The latest paper of my recently graduated PhD student Laura Ketzer is now in print – and as a side note, an all-female author list spanning two different research groups at the AIP:
Plot of LX/Lbol as a function of Rossby number for all stars in the sample by Wright et al. (2011a) with measured rotation periods. Field stars are marked as grey circles, while cluster stars with ages below ∼1 Gyr are shown in five different-coloured age bins. The location of K2-198, which is marked with a yellow star, indicates that the star has already dropped out of the saturated regime and concurs with an age between ∼150 and 600 Myr. (from Ketzer et al. 2024)
Ketzer, L.; Poppenhaeger, K.; Baratella, M.; Ilin, E., Monthly Notices of the Royal Astronomical Society, Volume 527, Issue 1, pp.374-385 (2024)
Planets orbiting young stars are thought to experience atmospheric evaporation as a result of the host stars’ high-magnetic activity. We study the evaporation history and expected future of the three known transiting exoplanets in the young multiplanet system K2-198. Based on spectroscopic and photometric measurements, we estimate an age of the K-dwarf host star between 200 and 500 Myr, and calculate the high-energy environment of these planets using eROSITA X-ray measurements. We find that the innermost planet K2-198c has likely lost its primordial envelope within the first few 10s of Myr regardless of the age at which the star drops out of the saturated X-ray regime. For the two outer planets, a range of initial envelope mass fractions is possible, depending on the not-yet-measured planetary mass and the stars’ spin-down history. Regarding the future of the system, we find that the outermost planet K2-198b is stable against photoevaporation for a wide range of planetary masses, while the middle planet K2-198d is only able to retain an atmosphere for a mass range between ~7 and 18 M⊕. Lower mass planets are too susceptible to mass-loss, and a very thin present-day envelope for higher mass planets is easily lost with the estimated mass-loss rates. Our results support the idea that all three planets started out above the radius valley in the (sub-)Neptune regime and were then transformed into their current states by atmospheric evaporation, but also stress the importance of measuring planetary masses for (young) multiplanet systems before conducting more detailed photoevaporation simulations.
Two new papers that my group contributed to: a study of flares on the Sun, but with the Sun seen as a star, which provides a “translation table” between stellar and solar flare observations; and spectroscopic observation of the great dimming of Beutelgeuse, which happened in 220 and was observed (among others) with the AIP’s STELLA telescope on Tenerife.
Pietrow, A. G. M.; Cretignier, M.; Druett, M. K.; Alvarado-Gómez, J. D.; Hofmeister, S. J.; Verma, M.; Kamlah, R.; Baratella, M.; Amazo-Gómez, E. M.; Kontogiannis, I.; Dineva, E.; Warmuth, A.; Denker, C.; Poppenhaeger, K.; Andriienko, O.; Dumusque, X.; Löfdahl, M. G., Astronomy & Astrophysics, Volume 682, id.A46, 20 pp. (2024)
Context. Stellar flares cannot be spatially resolved, which complicates ascertaining the physical processes behind particular spectral signatures. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument and in combination with spatially resolved observations.
Aims: We aim to understand the disk-integrated spectral behaviors of a confined X2.2 flare and its eruptive X9.3 successor, which had energies of 2.2 × 1031 erg and 9.3 × 1031 erg, respectively, as measured by Sun-as-a-star observations with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N).
Methods: The behavior of multiple photospheric (Na D1 & D2, Mg I at 5173 Å, Fe I at 6173 Å, and Mn I at 4031 Å) and chromospheric (Ca II H & K, Hα, Hβ, and He ID3) spectral lines were investigated by means of activity indices and contrast profiles. A number of different photospheric lines were also investigated by means of equivalent widths, and radial velocity measures, which were then related to physical processes directly observed in high-resolution observations made with the Swedish 1-m Solar Telescope (SST) and the Atmospheric Imaging Assembly (AIA) on board of the Solar Dynamics Observatory (SDO).
Results: Our findings suggest a relationship between the evolving shapes of contrast profile time and the flare locations, which assists in constraining flare locations in disk-integrated observations. In addition, an upward bias was found in flare statistics based on activity indices derived from the Ca II H & K lines. In this case, much smaller flares cause a similar increase in the activity index as that produced by larger flares. Hα-based activity indices do not show this bias and are therefore less susceptible to activity jitter. Sodium line profiles show a strongly asymmetric response during flare activity, which is best captured with a newly defined asymmetrical sodium activity index. A strong flare response was detected in Mn I line profiles, which is unexpected and calls for further exploration. Intensity increases in Hα, Hβ, and certain spectral windows of AIA before the flare onset suggest their potential use as short-term flare predictors.
Jadlovský, Daniel; Granzer, Thomas; Weber, Michael; Kravchenko, Kateryna; Krtička, Jiří; Dupree, Andrea K.; Chiavassa, Andrea; Strassmeier, Klaus G.; Poppenhäger, Katja, eprint arXiv:2312.02816, accepted for publication in Astronomy & Astrophysics (2024)
Betelgeuse, a red supergiant star of semi-regular variability, reached a historical minimum brightness in February 2020, known as the Great Dimming. Even though the brightness has returned to the values prior to the Great Dimming now, it continues to exhibit highly unusual behavior. Our goal is to study long-term dynamics of the photosphere, including during the Great Dimming. We applied the tomographic method, which allows different layers in the stellar atmosphere to be probed in order to reconstruct depth-dependent velocity fields. The method is based on the construction of spectral masks by grouping spectral lines from specific optical depths. These masks are cross-correlated with the observed spectra to recover the velocity field inside each atmospheric layer. We obtained about 2800 spectra over the past 15 years that were observed with the STELLA robotic telescope in Tenerife. We analyzed the variability of five different layers of Betelgeuse’s photosphere. We found phase shift between the layers, as well as between the variability of velocity and photometry. The time variations of the widths of the cross-correlation function reveal propagation of two shockwaves during the Great Dimming. For about two years after the dimming, the timescale of variability was different between the inner and outer photospheric layers. By 2022, all the layers were pulsating with higher frequency corresponding with the first overtone. The combination of the extensive high-resolution spectroscopic data set with the tomographic method revealed the variable velocity fields in the photosphere of Betelgeuse, for the first time in such detail. Our results demonstrate that powerful shocks are the triggering mechanism for episodic mass-loss events, which may be the missing component to explain the mass-loss process in red supergiants.
New papers: Science with the ANDES spectrograph for the ELT ?>
My group is contributing the the second-light instrument suite for the Extremely Large Telescope: we are developing and building the optical-ultraviolet (UBV) part of the high-resolution spectrograph ANDES. And of course, we are also highly interested in the scientific avenues that will open up with this instrument. The two recent White Papers on exoplanetary and stellar science with ANDES lay out many of the exciting ideas that we can explore:
Stellar versus planetary rest frame for a hot, i.e. short-orbit planet (red arrow) and a temperate planet (green arrow) in the habitable zone around the host star. For the temperate planet, the planetary and stellar (black arrow) rest frames are much closer together and changes in stellar activity may distort the planetary atmospheric signal (Palle et al. 2023; adapted from Bourrier et al, 2020).
In the past decade the study of exoplanet atmospheres at high-spectral resolution, via transmission/emission spectroscopy and cross-correlation techniques for atomic/molecular mapping, has become a powerful and consolidated methodology. The current limitation is the signal-to-noise ratio during a planetary transit. This limitation will be overcome by ANDES, an optical and near-infrared high-resolution spectrograph for the ELT. ANDES will be a powerful transformational instrument for exoplanet science. It will enable the study of giant planet atmospheres, allowing not only an exquisite determination of atmospheric composition, but also the study of isotopic compositions, dynamics and weather patterns, mapping the planetary atmospheres and probing atmospheric formation and evolution models. The unprecedented angular resolution of ANDES, will also allow us to explore the initial conditions in which planets form in proto-planetary disks. The main science case of ANDES, however, is the study of small, rocky exoplanet atmospheres, including the potential for biomarker detections, and the ability to reach this science case is driving its instrumental design. Here we discuss our simulations and the observing strategies to achieve this specific science goal. Since ANDES will be operational at the same time as NASA’s JWST and ESA’s ARIEL missions, it will provide enormous synergies in the characterization of planetary atmospheres at high and low spectral resolution. Moreover, ANDES will be able to probe for the first time the atmospheres of several giant and small planets in reflected light. In particular, we show how ANDES will be able to unlock the reflected light atmospheric signal of a golden sample of nearby non-transiting habitable zone earth-sized planets within a few tenths of nights, a scientific objective that no other currently approved astronomical facility will be able to reach.
Roederer, Ian U.; Alvarado-Gómez, Julián D.; Allende Prieto et al. (including Poppenhaeger, K.); eprint arXiv:2311.16320; Submitted to Experimental Astronomy.
The ArmazoNes high Dispersion Echelle Spectrograph (ANDES) is the optical and near-infrared high-resolution echelle spectrograph envisioned for the European Extremely Large Telescope (ELT). We present a selection of science cases, supported by new calculations and simulations, where ANDES could enable major advances in the fields of stars and stellar populations. We focus on three key areas, including the physics of stellar atmospheres, structure, and evolution; stars of the Milky Way, Local Group, and beyond; and the star-planet connection. The key features of ANDES are its wide wavelength coverage at high spectral resolution and its access to the large collecting area of the ELT. These features position ANDES to address the most compelling and potentially transformative science questions in stellar astrophysics of the decades ahead, including questions which cannot be anticipated today.
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 ?>
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.
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)
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).
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 ?>
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 ?>
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.
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.
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.
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