Category Archives: Publications

Weakened Magnetic Braking in the Exoplanet Host Star 51 Peg

The consistently low activity level of the old solar analog 51 Peg not only facilitated the discovery of the first hot Jupiter, but also led to the suggestion that the star could be experiencing a magnetic grand minimum. However, the 50 yr time series showing minimal chromospheric variability could also be associated with the onset of weakened magnetic braking (WMB), where sufficiently slow rotation disrupts cycling activity and the production of large scale magnetic fields by the stellar dynamo, thereby shrinking the Alfvén radius and inhibiting the efficient loss of angular momentum to magnetized stellar winds. In this Letter, we evaluate the magnetic evolutionary state of 51 Peg by estimating its wind braking torque. We use new spectropolarimetric measurements from the Large Binocular Telescope to reconstruct the large-scale magnetic morphology, we reanalyze archival X-ray measurements to estimate the mass-loss rate, and we detect solar-like oscillations in photometry from the Transiting Exoplanet Survey Satellite, yielding precise stellar properties from asteroseismology. Our estimate of the wind braking torque for 51 Peg clearly places it in the WMB regime, driven by changes in the mass-loss rate and the magnetic field strength and morphology that substantially exceed theoretical expectations. Although our revised stellar properties have minimal consequences for the characterization of the exoplanet, they have interesting implications for the current space weather environment of the system.

ZDI maps of the radial, meridional, and azimuthal field components of 51 Peg. Contours are shown with a step of 0.5 G. The dotted line corresponds to the lowest visible latitude. The vertical bars at the bottom of each panel show the central longitude of each LBT observation.

Read more: Metcalfe, T., Strassmeier, K. G., Ilyin, I. V. et al. 2024, ApJ 960, L6

AIP press release: link

LBTO news: link

The PEPSI Exoplanet Transit Survey (PETS) IV: Assessing the atmospheric chemistry of KELT-20b

Most ultra hot Jupiters (UHJs) show evidence of temperature inversions, in which temperature increases with altitude over a range of pressures. Temperature inversions can occur when there is a species that absorbs the stellar irradiation at a relatively high level of the atmospheres. However, the species responsible for this absorption remains unidentified. In particular, the UHJ KELT-20b is known to have a temperature inversion. Using high resolution emission spectroscopy from LBT/PEPSI we investigate the atomic and molecular opacity sources that may cause the inversion in KELT-20b, as well as explore its atmospheric chemistry. We confirm the presence of Fe I with a significance of 17𝜎. We also report a tentative 4.3𝜎 detection of Ni I. A nominally 4.5𝜎 detection of Mg I emission in the PEPSI blue arm is likely in fact due to aliasing between the Mg I cross-correlation template and the Fe I lines present in the spectrum. We cannot reproduce a recent detection of Cr I, while we do not have the wavelength coverage to robustly test past detections of Fe II and Si I. Together with non-detections of molecular species like TiO, this suggests that Fe I is likely to be the dominant optical opacity source in the dayside atmosphere of KELT-20b and may be responsible for the temperature inversion. We explore ways to reconcile the differences between our results and those in literature and point to future paths to understand atmospheric variability.

Shifted and combined cross correlation function (CCF, left) and injection-recovery test (right) for Ni I. This 4.3𝜎 detection falls within our tentative detection range. The vertical and horizontal dashed lines in all CCF maps represent the 𝐾𝑝 (radial velocity semi amplitude of the planet) and 𝑣𝑠𝑦𝑠 (system radial velocity) parameters for which we should expect to find a signal.

Read more: Petz, S., Johnson, M., Asnodkar, A. P. et al. 2023, MNRAS, in press  (arXiv:2301.09352)

Asteroseismology and Spectropolarimetry of the Exoplanet Host Star lambda Serpentis

The bright star lam Ser hosts a hot Neptune with a minimum mass of 13.6 MEarth and a 15.5 day orbit. It also appears to be a solar analog, with a mean rotation period of 25.8 days and surface differential rotation very similar to the Sun. We detect solar-like oscillations in time series photometry from the Transiting Exoplanet Survey Satellite (TESS), and we derive precise asteroseismic properties from detailed modeling. We obtain new spectropolarimetric data, and we use them to reconstruct the large-scale magnetic field morphology. We reanalyze the complete time series of chromospheric activity measurements from the Mount Wilson Observatory, and we present new X-ray and ultraviolet observations from the Chandra and Hubble space telescopes. Finally, we use the updated observational constraints to assess the rotational history of the star and to estimate the wind braking torque. We conclude that the remaining uncertainty on stellar age currently prevents an unambiguous interpretation of the properties of lam Ser, and that the rate of angular momentum loss appears to be higher than for other stars with similar Rossby number. Future asteroseismic observations may help to improve the precision of the stellar age.

Stokes V polarization profile for lam Ser from PEPSI/LBT observations on 2021 May 24. The mean profile is shown as a black line, with uncertainties indicated by the gray shaded area. The red and blue lines are model profiles assuming dipole geometry and fixed inclination i = 50 degrees with different obliquity angles beta.

Read more: Metcalfe et al. 2023, AJ, in press (arXiv 2308.09808).

Searching for magnetic fields in pulsating A-type stars

Numerous δ Sct and γ Dor pulsators are identified in the region of the Hertzsprung-Russell diagram that is occupied by chemically peculiar magnetic Ap stars. The connection between δ Sct and γ Dor pulsations and the magnetic field in Ap stars is however not clear: theory suggests for magnetic Ap stars some critical field strengths for pulsation mode suppression by computing the magnetic damping effect for selected p and g modes. To test these theoretical considerations, we obtained PEPSI spectropolarimetric snapshots of the typical Ap star HD 340577, for which δ Sct- like pulsations were recently detected in TESS data, and the γ Dor pulsator HR 8799, which is a remarkable system with multiple planets and two debris disks. Our measurements reveal the presence of a magnetic field with a strength of several hundred Gauss in HD 340577. The measured mean longitudinal field would be the strongest field measured so far in a δ Sct star if the pulsational character of HD 340577 is confirmed spectroscopically. No magnetic field is detected in HR 8799.

LSD Stokes I, V, and diagnostic null N spectra (from bottom to top) calculated for HD340577, for the mask with all lines and the masks for individual ions. The numbers in brackets relate to the number of lines used in each individual mask. The grey horizontal lines in the Stokes V and N spectra indicate the ±1σ error bars. Stokes V spectra are highlighted in red.

Read more: Hubrig et al. 2023, MNRAS 526, L83; arXiv 2308.09441

Zeeman Doppler Imaging of ksi Boo A and B

A magnetic-field surface map for both stellar components of the young visual binary ksi Boo AB (A: G8V, B: K5V) is presented. Employed are high resolution Stokes-V spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT). Stokes V line profiles are inverted with our iMAP software and compared to previous inversions. We employed an iterative regularization scheme without the need of a penalty function and incorporated a three-component description of the surface magnetic-field vector. The spectral resolution of our data is 130,000 (0.040-0.055A) and have signal-to-noise ratios (S/N) of up to three thousand per pixel depending on wavelength. A singular-value decomposition (SVD) of a total of 1811 spectral lines is employed for averaging Stokes-V profiles. Our mapping is accompanied by a residual bootstrap error analysis. Magnetic flux densities of the radial field component of up to plus/minus 115 ± 5 G were reconstructed for ksi Boo A while up to plus/minus 55 ± 3G were reconstructed for ksi Boo B. ksi Boo A’s magnetic morphology is characterized by a very high latitude, nearly polar, spot of negative polarity and three low-to-mid latitude spots of positive polarity while ksi Boo B’s morphology is characterized by four low-to-mid latitude spots of mixed polarity. No polar magnetic field is reconstructed for the cooler ksi Boo B star. Both our maps are dominated by the radial field component, containing 86 and 89 percent of the magnetic energy of ksi Boo A and B, respectively. We found only weak azimuthal and meridional field densities on both stars (plus/minus 15-30 G), about a factor two weaker than what was seen previously for ksi Boo A. The phase averaged longitudinal field component and dispersion is +4.5 ± 1.5G for ksi Boo A and -5.0 ± 3.0 G for ksi Boo B.

Representative PEPSI spectra used in the study (panel a). Zeeman-Doppler images of ξ Boo A (panel b) and ξ Boo B (panel c) in orthographic projection. φ is the rotational phase. Magnetic field strength is color coded and identified in the right bars in units of Gauss. Positive polarity is depicted in red, negative polarity in blue. The length of the surface dashes is proportional to field strength.

Read more: K. G. Strassmeier, T. A. Carroll, & I. V. Ilyin 2023, A&A, 674, 118; (arXiv 2305.07470)

Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and Tau Cet

During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking has previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf τ Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Lyα measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4–9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.

Stokes V polarization profile for τ Cet from LBT observations on 2022 September 18. The mean profile is shown as a black line with uncertainties indicated by the gray shaded area. The dashed blue line is an axisymmetric model profile assuming dipole morphology with the inclination fixed at i = 20 deg.

Read more: Metcalfe, T., Strassmeier, K., Ilyin, I., et al. 2023, ApJ Letters, in press (arXiv:2304.09896)

The PEPSI Exoplanet Transit Survey. III: The detection of Fe I, Cr I and Ti I in the atmosphere of MASCARA-1 b through high-resolution emission spectroscopy

Hot giant planets like MASCARA-1 b are expected to have thermally inverted atmospheres, that makes them perfect laboratory for the atmospheric characterization through high-resolution spectroscopy. Nonetheless, previous attempts of detecting the atmosphere of MASCARA-1 b in transmission have led to negative results. In this paper we aim at the detection of the optical emission spectrum of MASCARA-1 b. We used the high-resolution spectrograph PEPSI to observe MASCARA-1 (spectral type A8) near the secondary eclipse of the planet. We cross-correlated the spectra with synthetic templates computed for several atomic and molecular species. We obtained the detection of FeI, Cr I and Ti I in the atmosphere of MASCARA-1 b with a S/N ~7, 4 and 5 respectively, and confirmed the expected systemic velocity of ~13 km/s and the radial velocity semi-amplitude of MASCARA-1 b of ~200 km/s. The detection of Ti is of particular importance in the context of the recently proposed Ti cold-trapping below a certain planetary equilibrium temperature. We confirm the presence of an the atmosphere around MASCARA-1 b through emission spectroscopy. We conclude that the atmospheric non detection in transmission spectroscopy is due to the high gravity of the planet and/or to the overlap between the planetary track and its Doppler shadow.

Atmospheric planetary signal obtained with the Ti I template. The left column shows the Kp–vsys bi-dimensional map, with the color bar encoding the significance of the detection. The cross in white dashes marks the position where the planetary signal is expected. The central (right) column shows the cut along the vertical (horizontal) direction in the map and corresponding to the peak detection, together with its gaussian best fit (blue dashed line).

Read more: G. Scandariato, F. Borsa, A.S. Bonomo, et al. 2023, A&A, in press (eprint arXiv:2304.03328)

VPNEP: Detailed characterization of TESS targets around the Northern Ecliptic Pole

We embarked on a high-resolution optical spectroscopic survey of bright Transiting Exoplanet Survey Satellite (TESS) stars around the Northern Ecliptic Pole (NEP), dubbed the Vatican-Potsdam-NEP (VPNEP) survey. Our NEP coverage comprises 1067 stars, of which 352 are bona fide dwarf stars and 715 are giant stars, all cooler than spectral type F0 and brighter than V=8.5. Our aim is to characterize these stars for the benefit of future studies in the community. We analyzed the spectra via comparisons with synthetic spectra. Particular line profiles were analyzed by means of eigen-profiles, equivalent widths, and relative emission-line fluxes (when applicable). Two R=200 000 spectra were obtained for each of the dwarf stars with the Vatican Advanced Technology Telescope (VATT) and the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), with typically three R=55 000 spectra obtained for the giant stars with STELLA and the STELLA Echelle Spectrograph (SES). Combined with V-band magnitudes, Gaia eDR3 parallaxes, and isochrones from the Padova and Trieste Stellar Evolutionary Code, the spectra can be used to obtain radial velocities, effective temperatures, gravities, rotational and turbulence broadenings, stellar masses and ages, and abundances for 27 chemical elements, as well as isotope ratios for lithium and carbon, line bisector spans, convective blue-shifts (when feasible), and levels of magnetic activity from Halpha, Hbeta, and the Ca II infrared triplet. In this initial paper, we discuss our analysis tools and biases, presenting our first results from a pilot sub-sample of 54 stars (27 bona-fide dwarf stars observed with VATT+PEPSI and 27 bona-fide giant stars observed with STELLA+SES) and making all reduced spectra available to the community.

Data delivery example: 8000-Å CN region of HR 5844. Diamond symbols are the observed spectrum and the lines are fits with three different 12C/13C isotope ratios, using the CN line list of Plez. The best average from eight fits is 12C/13C=17.8 using a relative nitrogen abundance of 0.50±0.05 and a relative iron abundance of –0.25±0.03. Average S/N per pixel of this PEPSI spectrum in this wavelength range is 400.

Read more: Strassmeier et al. 2023, A&A, 671, A7 (arXiv:2302.01794)

Press release: To new worlds with quantitative spectroscopy

The PEPSI Exoplanet Transit Survey (PETS). II. A Deep Search for Thermal Inversion Agents in KELT-20 b/MASCARA-2 b with Emission and Transmission Spectroscopy

Recent observations have shown that the atmospheres of ultra hot Jupiters (UHJs) commonly possess temperature inversions, where the temperature increases with increasing altitude. Nonetheless, which opacity sources are responsible for the presence of these inversions remains largely observationally unconstrained. We used LBT/PEPSI to observe the atmosphere of the UHJ KELT-20 b in both transmission and emission in order to search for molecular agents which could be responsible for the temperature inversion. We validate our methodology by confirming previous detections of Fe I in emission at 16.9 σ. Our search for the inversion agents TiO, VO, FeH, and CaH results in nondetections. Using injection-recovery testing we set 4σ upper limits upon the volume mixing ratios for these constituents as low as ∼1×10−9 for TiO. For TiO, VO, and CaH, our limits are much lower than expectations from an equilibrium chemical model, while we cannot set constraining limits on FeH with our data. We thus rule out TiO and CaH as the source of the temperature inversion in KELT-20 b, and VO only if the line lists are sufficiently accurate.

Shifted and combined CCFs for emission from Fe I. We detect the Fe I emission at a significance of 16.9σ, reproducing previous results. We were able to recover the signal near the expected planetary radial velocity. Yan et al. (2022b) obtained a 7.7σ detection, and Borsa et al. (2022) at 7.1σ with the Calar Alto 3.5m and the TNG 3.6m, respectively.

Read more: Marshall et al. 2023, AJ, in press (arXiv:2205.12162)

Untangling the Sources of Abundance Dispersion in Low-Metallicity Stars

We measured abundances of 12 elements (Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni) in a sample of 86 metal-poor subgiant stars in the solar neighborhood. Abundances are derived from high-resolution spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope, modeled using iSpec and MOOG. By carefully quantifying the impact of photon-noise (< 0.05 dex for all elements) we robustly measure the intrinsic scatter of abundance ratios. At fixed [Fe/H] the RMS intrinsic scatter in [X/Fe] ranges from 0.04 dex (Cr) to 0.16 dex (Na), with a median of 0.08 dex. Scatter in [X/Mg] is similar, and accounting for [alpha/Fe] only reduces the overall scatter moderately. We consider several possible origins of the intrinsic scatter with particular attention to fluctuations in the relative enrichment by core-collapse supernovae (CCSN) and Type Ia supernovae (SNIa) and stochastic sampling of the CCSN progenitor mass distribution. The stochastic sampling scenario provides a good quantitative explanation of our data if the effective number of CCSN contributing to the enrichment of a typical sample star is N approx 50. At the median metallicity of our sample, this interpretation implies that the CCSN ejecta are mixed over a gas mass 10^5 MSun before forming stars. The scatter of elemental abundance ratios is a powerful diagnostic test for simulations of star formation, feedback, and gas mixing in the early phases of the Galaxy.

Spectral section from 5495−5535 A of three stars with [Fe/H] near −2 (green), −1.5 (blue), and −1.0 (purple). Line features for Fe I, Ti I, Ca II, Sc II, and Mg I are labeled. Fluxes are normalized, with the [Fe/H] ≈ −1.5 spectrum offset by 0.5 and the [Fe/H] ≈ −2 spectrum offset by 1.

Read more:  Griffith et al. 2022, ApJ, in press  (arXiv:2210.01821)