ASASSN-18aan: An eclipsing SU UMa-type cataclysmic variable with a 3.6-hr orbital period and a late G-type secondary star

We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period (Porb) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-Porb SU UMa-type dwarf novae. The estimated mass ratio, [q = M2/M1 = 0.278(1)], is almost identical to the upper limit of tidal instability by the 3 : 1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3 : 1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-Porb WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-q systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-q systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass M2 of around 0.18 M⊙, smaller than expected for the orbital period and the secondary’s spectral type. We also see indications of enhanced sodium abundance in the secondary’s spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system’s Porb, which may account for the occurrence of a superoutburst at this relatively long period.

Assessment and Progress of Polyanionic Cathodes in Aqueous Sodium Batteries

Aqueous sodium‐ion batteries are expected to be competitive technologies for large-scale energy storage applications due to the extraordinary sodium abundance, low manufacturing cost and high safety. However, only a few...

Leveraging HST with MUSE – I. Sodium abundance variations within the 2-Gyr-old cluster NGC 1978

ABSTRACT Nearly all of the well-studied ancient globular clusters (GCs), in the Milky Way and in nearby galaxies, show star-to-star variations in specific elements (e.g. He, C, N, O, Na, and Al), known as ‘multiple populations’ (MPs). However, MPs are not restricted to ancient clusters, with massive clusters down to ∼2 Gyr showing signs of chemical variations. This suggests that young and old clusters share the same formation mechanism but most of the work to date on younger clusters has focused on N variations. Initial studies even suggested that younger clusters may not host spreads in other elements beyond N (e.g. Na), calling into question whether these abundance variations share the same origin as in the older GCs. In this work, we combine Hubble Space Telescope (HST) photometry with Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy of a large sample of red giant branch (RGB) stars (338) in the Large Magellanic Cloud cluster NGC 1978, the youngest globular to date with reported MPs in the form of N spreads. By combining the spectra of individual RGB stars into N-normal and N-enhanced samples, based on the ‘chromosome map’ derived from HST, we search for mean abundance variations. Based on the NaD line, we find a Na difference of Δ[Na/Fe] = 0.07 ± 0.01 between the populations. While this difference is smaller than typically found in ancient GCs (which may suggest a correlation with age), this result further confirms that the MP phenomenon is the same, regardless of cluster age and host galaxy. As such, these young clusters offer some of the strictest tests for theories on the origin of MPs.

The relationship between globular cluster parameters and abundance variations

We discuss a meta-analysis of the association of abundance variations in globular cluster stars with the present-day stellar mass and metallicity of globular clusters. Using data for 42 globular clusters that are well-sampled from either or both of prior literature studies and the APOGEE survey, we confirm prior findings that increasing aluminum abundance variations in globular clusters are positively correlated with increasing present-day stellar mass or decreasing metallicity. We also demonstrate that the ratio of aluminum abundance variations to either nitrogen abundance variations or sodium abundance variations is itself positively correlated with decreasing metallicity and increasing stellar mass of globular clusters. This suggests that there were at least two non-supernovae chemical polluters that were active in the early universe.

Are Galactic globular cluster AGB stars rich or poor in Sodium? Sodium abundance of AGB and RGB stars in NGC 2808

A spectroscopic study comparing the [Na/Fe] distributions of RGB and AGB stars in the Galactic globular cluster (GC) NGC 6752 found that there was no Na-rich, 2nd-generation star along the early-AGB of this cluster. This came as a surprise since in this GC, as well as other Galactic GCs studied so far, 1st- and 2nd-generation stars have usually been found from the main sequence turnoff up to the red giant branch. To investigate whether the failure of a significant fraction of stars to ascend the AGB also happens to other GCs, we studied a sample of AGB and RGB stars in NGC 2808 observed at the ESO/VLT with FLAMES. Contrary to NGC 6752, we find that the AGB and RGB stars we studied in NGC 2808 have comparable [Na/Fe] dispersions.

Globular cluster star classification: Application to M13

Starting from recent determination of Fe, O, Na abundances on a restricted sample (N = 67) of halo and thick disk stars, a natural and well motivated selection criterion is defined for the classification globular cluster stars. An application is performed to M13 using a sample (N = 113) for which Fe, O, Na abundances have been recently inferred from observations. A comparison is made between the current and earlier M13 star classifications. Both O and Na empirical differential abundance distributions are determined for each class and for the whole sample (with the addition of Fe in the last case) and compared with their theoretical counterparts due to cosmic scatter obeying a Gaussian distribution whose parameters are inferred from related subsamples. The occurrence of an agreement between the empirical and theoretical distributions is interpreted as absence of significant chemical evolution and vice versa. The procedure is repeated with regard to four additional classes depending on whether oxygen and sodium abundance is above (stage CE) or below (stage AF) a selected threshold. Both O and Na empirical differential abundance distributions, related to the whole sample, exhibit a linear fit for the AF and CE stage. Within the errors, the oxygen slope for the CE stage is equal and of opposite sign with respect to the sodium slope for AF stage, while the contrary holds when dealing with the oxygen slope for the AF stage with respect to the sodium slope for the CE stage. In the light of simple models of chemical evolution applied to M13, oxygen depletion appears to be mainly turned into sodium enrichment for [O/H]? -1.35 and [Na/H]? -1.45, while one or more largely preferred channels occur for [O/H]< -1.35 and [Na/H]> -1.45. In addition, the primordial to the current M13 mass ratio can be inferred from the true sodium yield in units of the sodium solar abundance. Though the above results are mainly qualitative due to large (-+1.5 dex) uncertainties in abundance determination, still the exhibited trend is expected to be real. The proposed classification of globular cluster stars may be extended in a twofold manner, namely to: (i) elements other than Na and Fe and (ii) globular clusters other than M13.