Catalogue of averaged stellar effective magnetic fields – II. Re-discussion of chemically peculiar A and B stars

This paper presents a catalogue and the method of determining averaged quadratic effective magnetic fields  〈 B _e〉  for 1212 main-sequence and giant stars, and 11 white dwarf stars. The catalogue includes stars that are members of several open clusters. We have compiled measurements of the longitudinal magnetic field for those stars, which were scattered in the existing literature. A new parameter, magnetization (MA), has been defined, and we present values of MA for stars of various spectral classes. Our sample includes a subset of 610 chemically peculiar early-type stars. We confirm the conclusion of our previous study that the number distribution of all chemically peculiar stars versus the averaged magnetic field strength is described by a decreasing exponential function. Relations of this type also hold for stars of all the analysed subclasses of chemical peculiarity. Magnetization tends to correlate with the effective temperature only at high MA, for He-weak and He-rich stars.     

New classification and basic stellar parameter of SW Equulei

We review the classification of the variable star SW Equulei, formerly classified as a close binary of the W UMa type with a period of 0.83632 d and components of spectral types O–A or F–G and later. New observations indicate that the light curve is not that of an eclipsing binary but of a pulsating star with a period half of the old value. Further examination shows that SW Equulei is a double mode star of the RRd Lyrae type with a fundamental period of P = 0.560727 d and an overtone of P = 0.4181777 d at a distance of ≈ 7.1 kpc. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A photometric and spectroscopic study of the brightest northern Cepheids — II. Fundamental physical parameters

We present a new Cepheid reddening and effective temperature scale based on the uvby photometry published in the first paper of this series. Using all available information about the companion stars in Cepheids with bright blue secondaries, we remove their light from the observed light and colour curves. The resulting corrections are as large as 0.05–0.15 mag in several cases for different colour indices. A new photometric approach based on the ( b  −  y ) versus ( B  − V ) two-colour diagram is tested with three other previous calibrations taken from the literature. Two uvby relations in earlier studies turn out to be the most reliable and consistent, and so they are used in deriving colour excesses. We determine systematically higher reddenings for Cepheids with a significant secondary light correction. The dereddened Stro¨mgren colours are calibrated in terms of T _eff and log  g using the most recent synthetic colour grids. Our temperature scale is very close to that of Kraft, which is supported by other recent temperature determinations using the infrared flux method or Geneva photometry. The photometric gravities fit some of the earlier theoretical and observational (mainly spectroscopic) results very well.     

主序星的Ⅴ波段经验质光关系

恒星质量是恒星物理以及恒星系统动力学研究中一个不可或缺的参量.双星轨道拟合是获取恒星(动力学)质量的最可靠途径,而绝大部分恒星的质量仍然需要通过恒星质光关系来估计,因此,通过拟合恒星动力学质量和光度数据得到经验质光关系的工作具有重要意义.尽管主序星的Ⅴ波段质光关系由于金属丰度的影响而具有一定的弥散性,但有研究表明这种影响主要限于恒星质量小于 0.6M_⊙的情况.对于较大质量的主序星,近年来的观测拟合研究积累了比较充分的动力学质量和Ⅴ波段光度数据,从而为显著改进上述质光关系提供了可能.利用一个能合理分配两个不同量纲观测量权重的拟合方法,根据 203 颗恒星的动力学质量和光度数据给出了主序星的Ⅴ波段经验质光关系,该结果对此前结果的改进不仅具有统计显著性,而且其对恒星质量估计的相对误差已达到约 5%.因此,该结果不仅可以用于开展有关恒星物理或恒星系统动力学方面的统计性研究,而且对具体实际多星系统的长期动力学研究和短期定位研究等也有应用价值.     

Empirical calibration of the near-infrared Ca ii triplet – II. The stellar atmospheric parameters

We present a homogeneous set of stellar atmospheric parameters ( T _eff, log  g , [Fe/H]) for a sample of about 700 field and cluster stars which constitute a new stellar library in the near-IR developed for stellar population synthesis in this spectral region ( λ 8350–9020) . Having compiled the available atmospheric data in the literature for field stars, we have found systematic deviations between the atmospheric parameters from different bibliographic references. The Soubiran, Katz & Cayrel sample of stars with very well determined fundamental parameters has been taken as our standard reference system, and other papers have been calibrated and bootstrapped against it. The obtained transformations are provided in this paper. Once most of the data sets were on the same system, final parameters were derived by performing error weighted means. Atmospheric parameters for cluster stars have also been revised and updated according to recent metallicity scales and colour–temperature relations.     

Principal component analysis for spectral indices of stellar populations

We apply the method of principal component analysis to a sample of simple stellar populations to select some age-sensitive spectral indices. Besides the well-known age-sensitive index, H β , we find that some other spectral indices have great potential to determine the age of stellar populations, such as G4300, Fe4383, C₂4668, and Mg b . In addition, we find that the sensitivity to age of these spectral indices depends on the metallicity of the simple stellar population (SSP): H β and G4300 are more suited to determine the age of the low-metallicity stellar population, C₂4668 and Mg b are more suited to the high-metallicity stellar population. The results suggest that the principal component analysis method provides a more objective and informative alternative to diagnostics by individual spectral lines.     

Automated estimation of stellar fundamental parameters from low resolution spectra: the PLS method

PLS (Partial Least Squares regression) is introduced into an automatic esti-mation of fundamental stellar spectral parameters. It extracts the most correlative spec-tral component to the parameters (Teff, log g and [Fe/H]), and sets up a linear regres-sion function from spectra to the corresponding parameters. Considering the properties of stellar spectra and the PLS algorithm, we present a piecewise PLS regression method for estimation of stellar parameters, which is composed of one PLS model for Teff, and seven PLS models for log g and [Fe/H] estimation. Its performance is investigated by large experiments on flux calibrated spectra and continuum normalized spectra at dif-ferent signal-to-noise ratios (SNRs) and resolutions. The results show that the piecewise PLS method is robust for spectra at the medium resolution of 0.23 nm. For low resolu-tion 0.5 nm and 1 nm spectra, it achieves competitive results at higher SNR. Experiments using ELODIE spectra of 0.23 nm resolution illustrate that our piecewise PLS models trained with MILES spectra are efficient for O ～ G stars: for flux calibrated spectra, the systematic offsets are 3.8%, 0.14dex, and -0.09 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.44 dex and 0.38 dex, respectively; for continuum normalized spectra, the systematic offsets are 3.8%, 0.12dex, and -0.13 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.49 dex and 0.41 dex, respectively. The PLS method is rapid, easy to use and does not rely as strongly on the tightness of a parameter grid of templates to reach high precision as Artificial Neural Networks or minimum distance methods do.     

Automated spectral classification using template matching

An automated spectral classification technique for large sky surveys is pro-posed. We firstly perform spectral line matching to determine redshift candidates for an observed spectrum, and then estimate the spectral class by measuring the similarity be-tween the observed spectrum and the shifted templates for each redshift candidate. As a byproduct of this approach, the spectral redshift can also be obtained with high accuracy. Compared with some approaches based on computerized learning methods in the liter-ature, the proposed approach needs no training, which is time-consuming and sensitive to selection of the training set. Both simulated data and observed spectra are used to test the approach; the results show that the proposed method is efficient, and it can achieve a correct classification rate as high as 92.9%, 97.9% and 98.8% for stars, galaxies and quasars, respectively.     

Physical properties of active stars and stellar systems

We study spectra of 18 active stars and stellar systems, drawn from lists of known active binaries or newly discovered extreme-ultraviolet and soft X-ray sources. Using spectral synthesis and template modelling, we measure (often for the first time) a variety of physical properties of the stars, including spectral types, luminosities and v  sin  i values. We report direct observation of previously undetected secondary components in three systems, and refine the orbital elements for two of these (HR 7275 and HD 217344).