arXiv subjects

Klaus-Peter Schröder

Publications and source records attributed to Klaus-Peter Schröder.

9 recordsLinked to original sources

arXiv2026

On the stellar parameter dependence of the combined Fe I and Fe II chromospheric emission-line in the wings of the Ca II K line

We present an analysis of the chromospheric emission of Fe I + Fe II in the wing of the Ca II K line of 21 G and K giant stars. Stellar parameters and absolute chromospheric fluxes were obtained by comparison with PHOENIX models. The iron-blend emission shows a proportionality with the Ca II K flux for stars with similar stellar parameters and also for individual stars observed at different epochs, confirming that both diagnostics seems to respond to the same underlying chromospheric conditions. We found a dependence of the iron-blend emission on stellar parameters by fitting a power-law relation. The exponent of the effective temperature statistically matches with that found for Ca II K; which, together with a metallicity dependence, indicate that the iron-blend may act as an indirect tracer of chromospheric thermal conditions. The chromospheric emission flux ratio (Fe I + Fe II)/Ca II K exhibits a distinct slope change around log g approx. 2.5 dex; which causes the gravity dependence to be different for stars with gravities lower and higher than 2.5 dex. This behavior suggests a transition between regimes where the two diagnostics probe similar physical conditions and where they begin to diverge in the sensitivity to the chromospheric extension. We discuss the implications of this behavior for the thermal structure of the lower chromosphere and for the formation heights of both diagnostics.

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arXiv2023

Bright Spectroscopic Binaries: III. Binary systems with orbital periods of P > 500 days

We present an analysis of nine bright spectroscopic binaries (HD 1585, HD 6613, HD 12390, HD 39923, HD 55201, HD 147430, HD 195543, HD 202699, HD 221643), which have orbital periods of P > 500 days. These well-separated binaries are the last stars of our sample that we observed with the TIGRE telescope obtaining intermediate-resolution spectra of R = 20,000. We applied the same method as described in our previous publication of this series. For the analysis of the radial velocity curves, we used the toolkit RadVel, which allowed us to determine all orbital parameters. Recently published orbital solutions of some systems from Gaia DR3 agree with our results. However, our solutions have much smaller uncertainties. We determined the basic stellar parameters of the primary stars with our automatic script using iSpec. The parameter determination allowed us to place all nine stars in the Hertzsprung-Russell diagram. We found that all stars have already evolved to the giant phase. A comparison with stellar evolution tracks of the Eggleton code was applied to determine the stellar masses and ages. As a result of our analysis, we were able to estimate the masses of the secondary stars and the orbital inclinations of the binary systems.

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arXiv2022

Bright Spectroscopic Binaries: II. A study of five systems with orbital periods of P < 500 days

We present a detailed analysis of five bright spectroscopic binary systems (HD 18665, HD 27131, HD 171852, HD 215550, HD 217427) that have orbital periods of P < 500 days. We determined the complete set of orbital parameters using the toolkit RadVel by analyzing the observed radial velocity curves. To study the properties of the five systems, we also analyzed the intermediate resolution spectra (R = 20,000) observed with the TIGRE telescope and determined the stellar parameters of the primary stars using the toolkit iSpec. With Gaia Early Data Release 3 parallaxes, a correction for interstellar extinction using the 3D dust map, and bolometric corrections, we placed the stars in the Hertzsprung-Russell diagram and compared the positions with stellar evolution tracks calculated with the Eggleton code to determine the masses and ages of the primary stars. They have all evolved to the giant phase. Finally, we were able to determine the masses of the secondary stars and to estimate the orbital inclinations i of the binary systems.

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arXiv2020

Bright Spectroscopic Binaries: I. Orbital parameters of five systems with periods of P < 365 days

In a well-aimed search based on the recently published Gaia~DR2 radial velocity data and performed with the robotic TIGRE telescope, we have discovered 19 bright ($m_V<7.66$~mag) spectroscopic binary stars. We present the radial velocity curves of five of these stars (HD~20656, HD~27259, HD~98812, HD~150600, HD~193082) with periods of $P<365$~days. We determined their orbital parameters using the toolkit RadVel. The complete set of basic stellar parameters (effective temperature, surface gravity, metallicity, mass and age) of the primary stars are determined using the observed intermediate resolution spectra ($R\approx20,000$) with iSpec and a comparison of the position in the HRD with stellar evolution tracks. All five primary stars have already evolved to the giant phase and possess low-mass secondaries, which do not noticeably affect the spectra. Finally, we determined the minimum masses of the secondary stars of the binary systems and present conclusions about the properties of the binary systems.

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arXiv2020

Time series of optical spectra of Nova V659 Sct

With our robotic 1.2 m TIGRE telescope, we were able to obtain eight optical spectra with intermediate resolution (R = 20,000) of the Nova V659 Sct during different phases of its outburst. We present a list of the lines found in the Nova spectra. The most common features are H I, O I, Na I, Fe II and Ca II. Studying the spectral evolution of the strong features we found that the absorption features move to higher expansion velocities before disappearing and the emission features show (different) asymmetries. Thanks to the intermediate spectral resolution we identified and analysed the interstellar medium absorption features present in the spectra. We detected atomic absorption features of Na I and Ca II. The sodium D lines show more complex substructures with three main absorption features at around a velocity of -10, 30 and 85 km/s. We identified several DIBs in the Nova V659 Sct spectra and determined their velocities and equivalent widths.

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arXiv2019

Magnetic activity and evolution of the four Hyades K giants

We determine the exact physical parameters of the four Hyades cluster K giants, using their parallaxes and atmospheric modeling of our red-channel TIGRE high-resolution spectra. Performing a comparison with well-tested evolutionary tracks, we derive exact masses and evolutionary stages. At an age of 588 ($\pm 60$) Myrs and with a metallicity of Z=0.03 (consistent with the spectroscopic abundances), we find HD~27371 and HD~28307, the two less bright K giants, at the onset of central helium-burning, entering their blue loops with a mass of 2.62 $M_{\odot}$, while the slightly brighter stars HD~28305 and HD~27697 are already exiting their blue loop. Their more advanced evolution suggests a higher mass of 2.75 $M_{\odot}$. Notably, this pairing coincides with the different activity levels, which we find for these four stars from chromospheric activity monitoring with TIGRE and archival Mount Wilson data as well as from ROSAT coronal detections: The two less evolved K giants are the far more active pair, and we confidently confirm their rotation with periods of about 142 days. This work therefore provides some first, direct evidence of magnetic braking during the 130~Myrs lasting phase of central helium-burning, similar to what has long been known to occur to cool main-sequence stars.

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arXiv2019

Constraint on the axion-electron coupling constant and the neutrino magnetic dipole moment by using the tip-RGB luminosity of fifty globular clusters

The current constraints in the neutrino magnetic dipole moment and axion-electron coupling constant ($\mathrm{μ_ν\leq2.2\times10^{-12}μ_{B}}$ and $\mathrm{α_{ae}\leq0.5\times10^{-26}}$) are tested against the observational calibration on the tip-RGB of fifty galactic globular clusters, covering $\mathrm{-1.95\leq[M/H]\leq+0.04}$. When each energy sink is introduced into stellar models separately, the current known constraints predict a tip-RGB bolometric luminosity that does not conflict the calibration, while when they are assumed to occur simultaneously, these constraints can be lowered down by 50\% and 75\%.

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arXiv2009

Dynamo-generated magnetic fields in fast rotating single giants

Red giants offer a good opportunity to study the interplay of magnetic fields and stellar evolution. Using the spectro-polarimeter NARVAL of the Telescope Bernard Lyot (TBL), Pic du Midi, France and the LSD technique, we began a survey of magnetic fields in single G-K-M giants. Early results include 6 MF-detections with fast rotating giants, and for the first time a magnetic field was detected directly in an evolved M-giant: EK Boo. Our results could be explained in the terms of $α$--$ω$ dynamo operating in these giants.

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arXiv2005

What a Local Sample of Spectroscopic Binaries can tell us about the Field-Binary Population

We study a sample of spectroscopic binaries (SBs) in the local solar neighbourhood to 100 pc and to an absolute magnitude of 4 in an attempt to find the distributions of the period, primary mass and the mass ratio as well as the IMF of the local population of field binaries. The sample was collated using available SB data and the Hipparcos catalogue, the latter being used for distances and to refer numbers of objects to fractions of the local stellar population as a whole. We use the better-determined double-lined SBs (SB2s) to calibrate a Monte-Carlo approach to modelling the mass ratio distribution of the single-lined SBs (SB1s) from their mass functions and primary masses. While a complete sample is not possible, given the data available, we are able to address important questions of incompleteness and parameter-specific biases by comparing subsamples of SBs with different ranges in parameter space. Our results show a clear peak in the mass ratio distribution of field binaries near unity. This is dominated by the SB2s, but the flat distribution of the SB1s is inconsistent with their components being chosen independently at random from a steep IMF.

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