arXiv subjects

Takashi Tsuji

Publications and source records attributed to Takashi Tsuji.

26 records · Page 2Linked to original sources

arXiv2002

Water Observed in Red Giant and Supergiant Stars - Manifestation of a Novel Picture of the Stellar Atmosphere or else Evidence against the Classical Model Stellar Photosphere

We detected the H2O 6.3 micron bands in more than 30 normal red giants stars from K5III to M8III as well as in some early M supergiants on the SWS spectra retrieved from the ISO Data Archive. This result, however, shows serious inconsistency with the present model photospheres which predict H2O only in the latest M (super)giant stars. Also H2O was once discovered in the early M (super)giant stars nearly 40 years ago with the balloon-borne telescope named Stratoscope II. This discovery was so unexpected at that time that it was not understood correctly and overlooked for a long time. Now, we reflect on our ignorance of this important discovery during the 40 years and should consider more seriously the meaning of the rediscovery of water in so many red (super)giant stars with ISO. In view of the detection of H2O in emission in the early M supergiant mu Cep as well as in late M giants with ISO, H2O should be in the outer atmosphere rather than in the photosphere. We propose that the absorption as well as emission of H2O should be originating in a new component, which we refer to as molecular sphere. Since the extend atmospheres of red (super)giants radiate efficiently in the infrared, it is not surprising if the true structure of the atmospheres of these stars can first be revealed with the infrared observations especially from space.

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arXiv2002

Unified Cloudy Models of L and T Dwarfs - Physical Basis of the Spectral Classification in the Substellar Regime

Based on a simple thermodynamical argument, we proposed a unified cloudy model (UCM) with a warm dust cloud deep in the photosphere. We showed, for the first time, that a single grid of model photospheres (Teff between 800 and 2600K) offers a natural explanation not only for the division of dwarfs cooler than M into the two distinct types L and T but also for the changes of the spectra and colors along the L -- T spectral sequence. Based on our model analysis of the observed IR colors, we propose an effective temperature scale and compare it with the empirical one. For detailed analyses of the spectra, the chemical composition is an important basic stellar parameter besides Teff, and the observed infrared spectrum of the L dwarf prototype GD165B can finally be fitted with the predicted one based on our UCM being updated with the use of the new solar carbon and oxygen abundances. We confirm that our UCMs provide consistent interpretation of the spectra and colors of L and T dwarfs, and thus will provide the physical basis for the spectral classification of the dwarfs cooler than M extending to the substellar regime.

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arXiv2002

Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs

We address the problem of how dust forms and how it could be sustained in the static photospheres of cool dwarfs for a long time. In the cool and dense gas, dust forms easily at the condensation temperature, T_cond, and the dust can be in detailed balance with the ambient gas so long as it remains smaller than the critical radius, r_cr. However, dust will grow larger and segregate from the gas when it will be larger than r_cr somewhere at the lower temperature, which we refer to as the critical temperature, T_cr. Then, the large dust grains will precipitate below the photosphere and only the small dust grains in the region of T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud is formed. Incorporating the dust cloud, non-grey model photo- spheres in radiative-convective equilibrium are extended to T_eff as low as 800K. Observed colors and spectra of cool dwarfs can consistently be accounted for by a single grid of our cloudy models. This fact in turn can be regarded as supporting evidence for our basic assumption on the cloud formation.

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arXiv2001

H and K Band Methane Features in an L Dwarf, 2MASS 0920+35

We have obtained near-infrared spectra of three L dwarfs discovered by 2MASS, 1506+13 (L3), 1507-16 (L5), and 0920+35 (L6.5). From the comparison of the H and K band spectra of these L dwarfs, we have found the presence of methane absorption in 0920+35. This implies that detectable methane absorption in the H and K bands, usually considered the signature of a T dwarf, can be present in objects classified optically as late L. Methane detection in L dwarfs is consistent with the presence of a dust layer deep in the atmosphere as the unified model of Tsuji suggests.

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arXiv2001

Water in K and M giant stars unveiled by ISO

Based on the spectra obtained with Infrared Space Observatory, ISO, we detected the 6.3 micron bands of water in the late K giant Aldebaran (alpha Tau) and several early M giant stars between M0 and M3.5, which have been deemed to be too warm for tri-atomic H2O molecule to reside in their photospheres. The water column densities range 2E+17 - 2E+18 molecules/cm**2 in our sample of K and M giant stars and the excitation temperatures are 1500 K or higher. Thus, the water bands are not originating in cool stellar winds either. The presence of water in the K and early M giant stars was quite unexpected from the traditional picture of the atmosphere of the red giant star consisting of the photosphere, hot chromosphere, and cool wind. We confirm that a rather warm molecule forming region (referred to as MOLsphere) should exist as a new component of the atmosphere of red giant stars and that this should be a general phenomenon in late-type stars. ~

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arXiv2001

Unified Model Photospheres for Ultracool Dwarfs of the Types L and T

We show that the two groups of ultracool dwarfs classified as L and T types can be understood as a temperature effect in a unique sequence of the model photospheres in which dust always exists but only in the restricted region deep in the photosphere. Under the high density of the photospheres of cool dwarfs, dust forms easily at the condensation temperature T_cond (about 2000 K), but the dust will soon grow larger than the critical radius r_cr at the critical temperature T_cr (about 1800 K). The large dust grains with r > r_cr will soon segregate from the gas and precipitate below the photosphere. For this reason, dust exists only in the limited region of T_cr < T < T_cond in the photo- sphere which we referred to as an active dust zone, and this means that a cloud layer is formed deep in the photosphere rather than in the cooler surface region. With the simple assumption that only small dust grains (r < r_cr) can be sustained in the photosphere, a self- consistent non-grey model photosphere in radiative-convective equilibrium can be developed without any other ad-hoc assumption. We show that the spectra and colours of L (the cloud layer appears in the optically thin region) and T (the dust cloud is situated too deep to be observable) dwarfs can consistently be reproduced by a single grid of our unified model photospheres.

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arXiv1999

Near-Infrared Spectroscopy of the Cool Brown Dwarf, SDSS 1624+00

Using the Subaru Telescope, we have obtained multiple near-infrared spectra of the cool brown dwarf, SDSS 1624+00, in search of spectral variability in an 80 minute time span. We have found the suspected variability of water vapor absorption throughout the observations, which requires confirmation by a longer time baseline. After coadding the spectra, we have obtained a high-quality spectrum covering 1.05 to 1.8 um. Three kinds of spectral indicators, the water vapor bands, methane band, and KI lines in J band, suggest that SDSS 1624+00 is warmer and dustier than Gl 229B.

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arXiv1997

Spectral evidence for dust in late-type M dwarfs

We have found a dramatic change in the spectra of late-type M dwarfs at wavelengths shorter than 0.75 microns. Comparisons with synthetic spectra illustrate that this change is well explained by the presence of dust. It thus seems that a substantial part of the long-standing discrepancies between late-type M dwarf models and observations arises from dust as recently proposed by Tsuji, Ohnaka and Aoki (1996a).

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