Mapping EK Draconis with PEPSI

Aims. We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Methods. Changes in spectral line profiles are inverted to a stellar surface temperature map using our iMap code. The long-term photometric record is employed to compare our map with previously published maps. Results. Four cool spots were reconstructed, but no polar spot was seen. The temperature difference to the photosphere of the spots is between 990 and 280 K. Two spots are reconstructed with a typical solar morphology with an umbra and a penumbra. For the one isolated and relatively round spot (spot A), we determine an umbral temperature of 990 K and a penumbral temperature of 180 K below photospheric temperature. The umbra to photosphere intensity ratio of EK Dra is approximately only half of that of a comparison sunspot. A test inversion from degraded line profiles showed that the higher spectral resolution of PEPSI reconstructs the surface with a temperature difference that is on average 10% higher than before and with smaller surface areas by ~10–20%. PEPSI is therefore better suited to detecting and characterising temperature inhomogeneities. With ten more years of photometry, we also refine the spot cycle period of EK Dra to 8.9 ± 0.2 yr with a continuing long-term fading trend. Conclusions. The temperature morphology of spot A so far appears to show the best evidence for the existence of a solar-like penumbra for a starspot. We emphasise that it is more the non-capture of the true umbral contrast rather than the detection of the weak penumbra that is the limiting factor. The relatively small line broadening of EK Dra, together with the only moderately high spectral resolutions previously available, appear to be the main contributors to the lower-than-expected spot contrasts when comparing to the Sun.

SN 2014J in M82: new insights on the spectral diversity of Type Ia supernovae

ABSTRACT We present extensive spectroscopic observations for one of the closest Type Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 d before to 473.2 d after B-band maximum light. The diffuse interstellar band features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as Av ∼ 1.9 ± 0.6 mag. Spectroscopically, SN 2014J can be put into the high-velocity (HV) subgroup in Wang’s classification with a velocity of Si ii λ 6355 at maximum light of $v$0 = 1.22 ± 0.01 × 104 km s−1 but has a low velocity gradient (LVG, following Benetti’s classification) of $\dot{v}=41\pm 2$ km s−1 d−1, which is inconsistent with the trend that HV SNe Ia generally have larger velocity gradients. We find that the HV SNe Ia with LVGs tend to have relatively stronger Si iii (at ∼4400 Å) absorptions in early spectra, larger ratios of S ii λ 5468 to S ii λ 5640, and weaker Si ii 5972 absorptions compared to their counterparts with similar velocities but high velocity gradients. This shows that the HV+LVG subgroup of SNe Ia may have intrinsically higher photospheric temperature, which indicates that their progenitors may experience more complete burning in the explosions relative to the typical HV SNe Ia.

Super-luminous Type II supernovae powered by magnetars

Magnetar power is believed to be at the origin of numerous super-luminous supernovae (SNe) of Type Ic, arising from compact, hydrogen-deficient, Wolf-Rayet type stars. Here, we investigate the properties that magnetar power would have on standard-energy SNe associated with 15–20 M⊙ supergiant stars, either red (RSG; extended) or blue (BSG; more compact). We have used a combination of Eulerian gray radiation-hydrodynamics and non-LTE steady-state radiative transfer to study their dynamical, photometric, and spectroscopic properties. Adopting magnetar fields of 1, 3.5, 7 × 1014 G and rotational energies of 0.4, 1, and 3 × 1051 erg, we produce bolometric light curves with a broad maximum covering 50–150 d and a magnitude of 1043–1044 erg s−1. The spectra at maximum light are analogous to those of standard SNe II-P but bluer. Although the magnetar energy is channelled in equal proportion between SN kinetic energy and SN luminosity, the latter may be boosted by a factor of 10–100 compared to a standard SN II. This influence breaks the observed relation between brightness and ejecta expansion rate of standard Type II SNe. Magnetar energy injection also delays recombination and may even cause re-ionization, with a reversal in photospheric temperature and velocity. Depositing the magnetar energy in a narrow mass shell at the ejecta base leads to the formation of a dense shell at a few 1000 km s−1, which causes a light-curve bump at the end of the photospheric phase. Depositing this energy over a broad range of mass in the inner ejecta, to mimic the effect of multi-dimensional fluid instabilities, prevents the formation of a dense shell and produces an earlier-rising and smoother light curve. The magnetar influence on the SN radiation is generally not visible prior to 20–30 d, during which one may discern a BSG from a RSG progenitor. We propose a magnetar model for the super-luminous Type II SN OGLE-SN14-073.

The Effect of Scattering on the Temperature Stratification of 3D Model Atmospheres of Metal-Poor Red Giants

We study the effects of different approximations of scattering in 3D radiation-hydrodynamics simulations on the photospheric temperature stratification of metal-poor red giant stars. We find that assuming a Planckian source function and neglecting the contribution of scattering to extinction in optically thin layers provides a good approximation of the effects of coherent scattering on the photospheric temperature balance.

Short-Term Wind and Photospheric Activity in Be Stars Deduced from Campaigns with the IUE Spacecraft

From 1985–96 twelve multiwavelength campaigns on 15 Be stars were carried through using data from the IUE spacecraft to study the phenomenon of short-term spectroscopic and photometric variability. Highlights from recent work on this database are presented here. Three classes of variability have been identified: 1) The FUV flux and wind strength are correlated, 2) The FUV flux varies cyclically but wind variability if present does not correlate with the flux, and 3) The wind strength cyclically varies but does not correlate with light variations. For Class 1 the period is usually less than the star’s expected rotational period, but for Classes 2 & 3, the period is sometimes close to Prot. We have employed a cross-correlation technique to extract information on the nature of the line profile variability (lpv). Evidence for nonradial pulsations (NRP) in low-order modes is found for objects in Classes 1 & 2, the light variations appear to be caused by a modulation of the star's photospheric temperature, and the hot crest of the NRP wave is in front when the star is bright. For Class 1 stars the wind is enhanced over the hot crest. The mass loss in Class 3 may originate from a localized active region on the star and the importance of NRP in these stars remains unknown.

Effects of Photospheric Temperature Inhomogeneities on Lithium abundance Determinations (2D)

Based on detailed 2D radiation hydrodynamics (RHD) simulations, we have investigated the effects of photospheric temperature inhomogeneities induced by convection on spectroscopic determinations of the lithium abundance. Computations have been performed both for the solar case and for a metal-poor dwarf. NLTE effects are taken into account, using a five-level atomic model for Li I. Comparisons are presented with traditional 1D models having the same effective temperature and gravity. The net result is that, while LTE results differ dramatically between 1D and 2D models, especially in the metal-poor case, this does not remain true when NLTE effects are included: 1D/2D differences in the inferred NLTE Li abundance are always well below 0.1 dex. The present computations still assume LTE in the continuum. New computations removing this assumption are planned for the near future.

Observations of the Latitudinal Variation of the Solar Radiance of Non Active Regions of the Sun

Kuhn et al. (1988) have found that there are variations in the photospheric temperature with the solar cycle that depend on solar latitude. This would be an independent mechanism, other than the effects of sunspots and faculae, contributing to the change in solar irradiance (Kuhn 1991). It is important to pursue this investigation as such variations would be related to the transport of energy through the convection zone, and thus give a good indication of its structure and evolution.