Red giant asteroseismic binaries in the Kepler field

Abstract

Systems in which two oscillating stars are observed in the same light curve, so-called asteroseismic binaries (ABs), arise from either chance alignments or gravitationally bound stars. In the latter case, ABs offer a novel way to find binary systems and combine asteroseismology and orbital dynamics to determine precise stellar parameters for both stars. Such systems provide valuable tests to stellar models and scaling relations. While population synthesis studies predict approximately 200 ABs in the Kepler long-cadence data, only a few have been detected to date. In this work, we aim to (1) expand the sample of ABs in Kepler data, (2) estimate global asteroseismic parameters for both stars in each AB, and (3) assess whether these pairs are gravitationally bound. We analysed 40 well-resolved ABs identified in Kepler long-cadence data, and matched these solar-like oscillators with Gaia DR3 sources using spectroscopic estimates of $ν_{\rm max}$. To assess whether each pair is gravitationally bound, we checked their projected separation and parallax consistency, and compared observed total orbital velocity differences from astrometry with theoretical predictions from Keplerian orbits. We find that most ABs appear to be chance alignments. However, two systems, KIC 6501237 and KIC 10094545, show orbital velocities, seismic masses, and evolutionary stages consistent with a wide binary configuration, with probabilities of ~50% and ~25%, respectively. Furthermore, eleven ABs are likely spatially unresolved binaries based on Gaia multiplicity indicators. Our findings suggest that most seismically resolved ABs in the Kepler field are not gravitationally bound, in contrast to earlier population synthesis predictions. Remarkably, the two wide binary candidates identified here are promising benchmarks for asteroseismic calibration. Spectroscopic follow-up is necessary to confirm their binary nature.

Figures (10)

• Figure 1: Image of the asteroseismic binary KIC 8004637 from the Two Micron All Sky Survey in the JHK$_{\text{s}}$ bands (top panel) and its Fourier spectrum from Kepler data (bottom panel). Given the small spatial separation of these stars in the sky, their oscillations are detected in the same light curve.
• Figure 2: Power density spectrum and peak-bagging results of KIC 6501237. In the top panel, the blue and green dotted lines indicate $\nu_{\text{max, A}}$ and $\nu_{\text{max, B}}$, respectively. The four granulation components are represented by dashed grey lines, while the white noise is shown as a dot-dashed line. The total background model is depicted as a solid red line. The lower panels show the fitted oscillation modes of both solar-like oscillators in orange. Radial, quadrupole, and octupole modes are marked by red circles, green squares and yellow hexagons.
• Figure 3: Gaia colour-magnitude diagram of the AB-member candidates. Grey points represent Kepler stars from GodoyRivera25, with extinction- and reddening-corrected photometry, plotted using the black axes. The light blue axes are used to plot the Gaia DR3 photometry of our sample that lacks $E(BP-RP)$ and $A_{G}$ corrections. The light blue line depicts the borders of the giant-stars region as defined by GodoyRivera25, adjusted to account for the missing photometric corrections. AB-member candidates classified as giants according to this criterion are shown in cyan, while those excluded by the criteria are shown as blue circles.
• Figure 4: Comparison of asteroseismic and spectroscopic $\nu_{\rm max}$ estimates of the Kepler-Gaia matched sources described in Sect. \ref{['subsec:source_identification']}. Stars with atmospheric parameters from APOGEE, GSP-Spec, and GSP-Phot are shown in green, purple, and pink, respectively. Circles correspond to ABs where both solar-like oscillators of an AB were matched to a Gaia source. Squares depict ABs where only one of the two oscillators was matched. The bottom panel displays the difference in units of radial order, i.e. normalised by $\Delta\nu$. Grey dotted lines represent differences of $\pm1\Delta\nu$.
• Figure 5: Summary of binarity criteria. Top row: AB systems with both components successfully matched to Gaia sources (Table \ref{['table:Full_identification_summary']}). Bottom row: all possible pairs from AB-member candidate sets, excluding those in the top row. Left panels illustrate angular separation as a function of parallax, with limits from Eq. \ref{['eq:max_s']} ($s < 1$ pc) as grey hatched region. Centre panels show parallax consistency; black dashed lines mark the criteria from Eq. \ref{['eq:parallax_diff']}. Right panels show the total orbital velocity difference versus projected separation (log-log scale), with theoretical $\Delta V_{\text{tot}}$ values from Eq. \ref{['eq:total_mass']} for M$_{\rm tot} = 3$, 4, and 5 M$_\odot$. Crosses, circles and stars represent pairs that only satisfy one, two, or three binary conditions, respectively. Well-known bona fide wide binaries 16 Cygni, HD 80806/07, HIP 99727/29, and HAT-P-4, are shown as pink, orange, green, and purple diamonds, respectively. Genuine wide binary candidates are expected to lie within the white area of each plot. KIC 6501237 and KIC 10094545, highlighted as blue and orange stars, respectively, are identified as likely genuine wide binaries.