Three Red Giants With Substellar-Mass Companions

Abstract

We present three giant stars from the ongoing Penn State-Toruń Planet Search with the Hobby-Eberly Telescope, which exhibit radial velocity variations that point to a presence of planetary -mass companions around them. BD+49 828 is a M = 1.52 ± 0.22 M K0 giant with a m sin i = 1.6 +0.4 −0.2 M J minimum mass companion in a = 4.2 +0.32 −0.2 AU (2590 +300 −180 d), e = 0.35 +0.24 −0.10 orbit. HD 95127, a log L/L = 2.28 ± 0.38, R = 20 ± 9 R , M = 1.20 ± 0.22 M K0 giant has a msini=5.01 +0.61 −0.44 M J minimum mass companion in a = 1.28 +0.01 −0.01 AU (482 +5 −5 d), e = 0.11 +0.15 −0.06 orbit. Finally, HD 216536, is a M = 1.36 ± 0.38 M K0 giant with a m sin i = 1.47 +0.20 −0.12 M J minimum mass companion in a = 0.609 +0.002 −0.002 AU (148.6 +0.7−0.7 d), e = 0.38 +0.12 −0.10 orbit. Both, HD 95127 b and HD 216536 b in their compact orbits, are very close to the engulfment zone and hence prone to ingestion in the near future. BD+49 828 b is among the longest period planets detected with the radial velocity technique until now and it will remain unaffected by

Precise radial velocities of giant stars

“…The spectral characteristics of solartype main-sequence (MS) stars are favorable for RV measurements, which has made these stars the targets of the majority of the RV planet searches. However, a growing number of research groups are successfully searching for planets around evolved subgiant and giant stars (e.g., Döllinger et al 2009;Johnson et al , 2011Sato et al 2013;Jones et al 2015a,b;Niedzielski et al 2015;Reffert et al 2015;Wittenmyer et al 2016). …”

The Penn State − Toruń Centre for Astronomy Planet Search stars

“…With our sample and the available data on other samples of evolved stars that were searched for planets, we are in position to discuss one of the most intriguing features of the currently available sample of exoplanets around stars that are past the MS: the suspicious growth of planetary masses with evolutionary stage of theirs host (Niedzielski et al 2015b). …”

“…Although generally not true, this is somewhat justified with the increase (Niedzielski et al 2015b) of average host mass from dwarfs (M/M = 0.997 ± 0.016), through subgiants (M/M = 1.446 ± 0.031), to giants (M/M = 1.885 ± 0.091) or bright giants (M/M = 1.464 ± 0.12). The stellar mass increase for evolved stars with planets is not a physical phenomenon, and it only reflects selection effects caused by the most important scientific driver for planet searches beyond the MS -the search for planets around stars more massive than the Sun.…”

“…If systematic effects are present in addition to large uncertainties in stellar mass estimates, the problem may be even more severe as masses for some types of stars may be systematically incorrect. In this context it is important to note that one of the most striking features of known planetary systems around evolved stars is a significant stellar and companion-mass increase for more evolved stars (Niedzielski et al 2015b). An average dwarf 1 with a planetary system usually is a solar-mass F or G spectral type star.…”

“…An average dwarf 1 with a planetary system usually is a solar-mass F or G spectral type star. An average subgiant known to possess a planetary system is already a star with a mass of ≈1.5 M (MS spectral type A-F), and an 1 After Niedzielski et al (2015b), we call for simplicity stars with log g = 4.5 ± 0.5 dwarf stars, those with log g = 3.5 ± 0.5 subgiants, stars with log g = 2.5 ± 0.5 giants, and those with log g = 1.5 ± 0.5 bright giants.…”