The relation between the zonal jets and ammonia anomalies in Jupiter

<div> <div>Juno's six‐channel MWR measurements might reveal information about the structure of the wind profile below the cloud level. These measurements are used to calculate the nadir brightness temperature (T<sub>b</sub>), a profile determined by temperature and by the opacity of the atmosphere. This opacity for the relevant frequencies of the MWR is determined mostly by ammonia abundance. The T<sub>b</sub> vary considerably between the different channels (indicating on different depths) and between latitudes. Here, we take the <!-- mathfontold --> T<sub>b</sub> as an indicator for ammonia concentration and examine the relation to the zonal jets. We find that different theoretical mechanisms can explain this relation at different latitudes. At the equatorial region, the superrotation is accompanied by vertical upwelling. This vertical advection, driven by a convergence of eddy fluxes directed perpendicular to the axis of rotation, is shown to explain the equatorial ammonia enrichment. At the mid-latitudes, assuming that the ammonia is enriched with depth, alternating Ferrel-like cells framed by opposite vertical velocities redistributes the ammonia, maximizing its meridional gradient where the jet peaks. This hypothesis is well apparent in the data, using both correlation analysis and theoretical arguments. We find that dynamical reasoning, suggesting on vertical velocities through the cloud-level zonal jets, can explain the latitudinal variations in <!-- mathfontold --> T<sub>b,</sub> under the assumption that they are caused by ammonia abundance anomalies.</div> </div>

The discovery of lambda Bootis stars - the Southern Survey II

ABSTRACT The λ Boo stars are chemically peculiar A-type stars whose abundance anomalies are associated with the accretion of metal-poor material. We searched for λ Boo stars in the Southern hemisphere in a targeted spectroscopic survey of metal-weak and emission-line stars. Obtaining spectra for 308 stars and classifying them on the MK system, we found or co-discovered 24 new λ Boo stars. We also revised the classifications of 11 known λ Boo stars, one of which turned out to be a chemically normal rapid rotator. We show that stars previously classified in the literature as blue horizontal branch stars or emission-line A stars have a high probability of being λ Boo stars, although this conclusion is based on small-number statistics. Using WISE infrared fluxes, we searched our targets for infrared excesses that might be attributable to protoplanetary or debris discs as the source of the accreted material. Of the 34 λ Boo stars in our sample, 21 at various main-sequence ages have infrared excesses, confirming that not all λ Boo stars are young.

Mass transfer of low-mass binaries and chemical anomalies among unevolved stars in globular clusters

ABSTRACT While it is well known that mass transfer in binaries can pollute the surfaces of the accretors, it is still unclear whether this mechanism can reproduce the observed chemical inhomogeneities in globular clusters. We study the surface abundances of the accretors in low-mass binaries, as a first step towards understanding whether mass transfer in low-mass binaries is one of the potential origins of the aforementioned abundance anomalies in globular clusters. We use the mesa (Modules for Experiments in Stellar Astrophysics) code to calculate binary evolutionary models with different initial donor masses between 0.9 and 1.9 $\rm {M}_\odot$ for an initial metallicity of Z = 0.0034. The results show that in some low-mass binary systems, the accretors exhibit peculiar chemical patterns when they are still unevolved stars, e.g. C and O depletion; Na and N enhancement; and constant Mg, Al, and C+N+O. The abundance patterns of the accretors are significantly different from their initial abundances (or that of normal single stars), and can match the observed populations. These abundance patterns strongly depend not only on the initial parameters of binaries (donor mass, mass ratio, and orbital period), but also on the assumptions regarding mass-transfer efficiency and angular momentum loss. These results support the hypothesis that mass transfer in low-mass binaries is, at least, partly responsible for the unevolved anomalous stars in globular clusters. More work on binary evolutionary models and binary population synthesis is required to fully evaluate the contribution of this scenario.

Photospheric nitrogen abundances and carbon 12C$/$13C ratios of red giant stars

Nitrogen abundances and carbon isotope ratios (12C$/$13C) in the atmospheres of red giants are known to be influenced by dredge-up of H-burning products, and serve as useful probes to study the nature of evolution-induced envelope mixing. We determined the [N/Fe] and 12C$/$13C ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the 12CN and 13CN lines in the ∼8002–8005 Å region, with the aim of investigating how these quantities are related to other similar mixing-affected indicators which were already reported in our previous work. It was confirmed that [N/Fe] values are generally supersolar (typically by several tenths of a dex, though widely differing from star to star), anti-correlated with [C/Fe], and correlated with [Na/Fe], as expected from theory. As seen from their dependence upon stellar parameters, it appears that mixing tends to be enhanced with an increase of stellar luminosity (or mass) and rotational velocity, which is also reasonable from the theoretical viewpoint. In contrast, the resulting 12C$/$13C ratios turned out to be considerably diversified in the range of ∼5–50 (with a peak around ∼20), without showing any systematic dependence upon C or N abundance anomalies caused by the mixing of CN-cycled material. It thus appears that our understanding of the photospheric 12C$/$13C ratios in red giants is still incomplete, requiring more observational studies.

Statistical analysis of roAp, He-weak, and He-rich stars

To enlarge our data base of chemically peculiar stars, we compiled published data concerning the He-weak and He-rich stars observed by high-resolution spectroscopy techniques during last decades. Twenty He-weak and 28 He-rich stars have been added to the data base. We have also distinguished roAp stars from stars previously identified as Ap stars. To deepen our knowledge on statistical overview of the abundance anomalies versus the physical parameters of stars, we compared our data with previous compilations. We applied statistical tests on our data and found interesting correlations for effective temperature and surface gravity for all type of stars and a few correlations for projected rotation velocity only for He-rich stars. Because of the lack of the data, we could not check whether being a member of binary system is affecting on chemical peculiarities of those stars.