Okayama Astrophysical Observatory Research Articles

A planetary companion orbiting the intermediate-mass G Giant HD 173416

The discovery of a planetary companion to the intermediate-mass late-type giant star HD173416 from precise Doppler surveys of G and K giants at Xinglong station and Okayama Astrophysical Observatory (OAO) is presented in this letter. The planet has a minimum mass of 2.7 MJ, an eccentricity of 0.21, a semimajor axis of 1.16 AU and an orbital period of 324 days.

Planetary Companions to Evolved Intermediate-Mass Stars: 14 Andromedae, 81 Ceti, 6 Lyncis, and HD167042

Abstract We report on the detection of four extrasolar planets orbiting evolved intermediate-mass stars from a precise Doppler survey of G-K giants at Okayama Astrophysical Observatory. All of the host stars are considered to be formerly early F-type or A-type dwarfs when they were on the main sequence. 14 And (K0 III) is a clump giant with a mass of 2.2$M_{\odot}$ and has a planet of minimum mass $m_2$sin$i$$=$ 4.8$M_{\rm J}$ in a nearly circular orbit with a 186d period. This is one of the innermost planets around evolved intermediate-mass stars, and such planets have only been discovered in clump giants. 81 Cet (G5 III) is a clump giant with 2.4$M_{\odot}$ hosting a planet of $m_2$sin$i$$=$ 5.3$M_{\rm J}$ in a 953d orbit with an eccentricity of $e=$ 0.21. 6 Lyn (K0 IV) is a less-evolved subgiant with 1.7$M_{\odot}$, and has a planet of $m_2$sin$i$$=$ 2.4$M_{\rm J}$ in a 899d orbit with $e=$ 0.13. HD167042 (K1 IV) is also a less-evolved star with 1.5$M_{\odot}$ hosting a planet of $m_2$sin$i$$=$ 1.6$M_{\rm J}$ in a 418d orbit with $e=$ 0.10. This planet was independently announced by Johnson et al. (2008, ApJ, 675, 784). All of the host stars have solar or sub-solar metallicity, which supports the lack of a metal-rich tendency in planet-harboring giants in contrast to the case of dwarfs.

Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Delphini, ξ Aquilae, and HD 81688

Abstract We report on the detection of 3 new extrasolar planets from a precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, 18 Del (G6 III), $\xi $Aql (K0 III) and HD81688 (K0 III-IV), are located in the clump region on the HR diagram with estimated masses of 2.1-2.3$M_{\odot}$. 18Del b has a minimum mass of $10.3 M_{\rm J}$ and resides in a nearly circular orbit with period of 993d, which is the longest one around evolved stars. $\xi $ Aql b and HD81688 b have minimum masses of 2.8 and 2.7$M_{\rm J}$, and reside in nearly circular orbits with periods of 137 and 184d, respectively, which are the shortest ones around evolved stars. All of the substellar companions ever discovered around intermediate-mass (1.7-3.9$M_{\odot}$) clump giants have semimajor axes larger than 0.68AU, suggesting a lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5AU (even up to 1AU, depending on the metallicity and adopted models) around 2-3$M_{\odot}$ stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars. Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial, or due to engulfment by central stars.

ABUNDANCE PATTERN OF METAL-RICH STARS FROM 14 OLD AND 24 YOUNG STARS

The metallicities and abundance ratios, [X/Fe], of 12 elements—C, Na, Mg, Si, S, Ca, Ti, Cr, Ni, Y, Zr, and Ce—are determined for 14 old and 24 young metal-rich stars based on high resolution, high S/N spectra obtained with the HIDES spectrograph attached to the 1.88 m telescope of Okayama Astrophysical Observatory (Okayama, Japan). The results show that there is no any significant difference in the [X/Fe] versus [Fe/H] trend for these elements between the two groups of stars. This is consistent with the kinematics of the two groups of stars being similar to that of the thin disk. In connection with the abundance analysis, we found that iron abundances from Fe II lines are generally higher than those from Fe I lines for cool metal-rich stars. In particular, the [S/Fe] ratio strongly depends on stellar temperature. In view of this, the chemical evolution of the element sulfur can only be traced by selecting solar-type stars within a critically narrow temperature range. Inspecting a large sample of metal-rich stars, we found that young metal-rich stars generally have kinematics similar to that of the local thin disk, while old metal-rich stars show two different kinematic distributions, one with V LSR ~ −10kms−1 and the other with V LSR ~ −50kms−1. Abundances of this new population of old metal-rich stars with a slight lag in the Galactic rotation have already been investigated by the present authors. We compared the abundance patterns for the three groups of metal-rich stars, and did not find any significant difference in abundance ratios for the elements investigated. These results indicate an inhomogeneous metallicity enhancement but similar nucleosynthesis history for the Galactic evolution of the thin disk from the beginning to the present.

A Substellar Companion to the Intermediate‐Mass Giant 11 Comae

We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong Station and Okayama Astrophysical Observatory (OAO) reveal Keplerian velocity variations with an orbital period of 326.03 ± 0.32 days, a semiamplitude of 302.8 ± 2.6 m s−1, and an eccentricity of 0.231 ± 0.005. Adopting a stellar mass of 2.7 ± 0.3 M☉, the minimum mass of the companion is 19.4 ± 1.5 MJ, well above the deuterium-burning limit, and the semimajor axis is 1.29 ± 0.05 AU. This is the first result from a joint planet-search program between China and Japan aimed at revealing the statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet-search program at OAO. The program's current detection rate of brown dwarf candidates seems to be comparable to the rate of such detections around solar-type stars with orbital separations of 3 AU.

A Korea-Japan planet search program: Current status and discovery of a brown dwarf candidate

AbstractSince 2005, we have been carrying out a precise radial velocity survey of about 190 intermediate-mass (1.5-5 M⊙) G and K giants at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea and Okayama Astrophysical Observatory (OAO) in Japan, which aims to reveal statistical properties of planetary systems around intermediate-mass stars. We have finished the first screening of 120 stars so far and have identified 5 candidates with large periodic radial velocity variations. One of the candidates turned out to be orbited by a brown dwarf mass companion with minimum mass of 37.6 MJup and semimajor axis of 1.71 AU. The primary star has a mass of 3.9 M⊙, which ranks among the most massive stars with substellar companions. Our discovery may support the current view obtained from results of planet searches around intermediate-mass stars that massive substellar companions tend to form around massive stars.

A Planetary Companion to the Hyades Giant ε Tauri

Wereportthedetection of anextrasolarplanet orbitingTau,oneof thegiantstarsintheHyadesopencluster.This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594:9 � 5:3 days, a semiamplitude of 95:9 � 1: 8ms � 1 , and an eccentricity of 0:151 � 0:023. The minimum mass of the companion is 7:6 � 0:2MJ,andthesemimajoraxisis1:93 � 0:03AUadoptingastellarmassof 2:7 � 0:1M� .Theageof 625Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 Mfor thehoststarisrobustlydeterminedbyisochronefitting,whichmakesthestartheheaviestamongplanet-harboringstars. Puttingtogetherthefactthatnoplanetshavebeenfoundaroundabout100low-massdwarfsinthecluster,thefrequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones. Subject headingg open clusters and associations: individual (Hyades) — planetary systems — stars: individual (� Tauri) — techniques: radial velocities

Behavior of Li abundances in solar-analog stars

Context. It is known that the surface lithium abundances of field solar-analog G dwarfs show a large dispersion of >2 dex (among which our Sun is located at the lower end) despite the similarity of stellar parameters, and planet-host stars tend to show comparatively lower Li abundances in the narrow Teff range. Aims. To investigate the reason for these phenomena, an extensive study of Li abundances and their dependence on stellar parameters was carried out for a homogeneous sample of 118 selected solar analogs based on high-dispersion spectra obtained at Okayama Astrophysical Observatory. Methods. The atmospheric parameters were spectroscopically determined by using the equivalent widths of Fe i and Fe ii lines, the ages/masses were estimated from stellar evolutionary tracks, and the width of the macrobroadening (rotation plus macroturbulence) function as well as Li abundances (ALi) were established by spectrum-fitting analyses. Results. The resulting ALi vs. Teff relation revealed a characteristic inverse-triangle-like distribution enclosed by two clear-cut boundaries (the slanted one running from ∼5900 K to ∼5800 K and the vertical one at ∼5700 K), while the Sun is located around its lowest apex. More significantly, ALi in this region of large dispersion was found to closely correlate with the macrobroadening width (vr+m), which is considered to be the most important parameter. Conclusions. With a reasonable assumption that the difference of rotational velocity is mainly responsible for the variety of vr+m ,w e may conclude that the stellar angular momentum plays the decisive role in determining the surface Li abundances of solar-analog stars in the Teff range of ∼5900–5700 K. The low-Li tendency of planet-host stars may thus be interpreted in terms of rotational characteristics.

Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Scuti (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova

We present low-resolution (R ~ 90) and medium-resolution (R ~ 2500) spectropolarimetry of nova V475 Sct taken with the HBS instrument, mounted on the 0.91 m telescope at the Okayama Astrophysical Observatory, and taken with the Faint Object Camera and Spectrograph, mounted on the 8.2 m Subaru telescope. We estimated the interstellar polarization toward the nova from the steady continuum polarization components and Hα line emission components. After subtracting the interstellar polarization component from the observations, we found that the Hα emission seen on 2003 October 7 was clearly polarized. In the polarized flux spectrum the Hα emission had a distinct red wing extending to ~+4900 km s-1 and a shoulder around +3500 km s-1, showing a constant position angle (P.A.) of linear polarization θ* 155° ± 15°. This suggests that the nova had an asymmetric outflow with a velocity of vwind 3500 km s-1 or more, which is 6 times higher than the expansion velocity of the ionized shell at the same epoch. Such a high-velocity component has not previously been reported for a nova in the moderately fast speed class. Our observations suggest the occurrence of violent mass-loss activity in the nova binary system even during the common-envelope phase. The P.A. of the polarization in the Hα wing is in good agreement with that of the continuum polarization found on 2003 September 26 (p* 0.4%-0.6%), which disappeared within the following 2 days. The uniformity of the P.A. between the continuum polarization and the wing polarization on October 7 suggests that the axis of the circumstellar asymmetry remained nearly constant during the period of our observations.

TRISPEC: A Simultaneous Optical and Near‐Infrared Imager, Spectrograph, and Polarimeter

ABSTRACTWe present the design, construction, and performance of TRISPEC (Triple Range Imager and Spectrograph), a simultaneous optical and near‐infrared imager, spectrograph, and polarimeter. This instrument splits the incoming light from a telescope into the three beams—one optical channel (0.45–0.90 μm) and two infrared channels (0.90–1.85 and 1.85–2.5 μm)—by means of two dichroic mirrors. It is capable of simultaneous three‐band imaging or spectroscopy, with or without polarimetry. Low‐resolution grisms (R∼70–360) cover a wide wavelength range of 0.46–2.5 μm, simultaneously. The instrument employs three arrays—one 512 × 512 SITe CCD and two 256 × 256 SBRC InSb arrays—to cover the three channels. It has been commissioned in the imaging, spectroscopic, and polarimetric modes on the United Kingdom Infrared Telescope, the University of Hawaii 2.2 m telescope, and the Okayama Astrophysical Observatory 1.88 m telescope since 1999 June.

CURRENT STATUS OF THE INSTRUMENTS, INSTRUMENTATION AND OPEN USE OF OKAYAMA ASTROPHYSICAL OBSERVATORY

Current instrumentation activities and the open user status of Okayama Astrophysical Observatory (OAO) are reviewed. There are two telescopes in operation and one telescope under reforming at OAO. The 188cm telescope is provided for open use for more than 200 nights in a year. The typical over-subscription rate of observation proposals for the 188cm telescope is <TEX>${\~}$</TEX> 1.5 - 2. The 50cm telescope is dedicated to <TEX>$\gamma$</TEX>-ray burst optical follow-up observation and is operated in collaboration with Tokyo Institute of Technology. The 91cm telescope will become a new very wide field near-infrared camera in two years. The high-dispersion echelle spectrograph (HIDES) is the current primary instrument for the open use of the 188cm telescope. Two new instruments, an infrared multi-purpose camera (ISLE) and an optical low-dispersion spectrograph (KOOLS), are now under development. They will be open as common use instruments in 2006.

OKAYAMA PLANET SEARCH PROGRAM

We have carried out a precise Doppler survey of G-type giants aiming to unveil the properties of planetary systems in intermediate-mass stars (<TEX>$1.5-5M_{\bigodot}$</TEX>). G-type giants are promising targets for Doppler planet searches around massive stars, because they are slow-rotators and have many sharp absorption lines in their spectra and their surface activities are relatively low in contrast to their younger counterparts on the main-sequence (B-A stars). We are now monitoring radial velocities of about 300 late G-type (including early K-type) giants using HIgh Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. We have achieved a Doppler precision of about 6-7 m/s over a time span of 3 years using an iodine absorption cell. We found that most of the targets have radial velocity scatters of <TEX>${\sigma}{\~} 10-20 m\;s^{-1}$</TEX> over 1-3 years, with the most stable reaching levels of 6-8 m <TEX>$s^{-1}$</TEX>. Up to now, we have succeeded in discovering the first extrasolar planet around a G-type giant star HD 104985, and also found several candidates showing significant radial velocity variations, suggesting the existence of stellar and substellar companions. Observations have continued to establish their variability.

AN EAST-ASIAN EXTRA-SOLAR PLANET SEARCH NETWORK

We are undertaking an extra-solar planet search around G-type giant stars by means of Doppler technique using an iodine absorption cell installed to the high dispersion echelle spectrograph for the 188 cm reflector at Okayama Astrophysical Observatory (Okayama Planet Search Program, OPSP). Having detected the first planet candidate (Sato et al. 2003)the search has been proved very promising. Taking advantage of this success, we are trying to develop OPSP to an international collaborative work. We here report the current status of our efforts for establishing such collaborations, namely, those with Chinese and Korean astronomers. We also propose to establish an East-Asian network to search for extra-solar planets around G-type giant stars with the transit detecting technique as well as the Doppler technique, asking other persons/groups to join us to enjoy the planet search.

OKAYAMA ASTROPHYSICAL OBSERVATORY WIDE-FIELD CAMERA

We present the design, expected performance, and current status of the wide field near-infrared camera (OAOWFC) now being developed at Okayama Astrophysical Observatory, NAOJ, NINS. OAOWFC is a near-infrared survey telescope whose effective aperture is 91cm. It works at Y, J, H, and <TEX>$K_s$</TEX> bands and is dedicated to the survey of long period variable stars in the Galactic plane. The field of view is <TEX>$0.95 {\times} 0.95 deg^2$</TEX> which is covered by one HAWAII-2 RG detector of 2048 <TEX>${\times}$</TEX> 2048 pixels with the pixel size of <TEX>$18.5 {\mu}m\;{\times}\;18.5{\mu}m$</TEX>, that results in the sampling pitch of 1.6 arcsec/pixel. OAOWFC can sweep the area of <TEX>$840 deg^2$</TEX> every 3 weeks, attaining a limiting magnitude of 13 in <TEX>$K_s$</TEX> band. It allows us to observe long period variables embedded in the Galactic plane where interstellar extinction is severe in optical.

High-Dispersion Spectra Collection of Nearby F–K Stars at Okayama Astrophysical Observatory: A Basis for Spectroscopic Abundance Standards

Abstract Towards constructing a system of high-dispersion spectroscopic standard stars with well-established abundances, so that adequate reference stars of similar properties may be found for precise differential abundance studies of any near-solar type stars, we have collected high-resolution ($R \sim 70000$) echelle spectra for 160 F–K dwarfs and subgiants of the solar neighborhood in three wavelength regions (5000–6200 Å, 5800–7000 Å, and 7600–8800 Å) at Okayama Astrophysical Observatory. This spectra database is open to the public.

Stellar Parameters and Photospheric Abundances of Late-G Giants: Properties of the Targets of the Okayama Planet Search Program

Abstract Towards clarifying the properties of late-G giants, for which we are currently conducting a planet-search project at Okayama Astrophysical Observatory, an extensive spectroscopic study has been performed for our first target sample of fifty-seven G6–G9 III stars, in order to establish the atmospheric parameters ($T_{\mathrm{eff}}, \log g, v_\mathrm{t}$, and $\mathrm{[Fe/H]}$), the stellar mass along with the evolutionary status, and the photospheric abundances of various elements. It was confirmed that the conventional spectroscopic method of parameter determination using Fe I / II lines with the assumption of LTE works successfully for these evolved stars. We determined the abundances (relative to the Sun) of 19 elements, and examined how their $\mathrm{[X/Fe]}$ ratios behave themselves with the run of $\mathrm{[Fe/H]}$. While the trends appear to be similar to those exhibited by disk dwarfs for a number of elements, some elements (C, O, Na) showed appreciable anomalies, which may be interpreted as being due to a dredge-up of nuclear-processed material. Since the $\mathrm{[Fe/H]}$ values of these stars tend to be somewhat biased towards a subsolar regime, some mechanism of slightly reducing the metallicity might be suspected.

Forty Years of Spectroscopic Stellar Astrophysics in Japan

Abstract The development of Japanese spectroscopic stellar astrophysics in the recent 40 years is reviewed from an observational point of view. In this article, the research activities are provisionally divided into four fields: hot stars, hot emission-line (Be) stars, cool stars, and other stars. Historical developments of the observational facilities at Okayama Astrophysical Observatory (spectrographs and detectors) are also summarized in connection with the progress in scientific research activities.

Spectroscopic Study on the Atmospheric Parameters of Nearby F–K Dwarfs and Subgiants

Abstract Based on a collection of high-dispersion spectra obtained at Okayama Astrophysical Observatory, the atmospheric parameters ($T_{\mathrm{eff}}, \log g, v_{\mathrm{t}}$, and $\mathrm{[Fe/H]}$) of 160 mid-F through early-K stars were extensively determined by the spectroscopic method using the equivalent widths of Fe I and Fe II lines along with the numerical technique of Takeda et al. (2002, PASJ, 54, 451). The results are comprehensively discussed and compared with the parameter values derived by different approaches (e.g., photometric colors, theoretical evolutionary tracks, Hipparcos parallaxes, etc.) as well as with the published values found in various literature. It has been confirmed that our purely spectroscopic approach yields fairly reliable and consistent results.

Ground-based observation of the Venus 1.27-μm O 2 airglow

We conducted near-infrared imaging spectroscopy of the nightside of Venus with a 1.88-m telescope at Okayama Astrophysical Observatory, Japan. The 1.3-μm region spectra show 1.27-μm O 2 airglow feature and thermal emission from the lower atmosphere. A bright airglow feature of 5 mega-Rayleigh (1 MR = 10 12 photons cm −2 s −1 into 4π sr) was found at around the anti-solar point, as in the previous studies. The hemispheric distribution of 1.3-μm thermal emission shows an equatorial depression similar to that of the 2.3-μm region. The hemispheric distribution of the rotational temperature shows an enhancement at around the anti-solar point overlapping the bright region. This may suggest adiabatic and/or chemical heating due to downward flow.

A Planetary Companion to the G-Type Giant Star HD 104985

We report the detection of a planetary-mass companion to the G9 III giant star HD 104985 from precise Doppler velocity measurements made using the High Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. The radial velocity variability of this star is best explained by an orbital motion with a period of 198.2 ± 0.3 days, a velocity semiamplitude of 161 ± 2 m s-1, and an eccentricity of 0.03 ± 0.02. Assuming a stellar mass of 1.6 M☉, we obtained a minimum mass and a semimajor axis of 6.3MJ and 0.78 AU, respectively, for the companion. A probable upper limit to the stellar mass of 3 M☉ yielded m2 sin i = 9.6MJ, which falls in the planetary-mass regime. This is the first discovery of a planetary companion orbiting a G-type giant star.