def trajectory_SgConst(Sg=<NUM_LIT:0.1>, delta_logt_dex=-<NUM_LIT>):
<EOL>logtimerev=np.arange(<NUM_LIT>,-<NUM_LIT>,delta_logt_dex)<EOL>logrho=np.linspace(<NUM_LIT:0>,<NUM_LIT>,len(logtimerev))<EOL>logT = (old_div(<NUM_LIT:1.>,<NUM_LIT>))*(logrho + <NUM_LIT> + np.log10(Sg))<EOL>pl.close(<NUM_LIT:3>);pl.figure(<NUM_LIT:3>);pl.plot(logrho,logT,label='<STR_LIT>'+str(Sg)+'<STR_LIT:$>')<EOL>pl.legend(loc=<NUM_LIT:2>);pl.xlabel('<STR_LIT>'); pl.ylabel('<STR_LIT>')<EOL>pl.close(<NUM_LIT:5>);pl.figure(<NUM_LIT:5>);pl.plot(logtimerev, logrho)<EOL>pl.xlabel('<STR_LIT>'); pl.ylabel('<STR_LIT>')<EOL>pl.xlim(<NUM_LIT:8>,-<NUM_LIT:6>)<EOL>pl.close(<NUM_LIT:6>);pl.figure(<NUM_LIT:6>);pl.plot(logtimerev)<EOL>pl.ylabel('<STR_LIT>'); pl.xlabel('<STR_LIT>')<EOL>T9=old_div(<NUM_LIT:10>**logT,<NUM_LIT>)<EOL>data=[logtimerev,T9,logrho]<EOL>att.writeTraj(filename='<STR_LIT>', data=data, ageunit=<NUM_LIT:2>, tunit=<NUM_LIT:1>, rhounit=<NUM_LIT:1>, idNum=<NUM_LIT:1>)<EOL>
setup trajectories for constant radiation entropy.

S_gamma/R where the radiation constant R = N_A*k
(Dave Arnett, Supernova book, p. 212)
This relates rho and T but the time scale for this
is independent.

Parameters
----------
Sg : float
    S_gamma/R, values between 0.1 and 10. reflect conditions in
    massive stars.  The default is 0.1.
delta_logt_dex : float
    Sets interval between time steps in dex of logtimerev.  The
    default is -0.01.