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Gas fractions and depletion times in galaxies with different degrees of interaction

We study the atomic gas content and the central stellar mass concentration for a sample of almost 1500 nearby galaxies to investigate the nature of starbursts and the influence of galaxy-galaxy interactions on star formation (SF). We used a sample of catalogued interacting and non-interacting galaxies in the S$^4$G survey - along with archival HI gas masses, stellar masses ($M_{\ast}$), and SF rates from $IRAS$ far-infrared fluxes - and calculate depletion times and gas fractions. We traced the central stellar mass concentration from the inner slope of the stellar component of the rotation curves. Starbursts are defined as galaxies with a factor $>4$ enhanced SF rate relative to a control sample of non-interacting galaxies which are $\pm 0.2$ dex in stellar mass and $\pm 1$ in $T$-type. Starbursts are mainly early-type ($T\lesssim 5$), massive spiral galaxies ($M_{\ast}\gtrsim 10^{10}M_{\odot}$) that are not necessarily interacting. For a given stellar mass bin, starbursts are characterised by lower gas depletion times, similar gas fractions, and larger central stellar mass concentrations than non-starburst galaxies. The global distributions of gas fraction and gas depletion time are not statistically different for interacting and non-interacting galaxies. However, in the case of currently merging galaxies, the median gas depletion time is a factor of $0.4 \pm 0.2$ that of control sample galaxies, and their SF rates are a factor of $1.9 \pm 0.5$ enhanced, even though the median gas fraction is similar. Starbursts present long-lasting SF in the circumnuclear regions that causes an enhancement of the central stellar density at $z\approx0$ in both interacting and non-interacting systems. Starbursts have low gas depletion timescales, yet similar gas fractions as normal main-sequence galaxies. Galaxy mergers cause a moderate enhancement of the SF efficiency (Abridged).