arXiv subjects

A. Vallenari

Publications and source records attributed to A. Vallenari.

149 records · Page 9Linked to original sources

arXiv2002

Cluster formation in the LMC

In this work we present preliminary results concerning the properties of about 50 clusters and associations located in a densely populated region at the Southern edge of the supershell LMC~4. The ages of the clusters and associations are derived. The cluster formation rate peaks at about 10-20 Myr ago corresponding to the age of the formation of LMC~4. Objects younger than 10 Myr are located close to CO clouds, indicating a possible trigger of the recent star formation by the interaction of the supershell with the interstellar medium. Isophotal analysis has been done for 50% of the clusters. A large number of objects present isophotal distortions commonly interpreted as sign of interaction. Cluster multiplets are coeval within 20 Myr, suggesting a common origin. This indicates that clusters possibly form in large groups. Our findings are in agreement with the unified view of multi-scale star formation, where a size-duration correlation is expected.

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arXiv2001

The Field population towards NGC 6553

We analyze the colour magnitude diagram of the field population in the direction of the bulge cluster NGC 6553 to discuss the stellar field component projected towards the cluster. In particular we investigate the nature of a peculiar feature, called ``red flare'' which appears as a faint turnoff, located 2 magnitudes about below the turnoff of the cluster, with a sparse trace of a sub-giant branch just above. A detailed modeling shows that the red flare is consistent with the expected location of the turnoff and sub-giant branch of the bulge population, once that the appropriate metallicity range and spatial distribution of the bulge stars are adopted. From the data, hints can be derived about the spatial shape and the basic stellar parameters of the bulge.

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arXiv2001

An optical and near IR study of the old open cluster NGC 2141

We report on CCD optical (B and V passbands) and near IR (J and K bands) observations in the region of the old open cluster NGC 2141. By combining the two sets of photometry (500 stars in common) we derive new estimates of the cluster fundamental parameters. We confirm that the cluster is 2.5 Gyrs old, but, with respect to previous investigations, we obtain a slightly larger reddening (E(B-V)=0.40), and a slightly shorter distance (3.8 kpc) from the Sun. Finally we present Luminosity Function (LF) in the V band, which is another age indicator. We provide a good fit for the age range inferred from isochrones by assuming the Kroupa et al. (1993) IMF up to $M_V$=5.0. We interpret the disagreemt at fainter magnitudes as an evidence of mass segregation.

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arXiv2000

The Galactic disk: study of four low latitude Galactic Fields

We present deep V and I photometry of stellar field in four previously unstudied low latitude regions of the Galactic disk. All observed fields are located at the western side of the Galactic Center in the direction of the Coalsack-Carina region. Two of them are suspected to cross the inner spiral arm. More than $10^5$ stars are detected in total, down to a magnitude of V $\sim 23.5$. Due to the low galactic latitude of the studied fields, the scale length and mainly the scale height of the thick disk are not strongly constrained by the observations. However a thin disk scale height of about 250$\pm 60$ pc seems to be favored. The data are very sensitive to the star formation rate of the thin disk. A decreasing star formation rate (SFR) is necessary to reproduce the distribution of the stars in the colour-magnitude diagrams (CMDs) as well as the luminosity functions. A constant or a strongly increasing SFR as derived using Hipparcos data for the solar neighborhood (Bertelli et al 1999) are marginally in agreement with the luminosity functions but they are at odds with the CMDs. The analysis of these data suggest that the solar neighborhood SFR cannot be considered as representative for the whole thin disk. From the star-counts the local neighborhood mass density in stars more massive than 0.1 M$_\odot$ is found to be 0.036--0.02 M$_\odot$ pc$^{-3}$. Finally, the location of inner spiral arm is discussed. We find evidence of a population younger than 10$^8$ yr distributed in a spiral arm at a distance of 1.3$\pm 0.2$ and 1.5 $\pm 0.2$\,Kpc in the directions l$\sim$\,292 and l$\sim$\,305 respectively. This result is consistent with the spiral arm pattern defined on the basis of pulsars and young associations.

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arXiv1998

Interacting star clusters in the LMC - Overmerging problem solved by cluster group formation

We present the tidal tail distributions of a sample of candidate binary clusters located in the bar of the Large Magellanic Cloud (LMC). One isolated cluster, SL 268, is presented in order to study the effect of the LMC tidal field. All the candidate binary clusters show tidal tails, confirming that the pairs are formed by physically linked objects. The stellar mass in the tails covers a large range, from $1.8\times 10^3$ to $3\times 10^4$\msun. We derive a total mass estimate for SL 268 and SL 356. At large radii, the projected density profiles of SL 268 and SL 356 fall off as $r^{-γ}$, with $γ= 2.27$ and $γ=3.44$, respectively. Out of 4 pairs or multiple systems, 2 are older than the theoretical survival time of binary clusters (going from a few $10^6$ yr to $10^8$ yr). A pair shows too large age difference between the components to be consistent with classical theoretical models of binary cluster formation (Fujimoto & Kumai 1997). We refer to this as the ``overmerging'' problem.A different scenario is proposed: the formation proceeds in large molecular complexes giving birth to groups of clusters over a few $10^7$ yr. In these groups the expected cluster encounter rate is larger, and tidal capture has higher probability. Cluster pairs are not born together through the splitting of the parent cloud, but formed later by tidal capture. For 3 pairs, we tentatively identify the star cluster group (SCG) memberships. The SCG formation, through the recent cluster starburst triggered by the LMC-SMC encounter, in contrast with the quiescent open cluster formation in the Milky Way can be an explanation to the paucity of binary clusters observed in our Galaxy.

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