Title: Search for Exoplanets around Northern Circumpolar Stars- II. The Detection of Radial Velocity Variations in M Giant Stars HD 36384, HD 52030, and HD 208742,
Abstract: We present the detection of long-period RV variations in HD 36384, HD 52030,
and HD 208742 by using the high-resolution, fiber-fed Bohyunsan Observatory
Echelle Spectrograph (BOES) for the precise radial velocity (RV) survey of
about 200 northern circumpolar stars. Analyses of RV data, chromospheric
activity indicators, and bisector variations spanning about five years suggest
that the RV variations are compatible with planet or brown dwarf companions in
Keplerian motion. However, HD 36384 shows photometric variations with a period
very close to that of RV variations as well as amplitude variations in the
weighted wavelet Z-transform (WWZ) analysis, which argues that the RV
variations in HD~36384 are from the stellar pulsations. Assuming that the
companion hypothesis is correct, HD~52030 hosts a companion with minimum mass
13.3 M_Jup$ orbiting in 484 days at a distance of 1.2 AU. HD~208742 hosts a
companion of 14.0 M_Jup at 1.5 AU with a period of 602 days. All stars are
located at the asymptotic giant branch (AGB) stage on the H-R diagram after
undergone the helium flash and left the giant clump.With stellar radii of 53.0
R_Sun and 57.2 R_Sun for HD 52030 and HD 208742, respectively, these stars may
be the largest yet, in terms of stellar radius, found to host sub-stellar
companions. However, given possible RV amplitude variations and the fact that
these are highly evolved stars the planet hypothesis is not yet certain.