Dust and Chemical Abundances of the Sagittarius dwarf Galaxy Planetary Nebula Hen2-436

Abstract

We have estimated elemental abundances of the planetary nebula (PN) Hen2-436 in the Sagittarius (Sgr) spheroidal dwarf galaxy using ESO/VLT FORS2, Magellan/MMIRS, and Spitzer/IRS spectra. We have detected candidates of [F II] 4790A, [Kr III] 6826A, and [P II] 7875A lines and successfully estimated the abundances of these elements ([F/H]=+1.23, [Kr/H]=+0.26, [P/H]=+0.26) for the first time. We present a relation between C, F, P, and Kr abundances among PNe and C-rich stars. The detections of F and Kr support the idea that F and Kr together with C are synthesized in the same layer and brought to the surface by the third dredge-up. We have estimated the N^2+ and O^2+ abundances using optical recombination lines (ORLs) and collisionally excited lines (CELs). The discrepancy between the abundance derived from the O ORL and that derived from the O CEL is >1 dex. To investigate the status of the central star of the PN, nebula condition, and dust properties, we construct a theoretical SED model with CLOUDY. By comparing the derived luminosity and temperature of the central star with theoretical evolutionary tracks, we conclude that the initial mass of the progenitor is likely to be ~1.5-2.0 Msun and the age is ~3000 yr after the AGB phase. The observed elemental abundances can be explained by a theoretical nucleosynthesis model with a star of initial mass 2.25 Msun, Z=0.008 and LMC compositions. We have estimated the dust mass to be 2.9x10^-4 Msun (amorphous carbon only) or 4.0x10^-4 Msun (amorphous carbon and PAH). Based on the assumption that most of the observed dust is formed during the last two thermal pulses and the dust-to-gas mass ratio is 5.58x10^-3, the dust mass-loss rate and the total mass-loss rate are <3.1x10^-8 Msun / yr and <5.5x10^-6 Msun / yr, respectively. Our estimated dust mass-loss rate is comparable to a Sgr dwarf galaxy AGB star with similar metallicity and luminosity.

14 pages, 6 figures, accepted for publication in ApJ

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