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

Abstract

Context. The Gaia Radial Velocity Spectrometer (RVS) provides the unique opportunity of a spectroscopic analysis of millions of stars at mediumresolution (λ/∆λ ∼11500) in the near-infrared (845 -872 nm). This wavelength range includes the Ca ii infrared triplet (IRT) at 850.03, 854.44, and 866.45 nm, which is a good diagnostics of magnetic activity in the chromosphere of late-type stars.Aims. Here we present the method devised for inferring the Gaia stellar activity index from the analysis of the Ca ii IRT in the RVS spectrum, together with its scientific validation. Methods. The Gaia stellar activity index is derived from the Ca ii IRT excess equivalent width with respect to a reference spectrum, taking the projected rotational velocity (v sin i) into account. Scientific validation of the Gaia stellar activity index is performed by deriving a R IRT index, largely independent of the photospheric parameters, and considering the correlation with the R HK index for a sample of stars. A sample of well studied PMS stars is considered to identify the regime in which the Gaia stellar activity index may be affected by mass accretion. The position of these stars in the colour-magnitude diagram and the correlation with the amplitude of the photometric rotational modulation is also scrutinised. Results. Gaia DR3 contains a stellar activity index derived from the Ca ii IRT for some 2 × 10 6 stars in the Galaxy. This represents a gold mine for studies on stellar magnetic activity and mass accretion in the solar vicinity. Three regimes of the chromospheric stellar activity are identified, confirming suggestions made by previous authors on much smaller R HK datasets. The highest stellar activity regime is associated with PMS stars and RS CVn systems, in which activity is enhanced by tidal interaction. Some evidence of a bimodal distribution in MS stars with T eff 5000 K is also found, which defines the two other regimes, without a clear gap in between. Stars with 3500 K T eff 5000 K are found to be either very active PMS stars or active MS stars with a unimodal distribution in chromospheric activity. A dramatic change in the activity distribution is found for T eff 3500 K, with a dominance of low activity stars close to the transition between partially-and fully-convective stars and a rise in activity down into the fully-convective regime.

GaiaData Release 3

“…Additional validation results are also given in the dedicated online documentation chapter. Validation on GSP-Phot, GSP-Spec, and ESP-CS-specific products are also found in their dedicated papers (Andrae et al 2022;Lanzafame et al 2022;Recio-Blanco et al 2022). Here, we briefly describe our validation procedures.…”

“…In Gaia DR3, ESP-CS estimates a stellar activity index activityindex_espcs and its uncertainties activityindex_espcs_uncertainty, in units of nanometres (nm), from the calcium infrared triplet (Ca ii IRT, at 849.8, 854.2, and 866.2 nm) in the RVS spectra, see Lanzafame et al (2022). These parameters and a further parameter, activityindex_espcs_input, are found in the astrophysical_parameters table for about 2 million sources.…”

“…Only results for sources with the RVS spectrum S/N ≥ 20 are found in the archive. The ESP-CS activity index is given as an enhancement factor in the core of Ca ii IRT lines with respect to a synthetic template representing the spectrum of an inactive star with the same T eff , log g, and [M/H]; see Lanzafame et al (2022) for details. Despite the fact that the method ensures that the photospheric contribution is removed from the activity index parameter, in principle, because of the contrast effect with the underlying continuum, the index derived gives a relative measure of the stellar activity at a given T eff .…”

“…In general, a value of the activity index of around 0.03 -0.05 separates the regimes in which the chromospheric activity or mass accretion dominate. The separation in terms of R IRT is discussed in Lanzafame et al (2022).…”

“…More details on the validation and use of the stellar and non-stellar products can be found in the accompanying papers II (Fouesneau et al 2022) and III (Delchambre et al 2022), respectively. More complete descriptions of specific products and methods can be found in the following papers: Delchambre (2018), Andrae et al (2022), Lanzafame et al (2022) and Recio-Blanco et al (2022), and the official online documentation 1 . The AP content of Gaia DR3 represents one of the most extensive homogeneous databases of APs to date for exploitation in many domains of astrophysics; see for example Gaia Collaboration et al (2022b), Gaia Collaboration et al (2022c), Gaia Collaboration et al (2022d.…”