Sustaining Star Formation in the Galactic Star Cluster M 36?

New Astronomy, 2012

We have carried out deep (V∼21 mag) UBVRI photometric study of the star cluster Stock 18. These along with archival Infrared data have been used to derive the basic cluster parameters and also to study the star formation processes in and around the cluster region. The distance to the cluster is derived as 2.8±0.2 kpc while its age is estimated as 6.0 ± 2.0 Myr. Present study indicates that interstellar reddening is normal in the direction of the cluster. The mass function slope is found to be -1.37±0.27 for the mass range 1< M/M ⊙ <11.9. There is no evidence found for the effect of mass segregation in main-sequence stars of the cluster. A young stellar population with age between 1-2 Myr have been found in and around the cluster region. The presence of IRAS and AKARI sources with MSX intensity map also show the youth of the Sh2-170 region.

Astronomy &amp; Astrophysics, 2012

The most massive star clusters include several generations of stars with a different chemical composition (mainly revealed by an Na-O anti-correlation) while low-mass star clusters appear to be chemically homogeneous. We are investigating the chemical composition of several clusters with masses of a few 10 4 M to establish the lower mass limit for the multiple stellar population phenomenon. Using VLT/FLAMES spectra we determine abundances of Fe, O, Na, and several other elements (α, Fe-peak, and neutron-capture elements) in the old open cluster Berkeley 39. This is a massive open cluster: M ∼ 10 4 M , approximately at the border between small globular clusters and large open clusters. Our sample size of about 30 stars is one of the largest studied for abundances in any open cluster to date, and will be useful to determine improved cluster parameters, such as age, distance, and reddening when coupled with precise, well-calibrated photometry. We find that Berkeley 39 is slightly metal-poor, [Fe/H] = -0.20, in agreement with previous studies of this cluster. More importantly, we do not detect any star-to-star variation in the abundances of Fe, O, and Na within quite stringent upper limits. The rms scatter is 0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can be entirely explained by the noise in the spectra and by uncertainties in the atmospheric parameters. We conclude that Berkeley 39 is a single-population cluster.

The Astrophysical Journal, 2013

Gas surface density, star formation rate surface density, and the maximum mass of young star clusters in a disk galaxy. II. The grand-design galaxy M 51 ABSTRACT We analyze the relationship between maximum cluster mass, and surface densities of total gas (Σ gas ), molecular gas (Σ H2 ), neutral gas (Σ HI ) and star formation rate (Σ SFR ) in the grand design galaxy M 51, using published gas data and a catalog of masses, ages, and reddenings of more than 1800 star clusters in its disk, of which 223 are above the cluster mass distribution function completeness limit. By comparing the 2-D distribution of cluster masses and gas surface densities, we find for clusters older than 25 Myr that M 3rd ∝ Σ 0.4±0.2 HI , where M 3rd is the median of the 5 most massive clusters. There is no correlation with Σ gas , Σ H2 , or Σ SFR . For clusters younger than 10 Myr, M 3rd ∝ Σ 0.6±0.1 HI , M 3rd ∝ Σ 0.5±0.2 gas ; there is no correlation with either Σ H2 or Σ SFR . The results could hardly be more different than those found for clusters younger than 25 Myr in M 33. For the flocculent galaxy M 33, there is no correlation between maximum cluster mass and neutral gas, but we have determined M 3rd ∝ Σ 3.8±0.3

Astronomy and Astrophysics, 2002

Based on new, accurate photometry, radial velocities, and proper motions for the intermediate-age open cluster NGC 3680, we identify individual single and binary cluster members and field stars in the colour-magnitude diagram (CMD). This basic step turns out to be crucial for a proper understanding of the cluster CMD: ∼60% of the stars are found to be field stars, and over 50% of the cluster stars are binaries. No bona f ide cluster star is found more than 1. m 5 below the turnoff, and cluster stars below 1.4 M are only found in binary systems. The total present mass of NGC 3680 is ∼100M , excluding any as yet unseen stellar remnants, and its half-mass radius is 3.3 (1.2 pc). Comparison with plausible IMFs indicates that only ∼3% of the original stars and < ∼ 10% of the mass now survive, ∼30% of the initial mass being in the form of massive stars that have now completed their evolution, and ∼60% in low-mass stars which may now be located in a distant cluster halo or perhaps have been lost entirely.

2009

2013

Homogeneous samples of photometric data, coupled with uniform methods of data analysis are essential to make statistical inferences based on the fundamental parameters of clusters. These studies can contribute to understanding the galactic disk, formation and evolution of clusters, molecular cloud fragmentation, star formation and evolution. In this work, we study a sample of young clusters viz. King 16, NGC 1931, NGC 637 and NGC 189 using photometric data from the Two Micron All Sky Survey (2MASS) (Skrutskie et al. 2006). The 2MASS covers 99.99 % of the sky in the near-infrared J (1.25 µm), H (1.65 µm) and Ks (2.16 µm) bands (henceforth Ks shall be refered to as K). Hence the 2MASS database has the advantages of being homogeneous, all sky (enabling the study of the outer regions of clusters where the low mass stars dominate) and covering near infrared wavelengths where young clusters can be well observed in their dusty environments. Dutra and Bica (2001) discovered 42 objects at in...

Astronomy & Astrophysics, 2007

We report the discovery of a new young stellar cluster and molecular cloud located in the far outer Galaxy, seen towards IRAS 06361-0142, and we characterise their properties. Near-infrared images were obtained with VLT/ISAAC through JHKs filters, millimetre line observations of CO(1-0) were obtained with SEST, and VLA 6 cm continuum maps obtained from archive data. The cloud and cluster are located at a distance of 7 kpc and a Galactocentric distance of 15 kpc, well in the far outer Galaxy. Morphologically, IRAS 06361-0142 appears as a cluster of several tens of stars surrounded by a nearly spherical nebular cavity centred at the position of the IRAS source. The cluster appears composed of low and intermediate-mass, young reddened stars with a large fraction having cleared the inner regions of their circumstellar discs responsible for (H - Ks) colour excess. The observations are compatible with a 4 Myr cluster with variable spatial extinction between Av = 6 and Av = 13.

Monthly Notices of the Royal Astronomical Society, 2012

We study the stellar cluster population in two adjacent fields in the nearby, face-on spiral galaxy, M83, using multi-wavelength WFC3/HST imaging. After automatic detection procedures, the clusters are selected through visual inspection to be centrally concentrated, symmetric, and resolved on the images, which allows us to differentiate between clusters and likely unbound associations. We compare our sample with previous studies and show that the differences between the catalogues are largely due to the inclusion of large numbers of diffuse associations within previous catalogues as well as the inclusion of the central starburst region, where the completeness limit is significantly worse than in the surrounding regions. We derive the size distribution of the clusters, which is well described by a log-normal distribution with a peak at ∼ 2.5 pc, and find evidence for an expansion in the half-light radius of clusters with age. The luminosity function of the clusters is well approximated by a power-law with index, −2, over most of the observed range, however a steepening is seen at M V = −9.3 and −8.8 in the inner and outer fields, respectively. Additionally, we show that the cluster population is inconsistent with a pure power-law mass distribution, but instead exhibits a truncation at the high mass end. If described as a Schechter function, the characteristic mass is 1.6 and 0.5 ×10 5 M ⊙ , for the inner and outer fields, respectively, in agreement with previous estimates of other cluster populations in spiral galaxies. Comparing the predictions of the mass independent disruption (MID) and mass dependent disruption (MDD) scenarios with the observed distributions, we find that both models can accurately fit the data. However, for the MID case, the fraction of clusters destroyed (or mass lost) per decade in age is dependent on the environment, hence, the age/mass distributions of clusters are not universal. In the MDD case, the disruption timescale scales with galactocentric distance (being longer in the outer regions of the galaxy) in agreement with analytic and numerical predictions. Finally, we discuss the implications of our results on other extragalactic surveys, focussing on the fraction of stars that form in clusters and the need (or lack thereof) for infant mortality.

Monthly Notices of the Royal Astronomical Society, 2013

We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 10 4 < M/M ⊙ < 10 8 , 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star-formation within the clusters. Such episodes of secondary (or continuous) star-formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star-formation histories within LMC/SMC intermediate age clusters (1-2 Gyr), also imply that many young massive clusters should show ongoing star-formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star-formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star-formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar IMF variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.

The Astrophysical …, 2011

We present new analysis of 11 intermediate-age (1-2 Gyr) star clusters in the Large Magellanic Cloud based on Hubble Space Telescope imaging data. Seven of the clusters feature main sequence turnoff (MSTO) regions that are wider than can be accounted for by a simple stellar population, whereas their red giant branches indicate a single value of [Fe/H]. The star clusters cover a range in present-day mass from about 1 × 10 4 M ⊙ to 2 × 10 5 M ⊙ . We compare radial distributions of stars in the upper and lower parts of the MSTO region, and calculate cluster masses and escape velocities from the present time back to a cluster age of 10 Myr. Our main result is that for all clusters in our sample with estimated escape velocities v esc 15 km s −1 at an age of 10 Myr, the stars in the brightest half of the MSTO region are significantly more centrally concentrated than the stars in the faintest half and more massive red giant branch and asymptotic giant branch stars. This is not the case for clusters with v esc 10 km s −1 at an age of 10 Myr. We argue that the wide MSTO region of such clusters is mainly caused by to a ∼ 200 − 500 Myr range in the ages of cluster stars due to extended star formation within the cluster from material shed by first-generation stars featuring slow stellar winds. Dilution of this enriched material by accretion of ambient interstellar matter is deemed plausible if the spread of [Fe/H] in this ambient gas was very small when the second-generation stars were formed in the cluster.

2003

The ages of 203 open clusters from the list of Dambis (1999) are computed in terms of the evolutionary tracks of Pols et al. (1998) with and without the allowance for convective overshooting. The vertical scaleheight of the cluster layer at Galactocentric distances R_0-1 kpc < R_g< R_0+ 1 kpc varies nonmonotonically with age, exhibiting a wavelike pattern similar to the one earlier found for the Cepheid population (Joeveer 1974), with a period of P_Z = 74 +/- 2 Myr and P_Z = 92 +/- 2 Myr if cluster ages are computed in terms of evolutionary models without and with overshooting, respectively. The period of vertical oscillations can be reconciled with the known local mass density only if cluster ages are computed with no or just mild overshooting: the overshooting-based ages imply a local mass density of rho = 0.075 +/- 0.003 M_Sun/pc^3, which is incompatibe with the recent Hipparcos-based estimate of rho = 0.102 M_Sun/pc^3 (Holmberg and Flynn 2000). Our results imply a maximum local dark-mass density of rho_DM <= 0.027 M_Sun/pc^3. At the time of their formation open clusters have, on the average, excess vertical kinetic energy and as a population are not in virial equilibrium; moreover, their initial vertical coordinates (at the time of birth) are strongly and positively correlated with initial vertical velocities (r = 0.81 +/-0.08), thus favoring a scenario where star formation in the disk is triggered by some massive objects falling onto the Galactic plane.

Astronomy and Astrophysics, 2010

Context. Although current facilities allow the study of Galactic star formation at high angular resolution, our current understanding of the high-mass star-formation process is still very poor. In particular, we still need to characterize the properties of clouds giving birth to high-mass stars in our own Galaxy and use them as templates for our understanding of extragalactic star formation. Aims. We present single-dish (sub)millimeter observations of gas and dust in the Galactic high-mass star-forming region G19.61-0.23, with the aim of studying the large-scale properties and physical conditions of the molecular gas across the region. The final aim is to compare the large-scale (about 100 pc) properties with the small-scale (about 3 pc) properties and to consider possible implications for extragalactic studies. Methods. We have mapped CO isotopologues in the J = 1 − 0 transition using the FCRAO-14m telescope and the J = 2 − 1 transition using the IRAM-30m telescope. We have also used APEX 870 µm continuum data from the ATLASGAL survey and FCRAO supplementary observations of the 13 CO J = 1 − 0 line from the BU-FCRAO Galactic Ring Survey, as well as the Spitzer infrared Galactic plane surveys GLIMPSE and MIPSGAL to characterize the star-formation activity within the molecular clouds. Results. We reveal a population of molecular clumps in the 13 CO(1-0) emission, for which we derived the physical parameters, including sizes and masses. Our analysis of the 13 CO suggests that the virial parameter (ratio of kinetic to gravitational energy) varies over an order of magnitude between clumps that are unbound and some that are apparently "unstable". This conclusion is independent of whether they show evidence of ongoing star formation. We find that the majority of ATLASGAL sources have MIPSGAL counterparts with luminosities in the range 10 4-5 10 4 L ⊙ and are presumably forming relatively massive stars. We compare our results with previous extragalactic studies of the nearby spiral galaxies M31 and M33; and the blue compact dwarf galaxy Henize 2-10. We find that the main giant molecular cloud surrounding G19.61-0.23 has physical properties typical for Galactic GMCs and which are comparable to the GMCs in M31 and M33. However, the GMC studied here shows smaller surface densities and masses than the clouds identified in Henize 2-10 and associated with super star cluster formation.

Monthly Notices of the Royal Astronomical Society, 2008

We present U BV I c CCD photometry of the young open cluster Stock 8 with the aim of studying its basic properties such as the amount of interstellar extinction, distance, age, stellar contents and initial mass function (IMF). We also studied the star formation scenario in this region. From optical data, the radius of the cluster is found to be ∼ 6 ′ (∼ 3.6 pc) and the reddening within the cluster region varies from E(B − V ) = 0.40 to 0.60 mag. The cluster is located at a distance of 2.05 ± 0.10 kpc. Using Hα slitless spectroscopy and 2MASS NIR data we identified Hα emission and NIR excess young stellar objects (YSOs), respectively. From their locations in the colour-magnitude diagrams, majority of them seem to have ages between 1 to 5 Myr. The spread in their ages indicate a possible non-coeval star formation in the cluster. Massive stars in the cluster region reveal an average age of 2 Myr. In the cluster region (r 6 ′ ) the slope of the mass function (MF), Γ, in the mass range ∼ 1.0 M/M ⊙ < 13.4 can be represented by a power law having a slope of −1.38 ± 0.12, which agrees well with Salpeter value (-1.35). In the mass range 0.3 M/M ⊙ < 1.0, the MF is also found to follow a power law with a shallower slope of Γ = −0.58±0.23 indicating a break in the slope of the IMF at ∼ 1M ⊙ .

Astronomy and Astrophysics

Based on new, accurate photometry, radial velocities, and proper motions for the intermediate-age open cluster NGC 3680, we identify individual single and binary cluster members and field stars in the colour-magnitude diagram (CMD). This basic step turns out to be crucial for a proper understanding of the cluster CMD: ∼60% of the stars are found to be field stars, and over 50% of the cluster stars are binaries. No bona f ide cluster star is found more than 1. m 5 below the turnoff, and cluster stars below 1.4 M are only found in binary systems. The total present mass of NGC 3680 is ∼100M , excluding any as yet unseen stellar remnants, and its half-mass radius is 3.3 (1.2 pc). Comparison with plausible IMFs indicates that only ∼3% of the original stars and < ∼ 10% of the mass now survive, ∼30% of the initial mass being in the form of massive stars that have now completed their evolution, and ∼60% in low-mass stars which may now be located in a distant cluster halo or perhaps have been lost entirely.

The Astronomical Journal, 2011

We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to ≈ 1.4 × 10 −8 M ⊙ yr −1 pc −2 in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused "high density mode", with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density mode", as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest subclusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 × 10 −5 M ⊙ yr −1 pc −2 ) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.