Modeling of the Initial Mass Function Using the Metropolis−Hastings Algorithm
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
More filters
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
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
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....
[...]
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
More filters
TL;DR: In this article, the median stellar age ranges between 2.5 10^5 and 5 10^6 years, with evidence of an age spread of the same entity within each cluster.
Abstract: [Abridged] Twenty-six high-luminosity IRAS sources believed to be collection of stars in the early phases of high-mass star formation have been observed in the NIR (J, H, K) to characterize the clustering properties of their young stellar population and gain insight into the initial conditions of star formation in these clusters (Initial Mass Function [IMF], Star Formation History [SFH]), and to deduce mean values for cluster ages.
K luminosity functions (KLFs) are compared with simulated ones from a model that generates populations of synthetic clusters starting from assumptions on the IMF, the SFH, and the Pre-MS evolution, and using the average properties of the observed clusters as boundary conditions
Twenty-two sources show evidence of clustering from a few up to several tens of objects, and a median cluster radius of 0.7 pc. A considerable number of cluster members present an infrared excess characteristic of young Pre-Main-Sequence objects. We find that the median stellar age ranges between 2.5 10^5 and 5 10^6 years, with evidence of an age spread of the same entity within each cluster. We also find evidence that older clusters tend to be smaller in size, in line with the fact that our clusters are on average larger than those around relatively older Herbig Ae/Be stars. The relationship of the mass of the most massive star in the cluster with both the clusters richness and their total stellar mass suggest that our modeled clusters may not be consistent with them resulting from random sampling of the IMF.
Our results are consistent with a star formation which takes place continuously over a period of time which is longer than a typical crossing time.
27 citations
TL;DR: In this article, the authors measured the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on HST-ACS data.
Abstract: [abridged] Theoretical investigations have suggested the presence of Intermediate Mass Black Holes (IMBHs, with masses in the 100-10000 Msun range) in the cores of some Globular Clusters (GCs). In this paper we present the first application of a new technique to determine the presence or absence of a central IMBH in globular clusters that have reached energy equipartition via two-body relaxation. The method is based on the measurement of the radial profile for the average mass of stars in the system, using the fact that a quenching of mass segregation is expected when an IMBH is present. Here we measure the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on HST-ACS data. The observations are compared to expectations from direct N-body simulations of the dynamics of star clusters with and without an IMBH. The mass segregation profile for NGC 2298 is quantitatively matched to that inferred from simulations without a central massive object over all the radial range probed by the observations, that is from the center to about two half-mass radii. Profiles from simulations containing an IMBH more massive than ~ 300-500 Msun (depending on the assumed total mass of NGC 2298) are instead inconsistent with the data at about 3 sigma confidence, irrespective of the IMF and binary fraction chosen for these runs. While providing a null result in the quest of detecting a central black hole in globular clusters, the data-model comparison carried out here demonstrates the feasibility of the method which can also be applied to other globular clusters with resolved photometry in their cores.
26 citations
10 citations
TL;DR: In this paper, the authors used the Monte Carlo method to make numerical simulation of the dynamical evolution of embedded clusters and the result is compared with the observations, and they found that, due to the random motions of massive stars, temporary mass segregation may exist at certain times in the course of evolution of a given embedded cluster, and this phenomenon may be very prominent in some of them.
6 citations