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The Gaia Radial Velocity Spectrometer provides the unique opportunity of aspectroscopic analysis of millions of stars at medium-resolution in thenear-infrared. This wavelength range includes the Ca II infrared triplet (IRT),which is a good diagnostics of magnetic activity in the chromosphere oflate-type stars. Here we present the method devised for inferring the Gaiastellar activity index together with its scientific validation. A sample ofwell studied PMS stars is considered to identify the regime in which the Gaiastellar activity index may be affected by mass accretion. The position of thesestars in the colour-magnitude diagram and the correlation with the amplitude ofthe photometric rotational modulation is also scrutinised. Three regimes of thechromospheric stellar activity are identified, confirming suggestions made byprevious authors on much smaller $R'_{\rm HK}$ datasets. The highest stellaractivity regime is associated with PMS stars and RS CVn systems, in whichactivity is enhanced by tidal interaction. Some evidence of a bimodaldistribution in MS stars with $T_{\rm eff}\ge$ 5000 K is also found, whichdefines the two other regimes, without a clear gap in between. Stars with 3500K$\le T_{\rm eff} \le$ 5000 K are found to be either very active PMS stars oractive MS stars with a unimodal distribution in chromospheric activity. Adramatic change in the activity distribution is found for $T_{\rm eff}\le$3500K, with a dominance of low activity stars close to the transition betweenpartially- and fully-convective stars and a rise in activity down into thefully-convective regime.

Gaia Data Release 3. Stellar chromospheric activity and mass accretion from Ca II IRT observed by the Radial Velocity Spectrometer