Heating of the solar and stellar coronae: a review

Despite great advances in observations and modelling, the problem of solar and stellar heating still remains one of the most challenging problems of space physics. To find a definite answer to what sort of mechanisms act to heat the plasma to a few million degrees requires a collaborative effort of small scales observations, large capacity numerical modelling and complicated theoretical approaches. A unique theory should incorporate aspects such as the generation of energy, its transport and dissipation. Up to now, the first two problems are rather clarified. However, the modality of transfer of magnetic or kinetic energy into heat is a question still awaiting for an answer. In the present paper we review the various popular heating mechanisms put forward in the existing extensive literature. The heating processes are, somewhat arbitrarily, classified as hydrodynamic, magnetohydrodynamic or kinetic based on the characteristics of the model medium. These mechanisms are further divided based on the time scales of the ultimate dissipation involved (i.e. AC and DC heating, turbulent heating). In particular, special attention is paid to discuss shock dissipation, mode coupling, resonant absorption, phase mixing, and, reconnection. Finally, we briefly review the various heating mechanisms proposed to heat other stars. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray spectroscopy of stellar coronae: History - present - future

Since in 1948 X-rays were detected from the solar corona, stellar coronae were among the first predicted non-solar X-ray sources. However, because of their relatively low X-ray luminosity, the first non-solar stellar corona was not detected in X-rays until 1974 - twelve years after the discovery of the first non-solar X-ray source. After the 1980s, with the advent of sensitive X-ray imaging instruments on board the EINSTEIN, EXOSAT, and later the ROSAT observatories, the study of stellar coronae has become a vastly growing field of research. These X-ray observations have demonstrated that X-ray emitting coronae are a common feature among stars on the cool side of the Hertzsprung-Russell diagram, with the probable exception of single very cool giant and supergiant stars and A-type dwarfs. The instruments on board these satellites provided for the first time a taste of what can be achieved with X-ray spectroscopy and with the advent of the EUVE (1992) and ASCA (1993), detailed spectroscopy of stellar coronae in the EUV and X-ray regimes got off to a real start. The observations have permitted the identification of coronal material at different temperatures whose existence relates to a range of possible magnetic loop structures in the hot outer atmospheres of stars. The higher spectral resolution of the next generation of spectrometers on board NASA's AXAF (1998), ESA's XMM (1999), and the Japanese ASTRO-E (2000) will improve the determination of coronal temperature structure, abundances, and densities from which loop geometries can be derived and will enable velocity diagnostics. This paper reviews our present knowledge of observational stellar X-ray spectroscopy up to EUVE and ASCA and briefly discusses the perspectives for coronal diagnostics offered by AXAF, XMM, and ASTRO-E.Dedicated to Cornelis de Jager     

The performance of a large echelle grating in a solar spectrometer

A number of tests of a large diffraction grating are described. It is shown that useful information about the performance of the grating in a spectrometer can be obtained from an analysis of the diffracted wavefront by the method of Stroke. Comparison of the instrumental profile at three wavelengths strongly suggests that the influence of changes in the angle of diffraction on the profile cannot be neglected. A resolving power of 6.9 × 10⁵ is attained at 6328 Å. At the same wavelength the first order Rowland ghosts in the ninth order of diffraction have an intensity of only 8 × 10^-4 of the parent line. The influence of stray light on the central intensities of deep Fraunhofer lines is shown to be small but by no means negligible.     

An analysis of Almagest magnitudes for the study of stellar evolution

Visual magnitude data in Ptolemy's Almagest are analysed by comparing them with modern photoelectric magnitudes on the Pogson scale, after taking extinction into account. The results show that a linear relationship exists between Almagest and Pogson scales, contrary to the findings of earlier authors, with one Almagest magnitude being equal to about 1.36 Pogson magnitudes. This result is used to transform Almagest magnitudes to the Pogson scale. A study is made of changes in the visual magnitudes of supergiant stars in the Almagest between classical and modern times (an interval of nearly 19 centuries). No evidence is found for any secular changes in the mean brightness of these stars, contrary to the conclusion of Mayer (Observatory 104, 77 (1984)). However, it is shown that the Almagest magnitudes for stars in the Milky Way are on average 0.34 Pogson magnitudes too faint, and about half the Almagest supergiants are affected by this. Finally, some evidence is cited for the visual atmospheric extinction in classical times being significantly less than today. If this is the case, and it is not taken into account, it would mimic an apparent supergiant brightening of about 0.1 magnitudes over this time interval.     

The stellar halo of the Galaxy

Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia.     

Light curve synthesis for binary systems: The stellar wind effect

Synthesis methods for light and radial velocity curves allow one to find the physical parameters of the components of binary systems of different types. We describe the basics of a new light curve synthesis algorithm for binaries that contain stars with extended expanding atmospheres. The algorithm can be used to study, e.g., light curves of binaries that contain Wolf-Rayet stars.     

The starburst-AGN connection: the role of stellar clusters in AGNs

Nuclear stellar clusters are a common phenomenon in spirals and in starburst galaxies, and they may be a natural consequence of the star formation processes in the central regions of galaxies. HST UV imaging of a few Seyfert 2 galaxies have resolved nuclear starbursts in Seyfert 2 revealing stellar clusters as the main components of the extended emission. However, we do not know whether stellar clusters are always associated with all types of nuclear activity. We present HST NUV and optical images to study the role that stellar clusters play in different types of AGNs. Also with these images, we study the circumnuclear dust morphology as a probe of the circumnuclear environment of AGNs.     

The photoionization mechanism of LINERs: stellar and nonstellar

We present high quality spectroscopic observations of a sample of 14 low ionization nuclear emission-line regions (LINERs). Starlight removal is achieved by the subtraction of a suitable absorption-line template galaxy, allowing accurate measurements of emission lines. We use these line fluxes to examine the possible excitation mechanisms of LINERs. We suggest that LINERs with weak [O I] 6300 emission ([O I]/H < 1/6) may beH II regions photoionized by unusually hot O-type stars. LINERs with [O I]/H < 1/6 may be powered by photoionization from a nonstellar continuum. This is supported by the detection of broad h emission, a correlation between line width and critical density, and point-like X-ray emission in several of these objects.     

The coupling coefficients of pulsation for radiative stellar models

The second order theory of coupling is discussed regarding the radial pulsation of stellar models which are constructed ignoring convection. The formula including the nonadiabatic effect is presented. Numerical values given for model classical cepheids are considerably greater than the adiabatic values.     

The gamma ray spectrometer for the Solar Maximum Mission

The Solar Maximum Mission Gamma Ray Experiment (SMM GRE) utilizes an actively shielded, multicrystal scintillation spectrometer to measure the flux of solar gamma rays. The instrument provides a 476-channel pulse height spectrum (with energy resolution of 7% at 662 keV) every 16.38 s over the energy range 0.3–9 MeV. Higher time resolution (2 s) is available in three windows between 3.5 and 6.5 MeV to study prompt gamma ray line emission at 4.4 and 6.1 MeV. Gamma ray spectral analysis can be extended to 15 MeV on command. Photons in the energy band from 300–350 keV are recorded with a time resolution of 64 ms. A high energy configuration also gives the spectrum of photons in the energy range from 10–100 MeV and the flux of neutrons 20 MeV. Both have a time resolution of 2 s. Auxiliary X-ray detectors will provide spectra with 1-sec time resolution over the energy range of 10–140 keV. The instrument is designed to measure the intensity, energy, and Doppler shift of narrow gamma ray lines as well as the intensity of extremely broadened lines and the photon continuum. The main objective is to use this time and spectral information from both nuclear gamma ray lines and the photon continuum in a direct study of the dynamics of the solar flare/particle acceleration phenomena.     

Gregor@night: The future high‐resolution stellar spectrograph for the GREGOR solar telescope

We describe the future night‐time spectrograph for the GREGOR solar telescope and present its science core projects. The spectrograph provides a 3‐pixel resolution of up to R = 87 000 in 45 échelle orders covering the wavelength range 390‐900 nm with three grating settings. An iodine cell can be used for high‐precision radial velocity work in the 500‐630 nm range. The operation of the spectrograph and the telescope will be fully automated without the presence of humans during night‐time and will be based on the successful STELLA control system. Future upgrades include a second optical camera for even higher spectral resolution, a Stokes‐V polarimeter and a link to the laser‐frequency comb at the Vacuum Tower Telescope. The night‐time core projects are a study of the angular‐momentum evolution of “The Sun in Time” and a continuation of our long‐term Doppler imaging of active stars (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ovoid structures in the solar system: The coronae of Venus and Miranda,and the domes on the Earth

A comparison of Venus and Miranda coronae, and the Earth ovoidal structures, suggests that Venusian coronae, thermal structures associated with important compressional stress fields, could be compared to Archaean gneiss domes. Among Miranda coronae, Inverness has some characteristics not explained by either the raiser or the sinker models, and which deserve further investigation.     

Application of neural networks to the study of stellar model solutions

Artificial neural networks (ANN) have different applications in Astronomy, including data reduction and data mining. In this work we propose the use ANNs in the identification of stellar model solutions. We illustrate this method, by applying an ANN to the 0.8M_⊙ star CG Cyg B. Our ANN was trained using 60,000 different 0.8M_⊙ stellar models. With this approach we identify the models which reproduce CG Cyg B’s position in the HR diagram. We observe a correlation between the model’s initial metal and helium abundance which, in most cases, does not agree with a helium to metal enrichment ratio ΔY/ΔZ = 2. Moreover, we identify a correlation between the model’s initial helium/metal abundance and both its age and mixing-length parameter. Additionally, every model found has a mixing-length parameter below 1.3. This means that CG Cyg B’s mixing-length parameter is clearly smaller than the solar one. From this study we conclude that ANNs are well suited to deal with the degeneracy of model solutions of solar type stars.     

Seismic study of stellar convective regions: the base of the convective envelope in low-mass stars

The possibility of observing solar-type oscillations on other stars is of great relevance to investigating the uncertain aspects of the internal structure of stars. One of these aspects is the convective overshoot that takes place at the borders of the envelopes of stars of mass similar to, or lower than, the Sun. It affects the temperature stratification, mixing, rotation and magnetic-field generation. Asteroseismology can provide an observational test for the studies of the structure of such overshoot regions.
The seismic study of the transition in the Sun, located at the base of the convection zone, has been successful in determining the characteristics of this layer in the Sun. In this work we consider the extension of the analysis to other solar-type stars (of mass between 0.85 and 1.2 M_⊙) in order to establish a method for determining the characteristics of their convective envelopes. In particular, we hope to be able to establish seismologically that a star does indeed possess a convective envelope, to measure the size of the convective region and also to constrain the properties of an overshoot layer at the bottom of the envelope. The limitations in terms of observational uncertainties and stellar characteristics, and the detectability of an overshoot layer, are discussed.