arXiv subjects

A. Buccino

Publications and source records attributed to A. Buccino.

17 recordsLinked to original sources

arXiv2026

Magnetic activity in cool stars: manifestations and relevance to exoplanets

Understanding stellar magnetic activity is central to exoplanet science in two ways: it sets the dynamic astrospheric boundary condition governing planetary space environments, and it is the primary obstacle to exoplanet detection and characterisation, since magnetically driven variability imprints correlated quasi-periodic signals across detection time series. In cool stars, MHD-dynamo-generated fields emerge at the photosphere as bipolar regions, drive chromospheric and coronal heating, modulate irradiance and wind, and power flares and coronal mass ejections. Spatial scales range from individual flux tubes to global coronal configurations, and temporal scales from minutes to decades and beyond, requiring observational and theoretical tools of correspondingly wide scope. We review observational manifestations and physical models of magnetic activity in stars with outer convective envelopes, addressed to the exoplanet community. We develop the solar-stellar connection through the 'Sun in Time' framework and a sequence of solar analogues serving as evolutionary snapshots of a solar-mass star over several Gyr. We survey photospheric, chromospheric, and coronal activity diagnostics across timescales, together with forward-modelling tools translating surface field distributions into signals at or above the level of exoplanet detection. Empirical rotation-activity relationships and their physical interpretation are examined across all three atmospheric layers. Surface reconstruction techniques are assessed for their diagnostic reach and limitations. The evolution of magnetism in solar-like stars is discussed as context for habitability and as a window to other worlds. We close with an account of how stellar magnetism sculpts the astrospheric environment and affects close-in exoplanets, followed by a synthesis of outstanding issues and an outlook on future prospects.

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arXiv2026

Star-Planet Interactions: Observational Techniques and Methods

This chapter summarizes the techniques and methods used to study star-planet interaction (SPI) from the observational point of view. SPI can produce a wide range of observational signatures, from localized stellar activity enhancements to changes in planetary atmospheric escape and transmission spectra. This chapter reviews the main observational techniques used to detect and characterize SPI, emphasizing the methodological challenges involved in separating planet-induced signals from intrinsic stellar variability. We discuss radial-velocity diagnostics, including cross-correlation, template matching, line-by-line methods, and activity indicators, highlighting their sensitivity to line-profile distortions and chromatic variability. We then review precision photometry as a tool to search for orbit-locked variability, flare modulation, and active-region occultations during planetary transits. Chromospheric diagnostics, including Ca II, Halpha, He I, and Na II lines, are presented as tracers of magnetic variability at different atmospheric heights and as potential probes of intermittent SPI signatures. We also discuss transmission spectroscopy as a complementary approach, since planetary atmospheric tracers such as H I Lyalpha, Balmer lines, C II, and He I can encode information about the stellar high-energy environment, stellar wind, and magnetic coupling. In addition, radio observations provide a promising avenue to probe magnetic SPI directly through coherent emission mechanisms. Finally, we examine time-series analysis techniques commonly employed in SPI searches, including generalized Lomb-Scargle periodograms, rolling periodograms, harmonic analyses, and bootstrap-based significance estimation. (abridged)

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arXiv2020

Three generations of stars: a possible case of triggered star formation

Evidence for triggered star formation linking three generations of stars is difficult to assemble, as it requires convincingly associating evolved massive stars with Hii regions that, in turn, would need to present signs of active star formation. We present observational evidence for triggered star formation relating three generations of stars in the neighbourhood of the star LS II +26 8. We carried out new spectroscopic observations of LS II +26 8, revealing that it is a B0 III-type star. We note that LS II +26 8 is located exactly at the geometric centre of a semi-shell-like Hii region complex. The most conspicuous component of this complex is the Hii region Sh2-90, which is probably triggering a new generation of stars. The distances to LS II +26 8 and to Sh2-90 are in agreement (between 2.6 and 3 kpc). Analysis of the interstellar medium on a larger spatial scale shows that Hii region complex lies on the northwestern border of an extended H2 shell. The radius of this molecular shell is about 13 pc, which is in agreement with what an O9V star (the probable initial spectral type of LS II +26 8 as inferred from evolutive tracks) can generate through its winds in the molecular environment. In conclusion, the spatial and temporal correspondences derived in our analysis enable us to propose a probable triggered star formation scenario initiated by the evolved massive star LS II +26 8 during its main sequence stage, followed by stars exciting the Hii region complex formed in the molecular shell, and culminating in the birth of YSOs around Sh2-90.

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arXiv2019

Gemini-GRACES high-quality spectra of Kepler evolved stars with transiting planets I. Detailed characterization of multi-planet systems Kepler-278 and Kepler-391

(abridged) Kepler-278 and Kepler-391 are two of the three evolved stars known to date on the RGB to host multiple short-period transiting planets. Moreover, these planets are among the smallest discovered around RGB stars. Here we present a detailed stellar and planetary characterization of these remarkable systems. Based on high-quality spectra from Gemini-GRACES for Kepler-278 and Kepler-391, we obtained refined stellar parameters and precise chemical abundances for 25 elements. Also, combining our new stellar parameters with a photodynamical analysis of the Kepler light curves, we determined accurate planetary properties of both systems. The precise spectroscopic parameters of Kepler-278 and Kepler-391, along with their high $^{12}\mathrm{C}/^{13}\mathrm{C}$ ratios, show that both stars are just starting their ascent on the RGB. The planets Kepler-278b, Kepler-278c, and Kepler-391c are warm sub-Neptunes, whilst Kepler-391b is a hot sub-Neptune that falls in the hot super-Earth desert and, therefore, it might be undergoing photoevaporation of its outer envelope. The high-precision obtained in the transit times allowed us not only to confirm Kepler-278c's TTV signal, but also to find evidence of a previously undetected TTV signal for the inner planet Kepler-278b. From the presence of gravitational interaction between these bodies we constrain, for the first time, the mass of Kepler-278b ($M_{\mathrm{p}}$ = 56 $\substack{+37\\-13}$ $M_{\mathrm{\oplus}}$) and Kepler-278c ($M_{\mathrm{p}}$ = 35 $\substack{+9.9\\ -21} $ $M_{\mathrm{\oplus}}$). Finally, our photodynamical analysis also shows that the orbits of both planets around Kepler-278 are highly eccentric ($e \sim$ 0.7) and, surprisingly, coplanar. Further observations of this system are needed to confirm the eccentricity values presented here.

astro-ph.EP↗

arXiv2019

The active RS CVn-type system SZ Pictoris

We study the short and long-term variability of the spectroscopic binary SZ Pictoris, a southern RS CVn-type system. We used mid-resolution echelle spectra obtained at CASLEO spanning 18 years, and photometric data from the ASAS database V-band and from the Optical Robotic Observatory (BVRI-bands) for similar time lapses. We separated the composite spectra into those corresponding to both components, and we were able to determine accurate orbital parameters, in particular an orbital period of 4.95 days. We also observed a photometric modulation with half the orbital period, due to the ellipticity of the stars. We also found cyclic activity with a period of ~ 2030 days, both in the photometry and in the Ca II flux of the secondary star of the system.

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arXiv2018

HD 38858: a solar type star with a $\sim$10.8 yr activity cycle. Searching for Balmer and metallic line variations

Context. The detection of chromospheric activity cycles in solar-analogue and twin stars, can be used to put in context the solar cycle. However, there is a relatively low percentage of these stars with activity cycles detected. It is well known that the cores of Ca II H&K lines are modulated by stellar activity. The behaviour of Balmer and other optical lines with stellar activity is not yet completely understood. Aims. To search for Ca II H&K, Balmer and Fe II line variations modulated by the stellar activity. In particular, we apply a novel strategy to detect possible shape variations in the Hα line. Methods. We analyse activity signatures in HD 38858 by using HARPS and CASLEO spectra obtained between 2003 and 2017. We calculate the Mount Wilson index (S$_{MW}$), log($\mathrm{R}'_\mathrm{HK}$) and the statistical moments of Ca II H&K, Balmer and other optical lines. We search for periodicities by using the generalized Lomb-Scargle periodogram. Results. We detect a long-term activity cycle of 10.8 yr in Ca II H&K and Hα, being HD 38858 one of the most similar stars to the Sun for which a cycle has been detected. In contrast, this cycle is marginally detected in the Fe II lines. Also, we detect a noticeable radial velocity variation, which seems to be produced by stellar activity. Conclusions. HD 38858 is the second solar-analogue star where we find a clear activity cycle which is replicated by Balmer lines. Spectral indexes based on the shape of Hα seems to be more realiable than the fluxes in the same line to detect activity variations. On the other hand, the cyclic modulation detected gives place to a radial velocity variation, previously associated to a super-Earth planet. Finally, due to the similarity of HD 38858 with the Sun, we recommend to continue monitoring this star.

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arXiv2018

$ζ^{1} + ζ^{2}$ Reticuli binary system: A puzzling chromospheric activity pattern

We perform, for the first time, a detailed long-term activity study of the binary system $ζ$ Ret. We use all available HARPS spectra obtained between the years 2003 and 2016. We built a time series of the Mount Wilson $S$ index for both stars, then we analyse these series by using Lomb-Scargle periodograms. The components $ζ^{1}$ Ret and $ζ^{2}$ Ret that belong to this binary system are physically very similar to each other and also similar to our Sun, which makes it a remarkable system. We detect in the solar-analogue star $ζ^{2}$ Ret a long-term activity cycle with a period of $\sim$10 yr, similar to the solar one ($\sim$11 yr). It is worthwhile to mention that this object satisfies previous criteria for a flat star and for a cycling star simultaneously. Another interesting feature of this binary system, is a high $\sim$0.220 dex difference between the averages log($\mathrm{R}'_\mathrm{HK}$) activity levels of both stars. Our study clearly shows that $ζ^{1}$ Ret is significantly more active than $ζ^{2}$ Ret. In addition, $ζ^{1}$ Ret shows an erratic variability in its stellar activity. In this work, we explore different scenarios trying to explain this rare behaviour in a pair of coeval stars, which could help to explain the difference in this and other binary systems. From these results, we also warn that for the development of activity-age calibrations (which commonly use binary systems and/or stellar clusters as calibrators) it should be taken into account the whole history of activity available of the stars involved.

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arXiv2017

The beta Pictoris association low-mass members: membership assessment, rotation period distribution, and dependence on multiplicity

Low-mass members of young stellar associations exhibit a wide spread of rotation periods. Such a spread originates from distributions of masses and initial rotation periods. However, multiplicity can also play a significant role. We investigate the role played by physical companions in shortening the primordial disc lifetime. We have compiled the most extensive list of low-mass members of the young 25-Myr beta Pictoris association. We have measured the rotation periods of about all members and used updated UVWXYZ components to assess their membership. We built the rotation period distribution distinguishing between bona fide members and candidate members and according to their multiplicity status. We found that single stars and components of multiple systems in wide orbits (>80 AU) have rotation periods that exhibit a well defined sequence arising from mass distribution. All components of multiple systems in close orbits (<80 AU) have rotation periods significantly shorter than their equal-mass single counterparts. A comparison with the younger 13 Myr h Per cluster and with the older 40-Myr open clusters/stellar associations NGC2547, IC2391, Argus, and IC2602 and the 130-Myr Pleiades shows that whereas the evolution of F-G stars is well reproduced by angular momentum evolution models, this is not the case for the slow K and early-M stars. Finally, we found that the amplitude of their light curves is correlated neither with rotation nor with mass. Once single stars and wide components of multiple systems are separated from close components of multiple systems, the rotation period distributions exhibit a well defined dependence on mass that allows to make a meaningful comparison with similar distributions of either younger or older associations/clusters. Such cleaned distributions allow to use the stellar rotation period as age indicator, meaningfully for F and G type stars.

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arXiv2016

On the age of the magnetically active WW Psa and TX Psa members of the beta Pictoris association

There are a variety of different techniques available to estimate the ages of pre-main-sequence stars. Components of physical pairs, thanks to their strict coevality and the mass difference, such as the binary system analysed in this paper, are best suited to test the effectiveness of these different techniques. We consider the system WW Psa + TX Psa whose membership of the 25-Myr beta Pictoris association has been well established by earlier works. We investigate which age dating technique provides the best agreement between the age of the system and that of the association. We have photometrically monitored WW Psa and TX Psa and measured their rotation periods as P = 2.37d and P = 1.086d, respectively. We have retrieved from the literature their Li equivalent widths and measured their effective temperatures and luminosities. We investigate whether the ages of these stars derived using three independent techniques are consistent with the age of the beta Pictoris association. We find that the rotation periods and the Li contents of both stars are consistent with the distribution of other bona fide members of the cluster. On the contrary, the isochronal fitting provides similar ages for both stars, but a factor of about four younger than the quoted age of the association, or about 30% younger when the effects of magnetic fields are included. We explore the origin of the discrepant age inferred from isochronal fitting, including the possibilities that either the two components may be unresolved binaries or that the basic stellar parameters of both components are altered by enhanced magnetic activity. The latter is found to be the more reasonable cause, suggesting that age estimates based on the Li content is more reliable than isochronal fitting for pre-main-sequence stars with pronounced magnetic activity.

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arXiv2016

The beta Pictoris association: Catalog of photometric rotational periods of low-mass members and candidate members

We intended to compile the most complete catalog of bona fide members and candidate members of the beta Pictoris association, and to measure their rotation periods and basic properties from our own observations, public archives, and exploring the literature. We carried out a multi-observatories campaign to get our own photometric time series and collected all archived public photometric data time series for the stars in our catalog. Each time series was analyzed with the Lomb-Scargle and CLEAN periodograms to search for the stellar rotation periods. We complemented the measured rotational properties with detailed information on multiplicity, membership, and projected rotational velocity available in the literature and discussed star by star. We measured the rotation periods of 112 out of 117 among bona fide members and candidate members of the beta Pictoris association and, whenever possible, we also measured the luminosity, radius, and inclination of the stellar rotation axis. This represents to date the largest catalog of rotation periods of any young loose stellar association. We provided an extensive catalog of rotation periods together with other relevant basic properties useful to explore a number of open issues, such as the causes of spread of rotation periods among coeval stars, evolution of angular momentum, and lithium-rotation connection.

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arXiv2016

Semi-empirical Modeling of the Photosphere, Chromosphere, Transition Region, and Corona of the M-dwarf Host Star GJ 832

Stellar radiation from X-rays to the visible provides the energy that controls the photochemistry and mass loss from exoplanet atmospheres. The important extreme ultraviolet (EUV) region (10--91.2~nm) is inaccessible and should be computed from a reliable stellar model. It is essential to understand the formation regions and physical processes responsible for the various stellar emission features in order to predict how the spectral energy distribution varies with age and activity levels. We compute a state-of-the-art semi-empirical atmospheric model and the emergent high-resolution synthetic spectrum of the moderately active M2~V star GJ~832 as the first of a series of models for stars with different activity levels. Using non-LTE radiative transfer techniques and including many molecular lines, we construct a one-dimensional simple model for the physical structure of the star's chromosphere, chromosphere-corona transition region, and corona. The synthesized spectrum for this model fits the continuum and lines across the UV to optical spectrum. Particular emphasis is given to the emission lines at wavelengths shorter than 300~nm observed with {\em HST}, which have important effects on the photochemistry in the exoplanet atmospheres. The FUV line ratios indicate that the transition region of GJ~832 is more biased to hotter material than that of the quiet Sun. The excellent agreement of our computed EUV luminosity with that obtained by two other techniques indicates that our model predicts reliable EUV emission from GJ~832. We find that unobserved EUV flux of GJ~832, which heats the outer atmospheres of exoplanets and drives their mass loss, is comparable to the active Sun.

astro-ph.SR↗

arXiv2016

The rotation - Lithium depletion correlation in the beta Pictoris association and LDB age determination

There is evidence in the 125-Myr Pleiades cluster, and more recently in the 5-Myr NGC 2264 cluster, that rotation plays a key role in the Lithium (Li) depletion processes among low-mass stars. Fast rotators appear to be less Li-depleted than equal-mass slow rotators. We intend to explore the existence of a Li depletion - rotation connection among the beta Pictoris members at an age of about 24 Myr, and to use such correlation either to confirm or to improve the age estimate based on the Lithium Depletion Boundary (LDB) modeling. We have photometrically monitored all the known members of the beta Pictoris association with at least one Lithium equivalent width (Li EW) measurement from the literature. We measured the rotation periods of 30 members for the first time and retrieved from the literature the rotation periods for other 36 members, building a catalogue of 66 members with measured rotation period and Li EW. We find that in the 0.3 < M < 0.8 Msun range, there is a strong correlation between rotation and Li EW. For higher mass stars, no significant correlation is found. For very low mass stars in the Li depletion onset, at about 0.1 Msun, data are too few to infer a significant correlation. The observed Li EWs are compared with those predicted by the Dartmouth stellar evolutionary models that incorporate the effects of magnetic fields. After decorrelating the Li EW from the rotation period, we find that the hot side of the LDB is fitted well by Li EW values corresponding to an age of 25$\pm$3 Myr in good agreement with independent estimates from the literature.

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arXiv2016

Discovery of an activity cycle in the solar-analog HD 45184. Exploring Balmer and metallic lines as activity proxy candidates

(Abridged) Most stellar activity cycles similar to that found in the Sun have been detected by using the chromospheric Ca II H&K lines as stellar activity proxies. However, it is unclear if such activity cycles could be identified using other optical lines. Aims. To detect activity cycles in solar-analog stars and determine if these can be identified through other optical lines, such as Fe II and Balmer lines. We study the solar-analog star HD 45184. Methods. We analyse the activity signatures of HD 45184 by using 291 HARPS spectra obtained between 2003 and 2014. In order to search for line-core fluxes variations, we focus on Ca II H&K and Balmer H$α$, H$β$ lines, which are usually used as optical chromospheric activity indicators. We calculate the HARPS-S index from Ca II H&K lines and convert it to the Mount-Wilson scale. In addition, we also consider as activity indicators the equivalent widths of Balmer lines. Moreover, we analyse the possible variability of Fe II and other metallic lines in the optical spectra. Results. We report for the first time a long-term 5.14-yr activity cycle in the solar-analog star HD 45184. This makes HD 45184 one of most similar stars to the Sun with known activity cycle. Such variation is also evident in the first lines of the Balmer series, which not always show a correlation with activity in solar-type stars. Notably, unlike the solar case, we also found that the equivalent widths of the high photospheric Fe II lines (4924 Å, 5018 Å and 5169 Å) are modulated ($\pm$ 2 mÅ) by the chromospheric cycle of the star. From short-term modulation of the S index we calculate a rotational period of 19.98 days, which agrees with its mean chromospheric activity level.

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arXiv2016

A temperature condensation trend in the debris-disk binary system Zet2 Ret

We explore condensation temperature Tc trends in the unique binary system Zet1 Ret - Zet2 Ret, to determine whether there is a depletion of refractories, which could be related to the planet formation process. The star Zet2 Ret hosts a debris disk which was detected by an IR excess and confirmed by direct imaging and numerical simulations, while Zet1 Ret does not present IR excess nor planets. We carried out a high-precision abundance determination in both components of the binary system via a line-by-line, strictly differential approach. The stellar parameters Teff , log g, [Fe/H] and vturb were determined by imposing differential ionization and excitation equilibrium of Fe I and Fe II lines, with an updated version of the program FUNDPAR. The star Zet1 Ret resulted slightly more metal rich than Zet2 Ret by 0.02 dex. In the differential calculation of Zet1 Ret using Zet2 Ret as reference, the abundances of the refractory elements resulted higher than the volatile elements, and the trend of the refractory elements with Tc showed a positive slope. These facts together show a lack of refractory elements in Zet2 Ret (a debris-disk host) relative to Zet1 Ret. The Tc trend would be in agreement with the proposed signature of planet formation (Melendez et al. 2009) rather than possible Galactic Chemical Evolution or age effects, which are largely diminished here. Then, following the interpretation of Melendez et al. (2009), we propose an scenario in which the refractory elements depleted in Zet2 Ret are possibly locked-up in the rocky material that orbits this star and produce the debris disk observed around this object. We estimated a lower limit of Mrock = 3 Me for the rocky mass of depleted material, which is compatible with a rough estimation of 3-50 Me of a debris disk mass around a solar-type star (Krivov et al. 2008). (abridged)

astro-ph.SR↗

arXiv2015

HD 80606: Searching the chemical signature of planet formation

(Abridged) Binary systems with similar components are ideal laboratories which allow several physical processes to be tested, such as the possible chemical pattern imprinted by the planet formation process. Aims. We explore the probable chemical signature of planet formation in the remarkable binary system HD 80606 - HD 80607. The star HD 80606 hosts a giant planet with 4 MJup detected by both transit and radial velocity techniques, being one of the most eccentric planets detected to date. We study condensation temperature Tc trends of volatile and refractory element abundances to determine whether there is a depletion of refractories that could be related to the terrestrial planet formation. Methods. We carried out a high-precision abundance determination in both components of the binary system, using a line-by-line strictly differential approach, using the Sun as a reference and then using HD 80606 as reference. We used an updated version of the program FUNDPAR, together with ATLAS9 model atmospheres and the MOOG code. Conclusions. From the study of Tc trends, we concluded that the stars HD 80606 and HD 80607 do not seem to be depleted in refractory elements, which is different for the case of the Sun. Then, the terrestrial planet formation would have been less efficient in the components of this binary system than in the Sun. The lack of a trend for refractory elements with Tc between both stars implies that the presence of a giant planet do not neccesarily imprint a chemical signature in their host stars, similar to the recent result of Liu et al. (2014). This is also in agreement with Melendez et al. (2009), who suggest that the presence of close-in giant planets might prevent the formation of terrestrial planets. Finally, we speculate about a possible planet around the star HD 80607.

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arXiv2014

Trumpeting M Dwarfs with CONCH-SHELL: a Catalog of Nearby Cool Host-Stars for Habitable ExopLanets and Life

We present an all-sky catalog of 2970 nearby ($d \lesssim 50$ pc), bright ($J< 9$) M- or late K-type dwarf stars, 86% of which have been confirmed by spectroscopy. This catalog will be useful for searches for Earth-size and possibly Earth-like planets by future space-based transit missions and ground-based infrared Doppler radial velocity surveys. Stars were selected from the SUPERBLINK proper motion catalog according to absolute magnitudes, spectra, or a combination of reduced proper motions and photometric colors. From our spectra we determined gravity-sensitive indices, and identified and removed 0.2% of these as interloping hotter or evolved stars. Thirteen percent of the stars exhibit H-alpha emission, an indication of stellar magnetic activity and possible youth. The mean metallicity is [Fe/H] = -0.07 with a standard deviation of 0.22 dex, similar to nearby solar-type stars. We determined stellar effective temperatures by least-squares fitting of spectra to model predictions calibrated by fits to stars with established bolometric temperatures, and estimated radii, luminosities, and masses using empirical relations. Six percent of stars with images from integral field spectra are resolved doubles. We inferred the planet population around M dwarfs using $Kepler$ data and applied this to our catalog to predict detections by future exoplanet surveys.

astro-ph.SR↗

arXiv2012

12 Years of Stellar Activity Observations in Argentina

We present an observational program we started in 1999, to systematically obtain mid-resolution spectra of late-type stars, to study in particular chromospheric activity. In particular, we found cyclic activity in four dM stars, including Prox-Cen. We directly derived the conversion factor that translates the known S index to flux in the Ca II cores, and extend its calibration to a wider spectral range. We investigated the relation between the activity measurements in the calcium and hydrogen lines, and found that the usual correlation observed is the product of the dependence of each flux on stellar color, and it is not always preserved when simultaneous observations of a particular star are considered. We also used our observations to model the chromospheres of stars of different spectral types and activity levels, and found that the integrated chromospheric radiative losses, normalized to the surface luminosity, show a unique trend for G and K dwarfs when plotted against the S index.

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