Low Temperature Rosseland Opacities

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Session 111 -- Stellar Interiors
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[111.07] Low Temperature Rosseland Opacities

R. A. Bell (U. Maryland)

The tables of Rosseland mean absorption coefficients, Kappa(R), by Rogers & Iglesias (ApJS 79, 507, 1992, referred to as OPAL) and Seaton et al. (1994, MNRAS, in press, referred to as OP) are in good general agreement. However, OPAL values are presented only for temperatures greater than 6000K. While OP values are available for temperatures as low as 3162 K, they must be regarded as uncertain. The reason for this is that the calculations of the line absorption include only atomic lines whereas molecular lines are expected to be important at temperatures of 6000K and lower. It is necessary to continue the Kappa(R) calculations to lower temperatures, if they are to be used for modelling cool stars. In addition to including molecular lines, such low temperature calculations also have the advantage that they can be made using line lists which have been checked, at least in part, by comparison of observed and calculated stellar spectra. The line lists used in the construction of the OPAL and OP Rosseland means have not been subjected to observational tests. In fact, it would be difficult to do so. Synthetic spectrum programs are readily adapted to calculations of Kappa(R), since an essential part of their work is to calculate absorption coefficients as a function of wavelength and optical depth. The SSG program was consequently used to calculate Kappa(R) for a variety of temperatures and R values (R=rho/T**3). Overlap with temperatures used by OPAL and OP allowed a comparison of results. It also helped with inclusion of the results in stellar interior programs, since a smooth transition could be made from one table to another. The molecular lines included in the calculations are H2O, CO, SiO, TiO, CN, CH, C2, NH, OH, MgH, SiH and C2. The H2O data used is that of Plez, Brett & Nordlund (A&A 256, 611, 1992) while sources of the remaining data are given by Bell & Gustafsson (MNRAS, 236, 653, 1989). The atomic data used for wavelengths < 3000 A and > 10000 A is that of Kurucz & Peytremann (SAO Spec Rept 362, 1975) and Kurucz (1991) while that used between 3000 and 10000 A is the Bell New list - see Bell & Paltoglou (MNRAS, 1994).

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