Tadashi Nakajima

Dust in the Photospheric Environment. II. Effect on the Near-Infrared Spectra of L and T Dwarfs*

We report an attempt to interpret the spectra of L and T dwarfs with the use of the unified cloudy model (UCM). For this purpose, we extend the grid of the UCMs to cases of log g = 4.5 and 5.5. The dust column density relative to the gas column density in the observable photosphere is larger at higher gravities, and molecular line intensity is generally smaller at higher gravities. The overall spectral energy distributions (SEDs) are fJ fK in early T dwarfs (L/T transition objects), and finally fJ > fH > fK in middle and late T dwarfs, where fJ,fH, and fK are the peak fluxes at J, H, and K bands, respectively, in fν units. This tendency is the opposite of what is expected for the temperature effect, but it can be accounted for as the effect of thin dust clouds formed deep in the photosphere together with the effect of the gaseous opacities, including H2 (collision-induced absorption), H2O, CH4, and K I. Although the UCMs are semiempirical models based on a simple assumption that thin dust clouds form in the region of Tcr T Tcond (Tcr ≈ 1800 K is only an empirical parameter, while Tcond ≈ 2000 K is fixed by the thermodynamical data), the major observations, including the overall SEDs and the strengths of the major spectral features, are consistently accounted for throughout L and T dwarfs. In view of the formidable complexities of the cloud formation, we hope that our UCM can be of some use as a guide for future modeling of ultracool dwarfs and for interpretation of observed data of L and T dwarfs.

SPECTRAL CLASSIFICATION AND EFFECTIVE TEMPERATURES OF L AND T DWARFS BASED ON NEAR-INFRARED SPECTRA

We have obtained near-infrared spectra of L dwarfs, L/T transition objects, and T dwarfs using the Subaru telescope. The resulting spectra are examined in detail to study their dependence on spectral types. One question is where the methane feature appears: we suggest that it appears at L8 and marginally at L6.5. The water bands at 1.1 and 1.4 μm do not necessarily show steady increase toward later L types but may show inversion at late L types. This does not necessarily imply that the spectral types do not represent a temperature sequence, but rather can be interpreted as the increasing water abundance being offset by the heavier dust extinction in the later L types. We confirm that the FeH 0.99 μm bands appear not only in the late L dwarfs but also in the early T dwarfs. We suggest that FeH could be dredged up by the surface convective zone induced by the steep temperature gradient as a result of the large opacity of the dust cloud itself, and replenished constantly by convection.We have obtained bolometric luminosities of the objects with known parallaxes in our sample, first by integrating the spectra between 0.87 and 2.5 μm, and second by the K-band bolometric correction. Apart from an L3 dwarf, the bolometric luminosities obtained by both methods agree well, and this implies that the K-band bolometric correction, which is obtained using the unified cloudy models, can be applied to obtain the bolometric luminosities and effective temperatures of the L and T dwarfs with known parallaxes from the literature. The relation between the effective temperature and spectral type derived from the K-band bolometric correction shows monotonic behavior throughout the L-T sequence.

Transition from L to T Dwarfs on the Color-Magnitude Diagram

The color-magnitude (CM) diagram of cool dwarfs and brown dwarfs based on the recent astrometry data is compared with the CM diagram transformed from the theoretical evolutionary tracks via the unified cloudy models (UCMs) of L and T dwarfs. A reasonable agreement between the models and observations is shown for the whole regime of ultracool dwarfs covering L and T dwarfs, and this is achieved, for the first time, with the use of a single grid of self-consistent nongray model photospheres including dust clouds (UCMs; 700 K Teff 2600 K). A distinct brightening at the J band in the early T dwarfs revealed by the recent parallax measurements is explained as a natural consequence of the migration of the thin dust cloud to the inner region of the photosphere and should not necessarily be evidence for Burgasser et al.s proposition that the dust cloud breaks up in the L/T dwarf transition. Also, the rapid bluing from the late L to the early T dwarfs is a direct result of the migration of the thin dust cloud from the optically thin (τ < 1) to thick (τ 1) regimes while Lbol and Teff lower only slightly. Thus, the theoretical evolutionary models, the cloudy models of the photospheres (UCMs), and the observed fundamental stellar parameters are brought into a consistent picture of the newly defined L and T dwarfs.

CCD Centroiding Experiment for the Japan Astrometry Satellite Mission (JASMINE) and In Situ Lunar Orientation Measurement (ILOM)

JASMINE and ILOM are space missions that are in progress at the National Astronomical Observatory of Japan. These two projects require a common astrometric technique to obtain precise positions of star images on solid state detectors to accomplish their objectives. We have carried out measurements of the centroid of artificial star images on a CCD to investigate the accuracy of the positions of the stars using an algorithm for estimating them from photon-weighted means of the stars. We find that the accuracy of the star positions reaches 1/300 pixel for one measurement.

AN INTERPRETATION OF FLAT DENSITY CORES OF CLUSTERS OF GALAXIES BY DEGENERACY PRESSURE OF FERMIONIC DARK MATTER: A CASE STUDY OF A1689

Flat density cores have been observed for a limited number of clusters of galaxies by strong gravitational lensing. Using a phenomenological equation of state (EOS) describing the full-to-partial degeneracy, we integrate the equation of hydrostatic equilibrium. The EOS is based on the assumption that the local kinetic energy of a classical particle induced by gravity dissolves the quantum statistical degeneracy. The density profile is uniquely determined by four parameters: the central density, ρ(0); the properties of a fermion, namely, the mass, m, and statistical weight, g; and the ratio of the total matter density and fermion density, δ. As a case study, we model the column density and two-dimensional (2D) encircled mass profiles of A1689, whose column density profile has been observationally obtained by Broadhurst et al. using gravitational lensing. The column density and 2D encircled profiles at the core are reasonably reproduced for models with a limited range of particle properties. In the case that previously unknown fermions with spin dominate the dark matter, the acceptable particle mass range is between 2 and 4 eV. In the case that the dark matter consists of a mixture of degenerate relic neutrinos and classical collisionless cold dark matter (CDM) particles, the mass range of neutrinos is between 1 and 2 eV, if the ratio of the two kinds of dark matter particles is fixed to its cosmic value. Both the pure fermionic dark matter models and neutrino-CDM-mixture models reproduce the observations equally well.

Near-infrared long-slit spectroscopy of IRAS 14348 - 1447

New observations of IRAS 14348 - 1447 were made with a near-infrared long-slit spectrograph, high spatial and velocity resolution making it possible to study the nature of the two distinct nuclei based on the line profiles, and to understand the three-dimensional geometry. The detection of a previously unknown emission feature at the rest wavelength 1.890 microns, and the possible identification of an allowed C II line are reported. The rather small radial-velocity difference of the two nuclei and optical morphology of the galaxy may indicate a near head-on collision of two spirals, viewed face-on. 19 refs.

CCD centroiding experiment for correcting a distorted image on the focal plane

ABSTRACT JASMINE (Japan Astrometry Satellite Mission for Infrared Exploration) and ILOM (In situ Lunar Orientation Measurement) are space missions that are in progress at the National Astronomical Observatory of Japan. These two projects require a common astrometric technique to obtain precise positions of star images on solid‐state detectors in order to accomplish their objectives. In the laboratory, we have carried out measurements of the centroid of artificial star images on a CCD array in order to investigate the precision of the positions of the stars, using an algorithm for estimating them from photon‐weighted means of the stars. In the calibration of the position of a star image at the focal plane, we have also taken into account the lowest order distortion due to optical aberrations, which is proportional to the cube of the distance from the optical axis. Accordingly, we find that the precision of the measurement for the positions of the stars reaches below 1/100 pixel for one measurement.

PARTIAL ADAPTIVE COMPENSATION AND PASSIVE INTERFEROMETRY WITH LARGE GROUND-BASED TELESCOPES

The possibilities for diffraction-limited imaging with large ground-basd optical telescopes have been investigated assuming the availability of a modest adaptive optics system. Using numerical simulations, we have examined the degree of partial wavefront compensation required for useful diffratcion-limited direct imaging as well as the performance improvement expected for both filled-aperture speckle imaging and non-redundant mask interferometric imaging in the presence of partial adaptive correction. A partially compensated point spread function will have a significant diffraction-limited central core if the residual rms wavefront error is less than two radians. In this regime (the sharp core regime) deconvolution is likely to permit diffraction-limited direct imaging. However, if the residual rms wavefront error is more than two radians (the speckle regime), conventional passive interferometric methods will be required to attain diffraction-limited resolution. To study the improvement in passive interferometry we have investigated the effect of a low order adaptive optics system, correcting the first 21 terms in the Zernike expansion of the wavefront perturbations, for the case of D/r0=25. By using the attenuation of the monochromatic speckle transfer function to define an effective measure of r0 we find that for Kolmogorov atmospheric turbulence characterized by a value of r0 of 0.2 m, the 21 term Zernike corrected value of r0 is 0.29 m. The quantitative improvement in the power spectrum transfer function at finite bandwidth can primarily be accounted for by this increase in effective r0 which reduces the redundancy of baselines caused by both the finite bandwidth and the spatial extent of the pupil. For optical aperture synthesis imaging, the higher value of r0 permits larger subapertures to be utilized which in turn permits wider bandwidths to be employed without geometric attenuation of the power spectrum. The derived values of r0 can be used to estimate the improvement in the coherence time, pi0 and the overall signal-to-noise ratio of the imaging procedure. These results predict that the signal-to-noise ratios of the power spectrum and bispectrum are increased by factors of ~3 and ~5 respectively for both filled-aperture speckle imaging and optical aperture synthesis imaging.

Japanese Astrometry Satellite Mission for Infrared Exploration

JASMINE is the name of a Japanese infrared (K-band) scanning astrometric satellite. JASMINE (I and/or II-project) is planned to be launched between 2013 and 2017 and will measure parallaxes and proper motions with the precision of 10μas at K≃ 12 - 15 mag. JASMINE will observe 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 obtain photometry of stars in K, J and H-bands. The main objective of JASMINE is to study the most fundamental structure and evolution of the disk and the bulge components of the Milky Way Galaxy.

Sensitivity of a Ground‐based Infrared Interferometer for Aperture Synthesis Imaging

Sensitivity limits of ground-based infrared interferometers using aperture synthesis are presented. The motivation for this analysis is to compare interferometers composed of multiple large telescopes and a single giant telescope with adaptive optics. In deriving these limits, perfect wave front correction by adaptive optics and perfect cophasing by fringe tracking are assumed. We consider the case in which n beams are pairwise combined at n(n)/2 detectors (nC2 interferometer) and the case in which all the n beams are combined at a single detector (nCn interferometer). Our analysis covers broadband observations by considering spectral dispersion of interference fringes. In the read-noise limit, the nCn interferometer with one-dimensional baseline configuration is superior to the nC2 interferometer, while in the background limit, the advantage of the one-dimensional nCn interferometer is small. As a case study, we compare the point-source sensitivities of interferometers composed of nine 10 m diameter telescopes and a 30 m diameter single telescope with adaptive optics between 1 and 10 μm for 10 σ detection in 1 hr. At J and H, the sensitivities of the interferometers are limited to 25-24 mag by read noise and OH airglow background, while that of the single telescope is limited to 28-26 mag by OH airglow background. Longward of 2 μm, the sensitivities of the interferometers and the single telescope are all limited by instrumental thermal background. At K, the sensitivities of the interferometers are around 23 mag, while that of the single telescope is 27 mag. At N, the sensitivities of the interferometers are around 10.7 mag, while that of the single telescope is 14.4 mag.