scholarly journals Bimodal Star Formation and Remnant-Dominated Galactic Models

1987 ◽  
Vol 117 ◽  
pp. 413-413
Author(s):  
Richard B. Larson
Keyword(s):  
Star Formation ◽  
Formation Rate ◽  
Mass Function ◽  
Current Data ◽  
Initial Mass Function ◽  
Solar Neighborhood ◽  
Gas Content ◽  
High Mass ◽  
Stellar Content ◽  
Stellar Initial Mass Function

Current data on the luminosity function of nearby stars allow the possibility that the stellar initial mass function (IMF) is double-peaked and that the star formation rate (SFR) has decreased substantially with time. It is then possible to account for all of the unseen mass in the solar vicinity as stellar remnants. A model for the solar neighborhood has been constructed in which the IMF is bimodal, the SFR is constant for the low-mass mode and strongly decreasing for the high-mass mode, and the mass in remnants is equal to the column density of unseen matter; this model is found to be consistent with all of the available constraints on the evolution and stellar content of the solar neighborhood. In particular, the observed chemical evolution is satisfactorily reproduced without infall. The total SFR in the model decreases roughly with the 1.4 power of the gas content, which is more plausible than the nearly constant SFR required by models with a monotonic IMF.

scholarly journals Formation of Stellar Clusters and the Importance of Thermodynamics for Fragmentation

2007 ◽  
Vol 3 (S246) ◽  
pp. 3-12
Author(s):  
Ralf S. Klessen ◽  
Paul C. Clark ◽  
Simon C. O. Glover
Keyword(s):  
Star Formation ◽  
Dynamic Properties ◽  
Mass Function ◽  
Mass Scale ◽  
Initial Mass Function ◽  
Solar Neighborhood ◽  
High Mass ◽  
Mass Star ◽  
Heating Regime ◽  
Effective Equation

AbstractWe discuss results from numerical simulations of star cluster formation in the turbulent interstellar medium (ISM). The thermodynamic behavior of the star-forming gas plays a crucial role in fragmentation and determines the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This holds for star formation in molecular clouds in the solar neighborhood as well as for the formation of the very first stars in the early universe. The thermodynamic state of the ISM is a result of the balance between heating and cooling processes, which in turn are determined by atomic and molecular physics and by chemical abundances. Features in the effective equation of state of the gas, such as a transition from a cooling to a heating regime, define a characteristic mass scale for fragmentation and so set the peak of the initial mass function of stars (IMF). As it is based on fundamental physical quantities and constants, this is an attractive approach to explain the apparent universality of the IMF in the solar neighborhood as well as the transition from purely primordial high-mass star formation to the more normal low-mass mode observed today.

scholarly journals The Stellar Populations in Local Group Dwarf Galaxies

1995 ◽  
Vol 164 ◽  
pp. 175-180
Author(s):  
Abhijit Saha
Keyword(s):  
Star Formation ◽  
Globular Clusters ◽  
Formation Rate ◽  
Stellar System ◽  
Mass Function ◽  
Open Clusters ◽  
Initial Mass Function ◽  
Star Formation History ◽  
Physical Parameters ◽  
Stellar Content

The aim of the study of the populations in a stellar system is to understand and be able to describe the stellar content of a system in terms of physical parameters such as the age, star formation history, chemical enrichment history, initial mass function (IMF), environment, and dynamical history of the system. This is done given an understanding of stellar evolution and the ability to express the outcome in “observer parameters”, particularly a color-magnitude diagram (CMD), kinematics, and metallicity. From this perspective, the simplest systems are the galactic clusters and the globular clusters, where all the component stars are coeval and of the same metallicity. The current state of knowledge for these are discussed by others in this conference. We proceed to the next level of complexity (where metallicities are not necessarily all the same, and nor are the stars all coeval), and try to decompose their stellar content, particularly in terms of star formation rate and metallicity. In this regard the two classes of objects that come to mind are the dwarf spheroidals, and the dwarf irregulars. Both these classes of objects are more massive than the open clusters and globular clusters, and show evidence of complexities in their star formation histories, without being so convolved as to make such a study intractable. As we shall see, recent studies along these lines have presented some puzzling problems. Moreover, these are the smallest independent galaxies, and the study of star formation in these is likely to shed light on the history and formation of larger and more complex galaxies.

scholarly journals Star Formation and Galactic Evolution: From Protogalaxies to Starbursts

1987 ◽  
Vol 115 ◽  
pp. 663-689 ◽  
Author(s):  
Joseph Silk
Keyword(s):  
Star Formation ◽  
Physical Mechanism ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Mass Function ◽  
Initial Mass Function ◽  
Galactic Evolution ◽  
Solar Neighborhood ◽  
Formation Theory ◽  
Formation Efficiency

Three topics in star formation theory are reviewed: the initial mass function, the star formation efficiency, and the star formation rate. A physical mechanism for bimodal star formation is developed. Applications are made to the solar neighborhood, to the inner galaxy, to starburst galaxies, and to past star formation in protodisks and in protoellipticals. Implications are drawn for galactic morphology, for chemical evolution, and for the present density of stellar remnants.

scholarly journals A stochastically sampled IMF alters the stellar content of simulated dwarf galaxies

2019 ◽  
Vol 492 (1) ◽  
pp. 8-21 ◽  
Author(s):  
Elaad Applebaum ◽  
Alyson M Brooks ◽  
Thomas R Quinn ◽  
Charlotte R Christensen
Keyword(s):  
Star Formation ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Mass Function ◽  
Initial Mass Function ◽  
Gas Content ◽  
Stellar Content ◽  
Full Spectrum ◽  
Cosmological Simulations ◽  
Order Of Magnitude

ABSTRACT Cosmological simulations are reaching the resolution necessary to study ultra-faint dwarf galaxies. Observations indicate that in small populations, the stellar initial mass function (IMF) is not fully populated; rather, stars are sampled in a way that can be approximated as coming from an underlying probability density function. To ensure the accuracy of cosmological simulations in the ultra-faint regime, we present an improved treatment of the IMF. We implement a self-consistent, stochastically populated IMF in cosmological hydrodynamic simulations. We test our method using high-resolution simulations of a Milky Way halo, run to z = 6, yielding a sample of nearly 100 galaxies. We also use an isolated dwarf galaxy to investigate the resulting systematic differences in galaxy properties. We find that a stochastic IMF in simulations makes feedback burstier, strengthening feedback, and quenching star formation earlier in small dwarf galaxies. For galaxies in haloes with mass ≲ 108.5 M⊙, a stochastic IMF typically leads to lower stellar mass compared to a continuous IMF, sometimes by more than an order of magnitude. We show that existing methods of ensuring discrete supernovae incorrectly determine the mass of the star particle and its associated feedback. This leads to overcooling of surrounding gas, with at least ∼10 per cent higher star formation and ∼30 per cent higher cold gas content. Going forwards, to accurately model dwarf galaxies and compare to observations, it will be necessary to incorporate a stochastically populated IMF that samples the full spectrum of stellar masses.

scholarly journals The Initial Mass Function of Massive Stars in Galaxies: Empirical Evidence

1986 ◽  
Vol 116 ◽  
pp. 451-466 ◽  
Author(s):  
John M. Scalo
Keyword(s):  
Formation Rate ◽  
Mass Function ◽  
Convincing Evidence ◽  
Initial Mass Function ◽  
Solar Neighborhood ◽  
High Mass ◽  
Mass Limit ◽  
Nearby Galaxies ◽  
Evolution Modeling ◽  
Excitation Conditions

Observational constraints on the form of the high-mass stellar IMF are reviewed. The evidence includes star counts in the solar neighborhood, individual and composite star clusters, and nearby galaxies, and arguments based on integrated light and chemical evolution modeling. There is no convincing evidence for any systematic variations of the shape of the high-mass IMF. However, the various determinations are very uncertain, and do not allow any firm estimate of the logarithmic slope of the upper IMF; the appropriate value is somewhere between −1.3 and −2.3, with region-to-region variations smaller than about ±0.5. A number of lines of evidence suggest that the lower mass limit or mode mass of the IMF increases with increasing star formation rate, reaching perhaps 10–15 m⊙ in some starburst galaxies. It is also possible that the upper mass limit depends on metallicity, based on variations in excitation conditions of HII regions.

scholarly journals Turbulent structure and star formation in a stratified, supernova-driven, interstellar medium

2006 ◽  
Vol 2 (S237) ◽  
pp. 358-362
Author(s):  
M. K. Ryan Joung ◽  
Mordecai-Mark Mac Low
Keyword(s):  
Star Formation ◽  
Interstellar Medium ◽  
Adaptive Mesh Refinement ◽  
Formation Rate ◽  
Three Dimensional ◽  
Adaptive Mesh ◽  
Mass Function ◽  
Initial Mass Function ◽  
Interstellar Turbulence ◽  
Self Gravity

AbstractWe report on a study of interstellar turbulence driven by both correlated and isolated supernova explosions. We use three-dimensional hydrodynamic models of a vertically stratified interstellar medium run with the adaptive mesh refinement code Flash at a maximum resolution of 2 pc, with a grid size of 0.5 × 0.5 × 10 kpc. Cold dense clouds form even in the absence of self-gravity due to the collective action of thermal instability and supersonic turbulence. Studying these clouds, we show that it can be misleading to predict physical properties such as the star formation rate or the stellar initial mass function using numerical simulations that do not include self-gravity of the gas. Even if all the gas in turbulently Jeans unstable regions in our simulation is assumed to collapse and form stars in local freefall times, the resulting total collapse rate is significantly lower than the value consistent with the input supernova rate. The amount of mass available for collapse depends on scale, suggesting a simple translation from the density PDF to the stellar IMF may be questionable. Even though the supernova-driven turbulence does produce compressed clouds, it also opposes global collapse. The net effect of supernova-driven turbulence is to inhibit star formation globally by decreasing the amount of mass unstable to gravitational collapse.

scholarly journals THE HIGH-MASS STELLAR INITIAL MASS FUNCTION IN M31 CLUSTERS

2015 ◽  
Vol 806 (2) ◽  
pp. 198 ◽  
Author(s):  
Daniel R. Weisz ◽  
L. Clifton Johnson ◽  
Daniel Foreman-Mackey ◽  
Andrew E. Dolphin ◽  
Lori C. Beerman ◽  
...  
Keyword(s):  
Mass Function ◽  
Initial Mass Function ◽  
High Mass ◽  
Initial Mass ◽  
Stellar Initial Mass Function
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