fbulge.f

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      SUBROUTINE fbulge(XI,XIDOT,FM,FD)
*
*
*       Gamma/eta potential for the bulge.
*       ----------------------------------
*
      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 gamma/eta model (Dehnen 1993).
*     
*     phi(r) = -GM/[a(2-g)] * [ 1 - (1+a/r)**(g-2) ]  (g # 2)
*     phi(r) = -GM/a*ln(1 + a/r)                      (g = 2)
*
      rr = sqrt(xi(1)**2 + xi(2)**2 + xi(3)**2)
      rrd = (xi(1)*xidot(1) + xi(2)*xidot(2) + xi(3)*xidot(3))/(rr*rr)
      rrd = rrd*(ar*gam + 3.0*rr)/(rr + ar)
      h3 = (gmb/rr**3)*(1.0 + ar/rr)**(gam-3.0)
*
      DO 10 k = 1,3
         fm(k) = -h3*xi(k)
         fd(k) = -h3*(xidot(k) - rrd*xi(k))
 10   CONTINUE
*
      RETURN
*
      END