Continuum feedback control of a Rayleigh- Taylor type instability

Continuum feedback control of Rayleigh-Taylor instabilit

The Magellanic System: What have we learnt from FUSE?

I review some of the findings on the Magellanic System produced by the Far Ultraviolet Spectroscopic Explorer (FUSE) during and after its eight years of service. The Magellanic System with its high-velocity complexes provides a nearby laboratory that can be used to characterize phenomena that involve interaction between galaxies, infall and outflow of gas and metals in galaxies. These processes are crucial for understanding the evolution of galaxies and the intergalactic medium. Among the FUSE successes I highlight are the coronal gas about the LMC and SMC, and beyond in the Stream, the outflows from these galaxies, the discovery of molecules in the diffuse gas of the Stream and the Bridge, an extremely sub-solar and sub-SMC metallicity of the Bridge, and a high-velocity complex between the Milky Way and the Clouds.Comment: A contributed paper to the FUSE Annapolis Conference "Future Directions in Ultraviolet Spectroscopy.", 5 pages. To appear as an AIP Conference Proceedin

Tracking The Post-BBN Evolution Of Deuterium

The primordial abundance of deuterium produced during Big Bang Nucleosynthesis (BBN) depends sensitively on the universal ratio of baryons to photons, an important cosmological parameter probed independently by the Cosmic Microwave Background (CMB) radiation. Observations of deuterium in high-redshift, low-metallicity QSO Absorption Line Systems (QSOALS) provide a key baryometer, determining the baryon abundance at the time of BBN to a precision of 5%. Alternatively, if the CMB-determined baryon to photon ratio is used in the BBN calculation of the primordial abundances, the BBN-predicted deuterium abundance may be compared with the primordial value inferred from the QSOALS, testing the standard cosmological model. In the post-BBN universe, as gas is cycled through stars, deuterium is only destroyed so that its abundance measured anytime, anywhere in the Universe, bounds the primordial abundance from below. Constraints on models of post-BBN Galactic chemical evolution follow from a comparison of the relic deuterium abundance with the FUSE-inferred deuterium abundances in the chemically enriched, stellar processed material of the local ISM.Comment: 8 pages, 5 figures, to appear in the Proceedings of the Future Directions in Ultraviolet Spectroscopy Conferenc

Highly Ionized Envelopes of High Velocity Clouds

We present recent results on highly ionized gas in Galactic High-Velocity Clouds (HVCs), originally surveyed in OVI (Sembach et al. 2003). In a new FUSE/HST survey of SiII/III/IV (Shull et al. 2009) toward 37 AGN, we detected SiIII (lambda 1206.500 A) absorption with a sky coverage fraction 81 +/- 5% (61 HVCs along 30 of 37 high-latitude sight lines). The SiIII (lambda 1206.500 A) line is typically 4-5 times stronger than OVI (lambda 1031.926 A). The mean HVC column density of perhaps 10^19 cm^-2 of low-metallicity (0.1 - 0.2 Z_sun) ionized gas in the low halo. Recent determinations of HVC distances allow us to estimate a total reservoir of ~10^8 M_sun. Estimates of infall velocities indicate an infall rate of around 1 M_sun yr^-1, comparable to the replenishment rate for star formation in the disk. HVCs appear to be sheathed by intermediate-temperature gas (10^4.0 - 10^4.5 K) detectable in SiIII and SiIV, as well as hotter gas seen in OVI and other high ions. To prepare for HST observations of 10 HVC-selected sight lines with the Cosmic Origins Spectrograph (COS), we compile FUSE/STIS spectra of these ions, plus FeIII, CIII, CIV, and SIV. Better constraints on the physical properties of HVC envelopes and careful treatment of HVC kinematics and infall rates should come from high-quality (S/N ~ 30-40) COS data.Comment: 3 pages, 1 figure, published in Future Directions in Ultraviolet Spectroscopy, Proceedings of the AIP Conference held October 20-22, 2008 in Annapolis, Marylan

Intergalactic Baryons in the Local Universe

Simulations predict that shocks from large-scale structure formation and galactic winds have reduced the fraction of baryons in the warm, photoionized phase (the Lya forest) from nearly 100% in the early universe to less than 50% today. Some of the remaining baryons are predicted to lie in the warm-hot ionized medium (WHIM) phase at T=10^5-10^7 K, but the quantity remains a highly tunable parameter of the models. Modern UV spectrographs have provided unprecedented access to both the Lya forest and potential WHIM tracers at z~0, and several independent groups have constructed large catalogs of far-UV IGM absorbers along ~30 AGN sight lines. There is general agreement between the surveys that the warm, photoionized phase makes up ~30% of the baryon budget at z~0. Another ~10% can be accounted for in collapsed structures (stars, galaxies, etc.). However, interpretation of the ~100 high-ion (OVI, etc) absorbers at z<0.5 is more controversial. These species are readily created in the shocks expected to exist in the IGM, but they can also be created by photoionization and thus not represent WHIM material. Given several pieces of observational evidence and theoretical expectations, I argue that most of the observed OVI absorbers represent shocked gas at T~300,000 K rather than photoionized gas at T<30,000 K, and they are consequently valid tracers of the WHIM phase. Under this assumption, enriched gas at T=10^5-10^6 K can account for ~10% of the baryon budget at z<0.5, but this value may increase when bias and incompleteness are taken into account and help close the gap on the 50% of the baryons still "missing".Comment: Invited review to appear in "Future Directions in Ultraviolet Spectroscopy", Oct 20-22, 2008, Annapolis, MD, M. E. Van Steenberg, ed. (April 2009). 8 pages, five figure

The Cosmic Origins Spectrograph and the Future of Ultraviolet Astronomy

I describe the capabilities of the Cosmic Origins Spectrograph, scheduled for May 2009 installation on the Hubble Space Telescope. With a factor-of-ten increase in far-UV throughput for moderate resolution spectroscopy, COS will enable a range of scientific programs that study hot stars, AGN, and gas in the interstellar medium, intergalactic medium, and galactic halos. We also plan a large-scale HST Spectroscopic Legacy Project for QSO absorption lines, galactic halos, and AGN outflows. Studies of next-generation telescopes for UV/O astronomy are now underway, including small, medium, and large missions to fill the imminent ten-year gap between the end of Hubble and a plausible launch of the next large mission. Selecting a strategy for achieving these goals will involve hard choices and tradeoffs in aperture, wavelength, and capability.Comment: To appear in Future Directions in Ultraviolet Astronomy (AIP Conf Proc

Distances to the high galactic latitude molecular clouds G192-67 and MBM 23-24

We report on distance determinations for two high Galactic latitude cloud complexes, G192-67 and MBM 23-24. No distance determination exists in the literature for either cloud. Thirty-four early type stars were observed towards the two clouds, more than half of which have parallaxes measured by the Hipparcos satellite. For the remaining stars we have made spectroscopic distance estimates. The data consist of high resolution echelle spectra centered on the Na I D lines, and were obtained over six nights at the Coude Feed telescope at Kitt Peak National Observatory. Interstellar absorption lines were detected towards some of the stars, enabling estimates of the distances to the clouds of 109 +/- 14 pc for G192-67, and of 139 +/- 33 pc for MBM 23-24. We discuss the relationship of these clouds to other ISM features such as the Local Hot Bubble and the local cavity in neutral hydrogen.Comment: 15 pages, 6 embedded figures, to be published in the ApJ Vol. 516, No.