arXiv subjects

Adriana Valio

Publications and source records attributed to Adriana Valio.

At least 19 recordsLinked to original sources

arXiv2026

Bimodality in Rotational Modulation of Planet-Hosting Stars

Stellar magnetic activity is governed by the interplay between rotation, convection, and the evolution of surface magnetic structures, yet the role of planetary systems in shaping these processes remains uncertain. Here, we analyze \textit{Kepler} photometry of more than 1,300 stars to investigate rotational modulation in stars with and without confirmed exoplanets. Using a time--frequency analysis, we measure the photometric proxy of rotational modulation dispersion, $S_{\rm phot}$, tracing the temporal coherence of surface magnetic features. Stars hosting confirmed exoplanets exhibit systematically enhanced $S_{\rm phot}$ values compared to stars without detected planets ($\Delta S_{\rm phot}=0.17 \pm 0.01$ rad d$^{-1}$; $p<10^{-25}$). More importantly, the $S_{\rm phot}$ distribution of planet hosts is bimodal, with peaks at $0.12$ and $0.44$ rad d$^{-1}$ (Hartigan's Dip Test $p<10^{-6}$; $\Delta \mathrm{BIC}=188.7$), a feature absent in the control sample. We interpret $S_{\rm phot}$ as a proxy for rotational modulation dispersion, reflecting spot evolution rather than true differential rotation. The two regimes correspond to stable magnetic coherence with long-lived active regions and rapidly evolving activity patterns. The presence of these distinct regimes exclusively among planet-hosting stars suggests that planetary systems may influence the temporal organization of stellar magnetic activity and indirectly affect stellar dynamos.

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arXiv2026

Where does the simplified Stellar Contamination Model fail in Exoplanet Transmission Spectroscopy?

Stellar photospheric heterogeneities (e.g., starspots, faculae) distort the stellar spectrum in transit and imprint wavelength-dependent biases on the planet-to-star radius ratio (Transit Light Source Effect, TLSE). The Rackham-TLSE (R-TLSE) prescription applies a disc-averaged correction based solely on filling factor and spectral contrast, but transmission spectroscopy also depends on limb darkening, active-region distribution, and transit geometry. We include these in a pixel-resolved framework, ECLIPSE-Xlambda, and run idealised noise-free model-model comparisons to R-TLSE. For LHS 1140 b, K2-18 b, and WASP-69 b, disc-averaged corrections differ from the pixel model by up to about 400 ppm in the optical for active hosts and non-equatorial transits, but stay below about 10 ppm in the near-infrared where limb darkening is weak. We then apply both approaches to the JWST/NIRISS SOSS spectrum of LHS 1140 b. With limb darkening set to zero, ECLIPSE-Xlambda recovers stellar-contamination parameters matching the reference R-TLSE solution, confirming consistency in the disc-averaged limit. With wavelength-dependent limb darkening, reproducing the short-wavelength slope via stellar contamination alone requires hot faculae (delta Tfac about 600 K; ffac about 0.35), equivalent to a circular facular region of radius about 0.6 Rstar (about 60% of the stellar radius) on the disc; such an extended unocculted region is physically unlikely even for an active M dwarf. Purely stellar contamination would therefore require extreme faculae, whereas a genuine atmospheric contribution complementing a more modest facular signal is more plausible. These results delineate the validity regime of R-TLSE and underscore the need for geometry-aware stellar-heterogeneity models including limb darkening in high-precision transmission spectroscopy.

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arXiv2025

Evaluating Habitability and Biosignature Detection on TOI-700 d: The Role of UV Environment and Atmospheric Pressure

M dwarfs have long been prime targets in the search for habitable exoplanets, owing to their abundance in the galaxy and the relative ease of detecting Earth-sized worlds within their narrower habitable zones. Yet, these low-mass stars can emit high-energy radiation that may gradually erode planetary atmospheres, raising concerns about long-term habitability. TOI-700, a relatively quiescent M dwarf that hosts four known planets, stands out due to its Earth-sized TOI-700 d in the star's habitable zone. Here, we assess whether a habitable environment can be sustained on TOI-700 d by analyzing different UV flux levels and atmospheric pressures. We focus on two atmospheric scenarios - one analogous to the Archean Earth and another representing a modern Earth-like environment - using a 1D photochemistry-climate model. Our results indicate that all simulated cases can maintain temperatures compatible with liquid water on the surface. However, the dominant photochemical pathways differ substantially with UV levels: under low-UV conditions, haze formation in the Archean-like atmosphere provides the main UV shielding, whereas under intensified UV, ozone production in the modern-like atmospheres can protect the surface from harmful doses. Interestingly, although haze can impede the detection of certain biosignatures, such as CH4, CO2 and O2, it also enhances the overall atmospheric signal by increasing scattering and transit depth, potentially aiding in revealing the presence of an atmosphere. These findings underscore the dual role of hazes as both a challenge for biosignature detection and a potential protection of surface habitability.

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arXiv2025

Starspot temperature of CoRoT-2 from multiwavelength observations with SPARC4

Measuring starspot temperatures is crucial for understanding stellar magnetic activity, as it affects stellar brightness variations, influences exoplanet transit measurements, and provides constraints on the physical conditions and energy transport in active regions, offering insights into stellar dynamos. Our goal is to determine the temperature of starspots on the active star CoRoT-2 to enhance our understanding of magnetic activity in young, solar-like stars. Multiwavelength observations were conducted using the SPARC4 instrument on the 1.6-m telescope at Pico dos Dias Observatory (Brazil), capturing simultaneous transit data in four photometric bands (g, r, i, and z). The ECLIPSE model, combined with MCMC fitting, was used to model spot characteristics during the planetary transit of CoRoT-2 b. The spot intensities were analyzed considering three different methods: the assumption of blackbody emission, the PHOENIX atmospheric model, and multiwavelength fitting assuming the same spot parameters for all wavelengths. Two starspots were detected in the residuals of the light curve, yielding temperature estimates of 5040 - 5280 K based on the three different methods. These values align more closely with the temperatures of solar penumbrae than with typical umbral temperatures, suggesting relatively moderate magnetic activity. The radius of the spots ranged from 0.34 - 0.61 the planetary radius, or equivalently (38 - 69)$\times10^6$m, much larger than sunspots. This study provides a method to estimate spot temperatures on active stars using multiband photometry, with results indicating penumbral-like temperatures on CoRoT-2. The methodology enhances precision in starspot temperature estimation, beneficial for studies of stellar activity and exoplanet characterization.

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arXiv2024

TOI-1173 A $b$: The First Inflated Super-Neptune in a Wide Binary System

Among Neptunian mass exoplanets ($20-50$ M$_\oplus$), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density ($ρ< 0.3$~g~cm$^{-3}$). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107~$b$, TOI-1420~$b$, and WASP-193 $b$. In this paper, we report the discovery of TOI-1173 A $b$, a low-density ($ρ= 0.195_{-0.017}^{+0.018}$~g~cm$^{-3}$) super-Neptune with $P = 7.06$ days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X and HIRES spectrographs and transit photometry from TESS, we determined a planet mass of $M_{\rm{p}} = 27.4\pm1.7\ M_{\oplus}$ and radius of $R_{\rm{p}} = 9.19\pm0.18\ R_{\oplus}$. TOI-1173 A $b$ is the first puffy Super-Neptune planet detected in a wide binary system (projected separation $\sim 11,400$~AU). We explored several mechanisms to understand the puffy nature of TOI-1173 A $b$, and showed that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 A $b$ likely has maintained its orbital stability over time and may have undergone von-Zeipel-Lidov-Kozai migration followed by tidal circularization given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 A $b$ will shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin-orbit analyses may elucidate the dynamical evolution of the system.

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arXiv2023

Impact of M-dwarf Stellar Wind and Photoevaporation on the Atmospheric Evolution of Small Planets

The evolution of a planet's atmosphere depends strongly on its host star's properties. When their host stars are younger, planets can experience stronger winds and EUV emissions. This is particularly true for planets orbiting M-dwarfs due to their close proximity to the host star. To determine if these planets retain an atmosphere, we consider the impacts from stellar wind and EUV fluxes in driving atmospheric escape throughout the planet's lifetime. For this, we determined the atmospheric mass loss due to stellar wind and photoevaporation on 4 planets in close orbit and 34 in their star's habitable zone (HZ). The M-dwarf host stars' wind velocity, density, and EUV flux were calculated through rotation period and X-ray flux scaling over time. The mass loss rate due to stellar wind and photoevaporation was then computed as a function of time and accumulated throughout the planet's age to determine the total atmospheric mass loss of the planet's initial H/He envelope. We find that for HZ planets at orbits $<$ 0.1 AU, stellar wind can only remove $\leq 1\%$ of the H/He envelope, while photoevaporation is essential for completely removing the H/He envelope of most targets. Moreover, due to either mechanism, most planets orbiting at $>$ 0.1 AU do not have their primordial envelope stripped. Overall, out of the 38 planets studied, 13 were predicted to have lost the primordial envelope due to photoevaporation, while 2 planets lost the envelope due to both stellar wind and photoevaporation.

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arXiv2023

Automatic detection of solar flares observed at 45 GHz by the POEMAS telescope

Every 11 years, the Sun goes through periods of activity, with the occurrence of many solar flares and mass ejections, both energetic phenomena of magnetic origin. Due to its effects on Earth, the study of solar activity is of paramount importance. POEMAS (Polarization of Millimeter Emission of Solar Activity) is a system of two telescopes, installed at CASLEO (El Leoncito Astronomical Complex) in Argentina, which monitors the Sun at two millimeter wavelengths (corresponding frequencies of 45 and 90 GHz). The objective of this work is to automatically detect solar flares observed by the polarimeter. First it is necessary to eliminate the background noise, caused mainly by instrumental problems, from the light curves of millimeter solar emission. The methodology used to exclude the noise proposed in this work is to use the tendency of time series. The subtraction of this model from the light curves provides the input to automate the detection of solar flares using artificial intelligence techniques. A Neural Network was trained to recognize patterns and analyze a dataset in order to identify solar flares. Previously, a total of 30 flares had been visually identified and analyzed in the POEMAS database between 2011/11/22 and 2013/12/10. The methodology presented here confirmed 87% of these events, moreover the neural network was able to identify at least 9 new events. As the neural network was trained to detect impulsive events (lasting less than 5 min), long duration bursts were not automatically detected, nor were they detected visually due to the background noise of the telescope. Visual inspection of the POEMAS data, when comparing with microwave data from the RSTN, allowed the identification of an additional 10 long-duration solar flares at 45 GHz. We discuss some problems encountered and possible solutions for future work.

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arXiv2023

Statistical analysis of the onset temperature of solar flares in 2010-2011

Understanding the physical processes that trigger solar flares is paramount to help with forecasting space weather and mitigating the effects on our technological infrastructure. A previously unknown phenomenon was recently identified in solar flares: the plasma temperature, derived from soft X-ray (SXR) data, at the onset of four flares, was revealed to be in the range 10-15 MK, without evidence of gradual heating. To investigate how common the hot-onset phenomenon may be, we extend this investigation to solar flares of B1.2- X6.9 classes recorded by the X-ray Sensor (XRS) on-board the GOES-14 and GOES-15 satellites between 2010 and 2011. For this statistical study, we employed the same methodology as in recent work, where the pre-flare SXR flux of each flare is obtained manually, and the temperature and emission measure values are obtained by the flux ratio of the two GOES/XRS channels using the standard software. From 3224 events listed in the GOES flare catalog for 2010-2011, we have selected and analyzed 745 events for which the flare heliographic location was provided in the list, to investigate center-to-limb effects of the hot-onset phenomenon. Our results show that 559 out of 745 flares (75%) exhibit an onset temperature above 8.6 MK (the first quartile), with respective log10 of the emission measure values between 46.0 - 47.25 cm-3, indicating that small amounts of plasma are quickly heated to high temperatures. These results suggest that the hot-onset phenomenon is very common in solar flares.

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arXiv2023

Solar Submillimeter Telescope next generation

The Solar Submillimeter Telescope (SST) is an unique instrument that has been observing the Sun daily since 2001 bringing a wealth of information and raising new questions about the particle acceleration and transport, and emission mechanisms during flares. We are now designing its successor, the SSTng, that will expand the scientific goals of the instrument, including non-solar source observations.

astro-ph.IM↗

arXiv2023

Trajectories of Coronal Mass Ejection from Solar-type Stars

The Sun and other solar-type stars have magnetic fields that permeate their interior and surface, extends through the interplanetary medium, and is the main driver of stellar activity. Stellar magnetic activity affects physical processes and conditions of the interplanetary medium and orbiting planets. Coronal mass ejections (CMEs) are the most impacting of these phenomena in near-Earth space weather, and consist of plasma clouds, with magnetic field, ejected from the solar corona. Precisely predicting the trajectory of CMEs is crucial in determining whether a CME will hit a planet and impact its magnetosphere and atmosphere. Despite the rapid developments in the search for stellar CMEs, their detection is still very incipient. In this work we aim to better understand the propagation of CMEs by analysing the influence of initial parameters on CME trajectories, such as position, velocities, and stellar magnetic field's configuration. We reconstruct magnetograms for Kepler-63 (KIC 11554435) and Kepler-411 (KIC 11551692) from spot transit mapping, and use a CME deflection model, ForeCAT, to simulate trajectories of hypothetical CMEs launched into the interplanetary medium from Kepler-63 and Kepler-411. We apply the same methodology to the Sun, for comparison. Our results show that in general, deflections and rotations of CMEs decrease with their radial velocity, and increase with ejection latitude. Moreover, magnetic fields stronger than the Sun's, such as Kepler-63's, tend to cause greater CME deflections.

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arXiv2023

The connection between starspots and superflares: a case study of two stars

How do the characteristics of starspots influence the triggering of stellar flares? Here we investigate the activity of two K-type stars, similar in every way from mass to rotation periods and planetary systems. Both stars exhibit about a hundred spots, however, Kepler-411 produced 65 superflares, while Kepler-210 presented none. The spots of both stars were characterized using the planetary transit mapping technique, which yields the intensity, temperature, and radius of starspots. The average radius was $(17\pm7) \times 10^3$ km and $(58 \pm 23) \times 10^3$ km, while the intensity ratio with respect to the photosphere was $(0.35\pm0.24)$ $I_{c}$ and $(0.64\pm0.15)$ $I_{c}$, and the temperature was $(3800 \pm 700)$ K and $(4180 \pm 240)$ K for spots of Kepler-411 and Kepler-210, respectively. Therefore, spots on the star with no superflares, Kepler-210, are mostly larger, less dark, and warmer than those on the flaring star, Kepler-411. This may indicate magnetic fields with smaller magnitude and complexity of the spots on Kepler-210 when compared to those on Kepler-411. Thus, starspot area appears not to be the main culprit of superflares triggering. Perhaps the magnetic complexity of active regions is more important.

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arXiv2023

The Effect of Stellar Contamination on Low-resolution Transmission Spectroscopy: Needs Identified by NASA's Exoplanet Exploration Program Study Analysis Group 21

Study Analysis Group 21 (SAG21) of NASA's Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, plages, granules, and flares. This SAG brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current research needs that can be addressed in this context to make the most of transit studies from current NASA facilities like HST and JWST. The analysis produced 14 findings, which fall into three Science Themes encompassing (1) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities ("The Sun as the Stellar Benchmark"), (2) how stars other than the Sun extend our knowledge of heterogeneities ("Surface Heterogeneities of Other Stars") and (3) how to incorporate information gathered for the Sun and other stars into transit studies ("Mapping Stellar Knowledge to Transit Studies"). In this invited review, we largely reproduce the final report of SAG21 as a contribution to the peer-reviewed literature.

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arXiv2022

A Genetic Algorithm to model Solar Radio Active Regions from 3D Magnetic Field Extrapolations

In recent decades our understanding of solar active regions (ARs) has improved substantially due to observations made with better angular resolution and wider spectral coverage. While prior AR observations have shown that these structures were always brighter than the quiet Sun at centimeter wavelengths, recent observations at millimeter and submillimeter wavelengths have shown ARs with well defined dark umbrae. Given this new information, it is now necessary to update our understanding and models of the solar atmosphere in active regions. In this work, we present a data-constrained model of the AR solar atmosphere, in which we use brightness temperature measurements of NOAA 12470 at three radio frequencies: 17 (NoRH), 100 and 230 GHz (ALMA). Based on our model, which assumes that the radio emission originates from thermal free-free and gyroresonance processes, we calculate radio brightness temperature maps that can be compared with the observations. The magnetic field at distinct atmospheric heights was determined in our modelling process by force-free field extrapolation using photospheric magnetograms taken by HMI/SDO. In order to determine the best plasma temperature and density height profiles necessary to match the observations, the model uses a genetic algorithm that modifies a standard quiet Sun atmospheric model. Our results show that the height of the transition region (TR) of the modelled atmosphere varies with the type of region being modelled: for umbrae the TR is located at 1080 +/- 20 km above the solar surface; for penumbrae, the TR is located at 1800 +/- 50 km; and for bright regions outside sunspots, the TR is located at 2000 +/- 100 km. With these results, we find good agreement with the observed AR brightness temperature maps. Our modelled AR can be used to estimate the emission at frequencies without observational coverage.

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arXiv2021

Subterahertz Radius and Limb Brightening of the Sun Derived from SST and ALMA

Measurements of the radius and limb brightening of the Sun provide important information about the solar atmosphere structure and temperature. The solar radius increases as the observation at radio frequency decreases, indicating that each emission originates higher in the atmosphere. Thus, different layers of the solar atmosphere can be probed by observing at multiple wavelengths. In this work, we determined the average radius and limb brightening at 100, 212, 230, and 405 GHz, using data from the Solar Submillimeter Telescope and ALMA's single-dish observations. For the first time, limb brightening values for frequencies of 212 and 405 GHz were estimated. At sub-THz frequencies, the observed limb brightening may affect the solar radius measurements. We use two different and well known approaches to determine the radius: the half-power method and the inflection-point method. We investigate how the antenna beam size and the limb brightening level, LB, can affect the radius measurements using both methods. Our results showed that the inflection-point method is the least affected by these parameters, and should thus be used for solar radius estimates at radio wavelengths. The measured average radii are 968"~$\pm$~3" (100 GHz), 963"~$\pm$~3" (212 GHz), 963"~$\pm$~2" (230 GHz), and 963"~$\pm$~5" (405 GHz). Finally, we used forward modeling to estimate the ranges of LB of the solar disk resulting in 5%-19% (100 GHz), 2%-12% (212 GHz), 6%-18% (230 GHz), and 3%-17% (405 GHz). Both radius and limb brightening estimates agree with previous measurements reported in the literature.

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arXiv2021

Kepler-411 star activity: connection between starspots and superflares

Stellar magnetic activity, just like that of the Sun, manifests itself in the form of flares and spots on the surface of the star. In the solar case, the largest flares originate from large active regions. In this work, we present a study of the activity of the star Kepler-411, including spot modeling from planetary transits. Our goal was to search for a connection between the area of starspots with the energy of superflares produced by this star. Kepler-411 is a K2V-type star with an average rotation period of 10.52 days, radius of 0.79 $R_{\odot}$ and a mass of 0.83 $M_{\odot} $, which was observed by the Kepler satellite for about 600 days. Transit mapping allowed for the characterization of 198 starspots with estimates of their radius and temperature. Kepler-411 starspots had an average radius of $(17 \pm 7)\times 10^3$ km and a mean temperature of $3800 \pm 700$ K. Visual inspection of the light curves of Kepler-411 yields the identification of 65 superflares. The detected superflares lasted from 8 to 260 min and their energy varied from $10^{33} - 10^{35}$ ergs. The power-law index of the flare frequency distribution as a function of energy is (-2.04 $\pm$ 0.13) for the flare on Kepler-411. A positive correlation between the area of starspots and the energy of superflares was found when considering the averages taken every 16 to 35 days, with the highest correlation occurring for averages every 21 days. This timing is probably related to the lifetime of the Kepler-411 spots.

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arXiv2021

Detection of Aerosols at Microbar Pressures in an Exoplanet Atmosphere

Formation of hazes at microbar pressures has been explored by theoretical models of exoplanet atmospheres to explain Rayleigh scattering and/or featureless transmission spectra, however observational evidence of aerosols in the low pressure formation environments has proved elusive. Here, we show direct evidence of aerosols existing at $\sim$1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) instruments on the Hubble Space Telescope. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering that covers 11 scale heights of spectral modulation. Drawing on the extensive studies of haze in our Solar System, we model the transmission spectrum based on a scaled version of Jupiter's haze density profile to show that WASP-69b transmission spectrum can be produced by scattering from an approximately constant density of particles extending throughout the atmospheric column from 40 millibar to microbar pressures. These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres.

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arXiv2021

Detection of an Atmosphere on a Rocky Exoplanet

We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH$_{4}$ in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H$_{2}$ mantle degassing, previously identified as a possibility for this planet by theoretical studies, and find that outgassing from ultrareduced magma could produce the observed atmosphere. In this way we use the observed exoplanet transmission spectrum to gain insights into magma composition for a terrestrial planet. The detection of an atmosphere on this rocky planet raises the possibility that the numerous powerfully irradiated Super-Earth planets, believed to be the evaporated cores of Sub-Neptunes, may, under favorable circumstances, host detectable atmospheres.

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arXiv2021

The subterahertz solar cycle: Polar and equatorial radii derived from SST and ALMA

At subterahertz frequencies -- i.e., millimeter and submillimeter wavelengths -- there is a gap of measurements of the solar radius as well as other parameters of the solar atmosphere. As the observational wavelength changes, the radius varies because the altitude of the dominant electromagnetic radiation is produced at different heights in the solar atmosphere. Moreover, radius variations throughout long time series are indicative of changes in the solar atmosphere that may be related to the solar cycle. Therefore, the solar radius is an important parameter for the calibration of solar atmospheric models enabling a better understanding of the atmospheric structure. In this work we use data from the Solar Submillimeter-wave Telescope (SST) and from the Atacama Large Millimeter/submillimeter Array (ALMA), at the frequencies of 100, 212, 230, and 405 GHz, to measure the equatorial and polar radii of the Sun. The radii measured with extensive data from the SST agree with the radius-vs-frequency trend present in the literature. The radii derived from ALMA maps at 230 GHz also agree with the radius-vs-frequency trend, whereas the 100-GHz radii are slightly above the values reported by other authors. In addition, we analyze the equatorial and polar radius behavior over the years, by determining the correlation coefficient between solar activity and subterahertz radii time series at 212 and 405 GHz (SST). The variation of the SST-derived radii over 13 years are correlated to the solar activity when considering equatorial regions of the solar atmosphere, and anticorrelated when considering polar regions. The ALMA derived radii time series for 100 and 230 GHz show very similar behaviors with those of SST.

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