Subject: Re: Ref. comment (Sec. 5.2) From: David Cohen Date: Sat, 12 Apr 2008 13:59:07 -0400 To: Skinner Steve CC: Stan Owocki , Richard Townsend , "Gagne, Marc" Steve, I think the short answer is that no theory is precise enough that there's a substantial difference between 1.5 Rstar and 1.8 Rstar or between 350 km/s or 550 km/s, etc. Here are some complicating factors - - In MCWS, the plasma on loops isn't moving radially away from the star, so the radiation force on it isn't the same as what you'd expect in a spherically symmetric, radially accelerated wind; - If the star is rotating rapidly, then there is also centrifugal acceleration on the wind trapped in closed field regions. So, for both these reasons, the standard beta velocity law of the wind doesn't hold precisely in the MCWS scenario. Ultimately, we'd have to do some simulations to see if we can get the right x-ray temperature distribution and also be consistent with the f/i constraints. Regarding the standard LDI wind shock scenario - - The preshock wind flow is generally faster than the corresponding beta-law wind (this is inherent to the nature of the instability). The post-shock gas is decelerated back down to roughly the velocity given by the beta-law wind, but this is just an approximate statement. So, there's enough ambiguity/scatter in this scenario too such that the shock velocities inferred from the x-ray temperatures, the post-shock kinematics inferred from the line widths, and the radial constraints inferred from the f/i ratio are all more or less consistent. At least plausibly so. Plus, as I believe I recently pointed out, an x-ray line width expressed as a velocity, implies a somewhat larger physical velocity in the context of a spherically symmetric flow because most of the emitting wind will have a substantial velocity component tangential to our line of sight. So, for several reasons, I'd be hesitant to say that an x-ray line with of X km/s implies a flow velocity of X km/s which implies a radius such that a beta-velocity law gives X km/s at that radius. Again, though, the big problem I see with the LDI scenario is simply the relatively small X-ray line widths. Empirically (zeta Pup, zeta Ori, several other O stars) the line widths are typically 50% v_inf, not 20%. And this is expected from detailed numerical simulations of the instability generated structure as well as from empirical, simplified models designed to explain X-ray profiles. Finally, I'll point out that the f/i constraint of 1.8 Rstar is made under the assumption that there's a *single* radius of formation. Leutenegger et al. (2006) showed that if you assume that the x-ray emitting plasma is distributed throughout the wind above some onset radius, Ro, that the Ro you derive from fitting data is significantly less than the single radius of formation you derive from the same data. In other words, the f/i we measure is probably consistent with x-ray turning on at 1.5 Rstar (or 1.3 or something) and then being generated at every location above that radius, but with an emissivity weighted by the local density-squared. Sorry for the long email. I will leave it up to you as to how to answer the referee and/or edit the relevant part of the paper. Overall, I'd say we might want to make some edits so that readers - like this referee - don't over-interpret the quantitative statements we make (e.g. thinking that the MCWS scenario necessarily implies the x-ray plasma is farther from the photosphere than does the standard LDI scenario). That's my 2+ cents. David Skinner Steve wrote: > > David: > > I'm making good progress on implementing the > referee's comments and should be done by Sat. > noon, so hopefully I can send out the revised > paper to all co-authors this weekend. > > Do you have any input on the following referee > comment? Specifically, is there a constradiction > between the postulated pre-shock wind speed > v = 550 km/s and the constraint on line > formation radius from f/i ratios? I don't think > there is a problem here. For a beta = 0.8 wind > velocity law, I get v = 550 km/s at a radius > r = 1.79 R_star, which is consistent with the > upper limit on line formation radius from Mg XI. > Given our imprecise knowledge of the wind acceleration > law close to the star, I dont think there is a > contradiction. > > Steve > > Referee comment: > Section 5.2, third paragraph: The end of this paragraph is not very precise. > It says that the wind acceleration zone must be "near the star". However, > considering the speed of the wind of 500 km/s, wouldn't this be still > further than the standard wind shock region constrained by f/i? > What exactly is "near" in this case? > ---------------------------------------------- > > > > ******************************************************* > * * > * Steve Skinner * > * * > * Center for Astrophysics and Space Astronomy (CASA) * > * 389 UCB * > * University of Colorado * > * Boulder, CO 80309-0389 USA * > * * > * Tel. 303-492-4202 * > * Fax. 303-492-4052 * > * E-mail skinners@casa.colorado.edu * > * Web: http://casa.colorado.edu/~skinners/ss.html * > * * > *******************************************************