We introduce a new polarimeter installed on the high-resolution fiber-fed echelle spectrograph (called BOES) of the 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory, Korea. The instrument is intended to measure stellar magnetic fields with high-resolution (R ∼ 60000) spectropolarimetric observations of intrinsic polarization in spectral lines. In this paper we describe the spectropolarimeter and present test observations of the longitudinal magnetic fields in some well-studied F-B main sequence magnetic stars (m v < 8.8 m ). The results demonstrate that the instrument has a high precision ability of detecting the fields of these stars with typical accuracies ranged from about 2 to a few tens of gauss.Subject headings: Astronomical instrumentation: polarimetry -magnetic fields -stars: magnetic stars IntroductionThe presence of intrinsic linear and circular polarizations in spectra of stellar objects provides an important information for diagnostics of their magnetism, wind surroundings, atmospheric inhomogeneities and other properties. For example, non-zero continuum linear polarization due to Thomson and Rayleigh scattering demonstrates the presence of non-symmetric patterns in the distribution of atmospheric or wind medium. The broad-band circular polarization as well as circular and linear polarizations in spectral lines exhibit information on the magnetic fields. The spectropolarimetric observation is therefore one of the most important tools for the experimental studies of
In an attempt of clarifying the connection between the photospheric abundance anomalies and the stellar rotation as well as of exploring the nature of "normal A" stars, the abundances of seven elements (C, O, Si, Ca, Ti, Fe, and Ba) and the projected rotational velocity for 46 A-type field stars were determined by applying the spectrum-fitting method to the high-dispersion spectral data obtained with BOES at BOAO. We found that the peculiarities (underabundances of C, O, and Ca; an overabundance of Ba) seen in slow rotators efficiently decrease with an increase of rotation, which almost disappear at v e sin i ∼ > 100 km s −1 . This further suggests that stars with sufficiently large rotational velocity may retain the original composition at the surface without being altered. Considering the subsolar tendency (by several tenths dex below) exhibited by the elemental abundances of such rapidly-rotating (supposedly normal) A stars, we suspect that the gas metallicity may have decreased since our Sun was born, contrary to the common picture of galactic chemical evolution.
With an aim of investigating the nature of evolution-induced mixing in the envelope of evolved intermediate-mass stars, we carried out an extensive spectroscopic study for 12 Cepheid variables of various pulsation periods (∼ 2-16 days) to determine the photospheric abundances of C, N, O, and Na, which are the key elements for investigating how the H-burning products are salvaged from the interior, based on 122 high-dispersion echelle spectra (∼ 10 per target) of wide wavelength coverage collected at Bohyunsan Astronomical Observatory. Having established the relevant atmospheric parameters corresponding to each phase spectroscopically from the equivalent widths of Fe i and Fe ii lines, we derived C, N, O, and Na abundances from C i 7111/7113/7115/7116/7119, O i 6155-8, N i 8680/8683/8686, and Na i 6154/6161 lines by using the spectrum-synthesis fitting technique, while taking into account the non-LTE effect. The resulting abundances of these elements for 12 program stars turned out to show remarkably small star-to-star dispersions ( 0.1-0.2dex) without any significant dependence upon the pulsation period: near-solar Fe ([Fe/H] ∼ 0.0), moderately underabundant C ([C/H] ∼ −0.3), appreciably overabundant N ([N/H] ∼ +0.4-0.5), and mildly supersolar Na ([Na/H] ∼ +0.2). We conclude the following implications from these observational facts: (1) These CNO abundance trends can be interpreted mainly as due to the canonical dredge-up of CN-cycled material, while any significant non-canonical deep mixing of ON-cycled gas is ruled out (though only a slight mixing may still be possible). (2) The mild but definite overabundance of Na suggests that the NeNa-cycle product is also dredged up. (3) The extent of mixing-induced peculiarities in the envelope of Cepheid variables is essentially independent on the absolute magnitude; i.e., also on the stellar mass.
The abundances of alkali elements (Li, Na, and K) were determined from the Li I 6708, Na I 5682/5688, and K I 7699 lines by taking into account the non-LTE effect for 24 sharp-lined A-type stars ($v _{\rm e}$ sin $i$$\lesssim$ 50 km s$^{-1}$, 7000 K $\lesssim$$T_{\rm eff}$$\lesssim$ 10000 K, many showing Am peculiarities to different degrees), based on high-dispersion and high-$S/N$ spectral data secured at BOAO (Korea) and OAO (Japan). We found a significant trend that $A$(Na) tightly scales with $A$(Fe) irrespective of $T_{\rm eff}$, which means that Na becomes enriched similarly to Fe in accordance with the degree of Am peculiarity. Regarding lithium, $A$(Li) mostly ranges between $\sim$ 3 and $\sim$ 3.5 (i.e., almost the same as or slightly less than the solar system abundance of 3.3) with a weak decreasing tendency with a lowering of $T_{\rm eff}$ at $T_{\rm eff}$$\lesssim$ 8000 K, though several stars exceptionally show distinctly larger depletion. The abundances of potassium also revealed an apparent $T_{\rm eff}$-dependence in the sense that $A$(K) in late-A stars tends to be mildly subsolar [possibly with a weak anti-correlation with $A$(Fe)] systematically decreasing from $\sim$ 5.0 ($T_{\rm eff}$$\sim$ 8500 K) to $\sim$ 4.6 ($T_{\rm eff}$$\sim$ 7500 K), while those for early-A stars remain near-solar around $\sim$ 5.0–5.2. These observational facts may serve as important constraints for any theory aiming to explain chemical anomalies of A-type stars.
An extensive non-LTE abundance analysis based on Na i 5890/5896 doublet lines was carried out for a large unbiased sample of ∼ 120 A-type main-sequence stars (including 23 Hyades stars) covering a wide v e sin i range of ∼ 10-300 km s −1 , with an aim to examine whether the Na abundances in such A dwarfs can be reliably established from these strong Na i D lines. The resulting abundances ([Na/H] 58 ), which were obtained by applying the T eff -dependent microturbulent velocities of ξ ∼ 2-4 km s −1 with a peak at T eff ∼ 8000 K (typical for A stars), turned out generally negative with a large diversity (from ∼ −1 to ∼ 0), while showing a sign of v e sin i-dependence (decreasing toward higher rotation). However, the reality of this apparently subsolar trend is very questionable, since these [Na/H] 58 are systematically lower by ∼ 0.3-0.6 dex than more reliable [Na/H] 61 (derived from weak Na i 6154/6161 lines for sharp-line stars). Considering the large ξ-sensitivity of the abundances derived from these saturated Na i D lines, we regard that [Na/H] 58 must have been erroneously underestimated, suspecting that the conventional ξ values are improperly too large at least for such strong high-forming Na i 5890/5896 lines, presumably due to the depth-dependence of ξ decreasing with height. The nature of atmospheric turbulent velocity field in midto-late A stars would have to be more investigated before we can determine reliable sodium abundances from these strong resonance D lines.
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