Luminosity and mass function of galactic open clusters I. NGC 4815

Astronomy and Astrophysics, 2005

We present wide-field deep UBVI photometry for the previously unstudied open cluster NGC 4852 down to a limiting magnitude I ∼ 24, obtained from observations taken with the Wide Field Imager camera on-board the MPG/ESO 2.2 m telescope at La Silla (ESO, Chile). These data are used to obtain the first estimate of the cluster basic parameters, to study the cluster spatial extension by means of star counts, and to derive the Luminosity (LF) and Mass Function (MF). The cluster radius turns out to be 5.0 ± 1.0 arcmin. The cluster emerges clearly from the field down to V = 20 mag. At fainter magnitudes, it is completely confused with the general Galactic disk field. The stars inside this region define a young open cluster (200 million years old) 1.1 kpc far from the Sun (m − M = 11.60, E(B − V) = 0.45). The Present Day Mass Functions (PDMF) from the V photometry is one of the most extended in mass obtained to date, and can be represented as a power-law with a slope α = 2.3 ± 0.3 and (the Salpeter MF in this notation has a slope α = 2.35), in the mass range 3.2 ≤ m m ≤ 0.6. Below this mass, the MF cannot be considered as representative of the cluster MF, as the cluster merges with the field and therefore the MF is the result of the combined effect of strong irregularities in the stellar background and interaction of the cluster with the dense Galactic field. The cluster total mass at the limiting magnitude results to be 2570 ± 210 M .

Monthly Notices of the Royal Astronomical Society, 2013

We present U BV RI photometry of stars in the field of the intermediate-age open cluster NGC 559. By determining the stellar membership probabilities derived through a photometric and kinematic study of the cluster, we identify the 22 most probable cluster members. These are used to obtain robust cluster parameters. The mean proper motion of the cluster is µ x = −3.29 ± 0.35, µ y = −1.24 ± 0.28 mas yr −1. The radial distribution of the stellar surface density gives a cluster radius of 4 ′ .5 ± 0 ′ .2 (3.2±0.2 pc). By fitting solar metallicity stellar isochrones to the colour-colour and colourmagnitude diagrams, we find a uniform cluster reddening of E(B − V) = 0.82 ± 0.02. The cluster has an age of 224 ± 25 Myr and is at a distance of 2.43 ± 0.23 kpc. From the optical and near-infrared two-colour diagrams, we obtain colour excesses in the direction of the cluster E(V − K) = 2.14 ± 0.02, E(J − K) = 0.37 ± 0.01, and E(B − V) = 0.76 ± 0.04. A total-to-selective extinction of R V = 3.5 ± 0.1 is found in the direction of the cluster which is marginally higher than the normal value. We derive the luminosity function and the mass function for the cluster main sequence. The mass function slope is found to be −2.12 ± 0.31. We find evidence of mass segregation in this dynamically relaxed cluster.

Astronomy and Astrophysics, 2004

We present a BVI photometric and astrometric catalogue of the open cluster NGC 3960, down to limiting magnitude V ∼ 22, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope at La Silla. The photometry of all the stars detected in our field of view has been used to estimate a map of the strong differential reddening affecting this area. Our results indicate that, within the region where the cluster dominates, the E(V − I) values range from 0.21 up to 0.78, with E(V − I) = 0.36 (E(B − V ) = 0.29) at the nominal cluster centroid position; color excesses E(V − I) up to 1 mag have been measured in the external regions of the field of view where field stars dominate. The reddening corrected color-magnitude diagram (CMD) allows us to conclude that the cluster has an age between 0.9 and 1.4 Gyr and a distance modulus of (V − MV )0 = 11.35. In order to minimize field star contamination, their number has been statistically subtracted based on the surface density map. The empirical cluster main sequence has been recovered in the V vs. V − I and in the J vs. J − KS planes, using optical and infrared data, respectively. From these empirical cluster main sequences, two samples of candidate cluster members were derived in order to obtain the luminosity distributions as a function of the V and J magnitudes. The Luminosity Functions have been transformed into the corresponding Mass Functions; for M > 1 M⊙, the two distributions have been fitted with a power law of index αV = 2.95 ± 0.53 and αJ = 2.81 ± 0.84 in V and in J, respectively, while the Salpeter Mass Function in this notation has index α = 2.35.

Astronomy and Astrophysics Supplement Series, 1996

We present multiple-epoch radial-velocity observations for 104 stars in a 10 × 10 field of the intermediate-age open cluster IC 4651 to V ∼ 14.5. Only 13 stars (13%) of the full sample are field stars. From the 44 single member stars we find a mean radial velocity of −30.76 ± 0.20 km s −1 , and the 12 single red-giant members yield a true radial-velocity dispersion of 0.74 km s −1. Of the 19 giant members, 7 (37%) are spectroscopic binaries with periods up to 5000 days, while 35 (52%) of the 67 main-sequence and turnoff members are binaries with periods less than ∼1000 days. Combined with our deep, accurate CCD Strömgren photometry in a ∼21 × 21 field of IC 4651 (Meibom 2000), these data substantially improve the definition of the cluster locus in the colour-magnitude diagram and the spatial structure of the cluster, although the photometry shows that IC 4651 contains at least twice as many stars on the upper main sequence as was believed when the radialvelocity survey was initiated. The single cluster members define a very tight sequence in the CMD, and two sets of isochrones from stellar models with convective overshooting (d/Hp = 0.2) have been fit to it. Our best estimate for the age of IC 4651 is 1.7 ± 0.15 Gyr, assuming [Fe/H] = 0.12 (Hyades) and E (b−y) = 0.071. Including the ∼650 stars newly discovered from the photometry, we estimate the present total mass of IC 4651 to be ∼630 M , excluding any undetected stellar remnants. The corresponding tidal cutoff radius is ∼22. IC 4651 shows evidence of moderate mass segregation: Most of the turn-off stars and nearly all the red giants are located at radii smaller than ∼7 , while the lower main-sequence stars are less centrally concentrated. The spatial distributions of cluster and field stars indicate that additional cluster stars are probably still to be found outside the fields studied so far. Comparison of the present mass function of IC 4651 with plausible initial mass functions indicates that the cluster initially contained at least ∼8300 stars with a total mass of ∼5300 M. Thus, of the original cluster stars only ∼7%, containing ∼12% of the initial mass, remain today. Of the initial cluster mass, ∼35% has been lost due to evolution of the most massive stars into white dwarfs or other remnants while the remaining ∼53%, comprising ∼93% of the original low-mass stars, appear to have migrated out of the observed field or been lost from the cluster altogether. IC 4651 is currently 1 kpc closer to the Galactic center than its "sister" cluster NGC 3680 (Nordström et al. 1997), but their Galactic orbital parameters indicate that the mean orbital radius of IC 4651 is in fact larger by 0.7 kpc, providing a plausible reason why it is much less advanced in its dynamical evolution than the coeval cluster NGC 3680.

Monthly Notices of the Royal Astronomical Society, 2013

We present Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) images of the merger remnant NGC 7252. In particular, we focus on the surface brightness profiles and effective radii R eff of 36 young massive clusters (YMCs) within the galaxy. All the clusters have masses exceeding 10 5 M ⊙ and are, despite the 64 Mpc distance to the galaxy, (partly) resolved on the HST images. Effective radii can be measured down to ∼ 2.5 pc, and the largest clusters have R eff approaching 20 pc. The median R eff of our sample clusters is ∼ 6 − 7 pc, which is larger than typical radii of YMCs (∼ 2.5 pc). This could be due to our sample selection (only selecting resolved sources) or to an intrinsic mass-radius relation within the cluster population. We find at least three clusters that have power-law profiles of the Elson, Fall, & Freeman (1987, "EFF") type extending out to 150 pc. Among them are the two most massive clusters, W3 and W30, which have profiles that extend to at least 500 and 250 pc, respectively. Despite their extended profiles, the effective radii of the three clusters are 17.2, 12.6 and 9.1 pc for W3, W26 and W30, respectively. We compare these extended profiles with those of YMCs in the LMC (R136 in 30 Dor), the Antennae galaxies (Knot S) and in the nearby spiral galaxy NGC 6946. Extended profiles seem to be a somewhat common feature, even though many nearby YMCs show distinct truncations. A continuous distribution between these two extremes, i.e. truncated or extremely extended, is the most likely interpretation. We suggest that the presence or absence of an extended envelope in very young clusters may be due to the gas distribution of the proto-cluster giant molecular cloud, in particular if the proto-cluster core becomes distinct from the surrounding gas before star formation begins.

The Astrophysical Journal, 2000

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well defined high degree of completeness. In particular, we consider a subsample of 89 clusters with B j band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalogue. After suitable magnitude corrections and uniform conversions to B j band, we compute cluster luminosities L Bj within several clustercentric distances, 0.5, 1.0, 1.5 h −1 Mpc and within the virialization radius R vir. In particular, we use the luminosity function and background counts estimated by Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue, which is the well-calibrated part of the COSMOS catalogue. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, L Bj , and galaxy velocity dispersion, σ v , with L Bj ∝ σ 2.1-2.3 v. Our estimate of typical value for the mass-to-light ratio is M/L Bj ∼ 250 h M /L. We do not find any correlation of M/L Bj with cluster morphologies, i.e. Rood-Sastry and Bautz-Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M ∝ L 1.2-1.3 Bj. We verify the robustness of this relation against a number of possible systematics. We verify that this increasing trend of M/L with cluster mass cannot be entirely due to a higher spiral fraction in poorer clusters, thus suggesting that a similar result would also be found by using R band galaxy magnitudes.

Chinese Astronomy and Astrophysics, 1998

The luminosity function of a star cluster evolves markedly during the pre-main sequence phase. With an assumed initial mass function and premain sequence tracks, we calculate a set of monochromatic luminosity functions which, when compared with observations, can be used to infer the age and star formation history (coeval versus intermittent) of a star cluster. Applied to the Trapezium cluster, our model suggests an age close to a million years, whereas in IC 348 the age estimate yields 4-6 million years and continual bursts of star formation seem to have occurred in this cluster. CCD imaging observations in the I-band are presented for NGC 663, for which an age of lo-30 million years is inferred. The initial mass function for NGC 663 in the range 2-7.1 M. has a slope of-0.77f 0.20, much shallower than that for the solar neighborhood field stars. We interpret this being due to mass segregation in the cluster. We interpret this as being due to mass segregation in the cluster.

Astronomy & Astrophysics, 2014

Context. The study of the galaxy stellar mass function (SMF) in relation to the galaxy environment and the stellar mass density profile, ρ ⋆ (r),is a powerful tool to constrain models of galaxy evolution. Aims. We determine the SMF of the z=0.44 cluster of galaxies MACS J1206.2-0847 separately for passive and star-forming (SF) galaxies, in different regions of the cluster, from the center out to approximately 2 virial radii. We also determine ρ ⋆ (r) to compare it to the number density and total mass density profiles. Methods. We use the dataset from the CLASH-VLT survey. Stellar masses are obtained by spectral energy distribution fitting with the MAGPHYS technique on 5-band photometric data obtained at the Subaru telescope. We identify 1363 cluster members down to a stellar mass of 10 9.5 M ⊙ , selected on the basis of their spectroscopic (∼ 1/3 of the total) and photometric redshifts. We correct our sample for incompleteness and contamination by non members. Cluster member environments are defined using either the clustercentric radius or the local galaxy number density. Results. The whole cluster SMF is well fitted by a double Schechter function, which is the sum of the two Schechter functions that provide good fits to the SMFs of, separately, the passive and SF cluster populations. The SMF of SF galaxies is significantly steeper than the SMF of passive galaxies at the faint end. The SMF of the SF cluster galaxies does not depend on the environment. The SMF of the passive cluster galaxies has a significantly smaller slope (in absolute value) in the innermost (≤ 0.50 Mpc, i.e., ∼ 0.25 virial radii), and in the highest density cluster region than in more external, lower density regions. The number ratio of giant/subgiant galaxies is maximum in this innermost region and minimum in the adjacent region, but then gently increases again toward the cluster outskirts. This is also reflected in a decreasing radial trend of the average stellar mass per cluster galaxy. On the other hand, the stellar mass fraction, i.e., the ratio of stellar to total cluster mass, does not show any significant radial trend. Conclusions. Our results appear consistent with a scenario in which SF galaxies evolve into passive galaxies due to density-dependent environmental processes, and eventually get destroyed very near the cluster center to become part of a diffuse intracluster medium. Dynamical friction, on the other hand, does not seem to play an important role. Future investigations of other clusters of the CLASH-VLT sample will allow us to confirm our interpretation.

Monthly Notices of the Royal Astronomical Society, 2012

We present BV R c I c CCD photometry in the fields of six Galactic open clusters toward the Perseus spiral arm. These data, complemented with J, H, and K S magnitudes from 2MASS, have been used to determine the ages, distances, and colour excesses E(B − V) for these clusters: 40 Myr, 3180 +440 −380 pc, 0.54 ± 0.03 mag (Berkeley 96); 250 Myr, 2410 +220 −200 pc, 0.77±0.06 mag (Berkeley 97); 70 Myr, 2490 +180 −170 pc, 0.51±0.05 mag (King 12); 160 Myr, 2830 +160 −150 pc, 0.88 ± 0.09 mag (NGC 7261); 280 Myr, 2450 +190 −170 pc, 0.24 ± 0.03 mag (NGC 7296); and 160 Myr, 2750 +220 −210 pc, 0.49 ± 0.02 mag (NGC 7788). We found gaps in the mass function of clusters Be 97, King 12, and NGC 7788 in the mass intervals of [1.3-1.5], [1.4-1.6], and [1.5-1.7] solar masses, respectively.

Monthly Notices of the Royal Astronomical Society, 2006

In this paper we present charge-coupled device (CCD) images in the Johnson B and V and Kron—Cousins I passbands for the previously unstudied open cluster NGC 5288. The sample consists of 15 688 stars reaching down to V∼ 20.5. The cluster appears to have a relatively small but conspicuous nucleus and a low-density extended coronal region. Star counts carried out in 25 × 25 pixel2 boxes distributed throughout the whole observed field allowed us to estimate the angular core and corona radii as∼1.3 and 6.3 arcmin, respectively. Our analysis suggests that NGC 5288 is moderately young and probably more metal-rich than the Sun. Adopting the theoretical metal content Z= 0.040, which provides the best global fit, we derive an age of 130+40−30 Myr. Simultaneously, we have obtained colour excesses E(B-V) = 0.75 and E(V-I) = 0.95 and an apparent distance modulus V-MV= 14.00. The law of interstellar extinction in the cluster direction is found to be normal. NGC 5288 is located at 2.1 ± 0.3 kpc f...