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 (Teff ≈ 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.