arXiv subjects

S. Cassisi

Publications and source records attributed to S. Cassisi.

At least 73 records · Page 4Linked to original sources

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XVI. The helium abundance of multiple populations

Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first- (1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host sub-stellar populations with different chemical composition. We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G stars are consistent with being enhanced in helium with respect to 1G. The maximum helium variation ranges from less than 0.01 to more than 0.10 in helium mass fraction and correlates with both the cluster mass and the color extension of the horizontal branch (HB). These findings demonstrate that the internal helium variation is one of the main (second) parameters governing the HB morphology.

astro-ph.SR↗

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XVII. Public Catalogue Release

In this paper we present the astro-photometric catalogues of 56 globular clusters and one open cluster. Astrometry and photometry are mainly based on images collected within the "HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation" (GO-13297, PI:~Piotto), and the "ACS Survey of Galactic Globular Clusters" (GO-10775, PI:~Sarajedini). For each source in the catalogues for which we have reliable proper motion we also publish a membership probability for separation of field and cluster stars. These new catalogues, which we make public in Mikulski Archive for Space Telescopes, replace previous catalogues by Paper VIII of this series.

astro-ph.SR↗

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XV. The dynamical clock: reading cluster dynamical evolution from the segregation level of blue straggler stars

The parameter A+, defined as the area enclosed between the cumulative radial distribution of blue straggler stars (BSSs) and that of a reference population, is a powerful indicator of the level of BSS central segregation. As part of the Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GCs), here we present the BSS population and the determination of A+ in 27 GCs observed out to about one half-mass radius. In combination with 21 additional clusters discussed in a previous paper this provides us with a global sample of 48 systems (corresponding to \sim 32\% of the Milky Way GC population), for which we find a strong correlation between A+ and the ratio of cluster age to the current central relaxation time. Tight relations have been found also with the core radius and the central luminosity density, which are expected to change with the long-term cluster dynamical evolution. An interesting relation is emerging between A+ and the ratio of the BSS velocity dispersion relative to that of main sequence turn-off stars, which measures the degree of energy equipartition experienced by BSSs in the cluster. These results provide further confirmation that BSSs are invaluable probes of GC internal dynamics and A+ is a powerful dynamical clock.

astro-ph.GA↗

arXiv2018

The search for multiple populations in Magellanic Cloud Clusters IV: Coeval multiple stellar populations in the young star cluster NGC 1978

We have recently shown that the $\sim2$ Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the sub-giant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Due to its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star-formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star-formation epochs have occurred within NGC 1978. First, we use UV CMDs to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of $1\pm20$ Myr between them. This is in tension with predictions from the AGB scenario for the origin of multiple populations. Second, we estimate the broadness of the main sequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of $\sim$65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate age clusters, while it fully supports predictions from the stellar rotation model.

astro-ph.SR↗

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XIV. Multiple stellar populations within M 15 and their radial distribution

In the context of the Hubble Space Telescope UV Survey of Galactic Globular Clusters (GCs), we derived high-precision, multi-band photometry to investigate the multiple stellar populations in the massive and metal-poor GC M 15. By creating for red-giant branch (RGB) stars of the cluster a 'chromosome map', which is a pseudo two-colour diagram made with appropriate combination of F275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads around two of the three already known stellar populations. These spreads cannot be produced by photometric errors alone and could hide the existence of (two) additional populations. This discovery increases the complexity of the multiple-population phenomenon in M 15. Our analysis shows that M 15 exhibits a faint sub-giant branch (SGB), which is also detected in colour-magnitude diagrams (CMDs) made with optical magnitudes only. This poorly-populated SGB includes about 5% of the total number of SGB stars and evolves into a red RGB in the m_F336W vs. m_F336W-m_F814W CMD, suggesting that M 15 belongs to the class of Type II GCs. We measured the relative number of stars in each population at various radial distances from the cluster centre, showing that all of these populations share the same radial distribution within statistic uncertainties. These new findings are discussed in the context of the formation and evolution scenarios of the multiple populations.

astro-ph.SR↗

arXiv2018

The Updated BaSTI Stellar Evolution Models and Isochrones: I. Solar Scaled Calculations

We present an updated release of the BaSTI (a Bag of Stellar Tracks and Isochrones) stellar model and isochrone library for a solar scaled heavy element distribution. The main input physics changed from the previous BaSTI release include the solar metal mixture, electron conduction opacities, a few nuclear reaction rates, bolometric corrections, and the treatment of the overshooting efficiency for shrinking convective cores. The new model calculations cover a mass range between 0.1 and 15 Msun, 22 initial chemical compositions between [Fe/H]=-3.20 and +0.45, with helium to metal enrichment ratio dY /dZ=1.31. The isochrones cover an age range between 20 Myr and 14.5 Gyr, take consistently into account the pre-main sequence phase, and have been translated to a large number of popular photometric systems. Asteroseismic properties of the theoretical models have also been calculated. We compare our isochrones with results from independent databases and with several sets of observations, to test the accuracy of the calculations. All stellar evolution tracks, asteroseismic properties and isochrones are made available through a dedicated Web site.

astro-ph.GA↗

arXiv2018

Effective temperatures of red giants in the APOKASC catalogue and the mixing length calibration in stellar models

Red giants in the updated APOGEE-Kepler catalogue, with estimates of mass, chemical composition, surface gravity and effective temperature, have recently challenged stellar models computed under the standard assumption of solar calibrated mixing length. In this work, we critically reanalyse this sample of red giants, adopting our own stellar model calculations. Contrary to previous results, we find that the disagreement between the effective temperature scale of red giants and models with solar calibrated mixing length disappears when considering our models and the APOGEE-Kepler stars with scaled solar metal distribution. However, a discrepancy shows up when alpha-enhanced stars are included in the sample. We have found that assuming mass, chemical composition and effective temperature scale of the APOGEE-Kepler catalogue, stellar models generally underpredict the change of temperature of red giants caused by alpha-element enhancements at fixed [Fe/H]. A second important conclusion is that the choice of the outer boundary conditions employed in model calculations is critical. Effective temperature differences (metallicity dependent) between models with solar calibrated mixing length and observations appear for some choices of the boundary conditions, but this is not a general result

astro-ph.SR↗

arXiv2018

The star formation history of the Sextans dwarf spheroidal galaxy: a true fossil of the pre-reionization era

We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to constrain the duration of the main burst of star formation to $\sim0.6$ Gyr. From the calculation of the mechanical luminosity released from supernovae (SNe) during the brief episode of star formation, there are strong indications that SNe could have played an important role in the fate of Sextans, by removing almost completely the gas component, so preventing a prolonged star formation.

astro-ph.GA↗

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XIII. ACS/WFC Parallel-Field Catalogues

As part of the Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters, 110 parallel fields were observed with the Wide Field Channel of the Advanced Camera for Surveys, in the outskirts of 48 globular clusters, plus the open cluster NGC 6791. Totalling about $0.3$ square degrees of observed sky, this is the largest homogeneous Hubble Space Telescope photometric survey of Galalctic globular clusters outskirts to date. In particular, two distinct pointings have been obtained for each target on average, all centred at about $6.5$ arcmin from the cluster centre, thus covering a mean area of about $23\,{\rm arcmin^{2}}$ for each globular cluster. For each field, at least one exposure in both F475W and F814W filters was collected. In this work, we publicly release the astrometric and photometric catalogues and the astrometrised atlases for each of these fields.

astro-ph.GA↗

arXiv2018

Confirming chemical clocks: asteroseismic age dissection of the Milky Way disk(s)

Investigations of the origin and evolution of the Milky Way disk have long relied on chemical and kinematic identification of its components to reconstruct our Galactic past. Difficulties in determining precise stellar ages have restricted most studies to small samples, normally confined to the solar neighbourhood. Here we break this impasse with the help of asteroseismic inference and perform a chronology of the evolution of the disk throughout the age of the Galaxy. We chemically dissect the Milky Way disk population using a sample of red giant stars spanning out to 2~kpc in the solar annulus observed by the {\it Kepler} satellite, with the added dimension of asteroseismic ages. Our results reveal a clear difference in age between the low- and high-$α$ populations, which also show distinct velocity dispersions in the $V$ and $W$ components. We find no tight correlation between age and metallicity nor [$α$/Fe] for the high-$α$ disk stars. Our results indicate that this component formed over a period of more than 2~Gyr with a wide range of [M/H] and [$α$/Fe] independent of time. Our findings show that the kinematic properties of young $α$-rich stars are consistent with the rest of the high-$α$ population and different from the low-$α$ stars of similar age, rendering support to their origin being old stars that went through a mass transfer or stellar merger event, making them appear younger, instead of migration of truly young stars formed close to the Galactic bar.

astro-ph.GA↗

arXiv2018

Ages of the Bulge globular clusters NGC 6522 and NGC 6626 (M28) from HST Proper-motion-cleaned Color-Magnitude Diagrams

Bulge globular clusters (GCs) with metallicities [Fe/H] <~ -1.0 and blue horizontal branches are candidates to harbor the oldest populations in the Galaxy. Based on the analysis of HST proper-motion-cleaned color-magnitude diagrams in filters F435W and F625W, we determine physical parameters for the old bulge globular clusters NGC 6522 and NGC 6626 (M28), both with well-defined blue horizontal branches. We compare these results with similar data for the inner halo cluster NGC 6362. These clusters have similar metallicities (-1.3 <= [Fe/H] <= -1.0) obtained from high resolution spectroscopy. We derive ages, distance moduli, and reddening values by means of statistical comparisons between observed and synthetic fiducial lines employing likelihood statistics and the Markov chain Monte Carlo method. The synthetic fiducial lines were generated using alpha-enhanced BaSTI and Dartmouth stellar evolutionary models, adopting both canonical (Y ~ 0.25) and enhanced (Y~ 0.30-0.33) helium abundances. RR Lyrae stars were employed to determine the HB magnitude level, providing an independent indicator to constrain the apparent distance modulus and the helium enhancement. The shape of the observed fiducial line could be compatible with some helium enhancement for NGC 6522 and NGC 6626, but the average magnitudes of RR Lyrae stars tend to rule out this hypothesis. Assuming canonical helium abundances, BaSTI and Dartmouth models indicate that all three clusters are coeval, with ages between ~ 12.5 and 13.0 Gyr. The present study also reveals that NGC 6522 has at least two stellar populations, since its CMD shows a significantly wide subgiant branch compatible with 14% +/- 2% and 86% +/- 5% for first and second generations, respectively.

astro-ph.SR↗

arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XII. The RGB Bumps of multiple stellar populations

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters is providing a major breakthrough in our knowledge of Globular Clusters (GCs) and their stellar populations. Among the main results, we discovered that all the studied GCs host two main discrete groups consisting of first generation (1G) and second generation (2G) stars. We exploit the multiwavelength photometry from this project to investigate, for the first time, the Red Giant Branch Bump (RGBB) of the two generations in a large sample of GCs. We identified, with high statistical significance, the RGBB of 1G and 2G stars in 26 GCs and found that their magnitude separation as a function of the filter wavelength follows comparable trends. The comparison of observations to synthetic spectra reveals that the RGBB luminosity depends on the stellar chemical composition and that the 2G RGBB is consistent with stars enhanced in He and N and depleted in C and O with respect to 1G stars. For metal-poor GCs the 1G and 2G RGBB relative luminosity in optical bands mostly depends on helium content, Y. We used the RGBB observations in F606W and F814W bands to infer the relative helium abundance of 1G and 2G stars in 18 GCs, finding an average helium enhancement $Δ$Y=0.011$\pm$0.002 of 2G stars with respect to 1G stars. This is the first determination of the average difference in helium abundance of multiple populations in a large number of clusters and provides a lower limit to the maximum internal variation of helium in GCs.

astro-ph.SR↗

arXiv2017

A Universal Transition in Atmospheric Diffusion for Hot Subdwarfs Near 18,000 K

In the color-magnitude diagrams (CMDs) of globular clusters, when the locus of stars on the horizontal branch (HB) extends to hot temperatures, discontinuities are observed at colors corresponding to ~12,000 K and ~18,000 K. The former is the "Grundahl jump" that is associated with the onset of radiative levitation in the atmospheres of hot subdwarfs. The latter is the "Momany jump" that has remained unexplained. Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained ultraviolet and blue spectroscopy of six hot subdwarfs straddling the Momany jump in the massive globular cluster omega Cen. By comparison to model atmospheres and synthetic spectra, we find that the feature is due primarily to a decrease in atmospheric Fe for stars hotter than the feature, amplified by the temperature dependence of the Fe absorption at these effective temperatures.

astro-ph.SR↗

arXiv2017

PLATO as it is: a legacy mission for Galactic archaeology

Deciphering the assembly history of the Milky Way is a formidable task, which becomes possible only if one can produce high-resolution chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric and spectroscopic surveys will soon provide us with a well-defined view of the current chemo-kinematical structure of the Milky Way, but will only enable a blurred view on the temporal sequence that led to the present-day Galaxy. As demonstrated by the (ongoing) exploitation of data from the pioneering photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way forward: solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age-initial-mass relation they adhere to. In this paper we identify five key outstanding questions relating to the formation and evolution of the Milky Way that will need precise and accurate ages for large samples of stars to be addressed, and we identify the requirements in terms of number of targets and the precision on the stellar properties that are needed to tackle such questions. By quantifying the asteroseismic yields expected from PLATO for red-giant stars, we demonstrate that these requirements are within the capabilities of the current instrument design, provided that observations are sufficiently long to identify the evolutionary state and allow robust and precise determination of acoustic-mode frequencies. This will allow us to harvest data of sufficient quality to reach a 10% precision in age. This is a fundamental pre-requisite to then reach the more ambitious goal of a similar level of accuracy, which will only be possible if we have to hand a careful appraisal of systematic uncertainties on age deriving from our limited understanding of stellar physics, a goal which conveniently falls within the main aims of PLATO's core science.

astro-ph.SR↗

arXiv2017

A Photometric Study of the Outer Halo Globular Cluster NGC 5824

Multi-wavelength CCD photometry over 21 years has been used to produce deep color-magnitude diagrams together with light curves for the variables in the Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are identified, bringing the total to 47, of which 42 have reliable periods determined for the first time. The color-magnitude diagram is matched using BaSTI isochrones with age of $13$~Gyr. and reddening is found to be $E(B-V) = 0.15 \pm0.02$; using the period-Wesenheit relation in two colors the distance modulus is $(m-M)_0=17.45 \pm 0.07$ corresponding to a distance of 30.9 Kpc. The observations show no signs of populations that are significantly younger than the $13$~Gyr stars. The width of the red giant branch does not allow for a spread in [Fe/H] greater than $σ= 0.05$ dex, and there is no photometric evidence for widened or parallel sequences. The $V, c_{UBI}$ pseudo-color magnitude diagram shows a bifurcation of the red giant branch that by analogy with other clusters is interpreted as being due to differing spectral signatures of the first (75\%) and second (25\%) generations of stars whose age difference is close enough that main sequence turnoffs in the color-magnitude diagram are unresolved. The cluster main sequence is visible against the background out to a radial distance of $\sim17$ arcmin. We conclude that NGC 5824 appears to be a classical Oosterhoff Type II globular cluster, without overt signs of being a remnant of a now-disrupted dwarf galaxy.

astro-ph.GA↗

arXiv2017

Kepler red-clump stars in the field and in open clusters: constraints on core mixing

Convective mixing in Helium-core-burning (HeCB) stars is one of the outstanding issues in stellar modelling. The precise asteroseismic measurements of gravity-modes period spacing ($ΔΠ_1$) has opened the door to detailed studies of the near-core structure of such stars, which had not been possible before. Here we provide stringent tests of various core-mixing scenarios against the largely unbiased population of red-clump stars belonging to the old open clusters monitored by Kepler, and by coupling the updated precise inference on $ΔΠ_1$ in thousands field stars with spectroscopic constraints. We find that models with moderate overshooting successfully reproduce the range observed of $ΔΠ_1$ in clusters. In particular we show that there is no evidence for the need to extend the size of the adiabatically stratified core, at least at the beginning of the HeCB phase. This conclusion is based primarily on ensemble studies of $ΔΠ_1$ as a function of mass and metallicity. While $ΔΠ_1$ shows no appreciable dependence on the mass, we have found a clear dependence of $ΔΠ_1$ on metallicity, which is also supported by predictions from models.

astro-ph.GA↗

arXiv2017

The chemical composition of the low-mass Galactic globular cluster NGC 6362

We present chemical abundances for 17 elements in a sample of 11 red giant branch stars in NGC 6362 from UVES spectra. NGC 6362 is one of the least massive globulars where multiple populations have been detected, yet its detailed chemical composition has not been investigated so far. NGC 6362 turns out to be a metal-intermediate ([Fe/H]=-1.07\pm0.01 dex) cluster, with its α- and Fe-peak elements content compatible with that observed in clusters with similar metallicity. It also displays an enhancement in its s-process element abundances. Among the light elements involved in the multiple populations phenomenon, only [Na/Fe] shows star-to-star variations, while [Al/Fe] and [Mg/Fe] do not show any evidence for abundance spreads. A differential comparison with M4, a globular cluster with similar mass and metallicity, reveals that the two clusters share the same chemical composition. This finding suggests that NGC 6362 is indeed a regular cluster, formed from gas that has experienced the same chemical enrichment of other clusters with similar metallicity.

astro-ph.SR↗

arXiv2016

Multiple populations along the asymptotic giant branch of the globular cluster M 4

Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and horizontal branch stellar evolution models. Here we have investigated this issue by employing the CUBI= (U-B)-(B-I) index, that previous studies have demonstrated to be very effective in separating multiple populations along both the red giant and asymptotic giant branch sequences. We confirm previous results that the RGB is intrinsically broad in the V-CUBI diagram, with the presence of two components which nicely correspond to the two populations identified by high-resolution spectroscopy. We find that AGB stars are distributed over a wide range of CUBI values, in close analogy with what is observed for the RGB, demonstrating that the AGB of M4 also hosts multiple stellar populations.

astro-ph.SR↗