Charles Jose Bonatto

Theoretical isochrones compared to 2MASS observations: Open clusters at nearly solar metallicity

We study classical disk clusters at nearly solar metallicity with ages in the range 200 Myr - 6 Gyr using 2MASS pho- tometry and theoretical isochrones. As a first step we employ J, H and K Johnsons Padova isochrones which, on a large scale, result in a good fit of the observed data. However, we find some significant deviations, especially in the red giant branch for intermediate age clusters and the low-mass main-sequence end in general. Subsequently, isochrones involving the 2MASS J, H and KS transmission curves were generated which account for differences of ≈0.02 of the above deviations, in the sense re- quired by the 2MASS observations. However, these corrections are not enough to fully account for the observed differences. We suggest that the remaining deviations should be accounted for by model atmosphere limitations, particularly the lack of molec- ular opacity data in low-temperature giants. Future models will incorporate these updated data in order to produce isochrones suitable for the whole range of stellar masses.

Globular cluster system and Milky Way properties revisited

Aims. Updated data of the 153 Galactic globular clusters are used to readdress fundamental parameters of the Milky Way, such as the distance of the Sun to the Galactic centre, the bulge and halo structural parameters, and cluster destruction rates. Methods. We build a reduced sample that has been decontaminated of all the clusters younger than 10 Gyr and of those with retrograde orbits and/or evidence of relation to dwarf galaxies. The reduced sample contains 116 globular clusters that are tested for whether they were formed in the primordial collapse. Results. The 33 metal-rich globular clusters ([Fe/H] ≥− 0.75) of the reduced sample basically extend to the Solar circle and are distributed over a region with the projected axial-ratios typical of an oblate spheroidal, ∆x : ∆y : ∆z ≈ 1. 0:0 . 9:0 .4. Those outside this region appear to be related to accretion. The 81 metal-poor globular clusters span a nearly spherical region of axial-ratios ≈1. 0:1 . 0:0 .8 extending from the central parts to the outer halo, although several clusters in the external region still require detailed studies to unravel their origin as accretion or collapse. A new estimate of the Sun’s distance to the Galactic centre, based on the symmetries of the spatial distribution of 116 globular clusters, is provided with a considerably smaller uncertainty than in previous determinations using globular clusters, RO = 7.2 ± 0.3 kpc. The metal-rich and metal-poor radial-density distributions flatten for RGC ≤ 2 kpc and are represented well over the full Galactocentric distance range both by a power-law with a core-like term and Sersic’s law; at large distances they fall off as ∼R −3.9 . Conclusions. Both metallicity components appear to have a common origin that is different from that of the dark matter halo. Structural similarities between the metal-rich and metal-poor radial distributions and the stellar halo are consistent with a scenario where part of the reduced sample was formed in the primordial collapse and part was accreted in an early period of merging. This applies to the bulge as well, suggesting an early merger affecting the central parts of the Galaxy. The present decontamination procedure is not sensitive to all accretions (especially prograde) during the first Gyr, since the observed radial density profiles still preserve traces of the earliest merger(s). We estimate that the present globular cluster population corresponds to ≤23 ± 6% of the original one. The fact that the volume-density radial distributions of the metal-rich and metal-poor globular clusters of the reduced sample follow both a core-like power-law, and Sersic’s law indicates that we are dealing with spheroidal subsystems at all scales.

A general catalogue of extended objects in the Magellanic System

We update the Small Magellanic Cloud (SMC), Bridge and Large Magellanic Cloud (LMC) catalogues of extended objects that were constructed by members of our group from 1995 to 2000. In addition to the rich subsequent literature for the previous classes, we now also include H I shells and supershells. A total of 9305 objects were cross-identified, while our previous catalogues amounted to 7900 entries, an increase of ≈12 per cent. We present the results in subcatalogues containing 1445 emission nebulae, 3740 star clusters, 3326 associations and 794 H I shells and supershells. Angular and apparent size distributions of the extended objects are analysed. We conclude that the objects, in general, appear to respond to tidal effects arising from the LMC, SMC and Bridge. Number-density profiles extracted along directions parallel and perpendicular to the LMC bar, can be described by two exponential-discs. A single exponential-disc fits the equivalent SMC profiles. Interestingly, when angular-averaged number-densities of most of the extended objects are considered, the profiles of both Clouds do not follow an exponential-disc. Rather, they are best described by a tidally truncated, core/halo profile, despite the fact that the Clouds are clearly disturbed discs. On the other hand, the older star clusters taken isolately, distribute as an exponential disc. The present catalogue is an important tool for the unambiguous identification of previous objects in current CCD surveys and to establish new findings.

Open clusters in dense fields: the importance of field-star decontamination for NGC 5715, Lyngå 4, Lyngå 9, Trumpler 23, Trumpler 26 and Czernik 37

Star clusters projected on to dense stellar fields in general require field-star (FS) decontam- ination to confirm their nature and derive intrinsic photometric and structural fundamental parameters. This work focuses on the open clusters (OCs) or candidates NGC 5715, Lynga4 , Lynga 9, Trumpler 23, Trumpler 26 and Czernik 37 which are projected within 317 ◦ � 2. ◦ 2 and |b| 2. ◦ 8, against crowded bulge and/or disc fields. To tackle these difficult objects we develop a colour-magnitude diagram FS-decontamination algorithm based on 2MASS J, (J − H) and (J − Ks) data, and respective uncertainties, to detect cluster star excesses over the background. On the other hand, colour-magnitude filters are used to build stellar radial density profiles and mass functions. The results convey compelling evidence that Lynga9 and Czernik 37 are intermediate-age OCs, and their fundamental parameters are measured for the first time. Trumpler 23 is a particularly challenging object, since besides high back- ground level, its field presents variable absorption in near-infrared bands. We confirm it to be an intermediate-age OC. Trumpler 26 is studied in more detail than in previous works, while NGC 5715 and Lynga 4 have fundamental parameters determined for the first time. These OCs are located 0.9-1.6 kpc within the solar circle, with ages similar to that of the Hyades. Structurally, they are well described by King profiles. In all cases, core and limiting radii are significantly smaller than those of nearby OCs outside the solar circle. We test the effect of background levels on cluster radii determinations by means of simulations. They indicate that for central clusters, radii may be underestimated by about 10-20 per cent, which suggests that the small sizes measured for the present sample reflect as well intrinsic properties related to dynamical evolution effects. The objects probably have been suffering important tidal ef- fects that may have accelerated dynamical evolution, especially in Czernik 37, the innermost object.

New Galactic star clusters discovered in the VVV survey

Context. VISTA Variables in the V´oa Lactea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. Aims. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. Methods. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated KS-band tile images for stellar overdensities. Subsequently, we examined the composite JHKS and ZJKS color images of each candidate. PSF photometry of 15 × 15 arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. Results. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar- metallicity Padova isochrones to the color-magnitude diagrams.

Probing disk properties with open clusters

Received -; accepted - Abstract. We use the open clusters (OCs) with known parameters available in the WEBDA database and in recently published papers to derive properties related to the disk structure such as the thin-disk scale height, displacement of the Sun above the Galactic plane, scale length and the OC age-distribution function. The sample totals 654 OCs, consisting basically of Trumpler types I to III clusters whose spatial distribution traces out the local geometry of the Galaxy. We find that the population of OCs with ages younger than 200Myr distributes in the disk following an exponential-decay profile with a scale height of zh = 48±3pc. For the clusters with ages in the range 200Myr to 1Gyr we derive zh = 150±27 pc. Clusters older than 1Gyr distribute nearly uniformly in height from the plane so that no scale height can be derived from exponential fits. Considering clusters of all ages we obtain an average scale height of zh = 57±3pc. We confirm previous results that zh increases with Galactocentric distance. The scale height implied by the OCs younger than 1Gyr outside the Solar circle is a factor � 1.4 2 larger than zh of those interior to the Solar circle. We derive the displacement of the Sun above the Galactic plane as z⊙ = 14.8±2.4 pc, which agrees with previous determinations using stars. As a consequence of the completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile, instead it falls off both for regions external to the Solar circle and more sharply towards the Galactic center. We simulate the effects of completeness assuming that the observed distribution of the number of OCs with a given number of stars above the background, measured in a restricted zone outside the Solar circle, is representative of the intrinsic distribution of OCs throughout the Galaxy. Two simulation models are considered in which the intrinsic number of observable stars are distributed: (i) assuming the actual positions of the OCs in the sample, and (ii) random selection of OC positions. As a result we derive completeness-corrected radial distributions which agree with exponential disks throughout the observed Galactocentric distance range 5-14 kpc, with scale lengths in the range RD = 1.5 1.9kpc, smaller than those inferred by means of stars. In particular we retrieve the expected exponential-disk radial profile for the highly depleted regions internal to the Solar circle. The smaller values of RD may reflect intrinsic differences in the spatial distributions of OCs and stars. We derive a number-density of Solar-neighbourhood (with distances from the Sun d⊙ � 1.3kpc) OCs of �⊙ = 795 ± 70kpc −3 , which implies a total number of (Trumpler types I to III) OCs of � 730 of which � 47% would already have been observed. Extrapolation of the completeness-corrected radial distributions down to the Galactic center indicates a total number of OCs in the range (1.8 3.7) × 10 5 . These estimates are upper-limits because they do not take into account depletion in the number of OCs by dynamical effects in the inner parts of the Galaxy. The observed and completeness-corrected age-distributions of the OCs can be fitted by a combination of two exponential-decay profiles which can be identified with the young and old OC populations, characterized by age scales of � 100Myr and � 1.9Gyr, respectively. This rules out evolutionary scenarios based on constant star-formation and OC-disruption rates. Comparing the number of observed embedded clusters and candidates in the literature with the expected fraction of very young OCs, derived from the observed age-distribution function, we estimate that 3.4-8% of the embedded clusters do actually emerge from the parent molecular clouds as OCs.

Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane

Received -; accepted - Abstract. We derive photometric, structural and dynamical evolution-related parameters of 11 nearby open clus- ters with ages in the range 70Myr to 7Gyr and masses in the range � 400M⊙ to � 5300M⊙. The clusters are homogeneously analysed by means of J, H and KS 2MASS photometry, which provides spatial coverage wide enough to properly take into account the contamination of the cluster field by Galaxy stars. Structural parameters such as core and limiting radii are derived from the background-subtracted radial density profiles. Luminosity and mass functions (MFs) are built for stars later than the turnoff and brighter than the 2MASS PSC 99.9% completeness limit. The total mass locked up in stars in the core and the whole cluster, as well as the correspond- ing mass densities, are calculated by taking into account the observed stars (evolved and main sequence) and extrapolating the MFs down to the H-burning mass limit, 0.08M⊙. We illustrate the methods by analysing for the first time in the near-infrared the populous open clusters NGC2477 and NGC2516. For NGC2477 we derive an age of 1.1±0.1Gyr, distance from the Sun d⊙ = 1.2±0.1kpc, core radius Rcore = 1.4±0.1pc, limiting radius Rlim = 11.6 ± 0.7pc and total mass mtot � (5.3 ± 1.6) × 10 3 M⊙. Large-scale mass segregation in NGC2477 is reflected in the significant variation of the MF slopes in different spatial regions of the cluster, and in the large number-density of giant stars in the core with respect to the cluster as a whole. For NGC2516 we derive an age of 160 ± 10Myr, d⊙ = 0.44 ± 0.02 kpc, Rcore = 0.6 ± 0.1pc, Rlim = 6.2 ± 0.2pc and mtot � (1.3 ± 0.2) × 10 3 M⊙. Mass-segregation in NGC2516 shows up in the MFs. Six of the 11 clusters present a slope break in the MF occurring at essentially the same mass as that found for the field stars in Kroupas universal IMF. The MF break is not associated to cluster mass, at least in the clusters in this paper. In two clusters the low-mass end of the MF occurs above the MF break. However, in three clusters the MF break does not occur, at least for the mass range m � 0.7M⊙. One possibility is dynamical evolution affecting the MF slope distribution. We also search for relations of structural and evolutionary parameters with age and Galactocentric distance. The main results for the present sample are: (i) cluster size correlates both with age and Galactocentric distance; (ii) because of size and mass scaling, core and limiting radii, and core and overall mass correlate; (iii) massive (m � 1000M⊙) and less-massive clusters follow separate correlation paths on the plane core radius and overall mass; (iv) MF slopes of massive clusters are restricted to a narrow range, while those of the less-massive ones distribute over a wider range. Core and overall MF flattening is related to the ratio (�) of age to relaxation time. For large values ofthe effects of large-scale mass segregation and low-mass stars evaporation can be observed in the MFs. In this sense, � appears to characterize the evolutionary state of the clusters. We conclude that appreciable slope flattenings in the overall MFs of the less-massive clusters take � 6 times longer to occur than in the core, while in the massive clusters they take a time � 13 times longer. We investigate cluster parameters equivalent to those determining the fundamental plane of ellipticals. These parameters are: overall mass, projected mass density and core radius. We conclude that in the present sample there is evidence of a fundamental plane. Larger samples are necessary to pin down this issue.

Mass functions and structure of the young open cluster NGC 6611

We use J , H  and K S  2MASS photometry to study colour–magnitude (CMDs) and colour–colour diagrams, structure and mass distribution in the ionizing open cluster NGC 6611. Reddening variation throughout the cluster region is taken into account followed by field-star decontamination of the CMDs. Decontamination is also applied to derive the density profile and luminosity functions in the core, halo and overall (whole cluster) regions. The field-star decontamination showed that the lower limit of the main sequence (MS) occurs at ≈

Probing the near-infrared stellar population of Seyfert galaxies

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Probing the age and structure of the nearby very young open clusters NGC 2244 and 2239

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