Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets

We derive atmospheric parameters and lithium abundances for 671 stars and include our measurements in a literature compilation of 1381 dwarf and subgiant stars. First, a "lithium desert" in the effective temperature (Teff) versus lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li side of the desert have experienced a short-lived period of severe surface lithium destruction as main-sequence or subgiant stars. Next, we search for differences in the lithium content of thin-disk and thick-disk stars, but we find that internal processes have erased from the stellar photospheres their possibly different histories of lithium enrichment. Nevertheless, we note that the maximum lithium abundance of thick-disk stars is nearly constant from [Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance distribution of known planet-host stars relative to otherwise ordinary stars appear when restricting the samples to narrow ranges of Teff or mass, but they are fully explained by age and metallicity biases. We confirm the lack of a connection between low lithium abundance and planets. However, we find that no low A_Li planet-hosts are found in the desert Teff window. Provided that subtle sample biases are not responsible for this observation, this suggests that the presence of gas giant planets inhibit the mechanism responsible for the lithium desert.Comment: ApJ, in press. Complete Tables 1 and 3 are available upon reques

Magnon valley Hall effect in CrI3-based vdW heterostructures

Magnonic excitations in the two-dimensional (2D) van der Waals (vdW) ferromagnet CrI3 are studied. We find that bulk magnons exhibit a non-trivial topological band structure without the need for Dzyaloshinskii-Moriya (DM) interaction. This is shown in vdW heterostructures, consisting of single-layer CrI3 on top of different 2D materials as MoTe2, HfS2 and WSe2. We find numerically that the proposed substrates modify substantially the out-of-plane magnetic anisotropy on each sublattice of the CrI3 subsystem. The induced staggered anisotropy, combined with a proper band inversion, leads to the opening of a topological gap of the magnon spectrum. Since the gap is opened non-symmetrically at the K+ and K- points of the Brillouin zone, an imbalance in the magnon population between these two valleys can be created under a driving force. This phenomenon is in close analogy to the so-called valley Hall effect (VHE), and thus termed as magnon valley Hall effect (MVHE). In linear response to a temperature gradient we quantify this effect by the evaluation of the temperature-dependence of the magnon thermal Hall effect. These findings open a different avenue by adding the valley degrees of freedom besides the spin, in the study of magnons

IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies

We present a joint analysis of the four most prominent sodium-sensitive features (NaD, NaI8190, NaI1.14, and NaI2.21), in the optical and Near-Infrared spectral range, of two nearby, massive (sigma~300km/s), early-type galaxies (named XSG1 and XSG2). Our analysis relies on deep VLT/X-Shooter long-slit spectra, along with newly developed stellar population models, allowing for [Na/Fe] variations, up to 1.2dex, over a wide range of age, total metallicity, and IMF slope. The new models show that the response of the Na-dependent spectral indices to [Na/Fe] is stronger when the IMF is bottom heavier. For the first time, we are able to match all four Na features in the central regions of massive early-type galaxies, finding an overabundance of [Na/Fe], in the range 0.5-0.7dex, and a bottom-heavy IMF. Therefore, individual abundance variations cannot be fully responsible for the trends of gravity-sensitive indices, strengthening the case towards a non-universal IMF. Given current limitations of theoretical atmosphere models, our [Na/Fe] estimates should be taken as upper limits. For XSG1, where line strengths are measured out to 0.8Re, the radial trend of [Na/Fe] is similar to [Mg/Fe] and [C/Fe], being constant out to 0.5Re, and decreasing by 0.2-0.3dex at 0.8Re, without any clear correlation with local metallicity. Such a result seems to be in contrast with the predicted increase of Na nucleosynthetic yields from AGB stars and TypeII SNe. For XSG1, the Na-inferred IMF radial profile is consistent, within the errors, with that derived from TiO features and the Wing-Ford band, presented in a recent paper.Comment: 22 pages, 8 figure, accepted for publication in MNRAS. The new Na-enhanced models will be available soon at http://miles.iac.es

The Shortest Period Detached Binary White Dwarf System

We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest period detached binary white dwarf (WD) system currently known. We targeted J0106-1000 as part of our radial velocity program to search for companions around known extremely low-mass (ELM, ~ 0.2 Msol) WDs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 740 km/s with an orbital period of 39.1 min. The mass function and optical photometry rule out a main-sequence star companion. Follow-up high-speed photometric observations obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the distorted primary but no eclipses. This is the first example of a tidally distorted WD. Modeling the lightcurve, we constrain the inclination angle of the system to be 67 +- 13 deg. J0106-1000 contains a pair of WDs (0.17 Msol primary + 0.43 Msol invisible secondary) at a separation of 0.32 Rsol. The two WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf star. J0106-1000 is the shortest timescale merger system currently known. The gravitational wave strain from J0106-1000 is at the detection limit of the Laser Interferometer Space Antenna (LISA). However, accurate ephemeris and orbital period measurements may enable LISA to detect J0106-1000 above the Galactic background noise.Comment: MNRAS Letters, in pres

A Study of the Near-Ultraviolet Spectrum of Vega

UV, optical, and near-IR spectra of Vega have been combined to test our understanding of stellar atmospheric opacities, and to examine the possibility of constraining chemical abundances from low-resolution UV fluxes. We have carried out a detailed analysis assuming Local Thermodynamic Equilibrium (LTE) to identify the most important contributors to the UV continuous opacity: H, H$^{-}$, C I, and Si II. Our analysis also assumes that Vega is spherically symmetric and its atmosphere is well described with the plane parallel approximation. Comparing observations and computed fluxes we have been able to discriminate between two different flux scales that have been proposed, the IUE-INES and the HST scales, favoring the latter. The effective temperature and angular diameter derived from the analysis of observed optical and near-UV spectra are in very good agreement with previous determinations based on different techniques. The silicon abundance is poorly constrained by the UV observations of the continuum and strong lines, but the situation is more favorable for carbon and the abundances inferred from the UV continuum and optical absorption lines are in good agreement. Some spectral intervals in the UV spectrum of Vega that the calculations do not reproduce well are likely affected by deviations from LTE, but we conclude that our understanding of UV atmospheric opacities is fairly complete for early A-type stars.Comment: 13 pages, 9 figures, to be published in Ap

An equatorial ultra iron-poor star identified in BOSS

We report the discovery of SDSS J131326.89-001941.4, an ultra iron-poor red giant star ([Fe/H] ~ -4.3) with a very high carbon abundance ([C/Fe]~ +2.5). This object is the fifth star in this rare class, and the combination of a fairly low effective temperature (Teff ~ 5300 K), which enhances line absorption, with its brightness (g=16.9), makes it possible to measure the abundances of calcium, carbon and iron using a low-resolution spectrum from the Sloan Digital Sky Survey. We examine the carbon and iron abundance ratios in this star and other similar objects in the light of predicted yields from metal-free massive stars, and conclude that they are consistent. By way of comparison, stars with similarly low iron abundances but lower carbon-to-iron ratios deviate from the theoretical predictions.Comment: 6 pages, 4 figures, accepted for publication in A&

Photoionized OVI absorbers toward the bright QSO HE 0515-4414

We report on detailed Monte Carlo inversion analysis of five OVI systems from the spectrum of the bright quasar HE 0515-4414 (z_e = 1.71). The associated system at z_a = 1.697 with the neutral hydrogen column density N(HI)= 4.4 10^{13} cm^{-2} shows pronounced absorption from highly ionized transitions of CIII, CIV, NV, OVI, SiIV, and probably SVI. We found that only a power law type ionizing spectrum (J_nu propto nu^{-1.5}) is consistent with the observed sample of the line profiles, i.e. the system is definitely intrinsic. The relative metal abundances give almost the solar pattern and the metallicity of about 5 times solar. The system originates in a thin shell of the line-of-sight thickness L <= 16 pc. Two OVI systems at z_a = 1.674 ([C/H] = -1.6) and 1.602 ([O/H] = -1.1), arising in intervening halos, have linear sizes of L = 3-14 kpc and 17 kpc, respectively. Absorption systems at z_a = 1.385 ([C/H] = -0.3, L = 1.7-2.5 kpc) and z_a = 1.667 ([C/H] = -0.5, L = 1 kpc) exhibit characteristics very similar to that observed in metal-enriched high velocity clouds in the Milky Way.These systems are probably embedded in extremely metal-poor halos with [C/H] < -2.4 (z_a = 1.667) and [C/H] < -3.7 (z_a = 1.385). We also found two additional extremely metal-poor Ly-alpha systems at z_a = 1.500 and 1.681 with, respectively, N(HI) = 1.7 10^{15} and 1.8 10^{15} cm^{-2} and [C/H] < -4.0 and <-3.0, - an indication that the distribution of metals in the metagalactic medium is utterly patchy. Our results show that the ionization states in the analyzed OVI absorbers, ranging from z = 1.4 to 1.7, can be maintained by photoionization only and that the fraction of the shock-heated hot gas with temperature T > 10^5 K is negligible in these systems.Comment: 16 pages, including 11 PostScript figures. Accepted for publication in A&

Polycrystalline Ni nanotubes under compression: a molecular dynamics study

Mechanical properties of nanomaterials, such as nanowires and nanotubes, are an important feature for the design of novel electromechanical nano-architectures. Since grain boundary structures and surface modifications can be used as a route to modify nanostructured materials, it is of interest to understand how they affect material strength and plasticity. We report large-scale atomistic simulations to determine the mechanical response of nickel nanowires and nanotubes subject to uniaxial compression. Our results suggest that the incorporation of nanocrystalline structure allows completely flexible deformation, in sharp contrast with single crystals. While crystalline structures at high compression are dominated by dislocation pinning and the multiplication of highly localized shear regions, in nanocrystalline systems the dislocation distribution is significantly more homogeneous. Therefore, for large compressions (large strains) coiling instead of bulging is the dominant deformation mode. Additionally, it is observed that nanotubes with only 70% of the nanowire mass but of the same diameter, exhibit similar mechanical behavior up to 0.3 strain. Our results are useful for the design of new flexible and light-weight metamaterials, when highly deformable struts are required.Fil: Rojas Nunez, J.. Universidad de Santiago de Chile; ChileFil: Baltazar, S. E.. Universidad de Santiago de Chile; ChileFil: Gonzalez, R. I.. Universidad Mayor; ChileFil: Bringa, Eduardo Marcial. Universidad Mayor; Chile. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Allende, S.. Universidad de Santiago de Chile; ChileFil: Kiwi, M.. Universidad de Santiago de Chile; ChileFil: Valencia, F. J.. Universidad de Santiago de Chile; Chile. Universidad Mayor; Chil