Investigation – CWRU

During the summer of 2011 and the 2011-2012 school year I worked with Dr. R. Earle Luck investigating the systematic differences in different stellar evolution models by fitting them to the same population of dwarf and giant stars that had Hipparcos parallaxes. This work comprised my undergraduate senior thesis project. We examined the stellar evolution models of the Padova group, the Dartmouth Stellar Evolution Database, and the Yonsei-Yale stellar evolution models. For the Padova models, we also compared the results from the isochrones to the mass tracks. By fitting the population of stars, corrected to their absolute magnitudes and with their temperatures obtained from spectroscopic data, using the same analysis method, and comparing the best fit stellar masses between the models we were able to show that using isochrone fitting methods to determine absolute stellar parameters is highly model-dependent. The fitting is especially skewed for giant stars, as the isochrone tracks criss-cross each other in the red giant branch, asymptotic giant branch, and horizontal branch. Even when comparing the results from stellar mass tracks to the results from the isochrones derived from those same mass tracks led to a lot of systematic differences between the two.

The takeaway messages here:

  • No two sets of stellar evolution models are the same.
  • If you try to fit your star(s) to stellar evolution models, be sure that you understand the differences that are built in to your models. The assumptions that the models make may not be appropriate for the stars that you are trying to fit.
  • Fitting a single star to an isochrone is very challenging and very prone to error unless you have a handle on all the degenerate properties in the models.
  • Unless you have a full, co-eval population of stars (i.e. a population where you can see the main sequence turnoff), you won’t be able to get accurate ages from your isochrones.
  • Age, metallicity, and interstellar reddening can cause similar looking effects on the main sequence. So, unless you have a good handle on one or more of the above, you can’t use isochrones to solve for the others.
  • No one can truly model all of the exciting physics that occurs in the atmospheres of stars (especially M dwarfs). While there are very sophisticated atmosphere codes that do an excellent job modeling the atmospheres of cool stars, they suffer from missing molecular lines and bands in the spectrum, which can make the photometry derived from those models suspect. Know your model’s shortcomings before you choose one!

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