Category Archives: Publications

Publications

Surface activity of a Rossby sequence of cool Hyades stars

We present quantitative surface-activity information for a sequence of 21 Hyades dwarf stars with effective temperatures all cooler than the red edge of the lithium dip and Rossby (Ro) numbers between 0.14 to 0.54 with respect to the Sun (Ro(Sun)=1). High-resolution high-S/N PEPSI Stokes-IV spectra and least-squares deconvolution of thousands of spectral lines per spectrum are employed for measuring surface magnetic fields, rotational velocities, lithium abundances, and chromospheric CaII IRT fluxes. Lithium abundances A(Li) range from 95 times solar on the warm end of the sample to 1/25 solar on the cool end. We confirm the tight relation with T(eff)  and extent it to K-M stars. A formal relation with rotational period and velocity in the sense higher A(Li) for faster rotators is present. Targets rotating faster than vsini of 6 km/s appear Li saturated. CaII IRT fluxes also show a relation with T(eff), P(rot) and vsini, but opposite to A(Li), in an inverse sense with higher radiative losses for the slower=cooler rotators. Disk-integrated, unsigned, magnetic-field strengths of 15.4±3.6(rms)G are measured for targets warmer than 5000K and 91±61(rms)G for targets cooler than this. These field strengths relate to P(rot), vsini, and Ro, but in a bi-modal fashion. We conclude that the Rossby-number dependency of the surface activity tracers on our Hyades dwarf sequence primarily originates from convective motions, expressed by its turnover time, and only to a smaller and sometimes inverse extent from surface rotation and its related extra mixing.

Lithium-activity-rotation relation versus normalized Rossby number (Ro_n). From left to right: A(Li) logarithmic lithium abundance, R‘(IRT) logarithmic radiative loss in the three CaII infrared-triplet lines, <|B|> disk-integrated, unsigned magnetic field strength in Gauss.

 

Read more: Strassmeier et al. 2025, A&A, in press; e-print arXiv:2510.21456

Publications

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). IV.High-Resolution Phased-Resolved Spectroscopy of The Ultra Hot Jupiter KELT-20 b

We present five datasets of high-resolution optical emission spectra of the ultra-hot Jupiter KELT-20 b with the PEPSI spectrograph. Using a Bayesian retrieval framework, we constrain its dayside pressure-temperature profile and abundances of Fe, Ni, and Ca, providing the first measurements for Ni and Ca for KELT-20 b in emission. We retrieve the pre- and post-eclipse datasets separately (corresponding to the evening and morning sides, respectively), and compare the constraints on their thermal structures and chemical abundances. We constrain lower abundances in the pre-eclipse datasets compared to the post-eclipse datasets. We interpret these results with an equilibrium chemistry model which suggests ∼ 10 − 30× supersolar refractory abundances. Due to the well-known degeneracy between absolute abundances and continuum opacities, the abundance ratios are more precise probes of the planetary abundances. Therefore we measure the abundance ratios [Ni/Fe] and [Ca/Fe] across these datasets and find they agree within 1σ. We constrain [Ni/Fe] to be consistent with solar within 2σ, and [Ca/Fe] to be 0.001-0.01× solar, not accounting for ionization.

Top-down view of KELT-20 b’s orbit as it passes behind its host star, moving counterclockwise from pre-eclipse to post-eclipse. The orange side depicts the hot day side, while the blue side corresponds to the cooler night side of the tidally-locked planet. The translucent gray side depicts the side of the planet that is outside of the view of the observer, who is positioned off the bottom of the page.

Read more: Bonidie et al. 2025, AJ, in press.  eprint arXiv:2511.09720

Publications

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). II.Phase-Resolved Cross-Correlation Transmission Spectroscopy of KELT-20b

ELT-20b is a well-studied exoplanet within the highly observable ultra hot Jupiter (UHJ) regime, yet its multidimensional atmospheric structure remains largely unconstrained. Recent advances in instrumentation and increased general circulation model (GCM) complexity have enabled observers to resolve the imprints of more intricate physical mechanisms in time-resolved data. We performed high-resolution cross-correlation transmission spectroscopy (HRCCTS) on a single transit time series of KELT-20b, observed with PEPSI on the LBT. We detect Fe I (11.9σ) and Fe II (23.7σ) and tentatively detect Na I (3.4σ) and Cr I (3.3σ) upon combining nineteen in-transit exposures.

Observability score of each neutral species considered in this study. Colored elements are more likely to be detected since their absorption lines are covered by the PEPSI bandpass ranges we use. Dark cells have fewer and/or shallower lines. Light gray cells were skipped because they don’t have significant lines in the PEPSI bandpass or did not have available opacities. Species with observability score of ∼ 0.2 or greater have been detected in at least one study, except for Ni I.

Read more: Lenhart et al. 2025, AJ, in press. eprint arXiv:2503.07719

Publications

PEPSI investigation, retrieval, and atlas of numerous giant atmospheres (PIRANGA) –III. Composition and winds in the atmosphere of TOI-1518b

Ultra-hot Jupiters (UHJs) orbit close to their host stars and experience extreme conditions, making them important laboratories to explore atmospheric composition and dynamics. Transmission spectroscopy is a useful tool to reveal chemical species and their vertical and longitudinal distribution in the atmosphere. We use transmission spectra from the PEPSI (Potsdam Echelle Polarimetric and Spectroscopic Instrument) spectrograph on the Large Binocular Telescope to search for species and measure their time-resolved wind velocities in the atmosphere of TOI-1518 b. We detect Fe I at 7.8 σ and Fe II at 8.9 σ, and tentatively detect Cr I at 4.4 σ and Ni I at 4.0 σ. The time-resolved wind velocities of Fe I show a velocity pattern that is consistent with the velocity pattern of Fe II . TOI-1518 b joins a small sample of UHJs for which time-resolved wind velocities have been measured.

Raw CCF for Fe I. Dark (black) regions are anticorrelated and light (white) regions are correlated. The horizontal blue lines show the ingress and egress of the transit, calculated using the transit duration determined by Duck et al.. The red dotted line shows the expected velocity of the atmospheric signal. , φ is the orbital phase, and Vsys = −11.17 ± 0.035 km s−1 is the systemic velocity offset in the LBT PEPSI frame (S. Petz et al. 2025 ). The dark shaded region is the Doppler shadow.

Read more: Basinger et al. (2025), MNRAS, 543, 4136

Publications

A search for Maunder-minimum candidate stars

Stars with very low levels of magnetic activity provide an opportunity for a more quantitative comparison with the Sun during its Maunder minimum. We employ spectra from the RAVE survey in a search for particularly low-activity stars with the goal of identifying candidates for so-called Maunder-minimum stars. Spectra were used to measure the relative flux in the cores of the Ca II infrared-triplet (IRT) lines. Those were converted to absolute emission-line fluxes and were corrected with target fluxes from high-resolution STELLA and ultra-high-resolution PEPSI spectra. Absolute Ca II IRT fluxes for a total of 78 111 RAVE dwarf stars are presented and compared with fluxes of the 123 stars from our high-resolution STELLA+PEPSI sample. RAVE fluxes appear higher than the STELLA and PEPSI fluxes by on average 19% for IRT-1, 21% for IRT-2, and 25% for IRT-3 due to their lower spectral resolution. Our sample also spans a metallicity [Fe/H] range relative to the Sun of −1.5 to +0.5 dex. We confirm the strong dependency of IRT fluxes on metallicity and quantify it to be at most ±14% in the B–V range 0.53–0.73. Without a metallicity correction, practically all very-low-activity RAVE dwarfs show a super-solar metallicity. After correcting for spectral resolution and for metallicity, we find 13 RAVE stars out of 13 326 (0.1%) that fall well below our empirical lower flux bound from high-resolution versus B–V. For solar B–V, this relates to a photospheric uncorrected radiative loss in the IRT lines of log RIRT = −4.13 (≈20% below the solar-minimum value in late 2016). However, 11 targets turned out to be evolved stars based on their Gaia DR3 parallaxes. Two stars, TIC 352227373 (G2V) and TYC 7560-477-1 (G7V), are our only Maunder-minimum candidates from the present search. Contrary to the initial suggestion from the Mount-Wilson H&K Survey, we conclude that such stars are very rare.

Metallicity-corrected RAVE Ca II IRT-1 fluxes (13 326 targets). Crosses are the possible candidate targets. Boldfaced crosses are the two MM candidates, TIC 352227373 (RAVE J191213.1- 760732) and TYC 7560-477-1 (RAVE J025410.6-383603).

Read more: Järvinen & Strassmeier 2025, A&A, 698, A93;  arXiv

Publications

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). I. The Ubiquity of Fe I Emission and Inversions in Ultra Hot Jupiter Atmospheres

We present high-resolution optical emission spectroscopy observations of the ultra hot Jupiters (UHJs) TOI-1431b and TOI-1518b using the PEPSI spectrograph on the LBT. We detect emission lines from Fe I with a significance of 5.68σ and 7.68σ for TOI 1431b and TOI-1518b, respectively. We also tentatively detect Cr I emission from TOI-1431b at 4.32σ. For TOI-1518 b, we tentatively detect Ni I, Fe II, and Mg I at significance levels ranging from 3−4σ. Detection of emission lines indicates that both planets possess temperature inversions in their atmospheres, providing further evidence of the ubiquity of stratospheres among UHJs. By analyzing the population of hot Jupiters, we compare models that predict the distribution of planets in the temperature-gravity space, and find a recent global circulation model suite from Roth et al. (2024) provides a reasonable match to the observed onset of inversions at Teq∼2000 K. The ubiquity of strong Fe I emission lines among UHJs, together with the paucity of detections of TiO, suggest that atomic iron is the dominant optical opacity source in their atmospheres and can be responsible for the inversions.

Left: Phase coverage of the observations used in this work. The solid inner circle shows the stellar surface, while the middle and outer circles show the orbits of TOI-1518 b and TOI-1431 b, respectively, to scale. The observer is off the bottom of the page, and the vertical dashed lines show the line of sight, such that transit occurs in the lower intersection of the dashed lines and planetary orbits, and the secondary eclipse in the upper intersection. The planets orbit counter clockwise. The colored points show the portions of the orbit where we obtained data. Right: signal-to-noise ratio of our observations as a function of orbital phase. The PEPSI red and blue arms are shown as the solid and dotted lines, respectively. The SNR values shown are, for each spectrum, the 95th quantile per-pixel signal to-noise ratios.

Read more: Petz et al. 2025, AJ, in press; arXiv

Publications

First Doppler image and starspot-corrected orbit for lambda Andromedae

Starspots on a rotating stellar surface impact the measured radial velocities and thereby limit the determination of precise orbital elements as well as astrophysical stellar parameters and even jeopardize the detection and characterization of (exo)planets. Phase-resolved high resolution optical spectra from PEPSI and STELLA were recorded over the course of 522 days in 2021-2022. Doppler imaging is used to reconstruct λ And’s starspots with very high resolution (R = 250 000) and high signal-to-noise ratio PEPSI spectra. The Doppler image reconstructs a dominating cool spot with an umbral temperature difference of ≈1000K with respect to the photosphere of 4660K and is likely surrounded by a moat-like velocity field. Three more weaker spots add to the total surface spottedness, which is up to 25% of the visible surface.

Our Doppler image (panel a) is compared with the RV residuals (panel b) and the chromospheric Ca II IRT (8542 Å) line core flux (panel c). The Doppler image is from PEPSI spectra only and shows four spots, or spotted regions, identified with letters A D. RVs correlate well with the location of the four cool spots and almost anti-correlate with the chromospheric Ca ii IRT emission. The shown RVs and activity data are from the same time-coverage than the Doppler image (May–June 2022).

Read  more: Adebali, et al. 2025, A&A, 695, A89

Publications

Testing the Rossby Paradigm: Weakened Magnetic Braking in early K-type Stars

We present new observational constraints on magnetic braking for an evolutionary sequence of six early K-type stars. To determine the wind braking torque for each of our targets, we combine spectropolarimetric constraints on the large-scale magnetic field, Lyα or X-ray constraints on the mass-loss rate, as well as uniform estimates of the stellar rotation period, mass, and radius. As identified previously from similar observations of hotter stars, we find that the wind braking torque decreases abruptly by more than an order of magnitude at a critical value of the stellar Rossby number. Given that all of the stars in our sample exhibit clear activity cycles, we suggest that weakened magnetic braking may coincide with the operation of a subcritical stellar dynamo.

PEPSI Stokes V polarization profile for HD166620 from LBT observations on 2024 July 5. The mean LSD profile is shown as a black line with uncertainties indicated by the gray shaded area.

Read more: Metcalfe et al. 2025 arXiv

Publications

PEPSI’s non-detection of escaping hydrogen and metal lines adds to the enigma of WASP-12b

WASP-12 b is an ultra-hot Jupiter (UHJ) of special interest for atmospheric studies since it is on an inspiraling orbit in an extreme environment of intense radiation and circumstellar gas. Previously claimed detections of active mass loss from this planet are controversial across the literature. To address this controversy, we obtained two new transit observations of WASP-12 b with the optical high-resolution PEPSI spectrograph on the Large Binocular Telescope. Contrary to previous work, we do not observe planetary H𝛼 absorption and rule out the amplitude of previously reported detections. Our non-detection may be limited by the sensitivity of our data or could indicate weaker mass loss than suggested by previous studies. We conclude that previous claims of H𝛼 absorption from the atmosphere of WASP-12 b should be reevaluated. Given the anticipated line strength of Balmer/optical features, observing the atmosphere of this faint target will require stacking more observations even with the largest telescope facilities available.

Non-detection of planetary atmospheric absorption around H𝛼 line (6562.83 Å) in Night 1 data. The top panel is the order-stitched, continuum normalized stellar spectra from the PEPSI pipeline where the colorbar represents flux after continuum normalization. The second panel is the spectra with the stellar component removed; red (values greater than zero) indicates excess emission while blue (values less than zero) indicates excess absorption. The third panel shows the spectra from the second panel after applying SYSREM. The fourth and bottom panel displays the difference between the second and third panels.

Read more: Pai Asnodkar et al. 2024, MNRAS, 535, 1829

Publications

Sun-as-a-Star Spectroscopic Observations of the 2017 August 21 Solar Eclipse

The solar eclipse of 2017 August 21 was observed with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) on the Large Binocular Telescope (LBT), which is located at Mt. Graham International Observatory (MGIO), Arizona, USA. At this location, a partial eclipse was observed with maximum obscuration of 61.6%. The 11-millimeter-aperture, binocular Solar Disk-Integrated (SDI) telescope, located on the kitchen balcony of the LBT building, feeds sunlight to PEPSI, which has recorded a total of 116 Sun-as-a-star spectra in the wavelength range of 5300 – 6300 Å, with a spectral resolution R=250,000 and S/N of about 733:1. The temporal evolution of the Fraunhofer Na I D doublet at 5890/5896 Å is analyzed using contrast profiles that illustrate subtle changes in the spectral line, not obvious in the intensity profiles.

Time-series of the radial velocity obtained from the Na D2 (blue) and Na D1 (red) lines observed during solar eclipse. The vertical lines indicate the time of maximum obscuration. The line-core velocities (top-left) are computed from a parabola fit to the line core, whereas the line-wing velocities reflect bisector velocities at certain distances from the line core, which are given in milli-Ångstrom in the lower-right corner of each panel. The relative formation heights are given in the corresponding color in the lower-left corner of each panel.

Read more: Dineva et al., 2024, Sol. Phys. 299, 123