Kepler Mission Stellar and Instrument Noise Properties (1107.5207v1)

Abstract: Kepler Mission results are rapidly contributing to fundamentally new discoveries in both the exoplanet and asteroseismology fields. The data returned from Kepler are unique in terms of the number of stars observed, precision of photometry for time series observations, and the temporal extent of high duty cycle observations. As the first mission to provide extensive time series measurements on thousands of stars over months to years at a level hitherto possible only for the Sun, the results from Kepler will vastly increase our knowledge of stellar variability for quiet solar-type stars. Here we report on the stellar noise inferred on the timescale of a few hours of most interest for detection of exoplanets via transits. By design the data from moderately bright Kepler stars are expected to have roughly comparable levels of noise intrinsic to the stars and arising from a combination of fundamental limitations such as Poisson statistics and any instrument noise. The noise levels attained by Kepler on-orbit exceed by some 50% the target levels for solar-type, quiet stars. We provide a decomposition of observed noise for an ensemble of 12th magnitude stars arising from fundamental terms (Poisson and readout noise), added noise due to the instrument and that intrinsic to the stars. The largest factor in the modestly higher than anticipated noise follows from intrinsic stellar noise. We show that using stellar parameters from galactic stellar synthesis models, and projections to stellar rotation, activity and hence noise levels reproduces the primary intrinsic stellar noise features.

• The paper decomposes Kepler data noise into intrinsic stellar variability (19.5 ppm), instrumental effects, and time-variant contributions affecting transit signals.
• The analysis shows that sharper point spread functions enhance photometric quality by lowering channel-dependent detector noise.
• The study highlights that increased stellar noise necessitates refined calibration and extended observations to improve exoplanet detection sensitivity.

Overview of "Kepler Mission Stellar and Instrument Noise Properties"

The paper "Kepler Mission Stellar and Instrument Noise Properties" provides a comprehensive analysis of the noise characteristics associated with the observations conducted by the Kepler Mission. The mission, designed primarily for discovering exoplanets and conducting asteroseismology, offers unique data sets characterized by high precision and long-duration time series photometric observations. This analysis focuses on understanding the contributions of different noise components which impact the effectiveness of detecting exoplanets through their transit signals.

Key Findings

1. Noise Decomposition:
   • The authors decompose the observed noise into four primary components: intrinsic stellar noise, Poisson plus readout noise, channel-dependent detector noise, and a time-variant noise related to observational quarters. Intrinsic stellar noise, at 19.5 ppm for solar-type stars at 12th magnitude, emerges as the dominant contributor, exceeding the pre-mission budget of 10 ppm by a substantial margin.
2. Instrumental Noise Analysis:
   • The channel-dependent noise, although present, was found to be lower in magnitude relative to the intrinsic stellar noise. The paper reveals that sharper point spread functions (due to better focus) yield superior photometric quality, contrary to typical expectations for undersampled PSFs in space-based photometry.
3. Temporal Variations and Quarters:
   • Systematic noise variations due to specific mission events or operational conditions during each quarter were documented. Quarters 2 and 3 exhibited higher excess noise due to factors like spacecraft safing events and initial calibration errors.
4. Effects of Galactic Latitude:
   • Intrinsic stellar noise showed a variable distribution with galactic latitude, displaying a moderate correlation where noise increased towards lower latitudes. The simulated contamination from known and unknown blended stars accounted for some of these variations, but did not fully explain the observed distribution.

Implications and Future Prospects

• Practical Impact on Exoplanet Detection:
  • The observed higher-than-expected intrinsic stellar variability has significant implications for the mission's goal of detecting Earth-like exoplanets. The increased noise levels necessitate extended observation periods or enhanced data processing techniques to achieve requisite detection sensitivity.
• Theoretical Implications:
  • These findings prompt a re-evaluation of theoretical models of stellar noise, particularly for solar-type stars, which have been found to vary more than anticipated. The results provide a new baseline for future space missions aiming at precise photometric measurements.
• Future Developments in Astrophysics:
  • The paper's advancement in understanding baseline noise properties sets the stage for improved calibration and noise reduction strategies that could enhance the capability of ongoing and future missions.

Simulation and Comparative Analysis

The authors employ synthetic population models using galactic synthesis codes to simulate expected stellar noise characteristics. Although there are discrepancies in precise noise levels, particularly at fainter magnitudes (e.g., Kp = 14.5), the comparison retains a reasonable resemblance, reinforcing the validity of the noise decomposition approach.

Conclusions

The Kepler Mission's ability to decipher subtle stellar variability has enabled substantial progress in both exoplanetary science and stellar astrophysics. While challenges remain due to unexpected noise levels, the analyses presented in this paper provide crucial insights necessary for optimizing future observational strategies and enhancing the interpretation of stellar photometric variability.