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The sun and giant planets are generally thought to have the same helium abundance as that in the solar nebula from which they were formed 4.6 billion years ago. In contrast, the interstellar medium reflects current galactic conditions. The departure of current abundances from the primordial and protosolar values may help trace the processes that drive the nucleosynthesis evolution of the galaxy and planetary interior formation and evolution. The Galileo probe measured the He abundance in situ the atmosphere of Jupiter, showing that He is only slightly depleted compared to the solar value. For Saturn, contradictory estimates from past Voyager observations make its He abundance very uncertain. Here, we use He 58.4 nm dayglow measured from the outer planets by the Voyager ultraviolet spectrometers to derive the He abundance in the atmosphere of Jupiter and Saturn. We also use the solar He 58.4 nm line measured by the Solar Heliospheric Observatory to derive the He abundance inside the focusing cone. Finally, we compare He abundances derived here with primordial and protosolar values, stressing the unique opportunity offered by inner heliosphere observations and future Voyager in situ local interstellar medium measurements to derive the He abundance in the very interstellar cloud in which we reside.
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Debris discs are typically revealed through excess emission at infrared wavelengths. Most discs exhibit excess at mid- and far-infrared wavelengths, analogous to the solar systems Asteroid and Edgeworth-Kuiper belts. Recently, stars with strong (1 per cent) excess at near-infrared wavelengths were identified through interferometric measurements. Using the HIgh Precision Polarimetric Instrument (HIPPI), we examined a sub-sample of these hot dust stars (and appropriate controls) at parts-per-million sensitivity in SDSS g (green) and r (red) filters for evidence of scattered light. No detection of strongly polarized emission from the hot dust stars is seen. We therefore rule out scattered light from a normal debris disk as the origin of this emission. A wavelength dependent contribution from multiple dust components for hot dust stars is inferred from the dispersion (difference in polarization angle in red and green) of southern stars. Contributions of 17 ppm (green) and 30 ppm (red) are calculated, with strict 3 sigma upper limits of 76 and 68 ppm, respectively. This suggests weak hot dust excesses consistent with thermal emission, although we cannot rule out contrived scenarios, e.g. dust in a spherical shell or face on discs. We also report on the nature of the local interstellar medium, obtained as a byproduct of the control measurements. Highlights include the first measurements of the polarimetric colour of the local interstellar medium and discovery of a southern sky region with a polarization per distance thrice the previous maximum. The data suggest the wavelength of maximum polarization is bluer than typical.
We have analyzed 17 early-type galaxies, 13 ellipticals and 4 S0s, observed with Suzaku, and investigated metal abundances (O, Mg, Si, and Fe) and abundance ratios (O/Fe, Mg/Fe, and Si/Fe) in the interstellar medium (ISM). The emission from each on-source region, which is 4 times effective radius, r_e, is reproduced with one- or two- temperature thermal plasma models as well as a multi-temperature model, using APEC plasma code v2.0.1. The multi-temperature model gave almost the same abundances and abundance ratios with the 1T or 2T models. The weighted averages of the O, Mg, Si, and Fe abundances of all the sample galaxies derived from the multi-temperature model fits are 0.83+-0.04, 0.93+-0.03, 0.80+-0.02, and 0.80+-0.02 solar, respectively, in solar units according to the solar abundance table by Lodders (2003). These abundances show no significant dependence on the morphology and environment. The systematic differences in the derived metal abundances between the version 2.0.1 and 1.3.1 of APEC plasma codes were investigated. The derived O and Mg abundances in the ISM agree with the stellar metallicity within a aperture with a radius of one r_e derived from optical spectroscopy. From these results, we discuss the past and present SN Ia rates and star formation histories in early-type galaxies.
We quantify the gas-phase abundance of deuterium and fractional contribution of stellar mass loss to the gas in cosmological zoom-in simulations from the Feedback In Realistic Environments project. At low metallicity, our simulations confirm that the deuterium abundance is very close to the primordial value. The chemical evolution of the deuterium abundance that we derive here agrees quantitatively with analytical chemical evolution models. We furthermore find that the relation between the deuterium and oxygen abundance exhibits very little scatter. We compare our simulations to existing high-redshift observations in order to determine a primordial deuterium fraction of 2.549 +/- 0.033 x 10^-5 and stress that future observations at higher metallicity can also be used to constrain this value. At fixed metallicity, the deuterium fraction decreases slightly with decreasing redshift, due to the increased importance of mass loss from intermediate-mass stars. We find that the evolution of the average deuterium fraction in a galaxy correlates with its star formation history. Our simulations are consistent with observations of the Milky Ways interstellar medium: the deuterium fraction at the solar circle is 85-92 per cent of the primordial deuterium fraction. We use our simulations to make predictions for future observations. In particular, the deuterium abundance is lower at smaller galactocentric radii and in higher mass galaxies, showing that stellar mass loss is more important for fuelling star formation in these regimes (and can even dominate). Gas accreting onto galaxies has a deuterium fraction above that of the galaxies interstellar medium, but below the primordial fraction, because it is a mix of gas accreting from the intergalactic medium and gas previously ejected or stripped from galaxies.
The Interstellar Medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic, molecular species, and dust particles. The neutral ISM is dominated by neutral hydrogen and has ionization fractions up to 8%. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars, because they formed at different times. Instead, the gas in the Solar vicinity is assumed to be well mixed and have Solar metallicity in traditional chemical evolution models. The ISM chemical abundances can be accurately measured with UV absorption-line spectroscopy. However, the effects of dust depletion, which removes part of the metals from the observable gaseous phase and incorporates it into solid grains, have prevented, until recently, a deeper investigation of the ISM metallicity. Here we report the dust-corrected metallicity of the neutral ISM measured towards 25 stars in our Galaxy. We find large variations in metallicity over a factor of 10 (with an average 55 +/- 7% Solar and standard deviation 0.28 dex) and including many regions of low metallicity, down to ~17% Solar and possibly below. Pristine gas falling onto the disk in the form of high-velocity clouds can cause the observed chemical inhomogeneities on scales of tens of pc. Our results suggest that this low-metallicity accreting gas does not efficiently mix into the ISM, which may help us understand metallicity deviations in nearby coeval stars.
It has long been suggested that helium nuclei in the intracluster plasma can sediment in the cluster gravitational potential well. Some theoretical estimates for the cores of relaxed clusters predict an excess of helium abundance by up to a factor of a few over its primordial value. The intracluster helium abundance cannot be measured directly. This presents a significant source of uncertainty for cosmological tests based on the X-ray derived cluster quantities, such as the gas mass, total mass, and gas mass fraction, all of which depend on the assumed helium abundance. We point out that cluster distances derived by combining the Sunyaev-Zeldovich (SZ) and X-ray data also depend on the helium abundance. This dependence can be used to measure the abundance, provided the distance is known independently. For example, if one adopts the WMAP H_0 value, then the recent H_0 measurement by Bonamente and collaborators, derived from SZ data on 38 clusters assuming a primordial helium abundance, corresponds to an abundance excess by a factor of 1.9+-0.8 within r~1 Mpc (using only their statistical errors). This shows that interesting accuracy is within reach. We also briefly discuss how the SZ and X-ray cluster data can be combined to resolve the helium abundance dependence for the d_a(z) cosmological test.
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