10.9 Bright Object Protection for M Dwarfs

Observations of M dwarfs with COS require careful planning to ensure the safety and longevity of the instrument. These stars are subject to irregular and unpredictable flaring events, which may increase the FUV and NUV light by possibly large factors. These flaring outbursts are thought to be caused by the rearrangement of magnetic fields in the outer stellar atmosphere through magnetic reconnection processes, which results in the liberation of potentially large amounts of energy. While flares occur on almost all stars in the cool half of the HR diagram, historically M dwarfs have exhibited extreme enhancements, as measured by the peak luminosity enhancement. U-band increases in the largest flares can be around 6 magnitudes, but there is evidence for more extreme behavior in the NUV and the FUV.

Observers in recent cycles have demonstrated additional interest in obtaining UV spectra of nearby M dwarfs to study exoplanets and the influence of the short wavelength radiation from the host star on close-in exoplanets. In searching for habitable zone planets around M dwarfs, for example, the impact of stellar flares upon its near environment is a critical component in our understanding, as these events represent the most powerful instance of energy release for non-degenerate, main sequence stars. Starting in Cycle 26, a clear set of procedures for determining the safety of M dwarfs were laid out (COS ISR 2017-01).

Clearing M dwarfs with the above procedures applies to science targets and their nearby companions, as well as to objects populating the field. For every observing program, the PI is therefore responsible for working with members of the COS Team in order to clear M dwarfs. In order to calculate a typical flare rate for such objects, a model SED alongside estimated line emission values are requested (see COS ISR 2017-01 for specific guidance for evaluating the suitability of M dwarfs for observation). The probability of flaring for a given M dwarf must be ≤ 10^-4 per hour, so observers who are observing targets for a duration longer than one hour should consult with their COS CS for further guidance.

In case the M dwarf does not clear the limits stated in the COS ISR 2017-01, the PI should consider either changing to an equivalent STIS mode as a first alternative, or in more drastic cases changing to a medium- or high-resolution grating of either COS or STIS. We recommend working together with the program CS to determine which alternative mode would be the best path forward. Instrument and mode changes usually require a Telescope Time Review Board (TTRB) request if they have not been stated in the Phase 1 file.

Observers should be aware that the current version of the ETC may give one or more bright-object warnings when running a model for the flaring state. Ultimately, as long as the count rates stay below the limits stated in Table 6 of the COS ISR 2017-01, the target will be cleared for observation. Note that this is only valid for M dwarfs. Other types of targets remain subject to the bright object limits stated in Table 10.1 of the COS IHB.

Also note that M dwarfs with unknown properties will be assumed to be the most active for the purposes of flare probability calculations.