A. P. Hatzes’s research while affiliated with Thüringer Landessternwarte Tautenburg and other places

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 K and 4500 K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CARMENES and CRIRES$^+$. By applying the cross-correlation technique, we identified the emission signatures of Fe I and CO, as well as a thermal inversion layer in the dayside atmosphere; no significant H$_2$O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from 3200 K to 4600 K, with an atmospheric metallicity consistent with that of the host star (0.36 dex). Only weak constraints could be placed on the C/O ratio ($>$ 0.15). The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes. An upper temperature limit of 2400 K and a maximum atmospheric temperature gradient of 700 K/log bar could be derived for the nightside. Comparison of the retrieved dayside T-p profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H$_2$O emission lines, and KELT-9b, where molecular features are largely absent.

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500,K and 4500,K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the thermochemical structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CAHA/CARMENES and VLT/CRIRES^+. By applying the cross-correlation technique to the high-resolution spectra, we identified the emission signatures of Fe i (S/N,=,4.3) and CO (S/N,=,6.3), as well as a thermal inversion layer in the dayside atmosphere; no significant H_2O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from approximately 3200,K to 4600,K, with an atmospheric metallicity consistent with that of the host star (0.36,dex). Only weak constraints could be placed on the C/O ratio, with a lower limit of 0.15. The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes in the atmosphere. An upper temperature limit of approximately 2400,K and a maximum atmospheric temperature gradient of about bar could be derived for the planetary nightside. Comparison of the retrieved dayside temperature-pressure profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H_2O emission lines, and KELT-9b, where molecular features are largely absent.

We report the discovery of TOI-3493 b, a sub-Neptune-sized planet on an 8.15-d orbit transiting the bright (V=9.3) G0 star HD 119355 (aka TIC 203377303) initially identified by NASA's TESS space mission. With the aim of confirming the planetary nature of the transit signal detected by TESS and determining the mass of the planet, we performed an intensive Doppler campaign with the HARPS spectrograph, collecting radial velocity measurements. We found that TOI-3493 b lies in a nearly circular orbit and has a mass of 9.0+/-1.2 M_earth and a radius of 3.22+/-0.08 R_earth, implying a bulk density of 1.47+/-0.23 g/cm^3, consistent with a composition comprising a small solid core surrounded by a thick H/He dominated atmosphere.

A multitude of spectral activity indicators are routinely computed nowadays from the spectra generated as part of planet-hunting radial velocity surveys. Searching for shared periods among them can help to robustly identify astrophysical quantities of interest, such as the stellar rotation period. However, this identification can be complicated due to the fact that many different peaks occurring in the periodograms. This is especially true in the presence of aliasing and spurious signals caused by environmental influences affecting the instrument. Our goal is to test a clustering algorithm to find signals with the same periodicity, (i.e. with the stellar rotation period) in the periodograms of a large number of activity indicators. On this basis, we have looked to evaluate the correlations between activity indicators and fundamental stellar parameters. We used generalised Lomb-Scargle periodograms to find periodic signals in 24 activity indicators, spanning the VIS and NIR channels of the CARMENES spectrograph. Common periods were subsequently determined by a machine learning algorithm for density-based spatial clustering of applications with noise ( DBSCAN The clustering analysis of the signals apparent in the spectral activity indicators is a powerful tool for the detection of stellar rotation periods. It is straightforward to implement and can be easily automated, so that large data sets can be analysed. For a sample of 136 stars, we were able to recover the stellar rotation period in a total of 59 cases, including 3 with a previously unknown rotation period. In addition, we analysed spurious signals frequently occurring at the period of one year and its integer fractions, concluding that they are likely aliases of one underlying signal. Furthermore, we reproduced the results of several previous studies on the relationships between activity indicators and the stellar characteristics.

A multitude of spectral activity indicators are routinely computed nowadays from the spectra generated as part of planet-hunting radial velocity surveys. Searching for shared periods among them can help to robustly identify astrophysical quantities of interest, such as the stellar rotation period. However, this identification can be complicated due to the fact that many different peaks occurring in the periodograms. This is especially true in the presence of aliasing and spurious signals caused by environmental influences affecting the instrument. Our goal is to test a clustering algorithm to find signals with the same periodicity, (i.e. with the stellar rotation period) in the periodograms of a large number of activity indicators. On this basis, we have looked to evaluate the correlations between activity indicators and fundamental stellar parameters. We used generalised Lomb-Scargle periodograms to find periodic signals in 24 activity indicators, spanning the VIS and NIR channels of the CARMENES spectrograph. Common periods were subsequently determined by a machine learning algorithm for density-based spatial clustering of applications with noise (DBSCAN). The clustering analysis of the signals apparent in the spectral activity indicators is a powerful tool for the detection of stellar rotation periods. It is straightforward to implement and can be easily automated, so that large data sets can be analysed. For a sample of 136 stars, we were able to recover the stellar rotation period in a total of 59 cases, including 3 with a previously unknown rotation period. In addition, we analysed spurious signals frequently occurring at the period of one year and its integer fractions, concluding that they are likely aliases of one underlying signal. Furthermore, we reproduced the results of several previous studies on the relationships between activity indicators and the stellar characteristics.

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 10^38 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 10^40 erg at optical wavelengths.

Aims: Previous estimates of planet occurrence rates in the CARMENES survey indicated increased numbers of planets on short orbits for M dwarfs with masses below 0.34\,M$_\odot$. Here we focused on the lowest-mass stars in the survey, comprising 15 inactive targets with masses under 0.16\,M$_\odot$. Methods: To correct for detection biases, we determined detection sensitivity maps for individual targets and the entire sample. Using Monte Carlo simulations, we estimated planet occurrence rates for orbital periods of 1\,d to 100\,d and minimum masses from 0.5\,M$_\oplus$ to 10\,M$_\oplus$. Results: The radial velocity (RV) data from CARMENES reveal four new planets around three stars in our sample, namely G~268--110\,b, G~261--6\,b, and G~192--15\,b and c. All three b planets have minimum masses of 1.03--1.52\,M$_\oplus$ and orbital periods of 1.43--5.45\,d, while G~192--15\,c is a 14.3\,M$_\oplus$ planet on a wide, eccentric orbit with $P \approx 1218$\,d and $e \approx 0.68$. Our occurrence rates suggest considerable dependencies with respect to stellar masses. For planets below 3\,M$_\oplus$ we found rates consistent with one planet per star across all investigated periods, but the rates decrease almost by an order of magnitude for larger planet masses up to 10\,M$_\oplus$. Compared to previous studies, low-mass stars tend to harbor more planets with $P <10$\,d. We also demonstrate that synthetic planet populations based on the standard core accretion scenario predict slightly more massive planets on wider orbits than observed. Conclusions: Our findings confirm that planet occurrence rates vary with stellar masses even among M dwarfs, as we found more planets with lower masses and on shorter orbits in our subsample of very low-mass stars compared to more massive M dwarfs. Therefore, we emphasize the need for additional differentiation in future studies.

Context. Current exoplanet surveys are focused on detecting small exoplanets orbiting in the liquid-water habitable zones of their host stars. Despite recent significant advancements in instrumentation, the main limitation in detecting these exoplanets is the intrinsic variability of the host star itself. Aims. Our aim is to investigate the wavelength dependence of high-precision radial velocities (RV), as stellar activity induced RVs should exhibit a wavelength dependence while the RV variation due to an orbiting planet will be wavelength independent. Methods. We used the chromatic index (CRX) to quantify the slope of the measured RVs as a function of logarithmic wavelength of the full CARMENES guaranteed time observations (GTO) data set spanning more than eight years of observations of over 350 stars. We investigated the dependence of the CRX in the full Carmenes GTO sample on 24 stellar activity indices in the visible and near-infrared channels of the CARMENES spectrograph and each star’s stellar parameters. We also present an updated convective turnover time scaling for the calculation of the stellar Rossby number for M dwarfs. Results. Our results show that approximately 17% of GTO stars show a strong or a moderate correlation between the CRX and RV. We can improve the measured RVs by a factor of up to nearly 4 in terms of the root mean square (rms) by subtracting the RV predicted by the CRX-RV correlation from the measured RVs. Mid-M dwarfs with moderate rotational velocities and moderate CRX-gradients, with quasi-stable activity features, have the best rms improvement factors. Conclusions. We conclude that the CRX is a powerful diagnostic in mitigation of stellar activity and the search for low mass rocky planets.

Aims. Previous estimates of planet occurrence rates in the CARMENES survey indicated increased numbers of planets on short orbits for M dwarfs with masses below 0.34M ⊙ . Here we focused on the lowest-mass stars in the survey, comprising 15 inactive targets with masses under 0.16 M ⊙ . Methods. To correct for detection biases, we determined detection sensitivity maps for individual targets and the entire sample. Using Monte Carlo simulations, we estimated planet occurrence rates for orbital periods of 1 d to 100 d and minimum masses from 0.5 M ⊕ to 10M ⊕ . We also compared the actual sample of known planets to model predictions. Results. The radial velocity (RV) data from CARMENES reveal four new planets around three stars in our sample, namely G 268– 110 b, G 261–6 b, and G 192–15 b and c. All three b planets have minimum masses of 1.03–1.52 M ⊕ and orbital periods of 1.43–5.45 d, while G 192–15 c is a 14.3 M ⊕ planet on a wide, eccentric orbit with P ≈ 1218 d and e ≈ 0.68. Our occurrence rates suggest considerable dependencies with respect to stellar masses. For planets below 3 M ⊕ we found rates consistent with one planet per star across all investigated periods, but the rates decrease almost by an order of magnitude for larger planet masses up to 10 M ⊕ . Compared to previous studies, low-mass stars tend to harbor more planets with P < 10 d. We also demonstrate that synthetic planet populations based on the standard core accretion scenario predict slightly more massive planets on wider orbits than observed. Conclusions. Our findings confirm that planet occurrence rates vary with stellar masses even among M dwarfs, as we found more planets with lower masses and on shorter orbits in our subsample of very low-mass stars compared to more massive M dwarfs. Therefore, we emphasize the need for additional differentiation in future studies.

Context. Magnetic field investigations of Sun-like stars, using Zeeman splitting of non-polarised spectra, in the optical and H-band have found significantly different magnetic field strengths for the same stars, the cause of which is currently unknown. Aims. We aim to further investigate this issue by systematically analysing the magnetic field of ξ Boo A, a magnetically active G7 dwarf, using spectral lines at different wavelengths. Methods. We used polarised radiative transfer accounting for the departures from local thermodynamic equilibrium to generate synthetic spectra. To find the magnetic field strengths in the optical, H-band, and K-band, we employed MCMC sampling analysis of high-resolution spectra observed with the spectrographs CRIRES ⁺ , ESPaDOnS, NARVAL, and UVES. We also determine the formation depth of different lines by calculating the contribution functions for each line employed in the analysis. Results. We find that the magnetic field strength discrepancy between lines in the optical and H-band persists even when treating the different wavelength regions consistently. In addition, the magnetic measurements derived from the K-band appear to more closely align with the optical. The H-band appears to yield magnetic field strengths ∼0.4 kG with a statistically significant variation while the optical and K-band is stable at ∼0.6 kG for observations spanning about two decades. The contribution functions reveal that the optical lines form at a significantly higher altitude in the photosphere compared to those in the H- and K-band. Conclusions. While we find that the discrepancy remains, the variation of formation depths could indicate that the disagreement between magnetic field measurements obtained at different wavelengths is linked to the variation of the magnetic field along the line of sight and between different structures, such as star spots and faculae, in the stellar photosphere.