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Journal ArticleDOI

Modeling of the Initial Mass Function Using the Metropolis−Hastings Algorithm

01 Aug 2011-The Astrophysical Journal (IOP Publishing)-Vol. 736, Iss: 2, pp 152
TL;DR: In this article, a stochastic model for the hierarchical fragmentation of a molecular cloud was developed, where the number of fragments, time between successive fragmentation steps, and mass of a fragment were considered as random variables, and fragment masses were generated using the Metropolis-Hastings algorithm.
Abstract: A stochastic model has been developed for the hierarchical fragmentation of a molecular cloud. Here, the number of fragments, time between successive fragmentation steps, and mass of a fragment are considered as random variables, and fragment masses are generated using the Metropolis-Hastings algorithm. The resulting mass spectra, computed at different projected distances and taking opacity into consideration, show a signature of mass segregation. The critical mass, mass spectrum, and mass segregation are consistent with the observations of young massive clusters in our Galaxy as well as in external galaxies.
Citations
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Journal ArticleDOI
TL;DR: In this article, a left truncated beta probability density function was introduced to demonstrate the advantage of introducing a four-parameter initial mass function for the stars and the presence of brown dwarfs.
Abstract: The initial mass function (IMF) for the stars is usually fitted by three straight lines, which means seven parameters. The presence of brown dwarfs (BD) increases to four the straight lines and to nine the parameters. Another common fitting function is the lognormal distribution, which is characterized by two parameters. This paper is devoted to demonstrating the advantage of introducing a left truncated beta probability density function, which is characterized by four parameters. The constant of normalization, the mean, the mode and the distribution function are calculated for the left truncated beta distribution. The normal-beta (NB) distribution which results from convolving independent normally distributed and beta distributed components is also derived. The chisquare test and the K-S test are performed on a first sample of stars and BDs which belongs to the massive young cluster NGC 6611 and on a second sample which represents the star’s masses of the cluster NGC 2362.

16 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effect of the evolution of the stellar initial mass function (IMF) with increasing redshift on the formation of massive clusters in star burst galaxies.
Abstract: Theoretical and indirect observational evidences suggest that stellar initial mass function (IMF) increases with redshift. On the other hand star formation rates (SFR) may be as high as 100 $M_{\odot}$ yr$^{-1}$ in star burst galaxies. These may lead to formation of massive clusters hence massive stars to make the integrated galactic stellar initial mass function (IGIMF) top heavy (i.e. proportion of massive stars is higher than less massive stars). We investigate the joint effect of evolving IMF and several measures of SFR in dependence of galaxy wide IMF. The resulting IGIMF have slopes $\alpha_{2,IGIMF}$ in the high mass regime, which is highly dependent on the minimum mass of the embedded cluster ($M_{ecl,min}$), star formation rates and mass spectrum indices of embedded clusters (viz. $\beta$). It is found that for z $\sim$ 0 - 2, $\alpha_{2,IGIMF}$ becomes steeper (i.e. bottom heavy), for z $\sim$ 2 - 4, $\alpha_{2,IGIMF}$ becomes flatter (i.e. top heavy ) and from z $\sim$ 4 onwards $\alpha_{2,IGIMF}$ becomes again steeper. The effects are faster for higher values of $\beta$. $\alpha_{2,IGIMF}$ is flatter also for higher values of $M_{ecl,min}$. All these effects might be counted for the joint effect of increasing temperature of the ambient medium as well as varying SFR with increasing redshift.

12 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate the evolution of the stellar initial mass function (IMF) and varying indices of β for the integrated galactic initial mass functions, in relation to several measures of star formation rates of galaxies at various redshifts by random simulation.
Abstract: Theoretical as well as observational studies suggest that the stellar initial mass func-tion (IMF) might become top heavy with increasing redshift. Embedded cluster massfunction is a power law having index β, whose value still remains controversial. Inthe present work, we investigate the effect of evolving IMF and varying indices ofβ for the integrated galactic initial mass function, in relation to several measures ofstar formation rates of galaxies at various redshifts by random simulation. The result-ing IGIMF is segmented power law at various redshifts having slopes α 1,IGIMF andα 2,IGIMF with a turnover at a characteristic mass m c ′ . These differ from the stel-lar initial mass functions with slopes α 1,IMF , α 2,IMF , and characteristic masses m c for different values of redshift z, β, minimum and maximum masses of the embeddedclusters.Key words: galaxies:star clusters: general - galaxies:evolution 1 INTRODUCTIONThe form of stellar initial mass function is of considerabledebate in the present era as it describes the nature ofstellar population, the ratio of high mass to low mass starsand influences the dynamical evolution of star clusters aswell as star formation history of the whole galaxy. Usuallyit is derived using observed luminosity function togetherwith an assumed mass-to-light ratio for the stars underconsideration. Generally, IMFs, as suggested by variousauthors, are either of Salpeter type (Salpeter 1955) orconsists of segmented power laws (Scalo 1986; Kroupa 2001;Chabrier 2003) with a turnover at some characteristic massm

10 citations


Cites background from "Modeling of the Initial Mass Functi..."

  • ...In the work by Chattopadhyay et al. (2011), the authors have considered the random fragmentation of young massive clouds in our Galaxy as well as in external galaxies....

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Journal ArticleDOI
TL;DR: In this article, the authors presented deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56*57" (14pc*14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud.
Abstract: We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56*57" (14pc*14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayears. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 Msun and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range 1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 Msun at a distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.

7 citations

Journal Article
TL;DR: In this paper, the results of numerical N-body calculations which simulate the dynamical evolution of young clusters as they emerge from molecular clouds have been presented, and the combination of these parameters which result in the production of bound stellar groups after the gas not used in star formation is completely dispersed.
Abstract: We present the results of numerical N-body calculations which simulate the dynamical evolution of young clusters as they emerge from molecular clouds. We follow the evolution of initially virialized stellar systems of 50 and, in some cases, 100 stars from the point in time immediately after the stars have formed in a cloud until a time long after all the residual star-forming gas has been dispersed from the system. By varying the star formation efficiency and the gas dispersal time for each model, we determined the combination of these parameters which result in the production of bound stellar groups after the gas not used in star formation is completely dispersed.

7 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors presented a survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST).
Abstract: {Aims.} We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case. {Methods.} The reddening, the age and the metallicity of the cluster are estimated by comparison of the observed Color Magnitude Diagram (CMD) with theoretical isochrones. {Results.} Under the most conservative assumptions the stellar mass of VdB0 is M > 2.4 x 10^4 M_sun, but our best estimates lie in the range ~ 4-9 x 10^4 M_sun. The CMD of VdB0 is best reproduced by models having solar metallicity and age = 25 Myr. Ages smaller than = 12 Myr and larger than = 60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of Red Super Giants (> 18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example. {Conclusions.} VdB0 is significantly brighter (>~ 1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius (r_h=7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31 it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~ 4 Gyr.

45 citations

Journal ArticleDOI
TL;DR: In this paper, the authors reported the discovery of a young planetary-mass brown dwarf in the rho Oph cloud core using a 1.5-2.4 micron low-resolution spectrum obtained using the NIRC instrument on the Keck I telescope.
Abstract: We report the discovery of a young planetary-mass brown dwarf in the rho Oph cloud core. The object was identified as such with the aid of a 1.5-2.4 micron low-resolution spectrum obtained using the NIRC instrument on the Keck I telescope. Based on the COND model, the observed spectrum is consistent with a reddened (Av ~ 15-16) brown dwarf whose effective temperature is in the range 1200-1800 K. For an assumed age of 1 Myr, comparison with isochrones further constrains the temperature to ~ 1400 K and suggests a mass of ~ 2-3 Jupiter masses. The inferred temperature is suggestive of an early T spectral type, which is supported by spectral morphology consistent with weak methane absorption. Based on its inferred distance (~ 100 pc) and the presence of overlying visual absorption, it is very likely to be a rho Oph cluster member. In addition, given the estimated spectral type, it may be the youngest and least massive T dwarf found so far. Its existence suggests that the initial mass function for the rho Oph star-forming region extends well into the planetary-mass regime.

43 citations

Journal ArticleDOI
TL;DR: In this paper, a survey of compact star clusters (apparent size 1.5 arcsec (>06 pc) was carried out and the authors derived cluster parameters based on the photometric data and multiband images by employing simple stellar population models.
Abstract: We have carried out a survey of compact star clusters (apparent size 015 arcsec (>06 pc) We derived cluster parameters based on the photometric data and multiband images by employing simple stellar population models The clusters have a wide range of ages from ~5 Myr (young objects associated with 24 um and/or Ha emission) to ~10 Gyr (globular cluster candidates), and possess mass in a range of 30 < log(m/M_sol) < 43 peaking at m ~ 4000 M_sol Typical age of these intermediate-mass clusters is in the range of 30 Myr < t < 3 Gyr, with a prominent peak at ~70 Myr These findings suggest a rich intermediate-mass star cluster population in M31, which appears to be scarce in the Milky Way galaxy

42 citations

Journal ArticleDOI
TL;DR: In this article, the authors show that star formation scenarios predicting general primordial mass segregation are inconsistent with observed segregation levels, and discuss the implications of their results for theories of star formation.
Abstract: Mass segregation is observed in many star clusters, including several that are less than a few Myr old. Time-scale arguments are frequently used to argue that these clusters must be displaying primordial segregation, because they are too young to be dynamically relaxed. Looking at this argument from the other side, the youth of these clusters and the limited time available to mix spatially distinct populations of stars can provide constraints on the amount of initial segregation that is consistent with current observations. We present n-body experiments testing this idea, and discuss the implications of our results for theories of star formation. For system ages less than a few crossing times, we show that star formation scenarios predicting general primordial mass segregation are inconsistent with observed segregation levels.

41 citations

Journal ArticleDOI
TL;DR: In this paper, the reliability of infrared (IR) emission to trace star formation in individual star-forming sites of M 33 was determined and a new method for testing the distribution function of massive stars in newly formed clusters.
Abstract: Aims. The aim of this paper is twofold: to determine the reliability of infrared (IR) emission to trace star formation in individual star-forming sites of M 33, and to outline a new method for testing the distribution function of massive stars in newly formed clusters. Methods. We select 24 μ m IR sources from the Spitzer survey of M 33 with Hα counterparts and show that the IR luminosities have a weak dependence on galactocentric radius. The IR and Hα luminosities are not correlated. Complementing the infrared photometry with GALEX-UV data, we estimate the bolometric luminosities to investigate how they are related to the Hα luminosities. We simulate a theoretical diagram for the expected bolometric-to-Hα luminosity ratio, L bol /L H\alpha , of young clusters as a function of the cluster luminosity. We then compare the observed L bol /L H\alpha ratios with the theoretical predictions. Results. In the log(L/L H\alpha ) plane, stellar clusters should be born along a curve that we call the cluster birthline . The birthline depends on the stellar initial mass function (IMF) at the high-mass end, but not on the cluster mass function. For an upper stellar mass limit of , the birthline is flat for erg s-1 because all clusters fully sample the IMF. It increases toward lower luminosities as the upper end of the IMF becomes incompletely sampled. Aging moves clusters above the birthline. The observations of M 33 show that young isolated clusters lie close to the theoretical birthline for a wide range of L bol . The observed L bol /L H\alpha ratio increases toward low L bol like the theoretical curve, indicating that luminosity is not proportional to Hα emission for low mass clusters. The best fit to the birthline is for a randomly sampled IMF, in which the mass of most massive star in a cluster is not strictly limited by the cluster's mass, but can have any value up to the maximum stellar mass with a probability determined by the IMF. We also find that the IR luminosity of young stellar clusters in M 33 is not proportional to their bolometric luminosity. This irregularity could be the result of low and patchy dust abundance. In M 33 dust absorbs and re-radiates in the IR only part of the UV light from young clusters.

39 citations