arXiv subjects

Masahide Takada-Hidai

Publications and source records attributed to Masahide Takada-Hidai.

24 records · Page 2Linked to original sources

arXiv2005

Nucleosynthetic signatures of the first stars

The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models have been suggested to explain the origin of the abundance pattern of the giant star HE 0107-5240, which hitherto exhibited the highest deficiency of heavy elements known. Here we report the discovery of HE 1327-2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE 0107-5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE 1327-2326, however, challenge existing theoretical understanding: none predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars.

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arXiv2004

A low upper-limit on the lithium isotope ratio in HD140283

We have obtained a high-S/N (900-1100), high-resolving-power (R=95000) spectrum of the metal-poor subgiant HD 140283 in an effort to measure its 6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026. This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by one of the authors. HD 140283 provides no support for the suggestion that Population II stars may preserve their 6Li on the portion of the subgiant branch where 7Li is preserved. However, this star does not defeat the suggestion either; being at the cool end of subgiant branch of the Spite plateau, it may be sufficiently cool that 6Li depletion has already set in, or the star may be sufficiently metal poor that little Galactic production of 6Li had occurred. Continued investigation of other subgiants is necessary to test the idea. We also consider the implications of the HD 140283 upper limit in conjunction with other measurements for models of 6Li production by cosmic rays from supernovae and structure formation shocks.

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arXiv2003

Non-LTE Analysis for the Sodium Abundances of Metal-Poor Stars in the Galactic Disk and Halo

We performed an extensive non-LTE analysis of the neutral sodium lines of Na {\sc i} 5683/5688, 5890/5896, 6154/6161, and 8183/8195 for disk/halo stars of F--K type covering a wide metallicity range ($-4 \la$ [Fe/H] $\la 0.4$), based on our own data as well as those collected from the literature. For comparatively metal-rich disk stars ($-1 \la$ [Fe/H] $\la 0$) where the weaker 6154/6161 lines are best abundance indicators, we confirmed [Na/Fe] $\sim$ 0 with an ``upturn'' (i.e., a shallow/broad dip around $-0.5 \la$ [Fe/H] $\la 0$) as already reported by previous studies. Regarding the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines suffering considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly ``subsolar'' [Na/Fe] values down to $\sim -0.4$ (with a somewhat large scatter of $\sim \pm 0.2$ dex) on the average at the typical halo metallicity of [Fe/H] $\sim -2$, while they appear to rise again toward a very metal-poor regime recovering a near-solar ratio of [Na/Fe] $\sim$ 0 ([Fe/H] $\sim -3$ to -4). These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.

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arXiv2002

Subaru high resolution spectroscopy of complex metal absorption lines of QSO HS1603+3820

We present a high resolution spectrum of the quasar, HS1603+3820 (z_em=2.542), observed with the High Dispersion Spectrograph (HDS) on Subaru Telescope. This quasar, first discovered in the Hamburg/CfA Quasar Survey, has 11 C IV lines at 1.96 < z_abs < 2.55. Our spectrum covers 8 of the 11 C IV lines at z_abs > 2.29 and resolves some of them into multiple narrow components with b < 25 km/s because of the high spectral resolution R=45000, while other lines show broad profiles (b > 65 km/s). We use three properties of C IV lines, specifically, time variability, covering factor, and absorption line profile, to classify them into quasar intrinsic absorption lines (QIALs) and spatially intervening absorption lines (SIALs). The C IV lines at 2.42 < z_abs < 2.45 are classified as QIALs in spite of their large velocity shifts from the quasar. Perhaps they are produced by gas clouds ejected from the quasar with the velocity of v_ej = 8000 km/s -- 10000 km/s. On the other hand, three C IV lines at 2.48 < z_abs < 2.55 are classified as SIALs, which suggests there exist intervening absorbers near the quasar. We, however, cannot rule out QIALs for the two lines at z_abs ~ 2.54 and 2.55, because their velocity shifts, 430 km/s blueward and 950 km/s redward of the quasar, are very small. The C IV line at z_abs ~ 2.48 consists of many narrow components, and has also corresponding low-ionization metal lines (Al II, Si II, and Fe II). The velocity distribution of these low-ionization ions is concentrated at the center of the system compared to that of high-ionization C IV ion. Therefore we ascribe this system of absorption lines to an intervening galaxy.

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arXiv2002

On the Abundance of Potassium in Metal-Poor Stars

Based on extensive statistical-equilibrium calculations, we performed a non-LTE analysis of the K {\sc i} 7699 equivalent-width data of metal-deficient stars for the purpose of clarifying the behavior of the photospheric potassium abundance in disk/halo stars. While the resulting non-LTE abundance corrections turned out to be considerably large, amounting to 0.2--0.7 dex, their effect on the [K/Fe] vs. [Fe/H] relation is not very important, since these corrections do not show any significant metallicity dependence. Hence, we again confirmed the results of previous LTE studies, that [K/Fe] shows a gradual systematic increase toward a lowered metallicity up to [K/Fe] $\sim$ 0.3--0.5 at [Fe/H] $\sim$ -1 to -2, such as in the case of $α$ elements.

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arXiv1996

The Lyman-alpha Forest at z~4: Keck HIRES Observations of Q 0000-26

This paper describes a study of the Lyman-alpha forest absorption clouds along the quasar sightline Q0000-26 (zem=4.1). The spectrum was obtained with the High Resolution Spectrometer on the 10m Keck telescope. We derive accurate H I column density and Doppler width distributions for the clouds from Voigt profile fitting. We also analyze simulated Lyman-alpha forest spectra of matching characteristics in order to gauge the effects of line blending/blanketing and noise in the data. The results are compared with similar studies at lower redshifts in order to study any possible evolution in the clouds' properties. We also estimate the mean intensity of the UV background at z=4 from an analysis of the proximity effect.

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