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 $\sim 11R_{\star }$ for $o$ Ceti and $\sim 9R_{\star }$ for R Leo. From population diagram analysis, we report the AlF column densities of $\sim 5.8\times 10^{15}$ cm${}^{-2}$ and $\sim 3\times 10^{15}$ 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 $f_{AlF/{\mathrm{H}}_2}\sim (2.5\pm 1.7)\times 10^{-8}$. This gives a lower limit on the F budget in $o$ Ceti and is compatible with the solar F budget $f_{F/{\mathrm{H}}_2}=(5\pm 2)\times 10^{-8}$. For R Leo, a fractional abundance $f_{AlF/{\mathrm{H}}_2}=(1.2\pm 0.5)\times 10^{-8}$ 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 $\sim (1.2-1.5)\times 10^{15}$ cm${}^{-2}$ in W Hya, $\sim (2.5-3.0)\times 10^{14}$ cm${}^{-2}$ in R Dor, and $\sim (0.6-1.0)\times 10^{16}$ cm${}^{-2}$ in IK Tau. These result in fractional abundances within a range of $f_{AlF/{\mathrm{H}}_2}\sim (0.1-4)\times 10^{-8}$ in W Hya, R Dor, and IK Tau.

11 pages, 9 figures

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