arXiv subjects

Evan M. Cook

Publications and source records attributed to Evan M. Cook.

3 recordsLinked to original sources

arXiv2022

A Bayesian Analysis of Physical Parameters for 783 Kepler (Near-)Contact Binaries: Extreme-Mass-Ratio Systems and a New Mass Ratio versus Period Lower Limit

Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform understanding of binary evolutionary pathways and end products. We use light curves and new optical spectroscopy to conduct a pilot study of ten (near-)contact systems in the long-period ($P$>0.5 d) tail of close binaries in the Kepler field. We use PHOEBE light curve models to compute Bayesian probabilities on five principal system parameters. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the light curves. Pilot study systems have extreme mass ratios $q$<0.32. Most are triples. Analysis of the unbiased sample of 783 0.15 d<$P$<2 d (near-)contact binaries results in 178 probable contact systems, 114 probable detached systems, and 491 ambiguous systems for which we report best-fitting and 16th/50th/84th percentile parameters. Contact systems are rare at periods $P$>0.5 d, as are systems with $q$>0.8. There exists an empirical mass ratio lower limit $q_{min}$($P$)$\approx$0.05--0.15 below which contact systems are absent, supporting a new set of theoretical predictions obtained by modeling the evolution of contact systems under the constraints of mass and angular momentum conservation. Pre-merger systems should lie at long periods and near this mass ratio lower limit, which rises from $q$=0.044 for $P$=0.74 d to $q$=0.15 at $P$=2.0 d. These findings support a scenario whereby nuclear evolution of the primary (more massive) star drives mass transfer to the primary, thus moving systems toward extreme $q$ and larger $P$ until the onset of the Darwin instability at $q_{min}$ precipitates a merger.

astro-ph.SR↗

arXiv2018

The First Post-Kepler Brightness Dips of KIC 8462852

We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process.

astro-ph.SR↗

arXiv2017

Prediction of a red nova outburst in KIC 9832227

We present the first identification of a candidate precursor for an imminent red nova. Our prediction is based on the example of the precursor to the red nova V1309 Sco, which was retrospectively found to be a contact binary with an exponentially decreasing period. We explore the use of this distinctive timing signature to identify precursors, developing the observational and analysis steps needed. We estimate that our Galaxy has roughly 1-10 observable precursors. Specifically, we lay out the observational case for KIC 9832227, which we identified as a tentative candidate two years ago (Molnar et al. 2015, AAS Meeting Abstracts 415.05). Orbital timing over the past two years has followed the tentative exponential fit. As of late 2015, the period time derivative went beyond the range found in other systems (dP/dt < 1x10^{-8}), a necessary criterion for a serious candidate. We estimate time of merger is the year 2022.2(7). Double absorption line spectra confirm directly the 0.458 d light curve period is a contact binary system and yield a mass ratio m_B/m_A = 0.228(3). Closer analysis of the Kepler timing data shows evidence of a component C with orbital period P_C = 590(8) days and m_C x sin i_C = 0.11 solar masses. An alternative interpretation of the long term timing trend, light travel time delay due to orbit around a distant component D, is ruled out by the spectroscopic data for any nondegenerate star. Additional measurements are needed to test further the merging hypothesis and to utilize fully this fortuitous opportunity.

astro-ph.SR↗