arXiv subjects

E. Stonkute

Publications and source records attributed to E. Stonkute.

12 recordsLinked to original sources

arXiv2026

SN 2024afyu interpreted as a Pair Instability Supernova

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.

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arXiv2025

Chromospherically active stars: Lithium and CNO abundances in northern RS CVn stars

Aims: We carried out a detailed investigation of Lithium and CNO abundances, including carbon isotope ratios, in RS CVn stars to assess the role of magnetic activity in the mixing of stellar atmospheres. Methods: We obtained high-resolution spectra at the Moletai Astronomical Observatory. Lithium abundances were determined by spectral synthesis of the 6707 A line and the CNO abundances using the C2 band heads at 5135 and 5635.5 A CN bands at 6470- 6490 A and 7980 to 8005 A, and the [O I] line at 6300 A. By fitting the 13CN band at 8004.7 A, we determined the carbon isotope ratios.Results. We determined the main atmospheric parameters and investigated the chemical composition of 32 RS CVn stars. Lithium abundances were determined for 13 additional stars using archival spectra. We report that *iot Gem and HD 179094 have carbon isotope ratios already affected by extra-mixing, even though they are in the evolutionary stage below the red giant branch luminosity bump. About half of the low-mass giants, for which the lithium abundance was determined, follow the first dredge-up predictions; however, other stars show reduced Lithium abundances, as predicted by thermohaline-induced mixing. The intermediate-mass stars show reduced Lithium abundances reduced, as predicted by rotation-induced mixing. Conclusions. In low-mass, chromospherically active RS CVn stars, extra-mixing of lithium and carbon isotopes may begin earlier than in normal giants. The Li-rich RS CVn giant V*OP And has large C/N and carbon isotope ratios and raises questions about the origin of its lithium enhancement.

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arXiv2025

Chromospherically active stars: Chemical composition of photospheres in 20 RS CVn stars

Context. Various element transport processes modify the photospheric chemical composition of low-mass stars during their evolution. The most prominent one is the first dredge-up that occurs at the beginning of the red giant branch. Then, various extra-mixing processes, such as those caused by thermohaline- and/or rotation-induced mixing, come into action. The extent of the influence of stellar magnetic activity on alterations in stellar chemical composition is among the least studied questions. Aims. To investigate how magnetic activity influences mixing in the atmospheres of magnetically active stars, we carried out a detailed study of C, N, and up to ten other chemical element abundances, as well as carbon isotope ratios in a sample of RS CVn stars. Methods. Using a differential model atmosphere method, we analysed high-resolution spectra that had been observed with the VUES spectrograph on the 1.65 m telescope at the Moletai Astronomical Observatory of Vilnius University. Abundances of other chemical elements were determined from equivalent widths or spectral syntheses of unblended spectral lines. Results. We determined the main atmospheric parameters and abundances of up to 12 chemical elements for a sample of 20 RS CVn giants that represented different evolutionary stages. We determined that *29 Dra, *b01 Cyg, and V* V834 Her, which are in the evolutionary stage below the red giant branch luminosity bump, already show evidence of extra-mixing in their lowered carbon isotope ratios. Conclusions. We provide observational evidence that in low-mass chromospherically active RS CVn stars, due to their magnetic activity, extra-mixing processes may start acting below the luminosity bump of the red giant branch.

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arXiv2024

OCCASO V. Chemical-abundance trends with Galactocentric distance and age

Context. Open clusters provide valuable information on stellar nucleosynthesis and the chemical evolution of the Galactic disc, as their age and distances can be measured more precisely with photometry than for field stars. Aims. Our aim is to study the chemical distribution of the Galactic disc using open clusters by analysing the existence of gradients with Galactocentric distance, azimuth or height from the plane and dependency with age. Methods. High-resolution spectra (R>60 000) of 194 stars belonging to 36 open clusters are used to determine atmospheric parameters and chemical abundances with two independent methods: equivalent widths and spectral synthesis. The sample has been complemented with 63 clusters with high-resolution spectroscopy from literature. Results. We measure local thermodynamic equilibrium abundances for 21 elements: α (Mg, Si, Ca, and Ti), odd-Z (Na and Al), Fe-peak (Fe, Sc, V, Cr, Mn, Co, Ni, Cu, and Zn), and neutron-capture (Sr, Y, Zr, Ba, Ce, and Nd). We also provide non-local thermodynamic equilibrium abundances for elements when corrections are available. We find inner disc young clusters enhanced in [Mg/Fe] and [Si/Fe] compared to other clusters of their age. For [Ba/Fe] we report an age trend flattening for older clusters (age<2.5 Ga). The studied elements follow the expected radial gradients as a function of their nucleosynthesis groups, which are significantly steeper for the oldest systems. For the first time, we investigate the existence of an azimuthal gradient, finding some hints of its existence among the old clusters (age>2 Ga).

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arXiv2023

The Gaia-ESO Survey: homogenisation of stellar parameters and elemental abundances

The Gaia-ESO Survey is a public spectroscopic survey that has targeted $\gtrsim10^5$ stars covering all major components of the Milky Way from the end of 2011 to 2018, delivering its public final release in May 2022. Unlike other spectroscopic surveys, Gaia-ESO is the only survey that observed stars across all spectral types with dedicated, specialised analyses: from O ($T_\mathrm{eff} \sim 30,000-52,000$~K) all the way to K-M ($\gtrsim$3,500~K). The physics throughout these stellar regimes varies significantly, which has previously prohibited any detailed comparisons between stars of significantly different type. In the final data release (internal data release 6) of the Gaia-ESO Survey, we provide the final database containing a large number of products such as radial velocities, stellar parameters and elemental abundances, rotational velocity, and also, e.g., activity and accretion indicators in young stars and membership probability in star clusters for more than 114,000 stars. The spectral analysis is coordinated by a number of Working Groups (WGs) within the Survey, which specialise in the various stellar samples. Common targets are analysed across WGs to allow for comparisons (and calibrations) amongst instrumental setups and spectral types. Here we describe the procedures employed to ensure all Survey results are placed on a common scale to arrive at a single set of recommended results for all Survey collaborators to use. We also present some general quality and consistency checks performed over all Survey results.

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arXiv2022

The Gaia-ESO survey: mapping the shape and evolution of the radial abundance gradients with open clusters

The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangle the scenarios of formation and evolution of the Galaxy. We used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance to iron ratio gradients, and their time evolution. We selected member stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw different behaviours for elements belonging to different channels. We found that the youngest clusters in the inner disc have lower metallicity than their older counterpart and they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggested that it might be a bias introduced by the standard spectroscopic analysis producing lower metallicities in low gravity stars. To delineate the shape of the `true' gradient, we should limit our analysis to stars with low surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slow and stationary formation of the thin disc in the latest Gyr. We found a secondary role of clusters' migration in shaping the gradient, with a more prominent role of migration for the oldest clusters.

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arXiv2021

Gaia-ESO Survey: Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851

Context. NGC 1851 is one of several globular clusters for which multiple stellar populations of the subgiant branch have been clearly identified and a difference in metallicity detected. A crucial piece of information on the formation history of this cluster can be provided by the sum of A(C+N+O) abundances. However, these values have lacked a general consensus thus far. The separation of the subgiant branch can be based on age and/or A(C+N+O) abundance differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen abundances for evolved giants in the globular cluster NGC1851 in order to check whether or not the double populations of stars are coeval. Methods. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT telescope, were analysed using a differential model atmosphere method. Results. We provide abundances of up to 29 chemical elements for a sample of 45 giants in NGC 1851. The investigated stars can be separated into two populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron abundance ratios [s/Fe]. No significant difference was determined in the mean values of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- and iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O) values between the two populations do not differ, additional evidence is given that NGC 1851 is composed of two clusters, the metal-rich cluster being by about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851 properties and the detailed abundances of chemical elements favour its formation in a dwarf spheroidal galaxy that was accreted by the Milky Way.

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arXiv2019

The Gaia-ESO survey: Calibrating a relationship between Age and the [C/N] abundance ratio with open clusters

In the era of large high-resolution spectroscopic surveys, high-quality spectra can contribute to our understanding of the Galactic chemical evolution, providing chemical abundances belonging to the different nucleosynthesis channels, and also providing constraints to stellar age. Some abundance ratios have been proven to be excellent indicators of stellar ages. We aim at providing an empirical relationship between stellar ages and [C/N] using, as calibrators, open star clusters observed by both the Gaia-ESO and APOGEE surveys. We use stellar parameters and abundances from the Gaia-ESO and APOGEE of the Galactic field and open cluster stars. Ages of star clusters are retrieved from the literature sources and validated using a common set of isochrones. We use the same isochrones to determine, for each age and metallicity, the surface gravity at which the first dredge-up and red giant branch bump occur. We study the effect of extra-mixing processes in our sample of giant stars, and we derive the mean [C/N] in evolved stars, including only stars without evidence of extra-mixing. Combining the Gaia-ESO and APOGEE samples of open clusters, we derive a linear relationship between [C/N] and logarithmic cluster ages. We apply our relationship to selected giant field stars in both Gaia-ESO and APOGEE. We find an age separation between thin and thick disc stars and age trends within their populations, with an increasing age towards lower metallicity populations. With such empirical relationship, we are able to provide an age estimate for giant stars in which C and N abundances are measured. Isochrone fitting is less sensitive for giant than dwarf stars at the turn off. The present method can be thus considered as an additional tool to give an independent estimate of the age of giant stars, with uncertainties in their ages comparable to those obtained using isochrone fitting for dwarf stars.

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arXiv2018

The Gaia-ESO Survey: the N/O abundance ratio in the Milky Way

The abundance ratio N/O is a useful tool to study the interplay of galactic processes, e.g. star formation efficiency, time-scale of infall and outflow loading factor We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to compare it with a set of chemical evolution models to understand the role of infall, outflow and star formation efficiency in the building-up of the Galactic disc. We use the abundances from idr2-3, idr4, idr5 data releases of the Gaia-ESO Survey both for Galactic field and open cluster stars.We determine membership and average composition of open clusters and we separate thin and thick disc field stars.We consider the effect of mixing in the abundance of N in giant stars. We compute a grid of chemical evolution models, suited to reproduce the main features of our Galaxy, exploring the effects of the star formation efficiency, the infall time-scale and the differential outflow. With our samples, we map the metallicity range -0.6<[Fe/H]<0.3 with a corresponding -1.2<log(N/O)<-0.2, where the secondary production of N dominates. Thanks to the wide range of Galactocentric distances covered by our samples, we can distinguish the behaviour of log(N/O) in different parts of the Galaxy. Our spatially resolved results allow us to distinguish differences in the evolution of N/O with Galactocentric radius. Comparing the data with our models, we can characterise the radial regions of our Galaxy. A shorter infall time-scale is needed in the inner regions, while the outer regions need a longer infall time-scale, coupled with a higher star formation efficiency. We compare our results with nebular abundances obtained in MaNGA galaxies, finding in our Galaxy a much wider range of log(N/O) than in integrated observations of external galaxies of similar stellar mass, but similar to the ranges found in studies of individual H ii regions.

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arXiv2017

The Gaia-ESO Survey: Churning through the Milky Way

We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H]$>0.1$) stars within 2 kpc of the Sun using a sample of more than 3,000 stars selected from iDR4 of the Gaia-ESO Survey. We investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. This was done using the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Analyzing the kinematics of the most metal-rich stars ([Fe/H]$>0.1$), we find that more than half have small eccentricities ($e<0.2$) or are on nearly circular orbits. Slightly more than 20\% of the metal-rich stars have perigalacticons $R_p>7$ kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by $\sim10$ km s$^{-1}$. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s$^{-1}$ slower than stars with lower [Mg/Fe] ratios. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.

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arXiv2016

On the metallicity dependance of the [Y/Mg] - age relation for solar type stars

Several recent studies of Solar twins in the Solar neighbourhood have shown a tight correlation between various elemental abundances and age, in particular [Y/Mg]. If this relation is real and valid for other types of stars as well as elsewhere in the Galaxy it would provide a very powerful tool to derive ages of stars without the need to resort to determining their masses (evolutionary stage) very precisely. The method would also likely work if the stellar parameters have relatively large errors. The studies presented in the recent literature span a narrow range of [Fe/H]. By studying a larger sample of Solar neighbourhood dwarfs with a much larger range in [Fe/H], we find that the relation between [Y/Mg] and age depends on the [Fe/H] of the stars. Hence, it appears that the [Y/Mg] - age relation is unique to Solar analogues.

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arXiv2016

The Gaia-ESO Survey: the selection function of the Milky Way field stars

The Gaia-ESO Survey was designed to target all major Galactic components (i.e., bulge, thin and thick discs, halo and clusters), with the goal of constraining the chemical and dynamical evolution of the Milky Way. This paper presents the methodology and considerations that drive the selection of the targeted, allocated and successfully observed Milky Way field stars. The detailed understanding of the survey construction, specifically the influence of target selection criteria on observed Milky Way field stars is required in order to analyse and interpret the survey data correctly. We present the target selection process for the Milky Way field stars observed with VLT/FLAMES and provide the weights that characterise the survey target selection. The weights can be used to account for the selection effects in the Gaia-ESO Survey data for scientific studies. We provide a couple of simple examples to highlight the necessity of including such information in studies of the stellar populations in the Milky Way.

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