FLUCTUATIONS AND FLARES IN THE ULTRAVIOLET LINE EMISSION OF COOL STARS: IMPLICATIONS FOR EXOPLANET TRANSIT OBSERVATIONS
- Center for Astrophysics and Space Astronomy, Boulder, CO 80303 (United States)
- NASA Nancy Grace Roman Fellow. (United States)
Variations in stellar flux can potentially overwhelm the photometric signal of a transiting planet. Such variability has not previously been well-characterized in the ultraviolet lines used to probe the inflated atmospheres surrounding hot Jupiters. Therefore, we surveyed 38 F-M stars for intensity variations in four narrow spectroscopic bands: two enclosing strong lines from species known to inhabit hot Jupiter atmospheres, C II λλ1334, 1335 and Si III λ1206; one enclosing Si IV λλ1393, 1402; and 36.5 Å of interspersed continuum. For each star/band combination, we generated 60 s cadence lightcurves from archival Hubble Space Telescope Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph time-tagged photon data. Within these lightcurves, we characterized flares and stochastic fluctuations as separate forms of variability. Flares: we used a cross-correlation approach to detect 116 flares. These events occur in the time-series an average of once per 2.5 hr, over 50% last 4 minutes or less, and most produce the strongest response in Si IV. If the flare occurred during a transit measurement integrated for 60 minutes, 90/116 would destroy the signal of an Earth, 27/116 Neptune, and 7/116 Jupiter, with the upward bias in flux ranging from 1% to 109% of quiescent levels. Fluctuations: photon noise and underlying stellar fluctuations produce scatter in the quiescent data. We model the stellar fluctuations as Gaussian white noise with standard deviation σ {sub x}. Maximum likelihood values of σ {sub x} range from 1% to 41% for 60 s measurements. These values suggest that many cool stars will only permit a transit detection to high confidence in ultraviolet resonance lines if the radius of the occulting disk is ≳1 R{sub J} . However, for some M dwarfs this limit can be as low as several R {sub ⊕}.
- OSTI ID:
- 22340248
- Journal Information:
- Astrophysical Journal, Supplement Series, Vol. 211, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0067-0049
- Country of Publication:
- United States
- Language:
- English
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