fhalo.f

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      SUBROUTINE fhalo(XI,XIDOT,FM,FD)
*
*
*       Logarithmic halo force.
*       -----------------------
*
      IMPLICIT REAL*8  (a-h,o-z)
      common/galaxy/ gmg,rg(3),vg(3),fg(3),fgd(3),tg,
     &               omega,disk,a,b,v02,rl2,gmb,ar,gam,zdum(7)
      REAL*8  xi(3),xidot(3),fm(3),fd(3)
*
*
*       Obtain force and first derivative for logarithmic potential.
      r2 = xi(1)**2 + xi(2)**2 + xi(3)**2 + rl2
      rrd = 2.0*(xi(1)*xidot(1) + xi(2)*xidot(2) + xi(3)*xidot(3))/r2
      h2 = v02/r2
*
*       Note softening in the logarithmic potential (cf. Binney & Tremaine).
      DO 10 k = 1,3
          fm(k) = -h2*xi(k)
          fd(k) = -h2*(xidot(k) - rrd*xi(k))
   10 CONTINUE
*
      RETURN
*
      END