arXiv subjects

Eduardo Bica

Publications and source records attributed to Eduardo Bica.

At least 37 records · Page 2Linked to original sources

arXiv2014

VISTA Variables in the Vía Láctea (VVV): Halfway Status and Results

The VISTA Variables in the Vía Láctea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.

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arXiv2013

Towards a census of the Galactic anticentre star clusters - III: tracing the spiral structure in the Outer Disk

In this paper we investigate the nature of 27 star cluster candidates, most of them projected towards the Galactic anticentre. We derive fundamental parameters for 20 confirmed clusters, among these 7 are new identifications. Four of the remaining are uncertain cases that require deeper photometry to establish their nature, and 4 are probably field fluctuations. In addition, we provide a partial census of the open clusters towards the Galactic anticentre. We also include in this study some interesting objects outside the anticentre region, in the second and third Galactic quadrants, mainly in the Perseus and Outer arms. These clusters confirm the extension of the Outer arm along the third quadrant. We also point out that the embedded cluster FSR 486, at a distance of 7.2 +/- 1.3 kpc from de Sun, is projected on the line of sight of the Local Group irregular dwarf galaxy IC 10. Thus, part of the unusual properties of IC 10 may be explained by a Galactic contamination. We point out the importance of embedded clusters in tracing the spiral structure.

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arXiv2012

Washington photometry of candidate star clusters in the Small Magellanic Cloud

We present for the first time Washington CT1 photometry for 11 unstudied or poorly studied candidate star clusters. The selected objects are of small angular size, contain a handful of stars, and are projected towards the innermost regions of the Small Magellanic Cloud (SMC). The respective Colour-Magnitude Diagrams (CMDs) were cleaned from the unavoidable star field contamination by taking advantage of a procedure which makes use of variable size Colour-Magnitude Diagram cells. This method has shown to be able to eliminate stochastic effects in the cluster CMDs caused by the presence of isolated bright stars, as well as, to make a finer cleaning in the most populous CMD regions. Our results suggest that nearly 1/3 of the studied candidate star clusters would appear to be genuine physical systems. In this sense, the ages previously derived for some of them mostly reflect those of the composite stellar populations of the SMC field. Finally, we used the spatial distribution in the SMC of possible non-clusters to statistically decontaminate that of the SMC cluster system. We found that there is no clear difference between both expected and observed cluster spatial distributions, although it would become more important at a 2σ\, level between a ~ 0.3deg and 1.2deg (the semi-major axis of a ellipse parallel to the SMC bar and with b/a = 1/2), if the asterisms were increased up to 20%.

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arXiv2012

Towards a census of the Galactic anticentre star clusters II: exploring lower overdensities

We investigate the nature of 48 low-level stellar overdensities (from Froebrich, Scholz, and Raftery catalogue - FSR07) projected towards the Galactic anticentre and derive fundamental parameters for the confirmed clusters, thus improving the open cluster (OC) census in that direction. Parameters are derived with field-star decontaminated photometry, colour-magnitude filters and stellar radial density profiles. Among the 48 targets, we identified 18 star clusters, 6 previously studied OCs, and 7 probable clusters that require deeper photometry to establish the nature. We discovered 7 new clusters, 6 of them forming an association of clusters with BPI 14, FSR 777, Kronberger 1, and Stock 8 in the region of the nebula IC 417 and related to the Aur OB2 association, and one embedded in the nebula Sh2-229. We also derive parameters for these three non-FSR07 clusters, because they are important in determining the structure of the Galactic anticentre. Thus, 58 objects are analysed in this work and we could derive fundamental parameters for 28 of them. The scenario in the IC 417 star forming region is consistent with a sequential event. FSR 888 and FSR 890 are embedded in Sh2-249 within the Gem OB1 association. According to the distance derived for these clusters and those in the association of clusters, both Aur OB2 and Gem OB1 are located in the Perseus arm.

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arXiv2012

From light to mass: accessing the initial and present-day Galactic globular cluster mass functions

The initial and present-day mass functions (ICMF and PDMF, respectively) of the Galactic globular clusters (GCs) are constructed based on their observed luminosities, the stellar evolution and dynamical mass-loss processes, and the mass-to-light ratio (MLR). Under these conditions, a Schechter-like ICMF is evolved for approximately a Hubble time and converted into the luminosity function (LF), which requires finding the values of 5 free parameters: the mean GC age (\tA), the dissolution timescale of a $10^5 \ms$ cluster ($τ_5$), the exponential truncation mass (\mc) and 2 MLR parametrising constants. This is achieved by minimising the residuals between the evolved and observed LFs, with the minimum residuals and realistic parameters obtained with MLRs that increase with luminosity (or mass). The optimum PMDFs indicate a total stellar mass of $\sim4\times10^7$ \ms\ still bound to GCs, representing $\sim15%$ of the mass in clusters at the beginning of the gas-free evolution. The corresponding ICMFs resemble the scale-free MFs of young clusters and molecular clouds observed in the local Universe, while the PDMFs follow closely a lognormal distribution with a turnover at $\mto\sim7\times10^4$\,\ms. For most of the GC mass range, we find an MLR lower than usually adopted, which explains the somewhat low \mto. Our results confirm that the MLR increases with cluster mass (or luminosity), and suggest that GCs and young clusters share a common origin in terms of physical processes related to formation.

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arXiv2012

Unveiling hidden properties of young star clusters: differential reddening, star-formation spread and binary fraction

Usually, important parameters of young, low-mass star clusters are very difficult to obtain by means of photometry, especially when differential reddening and/or binaries occur in large amounts. We present a semi-analytical approach that, applied to the Hess diagram of a young star cluster, is able to retrieve the values of mass, age, star-formation spread, distance modulus, foreground and differential reddening, and binary fraction. The global optimisation method known as adaptive simulated annealing (ASA) is used to minimise the residuals between the observed and simulated Hess diagrams of a star cluster. The simulations are realistic and take the most relevant parameters of young clusters into account. Important features of the simulations are: a normal (Gaussian) differential reddening distribution, a time-decreasing star-formation rate, the unresolved binaries, and the smearing effect produced by photometric uncertainties on Hess diagrams. Free parameters are: cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and binary fraction.

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arXiv2011

Deriving reliable fundamental parameters of PMS-rich star clusters affected by differential reddening

We present an approach that improves the search for reliable astrophysical parameters (e.g. age, mass, and distance) of differentially-reddened, pre-main sequence-rich star clusters. It involves simulating conditions related to the early-cluster phases, in particular the differential and foreground reddenings, and internal age spread. Given the loose constraints imposed by these factors, the derivation of parameters based only on photometry may be uncertain, especially for the poorly-populated clusters. We consider a wide range of cluster {\em (i)} mass and {\em (ii)} age, and different values of {\em (iii)} distance modulus, {\em (iv)} differential and {\em (v)} foreground reddenings. Photometric errors and their relation with magnitude are also taken into account. We also investigate how the presence of unresolved binaries affect the derived parameters. For each set of {\em (i)} - {\em (v)} we build the corresponding model Hess diagram, and compute the root mean squared residual with respect to the observed Hess diagram. The parameters that produce the minimum residuals between model and observed Hess diagrams are searched by exploring the full parameter space of {\em (i)} - {\em (v)} by means of {\em brute force}, which may be time consuming but efficient. Control tests show that an adequate convergence is achieved allowing for solutions with residuals 10% higher than the absolute minimum. Compared to a colour-magnitude diagram containing only single stars, the presence of 100% of unresolved binaries has little effect on cluster age, foreground and differential reddenings; significant differences show up in the cluster mass and distance from the Sun. Our approach shows to be successful in minimising the subjectiveness when deriving fundamental parameters of young star clusters.

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arXiv2011

New Galactic Star Clusters Discovered in the VVV Survey

VISTA Variables in the V\'ıa Láctea (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. 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. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated $K_{\rm S}$-band tile images for stellar overdensities. Subsequently, we examined the composite $JHK_{\rm S}$ and $ZJK_{\rm S}$ color images of each candidate. PSF photometry of $15\times15$ 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. 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 5\,Myr. 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.

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arXiv2011

A Fossil Bulge Globular Cluster revealed by VLT Multi-Conjugate Adaptive Optics

The globular cluster HP1 is projected on the bulge, very close to the Galactic center. The Multi-Conjugate Adaptive Optics (MCAO) Demonstrator (MAD) at the Very Large Telescope (VLT) allowed to acquire high resolution deep images that, combined with first epoch New Technology Telescope (NTT) data, enabled to derive accurate proper motions. The cluster and bulge field stellar contents were disentangled by means of this process, and produced unprecedented definition in the color-magnitude diagrams for this cluster. The metallicity of [Fe/H] ~ -1.0 from previous spectroscopic analysis is confirmed, which together with an extended blue horizontal branch, imply an age older than the halo average. Orbit reconstruction results suggest that HP1 is spatially confined within the bulge.

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arXiv2011

Constraining the star formation rate in the Solar neighbourhood with star clusters

This paper investigates the star formation rate (SFR) in the Solar neighbourhood. First, we build the local age distribution function (ADF) with an updated sample of 442 star clusters located at less than 1\,kpc from the Sun. Next, we define the SFR, compute the individual mass evolution of a population of artificial clusters covering the broad range of parameters observed in actual clusters, and assume 100\,\ms\ as the low-mass limit for effective cluster observation. This leads to a simulated ADF, which is compared to the low-noise Solar neighbourhood ADF. The best match corresponds to a non-constant SFR presenting two conspicuous excesses for ages $\le9$\,Myr and between 220-600\,Myr (the local starburst). The average formation rate is $\bar{SFR}\approx(2500\pm500)\,\mmy$, corresponding to the average surface formation rate $\bar{\ssfr}\approx(790\pm160)\,\mmk$. These values are consistent with the formation rate inferred from embedded clusters (ECs), but much lower ($\la16%$) than that implied by field stars. Both the local starburst and the recent star formation period require $SFR\sim2\times\bar{SFR}$ to be described. The simulations show that $91.2\pm2.7%$ of the clusters created in the Solar neighbourhood do not survive the first 10\,Myr, which is consistent with the rate of EC dissolution.

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arXiv2011

Star clusters or asterisms? 2MASS CMD and structural analyses of 15 challenging targets

(Abridged) Poorly-populated star clusters may have photometric and structural properties not much different from asterisms, to the point that, in some cases, widely-used databases present conflicting classifications. We investigate the nature of a sample of challenging targets that have been classified either as star clusters or asterisms in different studies. A few objects are studied for the first time.

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arXiv2011

Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\,II region NGC\,2174 (Sh2-252)

We investigate 5 embedded clusters (ECs) and the extended stellar group itself of the prominent H\,II region NGC\,2174 (Sh2-252), which presents scarce and heterogeneous information, coming from the optical and infrared. Considering the discrepant values of distance and age, the clusters and the H\,II region appear to be physically unrelated. The analysis is based on field-star decontaminated 2MASS photometry, which allows sampling the pre-main sequence (PMS). We find that Sh2-252A, C, E, NGC\,2175s, and Teu\,136 are small ECs (radius within $1.0 - 2.3$\,pc) characterised by a similar age ($\sim5$\,Myr), reddening ($\aV\sim1$), distance from the Sun ($\ds\sim1.4$\,kpc), and low mass ($60-200\,\ms$). This age is consistent with the H\,II region, the presence of O and B stars still in the MS, and the dominance ($\ga95%$ in number) of PMS stars in colour-magnitude diagrams (CMDs). NGC\,2175 is not a star cluster, but an extended stellar group that encompasses the ECs Sh2-252\,A and C. It contains $\sim36%$ of the member stars (essentially PMS) in the area, with the remaining belonging to the 2 ECs. CMDs of the overall star-forming region and the ECs provide $\ds=1.4\pm0.4$\,kpc for the NGC\,2174 complex, consistent with the value estimated for the physically-related association Gem\,OB1. Our uniform approach shows that NGC\,2174 and its related ECs (except, perhaps, for Teu\,136) are part of a single star-forming complex. CMD similarities among the ECs and the overall region suggest a coeval (to within $\pm5$\,Myr) star-forming event extending for several Myr. At least 4 ECs originated in the event, together with the off-cluster star formation that probably gave rise to the scattered stars of NGC\,2175.

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arXiv2011

From proper motions to star cluster dynamics: measuring velocity dispersion in deconvolved distribution functions

We investigate the effect that the usually large errors associated with ground-based proper motion (PM) components have on the determination of a star cluster's velocity dispersion (\sv). Rather than histograms, we work with PM distribution functions (PMDFs), taking the $1σ$ uncertainties formally into account. In this context, a cluster's intrinsic PMDF is broadened by the error distribution function (eDF) that, given the average error amplitude, has a width usually comparable to the cluster PMDF. Thus, we apply a Richardson-Lucy (RL) deconvolution to the PMDFs of a set of relatively nearby and populous open clusters (OCs), using the eDFs as point spread functions (PSFs). The OCs are NGC\,1039 (M\,34), NGC\,2477, NGC\,2516, NGC\,2682 (M\,67), and NGC\,7762. The deconvolved PMDFs are approximately Gaussian in shape, with dispersions lower than the observed ones by a factor of 4-10. NGC\,1039 and NGC\,2516, the nearest OCs of the sample, have deconvolved \sv\ compatible with those of bound OCs of mass $\sim10^3$\,\ms. NGC\,2477 and NGC\,2682 have deconvolved PMDFs with a secondary bump, shifted towards higher average velocities, which may be an artefact of the RL deconvolution when applied to asymmetric profiles. Alternatively, it may originate from cluster merger, large-scale mass segregation or, least probably, binaries.

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arXiv2010

Towards a census of the Galactic anticentre star clusters: colour-magnitude diagram and structural analyses of a sample of 50 objects

In this work we investigate the nature of 50 overdensities from the catalogue of Froebrich, Scholz, and Raftery (FSR) projected towards the Galactic anticentre, in the sector 160° \leq \ell \leq 200°. The sample contains candidates with |b| \leq 20° classified by FSR as probable open cluster (OC) and labelled with quality flags 2 and 3. Our main purpose is to determine the nature of these OC candidates and the fraction of these objects that are unknown OCs, as well as to derive astrophysical parameters (age, reddening, distance, core and cluster radii) for the clusters and to investigate the relationship among parameters. The analysis is based on 2MASS J, (J-H), and (J-Ks) colour-magnitude diagrams (CMDs), and stellar radial density profiles (RDPs) built with decontamination tools. The tools are a field star decontamination algorithm, used to uncover the cluster's intrinsic CMD morphology, and colour-magnitude filters to isolate stars with a high probability of being cluster members. Out of the 50 objects, 16 (32%) are star clusters. We show that 9 (18%) overdensities are new OCs (FSR 735, FSR 807, FSR 812, FSR 826, FSR 852, FSR 904, FSR 941, FSR 953, and FSR 955) and 7 (14%) are previously studied or catalogued OCs (KKC1, FSR 795, Cz 22, FSR 828, FSR 856, Cz 24, and NGC 2234). These are OCs with ages in the range 5 Myr to 1 Gyr, at distances from the Sun 1.28 \precnapprox d_Sun(kpc) \precnapprox 5.78 and Galactocentric distances 8.5 R_GC(kpc) \precnapprox 12.9. We also derive parameters for the previously analysed OCs Cz 22 and NGC 2234. Five (10%) candidates are classified as uncertain cases, and the remaining objects are probable field fluctuations.

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arXiv2010

Characterisation of 15 overlooked Ruprecht clusters with ages within 400Myr and 3Gyr

We derive fundamental, structural, and photometric parameters of 15 overlooked Ruprecht (hereafter Ru) star clusters by means of 2MASS photometry and field-star decontamination. Ru\,1, 10, 23, 26, 27, 34, 35, 37, 41, 54, 60, 63, 66, and 152 are located in the third Galactic quadrant, while Ru\,174 is in the first. With the constraints imposed by the field-decontaminated colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs), we derive ages in the range 400\,Myr --- 1\,Gyr, except for the older Ru\,37, with $\sim3$\,Gyr. Distances from the Sun are within $\rm1.5\la\ds(kpc)\la8.0$. The RDPs are well-defined and can be described by a King-like profile for most of the radial range, except for Ru\,23, 27, 41, 63, and 174, which present a conspicuous stellar density excess in the central region. The clusters dwell between (or close to) the Perseus and Sagittarius-Carina arms. We derive evidence in favour of cluster size increasing with distance to the Galactic plane ($\zgc$), which is consistent with a low frequency of tidal stress associated with high-$|\zgc|$ regions. The clusters are rather faint even in the near-infrared, with apparent integrated \jj\ magnitudes within $6.4\la m_J\la9.8$, while their absolute magnitudes are $-6.6\la M_J\la-2.6$. Extrapolation of the relation between $M_V$ and $M_J$, derived for globular clusters, suggests that they are low-luminosity optical clusters, with $-5\la M_V\la-1$.

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arXiv2010

The fate of the pre-main sequence-rich clusters Collinder197 and vdB92: dissolution?

We investigate the nature and possible evolution of the young Galactic star clusters Collinder 197 (Cr 197) and vdB 92. The colour-magnitude diagrams (CMDs) are basically characterised by a poorly-populated MS and a dominant fraction ($\ga75%$) of PMS stars, and the combined MS and PMS CMD morphology in both clusters consistently constrains the age to within $5\pm4$ Myr, with a $\sim10$ Myr spread in the star formation process. The MS$ + $PMS stellar masses are $\approx660^{+102}_{-59} \ms$ (Cr 197) and $\approx750^{+101}_{-51} \ms$ (vdB 92). Cr 197 and vdB 92 appear to be abnormally large, when compared to clusters within the same age range. They have irregular stellar radial density distributions (RDPs) with a marked excess in the innermost region, a feature that, at less than 10 Myr, is more likely related to the star formation and/or molecular cloud fragmentation than to age-dependent dynamical effects. The velocity dispersion of both clusters, derived from proper motions, is in the range $\sim15 - 22 \kms$. Both clusters appear to be in a super-virial state, with velocity dispersions higher than those expected of nearly-virialised clusters of similar mass and size. A possible interpretation is that Cr 197 and vdB 92 deviate critically from dynamical equilibrium, and may dissolve into the field. We also conclude that early cluster dissolution leaves detectable imprints on RDPs of clusters as massive as several $10^2 \ms$. Cr 197 and vdB 92 may be the link between embedded clusters and young stellar associations.

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arXiv2009

Hierarchical structures in the Large and Small Magellanic Clouds

We investigate the degree of spatial correlation among extended structures in the LMC and SMC. To this purpose we work with sub-samples characterised by different properties such as age and size, taken from the updated catalogue of Bica et al. or gathered in the present work. The structures are classified as star clusters or non-clusters (basically, nebular complexes and their stellar associations). The radius distribution functions follow power-laws ($dN/dR\propto R^{-α}$) with slopes and maximum radius ($R_{max}$) that depend on object class (and age). Non-clusters are characterised by $α\approx1.9$ and $R_{max}\la472$ pc, while young clusters (age $\la10$ Myr) have $α\approx3.6$ and $R_{max}\la15$ pc, and old ones (age $\ga600$ Myr) have $α\approx2.5$ and $R_{max}\la40$ pc. Young clusters present a high degree of spatial self-correlation and, especially, correlate with star-forming structures, which does not occur with the old ones. This is consistent with the old clusters having been heavily mixed up, since their ages correspond to several LMC and SMC crossing times. On the other hand, with ages corresponding to fractions of the respective crossing times, the young clusters still trace most of their birthplace structural pattern. Also, small clusters ($R<10$ pc), as well as small non-clusters ($R<100$ pc), are spatially self-correlated, while their large counterparts of both classes are not. The above results are consistent with a hierarchical star-formation scenario for the LMC and SMC.

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arXiv2009

Open cluster survival within the solar circle: Teutsch145 and Teutsch146

Teutsch145 and Teutsch146 are shown to be open clusters (OCs) orbiting well inside the Solar circle, a region where several dynamical processes combine to disrupt most OCs on a time-scale of a few 10^8yrs. BVI photometry from the GALILEO telescope is used to investigate the nature and derive the fundamental and structural parameters of the optically faint and poorly-known OCs Teutsch145 and 146. These parameters are computed by means of field-star decontaminated colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs). Cluster mass estimates are made based on the intrinsic mass functions (MFs). We derive the ages 200+100-50Myr and 400+/-100Myr, and the distances from the Sun 2.7+/-0.3kpc and 3.8+/-0.2kpc, respectively for Teutsch145 and 146. Their integrated apparent and absolute magnitudes are m_V ~ 12.4, m_V ~ 13.3, M_V ~- 5.6 and M_V ~- 5.3. The MFs (detected for stars with m>1Msun) have slopes similar to Salpeter's IMF. Extrapolated to the H-burning limit, the MFs would produce total stellar masses of ~1400Msun, typical of relatively massive OCs. Both OCs are located deep into the inner Galaxy and close to the Crux-Scutum arm. Since cluster-disruption processes are important, their primordial masses must have been higher than the present-day values. The conspicuous stellar density excess observed in the innermost bin of both RDPs might reflect the dynamical effects induced by a few 10^8yrs of external tidal stress.

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