T. Nakajima

National Astronomical Observatory of Japan, Edo, Tōkyō, Japan

Are you T. Nakajima?

Claim your profile

Publications (19)114.64 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the discovery of 15 new T2.5–T7.5 dwarfs (with estimated distances ∼24–93 pc), identified in the first three main data releases of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (LAS) (to date) to 28. These discoveries are confirmed by near-infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. Seven of the new T dwarfs have spectral types of T2.5–T4.5, five have spectral types of T5–T5.5, one is a T6.5p and two are T7–7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations), and find that three of these new T dwarfs may have unusual metallicity, two may have low surface gravity, and one may have high surface gravity. The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y−J with decreasing effective temperature, and some interesting colour changes in J−H and z−J (deserving further investigation) beyond T8. The LAS T dwarf sample from the first and second main data releases show good evidence for a good level of completion to J= 19. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity, Malmquist and Eddington bias, we estimate that there are 17 ± 4 ≥ T 4 dwarfs in the J≤ 19 volume of the LAS second data release. This value is most consistent with theoretical predictions if the substellar mass function exponent α (dN/dm∝m−α) lies between −1.0 and 0. This is consistent with the latest 2-Micron All Sky Survey (2MASS)/Sloan Digital Sky Survey (SDSS) constraint (which is based on lower number statistics) and is significantly lower than the α∼ 1.0 suggested by L dwarf field populations, which is possibly a result of the lower mass range probed by the T dwarf class.
    Full-text · Article · Oct 2008 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the technique and results of a survey of stars within 8 pc of the Sun with declinations δ > -35° (J2000.00). The survey, designed to find without color bias faint companions, consists of optical coronagraphic images of the 1' field of view centered on each star and infrared direct images with a 32'' field of view. The images were obtained through the optical Gunn r and z filters and the infrared J and K filters. The survey achieves sensitivities up to 4 absolute magnitudes fainter than the prototype brown dwarf, Gliese 229B. However, this sensitivity varies with the seeing conditions, the intrinsic brightness of the star observed, and the angular distance from the star. As a result, we tabulate sensitivity limits for each star in the survey. We used the criterion of common proper motion to distinguish companions and to determine their luminosities. In addition to the brown dwarf Gl 229B, we have identified six new stellar companions of the sample stars. Since the survey began, accurate trigonometric parallax measurements for most of the stars have become available. As a result, some of the stars we originally included should no longer be included in the 8 pc sample. In addition, the 8 pc sample is incomplete at the faint end of the main sequence, complicating our calculation of the binary fraction of brown dwarfs. We assess the sensitivity of the survey to stellar companions and to brown dwarf companions of different masses and ages.
    Full-text · Article · Dec 2007 · The Astronomical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present eight new T4.5–T7.5 dwarfs identified in the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) Data Release 1 (DR1). In addition we have recovered the T4.5 dwarf SDSS J020742.91+000056.2 and the T8.5 dwarf ULAS J003402.77−005206.7. Photometric candidates were picked up in two-colour diagrams over 190 deg2 (DR1) and selected in at least two filters. All candidates exhibit near-infrared spectra with strong methane and water absorption bands characteristic of T dwarfs and the derived spectral types follow the unified scheme of Burgasser et al.. We have found six new T4.5–T5.5 dwarfs, one T7 dwarf, one T7.5 dwarf and recovered a T4.5 dwarf and a T8.5 dwarf. We provide distance estimates which lie in the 15–85 pc range; the T7.5 and T8.5 dwarfs are probably within 25 pc of the Sun. We conclude with a discussion of the number of T dwarfs expected after completion of the LAS, comparing these initial results to theoretical simulations.
    Full-text · Article · Aug 2007 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context.We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys.Aims.We aim to show that our methodologies for searching the ~27 deg$^2$ of the LAS EDR are successful for finding both L and T dwarfs via cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs.Methods.Ultracool dwarfs are selected by combinations of their $\it YJH(K)$ UKIDSS colours and SDSS DR4 $z-J$ and $i-z$ colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4 m and 8 m-class facilities.Results.Our main result is the discovery following CH$_4$ imaging and spectroscopy of a T4.5 dwarf, ULAS J 1452+0655, lying ~80 pc distant. A further T dwarf candidate, ULAS J 1301+0023, has very similar CH$_4$ colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter L0 dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey $J$-band limit.Conclusions.Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky.
    Full-text · Article · May 2007 · Astronomy and Astrophysics
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have started a corongraphic search for brown dwarfs and planets around young nearby stars within 20 pc of the Sun, using the adaptive optics coronagraph, CIAO, on Subaru. The dynamic range we have achieved is ΔK = 13 at 2.5″ from the central star. For a typical target with K = 7 at 10 pc, the limiting absolute magnitude is MK = 20. We apply two kinematical age criteria to select M and K dwarfs statistically younger than 350Myr. The first criterion is a small velocity deviation from the velocity of LSR. The second is a (U, V,W ) velocity vector similar to a particular young moving group. The combination of the age and magnitude limits implies that the mass limit for giant planet detection is about 2 MJ. We show a sample image of a target field at 3 pc of the Sun with faint companion candidates, to be followed up for the common proper motion test. We give a brief description of our procedures for data acquisition, reduction, and analysis. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    No preview · Article · Dec 2005 · Astronomische Nachrichten
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the optical design for the Japanese astrometry satellite mission (JASMINE). In order to accomplish measurements of astrometric parameters with high accuracy, optics with a long focal length and a wide focal plane for astrometry is required. In 1977, Korsch proposed a three mirror system with a long focal length and a wide focal plane. The Korsch system is one of the convincing models. However, the centre of the field is totally vignetted because of the fold mirror. Therefore we consider the improved Korsch system in which the centre of the field is not vignetted. Finally we obtain the diffraction limited optical design with small distortion. Our project needs a common astrometric technique to obtain precise positions of star images on solid state detectors to accomplish the objectives. In order to determine the centres of stars, an image of the point source must be focused onto the CCD array with a spread of a few pixels. The distribution of photons (photoelectrons) over a set of pixels enables us to estimate positions of stars with subpixel accuracy. We modify the algorithm to estimate the real positions of stars from the photon weighted mean, which is originally developed by the FAME (Full-Sky Astrometric Mapping Explorer) group. Finally, we obtain the results from the experiment that the accuracy of estimation of distance between two stars is about a variance of 1/300 pixel, that is, the error for one measurement is about 1/300 pixel, which is almost an ideal result given by Poisson noise of photons. We also investigate the accuracy of estimation of positions with a different size of PSF. In this case also, we obtain that the accuracy of estimation is about a variance of 1/300 pixel.
    No preview · Article · Jan 2005
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a Japanese plan for infrared (z-band: 0.9μm) space astrometry (the JASMINE project). It will measure parallaxes, positions with the accuracy of 10 μas and proper motions with the accuracy of 10 μas/ year for stars brighter than z ∼ 14 mag. JASMINE can observe about one hundred million stars belonging to the disc and bulge components of our Galaxy, which are hidden by the interstellar dust extinction in optical bands. The number of stars with σπ/π < 0.1 in the direction of the Galactic central bulge is about 103 times larger than those observed in optical bands. The main objective of JASMINE is to provide very useful and important astrometric parameters for studying fundamental structures and evolutions of the disc and bulge components of the Milky Way Galaxy. Furthermore the astrometric parameters given by JASMINE will give us exact absolute luminosities and motions of many stars in the bulge and the disc far away from us and so it will promote the study of stellar physics. The information from infrared astrometry that JASMINE will provide is very useful also for investigating stars in star formation regions, gravitational lens effects due to disc stars, extra-solar planets, etc. We hope that JASMINE, which is due to be launched in around 2014, can be complementary to Gaia for surveying the bulge and the disc far away from us. Furthermore, we introduce a Nano-JASMINE project which uses a nano-satellite whose size is about 30 cm 3 and whose weight is a few kilograms. The objective of Nano-JASMINE is verification of the observing strategy adopted in JASMINE and examination of some important technical issues for the JASMINE project. It will be launched around 2006.
    Full-text · Article · May 2004 · Proceedings of the International Astronomical Union
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a Japanese future plan for IR space astrometry (the JASMINE-project). JASMINE is an infrared (K-band) scanning astrometric satellite. JASMINE (I and/or II-project) is planned to be launched between 2013 and 2015 and will measure parallaxes, positions and proper motions with the precision of 10 microarcsec at K ˜ 12 - 14 mag. JASMINE can observe about a few hundred million stars belonging to the disk and the bulge components of our Galaxy, which are hidden by interstellar dust extinction in optical bands. Furthermore JASMINE will also do the photometry of stars in the K, J and H-bands. The main objective of JASMINE is to study the fundamental structure and evolution of the disk and the bulge components of the Milky Way Galaxy. Furthermore its important objective is to investigate stellar physics.
    Full-text · Article · Jan 2002
  • T. Nakajima · H. Matsuhara

    No preview · Article · Jan 2000
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High-resolution, Keck Telescope echelle observations from 630 nm to 850 nm of seven Pleiads with spectral types from M5 to M6.5 reveal rather rapid rotation, with an average v sin i ~ 52 km s(-1) , and chromospheric activity in Hα emission. The activity in these stars is not any stronger than that of other Pleiades low-mass stars, despite the expected high contrast of Hα with their cool photospheres and their rapid rotation. This shows that the ``levelling off'' of Hα equivalent widths previously noted in low-mass stars in young clusters is not related to the conventional rotation-activity connection. None of the stars previously categorized as brown dwarf candidates have lithium signatures in their spectra. They are, therefore, very low-mass stars and not brown dwarfs. However, two stars, HHJ 339 and HHJ 430, 1 and 2 magnitudes above the Pleiades zero-age main sequence, do show absorption due to Li 1 at 670.8 nm and in the subordinate feature at 812.6 nm. These two stars are also rotating very rapidly. These facts strongly suggest that these stars are rather young. Their proper motions and radial velocities agree with those measured for the Pleiades as a whole. We discuss various explanations for these stars, none of which is completely satisfactory. In one scenario they represent very late star formation in the Pleiades cluster (implying a huge range in the ages of Pleiads). This seems unpalatable given the lack of matter dense enough to form stars in the Pleiades at present. Another possibility is that these stars formed in a nearby, more recent star formation site and drifted into the Pleiades. Although the cluster recently passed through a clump of young Taurus stars, we do not see how it could ``accrete'' two of them. In our most feasible explanation, we posit that a cloud which was a member of the ``Pleiades Supercluster'' recently formed stars, which are now scattered between us and the Pleiades. HHJ 339 and HHJ 430 could be members of this group whose motion has now brought them near the older open star cluster.
    Preview · Article · Dec 1996 · The Astronomical Journal
  • Source

    Preview · Article · Nov 1996
  • [Show abstract] [Hide abstract]
    ABSTRACT: Infrared broadband photometry of the cool brown dwarf Gliese 229B extending in wavelength from 0.8 to 10.5 micron is reported. These results are derived from both new data and reanalyzed, previously published data. Existing spectral data reported have been rereduced and recalibrated. The close proximity of the bright Gliese 229A to the dim Gliese 229B required the use of special techniques for the observations and also for the data analysis. We describe these procedures in detail. The observed luminosity between 0.8 and 10.5 micron is (4.9 +/- 0.6) x 10(exp -6) solar luminosity. The observed spectral energy distribution is in overall agreement with a dust-free model spectrum by Tsuji et al. for T(eff) approx. equal to 900 K. If this model is used to derive the bolometric correction, the best estimate of the bolometric luminosity is 6.4 x 10(exp -6) solar luminosity and 50% of this luminosity ties between 1 and 2.5 microns. Our best estimate of the effective temperature is 900 K. From the observed near-infrared spectrum and the spectral energy distribution, the brightness temperatures (T(sub B) are estimated. The highest, T(sub B) = 1640 K, is seen at the peak of the J band spectrum, while the lowest, T(sub B) is less than or equal to 600 K, is at 3.4 microns, which corresponds to the location of the fundamental methane band.
    No preview · Article · Nov 1996 · The Astronomical Journal
  • [Show abstract] [Hide abstract]
    ABSTRACT: Spectroscopic measurements of a cool brown dwarf, Gl 229B, reveal absorption features attributable to methane in the near infrared much like those of Jupiter. These features are not seen in any star. The presence of methane indicates that the surface temperature of Gl 229B is below 1000 kelvin. Features attributed to water vapor also indicate that Gl 229B is much cooler than any known star.
    No preview · Article · Jan 1996 · Science
  • Source
    G. Basri · G. Marcy · B. Oppenheimer · S. R. Kulkari · T. Nakajima

    Preview · Article · Jan 1996
  • Article: Gliese 229B
    K. Matthews · T. Nakajima · S. Kulkarni · B. Oppenheimer
    [Show abstract] [Hide abstract]
    ABSTRACT: IAUC 6280 available at Central Bureau for Astronomical Telegrams. IAUC 6280 available at Central Bureau for Astronomical Telegrams.
    No preview · Article · Dec 1995
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: BROWN dwarfs are starlike objects with masses less than 0.08 times that of the Sun, which are unable to sustain hydrogen fusion in their interiors1-4. They are very hard to detect, as most of the energy of gravitational contraction is radiated away within ~108 yr, leaving only a very low residual luminosity. Accordingly, almost all searches for brown dwarfs have been directed towards clusters of young stars-a strategy that has recently proved successful5,6. But there are only modest observable differences between young brown dwarfs and very lowmass stars, making it difficult to identify the former without appealing to sophisticated models7. Older brown dwarfs should have a more distinctive appearance, and if they are companions to nearby stars, their luminosity can be determined unambiguously. Here we report the discovery of a probable companion to the nearby star G1229, with no more than onetenth the luminosity of the least luminous hydro-gen-burning star. We conclude that the companion, G1229B, is a brown dwarf with a temperature of less than 1,200 K, and a mass ~20-50 times that of Jupiter.
    Full-text · Article · Nov 1995 · Nature
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Optical coronagraph images of the high proper motion astrometric binary Gl 105A reveal a very red companion, Gl 105C, located 3.27 arcsecs from Gl 105A at P.A. 287 deg. At this location, it is not clear whether Gl 105C can fully account for the astrometric perturbation of Gl 105A. Aperture photometry gives I(sub C) = 12.6 and R(sub C) - I(sub C) = 3.7 for Gl 105C, indicating that it is a very low mass M dwarf. Using the observed I(sub C), an empirical M(sub I) versus I-K relation, and an assumed distance of 8.2 pc to Gl 105A, M(sub K) = 9.7 is derived for Gl 105C. An empirical mass-M(sub K) relation for low-mass stars suggest a mass of 0.084 solar mass for Gl 105C, which is just above the minimum mass for stable hydrogen burning. Gl 105C was not detected in previous K-band searches; its detection demonstrates the usefulness of optical coronagraphy for identifying very low mass objects.
    Preview · Article · Jun 1995 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Brown dwarf companions have been searched for around stars within 10 pc of the Sun using the Johns-Hopkins University Adaptive Optics Coronagraph (AOC), a stellar coronagraph with an image stabilizer. The AOC covers the field around the target star with a minimum search radius of 1 sec .5 and a field of view of 1 arcmin sq. We have reached an unprecedented dynamic range of Delta m = 13 in our search for faint companions at I band. Comparison of our survey with other brown dwarf searches shows that the AOC technique is unique in its dynamic range while at the same time just as sensitive to brown dwarfs as the recent brown dwarf surveys. The present survey covered 24 target stars selected from the Gliese catalog. A total of 94 stars were detected in 16 fields. The low-latitude fields are completely dominated by background star contamination. Kolmogorov-Smirnov tests were carried out for a sample restricted to high latitudes and a sample with small angular separations. The high-latitude sample (b greater than or equal to 44 deg) appears to show spatial concentration toward target stars. The small separation sample (Delta Theta less than 20 sec) shows weaker dependence on Galactic coordinates than field stars. These statistical tests suggest that both the high-latitude sample and the small separation sample can include a substantial fraction of true companions. However, the nature of these putative companions is mysterious. They are too faint to be white dwarfs and too blue for brown dwarfs. Ignoring the signif icance of the statistical tests, we can reconcile most of the detections with distant main-sequence stars or white dwarfs except for a candidate next to GL 475. Given the small size of our sample, we conclude that considerably more targets need to be surveyed before a firm conclusion on the possibility of a new class of companions can be made.
    Full-text · Article · Jul 1994 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a Japanese future plan of the IR space astrometry(JASMINE-project). JASMINE is an infrared(K- band) scanning astrometric satellite. JASMINE(I and/or II- project) is planned to be launched between 2013 and 2015 and will measure parallaxes, positions and proper motions with the precision of 10 microarcsec at K=12∼14mag. JASMINE can observe about a few hundred million stars belonging to the disk and the bulge components of our Galaxy, which are hidden by the interstellar dust extinction in optical bands. Furthermore JASMINE will also measure the photometries of stars in K, J and H-bands. The main objective of JASMINE is to study the fundamental structure and evolution of the disk and the bulge components of the Milky Way Galaxy. Further- more its important objective is to investigate stellar physics.
    Preview · Article ·