Linking chromospheric activity and magnetic field properties for late-type dwarf stars

Abstract

Spectropolarimetric data allow for simultaneous monitoring of stellar chromospheric $\log {R^{\prime }_{\rm {HK}}}$ activity and the surface-averaged longitudinal magnetic field, Bl, giving the opportunity to probe the relationship between large-scale stellar magnetic fields and chromospheric manifestations of magnetism. We present $\log {R^{\prime }_{\rm {HK}}}$ and/or Bl measurements for 954 mid-F to mid-M stars derived from spectropolarimetric observations contained within the PolarBase database. Our magnetically active sample complements previous stellar activity surveys that focus on inactive planet-search targets. We find a positive correlation between mean $\log {R^{\prime }_{\rm {HK}}}$ and mean log |Bl|, but for G stars the relationship may undergo a change between $\log {R^{\prime }_{\rm {HK}}}\sim -4.4$ and −4.8. The mean $\log {R^{\prime }_{\rm {HK}}}$ shows a similar change with respect to the $\log {R^{\prime }_{\rm {HK}}}$ variability amplitude for intermediately-active G stars. We also combine our results with archival chromospheric activity data and published observations of large-scale magnetic field geometries derived using Zeeman Doppler Imaging. The chromospheric activity data indicate a slight under-density of late-F to early-K stars with $-4.75\le \log {R^{\prime }_{\rm HK}}\le -4.5$. This is not as prominent as the original Vaughan–Preston gap, and we do not detect similar under-populated regions in the distributions of the mean |Bl|, or the Bl and $\log {R^{\prime }_{\rm HK}}$ variability amplitudes. Chromospheric activity, activity variability and toroidal field strength decrease on the main sequence as rotation slows. For G stars, the disappearance of dominant toroidal fields occurs at a similar chromospheric activity level as the change in the relationships between chromospheric activity, activity variability and mean field strength.

Kepler main-sequence solar-like stars: surface rotation and magnetic-activity evolution

“…As the activity proxies estimated from K2 light curves and APOGEE spectra are both related to spots, they are expected to show the same behavior. Indeed, that is the case, except with the rotation sequence from Cao and Pinsonneault (2022), which is located at slightly shorter P rot than that from Brown et al (2021, see middle panel of Figure 4). To split the f spot sample into saturated and unsaturated regimes for Figure 4, we consider the transition at log Ro C2022, f spot = −0.677 as determined by Cao and Pinsonneault (2022) with their power-law model.…”

“…Reiners et al (2022) tracked down the transition from the saturated to unsaturated regime in the average magnetic field strength, ⟨B⟩, of M dwarfs (and some K) from CARMENES spectra (Figure 6). Similarly, to the magnetic activity proxies above, the transition identified by the authors at their Ro R2022 = 1.3 takes place near the lower edge of the P rot distribution of Kepler M dwarfs.…”

A Spectroscopic Analysis of a Sample of K2 Planet-host Stars: Stellar Parameters, Metallicities and Planetary Radii

“…Three stars in our sample had activity indices reported by Brown et al (2022), who compiled a database of chromospheric activity measurements and surface-averaged large-scale magnetic-field measurements for a sample of FGK mainsequence stars. These stars are K2-229, EPIC 202089657, and Kepler-409, with log R' HK = −4.73, −4.75, and −4.82, respectively, keeping in mind that the Sun has a value of log R' HK = −5.02, which varies by about ±0.01 dex over the Solar activity cycle (Lorenzo-Oliveira et al 2018).…”

HST Low Resolution Stellar Library

“…The dynamo action brought about by differential stellar rotation is one of the most commonly accepted mechanisms for magnetic field generations in main sequence stars (Hartmann & Noyes 1987;Fröhlich et al 2012;Quentin & Tout 2018). Chromospheric activity is generally associated with strong magnetic fields (Musielak & Bielicz 1982;Brown et al 2022). Since stellar rotation is expected to slow down over the lifetime of a star, activity can be presumed to decrease (Barry 1988).…”