STScI, 29 Mar How Stars Die: Infrared Spectroscopy of Dusty Carbon Stars in the Local Group G. C. Sloan A.A. Zijlstra, E. Lagadec, M. Matsuura, K.E. Kraemer, M.A.T. Groenewegen, I. McDonald, J.T. van Loon, J. Bernard-Salas, & P.R. Wood

STScI, 29 Mar Getting from there to here

STScI, 29 Mar The Local Group project Objective – Understand dust production of evolved stars as a function of metallicity Method – Use the Infrared Spectrograph on Spitzer to study carbon stars in nearby dwarf spheroidal galaxies The great simplification – Treat each of these complex systems as having a uniform metallicity Results – Sloan et al. (2012, ApJ, submitted)

STScI, 29 Mar Samples and metallicities Galaxy [Fe/H] ~ 0 Large Magellanic Cloud ~ –0.3 D = 50 kpc Small Magellanic Cloud ~ – kpc Fornax dSph ~ – kpc Sculptor dSph ~ – kpc Leo I dSph ~ – kpc Carina dSph ~ – kpc

STScI, 29 Mar M bol  mass  age  [Fe/H] Right: Fig. 14 from Revaz et al. (2009), based on evolutionary models Fornax – Most targets are younger than ~3 Gyr –Metallicities most like SMC and LMC Sculptor – Both targets are <2 Gyr old – [Fe/H] ~ –1.0

STScI, 29 Mar A carbon star IRAS (V1187 Ori) Szczerba et al. (2002)

STScI, 29 Mar Local Group spectra These targets are faint! Need Cornell’s optimal extraction algorithm (Lebouteiller et al. 2010) 10,000 extracted spectra publicly available:

STScI, 29 Mar Manchester Method Total warm amorphous carbon content Measured by the [6.4] – [9.3] color Need outflow velocity, gas-to-dust ratio to get mass-loss rate Calibrated with radiative transfer models (Groenewegen et al. 2007) Gaseous acetylene absorption strength at 7.5  m SiC dust emission strength at 11.3  m Introduced by Sloan et al. (2006) and Zijlstra et al. (2006) Applied to large comparison samples from the Galaxy, LMC, and SMC

STScI, 29 Mar Metallicity diagnostics Fornax follows the SMC (as expected) Sculptor and Leo I are (mostly) in the upper left MAG 29 in Sculptor is off-scale, with EW = 0.8  m and no SiC! (But even that can’t account for the expected free carbon) In more metal-poor samples: Acetylene bands strengthen SiC dust emission weakens Leads to a metallicity gradient in the figure SMC… LMC… Milky Way

STScI, 29 Mar Total mass-loss rates [6.4]–[9.3] scales with dust opacity (aka dust content) Multiply by outflow velocity to get dust-production rate Multiply by gas-to-dust ratio to get total mass-loss rate

STScI, 29 Mar Carbon-rich dust content Pulsation periods from the SAAO Fornax: Whitelock et al. (2009) Sculptor: Menzies et al. (2011) Leo I: Menzies et al. (2010) Their work is the key to making these comparisons possible Dust content increases with pulsation period Metallicity has little obvious influence

STScI, 29 Mar A closer look We may be seeing a decrease in dust content at the lowest metallicities Sculptor and Leo I are below the fitted line, at a 3.6  level (The Fornax data are consistent with our assumed metallicity)

STScI, 29 Mar C/O and metallicity After formation of CO molecules Assume C i scales with Z Assume  C independent of Z O = O i does depend on Z [O/Fe] = –0.25 [Fe/H] for –1.5 < [Fe/H] < 0.0 Melendez & Barbuy 2002, Fig. 5

STScI, 29 Mar Expected free carbon Take (C/O) ⊙ = 0.54 and  C = 0.56 O ⊙ Galaxy[Fe/H]C/OC free /C ⊙ Milky Way LMC– SMC– Sculptor– Four times more free carbon in Sculptor than the Milky Way? It’s not in the dust! And it’s not in the C 2 H 2

STScI, 29 Mar Consequences Observation: Little change in amorphous carbon dust content with metallicity (Z) But we expect much more free carbon at low Z –Because the 3  sequence and dredge-up should not depend on Z, and there’s less O to make CO Conclusion: The dredge-up must be truncated Consequence: When the free carbon exceeds some threshold, it triggers a superwind, which strips the envelope, ends life on the AGB, and produces a PN

STScI, 29 Mar Consequences 2 The mass-loss history and lifetime on the AGB will determine what a star can produce and inject back into the ISM

STScI, 29 Mar The End