XMM-Newton Large Program: magnetism of old suns ?>

XMM-Newton Large Program: magnetism of old suns

I’m very excited these days because the first observation from my recently awarded XMM-Newton large observing program keeps getting scheduled and then re-scheduled by the XMM team. If all goes well, the first data will be collected before the end of June. The observing program is about the magnetism of old sun-like stars; specifically, my co-investigators and I want to figure out if there is still a magnetic braking mechanism operating in old stars that are rotating anomalously quickly. The existence of these stars has made quite a splash in the community back in 2016 with Jen van Saders’ Nature paper; then, a lot of people argued if selection effects cause this result versus a true anomalous spin-down. Initially, I also thought that this is likely a selection effect: namely, in order to detect rotation in light curves, we need starspots, but the rotation of our own Sun would be very hard to detect in Kepler-style light curves. Meaning that old stars with well-detected rotation periods might have to be the ones that have prominent starspots, caused by fast rotation; i.e., we’re just sampling a tail of the natural distribution. However, what changed my mind was the paper by Oliver Hall et al. in 2021, who used asteroseismic rotation periods instead of starspot-driven ones, and they found the same result!

Very soft (<200 eV) X-ray image of the Vela supernova remnant with XMM-PN – on the left with low-energy detector noise, on the right after boutique data reduction (from Dennerl et al. 2004, arXiv:astro-ph/0407637).

This was on my mind a lot last summer, when I took 4 weeks of summer vacation, but after 2 weeks I just had to do some back-of-the-envelope scribbles about this because it was too interesting. In the fall I then prepared the XMM proposal, because my scribbles had the result that the lowest X-ray energies observable with XMM should be able to tell us whether there is still a normal corona for those anomalously fast rotators, or whether their magnetic dynamo has completely collapsed. It’s also quite interesting on the technical side, because we will have to use photon energies below 200 eV, where the XMM data needs some boutique data calibration techniques that are not included in the standard reduction pipeline, but they are fortunately described in a proceeding by Konrad Dennerl from 2004. This stuff is really up my alley!

I’m also pretty hyped that this project is my first XMM Large Program as a PI. Sometimes you write a proposal because you think “I might as well give this a shot”, but this one was of the type “I really think there’s a unique idea in this”. Makes me happy that the reviewers liked this one!

Conference: Heraeus-Seminar on the heliosphere, astrospheres and exoplanets ?>

Conference: Heraeus-Seminar on the heliosphere, astrospheres and exoplanets

Last week a huge delegation from my group attended the international Heraeus-Seminar “From the Heliosphere to Astrospheres – Lessons for Exoplanets and their Habitability” in Bad Honnef, Germany (conference website).

We had contributed talks given by my group members Judy Chebly and Nikoleta Ilic, plus contributed talks by my group’s guests Yu Xu and Florian Rünger; posters given by Eliana Amazo-Gomez and Joana Wokittel; and three (!) invited talks by Ekaterina Ilin, Julian Alvarado-Gomez and myself. Unfortunately I became sick the weekend before the conference and therefore had to sty home and give my talk over zoom – a pity because the Physics Center at Bad Honnef is known for its excellent food! Still, a great experience and a very interesting mix of topics that were selected by the organizers (see the program here).

Paper on host star activity and the exoplanet radius gap ?>

Paper on host star activity and the exoplanet radius gap

Different stellar activity histories cause different locations and depths of the exoplanet radius gap. (Ketzer & Poppenhaeger 2023)
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

Paper on tidal star-planet interactions ?>

Paper on tidal star-planet interactions

Stars with heavy, close-in planets are over-active compared to their co-eval companion stars, a consequence of tidal spin-up (from Ilic et al. 2022).

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

Instrumentation projects presented at SPIE ?>

Instrumentation projects presented at SPIE

My group is involved in two instrumentation projects that presented updates at the recent SPIE conference:

  • the ANDES spectrograph for the ELT, where we at the AIP are responsible for the design and the construction of its UBV arm;
  • and ARCUS, a concept for a high spectral resolution X-ray space telescope, where I am involved in the exoplanet science goals.

The SPIE proceedings for both projects can be found here:

Marconi et al., “ANDES, the high resolution spectrograph for the ELT: science case, baseline design and path to construction”, Proceedings of the SPIE, Volume 12184, id. 1218424 16 pp. (2022). https://ui.adsabs.harvard.edu/abs/2022SPIE12184E..24M/abstract

Smith et al., “Arcus: exploring the formation and evolution of clusters, galaxies, and stars”, Proceedings of the SPIE, Volume 12181, id. 1218121 10 pp. (2022). https://ui.adsabs.harvard.edu/abs/2022SPIE12181E..21S/abstract

Completed PhD theses ?>

Completed PhD theses

Two PhD students successfully defended their PhD theses after the summer break:

  • Dr. Grace Foster completed her thesis and defense on “X-ray studies of exoplanet systems” in August 2022;
  • Dr. Ekaterina Ilin completed her thesis and defense on “High lights: stellar flares as probes of magnetism in stars and star-planet systems” in September 2022.

 
Congratulations!

Grace has started a job as a data scientist in the US, and Ekaterina will begin a postdoc position in my group on her own funding which she was awarded for the analysis of her XMM-Newton observing program. For me, these are the first of my Potsdam-based PhD students who have completed their studies. Great fun and a great honor for me to be their guide on their academic journey!

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:

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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 ∼1×10−10 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.

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!