Detection of nitrogen gas in the β Pictoris circumstellar disc

摘要

Context. The gas composition of the debris disc surrounding β Pictoris is rich in carbon and oxygen relative to solar abundances. Two possible scenarios have been proposed to explain this enrichment. The preferential production scenario suggests that the produced gas may be naturally rich in carbon and oxygen, while the alternative preferential depletion scenario states that the enrichment has evolved to the current state from a gas with solar-like abundances. In the latter case, the radiation pressure from the star expels the gas outwards, leaving behind species that are less sensitive to stellar radiation such as C and O. Nitrogen is not sensitive to radiation pressure either as a result of its low oscillator strength, which would make it also overabundant under the preferential depletion scenario. The abundance of nitrogen in the disc may therefore provide clues to why C and O are overabundant. Aims. We aim to measure the nitrogen column density in the direction of β Pictoris (including contributions by the interstellar medium and circumstellar disc), and use this information to distinguish these different scenarios to explain the C and O overabundance. Methods. Using far-UV spectroscopic data collected by the Hubble Space Telescope’s Cosmic Origins Spectrograph (COS) instrument, we analysed the spectrum and characterised the NI triplet by modelling the absorption lines. Results. We measure the nitrogen column density in the direction of β Pictoris for the first time, and find it to be log( N _(NI)/1 cm~(2)) = 14.9 ± 0.7. The nitrogen gas is found to be consistent with solar abundances and Halley dust. We also measure an upper limit for the column density of MnII in the disc at log( N _(MnII/1 cm~(2)))_(CS)= 12.7~(+0.1)and calculate the column density of SIII~(**)in the disc to be log( N _(SIII~(★★))/1 cm~(2))_(CS_( X ))= 14.2 ± 0.1. Both results are in good agreement with previous studies. Conclusions. The solar nitrogen abundance supports the preferential production hypothesis, in which the composition of gas in β Pictoris is the result of photodesorption from icy grains that are rich in C and O or collisional vaporisation of C- and O-rich dust in the disc. It does not support the hypothesis that C and O are overabundant because C and O are insensitive to radiation pressure, which would cause them to accumulate in the disc.