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Eclipsing binary and white dwarf
features associated with K2 target
EPIC251248385
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Citation (published version): Stephanie Yoshida, Samuel Grunblatt, J.J. Hermes, James D
Armstrong, Jeffrey Coughlin, Michael Gully-Santiago. "Eclipsing Binary and White Dwarf Features Associated with K2 Target
EPIC251248385." Research Notes of the AAS, Volume 3, Issue 11, pp. 174 - 174. https://doi.org/10.3847/2515-5172/ab5861
https://hdl.handle.net/2144/40133
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Eclipsing Binary and White Dwarf Features
Associated with
K2
Target EPIC251248385
Stephanie Yoshida , Samuel Grunblatt , J. J. Hermes , James D. Armstrong , Jeffrey Coughlin , and Michael Gully-Santiago
Published 2019 November 19 • © 2019. The American Astronomical Society. All rights reserved. Research Notes of the AAS, Volume 3, Number 11
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Punahou School, 1601 Punahou Street, Honolulu, HI 96822, USA
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr. Honolulu, HI 96822, USA
Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
Institute for Astronomy, University of Hawaii, 34 Ohia Ku St., Makawao, HI 96768, USA SETI Institute, 189 Bernardo Ave, Suite 200 Mountain View, CA 94043, USA
NASA Ames Research Center, Moffett Blvd, Mountain View, CA 94035, USA Samuel Grunblatt https://orcid.org/0000-0003-4976-9980
Jeffrey Coughlin https://orcid.org/0000-0003-1634-9672
Michael Gully-Santiago https://orcid.org/0000-0002-4020-3457 Received 2019 November 13 Accepted 2019 November 18 Published 2019 November 19 1 2 3 4 5 6
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Export citation and abstract
White dwarfs (WDs) are the final stage of Sun-like stars, composed of stellar core material covered by a thin layer of hydrogen and/or helium. 97% of stars evolve into WDs (Farihi et al. 2007), yet planetesimals and minor planets have only been detected orbiting WDs recently (Vanderburg et al. 2015; Manser et al. 2019). WD eclipsing binary systems are valuable for determining ages of stellar companions and evolution effects on stellar binarity (Ashley et al. 2018). Such studies motivate systematic
searches for WDs with transits and/or eclipses using surveys like K2.
A 1000 WD lightcurve survey is predicted to reveal ~2 transiting planet candidates (Loeb & Maoz 2013). To test this, we surveyed 1033 lightcurves requested by the K2 Mission General Observer (GO) programs GO10006 (PI: Kilic), GO10014, GO11040, GO12040, GO12901 (Hermes), GO10019, GO12027 (Burleigh), GO10048, and
GO12037 (Redfield). Visual inspection of lightcurves resulted in identification of one faint candidate with symmetric, periodic eclipses: EPIC251248385. EPIC251248385 (R.A.: 17:46:52.518, decl.: −25:50:21.81, K = 19.324) was targeted as a possible WD via color selection from the VST/OmegaCAM Photometric H-Alpha Survey using the methods of Raddi et al. (2016). Gaia finds magnitude G = 19.5 and parallax
π = 2.7 ± 0.6 mas for this target (Gaia Collaboration et al. 2018), but does not report B or R , and thus EPIC251248385 is not in the Gentile Fusilo et al. (2019) catalog. Orbital period and uncertainty were determined through an iterative lightcurve folding process. Eclipse duration was measured from the best-fit folded lightcurve, assuming eclipses began once no normalized flux measurements reached >100% over a 30 minute span (Howell et al. 2014). The transit period was measured to be
2.612 ± 0.006 days with a 2.5 ± 0.5 hr transit duration. The lightcurve features indicate a stellar eclipsing binary due to their V-shape, and secondary eclipses ~50% of the primary depth. Three 600 s observations taken using the Goodman spectrograph on the SOAR telescope (Clemens et al. 2004) revealed a low signal-to-noise WD
Stephanie Yoshida et al 2019 Res. Notes AAS 3 174 https://doi.org/10.3847/2515-5172/ab5861
White dwarf stars ; Light curves ; Eclipsing binary stars
BibTeX RIS
p
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spectrum (Figure 1, left). However, images from SOAR revealed three similarly bright stars within one Kepler pixel (4'') of the target.
We used the PyKe keppca function (Still & Barclay 2012) to decompose the
EPIC251248385 flux into principal components. This revealed eclipses localized to the target pixel. A one-pixel aperture lightcurve produced 12% ± 3% primary eclipse depths (Figure 1, right). We also utilized the PyKe KeplerCBVCorrector function to remove 16 cotrending basis vectors (CBVs) from the lightcurve, revealing that the observed eclipses were not present in the CBVs. However, contamination from similarly bright stars within 4'' necessitated additional vetting to determine the eclipse source and depth.
16,600 s exposures of EPIC251248385 were taken using the Haleakala LCOGT between 2018 June 12 and 2019 March 23, resolving three similarly bright sources within 4'' of the target (Brown et al. 2013).
To determine the WD light fraction (f), we used the following:
FFiigguurree 11.. Left: EPIC251248385 spectrum taken by SOAR. Broad spectral lines at 4300 and 4800 Å indicate a hydrogen-dominated WD atmosphere. Right: aperture-optimized EPIC251248385 lightcurve compared with HD161756 lightcurve taken from literature measurements.
where T and B1 represent total source and background counts in a 4'' aperture, and S and B2 represent total source and background counts in a 1 6 aperture, respectively. We determined f = 0.39 ± 0.08.
Using f, the primary to secondary star radius ratio was calculated:
where δ represents transit depth, d dilution factor, equivalent to 1/f, R primary stellar radius, and R secondary stellar radius. We calculated R /R = 0.56 ± 0.24. As WDs typically have radii ≈ 0.013 R (Tremblay et al. 2016), this implies a secondary radius of 0.0073 ± 0.0031 R , suggesting EPIC 251248385 may be orbited by another WD or low-mass dwarf.
However, the observed eclipses may be independent of the targeted WD.
EPIC251248385 eclipses are 2.5 ± 0.5 hr, while known WD transits typically last minutes (Rivera et al. 2005; Vanderburg et al. 2015). Nearby eclipsing binaries may mimic localized signals through direct PRF contamination, CCD crosstalk, antipodal reflection off the Schmidt corrector, or along a column via charge transfer inefficiency (Coughlin et al. 2014, 2016). A bright eclipsing binary, HD161756, bled onto the relevant K2 detector, displaying a similar period (2.619 days) and eclipse depth (14%) as EPIC251248385 (Waelkens & Rufener 1983; Avvakumova et al. 2013). However, phased lightcurves of these targets revealed significantly different primary to secondary eclipse depth ratios, implying the observed eclipses are independent of HD161756 (Figure 1, right). Differences in eclipse phase, shape and duration may support this hypothesis. This does not rule out contamination via other instrumental effects (Coughlin et al. 2014).
We acknowledge the UH HI-STAR program for initiating this project, and financial support from NASA K2 Cycle 5 Grant 80NSSC18K0387. Observations were obtained from the NASA K2 Mission, and the SOAR telescope, sponsored by the MCTIC do Brasil, the U.S. NOAO, the University of North Carolina, and Michigan State
University.
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