The neutrino emission processes play an important role in
different astrophysical scenarios, in particular during the
stellar core collapse and the cooling of a protoneutron star.
Recently, several authors have studied some processes
such as neutrino-electron scattering (Smit et al. 1996
and Cernohorsky J. 1994), neutrino
coherent scattering off nuclei (Leinson 1992),
effects of nucleon spin fluctuations in the weak
interaction rates (Janka et al. 1996) or neutrino reactions
with strange matter (Reddy & Prakash 1997) in order to
obtain the rates to be used in transport calculations.
We will focus on the
thermal pair emission-absorption process
(TP in the next).
In the stellar core collapse
scenario, discrepancies in the efficiency of TP in the heating
of matter outside the neutrinosphere have been noticed
(see, e.g., Janka 1990 and references therein) and in order to
obtain meaningful calculations of the energy deposition rate it is
necessary to carefully consider
the angular dependence of the distribution function.
In some calculations involving neutrino transport, only the
and 
terms of the Legendre expansion of the collision
kernel are included as in Bruenn (1985) and Suzuki (1989).
This approach could suffice where the diffusion
approximation remains valid. The justification of that approach
relies on the fact that the 
and 
order
terms appear multiplying and order terms of the
Legendre expansion of the
neutrino distribution function (I) and they
vanish in the diffusion limit.
However, if a general closure relation is used which is
different to 
, these higher order
terms of I do not vanish, and their contribution becomes
especially important in the semi-transparent region.
In this paper we will also analyze the influence of
the different closure relations that have been considered in recent
years.
This work is organized as follows:
In Sect. 2 we present the Legendre expansion of the TP production
and absorption kernels and we give
explicit expressions for the to order terms.
In Sect. 3 we study how
these new terms affect the sources in the two-moment closure
transport equations. In Sect. 4 we discuss the effects of the new terms
and the influence of the closure relation.