Searching for chemical signatures of brown dwarf formation⋆
1 INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
2 Universidad Autónoma de Madrid, Dpto. Física Teórica, Módulo 15, Facultad de Ciencias, Campus de Cantoblanco, 28049 Madrid, Spain
Received: 22 November 2016
Accepted: 9 February 2017
Context. Recent studies have shown that close-in brown dwarfs in the mass range 35–55 MJup are almost depleted as companions to stars, suggesting that objects with masses above and below this gap might have different formation mechanisms.
Aims. We aim to test whether stars harbouring massive brown dwarfs and stars with low-mass brown dwarfs show any chemical peculiarity that could be related to different formation processes.
Methods. Our methodology is based on the analysis of high-resolution échelle spectra (R~ 57 000) from 2–3 m class telescopes. We determine the fundamental stellar parameters, as well as individual abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Zn for a large sample of stars known to have a substellar companion in the brown dwarf regime. The sample is divided into stars hosting massive and low-mass brown dwarfs. Following previous works, a threshold of 42.5 MJup was considered. The metallicity and abundance trends of the two subsamples are compared and set in the context of current models of planetary and brown dwarf formation.
Results. Our results confirm that stars with brown dwarf companions do not follow the well-established gas-giant planet metallicity correlation seen in main-sequence planet hosts. Stars harbouring massive brown dwarfs show similar metallicity and abundance distribution as stars without known planets or with low-mass planets. We find a tendency of stars harbouring less-massive brown dwarfs of having slightly higher metallicity, [XFe/Fe] values, and abundances of Sc ii, Mn i, and Ni i than the stars having the massive brown dwarfs. The data suggest, as previously reported, that massive and low-mass brown dwarfs might present differences in period and eccentricity.
Conclusions. We find evidence of a non-metallicity dependent mechanism for the formation of massive brown dwarfs. Our results agree with a scenario in which massive brown dwarfs are formed as stars. At high metallicities, the core-accretion mechanism might become efficient in the formation of low-mass brown dwarfs, while at lower metallicities low-mass brown dwarfs could form by gravitational instability in turbulent protostellar discs.
Key words: techniques: spectroscopic / stars: abundances / stars: late-type / planetary systems
Based on observations made with the Mercator Telescope; on observations made with the Nordic Optical Telescope; on data products from the SOPHIE archive; on data products from the ELODIE archive; and on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes ID 072. C-0488(E), 076.C-0155(A), 076.C-0429(A), 078.C-0133(A), 079.C-0329(A), 082.C-0333(A), 083.C-0174(A), 083.C-0413(A), 085. C-0019(A), 085.C-0393(A), 087.A-9029(A), 087.C-0831(A), 090.C-0421(A), 093.C-0409(A), 094.D-0596(A), 095.A-9029(C), 178.D-0361(B), 183.C-0972(A), 184.C-0639(A), and 188.C-0779(A).
© ESO, 2017