HI Observations Towards the Sagittarius Dwarf Spheroidal Galaxy

We have measured the 21-cm line of Galactic HI over more than 50 square degrees in the direction of the Sagittarius dwarf spheroidal galaxy. The data show no evidence of HI associated with the dwarf spheroidal which might be consider analogous to the Magellanic Stream as it is associated in both position and velocity with the Large Magellanic Cloud. Nor do the HI data show evidence for any disturbance in the Milky Way disk gas that can be unambiguously assigned to interaction with the dwarf galaxy. The data shown here limit the HI mass at the velocity of the Sagittarius dwarf to <7000 solar masses over some 18 square degrees between Galactic latitudes -13 degrees and -18 degrees.Comment: 5 pages, 4 figures; accepted for publication in Astronomy & Astrophysic

The Milky and its Gas: Cold Fountains and Accretion

The Milky Way is acquiring gas from infalling high-velocity clouds. The material enters a disk-halo interface that in many places is populated with HI clouds that have been ejected from the disk through processes linked to star formation. The Smith Cloud is an extraordinary example of a high-velocity cloud that is bringing $>10^6$ M$_{\odot}$ of relatively low metallicity gas into the Milky Way. It may be part of a larger stream, components of which are now passing through the disk.Comment: Presented at "Assembling the Puzzle of the Milky Way", Grand Bornand, April 201

What is the Shell Around R Coronae Borealis?

The hydrogen-deficient, carbon-rich R Coronae Borealis (RCB) stars are known for being prolific producers of dust which causes their large iconic declines in brightness. Several RCB stars, including R CrB, itself, have large extended dust shells seen in the far-infrared. The origin of these shells is uncertain but they may give us clues to the evolution of the RCB stars. The shells could form in three possible ways. 1) they are fossil Planetary Nebula (PN) shells, which would exist if RCB stars are the result of a final, helium-shell flash, 2) they are material left over from a white-dwarf merger event which formed the RCB stars, or 3) they are material lost from the star during the RCB phase. Arecibo 21-cm observations establish an upper limit on the column density of H I in the R CrB shell implying a maximum shell mass of $\lesssim$0.3 M$_{\odot}$. A low-mass fossil PN shell is still a possible source of the shell although it may not contain enough dust. The mass of gas lost during a white-dwarf merger event will not condense enough dust to produce the observed shell, assuming a reasonable gas-to-dust ratio. The third scenario where the shell around R CrB has been produced during the star's RCB phase seems most likely to produce the observed mass of dust and the observed size of the shell. But this means that R CrB has been in its RCB phase for $\sim$10$^{4}$ yr.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A