  Molecular line lists are important for modelling absorption and emission
processes in atmospheres of different astronomical objects, such as cool stars
and exoplanets. In order to be applicable for high temperatures, line lists for
molecules like methane must contain billions of transitions, which makes their
direct (line-by-line) application in radiative transfer calculations
impracticable. Here we suggest a new, hybrid line list format to mitigate this
problem, based on the idea of temperature-dependent absorption continuum.
  The line list is partitioned into a large set of relatively weak lines and a
small set of important, stronger lines. The weaker lines are then used either
to construct a temperature-dependent (but pressure-independent) set of
intensity cross sections or are blended into a greatly reduced set of
super-lines. The strong lines are kept in the form of temperature independent
Einstein A coefficients.
  A line list for methane is constructed as a combination of 17 million strong
absorption lines relative to the reference absorption spectra and a background
methane continuum in two temperature-dependent forms, of cross sections and
super-lines. This approach eases the use of large high temperature line lists
significantly as the computationally expensive calculation of pressure
dependent profiles only need to be performed for a relatively small number of
lines. Both the line list and cross sections were generated using a new 34
billion methane line list (34to10), which extends the 10to10 line list to
higher temperatures (up to 2000 K). The new hybrid scheme can be applied to any
large line lists containing billions of transitions. We recommend to use
super-lines generated on a high resolution grid based on resolving power (R =
1,000,000) to model the molecular continuum as a more flexible alternative to
the temperature dependent cross sections.
