Using linear non-adiabatic pulsation analysis, we explore the radial-mode (p-mode) stability of stars across a wide range of mass (
$0.2 \le M \le 50{\,\rm M_{{\odot }}}$
), composition (0 ≤ X ≤ 0.7, ...Z = 0.001, 0.02), effective temperature (3000 ≤ T
eff ≤ 40 000 K), and luminosity (0.01 ≤ L/M ≤ 100 000 solar units). We identify the instability boundaries associated with low- to high-order radial oscillations (0 ≤ n ≤ 16). The instability boundaries are a strong function of both composition and radial order (n). With decreasing hydrogen abundance we find that (i) the classical blue edge of the Cepheid instability strip shifts to higher effective temperature and luminosity, and (ii) high-order modes are more easily excited and small islands of high radial-order instability develop, some of which correspond with real stars. Driving in all cases is by the classical κ-mechanism and/or strange modes. We identify regions of parameter space where new classes of pulsating variable may, in future, be discovered. The majority of these are associated with reduced hydrogen abundance in the envelope; one has not been identified previously.
Context. The gravitational wave (GW) background in the range 0.01-30 mHz has been assumed to be dominated by unresolved radiation from double white dwarf binaries (DWDs). Recent investigations ...indicate that, at short periods, a number of DWDs should be resolvable sources of GW. Aims. We characterize the GW signal which would be detected by LISA from DWDs in the Galaxy. Methods. We have constructed a Galactic model in which we consider distinct contributions from the bulge, thin disc, thick disc, and halo, and subsequently executed a population synthesis approach to determine the birth rates, numbers, and period distributions of DWDs within each component. Results. In the Galaxy as a whole, our model predicts the current birth rate of DWDs to be 3.21×10-2 yr-1, the local density to be 2.2×10-4 pc-3 and the total number to be 2.76×108. Assuming SNIa are formed from the merger of two CO white dwarfs, the SNIa rate should be 0.0013 yr-1. The frequency spectra of DWD strain amplitude and number distribution are presented as a function of galactic component, DWD type, formation channel, and metallicity. Conclusions. We confirm that CO+He and He+He white dwarf (WD) pairs should dominate the GW signal at very high frequencies (log f Hz-1 > -2.3), while CO+CO and ONeMg WD pairs have a dominant contribution at log f Hz-1 ≤ -2.3. Formation channels involving two common-envelope (CE) phases or a stable Roche lobe overflow phase followed by a CE phase dominate the production of DWDs detectable by LISA at log f Hz-1 > -4.5. DWDs with the shortest orbital periods will come from the CE+CE channel. The Exposed Core plus CE channel is a minor channel. A number of resolved DWDs would be detected, making up 0.012% of the total number of DWDs in the Galaxy. The majority of these would be CO+He and He+He pairs formed through the CE+CE channel.
Abstract
SALT spectra of the helium-rich hot subdwarf EC 22536-5304 show strong absorption lines of triply-ionized lead. Analysis of the HRS spectrum and a follow-up SALT/RSS spectrum show ...EC 22536-5304 to have surface properties (temperature, gravity, helium/hydrogen ratio) similar to other heavy-metal subdwarfs. With a lead overabundance of 4.8 dex relative to solar, EC 22536-5304 is the most lead-rich intermediate helium subdwarf discovered so far.
ABSTRACT
A medium- and high-resolution spectroscopic survey of helium-rich hot subdwarfs is being carried out using the Southern African Large Telescope (SALT). Objectives include the discovery of ...exotic hot subdwarfs and of sequences connecting chemically peculiar subdwarfs of different types. The first phase consists of medium-resolution spectroscopy of over 100 stars selected from low-resolution surveys. This paper describes the selection criteria, and the observing, classification, and analysis methods. It presents 107 spectral classifications on the MK-like Drilling system and 106 coarse analyses (${T_{\rm eff}}, \log g, \log y$) based on a hybrid grid of zero-metal non-LTE and line-blanketed LTE model atmospheres. For 75 stars, atmospheric parameters have been derived for the first time. The sample may be divided into six distinct groups including the classical ‘helium-rich’ sdO stars with spectral types (Sp) sdO6.5–sdB1 (74) comprising carbon-rich (35) and carbon-weak (39) stars, very hot He-sdO’s with Sp ≲ sdO6 (13), extreme helium stars with luminosity class ≲5 (5), intermediate helium-rich subdwarfs with helium class 25–35 (8), and intermediate helium-rich subdwarfs with helium class 10–25 (6). The last covers a narrow spectral range (sdB0–sdB1) including two known and four candidate heavy-metal subdwarfs. Within other groups are several stars of individual interest, including an extremely metal-poor helium star, candidate double-helium subdwarf binaries, and a candidate low-gravity He-sdO star.
Abstract
The discovery of extremely zirconium- and lead-rich surfaces amongst a small subgroup of hot subdwarfs has provoked questions pertaining to chemical peculiarity in hot star atmospheres and ...about their evolutionary origin. With only three known in 2014, a limited search for additional ‘heavy-metal’ subdwarfs was initiated with the Subaru telescope. Five hot subdwarfs having intermediate to high surface enrichment of helium were observed at high-resolution and analyzed for surface properties and abundances. This paper reports the analyses of four of these stars. PG 1559+048 and FBS 1749+373, having only intermediate helium enrichment, show strong lines of triply ionized lead. PG 1559+048 also shows a strong overabundance of germanium and yttrium. With more helium-rich surfaces, Ton 414 and J17554+5012, do not show evidence of heavy-metal enrichment. This limited survey suggests that extreme enrichment of ‘heavy metals’ by selective radiative levitation in hot subdwarf atmospheres is suppressed if the star is too helium-rich.
Inhibition of histone deacetylase 6 (HDAC6) was shown to support axon growth on the nonpermissive substrates myelin-associated glycoprotein (MAG) and chondroitin sulfate proteoglycans (CSPGs). Though ...HDAC6 deacetylates α-tubulin, we find that another HDAC6 substrate contributes to this axon growth failure. HDAC6 is known to impact transport of mitochondria, and we show that mitochondria accumulate in distal axons after HDAC6 inhibition. Miro and Milton proteins link mitochondria to motor proteins for axon transport. Exposing neurons to MAG and CSPGs decreases acetylation of Miro1 on Lysine 105 (K105) and decreases axonal mitochondrial transport. HDAC6 inhibition increases acetylated Miro1 in axons, and acetyl-mimetic Miro1 K105Q prevents CSPG-dependent decreases in mitochondrial transport and axon growth. MAG- and CSPG-dependent deacetylation of Miro1 requires RhoA/ROCK activation and downstream intracellular Ca
increase, and Miro1 K105Q prevents the decrease in axonal mitochondria seen with activated RhoA and elevated Ca
These data point to HDAC6-dependent deacetylation of Miro1 as a mediator of axon growth inhibition through decreased mitochondrial transport.
► Pillar- and tube-typed mesoporous titania thin films (MTTFs) are successfully synthesized. ► The MTTFs were used, for the first time, for hydrogen generation from water splitting. ► The ...pillar-typed MTTFs generated more hydrogen than tube-typed MTTFs. ► This is the first fundamental study of the effects of TiO2 nanostructures on the production of hydrogen. ► The pillar-typed MTTFs will be a useful catalyst for photocatalytic applications.
We researched the structural effect of titania photocatalysts on the yield of hydrogen gas from water splitting. We synthesized mesoporous titania thin films (MTTFs) with two different structures (that is, pillar structure and tube structure, denoted as P-MTTFs and T-MTTFs, respectively) through an evaporation-induced self-assembling (EISA) process with the presence of a surfactant as a structure-directing agent. The synthesized MTTFs were carefully characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis spectra. After analysis of photocurrent and hydrogen generation, we concluded that with different structures, MTTFs generate photocurrents and hydrogen with different efficiencies. The results indicated that the T-MTTF had a higher net current (i.e., 1.05E−3eV) than the P-MTTF (i.e., 1.04E−3eV). However, the yield of hydrogen from T-MTTFs (i.e., 17.5μmol) was lower than that of P-MTTFs (i.e., 37.5μmol). Tube structure induced a concentration gradient of NaOH, which restrained the generation of oxygen and hydrogen.
Since its discovery about 50 yr ago, HD 144941 has generally been classified as a peculiar member of the extreme helium (EHe) supergiant stars, a very rare class of low-mass hydrogen-deficient stars. ...We report the detection of a strong longitudinal magnetic field based on spectropolarimetry with FORS2 on the ESO VLT with surface-averaged longitudinal field strengths as large as −9 kG. This is further constrained by the detection of Zeeman splitting of spectral lines to a field strength of at least 15 kG, explaining the recent finding of surface spots for this star. The quantitative analysis of the stellar atmosphere based on a hybrid non-local thermodynamic equilibrium approach and new optical spectra yields an effective temperature of 22 000 ± 500 K, a logarithmic surface gravity of 4.20 ± 0.10, and a surface helium fraction of 0.950 ± 0.002 by number. While the metal abundances are about a factor of 10 sub-solar in absolute number, the metal-to-hydrogen ratios are typical of massive early-type stars, indicating that helium fallback in a weak, fractionated stellar wind in the presence of a magnetic field took place – the canonical mechanism for the formation of the helium-strong phenomenon. Both the spectroscopic and the
Gaia
EDR3 parallax imply HD 144941 to be a luminous massive star. Kinematically, we argue that HD 144941 has reached its high Galactic latitude as a runaway star. We conclude that instead of being a comparatively high-gravity low-mass EHe star, HD 144941 is by far the most extreme member of the magnetic massive helium-strong stars, with almost all atmospheric hydrogen substituted by helium.
ABSTRACT
Hydrogen-deficient stars include the cool R CrB variable (RCBs) and hydrogen-deficient carbon (HdCs) giants through extreme helium stars (EHes) to the very hot helium-rich subdwarfs (He-sdO ...and O(He) stars) and white dwarfs. With surfaces rich in helium, nitrogen, and carbon, their origins have been identified with the merger of two white dwarfs. Using Gaia to focus on the EHes, we aim to identify progenitor populations and test the evolution models. Gaia DR3 measurements and ground-based radial velocities have been used to compute Galactic orbits using galpy. Each orbit has been classified by population; EHe stars are found in all of the thin disc, thick disc, halo, and bulge, as are RCB, HdC, and He-sdO stars. Spectral energy distributions were constructed for all EHes, to provide angular diameters, and hence radii and luminosities. The EHes fall into two luminosity groups divided at $L \approx 2500 \, {\rm L_{\odot }}$. This supports theory for the origin of EHes, and is the strongest confirmation so far in terms of luminosity. The lower luminosity EHes correspond well with the post-merger evolution of a double helium white dwarf binary. Likewise, the higher luminosity EHes match the post-merger evolution of a carbon/oxygen plus helium white dwarf binary. In terms of parent populations, current models predict that double white dwarf mergers should occur in all Galactic populations, but favour mergers arising from recent star formation (i.e. thin disc), whereas the statistics favour an older epoch (i.e. thick disc).
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