arXiv subjects

Adriana Valio

Publications and source records attributed to Adriana Valio.

35 records · Page 2Linked to original sources

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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arXiv2021

Kepler-411 Differential Rotation from Three Transiting Planets

The differential rotation of the Sun is a crucial ingredient of the dynamo theory responsible for the generation of its magnetic field. Currently, the rotation profile of a star that hosts one or more transiting planets can be estimated. By detecting the same spot in a later transit, it is possible to infer the stellar rotation period at that latitude. In this work, we apply for the first time transit spot mapping to determine the differential rotation of Kepler-411, a K2V-type star with an average rotation period of 10.52 days, radius of 0.79 R$_\odot$ and mass of 0.83 M$_\odot$. Kepler-411 hosts at least four planets, the inner planet is a super-Earth with a radius of 1.88 R$_\oplus$ and an orbital period of 3.0051 days, whereas the two larger transiting planets are mini Neptunes with radii of 3.27 and 3.31 R$_\oplus$, and periods of 7.834435 and 58.0204 days, respectively. Their orbits are such that they transit the star at latitudes of -11$^{\circ}$, -21$^{\circ}$, and -49$^{\circ}$. Analysis of the transit light curves of the three planets resulted in the detection of a total of 198 spots. For each transit latitude, the rotation period of the star was estimated and the differential rotation pattern estimated independently. Then a solar like differential rotation profile was fit to the three rotation periods at the distinct latitudes, the result agreed extremely well with the previous ones, resulting in a differential shear of $0.0500\pm0.0006$ rd/d or a relative differential rotation of $8.4\pm0.1$\%.

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arXiv2020

The Submillimeter Active Region Excess Brightness Temperature during Solar Cycles 23 and 24

We report the temporal evolution of the excess brightness temperature above solar active regions (ARs) observed with the Solar Submillimeter Telescope (SST) at 212 (λ = 1.4 mm) and 405 GHz (λ = 0.7 mm) during Cycles 23 and 24. Comparison with the sunspot number (SSN) yields a Pearson's correlation coefficient R = 0.88 and 0.74 for 212 and 405 GHz, respectively. Moreover, when only Cycle 24 is taken into account the correlation coefficients go to 0.93 and 0.81 for each frequency. We derive the spectral index α between SST frequencies and found a slight anti-correlation with the SSN (R = -0.25); however, since the amplitude of the variation is lower than the standard deviation we cannot draw a definite conclusion. Indeed, α remains almost constant within the uncertainties with a median value approximate to 0 characteristic of an optically thick thermal source. Since the origin of the AR submillimeter radiation is thermal continuum produced at chromospheric heights, the strong correlation between the excess brightness temperature and the magnetic cycle evolution could be related to the available free magnetic energy to be released in reconnection events.

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arXiv2020

Surface and oceanic habitability of Trappist-1 planets under the impact of flares

The discovery of potentially habitable planets around the ultracool dwarf star Trappist-1 naturally poses the question: could Trappist-1 planets be home to life? These planets orbit very close to the host star and are most susceptible to the UV radiation emitted by the intense and frequent flares of Trappist-1. Here we calculate the UV spectra (100 - 450 nm) of a superflare observed on Trappist-1 with the K2 mission. We couple radiative transfer models to this spectra to estimate the UV surface flux on planets in the habitable zone of Trappist-1 (planets $e$, $f$, and $g$), assuming atmospheric scenarios based on a pre-biotic and an oxygenic atmosphere. We quantify the impact of the UV radiation on living organisms on the surface and on a hypothetical planet ocean. Finally, we find that for non-oxygenic planets, UV resistant lifeforms would survive on the surface of planets f and g. Nevertheless, more fragile organisms (i.e. \textit{E. coli}) could be protected from the hazardous UV effects at ocean depths greater than 8m. If the planets have an ozone layer, any lifeforms studied here would survive in the HZ planets.

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arXiv2020

The evolutionary track of H/He envelope in the observed population of sub-Neptunes and Super-Earths planets

The observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a 'gap', is an exciting discovery because of the implications for planet evolutionary history. This gap appears to define a transition region in which sub-Neptune planets are believed to have lost their H/He envelope, potentially by photoevaporation or core powered mass loss, and have thus been transformed into bare cores terrestrial planets. Here we investigate the transition between sub-Neptunes and super-Earths using a real sample of observed small close-in planets and applying envelope evolution models of the H/He envelope together with the mass-radius diagram and a photoevaporation model. We find that photoevaporation can explain the H/He envelope loss of most super-Earths in 100Myr, although an additional loss mechanism appears necessary in some planets. We explore the possibility that these planets families have different core mass and find a continuum in the primordial population of the strongly irradiated super-Earths and the sub-Neptunes. Our analysis also shows that close-orbiting sub-Neptunes with R < 3.5 R_oplus typically lose ~ 30% of their primordial envelope.

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arXiv2020

Planetary transits at radio wavelengths: secondary eclipses of hot Jupiter extended atmospheres

When a planet transits in front of its host star, a fraction of its light is blocked, decreasing the observed flux from the star. The same is expected to occur when observing the stellar radio flux. However, at radio wavelengths, the planet also radiates, depending on its temperature, and thus modifies the transit depths. We explore this scenario simulating the radio lightcurves of transits of hot-Jupiters, Kepler-17b and WASP-12b, around solar-like stars. We calculated the bremsstrahlung radio emission at 17, 100, and 400 GHz originated from the star, considering a solar atmospheric model. The planetary radio emission was calculated modelling the planets in two scenarios: as a blackbody or with a dense and hot extended atmosphere. In both cases the planet radiates and contributes to the total radio flux. For a blackbody planet, the transit depth is in the order of 2-4% and it is independent of the radio frequency. Hot-Jupiters planets with atmospheres appear bigger and brighter in radio, thus having a larger contribution to the total flux of the system. Therefore, the transit depths are larger than in the case of blackbody planets, reaching up to 8% at 17 GHz. Also the transit depth is frequency-dependent. Moreover, the transit caused by the planet passing behind the star is deeper than when the planet transits in front of the star, being as large as 18% at 400GHz. In all cases, the contribution of the planetary radio emission to the observed flux is evident when the planet transits behind the star.

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arXiv2019

The Solar Radius at 37 GHz through Cycles 22 to 24

To better understand the influence of the activity cycle on the solar atmosphere, we report the time variation of the radius observed at 37 GHz ($λ$=8.1 mm) obtained by the Metsähovi Radio Observatory (MRO) through Solar Cycles 22 to 24 (1989-2015). Almost 5800 maps were analyzed, however, due to instrumental setups changes the data set showed four distinct behaviors, which requested a normalisation process to allow the whole interval analysis. When the whole period was considered, the results showed a positive correlation index of 0.17 between the monthly means of the solar radius at 37 GHz and solar flux obtained at 10.7 cm (F10.7). This correlation index increased to 0.44, when only the data obtained during the last period without instrumental changes were considered (1999-2015). The solar radius correlation with the solar cycle agrees with the previous results obtained at mm/cm wavelengths (17 and 48 GHz), nevertheless, this result is the opposite of that reported at submillimetre wavelengths (212 and 405 GHz).

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arXiv2019

Stellar Magnetic Activity and "Butterfly Diagram" of Kepler-63

Context. The study of young solar type stars is fundamental for a better understanding of the magnetic activity of the Sun. As a planet in transit crosses in front of its host star, a darkspot on the stellar surface may be occulted, causing a detectable variation in the light curve. Kepler-63 is a young solar-like star withan age of only 210 Myr that exhibit photometric variations compatible with spot signatures. Since its orbiting planet is in an almostpolar orbit, different latitudes of the star can be probed by the method of spot transit mapping. Methods. A total of 150 transits of Kepler-63b were observed in the short cadence light curve, corresponding to a total duration of about 4 years. Each transit light curve was fit by a model that simulates planetary transits and allows the inclusion of starspots on the surface of the host star. This enables the physical characterisation of the spots size, intensity, and location. We determine the spot position in a reference frame that rotates with the star, and thus obtain the latitudinal distribution of the spots. Results. A total of 297 spots were fit and their sizes, intensities, and positions determined. The latitude distribution of spots exhibits a bimodality with a lack of spots around 34°. The high latitude spots dominate the magnetic cycle of Kepler-63. For a mean stellar rotation period of 5.400d, 59 spots were found at approximately the same longitude and latitude on a later transit. Conclusions. Due to the geometry of the Kepler-63 system, we were able to build a starspot "butterfly diagram". It was also possible to infer Kepler-63 differential rotation from the presence of spots at different latitudes. This star was found to rotate almost rigidly with a period of 5.400d and relative shear close to 0.01% for latitudes less than 34°, whereas the high latitudes do not follow a well behaved pattern.

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arXiv2019

The effect of stellar activity on the spectroscopic stellar parameters of the young solar twin HIP 36515

Spectroscopic equilibrium allows us to obtain precise stellar parameters in Sun-like stars. It relies on the assumption of the iron excitation and ionization equilibrium. However, several works suggest that magnetic activity may affect chemical abundances of young active stars, calling into question the validity of this widely-used method. We have tested for the first time variations in stellar parameters and chemical abundances for the young solar twin HIP 36515 ($\sim$0.4 Gyr), along its activity cycle. This star has stellar parameters very well established in the literature and we estimated its activity cycle in $\sim$6 years. Using HARPS spectra with high resolving power (115 000) and signal-to-noise ratio ($\sim$270), the stellar parameters of six different epochs in the cycle were estimated. We found that the stellar activity is strongly correlated with the effective temperature, metallicity, and microturbulence velocity. The possibility of changes in the Li I 6707.8 Åline due to flares and star spots was also investigated. Although the core of the line profile shows some variations with the stellar cycle, it is compensated by changes in the effective temperature, resulting in a non variation of the Li abundance.

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arXiv2018

Solar polar brightening and radius at 100 and 230 GHz observed by ALMA

Polar brightening of the Sun at radio frequencies has been studied for almost fifty years and yet a disagreement persists between solar atmospheric models and observations. Some observations reported brightening values much smaller than the expected values obtained from the models, with discrepancies being particularly large at millimeter wavelengths. New clues to calibrate the atmospheric models can be obtained with the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we analyzed the lower limit of the polar brightening observed at 100 and 230 GHz by ALMA, during its Science Verification period, 2015 December 16-20. We find that the average polar intensity is higher than the disk intensity at 100 and 230 GHz, with larger brightness intensities at the South pole in eight of the nine maps analyzed. The observational results were compared with calculations of the millimetric limb brightnening emission for two semi-empirical atmospheric models, FAL- C (Fontenla et al. 1993) and SSC (Selhorst et al. 2005a). Both models presented larger limb intensities than the averaged observed values. The intensities obtained with the SSC model were closer to the observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz, respectively. This discrepancy may be due to the presence of chromospheric features (like spicules) at regions close to the limb.

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arXiv2018

Superflare UV flashes impact on Kepler-96 system: a glimpse of habitability when the ozone layer first formed on Earth

Kepler-96 is an active solar-type star harbouring a Super-Earth planet in close orbit. Its age of 2.3 Gyr is the same as the Sun when there was a considerable increase of oxygen in Earth's atmosphere due to micro-organisms living in the ocean. We present the analysis of superflares seen on the transit lightcurves of Kepler-96b. The model used here simulates the planetary transit in a flaring star. By fitting the observational data with this model, it is possible to infer the physical properties of the flares, such as their duration and the energy released. We found 3 flares within the energy range of superflares, where the biggest superflare observed was found to have an energy of 1.81$\times$10$^{35}$ ergs. The goal is to analyse the biological impact of these superflares on a hypothetical Earth in the habitable zone of Kepler-96 assuming this planet has protection via different scenarios: an Archean and Present-day atmospheres. Also, we compute the attenuation of the flare UV radiation through an Archean ocean. The conclusion is that considering the increase in the UV flux by the strongest superflare emission, {\it E. Coli} and {\it D. Radiodurans} could survive on the surface of the planet only if there was an ozone layer present on the planet atmosphere. However, they could escape from the hazardous UV effects at a depth of 28m and 12m below the ocean surface, respectively. For smaller superflares contribution, {\it D. Radiodurans} could survive in the surface even in an Archean atmosphere with no ozone.

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arXiv2017

Solar Radius at Subterahertz Frequencies and its Relation to Solar Activity

The Sun emits radiation at several wavelengths of the electromagnetic spectrum. In the optical band, the solar radius is 695,700 km and this is what defines the photosphere, the visible surface of the Sun. However, as the altitude increases, the electromagnetic radiation is produced at other frequencies, causing the solar radius to change as function of wavelength. These measurements enable a better understanding of the solar atmosphere and the radius dependence on the solar cycle is a good indicator of the changes that occur in the atmospheric structure. We measure the solar radius at the subterahertz frequencies of 0.212 and 0.405 THz -- i.e., the altitude where these emissions are primarily generated -- and also analyse the radius variation over the 11-year solar activity cycle. For this, we used radio maps of the solar disk for the period between 1999 and 2017, reconstructed from daily scans made by the Solar Submillimeter-wave Telescope (SST), installed at El Leoncito Astronomical Complex (CASLEO) in the Argentinean Andes. Our measurements yield radii of 966.5" $\pm$ 2.8" for 0.2 THz and 966.5" $\pm$ 2.7" for 0.4 THz. This implies a height of $(5.0 \pm 2.0 \times 10^6)$ m above the photosphere. Furthermore, we also observed strong anti-correlation between the radius variation and the solar activity at both frequencies.

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arXiv2017

Association of radio polar cap brightening with bright patches and coronal holes

Radio-bright regions near the solar poles are frequently observed in Nobeyama Radioheliograph (NoRH) maps at 17 GHz, and often in association with coronal holes. However, the origin of these polar brightening has not been established yet. We propose that small magnetic loops are the source of these bright patches, and present modeling results that reproduce the main observational characteristics of the polar brightening within coronal holes at 17 GHz. The simulations were carried out by calculating the radio emission of the small loops, with several temperature and density profiles, within a 2D coronal hole atmospheric model. If located at high latitudes, the size of the simulated bright patches are much smaller than the beam size and they present the instrument beam size when observed. The larger bright patches can be generated by a great number of small magnetic loops unresolved by the NoRH beam. Loop models that reproduce bright patches contain denser and hotter plasma near the upper chromosphere and lower corona. On the other hand, loops with increased plasma density and temperature only in the corona do not contribute to the emission at 17 GHz. This could explain the absence of a one-to-one association between the 17 GHz bright patches and those observed in extreme ultraviolet. Moreover, the emission arising from small magnetic loops located close to the limb may merge with the usual limb brightening profile, increasing its brightness temperature and width.

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arXiv2017

Using planetary transits to estimate magnetic cycles lengths in Kepler stars

Observations of various solar-type stars along decades revealed that they can have magnetic cycles, just like our Sun. An investigation of the relation between their cycle length and rotation period can shed light on the dynamo mechanisms operating in these stars. Previous works on this relation suggested that the stars could be separated into active and inactive branches, with the Sun falling between them. In this work, we determined short magnetic activity cycles for 6 active solar-type stars observed by the Kepler telescope. The method adopted here estimates the activity from the excess in the residuals of the transitlight curves. This excess is obtained by subtracting a spotless model transit from the light curve, and then integrating over all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of the complete time series. Finally, we investigate the rotation-cycle period relation for the stars analysed here and find that some active stars do not follow the behaviour proposed earlier, falling in the inactive branch. In addition, we also notice a considerable spread from other stars in the literature in the active/inactive branches.

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arXiv2017

The influence of eclipses in the stellar radio emission

Here we simulate the shape of a planetary transit observed at radio wavelengths. The simulations use a light curve of the K4 star HAT-P-11 and its hot Jupiter companion as proxy. From the HAT-P-11 optical light curve, a prominent spot was identified (1.10 Rp and 0.6 Ic). On the radio regime, the limb brighting of 30% was simulated by a quadratic function, and the active region was assumed to have the same size of the optical spot. Considering that the planet size is 6.35% of the the stellar radius, for the quiet star regions the transit depth is smaller than 0.5%, however, this value can increase to ~2% when covering an active region with 5.0 times the quiet star brightness temperature.

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arXiv2016

Stellar magnetic cycles in the solar-like stars Kepler-17 and Kepler-63

The stellar magnetic field plays a crucial role in the star internal mechanisms, as in the interactions with its environment. The study of starspots provides information about the stellar magnetic field, and can characterise the cycle. Moreover, the analysis of solar-type stars is also useful to shed light onto the origin of the solar magnetic field. The objective of this work is to characterise the magnetic activity of stars. Here, we studied two solar-type stars Kepler-17 and Kepler-63 using two methods to estimate the magnetic cycle length. The first one characterises the spots (radius, intensity, and location) by fitting the small variations in the light curve of a star caused by the occultation of a spot during a planetary transit. This approach yields the number of spots present in the stellar surface and the flux deficit subtracted from the star by their presence during each transit. The second method estimates the activity from the excess in the residuals of the transit lightcurves. This excess is obtained by subtracting a spotless model transit from the lightcurve, and then integrating all the residuals during the transit. The presence of long term periodicity is estimated in both time series. With the first method, we obtained $P_{\rm cycle}$ = 1.12 $\pm$ 0.16 yr (Kepler-17) and $P_{\rm cycle}$ = 1.27 $\pm$ 0.16 yr (Kepler-63), and for the second approach the values are 1.35 $\pm$ 0.27 yr and 1.27 $\pm$ 0.12 yr, respectively. The results of both methods agree with each other and confirm their robustness.

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arXiv2011

Transit Model of Planets with Moon and Ring System

Since the discovery of the first exoplanets, those most adequate for life to begin and evolve have been sought. Due to observational bias, however, most of the discovered planets so far are gas giants, precluding their habitability. However, if these hot Jupiters are located in the habitable zones of their host stars, and if rocky moons orbit them, then these moons may be habitable. In this work, we present a model for planetary transit simulation considering the presence of moons and planetary rings around a planet. The moon's orbit is considered to be circular and coplanar with the planetary orbit. The other physical and orbital parameters of the star, planet, moon, and rings can be adjusted in each simulation. It is possible to simulate as many successive transits as desired. Since the presence of spots on the surface of the star may produce a signal similar to that of the presence of a moon, our model also allows for the inclusion of starspots. The result of the simulation is a light curve with a planetary transit. White noise may also be added to the light curves to produce curves similar to those obtained by the CoRoT and Kepler space telescopes. The goal is to determine the criteria for detectability of moons and/or ring systems using photometry. The results show that it is possible to detect moons with radii as little as 1.3 R$_{\oplus}$ with CoRoT and 0.3 R_{\oplus}$ with Kepler.

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