Discovery of a Compact Companion to a Nearby Star

Abstract

Radial velocity (RV) searches for exoplanets have surveyed many of the nearest and brightest stars for long-term velocity variations indicative of a companion body. Such surveys often detect highamplitude velocity signatures of objects that lie outside the planetary mass regime, most commonly those of a low-mass star. Such stellar companions are frequently discarded as false-alarms to the main science goals of the survey, but high-resolution imaging techniques can be employed to either directly detect or place significant constraints on the nature of the companion object. Here, we present the discovery of a compact companion to the nearby star HD 118475. Our Anglo-Australian Telescope (AAT) RV data allow the extraction of the full Keplerian orbit of the companion, found to have a minimum mass of 0.445 M ⊙ . Follow-up speckle imaging observations at the predicted time of maximum angular separation rule out a main-sequence star as the source of the RV signature at the 3.3σ significance level, implying that the companion must be a low-luminosity compact object, most likely a white dwarf. We provide an isochrone analysis combined with our data that constrain the possible inclinations of the binary orbit. We discuss the eccentric orbit of the companion in the context of tidal circularization timescales and show that non-circular orbit was likely inherited from the progenitor. Finally, we emphasize the need for utilizing such an observation method to further understand the demographics of white dwarf companions around nearby stars.

Speckle Imaging Characterization of Radial Velocity Exoplanet Systems

“…Our results are broadly in line with similar efforts to combine imaging and RV data sets for the characterization of exoplanets, (e.g., Kane et al 2014;Wittrock et al 2016Wittrock et al , 2017Kane et al 2019b), brown dwarfs (e.g., Crepp et al 2016), and white dwarfs (e.g., Crepp et al 2018;Kane et al 2019a). In our case, we have identified systems where the suspected planetary signal is fully consistent with a stellar companion (e.g., HD 1388, HD 87359, HD 104304) or where no companion is detected and the majority of the remaining parameter space for a companion is sub-stellar and planetary (e.g., HD 24040, HIP 57050).…”

“…For RV detections of exoplanet candidates, the use of high resolution imaging can reveal the presence of stellar companions, thus resolving an inclination ambiguity to the companion mass or the nature of a long-term RV trend (Crepp et al 2012;Wittrock et al 2016;Kane et al 2019b). Such imaging can occasionally reveal the presence of exotic stellar companions, whose low luminosity and RV signature can mimic that of a planet (Crepp et al 2013;Kane et al 2019a).…”

Speckle Imaging Characterization of Radial Velocity Exoplanet Systems

“…Our results are broadly in line with similar efforts to combine imaging and RV data sets for the characterization of exoplanets, (e.g., Kane et al 2014;Wittrock et al 2016Wittrock et al , 2017Kane et al 2019b), brown dwarfs (e.g., Crepp et al 2016), and white dwarfs (e.g., Crepp et al 2018;Kane et al 2019a). In our case, we have identified systems where the suspected planetary signal is fully consistent with a stellar companion (e.g., HD 1388, HD 87359, and HD 104304) or where no companion is detected and the majority of the remaining parameter space for a companion is substellar and planetary (e.g., HD 24040 and HIP 57050).…”

“…For RV detections of exoplanet candidates, the use of highresolution imaging can reveal the presence of stellar companions, thus resolving an inclination ambiguity to the companion mass or the nature of a long-term RV trend (Crepp et al 2012;Wittrock et al 2016;Kane et al 2019b). Such imaging can occasionally reveal the presence of exotic stellar companions, whose low luminosity and RV signature can mimic that of a planet (Crepp et al 2013;Kane et al 2019a).…”

The Pan-Pacific Planet Search – VIII. Complete results and the occurrence rate of planets around low-luminosity giants

“…Crepp et al 2012;Rodigas et al 2016;Kane et al 2019b), even when the object is not seen (e.g. Kane et al 2019a;Hirsch et al 2019). However, with our targets generally falling at distances of 150-300 pc, we do not expect that any stellar companions could typically be resolved in Gaia imaging, though we note that HD 110238 has a Gaia detected companion with common proper motion which is the likely cause of the observed large-amplitude radial velocity variation in our PPPS data.…”