ABSTRACT
We report iron and titanium abundance measurements from high-resolution spectra in a volume-limited sample of 106 M0 and M0.5 dwarf stars. The sample includes stars north of the celestial ...equator and closer than 29 parsecs. The results imply that there is an M dwarf problem similar to the previously known G dwarf problem, in that the fraction of low-metallicity M dwarfs is not large enough to fit simple closed-box models of Galactic chemical evolution. This volume-limited sample avoids many of the statistical uncertainties present in a previous study using a brightness-limited sample of M dwarf stars.
This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were ...identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have Fe/H < −1.5, 105 have Fe/H < −2.0, and 4 have Fe/H < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ Eu/Fe ≤ +1.0 and Ba/Eu < 0) and 4 new r-II stars (with Eu/Fe > +1.0). Nineteen stars are found to exhibit a "limited-r" signature (Sr/Ba > +0.5, Ba/Eu < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, , and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.
The Type II Cepheids include most intrinsic variables with periods between 1 and about 50 days, except for the classical Cepheids and the shortest semiregular variables of type M. The Type II ...Cepheids may be divided in groups by period, such that the stars with periods beween 1 and 5 days (BL Her class), 10–20 days (W Vir class), and greater than 20 days (RV Tau class) have differing evolutionary histories. The chemical composition of Type II Cepheids reflects the material they were made from as modified by their internal nuclear evolution and mixing. Finally, RV Tau stars are affected by mass loss by dust and species attached to the dust. The populations to which the various classes of Type II Cepheids are assigned constitute important clues to the origin and evolution of the halo of our Galaxy and the dwarf spheroidal systems from which at least part of the halo seems to have been accreted.
A high-resolution spectroscopic analysis is presented for a new highly r-process-enhanced (Eu/Fe = 1.27, Ba/Eu = −0.65), very metal-poor (Fe/H = −2.09), retrograde halo star, RAVE J153830.9-180424, ...discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r-II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r-process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r-process census in a very metal-poor r-II star. J1538-1804 shows no signs of s-process contamination, based on its low Ba/Eu and Pb/Fe. As with many other r-process-enhanced stars, J1538-1804's r-process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r-process material to be quite old. This robust main r-process pattern is a necessary constraint for r-process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r-II stars. Additional r-I and r-II stars will be reported by the R-Process Alliance in the near future.
We used high-resolution, high signal-to-noise ratio spectra obtained with the Very Large Telescope and the UV-Visual Echelle Spectrograph to determine abundances of 17 elements in four red giants in ...the Sculptor (Scl) dwarf spheroidal galaxy. Our Fe/H-values range from -2.10 to -0.97, confirming previous findings of a large metallicity spread. We combined our data with similar data for five Scl giants studied recently by Shetrone et al. to form one of the largest samples of high-resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range in this galaxy. These properties allow us to establish trends of X/Fe with Fe/H for many elements X. The trends are significantly different from the trends seen in Galactic halo and globular cluster stars. This conclusion is evident for most of the elements from oxygen to manganese. We compare our Scl sample with the most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between X/Fe as seen in Scl compared with Galactic field stars indicate that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Scl, corroborating similar conclusions reached by Shetrone et al., Fulbright, and Tolstoy et al. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. This star and the Ba/Eu trend we see indicate that asymptotic giant branch stars must have played an important role in the evolution of the s-process elements in Scl. A very high percentage of such heavy-element stars are now known in dwarf spheroidals compared with the halo, further mitigating against the formation of the halo from such objects.
From chemical abundance analysis of stars in the Sagittarius dwarf spheroidal galaxy (Sgr), we conclude that the alpha -element deficiencies cannot be due to the Type Ia supernova (SN Ia) time-delay ...scenario of Tinsley. The critical evidence is the 0.4 dex deficiency of O/Fe, Mg/Fe, and other hydrostatic elements, contrasting with the normal trend of r-process Eu/Fe sub(r) with Fe/H. Supporting evidence comes from the hydrostatic element (O, Mg, Na, Al, Cu) X/Fe ratios, which are inconsistent with iron added to the Milky Way (MW) disk trends. Also, the ratio of hydrostatic to explosive (Si, Ca, Ti) element abundances suggests a relatively top-light IMF. The Cu/O trends in Sgr and the MW suggest that Cu yields increase with both metallicity and stellar mass, as expected from Cu production by the weak s-process in massive stars. Finally, we present an updated hyperfine splitting line list, an abundance analysis of Arcturus, and further develop our error analysis formalism.
We report progress in the calibration of a method to determine cool dwarf star metallicities using molecular band strength indices. The molecular band index to metallicity relation can be calibrated ...using chemical abundances calculated from atomic‐line equivalent width measurements in high‐resolution spectra. Building on previous work, we have measured Fe and Ti abundances in 32 additional M and K dwarf stars to extend the range of temperature and metallicity covered. A test of our analysis method using warm star–cool star binaries shows we can calculate reliable abundances for stars warmer than 3500 K. We have used abundance measurements for warmer binary or cluster companions to estimate abundances in six additional cool dwarfs. Adding stars measured in our previous work and others from the literature provides 76 stars with Fe abundance and CaH2 and TiO5 index measurements. The CaH2 molecular index is directly correlated with temperature. TiO5 depends on temperature and metallicity. Metallicity can be estimated to within ±0.3 dex within the bounds of our calibration, which extends from roughly
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to −1.0, with a limited extension to −1.5.
We present high-resolution echelle observations of SN 2011dh, which exploded in the nearby, nearly face-on spiral galaxy M51. Our data, acquired on three nights when the supernova was near maximum ...brightness, reveal multiple absorption components in Na I D and Ca II H and K, which we identify with gaseous material in the Galactic disk or low halo and in the disk and halo of M51. The M51 components span a velocity range of over 140 km s super(-1), extending well beyond the range exhibited by H I 21 cm emission at the position of the supernova. Since none of the prominent Na I or Ca II components appear to coincide with the peak in H I emission, the supernova may lie just in front of the bulk of the H I disk. The Na I/Ca II ratios for the components with the most extreme positive and negative velocities relative to the disk are ~1.0, similar to those for more quiescent components, suggesting that the absorption originates in relatively cool gas. Production scenarios involving a galactic fountain and/or tidal interactions between M51 and its companion would be consistent with these results. The overall weakness of Na I D absorption in the direction of SN 2011dh confirms a low foreground and host galaxy extinction for the supernova.
We present an analysis of interstellar absorption lines in high-resolution optical echelle spectra of SS Cyg obtained during an outburst in 2013 June and in archivalHubble Space TelescopeandFar ...Ultraviolet Spectroscopic Explorerdata. The CaiiK and NaiD lines toward SS Cyg are compared with those toward nearby B and A stars in an effort to place constraints on the distance to SS Cyg. We find that the distance constraints are not very robust from this method due to the rather slow increase in neutral gas column density with distance and the scatter in the column densities from one sight line to another. However, the optical absorption-line measurements allow us to derive a precise estimate for the line-of-sight reddening ofE(B - V) = 0.020 ± 0.005 mag. Furthermore, our analysis of the absorption lines of Oi, Siii, Pii, and Feiiseen in the UV spectra yields an estimate of the Hicolumn density and depletion strength in this direction.
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