The companion of HD 190228: Planet or brown dwarf?

Abstract

A detailed abundance analysis has been carried out from high-resolution, high signal-to-noise spectra of the G5IV star HD 190228, which is announced to harbor an extrasolar planet with MP sin i of 5.0 MJ and an orbital period of 1127 days. Based on the model atmosphere of Teff = 5180 K, log g = 3.7, ξt = 1.3 kms-1, we obtained [Fe/H] = -0.40, which puts it on the metal-poor tail of the metallicity distribution of the so far discovered 48 planet-harboring stars. The relative abundance, [X/Fe], indicates an overabundance of light elements (O, Na, Mg, Al, Si, S, Sc) by 0.1-0.2 dex and the solar abundance of heavier elements (K, Ca, Ti, V, Cr, Mn, Ni, Ba). These elements show no conspicuous anomalies. The solar [C/Fe] seems to be slightly smaller than the average (but within the scatter) of field stars of the same [Fe/H], and there is no clear trend of [X/H] with condensation temperature of the element. Neither the process of planet formation nor the stellar dilution during the subgiant stage seems to have polluted its chemical composition. The initially low metallicity may be explained by the suggestion that HD 190228 is accompanied by a brown dwarf instead of a planet.