arXiv subjects

Beate Stelzer

Publications and source records attributed to Beate Stelzer.

At least 19 recordsLinked to original sources

arXiv2026

The Volans-Carina association An X-ray Bridge Between Younger and Older Stellar Populations

Nearby young moving groups (NYMGs) provide benchmarks for studying the evolution of magnetic activity and planetary environments at early ages. The Volans-Carina association (VCA), as a pre-main sequence association at a distance of ~90 pc, occupies a sparsely sampled region of parameter space between younger nearby associations and older distant open clusters. We have re-evaluated the membership probabilities of the VCA catalog presented in Gagne et al. (2018a) with BANYAN as well as updated kinematics from Gaia DR3. Stellar parameters were derived from multi-wavelength spectral energy distributions, with the obtained bolometric luminosity and effective temperature being used in combination with pre-main sequence evolutionary models to obtain an isochronal age of the association. We joined data from the SRG (Spectrum Roentgen Gamma)/eROSITA and ROSAT all-sky surveys with a dedicated XMM-Newton Large Programme to construct a comprehensive X-ray catalog. Our updated census of the VCA comprises 29 highly probable members, 30 candidate members, and four uncertain objects. Isochronal analysis of the association yields an age of 80+-20 Myr, consistent with previous estimates. We detect X-ray emission from 56 objects, reaching ~90% completeness, one of the most complete X-ray censuses achieved for a NYMG. Low-mass members and candidates (<0.6 Msolar) have X-ray to bolometric luminosity ratios around the canonical saturation limit (Lx/Lbol~10^-3). X-ray luminosity functions show the VCA to be consistent with the similarly aged Pleiades (in the F5 to M3 range) and significantly more active than the older Hyades (in the F5 to K4 range). The combination of an updated membership census and a nearly complete X-ray census establishes the VCA as a particularly robust calibration point for studies of the early evolution of coronal X-ray activity.

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arXiv2026

Balmer decrements as a new diagnostic for period-bounce Cataclysmic Variable stars

Cataclysmic variable stars (CVs) evolve toward shorter orbital periods ($P_{\rm orb}$) until they reach a minimum $P_{\rm orb}$ near $P_{\rm orb}\sim80$ min. Beyond this point, the donor star becomes out of thermal equilibrium or increasingly degenerate, causing the system to "bounce back" to longer $P_{\rm orb}$ values. Such highly evolved systems are known as period-bouncers. Although 40-80\% of all CVs are expected to have reached this stage, period-bouncers come up for only 3-25\% of the observed CV population. This is likely a consequence of their intrinsic faintness associated with lower mass-transfer rates. Establishing new diagnostics to unveil this missing population is therefore crucial. Two samples of non-magnetic CVs with public SDSS optical spectra were constructed: one of short-period pre-bounce CVs and another of period-bounce CVs. For systems showing Balmer absorption from the white dwarf (WD), hydrogen-dominated atmosphere models were fitted and subtracted to correct for the WD component. H$\alpha$, H$\beta$, and H$\gamma$ fluxes were measured. We then investigated statistical relations between the Balmer decrements, the H$\beta$ line luminosity, and $P_{\rm orb}$, and compared the measured Balmer decrements with theoretical predictions from accretion disc models. Short-period pre-bounce CVs show flat Balmer decrements, that is Balmer line ratios close to unity. In contrast, systems near and beyond the period minimum exhibit progressively steeper decrements (H$\alpha$/H$\beta$>1 and H$\gamma$/H$\beta$<1). This behaviour is attributed to their lower mass accretion rates, as inferred from the H$\beta$ line luminosity. We fitted a linear logistic regression model to the diagram of H$\gamma$/H$\beta$ versus H$\alpha$/H$\beta$. We establish that this diagnostic diagram effectively separates period-bouncers from pre-bounce CVs.

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arXiv2026

The average X-ray spectrum of the volume-complete M-, F-, G-, and K-type star sample within 10 pc of the Sun

F, G, K and M type stars are the most abundant stellar population in the Milky Way and are expected to contribute to its diffuse X-ray emission. Yet their intrinsic average X-ray spectrum remains poorly constrained due to their faint X-ray luminosities, leaving their collective role in the X-ray background of the Milky Way uncertain. We analysed the volume-complete sample of M- (M0--M6) and FGK-type stars within 10 pc of the Sun using data from eROSITA all-sky survey aboard the Spectrum-Roentgen-Gamma (SRG) mission (eRASS:4). Individual spectra were normalized by exposure and distance and stacked to produce representative averages. The distance-normalized emission measures yield an average X-ray luminosity of $(2.6 \pm0.1)\times 10^{27}$ erg/s for M-type stars, and $(15\pm3)\times 10^{27}$ erg/s for F, G and K-type stars in 0.2--2.0 keV. The average spectra could be well described by a sum of three and two thermal models. Fitted temperatures and abundances remain consistent across M-star subgroups, while early-M stars are surprisingly on average less luminous than mid/late-M types. These results offer new insights into the collective X-ray properties of nearby stars, and provide motivation to explore the link with the unresolved soft X-ray background of the Galaxy.

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arXiv2025

The Sun as an X-ray star V.: A new method to retrieve coronal filling factors

Context. Stellar coronae are unresolved in X-rays, so inferences about their structure rely on spectral analysis. The "Sun-as-an-X-ray-star" (SaXS) approach uses the Sun as a spatially resolved template to interpret stellar spectra, but previous SaXS implementations were indirect and computationally heavy. Aims. We present a new SaXS implementation that converts solar emission measure distributions (EMDs) of distinct coronal region types into XSPEC spectral components and test whether broad-band X-ray spectra alone can recover their filling factors. Methods. We built XSPEC multi-temperature spectral models for four solar region types (background/quiet corona, active regions, cores, and flares) by using EMDs derived from Yohkoh/SXT data and translating each EMD bin into an isothermal apec component. These models were fit (using PyXspec) to two one-hour DAXSS spectra representative of quiescent (2022-06-29) and flaring (2022-04-25) states. Best-fit normalizations were converted into projected areas and filling factors and compared with near-coincident Hinode/XRT full-disk images. Results. Using the Yohkoh/SXT EMDs, the quiescent Sun spectrum is dominated by active region emission (filling factor ~22%), with the background corona poorly constrained. The flaring Sun spectrum is best described by a combination of active regions, cores, and flares with filling factors of ~47.5%, ~4.1%, and ~0.062%, respectively. The dominant components match spatial features seen in Hinode/XRT images. Limitations include the DAXSS low-energy cutoff (~0.7 keV) and the small, non-uniform Yohkoh EMD sample. Conclusions. Our SaXS implementation enables direct retrieval of coronal filling factors from broad-band X-ray spectra and provides a physically motivated alternative to ad hoc few-temperature fits, suitable for stellar X-ray analyses.

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arXiv2025

A systematic search for orbital periods of polars with TESS. Methods, detection limits, and results

Context. Determining the orbital periods of cataclysmic variable stars (CVs) is essential for confirming candidates and for the understanding of their evolutionary state. The Transiting Exoplanet Survey Satellite (TESS) provides month-long photometric data across nearly the entire sky that can be used to search for periodic variability in such systems. Aims. This study aims to identify and confirm the orbital periods for members of a recent compilation of magnetic CVs (known as polars) using TESS light curves. In addition, we set out to investigate their reliability, and hence the relevance of TESS for variability studies of CVs. Results. Ninety-five of the 217 polars in our sample have pipeline-produced TESS two-minute cadence light curves available. The results from our period search are overall in good agreement with the previously reported values. Out of the 95 analysed systems, 85 exhibit periods consistent with the literature values. Among the remaining ten objects, four are asynchronous polars, where TESS light curves resolve the orbital period, the white dwarf's spin period, and additional beat frequencies. For four systems, the periods detected from the TESS data differ from those previously reported. For two systems, a period detection was not possible. Our analysis of the flattened TESS light curves reveals a positive correlation between noise levels and TESS magnitude. Our noise level estimates resemble the rmsCDPP, a measure of white noise provided with the TESS pipeline products. However, our values for the noise level are systematically higher than the rmsCDPP indicating red noise and high-frequency signals hidden in the flattened light curves. Additionally, we present a statistical methodology to assess the reliability of period detections in TESS light curves. We find that for TESS magnitudes $\gtrsim$ 17, period detections become increasingly unreliable.

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arXiv2025

Far beyond the Sun: III. The magnetic cycle of $\boldsymbol\iota$ Horologii

We present a comprehensive investigation of the magnetic cycle of the young, active solar analogue $\iota$ Horologii ($\iota$ Hor) based on intensive spectropolarimetric monitoring using HARPSpol. Over a nearly three-year campaign, the technique of Zeeman-Doppler Imaging (ZDI) was used to reconstruct 18 maps of the large-scale surface magnetic field of the star. These maps trace the evolution of the magnetic field morphology over approximately 139 stellar rotations. Our analysis uncovers pronounced temporal evolution, including multiple polarity reversals and changes in field strength and geometry. We examine the evolution of the poloidal and toroidal field components, with the toroidal component showing strong modulation in concert with the chromospheric activity. Furthermore, for the first time, we reconstruct stellar magnetic butterfly diagrams which are used to trace the migration of large-scale magnetic features across the stellar surface, determining a magnetic polarity reversal timescale of roughly 100 rotations ($\sim773$ d). In addition, by tracking the field-weighted latitudinal positions, we obtain the first estimates of the large-scale flow properties on a star other than the Sun, identifying possible pole-ward and equator-ward drift speeds for different field polarities. These results provide critical insights into the dynamo processes operating in young solar-type stars and offer a direct comparison with the solar magnetic cycle.

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arXiv2025

The X-ray luminosity function of stars observed with SRG/eROSITA

Background diffuse X-ray emission is contributed in large part by the emission of point sources not individually resolved. While this is established since decades for the contribution of quasars to the diffuse emission above 1 keV energies, the possible contribution of undetected stars in the Milky Way to the softer band (0.5-1 keV) background emission is still poorly constrained. The overall unresolved X-ray flux from the stars is the product between the stellar spatial distribution in the Milky Way and the underlying X-ray luminosity function (XLF). In this work, we build the XLF of the stars and study its structure with respect to a set of main-sequence spectral types (F, G, K, M) and evolutionary stages (giants and white dwarfs). We build the XLF in volume-limited subsamples of increasing maximum distance and corresponding minimum X-ray luminosity using the HamStar catalog of X-ray emitting stars detected in the Western Galactic hemisphere with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) space observatory. We build an extinction-corrected Gaia color-magnitude diagram and decompose the XLF into the relative contribution of different groups of stars. We provide an empirical polynomial description of the XLF for the total sample and for the different stellar subgroups that can be used to estimate the unresolved contribution from the stars to the soft X-ray background of the Milky Way in a flux-limited survey.

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arXiv2025

eROSITA-selection of new period-bounce Cataclysmic Variables: First follow-up confirmation using TESS and SDSS

Between 40$\%$ and 80$\%$ of cataclysmic variables (CVs) are expected to have evolved past the period-minimum and contain a degenerate donor. However, observational surveys for CVs have only been able to detect a few of these highly evolved "period-bouncers", most likely due to the intrinsic faintness associated with their predicted low mass accretion rates. We have produced an initial selection of 137 high-likelihood period-bounce candidates from WD catalog based on our multiwavelength period-bouncer scorecard and selection cuts including X-ray data from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spektrum-Roentgen-Gamma spacecraft (SRG). We have laid out three main requirements (classification as a CV, determination of an orbital period and detection of a very late-type donor) that should result in the confirmation of several of these candidates. Our path for confirming these candidates has already produced its first successful result with the confirmation of GALEX J125751.4-283015 as a new period-bouncer. Several other candidates have already fulfilled at least one of our three requirements making their future confirmation likely. Our search for period-bouncers using the X-ray eROSITA emission of objects in optical WD catalogs has led to the confirmation of six new period-bouncers identified from the Gaia DR3 WD catalog (five previously known CVs and one WD candidate), a 18$\%$ increase that brings the present population to 39 systems. Both the selection method for period-bounce candidates and the confirmation path that we outlined will aid in future searches for new period-bounce candidates, contributing to the goal of resolving the discrepancy between the predicted high number of period-bouncers and the low number of these systems successfully observed to date.

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arXiv2024

A first systematic characterization of cataclysmic variables in SRG/eROSITA surveys

(abridged) We present an account of known cataclysmic variables (CVs) that were detected as X-ray sources in eROSITA X-ray surveys and have Gaia DR3 counterparts. We address standard CVs with main sequence donors and white dwarfs accreting via Roche-lobe overflow (RLOF) and related objects, the double degenerates (DDs), and the symbiotic stars (SySts). We discern between nonmagnetic (dwarf novae and nova-like objects) and magnetic CVs (polars and intermediate polars (IPs)). In the publically available eROSITA catalog from the recent DR1, typically 65\% of known cataloged and classified CVs are detected. This fraction rises to over 90\% if the stack of all \ero X-ray surveys (called S45 in this paper) is considered and the search volume is restricted to a radius of 500\,pc. We examine the various classes of CVs in various diagnostic diagrams relating X-ray and optical properties (luminosity, absolute magnitude, color, X-ray spectral hardness, and optical variability) and establish their average class properties. We derive spectral properties for the 22 brightest polars and confirm an increase in the ratio of soft to hard X-rays with increasing magnetic field in the accretion region. We report three new soft IPs and present a spectral analysis of all soft IPs. Their blackbody temperatures agree well with published values. The DDs represent the bluest and faintest subcategory but reach the same identification fraction as the standard CVs. The SySts are the most distant systems; only 20 (13\%) were detected as X-ray sources in S45, and 7 of those are first-time detections. We investigate their mean properties using an upper limit on the flux of the nondetected CVs. Their X-ray nondetection is indeed a distance effect.

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arXiv2024

The corona of a fully convective star with a near-polar flare

In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly rotating M7 dwarf, TIC 277539431, produce a flare at 81{\deg} latitude, the highest latitude flare located to date. This is in stark contrast to solar flares that occur much closer to the equator, typically below 30{\deg}. The mechanisms that allow flares at high latitudes to occur are poorly understood. We studied five Sectors of TESS monitoring, and obtained 36 ks of XMM-Newton observations to investigate the coronal and flaring activity of TIC 277539431. From the observations, we infer the optical flare frequency distribution, flare loop sizes and magnetic field strengths, the soft X-ray flux, luminosity and coronal temperatures, as well as the energy, loop size and field strength of a large flare in the XMM-Newton observations. We find that TIC 277539431's corona does not differ significantly from other low mass stars on the canonical saturated activity branch with respect to coronal temperatures and flaring activity, but shows lower luminosity in soft X-ray emission by about an order of magnitude, consistent with other late M dwarfs. The lack of X-ray flux, the high latitude flare, the star's viewing geometry, and the otherwise typical stellar corona taken together can be explained by the migration of flux emergence to the poles in rapid rotators like TIC 277539431 that drain the star's equatorial regions of magnetic flux, but preserve its ability to produce powerful flares.

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arXiv2024

The Sydney Radio Star Catalogue: properties of radio stars at megahertz to gigahertz frequencies

We present the Sydney Radio Star Catalogue, a new catalogue of stars detected at megahertz to gigahertz radio frequencies. It consists of 839 unique stars with 3,405 radio detections, more than doubling the previously known number of radio stars. We have included stars from large area searches for radio stars found using circular polarisation searches, cross-matching, variability searches, and proper motion searches as well as presenting hundreds of newly detected stars from our search of Australian SKA Pathfinder observations. The focus of this first version of the catalogue is on objects detected in surveys using SKA precursor instruments; however we will expand this scope in future versions. The 839 objects in the Sydney Radio Star Catalogue are distributed across the whole sky and range from ultracool dwarfs to Wolf-Rayet stars. We find that the radio luminosities of cool dwarfs are lower than the radio luminosities of more evolved sub-giant and giant stars. We use X-ray detections of 530 radio stars by the eROSITA soft X-ray instrument onboard the SRG spacecraft to show that almost all of the radio stars in the catalogue are over-luminous in the radio, indicating that the majority of stars at these radio frequencies are coherent radio emitters. The Sydney Radio Star Catalogue can be found in Vizier or at https://radiostars.org.

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arXiv2024

Cataclysmic variables around the period-bounce: An eROSITA-enhanced multi-wavelength catalog

Context. Cataclysmic variables with degenerate donors which have evolved past the period minimum, also known as period-bouncers, are predicted to make up a great portion of the cataclysmic variable population, between 40 and 70 percent. However, either due to shortcomings in the models or due to the intrinsic faintness of these strongly evolved systems, only a few have been confidently identified so far. Aims. We have compiled a multi-wavelength catalog of period-bouncers and cataclysmic variables around the period minimum from the literature in order to provide an in-depth characterization of the elusive subclass of period-bounce CVs that will help in the identification of new candidates. Methods. In this study we combine published or archival multi-wavelength data with new X-ray observations from the all-sky surveys carried out with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) onboard the Spektrum-Roentgen-Gamma spacecraft (SRG). Our catalog comprises 192 cataclysmic variables around the period minimum that were chosen as likely period bounce candidates based on reported short orbital periods and low donor mass. This sample helped us establish specific selection parameters that have been used to compile a scorecard which rates a systems likelihood of being a period-bouncer. Results. Our scorecard correctly assigns high scores to the already confirmed period-bouncers in our literature catalog and it identifies 80 additional strong period-bounce candidates present in the literature that have not been classified as such. We established two selection cuts based on the X-ray-to-optical flux ratio and the typical X-ray luminosity observed from the 8 already confirmed period-bouncers with eROSITA data. These X-ray selection cuts led to the categorization of 5 systems as new period-bouncers, increasing their population number to 22 systems.

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arXiv2023

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

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 XMM-Newton and Chandra telescopes and the eROSITA instrument onboard the Spectrum R\"ontgen Gamma satellite. The activity difference in the NLTT 41135/41136 system is a $3.44 \sigma$ 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.

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arXiv2023

New X-ray detections of magnetic period-bounce cataclysmic variables from XMM-Newton and SRG/eROSITA

A great portion of the cataclysmic variable population, between 40% and 70%, is predicted to be made up of period-bouncers, systems with degenerate donors that have evolved past the period minimum. However, due to their intrinsic faintness, only a few of these systems have been observed and confidently identified so far. We have searched for X-ray emission as a proof of accretion in order to confirm period-bounce cataclysmic variables. In a dedicated XMM-Newton observation of the period-bounce candidate SDSS J151415.65+074446.5 we discovered X-ray modulation at the binary orbital period confirming it as an accreting system. The X-ray light curve and the X-ray spectrum display characteristics of magnetic Polar-type systems allowing for the first time the determination of the X-ray luminosity and mass accretion rate for this system. Catalog data from eROSITA on the SRG satellite for V379 Vir and SDSS J125044.42+154957.4 allowed a first look into the X-ray behavior of period-bounce candidates with this new all-sky instrument. From the eROSITA measurements the X-ray luminosity and mass accretion rate were determined for the first time for SDSS J125044.42+154957.4, and the earlier result for V379 Vir from XMM-Newton was confirmed. All three cataclysmic variables with a magnetic white dwarf and very low-mass donor studied in this work present evidence for X-ray emission at a similar level of $L_{\rm x}\,{\rm [erg/s]} \approx 10^{29}$, which, together with the detection of X-ray orbital modulation in two of them, V379 Vir and SDSS J151415.65+074446.5, unambiguously proves the presence of accretion in these systems. The detection of these period-bouncers at faint X-ray luminosity levels with the all-sky X-ray survey eROSITA offers new prospects for the identification of additional period-bouncers, providing impetus for theoretical studies of binary evolution.

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arXiv2022

Stratospheric balloons as a platform for the next large far infrared observatory

Observations that require large physical instrument dimensions and/or a considerable amount of cryogens, as it is the case for high spatial resolution far infrared (FIR) astronomy, currently still face technological limits for their execution from space. Angular resolution and available observational capabilities are particularly affected. Balloon-based platforms promise to complement the existing observational capabilities by offering means to deploy comparatively large telescopes with comparatively little effort, including other advantages such as the possibility to regularly refill cryogens and to change and/or update instruments. The planned European Stratospheric Balloon Observatory (ESBO) aims at providing these additional large aperture FIR capabilities, exceeding the spatial resolution of Herschel, in the long term. The plans focus on reusable platforms performing regular flights and an operations concept that provides researchers with proposal-based access to observations. It thereby aims at offering a complement to other airborne, ground-based and space-based observatories in terms of access to wavelength regions, spatial resolution capability, and photometric stability. While the FIR capabilities are a main long-term objective, ESBO will offer benefits in other wavelength regimes along the way. Within the ESBO Design Study (ESBO DS), a prototype platform carrying a 0.5 m telescope for ultraviolet and visible light observations is being built and a platform concept for a next-generation FIR telescope is being studied. A flight of the UV/VIS prototype platform is estimated for 2021. In this paper we will outline the scientific and technical motivation for a large aperture balloon-based FIR observatory and the ESBO DS approach towards such an infrastructure. Secondly, we will present the technical motivation, science case, and instrumentation of the 0.5 m UV/VIS platform.

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arXiv2022

Stratospheric Balloons as a Complement to the Next Generation of Astronomy Missions

Observations that require large physical instrument dimensions and/or a considerable amount of cryogens, as it is for example the case for high spatial resolution far infrared astronomy, currently still face technological limits for their execution from space. The high cost and finality of space missions furthermore call for a very low risk approach and entail long development times. For certain spectral regions, prominently including the mid- to far-infrared as well as parts of the ultraviolet, stratospheric balloons offer a flexible and affordable complement to space telescopes, with short development times and comparatively good observing conditions. Yet, the entry burden to use balloon-borne telescopes is high, with research groups typically having to shoulder part of the infrastructure development as well. Aiming to ease access to balloon-based observations, we present the efforts towards a community-accessible balloon-based observatory, the European Stratospheric Balloon Observatory (ESBO). ESBO aims at complementing space-based and airborne capabilities over the next 10-15 years and at adding to the current landscape of scientific ballooning activities by providing a service-centered infrastructure for broader astronomical use, performing regular flights and offering an operations concept that provides researchers with a similar proposal-based access to observation time as practiced on ground-based observatories. We present details on the activities planned towards the goal of ESBO, the current status of the STUDIO (Stratospheric UV Demonstrator of an Imaging Observatory) prototype platform and mission, as well as selected technology developments with extensibility potential to space missions undertaken for STUDIO.

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arXiv2021

Towards a European Stratospheric Balloon Observatory -- The ESBO Design Study

This paper presents the concept of a community-accessible stratospheric balloon-based observatory that is currently under preparation by a consortium of European research institutes and industry. The planned European Stratospheric Balloon Observatory (ESBO) aims at complementing the current landscape of scientific ballooning activities by providing a service-centered infrastructure tailored towards broad astronomical use. In particular, the concept focuses on reusable platforms with exchangeable instruments and telescopes performing regular flights and an operations concept that provides researchers with options to test and operate own instruments, but later on also a proposal-based access to observations. It thereby aims at providing a complement to ground-, space-based, and airborne observatories in terms of access to wavelength regimes - particularly the ultraviolet (UV) and far infrared (FIR) regimes -, spatial resolution capability, and photometric stability. Within the currently ongoing ESBO Design Study (ESBO DS), financed within the European Union's Horizon 2020 Programme, a prototype platform carrying a 0.5-m telescope for UV and visible light observations is being built and concepts for larger following platforms, leading up to a next-generation FIR telescope are being studied. A flight of the UV/visible prototype platform is currently foreseen for 2021. We present the technical motivation, science case, instrumentation, and a two-stage image stabilization approach of the 0.5-m UV/visible platform. In addition, we briefly describe the novel mid-sized stabilized balloon gondola under design to carry telescopes in the 0.5 to 0.6 m range as well as the currently considered flight option for this platform. Secondly, we outline the scientific and technical motivation for a large balloon-based FIR telescope and the ESBO DS approach towards such an infrastructure.

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arXiv2021

Effects of flares on the habitable zones of M dwarfs accessible to TESS planet detections

Photometric space missions like Kepler and TESS continuously discover new exoplanets and advance the search for a second habitable world. The light curves recorded by these telescopes also reveal signs of magnetic activity, such as star spot modulation and flares, which can influence habitability. Searching for these characteristics, we analyzed TESS light curves of 112 M dwarfs selected according to the criterion that TESS can spot planet transits over their entire habitable zone (HZ). We detected 2532 flare events, occurring on 35 stars, thus the flare incidence rate is $\approx32\%$. For only $\approx11\%$ of our stars, we found rotation periods. We calculated bolometric flare energies and luminosities, flare energy frequency distributions (FFDs) and the bolometric flux reaching the HZ at the peak of the flare. We estimated the effects of flaring on the atmosphere of an Earth-like planet in the HZ in the view of both ozone depletion and the enabling of chemical reactions necessary to build ribonucleic acid (RNA). None of our targets exhibits highly energetic flares at a frequency large enough to trigger ozone depletion or RNA formation. The flux reaching the inner HZ edge during flare events goes up to $\approx 4.5$ times the quiescent solar flux at 1\,au.

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