Abstract
We report the detection of radial velocity (RV) variations in nine evolved G- and K-type giant stars. The observations were conducted at Okayama Astrophysical Observatory. Planets or planet ...candidates can best explain these regular variations. However, a coincidence of near 280 d variability among five of them prevents us from fully ruling out stellar origins for some of the variations, since all nine stars behave similarly in stellar properties. In the planet hypotheses for the RV variations, the planets (including one candidate) may survive close to the boundary of the so-called “planet desert” around evolved stars, having orbital periods between 255 and 555 d. Besides, they are the least-massive giant planets detected around G- and K-type giant stars, with minimum masses between 0.45 MJ and 1.34 MJ. We further investigated other hypotheses for our detection, yet none of them can better explain regular RV variation. With our detection, we can be convinced that year-long regular variation with amplitude down to 15 m s−1 for G- and K-type giant stars is detectable. Moreover, we performed simulations to further confirm the detectability of planets around these stars. Finally, we explored giant planets around intermediate-mass stars, and likewise found a four-Jupiter-mass gap (e.g., Santos et al. 2017, A&A, 603, A30), which is probably a boundary of the giant planet population.
Abstract
We present the discoveries of two giant plants orbiting the red-giant-branch star HD 112570 and the red-clump star HD 154391, based on the radial-velocity (RV) measurements from the Xinglong ...station and Okayama Astrophysical Observatory. Spectroscopic and asteroseismic analyses suggest that HD 112570 has a mass of 1.15 ± 0.12
M
☉
, a radius of 9.85 ± 0.23
R
☉
, a metallicity Fe/H of −0.46 ± 0.1, and
log
g
of 2.47 ± 0.1. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of
M
p
=
3.42
−
0.84
+
1.4
M
Jup
, a period of
P
=
2615
−
77
+
85
days, and a moderate eccentricity of
e
=
0.20
−
0.14
+
0.16
for the Jovian planet HD 112570 b. For HD 154391, it has a mass of 2.07 ± 0.03
M
☉
, a radius of 8.56 ± 0.05
R
☉
, a metallicity Fe/H of 0.07 ± 0.1, and
log
g
of 2.86 ± 0.1. The super-Jupiter HD 154391 b has a mass of
M
p
=
9.1
−
1.9
+
2.8
M
Jup
, a period of
P
=
5163
−
57
+
60
days, and an eccentricity of
e
=
0.20
−
0.04
+
0.04
. We found that HD 154391 b has one of the longest orbital periods among those ever discovered orbiting evolved stars, which may provide a valuable case in our understanding of planetary formation at wider orbits. Moreover, while a mass gap at 4
M
Jup
seems to be present in the population of giant stars, there appear to be no significant differences in the distribution of metallicity among giant planets with masses above or below this threshold. Finally, the origin of the abnormal accumulation near 2 au for planets around large evolved stars (
R
⋆
> 21
R
☉
), remains unclear.
Abstract
The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late M dwarfs ...via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M dwarfs. To search for companions around such “deprioritized” M dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 with pyramid wave-front sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM J1002+1459 B and LSPM J2204+1505 B) and two new candidates that are likely companions (LSPM J0825+6902 B and LSPM J1645+0444 B), as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between ∼2 and 20 au in total. A comparison of the colors with the spectral library suggests that LSPM J2204+1505 B and LSPM J0825+6902 B are located at the boundary between late M and early L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions (∼10–75
M
Jup
) at 10 au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with the James Webb Space Telescope to search for smaller-mass companions below the NIRC2 detection limits.
ABSTRACT We report on the detection and early characterization of a hot Jupiter in a three day orbit around K2-34 (EPIC 212110888), a metal-rich F-type star located in the K2 Cycle 5 field. Our ...follow-up campaign involves precise radial velocity (RV) measurements and high-contrast imaging using multiple facilities. The absence of a bright nearby source in our high-contrast data suggests that the transit-like signals are not due to light variations from such a companion star. Our intensive RV measurements show that K2-34b (EPIC 212110888b) has a mass of , confirming its status as a planet. We also detect the Rossiter-McLaughlin effect for K2-34b and show that the system has a good spin-orbit alignment ( degrees). High-contrast images obtained by the HiCIAO camera on the Subaru 8.2 m telescope reveal a faint companion candidate (CC) ( mag) at a separation of . Follow-up observations are needed to confirm that the CC is physically associated with K2-34. K2-34b appears to be an example of a typical "hot Jupiter," albeit one which can be precisely characterized using a combination of K2 photometry and ground-based follow-up.
ABSTRACT We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at the Okayama Astrophysical Observatory, ...Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81 0.13 M , a radius of 7.30 0.33 R , and solar metallicity. The planetary system is composed of two giant planets with minimum masses of and , orbital periods of days and days, and eccentricities of and , respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely, considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ∼0.15, which is just within 1.4 of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160°.
We report the detection of a double planetary system orbiting around the evolved intermediate-mass star HD 4732 from precise Doppler measurements at Okayama Astrophysical Observatory and Australian ...Astronomical Observatory. The star is a KO subgiant with a mass of 1.7 M sub(middot in circle) and solar metallicity. The planetary system is composed of two giant planets with minimum mass of m sin i = 2.4 M sub(J), orbital period of 360.2 days and 2732 days, and eccentricity of 0.13 and 0.23, respectively. Based on dynamical stability analysis for the system, we set the upper limit on the mass of the planets to be about 28 M sub(J) (i > 5degrees) in the case of coplanar prograde configuration.
We report on the detection of the Rossiter–McLaughlin effect for the eccentric, super-Neptune exoplanet HAT-P-11b, based on radial-velocity measurements taken with the High Dispersion Spectrograph ...mounted on the Subaru 8.2 m telescope, and simultaneous photometry with the LCOGT 2.0 m Faulkes Telescope North, both located in Hawaii. The observed radial velocities during a planetary transit of HAT-P-11b show a persistent blue-shift, suggesting a spin–orbit misalignment in the system. The best-fit value for the projected spin–orbit misalignment angle is
${\lambda}$
$=$
103
$^\circ$
$_{-18^\circ}^{+22^\circ}$
. This fact suggests that not only hot-Jupiters, but also super-Neptunes like HAT-P-11b had once experienced dynamical processes, such as planet–planet scattering or Kozai migration.
Abstract
We report the results of intensive monitoring of the variability in the Hα line for two F-type stars, τ Boo and υ And, over the last four years 2019–2022, in order to investigate their ...stellar magnetic activity. The 4 yr Hα line intensity data taken with the 1.88 m reflector at the Okayama Branch Office, Subaru Telescope, shows the existence of a possible ∼123 d magnetic activity cycle for τ Boo. The result of the Hα variability as another tracer of the magnetic activity on the chromosphere is consistent with previous studies of the Ca ii H&K line and suggests that the magnetic activity cycle persists in τ Boo. For υ And, we suggest a quadratic long-term trend in the Hα variability. Meanwhile, the short-term monitoring shows no significant period corresponding to specific variations likely induced by their hot Jupiter in both cases (≈3.31 and 4.62 d, respectively). In this Hα observation, we could not find any signature of the star–planet magnetic interaction. It is speculated that the detected magnetic activity variability of the two F-type stars is related to the stellar intrinsic dynamo.
We report on the detection of two substellar companions orbiting around evolved intermediate-mass stars by precise Doppler measurements at Subaru Telescope and Okayama Astrophysical Observatory. HD ...145457 is a K0 giant with a mass of 1.9
$\ M_{\odot}$
, and has a planet of minimum mass,
$m_2$
sin
$i =$
2.9
$\ M_{\rm J}$
, orbiting with a period of
$P=176$
d and eccentricity of
$e=0.11$
. HD 180314 is also a K0 giant with 2.6
$\ M_{\odot}$
, and hosts a substellar companion of
$m_2$
sin
$i =$
22
$\ M_{\rm J}$
, which falls in the brown-dwarf mass regime, in an orbit with
$P =$
396 d and
$e =$
0.26. HD 145457 b is one of the innermost planets and HD 180314 b is the seventh candidate of a brown-dwarf-mass companion found around evolved intermediate-mass stars.
We present the design and performance of the High Dispersion Spectrograph (HDS) of the Subaru Telescope. HDS is an echelle spectrograph located at the Nasmyth focus of the telescope. The collimated ...beam size is 272 mm, and the echelle is 300 mm by 840 mm in total size (
$31.6 \,\mathrm{gr} \,\mathrm{mm}^{-1}, R=2.8$
). HDS has two cross-dispersing gratings with
$400 \,\mathrm{gr} \,\mathrm{mm}^{-1}$
and
$250 \,\mathrm{gr} \,\mathrm{mm}^{-1}$
, which are optimized for the blue and red wavelength regions, respectively. The camera is of the catadioptric type system, consisting of three corrector lenses and a mirror. Two EEV-CCD’s with
$4100 \times 2048$
pixels and a pixel size of 13.5
${\mu \mathrm {m}}$
are used as the detector. A standard configuration with a
${0\rlap {.}{}^{\mathrm {\prime \prime }}4}$
slit gives a spectral resolution of
$R=90000$
, and a narrower slit width enables higher resolution of up to
$R \sim 160000$
. The spectrograph has sensitivities from 3000
${Å}$
to 1
${\mu \mathrm {m}}$
, and one exposure covers a range of 1500–2500
${Å}$
, depending on the wavelength region. The throughput of the telescope and the spectrograph, including the efficiency of the detector, is about 13% in 5000–6000
${Å}$
and about 8% at 4000
${Å}$
. The stability of the spectrograph and scattered light level are also reported.
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