arXiv subjects

L. Pulone

Publications and source records attributed to L. Pulone.

At least 55 records · Page 3Linked to original sources

arXiv2007

Stroemgren photometry of Galactic Globular Clusters. I. New Calibrations of the metallicity index

We present a new calibration of the Stroemgren metallicity index m1 using red giant (RG) stars in 4 globular clusters (GCs:M92,M13,NGC1851,47Tuc) with metallicity ranging from [Fe/H]=-2.2 to -0.7, marginally affected by reddening (E(B-V)<0.04) and with accurate u,v,b,y photometry.The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we adopted the u-y/v-y colors instead of the b-y.These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71,NGC288,NGC362,NGC6397, and NGC6752 is 0.04+/-0.03dex (sigma=0.11dex). We also apply the MIC relations to 85 field RGs with metallicity raning from [Fe/H]=-2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is-0.14+/-0.01dex (sig=0.17dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations. We apply the semi-empirical relations to the 9 GCs and find that the difference between photometric and spectroscopic metallicities is 0.04+/-0.03dex (sig=0.10dex).A similar agreement is found for the sample of field RGs, with a difference of -0.09+/-0.03dex (sig=0.19dex).The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11+/-0.03dex (sig=0.14dex) for the 9 GGCs and -0.24+/-0.03dex (sig=0.15dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2dex.

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arXiv2006

A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars

We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex) with respect to iron we find a true distance modulus of 13.70 (with a random error of 0.06 and a systematic error of 0.06), corresponding to a distance d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC-17, but differ significantly from the recent distance estimates based on cluster dynamics and on high amplitude Delta Scuti stars.

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arXiv2005

Reddening distribution across the center of the globular cluster Omega Centauri

We present new medium-band uvby Stromgren and broad-band VI photometry for the central regions of the globular cluster Omega Cen. From this photometry we have obtained differential reddening estimates relative to two other globular clusters (M13 and NGC288) using a metallicity-independent, reddening-free temperature index, [c]=(u-v)-(v-b) - 0.2(b-y), for hot horizontal-branch (HB) stars (T_e> 8,500K). We estimate color excesses of these hot HB stars using optical and near-infrared colors, and find clumpy extinction variations of almost a factor of two within the area of the cluster core. In particular, the greatest density of more highly reddened objects appears to be shifted along the right ascension axis when compared with less reddened ones. These findings complicate photometric efforts to investigate the star formation history of Omega Cen.

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arXiv2005

The Carina dSph galaxy: where is the edge?

Recent cosmological N-body simulations suggest that current empirical estimates of tidal radii in dSphs might be underestimated by at least one order of magnitude. To constrain the plausibility of this theoretical framework, we undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of several fields located at radial distances from the Carina center ranging from 0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same magnitudes and colors of old, Turn-Off stars detected across the center. We found that the (U-V,B-I) color-color plane is a robust diagnostic to split stars from background galaxies. Unfortunately, current U,I-band data are too shallow to firmly constrain the real extent of Carina.

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arXiv2005

On the anomalous red giant branch of the globular cluster Omega Cen

We present three different optical and near-infrared (NIR) data sets for evolved stars in the Galactic Globular Cluster Omega Cen The comparison between observations and homogeneous sets of stellar isochrones and Zero-Age Horizontal Branches provides two reasonable fits. Both of them suggest that the so-called anomalous branch has a metal-intermediate chemical composition (-1.1 < [Fe/H] < -0.8) and is located ~500 pc beyond the bulk of Omega Cen stars. These findings are mainly supported by the shape of the subgiant branch in four different color-magnitude diagrams (CMDs). The most plausible fit requires a higher reddening, E(B-V)=0.155 vs. 0.12, and suggests that the anomalous branch is coeval, within empirical and theoretical uncertainties, to the bulk of Omega Cen stellar populations. This result is supported by the identification of a sample of faint horizontal branch stars that might be connected with the anomalous branch. Circumstantial empirical evidence seems to suggest that the stars in this branch form a clump of stars located beyond the cluster.

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arXiv2005

Discovery of more than two thousand white dwarfs in the globular cluster Omega Centauri

We present deep multiband (F435W, F625W, and F658N) photometric data of the Globular Cluster Omega Cen collected with the Advanced Camera for Surveys on board of the Hubble Space Telescope. We identified in the (F435W-F625W, F435W) plane more than two thousand White Dwarf (WD) candidates using three out of nine available pointings. Such a large sample appears in agreement with predictions based on the ratio between WD and Horizontal Branch (HB) evolutionary lifetimes. We also detected ~ 1600 WDs in the (F658N-F625W, F625W) plane, supporting the evidence that a large fraction of current cluster WDs are $H_α$ bright.

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arXiv2005

White Dwarfs in Omega Centauri: Preliminary Evidence

We present accurate and deep multiband B,R,Halpha data for the globular cluster Omega Cen collected with the Advanced Camera for Surveys on HST. The photometric catalogue includes more than one million stars. By adopting severe selection criteria we identified more than 600 bona fide White Dwarfs (WDs). Empirical evidence suggests that a small sample of WDs are Halpha-bright. The comparison between WD isochrones and observations shows a reasonable agreement at fainter magnitudes and a mismatch at the brighter ones.

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arXiv2004

Search for the elusive optical counterpart of PSR J0537-6910 with the HST Advanced Camera for Surveys

We present the results of deep, high-resolution, multi-band optical observations of the field of the young (~ 5,000 yrs) 16 ms X-ray pulsar PSR J0537$-$6910 performed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Although a few new potential counterparts have been detected within or close to the revised Chandra X-ray error circle (~1 arcsec) of the pulsar, only two of them (with magnitudes m_{814W} ~ 23.9 and m_{814W} ~ 24.2) show indications of a peculiar spectrum which could be related to optical emission from the pulsar. This might be true also for a third, fainter, candidate detected only in one filter (with magnitude m_{814W} ~ 26.7). If either of the two brighter candidates is indeed the actual counterpart, the optical output of PSR J0537-6910 would make it similar to young Crab-like pulsars. If not, it would mean that PSR J0537-6910 is significantly underluminous with respect to all pulsars detected in the optical

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arXiv2004

The global mass function of M15

Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to determine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M15, located ~7' from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ~0.3 Msolar, as obtained by Paresce & De Marchi (2000) for a stellar field at 4.6' from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20" and at 4.6'), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. The model that simultaneously best fits the LF at various locations, the surface brightness profile and the velocity dispersion profile suggests that the GMF should resemble a segmented power-law with the following indices: x ~ 0.8 for stars more massive than 0.8 Msolar, x ~ 0.9 for 0.3 - 0.8 Msolar and x ~ -2.2 at smaller masses (Salpeter's IMF would have x=1.35). The best fitting model also suggests that the cluster mass is ~5.4 10^5 Msolar and that the mass-to-light ratio is on average M/L_V ~ 2.1, with M/L_V ~ 3.7 in the core. A large amount of mass (~ 44 %) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M15 without invoking the presence of an intermediate-mass black hole.

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arXiv2004

The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars

We present new and accurate Near-Infrared J and Ks-band data of the Large Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of the cluster. Current data allowed us to derive accurate mean K-band magnitudes for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the semi-empirical K-band Period-Luminosity-Metallicity relation we have recently derived, we find that the absolute distance to this cluster is 18.52 +- 0.005 (random) +- 0.117 (systematic). Note that the current error budget is dominated by systematic uncertainty affecting the absolute zero-point calibration and the metallicity scale.

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arXiv2003

Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.

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arXiv2003

The Carina Project: II. Stellar Populations

We present a new (V,B-V) Color-Magnitude Diagram (CMD) of the Carina dwarf spheroidal (dSph) that extends from the tip of the Red Giant Branch (RGB) down to about V=25 mag. Data were collected with the Wide Field Imager (WFI) available at 2.2m ESO/MPI telescope and cover an area of about 0.3 square degrees around the center of the galaxy. We confirm the occurrence of a substantial amount of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new well-defined blue plume of young Main-Sequence (MS) stars with an age at most of the order of 1 Gyr. At odds with previous results available in the literature we found thatstars along the RGB of the old and of the intermediate-age stellar populationindicate a mean metallicity roughly equal to Z=0.0004 and a small dispersion around this value. We find evidence for a smooth spatial distribution of the "intermediate-age" stellar population, which appears more centrally concentrated than the oldest one.

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arXiv2003

The Carina Project: I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy. Current data cover an area of about 0.3 square degrees around the center of the galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae stars, 15 are "bona fide" Anomalous Cepheids, one might be a Galactic field RR Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply for the first time accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitude and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae and Anomalous Cepheids are new identifications. Among the RR Lyrae sample 6 objects are new candidate double-mode variables. On the basis of their pulsation properties we estimated that two variables (V152, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of two more massive than fundamental mode RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He burning phases. The data for Carina, together with data available in the literature, strongly support the conclusion that dSph galaxies can barely be classified into the classical Oosterhoff dichotomy. The mean period of fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first overtones and total number of RR Lyrae is quite similar to that found in Oosterhoff type I clusters.

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arXiv2003

"The Carina Project": color magnitude diagram and radial distribution

We present B, V photometric data of the Carina Dwarf Spheroidal Galaxy collected with the Wide Field Imager (WFI) available at the 2.2m MPI/ESO telescope. We briefly discuss the main features of the color magnitude diagram and in particular the mix of stellar populations present in this galaxy. A preliminary analysis of the spatial distribution of these populations over a substantial fraction of the body of the galaxy is also presented.

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arXiv2002

HST observations of the metal rich globular clusters NGC6496 and NGC6352

Deep exposures of the metal-rich globular clusters NGC6496 and NGC6352 were obtained with the WFPC2 camera on board the HST through the F606W and F814W filters. The resulting colour-magnitude diagrams reach down to absolute magnitude M_814~10-10.5, approximately 5 magnitudes below the main sequence (MS) turn-off. The MS of the two clusters are sharp and well defined and their fiducial lines overlap almost exactly throughout this range. Their colour is, however, more than 0.1 mag redder than the MS fiducial line of the prototype metal-rich globular cluster 47 Tuc, after proper correction for the relative distances and reddening. This provides solid empirical evidence of a higher metal content, which is not surprising if these objects belong indeed to the bulge as their present location suggests. A good fit to the upper part of the MS of both clusters is obtained with a 10 Gyr-old theoretical isochrone from Baraffe et al. (1998) for a metallicity of [M/H]=-0.5, but at lower luminosity all models depart considerably from the observations, probably because of a deficiency in the treatment of the TiO opacity. The luminosity functions obtained from the observed CMD are rather similar to one another and show a peak at M_814~9. The present day mass functions (PDMF) are derived down to m~0.2 Msolar and are consistent with power-law indices alpha=0.7 for NGC6496 and alpha=0.6 for NGC6352. The PDMF of 47 Tuc is twice as steep in the same mass range (alpha=1.4). We investigate the origin of this discrepancy and show that it can be understood if the two clusters contain a considerably higher fraction of primordial binaries amongst their MS population, similar to that expected in the bulge. We briefly discuss the implications of this finding on the process of star and binary formation and on the universality of the IMF.

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arXiv2002

Multiwavelength Time Series Data of the LMC Cluster Reticulum

We present accurate multiwavelength UBVI time series data of the LMC cluster Reticulum. Data cover a time interval of ~ 6 yr and have been collected with SUSI1/2 and SOFI at NTT/ESO. For each band we collected approximately 30 short/long exposures and the total exposure times range from roughly 3500 (U, B), 6000s (V) to 8300s (I). The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of 0.02 magnitude from the tip of the Red Giant Branch well below the Turn-Off region. Even though this cluster presents a very low central density, we found a sizable sample of Blue Stragglers (BSs). We also selected stars with a variability index larger than 2, and interestingly enough we detected together with the RR Lyrae stars a large sample of variable stars around and below the TO region. Preliminary analysis on the luminosity variation indicate that these objects might be binary stars.

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arXiv2001

Mass segregation of different populations inside the cluster NGC6101

We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B,V,I CCD photometry for the stars located within 200" (~= 2 half-mass radii, r_h = 1.71') from the center of the cluster NGC 6101. Color-Magnitude Diagrams extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO (V = 20.05 +- 0.05) have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a sizeable population of 73 "blue stragglers", which had been already partly detected (27).They are considerably more concentrated than either the subgiant branch or the main sequence stars, and have the same spatial distribution as the horizontal branch stars (84% prob. from K-S test). An hypothesis on the possible BSS progeny is also presented. b) The HB is narrow and the bulk of stars is blue, as expected for a typical metal-poor globular cluster. c) The derived magnitudes for the HB and the MSTO, $V(ZAHB) = 16.59+-0.10, V(TO) = 20.05+-0.05, coupled with the values E(B-V) = 0.1, [Fe/H] = -1.80, Y = 0.23 yield a distance modulus (m-M)_V = 16.23 and an age similar to other ``old'' metal-poor globular clusters. In particular, from the comparison with theoretical isochrones, we derive for this cluster an age of 13 Gyrs. d) By using the large statistical sample of Red Giant Branch (RGB) stars, we detected with high accuracy the position of the bump in the RGB luminosity function. This observational feature has been compared with theoretical prescriptions, yielding a good agreement within the current theoretical and observational uncertainties.

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arXiv2001

VLT observations of the peculiar globular cluster NGC 6712, II: luminosity and mass functions

We have carried out extensive VLT-FORS1 observations covering a fair fraction of the intermediate metallicity globular cluster NGC 6712 in the V and R bands. We derive accurate colour-magnitude diagrammes (CMD) and luminosity functions (LFs) of the cluster main sequence (MS) for four overlapping fields extending from the centre of the cluster out to a radius of ~10', well beyond the nominal tidal radius, and for a control field at ~42' distance. The LFs extend from the cluster turn-off (TO) at M_R 4 to the point at which the incompleteness drops below 50% (corresponding to R 23 or M_R 7.5) for most fields studied. Cluster stars become indistinguishable from field stars at r 5'. The shape of the cluster's LF and its variation with distance from the centre in these ranges are well described by a standard multi-mass static model having the following parameters: core radius r_c=1', half-light radius r_hl=1.8', tidal radius r_t=5.2', concentration ratio c=0.7, and a power-law global mass function (MF) with index alpha 0.9 for masses smaller than 0.8 Msolar, i.e. for all detected MS stars, and alpha -2.35 for evolved objects. The MF obtained in this way is consistent with that found in a preliminary investigation of this cluster with the VLT Test Camera and confirms that this is the only globular cluster known so far for which the global MF drops with decreasing mass below the TO. Possible reasons for this unique characteristic are discussed with the most likely associated with its extreme vulnerability to tidal disruption.

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