arXiv subjects

Giuseppina Micela

Publications and source records attributed to Giuseppina Micela.

At least 19 recordsLinked to original sources

arXiv2026

X-ray Activity of the RS CVn-type Star {\sigma} Gem with the First-Year Observations of Einstein Probe

Context. Stellar flares are energetic events driven by the sudden release of magnetic energy in the stellar atmosphere. Studying these flares is crucial for understanding their impact on exoplanets, the circumstellar environment, and stellar evolution itself. The launch of the Einstein Probe (EP) offers a unique opportunity to systematically detect such events. Aims. We present a systematic analysis of the flaring activity of the active RS CVn-type binary {\sigma} Gem, utilizing the first-year monitoring data from the Wide-field X-ray Telescope (WXT) aboard EP. Our goals are to demonstrate the unique capability of EP in monitoring stellar X-ray activity and detecting flares, by identifying and characterizing extreme X-ray flares on {\sigma} Gem and estimating their occurrence rate. Methods. We developed a data-processing pipeline to select and extract EP-WXT observations, producing a background-subtracted, vignetting-corrected light curve. We employed the Bayesian Blocks method to detect significant flares in the long-term X-ray light curve. For each identified flare, we performed light curve and spectral fitting to derive the flare parameters. Results. Between October 2024 and April 2025, WXT detected 6 distinct flares from {\sigma} Gem. Their durations ranged from 21 hours to 3 days, with peak X-ray luminosities (0.5-4 keV) of 3.7 * 10^31 to 7.0 * 10^32 erg/s and total energies of 1.1 * 10^36 to 4.4 * 10^37 erg, placing them among the "superflare" class. Conclusions. Using {\sigma} Gem as a case study, we demonstrate an analysis process for flare detection and analysis with EP-WXT data, which provides new statistical constraints on its flaring behavior. Applying this methodology to the growing EP stellar archive promises to yield a vast sample of X-ray flares, which will significantly advance our understanding of stellar magnetic activity.

astro-ph.HE↗

arXiv2025

Empirical prediction of plasma emission measure distributions and X-EUV spectra of late-type stars

High-energy emission spectra from the outer atmospheres of late-type stars represent an important feature of the stellar activity in several contexts, such as the photoevaporation and photochemistry of planetary atmospheres or the modeling of irradiated circumstellar disks in young objects. An accurate determination of these spectra in the EUV and soft X-ray (XUV) band requires high-resolution spectroscopy, that is rarely feasible with current instrumentation. We employed a relatively large set of plasma emission measure distributions (EMDs) as a function of temperature, derived from FUV and X-ray emission line spectra acquired with the Hubble Space Telescope and Chandra or XMM-Newton, in order to devise a relatively simple recipe for predicting EMDs and XUV spectra of stars of different spectral types, activity levels, and plasma metallicity. We show that the EMDs in the range of temperatures between 10^4 K and 10^7.5 K can be described using the stellar surface X-ray flux as a control parameter, but this parameterization also depends on the spectral type. In particular, we find that M-type stars show slightly lower emission measure at temperatures below ~10^5 K and higher emission measures for T > ~10^6.5 K with respect to FGK stars with similar surface X-ray fluxes. We evaluated the uncertainties in the broad-band EUV and X-ray fluxes derived from synthetic EMDs and spectra, considering in the error budget also the limited knowledge of the chemical abundances in stellar outer atmospheres.

astro-ph.SR↗

arXiv2025

On the synergetic use of Ariel and JWST for exoplanet atmospheric science

This paper explores the potential for strategic synergies between the JWST and the Ariel telescopes, two flagship observatories poised to revolutionise the study of exoplanet atmospheres. Both telescopes have the potential to address common fundamental questions about exoplanets-especially concerning their nature and origins-and serve a growing scientific community. With their operations now anticipated to overlap, starting from 2030, there is a unique opportunity to enhance the scientific outputs of both observatories through coordinated efforts. In this report, authored by the Ariel-JWST Synergy Working Group, part of the Ariel Consortium Science Team, we summarise the capabilities of JWST and Ariel; we highlight their key differences, similarities, synergies, and distinctive strengths. Ariel is designed to conduct a broad survey of exoplanet atmospheres but remains highly flexible, allowing the mission to integrate insights from JWST's discoveries. Findings from JWST, including data from initiatives shaped by NASA's decadal survey priorities and community-driven research themes, will inform the development of Ariel's core survey strategy. Conversely, Ariel's ability to perform broad-wavelength coverage observations for bright targets provides complementary avenues for exoplanet researchers, particularly those interested in time-domain observations and large-scale atmospheric studies. This paper identifies key pathways for fostering JWST-Ariel synergies, many of which can be initiated even before Ariel's launch. Leveraging their complementary designs and scopes, JWST and Ariel can jointly address fundamental questions about the nature, formation, and evolution of exoplanets. Such strategic collaboration has the potential to maximise the scientific returns of both observatories and lay the foundation for future facilities in the roadmap to exoplanet exploration.

astro-ph.IM↗

arXiv2024

PAStar: a model for stellar surface from the Sun to active stars

Context. The characterization of exoplanets requires a good description of the host star. Stellar activity acts as a source of noise which can alter planet radii as derived from the transit depth or atmospheric characterization. Aims. Here, we propose PAStar, a model to describe photospheric activity in the form of spots and faculae which could be applied to a wide range of stellar observations, from photometric to spectroscopic time series, to be able to correctly extract planetary and stellar properties. Methods. The adopted stellar atmosphere is a combination of three components, the quiet photosphere, spots and faculae. The model takes into account the effects of star inclination, doppler shifts due to stellar rotation as well as for limb darkening, independent for each component. Several synthetic products have been presented to show the capabilities of the model. Results. The model is able to retrieve the input surface inhomogeneities configuration through photometric or spectroscopic observations. The model has been validated against optical solar data and compared to alternative stellar surface activity models; e.g. SOAP code. The Sun is a unique laboratory to test stellar models because of the possibility to relate unambiguously flux variations to surface inhomogeneities configuration. This validation has been done by analyzing a photometric time series from the VIRGO photometer on board of SOHO mission. Results have been compared to real solar images from the HMI instrument on board of SDO to confirm the goodness of the results in terms of surface inhomogeneities position and dimensions. Conclusions. The description of stellar activity is a fundamental step in several astrophysical contexts and it is covered by the method we have presented. Our model offers a flexible and valuable tool to describe the activity of stars when it is dominated by spots and faculae.

astro-ph.SR↗

arXiv2024

Spot Modeling through Multiband Photometry Analysis of V1298 Tau

Context. Stellar activity consists of different phenomena, mainly spots and faculae, and it is one of the main sources of noise in exoplanetary observations because it affects both spectroscopic and photometric observations. If we want to study young active planetary systems we need to model the activity of the host stars in order to remove astrophysical noise from our observational data. Aims. We modelled the contribution of stellar spots in photometric observations. Through the use of multiband photometry, we aim to extract the geometric properties of the spots and constrain their temperature. Methods. We analyzed multiband photometric observations acquired with the 80 cm Marcon telescope of the Osservatorio Polifunzionale del Chianti of V1298 Tau, assuming that the photometric modulation observed in different bands should be due to cold spots. Results. We constrained the effective temperature of the active regions present on the surface of V1298 Tau, which is composed by the contemporary presence of spots and faculae. We tested our hypothesis on solar data, verifying that we measure the size of the dominant active region and its averaged effective temperature.

astro-ph.SR↗

arXiv2024

A reanalysis of the LHS 1140 b atmosphere observed with the Hubble Space Telescope

The super-Earth LHS 1140 b is an interesting target for atmospheric observations since it is close to the habitable zone of its star and falls in the gap of the radius distribution of small exoplanets, in the region thought to correspond to the transition between planets with and without atmospheres. Observations of the primary transit with WFC3 on board of the Hubble Space Telescope (HST) revealed a modulation in the planet transmission spectrum compatible with the presence of water; however this modulation may be also due to stellar activity-related phenomena. Here we present a detailed analysis of the WFC3/HST observations to probe the nature of this modulation and to understand if it can be attributable to the presence of unocculted spots on the stellar surface. Our analysis strongly suggests that LHS1140 is a rather quiet star with subsolar [Fe/H] and enriched in alpha elements. Therefore, we rule out the possibility that the planetary spectrum is affected by the presence of spots and faculae. This analysis shows the importance of a proper modelling of the stellar spectrum when analyzing transit observations. Finally, we modelled the planetary atmosphere of LHS1140 b to retrieve its atmospheric composition. However, the low resolution and the narrow spectral range of HST observations prevented us from definitively determining whether the spectral features are attributable to the presence of water or of other molecules in the planetary atmosphere.

astro-ph.EP↗

arXiv2024

Signatures of X-ray dominated chemistry in the spectra of exoplanetary atmospheres

High-energy radiation from stars impacts planetary atmospheres deeply affecting their chemistry, providing departures from chemical equilibrium. While the upper atmospheric layers are dominated by ionizations induced by extreme ultraviolet radiation, deeper into the atmosphere molecular abundances are controlled by a characteristic X-ray dominated chemistry, mainly driven by an energetic secondary electron cascade. In this work, we aim at identifying molecular photochemically induced fingerprints in the transmission spectra of a giant planet atmosphere. We have developed a numerical code capable of synthesizing transmission spectra with arbitrary spectral resolution, exploiting updated infrared photoabsorption cross sections. Chemical mixing ratios are computed using a photochemical model, tailored to investigate high energy ionization processes. We find that in case of high levels of stellar activity, synthetic spectra in both low and high resolutions show significant, potentially observable out-of-equilibrium signatures arising mainly from CO, CH$_4$, C$_2$H$_2$, and HCN.

astro-ph.EP↗

arXiv2024

The enigmatic dance of the HD 189733A system: Does the planet accrete onto the star?

Several studies suggest that the emission properties of a star can be affected by its interaction with a nearby planet. However, the actual observability of these effects remains a subject of debate. An example is the HD189733A system, where some characteristics of its emissions have been interpreted as indicative of ongoing interactions between the star and its planet. Other studies attribute these characteristics to the coronal activity of the star. In this work, we investigate whether the observed stellar X-ray flare events, which appear to be in phase with the planetary period in the HD189733A system, could be attributed to the accretion of the planetary wind onto the stellar surface or resulted from an interaction between the planetary and stellar winds. We developed a 3D MHD model that describes the system HD189733A, including the central host star and its hot Jupiter, along with their respective winds. The effects of gravity and the magnetic fields of both star and planet are taken into account. In the cases examined in this study, the accretion scenario is only viable when the stellar and planetary magnetic field strengths are at 5 G and 1 G, respectively. In this case, the Rayleigh-Taylor instabilities (RTIs) lead to the formation of an accretion column connecting star and planet. Once formed the column remains stable for the entire simulation. The accretion column yields an accretion rates of about 1e12 g/s and shows a mean density of about 1e7 cm^-3. In the other cases, the accretion column does not form because the RTI is suppressed by the stronger magnetic field intensities assumed for both the star and the planet. We synthesized the emission resulting from the shocked planetary wind and, its total X-ray emission ranges from 5e23 to 1e24 erg/s. In the case of accretion, the emission originating from the hot spot cannot be distinguished from the coronal activity.

astro-ph.SR↗

arXiv2023

Correcting Exoplanet Transmission Spectra for Stellar Activity with an Optimised Retrieval Framework

The chromatic contamination that arises from photospheric heterogeneities e.g. spots and faculae on the host star presents a significant noise source for exoplanet transmission spectra. If this contamination is not corrected for, it can introduce substantial bias in our analysis of the planetary atmosphere. We utilise two stellar models of differing complexity, StARPA and ASteRA, to explore the biases introduced by stellar contamination in retrieval under differing degrees of stellar activity. We use the retrieval framework TauREx3 and a grid of 27 synthetic, spot-contaminated transmission spectra to investigate potential biases and to determine how complex our stellar models must be in order to accurately extract the planetary parameters from transmission spectra. The input observation is generated using the more complex model (StARPA), in which the spot latitude is an additional, fixable parameter. This observation is then fed into a combined stellar-planetary retrieval which contains a simplified stellar model (ASteRA). Our results confirm that the inclusion of stellar activity parameters in retrieval minimises bias under all activity regimes considered. ASteRA performs very well under low to moderate activity conditions, retrieving the planetary parameters with a high degree of accuracy. For the most active cases, characterised by larger, higher temperature contrast spots, some minor residual bias remains due to ASteRA neglecting the interplay between the spot and the limb darkening effect. As a result of this, we find larger errors in retrieved planetary parameters for central spots (0 degrees) and those found close to the limb (60 degrees) than those at intermediate latitudes (30 degrees).

astro-ph.EP↗

arXiv2023

Hyades Member K2-136c: The Smallest Planet in an Open Cluster with a Precisely Measured Mass

K2-136 is a late-K dwarf ($0.742\pm0.039$ M$_\odot$) in the Hyades open cluster with three known, transiting planets and an age of $650\pm70$ Myr. Analyzing K2 photometry, we found that planets K2-136b, c, and d have periods of $8.0$, $17.3$, and $25.6$ days and radii of $1.014\pm0.050$ R$_\oplus$, $3.00\pm0.13$ R$_\oplus$, and $1.565\pm0.077$ R$_\oplus$, respectively. We collected 93 radial velocity measurements (RVs) with the HARPS-N spectrograph (TNG) and 22 RVs with the ESPRESSO spectrograph (VLT). Analyzing HARPS-N and ESPRESSO data jointly, we found K2-136c induced a semi-amplitude of $5.49\pm0.53$ m s$^{-1}$, corresponding to a mass of $18.1\pm1.9$ M$_\oplus$. We also placed $95$% upper mass limits on K2-136b and d of $4.3$ and $3.0$ M$_\oplus$, respectively. Further, we analyzed HST and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has $\sim$75% the radius of Neptune but is similar in mass, yielding a density of $3.69^{+0.67}_{-0.56}$ g cm$^{-3}$ ($\sim$2-3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets' atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time.

astro-ph.EP↗

arXiv2023

TOI-1695 b: A Water World Orbiting an Early M Dwarf in the Planet Radius Valley

Characterizing the bulk compositions of transiting exoplanets within the M dwarf radius valley offers a unique means to establish whether the radius valley emerges from an atmospheric mass loss process or is imprinted by planet formation itself. We present the confirmation of such a planet orbiting an early M dwarf ($T_{\rm mag} = 11.0294 \pm 0.0074, M_s = 0.513 \pm 0.012\ M_\odot, R_s = 0.515 \pm 0.015\ R_\odot, T_{\rm eff} =3690\pm 50 K$): TOI-1695 b ($P = 3.13$ days, $R_p = 1.90^{+0.16}_{-0.14}\ R_\oplus$). TOI-1695 b's radius and orbital period situate the planet between model predictions from thermally-driven mass loss versus gas depleted formation, offering an important test case for radius valley emergence models around early M dwarfs. We confirm the planetary nature of TOI-1695 b based on five sectors of TESS data and a suite of follow-up observations including 49 precise radial velocity measurements taken with the HARPS-N spectrograph. We measure a planetary mass of $6.36 \pm 1.00\ M_\oplus$, which reveals that TOI-1695 b is inconsistent with a purely terrestrial composition of iron and magnesium silicate, and instead is likely a water-rich planet. Our finding that TOI-1695 b is not terrestrial is inconsistent with the planetary system being sculpted by thermally driven mass loss. We present a statistical analysis of seven well-characterized planets within the M dwarf radius valley demonstrating that a thermally-driven mass loss scenario is unlikely to explain this population.

astro-ph.EP↗

arXiv2022

Kepler-102: Masses and Compositions for a Super-Earth and Sub-Neptune Orbiting an Active Star

Radial velocity (RV) measurements of transiting multiplanet systems allow us to understand the densities and compositions of planets unlike those in the Solar System. Kepler-102, which consists of 5 tightly packed transiting planets, is a particularly interesting system since it includes a super-Earth (Kepler-102d) and a sub-Neptune-sized planet (Kepler-102e) for which masses can be measured using radial velocities. Previous work found a high density for Kepler-102d, suggesting a composition similar to that of Mercury, while Kepler-102e was found to have a density typical of sub-Neptune size planets; however, Kepler-102 is an active star, which can interfere with RV mass measurements. To better measure the mass of these two planets, we obtained 111 new RVs using Keck/HIRES and TNG/HARPS-N and modeled Kepler-102's activity using quasi-periodic Gaussian Process Regression. For Kepler-102d, we report a mass upper limit of M$_{d} < $5.3 M$_{\oplus}$ [95\% confidence], a best-fit mass of M$_{d}$=2.5 $\pm$ 1.4 M$_{\oplus}$, and a density of $ρ_{d}$=5.6 $\pm$ 3.2 g/cm$^{3}$ which is consistent with a rocky composition similar in density to the Earth. For Kepler-102e we report a mass of M$_{e}$=4.7 $\pm$ 1.7 M$_{\oplus}$ and a density of $ρ_{e}$=1.8 $\pm$ 0.7 g/cm$^{3}$. These measurements suggest that Kepler-102e has a rocky core with a thick gaseous envelope comprising 2-4% of the planet mass and 16-50% of its radius. Our study is yet another demonstration that accounting for stellar activity in stars with clear rotation signals can yield more accurate planet masses, enabling a more realistic interpretation of planet interiors.

astro-ph.EP↗

arXiv2022

Short term variability of DS Tuc A observed with TESS

Impulsive short term variations occur in all kinds of solar-type stars. They are the results of complex phenomena such as the stellar magnetic field reconnection, low-level variability or in some cases even star-planet interactions. The radiation arising from these events is often highly energetic and, in stars hosting planets, may interact with the planetary atmospheres. Studying the rate of these energetic phenomena is fundamental to understand their role in modifying the chemical composition or, in some extreme cases, to the disruption of the planetary atmospheres. Here, we present a new procedure developed to identify the impulsive events in TESS light curves. Our goal is to have a simple and effective tool to study the short-term activity of a star using only its light curve, in order to derive its distribution and energetic. As our first case, we studied the system DS Tuc. Our technique consists of fitting the TESS light curves using iteratively Gaussian processes in order to remove all the long-term stellar activity contributions. Then, we identify the impulsive events and, derive amplitudes, time scales and the amount of energy emitted. We validate our procedure using the AU Mic TESS light curves obtaining results consistent with those presented in the literature. We estimate the frequency distribution of energetic events for DS Tuc. In particular, we find that there are approx 2 events per day with energy greater than 2x10^32 erg. We find evidence for a favoured stellar phase for short term activity on AU Mic, and also indications of short term activity in phase with the planetary orbit. For DS Tuc we find that the events distribution is not equally spaced in time but often grouped. The resulting distribution may be used to estimate the impact of short term variability on planetary atmosphere chemical compositions.

astro-ph.SR↗

arXiv2021

On The Synergy Between Ariel And Ground-Based High-Resolution Spectroscopy

Since the first discovery of an extra-solar planet around a main-sequence star, in 1995, the number of detected exoplanets has increased enormously. Over the past two decades, observational instruments (both onboard and on ground-based facilities) have revealed an astonishing diversity in planetary physical features (i. e. mass and radius), and orbital parameters (e.g. period, semi-major axis, inclination). Exoplanetary atmospheres provide direct clues to understand the origin of these differences through their observable spectral imprints. In the near future, upcoming ground and space-based telescopes will shift the focus of exoplanetary science from an era of 'species discovery' to one of 'atmospheric characterization'. In this context, the Atmospheric Remote-sensing Infrared Exoplanet Large (Ariel) survey, will play a key role. As it is designed to observe and characterize a large and diverse sample of exoplanets, Ariel will provide constraints on a wide gamut of atmospheric properties allowing us to extract much more information than has been possible so far (e.g. insights into the planetary formation and evolution processes). The low resolution spectra obtained with Ariel will probe layers different from those observed by ground-based high resolution spectroscopy, therefore the synergy between these two techniques offers a unique opportunity to understanding the physics of planetary atmospheres. In this paper, we set the basis for building up a framework to effectively utilise, at near-infrared wavelengths, high-resolution datasets (analyzed via the cross-correlation technique) with spectral retrieval analyses based on Ariel low-resolution spectroscopy. We show preliminary results, using a benchmark object, namely HD 209458 b, addressing the possibility of providing improved constraints on the temperature structure and molecular/atomic abundances.

astro-ph.EP↗

arXiv2021

Fundamental physics with ESPRESSO: Precise limit on variations in the fine-structure constant towards the bright quasar HE 0515$-$4414

The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515$-$4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure constant, $α$. We observed HE 0515$-$4414 for 16.1$\,$h with the Very Large Telescope and present here the first constraint on relative variations in $α$ with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: $Δα/α= 1.3 \pm 1.3_{\rm stat} \pm 0.4_{\rm sys}\,{\rm ppm}$. The statistical uncertainty (1$σ$) is similar to the ensemble precision of previous large samples of absorbers, and derives from the high S/N achieved ($\approx$105 per 0.4$\,$km$\,$s$^{-1}$ pixel). ESPRESSO's design, and calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum ($R=145000$) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of $Δα/α$. The evidence for the narrow components is corroborated by their correspondence with previously detected molecular hydrogen and neutral carbon. The main remaining systematic errors arise from ambiguities in the absorption profile modelling, effects from redispersing the individual quasar exposures, and convergence of the parameter estimation algorithm. All analyses of the spectrum, including systematic error estimates, were initially blinded to avoid human biases. We make our reduced ESPRESSO spectrum of HE 0515$-$4414 publicly available for further analysis. Combining our ESPRESSO result with 28 measurements, from other spectrographs, in which wavelength calibration errors have been mitigated, yields a weighted mean $Δα/α= -0.5 \pm 0.5_{\rm stat} \pm 0.4_{\rm sys}\,$ppm at redshifts 0.6-2.4.

astro-ph.CO↗

arXiv2021

Extreme ultraviolet and X-ray driven photochemistry of gaseous exoplanets

The interaction of exoplanets with their host stars causes a vast diversity in bulk and atmospheric compositions, and physical and chemical conditions. Stellar radiation, especially at the shorter wavelengths, drives the chemistry in the upper atmospheric layers of close orbiting gaseous giants, providing drastic departures from equilibrium. In this study, we aim at unfolding the effects caused by photons in different spectral bands on the atmospheric chemistry, with particular emphasis on the molecular synthesis induced by X-rays. This task is particularly difficult because the characteristics of chemical evolution emerge from many feedbacks on a wide range of time scales, and because of the existing correlations among different portions of the stellar spectrum. The weak X-ray photoabsorption cross-sections of the atmospheric constituents boost the gas ionization to pressures inaccessible to vacuum and extreme ultraviolet photons. Although X-rays interact preferentially with metals, they produce a secondary electron cascade able to ionize efficiently hydrogen and helium bearing species, giving rise to a distinctive chemistry.

astro-ph.EP↗

arXiv2021

Hot Jupiters accreting onto their parent stars: effects on the stellar activity

Hot Jupiters (HJs) are massive gaseous planets orbiting close to their host stars. Due to their physical characteristics and proximity to the central star, HJs are the natural laboratories to study the process of star-planet interaction (SPI). Phenomena related to SPI may include the inflation and the evaporation of planetary atmospheres, the formation of cometary tails and bow shocks and magnetospheric interaction between the magnetic field of the planet and that of the star. Several works suggest that some systems show enhanced stellar activity in phase with the planetary rotation period. In this work, we use a 3D magneto-hydrodynamic model that describes a system composed of a star and an HJ and that includes the corresponding planetary and stellar winds. The aim is to investigate whether the material evaporating from the planet interacts with the stellar extended corona, and generates observable features. Our simulation shows that, in some conditions, the planetary wind expands and propagates mainly along the planetary orbit. Moreover, part of the planetary wind collides with the stellar wind and a fraction of the planet's outflow is funnelled by the stellar magnetic field and hits the stellar surface. In both events, the material is heated up to temperatures of a few MK by a shock. These phenomena could manifest in the form of enhanced stellar activity at some orbital phases of the planet.

astro-ph.EP↗

arXiv2021

Hiding in plain sight: observing planet-starspot crossings with the James Webb Space Telescope

Transiting exoplanets orbiting active stars frequently occult starspots and faculae on the visible stellar disc. Such occultations are often rejected from spectrophotometric transits, as it is assumed they do not contain relevant information for the study of exoplanet atmopsheres. However, they can provide useful constraints to retrieve the temperature of active features and their effect on transmission spectra. We analyse the capabilities of the James Webb Space Telescope in the determination of the spectra of occulted starspots, despite its lack of optical wavelength instruments on board. Focusing on K and M spectral types, we simulate starspots with different temperatures and in different locations of the stellar disc, and find that starspot temperatures can be determined to within a few hundred kelvins using NIRSpec/Prism and the proposed NIRCam/F150W2$+$F322W2's broad wavelength capabilities. Our results are particularly promising in the case of K and M dwarfs of mag$_K \leq 12.5$ with large temperature contrasts.

astro-ph.EP↗