arXiv subjects

F. R. Ferraro

Publications and source records attributed to F. R. Ferraro.

At least 199 records · Page 11Linked to original sources

arXiv2004

The distance of M33 and the stellar population in its outskirts

We present deep V,I photometry of two $9.4' x 9.4' field in the outer regions of the M33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip (I{TRGB}=20.72 +- 0.08) from which we derived a new estimate of the distance modulus of M33, (m-M)_0=24.64 +- 0.15, corresponding to a distance D=847 +- 60 Kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity distributions are very similar over a range of distances from the galactic center 10' <= R <= 33', and are characterized by a well defined peak at [M/H] ~ -0.7 ([Fe/H] ~ -1.0, in the Zinn & West scale) and a weak metal-poor tail reaching [M/H] ~ -2.0. Our observations demonstrate that Red Giant Branch and Asymptotic Giant Branch stars have a radial distribution that is much more extended than the young MS stars associated with the star-forming disc.

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arXiv2004

The calibration of the RGB Tip as a Standard Candle: Extension to Near Infrared colors and higher metallicity

New empirical calibrations of the Red Giant Branch Tip in the I,J,H and K bands based on two fundamental pillars, namely omega Centauri and 47 Tucanae, have been obtained by using a large optical and near infrared photometric database. Our best estimates give M_I(TRGB)= -4.05 pm 0.12, M_J(TRGB)= -5.20 pm 0.16, M_H(TRGB)= -5.94 pm 0.18 and M_K(TRGB) = -6.04 pm 0.16 at [M/H]=-1.5 (omega Cen) and M_I(TRGB)= -3.91 pm 0.13, M_J(TRGB)= -5.47 pm 0.25, M_H(TRGB)= -6.35 pm 0.30 and M_K(TRGB)= -6.56 pm 0.20 at [M/H]=-0.6 (47 Tuc). With these new empirical calibrations we also provide robust relations of the TRGB magnitude in I, J, H and K bands as a function of the global metallicity ([M/H]). It has also been shown that our calibrations self-consistently provide a distance modulus of the Large Magellanic Cloud in good agreement with the standard value ((m-M)_0 = 18.50)).

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arXiv2004

The Red Giant Branch in Near-Infrared Colour-Magnitude Diagrams. II: The luminosity of the Bump and the Tip

We present new empirical calibrations of the Red Giant Branch (RGB) Bump and Tip based on a homogeneous near-Infrared database of 24 Galactic Globular Clusters. The luminosities of the RGB Bump and Tip in the J, H and K bands and their dependence on the cluster metallicity have been studied, yielding empirical relationships. By using recent transformations between the observational and theoretical planes, we also derived similar calibrations in terms of bolometric luminosity. Direct comparison between updated theoretical models and observations show an excellent agreement. The empirical calibration of the RGB Tip luminosity in the near-Infrared passbands presented here is a fundamental tool to derive distances to far galaxies beyond the Local Group, in view of using the new ground-based adaptive optics facilities and, in the next future, the James Webb Space Telescope.

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arXiv2004

The Red Giant Branch in the Near-Infrared Colour-Magnitude Diagrams. I: The calibration of photometric indices

We present new high-quality near-infrared photometry of 10 Galactic Globular Clusters spanning a wide metallicity range (-2.12<[Fe/H]<-0.49): five clusters belong to the Halo (namely, NGC288, NGC362, NGC6752, M15 and M30) and five (namely, NGC6342, NGC6380, NGC6440, NGC6441 and NGC6624) to the Bulge. By combining J, H and K observations with optical data, we constructed Colour-Magnitude Diagrams in various planes (K,J-K), (K,V-K), (H,J-H), and (H,V-H). A set of photometric indices (colours, magnitudes and slopes) describing the location and the morphology of the Red Giant Branch (RGB) have been measured. We have combined this new data set with those already published by Ferraro et al.(2000) and Valenti et al. (2004), and here we present an updated calibration of the various RGB indices in the 2MASS photometric system, in terms of the cluster metallicity.

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arXiv2004

Spectral Types and Masses of White Dwarfs in Globular Clusters

White dwarfs in globular clusters offer additional possibilities to determine distances and ages of globular clusters, provided their spectral types and masses are known. We therefore started a project to obtain spectra of white dwarfs in the globular clusters NGC 6397 and NGC 6752. All observed white dwarfs show hydrogen-rich spectra and are therefore classified as DA. Analysing the multi-colour photometry of the white dwarfs in NGC 6752 yields an average gravity of log g = 7.84 and 0.53 Msolar as the most probable average mass for globular cluster white dwarfs. Using this average gravity we try to determine independent temperatures by fitting the white dwarf spectra. While the stellar parameters determined from spectroscopy and photometry usually agree within the mutual error bars, the low resolution and S/N of the spectra prevent us from setting constraints stronger than the ones derived from the photometry alone. For the same reasons the white dwarf spectra obtained for NGC 6397 unfortunately do not provide an independent distance estimate of sufficient accuracy to distinguish between the long and short distance scale for globular clusters.

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arXiv2004

Discovery of an anomalous Sub Giant Branch in the Color Magnitude Diagram of omega Centauri

Using deep high-resolution multi-band images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the Color Magnitude Diagram of omega Cen. This feature appears as a narrow, well-defined Sub Giant Branch (SGB-a), which merges into the Main Sequence of the dominant cluster population at a magnitude significantly fainter than the cluster Turn-Off (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous Red Giant Branch (RGB-a) metal-rich population discovered by Lee et al. (1999) and Pancino et al. (2000). However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within omega Cen. In fact, its TO magnitude, shape and extension are not compatible with a young, metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extra-cluster origin of the most metal rich stars, as suggested by Ferraro, Bellazzini & Pancino (2002). Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal rich population of omega Cen.

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arXiv2004

The contribution of primordial binaries to the blue straggler population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue straggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution in the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise beyond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be explained within a purely collisional scenario in which blue stragglers are generated exclusively in the core through direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generated in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-transfer.

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arXiv2004

IR photometric properties of Red Giants in omega Cen

We present a near-infrared J and K photometric catalog containing more than 73,000 stars in the central region of the giant Globular Cluster omega Centauri. This is the largest IR data-set ever published for this cluster and has been used to completely characterize the morphology and the properties of the Red Giant Branch (RGB). In particular, we concentrated our attention on (i) the anomalous RGB (RGB-a), recently discovered in this cluster and (ii) the RGB of the dominant metal poor population (RGB-MP) in both the infrared (K,J-K) and optical-infrared (K,V-K) color magnitude diagrams. The full set of morphological parameters and photometric indices has been measured and compared with the empirical relations by Ferraro et al. (2000). We find that the detailed photometric properties of the RGB-a are in full agreement with the recent spectroscopic metallicity estimates, that place it at the metal-rich extreme of the stellar population mix in omega Centauri.

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arXiv2004

Wide Field Photometry of the Galactic Globular Cluster M22

We present wide field photometry of the Galactic Globular Cluster M~22 in the B, V and I passbands for more than 186,000 stars. The study is complemented by the photometry in two narrowband filters centered on H$_α$ and the adjacent continuum, and by infrared J, H and K magnitudes derived from the 2 MASS survey for $\sim$2000 stars. Profiting from this huge database, we completely characterized the evolved stellar sequences of the cluster by determining a variety of photometric parameters, including new photometric estimates of the mean metallicity, reddening and distance to the cluster. In particular, from our multi-wavelength analysis, we re-examined the long-standing metallicity spread problem in M~22. According to our dataset, we conclude that most of the observed width of the red giant branch must be due to differential reddening, which amounts to a maximum of $ΔE(B-V)\simeq0.06$, although the presence of a small metallicity spread cannot be completely ruled out. More specifically, the maximum metallicity spread allowed by our data is of the order of $Δ$[Fe/H]$\simeq 0.1÷0.2$ dex, i.e., not much more than what allowed by the photometric errors. Finally, we identified most of the known variable stars and peculiar objects in our field of view. In particular, we find additional evidence supporting previous optical identifications of the central star of the Planetary Nebula IRAS 18333-2357, which is associated with M~22.

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arXiv2004

TNG Near-IR Photometry of five Galactic Globular Clusters

We present near--infrared J and K observations of giant stars in five metal-poor Galactic Globular Clusters (namely M3, M5, M10, M13 and M92) obtained at the Telescopio Nazionale Galileo (TNG). This database has been used to determine the main photometric properties of the red giant branch (RGB) from the (K,J-K) and, once combined with the optical data, in the (K,V-K) Color Magnitude Diagrams. A set of photometric indices (the RGB colors at fixed magnitudes) and the major RGB evolutionary features (slope, bump, tip) have been measured. The results have been compared with the relations obtained by Ferraro et al. 2000 and with the theoretical expectations, showing a very good agreement.

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arXiv2003

The chemical composition of the peculiar companion to the millisecond pulsar in NGC 6397

We present the chemical composition of the bright companion to the millisecond pulsar J1740-5340 in NGC 6397, based on high resolution spectra. Though the large rotation velocity of the star broadens the lines and complicates the analysis, the derived abundances are found fully compatible with those of normal unperturbed stars in NGC 6397, with the exception of a few elements (Li, Ca, and C). The lack of C suggests that the star has been peeled down to regions where incomplete CNO burning occurs, favouring a scenario where the companion is a turn-off star which has lost most of its mass. In addition we found an unexpected large Li abundance, which suggests that fresh 7Li has been produced on the stellar surface.

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arXiv2003

The puzzling dynamical status of the core of the globular cluster NGC 6752

We have used high resolution WFPC2-HST and ground based Wide Field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the 9 innermost X-ray sources detected by Chandra is located only 1.9" off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core collapse "bounce". Taking advantage from our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: 1) the overall cluster gravitational potential which would require a central projected mass to light ratio of order 6-7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC6752; 2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100-$200 Msun) mass.

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arXiv2003

The multiple stellar population in omega Centauri: spatial distribution and structural properties

We present a detailed analysis of the spatial distribution of the stellar populations in the Galactic globular cluster omega Centauri. Taking advantage of the large photometric catalog published by Pancino et al. (2000), we confirm that metal-rich populations have a spatial distribution which is significantly different from the metal-poor dominant population. In particular: (i) the different sub-populations have different centroids and (ii) the metal-poor population is elongated along the E-W direction, while the metal-rich populations are oriented along the orthogonal direction, i.e., N-S. The evidence presented here can partially explain the weird spatial metallicity segregation found by Jurcsik (1998), and further supports the hyphothesis that different sub-populations in omega Centauri might have had different origins.

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arXiv2003

Building up the Galactic Halo: The Sagittarius dSph and the globular cluster M22

The Sagittarius dwarf spheroidal (Sgr) and a few paculiar Galactic Globular Clusters (GGC) can be considered as building blocks of theGalactic Halo. We present a series of results based on a wide field photometry of the Sgr system and the GGC M22. In particular for Sgr we present: (1) the detection of the RGB-bump, a feature that together with the shape of the RGB can be used to constrain the allowed range of age and metallicity and (2) the detection of a clear blue horizontal branch which probes the existance of a metal poor population. In the case of M22 we present clear evidence that differential reddening is the primary cause for the observed spread of the evolutionary sequences.

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arXiv2003

Tracing the Sgr Stream with 2MASS. Detection of Stream stars around Outer Halo Globular Clusters

We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr Stream and that they were previous members of the Sgr dSph galaxy.

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arXiv2003

The complex Halpha line profile of the bright companion to PSR J1740-5340 in NGC 6397

We present a detailed study of the Halpha and He I spectral features of COM J1740-5340 (the companion to PSR J1740-5340 in the Galactic Globular Cluster NGC 6397), exploiting a series of high resolution spectra obtained at different orbital phases. The Halpha absorption line shows a complex two-component structure, revealing that optically thin hydrogen gas resides outside the Roche lobe of COM J1740-5340. The line morphology precludes the existence of any residual disk around the millisecond pulsar, and suggests the presence of a stream of material going from the companion toward the neutron star. This material never reaches the neutron star surface, being driven back by the pulsar radiation far beyond COM J1740-5340. By analyzing the He I absorption lines as a function of orbital phase, we infer the presence of an overheated longitudinal strip (about 150 times narrower than it is long) on the COM J1740-5340 surface facing the radio pulsar.

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arXiv2003

A near-infrared spectroscopic screening of the red giant populations in omega Centauri

Near-infrared spectra of 21 giants in omega Centauri, spanning the whole range of metallicities observed in this cluster, are presented. This work is part of a coordinated photometric and spectroscopic campaign in the optical and in the infrared, aimed at studying the complex stellar population of omega Centauri and understanding its formation and chemical evolution. By analyzing the several CO and OH molecular bands and atomic lines in the spectra of the selected giants, metal abundances and abundance ratios have been obtained. The existence of three major metallicity regimes at [Fe/H]=-1.6, -1.2 and [Fe/H]<-0.5 has been confirmed. The most metal-rich stars in our sample show a lower (if any) alpha-enhancement when compared to the more metal-poor components, suggesting that they should have formed in a medium significantly polluted by type Ia supernova ejecta. Isotopic carbon abundances have been also inferred, providing an average 13C/12C=4, which clearly indicates that extra-mixing processes occurred in the stellar interiors during the ascent on the Red Giant Branch.

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arXiv2003

M75, a Globular Cluster with a Trimodal Horizontal Branch. II. BV Photometry of the RR Lyrae Variables

We present new BV CCD photometry, light curves, and ephemerides for 9 previously known, 29 newly detected RR Lyrae variables and one newly detected variable of an unknown type in the globular cluster M75. The photometry used for the detection of the additional variables was obtained with the image subtraction package ISIS. The data were acquired on an observing run in July 1999 and range over 7 observing nights. Estimates of fundamental photometric parameters are presented including intensity- and magnitude-averaged B and V magnitudes, magnitude-averaged colors, pulsation periods, and pulsation amplitudes. The mean period of the RRab variables, = 0.5868 d, and the number fraction of RRc stars, N_c/N_RR = 0.342, are both large for an Oosterhoff type I (OoI) globular cluster, suggesting that M75 may be Oosterhoff-intermediate. Possible conflicts between Oosterhoff-type determination based on the A_V - logP and A_B - logP diagrams are discussed. The physical parameters of the RRc and RRab variables, as obtained from Fourier decomposition of their light curves, do not show any clear deviation from normal OoI behavior.

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