arXiv subjects

M. Salaris

Publications and source records attributed to M. Salaris.

At least 109 records · Page 6Linked to original sources

arXiv2016

A Young Cluster With an Extended Main Sequence Turnoff: Confirmation of a Prediction of the Stellar Rotation Scenario

We present Hubble Space Telescope photometry of NGC1850, a ~100 Myr, ~10^5 Msun cluster in the Large Magellanic Cloud. The colour magnitude diagram clearly shows the presence of an extended main sequence turnoff (eMSTO). The use of non-rotating stellar isochrones leads to an age spread of ~40 Myr. This is in good agreement with the age range expected when the effects of rotation in MSTO stars are wrongly interpreted in terms of age spread. We also do not find evidence for multiple, isolated episodes of star-formation bursts within the cluster, in contradiction to scenarios that invoke actual age spreads to explain the eMSTO phenomenon. NGC 1850 therefore continues the trend of eMSTO clusters where the inferred age spread is proportional to the age of the cluster. While our results confirm a key prediction of the scenario where stellar rotation causes the eMSTO feature, direct measurements of the rotational rate of MSTO stars is required to definitively confirm or refute whether stellar rotation is the origin of the eMSTO phenomenon or if it is due to an as yet undiscovered effect.

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arXiv2015

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. V. Constraints on Formation Scenarios

We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertained for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating massive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to AGB stars as producers of the material for next generation stars encounters severe difficulties, specifically concerning the mass budget problem and the detailed chemical composition of second generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revision of the cross section of a critical reaction rate destroying sodium, or alternatively by a more extensive exploration of the vast parameter space controlling the evolutionary behavior of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

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arXiv2015

Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae

We used ultra-deep UV observations obtained with the Hubble Space Telescope to search for optical companions to binary millisecond pulsars (MSPs) in the globular cluster 47 Tucanae. We identified four new counterparts (to MSPs 47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs 47TucU and 47TucW). In the color magnitude diagram, the detected companions are located in a region between the main sequence and the CO white dwarf cooling sequences, consistent with the cooling tracks of He white dwarfs of mass between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling age, temperature and pulsar mass (as a function of the inclination angle) have been derived and discussed. For 47TucU we also found that the past accretion history likely proceeded in a sub-Eddington rate. The companion to the redback 47TucW is confirmed to be a non degenerate star, with properties particularly similar to those observed for black widow systems. Two stars have been identified within the 2-sigma astrometric uncertainty from the radio positions of 47TucH and 47TucI, but the available data prevent us from firmly assessing whether they are the true companions of these two MSPs.

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arXiv2015

New insights on the Galactic Bulge Initial Mass Function

We have derived the Galactic bulge initial mass function of the SWEEPS field in the mass range 0.15 $< M/M_{\odot}<$ 1.0, using deep photometry collected with the Advanced Camera for Surveys on the Hubble Space Telescope. Observations at several epochs, spread over 9 years, allowed us to separate the disk and bulge stars down to very faint magnitudes, F814W $\sim$ 26 mag, with a proper-motion accuracy better than 0.5 mas/yr. This allowed us to determine the initial mass function of the pure bulge component uncontaminated by disk stars for this low-reddening field in the Sagittarius window. In deriving the mass function, we took into account the presence of unresolved binaries, errors in photometry, distance modulus and reddening, as well as the metallicity dispersion and the uncertainties caused by adopting different theoretical color-temperature relations. We found that the Galactic bulge initial mass function can be fitted with two power laws with a break at M $\sim$ 0.56 $M_{\odot}$, the slope being steeper ($α$ = -2.41$\pm$0.50) for the higher masses, and shallower ($α$ = -1.25$\pm$0.20) for the lower masses. In the high-mass range, our derived mass function agrees well with the mass function derived for other regions of the bulge. In the low-mass range however, our mass function is slightly shallower, which suggests that separating the disk and bulge components is particularly important in the low-mass range. The slope of the bulge mass function is also similar to the slope of the mass function derived for the disk in the high-mass regime, but the bulge mass function is slightly steeper in the low-mass regime. We used our new mass function to derive stellar M/L values for the Galactic bulge and we obtained 2.1 $<M/L_{F814W}<$ 2.4 and 3.1 $< M/L_{F606W}<$ 3.6 according to different assumptions on the slope of the IMF for masses larger than 1 $M_{\odot}$.

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arXiv2015

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. IV. Helium content and relative age of multiple stellar populations within NGC 6352

In this paper we combine WFC3/UVIS F275W, F336W, and F438W data from the "UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation" (GO-13297) HST Treasury program with F606W, F625W, F658N, and F814W ACS archive data for a multi-wavelength study of the globular cluster NGC 6352. In the color-magnitude and two-color diagrams obtained with appropriate combination of the photometry in the different bands we separate two distinct stellar populations and trace them from the main sequence to the subgiant, red giant, horizontal and asymptotic giant branches. We infer that the two populations differ in He by Delta Y=0.029+/-0.006. With a new method, we also estimate the age difference between the two sequences. Assuming no difference in [Fe/H] and [alpha/Fe], and the uncertainties on Delta Y, we found a difference in age between the two populations of 10+/-120 Myr. If we assume [Fe/H] and [alpha/Fe] differences of 0.02 dex (well within the uncertainties of spectroscopic measurements), the total uncertainty in the relative age rises to ~300 Myr.

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arXiv2015

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819. I. The bottom of the white dwarf cooling sequence

We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050+/- 0.075. This implies an age of 2.25+/-0.20 Gyr, consistent with the age of 2.25+/-0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ~0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ~70 arcmin sq.) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.

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arXiv2014

Variable stars in two open clusters within the Kepler 2-Campaign-0 field: M 35 and NGC 2158

We present a multi-year survey aimed at collecting (1) high-precision (~5 milli-mag), (2) fast-cadence (~3 min), and (3) relatively long duration (~10 days) multi-band photometric series. The goal of the survey is to discover and characterize efficiently variable objects and exoplanetary transits in four fields containing five nearby open clusters spanning a broad range of ages. More in detail, our project will (1) constitute a preparatory survey for HARPS-N@TNG, which will be used for spectroscopic follow-up of any target of interest that this survey discovers or characterizes, (2) measure rotational periods and estimate the activity level of targets we are already monitoring with HARPS and HARPS-N for exoplanet transit search, and (3) long term characterization of selected targets of interest in open clusters within the planned K2 fields. In this first paper we give an overview of the project, and report on the variability of objects within the first of our selected fields, which contains two open clusters: M 35 and NGC 2158. We detect 519 variable objects, 273 of which are new discoveries, while the periods of most of the previously known variables are considerably improved.

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arXiv2014

The temperature distribution of horizontal branch stars: methods and first results

As part of a large project aimed at characterizing the ultraviolet (UV) properties of globular clusters, we present here a theoretical and observational analysis aimed at setting the framework for the determination of horizontal branch (HB) temperature distributions. Indeed this is a crucial information to understand the physical parameters shaping the HB morphology in globular clusters and to interpret the UV emission from unresolved stellar systems. We found that the use of zero age HB color-Teff relations is a robust way to derive effective temperatures of individual HB stars. We investigated the most suitable colors for temperature estimates, and the effect on the color-Teff relations of variations of the initial chemical composition, and of the evolution off the zero age horizontal branch. As a test case, we applied our color-Teff calibrations to the Galactic globular cluster M15. The photometry of M15 has been obtained with the Wide Field and Planetary Camera 2 on board the HST. The HB of M15 turned out to have a multimodal distribution, with a main component peaked at Teff~8,000 K and confined below Teff~10,000 K. The second component is peaked at Teff~14,000 K and extends up to Teff~20,000 K. The vast majority (~95%) of the HB stars in M15 is below 20,000 K, in agreement with the lack of a well populated extreme HB observed in other metal-poor globular clusters. We also verified that the temperatures derived with our analysis are consistent with spectroscopic estimates available in the literature.

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arXiv2014

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. II. The seven stellar populations of NGC7089 (M2)

We present high-precision multi-band photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic GCs GO-13297, with chemical abundances by Yong et al.(2014), and compare the photometry with models in order to analyze the multiple stellar sequences we identified in the color-magnitude diagram (CMD). We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different $s$-process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to Delta Y~0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.

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arXiv2014

The Na-O anticorrelation in horizontal branch stars. V. NGC 6723

We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae variables, and 17 blue horizontal branch (BHB) stars in the low-concentration, moderately metal-rich globular cluster NGC6723 ([Fe/H]=-1.22+/-0.08 from our present sample). The spectra were optimized to derive O and Na abundances. In addition, we obtained abundances for other elements, including N, Fe, Mg, Ca, Ni, and Ba. We used these data to discuss the evidence of a connection between the distribution of stars along the horizontal branch (HB) and the multiple populations that are typically present in globular clusters. We found that all RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these stars probably belong to the first stellar generation in this cluster. Only the four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and they probably belong to the second generation. While our sample is not fully representative of the whole HB population in NGC6723, our data suggest that in this cluster only HB stars warmer than ~9000 K, that is one fourth of the total, belong to the second generation, if at all. Since in many other clusters this fraction is about two thirds, we conclude that the fraction of first/second generation in globular clusters may be strongly variable. In addition, the wide range in colour of chemically homogeneous first-generation HB stars requires a considerable spread in mass loss (>0.10 Mo). The reason for this spread is yet to be understood. Finally, we found a high Ba abundance, with a statistically significant radial abundance gradient.

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arXiv2014

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations

In this paper we describe a new UV-initiative HST project (GO-13297) that will complement the existing F606W and F814W database of the ACS Globular Cluster (GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W and F438W. This "magic trio" of filters has shown an uncanny ability to disentangle and characterize multiple-population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In the present paper we will describe the data base and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and proper motions. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.

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arXiv2014

WFPC2 UV survey of Galactic Globular Clusters. The Horizontal Branch temperature distribution

Ultraviolet observations are the best tool to study hot stellar populations which emit most of their light at short wavelengths. As part of a large project devoted to the characterization of the UV properties of Galactic globular clusters (GGCs), we collected mid/far UV and optical images with the WFPC2@HST for 31 GGCs. These clusters cover a wide range in metallicity and structural parameters, thus representing an ideal sample for comparison with theoretical models. Here we present the first results from an ongoing analysis aimed at deriving the temperature distribution of Horizontal Branch stars in GGCs.

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arXiv2014

The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54

The cosmological Li problem is the observed discrepancy between Li abundance, A(Li), measured in Galactic dwarf, old and metal-poor stars (traditionally assumed to be equal to the initial value A(Li)_0), and that predicted by standard Big Bang Nucleosynthesis calculations (A(Li)_{BBN}). Here we attack the Li problem by considering an alternative diagnostic, namely the surface Li abundance of red giant branch stars that in a colour magnitude diagram populate the region between the completion of the first dredge-up and the red giant branch bump. We obtained high-resolution spectra with the FLAMES facility at the Very Large Telescope for a sample of red giants in the globular cluster M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=+0.93+-0.11 dex, translating -- after taking into account the dilution due to the dredge up-- to initial abundances (A(Li)_0) in the range 2.35--2.29 dex, depending on whether or not atomic diffusion is considered. This is the first measurement of Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0 in old stars obtained so far. The A(Li)_0 estimated in M54 is lower by ~0.35 dex than A(Li)_{BBN}, hence incompatible at a level of ~3sigma. Our result shows that this discrepancy is a universal problem concerning both the Milky Way and extra-galactic systems. Either modifications of BBN calculations, or a combination of atomic diffusion plus a suitably tuned additional mixing during the main sequence, need to be invoked to solve the discrepancy.

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arXiv2014

First evidence of fully spatially mixed first and second generations in globular clusters: the case of NGC 6362

We present the first evidence of multiple populations in the Galactic globular cluster NGC 6362. We used optical and near-UV Hubble Space Telescope and ground based photometry, finding that both the sub giant and red giant branches are split in two parallel sequences in all color magnitude diagrams where the F336W filter (or U band) is used. This cluster is one of the least massive globulars (M_tot~5x10^4 M_sun) where multiple populations have been detected so far. Even more interestingly and at odds with any previous finding, we observe that the two identified populations share the same radial distribution all over the cluster extension. NGC 6362 is the first system where stars from different populations are found to be completely spatially mixed. Based on N-body and hydrodynamical simulations of multiple stellar generations, we argue that, to reproduce these findings, NGC 6362 should have lost up to the 80% of its original mass

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arXiv2014

First Detection of the White-Dwarf Cooling Sequence of the Galactic Bulge

We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar Planet Search field (3'x3'), together with three more Advanced Camera for Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and F814W filters, approximately every two weeks for two years, with the principal aim of detecting a hidden population of isolated black holes and neutron stars through astrometric microlensing. Proper motions were measured with an accuracy of ~0.1 mas/yr (~4 km/s) at F606W~25.5 mag, and better than ~0.5 mas/yr (20 km/s) at F606W~28 mag, in both axes. Proper-motion measurements allowed us to separate disk and bulge stars and obtain a clean bulge color-magnitude diagram. We then identified for the first time a white dwarf (WD) cooling sequence in the Galactic bulge, together with a dozen candidate extreme horizontal branch stars. The comparison between theory and observations shows that a substantial fraction of the WDs (30%) are systematically redder than the cooling tracks for CO-core H-rich and He-rich envelope WDs. This evidence would suggest the presence of a significant number of low-mass WDs and WD - main sequence binaries in the bulge. This hypothesis is further supported by the finding of two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.

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arXiv2014

Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario

A new scenario --early disc accretion-- has been recently proposed to explain the discovery of multiple stellar populations in Galactic globular clusters. According to this model, the existence of well defined (anti)-correlations amongst light element abundances (i.e. C, N, O, Na) in the photospheres of stars belonging to the same cluster (and the associated helium enrichment), is caused by accretion of the ejecta of short lived interacting massive binary systems (and single fast rotating massive stars) on fully convective pre-main sequence low- and very low-mass stars, during the early stages of the cluster evolution. We investigated the constraints provided by considering simultaneously the observed spread of lithium and oxygen (and when possible also sodium) abundances for samples of turn-off stars in NGC6752, NGC6121 (M4), and NGC104 (47Tuc), and the helium abundance of their multiple main sequences. These observations provide a very powerful test for the accretion scenario, because the observed O, Li and He abundance distributions at the turn off can be used to constrain the composition (and mass) of the accreted matter, and the timescales of the polluting stars. In case of NGC6752 we could not find a physically consistent solution. In case of M4, spectroscopic errors are too large compared to the intrinsic spread, to constrain the properties of the accreted matter. As for 47Tuc, we could find a physically consistent solution for the abundances of He and O (and Na) in the accreted gas, and predict the abundances of these elements in the accreted matter only if pollution happens with timescales of ~1 Myr, hence polluters are objects with masses of the order of several tens of solar masses (abridged).

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arXiv2014

Detailed AGB evolutionary models and near infrared colours of intermediate-age stellar populations: Tests on star clusters

We investigate the influence of Asymptotic Giant Branch stars on integrated colours of star clusters of ages between ~100 Myr and a few gigayears, and composition typical for the Magellanic Clouds. We use state-of-the-art stellar evolution models that cover the full thermal pulse phase, and take into account the influence of dusty envelopes on the emerging spectra. We present an alternative approach to the usual isochrone method, and compute integrated fluxes and colours using a Monte Carlo technique that enables us to take into account statistical fluctuations due to the typical small number of cluster stars. We demonstrate how the statistical variations in the number of Asymptotic Giant Branch stars and the temperature and luminosity variations during thermal pulses fundamentally limit the accuracy of the comparison (and calibration, for population synthesis models that require a calibration of the Asymptotic Giant Branch contribution to the total luminosity) with star cluster integrated photometries. When compared to observed integrated colours of individual and stacked clusters in the Magellanic Clouds, our predictions match well most of the observations, when statistical fluctuations are taken into account, although there are discrepancies in narrow age ranges with some (but not all) set of observations.

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arXiv2014

The Na-O anticorrelation in horizontal branch stars. IV. M22

We obtained high-resolution spectra for 94 candidate stars belonging to the HB of M22 with FLAMES. The HB stars we observed span a restricted temperature range (7,800<Teff<11,000 K), where about 60% of the HB stars of M22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor and O-rich with abundances that match those determined for the primordial group of RGB stars from previous studies. Group 2 (23 stars) is still metal-poor, but it is N- and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation. The third group (20 stars) is metal-rich, Na-rich, and O-rich and likely corresponds to the most O-rich component of the previously found metal-rich RGB population. We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. The metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y=0.338\pm 0.014\pm 0.05). Our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M22.

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