|1- Sheikhi, N., Hasheminia, M., Khalaj, P., Haghi, H., Hasani Zonoozi, A., Baumgardt, H., "The binary fraction and mass segregation in Alpha Persei open cluster", MNRAS, 457: (1), 1028-1036, (2016).|
We have obtained membership probabilities of stars within a field of ∼3deg from the centre of the open cluster Alpha Persei using proper motions and photometry from the PPMXL and Wide-field Infrared Survey Explorer catalogues. We have identified 810 possible stellar members of Alpha Persei. We derived the global and radial present-day mass function (MF) of the cluster and found that they are well matched by two-stage power-law relations with different slopes at different radii. The global MF of Alpha Persei shows a turnover at m = 0.62 M⊙ with low- and high-mass slopes of αlow = 0.50 ± 0.09 (0.1 < m/ M⊙ < 0.62) and αhigh = 2.32 ± 0.14 (0.62 ≤ m/ M⊙ < 4.68), respectively. The high-mass slope of the cluster increases from 2.01 inside 1 ∘.10 to 2.63 outside 2 ∘.2, whereas the mean stellar mass decreases from 0.95 to 0.57 M⊙ in the same regions, signifying clear evidence of mass segregation in the cluster. From an examination of the high-quality colour–magnitude data of the cluster and performing a series of Monte Carlo simulations, we obtained a binary fraction of fbin = 34 ± 12 per cent for stars with 0.70 < m/ M⊙ < 4.68. This is significantly larger than the observed binary fraction, indicating that this open cluster contains a large population of unresolved binaries. Finally, we corrected the MF slopes for the effect of unresolved binaries and found low- and high-mass slopes of αlow = 0.89 ± 0.11 and αhigh = 2.37 ± 0.09 and a total cluster mass of 352 M⊙ for Alpha Persei.
|2- 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.
|3- Naddaf Moghaddam, M. H., Haghi, H., "The effect of external tidal field on life-time of star clusters in collisional and collision-less codes ", Applied Physics (YSF), 7333254, 1-4, (2015).|
The results of several performed N-body simulations using a collisionless N-body code named NMODY are reported here which aim at studying the evolution and life-time of star clusters under influence of an external tidal field. The code does not include stellar evolution, two body relaxation, and external tidal field as well. Having added the external tidal field to the code, we simulated our model clusters which are moving in circular orbits, in different galactocentric distances with various half-mass radius and initial mass around the Galaxy. Thereafter the results are compared with those of claimed by collisional code Nbody6. As the relaxation plays a main role in evolution of collisional systems, the main focus is to study how using an either collisional or collisionless code, changes the life-time of clusters. Finally the obtained relations between life-time of globular clusters with their initial half-mass radius are presented which numerically and virtually examine the strength of evolutionary mechanisms of globular clusters.
|4- 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.
|5- 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.
|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).|