Dennis Jack

We present an analysis of the chromospheric emission of Fe I + Fe II in the wing of the Ca II K line of 21 G and K giant stars. Stellar parameters and absolute chromospheric fluxes were obtained by comparison with PHOENIX models. The iron-blend emission shows a proportionality with the Ca II K flux for stars with similar stellar parameters and also for individual stars observed at different epochs, confirming that both diagnostics seems to respond to the same underlying chromospheric conditions. We found a dependence of the iron-blend emission on stellar parameters by fitting a power-law relation. The exponent of the effective temperature statistically matches with that found for Ca II K, which, together with a metallicity dependence, indicate that the iron-blend may act as an indirect tracer of chromospheric thermal conditions. The chromospheric emission flux ratio (Fe I + Fe II)/Ca II K exhibits a distinct slope change around log 𝑔 ≈ 2.5 dex, which causes the gravity dependence to be different for stars with gravities lower and higher than 2.5 dex. This behavior suggests a transition between regimes where the two diagnostics probe similar physical conditions and where they begin to diverge in the sensitivity to the chromospheric extension. We discuss the implications of this behavior for the thermal structure of the lower chromosphere and for the formation heights of both diagnostics.

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.

In this fourth publication of our series, we present the analysis of a further nine bright spectroscopic binaries (HD 9728, HD 21835, HD 32575, HD 41118, HD 59539, HD 88748, HD 148434, HD 177698, HD 189985) discovered with the TIGRE telescope. We obtained intermediate resolution spectra, which we used to determine the radial velocity variations and then the orbital parameters of the binary systems. The results were compared with Gaia DR3 orbits that are available for some of the binary stars. They are in good agreement, but with our analysis we could improve the orbital solutions and parameters. We further used the spectra to determine the stellar parameters of the primary stars. A comparison with evolution tracks allowed us to obtain an estimate of their masses and ages. All primaries turn out to be evolved, bright K giants, ascending on the RGB, Helium burning, or at the foot of the AGB. We were also able to estimate the minimum masses of the secondary stars. For some cases, we could determine the actual mass of the secondary using the inclinations from the Gaia DR3 non‐single stars catalogue. We found that the secondary stars of these systems are all low‐mass stars.

The emission lines of the Ca II H&K doublet present one of the most important channels of radiative cooling for the chromospheres of cool stars. Although most other line emissions (Mg II h&k and numerous iron lines) populate the far-UV and require a very competitive space-born observing time, the Ca II H&K lines in the optical UV are easily accessible by ground-based spectroscopy with relatively cheap instrumentation, and the observational data are plentiful. At the same time, in order to include realistic mechanical and magnetic heating, advanced chromospheric models now require radiative cooling losses in absolute terms and therefore call for a precise surface flux scale, which could be provided by matching a photospheric Ca II H&K line profile computed by photospheric models. However, a major obstacle here is the significant ambiguity in parameter space in the face of a very sensitive dependence of cool stellar optical UV surface fluxes on the effective temperature. Consequently, we have developed a rigorous method by which precise physical parameters, most notably the effective temperature, are first determined by ISPEC tools working on high signal-to-noise spectra and based on a suitable line list and reference continuum. However, crosstalk and navigation of multiple local chi-square minima (best solutions) in parameter space must be considered. Only with the optimal set of parameters is a single PHOENIX model calculated, which defines the spectral surface flux scale in the Ca II H&K line region. In this paper, we discuss the results of the absolute measurements of Ca II K line fluxes for 32 representative cool stars and the accuracy of this approach. Finally, we show a comparison of the Ca II K chromospheric fluxes with line widths, and a relation of chromospheric flux with effective temperature and gravity following the logarithmic form log F_{Ca II} = α′ log g + β′ log T_eff + C′.

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.

We present an analysis of nine bright spectroscopic binaries (HD 1585, HD 6613, HD 12390, HD 39923, HD 55201, HD 147430, HD 195543, HD 202699, HD 221643), which have orbital periods of days. These well‐separated binaries are the last stars of our sample that we observed with the TIGRE telescope obtaining intermediate‐resolution spectra of . We applied the same method as described in our previous publication of this series. For the analysis of the radial velocity curves, we used the toolkit RadVel, which allowed us to determine all orbital parameters. Recently published orbital solutions of some systems from Gaia DR3 agree with our results. However, our solutions have much smaller uncertainties. We determined the basic stellar parameters of the primary stars with our automatic script using iSpec. The parameter determination allowed us to place all nine stars in the Hertzsprung–Russell diagram. We found that all stars have already evolved to the giant phase. A comparison with stellar evolution tracks of the Eggleton code was applied to determine the stellar masses and ages. As a result of our analysis, we were able to estimate the masses of the secondary stars and the orbital inclinations of the binary systems.

We present an analysis of nine bright spectroscopic binaries (HD 1585, HD 6613, HD 12390, HD 39923, HD 55201, HD 147430, HD 195543, HD 202699, HD 221643), which have orbital periods of P > 500 days. These well-separated binaries are the last stars of our sample that we observed with the TIGRE telescope obtaining intermediate-resolution spectra of R = 20,000. We applied the same method as described in our previous publication of this series. For the analysis of the radial velocity curves, we used the toolkit RadVel, which allowed us to determine all orbital parameters. Recently published orbital solutions of some systems from Gaia DR3 agree with our results. However, our solutions have much smaller uncertainties. We determined the basic stellar parameters of the primary stars with our automatic script using iSpec. The parameter determination allowed us to place all nine stars in the Hertzsprung-Russell diagram. We found that all stars have already evolved to the giant phase. A comparison with stellar evolution tracks of the Eggleton code was applied to determine the stellar masses and ages. As a result of our analysis, we were able to estimate the masses of the secondary stars and the orbital inclinations of the binary systems.

The November 2019 eclipse of γ Per was a rare opportunity to seek evidence for a chromosphere of the G8 giant, hitherto suspected but not detected. Twenty-nine years after the only other observed eclipse, we aim to find chromospheric absorption in the strong Ca II H&K lines, and to determine its column densities and scale height. Using the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE) in Guanajuato (central Mexico) before, during and after the 8 days of total eclipse, we obtained good S/N spectra of the G8 giant alone and composite spectra of the partial phases, near eclipse and far from eclipse. In the near UV of the Ca II H&K and H_ϵ lines, the G giant spectrum that was adequately scaled was subtracted from the composite spectra in partial phases, near and far from eclipse, to obtain the A3 companion spectra with and without traces of chromospheric absorption. In addition, we used PHOENIX full non-local thermodynamic equilibrium model atmospheres on the blue A star spectrum, iSpec spectral analysis of the red G giant spectrum, and evolution tracks to study both components of γ Per. For the first time, we present evidence for this rare type of a not very extended G giant chromosphere, reaching out about half of an A-star radius (~1.5 Gm) with a scale height of only 0.17 Gm. By its location in the Hertzsprung-Russell diagram, the γ Per G8 giant is very close to the onset of more extended chromospheres. Furthermore, we show that this giant has a rather inactive chromosphere, and a recent 5 ksec XMM pointing reveals only a very faint, low-energy corona. While the γ Per primary has a mass of ~3.6 M_⊙, and its A3 companion has one of ~2.4 M_⊙, the latter is too cool (8400 ± 300 K), which is too evolved on the main sequence to be the same age as the primary. The high eccentricity of the 5329.08 days long-period orbit may therefore be reminiscent of a rare capture event. Using the eclipse method, we resolve a pivotal case of a G giant chromosphere, which seems to represent a low-gravity analogue of the inactive Sun. A systematic change of giant chromospheric extent by Hertzsprung-Russell diagram position is confirmed. Compared to the solar chromosphere, the density scale height increases with gravity by ∝ ɡ^−1.5.

The so-called great dimming event of alpha Ori in late 2019 and early 2020 sparked our interest in the behaviour of chromospheric activity during this period. To study the timeline of chromospheric activity, we derive a S_MWO time series of TIGRE and Mount Wilson values, and we compare this long time series with photometric data from the AAVSO database. In addition, we determine the absolute and normalised excess flux of the Ca II H&K lines. To do so, we estimate the changing effective temperature from TIGRE spectra and find a clear drop of about 80 K between November 2019 and February 2020, which coincides with the minimum of visual brightness. During the same period, the S-index increased significantly, yet this is a mere contrast effect, because the normalised excess flux of the Ca II H&K lines did not change significantly. However, the latter dropped immediately after this episode. Comparing the combined S_MWO values and visual magnitude time series, we find a similar increase in the S-index during another noticeable decrease in the visual magnitude of alpha Ori, which took place in 1984 and 1985. To also probe the dynamics of the upper photosphere, we analysed the lines in 6251-6263 A and found core distance varies which shows a relation with the great dimming event. This type of variation could be caused by rising and sinking cool plumes as a temporary spill-over of convection on alpha Ori. Based on our study, we conclude that the cause for the great dimming is located in the photosphere. Furthermore, the long-term spectroscopic and photometric time series suggests that this great dimming does not appear to be a unique phenomenon, but rather that such dimmings do occur more frequently, which motivates further monitoring of alpha Ori with facilities such as TIGRE.