Behavior of [S/Fe] in Very Metal-Poor Stars from the S I 1.046 micron Lines Revisited

With an aim of establishing how the [S/Fe] ratios behave at the very low metallicity regime down to [Fe/H] ~ -3, we conducted a non-LTE analysis of near-IR S ~ {\sc i} triplet lines (multiplet 3) at 10455-10459 A for a dozen of very metal-poor stars (-3.2 <[Fe/H] < -1.9) based on the new observational data obtained with IRCS+AO188 of the Subaru Telescope. It turned out that the resulting [S/Fe] values are only moderately supersolar at [S/Fe] ~ +0.2-0.5 irrespective of the metallicity. While this "flat" tendency is consistent with the trend recently corroborated by Spite et al. (2011, A&A, 528, A9) based on the S I 9212/9228/9237 lines (multiplet 1), it disaffirms the possibility of conspicuously large [S/Fe] (up to ~+0.8) at [Fe/H] ~ -3 that we once suggested in our first report on the S abundances of disk/halo stars using S I 10455-10459 lines (Takeda & Takada-Hidai 2011, PASJ, 63, S537). Given these new observational facts, we withdraw our previous argument, since we consider that [S/Fe]'s of some most metal-poor objects were overestimated in that paper; the likely cause for this failure is also discussed.Comment: 7 pages, 5 figures, accepted for publication in PASJ, Vol. 64, No. 3 (2012

Chromospheres in Metal-Poor Stars Evidenced from the He I 10830A Line

Based on the near-IR spectra of 33 late-type stars in the wide metallicity range (mainly dwarfs and partly giants) obtained with IRCS+AO188 of the Subaru Telescope, we confirmed that He I 10830A line is seen in absorption in almost all moderately to extremely metal-poor stars of thick disk and halo population (from [Fe/H]~ -0.5 down to [Fe/H]~ -3.7), the strength of which is almost constant irrespective of the metallicity. This is an evidence that chromospheric activity at a basal level persists even for such old stars, despite that their rotations are considered to be slowed down and incapable of sustaining a dynamo, suggesting that some kind of chromospheric heating mechanism independent of rotation/magnetism (e.g., acoustic heating) may take place.Comment: 8 pages, 6 figures, Pub. Astron. Soc. Japan, Vol. 63 (2011), in pres

Exploring the [S/Fe] Behavior of Metal-Poor Stars with the S I 1.046 micron Lines

In an attempt of clarifying the [S/Fe] behavior with the run of [Fe/H] in the metal-poor regime which has been a matter of debate, an extensive non-LTE analysis of near-IR S I triplet lines (multiplet 3) at 1.046 micron was carried out for selected 33 halo/disk stars in a wide metallicity range of [Fe/H] ~-3.7 to ~+0.3, based on the spectral data collected with IRCS+AO188 of the Subaru Telescope. We found an evidence of considerably large [S/Fe] ratio amounting to ~+0.7-0.8 dex at very low metallicity of [Fe/H] ~-3, which makes marked contrast with other alpha-elements (Mg, Si, Ca, Ti) flatly showing moderately supersolar [alpha/Fe] of ~0.3 dex. Meanwhile, a locally-flat tendency of [S/Fe] at ~+0.3 is seen at -2.5 <[Fe/H]< -1.5. These results may suggest that the nature of [S/Fe] in metal-poor halo stars is not so simple as has been argued (i.e., neither being globally flat independent of [Fe/H] nor monotonically increasing with a decrease in [Fe/H]), but rather complicated with a local plateau around [Fe/H] ~-2 followed by a discontinuous jump between the narrow interval of -3 <[Fe/H]< -2.5.Comment: 11 pages, 6 figures, 2 tables and 2 electronic data tables (appended as auxiliary files); accepted for publication in PASJ (to appear in vol. 63, No. 2, 2011

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.Comment: plain TeX containing 23 PS pages, 3 PS tables, and 9 PS figures, ApJ, Dec 1, 1996 issue replacing an earlier version which contains an corrupted table

Non-LTE Line-Formation and Abundances of Sulfur and Zinc in F, G, and K Stars

Extensive statistical-equilibrium calculations on neutral sulfur and zinc were carried out, in order to investigate how the non-LTE effect plays a role in the determination of S and Zn abundances in F, G, and K stars. Having checked on the spectra of representative F-type stars (Polaris, Procyon, and alpha Per) and the Sun that our non-LTE corrections yield a reasonable consistency between the abundances derived from different lines, we tried an extensive non-LTE reanalysis of published equivalent-width data of S I and Zn I lines for metal-poor halo/disk stars. According to our calculations, S I 9212/9228/9237 lines suffer significant negative non-LTE corrections amounting to <~ 0.2--0.3 dex, while LTE is practically valid for S I 8683/8694 lines. Embarrassingly, as far as the very metal-poor regime is concerned, a marked discordance is observed between the [S/Fe] values from these two abundance indicators, in the sense that the former attains a nearly flat plateau (or even a slight downward bending) while the latter shows an ever-increasing trend with a further lowering of metallicity. The reason for this discrepancy is yet to be clarified. Regarding Zn, we almost confirmed the characteristic tendencies of [Zn/Fe] reported from recent LTE studies (i.e., an evident/slight increase of [Zn/Fe] with a decrease of [Fe/H] for very metal-poor/disk stars), since the non-LTE corrections for the Zn I 4722/4810 and 6362 lines (tending to be positive and gradually increasing towards lower [Fe/H]) are quantitatively of less significance (<~ 0.1 dex).Comment: 33 pages, 7 figures, PASJ, Vol. 57, No. 5 (2005) in pres

Spectroscopic and asteroseismic analysis of the remarkable main-sequence A star KIC 11145123

A spectroscopic analysis was carried out to clarify the properties of KIC 11145123 -- the first main-sequence star with a determination of core-to-surface rotation -- based on spectra observed with the High Dispersion Spectrograph (HDS) of the Subaru telescope. The atmospheric parameters ($T_{\rm eff} = 7600$ K, $\log g = 4.2$, $\xi = 3.1$ km s$^{-1}$ and ${\rm [Fe/H]} = -0.71$ dex), the radial and rotation velocities, and elemental abundances were obtained by analysing line strengths and fitting line profiles, which were calculated with a 1D LTE model atmosphere. The main properties of KIC 11145123 are: (1) A low ${\rm [Fe/H]} = -0.71\pm0.11$ dex and a high radial velocity of $-135.4 \pm 0.2$ km s$^{-1}$. These are remarkable among late-A stars. Our best asteroseismic models with this low [Fe/H] have slightly high helium abundance and low masses of 1.4 M$_\odot$. All of these results strongly suggest that KIC 11145123 is a Population II blue straggler; (2) The projected rotation velocity confirms the asteroseismically predicted slow rotation of the star; (3) Comparisons of abundance patterns between KIC 11145123 and Am, Ap, and blue stragglers show that KIC 11145123 is neither an Am star nor an Ap star, but has abundances consistent with a blue straggler. We conclude that the remarkably long 100-d rotation period of this star is a consequence of it being a blue straggler, but both pathways for the formation of blue stragglers -- merger and mass loss in a binary system -- pose difficulties for our understanding of the exceedingly slow rotation. In particular, we show that there is no evidence of any secondary companion star, and we put stringent limits on the possible mass of any such purported companion through the phase modulation (PM) technique.Comment: 19 pages, of which the final 7 are appendixed data tables. Ten figures, some of which do require colour. Accepted for publication in MNRA

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 $\alpha$ elements.Comment: 16 pages (including 2 tables and 4 figures), to appear in PASJ, Vol.54, No.2 (using the style file pasj00.cls

On the first δ Sct–roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars

Abstract Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spectrum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of 2.8 ± 0.5 kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one δ Sct–roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic damping in suppressing low-overtone p modes in Ap stars. We compute the magnetic damping effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths ≲ 4 kG, while other low-overtone p modes are driven for polar field strengths up to ∼1.5 kG. We find that the high-order g modes commonly observed in γ Dor stars are heavily damped by polar fields stronger than 1–4 kG, with the damping being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as γ Dor stars. We use our helium-depleted models to calculate the δ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at ∼5 × 104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars