The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally ...important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1.
Context.
Precise continuum normalisation of merged échelle spectra is a demanding task that is necessary for various detailed spectroscopic analyses. Automatic methods have limited effectiveness due ...to the variety of features present in the spectra of stars. This complexity often leads to the necessity for manual normalisation which is highly time-consuming.
Aims.
The aim of this work is to develop a fully automated normalisation tool that works with order-merged spectra and offers flexible manual fine-tuning, if necessary.
Methods.
The core of the proposed method uses the novel, fully convolutional deep neural network (SUPP Network) that was trained to predict a pseudo-continuum. The post-processing step uses smoothing splines that give access to regressed knots, which are useful for optional manual corrections. The active learning technique was applied to deal with possible biases that may arise from training with synthetic spectra and to extend the applicability of the proposed method to features absent in this kind of spectra.
Results.
The developed normalisation method was tested with high-resolution spectra of stars with spectral types from O to G, and gives a root mean squared (RMS) error over the set of test stars equal to 0.0128 in the spectral range from 3900 Å to 7000 Å and 0.0081 in the range from 4200 Å to 7000 Å. Experiments with synthetic spectra give a RMS of the order of 0.0050.
Conclusions.
The proposed method leads to results that are comparable to careful manual normalisation. Additionally, this approach is general and can be used in other fields of astronomy where background modelling or trend removal is a part of data processing.
Abstract
We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), ...and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars
μ
Lep and
β
Scl, and to the superficially normal star
ν
Cap. In the spectra of the six stars, the Hg
ii
line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of
P
orb
= 26.12022 ± 0.00004 day.
The asteroseismic and planetary studies, like all research related to stars, need precise and accurate stellar atmospheric parameters as input. We aim at deriving the effective temperature (T
eff), ...the surface gravity (log g), the metallicity (Fe/H), the projected rotational velocity (v sin i) and the MK type for 169 F-, G-, K- and M-type Kepler targets which were observed spectroscopically from the ground with five different instruments. We use two different spectroscopic methods to analyse 189 high-resolution, high-signal-to-noise spectra acquired for the 169 stars. For 67 stars, the spectroscopic atmospheric parameters are derived for the first time. KIC 9693187 and 11179629 are discovered to be double-lined spectroscopic binary systems. The results obtained for those stars for which independent determinations of the atmospheric parameters are available in the literature are used for a comparative analysis. As a result, we show that for solar-type stars the accuracy of present determinations of atmospheric parameters is ±150 K in T
eff, ±0.15 dex in Fe/H and ±0.3 dex in log g. Finally, we confirm that the curve-of-growth analysis and the method of spectral synthesis yield systematically different atmospheric parameters when they are applied to stars hotter than 6000 K.
ABSTRACT
Photometric and spectroscopic time-series of chemically peculiar mercury–manganese (HgMn) stars show variability, which in most cases is interpreted as the effect of binarity or surface ...spots. Until recently, pulsations have not been detected in these objects. However, the analysis of the Transiting Exoplanet Survey Satellite (TESS) photometric time-series of a large sample of HgMn stars resulted in a small sample of candidates for pulsating variables. In this paper, we present a thorough analysis of one of them, HD 29589. High-resolution spectroscopic data were used to determine the atmospheric parameters and a detailed chemical composition of the star. The obtained effective temperature, Teff = 14 400 ± 200 K, places this star among the high-temperature HgMn stars. The determined abundance pattern is typical for this class of objects. Interferometric data were used to discuss the properties of the secondary in the HD 29589 system. More than 20 independent frequencies were identified in the TESS time-series of this star, which we interpret as high-order g-modes. We found regular period spacing, which is due to the presence of consecutive prograde dipole modes. The exact matching of the six frequencies in the series and the position of the star in the Hertzsprung–Russell diagram allowed us to constrain the internal structure of HD 29589. We derived the overshooting parameter, fov = 0.028−0.03, mass M = 3.4 M⊙ and metallicity Z = 0.008. A detailed analysis of pulsating HgMn stars will allow us to understand the influence of the chemical peculiarity on the pulsation characteristics of these objects.
ABSTRACT
The main-sequence mercury–manganese (HgMn) stars are known to exhibit large overabundances of exotic elements and, similar to magnetic Ap/Bp stars, are spectrum variables, implying the ...presence of an inhomogeneous element distribution over the stellar surface. A number of magnetic field studies have been attempted in the last decades, indicating that magnetic fields in HgMn stars, if they exist, should be rather weak. The presence of tangled magnetic fields was suggested by several authors who detected quadratic magnetic fields using the moment technique. We employ the least-squares deconvolution technique to carry out a sensitive search for weak magnetic fields in spectropolarimetric observations of three HgMn stars, HD 221507, HD 65949, and HD 101189, which have different fundamental parameters and spectral characteristics. A definite weak longitudinal field is discovered in HD 221507 and HD 65949 on single epochs, while only marginal field detections were achieved for HD 101189. The new measurements indicate that the structure of the magnetic fields is probably rather complex: our analysis reveals the presence of reversed Stokes V profiles at the same observational epoch if individual elements are used in the measurements. This is the first observational evidence that individual elements sample distinct local magnetic fields of different polarity across the stellar surface.
We present a spectroscopic survey of known and candidate γ Doradus stars. The high-resolution, high signal-to-noise spectra of 52 objects were collected by five different spectrographs. The spectral ...classification, atmospheric parameters (T
eff, log g, ξ), vsin i and chemical composition of the stars were derived. The stellar spectral and luminosity classes were found between G0–A7 and IV–V, respectively. The initial values for T
eff and log g were determined from the photometric indices and spectral energy distribution. Those parameters were improved by the analysis of hydrogen lines. The final values of T
eff, log g and ξ were derived from the iron lines analysis. The T
eff values were found between 6000 K and 7900 K, while log g values range from 3.8 to 4.5 dex. Chemical abundances and vsin i values were derived by the spectrum synthesis method. The vsin i values were found between 5 and 240 km s−1. The chemical abundance pattern of γ Doradus stars were compared with the pattern of non-pulsating stars. It turned out that there is no significant difference in abundance patterns between these two groups. Additionally, the relations between the atmospheric parameters and the pulsation quantities were checked. A strong correlation between the vsin i and the pulsation periods of γ Doradus variables was obtained. The accurate positions of the analysed stars in the Hertzsprung–Russell diagram have been shown. Most of our objects are located inside or close to the blue edge of the theoretical instability strip of γ Doradus.
Abstract
We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS ...2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these and the singly periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor of 6 smaller than that in the B filter, which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 $\mu$mag, respectively, in two of the best characterized non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.
ABSTRACT
If γ Dor-type pulsations are driven by the convective blocking mechanism, a convective envelope at a sufficient depth is essential. There are several hot γ Dor and hybrid star candidates in ...which there should not be an adequate convective envelope to excite the γ Dor-type oscillations. The existence of these hot objects needs an explanation. Therefore, we selected, observed, and studied 24 hot γ Dor and hybrid candidates to investigate their properties. The atmospheric parameters, chemical abundances, and vsin i values of the candidates were obtained using medium-resolution (R = 46 000) spectra taken with the Fibre-fed Échelle Spectrograph instrument mounted at the Nordic Optical Telescope. We also carried out frequency analyses of the Kepler long- and short-cadence data to determine the exact pulsation contents. We found only five bona fide hot γ Dor and three bona fide hot hybrid stars in our sample. The other 16 stars were found to be normal γ Dor, δ Sct, or hybrid variables. No chemical peculiarity was detected in the spectra of the bona fide hot γ Dor and hybrid stars. We investigated the interplay between rotation and pulsational modes. We also found that the hot γ Dor stars have higher Gaia luminosities and larger radii compared to main-sequence A–F stars.
Spectroscopic study of the open cluster NGC 6811 Molenda-Żakowicz, J; Brogaard, K; Niemczura, E ...
Monthly notices of the Royal Astronomical Society,
12/2014, Letnik:
445, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The NASA space telescope Kepler has provided unprecedented time series observations which have revolutionized the field of asteroseismology, i.e. the use of stellar oscillations to probe the interior ...of stars. The Kepler-data include observations of stars in open clusters, which are particularly interesting for asteroseismology. One of the clusters observed with Kepler is NGC 6811, which is the target of this paper. However, apart from high-precision time series observations, sounding the interiors of stars in open clusters by means of asteroseismology also requires accurate and precise atmospheric parameters as well as cluster membership indicators for the individual stars. We use medium-resolution (R ∼ 25 000) spectroscopic observations, and three independent analysis methods, to derive effective temperatures, surface gravities, metallicities, projected rotational velocities and radial velocities, for 15 stars in the field of the open cluster NGC 6811. We discover two double-lined and three single-lined spectroscopic binaries. Eight stars are classified as either certain or very probable cluster members, and three stars are classified as non-members. For four stars, cluster membership could not been assessed. Five of the observed stars are G-type giants which are located in the colour–magnitude diagram in the region of the red clump of the cluster. Two of these stars are surely identified as red clump stars for the first time. For those five stars, we provide chemical abundances of 31 elements. The mean radial velocity of NGC 6811 is found to be +6.68 ± 0.08 km s−1 and the mean metallicity and overall abundance pattern are shown to be very close to solar with an exception of Ba which we find to be overabundant.
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