[mesa-users] Vink_wind_eta

[Zsolt Keszthelyi]

Hi Bill, 

thanks for your reply. 

I think the issue is that eta is used in different contexts with different meanings in the literature. 


1. In the context of RGB / AGB stars (Reimers 1975, Blöcker 1995) eta is a scaling factor to match observed mass loss rates with their empirical mass loss formulas.  


2. On the other hand, in the context of WR stars (Cassinelli 1991 and references therein) eta was introduced with a physical meaning, to describe the wind with the ratio of stellar wind momentum flux and the photon momentum flux, eta = Mdot * vinf / (L / c).

This eta is used in the context of OB stars (e.g., Lamers & Leitherer 1993, Puls et al. 1996) as the performance number.



Even though the idea of eta as a scaling factor is stated (and used), a user was looking for eta in the Vink paper, so that is why I think calling it simply 'wind_scaling_factor' might make it easier for the users. 


Also my understanding is that you must set either 'RGB_wind_scheme' or 'AGB_wind_scheme' to apply one of the wind recipes, however the Kudritzki, Vink (OB) and Nugis & Lamers, Gräfener & Hamann (WR) prescriptions are not valid for these stars. This might also be a source of confusion since in a previous email the vink_wind_eta and RGB mass loss was mentioned together. 


I hope this is helpful. 

Cheers, 
 
 Zsolt 















> Von: Bill Paxton <paxton at kitp.ucsb.edu>
> An: Zsolt Keszthelyi <Z.Keszthelyi at campus.lmu.de>
> Kopie: mesa-users at lists.sourceforge.net users <mesa-users at lists.sourceforge.net>
> Datum: Tue May 26 06:46:15 CEST 2015
> 
> 
> On May 23, 2015, at 4:25 AM, Zsolt Keszthelyi wrote:
> 
> > It's a little bit unlucky nomenclature though, since in stellar wind physics the
> eta is the so-called wind efficiency / performance number and has nothing to do with
> a numerical scaling factor. (It might be useful to change it in the next version.)
> 
> 
> Hi,
> 
> In star/defaults/controls.defaults we describe the various etas like this for Reimers
> and Blocker winds:
> 
>          !### Reimers_wind_eta
>          ! Parameter for mass loss by Reimers wind prescription.
>          ! Reimers mdot is `eta*4d-13*L*R/M` (Msun/year), with L, R, and M in solar
> units.
> 
>          !### Blocker_wind_eta = 0
>          ! Parameter for mass loss by Blocker's wind prescription.
>          ! Blocker mdot is `eta*4.83d-9*M**-2.1*L**2.7*4d-13*L*R/M` (Msun/year),
> 
> Having stated that idea twice, we just hope the reader will make the leap to understand
> that they work similarly for the remaining kinds of winds.
> 
> So we use "parameter for mass loss" where you would use "wind efficiency / performance
> number".
> 
> In practice of course it is a numerical scaling factor for the strength of the wind.
>  At least it looks like that to me.  And it does that in the code.
> 
> If I'm missing something important about the role of eta in stellar wind physics,
> please educate me.  
> 
> Thanks,
> Bill
> 
> 
> 
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