Stellar collisions in globular clusters: Constraints on the initial mass function of the first generation of stars

Dib, Sami; Kravtsov, V. V.; Haghi, Hosein; Zonoozi, Akram Hasani; Belinchón, José Antonio

doi:10.1051/0004-6361/202243556

“…This result has been predicted by Dib & Basu (2018) who attributed it to the existence of cluster-to-cluster variations of the IMF in Galactic clusters (see also Dib 2022). Variations of the IMF between galaxies have also been reported (Gunawardhana et al 2011;Geha et al 2013;Martín-Navarro et al 2015;Gennaro et al 2018), and these have been successfully interpreted as being due to variations in the average galactic gas-phase metallicity and SFR, and to the existence of various levels of IMF variations in their population of clusters (Yan et al 2017;Jerábková et al 2018;Dib et al 2022).…”

Section: Introductionsupporting

confidence: 57%

“…As Guszejnov et al (2019) stated, either the present-day mass function of these galaxies is problematic or the universality of the IMF in clusters is in question. Along this same idea, Dib & Basu (2018) and Dib et al (2022) showed how the presence of significant cluster-tocluster of the IMF can generate a bottom-heavy mass function in elliptical galaxies. Additionally, Dib et al (2022) showed that accounting for IMF variations in clusters coupled with a metallicity dependence of the IMF can help reproduce the galaxy-wide IMF of ultrafaint dwarf galaxies.…”

Section: Discussionmentioning

confidence: 93%

“…In M31, the results of Weisz et al (2015) also hint to the existence of cluster-tocluster variations of the slope of the IMF in the intermediate-to high-mass regime. There are also indications that the shape of the IMF of massive clusters that are the progenitors to presentday globular clusters possess some degree of variations, both in their characteristic mass and slope at the high-mass end (Wang et al 2020;Dib et al 2022;Kravtsov et al 2022;Wirth et al 2022) When attempting to investigate whether the slope of the IMF in the intermediate-to high-mass range truly varies, it is necessary to compare clusters whose IMFs have been derived using the same methodology; that is, where photometric or spectroscopic measurements have been derived using the same calibrations and instruments and that have accounted for the effects of reddening and cluster membership using the same procedures. Such homogenous samples of clusters for which the IMFs have been derived in a similar fashion are rare.…”

Section: Introductionmentioning

confidence: 99%

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Variation of the High-mass Slope of the Stellar Initial Mass Function: Theory Meets Observations

Dib

2023

ApJ

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We present observational evidence of a correlation between the high-mass slope of the stellar initial mass function (IMF) in young star clusters and their stellar surface density, σ *. When the high-mass end of the IMF is described by a power law of the form dN / d log M * ∝ M * − Γ , the value of Γ is seen to decrease weakly with increasing σ *, following a Γ = 1.31 σ * − 0.095 relation. We also present a model that can explain these observations. The model is based on the idea that the coalescence of protostellar cores in a protocluster-forming clump is more efficient in high-density environments where cores are more closely packed. The efficiency of the coalescence process is calculated as a function of the parental clump properties, in particular the relation between its mass and radius as well as its core formation efficiency. The main result of this model is that the increased efficiency of the coalescence process leads to shallower slopes of the IMF, in agreement with the observations of young clusters, and the observations are best reproduced with compact protocluster-forming clumps. These results have significant implications for the shape of the IMF in different Galactic and extragalactic environments and have very important consequences for galactic evolution.

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“…Initial mass function (IMF) describes the mass distribution of stars that form together at the birth time. It is a crucial parameter in astrophysics (Dib et al 2022), influencing most observable properties of star clusters and galaxies. The IMF serves as a critical input parameter for theoretical models or for deriving other physical parameters based on observations (Romano et al 2005;Gu et al 2022).…”

Section: Introductionmentioning

confidence: 99%

On the Determination of Stellar Mass and Binary Fraction of Open Clusters within 500 pc from the Sun

Jiang 蒋,

Zhong 钟,

Qin 秦

et al. 2024

ApJ

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We investigated the stellar mass function and the binary fraction of 114 nearby open clusters (OCs) using the high-precision photometric data from Gaia Data Release 3 (Gaia DR3). We estimated the mass of member stars by using a ridge line that is better in line with the observed color–magnitude diagram, thus obtaining more accurate stellar mass and binary mass ratio (q) at the low-mass region. By analyzing the present-day mass function (PDMF) of star clusters, we found that 108 OCs follow a two-stage power-law distribution, whereas six OCs present a single power-law PDMF. Subsequently, we fitted the high(low)-mass index of PDMF ( dN / dm ∝ m − α ), denoted as α h(α l), and segmentation point m c. For our cluster sample, the median values of α h and α l are 2.65 and 0.95, respectively, which are approximately consistent with the initial mass function results provided by Kroupa. We utilized the cumulative radial number distribution of stars with different masses to quantify the degree of mass segregation. We found a significant positive correlation between the state of dynamical evolution and mass segregation in OCs. We also estimated the fraction of binary stars with q ≥ 0.5, ranging from 6% to 34% with a median of 17%. Finally, we provided a catalog of 114 nearby cluster properties, including the total mass, the binary fraction, the PDMF, and the dynamical state.

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The galaxy-wide stellar initial mass function in the presence of cluster-to-cluster IMF variations

Dib

¹

2022

A&A

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We calculate the stellar integrated galactic initial mass function (IGIMF) in the presence of cluster-to-cluster variations of the IMF. Variations of the IMF for a population of coeval clusters that populate the initial cluster mass function (ICLMF) are taken into account in the form of Gaussian distribution functions of the IMF parameters. For the tapered power-law function used in this work, these are the slope at the high-mass end, Γ, the slope at the low-mass end, γ, and the characteristic mass M ch . The level of variations is modeled by varying the width of the Gaussian distributions. The reference values are the standard deviations of the parameters observed for the population of young clusters in the present-day Milky Way, which are σ Γ = 0.6, σ γ = 0.25, and σ M ch = 0.27 M ⊙ . We find that increasing the levels of dispersion for γ and Γ tends to moderately flatten the IGIMF at the low and high-mass end, respectively. The characteristic mass of the IGIMF is, however, strongly impacted by variations in M ch . Increasing the value of σ M ch shifts the peak of the IGIMF to lower masses, rendering the IGIMF more bottom heavy. This can provide a simple explanation for the bottom-heavy stellar mass function that is inferred for early-type galaxies since these are likely the result of a merger of disk galaxies where the physical conditions of the star-forming gas may vary significantly both in time and space in the merging system. The effect of IMF variations on the IGIMF is compared to the effects of other processes and sources of systematic variations such as those due to variations in the shape of ICLMF, the gas-phase metallicity, and the galactic star formation rate (SFR) which can potentially affect the maximum mass of stellar clusters in a galaxy and set the mean value of the characteristic mass in clusters. For the various dependencies we have explored, we found that the effect of IMF variations is a dominant factor that always affects the characteristic mass of the IGIMF. For the regimes at low metallicity where the IGIMF resembles a single power law, an increased level of IMF variations renders the IGIMF steeper and more bottom heavy, especially at low SFRs. On the other hand, variations in the IMF in the high mass regime can be easily dominated by variations in the slope of the ICLMF. We compare our results of the metallicity and SFR-dependent IGIMF to a sample of Milky Way ultra-faint dwarf (UFD) satellite galaxies that have available metallicity measurements. The present-day stellar mass function of these galaxies is a good analog to the IGIMF at the time their overall population of stars formed. We show that the slope of the stellar mass function of the UFD galaxies measured for stars in the mass range [0.4,0.8] M ⊙ can only be reproduced when IMF variations of the same order as those measured in the present-day Milky Way are included. Our results suggest that the inclusion of IMF variations in models of galaxy formation and evolution is of vital importance in order to improve our understanding of ...

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Stellar collisions in globular clusters: Constraints on the initial mass function of the first generation of stars

Cited by 4 publications

References 98 publications

Variation of the High-mass Slope of the Stellar Initial Mass Function: Theory Meets Observations

Variation of the High-mass Slope of the Stellar Initial Mass Function: Theory Meets Observations

On the Determination of Stellar Mass and Binary Fraction of Open Clusters within 500 pc from the Sun

The galaxy-wide stellar initial mass function in the presence of cluster-to-cluster IMF variations

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