Showing papers on "Mass segregation published in 2007"

The Stellar Populations of Praesepe and Coma Berenices

Abstract: We present the results of a stellar membership survey of the nearby open clusters Praesepe and Coma Berenices. We have combined archival survey data from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions and photometry for ~5 million sources over 300 deg^2. Of these sources, 1010 stars in Praesepe and 98 stars in Coma Ber are identified as candidate members with probability >80%; 442 and 61 are identified as high-probability candidates for the first time. We estimate that this survey is >90% complete across a wide range of spectral types (F0-M5 in Praesepe, F5-M6 in Coma Ber). We have also investigated the stellar mass dependence of each cluster's mass and radius in order to quantify the role of mass segregation and tidal stripping in shaping the present-day mass function and spatial distribution of stars. Praesepe shows clear evidence of mass segregation across the full stellar mass range; Coma Ber does not show any clear trend, but low number statistics would mask a trend of the same magnitude as in Praesepe. The mass function for Praesepe (τ ~ 600 Myr; M ~ 500 M_⊙) follows a power law consistent with that of the field present-day mass function, suggesting that any mass-dependent tidal stripping could have removed only the lowest mass members (<0.15 M_⊙). Coma Ber, which is younger but much less massive (τ ~ 400 Myr; M ~ 100 M_⊙), follows a significantly shallower power law. This suggests that some tidal stripping has occurred, but the low-mass stellar population has not been strongly depleted down to the survey completeness limit (~0.12 M_⊙).

The structure of the nuclear stellar cluster of the Milky Way

Abstract: Aims. The centre of the Milky Way is the nearest nucleus of a galaxy and offers a unique possibility to study the structure and dynamics of a dense stellar cluster around a super-massive black hole. Methods. We present high-resolution seeing limited and AO NIR imaging observations of the stellar cluster within about one parsec of Sgr A*, the massive black hole at the centre of the Milky Way. Stellar number counts and the diffuse background light density were extracted from these observations in order to examine the structure of the nuclear stellar cluster. A detailed map of the variation of interstellar extinction in the central ∼0.5 pc of the Milky Way is presented and used to correct the stellar number counts and diffuse light density. Results. Our findings are as follows: (a) a broken-power law provides an excellent fit to the overall structure of the GC nuclear cluster. The power-law slope of the cusp is Γ = 0.19 ±0.05, the break radius is R break = 6.0" ± 1.0" or 0.22 + 0.04 pc, and the cluster density decreases with a power-law index of Γ = 0.75 ± 0.1 outside of R break . (b) Using the best velocity dispersion measurements from the literature, we derive higher mass estimates for the central parsec than assumed until now. The inferred density of the cluster at the break radius is 2.8 ± 1.3 x 10 6 M ⊙ pc -3 . This high density agrees well with the small extent and flat slope of the cusp. Possibly, the mass of the stars makes up only about 50% of the total cluster mass. (c) Possible indications of mass segregation in the cusp are found (d) The cluster appears not entirely homogeneous. Several density clumps are detected that are concentrated at projected distances of R = 3" and R = 7" from Sgr A*. (e) There appears to exist an under-density of horizontal branch/red clump stars near R = 5", or an over-density of stars of similar brightness at R = 3" and R = 7". (f) The extinction map in combination with cometary-like features in an L'-band image may provide support for the assumption of an outflow from Sgr A*.

The structure of the nuclear stellar cluster of the Milky Way

Abstract: We present high-resolution seeing limited and AO NIR imaging observations of the stellar cluster within about one parsec of Sgr A*, the massive black hole at the centre of the Milky Way. Stellar number counts and the diffuse background light density were extracted from these observations in order to examine the structure of the nuclear stellar cluster.Our findings are as follows: (a) A broken-power law provides an excellent fit to the overall structure of the GC nuclear cluster. The power-law slope of the cusp is $\Gamma=0.19\pm0.05$, the break radius is $R_{\rm break} = 6.0'' \pm 1.0''$ or $0.22\pm0.04$ pc, and the cluster density decreases with a power-law index of $\Gamma=0.75\pm0.1$ outside of $R_{\rm break}$. (b) Using the best velocity dispersion measurements from the literature, we derive higher mass estimates for the central parsec than assumed until now. The inferred density of the cluster at the break radius is $2.8\pm1.3\times 10^{6} {\rm M_{\odot} pc^{-3}}$. This high density agrees well with the small extent and flat slope of the cusp. Possibly, the mass of the stars makes up only about 50% of the total cluster mass. (c) Possible indications of mass segregation in the cusp are found (d) The cluster appears not entirely homogeneous. Several density clumps are detected that are concentrated at projected distances of $R=3''$ and $R=7''$ from Sgr A*.(e) There appears to exist an under-density of horizontal branch/red clump stars near $R=5''$, or an over-density of stars of similar brightness at $R=3''$ and $R=7''$. (f) The extinction map in combination with cometary-like features in an L'-band image may provide support for the assumption of an outflow from Sgr A*.

A Dynamical Origin for Early Mass Segregation in Young Star Clusters

Abstract: Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. In this Letter we demonstrate that mergers of small clumps that are initially mass segregated, or in which mass segregation can be produced by two-body relaxation before they merge, generically lead to larger systems that inherit the progenitor clumps' segregation. We conclude that clusters formed in this way are naturally mass segregated, accounting for the anomalous observations and suggesting that this process of prompt mass segregation due to initial clumping should be taken into account in models of cluster formation and dynamics.

A Spitzer Census of the IC 348 Nebula

Abstract: Spitzer mid-infrared surveys enable an accurate census of young stellar objects by sampling large spatial scales, revealing very embedded protostars, and detecting low-luminosity objects Taking advantage of these capabilities, we present a Spitzer-based census of the IC 348 nebula and embedded star cluster, covering a 25 pc region and comparable in extent to the Orion Nebula Our Spitzer census supplemented with ground-based spectra has added 42 Class II T Tauri sources to the cluster membership and identified ~20 Class 0/I protostars The population of IC 348 likely exceeds 400 sources after accounting statistically for unidentified diskless members Our Spitzer census of IC 348 reveals a population of Class I protostars that is anticorrelated spatially with the Class II/III T Tauri members, which comprise the centrally condensed cluster around a B star The protostars are instead found mostly at the cluster periphery about ~1 pc from the B star and spread out along a filamentary ridge We further find that the star formation rate in this protostellar ridge is consistent with that rate which built the older exposed cluster, while the presence of 15 cold, starless, millimeter cores intermingled with this protostellar population indicates that the IC 348 nebula has yet to finish forming stars Moreover, we show that the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced by its smooth radial profile and confirmed mass segregation, is likely relaxed While it seems apparent that the current cluster configuration is the result of dynamical evolution and its primordial structure has been erased, our finding of a filamentary ridge of Class I protostars supports a model in which embedded clusters are built up from numerous smaller subclusters Finally, the results of our Spitzer census indicate that the supposition that star formation must progress rapidly in a dark cloud should not preclude these observations that show it can be relatively long lived

The Discovery of Two Extremely Low Luminosity Milky Way Globular Clusters

Abstract: We report the discovery of two extremely low luminosity globular clusters in the Milky Way halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at ~40-50 kpc and appear to have old stellar populations and luminosities of only MV ~ -1 mag. Their observed sizes of ~3 pc are well within the expected tidal limit of ~10 pc at that distance. Together with Palomar 1, AM 4, and Whiting 1, these new clusters are the lowest luminosity globular clusters orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only ~70 and ~55 Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as ~0.1tHubble. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.

Intermediate to low-mass stellar content of Westerlund 1

Abstract: We have analysed near-infrared NTT/SofI observations of the starburst cluster Westerlund 1, which is among the most massive young clusters in the Milky Way. A comparison of colour-magnitude diagrams with theoretical main-sequence and pre-main sequence evolutionary tracks yields improved extinction and distance estimates of A_Ks = 1.13+-0.03 mag and d = 3.55+-0.17 kpc (DM = 12.75+-0.10 mag). The pre-main sequence population is best fit by a Palla & Stahler isochrone for an age of 3.2 Myr, while the main sequence population is in agreement with a cluster age of 3 to 5 Myr. An analysis of the structural parameters of the cluster yields that the half-mass radius of the cluster population increases towards lower mass, indicative of the presence of mass segregation. The cluster is clearly elongated with an eccentricity of 0.20 for stars with masses between 10 and 32 Msun, and 0.15 for stars with masses in the range 3 to 10 Msun. We derive the slope of the stellar mass function for stars with masses between 3.4 and 27 Msun. In an annulus with radii between 0.75 and 1.5 pc from the cluster centre, we obtain a slope of Gamma = -1.3. Closer in, the mass function of Westerlund 1 is shallower with Gamma = -0.6. The extrapolation of the mass function for stars with masses from 0.08 to 120 Msun yields an initial total stellar mass of ~52,000 Msun, and a present-day mass of 20,000 to 45,000 Msun (about 10 times the stellar mass of the Orion Nebula Cluster, and 2 to 4 times the mass of the NGC 3603 young cluster), indicating that Westerlund 1 is the most massive starburst cluster identified to date in the Milky Way.

Past and Present Star Formation in the SMC: NGC 346 and its Neighborhood*

Abstract: In the quest to understand how star formation occurs and propagates in the low-metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACS images of the young and massive star-forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ?10 Gyr in the past until ?150?Myr in the field of the SMC. The youngest stellar population (~3 ? 1?Myr) is associated with the NGC 346 cluster. It includes a rich component of low-mass pre-main-sequence stars mainly concentrated in a number of subclusters spatially colocated with CO clumps previously detected by Rubio and coworkers. Within our analysis uncertainties, these subclusters appear coeval with each other. The most massive stars appear concentrated in the central subclusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate-age star cluster, BS 90, formed ~4.3 ? 0.1?Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

CCD BV survey of 42 open clusters

Abstract: Aims. We present results of a photometric survey whose aim was to derive structural and astrophysical parameters for 42 open clusters. While our sample is definitively not representative of the total open cluster sample in the Galaxy, it does cover a wide range of cluster parameters and is uniform enough to allow for simple statistical considerations. Methods. BV wide-field CCD photometry was obtained for open clusters for which photometric, structural, and dynamical evolution parameters were determined. The limiting and core radii were determined by analyzing radial density profiles. The ages, reddenings, and distances were obtained from the solar metallicity isochrone fitting. The mass function was used to study the dynamical state of the systems, mass segregation effect and to estimate the total mass and number of cluster members. Results. This study reports on the first determination of basic parameters for 11 out of 42 observed open clusters. The angular sizes for the majority of the observed clusters appear to be several times larger than the catalogue data indicate. The core and limiting cluster radii are correlated and the latter parameter is 3.2 times larger on average. The limiting radius increases with the cluster's mass, and both the limiting and core radii decrease in the course of dynamical evolution. For dynamically not advanced clusters, the mass function slope is similar to the universal IMF slope. For more evolved systems, the effect of evaporation of low-mass members is clearly visible. The initial mass segregation is present in all the observed young clusters, whereas the dynamical mass segregation appears in clusters older than about $\log({age})$ = 8. Low-mass stars are deficient in the cores of clusters older than $\log({age})$ = 8.5 and not younger than one relaxation time.

The Core Binary Fractions of Star Clusters from Realistic Simulations

Abstract: We investigate the evolution of binary fractions in star clusters using N-body models of up to 100,000 stars. Primordial binary frequencies in these models range from 5% to 50%. Simulations are performed with the NBODY4 code and include a full mass spectrum of stars, stellar evolution, binary evolution, and the tidal field of the Galaxy. We find that the overall binary fraction of a cluster almost always remains close to the primordial value, except at late times when a cluster is near dissolution. A critical exception occurs in the central regions, where we observe a marked increase in binary fraction with time—a simulation starting with 100,000 stars and 5% binaries reached a core binary frequency as high as 40% at the end of the core-collapse phase (occurring at 16 Gyr with ~20,000 stars remaining). Binaries are destroyed in the core by a variety of processes as a cluster evolves, but the combination of mass segregation and creation of new binaries in exchange interactions produces the observed increase in relative number. We also find that binaries are cycled into and out of cluster cores in a manner that is analogous to convection in stars. For models of 100,000 stars we show that the evolution of the core radius up to the end of the initial phase of core collapse is not affected by the exact value of the primordial binary frequency (for frequencies of 10% or less). We discuss the ramifications of our results for the likely primordial binary content of globular clusters.

Near-IR imaging of Galactic massive clusters : Westerlund 2

Abstract: Context. Most stars in the Galaxy were formed in massive clusters. To understand nature's favorite mode of star formation and the initial stages of the life of most stars one needs to characterize the youngest and resolved massive clusters in the Milky Way. Unfortunately young massive clusters are challenging observational targets as they are rare, hence found at large distances, are still embedded in their parental molecular cloud, and are swamped by relatively bright nebulae. Aims. In this paper we propose to use deep subarcsec resolution NIR data to derive the basic parameters of the unstudied population of massive cluster Westerlund 2. Methods. We present deep JHK s images (∼0.6" seeing) and photometry of Westerlund 2. This is the most complete photometric census of the cluster's population to date. Results. We detect a total of 4701, 5724, and 5397 sources in the J, H, and K s bands respectively. By comparison with main-sequence and pre-main-sequence model tracks, we determine an average visual extinction toward the cluster of 5.8 mag, a likely distance of 2.8 kpc, and an age of 2.0 ± 0.3 Myr for the core of the cluster. Although we have the sensitivity to reach beyond the hydrogen burning limit in the cluster, we are only complete to about 1 M ⊙ due to source confusion. We find no evidence for a top-heavy MF, and the slope of the derived mass function is -1.20 ± 0.16. Based on the extrapolation of a field IMF, we roughly estimate the total mass of the cluster to be about 10 4 M ⊙ . We find compelling evidence for mass segregation in this cluster.

A comprehensive nbody study of mass segregation in star clusters: energy equipartition and escape

Abstract: We address the dynamical evolution of an isolated self-gravitating system with two stellar mass groups. We vary the individual ratio of the heavy to light bodies, μ from 1.25 to 50 and alter also the fraction of the total heavy mass M h from 5 to 40 per cent of the whole cluster mass. Clean-cut properties of the cluster dynamics are examined, like core collapse, the evolution of the central potential, as well as escapers. We present in this work collisional N-body simulations, using the high-order integrator NBODY6++ with up to Ν * = 2 x 10 4 particles improving the statistical significancy of the lower-N * simulations by ensemble averages. Equipartition slows down the gravothermal contraction of the core slightly. Beyond a critical value of μ ≈ 2, no equipartition can be achieved between the different masses; the heavy component decouples and collapses. For the first time, the critical boundary between Spitzer-stable and Spitzer-unstable systems is demonstrated in direct N-body models. We also present the measurements of the Coulomb logarithm and discuss the relative importance of the evaporation and ejection of escapers.

Mass Segregation in Very Young Open Clusters: A Case Study of NGC 2244 and NGC 6530

Abstract: We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (~2-4 Myr old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 yr, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is at least partially due to the way in which star formation has proceeded in these complex star-forming regions ("primordial" mass segregation). Based on arguments related to the clusters' published initial mass functions, in conjunction with our new measurements of their internal velocity dispersions (~35 and 8 km s-1 for NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of the dissolution of NGC 2244 on very short timescales, while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation. However, this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic center.

The origin of the Arches stellar cluster mass function

Abstract: We investigate the time evolution of the mass distribution of pre-stellar cores (PSCs) and their transition to the initial stellar mass function (IMF) in the central parts of a molecular cloud (MC) under the assumption that the coalescence of cores is important. Our aim is to explain the observed shallow IMF in dense stellar clusters such as the Arches cluster. The initial distributions of PSCs at various distances from the MC center are those of gravitationally unstable cores resulting from the gravo-turbulent fragmentation of the MC. As time evolves, there is a competition between the PSCs rates of coalescence and collapse. Whenever the local rate of collapse is larger than the rate of coalescence in a given mass bin, cores are collapsed into stars. With appropriate parameters, we find that the coalescence-collapse model reproduces very well all the observed characteristics of the Arches stellar cluster IMF; Namely, the slopes at high and low mass ends and the peculiar bump observed at � 5 6 M⊙. Our results suggest that today’s IMF of the Arches cluster is very similar to the primordial one and is prior to the dynamical effects of mass segregation becoming important.

Star formation in young star cluster NGC 1893

Abstract: We present a comprehensive multiwavelength study of the star-forming region NGC 1893 to explore the effects of massive stars on low-mass star formation. Using near-infrared colours, slitless spectroscopy and narrow-band Hα photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The V, (V- I) colour-magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 and 5 Myr. The spread in the ages of the YSOs may indicate a non-coeval star formation in the cluster. The slope of the K-band luminosity function for the cluster is estimated to be 0.34 ± 0.07, which agrees well with the average value (∼0.4) reported for young clusters. For the entire observed mass range 0.6 < M/M ⊙ ≤ 17.7 the value of the slope of the initial mass function, 'Γ', comes out to be -1.27 ± 0.08, which is in agreement with the Salpeter value of - 1.35 in the solar neighbourhood. However, the value of 'Γ' for pre-main-sequence phase stars (mass range 0.6 < M/M ⊙ ≤ 2.0) is found to be -0.88 ± 0.09 which is shallower than the value (-1.71 ± 0.20) obtained for main-sequence stars having mass range 2.5 < M/M ⊙ ≤ 17.7 indicating a break in the slope of the mass function at ∼2M ⊙ . Estimated 'Γ' values indicate an effect of mass segregation for main-sequence stars, in the sense that massive stars are preferentially located towards the cluster centre. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore, the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.

N‐body models of rotating globular clusters

Abstract: In this paper we examine the dynamical evolution of rotating globular clusters with direct N-body models. Our initial models are rotating King models, and we obtain results both for equal-mass systems and for systems composed of two mass components. Previous investigations using a Fokker–Planck solver have shown that rotation has a noticeable influence on stellar systems such as globular clusters that evolve by two-body relaxation. In particular, it accelerates their dynamical evolution through the gravogyro instability. We have validated the occurrence of the gravogyro instability with direct N-body models. In the case of systems composed of two mass components, mass segregation takes place, a process that competes with the rotation in the acceleration of the core collapse. The ‘accelerating’ effect of rotation was detected in our isolated two-mass N-body models. Finally, we look at rotating N-body models in a tidal field within the tidal approximation. It turns out that rotation increases the escape rate significantly. A difference between retrograde- and prograde-rotating stellar clusters, with respect to the orbit of the cluster around the Galaxy, occurs. This difference is the result of the presence of a ‘third integral’ and chaotic scattering, respectively.

The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo

Abstract: We performed a deep wide field optical survey of the young (~100-150 Myr) open cluster Blanco1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.The survey covers 2.3 square degrees in the I and z-bands down to I ~ z ~ 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I,I-z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. We find that our survey should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range, including 30-40 brown dwarfs. The candidate's radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison between the Blanco1 mass function, other young open clusters' MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution.

Star formation in young star cluster NGC 1893

Abstract: We present a comprehensive multi-wavelength study of the star-forming region NGC 1893 to explore the effects of massive stars on low-mass star formation. Using near-infrared colours, slitless spectroscopy and narrow-band $H\alpha$ photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The $V, (V-I)$ colour-magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 to 5 Myr. The spread in the ages of the YSOs may indicate a non-coeval star formation in the cluster. The slope of the KLF for the cluster is estimated to be $0.34\pm0.07$, which agrees well with the average value ($\sim 0.4$) reported for young clusters. For the entire observed mass range $0.6 < M/M_\odot \le 17.7$ the value of the slope of the initial mass function, $`\Gamma$', comes out to be $-1.27\pm0.08$, which is in agreement with the Salpeter value of -1.35 in the solar neighborhood. However, the value of $`\Gamma$' for PMS phase stars (mass range $0.6 < M/M_\odot \le 2.0$) is found to be $-0.88\pm0.09$ which is shallower than the value ($-1.71\pm0.20$) obtained for MS stars having mass range $2.5 < M/M_\odot \le 17.7$ indicating a break in the slope of the mass function at $\sim 2 M_\odot$. Estimated $`\Gamma$' values indicate an effect of mass segregation for main-sequence stars, in the sense that massive stars are preferentially located towards the cluster center. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.

The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_☉

Abstract: Aims. We performed a deep wide field optical survey of the young (~100−150 Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range. Methods. The survey covers 2.3 square degrees in the I and z-bands down to I ≃ z ≃ 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I, I − z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. Results. We find that our survey should contain about 57% of the cluster members in the 0.03−0.6 M_☉ mass range, including 30–40 brown dwarfs. The candidate’s radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03 M_☉ and 0.6 M_☉, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index α = 0.69 ± 0.15. Over the whole mass domain, from 0.03 M_☉ to 3 M_☉, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_☉ and σ = 0.58 ± 0.06. Conclusions. Comparison between the Blanco 1 mass function, other young open clusters’ MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution.

Old open clusters in the inner Galaxy: FSR 1744, FSR 89 and FSR 31

Abstract: Context. We examine the dynamical survival of intermediate-age/old open clusters in the inner Galaxy. Aims. We aim to establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR 1744, FSR 89 and FSR 31 to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. The current status of the issue dealing with the small number of detected open clusters in the inner Galaxy is discussed. Methods. Properties of the objects are investigated with 2MASS colour–magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic diagrams of structural parameters are used to separate dynamical from highbackground effects affecting such centrally projected open clusters. Results. FSR 1744, FSR 89 and FSR 31 are Gyr-class open clusters located at Galactocentric distances 4.0–5.6 kpc. Compared to nearby open clusters, they have small core and limiting radii. Conclusions. With respect to the small number of open clusters observed in the inner Galaxy, the emerging scenario in the nearinfrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are mass loss by stellar evolution, mass segregation and evaporation. Externally they are tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR 1744, FSR 89 and FSR 31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.

Old open clusters in the inner Galaxy: FSR1744, FSR89 and FSR31

Abstract: We establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR1744, FSR89 and FSR31. This work intends to provide clues to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. Properties of the objects are investigated by means of 2MASS colour-magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic-diagrams for structural parameters are used to help disentangle dynamical from high-background effects affecting such centrally projected open clusters. FSR1744, FSR89 and FSR31 are Gyr-class OCs located at Galactocentric distances 4.0 - 5.6kpc. Compared to nearby OCs, they have small core and limiting radii. With respect to the small number of OCs observed in the inner Galaxy, the emerging scenario in the near-infrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are, e.g. mass loss by stellar evolution, mass segregation and evaporation. Externally they are, e.g. tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR1744, FSR89 and FSR31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.

Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries

Abstract: We study the first ∼100 Myr of the evolution of isolated star clusters initially containing 144 179 stars, including 13 107 (10 per cent) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high-precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (≤100 Myr) star clusters in the large Magellanic Cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (∼10 per cent). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as a = - 1.8 in the central part of the cluster (where the initial Salpeter mass function had a = -2.35). About 6-12 per cent of the neutron stars were retained in our simulations; the fraction of retained black holes is 40-70 per cent. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an X-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find ∼11 potential X-ray binaries containing a black hole. Binaries consisting of two white dwarfs are quite common, but few (20-30) are sufficiently close that they will merge within a Hubble time due to the emission of gravitational radiation. Clusters with shorter relaxation times tend to produce fewer merging white dwarf binaries. The white dwarf binaries that do merge are all sufficiently massive to produce a Type Ia supernova. The densest cluster produces about twice as many blue stragglers as a field population containing the same number of binaries, and these blue stragglers are more massive, bluer and brighter than in less dense clusters.

Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K 29 and NGC 290 and their field population

Abstract: Aims. We determine the age, metallicity and initial mass function of three clusters, namely NGC 265, K 29, NGC 290, located in the main body of the Small Magellanic Cloud. In addition, we derive the history of star formation in the companion fields. Methods. We make use of ACS/WFC HST archive data. For the clusters, the age and metallicity are derived fitting the integrated luminosity function with single synthetic stellar population by means of the x 2 minimization. For the companion fields, the history of star formation is derived using the x 2 minimization together with the downhill-simplex method. Results. For the clusters we find the following ages and metallicities: NGC 265 has log(Age) = 8.5 ± 0.3 yr and metallicity 0.004 ± 0.003 (or [Fe/H] = -0.62); K 29 has log(Age) = 8.2 ± 0.2 yr and metallicity Z = 0.003 ± 0.002 (or [Fe/H] = -0.75); NGC 290 has log(Age) = 7.8 ± 0.5 yr and metallicity 0.003 ± 0.002 (or [Fe/H] = -0.75). The superior quality of the data allows the study of the initial mass function down to M ∼ 0.7 M ⊙ . The initial mass function turns out to be in agreement with the standard Kroupa model. The comparison of the NGC 265 luminosity function with the theoretical ones from stellar models both taking overshoot from the convective core into account and neglecting it, seems to suggest that a certain amount of convective overshoot is required. However, this conclusion is not a strong one because this cluster has a certain amount of mass segregation which makes it difficult to choose a suitable area for this comparison. The star formation rate of the field population presents periods of enhancements at 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relatively quiescent at ages older than 6 Gyr. This result suggests that at older ages, the tidal interaction between the Magellanic Clouds and the Milky Way was not able to trigger significant star formation events.

BATC 13 Band Photometry of the Open Cluster NGC 7789

Abstract: We present 13 band CCD intermediate-band spectrophotometry of a field centered on the open cluster NGC 7789 from 400 to nearly 1000 nm, taken with the Beijing-Arizona-Taiwan-Connecticut (BATC) Multi-Color Survey photometric system. By comparing observed spectral energy distributions of NGC 7789 stars with theoretical ones, the fundamental parameters of this cluster are derived: an age of 1.4 ± 0.1 Gyr, a distance modulus (m - M)0 = 11.27 ± 0.04, a reddening E(B - V) = 0.28 ± 0.02, and a metallicity with the solar composition Z = 0.019. When the surface density profile for member stars with limiting magnitudes of 19.0 in the BATC e band (λeff = 4925 A) is fitted by a King model, a core radius Rc = 7.52' and a tidal radius Rt = 28.84' are derived for NGC 7789. The observed mass function (MF) for main-sequence (MS) stars of NGC 7789 with masses from 0.95 to 1.85 M⊙ is fitted with a power-law function (m) ∝ mα, and a slope α = -0.96 is derived. Strong mass segregation in NGC 7789 is reflected in the significant variation of the concentration parameters C0 = log(Rt/Rc) for member stars of NGC 7789 within different mass ranges: C0 = 1.02 for the most massive stars and C0 = 0.37 for the lowest mass MS stars. Strong mass segregation in NGC 7789 is also indicated in the significant variation of the slopes α in different spatial regions of the cluster: the MF for stars within the core region has α = -0.71, much flatter than that for stars in external regions of the cluster (α = -1.20).

Capture-formed Binaries via Encounters with Massive Protostars

Abstract: Most massive stars are found in the center of dense clusters and have a companion fraction much higher than their lower mass siblings; the massive stars of the Trapezium core in Orion have ~1.5 companions each. This high multiplicity could be a consequence of formation via a capture scenario, or it could be due to fragmentation of the cores that form the massive stars. During stellar formation circumstellar disks appear to be nearly ubiquitous. Their large radii compared to stellar sizes increase the interaction radius significantly, suggesting that disk interactions with neighboring stars could assist in capturing binary companions. This mechanism has been studied for stars of approximately solar mass and found to be inefficient. In this paper we present simulations of interactions between a 22 M☉ star-disk system and less massive impactors in order to study the disk-assisted capture formation of binaries in a regime suited to massive stars. The formation of binaries by capture is found to be much more efficient for massive capturers. We discuss the effects of a mass-dependent velocity dispersion and mass segregation on the capture rates and consider the long-term survival of the resulting binaries in a dense cluster.

Unveiling the core of the globular cluster m15 in the ultraviolet

Abstract: We have obtained deep FUV and NUV images of the inner region of the dense globular cluster M15 with the HST ACS. The FUV - NUV color-magnitude diagram shows a well-defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main-sequence turnoff is clearly visible at FUV 23.5 mag and FUV - NUV 3 mag, and the main-sequence stars form a prominent track that extends at least 2 mag below the main-sequence turnoff. As such, this is the deepest FUV - NUV color-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colors and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variable candidates, one known and one probable SX Phoenicis star, and the well-known low-mass X-ray binary AC 211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyrae stars, and SX Phoenicis exhibit massive variability amplitudes in this wave band (several magnitudes).

A detailed study of the enigmatic cluster M82F

Abstract: We present a detailed study of the stellar cluster M82F, using multiband high-resolution Hubble Space Telescope (HST) imaging and deep ground-based optical slit and integral field spectroscopy. Using the imaging, we create colour maps of the cluster and surrounding region in order to search for substructure. We find a large amount of substructure, which we interpret as the result of differential extinction across the projected face of the cluster. With this interpretation, we are able to construct a spatially resolved extinction map across the cluster which is used to derive the intrinsic flux distribution. Fitting cluster profiles (King and Elson, Fall & Freeman) to the intrinsic images, we find that the cluster is 15-30 per cent larger than previous estimates, and that no strong evidence of mass segregation in this cluster exists. Using the optical spectra, we find that the age of M82F is 60-80 Myr and from its velocity conclude that the cluster is not physically associated with a large H II region that it is projected upon, both in agreement with previous studies. The reconstructed integral field maps show that that majority of the line emission comes from a nearby H II region. The spatial dependence of the line widths (implying the presence of multiple components) measured corresponds to the extinction map derived from photometry, indicating that the gas/dust clouds responsible for the extinction are also partially ionized. Even with the wealth of observations presented here, we do not find a conclusive solution to the problem of the high light-to-mass ratio previously found for this cluster and its possible top-heavy stellar initial mass function.

The present day mass function in the central region of the Arches cluster

Abstract: We study the evolution of the mass function in young and dense star clusters by means of direct N-body simulations. Our main aim is to explain the recent observations of the relatively flat mass function observed near the centre of the Arches star cluster. In this region, the power-law index of the mass function for stars more massive than about 5–6 M⊙ is larger than the Salpeter value by about unity, whereas further out, and for the lower mass stars, the mass function resembles the Salpeter distribution. We show that the peculiarities in the Arches mass function can be explained satisfactorily without primordial mass segregation. We draw two conclusions from our simulations: (i) the Arches initial mass function is consistent with a Salpeter slope down to ∼1 M⊙, and (ii) the cluster is about half-way towards core collapse. The cores of other star clusters with characteristics similar to those of the Arches are expected to show similar flattening in the mass functions for the high-mass (≳5 M⊙) stars.

The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90

Abstract: We present the results of our investigation of the intermediate-age star cluster BS 90, located in the vicinity of the H II region N66 in the SMC, observed with HST ACS. The high-resolution data provide a unique opportunity for a very detailed photometric study performed on one of the rare intermediate-age rich SMC clusters. The complete set of observations is centered on the association NGC 346 and contains almost 100,000 stars down to V 28 mag. In this study we focus on the northern part of the region, which covers almost the whole stellar content of BS 90. We construct its stellar surface density profile and derive structural parameters. Isochrone fits on the CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function is constructed and the present-day mass function of BS 90 has been obtained using the mass-luminosity relation, derived from the isochrone models. We found a slope between -1.30 and -0.95, comparable to or somewhat shallower than a typical Salpeter IMF. Examination of the radial dependence of the mass function shows a steeper slope at larger radial distances, indicating mass segregation in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that the cluster is mass segregated due to its dynamical evolution. From the isochrone model fits we derive a metallicity for BS 90 of [Fe/H] = -0.72, which adds an important point to the age-metallicity relation of the SMC. We discuss our findings on this relation in comparison to other SMC clusters.

The Origin of the Initial Mass Function

Abstract: We review recent advances in our understanding of the origin of the initial mass function (IMF). We emphasize the use of numerical simulations to investigate how each physical process involved in star formation affects the resulting IMF. We stress that it is insufficient to just reproduce the IMF, but that any successful model needs to account for the many observed properties of star-forming regions, including clustering, mass segregation, and binarity. Fragmentation involving the interplay of gravity, turbulence, and thermal effects is probably responsible for setting the characteristic stellar mass. Low-mass stars and brown dwarfs can form through the fragmentation of dense filaments and disks, possibly followed by early ejection from these dense environments, which truncates their growth in mass. Higher-mass stars and the Salpeter-like slope of the IMF are most likely formed through continued accretion in a clustered environment. The effects of feedback and magnetic fields on the origin of the IMF are still largely unclear. Finally, we discuss a number of outstanding problems that need to be addressed in order to develop a complete theory for the origin of the IMF.