Star formation initial mass function

STOCHASTIC PROCESSES IN ASTROPHYSICS B. Star Formation—Initial Mass Function... 

Nowadays, the star formation history (SFH), initial mass function (IMF) and detailed chemical properties have been determined for many dwarfs, both in the Local Group and outside it (e.g. Grebel, Shetrone, Tolstoy, these proceedings). This in principle allows us to base theories of late-type galaxy formation and evolution on firmer grounds, by reducing the free parameter space. 

Salpeter introduces the Initial Mass Function for star formation. 

Present-day mass function (PDMF) of stars in the galaxy compared to the initial mass function (IMF). PDMF is the number of stars of a given mass in the galaxy today, whereas IMF is the number of stars of a given mass produced in a single episode of star formation. The difference in the two curves at high stellar mass reflects absence of the stars that have exhausted their nuclear fuel and died over galactic history from the PDMF. After Basu and Rana (1992). 

The abundance ratios of heavy elements are sensitive to the initial mass function (IMF), the star formation history, and variations in stellar nucleosynthesis with, e.g., metallicity. In particular, comparison of abundances of elements produced in stars with relatively long lifetimes (such as C, N, Fe, and the s-process elements) with those produced in short-lived stars (such as O) probe the star formation history. Below, I review the accumulated data on C, N, S, and Ar abundances (relative to O) in spiral and irregular galaxies, covering two orders of magnitude in metallicity (as measured by O/H). The data are taken from a variety of sources on abundances for H II regions in the literature. 

YSO) classiflcation (Lada 1987). However, there is not as much knowledge on the formation of higher mass stars (>8Mq) because they are rare and distant, and it is the formation of the latter which controls the chemical and dynamical evolution of galaxies. Also the initial mass function (IMF), the distribution of stellar masses at birth which is one of the fundamentally important parameters in astrophysics, is not well constrained because of the lack of sufficiently resolved regions on which to study its emergence. 

The next problem was to find internally constitent values of physical parameters of stellar populations of different age and composition. For this purpose I developed a model of physical evolution of stellar populations (Einasto 1971). When I started the modelling of physical evolution of galaxies I was not aware of similar work by Beatrice Tinsley (1968). When my work was almost finished I had the opportunity to read the PhD thesis by Beatrice. Both studies were rather similar, in some aspects my model was a bit more accurate (evolution was calculated as a continuous function of time whereas Beatrice found it for steps of 1 Gyr, also some initial parameters were different). Both models used the evolutionary tracks of stars of various composition (metallicity) and age, and the star formation rate by Salpeter (1955). I accepted a low-mass limit of star formation, Mo 0.03 Msun, whereas Beatrice used a much lower mass limit to get higher mass-to-luminosity ratio for elliptical galaxies. My model... 

When this is done, some parametric form for the mass spectrum has to be assumed. The initial approximation, that of a power law, is referred to as Salpeter mass function, following Salpeter. This approximation, of course, cannot apply over the entire range of possible masses, since the lower masses produce divergence in the total population. It is usual to specify three parameters the upper and lower mass cutoffs and the exponent. While not useful in a fundamental way for explicating the origin of the mass spectrum, it is a convenient parametrization for models of star formation and the populations of external galaxies. 

Determination of the mass spectra of the stars in their initial state is crucial in understanding the star formation process. The observational approach for such a study is first to define the presmt day luminosity function firom the embedded sources, i.e. the candidates of the young stellar objects, in the molecular clouds. Thanhs to the progress of the infrared array camera, large scale mapping in the infrared is now conceivable. 

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