Stellar mergers are a brief but common phase in the evolution of binary star systems(1,2). These events have many astrophysical implications; for example, they may lead to the creation of atypical stars (such as magnetic stars(3), blue stragglers(4) and rapid rotators(5)), they play an important part in our interpretation of stellar populations(6) and they represent formation channels of compact-object mergers(7). Although a handful of stellar mergers have been observed directly(8,9), the central remnants of these events were shrouded by an opaque shell of dust and molecules(10), making it impossible to observe their final state (for example, as a single merged star or a tighter, surviving binary(11)). Here we report observations of an unusual, ring-shaped ultraviolet ('blue') nebula and the star at its centre, TYC 2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC 2597-735-1. The spectrum of TYC 2597-735-1 and its proximity to the Galactic plane suggest that it is an old star, yet it has abnormally low surface gravity and a detectable long-term luminosity decay, which is uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits H alpha emission, radial-velocity variations, enhanced ultraviolet radiation and excess infrared emission-signatures of dusty circumstellar disks(12), stellar activity(13) and accretion(14). Combined with stellar evolution models, the observations suggest that TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, enabling the direct study of the merging process.

G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R = 15000) spectrum obtained with the high-resolution spectrograph on the Hobby-Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate-resolution (R similar to 4000) spectrum of the CaII triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium's 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with [Fe/H] = -0.98 +/- 0.05. G1 also shows signs of alpha-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs - which suggests that G1's formation is intimately connected with GC formation. G1's high [Na/Fe] also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1's chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.

We use photometric acid spectroscopic observations of the eclipsing binary E32 in the globular cluster 47 Tuc to derive the masses, radii, and luminosities of the component stars. The system has an orbital period of 40.9 d, a markedly eccentric orbit with e = 0.24, and is shown to be a member of or a recent escaper from the cluster. We obtain M-p = 0.862 +/- 0.005 M-circle dot, R-p = 1.183 +/- 0.003 R-circle dot, L-p = 1.65 +/- 0.05 L-circle dot for the primary and M-s = 0.827 +/- 0.005 M-circle dot, R-s = 1.004 +/- 0.004 R-circle dot, L-s = 1.14 +/- 0.04 L-circle dot for the secondary. Based on these data and on an earlier analysis of the binary V69 in 47 Tuc, we measure the distance to the cluster from the distance moduli of the component stars, and, independently, from a colour - surface brightness calibration. We obtain 4.55 +/- 0.03 and 4.50 +/- 0.07 kpc, respectively - values compatible within 1 sigma with recent estimates based on Gaia DR2 parallaxes. By comparing the M-R diagram of the two binaries and the colour-magnitude diagram of 47 Tuc to Dartmouth model isochrones we estimate the age of the cluster to he 12.0 +/- 0.5 Gyr, and the helium abundance of the cluster to be Y approximate to 0.25.

Ion microprobe elemental and isotopic determinations can be precise but difficult to quantify. Error is introduced when the reference material and the sample to be analysed have different compositions. Mitigation of such matrix effects' is possible using ion implants. If a compositionally homogeneous reference material is available which is matrix-appropriate' (i.e., close in major element composition to the sample to be analysed, but having an unknown concentration of the element, E, to be determined) then ion implantation can be used to introduce a known amount of an E isotope, calibrating the E concentration and producing a matrix-appropriate calibrator. Nominal implant fluences (ions cm(-2)) are inaccurate by amounts up to approximately 30%. However, ion implantation gives uniform fluences over large areas; thus, it is possible to co-implant' an additional reference material of any bulk composition having known amounts of E, independently calibrating the implant fluence. Isotope ratio measurement standards can be produced by implanting two different isotopes, but permil level precision requires postimplant calibration of the implant isotopic ratio. Examples discussed include (a) standardising Li in melilite; (b) calibrating a Mg-25 implant fluence using NIST SRM 617 glass and (c) using Si co-implanted with Mg-25 alongside NIST SRM 617 to produce a calibrated measurement of Mg in Si.

We performed an in-depth exploration of the Al-Mg system for presolar graphite, SiC, and Si3N4 grains found to contain large excesses of Mg-26, indicative of the initial presence of live Al-26. Ninety of the more than 450 presolar grains processed in this study contain well-correlated delta Mg-26/Mg-24 and Al-27/Mg-24 ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial Al-26 Al-27 ratios that, on average, are similar to 1.5-2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative delta Mg-26/Mg-24 intercepts of the isochron lines. Al contamination is potentially due to etching of the grains' surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial Al-26/Al-27 ratios. The presolar grains with the highest Al-26/Al-27 ratios are from Type II supernovae whose isochronderived initial Al-26/Al-27 ratios greatly exceed those predicted in the He/C and He/N zones of SN models.

Presolar stardust is present at low levels in meteorites and cometary dust and identified as ancient stellar matter by unusual isotopic compositions reflecting nuclear processes in stellar interiors and galactic chemical evolution. Most grains originated in winds from asymptotic giant branch (AGB) stars and supernova and their isotopic compositions provide important constraints on models of evolution and nucleosynthesis in these environments. The presolar grains from AGB stars appear to have formed in a lower-mass population of stars than predicted by GCE models. A merger of the Milky Way with a dwarf galaxy some 1Gyr before the birth of the Solar System may explain this and other grain observations and the data thus can provide a unique window into the presolar history of the solar neighborhood.

Extreme excesses of C-13 (C-12/C-13 < 10) and N-15 (N-14/N-15 < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron-to micron-sized C-13-and N-15-enriched presolar SiC grains (C-12/C-13 < 16 and N-14/N-15 < similar to 100) from Murchison, and their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains are enriched in C-13 and N-15, but with quite diverse Si isotopic signatures. Four grains with Si-29,Si-30 excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton-and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with Si-30 excesses and Si-29 depletions show lower-than-solar S-34/S-32 ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that N-15-enriched AB grains (similar to 50 < 14N/15N < similar to 100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower Al-26/Al-27, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments.

The X-Ray Spectrometer (XRS) that flew on the MESSENGER spacecraft measured X-rays from the surface of Mercury in the energy range similar to 1-10 keV. Detection of characteristic K-alpha-line emissions from Mg, Al, Si, S, Ca, Ti, and Fe yielded the surface abundances of these geologically important elements. Spatial resolution as fine as similar to 40 km (across track) was possible at periapsis for those elements for which counting statistics were not a limiting factor. Four years of orbital observations have made it possible to generate from XRS spectra detailed elemental composition maps that cover a majority of Mercury's surface. Converting measurements to compositions requires a thorough understanding of the XRS instrument capabilities. The ground and flight calibration measurements presented here are necessary for the reduction and analysis of the X-ray data from the MESSENGER mission. (c) 2016 Elsevier Ltd. All rights reserved.