Digitization of sunspot drawings by Spörer made in 1861–1894

Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Spörer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33‐year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New spectroscopic and polarimetric observations of the A0 supergiant HD 92207

Our recent search for the presence of a magnetic field in the bright early A‐type supergiant HD 92207 using FORS 2 in spectropolarimetric mode revealed the presence of a longitudinal magnetic field of the order of a few hundred Gauss. However, the definite confirmation of the magnetic nature of this object remained pending due to the detection of shortterm spectral variability probably affecting the position of line profiles in left‐ and right‐hand polarized spectra. We present new magnetic field measurements of HD 92207 obtained on three different epochs in 2013 and 2014 using FORS 2 in spectropolarimetric mode. A 3σ detection of the mean longitudinal magnetic field using the entire spectrum, 〈B_z〉_all = 104 ± 34 G, was achieved in observations obtained in 2014 January. At this epoch, the position of the spectral lines appeared stable. Our analysis of spectral line shapes recorded in opposite circularly polarized light, i.e. in light with opposite sense of rotation, reveals that line profiles in the light polarized in a certain direction appear slightly split. The mechanism causing such a behaviour in the circularly polarized light is currently unknown. Trying to settle the issue of short‐term variability, we searched for changes in the spectral line profiles on a time scale of 8–10 min using HARPS polarimetric spectra and on a time scale of 3–4 min using time series obtained with the CORALIE spectrograph. No significant variability was detected on these time scales during the epochs studied. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Roche volume filling and the dissolution of open star clusters

From direct N‐body simulations we find that the dynamical evolution of star clusters is strongly influenced by the Roche volume filling factor. We present a parameter study of the dissolution of open star clusters with different Roche volume filling factors and different particle numbers. We study both Roche volume underfilling and overfilling models and compare with the Roche volume filling case. We find that in the Roche volume overfilling limit of our simulations two‐body relaxation is no longer the dominant dissolution mechanism but the changing cluster potential. We call this mechanism “mass‐loss driven dissolution” in contrast to “two‐body relaxation driven dissolution” which occurs in the Roche volume underfilling regime. We have measured scaling exponents of the dissolution time with the two‐body relaxation time. In this experimental study we find a decreasing scaling exponent with increasing Roche volume filling factor. The evolution of the escaper number in the Roche volume overfilling limit can be described by a log‐logistic differential equation. We report the finding of a resonance condition which may play a role for the evolution of star clusters and may be calibrated by the main periodic orbit in the large island of retrograde quasiperiodic orbits in the Poincaré surfaces of section. We also report on the existence of a stability curve which may be of relevance with respect to the structure of star clusters. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

3XMM J185246.6+003317 as transient neutron star

We performed an X‐ray timing and spectral analysis of the variable source 3XMM J185246.6+003317 to investigate its physical nature. The data from all observations of 3XMMJ185246.6+003317 conducted by XMM‐Newton EPIC MOS1 and MOS2 with the same instrumental setup in 2004–2009 were reprocessed to form a homogenous data set of solar barycenter corrected photon arrival times and high S/N spectra of 3XMM J185246.6+003317. A Bayesian method for the search, detection, and estimation of the parameters of a periodic signal of unknown shape was employed, as developed by Gregory & Loredo (1992, 1993). The results show that 3XMM J185246.6+003317 is a transient neutron star with the genuine spin‐period of 23.11722 (23.11711–23.11727) s and its derivative of 5.3(0.3–5.5)×10^–11 s s^–1, implying a characteristic age of 7 (6–104) kyr, if the period derivative can be ascribed to the genuine spin‐down rate of the neutron star. The rotational‐phase averaged X‐ray spectra at the different brightness periods can be fitted with a highly absorbed blackbody model with different temperatures. The phase‐folded light curves in different energy bands with high S/N ratio show a double‐peaked profile; the variations depend on time and energy, indicating that radiation emerges from at least two emitting areas. The spectra at the phases corresponding to the maxima in the phase‐folded light curve show different spectral parameters of absorbed blackbody radiation, i.e. the hotter one has a smaller size. The source is detected only from September 2008 to April 2009 with persistently decreasing brightness, but not before, even though it was observed by XMM. Hence, it is a transient neutron star or a binary system hosting it. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Symbiotic stars with spectral indication of bipolar ejection and stellar wind

Four out of a total of five symbiotic systems whose optical spectral lines contain satellite components indicating collimated ejection have an additional absorption of P Cyg type with different velocity. These systems are Z And, Hen 3‐1341, StHα 190, and BF Cyg. It is shown that the line profiles of every of these systems can be interpreted in the framework of a model of a collimated stellar wind from the hot compact component proposed initially to interpret the line profiles of Z And, observed during its last active phase after 2000. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Combining optical spectroscopy and interferometry

Modern optical spectrographs and optical interferometers push the limits in the spectral and spatial regime, providing important new tools for the exploration of the Universe. In this contribution I outline the complementary nature of spectroscopic and interferometric observations and discuss different strategies for combining such data. Most remarkable, the latest generation of “spectro‐interferometric” instruments combine the milliarcsecond angular resolution achievable with interferometry with spectral capabilities, enabling direct constraints on the distribution, density, kinematics, and ionization structure of the gas component in protoplanetary disks. I will present some selected studies from the field of star‐ and planet formation and hot star research in order to illustrate these fundamentally new observational opportunities. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical properties of twin kilohertz quasi‐periodic oscillations in neutron star low‐mass X‐ray binaries

We collect the data of twin kilohertz quasi‐periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low‐mass X‐ray binary (LMXB) sources, then we analyze the centroid frequency (ν) distribution of twin kHz QPOs (lower frequency ν₁ and upper frequency ν₂) both for Atoll and Z sources. For the data without shift‐and‐add, we find that Atoll and Z sources show different distributions of ν₁, ν₂ and ν₂/ν₁, but the same distribution of Δν (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of Δν is quite different from a constant value, so is ν 2/ν₁ from a constant ratio. The weighted mean values and maxima of ν₁ and ν₂ in Atoll sources are slightly higher than those in Z sources. We also find that shift‐and‐add technique can reconstruct the distributions of ν₁ and Δν. The K‐S test results of ν₁ and Δν between Atoll and Z sources from data with shift‐and‐add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root‐meansquared (rms) amplitude of 4U 0614+09 with data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency (ν_s) distribution of 28 NS‐LMXBs show a bigger mean value (∼408 Hz) than that (∼281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the kHz QPOs and NS spin frequency ν_s, we find the approximate correlations of the mean values of Δν with NS spin and its half, respectively. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparison of chaos detection methods in the circular restricted three‐body problem

In this study, a sample of orbits is considered in the framework of the planar circular restricted three‐body problem. In order to separate ordered from chaotic orbits three numerical methods are compared: the Largest Lyapunov Characteristic Exponent (LLCE) and the Smaller Alignment Index (SALI) provide a fairly good characterization of the chaotic motions, while the computational time required is of the same order; the Correlation Dimension (CD) has the advantage of correctly classifying sticky orbits, but at the expense of a longer computational time. In order to classify a given orbit, any pair of the three methods can be considered, but LLCE and SALI are recommended due to their speed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relative sea‐level observations in western Scotland since the Last Glacial Maximum for testing models of glacial isostatic land movements and ice‐sheet reconstructions

Observations of relative sea‐level change and local deglaciation in western Scotland provide critical constraints for modelling glacio‐isostatic rebound in northern Britain over the last 18 000 years. The longest records come from Skye, Arisaig and Knapdale with a shorter, Holocene, record from Kintail. Biostratigraphic (diatom, pollen, dinoflagellate, foraminifera and thecamoebian), lithological and radiocarbon analyses provide age and elevation parameters for each sea‐level index point. All four sites reveal relative sea‐level change that is highly non‐monotonic in time as the local vertical component of glacio‐isostatic rebound and eustasy (or global meltwater influx) dominate at different periods. Copyright © 2006 John Wiley & Sons, Ltd.     

Weichselian sediments at Foss‐Eikeland,Jæren (southwest Norway): sea‐level changes and glaciation history

The Jæren area in southwestern Norway has experienced great changes in sea‐levels and sedimentary environments during the Weichselian, and some of these changes are recorded at Foss‐Eikeland. Four diamictons interbedded with glaciomarine and glaciofluvial sediments are exposed in a large gravel pit situated above the post‐glacial marine limit. The interpretation of these sediments has implications for the history of both the inland ice and the Norwegian Channel Ice Stream. During a Middle Weichselian interstadial, a large glaciofluvial delta prograded into a shallow marine environment along the coast of Jæren. A minor glacial advance deposited a gravelly diamicton, and a glaciomarine diamicton was deposited during a following marine transgression. This subsequently was reworked by grounded ice, forming a well‐defined boulder pavement. The boulder pavement is followed by glaciomarine clay with a lower, laminated part and an upper part of sandy clay. The laminated clay probably was deposited under sea‐ice, whereas more open glaciomarine conditions prevailed during deposition of the upper part. The clay is intersected by clastic dykes protruding from the overlying, late Weichselian till. Preconsolidation values from the marine clay suggest an ice thickness of at least 500 m during the last glacial phase. The large variations in sea‐level probably are a combined effect of eustasy and glacio‐isostatic changes caused by an inland ice sheet and an ice stream in the Norwegian Channel. Copyright © 2002 John Wiley & Sons, Ltd.