[Mesa-users] Qneu for r_h1_h1_ec_h2

[Meisel, Zach]

Hi Thomas,

This isn’t exactly my wheelhouse, so it is a good chance for me to embarrass myself, but here is my understanding from the same Bahcall paper referred to earlier:

The pep reaction using laboratory masses has a Q-value of 1.442 MeV. So, if that reaction were to occur on earth, 1.442MeV of energy would be carried away from the reaction somehow. However, the particles fusing in a stellar environment have some thermal energy that they bring into the reaction. So the amount of energy that can be released is actually a bit larger than the laboratory Q-value. Bahcall ballparks that the thermal energy for the pep reaction will be the same as for the pp reaction, which is a few keV in the sun. This means the actual energy release from a solar pep reaction is more like 1.445MeV.

In principle, the energy release could be carried away as kinetic energy in reaction products or photons from a photon-producing reaction or from a de-excitation of an excited state created in a reaction. However, there is no excited state of the deuteron, so that’s out. So energy is going to be carried away by the neutrino and the recoil of the deuteron. Using conservation of momentum and energy, while using the non-relativistic approximation (which is good enough), the kinetic energy carried away by the deuteron will be the Q-value squared divided by twice the mass of the deuteron: KEd = Q^2/(2*2*931.5MeV) ~ 0.6keV.  This means essentially all of the energy released in the reaction is going to be carried away by the neutrino (certainly all within the uncertainty of the approximation for the thermal energy shift)

So, the pep reaction is always going to act as a heat sink, with energy being liberated from the star by an escaping neutrino. The thermal shift increases the amount of energy released just a little bit.  I’m not sure where Q-Qneu comes into the work you’re doing, but this is just the energy shift that Bahcall describes in his paper.

-Zach

From: Thomas Steindl<mailto:thomas.steindl at uibk.ac.at>
Sent: Friday, June 5, 2020 11:13 AM
To: Meisel, Zach<mailto:meisel at ohio.edu>
Cc: mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Qneu for r_h1_h1_ec_h2

Hi Zach,

Thank you for the answer and the provided reference!

So the net energy of the pep reaction is indeed negative?

Thomas


Am 05.06.2020 um 14:27 schrieb Meisel, Zach <meisel at ohio.edu<mailto:meisel at ohio.edu>>:

Hi Thomas,

See Bahcall PRC 1997 (https://ui.adsabs.harvard.edu/abs/1997PhRvC..56.3391B/abstract) for the 1.445MeV neutrino energy loss.

-Zach

----
Zach Meisel
Assistant Professor of Physics & Astronomy
Director, Edwards Accelerator Laboratory
Ohio University
Email: meisel at ohio.edu<mailto:meisel at ohio.edu>
Web: http://inpp.ohio.edu/~meisel

From: Thomas Steindl<mailto:thomas.steindl at uibk.ac.at>
Sent: Friday, June 5, 2020 5:04 AM
To: mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>
Subject: [Mesa-users] Qneu for r_h1_h1_ec_h2

Hello all,

I am supervising a bachelor student and we have been looking at the pp reactions and are puzzled by the pep reaction r_h1_h1_ec_h2. We found that Q - Qneu is negative as Q is 1.44206 MeV and Qneu is 1.445 MeV. However, I can only find Qneu = 1.44 MeV in the literature which would set Q-Qneu positive. I hope somebody can help me figure out what happens here. As always, thank you for any help!

Thomas
__________________________________________________________________

Thomas Steindl, MSc
University of Innsbruck | Institute for Astro- and Particle Physics
Tel: +43 512507 52031
Mail: thomas.steindl at uibk.ac.at<mailto:thomas.steindl at uibk.ac.at>

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