New observations made with APEX and other telescopes reveal that the star that European astronomers saw appear in the sky in 1670 was not a nova, but a much rarer, violent breed of stellar collision. It was spectacular enough to be easily seen with the naked eye during its first outburst, but the traces it left were so faint that very careful analysis using submillimetre telescopes was needed before the mystery could finally be unravelled more than 340 years later.
The lead author of the new study, Tomasz Kamiński (ESO and the Max Planck Institute for Radio Astronomy, Bonn, Germany) explains: “For many years this object was thought to be a nova, but the more it was studied the less it looked like an ordinary nova — or indeed any other kind of exploding star.”
The results appear online in the journal Nature on 23 March 2015.
The St. Patrick’s Day geomagnetic storm, the most intense since fall of 2013, spurred dazzling aurora before and after sunset, and then finally faded.
On Tuesday evening, forecasts indicated a slight chance aurora would be viewable as far south as the Mid-Atlantic, but the farthest south we’ve seen reports were in Illinois, Ohio and northern New Jersey – which is nonetheless quite unusual.
Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band and went into a long-lasting minimum. In 2010 the star suffered from a similar fading, although less deep. These events in RW Aur A are very unusual among the CTTS, and have been attributed to occultations by passing dust clouds. Aims. We want to find out if any spectral changes took place after the last fading of RW Aur A with the intention to gather more information on the occulting body and the cause of the phenomenon. Methods. We collected spectra of the two components of RW Aur. Photometry was made before and during the minimum. Results. The overall spectral signatures reflecting emission from accretion flows from disk to star did not change after the fading. However, blue-shifted absorption components related to the stellar wind had increased in strength in certain resonance lines, and the profiles and strengths, but not fluxes, of forbidden lines had become drastically different. Conclusions. The extinction through the obscuring cloud is grey indicating the presence of large dust grains. At the same time, there are no traces of related absorbing gas. The cloud occults the star and the interior part of the stellar wind, but not the wind/jet further out. The dimming in 2014 was not accompanied by changes in the accretion flows at the stellar surface. There is evidence that the structure and velocity pattern of the stellar wind did change significantly. The dimmings could be related to passing condensations in a tidally disrupted disk, as proposed earlier, but we also speculate that large dust grains have been stirred up from the inclined disk into the line-of-sight through the interaction with an enhanced wind.
Authors: P. P. Petrov, G. F. Gahm, A. A. Djupvik, E. V. Babina, S. A. Artemenko, K. N. Grankin
Classical Cepheid variable stars are crucial calibrators of the cosmic distance scale thanks to a relation between their pulsation periods and luminosities. Their archetype, δ Cephei, is an important calibrator for this relation. In this paper, we show that δ Cephei is a spectroscopic binary based on newly-obtained highprecision radial velocities. We combine these new data with literature data to determine the orbit, which has period 2201 days, semi-amplitude 1.5 km s−1 , and high eccentricity (e = 0.647). We re-analyze Hipparcos intermediate astrometric data to measure δ Cephei’s parallax ($ = 4.09 ± 0.16 mas) and find tentative evidence for an orbital signature, although we cannot claim detection. We estimate that Gaia will fully determine the astrometric orbit. Using the available information from spectroscopy, velocimetry, astrometry, and Geneva stellar evolution models (MδCep ∼ 5.0 − 5.25 M ), we constrain the companion mass to within 0.2 < M2 < 1.2 M . We discuss the potential of ongoing and previous interactions between the companion and δ Cephei near pericenter passage, informing reported observations of circumstellar material and bow-shock. The orbit may have undergone significant changes due to a Kozai-Lidov mechanism driven by the outer (visual and astrometric) companion HD 213307. Our discovery of δ Cephei’s nature as a spectroscopic binary exposes a hidden companion and reveals a rich and dynamical history of the archetype of classical Cepheid variables.
Authors: Richard I. Anderson, Johannes Sahlmann, Berry Holl, Laurent Eyer, Lovro Palaversa, Nami Mowlavi, Maria Süveges, Maroussia Roelens