Absolute Properties of the Detached Eclipsing Binary EPIC 202674012 (HD 149946)

We present a full set of high-precision orbital and physical parameters of a detached eclipsing binary (DEB) EPIC 202674012 (HD 149946, ASAS J163903-2847.2; hereafter E2026), based on the K2 photometry and radial velocities (RVs) from our own observations.

E2026 has been discovered as a DEB by the All-Sky Automated Survey⁶ (ASAS; Pojmański 2002). It has also been observed by the Kepler spacecraft during campaign C2 of the K2 mission. Recently, (Maxted & Hutcheon 2018, hereafter MH18) analyzed the K2 light curve (LC), combined it with RVs obtained from four publicly available spectra, and derived a preliminary set of absolute parameters with a precision of ∼10% in masses or radii.

We observed E2026 between 2012 and 2015, obtaining 15 high-resolution spectra with 3 stable instruments. Five spectra were taken with the CHIRON spectrograph ("slicer" mode, R ∼ 90,000; Schwab et al. 2012; Tokovinin et al. 2013) attached to the SMARTS-1.5 m telescope (CTIO, Chile). Six observations come from the CORALIE spectrograph (R ∼ 70,000) behind the 1.2 m Euler telescope (La Silla, Chile). Finally, four spectra were obtained with the FEROS instrument (R ∼ 48,000; Kaufer et al. 1999) at the MPG-2.2 m telescope (La Silla, Chile). These are the spectra previously used by MH18. All data were reduced and calibrated with dedicated pipelines (Tokovinin et al. 2013; Jordán et al. 2014; Brahm et al. 2017).

We measured RVs with our own implementation of the TODCOR technique (Zucker & Mazeh 1994), with synthetic spectra used as templates. Measurement errors were estimated with a bootstrap procedure (Hełminiak et al. 2012), sensitive to the signal-to-noise ratio and rotational broadening of spectral lines. Our RVs with errors, and residuals of the fit are available in a machine-readable form.

The orbital parameters were found by fitting a double-Keplerian orbit to our RVs with a Levenberg–Marquardt algorithm, implemented in our own code V2FIT (Konacki et al. 2010). In the fit we fixed the orbital period P to the value given by MH18. Additionally, we found our values of e and ω in excellent agreement with and with comparable errors to MH18.

Using the code JKTABSDIM,⁷ we combined our spectroscopic and MH18's photometric results in order to derive the absolute stellar parameters. We estimated the distance, using apparent total magnitudes of the binary, ${T}_{\mathrm{eff}}$ from MH18, and by finding $E(B-V)$ that minimizes the spread between distances individually calculated for each band. The age of E2026 was also assessed, by comparison with solar-composition MESA isochrones (Dotter 2016). All parameters derived by us and MH18 are listed in Table 1. Thanks to the superb K2 photometry, and our precise RVs, we reached 0.81%–0.84% precision in masses, and 0.32%–0.47% in radii—sufficient for meaningful and stringent tests of stellar structure and evolution. This makes E2026 a well-studied system.

Table 1.  The Full Set of Parameters of E2026

Parameter	Primary	Secondary	Meaning
	Systemic
From the LC analysis (MH18)
P [day]	23.30962 ± 0.00005		Orbital period
i [°]	88.65 ± 0.01		Orbital inclination
e	0.027 ± 0.002		Orbital eccentricity (from LC)
ω [°]	259.7 ± 0.6		Argument of pericenter (from LC)
r	0.04596 ± 0.00008	0.0258 ± 0.0001	Fractional radius (R/a)
${T}_{\mathrm{eff}}$ [K]	6250 ± 285	6150 ± 285	Effective temperature (color-based)
From the RV analysis (this work)
${T}_{{\rm{P}}}$ [BJD-2450000]	5871.31 ± 0.22		Time of pericenter passage
e	0.0272 ± 0.0013		Orbital eccentricity (from RVs)
ω [°]	259 ± 3		Argument of pericenter (from RVs)
K [km s−1]	45.34 ± 0.17	53.35 ± 0.21	RV semi-amplitude
γ [km s−1]	9.72 ± 0.17		Center-of-mass velocity
${{\rm{\Delta }}}_{\mathrm{COR}-\mathrm{FER}}$ [km s−1]	−0.05 ± 0.35	0.09 ± 0.37	Zero-point shift (CORALIE versus FEROS)
${{\rm{\Delta }}}_{\mathrm{COR}-\mathrm{CHI}}$ [km s−1]	−0.08 ± 0.34	−0.24 ± 0.35	Zero-point shift (CORALIE versus CHIRON)
q	0.805 ± 0.004		Mass ratio (secondary/primary)
$M{\sin }^{3}(i)$ [${M}_{\odot }$]	1.406 ± 0.012	1.131 ± 0.009	Minimum mass
$a\,\sin (i)$ [${R}_{\odot }]$	46.85 ± 0.12		Projected physical major semi-axis
rms [km s−1]	0.10	0.34	Root mean square of the RV fit
Absolute and physical
M [${M}_{\odot }$]	1.407 ± 0.012	1.132 ± 0.009	Absolute mass
R [${R}_{\odot }$]	2.154 ± 0.007	1.209 ± 0.006	Absolute radius
a [${R}_{\odot }$]	46.86 ± 0.12		Absolute physical major semi-axis
$\mathrm{log}(g)$	3.920 ± 0.002	4.327 ± 0.004	Surface gravity
$\mathrm{log}(L/{L}_{\odot })$	0.80 ± 0.08	0.27 ± 0.08	Luminosity
${v}_{\mathrm{synch}}$ [km s−1]	4.673 ± 0.015	2.623 ± 0.012	Synchronous rotation velocity
$E(B-V)$ [mag]	0.11 (assumed)		Color excess
d [pc]	253 ± 12		Distancea
τ [Gyr]	2.85 ± 0.30		Isochrone-estimated ageb

Notes.

^aWeighted average from five ($B,V,J,H,K$ bands) surface brightness-${T}_{\mathrm{eff}}$ relations (Kervella et al. 2004). ^bAssuming $[M/H]=0.0\pm 0.1$ dex. (This table is available in its entirety in FITS format.)

K.G.H. is supported by the Polish National Science Center through grant No. 2016/21/B/ST9/01613. This research made use of data collected at ESO under programmes 089.D-0097 and 091.D-0145, and through CNTAC proposals CN2012A-021, CN2012B-036 and CN2015A-074.

Facilities: MPG-2.2 m/FEROS - , Euler/CORALIE - , SMARTS-1.5 m/CHIRON - , Kepler. -