We have modelled the circumstellar envelope of o Ceti ( Mira ) using new observational constraints .
These are obtained from photospheric light scattered in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6 \mbox { $ \mu$m } : absolute fluxes , the radial dependence of the scattered intensity , and two line ratios .
Further observational constraints are provided by ISO observations of far-IR emission lines from highly excited rotational states of the ground vibrational state of CO , and radio observations of lines from rotational levels of low excitation of CO. A code based on the Monte-Carlo technique is used to model the circumstellar line emission .

The vibrational-rotational lines are sensitive to the radiation field , whereas the pure rotational lines , such as the rotational lines of low excitation measured at radio wavelengths and the rotational lines from highly excited states observed with ISO , are usually more sensitive to the temperature structure .
These rotational lines have been the prime probe in most earlier investigations .

We find that it is possible to model the radio and ISO fluxes , as well as the highly asymmetric radio-line profiles , reasonably well with a spherically symmetric and smooth stellar wind model .
However , it is not possible to reproduce the observed NIR line fluxes consistently with a ‘ standard model ’ of the stellar wind .
This is probably due to incorrectly specified conditions of the inner regions of the wind model , since the stellar flux needs to be larger than what is obtained from the standard model at the point of scattering , i.e. , the intermediate regions at approximately 100 - 400 R _ { * } ( 2 \arcsec - 7 \arcsec ) away from the star .
Thus , the optical depth in the vibrational-rotational lines from the star to the point of scattering has to be decreased .
This can be accomplished in several ways .
For instance , the gas close to the star ( within approximately 2 \arcsec ) could be in such a form that light is able to pass through , either due to the medium being clumpy or by the matter being in radial structures ( which , further out , developes into more smooth or shell-like structures ) .
Further observations of the gas in the stellar wind close to Mira are required to resolve this problem .

The model circumstellar envelope , which reproduces the observables reasonably well , has a mass-loss rate of 2.5 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } , and a turbulent velocity of 1.5 km s ^ { -1 } , given a terminal expansion velocity of the wind of 2.5 km s ^ { -1 } .