reference for shape H2-self-shielding function

[Leonard Romano]

Dear enzo-developers,

This question might be more related to Grackle, but I had trouble signing up for the mailing list so I am asking here instead.
I noticed that in grackle (and I have been told the same is true for enzo), the exponent in the first term of the H2-self-shielding function is slightly different, than in the commonly cited form from Draine & Bertoldi '96 (ApJ, 468, 269). The corresponding code in grackle is in line 1434 in the file "solve_rate_cool_g.F":

f_shield = 0.965_DKIND / (1._DKIND + x/b_doppler)**1.1+
     &           0.035_DKIND * exp(-8.5E-4_DKIND * sqrt(1._DKIND +x))/
     &           sqrt(1._DKIND + x)

The exponent here is 1.1, whereas in Draine & Bertoldi '96 it's 2. If anyone knows any references for this different shape, or knows whether or not this is a typo, it would be very helpful!

Best,
Leonard

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Leonard Romano, B.Sc.（レオナルド・ロマノ）
Physics Department
Technical University of Munich (TUM), Germany
Theoretical Astrophysics Group 
Department of Earth and Space Science
Graduate School of Science, Osaka University, Japan
he / him / his
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