On 2017 August 17, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively-powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the Universe.

Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific r-process event, such as a neutron star merger. To investigate the nature of this event, we present high-resolution Magellan/MIKE spectroscopy of the brightest star in this galaxy. The high signal-to-noise allows us to determine the abundances of 41 elements, including the radioactive actinide element Th and first ever detections of third r-process peak elements (Os and Ir) in a star outside the Milky Way. The observed neutron-capture element abundances closely match the solar r-process component, except for the first r-process peak which is significantly lower than solar but matches other r-process enhanced stars. The ratio of first peak to heavier r-process elements implies the r-process site produces roughly equal masses of high and low electron fraction ejecta, within a factor of 2. We compare the detailed abundance pattern to predictions from nucleosynthesis calculations of neutron star mergers and magneto-rotationally driven jet supernovae, finding that nuclear physics uncertainties dominate over astrophysical uncertainties. We measure $\log\mbox{Th/Eu} = -0.84 \pm 0.06\,\text{(stat)} \pm 0.22\,\text{(sys)}$, somewhat lower than all previous Th/Eu observations. The youngest age we derive from this ratio is $21.7 \pm 2.8\,\text{(stat)} \pm 10.3\,\text{(sys)}$ Gyr, indicating that current initial production ratios do not well describe the r-process event in Reticulum II. The abundance of light elements up to Zn are consistent with extremely metal-poor Milky Way halo stars. They may eventually provide a way to distinguish between neutron star mergers and magneto-rotationally driven jet supernovae, but this would require more detailed knowledge of the chemical evolution of Reticulum II.

From Actinides to Zinc: Using the Full Abundance Pattern of the Brightest Star in Reticulum II to Distinguish between Different

http://www.cell.com/article/S2666675822000200/pdf

Overview of the LAMOST survey in the first decade

The outer Galactic disc contains some features such as the warp and flare, whose origin is still debated. The Gaia data provide an excellent opportunity to probe the Galactic disc at large distances and study these features. We derive the density distributions of the average (old) whole population and the supergiants (representative of a young population), and we use them to constrain their warp and flare. By comparing the results, we study how the properties of these phenomena depend on the studied population. We used Lucy's deconvolution method to recover corrected star counts as a function of distance, from which we derive the density distribution. We find that supergiants have an asymmetric warp, reaching a maximum amplitude of $z_w=0.658$ kpc and minimum amplitude of $z_w=-0.717$ kpc at a distance of $R=[19.5,20]$ kpc, which is almost twice as high as the amplitude of the whole population of the disc. We find a significant flare of the whole population, especially in the thick disc. The scale height increases from $h_{z,thick}\approx 0.8$ kpc and $h_{z,thin}\approx 0.3$ kpc in the solar neighbourhood, to $h_{z,thick} \approx 3$ kpc and $h_{z,thin}\approx 0.7$ kpc in the remote regions of the Milky Way ($R\approx 18$ kpc). The supergiants' population has only a small flare.

/pdf/warp-and-flare-of-the-galactic-disc-revealed-with-2afpsbwl.pdf

Warp and flare of the Galactic disc revealed with supergiants by Gaia EDR3

Context. Mapping the Galactic spiral structure is a di ﬃ cult task since the Sun is located in the Galactic plane and because of dust extinction. For these reasons, molecular masers in radio wavelengths have been used with great success to trace the Milky Way spiral arms. Recently, Gaia parallaxes have helped in investigating the spiral structure in the Solar extended neighborhood. Aims. In this paper, we propose to determine the location of the spiral arms using Cepheids since they are bright, young supergiants with accurate distances (they are the ﬁrst ladder of the extragalactic distance scale). They can be observed at very large distances; therefore, we need to take the Galactic warp into account. Methods. Thanks to updated mid-infrared photometry and to the most complete catalog of Galactic Cepheids, we derived the pa- rameters of the warp using a robust regression method. Using a clustering algorithm, we identiﬁed groups of Cepheids after having corrected their Galactocentric distances from the (small) e ﬀ ects of the warp. Results. We derived new parameters for the Galactic warp, and we show that the warp cannot be responsible for the increased disper- sion of abundance gradients in the outer disk reported in previous studies. We show that Cepheids can be used to trace spiral arms, even at large distances from the Sun. The groups we identify are consistent with previous studies explicitly deriving the position of spiral arms using young tracers (masers, OB(A) stars) or mapping overdensities of upper main-sequence stars in the Solar neighborhood thanks to Gaia data.

/pdf/tracing-the-milky-way-warp-and-spiral-arms-with-classical-1t0mgmzm.pdf

Tracing the Milky Way warp and spiral arms with classical Cepheids

We present analysis of the spatial density structure for the outer disk from 8$-$14 \,kpc with the LAMOST DR5 13534 OB-type stars. We observe similar flaring on north and south sides of the disk implying that the structure is symmetrical about the Galactic plane, for which the scale height is from 0.14 to 0.5 \,kpc in the north and south sides. We reveal that the thickness of the OB stellar disk is similar but flaring is slightly stronger compared to the thin disk as traced by red giant branch stars, possibly implying that secular evolution is not the main contributor to the flaring but perturbation scenarios such as interactions with passing dwarf galaxies should be more possible. When comparing the OB stellar disk of the north and south side with the gas disk, the former one is slightly thicker than the later one by $\approx$ 33 and 9 \,pc for the median value, meaning that one could tentatively use young OB-type stars to trace the gas properties. Meanwhile, we unravel that the radial scale length of the young OB stellar disk is 1.17 $\pm$ 0.05 \,kpc, which is shorter than that of the gas disk, confirming that the gas disk is more extended than stellar disk. What is more, by considering the mid$-$plane displacements ($Z_{0}$) in our density model we find that almost all of $Z_{0}$ are within 100 \,pc with the increasing trend as Galactocentric distance increases, which looks like the warp signal in young stellar population but the significance and scenarios will need to be investigated in more detail.

The Flare and Warp of the Young Stellar Disk Traced with LAMOST DR5 OB-type Stars

Study of the puzzling abundance pattern of the neutron-capture elements in LMC star J053253.51-695915.1

The Flare and Warp of the Young Stellar Disk traced with LAMOST DR5 OB-type stars

Based on the spectra with high resolution and a high signal-to-noise ratio, we investigate the enrichment history of the s-process element in seven barium (Ba) stars by measuring their Ba odd isotope fraction. It is found that the relative contributions of the s-process to their Ba abundance are 91.4±25.7%, 91.4±34.3%, 82.9±28.5%, 77.1±31.4%, and 71.4±37.1% for REJ 0702+129, HD 13611, BD+80∘670, HR 5692, and HD 202109, respectively. Our results suggest that these five Ba stars have a prominent s-process signature, which indicates that their heavy elements mainly come from their former AGB companions (now WDs) by mass transfer, while the r-process contribution can naturally be explained by the evolution of the Milky Way. The s-process contribution of BD+80∘670 is 51.4±31.4%, which is the lowest among our seven sample stars. Considering its lower values of both [Ba/Nd] and [Ba/Eu], we suspect that BD+68∘1027 is likely to be a r-rich Ba star and has similar origins to the CEMP-r/s stars. HD 218356 has an unreasonable s-process contribution over 100%. Combining its stellar atmospheric parameters and the evolutionary stage, we speculate that HD 218356 is a more evolved extrinsic Ba star, and its massive companion should have the largest s-process efficiency in our samples.

/pdf/the-barium-odd-isotope-fractions-in-seven-ba-stars-2bx9hs7j.pdf

Fang Wen

Papers

The Flare and Warp of the Young Stellar Disk Traced with LAMOST DR5 OB-type Stars

Study of the puzzling abundance pattern of the neutron-capture elements in LMC star J053253.51-695915.1

The Flare and Warp of the Young Stellar Disk traced with LAMOST DR5 OB-type stars

The Barium Odd Isotope Fractions in Seven Ba Stars

Pollution tracks of r-process material in [Sr/Ba] vs. [Ba/Fe] space for the early Galaxy