paper (ps, preprint, 0.8M)
The identification of two new Planetary Nebulae in the Sagittarius Dwarf
Spheroidal Galaxy (Sgr) is presented. This brings the total number to four.
Both new PNe were previously classified as Galactic objects. The first,
StWr 2-21, belongs to the main body of Sgr, from its velocity and location.
The second, the halo PN BoBn 1, has a location, distance and velocity in
agreement with the leading tidal tail of Sgr. We estimate that 10 per cent
of the Galactic halo consists of Sgr debris. The specific frequency of PNe
indicates a total luminosity of Sgr, including its tidal tails, of
MV=-14.1. StWr2-21 shows a high abundance of [O/H] =-0.23, which
confirms the high-metallicity population in Sgr uncovered by Bonaficio et
al. (2004). The steep metallicity--age gradient in Sgr is due to ISM
removal during the Galactic plane passages, ISM reformation due to stellar
mass loss, and possibly accretion of metal-enriched gas from our Galaxy.
The ISM re-formation rate of Sgr, from stellar mass loss, is 5 times
10-4 Msun yr-1, amounting to ~ 106 Msun
per orbital period.
HST images of three of the PNe reveal well-developed bipolar
morphologies, and provide clear detections of the central stars. All three
stars with deep spectra show WR-lines, suggesting that the progenitor mass
and metallicity determines whether a PN central star develops a WR spectrum.
One Sgr PN belongs to the class of IR-[WC] stars. Expansion velocities are
determined for three nebulae. Comparison with hydrodynamical models
indicates an initial density profile proportional to r-3. This is
evidence for increasing mass-loss rates on the AGB. Peak mass-loss rates are
indicated of ~10-4 Msun yr-1.
The IR-[WC] PN, He 2-436, provides the sole direct detection of dust in
a dwarf spheroidal galaxy, to date.