[Paper Review] The Variable Wavelength Dependence of the Dipping event of KIC 8462852

Eva H. L. Bodman, Jason T. Wright+2 more|arXiv (Cornell University)|Jun 22, 2018

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This study analyzes the variable wavelength dependence of the 2017 dimming events (the 'Elsie dip family') in KIC 8462852 using multi-band photometry from the Las Cumbres Observatory. It finds that the wavelength dependence varies over time, indicating heterogeneous dust in size and/or composition, with occulting dust grains likely smaller than 0.5 μm, consistent with newly formed, optically thin dust, while secular dimming shows a transition toward gray extinction over time.

ABSTRACT

First observed with the Kepler mission, KIC 8462852 undergoes unexplained dimming events, "dips," on the timescale of days which were again observed from the ground from May to December 2017. Monitored with multi-band photometry by the Los Cumbres Observatory, all four dips of the "Elsie dip family" display clear wavelength dependence. We measure how the wavelength dependence changes over the whole dimming event, including the dimming between the dips and the brightening event (the `blip') which occurs after the dips. We find that a single wavelength dependence does not fit the entire light curve and the dimming occurring between the dips is non-gray and varies in time. Because of the non-gray dimming between the dips, we measure the wavelength dependence of the dips separately and without the extra depth from this dimming. Such measurements yield a different estimate of the wavelength dependence the wavelength dependence of the dips but remains consistent with the previous measurement except for Elsie (the first dip), which is surrounded by dimming with strong wavelength dependence. We find the range of the wavelength dependence variation of the entire 2017 light curve is consistent with optically-thin dust with an average radius of $r<1\,μ$m and the dust causing just the dips being $r<0.5\,μ$m. Since the dependence is time-dependent, the dust occulting the star must be heterogeneous in size, composition, or both and the distributions of these properties along the line of sight must change over time.

Motivation & Objective

• To investigate how the wavelength dependence of the 2017 dimming events in KIC 8462852 changes over time.
• To disentangle the contributions of short-term dips and long-term secular dimming to the observed color variations.
• To measure the wavelength dependence of individual dips (Elsie, Celeste, Skara Brae, Angkor) after removing the influence of secular dimming.
• To estimate the grain size of the occulting dust based on the measured wavelength dependence, assuming various compositions.
• To assess whether the observed dimming is consistent with circumstellar dust or other mechanisms such as interstellar extinction.

Proposed method

• Multi-band photometry was collected using the Las Cumbres Observatory global network across multiple optical bands (B, r′, i′) during the 2017 dip events.
• The light curve was decomposed into individual dips and secular dimming components to isolate the wavelength dependence of each.
• Depth ratios between different bands (e.g., B/i′, r′/i′) were calculated to quantify the wavelength dependence of each dip, both with and without secular dimming.
• The wavelength dependence was modeled assuming optically thin dust extinction, and grain sizes were estimated for various compositions (e.g., carbon, silicates, ice).
• Statistical uncertainties were propagated through the fitting process to assess confidence in grain size estimates.
• The results were compared to prior measurements from Boyajian et al. (2018) and Deeg et al. (2018), particularly for the Elsie dip.

Experimental results

Research questions

• RQ1 How does the wavelength dependence of the dips in KIC 8462852 vary over the course of the 2017 dimming events?
• RQ2 To what extent does the secular dimming influence the measured wavelength dependence of the individual dips?
• RQ3 What grain size and composition are consistent with the observed wavelength dependence of the dips and secular dimming?
• RQ4 Is the wavelength dependence of the dips consistent with optically thin dust, and does it support a circumstellar origin?
• RQ5 How do the results compare to previous measurements when secular dimming is excluded from the analysis?

Key findings

• The wavelength dependence of the 2017 dip family is time-variable, indicating changing dust properties along the line of sight or over time.
• The secular dimming exhibits a transition from strong wavelength dependence (non-gray) near the Elsie dip to more gray behavior over time, suggesting a shift toward larger grain sizes.
• After removing secular dimming, the depth ratios for the dips (e.g., B/i′ = 1.665, r′/i′ = 1.292 for Elsie) are more consistent with prior measurements and indicate sub-micron dust.
• The occulting dust causing the dips is consistent with grains smaller than 0.5 μm, while the secular dimming may involve larger grains, though with large uncertainties.
• The dust responsible for the dips is likely newly created, as sub-micron grains would otherwise be blown out by radiation pressure from the F-type star.
• The post-Angkor secular dimming is consistent with gray extinction (implying larger grains), but uncertainties are large, leaving open the possibility of a different dust population.