[Paper Review] The Occurrence of Non-Pulsating Stars in the gamma Dor and delta Sct Pulsation Instability Regions: Results from Kepler Quarter 14-17 Data

Joyce Ann Guzik, Paul A. Bradley+4 more|arXiv (Cornell University)|Jan 31, 2015

Stellar, planetary, and galactic studies17 references4 citations

TL;DR

This study analyzes Kepler Q14–17 data to investigate why 34 stars in the γ Dor and δ Sct pulsation instability regions show no detectable variability down to 20 ppm. The authors find that inaccurate stellar parameters—particularly an overestimated T_eff and log g in the Kepler Input Catalog—likely explain why these stars appear in the instability strips despite being non-pulsating, suggesting that revised parameters could resolve the discrepancy.

ABSTRACT

In our 2013 Astronomical Review article, we discussed the statistics of variability for 633 faint spectral type A-F stars observed by the Kepler spacecraft during Quarters 6-13. We found six stars that showed no variability with amplitude 20 ppm or greater in the range 0.2 to 24.4 cycles/day, but whose positions in the log g--Teff diagram place them in the delta Sct or gamma Dor pulsation instability regions established from pre-Kepler ground-based observations. Here we present results for 2137 additional stars observed during Quarters 14-17, and find 34 stars that lie within the instability regions. In Paper I, we included a +229 K offset to the Kepler Input Catalog Teff to take into account an average systematic difference between the KIC values and the Teff derived from SDSS color photometry for main-sequence F stars (Pinsonneault et al. 2012). Here we compare the KIC Teff value and the Teff derived from spectroscopy taken by the LAMOST instrument (Molenda-Zakowicz et al. 2013, 2014) for 54 stars common to both samples. We find no trend to support applying the offset, but instead find that a small average temperature decrease relative to the KIC Teff may be more appropriate for the stars in our spectral-type range. If the offset is omitted, only 17 of our 34 `constant' stars fall within the instability regions. For the two `constant' stars also observed by LAMOST, the LAMOST Teff values are cooler than the KIC Teff by several hundred K, and would move these stars out of the instability regions. It is possible that a more accurate determination of their Teff and log g would move some of the other `constant' stars out of the instability regions. However, if average (random) errors in Teff are taken into account, 15 to 52 stars may still persist within the instability regions. Explanations for these `constant' stars, both theoretical and observational, remain to be investigated.

Motivation & Objective

• To identify and characterize non-pulsating stars within the γ Dor and δ Sct pulsation instability regions using high-precision Kepler photometry.
• To investigate the discrepancy between theoretical instability boundaries and observed non-variability in stars that should pulsate.
• To assess the reliability of stellar parameters (T_eff, log g) in the Kepler Input Catalog (KIC) for A-F type main-sequence stars.
• To evaluate whether improved T_eff and log g from LAMOST spectroscopy can resolve the apparent paradox of non-pulsating stars in instability regions.
• To explore the implications for stellar pulsation theory, including potential limitations in current instability strip boundaries or missing pulsation mechanisms.

Proposed method

• Analyzed long-cadence Kepler light curves from Quarters 14–17 for 2137 A-F type main-sequence stars.
• Identified 'constant' stars as those with no significant frequencies (amplitude ≥ 20 ppm) in the 0.2–24.4 cycles/day range.
• Compared KIC stellar parameters (T_eff, log g) with LAMOST spectroscopic T_eff and log g for 54 common stars to assess systematic offsets.
• Applied a +229 K offset to KIC T_eff based on prior work (Pinsonneault et al., 2012), then re-evaluated instability region membership without it.
• Used Fourier analysis to detect potential high-frequency pulsations above the long-cadence Nyquist limit, ruling out aliasing as an explanation.
• Evaluated the impact of parameter uncertainties on instability strip membership using error bars (±290 K for T_eff, 0.3 dex for log g).

Experimental results

Research questions

• RQ1 Why do 34 stars in the γ Dor and δ Sct instability regions show no detectable pulsations in Kepler Q14–17 data?
• RQ2 To what extent do systematic errors in KIC T_eff and log g values explain the apparent mismatch between theoretical instability strips and observed non-variability?
• RQ3 How do LAMOST-derived spectroscopic T_eff and log g values compare to KIC values, and what impact do they have on instability region membership?
• RQ4 What fraction of the 'constant' stars in the instability region remain so after correcting for parameter uncertainties and systematic offsets?
• RQ5 Could alternative pulsation mechanisms or stellar evolution effects (e.g., diffusive settling) explain the lack of pulsations in these stars?

Key findings

• Of 2137 stars observed in Q14–17, 990 (46%) were classified as 'constant'—a lower fraction than the 60% found in the earlier Q6–13 sample due to target selection bias.
• Thirty-four 'constant' stars lie within the γ Dor and δ Sct instability regions as defined by pre-Kepler ground-based observations.
• LAMOST spectroscopy for 54 common stars shows no systematic trend supporting the +229 K offset to KIC T_eff; instead, LAMOST T_eff is slightly cooler on average.
• Without the +229 K offset, only 17 of the 34 'constant' stars remain within the instability regions, suggesting the offset may be unwarranted for this sample.
• For two stars with LAMOST data, the cooler LAMOST T_eff values move them outside the instability strips, indicating that parameter errors likely explain their apparent location.
• Even after accounting for parameter uncertainties (±290 K in T_eff, 0.3 dex in log g), 15 to 52 'constant' stars still lie within the instability regions, indicating a persistent puzzle.