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.
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COS Instrument Handbook
- Acknowledgments
- Chapter 1: An Introduction to COS
- Chapter 2: Proposal and Program Considerations
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
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Chapter 5: Spectroscopy with COS
- 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- • 5.11 Spectroscopy with Available-but-Unsupported Settings
- • 5.12 FUV Detector Lifetime Positions
- • 5.13 Spectroscopic Use of the Bright Object Aperture
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
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Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
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Chapter 10: Bright-Object Protection
- • 10.1 Introduction
- • 10.2 Screening Limits
- • 10.3 Source V Magnitude Limits
- • 10.4 Tools for Bright-Object Screening
- • 10.5 Policies and Procedures
- • 10.6 On-Orbit Protection Procedures
- • 10.7 Bright Object Protection for Solar System Observations
- • 10.8 SNAP, TOO, and Unpredictable Sources Observations with COS
- • 10.9 Bright Object Protection for M Dwarfs
- Chapter 11: Data Products and Data Reduction
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Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
- • 12.16 Cycle 28 Calibration Program
- • 12.17 Cycle 29 Calibration Program
- • 12.18 Cycle 30 Calibration Program
- • 12.19 Cycle 31 Calibration Program
- Chapter 13: COS Reference Material
- • Glossary