arXiv subjects

Giovanni Carraro

Publications and source records attributed to Giovanni Carraro.

158 records · Page 9Linked to original sources

arXiv1999

The Age of the Galactic Disk

I review different methods devised to derive the age of the Galactic Disk, namely the Radio-active Decay (RD), the Cool White Dwarf Luminosity Function (CWDLF), old opne clusters (OOC) and the Color Magnitude Diagram (CMD) of the stars in the solar vicinity. I argue that the disk is likely to be 8-10 Gyr old. Since the bulk of globulars has an age around 13 Gyr, the possibility emerges that the Galaxy experienced a minimum of Star Formation at the end of the halo/bulge formation. This minimum might reflect the time at which the Galaxy started to acquire material to form the disk inside-out.

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arXiv1999

Near-infrared photometry of the intermediate age open clusters IC 166 and NGC 7789

We present and discuss new photometric data obtained with an IR camera in the J and K pass-bands for the intermediate age open clusters IC 166 (936 stars in total) and NGC 7789 (1030 stars in total). Ic 166 is a poorly studied open cluster for which no IR data was availabele previoulsy, while NGC 7789 is a well studied open cluster. We show that IC 166 is of intermediate age (about 1.0 Gyr), whit a reddening E(V-K) =1.3 (corresponding to E(B-V) = 0.50) and a true distance modulus (m-M)o = 13.25. These values are significantly different from previous determinations. NGC 7789 is found to be a 1.4 Gyr old open cluster. The metallicity derived with the method developed by Tiede et al (1997) is found to be closer to the spectroscopic estimate than previous photometric studies.

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arXiv1999

Galaxy Formation and Evolution. II. Energy Balance, Star Formation and Feed-back

In this paper we present a critical discussion of the algorithms commonly used in N-body simulations of Galaxy Formation to deal with the energy equation governing heating and cooling, to model star formation and the star formation rate, and to account for energy feed-back from stars. First, we propose our technique for solving the energy equation in presence of heating and cooling, which includes some difference with respect to the standard semi-implicit technique. Second, we examine the current criteria for the onset of the star formation activity. We suggest a new approach, in which star formaiton is let depend on the total mass density - baryonic (gas and stars) and dark matter - of the system and on the metal-dependent cooling efficiency. Third, we check and discuss the separate effects of energy (and mass) feed-back from several sources - namely supernovae, stellar winds from massive stars, and UV flux from the same objects. All the simulations are performed in the framework of the formation and evolution of a disk galaxy. We show that the inclusion of these physical phenomena has a signigicant impact on the evolution of the galaxy model.

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arXiv1999

Near-infrared photometry of the young open clusters NGC 1893 and Berkeley 86

We present photometry in the J and K near-infrared bands for two regions centered on the young open clusters NGC 1893 and Berkeley 86. We study 700 stars down to K = 17 in the field of NGC 1893, and about 2000 stars in the field of Berkeley 86 down to K$\sim$ 16.5, for which near-infrared photometry was insofar not available. Coupling J-K data with UBV photometry taken from literature, we produce reddening corrected colour-magnitude diagrams. We find that our data are consistent with previous determinations: the clusters are roughly coeval with an age between 4 and 6 million years. The mean reddening (measured as E(J-K)) values turn out to be 0.35 and 0.50 for NGC 1893 and Berkeley 86, respectively.Using colour-colour plots we discuss the presence of candidate pre-main sequence stars showing infrared excess. Candidates are found in both cluster regions, confirming the young age of these clusters.

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arXiv1999

Is the galactic disk older than the halo?

Aim of this study is to infer the age of the Galactic Disk by means of the ages of old open clusters, and comment on some recent claims that the Galactic Disk can be older than the Halo. To this purpose, we analyze the Color-Magnitude Diagrams (CMDs) of six very old clusters, namely NGC 188, NGC 6791, Collinder 261, Melotte 66, Berkeley 39 and Berkeley 17, and determine their ages. For each cluster we use the most recent photometric and spectroscopic data and metallicity estimates. The ages are derived from the isochrone fitting method using the stellar models of the Padua library (Bertelli et al . 1994, Girardi et al. 1999). We find that the ages of these clusters fall in the range 4 to 9-10 Gyr: Melotte 66 is the youngest whereas NGC 6791 and Berkeley 17 have ages of about 9-10 Gyr. Previous estimates for Berkeley 17 indicated an age as old as 12 Gyr, almost falling within the range of classical globular clusters. In our analysis, this cluster is always very old but perhaps somewhat younger than in previous studies. However we call attention on the fact that the above ages are to be taken as provisional estimates, because of the many uncertaintes still affecting stellar models in the mass range 1.0 to 1.5 Mo. Despite this drawback of extant theory of stellar structure, if NGC 6791 and Berkeley 17 set the limit to the age of the Galactic Disk, this component of the Milky Way can be as old as 9-10 Gyr, but surely younger than the Galactic Halo, at least as inferred from recent determinations of the age of globular clusters. Finally, it is worth recalling that open clusters can only provide a lower limit to the age of the Galactic Disk, while other indicators - like White Dwarfs - are perhaps more suited to this task.

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arXiv1999

Parallel TreeSPH: A Tool for Galaxy Formation

We describe a new implementation of a parallel Tree-SPH code with the aim to simulate Galaxy Formation and Evolution. The code has been parallelized using SHMEM, a Cray proprietary library to handle communications between the 256 processors of the Silicon Graphics T3E massively parallel supercomputer hosted by the Cineca Super-computing Center (Bologna, Italy). The code combines the Smoothed Particle Hydrodynamics (SPH) method to solve hydro-dynamical equations with the popular Barnes and Hut (1986) tree-code to perform gravity calculation with a $N \times logN$ scaling, and it is based on the scalar Tree-SPH code developed by Carraro et al (1998)[MNRAS 297, 1021]. Parallelization is achieved distributing particles along processors according to a work-load criterium. Benchmarks, in terms of load-balance and scalability, of the code are analised and critically discussed against the adiabatic collapse of an isothermal gas sphere test using $2 \times 10^{4}$ particles on 8 processors. The code results balanced at more than 95% level. Increasing the number of processors, the load balance sligthly worsens. The deviation from perfect scalability at increasing number of processors is negligible up to 64 processors. Additionally we have incorporated radiative cooling, star formation, feed-back and an algorithm to follow the chemical enrichment of the interstellar medium.

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arXiv1998

Near IR photometry of the old open clusters Berkeley 17 and Berkeley 18. Probing the age of the galactic disc

We report on near IR ($J$ and $K$ bands) observations of two $8 \times 8 \~(arcmin)^{2}$ regions centered on the old open clusters Berkeley~17 and Berkeley~18, for which only optical photometry (in $B$,$V$ and $I$ bands) exists. $J$ and $K$ photometry allows us to obtain an independent estimate of cluster metallicity by means of the relationship between the spectroscopic metallicity and the Red Giant Branch (RGB) slope calibrated by Tiede et al (1997). From the analysis of the colour magnitude diagram (CMD) and luminosity function (LF), Berkeley~17 turns out to have a metal content $[Fe/H]~\sim~-0.35$. It is $9~Gyr$ old, suffers from a reddening $E(B-V) = 0.58 mag$ and has an heliocentric distance of $2.5 kpc$. Berkeley~17 comes out to be substantially younger than in previous work (age $\approx 13~Gyr$). On the other hand Berkeley~18 is found to have solar metal abundance, and to be younger than Berkeley~17, with an age of about $4~Gyr$. While we confirm Kaluzny (1997) reddening estimate, we significantly revise the distance of the cluster, which lies $4.5 kpc$ from the Sun. These results on two open clusters believed to be between the oldest put constraints on the age and the evolution of the Galactic Disc. The absence of clusters older than $8-9~Gyr$ suggests the possibility that the Galaxy underwent a star formation minimum between 13 and $10~Gyr$ ago.

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arXiv1998

Bright Stars and Metallicity Spread in the Globular Cluster Omega Centauri

The globular cluster Omega Centauri (NGC 5139) is the most massive and brightest cluster in our Galaxy. It has also a moderately high mass to light ratio (3.6) and an anomalous flattening (0.83) for a globular cluster. This cluster is also very interesting because it is one of a few examples of globular clusters with a measurable spread in the metal abundance (see Da Costa & Willumsen 1981, Norris et al. 1996, and Suntzeff and Kraft 1996 and references therein) and then it offers a unique, big sample of nearby stars having all the same distance and reddening but showing different metallicity (and age ?) effects. A recent paper by Norris et al. (1997) shows also an interesting correlation between kinematics and metal abundance.

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arXiv1998

Comparison of two different extinction laws with Hipparcos observations

Interstellar absorption in the galactic plane is highly variable from one direction to another. In this paper colour excesses and distances from a new open cluster sample are used to investigate the spatial distribution of the interstellar extinction. An inverse method (Tarantola & Valette, 1982) is used to construct the extinction map in the galactic plane below $|b| < 10^{o}$. The $A_{v} (r,l) $ diagrams are compared with those derived from individual stars (Arenou et al. 1992, Neckel & Klare 1980). An analytic expression for the interstellar extinction as a function of galactic longitude and distance in the solar neighborhood is given. The comparison of the model predictions with Hipparcos observations in the 4-dimensional space of ($V$, $B-V$, $H_v$, $r$) shows that our extinction model provides a better fit to the data. However, a new and more detailed extinction model is still lacking.

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arXiv1998

NBODY/SPH simulations of individual galaxies

We present preliminary results on galactic Dark Matter (DM), halo structure, and galaxy evolution. We show how during the first Gyr of the evolution of a $10^{10} M_{\odot}$ dwarf elliptica feed-back from stars (SNæ~ and stellar winds) leads to an extended constant density isothermal core with radius of 0.15 the virial radius $R_{200}$. We also present first results on galaxy merging as a possibile scenario to form ellipticals, studying in particular how the details of the merging evolution vary as a function of the mass ratio of the interacting galaxies.

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arXiv1998

A new parallel TreeSPH code

In this report we describe a parallel implementation of a Tree-SPH code realized using the SHMEM libraries in the Cray T3E supercomputer at CINECA. We show the result of a 3D test to check the code performances against its scalar version. Finally we compare the load balancing and scalability of the code with PTreeSPH (Davé et al 1997), the only other parallel Tree-SPH code present in the literature.

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arXiv1997

Galaxy Formation and Evolution: I. The Padua TreeSPH code (PD-SPH)

In this paper we report on PD-SPH the new tree-sph code developed in Padua. The main features of the code are described and the results of a new and independent series of 1-D and 3-D tests are shown. The paper is mainly dedicated to the presentation of the code and to the critical discussion of its performances. In particular great attention is devoted to the convergency analysis. The code is highly adaptive in space and time by means of individual smoothing lengths and individual time steps. At present it contains both dark and baryonic matter, this latter in form of gas and stars, cooling, thermal conduction, star formation, and feed-back from Type I and II supernovae, stellar winds, and ultraviolet flux from massive stars, and finally chemical enrichment. New cooling rates that depend on the metal abundance of the interstellar medium are employed, and the differences with respect to the standard ones are outlined. Finally, we show the simulation of the dynamical and chemical evolution of a disk-like galaxy with and without feed-back. The code is suitably designed to study in a global fashion the problem of formation and evolution of elliptical galaxies, and in particular to feed a spectro-photometric code from which the integrated spectra, magnitudes, and colors (together with their spatial gradients) can be derived.

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arXiv1997

UBVRI photometry and spectroscopy of the young open cluster Haffner 18

UBVRI CCD photometry of a $2.^{\prime}1 \times 3.^{\prime}3$ field centered on the young open cluster Haffner~18 is presented and discussed. Spectroscopic classification of seven stars is also provided. 44 clusters members are identified, the earliest type being O6. The distance to the cluster is found to be 6.3 kpc, corresponding to a galactocentric distance of 12.7 kpc (for a Sun galactocentric distance of 8.5 kpc). Excellent fit to the observed main sequence is achieved by a solar composition isochrone of 2$\times 10^6$ years reddened by E$_{B-V}$=0.62 mag. Differential reddening of intra-cluster origin is present. Pre-main sequence members are likely to be present over the 6 mag range explored by our observations (reaching down to earliest A spectral types). The presence of differential reddening and pre-main sequence members agrees with the evidence for a bright parent nebulosity embedding the whole cluster. The radial velocity of the cluster is consistent with the Hron (1987) model of galactic rotation.

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arXiv1997

The spatial distribution and luminosity function of the open cluster NGC 4815

NGC 4815 is a distant and populous open cluster, which lies in the galactic plane in a region of strong absorption. As a consequence, its membership, spatial distribution and luminosity function are not well determined. In this paper, we present an algorithm which uses both positional and photometric data simultaneously to search for open cluster members. The contribution of the field stars is estimated by our Galaxy model software. The method has been developed for deep CCD photometric observations, but it can also be used to investigate the membership in an n-dimensional space, including proper motions, radial velocity and metallicity. A clean and well defined colour-magnitude diagram (CMD) for NGC 4815 has been obtained after decontaminating from field stars. We have investigated the spatial distribution of cluster members and the luminosity function. We have shown that the projected surface distribution of stars is well-represented by an exponentially decreasing function with a scale length of 1.67 $\pm$ 0.06 arcmin. We have found evidence for mass segregation. The luminosity function for NGC 4815 is similar to that of the Hyades cluster, and shows a gap at $V$ = 15 mag.

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