The planetary nebula population of the Sagittarius dwarf spheroidal galaxy

Abstract

The identification of two new planetary nebulae (PNe) 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. StWr 2-21 shows a high abundance of [O/H] =−0.23, which confirms the high-metallicity population in Sgr uncovered by Bonifacio et al. The steep metallicity–age gradient in Sgr is due to interstellar medium (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 × 10−4 M⊙ yr−1, amounting to ∼106 M⊙ per orbital period. Hubble Space Telescope 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 of ρ∝r−3. This is evidence for increasing mass-loss rates on the asymptotic giant branch. Peak mass-loss rates are indicated of ∼10−4 M⊙ yr−1. The IR-[WC] PN, He 2-436, provides the sole direct detection of dust in a dwarf spheroidal galaxy, to date.