Axion spectra and the associated x-ray spectra of low-mass stars

Abstract

For the first time, we predict the axion spectra and their associated luminosities for several low-mass stars -- one and two solar masses stars in the main sequence and post-main sequence stages of evolution. Equally, we also compute the x-ray excess emission resulting from the conversion of axions back to photons in the presence of a strong magnetic field in the envelope of these stars. Hence, a given star will have a unique axion spectrum and $L_a$ axion luminosity. And if such star has a strong magnetic field in its stellar envelope, it will also show a characteristic x-ray spectrum and $L_{a\gamma}$ x-ray luminosity. Such radiation will add up to the x-ray electromagnetic spectrum and $L_X$ luminosity of the star. The present study focuses on axion models with an axion-photon coupling constant, $5\;10^{-11} {\rm GeV}^{-1}$, a value just below the most recent upper limit of $6.6\;10^{-11} {\rm GeV}^{-1}$ found by CAST and IAXO helioscopes. The range of axion parameters discussed here spans many axion models' parameter space, including the DFSZ and KSVZ models. We found that axions with a mass in the range $10^{-7}$ to $10^{-5}\; {\rm eV}$ and an axion-photon coupling constant of $5\;10^{-11} {\rm GeV}^{-1}$ produce an axion emission spectra with an averaged axion energy that varies from 1 to 5 KeV, and an $L_{a}$ ranging from $10^{-6} $ to $7\;10^{-3} \;L_\odot$. We also predict that $L_{a\gamma}$ varies from $5\;10^{-8}$ to $10^{-5} \;L_\odot$ for stars with an averaged magnetic field of $3 \;10^{4}\;{\rm G}$ in their atmospheres. Most of these $L_{a\gamma}$ predictions are larger than the $L_X$ observed in some stars. Therefore, such preliminary results show the potential of the next generation of stellar x-ray missions to constrain several classes of axion models.