Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca ...II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The
XMM-Newton
satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star
ɛ
Eridani, previously observed on two occasions: in 2003 and in early 2015, both by
XMM-Newton
. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for
ɛ
Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term
XMM-Newton
lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 10
28
erg s
−1
, with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of
ɛ
Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of
ɛ
Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of
ɛ
Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of
ɛ
Eridani. Our analysis also revealed that the X-ray cycle of
ɛ
Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of
ɛ
Eridani is thus explained by the high percentage of magnetic structures on its surface.
The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the ...so-called excess flux, and its dimensionless indicator R+HK R HK + $R^{+}_{\rm HK}$ in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK R HK + $R^{+}_{\rm HK}$ values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK R HK + $R^{+}_{\rm HK}$ approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK R HK + $R^{+}_{\rm HK}$ , rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.
ABSTRACT
We determine the exact physical parameters of the four Hyades cluster K giants, using their parallaxes and atmospheric modelling of our red-channel TIGRE high-resolution spectra. Performing ...a comparison with well-tested evolutionary tracks, we derive exact masses and evolutionary stages. At an age of 588 (±60) Myr and with a metallicity of Z = 0.03 (consistent with the spectroscopic abundances), we find HD 27371 and HD 28307, the two less bright K giants, at the onset of central helium burning, entering their blue loops with a mass of 2.62 M⊙, while the slightly brighter stars HD 28305 and HD 27697 are already exiting their blue loop. Their more advanced evolution suggests a higher mass of 2.75 M⊙. Notably, this pairing coincides with the different activity levels, which we find for these four stars from chromospheric activity monitoring with TIGRE and archival Mount Wilson data as well as from ROSAT coronal detections. The two less evolved K giants are the far more active pair, and we confidently confirm their rotation with periods of about 142 d. This work therefore provides some first, direct evidence of magnetic braking during the 130 Myr lasting phase of central helium-burning, similar to what has long been known to occur to cool main-sequence stars.
Previous studies have revealed a 120 day activity cycle in the F-type star τ Boo, which represents the shortest activity cycle discovered until now. The question arises as to whether or not ...short-term activity cycles are a common phenomenon in F-type stars. To address this question, we analyse S-index time series of F-type stars taken with the TIGRE telescope to search for periodic variations with a maximal length of 2 years using the generalised Lomb-Scargle periodogram method. In our sample, we find four F-type stars showing periodic variations shorter than one year. However, the amplitude of these variations in our sample of F-star type stars appears to be smaller than that of solar-type stars with well-developed cyclic activity, and apparently represents only a part of the total activity. We conclude that among F-stars, the time-behaviour of activity differs from that of the Sun and cooler main sequence stars, as short-term cyclic variations with shallow amplitude of the cycle seem to prevail, rather than cycles with 10+ years periods and a larger cycle amplitude.
Stellar activity cycles and rotation periods are important parameters for characterising the stellar dynamo,
which operates in late-type main-sequence stars. However, the number of stars with ...well-known cycle and rotation periods
is rather low, so new detections are still important. To find activity cycles and rotation periods, we utilised the TIGRE telescope to monitor stars for periodic variations
in chromospheric activity indicators. We employed the widely used Ca ii H K lines and the Ca ii infrared triplet lines as stellar activity indicators.
To verify a periodic variation and to determine the corresponding period,
we performed a frequency analysis via the generalised Lomb-Scargle method of the taken time series. We studied Ca ii data of the G5V star HD 111395 and derive an activity cycle period of 949pm 5 d (approx 2.6 years).
This cycle is coincident with coronal measurements from the X-ray telescope eROSITA on board SRG. Furthermore,
the TIGRE Ca ii time series show a long-term trend that
indicates an additional long-term cycle. Using the few available literature
S-index data points, we estimate a probable cycle length of 12-15 years for this
potential long-term cycle. Finally, we determined rotation periods from each observation season. We computed a mean rotation period
of 16.76pm 0.36 d averaged over all observation seasons and chromospheric indicators. However, we also find a strong variation
in the mean seasonal rotation
periods, which follows the derived cycle period; therefore, we interpret this behaviour
as a sign of surface differential rotation.
ABSTRACT
We present a study of high-resolution spectra of RS Canum Venaticorum (RS CVn), a prototype of active binary systems. Our data were obtained from 1998 to 2017 using different telescopes. We ...analyse the chromospheric activity indicators $\rm{Ca~{\small II}}$ IRT, Hα, $\rm{Na~{\small I}}$ D1, D2 doublet, $\rm{He~{\small I}}$ D3, and Hβ using a spectral subtraction technique. The chromospheric emission stems mainly from the K2 IV primary star, while the F5 V secondary star only shows weak emission features in a few of our spectra. We find excess absorption features in the subtracted Hα lines and other activity indicators from spectra taken near primary eclipse, which we ascribe to prominence-like material associated with the primary star. We estimate size limits of these tentative prominences based on the geometry of the binary system, and investigate the physical properties of the strongest prominence. An optical flare, characterized by $\rm{He~{\small I}}$ D3 line emission, together with stronger emission in other activity lines, was detected. The flare energy is roughly comparable to strong flares observed on other RS CVn-type stars. The chromospherically active longitudes of RS CVn most frequently appear near the two quadratures of the system and display changes between observing runs, which indicates an ongoing evolution of its active regions.
Aims.
The so-called great dimming event of
α
Ori in late 2019 and early 2020 sparked our interest in the behaviour of chromospheric activity during this period.
α
Ori was already part of the ...long-term monitoring program of our TIGRE telescope to study the stellar activity of giant stars, and therefore regular measurements of
α
Ori have been taken since 2013.
Methods.
In the context of this study, we determined the TIGRE
S
-index values and, using a set of calibration stars, converted these to the Mount Wilson
S
-index scale, which allows us to combine our TIGRE activity measurements with the
S
MWO
values taken during the landmark Mount Wilson program some decades earlier and to compare that extended time series with the visual and V magnitude photometric data from the AAVSO database. In addition, we determined the absolute and normalised excess flux of the Ca
II
H&K lines. To understand the activity in absolute terms, we also assessed the changes in effective temperature using the TiO bands covered by our TIGRE spectra.
Results.
We find a clear drop in effective temperature by about 80 K between November 2019 and February 2020, which coincides with the minimum of visual brightness. In addition, the effective (luminous) photospheric area of
α
Ori also shrank. This might be related to a temporary synchronisation of several large convective cells in cooling and sinking down. 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
α
Ori which took place in 1984 and 1985. These two episodes of dimming therefore seem to share a common nature. To probe the dynamics of the upper photosphere, we further analysed the closely neighbouring lines of V
I
and Fe
I
at 6251.82 and 6251.56 Å respectively. Remarkably, their core distance varies, and once converted to radial velocity, shows a relation with the great dimming event, as well as with the consecutive, weaker dimming episode in the observing season of 2020 and 2021. This type of variation could be caused by rising and sinking cool plumes as a temporary spill-over of convection on
α
Ori.
Conclusions.
As the effective temperature of
α
Ori is variable, the S -index, computed relative to a near-ultraviolet (NUV) continuum, is only of restricted use for any monitoring study of the chromospheric activity of
α
Ori. It is therefore important to consider the effective temperature variability and derive the normalised Ca
II
H&K flux to study the chromospheric long-term changes in absolute terms. In fact, the Ca
II
H&K normalised excess flux time series shows that the chromospheric emission of
α
Ori did not change significantly between November 2019 and February 2020, but then beyond the great dimming minimum it does vary. Hence, this delay of the chromospheric reaction suggests that the cause for the great dimming is located in the photosphere. An investigation of the long-term spectroscopic and photometric time series of
α
Ori suggests that the great dimming in 2019 and 2020 does not appear to be a unique phenomenon, but rather that such dimmings do occur more frequently, which motivates further monitoring of
α
Ori with facilities such as TIGRE.
ABSTRACT
This work presents and tests a reliable, but nonetheless fast, method for determining the physical parameters of large stellar samples with moderate-resolution spectra, with extensive host ...star–exoplanet studies in mind. The proposed strategy complements spectral synthesis for obtaining spectroscopically sensitive parameters (i.e. effective temperature and rotation velocity) through other data to keep less critical quantities fixed. We test this approach on a sample of 25 bright (4–7 mag), cool main-sequence stars, for which rotation periods are known from chromospheric activity monitoring. On the basis of good-quality (signal-to-noise ratio 70–80) Tracking and Imaging Gamma-Ray Experiment–Heidelberg Extended Range Optical Spectrograph (TIGRE–HEROS) spectra with a modest spectral resolving power of R = 21 000, we employ the fast iSpec tool. With gravities calculated and approximate metallicities taken from uvby photometry (Geneva–Copenhagen catalogue), spectral synthesis is focused on refining the crucial effective temperature. Finally, rotational velocities are fitted. However, these suffer from cross-talk with gravity and convective turbulence. We find that prescribing macroscopic turbulent velocities for most stars within 2–3 km s−1 (with 4–6 km s−1 for only our three warmest stars) and microscopic turbulent velocities within 0.7–1.5 km s−1 (turbulence increasing with effective temperature, from under 5000 K to 6300 K) results in a satisfactory match (with residuals of 2.5 km s−1) of the period-related, very small rotation velocities of our sample stars. With this prescription, the fast spectral synthesis method described yields effective temperatures similar to intensive atmospheric modelling of high-resolution spectra.
Context. Historic stellar activity data based on chromospheric line emission using O.C. Wilson’s S-index reach back to the 1960ies and represent a very valuable data resource both in terms of ...quantity and time-coverage. However, these data are not flux-calibrated and are therefore difficult to compare with modern spectroscopy and to relate to quantitative physics. Aims. In order to make use of the rich archives of Mount Wilson and many other S-index measurements of thousands of main sequence stars, subgiants and giants in terms of physical Ca ii H+K line chromospheric surface fluxes and the related R-index, we seek a new, simple but reliable conversion method of the S-indices. A first application aims to obtain the (empirical) basal chromospheric surface flux to better characterise stars with minimal activity levels. Methods. We collect 6024 S-indices from six large catalogues from a total of 2530 stars with well-defined parallaxes (as given by the Hipparcos catalogue) in order to distinguish between main sequence stars (2133), subgiants (252) and giants (145), based on their positions in the Hertzsprung-Russell diagram. We use the spectra of a grid of PHOENIX model atmospheres to obtain the photospheric contributions to the S-index. To convert the latter into absolute Ca ii H+K chromospheric line flux, we first derive new, colour-dependent photospheric flux relations for, each, main sequence, subgiant and giant stars, and then obtain the chromospheric flux component. In this process, the PHOENIX models also provide a very reliable scale for the physical surface flux. Results. For very large samples of main sequence stars, giants and subgiants, we obtain the chromospheric Ca ii H+K line surface fluxes in the colour range of 0.44 < B − V < 1.6 and the related R-indices. We determine and parametrize the lower envelopes, which we find to well coincide with historic work on the basal chromospheric flux. There is good agreement in the apparently simpler cases of inactive giants and subgiants, and distinguishing different luminosity classes proves important. Main sequence stars, surprisingly, show a remarkable lack of inactive chromospheres in the B − V range of 1.1 to 1.5. Finally, we intoduce a new, “pure” and universal activity indicator: a derivative of the R-index based on the non-basal, purely activity-related Ca ii H+K line surface flux, which puts different luminosity classes on the same scale. Conclusions. The here presented conversion method can be used to directly compare historical S-indices with modern chromospheric Ca ii H+K line flux measurements, in order to derive activity records over long periods of time or to establish the long-term variability of marginally active stars, for example. The numerical simplicity of this conversion allows for its application to very large stellar samples.
To continue our study on chromospheric activity and detection for possible prominence events of the very active RS Canum Venaticorum star SZ Piscium (SZ Psc), long-term high-resolution spectroscopic ...observations were obtained during several observing runs from 2014 to 2018. Based on the spectral subtraction technique, the chromospheric emission of the Ca ii IRT (λ8662, λ8542, and λ8498), H , Na i D1, D2 doublet, Hβ, and Ca ii H & K lines is mainly associated with the K1 IV primary star of the SZ Psc system, in good agreement with the previous studies, and the F8 V secondary star also shows some chromospheric emission, implying its active chromosphere. Moreover, an optical flare characterized by the He i D3 line emission together with stronger emission in the other indicators was detected. Furthermore, two chromospheric active longitudes around the two quadratures of the system were identified for most of the time, and the chromospheric activity shows significant changes during a few orbital cycles. The chromospheric activity level seems to show a long-term variation during our observations. There were some excess absorption features in the subtracted H line and the other activity indicators, which would be caused by prominence-like materials associated with the K1 IV primary star of the system. Prominence materials could absorb the chromospheric emission and continuum from the K1 IV primary star and even the F8 V secondary one.
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