A. Hempelmann

Simultaneous X-ray and optical spectroscopy of the Oef supergiant λ Cephei

Context. Probing the structures of stellar winds is of prime importance for the understanding of massive stars. Based on their optical spectral morphology and variability, the stars of the Oef cl ass have been suggested to feature large-scale structures i n their wind. Aims. High-resolution X-ray spectroscopy and time-series of X-ray observations of presumably-single O-type stars can help us understand the physics of their stellar winds. Methods. We have collected XMM-Newton observations and coordinated optical spectroscopy of the O6 Ief starλ Cep to study its X-ray and optical variability and to analyse its high-resol ution X-ray spectrum. We investigate the line profile variab ility of the Heii λ 4686 and Hα emission lines in our time series of optical spectra, includ ing a search for periodicities. We further discuss the varia bility of the broadband X-ray flux and analyse the high-resoluti on spectrum ofλ Cep using line-by-line fits as well as a code designed to fit the full high-resolution X-ray spectrum consistently . Results. During our observing campaign, the Heiiλ 4686 line varies on a timescale of∼ 18 hours. On the contrary, the Hα line profile displays a modulation on a timescale of 4.1 days which is likely the rotation period of the star. The X-ray flux varie s on timescales of days and could in fact be modulated by the same 4.1 days period as Hα, although both variations are shifted in phase. The high-resolution X-ray spectrum reveals broad and skewed emission lines as expected for the X-ray emission from a distribution of wind-embedded shocks. Most of the X-ray emission arises within less than 2 R∗ above the photosphere. Conclusions. The properties of the X-ray emission ofλ Cep generally agree with the expectations of the wind-embedded shock model. There is mounting evidence for the existence of large-scale structures that modulate the Hα line and about 10% of the X-ray emission ofλ Cep.

What do the Mt. Wilson stars tell us about solar activity

We relate the evolutionary status and mass of the Mt. Wilson project stars with the type and strength of stellar activity as established in decades of monitoring their chromospheric Ca II K line emission. We specifically derive their positions in the Hertzsprung-Russell-diagram (HRD) from Hipparcos parallaxes and SIMBAD B −V data, considering and correcting for the effects of different individual stellar metallicities, and place different activity groups of the Mt. Wilson stars on a common set of Z = 0.02 evolution tracks to obtain a quantitative picture of their relative evolutionary status and mass distribution. We find that, first, the downturn in stellar activity does not depend on absolute age but instead decreases with the relative age as stars advance on the main sequence and thus confirm theoretical expectations, while the most active of the irregularly variable stars are found to scatter around the zero-age main-sequence (ZAMS). Moderately active stars, both with clear cycles like the Sun and those without a dominant activity period, populate the 2nd quarter of main-sequence (MS) evolution. Almost inactive stars are mostly in their 3rd quarter of MS evolution and seem to represent stellar analogues of the solar Maunder minimum state. Totally inactive stars are all in the final quarter of their MS evolution and make up for over 70% of the Mt. Wilson stars that far evolved (the remainders being only weakly active). Most of these are more massive and younger than the Sun. Accordingly, less massive stars did not have enough time to significantly decrease their activity, since they generally evolve more slowly. We find, second, that the Sun is near an apparent upper mass limit for cyclic activity on the MS, because there are no cyclic MS stars much above one solar mass, at least not in the Mt. Wilson sample. Once put in proper perspective with the other Mt. Wilson stars, the Sun indeed ought to be approaching a gradual transition from moderate cyclic activity to a weak, Maunder-minimum-type state, as historic Maunder-type minima seem to indicate already. In addition, the apparent upper mass limit for MS stars to solar-like cyclic activity, not much above one solar mass, is providing dynamo theory with an interesting new challenge.

Chromospheric activity and evolutionary age of the Sun and four solar twins

The activity levels of the solar-twin candidates HD 101364 and HD 197027 are measured and compared with the Sun, the known solar twin 18 Sco, and the solar-like star 51 Peg. Furthermore, the absolute ages of these five objects are estimated from their positions in the HR diagram and the evolutionary (relative) age compared with their activity levels. To represent the activity level of these stars, the Mount Wilson S-indices were used. To obtain consistent ages and evolutionary advance on the main sequence, we used evolutionary tracks calculated with the Cambridge Stellar Evolution Code. From our spectroscopic observations of HD 101364 and HD 197027 and based on the established calibration procedures, the respective Mount Wilson S-indices are determined. We find that the chromospheric activity of both stars is comparable with the present activity level of the Sun and that of 18 Sco, at least for the period in consideration. Furthermore, the absolute age of HD 101364, HD 197027, 51 Peg, and 18 Sco are found to be 7.2, 7.1, 6.1, and 5.1 Gyr, respectively. With the exception of 51 Peg, which has a significantly higher metallicity and a mass higher by about 10% than the Sun, the present Sun and its twins compare relatively well in their activity levels, even though the other twins are somewhat older. Even though 51 Peg has a similar age of 6.1 Gyr, this star is significantly less active. Only when we compare it on a relative age scale (which is about 20% shorter for 51 Peg than for the Sun in absolute terms) and use the higher-than-present long-term S

Time series of high-resolution spectra of SN 2014J observed with the TIGRE telescope

_{\rm{MWO}}

Wavelet analysis of stellar differential rotation. II. The Sun in ultraviolet

average of 0.18 for the Sun, does the S-index show a good correlation with evolutionary (relative) age. This shows that in the search for a suitably similar solar twin, the relative main-sequence age matters for obtaining a comparable activity level.

The Data Reduction Pipeline of the Hamburg Robotic Telescope

We present a time series of high resolution spectra of the Type Ia supernova 2014J, which exploded in the nearby galaxy M82. The spectra were obtained with the HEROS echelle spectrograph installed at the 1.2 m TIGRE telescope. We present a series of 33 spectra with a resolution of R = 20, 000, which covers the important bright phases in the evolution of SN 2014J during the period from January 24 to April 1 of 2014. The spectral evolution of SN 2014J is derived empirically. The expansion velocities of the Si II P-Cygni features were measured and show the expected decreasing behaviour, beginning with a high velocity of 14,000 km/s on January 24. The Ca II infrared triplet feature shows a high velocity component with expansion velocities of > 20, 000 km/s during the early evolution apart from the normal component showing similar velocities as Si II. Further broad P-Cygni profiles are exhibited by the principal lines of Ca II, Mg II and Fe II. The TIGRE SN 2014J spectra also resolve several very sharp Na I D doublet absorption components. Our analysis suggests interesting substructures in the interstellar medium of the host galaxy M82, as well as in our Milky Way, confirming other work on this SN. We were able to identify the interstellar absorption of M82 in the lines of Ca II H & K at 3933 and 3968 A as well as K I at 7664 and 7698 A. Furthermore, we confirm several Diffuse Interstellar Bands, at wavelengths of 6196, 6283, 6376, 6379 and 6613 A and give their measured equivalent widths.

Wavelet analysis of stellar differential rotation - III. The Sun in white light

Among the several methods to study differential rotation on stellar surfaces the observation of a stellar butterfly diagram, i.e. monitoring the rotation period over a complete activity cycle, is the only method which canbe applied to stars rotating as slow as the Sun. However, the method requires active belts on the stellar surface which are moving continously in stellar latitude during a cycle. In addition, an appropriate activity tracer showing rotational modulation is needed. Tracers which even work for slowly rotating, i.e. weakly active, stars are the emission cores of certain spectral lines. However, the question is whether surface differential rotation can be determined from this kind of observations and furthermore, which method of analysis is best suited to yield a correct result. We investigate observations of the Sun as a star, i.e. disk-integrated measurements, to answer these questions. In a Paper I it was demonstrated that time-frequency analysis using the wavelet transform is possibly a suitable method for monitoring the stellar butterfly diagram. It was also shown that - in comparison with Fourier analysis - wavelet analysis of disk-integrated solar Ca II K line core emission measures yield a much more realistic pattern of the solar differential rotation. In this Paper II disk-integrated solar measures in Mg II h+k and Lyman a taken from public UARLS Solstice data are analysed using the same methods as in Paper I. In these data solar rotational modulation is much more pronounced than in earlier Ca II K time-series. Wavelet analysis yields the following results: from the beginning of the time-series in 1991 until the end of solar cycle 22 the period of rotational modulation remains stable at 26.9-27.0 days. It is followed by a jump to 27.6 days when the new cycle 23 starts. Then the period rapidly decreases to 26.9 days again until the end of the time-series in 1999. However, the analysis is hampered by frequent period splitting into two modes. It can be shown that this kind of splitting results from phase jumps in the time-series coming from active region growth and decay at 180° shifted solar longitudes.

High-resolution spectroscopic monitoring of SV Camelopardalis I. Orbits, absolute masses and radii of the components

The fully automatic reduction pipeline for the blue channel of the HEROS spectrograph of the Hamburg Robotic Telescope (HRT) is presented. This pipeline is started automatically after finishing the night-time observations and calibrations. The pipeline includes all necessary procedures for a reliable and complete data reduction, that is, Bias, Dark, and Flat Field correction. Also the order definition, wavelength calibration, and data extraction are included. The final output is written in a fits-format and ready to use for the astronomer. The reduction pipeline is implemented in IDL and based on the IDL reduction package REDUCE written by Piskunov and Valenti (2002).

Measuring rotation periods of solar-like stars using TIGRE. A study of periodic CaII H+K S-index variability

Future space projects like KEPLER will deliver a vast quantity of high precision light curves of stars. This paper describes a test concerning the observability of rotation and even dierential rotation of slowly rotating stars from such data. Two published light curves of solar total irradiance measures are investigated: the Nimbus-7 Earth Radiation Budget (ERB) observations between 1978 and 1993 and the Active Cavity Radiometer Irradiance Monitor I (ACRIM I) measurements between 1980 and 1989. Light curve analysis show that oscillations on time-scales comparable to solar rotation but of a complex pattern are visible. Neither Fourier analysis nor time-frequency Wavelet analysis yield the true rotation period during the more active phases of the solar cycle. The true rotation period dominates only for a short time during solar minimum. In the light of this study even space-born broad band photometry may turn out an inappropriate instrument to study stellar butterfly diagrams of stars rotating as slow as the Sun. However, it was shown in Papers I and II of this series that chromospheric tracers like Lyman, Mg II h+k and CaII H+K are appropriate instruments to perform this task.

Four-month chromospheric and coronal activity cycle in τ Boötis

This study is a first part of an analysis of newly obtained time series of high-resolution ´ echelle spectra of the RS CVn star SV Cam. We investigate the radial velocity variations of the spectroscopic binary and derive an improved orbital solution for both components. The results allow us to estimate the masses of the primary to 1.1 M and of the secondary to 0.7 M and, from a previous analysis of the lightcurve of SV Cam, we derive the stellar radii of 1.18 R for the primary and of 0.76 R for the secondary. From a compilation of eclipse minimum times spanning a period of over 100 years we compute the orbital elements of a hypothetical third body belonging to the SV Cam system. We obtain a period of 52.3 years which is slightly longer than derived by authors before and a much higher eccentricity of at least 0.42. From the mass function there follows a lower limit to the third body mass of 0.17 M. The applied procedure also provides an optimized period of eclipses of 0: d 59307180.