sigma0 = 2d4*msun/pc^2
r0     = 0.3*kpc
;
nr     = 1000
r      = (1+findgen(nr))/float(nr)*10.*r0
M      = 2.d0*!dpi*sigma0*r0^2*(1-(1+r/r0)*exp(-r/r0))
vc     = sqrt(g*m/r)
;
vc0    = 200.*km
;
vct    = sqrt(vc0^2+vc^2)
;
r      = 1.*kpc
M      = 2.d0*!dpi*sigma0*r0^2*(1-(1+r/r0)*exp(-r/r0))
vc     = sqrt(g*m/r)
vct    = sqrt(vc0^2+vc^2)
print,' at r = ',r/kpc,' vc = ',vct/km,' km/s'
;
r      = 10.*kpc
M      = 2.d0*!dpi*sigma0*r0^2*(1-(1+r/r0)*exp(-r/r0))
vc     = sqrt(g*m/r)
vct    = sqrt(vc0^2+vc^2)
print,' at r = ',r/kpc,' vc = ',vct/km,' km/s'