@article{MallonnPoppenhaegerGranzeretal.2022,
  author    = {Mallonn, Matthias and Poppenh{\"a}ger, Katja and Granzer, Thomas and Weber, Michael and Strassmeier, Klaus G.},
  title     = {Detection capability of ground-based meter-sized telescopes for shallow exoplanet transits},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {657},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202140599},
  pages     = {10},
  year      = {2022},
  abstract  = {Meter-sized ground-based telescopes are frequently used today for the follow-up of extrasolar planet candidates. While the transit signal of a Jupiter-sized object can typically be detected to a high level of confidence with small telescope apertures as well, the shallow transit dips of planets with the size of Neptune and smaller are more challenging to reveal. We employ new observational data to illustrate the photometric follow-up capabilities of meter-sized telescopes for shallow exoplanet transits. We describe in detail the capability of distinguishing the photometric signal of an exoplanet transit from an underlying trend in the light curve. The transit depths of the six targets we observed, Kepler-94b, Kepler-63b, K2-100b, K2-138b, K2-138c, and K2-138e, range from 3.9 ppt down to 0.3 ppt. For five targets of this sample, we provide the first ground-based photometric follow-up. The timing of three targets is precisely known from previous observations, and the timing of the other three targets is uncertain and we aim to constrain it. We detect or rule out the transit features significantly in single observations for the targets that show transits of 1.3 ppt or deeper. The shallower transit depths of two targets of 0.6 and 0.8 ppt were detected tentatively in single light curves, and were detected significantly by repeated observations. Only for the target of the shallowest transit depth of 0.3 ppt were we unable to draw a significant conclusion despite combining five individual light curves. An injection-recovery test on our real data shows that we detect transits of 1.3 ppt depth significantly in single light curves if the transit is fully covered, including out-of-transit data toward both sides, in some cases down to 0.7 ppt depth. For Kepler-94b, Kepler-63b, and K2-100b, we were able to verify the ephemeris. In the case of K2-138c with a 0.6 ppt deep transit, we were able to refine it, and in the case of K2-138e, we ruled out the transit in the time interval of more than ±1.5 σ of its current literature ephemeris.},
  language  = {en}
}
@article{FosterPoppenhaegerIlićPetkovićetal.2022,
  author    = {Foster, Mary Grace and Poppenh{\"a}ger, Katja and Ilić Petković, Nikoleta and Schwope, Axel},
  title     = {Exoplanet X-ray irradiation and evaporation rates with eROSITA},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {661},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202141097},
  pages     = {11},
  year      = {2022},
  abstract  = {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 on board the Spectrum Roentgen Gamma mission, as well as on 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, that is, 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 of which are characterised for the first time based on 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. This makes them suitable targets for follow-up observations.},
  language  = {en}
}
@article{Poppenhaeger2022,
  author    = {Poppenh{\"a}ger, Katja},
  title     = {Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {512},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {2},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stac507},
  pages     = {1751 -- 1764},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}
@article{Poppenhaeger2019,
  author    = {Poppenh{\"a}ger, Katja},
  title     = {How stars and planets interact},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {340},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  number    = {4},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201913619},
  pages     = {329 -- 333},
  year      = {2019},
  abstract  = {The architecture of exoplanetary systems is often different from the solar system, with some exoplanets being in close orbits around their host stars and having orbital periods of only a few days. In analogy to interactions between stars in close binary systems, one may expect interactions between the star and the exoplanet as well. From theoretical considerations, effects on the host star through tidal and magnetic interaction with the exoplanet are possible; for the exoplanet, some interesting implications are the evaporation of the planetary atmosphere and potential effects on the planetary magnetism. In this review, several possible interaction pathways and their observational prospects and existing evidence are discussed. A particular emphasis is put on observational opportunities for these kinds of effects in the high-energy regime.},
  language  = {en}
}
@article{FosterPoppenhaeger2022,
  author    = {Foster, Grace and Poppenh{\"a}ger, Katja},
  title     = {Identifying interesting planetary systems for future X-ray observations},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {343},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  number    = {4},
  publisher = {Wiley-VCH},
  address   = {Berlin},
  issn      = {1521-3994},
  doi       = {10.1002/asna.20220007},
  pages     = {7},
  year      = {2022},
  abstract  = {X-ray observations of star-planet systems are important to grow our understanding of exoplanets; these observations allow for studies of photoevaporation of the exoplanetary atmosphere, and in some cases even estimations of the size of the outer planetary atmosphere. The German-Russian eROSITA instrument onboard the SRG (Spectrum Roentgen Gamma) mission is performing the first all-sky X-ray survey since the 1990s, and provides X-ray fluxes and spectra of exoplanet host stars over a much larger volume than was accessible before. Using new eROSITA data as well as archival data from XMM-Newton, Chandra, and ROSAT, we estimate mass-loss rates of exoplanets under an energy-limited escape scenario and identify several exoplanets with strong X-ray irradiation and expected mass loss that are amenable to follow-up observations at other wavelengths. We model sample spectra using a toy model of an exoplanetary atmosphere to predict what exoplanet transit observations with future X-ray missions such as Athena will look like and estimate the observable X-ray transmission spectrum for a typical hot Jupiter-type exoplanet.},
  language  = {en}
}
@article{IlinPoppenhaegerAlvaradoGomez2022,
  author    = {Ilin, Ekaterina and Poppenh{\"a}ger, Katja and Alvarado-G{\´o}mez, Juli{\´a}n David},
  title     = {Localizing flares to understand stellar magnetic fields and space weather in exo-systems},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {343},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  number    = {4},
  publisher = {Berlin},
  address   = {Wiley-VCH},
  issn      = {1521-3994},
  doi       = {10.1002/asna.20210111},
  pages     = {7},
  year      = {2022},
  abstract  = {Stars are uniform spheres, but only to first order. The way in which stellar rotation and magnetism break this symmetry places important observational constraints on stellar magnetic fields, and factors in the assessment of the impact of stellar activity on exoplanet atmospheres. The spatial distribution of flares on the solar surface is well known to be nonuniform, but elusive on other stars. We briefly review the techniques available to recover the loci of stellar flares, and highlight a new method that enables systematic flare localization directly from optical light curves. We provide an estimate of the number of flares we may be able to localize with the Transiting Exoplanet Survey Satellite, and show that it is consistent with the results obtained from the first full sky scan of the mission. We suggest that nonuniform flare latitude distributions need to be taken into account in accurate assessments of exoplanet habitability.},
  language  = {en}
}
@article{EigmuellerChaushevGillenetal.2019,
  author    = {Eigm{\"u}ller, Philipp and Chaushev, Alexander and Gillen, Edward and Smith, Alexis and Nielsen, Louise D. and Turner, Oliver and Csizmadia, Szilard and Smalley, Barry and Bayliss, Daniel and Belardi, Claudia and Bouchy, Francois and Burleigh, Matthew R. and Cabrera, Juan and Casewell, Sarah L. and Chazelas, Bruno and Cooke, Benjamin F. and Erikson, Anders and Gansicke, Boris T. and Guenther, Maximilian N. and Goad, Michael R. and Grange, Andrew and Jackman, James A. G. and Jenkins, James S. and McCormac, James and Moyano, Maximiliano and Pollacco, Don and Poppenh{\"a}ger, Katja and Queloz, Didier and Raynard, Liam and Rauer, Heike and Udry, Stephane and Walker, Simon. R. and Watson, Christopher A. and West, Richard G. and Wheatley, Peter J.},
  title     = {NGTS-5b},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {625},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201935206},
  pages     = {9},
  year      = {2019},
  abstract  = {Context. Planetary population analysis gives us insight into formation and evolution processes. For short-period planets, the sub-Jovian desert has been discussed in recent years with regard to the planet population in the mass/period and radius/period parameter space without taking stellar parameters into account. The Next Generation Transit Survey (NGTS) is optimised for detecting planets in this regime, which allows for further analysis of the sub-Jovian desert. Aims. With high-precision photometric surveys (e.g. with NGTS and TESS), which aim to detect short period planets especially around M/K-type host stars, stellar parameters need to be accounted for when empirical data are compared to model predictions. Presenting a newly discovered planet at the boundary of the sub-Jovian desert, we analyse its bulk properties and use it to show the properties of exoplanets that border the sub-Jovian desert. Methods. Using NGTS light curve and spectroscopic follow-up observations, we confirm the planetary nature of planet NGTS-5b and determine its mass. Using exoplanet archives, we set the planet in context with other discoveries. Results. NGTS-5b is a short-period planet with an orbital period of 3.3569866 +/- 0.0000026 days. With a mass of 0.229 +/- 0.037 M-Jup and a radius of 1.136 +/- 0.023 R-Jup, it is highly inflated. Its mass places it at the upper boundary of the sub-Jovian desert. Because the host is a K2 dwarf, we need to account for the stellar parameters when NGTS-5b is analysed with regard to planet populations. Conclusions. With red-sensitive surveys (e.g. with NGTS and TESS), we expect many more planets around late-type stars to be detected. An empirical analysis of the sub-Jovian desert should therefore take stellar parameters into account.},
  language  = {en}
}
@article{KelesKitzmannMallonnetal.2020,
  author    = {Keles, Engin and Kitzmann, Daniel and Mallonn, Matthias and Alexoudi, Xanthippi and Fossati, Luca and Pino, Lorenzo and Seidel, Julia Victoria and Caroll, Thorsten A. and Steffen, M. and Ilyin, Ilya and Poppenh{\"a}ger, Katja and Strassmeier, Klaus G. and von Essen, Carolina and Nascimbeni, Valerio and Turner, Jake D.},
  title     = {Probing the atmosphere of HD189733b with the Na i and K i lines},
  series = {Monthly Notices of the Royal Astronomical Society},
  volume    = {498},
  journal   = {Monthly Notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  doi       = {10.1093/mnras/staa2435},
  pages     = {1033},
  year      = {2020},
  abstract  = {High spectral resolution transmission spectroscopy is a powerful tool to characterize exoplanet atmospheres. Especially for hot Jupiters, this technique is highly relevant, due to their high-altitude absorption, e.g. from resonant sodium (Na i) and potassium (K i) lines. We resolve the atmospheric K i absorption on HD189733b with the aim to compare the resolved K i line and previously obtained high-resolution Na i-D line observations with synthetic transmission spectra. The line profiles suggest atmospheric processes leading to a line broadening of the order of ∼10 km/s for the Na i-D lines and only a few km/s for the K i line. The investigation hints that either the atmosphere of HD189733b lacks a significant amount of K i or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences we see in the resolved absorption lines.},
  language  = {en}
}
@article{AlexoudiMallonnKelesetal.2020,
  author    = {Alexoudi, Xanthippi and Mallonn, Matthias and Keles, Engin and Poppenh{\"a}ger, Katja and von Essen, Carolina and Strassmeier, Klaus},
  title     = {Role of the impact parameter in exoplanet transmission spectroscopy},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {640},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202038080},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-605378},
  pages     = {9},
  year      = {2020},
  abstract  = {Context Transmission spectroscopy is a promising tool for the atmospheric characterization of transiting exoplanets. Because the planetary signal is faint, discrepancies have been reported regarding individual targets. Aims We investigate the dependence of the estimated transmission spectrum on deviations of the orbital parameters of the star-planet system that are due to the limb-darkening effects of the host star. We describe how the uncertainty on the orbital parameters translates into an uncertainty on the planetary spectral slope. Methods We created synthetic transit light curves in seven different wavelength bands, from the near-ultraviolet to the near-infrared, and fit them with transit models parameterized by fixed deviating values of the impact parameter b. First, we performed a qualitative study to illustrate the effect by presenting the changes in the transmission spectrum slope with different deviations of b. Then, we quantified these variations by creating an error envelope (for centrally transiting, off-center, and grazing systems) based on a derived typical uncertainty on b from the literature. Finally, we compared the variations in the transmission spectra for different spectral types of host stars. Results Our simulations show a wavelength-dependent offset that is more pronounced at the blue wavelengths where the limb-darkening effect is stronger. This offset introduces a slope in the planetary transmission spectrum that becomes steeper with increasing b values. Variations of b by positive or negative values within its uncertainty interval introduce positive or negative slopes, thus the formation of an error envelope. The amplitude from blue optical to near-infrared wavelength for a typical uncertainty on b corresponds to one atmospheric pressure scale height and more. This impact parameter degeneracy is confirmed for different host types; K stars present prominently steeper slopes, while M stars indicate features at the blue wavelengths. Conclusions We demonstrate that transmission spectra can be hard to interpret, basically because of the limitations in defining a precise impact parameter value for a transiting exoplanet. This consequently limits a characterization of its atmosphere.},
  language  = {en}
}
@article{IlinPoppenhaeger2022,
  author    = {Ilin, Ekaterina and Poppenh{\"a}ger, Katja},
  title     = {Searching for flaring star-planet interactions in AU Mic TESS observations},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {513},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {3},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stac1232},
  pages     = {4579 -- 4586},
  year      = {2022},
  abstract  = {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 d 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.},
  language  = {en}
}