Stellar Spectra

Traditional calculations of stellar "habitable zones" [HZs] have represented the stellar emission spectrum by blackbodies. In so doing, these calculations have probably been adequate to indicate crudely the scale of the HZ around a variety of stars and to conclude that one should seek a star between 0.4 and 1.4 M(sun) in a region of the Galaxy similar to our own. However, to anticipate theoretically what missions like Terrestrial Planet Finder (TPF) are likely to encounter, we must greatly refine our representation of the primary radiative input to any planet, namely the energy distribution of the host star.

The signatures of the biogenic materials are well detected in the infrared and this will be the focus of TPF. To explore parameter space adequately we must examine the influence of stellar effective temperature, gravity, and metallicity on planetary emission and absorption fingerprints, and determine at what spectral resolutions these signatures become definitive indicators of habitable enviroments.

Over the past decade there has been a major effort to establish accurate, detailed, absolute calibration stars in the infrared, spanning the 1-30 µm range, with extensions to 300 µm to support specific space-based instruments. No star radiates as a blackbody and stellar spectral energy distributions (SEDs) are invariably mutilated, even at low spectral resolution, by strong absorption bands.

Recent infrared missions have left a wealth of low and moderate resolution spectra of normal stars. In order to interpret these spectra correctly there has been a corresponding growth in the creation of model grids for a wide diversity of stars, with associated synthetic spectra with resolutions anywhere from hundreds to hundreds of thousands. Within the context of this vigorous modeling, with its focus on IR fidelity, there are now grids of models that that have established reputations for particular types of star.

The time is, therefore, ripe to upgrade the representations of the host stellar radiation fields that will serve as inputs to the theoretical modeling codes for planetary atmospheres. The importance of undergoing this change from blackbodies is not merely to assess the correct integrated luminosity incident on a planetary atmosphere. Much more critical is the question of the mid-ultraviolet input radiation because the intensity of UV radiation incident on a planet profoundly affects the rate of chemical reactions (in early times) and of genetic mutations (once life has emerged). The profile of ozone abundance through planetary atmospheres is likewise crucial to the production of biogenic spectral signatures and to the survival of life forms.

Such considerations lead to the conclusion that, for a rigorous discussion of those factors that influence the detectability of biogenic signatures, one must use state-of-the-art representations of the potential stellar radiation fields of the stellar photospheres (surface layers). However, of greater impact on the UV component of stellar radiation incident on a planet is the presence and intensity of chromospheric emission which, even in average stars like our sun, can grossly exceed the expected photospheric emission. Consequently, our current approach is to blend well-validated theoretical model atmospheric spectra of stellar photospheres with empirical UV spectra of dwarf stars spanning the range at least from F (hotter then the sun), through G (solar), into the cooler K-types. These spectra we obtain from previous (International Ultraviolet Explorer) and present (Hubble Space Telescope Imaging Spectrograph) missions. It is these composite, two-component stellar spectra that will be input to the complex machinery of the VPL.