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Project Progress

The habitability of terrestrial planets orbiting low-luminosity yet highly-active M stars, still hinges largely on the fact that such planets will be irradiated by intense and frequent UV flares. The severity of the resulting fluctuating atmospheric composition and UV levels at the planetary surface is usually assumed to be large, but has never been examined. We are taking a few of the apparently severe problems with the radiation and stellar wind environment of M stars (and, to a lesser extent, the young Sun) and examining them quantitatively. These include 1. The erosion or creation of an ozone layer, 2. Whether an atmosphere will be constantly driven far from its time-independent state; and 3. The magnitude of the UV variations at the surface.

The effects of a single flare on an orbiting planet

To address these issues we have used data on a large stellar flare output by AD Leonis to create time-resolved stellar spectra during that flare to use as input of a time dependent 1-D coupled radiative—convective/photochemical model. This model was then used to simulate the effect of that flare on an Earth-like planet in the habitable zone of AD Leonis. The planetary atmospheric and surface consequences of this single flare are being prepared for publication.

Synthetic flare time series’

In preparation for full-scale photochemistry calculations with realistic flare flux variations, we have prepared a suite of synthetic time series’ giving the flux of radiation from a flaring M star, using statistical sampling of large numbers of flares of various energies, durations, rise/fall ratios, distributed randomly (a Poisson process). We match the exponent of a power-law relation between flare rate and flare energy, and the mean time between flares of a given amplitude, to available data for a number of M flare stars that are candidates for future spectrally resolved detection missions. The observed characteristics of the most frequently-flaring M stars led to the construction of time series’ of flare amplitudes lasting many days with a resolution of about a fraction of a second, set by considerations of characteristic chemical timescales with much larger photodissociation rates than ever encountered on Earth.

An example of a set of four independent realizations of time series’ whose amplitude-frequency statistical distribution was set to match EUVE observations of the star AD Leo is shown in Fig. 2.

A library of data files containing a large number of these time series’ will be made available to the general community, supplemented with spectral time variation data generated from a not-yet-set prescription. These should be useful to researchers studying the effects of energetic particle events or EUVE and X-ray flares on atmospheric loss, or other groups studying irradiation of atmospheres.

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