Hans-Erich Fröhlich

Leibniz Institute for Astrophysics Potsdam

Chemically peculiar (CP) stars with a measurable magnetic field comprise the group of mCP stars. The pulsating members define the subgroup of rapidly oscillating Ap (roAp) stars, of which Alpha Circini is the brightest member. Hence, Alpha Circini allows the application of challenging techniques, such as interferometry, very high temporal and spect...

Context. Chemically peculiar (CP) stars with a measurable magnetic field comprise the group of mCP stars. The pulsating members define the subgroup of rapidly oscillating Ap (roAp) stars, of which α Cir is the brightest member. Hence, α Cir allows the application of challenging techniques, such as interferometry, very high temporal and spectral res...

Helioseismology can provide strong constraints on the evolution of Newton's constant over cosmic time. We make use of the best possible estimate of 8640 days of low-$\ell$ BISON data, corrected for the solar cycle variation, to obtain a new constraint on an evolving gravitational constant. In particular, by means of a Bayesian analysis we conclude...

We report on an analysis of high-precision, multi-colour photometric observations of the rapidly-oscillating Ap (roAp) star $\alpha$ Cir. These observations were obtained with the BRITE-Constellation, which is a coordinated mission of five nanosatellites that collects continuous millimagnitude-precision photometry of dozens of bright stars for up t...

The helioseismic determination of the solar age has been a subject of several studies because it provides us with an independent estimation of the age of the solar system. We present the Bayesian estimates of the helioseismic age of the Sun, which are determined by means of calibrated solar models that employ different equations of state and nuclea...

Context. The magnetic chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of local magnetic fields on the stellar surface because they produce inhomogeneities (spots) that can be investigated in detail as the star rotates. Aims. We studied the inhomogeneous surface structure of the...

We present photometric spot modeling of the nearly four-year long light-curve of the Kepler target KIC 5955122 in terms of persisting dark circular surface features. With a Bayesian technique, we produced a plausible surface map that shows dozens of small spots. After some artifacts are removed, the residuals are at $\pm 0.16$\,mmag. The shortest r...

We investigate different amplitude scaling relations adopted for the asteroseismology of stars that show solar-like oscillations. Amplitudes are among the most challenging asteroseismic quantities to handle because of the large uncertainties that arise in measuring the background level in the star's power spectrum. We present results computed by me...

We report on cross-matching the ROSAT All-Sky Survey and the XMM-Newton catalogs with the Kepler Input Catalog (KIC). For several stars associated with X-ray sources, we provide also an access to our high-resolution spectroscopic observations.

We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, aimed at determining their activity level, spot distribution, and differential rotation. Both stars were discovered by us to be young stars and were observed by the NASA Kepler mission. The stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]) were deriv...

A method for estimating the true meteor rate \lambda\ from a small number of observed meteors n is derived. We employ Bayesian inference with a Poissonian likelihood function. We discuss the choice of a suitable prior and propose the adoption of Jeffreys prior, P(\lambda)=\lambda^{-0.5}, which yields an expectation value E(\lambda) = n+0.5 for any...

We present a spectroscopic/photometric analysis of the rapid rotator KIC8429280, discovered by ourselves as a very young star and observed by the Kepler mission. We use spectroscopic/photometric ground-based data to derive stellar parameters, and we adopt a spectral subtraction technique to highlight the chromospheric emission in the cores of Halph...

We compare surface maps of the chemically peculiar star HD 50773 produced with a Bayesian technique and based on high quality CoRoT photometry with those derived from rotation phase resolved spectropolarimetry. The goal is to investigate the correlation of surface brightness with surface chemical abundance distribution and the stellar magnetic fiel...

We apply a robust spot model to fit the light curve of CoRoT-2a. The spots are assumed to be long-living and each has its own rotation period. A model with three circular spots reproduces the basic features of the longitude-time spot coverage map. One of the spots exhibits a noticeably lower rotational frequency than the other two. From the rotatio...

One-dimensional similarity solutions for a collapsing, homogeneous, infinitely long cylinder that is subject to am-bipolar diffusion are given. There is an analytical solution with infinite density being reached after 4.3 free-fall times. In that case the magnetic field strength B z on the axis scales like B z ∝ ρ 2/3 with density ρ. The analytical...

We have performed a long-term photometry of the young solar analogue EK Draconis (HD 129333) using 1030 blue-sensitive Sky-Patrol plates taken at Sonneberg Observatory from 1958 onwards. A secular dimming of 0.0057±0.0008 mag/y beginning around 1975 has been found. Hence, the recent trend first observed with Automatic Photoelectric Telescopes (APTs...

We performed long-term photometry of the young solar analog EK Draconis (HD 129333) using 1030 blue-sensitive Sky-Patrol plates taken at Sonneberg Observatory from 1958 onwards. A secular dimming of 0.0057 ± 0.0008 mag/y beginning around 1975 was found. Hence, the recent trend first observed with Automatic Photoelectric Telescopes (APTs) is confirm...

The time-dependent vertical structure of cold accretion disks and their thermal stability have been studied with turbulent heat transfer being included. A strong turbulent heat transport enforces a nearly adiabatic stratification and the disk evolves quasi-homologously, although homology is broken by the outer boundary condition. The radiative ener...

The time-dependent vertical structure of cold accretion disks and their thermal stability have been studied with turbulent heat transfer being included. A strong turbulent heat transport enforces a nearly adiabatic stratification and the disk evolves quasi-homologously, although homology is broken by the outer boundary condition. The radiative ener...

Provided the opacity goes like kappaproptorho^{1-2 q/3} T^q, an analytical solution for an inviscid thick disk is given. The disk is subject to a central point mass being the sole source for the radiation field. Radiation transport is treated within the diffusion approximation. Self-gravity and radiation pressure are ignored, otherwise no self-simi...

The time-dependent vertical structure of a cold accretion disk is studied semi-analytically and a stability criterion has been derived. Turbulent heat transfer is able to stabilize thermally if the Prandtl number as well as the viscosity alpha is lower than, say, 0.1. However, for very low values of these parameters numerical simulations reveal the...

A self-similar solution for the vertical structure of a thin accretion disk - apart from thermal equilibrium - is described. The main assumptions made are that the energy loss is due to radiation and that throughout the disk the Rosseland mean of the opacity can be fitted by the same power-law dependency on density and temperature.

Im Auftrag des Wissenschaftlichen Rates des Zentralinstituts für Astrophysik im Zusammenwirken mit allen astronomischen Einrichtungen auf dem Gebiet der ehemaligen DDR. Contents: 1. Die Struktur der Astronomie in der DDR. 2. Die instrumentelle Ausrüstung der Astronomie in der DDR. Beobachtungstechnik. Auswertetechnik. Rechentechnik. 3. Sachgebiete....

J-magnitude limits that have to be reached to detect a faint cluster against a noisy background of field galaxies are given. The detectability distinctly depends on redshift (up to z approximately equal to 1) and richness.

An unbiased estimating function for Hubble's constant is proposed appropriate to the case when both radial velocity and absolute magnitude contribute to the scatter. The approach used is based on a modification of the formulation of Aaronson et al. (1981). Also considered is the estimation of the reciprocal of Hubble's constant, the 'world age'.

Burnt-out Population III remnants are thought to have acted as protogalactic seeds. Once tepid pancakes have formed such seeds (≈106M⊙) could be subject to considerable growing by quasi-stationary accretion. Ausgebrannte Population-III-Objekte könnten einst als protogalaktische „Keime” gedient haben. Im Innern von „lauwarmen Plinsen” erfahren diese...

Das interstellare UV-Strahlungsfeld scheint hinreichend stark zu sein, kleine Staubteilchen aus der galaktischen Ebene hinauszublasen. Einige Schlußfolgerungen, die sich daraus für die chemische Entwicklung ergeben könnten, werden kurz erörtert. It is shown that small interstellar dust particles should leave the galactic plane under the influence o...