Heavy-element Rydberg transition line emission from the post-giant-evolution star HD 101584

Abstract

Context. We report the detection of two lines at millimetre wavelengths towards the immediate surroundings of the post-giant and most likely post-common-envelope star HD 101584 using high-angular-resolution ALMA observations. The circumstellar environment of this object is rich in different molecular species, but we find no viable identifications in terms of molecular lines.
Aims: We aim to determine whether or not these lines can be attributed to the Rydberg transitions - X30α and X26α - of neutral atoms of elements heavier than carbon.
Methods: A simple model in strict local thermodynamic equilibrium for a warm-gas environment of the moderate-temperature star (T_eff ≈ 8500 K) was constructed to corroborate our findings. A geometrically thin, disc-like geometry seen face-on was chosen and a distance of 1 kpc.
Results: The observed flux densities of the lines and the continuum at 232 and 354 GHz can be reproduced using ≈10⁻³ M_⊙ of gas at a temperature of ≈2800 K and a hydrogen density of ≈10¹² cm⁻³, assuming solar abundances for the elements. The gas lies within a distance of about 5 au from the star (assuming a distance of 1 kpc). The ionisation fraction is low, ≈3 × 10⁻⁵. The origin of such a region is not clear, but it may be related to a common-envelope-evolution phase. With these conditions, the line emissions are dominated by Rydberg transitions within the stable isotopes of Mg. A turbulent velocity field in the range 5.5-7.5 km s⁻¹ is required to fit the Gaussian line shapes. An upper limit to the average magnetic field in the line-emitting region of 1 G is set using the Zeeman effect in these lines.
Conclusions: We speculate that Rydberg transitions of heavy elements may be an interesting probe for the close-in environments of other moderate-temperature objects like AGB stars, red supergiants, yellow hypergiants, and binaries of various types.