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Yoichi Takeda

Publications and source records attributed to Yoichi Takeda.

At least 19 recordsLinked to original sources
arXiv2026

On the photospheric abundances of the B-type fast rotator Regulus

A spectroscopic study of Regulus (late B-type rapidly-rotating star) was conducted with an aim of investigating its photospheric chemical abundances (especially for light elements), which has barely been challenged so far because of the considerable difficulty of abundance determination for such a very fast rotator (>~300 km/s). The atmospheric parameters were determined to be T_eff = 12345K (effective temperature) and log g = 3.58 (surface gravity) based on the spectral energy distribution, while the microturbulence was estimated as v_t = 0.5 km/s. The abundances of 14 elements (He, C, N, O, Ne, Mg, Si, S, Ca, Ti, Cr, Mn, Fe, and Ni) were derived by applying the spectrum-fitting technique, where the non-LTE effect was taken into account for lighter elements from He to Ca. The resulting relative-to-the-Sun abundances ([X/H]) revealed a remarkable trend that, while most elements indicate near-solar abundances (within +/-<~0.3dex), only C shows a marked deficiency ([C/H]~ -1.4) which is hard to explain. For example, a simple scenario of C underabundance caused by mixing of CN-cycled product (e.g., due to rapid rotation or binary mass transfer) is unlikely because any N enrichment is not observed. This problem remains yet to be further investigated.

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arXiv2026

Photospheric abundances of the rapidly-rotating A-type star Altair

Altair is an A-type star known to have an appreciably oblate shape owing to its very fast rotation (~300 km/s). Despite of numerous publications on this star, its chemical abundances have been scarcely investigated so far, presumably because of the practical difficulty that spectral lines are considerably broadened by rapid rotation and badly blended with each other. Motivated by this situation, a spectroscopic analysis was conducted to study the photospheric abundances of Altair by using the synthetic spectrum-fitting technique, in order to clarify whether or not any chemical peculiarities exist. The microturbulent velocity was determined to be 2.9 (+/-0.9) km/s by requiring that the metallicity does not show any systematic region-dependence. Then, the abundances of 17 elements (C, N, O, Mg, Al, Si, S, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Sr, Ba) were derived, where the non-LTE effect was taken into consideration as much as possible. The results revealed considerable region-by-region dispersion (several tenths dex or even more), reflecting the difficulty of reliable abundance determination for such a very rapid rotator. Nevertheless, the differential mean abundances relative to the Sun turned out to fall within -0.5 < [X/H] < +0.3 for all elements without any dependence upon the atomic number. Accordingly, we may conclude that (1) no appreciable anomalies of chemical abundance patterns exist in the atmosphere of Altair, (2) but its global metallicity is likely to be slightly subsolar (~-0.2 dex on the average).

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arXiv2025

Barium abundances of A--F--G type stars in the Hyades cluster

With an aim of clarifying the extent and parameter-dependence of compositional anomaly of barium in A-type stars, Ba abundances were spectroscopically determined based on BaII 6141/6496 lines for 89 (23 A-type and 66 F--G-type) main-sequence stars belonging to the members of Hyades cluster by taking into account the non-LTE effect and the hyper-fine-structure effect. While the non-LTE effect tends to strengthen lines in G stars, it acts in the direction of line weakening in the regime of A stars due to increasing imortance of overionization. The Ba abundances of G stars turned out almost constant ( = 2.33), indicating that the primordial composition of Ba in Hyades is mildly supersolar by ~+0.2dex. In contrast, A-type stars show Ba overabundances of considerably large dispersion (0~<[Ba/H]~<2). Since this Ba excess tends to increase with an increase/decrease in Teff/vsini, these two parameters may be essential for producing or controling the anomaly. Regarding Hyades F-type stars, their Ba abundances are not uniform but show a broad depression (by <~0.3dex) around Teff~6500K, interestingly coinciding with the location of Li-dip.

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arXiv2025

Atmospheric parameters and chemical abundances of the A-type eclipsing binary system RR Lyncis A and B

A spectroscopic study was carried out for the double-line A-type eclipsing binary system RR Lyn A+B based on the disentangled spectra, with an aim of clarifying the differences in photospheric chemical compositions between the components, where T_eff (effective temperature) and v_t (microturbulence) were determined from Fe lines. The resulting abundances of 30 elements revealed the following characteristics. (1) The brighter/hotter A shows metal-rich trends of classical Am stars; i.e., heavier elements generally show overabundances tending to increase towards higher Z (atomic number) with exceptionally large deficit of Sc, while light elements such as CNO show underabundances. (2) Meanwhile, the abundances of fainter/cooler B are closer to the solar composition ([X/H]~0 for intermediate Z elements such as Fe group) though [X/H] does exhibit a slightly increasing tendency with Z, which suggests that B is a kind of marginal Am star with almost normal metallicity. This consequence is in contrast to the results of previous studies, which reported B to be of metal-deficient nature similar to lambda Boo stars. Such distinctions of chemical abundances between A and B may serve as a key to understanding the condition for the emergence of Am phenomenon.

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arXiv2025

Formation of Al II lines and photospheric aluminium abundances in B-type stars

Aluminium abundances of B-type stars were spectroscopically determined in order to get information about the galactic gas composition at the time of their formation. For this purpose, two AlII lines at 6243 and 4663A were employed. The non-LTE effect of these AlII lines generally acts in the direction of weakening (i.e., profile becomes shallower) caused by a decrease of line opacity (due to overionization) along with an enhanced line source function (overexcitation), and this effect tends to become progressively larger with an increase in Teff as well as with a decrease in log g (surface gravity). Regarding the AlII 6243 line, while the non-LTE calculation qualitatively reproduces its overall behavior (e.g., transition from absorption to emission at early B-type), some Teff-dependent systematic trend remains unremoved in the non-LTE abundances of normal stars, which means that non-LTE corrections evaluated for this line are quantitatively insufficient. Meanwhile, for the case of the Al II 4663 line, which is more advantageous than the 6243 line in the sense that it is stronger without showing any emission, the resulting non-LTE abundances of ordinary B stars are almost constant at the solar abundance (A~6.5) over the wide Teff range (~10000-20000K), suggesting that the abundances derived from this line are successfully non-LTE-corrected and trustable. Therefore, according to the results from the AlII 4663 line, we may conclude that the Al abundance of the galactic gas in the recent past (several times ~10^7-10^8 yr ago) is almost consistent with the solar composition. As to the Al abundances of HgMn stars (Teff<15000K), our analysis confirmed that this element is conspicuously deficient (by ~0.5-2 dex in comparison with the Sun) in the photosphere of these chemically peculiar stars, as already reported in previous studies.

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arXiv2025

Spectroscopic study of the late B-type eclipsing binary system AR Aurigae A and B: Towards clarifying the differences in atmospheric parameters and chemical abundances

AR Aur A+B is a close binary of astrophysical interest, because dissimilar surface compositions are reported between similar late B-type dwarfs. A new spectroscopic study on this system was carried out based on the disentangled spectra, in order to determine their atmospheric parameters and elemental abundances, The effective temperature and microturbulence (determined from the equivalent widths of Fe II lines) turned out (11150K, 0.9km/s) and (10650K, 0.1km/s) for A and B. The chemical abundances of 28 elements were derived while taking into account the non-LTE effect for Z<=15 elements (Z: atomic number). The following trends were elucidated for [X/H] (abundance of X relative to the Sun): (1) Qualitatively, [X/H] shows a rough global tendency of increasing with Z, with the gradient steeper for A than for B. (2) However, considerable dispersion is involved for A, since prominently large peculiarities are seen in specific elements reflecting the characteristics of HgMn stars (e.g., very deficient N, Al, Sc, Ni; markedly overabundant P, Mn). (3) In contrast, the Z-dependence of [X/H] for B tends to be nearly linear with only a small dispersion. These observational facts may serve as a key to understanding the critical condition for the emergence of chemical anomaly.

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arXiv2024

Phosphorus Abundances of B-Type Stars in the Solar Neighborhood

Phosphorus abundances of ~80 apparently bright sharp-lined early-to-late B-type stars on the upper main sequence are determined by applying the non-LTE analysis to the P II line at 6043.084 A, with an aim of getting information on the P abundance of the galactic gas (from which these young stars were formed) in comparison with the reference solar abundance (A_sun = 5.45). These sample stars turned out to be divided into two distinct groups with respect to their P abundances: (1) chemically peculiar late B-type stars of HgMn group show considerable overabundances of P (supersolar by ~0.5--1.5 dex), the extent of which progressively increases with T_eff. (2) In contrast, the P abundances of normal B-type stars are comparatively homogeneous, though a notable difference is observed between the LTE and non-LTE cases. Although their LTE abundances are near-solar, a slight gradual trend with T_eff is observed. However, after applying the negative non-LTE corrections (amounting ~0.1-0.5 dex), this T_eff-dependence is successfully removed, but the resulting non-LTE abundances (their mean is ~5.20) are appreciably underabundant relative to the Sun by ~0.2--0.3 dex. The cause of this systematic discrepancy (contradicting the galactic chemical evolution) is yet to be investigated.

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arXiv2024

Measurement of Solar Differential Rotation by Absolutely Calibrated Iodine-Cell Spectroscopy

The iodine-cell technique, which is known to be efficient in precisely establishing Doppler velocity shifts, was once applied by the author to measuring the solar differential rotation based on full-disk spectroscopic observations (Takeda and Ueno, Sol. Phys. 270, 447, 2011). However, the data reduction procedure (in simple analogy with the stellar case) adopted therein was not necessarily adequate, because specific characteristic involved with the disk-resolved Sun (i.e., center-limb variation of line strengths) was not properly taken into consideration. Therefore, this problem is revisited based on the same data but with an application to theoretical spectrum fitting, which can yield absolute heliocentric radial velocities (v_obs) in a consistent manner as shown in the study of solar gravitational redshift (Takeda and Ueno, Sol. Phys. 281, 551, 2012). Likewise, instead of converting v_obs into omega (angular velocity) at each disk point, which suffers considerable errors especially near the central meridian, omega was derived this time by applying the least squares analysis to a dataset comprising v_obs values at many points. This new analysis resulted in omega (deg/day) = 13.92 (+/- 0.03) -1.69(+/- 0.34)(sin psi)^2 -2.37(+/- 0.62) (sin psi)^4 (psi: the heliographic latitude) along with the gravitational redshift of 675 m/s, which are favorably compared with previous publications. In addition, how the distribution of observing points on the disk affects the result is also examined, which reveals that rotation parameters may suffer appreciable errors depending on cases.

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arXiv2024

On the chemical abundance differences between the solar twin visual binary system 16 Cygni A and B

The visual binary system 16~Cyg~A+B consists of similar solar twins, but a planetary companion is detected only in B. An intensive spectroscopic differential analysis is carried out to the Sun, 16~Cyg~A, and 16~Cyg~B, with particular attentions being paid to (i) precisely establishing the differential atmospheric parameters/metallicity between A and B, and (ii) determining the important CNO abundances based on the lines of CH, NH, and OH molecules. The following results are obtained. (1) The Fe abundances (relative to the Sun) are [Fe/H]^A=+0.09 and [Fe/H]^B=+0.06 (i.e., A is slightly metal-rich than B by +0.03~dex). This lends support to the consequences of recently published papers, while the conclusion once derived by the author (almost the same metallicity for A and B) is acknowledged to be incorrect. (2) The differential abundances (Delta[X/H]) of volatile CNO with low T_c (condensation temperature) are apparently lower than those of refractory Fe group elements of higher T_c, leading to a positive gradient in the Delta[X/H] vs. T_c relation being more conspicuous for A than B. This is qualitatively consistent with previous studies, though the derived slope is quantitatively somewhat steeper than that reported by other authors.

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arXiv2023

Photospheric aluminium abundances of A-type main-sequence stars

Although anomalous surface abundances are often observed in A-type main-sequence stars (known as chemically peculiar stars; e.g., metallic line stars or Am stars), our understanding about the behavior of aluminium is still insufficient. Actually, even whether Al is overabundant or underabundant in Am stars is not clarified. This is presumably because most of the previous studies employed the Al I 3944/3961 lines with the assumption of LTE, despite that a considerable non-LTE effect is expected in this resonance doublet. With an aim to shed light on this issue, extensive statistical-equilibrium calculations on Al I/Al II were carried out for a wide range of atmospheric parameters, based on which the non-LTE Al abundances were determined by applying the spectrum-fitting technique to the Al I 3944/3961 lines for 63 A-type dwarfs (7000 < Teff < 10000 K) of comparatively lower rotational velocities (vsini < 100 km/s). The following results were obtained. (1) The non-LTE corrections (Delta) are positive (reflecting the importance of overionization) and significantly large (0.3 < Delta < 1.0 dex depending on Teff; generally Delta_3944 < Delta_3961). (2) By applying these corrections (and indispensable inclusion of Balmer line wings as background opacity), consistent non-LTE abundances for both lines could be obtained, and the serious zero-point discrepancy (considerably negative [Al/H] for normal metallicity stars of [Fe/H]~ 0) found in old studies has been settled. (3) Al abundances of A-type stars are almost in proportion to [Fe/H] (tending to be overabundant in Am stars) with an approximate relation of [Al/H]~1.2[Fe/H]. which is qualitatively consistent with the prediction of the diffusion theory (suggesting an Al excess in the photosphere of Am stars).

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arXiv2023

Does the A-type Metallic-line Star IW Persei Have Non-Uniform Chemical Anomaly on the Surface?

IW Per, a single-lined spectroscopic binary with a short period of 0.92d, is known to be a A-type metallic-line (Am) star showing anomalous line strengths of specific elements. Previously, Kim (1980) reported that its equivalent widths of CaII 3934, SrII 4215, and ScII 4320 lines (important key lines characterizing the Am anomaly) show cyclic variations in accordance with the rotation phase, implyig that the chemical peculiarities on the surface are not uniform but of rather patchy distribution, though no trial of reconfirmation seems to have been done so far. In order to check the validity of this finding, 10 high-dispesion spectra of IW Per covering different phases were analyzed for these lines by using the spectrum-fitting technique to determine the abundances of Ca, Sr, and Sc and the corresponding equivalent widths. It turned out, however, that no firm evidence of such phase-dependent line-strength variations could be found, suggesting that significant chemical inhomogeneity on the surface of IW Per is unlikely to exist, at least as regards to the period of our observations (2010 December). Meanwhile, the abundances of O, Si, Ca, Ba, and Fe resulting from the 6130-6180A region corroborate that IW Per is a distinct Am star despite that its rotational velocity (~100 km/s) is near to the existent limit of Am phenomenon.

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arXiv2023

Spectroscopic comparative study of the red giant binary system gamma Leonis A and B

Gamma~Leo is a long-period visual binary system consisting of K0III (A) and G7III (B) giants, in which particular interest is attracted by the brighter A since the discovery of a planet around it. While detailed spectroscopic comparative study of both components would be worthwhile (e.g., for probing any impact of planet formation on chemical abundances), such a research seems to have been barely attempted as most available studies tend to be biased toward A. Given this situation, the physical properties of A and B along with their differences were investigated based on high-dispersion spectra in order to establish their stellar parameters, evolutionary status, and surface chemical compositions. The following results were obtained. (1) The masses were derived as ~1.7Msun and ~1.6Msun for A and B, respectively, both of which are likely to be in the stage of red clump giants after He-ignition. The mass of the planet around A has also been revised as m*sin(i) = 10.7M_Jupiter (increased by ~20%). (2) These are normal giants of subsolar metallicity ([Fe/H]~-0.4) belonging to the thin-disk population. (3) A as well as B show moderate C deficiency and N enrichment, which are in compatible with the prediction from the standard stellar evolution theory. (4) The chemical abundances of 26 elements are practically the same within <~0.1dex for both components, which implies that the surface chemistry is not appreciably affected by the existence of a planet in A.

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arXiv2023

Surface magnetic field of the A-type metallic-line star omicron Pegasi revisited

The bright A-type metallic-line star o Peg was reported in the early 1990s to have a surface magnetic field of ~2kG by analyzing the widths and strengths of spectral lines. In respect that those old studies were of rather empirical or approximate nature and the quality of observational data was not sufficient, this problem has been newly reinvestigated based on physically more rigorous simulations of line flux profiles, along with the observed equivalent widths (W) and full-widths at half-maximum (h) of 198 Fe I and 182 Fe II lines measured from the high-quality spectra. Given the Fe abundance derived from the conventional analysis, theoretical W and h values calculated for various sets of parameters were compared with the observed ones, which lead to the following conclusion regarding (mean field strength). (1) An analysis of W yielded ~1-1.5kG from Fe II lines with the microturbulence of vt~1.5km/s. (2) A comparison of h resulted in ~1.5-2kG as well as the projected rotational velocity of vsini~5km/s. (3) Accordingly, the existence of mean magnetic field on the order of ~1-2kG in o Peg was confirmed, which is almost consistent with the consequence of the previous work.

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arXiv2022

Spectroscopic determination of C, N, and O abundances of solar-analog stars based on the lines of hydride molecules

Photospheric C, N, and O abundances of 118 solar-analog stars were determined by applying the synthetic-fitting analysis to their spectra in the blue or near-UV region comprising lines of CH, NH, and OH molecules, with an aim of clarifying the behaviors of these abundances in comparison with [Fe/H]. It turned out that, in the range of -0.6<[Fe/H]<+0.3, [C/Fe] shows a marginally increasing tendency with decreasing [Fe/H] with a slight upturn around [Fe/H]~0, [N/Fe] tends to somewhat decrease towards lower [Fe/H], and [O/Fe] systematically increases (and thus [C/O] decreases) with a decrease in [Fe/H]. While these results are qualitatively consistent with previous determinations mostly based on atomic lines, the distribution centers of these [C/Fe], [N/Fe], and [O/Fe] at the near-solar metallicity are slightly negative by several hundredths dex, which is interpreted as due to unusual solar abundances possibly related to the planetary formation of our solar system. However, clear anomalies are not observed in the [C,N,O/Fe] ratios of planet-host stars. Three out of four very Be-deficient stars were found to show anomalous [C/Fe] or [N/Fe] which may be due to mass transfer from the evolved companion, though its relation to Be depletion mechanism is still unclear.

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arXiv2022

Mass evaluation for red giant stars based on the spectroscopically determined atmospheric parameters

The mass (M) of a star can be evaluated from its spectroscopically determined effective temperature (T _eff) and metallicity ([Fe/H]) along with the luminosity (L; derived from parallax), while comparing them with grids of theoretical evolutionary tracks. It has been argued, however, that such a track-based mass (M_trk) may tend to be overestimated for the case of red giants. Meanwhile, there is an alternative approach of evaluating mass (M_gLT) directly from surface gravity (g), L, and T_eff. The practical reliability of M_gLT was examined for ~100 benchmark giants in the Kepler field, for which atmospheric parameters are already determined and the reliable mass (M_seis) along with the evolutionary status are known from asteroseismology. In addition, similar check was also made for the accuracy of M_trk for comparison. It turned out that, while a reasonable correlation is seen between M_gLT and M_seis almost irrespective of the stellar property, its precision is rather insufficient because log(M_gLT/M_seis) distributes rather widely within ~+/-0.2--0.3dex. In contrast, the reliability of M_trk was found to depend on the evolutionary status. Although M_trk and M_seis are satisfactorily consistent with each other (typical dispersion of log(M_trk}/M_seis) is within ~+/-0.1dex) for H-burning red giants as well as He-burning 2nd clump giants of higher mass, M_trk tends to be considerably overestimated as compared to M_seis by up to ~<0.4~dex for He-burning 1st clump giants of lower mass. Accordingly, M_gLT and M_trk are complementary with each other in terms of their characteristic merit and demerit.

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arXiv2022

Surface C, N, O, and Na abundances of RR Lyrae variables implying the nature of internal mixing in low-mass stars

Photospheric abundances of C, N, O, and Na were determined by applying the synthetic spectrum-fitting technique to 34 snap-shot high-dispersion spectra of 22 RR Lyr stars covering a metallicity range of -1.8 <[Fe/H] < 0.0, with an aim of investigating the mixing mechanism in the interior of low-mass giant stars by examining the abundance anomalies of these elements possibly affected by the evolution-induced dredge-up of nuclear burning products. Special attention was paid to check the recent theoretical stellar evolution simulations indicating the importance of thermohaline mixing in low-mass stars (M <~1 M_sun), which is expected to be more significant as the metallicity is lowered. By inspecting the resulting abundances in comparison with those of unevolved metal-poor dwarfs at the same metallicity, the deficiency in C as well as enrichment in N was confirmed (while O is almost unchanged), the extent of peculiarities tending to increase with a decrease in [Fe/H]. Accordingly, the [C/N] ratio turned out to progressively decrease towards lower metallicity from ~0 (Fe/H]~0) to ~-1 ([Fe/H]~-1.5), which is reasonably consistent with the theoretical prediction in the presence of thermohaline mixing. However, these RR Lyr stars do not show any apparent Na anomaly (i.e., essentially the same [Na/Fe] vs. [Fe/H] trends as those of dwarfs), despite that metallicity-dependent overabundance in Na is theoretically expected for the case of non-canonical mixing. This inconsistency between C/N and Na may suggest a necessity of further improvement in the current theory.

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arXiv2021

Regular Radial Velocity Variations in Nine G- and K-type Giant Stars: Eight Planets and One Planet Candidate

We report the detection of radial velocity variations in nine evolved G- and K-type giant stars. The observations were conducted at Okayama Astrophysical Observatory. Planets or planet candidates can best explain these regular variations. However, a coincidence of near 280-day variability among five of them prevents us from fully ruling out stellar origins for some of the variations, since all nine stars behave similarly in stellar properties. In the planet hypotheses to the RV variations, the planets (including one candidate) may survive close to the boundary of the so-called "planet desert" around evolved stars, having orbital periods between 255 and 555 days. Besides, they are the least-massive giant planets detected around G- and K-type giant stars, with minimum masses between 0.45$M_{\rm{J}}$ and 1.34$M_{\rm{J}}$. We further investigated other hypotheses for our detection, yet none of them can better explain regular RV variation. With our detection, it is convinced that year-long regular variation with amplitude down to 15 $\rm{m\ s^{-1}}$ for G- and K-type giant stars is detectable. Moreover, we performed simulations to further confirm the detectability of planets around these stars. Finally, we explored giant planets around intermediate-mass stars, and likewise found a 4 Jupiter mass gap (e.g. Santo et al. 2017), which is probably a boundary of the giant planet population.

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arXiv2021

Center--limb variation of solar photospheric microturbulence

Microturbulence (ξ) is a key parameter introduced in stellar spectroscopy to explain the strength of saturated lines by formally incorporating an additional thermal broadening term in the line opacity profile. Although our Sun can serve as an important testing bench to check the usual assumption of constant ξ, the detailed behavior of how ξvaries from the disk center through the limb seems to have never been investigated so far. In order to fill this gap, local ξvalues on the solar disk were determined from the equivalent widths of 46 Fe I lines at 32 points from the center to the limb by requiring the consistency between the abundances derived from lines of various strengths. The run of ξwith θ(angle between line of sight and the surface normal) was found to be only gradual from ~1.0km/s (at sinθ= 0: disk center) to ~1.3km/s (at sinθ~ 0.7: two-thirds of radial distance); but thereafter increasing more steeply up to ~2km/s (at sinθ= 0.97: limb). This result further suggests that the microturbulence derived from the flux spectrum of the disk-integrated Sun is by ~20% larger than that of the disk-center value, which is almost consistent with the prediction from 3D hydrodynamical model atmospheres.

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