Chromospheric activity on late-type star DM UMa using high-resolution spectroscopic observations

Abstract

We present new 14 high-resolution echelle spectra to discuss the level of chromospheric activity of DM UMa in He i D 3 , Na i D 1 , D 2 , H α , and Ca ii infrared triplet lines (IRT). It is the first time to discover the emissions above the continuum in the He i D 3 lines on Feb. 9 and 10, 2015. The emission on Feb. 9 is the strongest one ever detected for DM UMa. We analyzed these chromospheric active indicators by employing the spectral subtraction technique. The subtracted spectra reveal weak emissions in the Na i D 1 , D 2 lines, strong emission in the H α line, and clear excess emissions in the Ca ii IRT lines. Our values for the EW 8542 /EW 8498 ratio are on the low side, in the range of 1.0 -1.7. There are also clear phase variations of the level of chromospheric activity in equivalent width (EW) light curves in these chromospheric active lines (especially the H α line). These phenomena might be explained by flare events or rotational modulations of the level of chromospheric activity.

Stellar Chromospheric Variability

“…A range of other spectral activity indicators have also been explored and found useful as tracers of chromospheric magnetic activity (see Figure 6), flares and plages, including Hα [4,110] and Hβ emission [111], He I D 3 (587.59 nm) and 1.08 µm absorption [112][113][114][115], the Na I D 1 and D 2 lines, the infrared Ca II triplet absorption lines (centred at 8498.062 Å, 8542.144 Å and 8662.170 Å), and extreme ultraviolet (UV; 200.5-300.5 nm) resonance lines associated with surface effective temperatures T eff < 2 × 10 4 K, including C I, C II, Co II, Cr II, Fe II, Mn II, Ni II, O I, Si II and Ti II [116], as well as Mg II h and k [117][118][119]. Of these, Fe II (298.5 nm) is the dominant contributor to solar irradiance variations in high-ionisation environments [114].…”

“…The Ca II infrared triplet lines are of particular interest given that they offer a number of distinct advantages with respect to the workhorse Ca II H and K lines. The ratio of the equivalent widths (EWs) of the Ca II triplet lines at 8542 Å and 8498 Å, EW 8542 /EW 8498 , is an indicator of chromospheric activity that is particularly suitable to distinguish between the occurrence of plages and prominences [113]. Zhang et al [113] recorded these line ratios for the chromospherically active, late-type spectroscopic RS CVn binary system DM UMa.…”

“…The ratio of the equivalent widths (EWs) of the Ca II triplet lines at 8542 Å and 8498 Å, EW 8542 /EW 8498 , is an indicator of chromospheric activity that is particularly suitable to distinguish between the occurrence of plages and prominences [113]. Zhang et al [113] recorded these line ratios for the chromospherically active, late-type spectroscopic RS CVn binary system DM UMa. They found EW 8542 /EW 8498 ratios of 1.0-1.7, which implies the presence of optically thick plages.…”

“…Correlations between starspot and plage behaviour often manifest themselves as a dependence of the chromospheric variability on orbital phase or longitude [113,190]. This type of chromospheric variability reflects a star's rotational modulation.…”

“…They feature active chromospheres-as traced by strong Ca II H and K emission-and large photospheric starspots. The latter may cover as much as 50% of the stellar surface [113,180,266]. They are likely accompanied by co-located plages, as traced by increasing Hα EWs caused by increased electron densities ( [186], and references therein).…”