Katja Poppenhaeger

Laura Ketzer, one of my PhD students, recently published a detailed study on the fate of small exoplanets – super-Earths and mini-Neptunes – under different time evolutions of the magnetic activity of the host star. The so-called radius gap manisfests itself differently for stars that experience an early versus a late spin-down. Also in samples of mixed stellar spin-down histories the features of the gap as a whole change with age, so that uniform age samples, such as open stellar clusters, should display different locations of the gap in the diagram of exoplanetary radius versus irradiation.

Ketzer, L. and Poppenhaeger, K., “The influence of host star activity evolution on the population of super-Earths and mini-Neptunes”

The detected exoplanet population displays a dearth of planets with sizes of about two Earth radii, the so-called radius gap. This is interpreted as an evolutionary effect driven by a variety of possible atmospheric mass-loss processes of exoplanets. For mass loss driven by an exoplanet’s irradiation by stellar X-ray and extreme-UV photons, the time evolution of the stellar magnetic activity is important. It is known from observations of open stellar clusters that stars of the same age and mass do not all follow the same time evolution of activity-induced X-ray and extreme-UV luminosities. Here, we explore how a realistic spread of different stellar activity tracks influences the mass loss and radius evolution of a simulated population of small exoplanets and the observable properties of the radius gap. Our results show qualitatively that different saturation time-scales, i.e. the young age at which stellar high-energy emission starts to decline, and different activity decay tracks over moderate stellar ages can cause changes in the population density of planets in the gap, as well as in the observable width of the gap. We also find that while the first 100 million years of mass loss are highly important to shape the radius gap, significant evolution of the gap properties is expected to take place for at least the first 500-600 million years, i.e. the age of the Hyades cluster. Observations of exoplanet populations with defined ages will be able to shed more light on the radius gap evolution.

Monthly Notices of the Royal Astronomical Society, Volume 518, Issue 2, pp.1683-1706 (2023); https://ui.adsabs.harvard.edu/abs/2023MNRAS.518.1683K/abstract

My PhD student Nikoleta Ilic has published a project we have been working on for the past two years: the identification of tidal star-planet interaction in a sample of planet-hosting stars. The tidal interaction leads to a spin-up of the host stars, which we identified through comparisons with co-eval companion stars in wide orbits around the star-planet systems.

The paper was also featured in a joint press release of the AIP and the Harvard-Smithsonian Center for Astrophysics: https://chandra.harvard.edu/press/22_releases/press_110222.html

Ilic, N. search by orcid ; Poppenhaeger, K. search by orcid ; Hosseini, S. Marzieh: “Tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems”

Tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. In cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star’s rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. Observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. Here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. We also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation-activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction that would be expected to affect the stellar activity, but not necessarily the rotation. We conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence.

Monthly Notices of the Royal Astronomical Society, Volume 513, Issue 3, pp.4380-4404 (2022), https://ui.adsabs.harvard.edu/abs/2022MNRAS.513.4380I/abstract

The second paper of the PEPSI Exoplanet Transit Survey (PETS) performs a search for possible atomic and molecular species that may drive the thermal inversion in the atmospheres of ultrahot Jupiters, and it rules out several species in the case of KELT-20 b/MASCARA-2 b:

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

Johnson, Marshall C.; Wang, Ji; Pai Asnodkar, Anusha; Bonomo, Aldo S.; Gaudi, B. Scott; Henning, Thomas; Ilyin, Ilya; Keles, Engin; Malavolta, Luca; Mallonn, Matthias; Molaverdikhani, Karan; Nascimbeni, Valerio; Patience, Jennifer; Poppenhaeger, Katja; Scandariato, Gaetano; Schlawin, Everett; Shkolnik, Evgenya; Sicilia, Daniela; Sozzetti, Alessandro; Strassmeier, Klaus G.; Veillet, Christian; Yan, Fe

Abstract: 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 a previous detection of Fe I in emission at 15.1σ; however, we are unable to reproduce published detections of Fe II, Cr I, or Si I. We attribute the non-detection of Si I to the lack of lines in our bandpass, but the non-detections of Fe II and Cr I are puzzling due to our much higher signal-to-noise ratio than previous works. Our search for the inversion agents TiO, VO, FeH, and CaH results in non-detections. Using injection-recovery testing we set 4σ upper limits upon the volume mixing ratios for these constituents as low as ∼1×10−10 for TiO. For TiO, VO, and CaH, our limits are much lower than expectations from an equilibrium chemical model, while FeH is lower than the expectations only from a super-Solar metallicity model. We thus rule out TiO, VO, and CaH as the source of the temperature inversion in KELT-20 b, while FeH is disfavored only if KELT-20 b possesses a high-metallicity atmosphere.

17 pages, 11 figures. Submitted to AAS Journals.

One possible manifestation of (magnetic) star-planet interactions is flare triggering by a planet passing by some active region of the star and disturbing the magnetic field structure of the active region enough that a flare is triggered earlier than it usually would have occurred. My PhD student Ekaterina Ilin recently studied the many TESS-observed flares of the young planet-hosting star AU Mic to test whether there is any significant correlation of flare timing with the orbit of the exoplanet or the synodic period. While there is some possible periodicity of larger flares with the planet’s orbital period, the effect is not significant at 3 sigma level. Should the effect be real and not a statistical fluctuation, an increased observing time by a factor of 2-3 should yield a solid detection!

Searching for flaring star-planet interactions in AU Mic TESS observations

Ilin, Ekaterina; Poppenhäger, Katja

Planets that closely orbit magnetically active stars are thought to be able to interact with their magnetic fields in a way that modulates stellar activity. This modulation in phase with the planetary orbit, such as enhanced X-ray activity, chromospheric spots, radio emission, or flares, is considered the clearest sign of magnetic star-planet interaction (SPI). However, the magnitude of this interaction is poorly constrained, and the intermittent nature of the interaction is a challenge for observers. AU Mic is an early M dwarf, and the most actively flaring planet host detected to date. Its innermost companion, AU Mic b, is a promising target for magnetic SPI observations. We used optical light curves of AU Mic obtained by the Transiting Exoplanet Survey Satellite to search for signs of flaring SPI with AU Mic b using a customized Anderson-Darling test. In the about 50 days of observations, the flare distributions with orbital, rotational, and synodic periods were generally consistent with intrinsic stellar flaring. We found the strongest deviation (p=0.07,n=71) from intrinsic flaring with the orbital period of AU Mic b, in the high energy half of our sample (ED>1 s). If it reflects the true SPI signal from AU Mic b, extending the observing time by a factor of 2−3 will yield a >3 sigma detection. Continued monitoring of AU Mic may therefore reveal flaring SPI with orbital phase, while rotational modulation will smear out due to the star’s strong differential rotation.

eprint arXiv:2204.14090, accepted for publication in Monthly Notices of the Royal Astronomical Society, April 2022

Our group is in a collaboration with other groups from Germany, Italy and the US to perform the PEPSI Exoplanet Transit Survey (PETS), which investigates exoplanet atmospheres in transmission and reflection observations. The first paper of the paper series is now published, and it’s an investigation of the nature of 55 Cnc e, a small rocky exoplanet, performed by my postdoc Engin Keles.

The PEPSI exoplanet transit survey (PETS) I: investigating the presence of a silicate atmosphere on the super-earth 55 Cnc e

Keles, Engin; Mallonn, Matthias; Kitzmann, Daniel; Poppenhaeger, Katja; Hoeijmakers, H. Jens; Ilyin, Ilya; Alexoudi, Xanthippi; Carroll, Thorsten A.; Alvarado-Gomez, Julian; Ketzer, Laura; Bonomo, Aldo S.; Borsa, Francesco; Gaudi, B. Scott; Henning, Thomas; Malavolta, Luca; Molaverdikhani, Karan; Nascimbeni, Valerio; Patience, Jennifer; Pino, Lorenzo; Scandariato, Gaetano; Schlawin, Everett; Shkolnik, Evgenya; Sicilia, Daniela; Sozzetti, Alessandro; Foster, Mary G.; Veillet, Christian; Wang, Ji; Yan, Fei; Strassmeier, Klaus G.

The study of exoplanets and especially their atmospheres can reveal key insights on their evolution by identifying specific atmospheric species. For such atmospheric investigations, high-resolution transmission spectroscopy has shown great success, especially for Jupiter-type planets. Towards the atmospheric characterization of smaller planets, the super-Earth exoplanet 55 Cnc e is one of the most promising terrestrial exoplanets studied to date. Here, we present a high-resolution spectroscopic transit observation of this planet, acquired with the PEPSI instrument at the Large Binocular Telescope. Assuming the presence of Earth-like crust species on the surface of 55 Cnc e, from which a possible silicate-vapor atmosphere could have originated, we search in its transmission spectrum for absorption of various atomic and ionized species such as Fe , Fe +, Ca , Ca +, Mg, and K , among others. Not finding absorption for any of the investigated species, we are able to set absorption limits with a median value of 1.9 × RP. In conclusion, we do not find evidence of a widely extended silicate envelope on this super-Earth reaching several planetary radii.

Monthly Notices of the Royal Astronomical Society, Volume 513, Issue 1, pp.1544-1556, June 2022