# [mesa-users] Clarification: velocity of wind

Josiah Schwab jwschwab at ucsc.edu
Sat Feb 4 16:51:35 EST 2017

```Hi Sanskriti,

> Now the question is, what exactly v_wind is ? Does it completely
> represent the speed of wind at all?

When you say v_wind, I'm assuming you mean the value of the history
column v_wind_Km_per_s.

If you look in history_columns.list, which is located in
\$MESA_DIR/star/defaults (or you may have a local copy in your work
directory), you'll see the comment:

v_wind_Km_per_s ! Km/s

! = 1d-5*s% opacity(1)*max(0d0,-s% mstar_dot)/ &
!        (4*pi*s% photosphere_r*Rsun*s% tau_base)
! Lars says:
! wind_mdot = 4*pi*R^2*rho*v_wind
! tau = integral(opacity*rho*dr) from R to infinity
! so tau = opacity*wind_mdot/(4*pi*R*v_wind) at photosphere
! or v_wind = opacity*wind_mdot/(4*pi*R*tau) at photosphere

Another way to find out this information is to grep for the name of the
column in \$MESA_DIR/star/private/history.f90.  This lets you see exactly
how the quantity is evaluated.

\$ grep -A 2 v_wind history.f90
case(h_v_wind_Km_per_s)
val = 1d-5*s% opacity(1)*max(0d0,-s% mstar_dot)/ &
(4*pi*s% photosphere_r*Rsun*s% tau_base)

(In this case you learn exactly the same thing as the comment, but not
every history column has such an extensive comment.)

Hope that helps,
Josiah

```