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How Stars Die: Infrared Spectroscopy of Dusty Carbon Stars in the Local Group

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. Getting from there to here. The Local Group project.

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How Stars Die: Infrared Spectroscopy of Dusty Carbon Stars in the Local Group

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  1. 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 2012

  2. Getting from there to here STScI, 29 Mar 2012

  3. 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) http://isc.astro.cornell.edu/~sloan/library/ STScI, 29 Mar 2012

  4. Samples and metallicities Galaxy [Fe/H] ~ 0 Large Magellanic Cloud ~ –0.3 D = 50 kpc Small Magellanic Cloud ~ –0.7 60 kpc Fornax dSph ~ –0.3-0.8 150 kpc Sculptor dSph ~ –1.0 87 kpc Leo I dSph ~ –1.4 280 kpc Carina dSph ~ –1.7 100 kpc STScI, 29 Mar 2012

  5. Mbol 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 2012

  6. A carbon star IRAS 05373-0810 (V1187 Ori) Szczerba et al. (2002) STScI, 29 Mar 2012

  7. Local Group spectra • These targets are faint! • Need Cornell’s optimal extraction algorithm (Lebouteiller et al. 2010) • 10,000 extracted spectra publicly available: http://cassis.astro.cornell.edu STScI, 29 Mar 2012

  8. Manchester Method Introduced by Sloan et al. (2006) and Zijlstra et al. (2006) Applied to large comparison samples from the Galaxy, LMC, and SMC 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 mm SiC dust emission strength at 11.3 mm STScI, 29 Mar 2012

  9. Metallicity diagnostics In more metal-poor samples: Acetylene bands strengthen SiC dust emission weakens Leads to a metallicity gradient in the figure SMC… LMC… Milky Way Fornaxfollows the SMC (as expected) Sculptorand Leo Iare (mostly) in the upper left MAG 29 in Sculptor is off-scale, with EW = 0.8 mm and no SiC! (But even that can’t account for the expected free carbon) STScI, 29 Mar 2012

  10. 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 2012

  11. Carbon-rich dust content Dust content increases with pulsation period Metallicity has little obvious influence 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 STScI, 29 Mar 2012

  12. 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 3.6s level (The Fornax data are consistent with our assumed metallicity) STScI, 29 Mar 2012

  13. C/O and metallicity After formation of CO molecules • Assume Ci scales with Z • Assume dC independent of Z • O = Oidoes depend on Z [O/Fe] = –0.25 [Fe/H] for –1.5 < [Fe/H] < 0.0 Melendez & Barbuy 2002, Fig. 5 STScI, 29 Mar 2012

  14. Expected free carbon Take (C/O)⊙ = 0.54 and dC = 0.56 O⊙ Four times more free carbon in Sculptor than the Milky Way? It’s not in the dust! And it’s not in the C2H2 STScI, 29 Mar 2012

  15. Consequences • Observation: Little change in amorphous carbon dust content with metallicity (Z) • But we expect much more free carbon at low Z • Because the 3a 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 2012

  16. Consequences2 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 2012

  17. The End STScI, 29 Mar 2012

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