One of the most important improvements of the new models compared to the [Allard & Hauschildt(1995)Allard and Hauschildt] models (hereafter, AH95) is that new molecular line data have recently become available. The first improvement of our molecular line list is the addition of the HITRAN92 [Rothman et al.(1992)Rothman, Gamache, Tipping, Rinsland, Smith, Chris Benner, Malathy Devi, Flaub, Camy-Peyret, Perrin, Goldman, Massie, Brown, and Toth] database, first incorporated in our ``Extended'' model grid (Allard & Hauschildt, unpublished). For the molecules with lines available from other sources (e.g., water vapor and CO) we prefer to use other sources in model calculations because the HITRAN92 lists are fairly incomplete (they have been prepared for the conditions of the Earth atmosphere) although the line data are of very high quality. The OH molecule is an exception because the vibrational bands present in the HITRAN92 database are not present in the Kurucz CD 15 [Kurucz(1993b)Kurucz] dataset listing electronic transitions of OH. Therefore, we use both HITRAN92 and CD 15 OH line data simultaneously in the model calculations.
We have replaced the vibration-rotation lines of CO [Kurucz(1993b)Kurucz] used in AH95 with the more recent CO line list calculated by Goorvich [Goorvitch & Chackerian(1994a)Goorvitch and Chackerian,Goorvitch & Chackerian(1994b)Goorvitch and Chackerian]. This list is more accurate than the old CO line data. However, we have kept the electronic CO transitions available on Kurucz's CD 15. Similarly, we have replaced the Kurucz CN line list (CD 15) with a more recent calculation by Jørgensen [Jørgensen & Larsson(1990)Jørgensen and Larsson]. CN lines are comparatively weak in most of the models presented here. In addition, we have added line lists for YO [Littleton(1987)Littleton] and ZrO [Littleton & Davis(1985)Littleton and Davis] as well as H3+ partition functions and lines [Neale & Tennyson(1995)Neale and Tennyson,Neale, Miller, & Tennyson(1996)Neale, Miller, and Tennyson].
Another improvement over the ``Base'' model grid [Allard & Hauschildt(1995)Allard and Hauschildt] is the replacement of the straight mean TiO opacities with the list of TiO lines computed by Jørgensen [Jørgensen(1994)Jørgensen]. This list includes a total of more than 12 million lines for the most important band systems of 5 TiO isotopes. On average, the opacity of TiO lines is about a factor of 2 smaller than the old straight mean opacities. For any given effective temperature this will result in weaker TiO features when compared to the old straight means.
The absorption coefficient of water has been a long standing problem for modelers of cool dwarf atmospheres. In our Base model grid we have used the straight means of the [Ludwig(1971)Ludwig] water opacity tables. These are known to overestimate the water opacity significantly at higher (and for M dwarfs more important) temperatures [Schryber, Miller, & Tennyson(1995)Schryber, Miller, and Tennyson] but they are accurate at lower temperatures. The currently available list of water lines of Miller & Tennyson [, MT-H2O,]h2olet is incomplete and therefore some water opacity will be missing in models with . Comparison with high-resolution observations in spectral regions where the MT-H2O list should be complete show that its accuracy is very high [Jones et al.(1995)Jones, Longmore, Allard, Hauschildt, Miller, and Tennyson]. In the models presented here, we have used the MT-H2O list. Recently, a new water vapor line list with more than 300 million lines was published by [Partridge & Schwenke(1997)Partridge and Schwenke]. A new and improved version of the Miller & Tennyson list is currently being calculated (Viti et al, in preparation). In a subsequent publication we will show the differences in the model structure and the synthetic spectra for low effective temperature models calculated using different water vapor line lists.