Astrophysical calibration of the oscillator strengths of YJ-band absorption lines in classical Cepheids
S. S. ElguetaNoriyuki MatsunagaMingjie JianDaisuke TaniguchiNaoto KobayashiKei FukueSatoshi HamanoHiroaki SameshimaShunsuke KondoAkira AraiYuji IkedaHideyo KawakitaShogo OtsuboYuki SarugakuChikako YasuiTakuji Tsujimoto
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Newly-developed spectrographs with increased resolving powers, particularly those covering the near-IR range, allow the characterization of more and more absorption lines in stellar spectra. This includes the identification and confirmation of absorption lines and the calibration of oscillator strengths. In this study, we provide empirical values of loggf based on abundances of classical Cepheids obtained with optical spectra in Luck (2018), in order to establish the consistency between optical and infrared abundance results. Using time-series spectra of classical Cepheids obtained with WINERED spectrograph (0.97-1.35 $\mu$ m, R ~28000, we demonstrate that we can determine the stellar parameters of the observed Cepheids, including effective temperature (Teff), surface gravity (logg), microturbulence, and metallicity. With the newly calibrated relations of line-depth ratios (LDRs), we can achieve accuracy and precision comparable to optical studies (Luck 2018), with uncertainties of 90K and 0.108 dex for Teff, and log g, respectively. Finally, we created a new atlas of absorption lines, featuring precise abundance measurements of various elements found in the atmosphere of Cepheids (including neutron-capture elements), with loggf values that have been astrophysically calibrated.Keywords:
Surface gravity
Microturbulence
Effective temperature
Microturbulence
Effective temperature
Surface gravity
Star (game theory)
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We compare observations of the eclipsing binary system CM Draconis (hereafter CM Dra) with synthetic spectra computed using the stellar atmosphere code phoenix. High resolution infrared spectroscopic observations of six 0.05 μm wide regions between 1.51 and 2.45μm, combined with previous work, particularly CM Dra's accurately known surface gravity, enable us to estimate its metallicity using detailed spectral synthesis. We find significant discrepancies between the observed and synthetic spectra throughout most of the region emphasizing the need for higher quality atomic data in the infrared. Nevertheless, the CO bands beyond 2.3μm seem to be well modelled and metal sensitive, and thus high resolution spectra should be a most powerful diagnostic tool for spectroscopic analyses for M dwarfs and brown dwarfs. The CO bands indicate a metallicity of around −1dex for CM Dra. This result is supported by observations of two M dwarfs of similar spectral type, GJ 699 (Barnard's star) and GJ 725B. This result supports inferences from previous infrared work, although it does not agree with standard evolutionary models or optical analyses, which both suggest an abundance for CM Dra close to that of the Sun.
Surface gravity
Effective temperature
Spectral resolution
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We show that the UV spectrum (1280-3200 A) of the "superficially normal" A-star Vega, as observed by the IUE satellite at a resolution comparable to the star's rotational broadening width, can be fit remarkably well by a single-temperature synthetic spectrum based on LTE atmosphere models and a newly constructed UV line list. If Vega were a normal, equator-on, slow-rotating star, then its spectrum and our analysis would indicate a temperature of Teff ~ 9550 K, surface gravity of log g ~ 3.7, general surface metallicity of [m/H] ~ -0.5, and a microturbulence velocity of v(turb) ~ 2.0 km/s. Given its rapid rotation and nearly pole-on orientation, however, these parameters must be regarded as representing averages across the observed hemisphere. Modeling the complex UV line spectrum has allowed us to determine the specific surface abundances for 17 different chemical elements, including CNO, the light metals, and the iron group elements. The resultant abundance pattern agrees in general with previous results, although there is considerable scatter in the literature. Despite its peculiarities, Vega has turned out to provide a powerful test of the extent of our abilities to model the atmospheric properties of the early A-stars, particularly the detailed UV line spectrum. The value of the measurements from this pilot study will increase as this analysis is extended to more objects in the rich high-dispersion IUE data archive, including both normal and peculiar objects.
Microturbulence
Vega
Effective temperature
Surface gravity
Line (geometry)
Rotation period
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A determination of the surface gravity and an abundance analysis of helium in a sample of 17He-rich and 5 normal, reference stars is presented. These results are derived from low resolution CCD spectra, but each star was measured at least 6 times in order to obtain a significant average spectrum for the spectroscopic variables. The helium abundances derived from the models used are very close to 0.1 for normal, reference stars and are larger for the others, clearly indicating the He-rich phenomenon in them. NLTE effects, errors on the microturbulence value or on the surface gravity do not influence the estimated helium abundances. Nevertheless, synthesized Geneva colours are affected by the He-rich peculiarity, especially the [U-B] index which systematically changes by -0.025 mag per 0.1 of He abundance for the coolest stars in the sample. We cannot confirm the correlation between the evolutionary state and the helium abundance reported previously (Zboril et al. 1994), although we used a more reliable technique of log g determination. All He-rich objects lie within themain sequence: their surface gravities are all inside the range 4.1 < log g < 4.5, with no more than three objects having log g < 4.25. We find a significant spread of helium abundances in this range of surface gravities, from the solar value∼ 0.1 up to about 0.4. Some of the programme stars (including reference stars) present emission in their Balmer lines and therefore some kind of stellar activity. Strong helium overabundance often coexists with emission and stellar activity.
Balmer series
Microturbulence
Surface gravity
Effective temperature
Horizontal branch
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We report spectral chemical abundances for 12 elements in Kapteyn's Star (HD 33793), a subdwarf M star. We use stellar temperature and gravity parameters derived using the recent interferometric measurement of its radius. We find [Fe/H]=−1.13 ± 0.01, [Ti/H]=−0.90 ± 0.03, and a weighted mean metallicity [M/H]=−0.98 ± 0.10, where the uncertainties in temperature, gravity and microturbulence are included in the latter. This value falls about mid-way between the metallicity found in the only previous high-resolution analysis, [M/H]=−0.55 ± 0.15, and the value derived from low-resolution spectra of TiO and CaH features, [M/H]=−1.5 ± 0.5. It is essentially identical to that found using low-resolution infrared spectra of water lines, [M/H]=−1.0 ± 0.3. This abundance determination for a nearby subdwarf M star using well-determined stellar parameters and a spectrum with a high signal-to-noise ratio will provide an anchor point for abundance studies of similar stars where the situation is not so favourable.
Microturbulence
Subdwarf
Effective temperature
Surface gravity
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We have tested current models for the atmospheres (including photosphere and low chromosphere) of late-type stars using the D resonance lines of neutral sodium as a di- agnostic. To this end, we have measured the equivalent widths of the D lines for a sample of 39 dwarf and 45 giant late-type stars observed with high spectral resolution. We constructed photo- spheric models over a grid in effective temperature and sur- face gravity spanning the spectral types F to M, and luminosity classes V and III of the sample stars. The model photospheres were extended into the chromosphere by assuming a suitable scaling from the Sun, and theoretical Nai D equivalent widths were computed over the grid of models including the deviations from local thermodynamic equilibrium. By taking into account both the experimental errors and the possible variations of stellar parameters (effective temperature, surface gravity, sodium abundance and microturbulence), the comparison between observed and computed equivalent widths allows us to state that the model atmospheres we have used can reproduce the observations for the two luminosity classes and for all the spectral types except for the M-type stars. We have discussed the importance of line blanketing in the spectral analysis of these stars, but at present we cannot conclude that this effect would reduce the discrepancy.
Microturbulence
Surface gravity
Effective temperature
Photosphere
Chromosphere
Blanketing
Giant star
Equivalent width
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Accurate surface gravities are needed for studies of stellar atmospheres and to determine stellar masses. It is suggested that measurements of gravities for late type stars are usually subject to great uncertainties, especially as the results are sensitive to the assumed effective temperature. A method is therefore proposed using the damping wings of metal lines to measure the gravity; and weak lines from the same lower level as the strong line to determine the metal abundance. Low excitation lines of Pel are used to demonstrate the method and new oscillator strengths are given for 51 transitions from the 0.00-0.12 and 0.86-0.99 eV levels. Using the furnace absorption technique the precision obtained is about 0.02 dex. Comparison with other authors' results shows systematic discrepancies between emission and absorption methods. The oscillator strengths are used to interpret the observed line strengths and profiles in the solar spectrum. From 5 model atmospheres a microturbulence of 0.8 km/sec is found, and iron abundances of 7.5-7.7. Damping constants are found for strong and medium-strong lines and the values are close to the predictions of classical theories in contrast with the greater damping predicted by more recent calculations. Using the measured oscillator strengths and damping constants the spectrum of Arcturus is used to derive an iron abundance of 6.99 and a microturbulence of 2.1 km/sec. The abundance is sensitive to the effective temperature, and 4400K is proposed from the observed flux distribution. The gravity determined from the profile of the line FeI 5269.5A is however insensitive to the adopted temperature and the measured value is log g = 1.45 p 0.15 corresponding to a mass of 0.56 M(sun) for a radius of 23 R(sun). The gravity and mass are greater than the results of other methods which assume, and are sensitive to, lower effective temperatures. It is concluded that metallic line profiles in the spectra of cool stars can yield precise gravities which are insensitive to the assumption of LTE or to the temperature in the atmosphere.
Microturbulence
Surface gravity
Voigt profile
Line (geometry)
Effective temperature
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We present a new direct spectroscopic calibration for a fast estimation of the stellar metallicity [Fe/H]. This calibration was computed using a large sample of 451 solar-type stars for which we have precise spectroscopic parameters derived from high quality spectra. The new [Fe/H] calibration is based on weak Fe I lines, which are expected to be less dependent on surface gravity and microturbulence, and require only a pre-determination of the effective temperature. This temperature can be obtained using a previously presented line-ratio calibration. We also present a simple code that uses the calibrations and procedures presented in these works to obtain both the effective temperature and the [Fe/H] estimate. The code, written in C, is freely available for the community and may be used as an extension of the ARES code. We test these calibrations for 582 independent FGK stars. We show that the code can be used as a precise and fast indicator of the spectroscopic temperature and metallicity for dwarf FKG stars with effective temperatures ranging from 4500 K to 6500 K and with [Fe/H] ranging from –0.8 dex to 0.4 dex.
Microturbulence
Effective temperature
Surface gravity
Code (set theory)
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High resolution, high -- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters (Teff, , and ξ) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the and lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 Å line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower Teff and values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-Teff scale used. The α-elements have abundance ratios consistent with the solar values for all the program stars, as expected for “normal” disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature.
Microturbulence
Subgiant
Effective temperature
Surface gravity
Line (geometry)
Equivalent width
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