Celebration barbecue ?>

Celebration barbecue

2023 barbecue

This week we had a really nice afternoon barbecue with a bunch of people I like to call “star-planet group and friends” – i.e. my own group plus other colleagues mainly from the solar group and the MHD group. We had a lot to celebrate:

  • my postdoc Ekaterina Ilin will start a new postdoctoral position in Groningen very soon, where she will work with Harish Vedantham on radio signatures of star-planet interactions;
  • Prachi Rahate and Joana Wokittel have successfully defended their Master theses (on stellar surfaces and telluric line treatments, respectively);
  • Nikoleta Ilic and Laura Ketzer have handed in their PhD theses (on star-planet interactions and evaporation of young exoplanets, respectively);
  • plus a few other successes of colleagues who got offers for new positions or aced their exams (congratulations, everyone!).

We had a lot of fun and ate enormous amounts of vegan and meaty grill food; as always, a good time with people at AIP!

Paper on tidally-induced activity in an M dwarf / brown dwarf pair ?>

Paper on tidally-induced activity in an M dwarf / brown dwarf pair


The activity of NLTT 41135 (red) is significally elevated over random activity scatter in other M dwarf pairs (black) and can therefore be traced back to the influence of the brown dwarf orbiting it (from Ilic et al. 2023).
My PhD student Nikoleta Ilic has investigated a really interesting object, the close M dwarf / brown dwarf pair NLTT 41135, which forms a hierarchical wide triple system with the M dwarf NLTT 41136. Nikoleta found an elevated activity of about an order of magnitude in X-rays that is most likely caused by tidal interaction between the brown dwarf and the M dwarf. Nikoleta was able to quantify this with the help of other wide M dwarf systems from the eROSITA survey.

Paper information:

“The first evidence of tidally induced activity in a brown dwarf-M dwarf pair: a Chandra study of the NLTT 41135/41136 system”

Ilić, Nikoleta; Poppenhaeger, Katja; Dsouza, Desmond; Wolk, Scott J.; Agüeros, Marcel A.; Stelzer, Beate

The magnetic activity of low-mass stars changes as they age. The primary process decreasing the stellar activity level is the angular momentum loss via magnetized stellar wind. However, processes like tidal interactions between stars and their close companions may slow down the braking effect and the subsequent decrease of the activity level. Until now, the tidal impact of substellar objects like brown dwarfs on the evolution of their central stars has not been quantified. Here, we analyse the X-ray properties of NLTT 41135, an M dwarf tightly orbited by a brown dwarf, to determine the impact of tidal interactions between them. We find that NLTT 41135 is more than an order of magnitude brighter in the X-ray regime than its stellar companion, NLTT 41136, also an M dwarf star, with whom it forms a wide binary system. To characterize the typical intrinsic activity scatter between coeval M dwarf stars, we analyse a control sample of 25 M dwarf wide binary systems observed with the XMM-Newton and Chandra telescopes and the eROSITA instrument onboard the Spectrum Röntgen Gamma satellite. The activity difference in the NLTT 41135/41136 system is a 3.44σ outlier compared to the intrinsic activity scatter of the control systems. Therefore, the most convincing explanation for the observed activity discrepancy is tidal interactions between the M dwarf and its brown dwarf. This shows that tidal interactions between a star and a substellar companion can moderately alter the expected angular-momentum evolution of the star, making standard observational proxies for its age, such as X-ray emission, unreliable.

Monthly Notices of the Royal Astronomical Society, Volume 524, Issue 4, pp.5954-5970 (2023)

EAS conference in Krakow ?>

EAS conference in Krakow

A good chunk of my research group plus myself spent last week in Krakow/Poland at the annual meeting of the European Astronomical Society (EAS). First time for me: we all took a “Eurocity” train that goes directly from Berlin to Krakow and takes 7 hours. Ecofriendly, and I was actually less wrung out after the train trip than I would have been after two flights (since there are no direct plane connections between Berlin and Krakow).

The conference itself was quite a lot of fun: held in a nice modern conference center, good food and drink during the breaks, and I met a whole bunch of people I hadn’t talked to in a while (hi Nick, Mario and Sabine!). I gave an invited review on Star-Planet Interactions, and my folks gave contributed talks on AU Mic’s space weather (Julián Alvarado-Gómez), flares triggered by star-planet interactions (Ekaterina Ilin), stellar wind simulations (Judy Chebly), and magnetic variability of massive stars (Silva Järvinen).

Group photo:

My crew at EAS 2023 (left to right: Ekaterina Ilin, Katja Poppenhäger, Judy Chebly, Silva Järvinen, Julián Alvarado-Gómez).

And individual photos below the fold.
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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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