|1- Haghi, H., Hasani Zonoozi, A., Kroupa, P., Banerjee, S., Baumgardt, H., "Possible smoking-gun evidence for initial mass segregation in re-virialized post-gas expulsion globular clusters", MNRAS, 454: (4), 3872-3885, (2015).|
We perform a series of direct N-body calculations to investigate the effect of residual gas expulsion from the gas-embedded progenitors of present-day globular clusters (GCs) on the stellar mass function (MF). Our models start either tidally filling or underfilling, and either with or without primordial mass segregation. We cover 100 Myr of the evolution of modelled clusters and show that the expulsion of residual gas from initially mass-segregated clusters leads to a significantly shallower slope of the stellar MF in the low- (m ≤ 0.50 M⊙) and intermediate-mass (≃ 0.50–0.85 M⊙) regime. Therefore, the imprint of residual gas expulsion and primordial mass segregation might be visible in the present-day MF. We find that the strength of the external tidal field, as an essential parameter, influences the degree of flattening, such that a primordially mass-segregated tidally filling cluster with rh/rt ≥ 0.1 shows a strongly depleted MF in the intermediate stellar mass range. Therefore, the shape of the present-day stellar MF in this mass range probes the birth place of clusters in the Galactic environment. We furthermore find that this flattening agrees with the observed correlation between the concentration of a cluster and its MF slope, as found by de Marchi et al.. We show that if the expansion through the residual gas expulsion in primordial mass segregated clusters is the reason for this correlation then GCs most probably formed in strongly fluctuating local tidal fields in the early proto-Milky Way potential, supporting the recent conclusion by Marks & Kroupa.
|2- Haghi, H., Hasani Zonoozi, A., Taghavi, S., "Galactic orbital motions of star clusters: static versus semicosmological time-dependent Galactic potentials", MNRAS, 450, 2812-2821 , (2015).|
In order to understand the orbital history of Galactic halo objects, such as globular clusters, authors usually assume a static potential for our Galaxy with parameters that appear at the present-day. According to the standard paradigm of galaxy formation, galaxies grow through a continuous accretion of fresh gas and a hierarchical merging with smaller galaxies from high redshift to the present day. This implies that the mass and size of disc, bulge, and halo change with time. We investigate the effect of assuming a live Galactic potential on the orbital history of halo objects and its consequences on their internal evolution. We numerically integrate backwards the equations of motion of different test objects located in different Galactocentric distances in both static and time-dependent Galactic potentials in order to see if it is possible to discriminate between them. We show that in a live potential, the birth of the objects, 13 Gyr ago, would have occurred at significantly larger Galactocentric distances, compared to the objects orbiting in a static potential. Based on the direct N-body calculations of star clusters carried out with collisional N-body code, NBODY6, we also discuss the consequences of the time-dependence of a Galactic potential on the early- and long-term evolution of star clusters in a simple way, by comparing the evolution of two star clusters embedded in galactic models, which represent the galaxy at present and 12 Gyr ago, respectively. We show that assuming a static potential over a Hubble time for our Galaxy as it is often done, leads to an enhancement of mass-loss, an overestimation of the dissolution rates of globular clusters, an underestimation of the final size of star clusters, and a shallower stellar mass function.
|3- Haghi, H., Hoseini-Rad, S. M., Hasani Zonoozi, A., H. W. Küpper, A., "The effect of primordial mass segregation on the size scale of globular clusters ", MNRAS, 444, 3699-3708 , (2014).|
We use direct N-body calculations to investigate the impact of primordial mass segregation on the size scale and mass-loss rate of star clusters in a galactic tidal field. We run a set of simulations of clusters with varying degrees of primordial mass segregation at various galactocentric radii and show that, in primordially segregated clusters, the early, impulsive mass-loss from stellar evolution of the most massive stars in the innermost regions of the cluster leads to a stronger expansion than for initially non-segregated clusters. Therefore, models in stronger tidal fields dissolve faster due to an enhanced flux of stars over the tidal boundary. Throughout their lifetimes, the segregated clusters are more extended by a factor of about 2, suggesting that (at least) some of the very extended globular clusters in the outer halo of the Milky Way may have been born with primordial mass segregation. We finally derive a relation between star–cluster dissolution time, Tdiss, and galactocentric radius, RG, and show how it depends on the degree of primordial mass segregation.
|4- Hasani Zonoozi, A., Haghi, H., H. W. Küpper, A., Baumgardt, H., J. Frank, M., Kroupa, P., "Direct N-body simulations of globular clusters – II. Palomar 4", MNRAS, 440, 3172-3183 , (2014).|
We use direct N-body calculations to study the evolution of the unusually extended outer halo globular cluster Palomar 4 (Pal 4) over its entire lifetime in order to reproduce its observed mass, half-light radius, velocity dispersion and mass function slope at different radii. We find that models evolving on circular orbits, and starting from a non-mass segregated, canonical initial mass function (IMF) can reproduce neither Pal 4s overall mass function slope nor the observed amount of mass segregation. Including either primordial mass segregation or initially flattened IMFs does not reproduce the observed amount of mass segregation and mass function flattening simultaneously. Unresolved binaries cannot reconcile this discrepancy either. We find that only models with both a flattened IMF and primordial segregation are able to fit the observations. The initial (i.e. after gas expulsion) mass and half-mass radius of Pal 4 in this case are about 57 000 M⊙ and 10 pc, respectively. This configuration is more extended than most globular clusters we observe, showing that the conditions under which Pal 4 formed must have been significantly different from that of the majority of globular clusters. We discuss possible scenarios for such an unusual configuration of Pal 4 in its early years.
|5- Derakhshani , K., Haghi, H., "ERRATUM: THE MOND EXTERNAL FIELD EFFECT ON THE DYNAMICS OF THE GLOBULAR CLUSTERS:
GENERAL CONSIDERATIONS AND APPLICATION TO NGC 2419 (ApJ , 783, 48) ", ApJ , 785, 166-, (2014). (Comment- Hosein Haghi’s name was left off the author list in the original article.
The abstract of the original article (ApJ , 783, 48) can be found as follows)|
In this paper, we investigate the external field effect in the context of the MOdified Newtonian Dynamics (MOND) on the surface brightness and velocity dispersion profiles of globular clusters (GCs). Using N-MODY, which is an N-body simulation code with a MOND potential solver, we show that the general effect of the external field for diffuse clusters, which obey MOND in most of their parts, is that it pushes the dynamics toward the Newtonian regime. On the other hand, for more compact clusters, which are essentially Newtonian in their inner parts, the external field is effective mainly in the outer parts of compact clusters. As a case study, we then choose the remote Galactic GC NGC 2419. By varying the cluster mass, half-light radius, and mass-to-light ratio, we aim to find a model that will reproduce the observational data most effectively, using N-MODY. We find that even if we take the Galactic external field into account, a Newtonian Plummer sphere represents the observational data better than MOND to an order of magnitude in terms of the total χ2 of surface brightness and velocity dispersion.
|1- Haghi, H., Kroupa, P., Hasani Zonoozi, A., Banerjee, S., "Possible smoking-gun evidence for initial mass segregation in re-virialized post-gas expulsion star-burst clusters ", Modelling and Observing Dense Stellar Clusters in Chile,Universidad de Concepción, Chile
, 29-29, (2015).|
In the present study, we have carried out a series of direct N-body calculations to investigate the effect of residual-gas expulsion from the globular clusters' embedded progenitors on the stellar mass function of different models on circular orbits, starting either tidally filling or underfilling, and either with or without primordial mass segregation. We covered the first 100 Myr of the evolution of modeled clusters and showed for the first time that the expulsion of residual gas from initially mass-segregated models leads to the signi_cantly shallower slope of the stellar mass function in the low- (≃M _ 0.5M⨀) and intermediate-mass ( 0.5 <= 0.8 M⨀) regime. Therefore the imprint of residual gas expulsion, as a direct evidence of primordial segregation, might be visible in the present day MF. We also found that the strength of the external tidal _led, as an essential parameter, inuences the degree of attening in the intermediate-mass range, such that the MFs of primordially mass-segregated tidally-filliing clusters with r h =rt values larger than 0.1 show a strongly depleted mass function in the intermediate stellar mass range, while the slower mass-loss rate of clusters initially lying inside their tidal radii, takes a longer time to lose a given amount of mass. Therefore, the shape of present day MF in intermediate stellar mass range probes the birth place of clusters in Galactic environment.
|2- Haghi, H., "How does the gas expulsion phase affect the initial conditions of star clusters?
", Workshop,The Early Life of Stellar Clusters: Formation and Dynamics, 25-25, (2014).|
The study of stellar clusters has played an important rule in developing of our knowledge about the universe. Since most stars in the galactic disc may originate in star clusters, these systems can therefore be investigated as the fundamental building blocks of galaxies to understand the origins of the properties of the galactic stellar population, such as the galactic stellar mass function. Detailed knowledge of the initial condition of globular clusters, is necessary to understand the evolution of stellar system including all physical processes that may happen during their evolution. Zonoozi et al. (2011, 2014), have been found that dynamical mass segregation alone cannot explain the mass function flattening in the cluster centre when starting from a canonical Kroupa IMF, and that a very high degree of primordial mass segregation would be necessary to explain this discrepancy. We concluded that such initial conditions for Pal 14 and Pal 4 might be obtained by a violent early gas-expulsion phase from an embedded cluster born with mass segregation and a canonical IMF for low-mass stars. After modelling some realistic Galactic clusters and finding the initial conditions, as a next stage we need to understand how this connects to what we know of star formation. So the t=0 condition which we constrained is the state of the cluster after re-virialisation and after gas expulsion. But how does the prior phase work, and what are the possible birth configurations, given the t=0 boundary condition? Which birth conditions do gas-expulsion computations covering the first 100 Myr of a Pal 14/4 type cluster require for the post-gas expulsion re-virialised cluster to match up with the initial conditions found for Pal 14/4? Did gas expulsion even play a dynamical role for Pal 14/4? This work would require many more stars in the N-body models, but covers a much shorter time, and would be done without binaries (as a first step). In my talk I therefore try to answer these questions.
|3- Haghi, H., "The remote Galactic globular clusters as a tool to test
gravity models and direct N-body simulation", Dynamics and kinetic theory of
self-gravitating systems, IHP Gravasco Trimester, Paris, (2013).|
|4- Haghi, H., "How primordial mass segregation can increase the size scale
of the star clusters", Vlasov-Poisson : the numerical approach and its limits, IHP Gravasco Trimester, Paris, (2013).|
|5- Haghi, H., "Direct N-body simulations of globular clusters", 1st Doha International Astronomy Conference, Doha, Qatar, (2013).|