arXiv subjects

K. G. Strassmeier

Publications and source records attributed to K. G. Strassmeier.

At least 109 records · Page 6Linked to original sources

arXiv2010

Ellipsoidal primary of the RS CVn binary zeta And: Investigation using high-resolution spectroscopy and optical interferometry

We have obtained high-resolution spectroscopy, optical interferometry, and long-term broad band photometry of the ellipsoidal primary of the RS CVn-type binary system zeta And. Based on the optical interferometry the apparent limb darkened diameter of zeta And is 2.55 +/- 0.09 mas using a uniform disk fit. The Hipparcos distance and the limb-darkened diameter obtained with a uniform disk fit give stellar radius of 15.9 +/- 0.8 Rsolar, and combined with bolometric luminosity, it implies an effective temperature of 4665 +/- 140 K. The temperature maps obtained from high resolution spectra using Doppler imaging show a strong belt of equatorial spots and hints of a cool polar cap. The equatorial spots show a concentration around the phase 0.75. This spot configuration is reminiscent of the one seen in the earlier published temperature maps of zeta And. Investigation of the Halpha line reveals both prominences and cool clouds in the chromosphere. Long-term photometry spanning 12 years shows hints of a spot activity cycle, which is also implied by the Doppler images, but the cycle length cannot be reliably determined from the current data.

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arXiv2009

Development of the opto-mechanical design for ICE-T

ICE-T (International Concordia Explorer Telescope) is a double 60 cm f/1.1 photometric robotic telescope, on a parallactic mount, which will operate at Dome C, in the long Antarctic night, aiming to investigate exoplanets and activity of the hosting stars. Antarctic Plateau site is well known to be one of the best in the world for observations because of sky transparency in all wavelengths and low scintillation noise. Due to the extremely harsh environmental conditions (the lowest average temperature is -80$^\circ$C) the criteria adopted for an optimal design are really challenging. Here we present the strategies we have adopted so far to fulfill the mechanical and optical requirements.

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arXiv2009

Multiple and changing cycles of active stars II. Results

We study the time variations of the cycles of 20 active stars based on decades-long photometric or spectroscopic observations. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Fifteen stars definitely show multiple cycles; the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics, and which vary in parallel, indicating that a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Stellar activity cycles are generally multiple and variable.

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arXiv2009

Zeeman-Doppler Imaging of II Peg - Magnetic field restructuring from 2004 to 2007

We present Zeeman-Doppler images of the active K2 star II Peg for the years 2004 and 2007. The surface magnetic field was reconstructed with our new ZDI code "iMap" which provides a full polarized radiative transfer driven inversion to simultaneously reconstruct the surface temperature and magnetic vector field distribution. II Peg shows a remarkable large scale magnetic field structure for both years. The magnetic field is predominantly located at high latitudes and is arranged in active longitudes. A dramatic evolution in the magnetic field structure is visible for the two years, where a dominant and largely unipolar field in 2004 has developed into two distinct and large scale bipolar structures in 2007.

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arXiv2009

Zeeman-Doppler Imaging : Old Problems and New Methods

Zeeman-Doppler Imaging (ZDI) is a powerful inversion method to reconstruct stellar magnetic surface fields. The reconstruction process is usually solved by translating the inverse problem into a regularized least-square or optimization problem. In this contribution we will emphasize that ZDI is an inherent non-linear problem and the corresponding regularized optimization is, like many non-linear problems, potentially prone to local minima. We show how this problem will be exacerbated by using an inadequate forward model. To facilitate a more consistent full radiative transfer driven approach to ZDI we describe a two-stage strategy that consist of a principal component analysis (PCA) based line profile reconstruction and a fast approximate polarized radiative transfer method to synthesize local Stokes profiles. Moreover, we introduce a novel statistical inversion method based on artificial neural networks (ANN) which provide a fast calculation of a first guess model and allows to incorporate better physical constraints into the inversion process.

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arXiv2008

First measurement of the magnetic field on FK Com and its relation to the contemporaneous starspot locations

In this study we present simultaneous low-resolution longitudinal magnetic field measurements and high-resolution spectroscopic observations of the cool single giant FK Com. The variation of the magnetic field over the rotational period of 2.4 days is compared with the starspot location obtained using Doppler imaging techniques, V-band photometry and V-I colours. The chromospheric activity is studied simultaneously with the photospheric activity using high resolution observations of the Halpha, Hbeta and Hgamma line profiles. Both the maximum (272 +/- 24 G) and minimum (60 +/- 17 G) in the mean longitudinal magnetic field, , are detected close to the phases where cool spots appear on the stellar surface. A possible explanation for such a behaviour is that the active regions at the two longitudes separated by 0.2 in phase have opposite polarities.

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arXiv2008

The chromospherically active binary star EI Eridani II. Long-term Doppler imaging

Data from 11 years of continuous spectroscopic observations of the active RS CVn-type binary star EI Eridani - gained at NSO/McMath-Pierce, KPNO/Coude Feed and during the MUSICOS 98 campaign - were used to obtain 34 Doppler maps in three spectroscopic lines for 32 epochs, 28 of which are independent of each other. Various parameters are extracted from our Doppler maps: average temperature, fractional spottedness, and longitudinal and latitudinal spot-occurrence functions. We find that none of these parameters show a distinct variation nor a correlation with the proposed activity cycle as seen from photometric long-term observations. This suggests that the photometric brightness cycle may not necessarily be due to just a cool spot cycle. The general morphology of the spot pattern remains persistent over the whole period of 11 years. A large cap-like polar spot was recovered from all our images. A high degree of variable activity was noticed near latitudes of approx. 60-70 degrees where the appendages of the polar spot emerged and dissolved.

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arXiv2008

The chromospherically active binary star EI Eridani I. Absolute dimensions

We present a detailed determination of the astrophysical parameters of the chromospherically active binary star EI Eridani. Our new radial velocities allow to improve the set of orbital elements and reveal long-term variations of the barycentric velocity. A possible third-body orbit with a period of approximately 19 years is presented. Absolute parameters are determined in combination with the Hipparcos parallax. EI Eri's inclination angle of the rotational axis is confined to 56.0 plus/minus 4.5 degrees, its luminosity class IV is confirmed by its radius of 2.37 plus/minus 0.12 R_Sun. A comparison to theoretical stellar evolutionary tracks suggests a mass of 1.09 plus/minus 0.05 M_Sun and an age of approximately 6.15 Gyr. The present investigation is the basis of our long-term Doppler imaging study of its stellar surface.

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arXiv2008

A fast method for Stokes profile synthesis -- Radiative transfer modeling for ZDI and Stokes profile inversion

The major challenges for a fully polarized radiative transfer driven approach to Zeeman-Doppler imaging are still the enormous computational requirements. In every cycle of the iterative interplay between the forward process (spectral synthesis) and the inverse process (derivative based optimization) the Stokes profile synthesis requires several thousand evaluations of the polarized radiative transfer equation for a given stellar surface model. To cope with these computational demands and to allow for the incorporation of a full Stokes profile synthesis into Doppler- and Zeeman-Doppler imaging applications as well as into large scale solar Stokes profile inversions, we present a novel fast and accurate synthesis method for calculating local Stokes profiles. Our approach is based on artificial neural network models, which we use to approximate the complex non-linear mapping between the most important atmospheric parameters and the corresponding Stokes profiles. A number of specialized artificial neural networks, are used to model the functional relation between the model atmosphere, magnetic field strength, field inclination, and field azimuth, on one hand and the individual components (I,Q,U,V) of the Stokes profiles, on the other hand. We performed an extensive statistical evaluation and show that our new approach yields accurate local as well as disk-integrated Stokes profiles over a wide range of atmospheric conditions. The mean rms errors for the Stokes I and V profiles are well below 0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in the range of 1%. Our approach does not only offer an accurate approximation to the LTE polarized radiative transfer it, moreover, accelerates the synthesis by a factor of more than 1000.

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arXiv2008

First time-series optical photometry from Antarctica

Beating the Earth's day-night cycle is mandatory for long and continuous time-series photometry and had been achieved with either large ground-based networks of observatories at different geographic longitudes or when conducted from space. A third possibility is offered by a polar location with astronomically-qualified site characteristics. Aims. In this paper, we present the first scientific stellar time-series optical photometry from Dome C in Antarctica and analyze approximately 13,000 CCD frames taken in July 2007. We conclude that high-precision CCD photometry with exceptional time coverage and cadence can be obtained at Dome C in Antarctica and be successfully used for time-series astrophysics.

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arXiv2008

Chromospheric features of LQ Hydrae from H-alpha line profiles

We analyze the H-alpha spectral variability of the rapidly-rotating K1-dwarf LQ Hya using high-resolution H-alpha spectra recorded during April-May 2000. Chromospheric parameters were computed from the H-alpha profile as a function of rotational phase. We find that all these parameters vary in phase, with a higher chromospheric electron density coinciding with the maximum H-alpha emission. We find a clear rotational modulation of the H-alpha emission that is better emphasized by subtracting a reference photospheric template built up with a spectrum of a non-active star of the same spectral type. A geometrical plage model applied to the H-alpha variation curve allows us to derive the location of the active regions that come out to be close in longitude to the most pronounced photospheric spots found with Doppler imaging applied to the photospheric lines in the same spectra. Our analysis suggests that the H-alpha features observed in LQ Hya in 2000 are a scaled-up version of the solar plages as regards dimensions and/or flux contrast. No clear indication of chromospheric mass motions emerges.

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arXiv2008

Zeeman-Doppler Imaging of Late-Type Stars -- The Surface Magnetic Field of II Peg

Late-type stars in general possess complicated magnetic surface fields which makes their detection and in particular their modeling and reconstruction challenging. In this work we present a new Zeeman-Doppler imaging code which is especially designed for the application to late-type stars. This code uses a new multi-line cross-correlation technique by means of a principal component analysis to extract and enhance the quality of individual polarized line profiles. It implements the full polarized radiative transfer equation and uses an inversion strategy that can incorporate prior knowledge based on solar analogies. Moreover, our code utilizes a new regularization scheme which is based on local maximum entropy to allow a more appropriate reproduction of complex surface fields as those expected for late-type stars. In a first application we present Zeeman-Doppler images of II Pegasi which reveal a surprisingly large scale surface structure with one predominant (unipolar) magnetic longitude which is mainly radially oriented.

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arXiv2007

Solar Physics and the Solar-Stellar Connection at Dome C

Solar magnetic fields evolve on many time-scales, e.g., the generation, migration, and dissipation of magnetic flux during the 22-year magnetic cycle of the Sun. Active regions develop and decay over periods of weeks. The build-up of magnetic shear in active regions can occur within less than a day. At the shortest time-scales, the magnetic field topology can change rapidly within a few minutes as the result of eruptive events such as flares, filament eruptions, and coronal mass ejections. The unique daytime seeing characteristics at Dome C, i.e., continuous periods of very good to excellent seeing during almost the entire Antarctic summer, allow us to address many of the top science cases related to the evolution of solar magnetic fields. We introduce the Advanced Solar Photometric Imager and Radiation Experiment and present the science cases for synoptic solar observations at Dome C. Furthermore, common science cases concerning the solar-stellar connection are discussed in the context of the proposed International Concordia Explorer Telescope.

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arXiv2007

Anti-solar differential rotation and surface flow pattern on UZ Librae

We re-investigate UZ Librae spectra obtained at KPNO in 1998 and 2000. From the 1998 data we compose 11 consecutive Doppler images using the Ca I-6439, Fe I-6393 and Fe I-6411 lines. Applying the method of average cross-correlation of contiguous Doppler images we find anti-solar differential rotation with a surface shear of alpha ~ -0.03. The pilot application of the local correlation tracking technique for the same data qualitatively confirms this result and indicates complex flow pattern on the stellar surface. From the cross-correlation of the two available Doppler images in 2000 we also get anti-solar differential rotation but with a much weaker shear of alpha ~ -0.004.

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arXiv2007

Surface velocity network with anti-solar differential rotation on the active K-giant $σ$ Geminorum

We demonstrate the power of the local correlation tracking technique on stellar data for the first time. We recover the spot migration pattern of the long-period RS CVn-type binary $σ$ Gem from a set of six Doppler images from 3.6 consecutive rotation cycles. The resulting surface flow map suggests a weak anti-solar differential rotation with $α\approx-0.0022\pm0.0016$, and a coherent poleward spot migration with an average velocity of $220\pm10$ m s$^{-1}$. This result agrees with our recent findings from another study and could also be confirmed theoretically.

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arXiv2006

Challenges in optics for Extremely Large Telescope instrumentation

We describe and summarize the optical challenges for future instrumentation for Extremely Large Telescopes (ELTs). Knowing the complex instrumental requirements is crucial for the successful design of 30-60m aperture telescopes. After all, the success of ELTs will heavily rely on its instrumentation and this, in turn, will depend on the ability to produce large and ultra-precise optical components like light-weight mirrors, aspheric lenses, segmented filters, and large gratings. New materials and manufacturing processes are currently under study, both at research institutes and in industry. In the present paper, we report on its progress with particular emphasize on volume-phase-holographic gratings, photochromic materials, sintered silicon-carbide mirrors, ion-beam figuring, ultra-precision surfaces, and free-form optics. All are promising technologies opening new degrees of freedom to optical designers. New optronic-mechanical systems will enable efficient use of the very large focal planes. We also provide exploratory descriptions of "old" and "new" optical technologies together with suggestions to instrument designers to overcome some of the challenges placed by ELT instrumentation.

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arXiv2005

Binarity, activity and metallicity among late-type stars I. Methodology and application to HD 27536 and HD 216803

We present the first in a series of papers that attempt to investigate the relation between binarity, magnetic activity, and chemical surface abundances of cool stars. In the current paper, we lay out and test two abundance analysis methods and apply them to two well-known, active, single stars, HD 27536 (G8IV-III) and HD 216803 (K5V), presenting photospheric fundamental parameters and abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. The abundances from the two methods agree within the errors for all elements except calcium in \hdeen, which means that either method yields the same fundamental model parameters and the same abundances. Activity is described by the radiative loss in the Ca II H & K lines with respect to the bolometric luminosity, through the activity index R_{HK}. Binarity is established by very precise radial velocity (RV) measurements using HARPS spectra. The spectral line bisectors are examined for correlations between RV and bisector shape to distinguish between the effects of stellar activity and unseen companions. We show that HD 27536 exhibit RV variations mimicking the effect of a low-mass (m ~ 4M_J) companion in a relatively close (a ~ 1AU) orbit. The variation is strongly correlated with the activity, and consistent with the known photometric period P = 306.9 d, demonstrating a remarkable coherence between R_{HK} and the bisector shape, i.e. between the photosphere and the chromosphere. We discuss the complications involved in distinguishing between companion and activity induced RV variations.

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arXiv2004

Multi-Site observations of SU Aurigae

We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy) campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in the Balmer lines and in Na D. Our observations support the suggestion that the wind and infall signatures are out of phase on SU Aur. We present Doppler images of SU Aur that have been obtained from least-squares deconvolved profiles. Images taken about one rotation apart show only limited overlap, in particular at low latitudes. This is in part due to limitations in signal-to-noise, and in part due to line profile deformations that arise from short-lived and/or non-surface features. The agreement at high latitudes is better and suggests that at least some longer-lived features are present. The analysis of Stokes V profiles yields a marginal magnetic field detection during one of the phases.

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