arXiv subjects

E. Bica

Publications and source records attributed to E. Bica.

At least 73 records · Page 4Linked to original sources

arXiv2006

Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960

We derive accurate parameters related to the CMD, structure and dynamical state of M52 and NGC3960, whose fields are affected by differential reddening. Compared to open clusters in different dynamical states studied with similar methods, the core and overall parameters of M52 are consistent with an open cluster more massive than 1000Mo and ~60Myr old, with some mass segregation in the inner region. The core of NGC3960 is in an advanced dynamical state with strong mass segregation in the core/halo region, while the somewhat flat overall MF (x~1.07) suggests low-mass star evaporation. The excess stellar density in the core may suggest post-core collapse. The dynamical evolution of NGC3960 may have been accelerated by the tidal Galactic field, since it lies \~0.5kpc inside the Solar circle.

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arXiv2006

AL 3 (BH 261): a new globular cluster in the Galaxy

AL~3 (BH 261), previously classified as a faint open cluster candidate, is shown to be a new globular cluster in the Milky Way, by means of B, V and I Color-Magnitude Diagrams. The main feature of AL~3 is a prominent blue extended Horizontal Branch. Its Color-Magnitude Diagrams match those of the intermediate metallicity cluster M~5. The cluster is projected in a rich bulge field, also contaminated by the disk main sequence. The globular cluster is located in the Galactic bulge at a distance from the Sun d$_{\odot}$ = 6.0$\pm$0.5 kpc. The reddening is E(B-V)=0.36$\pm$0.03 and the metallicity is estimated to be [Fe/H] $\approx$ -1.3$\pm$0.25. AL~3 is probably one of the least massive globular clusters of the Galaxy.

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arXiv2006

Detection of Ks-excess stars in the 14Myr open cluster NGC4755

We derive the structure, distribution of MS and PMS stars and dynamical state of the young open cluster NGC 4755. We explore the possibility that, at the cluster age, some MS and PMS stars still present infrared excesses related to dust envelopes and proto-planetary discs. The radial density profile follows King's law with a core radius $\rm\rc=0.7\pm0.1 pc$ and a limiting radius $\rm\rl=6.9\pm0.1 pc$; the cluster age is $\rm14\pm2 Myr$. Field-star decontamination reveals a low-MS limit at $\rm\approx1.4 \ms$. The core MF ($χ=0.94\pm0.16$) is flatter than the halo's ($χ=1.58\pm0.11$). NGC 4755 contains $\rm\sim285$ candidate PMS stars of age $\rm\sim1 - 15 Myr$, and a few evolved stars. The mass locked up in PMS, MS and evolved stars amounts to $\rm\sim1150 \ms$. Proper motions show that \ks-excess MS and PMS stars are cluster members. \ks-excess fractions in PMS and MS stars are $\rm5.4\pm2.1%$ and $\rm3.9\pm1.5%$ respectively, consistent with the cluster age. The core is deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halo but they are scarce in the core. Compared to open clusters in different dynamical states studied with similar methods, NGC 4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass. On the other hand, the flatter core MF probably originates from primordial processes related to parent molecular cloud fragmentation and mass segregation over $\rm\sim14 Myr$. Star formation in NGC 4755 began $\rm\approx14 Myr$ ago and proceeded for about the same length of time. Detection of \ks-excess emission in member MS stars suggests that some circumstellar dust discs survived for $\rm\sim10^7 yr$, occurring both in some MS and PMS stars for the age and spread observed in NGC 4755.

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arXiv2005

Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr

Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Cluster reddening values were estimated by interpolation, using the available extinction maps. We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta). The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr (NGC1932 and SL709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.

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arXiv2005

SOAR BVI Photometry of the Metal-Poor Bulge Globular Cluster NGC 6642

We present BVI photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Telescope. The Colour Magnitude Diagrams (CMD) reach $\approx$1.5mag in V below the main sequence turn-off. A comparison of the overall sequences, and in particular the Red Giant Branch slope of NGC 6642 with that of M~5 indicates that the two clusters must have a similar metallicity of [Fe/H]$\approx$-1.3. We also obtain for NGC 6642 a reddening E(B-V)=0.42$\pm$0.03, and a distance from the Sun of d$_{\odot}$=7.2$\pm$0.5 kpc. Therefore NGC 6642 is a moderately metal-poor globular cluster, spatially located in the bulge, at a galactocentric distance of R$_{\rm GC}$$\approx$1.7 kpc. The comparison of CMDs of NGC 6642 with those of M~5 shows that there is a very good match of magnitude difference between turn-off and horizontal branch, suggesting comparable ages. M~5 has an age typical of the halo globulars, and consequently NGC 6642 is coeval with the halo. NGC 6642 is a good candidate to be one of the few genuine metal-poor and old {\it bulge} clusters, and might be one of the most ancient fossils in the Galaxy.

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arXiv2005

Probing disk properties with open clusters

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. The sample totals 654 OCs, consisting basically of Trumpler types I to III clusters. Because of the completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile. 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. 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 $1.5 - 1.9\,kpc$. In particular we retrieve the expected exponential-disk radial profile for the highly depleted regions internal to the Solar circle. Extrapolation of the completeness-corrected radial distributions down to the Galactic center indicates a total number of OCs in the range $\rm(1.8 - 3.7)\times10^5$. 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 characterized by age scales of $\rm\sim100\,Myr$ and $\rm\sim1.9\,Gyr$, respectively. This rules out evolutionary scenarios based on constant star-formation and OC-disruption rates. We estimate that 3.4--8\% of the embedded clusters do actually emerge from the parent molecular clouds as OCs.

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arXiv2005

Integrated spectral analysis of 18 concentrated star clusters in the Small Magellanic Cloud

We present in this study flux-calibrated integrated spectra in the range 3600-6800A for 18 concentrated SMC clusters. Cluster reddening values were estimated by interpolation between the extinction maps of Burstein & Heiles (1982, AJ, 87, 1165) and Schlegel et al. (1998, ApJ, 500, 525). The cluster parameters were derived from the template matching procedure by comparing the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known parameters and from the equivalent width (EW) method. In this case, new calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines. A very good agreement between ages derived from both methods was found. The final cluster ages obtained from the weighted average of values taken from the literature and the present measured ones range from 15 Mr (e.g. L51) to 7 Gyr (K3). Metal abundances have been derived for only 5 clusters from the present sample, while metallicity values directly averaged from published values for other 4 clusters have been adopted. Combining the present cluster sample with 19 additional SMC clusters whose ages and metal abundances were put onto a homogeneous scale, we analyse the age and metallicity distributions in order to explore the SMC star formation history and its spatial extent. By considering the distances of the clusters from the SMC centre instead of their projections onto the right ascension and declination axes, the present age-position relation suggests that the SMC inner disk could have been related to a cluster formation episode which reached the peak ~2.5 Gyr ago. Evidence for an age gradient in the inner SMC disk is also presented.

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arXiv2005

Structure and stellar content analysis of the open cluster M11 with 2MASS photometry

An overall analysis of the structure and stellar content of M11 is presented, thanks to the wide-angle 2MASS spatial coverage. We derive photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions. Photometric parameters basically agree with previous ones mostly based on the optical. We obtained a core radius of 1.23pc and a tidal radius of 29pc. In particular, the cluster is populous enough so that the tidal radius could be obtained by fitting the three-parameter King profile to the radial distribution of stars. We analyzed the spatial distribution of mass functions, finding that the the slope changes from -0.73 in the core to +2.88 in the outer halo. The spatial distribution of mass function slopes derived from 2MASS agrees with that derived from optical CCD data, which further confirms the reliability of 2MASS data for future analyses of this kind at comparable observational limits. We detect mass segregation up to distances from the center of ~20arcmin. We emphasize that the mass function slope in the core is flatter than anywhere else as a consequence of mass segregation. The derived total cluster mass is ~11000 solar masses.

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arXiv2005

Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188

Luminosity and mass functions in the old open cluster NGC 188 are analysed by means of J and H 2MASS photometry. Within the uncertainties, the observed projected radial density profile of NGC 188 departs from the two-parameter King model in two inner regions, which reflects the non-virialized dynamical state and possibly, some degree of non-sphericity in the spatial shape of this old open cluster. Fits with two and three-parameter King models to the radial distribution of stars resulted in a core radius of 1.3 pc and a tidal radius of 21 pc. The present 2MASS analysis resulted in significant slope variations with distance in the mass function $ϕ(m)\propto m^{-(1+χ)}$, being flat in the central parts ($χ=0.6\pm0.7$) and steep in the cluster outskirts ($χ=7.2\pm0.6$). The overall mass function has a slope $χ=1.9\pm0.7$, slightly steeper than a standard Salpeter mass function. Solar metallicity Padova isochrone fits to the near-infrared colour-magnitude diagram of NGC 188 resulted in an age of $7.0\pm1.0$ Gyr. The best fit, obtained with the 7.1 Gyr isochrone, produced a distance modulus $\mM=11.1\pm0.1$, $\ebv=0.0$, and a distance to the Sun $\ds=1.66\pm0.08$ kpc. The observed stellar mass (in the range $0.98 \ms - 1.08 \ms$) in NGC 188 is $\mobs=380\pm12 \ms$. A simple extrapolation of the observed overall mass function to stars with $0.08 \ms$ resulted in a total present mass of $\mtot\sim(1.8\pm0.7)\times10^4 \ms$. On the other hand, for a more realistic initial mass function which flattens in the low-mass range, the total mass in NGC 188 drops to $\mtot\sim(3.8\pm1.6)\times10^3 \ms$.

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arXiv2005

Proper motion measurements as indicators of binarity in open clusters

We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using UCAC2 proper motion data and 2MASS photometry. For each cluster we consider the projected velocity distributions in the core and off-core regions separately. In the projected velocity distribution of all sample clusters we find a well-defined low-velocity peak, as well as an excess in the number of stars at larger velocities. The low-velocity peak is accounted for by the random motion of the single stars, while the high-velocity excess can be attributed to the large velocity changes produced by a significant fraction of unresolved binaries in a cluster. We derive kinematic parameters of the single-star distribution, in particular the projected velocity dispersion. The relatively large velocity dispersions derived in this work may reflect the non-virialized state of the clusters. Based on the relative number of high-velocity (binary) and single stars, we inferred for the sample clusters unresolved binary fractions in the range $15%-54%$, for both core and off-core regions. The present results suggest that care must be taken when applying proper-motion filters to sort out members, especially binaries in a star cluster. This paper shows that proper motions turn out to be a useful tool for identifying high-velocity stars as unresolved binary cluster members, and as a consequence, map and quantify the binary component in colour-magnitude diagrams.

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arXiv2004

Photometric and spectroscopic study of low mass embedded star clusters in reflection nebulae

An analysis of the candidate embedded stellar systems in the reflection nebulae vdBH-RN26, vdBH-RN38, vdBH-RN53a, GGD20, ESO95-RN18 and NGC6595 is presented. Optical spectroscopic data from CASLEO (Argentina) in conjunction with near infrared photometry from the 2MASS Point Source Catalogue were employed. The analysis is based on source surface density, colour-colour and colour-magnitude diagrams together with theoretical pre-main sequence isochrones. We take into account the field population affecting the analysis by carrying out a statistical subtraction. The fundamental parameters for the stellar systems were derived. The resulting ages are in the range 1-4 Myr and the objects are dominated by pre-main sequence stars. The observed masses locked in the clusters are less than 25 solar masses. The studied systems have no stars of spectral types earlier than B, indicating that star clusters do not necessarily evolve through an HII region phase. The relatively small locked mass combined with the fact that they are not numerous in catalogues suggests that these low mass clusters are not important donors of stars to the field populations.

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arXiv2004

The Metal Content of the Bulge Globular Cluster NGC 6528

High resolution spectra of five stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. Out of the five stars, two of them showed evidence of binarity. The target stars belong to the horizontal and red giant branch stages, at 4000<T_eff<4800 K. Multiband V,I,J,H,Ks photometry was used to derive initial effective temperatures and gravities. The main purpose of this study is the determination of metallicity and elemental ratios for this template bulge cluster, as a basis for the fundamental calibration of metal-rich populations. The present analysis provides a metallicity [Fe/H] = -0.1\pm0.2 and the alpha-elements O, Mg and Si, show [alpha/Fe] ~ +0.1, whereas Ca and Ti are around the solar value or below, resulting in an overall metallicity Z ~ Zsun.

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arXiv2004

Discovery of three optical open clusters in the Galaxy

We report the discovery of three optical open clusters in the Milky Way. Two clusters are in Scutum (Cluster1 at l=18.44 degrees and b=-0.42 degrees, and Cluster2 at l=19.60 degrees and b=-1.02 degrees), thus projected not far from the Galactic center direction, and the other is in Canis Major (Cluster3 at l=235.61 degrees and b=-4.10 degrees), near the anti-center direction. Cluster3 is less populous than Clusters 1 and 2, but presents evidence of being a physical system. The objects were found optically by inspecting maps obtained from the Guide Star Catalogue and images from the Digitized Sky Survey. No previous identification of cluster has been reported in each area so far. The analysis was carried out with 2MASS photometry in J and H. For Cluster1 we derive an age of 25 Myr, a reddening E(B-V)=2.18 and a distance from the Sun 1.64kpc; for Cluster2, age of 500Myr, E(B-V)=0.91 and distance 2.19kpc; finally for Cluster3, age 32-100Myr, E(B-V)=0.94 and distance of 3.93kpc. Luminosity and mass functions are derived for Clusters1 and 2 which, in turn, allowed us to estimate their observed masses as 147 and 89 solar masses, respectively. Estimated total masses, by extrapolating the mass functions to 0.08 solar mass, amount to 382 and 614 solar masses, for the two clusters. Cluster3 has an observed mass of 55 solar masses. The present results indicate that further searches in the optical might still reveal new open clusters, and more so in infrared bands.

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arXiv2003

Gemini-Phoenix infrared high-resolution abundance analysis of five giants in the bulge globular cluster NGC 6553

A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was carried out using high resolution infrared spectra in the H band, obtained at the Gemini-South 8m telescope. JK photometry collected at ESO and VI photometry from the Hubble Space Telescope are used to derive effective temperatures. The present analysis provides a metallicity [Fe/H] = -0.20 +/- 0.10. An overabundance of oxygen of [O/Fe] = +0.20 is found from IR OH lines.

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arXiv2003

NTT infrared imaging of star cluster candidates towards the central parts of the Galaxy

We address the issue whether the central parts of the Galaxy harbour young clusters other than Arches, Quintuplet and the Nuclear Young Cluster. A large sample of centrally projected cluster candidates has been recently identified from the 2MASS J, H and Ks Atlas. We provide a catalogue of higher angular resolution and deeper images for 57 2MASS cluster candidates, obtained with the near-IR camera SOFI at the ESO NTT telescope. We classify 10 objects as star clusters, some of them deeply embedded in gas and/or dust clouds. Three other objects are probably star clusters, although the presence of dust in the field does not exclude the possibility of their being field stars seen through low-absorption regions. Eleven objects are concentrations of stars in areas of little or no gas, and are classified as dissolving cluster candidates. Finally, 31 objects turned out to be the blend of a few bright stars, not resolved as such in the low resolution 2MASS images. By combining the above results with other known objects we provide an updated sample of 42 embedded clusters and candidates projected within 7 degrees. As a first step we study Object 11 of Dutra & Bica (2000) projected at approximately 1 degree from the nucleus. We present H and Ks photometry and study the colour-magnitude diagram and luminosity function. Object 11 appears to be a less massive cluster than Arches or Quintuplet, and it is located at a distance from the Sun d=8 kpc, with a visual absorption Av=15.

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arXiv2003

On the calibration of the COBE/IRAS dust emission reddening maps

In this work we study the spectral properties (3600 - 6800 A) of the nuclear region of early-type galaxies at low (|b|<25, intermediate (including surroundings of the Magellanic Clouds) and high (South Polar Cap) Galactic latitudes. We determine the E(B-V) reddening values of the galaxies by matching their continuum distribution with respect to those of reddening-free spectral galaxy templates with similar stellar populations. We also compare the spectroscopic reddening value of each galaxy with that derived from 100 micron dust emission (E(B-V)_{FIR}) in its line of sight, and we find that there is agreement up to E(B-V)=0.25. Beyond this limit E(B-V)_{FIR} values are higher. Taking into account the data up to E(B-V) \approx 0.7, we derive a calibration factor of 0.016 between the spectroscopic E(B-V) values and Schlegel et al.'s (1998) opacities. By combining this result with an A_K extinction map built within ten degrees of the Galactic centre using Bulge giants as probes (Dutra et al. 2003), we extended the calibration of dust emission reddening maps to low Galactic latitudes down to |b|=4 and E(B-V)= 1.6 (A_V \approx 5). According to this new calibration, a multiplicative factor of approximately 0.75 must be applied to the COBE/IRAS dust emission reddening maps.

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arXiv2003

New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS

We carried out a survey of infrared star clusters and stellar groups on the 2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on the directions of known optical and radio nebulae. The present study provides 167 new infrared clusters, stellar groups and candidates. Combining the two studies for the whole Milky Way, 346 infrared clusters, stellar groups and candidates were discovered, whereas 315 objects were previously known. They constitute an important new sample for future detailed studies.

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arXiv2003

V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528

New near-infrared observations of NGC6528 are presented. The JHK observations complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that they sample a larger area, contain a more numerous sample of red giant stars, and include the K band. Also, archival HST data sets (separated by 6.093 years) were used to proper-motion decontaminate the near-infrared sample and extract a clean VJHK catalogue. Using the present wide colour baseline, we compared the cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104 and derived new estimates of reddening and distance, E(B-V)=0.55 and (m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned red giant branch were used to derive a photometric estimate of the cluster metallicity. The average of 10 metallicity indicators yields a mean value of [M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta & Gratton (1997) revised metallicity scale, respectively. The best isochrone fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02 isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close to the mean of stars in the Baade's Window. Five AGB variable star candidates, whose membership has to be confirmed spectroscopically, are bolometrically as bright as the known long period variable stars in NGC6553. As discussed in Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate age' population is not needed to account for the brightest stars in external galaxies such as M32.

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