arXiv subjects

Silvia Rossi

Publications and source records attributed to Silvia Rossi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Targeting Bright Metal-poor Stars in the Disk and Halo Systems of the Galaxy

We present the results of spectroscopic follow-up for 1897 low-metallicity star candidates, selected from the Best & Brightest (B&B) Survey, carried out with the GMOS-N/S (Gemini North/South telescopes) and Goodman (SOAR Telescope) spectrographs. From these low-resolution ($R \sim 2000$) spectra, we estimate stellar atmospheric parameters, as well as carbon and magnesium (representative of $α$ elements) abundance ratios. We confirm that $56\%$ of our program stars are metal-poor ([Fe/H] $< -1.0$), $30\%$ are very metal-poor (VMP; [Fe/H] $< -2.0$) and $2\%$ are extremely metal-poor (EMP; [Fe/H] $< -3.0$). There are 191 carbon-enhanced metal-poor (CEMP) stars, resulting in CEMP fractions of $19\%$ and $43\%$ for the VMP and EMP regimes, respectively. A total of 94 confirmed CEMP stars belong to Group I ($A({\rm C}) \gtrsim 7.25$) and 97 to Group II ($A({\rm C}) \lesssim 7.25$) in the Yoon-Beers $A$(C)$-$[Fe/H] diagram. Moreover, we combine these data with Gaia EDR3 astrometric information to delineate new target-selection criteria, which have been applied to the Goodman/SOAR candidates, to more than double the efficiency for identification of bona-fide VMP and EMP stars in comparison to random draws from the B&B catalog. We demonstrate that this target-selection approach can achieve success rates of $96\%$, $76\%$, $28\%$ and $4\%$ for [Fe/H] $\leq -1.5$, $\leq -2.0$, $\leq -2.5$ and $\leq -3.0$, respectively. Finally, we investigate the presence of dynamically interesting stars in our sample. We find that several VMP/EMP ([Fe/H] $\leq -2.5$) stars can be associated with either the disk system or halo substructures like Gaia-Sausage/Enceladus and Sequoia.

astro-ph.GA↗

arXiv2021

Searching for Extragalactic Exoplanetary Systems: the Curious Case of BD+20 2457

Planets and their host stars carry a long-term memory of their origin in their chemical compositions. Thus, identifying planets formed in different environments improves our understating of planetary formation. Although restricted to detecting exoplanets within the solar vicinity, we might be able to detect planetary systems that formed in small external galaxies and later merged with the Milky Way. In fact, Gaia data have unequivocally shown that the Galaxy underwent several significant minor mergers during its first billion years of formation. The stellar debris of one of these mergers, Gaia-Enceladus (GE), is thought to have built up most of the stellar halo in the solar neighborhood. In this Letter, we investigate the origin of known planet-host stars combining data from the NASA Exoplanet Archive with Gaia EDR3 and large-scale spectroscopic surveys. We adopt a kinematic criterion and identify 42 stars associated with the Milky Way's thick disk and one halo star. The only halo star identified, BD+20 2457, known to harbor two exoplanets, moves on a retrograde and highly eccentric orbit. Its chemical abundance pattern situates the star at the border between the thick disk, the old halo, and accreted populations. Given its orbital parameters and chemical properties, we suggest that BD+20 2457 is likely formed in the protodisk of the Galaxy, but we do not exclude the possibility of the star belonging to the debris of GE. Finally, we estimate a minimum age and mass limit for the star, which has implications for its planetary system and will be tested with future Transiting Exoplanet Survey Satellite observations.

astro-ph.EP↗

arXiv2021

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

We analyze the dynamical properties of $\sim$1500 very metal-poor (VMP; [Fe/H] $\lesssim -2.0$) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past thirty years, are supplemented by a number of calibration stars and other small samples, along with astrometric information from $Gaia$ DR2. We apply a clustering algorithm to the 4-D energy-action space of the sample, and identify a set of 38 Dynamically Tagged Groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as $Gaia$-Sausage/Enceladus (GSE), Sequoia, the Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and $\sim$300 individual $r$-process-enhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which $r$-process nucleosynthesis took place, and then were disrupted by the Milky Way.

astro-ph.GA↗

arXiv2021

The Road to a Successful HRI: AI, Trust and ethicS-TRAITS

The aim of this workshop is to give researchers from academia and industry the possibility to discuss the inter-and multi-disciplinary nature of the relationships between people and robots towards effective and long-lasting collaborations. This workshop will provide a forum for the HRI and robotics communities to explore successful human-robot interaction (HRI) to analyse the different aspects of HRI that impact its success. Particular focus are the AI algorithms required to implement autonomous interactions, and the factors that enhance, undermine, or recover humans' trust in robots. Finally, potential ethical and legal concerns, and how they can be addressed will be considered. Website: https://sites.google.com/view/traits-hri

cs.RO↗

arXiv2021

I Know What You Would Like to Drink: Benefits and Detriments of Sharing Personal Info with a Bartender Robot

This paper introduces benefits and detriments of a robot bartender that is capable of adapting the interaction with human users according to their preferences in drinks, music, and hobbies. We believe that a personalised experience during a human-robot interaction increases the human user's engagement with the robot and that such information will be used by the robot during the interaction. However, this implies that the users need to share several personal information with the robot. In this paper, we introduce the research topic and our approach to evaluate people's perceptions and consideration of their privacy with a robot. We present a within-subject study in which participants interacted twice with a robot that firstly had not any previous info about the users, and, then, having a knowledge of their preferences. We observed that less than 60\% of the participants were not concerned about sharing personal information with the robot.

cs.RO↗

arXiv2021

The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82

We report photometric estimates of effective temperature, $T_{\rm eff}$, metallicity, [Fe/H], carbonicity, [C/Fe], and absolute carbon abundances, $A{\rm (C)}$, for over 700,000 stars from the Southern Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: 1) halo main-sequence turnoff stars and 2) K-dwarf stars of mass $0.58 < M/M_{\odot} <0.75$ in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (SPHINX) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperature, $4250 < T_{\rm eff} \textrm{(K)} < 7000$. The use of multiple narrow-band S-PLUS filters enables SPHINX to achieve substantially lower levels of "catastrophic failures" (large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way's K-dwarf halo metallicity distribution function (MDF), $λ_{10, \textrm{[Fe/H]}} = 0.85 \pm 0.21$, over the metallicity range $-2.5 < \textrm{[Fe/H]} < -1.0$; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters $α=2.8$ and $β=4.2$. S-PLUS photometry obtains absolute carbon abundances with a precision of $\sim 0.35$dex for stars with $T_{\rm eff} < 6500$K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon-Beers morphological groups in the $A$(C)-[Fe/H] space, and derive the CEMP frequencies.

astro-ph.GA↗

arXiv2021

Spatio-temporal Graph-RNN for Point Cloud Prediction

In this paper, we propose an end-to-end learning network to predict future frames in a point cloud sequence. As main novelty, an initial layer learns topological information of point clouds as geometric features, to form representative spatio-temporal neighborhoods. This module is followed by multiple Graph-RNN cells. Each cell learns points dynamics (i.e., RNN states) by processing each point jointly with the spatio-temporal neighbouring points. We tested the network performance with a MINST dataset of moving digits, a synthetic human bodies motions and JPEG dynamic bodies datasets. Simulation results demonstrate that our method outperforms baseline ones that neglect geometry features information.

cs.CV↗

arXiv2020

Towards Transparency of TD-RL Robotic Systems with a Human Teacher

The high request for autonomous and flexible HRI implies the necessity of deploying Machine Learning (ML) mechanisms in the robot control. Indeed, the use of ML techniques, such as Reinforcement Learning (RL), makes the robot behaviour, during the learning process, not transparent to the observing user. In this work, we proposed an emotional model to improve the transparency in RL tasks for human-robot collaborative scenarios. The architecture we propose supports the RL algorithm with an emotional model able to both receive human feedback and exhibit emotional responses based on the learning process. The model is entirely based on the Temporal Difference (TD) error. The architecture was tested in an isolated laboratory with a simple setup. The results highlight that showing its internal state through an emotional response is enough to make a robot transparent to its human teacher. People also prefer to interact with a responsive robot because they are used to understand their intentions via emotions and social signals.

cs.RO↗

arXiv2020

Spherical clustering of users navigating 360° content

In Virtual Reality (VR) applications, understanding how users explore the omnidirectional content is important to optimize content creation, to develop user-centric services, or even to detect disorders in medical applications. Clustering users based on their common navigation patterns is a first direction to understand users behaviour. However, classical clustering techniques fail in identifying these common paths, since they are usually focused on minimizing a simple distance metric. In this paper, we argue that minimizing the distance metric does not necessarily guarantee to identify users that experience similar navigation path in the VR domain. Therefore, we propose a graph-based method to identify clusters of users who are attending the same portion of the spherical content over time. The proposed solution takes into account the spherical geometry of the content and aims at clustering users based on the actual overlap of displayed content among users. Our method is tested on real VR user navigation patterns. Results show that our solution leads to clusters in which at least 85% of the content displayed by one user is shared among the other users belonging to the same cluster.

cs.MM↗

arXiv2019

Integrated Spectra of Milky Way Globular Cluster

Integrated spectra of Milky Way globular clusters (GCs) are reproduced by computing synthetic spectra taking into account individual element abundances. Five clusters were selected from their location in the Galactic bulge, for which integrated spectra were available in the WiFeS Atlas of Galactic Globular Cluster Spectra project. Our aim is to further study the oldest GC located in the Galactic bulge, with metallicity in the range $-$1.6$<$[Fe/H]$<$$-$0.7. We also include the halo cluster NGC 6752 for comparison purposes. We reproduce the full spectra in the range 4500-9000 $\mathring{A}$ available in these observed spectra, as well as individual lines of Na, Mg, Al, Si, Ca, Ti, Ba, and Eu. We report a list of lines that are suitable for abundance derivation, and by adopting these abundances we are able to fit the damping parameters that define the wings of strong lines of well-known triplets of Mg I and Ca II. Finally, the effect of multiple stellar populations through enhanced Na abundances is tested.

astro-ph.SR↗

arXiv2019

Emotional Distraction for Children Anxiety Reduction During Vaccination

Social assistive robots are starting to be widely used in pediatric health-care environments with the aim of distracting and entertaining children, and so of reducing a possible state of anxiety. In this paper, we present some initial results of a study (N=69) conducted in a Health-Vaccines Center, where the distraction role of a social robot, which interacts with a child showing an emotional behavior, is compared with the same not showing any emotional social cue. Outcome criteria for the evaluation of the intervention included the parents reported level of anxiety before, during and after the procedure.

cs.RO↗

arXiv2018

The R-Process Alliance: Spectroscopic Follow-up of Low-Metallicity Star Candidates from the Best & Brightest Survey

We present results from an observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R ~ 1, 200 - 2, 000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters (Teff, log g, and [Fe/H]), as well as carbon and alpha-element abundances. We find that 69% of the observed stars have [Fe/H] <= -1.0, 39% have [Fe/H] <= -2.0, and 2% have [Fe/H] <= -3.0. There are also 133 carbon-enhanced metal-poor (CEMP) stars in this sample, with 97 CEMP Group I and 36 CEMP Group II stars identified in the A(C) versus [Fe/H] diagram. A subset of the confirmed low-metallicity stars were followed-up with high-resolution spectroscopy, as part of the R-process Alliance, with the goal of identifying new highly and moderately r-process-enhanced stars. Comparison between the stellar atmospheric parameters estimated in this work and from high-resolution spectroscopy exhibit good agreement, confirming our expectation that medium-resolution observing campaigns are an effective way of selecting interesting stars for further, more targeted, efforts.

astro-ph.SR↗

arXiv2018

Spectroscopic Validation of Low-Metallicity Stars from RAVE

We present results from a medium-resolution (R ~ 2, 000) spectroscopic follow-up campaign of 1,694 bright (V < 13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow-up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and α-element abundances, which are crucial for con- sidering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have [Fe/H] <= -1.0, 61% have [Fe/H] <= -2.0, and 3% have [Fe/H] <= -3.0 (with four stars at [Fe/H] <= -3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP Group I, 131 CEMP Group II, and 6 CEMP Group III stars from the A(C) vs. [Fe/H] diagram. Inspection of the [alpha/C] abundance ratios reveals that five of the CEMP Group II stars can be classified as "mono-enriched second-generation" stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy, to reveal their full chemical abundance patterns for further studies.

astro-ph.SR↗

arXiv2016

Bright Metal-Poor Stars from the Hamburg/ESO Survey. II. A Chemodynamical Analysis

We obtain estimates of stellar atmospheric parameters for a previously published sample of 1777 relatively bright (9 < B < 14) metal-poor candidates from the Hamburg/ESO Survey. The original Frebel et al. analysis of these stars was only able to derive estimates of [Fe/H] and [C/Fe] for a subset of the sample, due to limitations in the methodology then available. A new spectroscopic analysis pipeline has been used to obtain estimates of Teff, log g, [Fe/H], and [C/Fe] for almost the entire dataset. This sample is very local - about 90% of the stars are located within 0.5 kpc of the Sun. We consider the chemodynamical properties of these stars in concert with a similarly local sample of stars from a recent analysis of the Bidelman & MacConnell 'weak-metal' candidates by Beers et al. We use this combined sample to identify possible members of the suggested halo stream of stars by Helmi et al. and Chiba & Beers, as well as stars that may be associated with stripped debris from the putative parent dwarf of the globular cluster Omega Centauri, suggested to exist by previous authors. We identify a clear increase in the cumulative frequency of carbon-enhanced metal-poor (CEMP) stars with declining metallicity, as well as an increase in the fraction of CEMP stars with distance from the Galactic plane, consistent with previous results. We also identify a relatively large number of CEMP stars with kinematics consistent with the metal-weak thick-disk population, with possible implications for its origin.

astro-ph.SR↗

arXiv2015

The Fractions of Inner- and Outer-Halo Stars in the Local Volume

We obtain a new determination of the metallicity distribution function (MDF) of stars within $\sim5$-$10$ kpc of the Sun, based on recently improved co-adds of $ugriz$ photometry for Stripe 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] $\approx -1.4$ and $-1.9$, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically- and kinematically-distinct properties of the inner- and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of $\sim35\%$-$55\%$ of halo stars in the local volume.

astro-ph.GA↗

arXiv2015

The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F$_{\rm BSS/BHB}$, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for F$_{\rm BSS/BHB}$ is $\sim~$4.0 in the thick disk (at 3 kpc $<$ $|$Z$|$ $<$ 4 kpc), declining to on the order of $\sim~1.5-2.0$ in the inner-halo region; this ratio continues to decline to $\sim~$1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5$\%$ of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.

astro-ph.SR↗

arXiv2014

Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy

A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R~3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the SEGUE Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and [Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities -1.8 < [Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy vs. angular momentum diagram. A comparison is made with recent results for a similar-size sample of RAVE stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.

astro-ph.GA↗

arXiv2014

Seven New Carbon-Enhanced Metal-Poor RR Lyrae Stars

We report estimated carbon-abundance ratios, [C/Fe], for seven newly-discovered carbon-enhanced metal-poor (CEMP) RR Lyrae stars. These are well-studied RRab stars that had previously been selected as CEMP candidates based on low-resolution spectra. For this pilot study, we observed eight of these CEMP RR Lyrae candidates with the Wide Field Spectrograph (WiFeS) on the ANU 2.3m telescope. Prior to this study, only two CEMP RR Lyrae stars had been discovered: TY Gru and SDSS J1707+58. We compare our abundances to new theoretical models of the evolution of low-mass stars in binary systems. These simulations evolve the secondary stars, post accretion from an AGB donor, all the way to the RR Lyrae stage. The abundances of CEMP RR Lyrae stars can be used as direct probes of the nature of the donor star, such as its mass, and the amount of material accreted onto the secondary. We find that the majority of the sample of CEMP RR Lyrae stars is consistent with AGB donor masses of around 1.5 - 2.0 solar masses and accretion masses of a few hundredths of a solar mass. Future high-resolution studies of these newly-discovered CEMP RR Lyrae stars will help disentangle the effects of the proposed mixing processes that occur in such objects.

astro-ph.SR↗