Capturing a Tetratomic Nitrogen Ring cyclo-N4 inside an Aza[ 82 ]fullerene

Synthesis of polymeric nitrogen compounds is a formidable task due to the proneness of nitrogen to the formation of N ≡ N triple bond, one of the strongest chemical bonds known. Here, we report an arc-discharge approach to successfully stabilize the elusive four-membered nitrogen ring (cyclo-N4) in an unprecedented endohedral metallofullerene Dy2N4@C81N (Dy-I). Its molecular structure has been unambiguously determined by X-ray crystallography to show a covalently bonded cyclo-N4 plane bridging two dysprosium ions inside an aza[82]fullerene cage, highlighting the stabilization of cyclo-N4 as a concurrent result of fullerene encapsulation and metal coordination. Our computational results further reveal a six-center-one-electron (6c-1e) bond delocalized over the inverse-sandwich Dy-N4-Dy cluster. This chemical peculiarity stems from the diffuse radical character of the highly anionic cyclo-N43− ligand, which is confirmed by electron paramagnetic resonance (EPR) spectrum of Y2N4@C81N (Y-I).

On the rotational velocity of Sirius A

ABSTRACT With an aim of getting information on the equatorial rotation velocity (ve) of Sirius A separated from the inclination effect (sin i), a detailed profile analysis based on the Fourier transform technique was carried out for a large number of spectral lines, while explicitly taking into account the line-by-line differences in the centre–limb behaviours and the gravity darkening effect (which depend on the physical properties of each line) based on model calculations. The simulations showed that how the first-zero frequencies (q1) of Fourier transform amplitudes depends on ve is essentially determined by the temperature-sensitivity parameter (K) differing from line to line, and that Fe i lines (especially those of very weak ones) are more sensitive to ve than Fe ii lines. The following conclusions were drawn by comparing the theoretical and observed q1 values for many Fe i and Fe ii lines: (1) The projected rotational velocity (vesin i) for Sirius A is fairly well established at 16.3(±0.1) km s−1 by requiring that both Fe i and Fe ii lines yield consistent results. (2) Although precise separation of ve and i is difficult, ve is concluded to be in the range of $16 \le v_{\rm e} \lesssim$ 30–40 km s−1, which corresponds to $25^{\circ } \lesssim i \le 90^{\circ }$. Accordingly, Sirius A is an intrinsically slow rotator for an A-type star, being consistent with its surface chemical peculiarity.

A search for heavy-metal stars: abundance analyses of hot subdwarfs with Subaru★

The discovery of extremely zirconium- and lead-rich surfaces amongst a small subgroup of hot subdwarfs has provoked questions pertaining to chemical peculiarity in hot star atmospheres and about their evolutionary origin. With only three known in 2014, a limited search for additional ‘heavy-metal’ subdwarfs was initiated with the Subaru telescope. Five hot subdwarfs having intermediate to high surface enrichment of helium were observed at high-resolution and analyzed for surface properties and abundances. This paper reports the analyses of four of these stars. PG 1559+048 and FBS 1749+373, having only intermediate helium enrichment, show strong lines of triply ionized lead. PG 1559+048 also shows a strong overabundance of germanium and yttrium. With more helium-rich surfaces, Ton 414 and J17554+5012, do not show evidence of heavy-metal enrichment. This limited survey suggests that extreme enrichment of ‘heavy metals’ by selective radiative levitation in hot subdwarf atmospheres is suppressed if the star is too helium-rich.

Modelling depletion by re-accretion of gas from a dusty disc in post-AGB stars

Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically peculiar, showing underabundances of refractory elements in their photospheres that correlate with condensation temperature. The aim of this paper is to investigate how accretion from a circumbinary disc can cause this phenomenon of depletion and how this impacts the evolution of post-AGB stars. We used the MESA code to evolve stars in the post-AGB phase, while including accretion of metal-poor gas. We compared the models to a sample of 58 observed disc-type post-AGB stars with chemical abundance data. For each of these stars, we estimated the luminosity and the mass using the Gaia distance. We modelled the accretion rate onto the binary from a viscously evolving disc for a range of initial accretion rates and disc masses. We find that large initial accretion rates (≳3 × 10−7 M⊙ yr−1) and large initial disc masses (∼10−2 M⊙) are needed to reproduce the observed depleted post-AGB stars. Based on these high accretion rates, the evolution timescale of post-AGB stars can be significantly extended by a factor between two and five. We distinguish depletion patterns that are unsaturated (plateau profile) from those that are saturated, and we expect that post-red giant branch (post-RGB) stars are much more likely to show an unsaturated abundance pattern compared to post-AGB stars. Finally, because of the slower evolution of the low-mass post-RGB stars, we find that these systems can become depleted at lower effective temperatures (<5000 K). We conclude that accretion from a circumbinary disc successfully accounts for the chemical peculiarity of post-AGB stars.

Extremely peculiar spectrum of the primary component of the eclipsing binary HD 66051

HD 66051 is an eclipsing and spectroscopic double-lined binary (SB2), hosting two chemically peculiar stars: a highly peculiar B star as primary and an Am star as secondary. The investigation of the new high-resolution UVES spectrum of HD 66051 allowed us to decide on the chemical peculiarity type of both components with more reliability. An analysis of TESS photometric time series data will further specify the physical parameters of the stars and the orbital parameters of the system.

Peculiar Metal Poor Stars as Guides to Key Processes in the Early Halo

Large photometric or spectroscopic surveys are used to sort stars into populations and define the main trends that characterise them, as diagnostics of their origin. Stars falling off the trends defined by the ‘normal’ stars are called ‘peculiar’ and typically eliminated in discussions of Galactic structure and evolution. In our programme on extremely metal-poor halo giants, we have recently focused on the small subgroup that is strongly enhanced in r-process elements, asking whether the chemical peculiarity is intrinsic to these stars or due to local surface pollution caused by mass transfer from a binary companion. Precise radial-velocity monitoring over several years turns out to disprove the binary hypothesis and has led to new insight in the processes of chemical enrichment in the early Galactic halo. An ongoing analogous programme on carbon-enhanced metal-poor giants is briefly described at the end.

The magnetic field of the B1/B2V star σ Lup

The ultraviolet stellar wind lines of the photometrically periodic variable early B-type star σ Lupi were found to behave very similarly to what has been observed in known magnetic B stars, although no periodicity could be determined. AAT spectropolarimetric measurements with SEMPOL were obtained. We detected a longitudinal magnetic field with varying strength and amplitude of about 100 G with error bars of typically 20 G. This type of variability supports an oblique magnetic rotator model. We fold the equivalent width of the 4 usable UV spectra in phase with the well-known photometric period of 3.019 days, which we identify with the rotation period of the star. The magnetic field variations are consistent with this period. Additional observations with ESPaDOnS attached to the CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period. Like in the other magnetic B stars the wind emission likely originates in the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.0182 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field. No helium or other chemical peculiarity is known for this object.

Magnetic fields in O-type stars measured with FORS 1 at the VLT

The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Hα emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Here we present the results of our studies of magnetic fields in O-type stars, carried out over the last years.

Decoupling of Helium in The Winds of Chemically Peculiar Stars

We calculate hydrodynamic multicomponent models of radiatively driven winds in B type stars. Our wind models consist of four components, namely hydrogen, helium, accelerated ions, and electrons. We solve equations of continuity, motion, and energy for all components and we take into account mutual collisions. The resulting models show large heating caused by friction. Results of our calculations show that the explanation of helium chemical peculiarity by the helium decoupling from the mean wind is unlikely.