Cool Stars 21 conference in Toulouse ?>

Cool Stars 21 conference in Toulouse

This year the Cool Stars conference – the most important conference for the stellar half of my group’s research – took place in person again, for the first time since the pandemic started. Toulouse hosted the meeting this year, and it was a really good conference, lots of new scientific insights. A pretty big delegation from my group attended: PhD students Laura Ketzer, Ekaterina Ilin, Nikoleta Ilic, Judy Chebly, and guest student Yu Xu; postdocs Eliana Amazo-Gomez and Julián Alvarado-Gomez; and myself. Liz Cole-Codikara, Dario Fritzewski and David Gruner from Sydney Barnes’ subgroup also attended. We scored quite a number of talk slots: Eliana, Nikoleta and myself gave plenary talks, and Julián and Judy gave talks in the splinter sessions. Laura, Ekaterina and Yu presented posters. We also met some previous group members and old friends, such as Alex Gillet, who did his Master’s thesis in my group and is now a PhD student in Antoine Strugarek’s group, and Moritz Günther, Nick Wright, and Ofer Cohen, whom I was office mates with during my postdoc time. Below the fold are some photos from the talks and the conference dinner.

The (almost complete) group at the welcome reception:


PETS paper II: Thermal Inversion Agents in KELT-20 b/MASCARA-2 b ?>

PETS paper II: Thermal Inversion Agents in KELT-20 b/MASCARA-2 b

Detection of Fe I and nondetections of Si I, Fe II and Cr I in the atmosphere of KELT-20b (MASCARA-2b), from Johnson et al. (2022).

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 1×1010 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.

Paper on AU Mic and planet-triggered flares ?>

Paper on AU Mic and planet-triggered flares

Cartoon depicting the flare observability for AU Mic (from Ilin & Poppenhaeger 2022).

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

New and old group members ?>

New and old group members

We have a few new students in the group this spring (welcome to all of you!):

– Julia Schötzig, a bachelor student from TH Wildau, working on exoplanet orbital obliquities from PEPSI data (mentored by Dr. Engin Keles)
– Desmond Dsouza, a master student from Potsdam University, working on X-ray activity of wide binary stars (mentored by Nikoleta Ilic)
– Joana Wokittel, a master student from Humboldt University, working on telluric corrections for exoplanet transmission spectra (working directly with me)
– Cole Kilby, a master student from Potsdam University, working on machine learning approaches for exoplanet high-resolution spectra (mentored by Dr. Thorsten Carroll)
– Yu Xu, a PhD student from Peking University visiting the AIP for 2 years, working on coronal mass ejections (mentored by Dr. Julián Alvarado-Gomez).

A few people are in the process of moving on:

– Grace Foster has handed in her PhD thesis and will likely have her defense this summer, and then move on to a career in the US
– Dr. Silva Järvinen has moved over to the AIP’s high-resolution spectroscopy group for this year, where she will be the Instrument Scientist for the ELT-ANDES UBV spectrograph that I am the PI for.

And someone will continue staying in the group for a bit longer:

– Ekaterina Ilin has leveraged her accepted priority-A observations with XMM-Newton into a 2-year postdoctoral funding grant from the DLR and will therefore stay with my group for 2 more years. A really nice success!

ELT instrumentation: the ANDES-UBV spectrograph a AIP ?>

ELT instrumentation: the ANDES-UBV spectrograph a AIP

Members of the AIP’s ANDES-UBV team at the Florence kick-off meeting 2022: Katja Poppenhaeger, Joar Brynnel, Jörg Weingrill, Olga Bellido, Julián Alvarado-Gomez (left to right)

The AIP is part of the ANDES consortium, with ANDES being the planned high-resolution, wide wavelength coverage spectrograph for the Extremely Large Telescope (ELT). The AIP is in charge of developing and building the UBV spectrograph of ANDES, and I am leading the UBV development as the PI. The whole ANDES spectrograph will have at least three individual spectrographs, one covering the UBV bands, one covering RIZ, and one covering YJH. An additional K-band spectrogrpah is still under discussion.

ANDES will have lots of exciting science cases, many of them covered in the community white paper from 2013 (link), and a new white paper will be coming up in the next years.

2 weeks ago, after Covid restrictions loosened in many European countries, the kick-off meeting was held in person in Florence, and 5 of the AIP team members including myself attended the meeting. It was very informative, and also really helpful to finally meet our international consortium colleagues in person!

First paper of the PEPSI Exoplanet Transit Survey (PETS) publication series ?>

First paper of the PEPSI Exoplanet Transit Survey (PETS) publication series

Non-detection of ionized calcium aroung the rocky planet 55 Cnc e (from Keles et al. 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

Paper on tidal star-planet interaction ?>

Paper on tidal star-planet interaction

One of the major research questions in my group has reached a milestone – my PhD student Nikoleta Ilic led an investigation of the magnetic activity of planet-hosting stars that are located in wide multiple system with one or more other stars. Such systems allow us to test for an overactivity of the planet host star due to star-planet interaction (SPI) by using the other star(s) in the system as a negative, same-age control. Nikoleta found a significant correlation of excess stellar activity with the expected tidal interaction strength between a star and its planet. This has been very difficult to show unambiguously in previous studies because of the various biases from planet detection methods which can cause a similar trend in activity levels if not corrected for. So I am very happy that this bias-controlled sample actually shows significant SPI effects!

The activity difference of planet-hosting stars relative to their same-age stellar companions correlates with the tidal interaction strength between planet and star. (Ilic et al. 2022)

Tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems

Ilic, N.; Poppenhaeger, K.; Hosseini, S. Marzieh

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 which 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, Advance Access, April 2022

Paper on AU Mic’s space weather ?>

Paper on AU Mic’s space weather

The Alfvén surface of AU Mic (double-lobed shadow) encloses the orbits of the two innermost planets in this simulation (Alvarado-Gomez et al. 2022).

A new study by our Schwarzschild Fellow Dr. Julián Alvarado-Gomez shows that the space weather of AU Mic makes the survival of exoplanet atmospheres very challenging.

Simulating the Space Weather in the AU Mic System: Stellar Winds and Extreme Coronal Mass Ejections

Alvarado-Gómez, Julián D.; Cohen, Ofer; Drake, Jeremy J.; Fraschetti, Federico; Poppenhaeger, Katja; Garraffo, Cecilia; Chebly, Judy; Ilin, Ekaterina; Harbach, Laura; Kochukhov, Oleg

Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the so-called “evaporation valley” in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation of this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic coronal mass ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict extreme space weather for AU Mic and its planets. This includes sub-Alfvénic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within 102-105 times the dynamic pressure experienced by Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of exoplanetary atmospheres (if any) in this system.

The Astrophysical Journal, Volume 928, Issue 2, id.147, 12 pp., April 2022

Paper on solar H alpha emission ?>

Paper on solar H alpha emission

My group recently collaborated with the solar physics group and in particular Dr. Andrea Dierke for an investigation of solar H alpha emission. The paper is now published!

ChroTel Hα filtergram of the Sun on 2013 April 13
(top left) and the corresponding mask for bright
(bottom left) and dark features (bottom right).
For comparison, we display the corresponding
continuum map from HMI/SDO (top right). (Dierke et al. 2022)

Solar H alpha excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope

Diercke, A.; Kuckein, C.; Cauley, P. W.; Poppenhäger, K.; Alvarado-Gómez, J. D.; Dineva, E.; Denker, C.

The chromospheric H-alpha spectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers (i.e, CaII K) are typically used to monitor solar activity. We used observations of full-disk H-alpha filtergrams of the Chromospheric Telescope (ChroTel) to extract the imaging H-alpha excess and deficit, which are related to bright features (plage regions) and dark absorption features (filaments and sunspots), respectively. The aim of this study is to introduce the imaging H-alpha excess and deficit as tracers of solar activity and compare them to other established indicators: the relative sunspot number, the F10.7cm radio flux, and the MgII index. The H-alpha excess and deficit follow the behavior of the solar activity over the course of the cycle, whereby the peak of the H-alpha deficit is shortly after the solar maximum. The H-alpha excess is closely correlated to the chromospheric MgII index. The highest correlation of the H-alpha deficit is found with the F10.7cm radio flux. The H-alpha deficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for H-alpha excess regions for solar minimum and maximum, whereby the shape of the distributions is very similar, but with different amplitudes. Furthermore, we investigate whether the area coverage fraction or the changing H-alpha excess in the active regions dominates temporal variability in solar H-alpha observations. The area coverage fraction and the H-alpha excess are strongly correlated, whereas the weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces.

eprint arXiv:2203.04357, accepted for publication in Astronomy & Astrophysics, March 2022

Paper on helium transits of exoplanets ?>

Paper on helium transits of exoplanets

Stellar coronae with different iron abundance cause different levels of helium ionization which is important for observing helium transits of exoplanets. (From Poppenhaeger 2022)

During the first Covid lockdown I started what I call my little pandemic project. But then all the real-life consequences of living through a worldwide pandemic set in, and it took until early this year to wrap up my project. It is finally done! And it is the first single-author paper I have ever written.

Here’s some information on the paper:

Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances
Poppenhaeger, K.

Transit observations in the helium triplet around 10 830 Angstrom are a successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionization and recombination of helium in the exoplanetary atmosphere. This ionization is caused by stellar photons at extreme ultraviolet (EUV) wavelengths; however, no currently active telescopes can observe this part of the stellar spectrum. The relevant part of the stellar EUV spectrum consists of individual emission lines, many of them being formed by iron at coronal temperatures. The stellar iron abundance in the corona is often observed to be depleted for high-activity low-mass stars due to the first ionization potential (FIP) effect. I show that stars with high versus low coronal iron abundances follow different scaling laws that tie together their X-ray emission and the narrow-band EUV flux that causes helium ionization. I also show that the stellar iron to oxygen abundance ratio in the corona can be measured reasonably well from X-ray CCD spectra, yielding similar results to high-resolution X-ray observations. Taking coronal iron abundance into account, the currently observed large scatter in the relationship of EUV irradiation with exoplanetary helium transit depths can be reduced, improving the target selection criteria for exoplanet transmission spectroscopy. In particular, previously puzzling non-detections of helium for Neptunic exoplanets are now in line with expectations from the revised scaling laws.

Monthly Notices of the Royal Astronomical Society, Volume 512, Issue 2, pp.1751-1764, May 2022