Spectroscopic study of IRAS 19285+0517 (PDS 100): A rapidly rotating Li-rich K giant

We report on photometry and high-resolution spectroscopy for IRAS 19285+0517. The spectral energy distribution based on visible and near-infrared photometry and far-infrared fluxes shows that the star is surrounded by dust at a temperature of T-d similar to 250 K. Spectral line analysis shows that the star is a K giant with a projected rotational velocity of upsilon sin i = 9 +/- 2 km s(-1). We determined the atmospheric parameters: T-eff = 4500 K, log g = 2.5, xi(t) = 1.5 km s(-1), and [Fe/H] =0.14 dex. The LTE abundance analysis shows that the star is Li-rich [log epsilon(Li) = 2.5 +/- 0.15], but with essentially normal C, N, O, and metal abundances. Spectral synthesis of molecular CN lines yields the carbon isotopic ratio C-12/C-13 = 9 +/- 3, a signature of post-main-sequence evolution and dredge-up on the red giant branch (RGB). Analysis of the Li resonance line at 6707 Angstrom for different Li-6/Li-7 ratios shows that the Li profile can be best fitted with a predicted profile for pure Li-7. Far-infrared excess, high Li abundance, and rapid rotation suggest that a planet has been swallowed or, perhaps, that an instability in the RGB outer layers triggered a sudden enrichment of Li and caused mass loss.