We re-evaluate the CO dipole moment function in order to obtain more accurate isotope ratios for the solar photosphere using previous infrared observations. We used a new set of dipole moments from HITEMP which were accurately determined by both semi-empirical and ab initio methods. Preliminary values of isotope ratios using the new dipole moments are in better agreement with the inferred photosphere values from Genesis, showing that the solar photosphere is isotopically similar to primitive inclusions in meteorites.

Different from previous triennial reports, this report covers the activities of IAU Commission 36 ‘Theory of Stellar Atmospheres’ over the past six years†, and will be the last report from the ‘old’ Commission 36. After the General Assembly in Honolulu (August 2015), a new Commission ‘Stellar and Planetary Atmospheres’ (C.G5, under Division G, ‘Stars and Stellar Physics’) has come into life, and will continue our work devoted to the outer envelopes of stars, as well as extend it to the atmospheres of planets (see Sect. 4).

The business meeting of IAU Commission 36 took place during the GA in Beijing on August 27th, and its major topic was the re-structuring of the IAU Divisions and consequences for our Commission. The meeting was conducted by the new president, Joachim Puls, since the past president (still in charge during the GA), Martin Asplund, could not participate.

The Kepler satellite provides a unique opportunity to study the detailed optical photometric variability of late-type stars with unprecedentedly long (several year) continuous monitoring and sensitivity to very small-scale variations. We are studying a sample of over two hundred cool (mid-A - late-K spectral type) stars using Kepler long-cadence (30 minute sampling) observations. These stars show a remarkable range of photometric variability, but in this paper we concentrate on rotational modulation due to starspots and flaring. Modulation at the 0.1% level is readily discernable. We highlight the rapid timescales of starspot evolution seen on solar-like stars with rotational periods between 2 and 7 days.

The solar “COmosphere” is an enigmatic region of cold gas (temperatures as low as ~3500 K) coexisting in the low chromosphere with plasma much hotter (~7000 K). This zone probably consists of patchy clouds of cool gas, seen readily in off-limb emissions of CO 4667 nm lines, threaded by hot gas entrained in long-lived magnetic filaments as well as transient shock fronts. The COmosphere was not anticipated in classical 1D models of the solar outer atmosphere, but is quite at home in the contemporary 3D highly dynamic view, which one might call the Magnetic Complexity Zone.

The main AB pair of the nearby Alpha Centauri triple system has one of the most extensive X-ray records of any cosmic object, stretching over three decades. The primary, α Cen A (G2V), is a near twin of the Sun, with a similarly soft (1–2 MK) corona. The secondary, α Cen B (K1V), is more active than the Sun, with a generally harder coronal spectrum. Here, spatially resolved measurements of the pair by Chandra's High Resolution Camera are compared, on a common basis, with previous pointings from ROSAT and XMM-Newton.

Growth orientation and type of internal structures are both observed to change abruptly as a function of growth temperature in catalyst free growth of gallium nitride nanowires. In the present work, corresponding temperature-dependent changes in the growth matrix substrate that can affect the availability of nucleation sites and influence the reactivity of constituent adatom materials in catalyst-free nanowire growth are investigated. The influence of Ga vapor pressure and an abrupt change in the availability of single versus molecular adatom constituents is identified as a possible controlling parameter.

The business meeting of Commission 36 was held during the General Assembly in Prague on 16 August. It was attended by about 15 members. The issues presented included a review of the work made by members of Commission 36, and the election of the new Organising Committee. We note that a comprehensive report on the activities of the commission during the last triennium has been published in Reports on Astronomy, Transactions IAU Volume XXVIA. The scientific activity of the members of the commission has been very intense, and has led to the publication of a large number of papers.

Fast spinning Hertzsprung gap giants display super-rotational broadening of UV “hot” lines like Fe XXI λ1354 and C IV λ1548, with FWHM's up to twice that expected from the photospheric v sin i. This possibly is the result of extended fossil magnetospheres enveloping the gap giants, a new type of stellar corona. The magnetospheric phase is short-lived, however, as the rapidly evolving giants develop a competing dynamo-generated surface field in the so-called Rapid Braking Zone.

We report on a large, multi-wavelength campaign to observe variability across the electromagnetic spectrum in the M dwarf flare star EV Lacertae, in 2001 September. The campaign involved X-ray (Chandra ACIS-S+HETG), UV (HST/STIS), and optical (McDonald) spectra, as well as optical photometry and multi-frequency radio (VLA) observations. EV Lac demonstrated both frequent and extreme variability during the course of the two day intensive recordings. Dispersed X-ray spectra confirm the metal underabundance seen in other active stars. The increase in continuum fluxes at short X-ray wavelengths during flare intervals compared to quiescent intervals signals the creation of high temperature plasma, a signature of the flare process. Multi-wavelength comparisons reveal interesting trends: X-ray flare frequencies are within the range predicted by optical observations, yet there is no correspondence between X-ray flares and optical flares in our data. Two UV flares occur during the rise stages of X-ray flares; a major radio flare is accompanied by a large optical flare, which has no apparent counterpart in the X-ray. The results give conflicting evidence for the applicability of the Neupert effect interpretation in stellar coronae.

We investigate the coronal structure of rapidly-rotating, solar-like stars using Chandra HETGS spectra of the short-period binary ER Vul, and by comparison with X-ray observations of the Sun and other dwarf stars. ER Vul consists of two solar-like (G0 + G5) dwarfs with rotation rates ~ 40 times that of the Sun. This binary is not interacting and these stars are the fastest rotating G dwarfs suitable for high resolution X-ray spectroscopy. X-ray (1.8-40 Å) spectra were obtained on 2001 March 29-30 along with 10.5 hours of simultaneous VLA monitoring at 3.6 and 20 cm. These spectra show hot, multi-temperature coronal emission with emission lines ranging in temperature from O VII (2 MK) to Fe XXIV (30 MK). ER Vul showed only low-level variability during the X-ray observation. Unlike the behaviour of longer period active binaries, no large, long-duration flares were detected, consistent with previous X-ray observations of this binary. No evidence for eclipses is seen in either the X-ray or radio emission. The coronal emission measure distribution and elemental abundances were derived for ER Vul.

Moderate-mass giants represent a touchstone for probing the mechanisms of magnetic activity among fast-rotating convective stars. Extreme ultraviolet and soft X-ray observations of such stars detect generally hot coronae: the Hertzsprung-gap giants (F5–G2), in particular, have remarkable high-excitation peaks (107 K, or hotter) in their emission-measure distributions. While the high-temperature coronal plasmas are reminiscent of violent solar flares, the high-energy spectra of the Hertzsprung-gap giants appear to be quite steady over time; in contrast to other hot-corona objects whose optical light curves carry the stamps of starspots, and whose high-excitation emissions are sporadically—and dramatically—variable. The constancy of the Hertzsprung-gap stars is particularly puzzling in light of high-dispersion FUV spectroscopy that reveals supersonic flows in their 105 K subcoronal “transition zones.”

Magnetic flux tubes are usually envisioned as small discrete structures sparsely distributed throughout an otherwise uniform “intertube” medium. An important distinction between the flux tubes and the surrounding atmosphere is the presence of a strong chromospheric temperature inversion at high pressures in the flux tubes, while the intertube component has only a mild, low pressure chromosphere, if any at all. This implies that the flux tubes will be enormously brighter in conventional chromospheric diagnostics than the intertube component. However, the unresolved magnetic elements cover perhaps only 10% of the “quiet” Sun at chromospheric heights. Consequently the intense K and k emission cores are severely diluted. The net result is a weak emission reversal that is not characteristic of either the flux tube or intertube chromosphere. Even in the thermal microwave continuum longward of 100 μm, the flux tubes can contribute significantly to low spatial resolution spectra. Consequently, the spatially averaged microwave emission is also not characteristic of either of the distinct components.

If magnetic flux tubes are indeed the dominant class of atmospheric inhomogeneity in the Sun and other cool stars, then single-component interpretations of spatially unresolved data can be completely misleading, especially for inferring important auxiliary quantities, for example chemical abundances, line broadening parameters and chromospheric energy budgets. In the latter case, chromospheric radiative cooling rates derived from empirical mean models could be overestimated by up to an order of magnitude.