Juergen H. M. M. Schmitt

The combined time series of the consistently calibrated chromospheric activity indicator, the S-index -- derived from Mount Wilson and TIGRE data -- now spans more than five decades of monitoring observations of the Sun and of more than one hundred solar-like stars. For the first time, these data allow us to probe the long-term stability of solar-type magnetic cycles as well as their possible transition into other behavioral forms of stellar activity in the time domain. Such variability studies are an important input for solar-stellar connection research and for our empirical understanding of the long-term perspective of solar activity as well as of a wider empirical picture of the coevolution of magnetic activity with stellar structure. We here present and discuss a representative selection of cool main-sequence stars, for which such data are available. The excellent quality of our TIGRE data now more than doubles the number of known cyclic stars. Grading them (i.e., excellent, good, fair, and poor) and putting them into an evolutionary perspective, we find that stars more active than the Sun are also more likely to show cyclic variability than stars below solar activity. Furthermore, more active stars tend to have more regular cycles, while the solar mean S-value fits the lower end of the S-index range found for cyclic stars. This is consistent with the occurrence of solar Maunder minima episodes, hinting at a sometimes already unstable solar cycle. We find eighteen stars with even lower activity and lower variability than the Sun, which provide a preview into the latter's distant future. For half of these stars, we can use sensitive X-ray observations to complement our chromospheric data and to explore the lower end of stellar activity also from a coronal perspective. These so-called ''Maunder minimum candidate stars'' exhibit what we refer to as ''remnant activity,'' which expresses itself in a minimal coronal flux and a minimal S-value, above which only rather small variations occur. Using accurate Gaia DR3 distances allows us to place our sample stars into an Hertzsprung-Russell diagram (HRD), which demonstrates that active cyclic stars and inactive noncyclic stars populate rather different regions in the HRD.

The rotation-activity connection is the standard paradigm for interpreting chromospheric and coronal activity in late-type stars, namely, stars with outer convection zones. This paradigm states that activity increases with decreasing rotation period until a saturation limit is reached. By scaling rotation periods with the convective turnover time via the Rossby number, Ro, saturation is expected to occur at a universal value across all spectral types. In our paper, we systematically investigate the relationship between rotation and activity as measured though X-ray emission for a large sample of late-type stars to test the universal applicability of this paradigm. To this end, we utilized TESS short-cadence space photometry to determine the rotation periods for late-type stars identified in the eROSITA all-sky survey. This combined dataset provides rotation and X-ray measurements for 14 004 stars, representing a sample size increase of more than an order of magnitude compared to previous studies. Our results show that F-type stars do not reach the “classical” saturation limit of L _X /L _bol ≈ 10 ⁻³ , instead exhibiting significantly lower L _x / L _bol values. We find that the convective turnover times derived from this sample closely agree with theoretical computations, supporting the idea that Rossby number-activity relations hold for all late-type main sequence stars. The lower level of activity in earlier spectral types (e.g., F-type and late A-type stars) is a physical consequence of their short convective turnover times, which prevent them from rotating rapidly enough to ever reach the saturation regime. We demonstrate that a simple model incorporating our derived turnover times versus color can successfully reproduce the observed characteristics of the eROSITA X-ray activity distribution, as measured by the L X /L _bol ratio and Gaia BP-RP color.

Giant stars can be magnetically active. A prominent example is HD 251108, which exhibited a giant flare recorded on 7 November 2022 at X-ray and optical wavelengths. We report on a spectroscopic campaign monitoring this star from November 2022 to March 2024, starting with the late phases of the giant flare and accumulating 149 medium-resolution spectra with the TIGRE telescope. HD 251108 shows periodic variations in radial velocity (RV), which we interpret as being caused by the star being part of a binary system, yet the photospheric lines do not show any obvious signs of a second component. Similar periodic changes are found in the effective temperature and in the equivalent widths (EWs) of various chromospheric emission lines with the same period as observed in RV, but with a phase shift. We find these effects could be caused by a plage rotating with the star if the star is rotating synchronously. Since the star shows strong Hα and even He I infrared triplet emission outside the flare, we find it to be hyperactive, even though the star only has moderate lithium abundance of A(Li) = 1.28 dex. We further investigated archival TESS observations of HD 251108 and found that flares with energy releases in excess of 10³⁸ erg at optical wavelengths are not unusual. Finally, considering the November 2022 event in a broader context, we argue that this event likely released more than 10⁴⁰ erg at optical wavelengths.

Stellar activity can be observed at different wavelengths in a variety of different activity indicators. We investigated the correlation between coronal and chromospheric emissions by combining X-ray data from stars detected in the eROSITA all-sky surveys (eRASS1 and eRASS:5) with Ca II infrared triplet (IRT) activity indices as published in the third Gaia data release (Gaia DR3). We specifically studied 24 300 and 43 200 stellar sources with reliable Ca II IRT measurement and X-ray detection in eRASS1 and eRASS:5, which is by far the largest stellar sample available so far. The largest detection fraction is obtained for highly active sources and stars of a late spectral type, while F-type and less active stars (as measured in the Ca II IRT) remain mostly undetected in X-rays. Also, the correlation is the strongest for late-type sources, while F-type stars show a rather weak correlation between the X-ray to bolometric flux ratio and the Ca II IRT activity index. The relation between the X-ray and Ca II IRT surface fluxes changes with the fractional X-ray flux without showing two separated branches as described in previous studies. For fast rotators, both activity indicators saturate at a similar Rossby number and the X-ray to bolometric flux ratio decreases faster than the IRT index for slower rotating stars. As a consequence, the ratio between X-ray and IRT fluxes is constant in the saturation regime and decreases for slow rotators.

Many giant stars are magnetically active, which causes rotational variability, chromospheric emission lines, and X-ray emission. Large outbursts in these emission features can set limits on the magnetic field strength and thus constrain the mechanism of the underlying dynamo. HD 251108 is a Li-rich active K-type giant. We find a rotational period of 21.3 days with color changes and additional long-term photometric variability. Both can be explained with very stable stellar spots. We followed the decay phase of a superflare for 28 days with NICER and from the ground. We track the flare decay in unprecedented detail in several coronal temperature components. With a peak flux around 10³⁴ erg s⁻¹ (0.5–4.0 keV) and an exponential decay time of 2.2 days in the early decay phase, this is one of the strongest flares ever observed, yet it follows trends established from samples of smaller flares, for example, for the relations between Hα and X-ray flux, indicating that the physical process that powers the flare emission is consistent over a large range of flare energies. We estimate a flare loop length about 2–4 times the stellar radius. No evidence is seen for abundance changes during the flare.

HD 110067 is a near (d = 32.22 pc, V = 8.43 mag) K0V star in Coma Ber that was recently discovered to host a six-planet system in stable resonances. The star has a very old age of 8+/−4 Gyr. However, by the nature of the respective evolution tracks (i.e. with masses of 0.78...0.85 M_⊙), which run parallel to the zero-age main sequence for ≈8 Gyr, such age estimates are impossible unless the mass and luminosity are independently known to very high precision. We demonstrate this point using physical parameters derived from two different spectroscopic methods. As an alternative age indicator, we looked at the emission in Ca II H&K using TIGRE/HEROS spectra (Guanajuato, Mexico) and 78 archive TNG/HARPS-N spectra from 2021 to 2024. Surprisingly, HD 110067 has a high and persistent activity level of S_MWO = 0.32. From the estimated empirical and Rossby number of 0.4, and with the parameterised spin-down timescale, we derive an activity age of ≈2.5(±0.8) Gyr. Similarly, a possible rotation period of 20 days, consistent with TESS photometric variations and our vsin(i), suggests Ro = 0.32 and an age of just 1.7 Gyr. Such a relatively young activity age is indeed consistent with a very small lithium signature (the equivalent width of the 6707.8 Å doublet is 1.1 ± 0.2 mÅ) and implies that HD110067 can be directly compared to its virtual twin σ Draconis, which has an even weaker lithium presence and an activity cycle around ⟨S⟩_MWO ≈ 0.22.

Teegarden’s star is a late-type M-dwarf planet host, typically showing only rather low levels of activity. In this paper we present an extensive characterisation of this activity at photospheric, chromospheric, and coronal levels. We specifically investigated TESS observations of Teegarden’s star, which showed two very large flares with an estimated flare fluence between 10²⁹ and 10³² erg comparable to the largest solar flares. We furthermore analysed nearly 300 CARMENES spectra and 11 ESPRESSO spectra covering all the usually used chromospheric lines in the optical from the Ca II H & K lines at 3930 Å to the He I infrared triplet at 10 830 Å. These lines show different behaviour: The He I infrared triplet is the only one absent in all spectra, some lines show up only during flares, and others are always present and highly variable. Specifically, the Hα line is more or less filled in during quiescence; however, the higher Balmer lines are still observed in emission. Many chromospheric lines show a correlation with Hα variability, which, in addition to stochastic behaviour, shows systematic behaviour on different timescales including the rotation period. Moreover, we found several flares and also report hints of an erupting prominence, which may have led to a coronal mass ejection. Finally, we present X-ray observations of Teegarden’s star (i.e. a discovery pointing obtained with the Chandra observatory) and an extensive study with the XMM-Newton observatory, which observed two large flares. One of these showed clear signatures of the Neupert effect, suggesting the production of hard X-rays in the system.

The planet GJ 367 b is a recently discovered high-density sub-Earth orbiting an M dwarf star. Its composition was modelled to be predominantly iron with a potential remainder of a hydrogen-helium envelope. Here we report an X-ray detection of this planet’s host star for the first time, using data from the spectro-imaging X-ray telescope eROSITA onboard the Spectrum-Roentgen-Gamma mission. We characterise the magnetic activity of the host star from the X-ray data and estimate its effects on a potential atmosphere of the planet. We find that despite the very low activity level of the host star the expected mass loss rates, both under core-powered and photoevaporative mass loss regimes, are so high that a potential primordial or outgassed atmosphere would evaporate very quickly. Since the activity level of the host star indicates that the system is several Gigayears old, it is very unlikely that the planet currently still hosts any atmosphere.