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L. Monaco

Publications and source records attributed to L. Monaco.

At least 19 recordsLinked to original sources

Something new under the Sun: A magnetically driven CH/CN anti-correlation

Spectroscopic monitoring surveys have shown that the magnetic cycle alters the photospheric structure of the Sun, perturbing the formation of molecular lines. In parallel, the study of Globular Clusters has remained centred on the multiple stellar populations conundrum, a phenomenon defined-most notably-by the anti-correlation between CH and CN molecular bands, serving to distinguish between first-population and second-population stars. We systematically monitored the behaviour of the CH and CN molecular bands as a function of the solar magnetic cycle. We demonstrate that the disc-integrated solar spectrum exhibits a distinct CH/CN anti-correlation driven by the magnetic cycle. Our results suggest that the spectroscopic and photometric anomalies defining the multiple populations conundrum may be physically linked to a differential spectral response to varying degrees of surface magnetic coverage.

astro-ph.SR

TOPoS VII. Age-metallicity relation in the Galactic halo and assembly of the Milky Way

One technique for determining stellar ages is to compare the position of a star in the Hertzsprung-Russell diagram to theoretical stellar evolutionary tracks. The sub-giant evolutionary stage is the one that is most sensitive to age and allows the most precise evolutionary age estimates. The TOPoS sample of stars with metallicities derived from low-resolution Sloan Digital Sky Survey spectra contains a large subset of sub-giant stars with precise parallaxes from the Gaia mission, for which evolutionary ages can be determined. Our aim is to use this stellar sample to investigate the age-metallicity relation in the Galactic halo. We use the Bayesian inference code SPInS and theoretical BaSTI stellar evolutionary tracks to determine the ages for TOPoS stars. There is a clear increase in metallicity with decreasing age, albeit with a considerable scatter at any given age. At ages larger than 8 Ga, the scatter is so large that in fact, over this range, age and metallicity appear to be uncorrelated. At any given age, the metallicity distribution is multi-modal, with up to three distinct peaks. These peaks trace three age-metallicity relations that we tentatively identify with the halo, thick-disc, and thin-disc. Our data demonstrate the important role of mergers in the evolution of the Galaxy, up to 8 Ga ago. In more recent times, the spread in metallicity drops. One possibility is that the major merger Gaia-Sausage-Enceladus may have perturbed the galaxies in the Milky Way vicinity in such a way as to decrease the merger rate. Chemical evolution models and cosmological models of the Local Group both support the importance of mergers in the early evolution of the Milky Way. Larger, unbiased samples, or at least with well-understood biases, of stars with accurate ages are required for a quantitative comparison between models and data.

astro-ph.GA

TYC 170-1218-1: A new r-process-enhanced extremely metal-poor star, rich in Th

Context . Extremely metal-poor (EMP) stars are formed from gas clouds enriched by one or a few supernova explosions belonging to the first stellar generation and this very limited number of sources of metal enrichment is suitable to produce peculiar chemical patterns that give birth to stars with anomalous chemical composition. Among the EMP stars, r-II stars are characterised by an over-abundance of the heavy elements with respect to iron. Aims . In the search for apparently young, metal-poor stars, we serendipitously selected TYC 170-1218-1, which turned out to be an EMP star, enhanced in neutron capture elements over iron. Our aim is to obtain a detailed chemical inventory for this exceptional object. Methods . We investigated high-resolution spectra observed with UVES at the VLT telescope and Mike at the Magellan Clay telescope. We derived the abundance of 33 elements using the MyGIsFOS code and an ATLAS 9 model atmosphere. Results . The star is an EMP with [Fe/H] = -3.52. It is enhanced in the $\alpha$ elements, as EMP stars usually are. It is an r-II star with [Eu/Fe] = +1.84 and [Th/Fe] = +1.85. The star is also poor in carbon with respect to iron. The quality of the spectra was insufficient for us to detect uranium. Kinematically the star belongs now to the Galactic halo, but it joined the Milky Way during the Sequoia accretion event.

astro-ph.GA

The ionization structure and chemical history in isolated H ii regions of dwarf galaxies with VIMOS/IFU II. The Leo A galaxy

Study the ionized gas in metal-poor environments is key to understanding the mechanisms regulating galaxy evolution. However, most of the previous studies of extragalactic HII regions rely on unresolved observations of gaseous structures. We study the south-western, spatially resolved, HII region of Leo A, one of the most studied isolated dwarf galaxies in the Local Group. Using archival VIMOS-IFU/VLT data, we explored its gaseous structure through optical emission lines to gain insights into the present-day drivers of gas physics in this dIrr, and we place constraints on the chemical evolution scenario responsible for this low chemical enrichment. The emission line maps reveal that the strongest emission comes from the south-west region. A stratified distribution of ionic species was detected, likely powered by the young star cluster at the nebular centre. HST/ACS data show that the brightest star is in the centre of both the HII region and the star cluster. Photoionization production rates derived indicate that this star can sustain the ionization budget to power the HII region, although subject to the assumed electron density. Using the direct method, we derived a metallicity of $12+\log(\mathrm{O/H})=7.29\pm0.06$ dex, increasing to $7.46\pm0.09$ dex after correcting for temperature fluctuations, placing Leo A in the low-mass end of the MZR. Chemical evolution models suggest that, under constant accretion, the stellar mass growth and metal enrichment over the last 10 Gyr are successfully reproduced by both leaky-box and gas-regulator models. Those results are similar to those found in SagDIG, supporting a picture in which the present-day evolution of Leo A is dominated by stellar feedback processes. The combination of mass loss mechanisms and accretion events reproduces its chemical evolution, suggesting that Leo A has evolved under a gas equilibrium regime across its lifetime.

astro-ph.GA

Unveiling the nature of HE 0107-5240

The vast majority of the most iron-poor stars in the Galaxy exhibit a strong carbon enhancement, with C/H ratios only about two orders of magnitude below solar. This unusual chemical composition likely reflects the properties of the gas cloud from which these stars formed, having been enriched by one, or at most a few, supernovae. A remarkable member of this stellar class, HE 0107-5240 with [Fe/H]=-5.56, has been identified as part of a binary system. To constrain its orbital parameters, radial velocity monitoring has been carried out using the ESPRESSO spectrograph. Radial velocities were derived using cross-correlation with a template, taking advantage of the strong G-band feature. Combining all observations yielded a high signal-to-noise spectrum, which has been used to refine our understanding of the stellar chemical composition. Additionally, a co-added UVES spectrum in the blue was used to complement the wavelength coverage of ESPRESSO. Observations of HE 0107-5240 over a span of more than four years have yielded a revised orbital period of about 29 years. Updated elemental abundances have been determined for Sc, Cr, Co, and, tentatively, Al, along with a new upper limit for Be. The iron abundance has been derived from ionised Fe lines. Significant upper limits have been established for Li, Si, and Sr. The star is confirmed to be a long-period binary. Iron abundances derived from neutral and ionised lines are consistent with local thermodynamical equilibrium (LTE) assumption, casting doubt on published deviation from LTE corrections for Fe for this star. The heavy elements Sr and Ba remain undetected, confirming the classification of HE 0107-5240 as a carbon enhanced metal-poor and non enhanced in heavy elements (CEMP-no) star and supporting the absence of an n-capture element plateau at the lowest metallicities.

astro-ph.SR

C and N abundances in globular clusters. I. The case of 47 Tuc and the effect of the first dredge-up

Globular clusters exhibit star-to-star chemical variations, traceable through both photometric and spectroscopic data. In this study, we investigate chemical variations in the globular cluster NGC104 (47Tucanae), aiming to trace multiple stellar populations across evolutionary phases and examine how the C/N anti-correlation evolves from the main sequence (MS) to the asymptotic giant branch (AGB). We also assess the impact of these populations on the interpretation of the V vs. V-I diagram. [C/Fe] and [N/Fe] abundances are obtained from CN and the CH features, while atmospheric parameters are homogeneously derived from photometry. The inferred abundances allow us to disentangle multiple populations along the CMD and refine cluster parameters. We find that MS stars are more C- and N-rich than their red giant branch, horizontal branch, and AGB counterparts. The C/N anti-correlation shifts during the sub-giant branch phase, coinciding with the first dredge-up. Interestingly, stars with different C and N abundances occupy distinct regions of the V vs V-I diagram, a pattern not attributable to differential reddening. Proper CMD fitting requires two isochrones with differing helium content, metallicity, and possibly age.

astro-ph.GA

The ionization structure and chemical history in isolated H II regions of dwarf galaxies with IFU I. The Sagittarius Dwarf Irregular Galaxy

Studying metal-poor galaxies is crucial for understanding the physical mechanisms driving the evolution of galaxies. Most observational works in dwarf galaxies employ integral field unit data to investigate gas physics in the entire galaxy body. However, previous studies have not explored the detailed spatially resolved properties of individual extragalactic HII regions. We study the only known HII region in the SagDIG dIrr, a metal-poor dwarf galaxy of the local universe, using VIMOS-IFU/VLT and EFOSC2/NTT archival data. We aim to probe the structure of the gaseous nebulae to (i) give insights into the physical processes shaping its gas dynamics, and (ii) relate these mechanisms to the low gas-phase metallicity content estimated with the direct method. The H$\beta$ emission line map shows two clumps as a biconic-like structure aligned across the same axis. The [OIII]$\lambda$5007 map, on the other hand, shows a more symmetric structure. Radial flux density profiles exhibit signatures of ionized stratification. By comparing the gas structure with the stellar population in the same region, the young stellar population ($<700$ Myr) are found closer to the edges of the biconic-like structure. We estimate 12+log(O/H) = $7.23\pm0.04$ dex with the direct method, considering the whole nebula. Temperature fluctuations are detected, as variations of $T_{e}$ from $\sim 23400$K to $\sim 22000$K, and oxygen abundances from 7.24$\pm0.05$ to 7.50$\pm0.14$ dex, from the centre to the outskirts, respectively. In addition, SagDIG is in line with the low-mass end of the MZR. The observed stratification, the distribution of young MS stars, and the metal-poor content of the HII region suggest that the evolution of SagDIG seems to be actively shaped by stellar feedback processes, likely as the result of extragalactic HII regions being subject to the same gas physics as HII regions of the Milky-Way.

astro-ph.GA

Status of the International Linear Collider

This paper is not a proposal for a CERN future project but provides information on the International Linear Collider (ILC) considered for Japan in order to facilitate the European Strategy discussion in a global context. It describes progress to date, ongoing engineering studies, updated cost estimate for the machine at $\sqrt{s}=250~\rm GeV$ and the situation in Japan. The physics of the ILC is not presented here, but jointly for all Linear Collider projects in a separate document ``A Linear Collider Vision for the Future of Particle Physics'' submitted for the forthcoming European Strategy deliberations.

hep-ex

A Sequoia stellar candidate with very high 7Li and 9Be

Aims. The metal-poor star BPM 3066 belongs to the retrograde halo and presents unexpectedly strong spectral features of lithium. To gain insight into the origin of this peculiar abundance, we investigate the chemistry and kinematic properties of this star. Methods. We performed a local thermodynamic equilibrium chemical abundance analysis of UVES/VLT high-resolution spectra of BPM 3066 using ATLAS9 and ATLAS12 model atmospheres and the MyGIsFOS code. We further characterised the orbital properties of the star by integrating its orbit and analysing its integrals of motion using the galpy code. Results. The star BPM 3066 shows an exceptional overabundance of both lithium and beryllium. The abundances are A(Li) = 3.0 and A(Be) = 2.1, which are respectively about 0.8 and 2.2 dex higher than the Li and Be abundances expected at [Fe/H] = -1.5, the metallicity of the star. The observed ratio 7Li/9Be is 7.9, which is close to that expected from a synthesis by spallation processes. Overabundances of Si, Al, and of the neutron capture elements Sr,Y, Zr, and Ba are also measured. Kinematically, BPM 3066 has an eccentric, strongly retrograde orbit, confined to a height lower than 1 kpc from the galactic plane, and it is a candidate member of the Sequoia/Thamnos accreted galaxy. Conclusions. The processes leading to the 7Li and 9Be synthesis could have occurred in the environment of a hypernova. This is supported by some abundance anomalies like the high value of Si, [Si/Fe]=1.2 and [Si/O]=1.1. However, the simultaneous high values of N, Na, Al, Sc, Ti, and Cu are at odds with the expectations from a hypernova. Alternatively, the abundances of BPM 3066 could result from the engulfing of rocky planets that were rich in spallated Li and Be. In both cases, it is remarkable that such an extreme abundance pattern has been found in a star belonging to the Sequoia/Thamnos accreted galaxy.

astro-ph.GA

MINCE III. Detailed chemical analysis of the UVES sample

Context. The MINCE (Measuring at Intermediate Metallicity Neutron-Capture Elements) project aims to provide high quality neutron-capture abundances measurements in several hundred stars at intermediate metallicity,-2.5 < [Fe/H] < -1.5. This project will shed light on the origin of the neutron-capture elements and the chemical enrichment of the Milky Way. Aims. The goal of this work is to chemically characterize the second sample of the MINCE project and compare the abundances with the galactic chemical evolution model at our disposal. Methods. We performed a standard abundance analysis based on 1D LTE model atmospheres on high-resolution and high-signal-to-noise-ratio UVES spectra. Results. We provide the kinematic classification (i.e., thin disk, thick disk, thin-to-thick disk, halo, Gaia Sausage Enceladus, Sequoia) of 99 stars and the atmospheric parameters for almost all stars. We derive the abundances for light elements (from Na to Zn) and neutron-capture elements (Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu) in a subsample of 32 stars in the metallicity range -2.5 < Fe/H] < -1.00. In the subsample of 32 stars, we identify 8 active stars exhibiting (inverse) P-Cygni profile and one Li-rich star, CD 28-11039. We find a general agreement between the chemical abundances and the stochastic model computed for the chemical evolution of the Milky Way halo for the elements Mg, Ca, Si, Ti, Sc, Mn, Co, Ni, Zn, Rb, Sr, Y, Zr, Ba, La, and Eu . Conclusions. The MINCE project has already significantly increased the number of neutron-capture elements measurements in the intermediate metallicity range. The results from this sample are in perfect agreement with the previous MINCE sample. The good agreement between the chemical abundances and the chemical evolution model of the Galaxy supports the nucleosynthetic processes adopted to describe the origin of the n-capture elements.

astro-ph.GA

SDSS J102915.14+172927.9: Revisiting the chemical pattern

Context: The small- to intermediate-mass ($M <0.8 M_\odot$), most metal-poor stars that formed in the infancy of the Universe are still shining today in the sky. They are very rare, but their discovery and investigation brings new knowledge on the formation of the first stellar generations. Aims: SDSS J102915.14+172927.9 is one of the most metal-poor star known to date. Since no carbon can be detected in its spectrum, a careful upper limit is important, both to classify this star and to distinguish it from the carbon-enhanced stars that represent the majority at these metallicities. Methods: We undertook a new observational campaign to acquire high-resolution UVES spectra. The new spectra were combined with archival spectra in order to increase the signal-to-noise ratio. From the combined spectrum, we derived abundances for seven elements (Mg, Si, Ca, Ti, Fe, Ni, and a tentative Li) and five significant upper limits (C, Na, Al, Sr, and Ba). Results: The star has a carbon abundance A(C) <4.68 and therefore is not enhanced in carbon, at variance with the majority of the stars at this Fe regime, which typically show A(C)> 6.0. A feature compatible with the Li doublet at 670.7 nm is tentatively detected. Conclusions: The upper limit on carbon implies $Z<1.915 \times 10^{-6}$, more than 20 times lower than the most iron-poor star known. Therefore, the gas cloud out of which the star was formed did not cool via atomic lines but probably through dust. Fragmentation of the primordial cloud is another possibility for the formation of a star with a metallicity this low.

astro-ph.GA

The true nature of HE0057-5959, the most metal-poor Li-rich star

The Li-rich stars are a class of rare objects with A(Li) higher than that of other stars in the same evolutionary stage. Their origin is still debated and valuable routes are the Cameron-Fowler mechanism, mass transfer process in a binary system or engulfment of small bodies. Metal-poor ([Fe/H]<-1 dex) stars are only a small fraction of the entire population of Li-rich stars. We observed with MIKE at the Magellan Telescope the metal-poor ([Fe/H]=-3.95+-0.11 dex) giant star HE0057-5959, deriving A(Li)=+2.09+-0.07 dex. Such A(Li) is higher by about 1 dex than that of other stars in the same evolutionary stage. A previous analysis of the same target suggested that its high A(Li) reflects a still ongoing First Dredge-Up process. We revise the nature of HE0057-5959 by comparing its stellar parameters and A(Li) with appropriate stellar evolution models. This comparison rules out that HE0057-5959 is caught during its First Dredge-Up, being this latter already ended according to the parameters of this star. Its A(Li), remarkably higher than the typical lithium plateau drawn by similar giant stars, demonstrates that HE0057-5959 joins the class of the rare metal-poor Li-rich stars. HE0057-5959 is the most metal-poor Li-rich star discovered so far. We consider different scenarios to explain this star. No internal mixing able to activate the Cameron-Fowler mechanism is known for metal-poor stars at this evolutionary stage. Also the engulfment of planets is disfavored because such metal-poor stars should not host planets. Finally, HE0057-5959 is one of the most Na-rich among the Li-rich stars and we found that a strong excess of Na is common to the three Li-rich stars with [Fe/H]<-3 dex. This finding could support the scenario of mass transfer from a massive companion star in a binary system, even if we found no evidence of radial velocity variations.

astro-ph.SR

MINCE II. Neutron capture elements

The MINCE (Measuring at Intermediate metallicity Neutron-Capture Elements) project aims to gather the abundances of neutron-capture elements but also of light elements and iron peak elements in a large sample of giant stars in this metallicity range. T The aim of this work is to study the chemical evolution of galactic sub-components recently identified (i.e. Gaia Sausage Enceladus (GSE), Sequoia). We used high signal-to-noise ratios, high-resolution spectra and standard 1D LTE spectrum synthesis to determine the detailed abundances. We could determine the abundances for up to 10 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu) in 33 stars. The general trends of abundance ratios [n-capture element/Fe] versus [Fe/H] are in agreement with the results found in the literature. When our sample is divided in sub-groups depending on their kinematics, we found that the run of [Sr/Ba] vs [Ba/H] for the stars belonging to the GSE accretion event shows a tight anti-correlation. The results for the Sequoia stars, although based on a very limited sample, shows a [Sr/Ba] systematically higher than the [Sr/Ba] found in the GSE stars at a given [Ba/H] hinting at a different nucleosynthetic history. Stochastic chemical evolution models have been computed to understand the evolution of the GSE chemical composition of Sr and Ba. The first conclusions are that the GSE chemical evolution is similar to the evolution of a dwarf galaxy with galactic winds and inefficient star formation. Detailed abundances of neutron-capture elements have been measured in high-resolution, high signal-to-noise spectra of intermediate metal-poor stars, the metallicity range covered by the MINCE project. These abundances have been compared to detailed stochastic models of galactic chemical evolution.

astro-ph.SR

Differential reddening in 48 globular clusters: An end to the quest for the intracluster medium

For decades, it has been theorized that a tenuous but detectable intracluster medium should be present in globular clusters, which is continuously replenished by the gas and dust ejected by bright giants and periodically cleared by interactions with the Galactic disk. However, dedicated searches, especially in infrared and radio wavelengths, have returned mostly upper limits, which are lower than theoretical expectations by several orders of magnitude. We profited from recent wide-field photometry for 48 Galactic globular clusters to compute high-resolution maps of differential reddening, which can be used to correct any photometric catalog in these areas for reddening variations. Using 3D reddening maps from the literature, we evaluated the amount of foreground extinction. This allowed us to estimate the masses of the intracluster medium in our sample clusters, with an accuracy of one order of magnitude. Our estimates agree with the few available literature detections and with theoretical expectations. Because the discrepancy between observations and expectations only concerns literature upper limits, we explored possible reasons why they could be underestimated and we show that two recent discoveries can explain the discrepancy. The first is the recent discovery that the intracluster medium in 47 Tuc is not centrally concentrated. This is also supported by our maps, which in the majority of cases do not show a central reddening concentration. The second is the discovery that the dust in metal-poor ([Fe/H] less than about -1 dex) globular clusters is dominated by iron grains rather than silicates, which undermines previous dust mass estimates from observed upper limits. We conclude that current evidence, including our maps, does not contradict theoretical expectations and the problem of the missing intracluster medium is no longer an issue.

astro-ph.GA

The galaxy cluster AC114 III. The role of galaxy clusters in the mass-metallicity relation

We study the role of galaxy clusters in the mass-metallicity relation by using a sample of galaxies belonging to the AC114 (z=0.317) galaxy cluster. The data was taken from archival VIMOS-MOS spectroscopy to estimate gas-phase metallicities by using strong-line methods. On the other hand, the data obtained from DECaLS DR10 and VIKING DR4 ESO surveys were used to estimate photometric masses. We found that passive galaxies dominate the population of AC114 in the cluster center, whereas the star-forming members tend to be located outside this region. The constructed MZR from the latter indicates that AC114 SF galaxies have a lower metallicity content than foreground galaxies (z up to 0.28) and the same or even lower metallicities with respect to background galaxies (spanning redshifts 0.34 to 0.70), producing slightly a shallower slope compared to the field MZR at z~0.3. Additionally, it shows a higher scatter of 0.17 dex, consistent with MZRs of galaxy clusters reported in the literature. The responsible mechanisms behind the scatter are consistent with simulations and observations where dynamic and environmental effects such as galaxy-galaxy interactions and ram-pressure stripping are shaping the chemical evolution of the AC114 SF members. The former mechanism can decrease gas-phase metallicities because of pristine gas infall to the galaxy's outskirts, and the latter increases gas-phase metallicities because of the stripping of the metal-poor gas located at the outskirts of the SF members. The AC114 SF members are metal-poorer than field galaxies at the same redshift, so the cluster dominated by galaxy-galaxy interactions is the best scenario that explains the slightly shallower slope and the higher scatter of the AC114-MZR compared to the field MZR at z~0.3.

astro-ph.GA

High-speed stars. II. An unbound star, young stars, bulge metal-poor stars, and Aurora candidates

The data from the Gaia satellite led us to revise our conception of the Galaxy structure and history. Hitherto unknown components have been discovered and a deep re-thinking of what the Galactic halo is is in progress. We selected from the Gaia catalogue stars with extreme transverse velocities with respect to the Sun ($|V_T| > 500 $ and observed them with FORS2 at the ESO VLT, to classify them using both their chemical and dynamical properties. Two apparently young stars, identified in paper\,I, were observed with UVES. We derived abundances for Na, Mg, Ca, Ti, Mn, and Fe, analysing the spectra with while for Ba we used line profile fitting. We computed actions from parallaxes and kinematical data. The stars span the metallicity range $ Fe/H -0.5$ with $ Fe/H = -1.6$. Star GHS143 has a total speed of about 1440 which is almost three times faster than the local escape velocity of 522 strongly implying this star is unbound to the Galaxy. Remarkably, this star is not escaping from the Galaxy, but it is falling into it. Ten stars are apparently young with masses in excess of 1.3M. Their interpretation as evolved blue stragglers is doubtful. The existence of a young metal-poor population is possible. The two stars observed with UVES show no lithium, suggesting they are blue stragglers. We detected a metal-poor population, confined to the bulge, that we call SpiteF, and argue that it is the result of a recent accretion event. We detect 102 candidates of the Aurora population that should have formed prior to the formation of the disc. Our sample is non-homogeneous and mainly retrograde. The stars are metal poor, and 23<!PCT!> have Fe/H -2.0$. Our selection is efficient at finding very metal-poor stars, but it selects peculiar populations.

astro-ph.GA

The Gaia RVS benchmark stars II. A sample of stars selected for their Gaia high radial velocity

The Gaia satellite has already provided the astronomical community with three data releases, and the Radial Velocity Spectrometer (RVS) on board Gaia has provided the radial velocity for 33 million stars. When deriving the radial velocity from the RVS spectra, several stars are measured to have large values. To verify the credibility of these measurements, we selected some bright stars with the modulus of radial velocity in excess of 500\ to be observed with SOPHIE at OHP and UVES at VLT. This paper is devoted to investigating the chemical composition of the stars observed with UVES. We derived atmospheric parameters using Gaia photometry and parallaxes, and we performed a chemical analysis using the code. We find that the sample consists of metal-poor stars, although none have extremely low metallicities. The abundance patterns match what has been found in other samples of metal-poor stars selected irrespective of their radial velocities. We highlight the presence of three stars with low Cu and Zn abundances that are likely descendants of pair-instability supernovae. Two stars are apparently younger than 1\,Ga, and their masses exceed twice the turn-off mass of metal-poor populations. This makes it unlikely that they are blue stragglers because it would imply they formed from triple or multiple systems. We suggest instead that they are young metal-poor stars accreted from a dwarf galaxy. Finally, we find that the star RVS721 is associated with the Gjoll stream, which itself is associated with the Globular Cluster NGC\,3201.

astro-ph.GA

The nature of medium-period variables on the extreme horizontal branch I. X-shooter study of variable stars in the globular cluster $\omega$ Cen

A fraction of the extreme horizontal branch stars of globular clusters exhibit a periodic light variability that has been attributed to rotational modulation caused by surface spots. These spots are believed to be connected to inhomogeneous surface distribution of elements. However, the presence of such spots has not been tested against spectroscopic data. We analyzed the phase-resolved ESO X-shooter spectroscopy of three extreme horizontal branch stars that are members of the globular cluster $\omega$ Cen and also display periodic light variations. The aim of our study is to understand the nature of the light variability of these stars and to test whether the spots can reproduce the observed variability. Our spectroscopic analysis of these stars did not detect any phase-locked abundance variations that are able to reproduce the light variability. Instead, we revealed the phase variability of effective temperature and surface gravity. In particular, the stars show the highest temperature around the light maximum. This points to pulsations as a possible cause of the observed spectroscopic and photometric variations. However, such an interpretation is in a strong conflict with Ritter's law, which relates the pulsational period to the mean stellar density. The location of the $\omega$ Cen variable extreme horizontal branch stars in HR diagram corresponds to an extension of PG 1716 stars toward lower temperatures or blue, low-gravity, large-amplitude pulsators toward lower luminosities, albeit with much longer periods. Other models of light variability, namely, related to temperature spots, should also be tested further. The estimated masses of these stars in the range of $0.2-0.3\,M_\odot$ are too low for helium-burning objects.

astro-ph.SR