Armagh Observatory & Planetarium

Hot stars born as rapid rotators are expected to induce meridional currents that mix hydrogen from the envelope into the core and return CNO-cycle processed material to the envelope, which should enhance the N at the surface at the expense of C and possibly also O depending on the ambient conditions. But the photospheric C and N abundances could also be influenced by mass transfer in a close binary system which spins up the mass gainer and deposits either processed or unprocessed material to its surface depending on just how much material has been peeled off the mass donor. We focus on the chemical composition of Be star photospheres to infer the present and past evolution of rapidly rotating early B stars. To mitigate the effects of gravity darkening and photospheric line blending on the abundances, we chose 8 Be stars with low vsin i that have good high-resolution FUV spectra in the IUE archive. We carried out a conventional NLTE abundance analysis of selected N iii, N i, and C iii lines in the FUV spectral region. We find clear evidence that the C iii 1176 Å multiplet is weak in the core region in most program stars, suggesting CNO processing. However, in all cases we infer a N abundance that is solar or less, raising a conundrum as to what happened to the “missing C.” Since a similar pattern of weak C yet normal N is also found in the mass gainer in some Algol binaries, there appears to be an emerging challenge to explain this apparent abundance anomaly. We speculate that the excess N from CNO processing might be converted into O (and perhaps on to Ne) by fusion with He in the hot but low-density regions either in the trail of ashes just outside the receding carbon-fusing core, or in He-shell flash regions, of a highly evolved mass loser in its final stage of mass transfer.

The Younger Dryas Impact Hypothesis (YDIH) posits that ~12,800 years ago Earth encountered the debris stream of a disintegrating comet, triggering hemisphere-wide airbursts, atmospheric dust loading, and the deposition of a distinctive suite of extraterrestrial (ET) impact proxies at the Younger Dryas Boundary (YDB). Until now, evidence supporting this hypothesis has come only from terrestrial sediment and ice-core records. Here we report the first discovery of similar impact-related proxies in ocean sediments from four marine cores in Baffin Bay that span the YDB layer at water depths of 0.5–2.4 km, minimizing the potential for modern contamination. Using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and laser ablation ICP-MS, we detect synchronous abundance peaks of metallic debris geochemically consistent with cometary dust, co-occurring with iron- and silica-rich microspherules (4–163 μm) that are predominantly of terrestrial origin with minor (<2 wt%) ET contributions. These microspherules were likely formed by low-altitude touchdown airbursts and surface impacts of comet fragments and were widely dispersed. In addition, single-particle ICP-TOF-MS analysis reveals nanoparticles (<1 μm) enriched in platinum, iridium, nickel, and cobalt. Similar platinum-group element anomalies at the YDB have been documented at dozens of sites worldwide, strongly suggesting an ET source. Collectively, these findings provide robust support for the YDIH. The impact event likely triggered massive meltwater flooding, iceberg calving, and a temporary shutdown of thermohaline circulation, contributing to abrupt Younger Dryas cooling. Our identification of a YDB impact layer in deep marine sediments underscores the potential of oceanic records to broaden our understanding of this catastrophic event and its climatological impacts.

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Context Massive stars (&gt;8 M⊙) play an important role in galactic evolution at all cosmic ages. A deeper understanding of the behaviour of mass loss in low metallicity environments is therefore required. This behaviour largely determines the path of a massive star throughout its life, and its final fate. A better understanding would allow us to predict the evolution of massive stars in the early Universe better. Aims We investigated the theoretical bi-stability jump, which predicts an increase in the mass-loss rates below Teff ≈25–21 kK. We further constrained the photospheric and wind parameters of a sample of late-O and B supergiants in the Large Magellanic Cloud. Methods We used the 1D non-local thermal equilibrium radiative transfer model CMFGEN in a grid-based approach and a fine-tuned spectroscopic fitting procedure that allowed us to determine the stellar and wind parameters of each star. We applied this method to ultra-violet data from the ULLYSES programme and to complementary optical data from the XShootU collaboration. We also used evolutionary models to obtain the evolutionary masses, and we compared them to the spectroscopic masses we derived. Results We derived physical parameters and wind properties of 16 late-O and B supergiants that span a wide temperature range of Teff ≈12–30 kK, surface gravity range of log (g/cm s−2) ≈1.8–3.1, and mass-loss rate range of Ṁ ˙≈ 10−7.6−10−5.7 M⊙ yr−1. We also compared our results to previous studies that attempted to investigate the metallicity dependence of the wind properties. Conclusions The photospheric and wind properties we derived are consistent with those of multiple previous studies. The evolutionary and spectroscopic masses for most of our sample are consistent within the uncertainties. Our results do not reproduce a bi-stability jump in any temperature range, but rather a monotonic decrease in the mass-loss rate at lower temperatures. We obtain a relation of the wind terminal velocity to effective temperature for supergiants in the Large Magellanic Cloud of ν∞/km s−1 = 0.076(±0.011)Teff/K − 884(±260). The mass-loss rates we derived disagree with the mass-loss rates predicted by any of the numerical recipes. This is also the case for the ratio of the terminal wind velocity to the escape velocity ν∞/νesc, and we derived the relation ν∞/νesc = 4.1(±0.8) log (Teff/K) −16.3(± 3.5). The wind parameters depend on the metallicity, based on a comparison with a previous study of the Small Magellanic Cloud, and the modified wind momentum-luminosity relation is log DmomLMC = 1.39(±0.54)log(Lbol/L⊙) + 20.4(±3.0).

Context. Mass loss governs the evolution of massive stars and shapes the stellar surroundings. To quantify the impact of the stellar winds, we need to know the exact mass-loss rates; however, empirical constraints on the rates are hampered by limited knowledge of their small-scale wind structure, also referred to as ‘wind clumping’. Aims. We aim to improve empirical constraints on the mass loss of massive stars by investigating the clumping properties of their winds, in particular, the relation between stellar parameters and wind structure. Methods. We analysed the optical and ultraviolet spectra of 25 O-type giants and supergiants in the Large Magellanic Cloud, using the model atmosphere code FASTWIND and a genetic algorithm. We derived the stellar and wind parameters, including detailed clumping properties, such as the amount of clumping, the density of the interclump medium, velocity–porosity of the medium, and wind turbulence. Results. We obtained stellar and wind parameters for 24 of our sample stars and found that the winds are highly clumped, with an average clumping factor of 〈fcl〉 = 33 14, an interclump density factor of 〈fic〉 = 0.2 0.1, and moderate-to-strong velocity-porosity effects of 〈fvel〉 = 0.6 0.2. The scatter around the average values of the wind-structure parameters is large. With the exception of a significant, positive correlation between the interclump density factor and mass loss, we find no dependence of clumping parameters on the mass-loss rate or stellar properties. Conclusions. In the luminosity range we investigate here, the empirical and theoretical mass-loss rates both have a scatter of about 0.5 dex (or a factor 3). Within this uncertainty, the empirical rates and theoretical predictions are in agreement. The origin of the scatter of the empirically inferred mass-loss rates requires further investigation. It is possible that our description of wind clumping is still not sufficient to capture effects of the structured wind, which could contribute to the scatter.

Abstract As part of the B-fields In Star-forming Region Observations survey, we present James Clerk Maxwell Telescope (JCMT) 850 μm polarimetric observations toward the Orion integral-shaped filament (ISF) that covers three portions known as OMC-1, OMC-2, and OMC-3. The magnetic field threading the ISF seen in the JCMT POL-2 map appears as a tale of three: pinched for OMC-1, twisted for OMC-2, and nearly uniform for OMC-3. A multiscale analysis shows that the magnetic field structure in OMC-3 is very consistent at all the scales, whereas the field structure in OMC-2 shows no correlation across different scales. In OMC-1, the field retains its mean orientation from large to small scales but shows some deviations at small scales. Histograms of relative orientations between the magnetic field and filaments reveal a bimodal distribution for OMC-1, a relatively random distribution for OMC-2, and a distribution with a predominant peak at 90∘ for OMC-3. Furthermore, the magnetic fields in OMC-1 and OMC-3 both appear to be aligned perpendicular to the fibers, which are denser structures within the filament, but the field in OMC-2 is aligned along with the fibers. All these suggest that gravity, turbulence, and magnetic field are each playing a leading role in OMC-1, 2, and 3, respectively. While OMC-2 and 3 have almost the same gas mass, density, and nonthermal velocity dispersion, there are on average younger and fewer young stellar objects in OMC-3, providing evidence that a stronger magnetic field will induce slower and less efficient star formation in molecular clouds.