Discovery of a White Dwarf Companion to HD 159062

TL;DR

This paper reports the discovery of a white dwarf companion to the nearby star HD 159062, using 14 years of radial velocity data and high-resolution imaging, revealing its properties and orbit.

Contribution

The study combines radial velocity and imaging data with Bayesian analysis to characterize a previously unknown white dwarf companion and determine its properties and orbit.

Findings

White dwarf companion has a mass of about 0.65 solar masses.

Orbital period of the white dwarf is approximately 250 years.

Cooling age of the white dwarf is around 8.2 billion years.

Abstract

We report on the discovery of a white dwarf companion to the nearby late G dwarf star, HD 159062. The companion is detected in 14 years of precise radial velocity (RV) data, and in high-resolution imaging observations. RVs of HD 159062 from 2003-2018 reveal an acceleration of $-13.3\pm0.12\ \rm{m s}^{-1}$, indicating that it hosts a companion with a long-period orbit. Subsequent imaging observations with the ShaneAO system on the Lick Observatory 3-meter Shane telescope, the PHARO AO system on the Palomar Observatory 5-meter telescope, and the NIRC2 AO system at the Keck II 10-meter telescope reveal a faint companion 2.7'' from the primary star. We performed relative photometry, finding $\Delta J = 10.09 \pm 0.38$ magnitudes, $\Delta K_{S} = 10.06 \pm 0.22$ magnitudes, and $\Delta L' = 9.67\pm0.08$ magnitudes for the companion from these observations. Analysis of the radial velocities, astrometry, and photometry reveals that the combined data set can only be reconciled for the scenario where HD 159062 B is a white dwarf. A full Bayesian analysis of the RV and imaging data to obtain the cooling age, mass, and orbital parameters of the white dwarf indicates that the companion is an old $M_{B} = 0.65^{+0.12}_{-0.04} M_{\odot}$ white dwarf with an orbital period of $P = 250^{+130}_{-76}$ years, and a cooling age of $\tau = 8.2^{+0.3}_{-0.5}$ Gyr.

Tables

Table 1. Table 1: Properties of HD 159062

HD 159062 Properties
R.A. (J2000)	17 30 16.4238
Dec. (J2000)	+47 24 07.922
$J$ mag.	$5.804\pm {0.018}^a$
$K_s$ mag.	$5.392\pm {0.021}^a$
W1 mag.	$5.397\pm {0.164}^b$
$\pi$ (mas)	$46.123\pm {0.024}^c$
d (pc)	$21.68\pm 0.01$
${\mu }_{\alpha }$ (mas yr-1)	$169.747\pm {0.061}^c$
${\mu }_{\delta }$ (mas yr-1)	$77.164\pm {0.058}^c$
$T_{\mathrm{eff}}$ (K)	$5283\pm {100}^d$
$\log g$ (dex)	$4.4\pm {0.1}^d$
[Fe/H] (dex)	$-0.31\pm {0.06}^d$
Mass (M⊙)	$0.76\pm {0.03}^d$
Radius (R⊙)	$0.76\pm {0.04}^d$
$\log R_{\mathrm{HK}}^{\prime }$	-4.97
$S_{\mathrm{HK}}$	$0.170\pm 0.002$
V$\sin i$ ($\mathrm{km}{s}^{-1}$)	$2.06\pm {0.5}^e$

Table 2. Table 2: Keck/HIRES Radial Velocity Data

Epoch (BJD)	RV ($m{s}^{-1}$)	$\sigma$ ($m{s}^{-1}$)	$S_{\mathrm{HK}}$
2452832.91668	89.22	1.62	$\cdots$
2453074.09155	83.23	1.68	$\cdots$
2453196.81631	76.88	1.58	$\cdots$
2453430.15779	65.68	0.93	0.166
2453547.94513	60.98	0.84	0.167
2453604.84622	57.70	1.03	0.168
2453807.13748	50.30	1.00	0.168
2453926.45063	43.33	0.57	0.168
2453927.88530	44.96	0.83	0.167
2454248.03230	31.71	1.44	0.170
2454249.95458	32.00	1.49	0.170
2454252.04380	32.81	1.02	0.172
2454255.93510	32.28	1.01	0.172
2454277.86337	31.99	1.33	0.171
2454278.90748	32.22	1.22	0.171
2454279.94547	31.42	1.29	0.164
2454285.90800	30.58	1.34	0.170
2454294.90713	30.60	1.30	0.173
2454634.43562	19.74	1.03	0.170
2454635.94255	19.00	1.43	0.170
2454636.93316	17.04	1.53	0.171
2454638.33247	18.76	0.99	0.171
2454640.38633	14.57	1.06	0.171
2454641.94950	19.55	1.47	0.172
2454644.08325	15.11	1.52	0.170
2454688.88580	17.38	1.47	0.175
2454689.89920	9.37	1.58	0.174
2454957.13845	3.77	1.67	0.178
2455401.78282	-16.12	1.40	0.172
2455722.86450	-22.42	1.56	0.176
2455782.89029	-26.95	1.55	0.170
2456164.72278	-40.66	1.54	0.172
2456451.97582	-50.77	1.03	0.169
2456475.78721	-49.97	1.43	0.173
2456709.08945	-54.30	1.43	0.172
2456883.74892	-65.35	1.49	0.171
2456910.83550	-66.37	1.62	0.171
2457061.17682	-66.74	1.43	0.169
2457211.95513	-76.17	1.40	0.170
2457671.70374	-91.12	1.72	0.167
2457831.16397	-94.97	1.59	0.167
2457853.14530	-91.51	1.92	0.170
2457970.94501	-103.25	1.86	0.169
2458263.03029	-108.99	1.22	0.168
2458349.72648	-110.69	1.58	0.167

Table 3. Table 3: Summary of imaging observations

Epoch (JD)	Instrument	Filter	Separation ($″$)	P.A. (deg)	Contrast ($\Delta$-mag)
2456102.89193	NIRC2	$K^{\prime }$	$2.594\pm 0.014$	$298.6\pm 0.1$	$\cdots$
2456942.70077	NIRC2	$K_s$	$2.637\pm 0.014$	$299.5\pm 0.1$	$\cdots$
2457498.99002	ShaneAO	$K_s$	$2.66\pm 0.03$	$301.5\pm 0.56$	$\cdots$
2457617.69501	ShaneAO	$K_s$	$2.68\pm 0.02$	$301.7\pm 0.47$	$\cdots$
2457911.93061	PHARO	$K_s$	$2.671\pm 0.004$	$301.3\pm 0.2$	$10.06\pm 0.22$
2458236.09489	NIRC2	$L^{\prime }$	$2.673\pm 0.003$	$301.27\pm 0.04$	$9.67\pm 0.08$
2458268.86233	ShaneAO	$J$	$2.67\pm 0.01$	$301.0\pm 0.24$	$10.09\pm 0.38$

Table 4. Table 4: MCMC Priors on Orbital Parameters

Parameter	Prior
mA	Gaussian, $0.80\pm 0.05M_{\odot }$
$m_B$	Uniform, $[0.24,1.44]M_{\odot }$
$\log P$	Uniform, $[{10}^2,{10}^4]$ yr
$\sqrt{e}\cos \omega$	Uniform, $[0.0,1.0]$
$\sqrt{e}\sin \omega$	Uniform, $[0.0,1.0]$
$\Omega$	Uniform, $[0,2\pi ]$
$M_0\left(t_0\right)$	Uniform, $[0,2\pi ]$
$\cos i$	Uniform, $[-1.0,1.0]$
$\pi$	Gaussian, $46.123\pm 0.024$ mas
${\gamma }_j-{\gamma }_k$	Gaussian, $0\pm 2m{s}^{-1}$

Table 5. Table 5: MCMC Results

Parameter	Median & 68% CI
Model	RV/Ast	RV/Ast/Phot
mA ($M_{☉}$)	$0.80\pm 0.05$	$0.80\pm 0.05$
mB ($M_{☉}$)	${0.65}_{-0.04}^{+0.14}$	${0.65}_{-0.04}^{+0.12}$
$\log P$ (yr)	${2.38}_{-0.15}^{+0.19}$	${2.40}_{-0.16}^{+0.18}$
$\sqrt{e}\cos \omega$	${0.61}_{-0.42}^{+0.16}$	${0.58}_{-0.46}^{+0.18}$
$\sqrt{e}\sin \omega$	$-{0.27}_{-0.10}^{+0.07}$	$-{0.27}_{-0.07}^{+0.07}$
$\Omega$ (∘)	${138}_{-5}^{+11}$	${137}_{-4}^{+9}$
$M_0\left(t_{i=0}\right)$ (∘)	${144}_{-28}^{+59}$	${147}_{-27}^{+65}$
$\cos i$	${0.60}_{-0.13}^{+0.23}$	${0.58}_{-0.12}^{+0.22}$
$\pi$ (mas)	$46.12\pm 0.02$	$46.12\pm 0.02$
$\tau$ (Gyr)	$\cdots$	${8.2}_{-0.5}^{+0.3}$
Derived Parameters
P (yr)	${238}_{-68}^{+128}$	${250}_{-76}^{+130}$
$e$	${0.44}_{-0.31}^{+0.30}$	${0.40}_{-0.28}^{+0.31}$
$\omega$ (∘)	$-{26}_{-29}^{+7}$	$-{26}_{-38}^{+7}$
$i$ (∘)	${53}_{-19}^{+9}$	${54}_{-18}^{+8}$
$T_{eff,B}$ (K)	$\cdots$	${4580}_{-160}^{+440}$
Instrumental Parameters
${\gamma }_k$ ($m{s}^{-1}$)	${934}_{-605}^{+492}$	${1018}_{-572}^{+431}$
${\gamma }_j$ ($m{s}^{-1}$)	${933}_{-605}^{+492}$	${1018}_{-573}^{+431}$
${\sigma }_{jit,k}$ ($m{s}^{-1}$)	${2.3}_{-1.5}^{+2.9}$	${2.3}_{-1.5}^{+2.9}$
${\sigma }_{jit,j}$ ($m{s}^{-1}$)	${1.35}_{-0.30}^{+0.33}$	${1.36}_{-0.30}^{+0.32}$

Equations

$$M_B=\frac{a_A{r}^2}{G}$$

$$M_B\ge \left|\frac{dRV}{dt}\right|{\rho }_{proj}^2{G}^{-1}\ge 0.24M_{\text{sun}}.$$

$$\begin{gathered} \log \mathcal{L}=-\sum _i\left[\frac{(v_i-v_m(t_i){)}^2}{2({\sigma }_i^2+{\sigma }_{\mathrm{jit}}^2)}+\log \sqrt{2\pi ({\sigma }_i^2+{\sigma }_{\mathrm{jit}}^2)}\right] \\ -\sum _j\left[\frac{(x_j-x_m(t_j){)}^2}{2{\sigma }_j^2}+\log \sqrt{2\pi {\sigma }_j^2}\right] \end{gathered}$$

Full text

††software: Emcee (Foreman-Mackey et al., 2013), VIP (Gomez Gonzalez et al., 2017), isoclassify (Huber et al., 2017), pyKLIP (Wang et al., 2015), photutils (Bradley et al., 2017)

1 Introduction

There are more than two hundred known white dwarfs within 25 pc of the Sun (Sion et al., 2014), most of which are kinematically consistent with the thin disk population. Recent Gaia DR2 catalog queries report 139 white dwarfs within 20 pc (Hollands et al., 2018) and 153 within 25 pc (Jiménez-Esteban et al., 2018) based on strict cuts in Gaia color-magnitude space. The DA spectral type, which is distinguished by Balmer lines in the spectra for T_{\rm eff}$$>5000 K, is the most common, with DA white dwarfs approximately twice as abundant as all other spectral types (Giammichele et al., 2012; Sion et al., 2014; Kilic et al., 2018).

Approximately one quarter of the known nearby white dwarfs reside in binary systems, most with a less-evolved companion but several in double-degenerate binaries (Holberg et al., 2016). This fraction is notably significantly lower than the field binary fraction. However, Toonen et al. (2017) perform population synthesis modeling including stellar multiplicity and evolution, and determine that the known WD/MS binary population is actually in reasonably good agreement with models. They determine that as high a fraction as 10–30% of single white dwarfs originate as binary systems in which the components merge during post-main sequence evolution. Low detection sensitivity to faint white dwarfs near bright, nearby main sequence stars also likely plays a role in the observed rate of WD/MS binaries (Toonen et al., 2017).

Understanding the population of nearby white dwarfs, and especially those in binary systems, is important for constraining the star formation history of the local neighborhood, as well as low-mass stellar evolution and white dwarf cooling models. Additionally, studying white dwarfs in binary systems can provide insights into the possible progenitors for Type 1a supernovae. Several nearby benchmark white dwarf-main sequence binary systems have been discovered with the combination of imaging and radial velocity data (e.g. Crepp et al., 2013, 2018), and a new detection of such a binary system is reported here.

HD 159062 is a bright nearby G9 dwarf star with low metallicity ([Fe/H]$=-0.31$ dex). It is likely fairly old; using its observed Ca II H&K emission diagnostic $R^{\prime}_{\rm HK}$, and the empirical relation between $R^{\prime}_{\rm HK}$, rotation, and age from Mamajek & Hillenbrand (2008) results in an age estimate of $\approx 7$ Gyr. HD 159062 is in the solar neighborhood at 21.7 pc (Gaia Collaboration et al., 2018), and as expected based on its age estimate, its kinematics are inconsistent with any nearby young clusters (Gagné et al., 2018). HD 159062 is consistent with an old main sequence G–K dwarf based on its $\log{g}$ and $T_{\rm eff}$. Its properties are detailed in Table 1, most of which derive from the literature and are indicated with superscripts on each parameter value.

Based on its [Fe/H] and [Mg/Fe] abundances as determined from high resolution spectroscopy, Fuhrmann et al. (2017a) demonstrate that HD 159062 is most likely a Population II star. They argue that its age is most likely greater than the estimates from chromospheric activity or standard isochronal analysis would indicate, $\tau\geq 12$ Gyr. The authors list HD 159062 as a candidate blue straggler, indicating that rejuvenation from winds from a more evolved stellar companion may be complicating the standard age diagnostics.

Fuhrmann et al. (2017b) note that HD 159062 has an anomalously high barium abundance of [Ba/Fe] $=+0.4\pm 0.01$ dex for a star of its type and metallicity. They argue that this may be further evidence of accretion of material onto HD 159062 via winds from an AGB companion which later become a white dwarf. Other abundance surveys of the solar neighborhood report more typical values for the Ba abundance of HD 159062; Mishenina et al. (2008) report [Ba/Fe] $=+0.15\pm 0.1$ and Reddy et al. (2006) report [Ba/Fe] $=+0.17\pm 0.12$.

There exist many previously studied examples of binary star systems in which the more massive component has evolved off the main sequence through the AGB phase and into a white dwarf. These systems are typically distinguished by enhanced abundances of s-process heavy elements in the spectrum of the less-evolved star, which are typically thought to be a result of contamination from winds or accretion from the evolved companion during its AGB phase. Barium stars and CH stars are two subsets of this category of binary systems.

Barium stars are typically observed to be G and K giants, in binary systems with a white dwarf. These binaries have typical orbital periods of 500 – $10^{4}$ days, indicating that accretion from stellar winds (and not Roche-lobe overflow) is the dominant mechanism for mass transfer (Boffin & Jorissen, 1988; Izzard et al., 2010; Van der Swaelmen et al., 2017).

The low-metallicity counterparts of Ba stars are CH stars, which are typically Population II stars with enhanced heavy-metal abundances, also due to accretion from an evolved binary companion. These types of stars were named for the lines arising from the CH molecule observed in their spectra. They are also formed via accretion from an evolved companion, and the observed binary period distribution for this class of stars is similar to that of Ba stars, with periods nearly always below $10^{4}$ days (Jorissen et al., 2016).

Recently, Escorza et al. (2019) published radial velocity observations and orbit fits for a compilation of 60 Ba and CH binary systems. Of these, 27 had well-measured orbits with periods ranging from hundreds to $>8000$ days. Other systems had incomplete orbital coverage, and so orbital periods were not estimated, but it can be inferred that these systems may have significantly longer orbital periods.

The inconsistent measurements of the metallicity of HD 159062 raise questions about whether or not the system could be an example of a mild Ba or CH binary. Either way, we report here that HD 159062 does definitively reside in a binary with an evolved companion.

In this paper, we report the discovery of the white dwarf companion to HD 159062, using 14 years of precise radial velocity data from Keck/HIRES as well as multi-epoch, multi-band imaging observations from the ShaneAO system at Lick Observatory, PHARO at Palomar Observatory and NIRC2 at Keck Observatory. By combining the RV and imaging data, we can constrain the orbit and cooling age of HD 159062 A and its white dwarf companion, HD 159062 B. We describe the observations and data reduction in §2, and the astrometry and common proper motion of the companion in §3. We demonstrate that HD 159062 B is not consistent with a brown dwarf or main sequence companion based on its dynamics and color in §4. We detail our joint Bayesian analysis of the combined data set in §5, and present the best orbital parameters. Finally, we discuss the implications for the system’s evolution, especially in the context of typical Ba star systems in §6.

2 Observations and Data Reduction

2.1 Radial Velocity Observations

Using HIRES at the 10 meter Keck Telescope (Vogt et al., 1994), we have obtained 45 radial velocity observations of HD 159062 since 2003 as part of the California Planet Survey (CPS). Several of these measurements were taken as double exposures, and were later binned on a 1-day cadence.

For HD 159062, we collected spectra through the B5 or C2 decker, with a width of $0\farcs 86$ and a spectral resolution of $R\approx 55,000$. The exposures were timed to yield a per-pixel SNR of $\approx 160$ at 550 nm. A template spectrum (without iodine) was taken through the B3 decker, with a width of $0\farcs 57$ and spectral resolution of $R\approx 66,000$, and with per-pixel SNR of $\approx 220$.

To reduce the spectra and extract RVs, we used the standard California Planet Survey pipeline (e.g. Howard et al., 2010; Howard & Fulton, 2016). In brief, starlight passes through a cell containing molecular iodine, imprinting a dense forest of absorption lines on the stellar spectrum. These lines are used as a high-fidelity wavelength calibration and PSF reference. The template spectrum is obtained without the iodine cell to use as a stellar spectral reference. This reference spectrum is deconvolved against the instrumental PSF, and the wavelength solution, instrumental PSF, and radial velocity are forward modeled for each observing epoch. Each spectrum is divided into $\sim 700$ “chunks”, and the radial velocity is extracted individually for each. The RVs and uncertainties reflect the weighted average and scatter of the results for the ensemble of spectral chunks.

The RV time series of HD 159062 shows a strong RV acceleration of $-13.29\pm 0.12$ $\rm m\,s^{-1}\,yr^{-1}$. Careful inspection of the radial velocity time series shows slight curvature about this linear trend, which provides greater leverage on the full orbit. Figure 1 displays the radial velocity time series, as well as a linear fit to the data. The RV dataset is provided in Table 2.1, along with the internal RV uncertainties.