Identification of a White Dwarf Companion in the V* HP Dra System

The Backyard Worlds: Planet 9 (BYW; Kuchner et al. 2017) project considers if the theoretical Planet 9 (Batygin & Brown 2016), exists by searching the WISE images Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010). The same technique is also applied to identify previously unknown low-mass stars and brown dwarfs near the vicinity of the solar system, many of which are included in the recent full-sky census of Kirkpatrick et al. (2021). BYW: P9 has seen success with the discovery of exotic objects such as extreme T-type subdwarfs (Schneider et al. 2020) and common proper motion companions to other stars (Rothermich et al. 2021). Here we present another area of expansion for the Backyard Worlds: Planet 9 project with the discovery of a white dwarf companion which was found to be associated with the HP Dra eclipsing binary system.

HP Draconis is a well-known eclipsing binary (Milone et al. 2010) derived orbital parameters; P = 10.76152 ± 0.00019 days, a = 26.79 ± 0.04 R_⊙, e = 0.0367 ± 0.0009, and ω = 0.693 ± 0.037. HP Draconis was found to be a variable system in the Hipparcos mission data (HIP 92835). The initial period of variation reported for HP Dra was incorrectly reported as 6.67 days Kurpinska-Winiarska et al. (1999). Further work by Kurpinska-Winiarska et al. (2000) established the period of 10.76154 days and Milone et al. (2005) confirmed this result at 10.76152 days, with only minor discrepancies. The spectrum of the central pair is listed as G5 in the Henry Draper catalog. Further work by Milone et al. (2005) has resulted in both components being re-classified as F9V. Various other properties have been derived from the system. For example Feltzing et al. (2001) deduced a [Fe/H] metallicity of −0.24 from the uvby color indices.

Milone et al. (2010) investigated the system for additional optical companions, and found no conclusive evidence for any additional companions in the system.

We identified a white dwarf companion co-moving with HP Dra. WDJ185454.97+511833.50 (DR22144465183642117888) was identified as having a similar proper motion and Gaia parallax (Gaia Collaboration et al. 2021) to the HP Dra system central pair at a separation of 14405 on the sky, indicating that they are likely associated.

Table 1. Properties of HP Dra Triple System

Parameter	HP Dra	WDJ185454.97+511833.50	References
Mass	M1: 1.133 ± 0.005 M⊙	0.45 ± 0.03 ${M}_{\odot }$	1
	M2: 1.094 ± 0.007 M⊙		1
Radius	R1: 1.371 ± 0.012 R⊙		1
	R2: 1.052 ± 0.010 R⊙		1
R.A. (degrees)	283.72300282352	283.72918798268	1
Decl. (degrees)	51.30864577245	51.30967553575	1
ϖ (mas)	12.6153 ± 0.0516	12.4974 ± 0.0723	1
μ ${}_{\alpha }$ (mas yr−1)	23.234 ± 0.074	21.955 ± 0.100	1
μ ${}_{\delta }$ (mas yr−1)	83.306 ± 0.064	83.644 ± 0.098	1
${G}_{\mathrm{Bp}}$ (mag)	8.111 ± 0.003	17.554 ± 0.022	1
G (mag)	7.826 ± 0.003	17.490 ± 0.003	1
${G}_{\mathrm{Rp}}$ (mag)	7.369 ± 0.004	17.175 ± 0.022	1

References. (1) Gaia EDR3 (Gaia Collaboration et al. 2021).

WDJ185454.97+511833.50 has a 99% chance of being a white dwarf, with a ${T}_{\mathrm{eff}}$ = 7087 ± 205 K and log g = 7.73 ± 0.07, and M = 0.45 ± 0.03 ${M}_{\odot }$ if it is a DA white dwarf (Gentile Fusillo et al. 2018). The values are slightly lower, but within the errors if the white dwarf is a DB white dwarf. Although the white dwarf has a lower than average mass, the mass is high enough to suggest single star evolution.

Using wdwarfdate (Kiman et al. 2021, in preparation) which combines the MIST isochrones with the Cummings et al. (2019) initial mass-final mass relation we determine the white dwarf cooling age to be ${1.15}_{-0.11}^{+0.14}$ Gyr, and the total age ${5.91}_{-2.86}^{+4.11}$ Gyr, with a likely initial mass of ${1.29}_{-0.22}^{+0.44}$ ${M}_{\odot }$, consistent with an earlier spectral type and higher mass than the common proper motion pair.

From this, calculations indicate that the wd is 1140 au from the central HP Dra pair. Assuming a circular orbit and taking the mass of the central pair at 2.4 solar masses this would give an orbital period for the white dwarf of ∼23,000 yr.

We present the discovery of white dwarf companion DR2 2144465183642117888 co-moving with the previously known eclipsing binary HP Dra. This has a wide projected orbital separation of ∼1140 au. This system joins a growing list of recent discoveries from the Backyard Worlds: Planet 9 Citizen Science community. HP DRa is now a triple star system. With the white dwarf companion now being evolved past the main sequence stage it is expected that it had a higher original mass than the central eclipsing pair.

Considering the proper motions using the central pair as a baseline. The PMs of the central pair are given at: 23.234 R.A. mas yr⁻¹ ± 0.074 and 83.306 decl. mas yr⁻¹ ± 0.064. Proposed companion: 21.955 R.A. mas yr⁻¹ ± 0.100 and 83.644 decl. mas yr⁻¹ ± 0.098.

The R.A. shows a 5.5% difference and decl. 0.41% difference respectively between the components in this system. This high agreement within such tight margins between the central pair and the proposed companion would indicate that they are likely associated. The central pair and proposed companion are located at 79.13 pc and 79.90 pc from the solar system respectively. This would indicate that the companion is, at present, on the other side of its orbit around the pair as seen from Earth.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement, This publication makes use of data products from the Wide-field Infrared Survey Explorer, (WISE) which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.