Kitsionas, S., Whitworth, Anthony Peter, Klessen, R. S. and Jappsen, A.K. 2006. The dependence of the IMF on the densitytemperature relation of prestellar gas. Proceedings of the International Astronomical Union 2 (S237) , p. 435. 10.1017/S1743921307002232 

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Abstract
It has been recently shown by several authors that fragmentation of prestellar gas (i.e. at densities from 104 to 1010 particles cm−3 and temperatures of order 1030 K) depends on the gas thermodynamics much more than it was anticipated in earlier studies, in which only an isothermal behaviour has been assumed for the gas. Here we review the results of a number of numerical hydrodynamic simulations (e.g. Li et al. 2003, Jappsen et al. 2005, Bonnell et al. 2006) in which departure from isothermality has been attempted by employing a polytropic equation of state (eos) with exponent different from unity. In particular, in these studies it has been shown that the dominant fragmentation scale of prestellar gas, and hence the peak of the initial mass function (IMF), depends on a polytropic exponent that changes value, from below to above unity, at a critical density (Larson 2005). Furthermore, this piecewise polytropic eos depends on the gas metallicity and fundamental constants. Therefore, the peak of the IMF depends, in turn, also on the gas metallicity and fundamental constants rather than on initial conditions, as it has been previously suggested (e.g. Larson 1995). Hence, we are for the first time in a position to infer theoretically the notion of a universal IMF (at least for its lowmass end). We also present two test cases in which a nonisothermal eos has been used in the context of smoothed particle hydrodynamic (SPH) numerical simulations. In the first case star formation is triggered by means of lowmass clump collisions. These calculations have shown that clump collisions can be a relatively efficient mechanism for the formation of solarmass protostars and their lowermass companions (efficiency greater or of order 2025%; Kitsionas & Whitworth 2006). We have also found that in such collisions protostars form mainly by fragmentation of dense filaments along which it is likely that pairs of protostars capture each other in close binaries surrounded by circumbinary discs. In the second case, the use of a polytropic eos with a varying exponent appropriate for the metallicity of starburst regions (Spaans & Silk 2000, 2005) is shown to be sufficient to obtain a top heavy IMF similar to that observed e.g. in the Galactic centre (Klessen, Spaans & Jappsen 2006). These are preliminary results in the direction of revisiting earlier isothermal calculations that were resolving all densities up to the opacity limit for fragmentation (e.g. Bate et al. 2002ab, 2003), this time also taking into account the thermal properties of the gas in the density range between 104 and 1010 particles cm−3. The next step would be to include selfconsistent radiation transport in the calculations, the first attempts for which are already in the making (e.g. Whitehouse & Bate 2004).
Item Type:  Article 

Date Type:  Publication 
Status:  Published 
Schools:  Physics and Astronomy 
Subjects:  Q Science > QB Astronomy 
Uncontrolled Keywords:  equation of state; hydrodynamics; method: numerical; stars: formation; stars: mass function 
Additional Information:  Pdf uploaded in accordance with publisher's policy at http://www.sherpa.ac.uk/romeo/issn/17439213/ (accessed 21/02/2014). 
Publisher:  Cambridge University Press 
ISSN:  17439213 
Date of First Compliant Deposit:  30 March 2016 
Last Modified:  04 Jun 2017 04:54 
URI:  http://orca.cf.ac.uk/id/eprint/46342 
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