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Sakamoto T.,Japan Spaceguard Association | Matsunaga N.,University of Tokyo | Hasegawa T.,Gunma Astronomical Observatory | Nakada Y.,University of Tokyo
Astrophysical Journal Letters | Year: 2012

We report the discovery of two Mira variable stars (Miras) toward the Sextans dwarf spheroidal galaxy (dSph). We performed optical long-term monitoring observations for two red stars in the Sextans dSph. The light curves of both stars in the I c band show large-amplitude (3.7 and 0.9 mag) and long-period (326 ± 15 and 122 ± 5 days) variations, suggesting that they are Miras. We combine our own infrared data with previously published data to estimate the mean infrared magnitudes. The distances obtained from the period-luminosity relation of the Miras (75.3+12.8 -10.9 and 79.8+11.5 - 9.9 kpc, respectively), together with the radial velocities available, support memberships of the Sextans dSph (90.0 ± 10.0 kpc). These are the first Miras found in a stellar system with a metallicity as low as [Fe/H] ∼ -1.9 than any other known system with Miras. © 2012. The American Astronomical Society. All rights reserved.

Maciejewski G.,Astrophysikalisches Institute und Universitats Sternwarte | Maciejewski G.,Nicolaus Copernicus University | Dimitrov D.,Bulgarian Academy of Science | Neuhauser R.,Astrophysikalisches Institute und Universitats Sternwarte | And 14 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

Transit timing analysis may be an effective method of discovering additional bodies in extrasolar systems that harbour transiting exoplanets. The deviations from the Keplerian motion, caused by mutual gravitational interactions between planets, are expected to generate transit timing variations of transiting exoplanets. In 2009, we collected nine light curves of eight transits of the exoplanet WASP-10b. Combining these data with those published, we have found that transit timing cannot be explained by a constant period but by a periodic variation. Simplified three-body models, which reproduce the observed variations of timing residuals, were identified by numerical simulations. We have found that the configuration with an additional planet with a mass of ∼0.1MJ and an orbital period of ∼5.23 d, located close to the outer 5:3 mean motion resonance, is the most likely scenario. If the second planet is a transiter, the estimated flux drop will be ∼0.3 per cent and can be observed with a ground-based telescope. Moreover, we present evidence that the spots on the stellar surface and the rotation of the star affect the radial-velocity curve, giving rise to a spurious eccentricity of the orbit of the first planet. We argue that the orbit of WASP-10b is essentially circular. Using the gyrochronology method, the host star was found to be 270 ± 80 Myr old. This young age can explain the large radius reported for WASP-10b. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Naito H.,Nagoya University | Mizoguchi S.,Sendai Astronomical Observatory | Arai A.,Kyoto Sangyo University | Tajitsu A.,Japan National Astronomical Observatory | And 9 more authors.
Astronomy and Astrophysics | Year: 2012

We present optical (B, V, R c, I c and y) and near-infrared (J, H, and K s) photometric and spectroscopic observations of a classical nova V1280 Scorpii for five years from 2007 to 2011. Our photometric observations show a declining event in optical bands shortly after the maximum light, which took about 250 days to recover. This event was most probably caused by dust formation. The event was accompanied by a short (∼30 days) re-brightening episode (∼2.5 mag in V), which suggests that there had been some re-ignition of the surface nuclear burning. After 2008, the y band observations show a very long plateau at around y = 10.5 for more than 1000 days until April 2011 (∼1500 days after the maximum light). The nova had taken a very long time (∼50 months) to enter the nebular phase, according to a clear detection of both [O III] 4959 and 5007 and is still continuing to generate the wind caused by H-burning. This finding suggests that historically V1280 Sco is evolving at its slowest ever measured rate. The interval from the maximum light (2007 February 16) to the beginning of the nebular phase is longer than any previously known slow novae: V723 Cas (18 months), RR Pic (10 months), or HR Del (8 months). It suggests that the mass of a white dwarf in the V1280 Sco system might be 0.6 M ⊙ or lower. The distance, based on our measurements of the expansion velocity combined with the directly measured size of the dust shell, is estimated to be 1.1 ± 0.5 kpc. © 2012 ESO.

Honda S.,Gunma Astronomical Observatory | Honda S.,Kyoto University | Aoki W.,Japan National Astronomical Observatory | Aoki W.,Graduate University for Advanced Studies | And 2 more authors.
Astrophysical Journal | Year: 2011

We report Zn abundances for 18 very metal-poor stars studied in our previous work, covering the metallicity range -3.2< [Fe/H] <-2.5. The [Zn/Fe] values of most stars show an increasing trend with decreasing [Fe/H] in this metallicity range, confirming the results found by previous studies. However, the extremely metal-poor star BS 16920-017([Fe/H] =-3.2) exhibits a significantly high [Zn/Fe] ratio ([Zn/Fe] = +1.0). Comparison of the chemical abundances of this object with HD 4306, which has similar atmospheric parameters to BS 16920-017, clearly demonstrates a deficiency of α elements and neutron-capture elements in this star, along with enhancements of Mn and Ni, as well as Zn. The association with a hypernova explosion that has been proposed to explain the high Zn abundance ratios found in extremely metal-poor stars is a possible explanation, although further studies are required to fully interpret the abundance pattern of this object. © 2011. The American Astronomical Society. All rights reserved.

Stonkute R.,Lithuanian Academy of Sciences | Arimoto N.,Japan National Astronomical Observatory | Arimoto N.,Graduate University for Advanced Studies | Hasegawa T.,Gunma Astronomical Observatory | And 4 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

We have surveyed a complete extent of Leo A - an apparently isolated gas-rich low-mass dwarf irregular galaxy in the Local Group. The B, V, and I passband CCD images (typical seeing ∼0.″8) were obtained with the Subaru Telescope equipped with the Suprime-Cam mosaic camera. The wide-field (20′ × 24′) photometry catalog of 38,856 objects (V ∼ 16-26 mag) is presented. This survey is also intended to serve as "a finding chart" for future imaging and spectroscopic observation programs of Leo A. © 2014. The American Astronomical Society. All rights reserved.

Moritani Y.,Kyoto University | Nogami D.,Kyoto University | Okazaki A.T.,Hokkai Gakuen University | Imada A.,Japan National Astronomical Observatory | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We report on optical high-dispersion spectroscopic monitoring observations of the Be/X-ray binary A0535+26/V725 Tau, carried out from 2005 November to 2009 March. The main aim of these monitoring observations is to study spectral variabilities in the Be disc, on both the short (a week or so) and long (more than hundreds of days) time-scales, by taking long-term frequent observations. Our four-year spectroscopic observations indicate that the V/R ratio, i.e. the relative intensity of the violet (V) peak with respect to the red (R) one, of the double-peaked Hα line profile varies with a period of 500 d. The Hβ line profile also varies in phase with the Hα profile. With these observations covering two full cycles of the V/R variability, we reconstruct the two-dimensional structure of the Be disc by applying the Doppler tomography method to the Hα and Hβ emission-line profiles, using a rigidly rotating frame with the V/R variability period. The resulting disc structure reveals non-axisymmetric features, which can be explained by a one-armed perturbation in the Be disc. It is the first time that an eccentric disc structure is directly detected by using a method other than the interferometric one. © 2010 The Authors. Journal compilation © 2010 RAS.

Miura R.,Japan National Astronomical Observatory | Miura R.,University of Tokyo | Okumura S.K.,Japan National Astronomical Observatory | Okumura S.K.,University of Tokyo | And 9 more authors.
Astrophysical Journal | Year: 2010

We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and 89 GHz continuum emission toward NGC 604, known as the supergiant HII region in the nearby galaxy M33. Our high spatial resolution images (4″.2 × 2Prime;.6, corresponding to 17 pc × 11 pc physical size) of CO emission allowed us to uncover 10 individual molecular clouds that have masses of (0.8-7.4) × 105 M ⊙ and sizes of 5-29 pc, comparable to those of typical Galactic giant molecular clouds. Moreover, we detected for the first time HCN emission in the two most massive clouds and 89 GHz continuum emission at the rims of the "Hα shells." The HCN and 89 GHz continuum emission are offset from the CO peak and are distributed in the direction of the central cluster. Three out of ten CO clouds are well correlated with the H α shells both in spatial and velocity domains, implying an interaction between molecular gas and the expanding HII region. The CO clouds show varieties in star formation efficiencies (SFEs), which are estimated from the 89 GHz emission and combination of Hα and Spitzer 24 μm data. Furthermore, we found that the SFEs decrease with increasing projected distance measured from the heart of the central OB star cluster in NGC 604, suggesting radial changes in the evolutionary stages of the molecular clouds in the course of stellar cluster formation. Our results provide further support to the picture of sequential star formation in NGC 604 initially proposed by Tosaki et al. with the higher spatially resolved molecular clouds, in which an isotropic expansion of the HII region pushes gases outward, which accumulates to form dense molecular clouds, and then induces massive star formations. © 2010. The American Astronomical Society.

Sadakane K.,Osaka Kyoiku University | Kambe E.,Japan National Astronomical Observatory | Sato B.,Tokyo Institute of Technology | Honda S.,Gunma Astronomical Observatory | And 2 more authors.
Publications of the Astronomical Society of Japan | Year: 2010

A detailed abundance analysis of the primary star of ε Aur before an eclipse was carried out using a very high signal-to-noise ratio optical-region spectrum. An A7 Iab supergiant HD 81471, presumably a member of the Vela OB1 association, was used as a reference. We obtained atmospheric parameters (T eff, logg, and ξt) to be (8025K, 1.0 and 10 km s -1) and (8050K, 1.0 and 7 kms-1) for ε Aur and HD 81471, respectively. The abundances of Mg, Si, S, Ca, Sc, Ti, Cr, and Fe were very close to the solar abundances in both stars. Light elements C and O were under-abundant, while N and Na were over-abundant in both stars, after correcting for non-LTE effects. Definite under-abundances of Sr were detected in both stars. Slight, but definite, over-abundances were found in ε Aur for the s-process elements Y, Zr, and Ba, when compared with the results of HD 81471. Enhancements in the abundances of N, Na, and s-process elements might indicate the occurences of thermal dredge-up and the s-process nucleosynthesis in ε Aur during the past evolution. © 2010. Astronomical Society of Japan.

Sadakane K.,Osaka Kyoiku University | Kambe E.,Japan National Astronomical Observatory | Hashimoto O.,Gunma Astronomical Observatory | Honda S.,University of Hyogo | Sato B.,Tokyo Institute of Technology
Publications of the Astronomical Society of Japan | Year: 2013

Frequent high-resolution spectroscopic observations of ε Aur during the 2009-2011 eclipse revealed temporal appearances of low-excitation absorption lines of neutral iron peak elements. Several 0 eV lines of Fe I were detected near the end of the first half of the eclipse. They had migrated toward shorter wavelength, became much strong after the third contact, and then disappeared after the fouth contact. Curiously, the widths of these temporal absorption lines became broader for about a 100-d period just before the third contact. The radial velocity measured for these lines follows that of the KI line at 7699 A°, implying that these lines originated in a rotating disk around the secondary star, which occulted the primary star. Weak and sharp absorption lines of low excitation neutral metals, such as Sc I, Ti I, VI, Cr I, and MnI, were observed only for a short period from the third contact to the fourth contact, and they were all blue-shifted by ∼40 km s-1. These observations show that the rotating disk around the secondary star is not axially symmetric, but suggest that a clump of cool gas concentrated in a region observed projected on the primary star near at the end of the eclipse. © 2013 Astronomical Society of Japan.

Hayato A.,RIKEN | Hayato A.,NASA | Yamaguchi H.,RIKEN | Tamagawa T.,RIKEN | And 12 more authors.
Astrophysical Journal | Year: 2010

We showthat the expansion of ejecta in Tycho's supernova remnant (SNR) is consistent with a spherically symmetric shell, based on Suzaku measurements of the Doppler broadened X-ray emission lines. All of the strong Kα line emissions show broader widths at the center than at the rim, while the centroid energies are constant across the remnant (except for Ca). This is the pattern expected for Doppler broadening due to expansion of the SNR ejecta in a spherical shell. To determine the expansion velocities of the ejecta, we applied a model for each emission-line feature having two Gaussian components separately representing red- and blueshifted gas, and inferred the Doppler velocity difference between these two components directly from the fitted centroid energy difference. Taking into account the effect of projecting a three-dimensional shell to the plane of the detector, we derived average spherical expansion velocities independently for the Kα emission of Si, S, Ar, and Fe, and Kβ of Si. We found that the expansion velocities of Si, S, and Ar ejecta of 4700 ± 100 km s-1 are distinctly higher than that obtained from Fe Kα emission, 4000 ±300 km s-1, which is consistent with segregation of the Fe in the inner ejecta. Combining the observed ejecta velocities with the ejecta proper-motion measurements by Chandra, we derived a distance to Tycho's SNR of 4 ± 1 kpc.

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