arXiv subjects

F. Fusi Pecci

Publications and source records attributed to F. Fusi Pecci.

At least 37 records · Page 2Linked to original sources

arXiv2007

The extended structure of the remote cluster B514 in M31. Detection of extra-tidal stars

We present a study of the density profile of the remote M31 globular cluster B514, obtained from HST/ACS observations. Coupling the analysis of the distribution of the integrated light with star counts we can reliably follow the profile of the cluster out to r~35", corresponding to ~130pc. The profile is well fitted, out to ~15 core radii, by a King Model having C=1.65. With an estimated core radius r_c=0.38", this corresponds to a tidal radius of r_t~17" (~65pc). We find that both the light and the star counts profiles show a departure from the best fit King model for r>~8" - as a surface brightness excess at large radii, and the star counts profile shows a clear break in correspondence of the estimated tidal radius. Both features are interpreted as the signature of the presence of extratidal stars around the cluster. We also show that B514 has a half-light radius significantly larger than ordinary globular clusters of the same luminosity. In the M_V vs. log r_h plane, B514 lies in a region inhabited by peculiar clusters, like Omega Cen, G1, NGC2419 and others, as well as by the nuclei of dwarf elliptical galaxies.

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arXiv2007

The fraction of binary systems in the core of thirteen low-density Galactic globular clusters

We used deep observations collected with ACS@HST to derive the fraction of binary systems in a sample of thirteen low-density Galactic globular clusters. By analysing the color distribution of Main Sequence stars we derived the minimum fraction of binary systems required to reproduce the observed color-magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6% within the core radius. The estimated global fractions of binary systems range from 10% to 50% depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.

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arXiv2007

An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters

We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M31 galaxy. We obtained low resolution spectra of 48 targets selected from the XSC of 2MASS, as in Galleti et al. (2005). The observed candidates have been robustly classified according to their radial velocity and by verifying their extended/point-source nature from ground-based optical images. Among the 48 observed candidates clusters we found 5 genuine remote globular clusters. One of them has been already identified independently by Mackey et al. (2007), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc<~R_p<~100 kpc from the center of M31, and have absolute integrated magnitude -9.5 40 kpc. At odds with the Milky Way, M31 appears to have a significant population of very bright globular clusters in its extreme outskirts.

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arXiv2006

Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process

We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc. We discovered that a sub-population of BSS shows a significant depletion of Carbon and Oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of mass-transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

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arXiv2006

ACS photometry of the globular cluster B514

We present deep F606W, F814W ACS photometry of the recently discovered globular cluster B514, the outermost known globular in the M31 galaxy. The cluster appears quite extended and member stars are unequivocally identified out to ~200 pc from the center. The Color Magnitude Diagram reveals a steep Red Giant Branch (RGB), and a Horizontal Branch (HB) extending blue ward of the instability strip, indicating that B514 is a classical old metal-poor globular cluster. The RGB locus and the position of the RGB Bump are both consistent with a metallicity [Fe/H] ~ -1.8, in excellent agreement with spectroscopic estimates. A preliminary estimate of the integrated absolute V magnitude (M_V< -9.1) suggests that B514 is among the brightest globulars of M31.

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arXiv2006

An updated survey of globular clusters in M 31. I. Classification and radial velocity for 76 candidate clusters

Aims - We present the first results of a large spectroscopic survey of globular clusters and candidate globular clusters in the nearby M~31 galaxy. The survey is aimed at the classification of known candidate M~31 clusters and at the study of their kinematic properties. Methods - We obtained low-resolution spectroscopy (lambda/Dlambda~800-1300) for 133 targets, including 76 yet-to-confirm candidate clusters (i.e. with no previous spectroscopic information), 55 already-confirmed genuine M31 clusters, and 2 uncertain candidates. Our observations allowed a reliable estimate of the target radial velocity, within a typical accuracy of ~+-20km/s. The observed candidates have been robustly classified according to their radial velocity and shape parameters that allowed us to confidently discriminate between point sources and extended objects even from low-spatial-resolution imagery. Results - In our set of 76 candidates clusters we found: 42 newly-confirmed bona-fide M~31 clusters, 12 background galaxies, 17 foreground Galactic stars, 2 HII regions belonging to M31 and 3 unclassified (possibly M31 clusters or foreground stars) objects. The classification of a few other candidates not included in our survey has been also reassessed on various observational bases. All the sources of radial velocity estimates for M31 known globular clusters available in the literature have been compared and checked, and a homogeneous general list has been obtained for 349 confirmed clusters with radial velocity. Conclusions - Our results suggest that a significant number of genuine clusters (~> 100) is still hidden among the plethora of known candidates proposed by various authors. Hence our knowledge of the globular cluster system of the M31 galaxy is still far from complete even in terms of simple membership.

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arXiv2005

A helium spread among the main sequence stars in NGC 2808

We studied the color distribution of the main sequence of the Globular Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The color distribution of main sequence stars is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. We estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may have suffered self-enrichment, with different stellar populations born from the ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the first generation. In addition to the Y=0.40 stars, roughly 30% of the stars should have Y distributed between 0.26-0.29 while 50% have primordial Y, to explain also the peculiar horizontal branch morphology. Three main stages of star formation are identified, the first with primordial helium content Y ~ 0.24, the second one born from the winds of the most massive AGBs of the first stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a long hiatus of time (several 10^7yr) between the second and third generation, star formation might have been inhibited by the explosion of late Supernovae II deriving from binary evolution.

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arXiv2005

The outermost cluster of M31

We report on the identification of a new cluster in the far halo of the M31 galaxy. The cluster, named Bologna 514 (B514) has an integrated magnitude M_V=-8.5 +- 0.6, and a radial velocity, as estimated from two independent low-resolution spectra, V_r=-456 +- 23 km/s, which fully confirms its membership to the M31 system. The observed integrated spectrum is very similar to those of classical globular clusters. Being located at ~ 4^o (~55 kpc in projected distance) from the center of the parent galaxy, B514 is by far the most remote M31 cluster ever discovered. Its projected position, near the galaxy major axis, and M31-centric velocity, similar to that observed in the outermost regions of the HI rotation curve, may indicate that it belongs to the subsystem of M31 clusters that has been recently proposed (Morrison et al. 2004) to be part of the dynamically-cold thin disc of the galaxy.

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arXiv2005

Massive young clusters in the disc of M31

We have studied the properties of a sample of 67 very blue and likely young massive clusters in M31 extracted from the Bologna Revised Catalog of globular clusters, selected according to their color [(B-V) < 0.45] and/or to the strength of their Hbeta spectral index (Hbeta > 3.5 A). Their existence in M31 has been noted by several authors in the past; we show here that these Blue Luminous Compact Clusters (BLCCs) are a significant fraction (>~ 15%) of the whole globular cluster system of M31. Compared to the global properties of the M31 globular cluster system, they appear to be intrinsically fainter, morphologically less concentrated, and with a shallower Balmer jump and enhanced $Hβ$ absorption in their spectra. Empirical comparison with integrated properties of clusters with known age as well as with theoretical SSP models consistently indicate that their typical age is less than ~2 Gyr, while they probably are not so metal-poor as deduced if considered to be old. Either selecting BLCCs by their (B-V) colors or by the strength of their Hbeta index the cluster sample turns out to be distributed onto the outskirts of M31 disc, sharing the kinematical properties of the thin, rapidly rotating disc component. If confirmed to be young and not metal-poor, these clusters indicate the occurrence of a significant recent star formation in the thin disc of M31, although they do not set constraints on the epoch of its early formation.

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arXiv2005

HST/WFPC2 Color-Magnitude Diagrams for Globular Clusters in M31

We report new HST/WFPC2 photometry for 10 globular clusters (GC) in M31 observed in F5555W(V) and F814W(I). Additionally we have reanalyzed HST archival data of comparable quality for 2 more GCs. Extraordinary care is taken to account for the effects of blended stellar images and required field subtraction. We thus reach 1 mag fainter than the horizontal branch (HB) even in unfavorable cases. We present the color-magnitude diagrams (CMDs) and discuss their main features also in comparison with the properties of the Galactic GCs. This analysis is augmented with CMDs previously obtained and discussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report the following significant results: 1. The locus of the red giant branches give reliable metallicity determinations which compare generally very well with ground-based integrated spectroscopic and photometric measures, as well as giving good reddening estimates. 2. The HB morphologies show the same behavior with metallicity as the Galactic GCs, with indications that the 2nd-parameter effect can be present in some GCs of our sample. However, at [Fe/H] ~ -1.7 we observe a number of GCs with red HB morphology such that the HB type versus [Fe/H] relation is offset from the MW and resembles that of the Fornax dwarf spheroidal galaxy. One explanation for the offset is that they are younger than their MW counterparts by 1-2 Gyr. 3. The Mv(HB)-[Fe/H] relationship has been determined and the slope (~0.20) is very similar to the values derived from RR Lyrae stars in the MW and the LMC. The zero-point of this relation based on the assumed distance modulus (m-M)o(M31)=24.47+/-0.03 is consistent with (m-M)o(LMC)=18.55.

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arXiv2003

2MASS NIR photometry for 693 candidate globular clusters in M31 and the Revised Bologna Catalogue

We have identified in the 2MASS database 693 known and candidate globular clusters in M31. The 2MASS J,H,K magnitudes of these objects have been transformed to the same homogeneous photometric system of existing near infrared photometry of M31 globulars, finally yielding J,H,K integrated photometry for 279 confirmed M31 clusters, 406 unconfirmed candidates and 8 objects with controversial classification. Of these objects 529 lacked any previous estimate of their near infrared magnitudes. The newly assembled near infrared dataset has been implemented into a revised version of the Bologna Catalogue of M31 globulars, with updated optical (UBVRI) photometry taken, when possible, from the most recent sources of CCD photometry available in the literature and transformed to a common photometric system. The final Revised Bologna Catalogue (available in electronic form) is the most comprehensive list presently available of confirmed and candidate M31 globular clusters, with a total of 1164 entries. In particular, it includes 337 confirmed GCs, 688 GC candidates, 10 objects with controversial classification, 70 confirmed galaxies, 55 confirmed stars, and 4 HII regions lying within ~3 deg. from the center of the M31 galaxy. Using the newly assembled database we show that the V-K color provides a powerful tool to discriminate between M31 clusters and background galaxies, and we identify a sample of 83 globular cluster candidates, which is not likely to be contaminated by misclassified galaxies.

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arXiv2003

Mass motions and chromospheres of RGB stars in the globular cluster NGC 2808

We present the results of the first observations, taken with FLAMES during Science Verification, of red giant branch (RGB) stars in the globular cluster NGC 2808. A total of 137 stars was observed, of which 20 at high resolution (R=47,000) with UVES and the others at lower resolution (R=19,000-29,000) with GIRAFFE in MEDUSA mode, monitoring ~ 3 mag down from the RGB tip. Spectra were taken of the Halpha, Na I D and Ca II H and K lines. This is by far the largest and most complete collection of such data in globular cluster giants, both for the number of stars observed within one cluster, and for monitoring all the most important optical diagnostics of chromospheric activity/mass motions.Evidence of mass motions in the atmospheres was searched from asymmetry in the profiles and coreshifts of the Halpha, Na I D and Ca II K lines, as well as from Halpha emission wings. We have set the detection thresholds for the onset of Halpha emission, negative Na D_2 coreshifts and negative K_3 coreshifts at log L/Lsun ~ 2.5, 2.9 and 2.8, respectively. These limits are significantly fainter than the results found by nearly all previous studies. Also the fraction of stars where these features have been detected has been increased significantly with respect to the previous studies. Our observations confirm the widespread presence of chromospheres among globular cluster giants, as it was found among Population I red giants. Some of the above diagnostics suggest clearly the presence of outward mass motions in the atmosphere of several stars.

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arXiv2003

Correlations of Globular Cluster Properties: Their Interpretations and Uses

Correlations among the independently measured physical properties of globular clusters (GCs) can provide powerful tests for theoretical models and new insights into their dynamics, formation, and evolution. We review briefly some of the previous work, and present preliminary results from a comparative study of GC correlations in the Local Group galaxies. The results so far indicate that these diverse GC systems follow the same fundamental correlations, suggesting a commonality of formative and evolutionary processes which produce them.

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arXiv2002

The Andromeda Project. I. Deep HST-WFPC2 V,I photometry of 16 fields toward the disk and the halo of the M31 galaxy. Probing the stellar content and metallicity distribution

HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the luminous nearby spiral galaxy M31, sampling the stellar content of the disk and the halo at different distances from the center, from ~ 20 to ~ 150 arcmin (i.e. ~ 4.5 to 35 kpc), down to limiting V and I magnitudes of ~ 27. The Color-Magnitude diagrams (CMD) show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of M31 generally show a blue plume of stars which we identify with main sequence members. Accordingly, the star formation rate over the last 0.5 Gyr appears to have varied dramatically with location in the disk. All the CMDs show a prominent Red Giant Branch (RGB) with a descending tip in the V band, characteristic of metallicity higher than 1/10 Solar. A red clump is detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions (MDs), obtained by comparison of the RGB stars with globular cluster templates, are basically similar in all the sampled fields: they all show a long, albeit scantly populated metal-poor tail and a main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas the fraction of metal-poor stars seems to be approximately constant in all fields, the fraction of very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures. This might indicate and possibly trace interaction effects with some companion, e.g. M32.

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arXiv2001

Deep HST-WFPC2 photometry of NGC 288. I. Binary Systems and Blue Stragglers

We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the Main Sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn(1997). We have unequivocally detected a significant population of binary systems which has a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r<1 r_h = 1.6 r_c) the binary fraction (f_b) lies in the range 0.08--0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable f_b lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r> 1 r_h), f_b must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of Blue Straggler Stars (BSS) is exceptionally high, suggesting that the BSS production mechanism via binary evolution can be very efficient. A large population of BSS is possible even in low density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSS in the Color Magnitude Diagram is not compatible with a rate of BSS production which has been constant in time, if it is assumed that all the BSS are formed by the merging of two stars.

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arXiv2001

Age as the Second Parameter in NGC 288 / NGC 362? I. Turnoff Ages: a Purely Differential Comparison

We present deep V,I photometry of the globular clusters NGC 288, NGC 362 and NGC 1851 obtained during a single observational run under strictly homogeneous conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a ``bridge'' to obtain the optimum relative match between the HBs of NGC 288 and NGC 362. In this way we can effectively remove the uncertainties associated with distance, reddening and inhomogeneities in the absolute calibration, thus obtaining a very robust, purely differential estimate of the age difference between these two clusters. According to the bridge test, NGC 288 is found to be older than NGC 362 by 2 +- 1 Gyr. This result is fully confirmed also by all classical differential age diagnostics, either based on the luminosity or color of the main sequence turnoff point.

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arXiv2001

Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

We revisit the "second parameter" pair of globular clusters NGC 288/NGC 362 on the basis of theoretical models for red giant branch (RGB) and horizontal branch (HB) stars. The results of the most extensive set of RGB/HB simulations computed so far for these clusters are presented for two different metallicities. Using several different analytical mass loss formulae for RGB stars, we derive relative "HB morphology ages." We compare them with the relative main-sequence turnoff ages derived by application of the "bridge test" by Bellazzini et al. (2001), who found that NGC 288 is 2+/-1 Gyr older than NGC 362. We find that adoption of a higher metallicity ([Fe/H] ~ -1.2), as favored by the Carretta & Gratton metallicity scale, makes age a much more plausible second parameter candidate for this pair than is the case when a lower metallicity ([Fe/H] ~ -1.5), closer to the Zinn & West scale, is adopted. However, while the different HB morphology of these two clusters can be reproduced by canonical HB models with [Fe/H] ~ -1.2 and an age difference of 2 Gyr, this explanation is not without difficulty. In particular, we confirm previous suggestions that canonical models are unable to reproduce the detailed HB morphology of NGC 288 at its red end, for as yet unknown reasons. Finally, we show that the mass dispersion on the HB of NGC 362 is substantially larger than for NGC 288, and suggest that there is a correlation between the mass dispersion on the HB phase and the central density of globular clusters. This is presumably related to the way environmental effects affect RGB mass loss. [Abridged]

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arXiv2001

A Survey of M31 Globular Clusters using WFPC2 on board HST

We report new, as yet unpublished HST/WFPC2 V, I photometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters, including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=--0.7 with a tail toward low metallicity, resembling the halo of NGC 5128; metal rich giants appear to be centrally concentrated.

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