arXiv subjects

V. Ripepi

Publications and source records attributed to V. Ripepi.

At least 37 records · Page 2Linked to original sources

arXiv2024

OGLE GD-CEP-0516: the most metal-poor lithium-rich Galactic Cepheid

Classical Cepheids (DCEPs) are important astrophysical objects not only as standard candles for the determination of the cosmic distance ladder but also as a test-bed for the stellar evolution theory, thanks to the connection between their pulsation (periods, amplitudes) and stellar (luminosity, mass, effective temperature, metallicity) parameters. We aim to unveil the nature of the Galactic DCEP OGLE-GD-CEP-0516 and other DCEPs showing an enhanced abundance of lithium in their atmospheres. We have collected a high-resolution spectrum for OGLE-GD-CEP-0516 with UVES@VLT. Accurate stellar parameters: effective temperature, gravity, micro- and macro-turbulence, radial velocity, and metal abundances were measured for this star, using spectral synthesis techniques based on LTE plane-parallel atmospheric model. We measured a chemical pattern with most elements under-abundant compared with the Sun, i.e. [Fe/H]\,=\,$-$0.54\,$\pm$\,0.16~dex, [C/H]\,=\,$-$0.45\,$\pm$\,0.05~dex, or [Mg/H]\,=\,$-$0.40\,$\pm$\,0.16~dex among others. In particular, we measured a lithium abundance A(Li)\,=\,3.06\,$\pm$\,0.10~dex for OGLE-GD-CEP-0516, which makes this object the sixth Li-rich object among the Milky Way DCEPs. Our results favour the scenario in which the six Galactic Li-rich DCEPs are first-crossing the instability strip having had slowly-rotating progenitors during their main sequence phase. This study explored the link between lithium abundance and the pulsation period in classical Cepheids. It found that brighter Cepheids, indicative of higher mass, show enhanced lithium abundance, contrary to predictions from evolutionary models considering rotation. Additionally, an analysis of lithium abundance versus [Fe/H] revealed a lack of significant correlation, contradicting expectations from galactic chemical evolution (GCE) models.

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arXiv2024

Cepheid Metallicity in the Leavitt Law (C-MetaLL) Survey. V. New multiband (grizJHKs) Cepheid light curves and period-luminosity relations

We present homogeneous multiband (grizJHKs) time-series observations of 78 Cepheids including 49 fundamental mode variables and 29 first-overtone mode variables. These observations were collected simultaneously using the ROS2 and REMIR instruments at the Rapid Eye Mount telescope. The Cepheid sample covers a large range of distances (0.5 - 19.7 kpc) with varying precision of parallaxes, and thus astrometry-based luminosity fits were used to derive PL and PW relations in optical Sloan (griz) and near-infrared (JHKs) filters. These empirically calibrated relations exhibit large scatter primarily due to larger uncertainties in parallaxes of distant Cepheids, but their slopes agree well with those previously determined in the literature. Using homogeneous high-resolution spectroscopic metallicities of 61 Cepheids covering -1.1 < [Fe/H] < 0.6 dex, we quantified the metallicity dependence of PL and PW relations which varies between $-0.30\pm0.11$ (in Ks) and $-0.55\pm0.12$ (in z) mag/dex in grizJHKs bands. However, the metallicity dependence in the residuals of the PL and PW relations is predominantly seen for metal-poor stars ([Fe/H] < -0.3 dex), which also have larger parallax uncertainties. The modest sample size precludes us from separating the contribution to the residuals due to parallax uncertainties, metallicity effects, and reddening errors. While this Cepheid sample is not optimal for calibrating the Leavitt law, upcoming photometric and spectroscopic datasets of the C-MetaLL survey will allow the accurate derivation of PL and PW relations in the Sloan and near-infrared bandpasses, which will be useful for the distance measurements in the era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time and upcoming extremely large telescopes.

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arXiv2023

Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey: IV. The metallicity dependence of Cepheid Period-Luminosity relations

Classical Cepheids (DCEPs) play a fundamental role in the calibration of the extra-galactic distance ladder which eventually leads to the determination of the Hubble constant($H_0$) thanks to the period-luminosity ($PL$) and period-Wesenheit ($PW$) relations exhibited by these pulsating variables. Therefore, it is of great importance to establish the dependence of $PL/PW$ relations on metallicity. We aim at quantifying the metallicity dependence of the Galactic DCEPs' $PL/PW$ relations for a variety of photometric bands ranging from optical to near-infrared. We gathered a literature sample of 910 DCEPs with available [Fe/H] values from high-resolution spectroscopy or metallicities from \gaia\ Radial Velocity Spectrometer. For all these stars, we collected photometry in the $G_{BP},G_{RP},G,I,V,J,H,K_S$ bands and astrometry from the \gaia\ DR3. These data have been used to investigate the metal dependence of both intercepts and slopes of a variety of $PL/PW$ relations at multiple wavelengths. We find a large negative metallicity effect on the intercept ($γ$ coefficient) of all the $PL/PW$ relations investigated in this work, while present data still do not allow us to draw firm conclusions regarding the metal dependence of the slope ($δ$ coefficient). The typical values of $γ$ are around $-0.4:-0.5$ mag/dex, i.e. larger than most of the recent determinations present in the literature. We carried out several tests which confirm the robustness of our results. As in our previous works, we find that the inclusion of global zero point offset of \gaia\ parallaxes provides smaller values of $γ$ (in an absolute sense). However, the assumption of the geometric distance of the LMC seems to indicate that larger values of $γ$ (in an absolute sense) would be preferred.

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arXiv2023

First spectroscopic investigation of Anomalous Cepheid variables

Anomalous Cepheids (ACEPs) are intermediate mass metal-poor pulsators mostly discovered in dwarf galaxies of the Local Group. However, recent Galactic surveys, including the Gaia DR3, found a few hundreds of ACEPs in the Milky Way. Their origin is not well understood. We aim to investigate the origin and evolution of Galactic ACEPs by studying for the first time the chemical composition of their atmospheres. We used UVES@VLT to obtain high-resolution spectra for a sample of 9 ACEPs belonging to the Galactic halo. We derived the abundances of 12 elements, including C, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Ni, Y, and Ba. We complemented these data with literature abundances for an additional three ACEPs that were previously incorrectly classified as type II Cepheids, thus increasing the sample to a total of 12 stars. All the investigated ACEPs have an iron abundance [Fe/H]$<-1.5$ dex as expected from theoretical predictions for these pulsators. The abundance ratios of the different elements to iron show that the ACEP's chemical composition is generally consistent with that of the Galactic halo field stars, except the Sodium, which is found overabundant in 9 out of the 11 ACEPs where it was measured, in close similarity with second-generation stars in the Galactic Globular Clusters. The same comparison with dwarf and ultra-faint satellites of the Milky Way reveals more differences than similarities so it is unlikely that the bulk of Galactic ACEPs originated in such a kind of galaxies which subsequently dissolved in the Galactic halo. The principal finding of this work is the unexpected overabundance of Sodium in ACEPs. We explored several hypotheses to explain this feature, finding that the most promising scenario is the evolution of low-mass stars in a binary system with either mass transfer or merging. Detailed modelling is needed to confirm this hypothesis.

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arXiv2023

A multi-wavelength analysis of BL Her stars: Models versus Observations

We present new theoretical period--luminosity (PL) and period--radius (PR) relations at multiple wavelengths (Johnson--Cousins--Glass and {\sl Gaia} passbands) for a fine grid of BL~Herculis models computed using {\sc mesa-rsp}. The non-linear models were computed for periods typical of BL~Her stars, i.e. $1\leq P ({\rm days}) \leq4$, covering a wide range of input parameters: metallicity ($-$2.0 dex $\leq$ [Fe/H] $\leq$ 0.0 dex), stellar mass (0.5--0.8 M$_{\odot}$), luminosity (50--300 L$_{\odot}$) and effective temperature (full extent of the instability strip; in steps of 50K). We investigate the impact of four sets of convection parameters on multi-wavelength properties. Most empirical relations match well with theoretical relations from the BL~Her models computed using the four sets of convection parameters. No significant metallicity effects are seen in the PR relations. Another important result from our grid of BL~Her models is that it supports combining PL relations of RR Lyrae and Type~II Cepheids together as an alternative to classical Cepheids for the extragalactic distance scale calibration.

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arXiv2023

Cepheid metallicity in the Leavitt law (C-MetaLL) survey -- III. Simultaneous derivation of the Gaia parallax offset and Period-Luminosity-Metallicity coefficients

Classical Cepheids (DCEPs) are the most important standard candles in the extra-galactic distance scale thanks to the Period-Luminosity ($\rm PL$), Period-Luminosity-Color ($\rm PLC$) and Period-Wesenheit ($\rm PW$) relations that hold for these objects. The advent of the {\it Gaia} mission, and in particular the Early Data Release 3 (EDR3) provided accurate parallaxes to calibrate these relations. In order to fully exploit {\it Gaia} measurements, the zero point (ZP) of {\it Gaia} parallaxes should be determined with an accuracy of a few $\rm μas$. The individual ZP corrections provided by the {\it Gaia} team depend on the magnitude and the position on the sky of the target. In this paper, we use an implicit method that relies on the Cepheid $\rm PL$ and $\rm PW$ relations to evaluate the ensemble {\it Gaia} parallax zero point. The best inferred estimation of the offset value needed to additionally correct (after the {\it Gaia} team correction) the {\it Gaia} parallaxes of the present DCEP sample, amounts to $\rm -22\pm 4\, μas$. This value is in agreement with the most recent literature values and confirms that the correction proposed by the {\it Gaia} team over-corrected the parallaxes.\\ As a further application of our results, we derive an estimate of the Large Magellanic Cloud distance ($\rm μ_0=18.49\pm 0.06\, mag$), in very good agreement with the currently accepted value obtained through geometric methods.

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arXiv2022

Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey: II.High-resolution spectroscopy of the most metal poor Galactic Cepheids

Classical Cepheids (DCEPs) are the first fundamental step in the calibration of the cosmological distance ladder. Furthermore, they represent powerful tracers in the context of Galactic studies. We have collected high-resolution spectroscopy with UVES@VLT for a sample of 65 DCEPs. The majority of them are the faintest DCEPs ever observed in the Milky Way. For each target, we derived accurate atmospheric parameters, radial velocities, and abundances for 24 different species. The resulting iron abundances range between +0.3 and $-$1.1 dex with the bulk of stars at [Fe/H]$\sim-0.5$ dex. Our sample includes the most metal-poor DCEPs observed so far with high-resolution spectroscopy. We complement our sample with literature data obtaining a complete sample of 637 DCEPs and use Gaia Early Data Release 3 (EDR3) photometry to determine the distance of the DCEPs from the Period-Wesenheit-Metallicity relation. Our more external data trace the Outer arm (at Galactocentric radius ($R_{GC})\sim$16--18 kpc) which appears significantly warped. We investigate the metallicity gradient of the Galactic disc using this large sample, finding a slope of $-0.060 \pm 0.002$ dex kpc$^{-1}$, in very good agreement with previous results based both on DCEPs and open clusters. We also report a possible break in the gradient at $R_{GC}$=9.25 kpc with slopes of $-0.063 \pm 0.007$ and $-0.079 \pm 0.003$ dex kpc$^{-1}$ for the inner and outer sample, respectively. The two slopes differ by more than 1 $σ$. A more homogeneous and extended DCEPs sample is needed to further test the plausibility of such a break.

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arXiv2022

Near-infrared observations of RR Lyrae and Type II Cepheid variables in the metal-rich bulge globular cluster NGC 6441

NGC 6441 is a bulge globular cluster with an unusual horizontal branch morphology and a rich population of RR Lyrae (RRL) and Type II Cepheid (T2C) variables that is unexpected for its relatively high metallicity. We present near-infrared (NIR, $JHK_s$) time-series observations of 42 RRL, 8 T2Cs, and 10 eclipsing binary candidate variables in NGC 6441. The multi-epoch observations were obtained using the FLAMINGOS-2 instrument on the 8-m Gemini South telescope. Multi-band data are used to investigate pulsation properties of RRL and T2C variables including their light curves, instability strip, period--amplitude diagrams, and period--luminosity and period--wesenheit relations (PLRs and PWRs) in $JHK_s$ filters. NIR pulsation properties of RRL variables are well fitted with theoretical models that have canonical helium content and mean-metallicity of NGC 6441 ([Fe/H]$=-0.44\pm0.07$~dex). The helium-enhanced RRL models predict brighter NIR magnitudes and bluer colors than the observations of RRL in the cluster. This suggests that these RRL variables in NGC 6441 are either not significantly helium enhanced as previously thought or the impact of such enhancement is smaller in NIR than the predictions of the pulsation models. We also use theoretical calibrations of RRL period--luminosity--metallicity (PLZ) relations to simultaneously estimate the mean reddening, $E(J-K_s)=0.26\pm0.06$~mag, and the distance, $d=12.67\pm0.09$~kpc, to NGC 6441. Our mean reddening value and the distance are consistent with the independent estimates from the bulge reddening map based on red clump stars and the latest Gaia data, respectively. Our distance and reddening values provide a very good agreement between the PLRs of T2Cs in NGC 6441 and those for RRL and T2Cs in Galactic globular clusters that span a broad range of metallicity.

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arXiv2022

Gaia Data Release 3: Pulsations in main sequence OBAF-type stars

The third Gaia data release provides photometric time series covering 34 months for about 10 million stars. For many of those stars, a characterisation in Fourier space and their variability classification are also provided. This paper focuses on intermediate- to high-mass (IHM) main sequence pulsators M >= 1.3 Msun) of spectral types O, B, A, or F, known as beta Cep, slowly pulsating B (SPB), delta Sct, and gamma Dor stars. These stars are often multi-periodic and display low amplitudes, making them challenging targets to analyse with sparse time series. All datasets used in this analysis are part of the Gaia DR3 data release. The photometric time series were used to perform a Fourier analysis, while the global astrophysical parameters necessary for the empirical instability strips were taken from the Gaia DR3 gspphot tables, and the vsini data were taken from the Gaia DR3 esphs tables. We show that for nearby OBAF-type pulsators, the Gaia DR3 data are precise and accurate enough to pinpoint them in the Hertzsprung-Russell diagram. We find empirical instability strips covering broader regions than theoretically predicted. In particular, our study reveals the presence of fast rotating gravity-mode pulsators outside the strips, as well as the co-existence of rotationally modulated variables inside the strips as reported before in the literature. We derive an extensive period-luminosity relation for delta Sct stars and provide evidence that the relation features different regimes depending on the oscillation period. Finally, we demonstrate how stellar rotation attenuates the amplitude of the dominant oscillation mode of delta Sct stars.

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arXiv2022

Gaia Data Release 3: Mapping the asymmetric disc of the Milky Way

With the most recent Gaia data release the number of sources with complete 6D phase space information (position and velocity) has increased to well over 33 million stars, while stellar astrophysical parameters are provided for more than 470 million sources, in addition to the identification of over 11 million variable stars. Using the astrophysical parameters and variability classifications provided in Gaia DR3, we select various stellar populations to explore and identify non-axisymmetric features in the disc of the Milky Way in both configuration and velocity space. Using more about 580 thousand sources identified as hot OB stars, together with 988 known open clusters younger than 100 million years, we map the spiral structure associated with star formation 4-5 kpc from the Sun. We select over 2800 Classical Cepheids younger than 200 million years, which show spiral features extending as far as 10 kpc from the Sun in the outer disc. We also identify more than 8.7 million sources on the red giant branch (RGB), of which 5.7 million have line-of-sight velocities, allowing the velocity field of the Milky Way to be mapped as far as 8 kpc from the Sun, including the inner disc. The spiral structure revealed by the young populations is consistent with recent results using Gaia EDR3 astrometry and source lists based on near infrared photometry, showing the Local (Orion) arm to be at least 8 kpc long, and an outer arm consistent with what is seen in HI surveys, which seems to be a continuation of the Perseus arm into the third quadrant. Meanwhile, the subset of RGB stars with velocities clearly reveals the large scale kinematic signature of the bar in the inner disc, as well as evidence of streaming motions in the outer disc that might be associated with spiral arms or bar resonances. (abridged)

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arXiv2022

KMHK 1762: Another star cluster in the Large Magellanic Cloud age gap

The star cluster (SC) age distribution of the Large Magellanic Cloud (LMC) exhibits a gap from $\sim$ 4 to 10 Gyr ago, with an almost total absence of SCs. Within this age gap, only two confirmed SCs have been identified hitherto. Nonetheless, the star field counterpart does not show the same characteristics, making the LMC a peculiar galaxy where star formation history and cluster formation history appear to differ significantly. We re-analyzed the color-magnitude diagram (CMD) of the KMHK 1762 SC by using the deep optical photometry provided by the "Yes, Magellanic Clouds Again" survey, to robustly assess its age. First, we partially removed foreground and/or field stars by means of parallaxes and proper motions obtained from the {\it Gaia} Early Data Release 3. Then, we applied the Automated Stellar Cluster Analysis package to the cleaned photometric catalogue to identify the isochrone that best matches the CMD of KMHK 1762. The estimated age of KMHK 1762 is $\log (t) = 9.74 \pm 0.15$ dex ($\sim$5.5 Gyr), that is more than 2 Gyr older than the previous estimation which was obtained with shallower photometry. This value makes KMHK 1762 the third confirmed age gap SC of the LMC. The physical existence of a quiescent period of the LMC SC formation is questioned. We suggest it can be the result of an observational bias, originated by the combination of shallow photometry and limited investigation of the LMC periphery.

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arXiv2022

Gaia Data Release 3: Cross-match of Gaia sources with variable objects from the literature

Context. In the current ever increasing data volumes of astronomical surveys, automated methods are essential. Objects of known classes from the literature are necessary for training supervised machine learning algorithms, as well as for verification/validation of their results. Aims.The primary goal of this work is to provide a comprehensive data set of known variable objects from the literature cross-matched with \textit{Gaia}~DR3 sources, including a large number of both variability types and representatives, in order to cover as much as possible sky regions and magnitude ranges relevant to each class. In addition, non-variable objects from selected surveys are targeted to probe their variability in \textit{Gaia} and possible use as standards. This data set can be the base for a training set applicable in variability detection, classification, and validation. MethodsA statistical method that employed both astrometry (position and proper motion) and photometry (mean magnitude) was applied to selected literature catalogues in order to identify the correct counterparts of the known objects in the \textit{Gaia} data. The cross-match strategy was adapted to the properties of each catalogue and the verification of results excluded dubious matches. Results.Our catalogue gathers 7\,841\,723 \textit{Gaia} sources among which 1.2~million non-variable objects and 1.7~million galaxies, in addition to 4.9~million variable sources representing over 100~variability (sub)types. Conclusions.This data set served the requirements of \textit{Gaia}'s variability pipeline for its third data release (DR3), from classifier training to result validation, and it is expected to be a useful resource for the scientific community that is interested in the analysis of variability in the \textit{Gaia} data and other surveys.

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arXiv2022

Gaia DR3: Specific processing and validation of all-sky RR Lyrae and Cepheid stars -- The RR Lyrae sample

Gaia DR3 publishes a catalogue of full-sky RR Lyrae stars (RRLs) observed during the initial 34 months of science operations, that were processed through the Specific Object Study (SOS) pipeline for Cepheids and RRLs (SOS Cep&RRL) observed by Gaia. The SOS Cep&RRL validation of DR3 candidate RRLs relies on tools that include the Period (P) G-amplitude diagram and the P-phi21 and -phi31 parameters of the G light curve Fourier decomposition, based on a sample of bona fide known RRLs (Gold Sample). The SOS processing led to a catalogue of 271779 RRLs listed in the vari_rrlyrae table of DR3. By dropping sources that clearly are contaminants, or have an uncertain classification we produce the final catalogue of SOS-confirmed DR3 RRLs containing 270905 sources (174947 fundamental mode, 93952 first overtone and 2006 double-mode RRLs) confirmed and fully characterised by the SOS Cep&RRL pipeline. They are distributed all over the sky, including 95 globular clusters and 25 Milky Way companions. RVS time series radial velocities are also published for 1096 RRLs and 799 Cepheids. Of the 270905 DR3 RRLs, 200294 are already known in the literature and 70611 are, to the best of our knowledge, new discoveries by Gaia. An estimate of the interstellar absorption is published for 142660 fundamental-mode RRLs from a relation based on the G-band amplitude and the pulsation period. Metallicities derived from the Periods and the phi31 Fourier parameters of the G-light curves are also released for 133559 RRLs. The final Gaia DR3 catalogue of confirmed RRLs almost doubles the DR2 RRLs catalogue. An increase of statistical significance, a better characterization of the RRLs pulsational and astrophysical parameters, and the improved astrometry published with Gaia EDR3, make the SOS Cep&RRL DR3 sample, the largest, most homogeneous and parameter-rich catalogue of All-Sky RRLs published so far.

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arXiv2022

Gaia DR3: Specific processing and validation of all-sky RR Lyrae and Cepheid stars -- The Cepheid sample

Context. Cepheids are pulsating stars that play a crucial role in several astrophysical contexts. Among the different types, the Classical Cepheids are fundamental tools for the calibration of the extragalactic distance ladder. They are also powerful stellar population tracers in the context of Galactic studies. The Gaia Third Data Release (DR3) publishes improved data on Cepheids collected during the initial 34 months of operations. Aims. We present the Gaia DR3 catalogue of Cepheids of all types, obtained through the analysis carried out with the Specific Object Study (SOS) Cep&RRL pipeline. Methods. We discuss the procedures adopted to clean the Cepheid sample from spurious objects, to validate the results, and to re-classify sources with a wrong outcome from the SOS Cep&RRL pipeline. Results. The Gaia DR3 includes multi-band time-series photometry and characterisation by the SOS Cep&RRL pipeline for a sample of 15,006 Cepheids of all types. The sample includes 4,663, 4,616, 321 and 185 pulsators, distributed in the LMC, SMC, M31 and M33, respectively, as well as 5 221 objects in the remaining All Sky sub-region which includes stars in the MW field/clusters and in a number of small satellites of our Galaxy. Among this sample, 327 objects were known as variable stars in the literature but with a different classification, while, to the best of our knowledge, 474 stars have not been reported before to be variable stars and therefore they likely are new Cepheids discovered by Gaia.

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arXiv2022

Gaia Data Release 3. Summary of the variability processing and analysis

Context. Gaia has been in operations since 2014. The third Gaia data release expands from the early data release (EDR3) in 2020 by providing 34 months of multi-epoch observations that allowed us to probe, characterise and classify systematically celestial variable phenomena. Aims. We present a summary of the variability processing and analysis of the photometric and spectroscopic time series of 1.8 billion sources done for Gaia DR3. Methods. We used statistical and Machine Learning methods to characterise and classify the variable sources. Training sets were built from a global revision of major published variable star catalogues. For a subset of classes, specific detailed studies were conducted to confirm their class membership and to derive parameters that are adapted to the peculiarity of the considered class. Results. In total, 10.5 million objects are identified as variable in Gaia DR3 and have associated time series in G, GBP, and GRP and, in some cases, radial velocity time series. The DR3 variable sources subdivide into 9.5 million variable stars and 1 million Active Galactic Nuclei/Quasars. In addition, supervised classification identified 2.5 million galaxies thanks to spurious variability induced by the extent of these objects. The variability analysis output in the DR3 archive amounts to 17 tables containing a total of 365 parameters. We publish 35 types and sub-types of variable objects. For 11 variable types, additional specific object parameters are published. An overview of the estimated completeness and contamination of most variability classes is provided. Conclusions. Thanks to Gaia we present the largest whole-sky variability analysis based on coherent photometric, astrometric, and spectroscopic data. Later Gaia data releases will more than double the span of time series and the number of observations, thus allowing for an even richer catalogue in the future.

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arXiv2022

Deep VLT photometry of the faint stellar system in the Large Magellanic Cloud periphery YMCA-1

We present FORS2@VLT follow-up photometry of YMCA-1, a recently discovered stellar system located 13\degr~from the Large Magellanic Cloud (LMC) center. The deep color-magnitude diagram (CMD) reveals a well-defined main sequence (MS) and a handful of stars in the post-MS evolutionary phases. We analyse the YMCA-1 CMD by means of the automated isochrone matching package {\tt ASteCA} and model its radial density profile with a Plummer function. We find that YMCA-1 is an old ($11.7^{+1.7}_{-1.3}$~Gyr), metal-intermediate ([Fe/H] $\simeq -1.12^{+0.21}_{-0.13}$~dex), compact (r$_{\rm h} = 3.5 \pm 0.5$ pc), low-mass (M $= 10^{2.45 \pm 0.02} M_{\odot}$) and low-luminosity (M$_V = -0.47 \pm 0.57$~mag) stellar system. The estimated distance modulus ($μ_0 = 18.72^{+0.15}_{-0.17}$~mag), corresponding to about 55~kpc, suggests that YMCA-1 is associated to the LMC, but we cannot discard the scenario in which it is a Milky Way satellite. The structural parameters of YMCA-1 are remarkably different compared with those of the 15 known old LMC globular clusters. In particular, it resides in a transition region of the M$_V$-r$_h$ plane, in between the ultra-faint dwarf galaxies and the classical old clusters, and close to SMASH-1, another faint stellar system recently discovered in the LMC surroundings.

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arXiv2022

The VMC survey -- XLVI. Stellar proper motions in the centre of the Large Magellanic Cloud

We present proper motion (PM) measurements within the central region of the Large Magellanic Cloud (LMC) using near-infrared data from the VISTA survey of the Magellanic Cloud system (VMC). This work encompasses 18 VMC tiles covering a total sky area of $\sim$28~deg$^2$. We computed absolute stellar PMs from multi-epoch observations in the $K_s$ filter over time baselines between $\sim$12 and 47 months. Our final catalogue contains $\sim$6,322,000 likely LMC member stars with derived PMs. We employed a simple flat-rotating disc model to analyse and interpret the PM data. We found a stellar centre of rotation ($α_0$ = 79.95 deg +0.22 -0.23, $δ_0$ = -69.31 deg +0.12 -0.11) that is in agreement with that resulting from Hubble Space Telescope data. The inferred viewing angles of the LMC disc (i = 33.5 deg +1.2 -1.3, $Θ$ = 129.8 deg +1.9 -1.9) are in good agreement with values from the literature but suggest a higher inclination of the central parts of the LMC. Our data confirm a higher rotation amplitude for the young ($\lesssim$0.5~Gyr) stars compared to the intermediate-age/old ($\gtrsim$1~Gyr) population, which can be explained by asymmetric drift. We constructed spatially resolved velocity maps of the intermediate-age/old and young populations. Intermediate-age/old stars follow elongated orbits parallel to the bar's major axis, providing first observational evidence for $x_1$ orbits within the LMC bar. In the innermost regions, the motions show more chaotic structures. Young stars show motions along a central filamentary bar structure.

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arXiv2022

Discovery of NES, an extended tidal structure in the North-East of the Large Magellanic Cloud

We report on the discovery of a new diffuse stellar sub-structure protruding for 5 degrees from the North-Eastern rim of the LMC disc. The structure, that we dub North-East Structure (NES), was identified by applying a Gaussian Mixture Model to a sample of strictly selected candidate members of the Magellanic System, extracted from the Gaia EDR3 catalogue. The NES fills the gap between the outer LMC disk and other known structures in the same region of the LMC, namely the Northern tidal arm (NTA) and the Eastern sub-structures (ES). Particularly noteworthy is that the NES is placed in a region where N-body simulations foresee a bending of the LMC disc due to tidal stresses induced by the MW. The velocity field in the plane of the sky indicates that the complex of tidal structures in the North-Eastern part of the LMC, including NES, is subject to coherent radial motions. Additional data, as well as extensive dynamical modeling, is required to shed light on the origin of NES as well as on the relationships with the surrounding substructures.

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