First detection of AlF line emission towards M-type AGB stars

Abstract

The nucleosynthesis production of fluorine (F) is still a matter of debate. Asymptotic giant branch (AGB) stars are one of the main candidates for F production. However, their contribution to the total F budget is not fully known due to the lack of observations. In this paper, we report the detection of AlF line emission, one of the two main carriers of F in the gas-phase in the outflow of evolved stars, towards five nearby oxygen-rich AGB stars, $o$ Ceti, R Leo, IK Tau, R Dor, and W Hya. From spatially resolved observations, we estimated the AlF emitting region with a radius $\sim11R_{\star}$ for $o$ Ceti and $\sim9R_{\star}$ for R Leo. From population diagram analysis, we report the AlF column densities of $\sim 5.8\times10^{15}$ cm$^{-2}$ and $\sim 3\times10^{15}$ cm$^{-2}$ for $o$ Ceti and R Leo, respectively, within these regions. For $o$ Ceti, we used the C$^{18}$O ($v=0$, $J=3-2$) observations to estimate the H$_2$ column density of the emitting region. We found a fractional abundance of $f_{\rm AlF/H_2}\sim(2.5\pm1.7)\times10^{-8}$. This gives a lower limit on the F budget in $o$ Ceti and is compatible with the solar F budget $f_{\rm F/H_2}=(5\pm2)\times10^{-8}$. For R Leo, a fractional abundance $f_{\rm AlF/H_2}=(1.2\pm0.5)\times10^{-8}$ is estimated. For other sources, we cannot precisely determine the emitting region based on the available data. Assuming an emitting region with a radius of $\sim 11R_{\star}$ and the rotational temperatures derived for $o$ Ceti and R Leo, we crudely approximated the AlF column density to be $\sim(1.2-1.5)\times10^{15}$ cm$^{-2}$ in W Hya, $\sim(2.5-3.0)\times10^{14}$ cm$^{-2}$ in R Dor, and $\sim(0.6-1.0)\times10^{16}$ cm$^{-2}$ in IK Tau. These result in fractional abundances within a range of $f_{\rm AlF/H_2}\sim(0.1-4)\times10^{-8}$ in W Hya, R Dor, and IK Tau.