Population of excited levels of Fe+, Ni+ and Cr+ in exocomets gaseous tails

Abstract

The star Beta Pic is widely known for harbouring a large population of exocomets, which create variable absorption signatures in the stellar spectrum as they transit the star. While the physical and chemical properties of these objects have long remained elusive, Vrignaud et al. (2024) recently introduced the exocomet curve of growth approach, enabling, for the first time, the estimate of exocometary column densities and excitation temperature using absorption measurements in many spectral lines. Using this new tool, we present a refined study of a Beta Pic exocomet observed on December 6, 1997 with the HST. We first show that the comet's signature in FeII lines is well explained by the transit of two gaseous components, with different covering factors and opacities. Then, we show that the studied comet is detected in the lines of other species, such as NiII and CrII. These species are shown to experience similar physical conditions than FeII (same radial velocity profile, same excitation temperature), hinting that they are well-mixed. Finally, using almost 100 FeII lines rising from energy levels between 0 and 33000 cm-1, we derive the complete excitation diagram of Fe+ in the comet. The transiting gas is found to be populated at an excitation temperature of 8190+-160 K, very close to the stellar effective temperature (8052 K). Using a model of radiative and collisional excitation, we show that the observed excitation diagram is compatible with a radiative regime, associated with a close transit distance (< 0.43 au) and a low electronic density (< 1e7 cm-3). In this regime, the excitation of Fe+ is controlled by the stellar flux, and do not depend on the local electronic temperature or density. These results allow us to derive the Ni+/Fe+ and Cr+/Fe+ ratios in the December 6, 1997 comet, at 8.5 +- 0.8 x 10-2 and 1.04 +- 0.15 x 10-2 respectively, close to solar abundances.