Code published: AltaiPony ?>

Code published: AltaiPony

The AltaiPony flare-finder on github (Ilin et al. 2022).

My PhD student Ekaterina Ilin investigates stellar flares in various space-based observations, and has developed a flare-finding code names AltaiPony already a while ago. We used this in several of her papers, for example for the flare census in open clusters (Ilin et al. 2019, Ilin et al. 2021) and the high latitude flares on M dwarfs (Ilin et al. 2021, blog post). After having it available on github already for a good while, Ekaterina has now also published her code in the peer-reviewed Journal for Open Source Software and it is linked in the Astrophysical Source Code Library. It is pip-installable and a very useful tool!

AltaiPony: Flare finder for Kepler, K2, and TESS light curves


Ilin, Ekaterina; Schmidt, Sarah J.; Poppenhäger, Katja; Davenport, James R. A.; Kristiansen, Martti H.; Omohundro, Mark

AltaiPony de-trend light curves from Kepler, K2, and TESS missions, and searches them for flares. The code also injects and recovers synthetic flares to account for de-trending and noise loss in flare energy and determines energy-dependent recovery probability for every flare candidate. AltaiPony uses K2SC (ascl:1605.012), AstroPy (ascl:1304.002) and lightkurve (ascl:1812.013) in addition to other common codes, and extensive documentation and tutorials are provided for the software.

Astrophysics Source Code Library, record ascl:2201.009, January 2022

New group members and completed Master’s theses ?>

New group members and completed Master’s theses

We have two new group members who joined us this fall: Prachi Rahate, who is a Master student and works with Dr. Eliana Amazo-Gomez on stellar rotation, and Dr. Andy Gallagher, who is a postdoc and works with Dr. Matthias Steffen on 3D NLTE stellar atmospheres. Welcome!

We also had two students successfully complete their Master theses over the summer: Marzie Hosseini completed her thesis on “Transits in the young system PTFO 8-8695, is there a planet?”, as well as Alexandre Gillet, who completed his thesis on the topic “Are there clouds on Ultra Hot Jupiters?”. We were able to have a socially distanced outdoors picnic to celebrate Marzie’s and Alex’s completion of the Master’s degree – it was really nice seeing (almost) the whole group together in person again.

Paper on polar flares on fast-rotating M dwarfs ?>

Paper on polar flares on fast-rotating M dwarfs

Small stars flare actively and expel particles that can alter and evaporate the atmospheres of planets that orbit them. New findings suggest that large superflares prefer to occur at high latitudes, sparing planets that orbit around the stellar equator. Credit: AIP/ J. Fohlmeister

My PhD student Ekaterina Ilin has discovered a new method to localize flares on fast-rotating stars, using the rotational modulation of flares that last longer than the stellar rotation. With her new method she found that flares

on fast-rotating M dwarfs occur close to the poles of the star, instead of neat to the equator like on our own Sun. This could be good news for exoplanets and their habitability, because particles expelled by the star during flares may miss the planets and harm their atmospheres less.

The AIP published a press release about Ekaterina’s paper.

Giant white-light flares on fully convective stars occur at high latitudes

Ilin, Ekaterina; Poppenhaeger, Katja; Schmidt, Sarah J.; Järvinen, Silva P.; Newton, Elisabeth R.; Alvarado-Gómez, Julián D.; Pineda, J. Sebastian; Davenport, James R. A.; Oshagh, Mahmoudreza; Ilyin, Ilya

White-light flares are magnetically driven localized brightenings on the surfaces of stars. Their temporal, spectral, and statistical properties present a treasury of physical information about stellar magnetic fields. The spatial distributions of magnetic spots and associated flaring regions help constrain dynamo theories. Moreover, flares are thought to crucially affect the habitability of exoplanets that orbit these stars. Measuring the location of flares on stars other than the Sun is challenging due to the lack of spatial resolution. Here we present four fully convective stars observed with the Transiting Exoplanet Survey Satellite that displayed large, long-duration flares in white-light which were modulated in brightness by the stars’ fast rotation. This allowed us to determine the loci of these flares directly from the light curves. All four flares occurred at latitudes between 55° and 81°, far higher than typical solar flare latitudes. Our findings are evidence that strong magnetic fields tend to emerge close to the stellar rotational poles for fully convective stars, and suggest that the impact of flares on the habitability of exoplanets around small stars could be weaker than previously thought.

Monthly Notices of the Royal Astronomical Society, Volume 507, Issue 2, pp.1723-1745 (2021)

Paper on the X-ray irradiation and evaporation of exoplanets ?>

Paper on the X-ray irradiation and evaporation of exoplanets

Exoplanets host stars detected on the German half of the eROSITA X-ray sky, shown as pink dots, other exoplanet host stars shown as small grey dots. Credit: AIP/K. Poppenhaeger

Our group’s first paper using data from the German-Russian eROSITA mission has been accepted for publication. My PhD student Grace Foster has investigated the X-ray irradiation and estimated the evaporation of exoplanets using data from eROSITA’s first and second all-sky scan.

The paper is available here:

Exoplanet X-ray irradiation and evaporation rates with eROSITA

Foster, G. ; Poppenhaeger, K. ; Ilic, N. ; Schwope, A.

High-energy irradiation is a driver for atmospheric evaporation and mass loss in exoplanets. This work is based on data from eROSITA, the soft X-ray instrument aboard SRG (Spectrum Roentgen Gamma) mission, as well as archival data from other missions, we aim to characterise the high-energy environment of known exoplanets and estimate their mass loss rates. We use X-ray source catalogues from eROSITA, XMM-Newton, Chandra and ROSAT to derive X-ray luminosities of exoplanet host stars in the 0.2-2 keV energy band with an underlying coronal, i.e. optically thin thermal spectrum. We present a catalogue of stellar X-ray and EUV luminosities, exoplanetary X-ray and EUV irradiation fluxes and estimated mass loss rates for a total of 287 exoplanets, 96 among them being characterised for the first time from new eROSITA detections. We identify 14 first time X-ray detections of transiting exoplanets that are subject to irradiation levels known to cause observable evaporation signatures in other exoplanets, which makes them suitable targets for follow-up observations.

eprint arXiv:2106.14550, accepted by A&A (2021)

XMM-Newton Workshop on exoplanets and their environments ?>

XMM-Newton Workshop on exoplanets and their environments

This week I am organizing the XMM-Newton 2021 virtual workshop on the topic of “A high-energy view of exoplanets and their environments” as the chair of the Scientific Organizing Committee. We have over 350 participants, which is fantastic! The program and workshop website is here:

The conference proceedings will be peer-reviewed and published in Astronomische Nachrichten/Astronomical Notes.

Paper on the day-side temperature of KELT-1b, a low-mass brown dwarf ?>

Paper on the day-side temperature of KELT-1b, a low-mass brown dwarf

TESS phase curve of KELT-1b, from von Essen et al. (2021)
In a recent work by Prof. Carolina von Essen, with large contributions from our group from the side of Dr. Matthias Mallonn (and minor contributions from myself), we investigate the TESS phase curve and the dayside temperature of KELT-1b, which is a bit too heavy to still be called a planet and is therefore classified as a low-mass brown dwarf.

From the abstract:

We present the detection and analysis of the phase curve of KELT-1b at optical wavelengths, analyzing data taken by the Transiting Exoplanet Survey Satellite (TESS) during cycle 2 and sector 17. With a mass of ~27 MJup, KELT-1b is an example of a low-mass brown dwarf. Due to the high mass and close proximity of its companion, the host star exhibits a TESS light curve that shows clear ellipsoidal variations. We modeled the data with a six-component model: secondary eclipse, phase curve accounting for reflected light and thermal emission, Doppler beaming, ellipsoidal variations, stellar activity, and the primary transit. We determined the secondary eclipse depth in the TESS bandpass to be 304 ± 75 parts-per-million (ppm). In addition, we measured the amplitude of the phase curve to be 128 ± 27 ppm, with a corresponding eastward offset between the region of maximum brightness and the substellar point of 19.2 ± 9.6 degrees, with the latter showing good agreement with Spitzer measurements. We determined a day-side brightness temperature in the TESS bandpass of 3201 ± 147 K that is approximately 200 K higher than the values determined from the Spitzer 3.6 and 4.5 μm data. By combining TESS and Spitzer eclipse depths, we derived a day-side effective temperature of Teff = 3010 ± 78 K. Previously published eclipse depths in the near-infrared suggest a much higher brightness temperature and this discrepancy cannot be explained by spectral models combined with the current data. We attribute those large eclipse depths to unmodeled ellipsoidal variations, which would typically be manifested as a deeper secondary eclipse in observations with insufficient phase coverage. A one-dimensional self-consistent atmospheric model is able to explain the TESS and Spitzer day-side brightness temperatures with thermal emission alone and no reflected light. The difference between the TESS and Spitzer brightness temperatures can be explained via CO absorption due to a non-inverted temperature profile. The night side data fix an upper limit of ~2000 K on the internal temperature of KELT-1 b.

“TESS unveils the optical phase curve of KELT-1b. Thermal emission and ellipsoidal variation from the brown dwarf companion along with the stellar activity”, von Essen, C.; Mallonn, M.; Piette, A.; Cowan, N. B.; Madhusudhan, N.; Agol, E.; Antoci, V.; Poppenhaeger, K.; Stassun, K. G.; Khalafinejad, S.; Tautvaišienė, G., Astronomy & Astrophysics, Volume 648, id.A71, 19 pp., April 2021.

Cool Stars 20.5 ?>

Cool Stars 20.5

Because Zenodo sometimes give a too-many-requests error during the conference, here’s a backup of my conference poster:

Katja’s Cool Stars 20.5 poster on stellar coronal abundances and helium observations of exoplanet atmospheres

Paper on stellar winds blowing around our nearest exoplanet neighbour ?>

Paper on stellar winds blowing around our nearest exoplanet neighbour

A recent work led by our Schwarzschild-Fellow Dr. Julián Alvarado-Gómez shows through numerical simulations that our nearest exoplanetary neighbour experiences a space weather environment similar to our own Earth. The planet is Proxima Centauri c, a planet a few times the size of the Earth, which orbits our nearest stellar neighbour Proxima Centauri in a roughly 5-year orbit.

Simulated stellar wind enviroment of the exoplanet Proxima Centauri c during a magnetic activity minimum of its host star, from Alvarado-Gómez et al. (2020).

From the abstract:

A new planet has been recently discovered around Proxima Centauri. With an orbital separation of ∼1.44 au and a minimum mass of about 7M , Proxima c is a prime direct imaging target for atmospheric characterization. The latter can only be performed with a good understanding of the space environment of the planet, as multiple processes can have profound effects on the atmospheric structure and evolution. Here, we take one step in this direction by generating physically realistic numerical simulations of Proxima’s stellar wind, coupled to a magnetosphere and ionosphere model around Proxima c. We evaluate their expected variation due to the magnetic cycle of the host star, as well as for plausible inclination angles for the exoplanet orbit. Our results indicate stellar wind dynamic pressures comparable to present-day Earth, with a slight increase (by a factor of 2) during high-activity periods of the star. A relatively weak interplanetary magnetic field at the distance of Proxima c leads to negligible stellar wind Joule heating of the upper atmosphere (about 10% of the solar wind contribution on Earth) for an Earth-like planetary magnetic field (0.3 G). Finally, we provide an assessment of the likely extreme conditions experienced by the exoplanet candidate Proxima d, tentatively located at 0.029 au with a minimum mass of 0.29 M.

“An Earth-like Stellar Wind Environment for Proxima Centauri c”, Alvarado-Gómez, Julián D.; Drake, Jeremy J.; Garraffo, Cecilia; Cohen, Ofer; Poppenhaeger, Katja; Yadav, Rakesh K.; Moschou, Sofia P., The Astrophysical Journal Letters, Volume 902, Issue 1, id.L9, 7 pp. (2020).

Also featured in a Science Update by the Harvard-Smithsonian Center for Astrophysics.



Paper on stellar flares in open clusters ?>

Paper on stellar flares in open clusters

Median fraction of stellar luminosity in flares plotted as a function of cluster age, from Ilin et al. (2020).

Ekaterina Ilin, a PhD student in my group, has recently published her work on stellar flares in three young and two middle-aged open clusters.

From the abstract: “Drawing from the complete K2 archive, we searched 3435∼80 day long light curves of 2111 open cluster members for flares using the open-source software packages K2SC to remove instrumental and astrophysical variability from K2 light curves, and AltaiPony to search and characterize the flare candidates. We confirmed a total of 3844 flares on high probability open cluster members with ages from zero age main sequence(Pleiades) to 3.6 Gyr (M67). We extended the mass range probed in the first study of this series to span from Sun-like stars to mid-Mdwarfs. We added the Hyades (690 Myr) to the sample as a comparison cluster to Praesepe (750 Myr), the 2.6 Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. We found that the flare energy distribution was similar in the entire parameter space, following a power law relation with exponent a=1.84−2.39. We confirmed that flaring rates declined with age, and declined faster for higher mass stars. Our results are in good agreement with most previous statistical flare studies. We found evidence that a rapid decline in flaring activity occurred in M1-M2 dwarfs around Hyades/Praesepe age, when these stars spun down to rotation periods of about 10 d, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. We conclude that somediscrepancies between our results and flare studies that used rotation periods for their age estimates could be explained by sample selection bias toward more active stars, but others may hint at limitations of using rotation as an age indicator without additional constraints from stellar activity.”

Flares in Open Clusters with K2. II. Pleiades, Hyades, Praesepe, Ruprecht 147, and M67“, Ilin, Ekaterina; Schmidt, Sarah J.; Poppenhäger, Katja; Davenport, James R. A.; Kristiansen, Martti H.; Omohundro, Mark, accepted by A&A, 2020.

Welcome to new group members ?>

Welcome to new group members

Two new group members are starting their research here in fall 2020: Judy Chebly is a new PhD student who works with Julián Alvarado-Gómez and myself on simulations of coronal mass ejections in stars-planet systems, and Dr. Eliana Amazo-Gomez is a new postdoc who works on stellar rotation and activity. We’re very happy to have them on board!