Detailed chemical abundances of two stars in the intermediate-age Large Magellanic Cloud (LMC) globular cluster NGC 1718 are presented, based on high-resolution spectroscopic observations with the MIKE spectrograph. The detailed abundances confirm NGC 1718 to be a fairly metal-rich cluster, with an average [ Fe/ H] similar to -0.55 +/- 0.01. The two red giants appear to have primordial O, Na, Mg and Al abundances, with no convincing signs of a composition difference between the two stars -hence, based on these two stars, NGC 1718 shows no evidence for hosting multiple populations. The Mg abundance is lower than Milky Way field stars, but is similar to LMC field stars at the same metallicity. The previous claims of very low [ Mg/ Fe] in NGC 1718 are therefore not supported in this study. Other abundances (Si, Ca, Ti, V, Mn, Ni, Cu, Rb, Y, Zr, La and Eu) all follow the LMC field star trend, demonstrating yet again that (for most elements) globular clusters trace the abundances of their host galaxy's field stars. Similar to the field stars, NGC 1718 is found to be mildly deficient in explosive a-elements, but moderately to strongly deficient in O, Na, Mg, Al and Cu, elements that form during hydrostatic burning in massive stars. NGC 1718 is also enhanced in La, suggesting that it was enriched in ejecta from metal-poor asymptotic giant branch stars.

A long-standing problem is identifying the elusive progenitors of Type Ia supernovae ( SNe Ia), which can roughly be split into Chandraksekhar and sub-Chandrasekhar-mass events. An important difference between these two cases is the nucleosynthetic yield, which is altered by the increased neutron excess in Chandrasekhar progenitors due to their pre-explosion simmering and high central density. Based on these arguments, we show that the chemical composition of the most metal-rich star in the Ursa Minor dwarf galaxy, COS 171, is dominated by nucleosynthesis from a low-metallicity, low-mass, sub-Chandrasekhar-mass SN Ia. Key diagnostic abundance ratios include Mn/Fe and Ni/Fe, which could not have been produced by a Chandrasekhar-mass SN Ia. Large deficiencies of Ni/Fe, Cu/Fe and Zn/Fe also suggest the absence of alpha-rich freeze-out nucleosynthesis, favoring low-mass white dwarf progenitors of SNe Ia, near 0.95 M-circle dot, from comparisons to numerical detonation models. We also compare Mn/Fe and Ni/Fe ratios to the recent yields predicted by Shen et al., finding consistent results. To explain the [Fe/H] at -1.35 dex for COS 171 would require dilution of the SN Ia ejecta with similar to 10(4) M-circle dot of material, which is expected for an SN remnant expanding into a warm interstellar medium with n similar to 1 cm(-3). In the future, finding more stars with the unique chemical signatures we highlight here will be important for constraining the rate and environments of sub-Chandrasekhar SNe Ia.

There is no consensus on the progenitors of Type. Ia supernovae (SNe Ia) despite their importance for cosmology and chemical evolution. We address this question using our previously published catalogs of Mg, Si, Ca, Cr, Fe, Co, and Ni abundances in dwarf galaxy satellites of the Milky Way (MW) to constrain the mass at which the white dwarf (WD) explodes during a typical SN. Ia. We fit a simple bi-linear model to the evolution of [X/Fe] with [Fe/H], where X represents each of the elements mentioned above. We use the evolution of [Mg/Fe] coupled with theoretical supernova yields to isolate what fraction of the elements originated in SNe. Ia. Then, we infer the [X/Fe] yield of SNe. Ia for all of the elements except Mg. We compare these observationally inferred yields to recent theoretical predictions for two classes of Chandrasekhar-mass (M-Ch) SN. Ia as well as sub-M-Ch SNe. Ia. Most of the inferred SN. Ia yields are consistent with all of the theoretical models, but [Ni/Fe] is consistent only with sub-M-Ch models. We conclude that the dominant type of SN. Ia in ancient dwarf galaxies is the explosion of a sub-M-Ch WD. The MW and dwarf galaxies with extended star formation histories have higher [Ni/Fe] abundances, which could indicate that the dominant class of SN. Ia is different for galaxies where star formation lasted for at least several Gyr.

We present the distance-calibrated spectral energy distribution (SED) of TRAPPIST-1 using a new medium-resolution (R similar to 6000) near-infrared (NIR) Folded-port InfraRed Echellette (FIRE) spectrum and its Gaia parallax. We report an updated bolometric luminosity (L-bol) of -3.216 +/- 0.016, along with semiempirical fundamental parameters: effective temperature T-eff = 2628 +/- 42 K, mass = 90 +/- 8 M-Jup, radius = 1.16 +/- 0.03 R-Jup, and log g = 5.21 +/- 0.06 dex. Its kinematics point toward an older age, while spectral indices indicate youth; therefore, we compare the overall SED and NIR bands of TRAPPIST-1 to field-age, low-gravity, and low-metallicity dwarfs of similar T-eff and L-bol. We find field dwarfs of similar T-eff and L-bol best fit the overall and band-by-band features of TRAPPIST-1. Additionally, we present new Allers & Liu spectral indices for the SpeX SXD and FIRE spectra of TRAPPIST-1, both classifying it as intermediate gravity. Examining T-eff, L-bol, and absolute JHKW1W2 magnitudes versus optical spectral type places TRAPPIST-1 in an ambiguous location containing both field and intermediate-gravity sources. Kinematics place TRAPPIST-1 within a subpopulation of intermediate-gravity sources lacking bona fide membership in a moving group with higher tangential and UVW velocities. We conclude that TRAPPIST-1 is a field-age source with subtle spectral features reminiscent of a low surface gravity object. To resolve the cause of TRAPPIST-1's intermediate-gravity indicators we speculate on two avenues that might be correlated to inflate the radius: (1) magnetic activity or (2) tidal interactions from planets. We find the M8 dwarf LHS 132 is an excellent match to TRAPPIST-1's spectral peculiarities along with the M9 beta dwarf 2MASS J10220489+0200477, the L1 beta 2MASS J10224821+5825453, and the L0 beta 2MASS J23224684-3133231, which have distinct kinematics, making all four intriguing targets for future exoplanet studies.

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.