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Project Progress

M. Cohen has developed complete far-UV to far-IR spectra of a variety of potential host stars from solar analogs to M-dwarfs in their quiescent phase (non-flaring). Optical and infrared spectra are represented using theoretical synthetic photospheres, augmented by, or compared directly with, observed spectra. The UV portions are from empirical low-resolution spectra taken by the International Ultraviolet Explorer that have been corrected and recalibrated for all known errors and artifacts. These have been used as input to codes that couple planetary atmospheric photochemistry with radiative/convective modeling to derive biosignatures run by Kasting and Segura of VPL. The assembly of further spectra of M-dwarf stars in active and quiescent phases is planned.

John Scalo has focused on stellar activity, notably in low-mass cool dwarf stars, as evidenced through their X-ray, FUV, and optical chromospheric characteristics. The current lack of empirical FUV spectra for M-dwarfs in quiescence or at times of only mild activity may be partially addressed by a new data set (Fuhrmeister, B., Schmitt, J. H. M. M., & Hauschildt, P. H. 2005. “PHOENIX model chromospheres of mid-to late-type dwarfs. Astron. Astrophys., in press; astro-ph/0505375). This combines empirical high-resolution optical spectra of 23 cool stars (spectra of 5 of these were recently published) with theoretical models of their chromospheric spectra. If we are able to secure these modeled spectra they can be directly compared with IUE spectra for stars in common, or used as direct substitutes in stars that lack IUE and other FUV spectra.

The results of this work, full wavelength range, calibrated stellar spectra for different spectral types are used by the VPL as input to the planetary models. They are also available to the scientific community as downloadable ascii spectra on the VPL website, at:
http://vpl.ipac.caltech.edu/spectra/stellar/frontpage.htm

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Figure 1. a) Spectra of stars of different spectral types at the top of the atmosphere of a planet on the habitable zone of each star. The F2V star is sigma Bootis located at 12 pc from Earth, its spectrum is shown at 1.68 AU from the star. Solar spectrum from Torr and Torr (J. Geophys. Res., 90, 6675, 985), Mount and Rottman (J. Geophys. Res., 88, 403, 1983), and Atmospheric Ozone 1985 Vol. 1 World Meteorogical Organization. Global Ozone Research and Monitoring Project. Report No. 16. he K2V star is epsilon Eridani 3.2 pc distant from Earth. The planet for this star is located at 0.53 AU form the star. AD Leonis and GJ 643 are chromospherically active M dwarfs with spectral ypes of M4.5V and M3.5V, respectively. The non-active star is a synthetic spectrum of an M5 star taken from the BaSeL website (http://www.astro.mat.uc.pt/BaSeL/). The planets around this stars are located at: 0.16 AU from AD Leo, 0.07 AU from GJ 643, and 0.07 AU from the non-active star. (Details on Segura et al. Astrobiology 3, 689, 2003 and Segura et al. Astrobiology, 2005 submitted)

b) Ultraviolet radiation from the stars shown in anel a). The spectra of the chromospherically active stars, AD Leo and GJ 643, correspond to the
minimun observed. (Except for the Sun all data as gathered by Martin Cohen at UC Berkeley)

Examples of full-wavelength stellar spectra compiled by this team for F, G, K and M stars (Plot provided by A. Segura)