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Lots of Dust from Massive Galactic WR Stars

Lots of Dust from Massive Galactic WR Stars. Tony Moffat – Univ. de Montr éal Sergey Marchenko – Science Systems and Applications Inc., Lanham, MD. M1-67/WR124. Introduction Early discovery in the history of IR astronomy: Excess hot-dust emission from a variety of mass-losing stars

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Lots of Dust from Massive Galactic WR Stars

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  1. Lots of Dust from Massive Galactic WR Stars Tony Moffat – Univ. de Montréal Sergey Marchenko – Science Systems and Applications Inc., Lanham, MD M1-67/WR124

  2. Introduction • Early discovery in the history of IR astronomy: • Excess hot-dust emission from a variety of mass-losing stars • Among them: massive WR, espec. of subtype WC9 (8) + some WC+O binaries • Recall - massive stars > 20 Mo: • O  LBV  WN  WC  SNIc (sometimes GRB)  BH • or sometimes: … WN  SNIb  NS/BH • No dust (except LBV?) before WC (40% C !) • WN have ~1.5% N (no dust from N anyway)

  3. But … there is a problem: • How to form dust around a WR in such a hostile environment? • Near the star, the radiation field heats the grains to T >> T(evaporation) • Further from the star, where the radiation is sufficiently diluted, the wind density is too low (need a factor 1000 denser than WR winds) The solution? Wind collision in a WC+O system  Compression  Formation of amorphous-C dust grains Wind shocks lack sufficient compression (?)

  4. WR140 (WC7pd + O5.5fc) = theRosetta Stone of massive binary systems (P = 8 ans, e = 0.9) with strong colliding winds Marchenko et al. (2003) Fahed et al. (2011)

  5.  Episodic dust formation during periastron passage of WR140 Marchenko & Moffat (2007) Williams et al. (1997)

  6. Optical light curves of WR140: • o quiescence • rapid var. arrow  dust <a> = 0.07 m Marchenko et al. 2003

  7. Among WC9 stars, direct orbital motion is rarely seen. The link between WC9d stars and binarity often comes from ``pinwheel``, images, e.g.: WR112, WC9d, P = 12 a (Marchenko et al. 2002, 2007) WR104, WC9d, P = 220 d (Tuthill et al. 1999)

  8. WR112 Dust properties from multiband NIR images (Marchenko et al. 2002): • dM/dt (dust) = 6% of dM/dt (total)  10-5 M/yr • ~20% reaches ISM • <a> = 0.5  0.1 m (expect 0.01 m)

  9. … and more pinwheels elsewhere, too, e.g. here in the Quintuplet Cluster near the Galactic center (Tuthill et al. 2006)

  10. … another example: WR48a, WC8d, Gemini/S MIR – a rather spectacular case (Marchenko & Moffat 2007):

  11. Recentanalysis of WR48a IR light-curves (Williams et al. 2011)  Evolution of dustemission: • Rel. slow variation with P ~ 32 a • Secondary short episodes (no periodicity) • Rate of fallfaster for shorter (as WR140)  formation & cooling of dust

  12. 3 processes for the formation and evolution of dust in the winds of WC+O systems: 1. Nucleation of new grains in the compression zone (T_condensation~ 1200 K, processpoorlyunderstood) 2. Growth of grains by accretionof C ions (and thus more efficient cooling) 3. Cooling of the grains when the grains move to larger distance from the stars In the case of WR48a: continuous dust formation … other than in WR140

  13. SED model (opt. thin) of the mini-eruption 1994-5: 4 d^2 F_ = M_d _ B(, T_g), where: B = Planck function T_g = grain temperature _ = grain emissivity d = our distance M_d = dust mass Fit results: T_g =1200 K, dM/dt (dust) = 1.4 10^{-7} ~ 1 % of dM/dt of the WR star!! … and this is just a mini-eruption! WR48a

  14. ISO/SWS MIR spectra (van der Hucht et al. 1996) • non-shifted narrow IS absorptions + • strong, red-shifted CS emissions •  IS & CS PAHs side-by-side (Marchenko et al. in prep.) Mean of 5 pinwheels (WR48a, 98a, 104, 112, 118) PAH template (high-ionization bar in the Orion HIIR)

  15. CS PAHs prove that complex molecules can form in a harsh environment: • required H for PAHs comes from the companion’s wind • 6.2/7.6 mu PAH emissions  PAH clusters with N_C > 50 • 0.2 mu red-shifted PAH emissions due to high T >~ 1000K and freshly formed • PAHs ~0.5% of total dust content = low cf. PNe, HIIR, etc.. (low survival rate in WC+O – e.g. WR112)

  16. Can single WC9 (8) stars make C dust? Case of CV Ser… yes, a binary (WC8d + O8-9IV, P = 29d) BUT: MOST satellite  dM/dt (WR) increases by 70% over P = 29d, if due to electron scattering David-Uraz et al. (2012, subm.)

  17. Just a small aside about … The Humble Space Telescope ... not to be confused with another HST (m b)

  18. BUT if dust is being created continuously in CV Ser’s wind  • Take one dust grain with (grain) = N m_C/(4/3 a^3) ~ 2 gm/cc  • N ~ 4 10^8 C-atoms for a ~ 0.1 m. • Then grain X-section = Q  a^2 = 6 10^{-10} cm^2 for Q ~ 2,  optical. • Then for 2N free electrons before combining to neutralize N C++ ions, equiv. free-electron X-section = 2N _e = 5 10^{-16} cm^2 • i.e. ~10^6 x smaller than one grain! • Change in eclipse depth of CV Ser can easily be due to grain formation with negligible change in dM/dt! • … if grains can really form this way – BIG QUESTION!

  19. Bottom line overall: ~1% of total dM/dt in dust-forming WC stars (“dustars”) comes out in carbon dust, i.e. ~10^{-6} Mo/a per star • But how many “dustars” are there at any given time in the Galaxy? • Current NIR surveys for new Gal WR stars: • Shara et al. (2009, 2012) – using narrow-band line photometry • Mauerhahn et al. (2009, 2011) – using broadband photometry • Many new WC9 (8) stars, espec. in the central regions of the Galaxy • If N(dustars) = 100 - 1000  dM/dt (total dust) ~ 10^{-4} – 10^{-3} Mo/a

  20. Conclusion Conventional sources of stars  dust in the Galaxy (Dweck 1985) AGB RG ~ 10-3 M/yreach Novae SNe PNe ~ 10-4 M/yreach Protostars PN: Egg nebula … and now add WCd stars with similar contributions!

  21. … and in pop III of the early Universe: • Massive WC stars (in binaries?) • first sources of heavy elements, even before Supernovae • providing first building blocks for the formation of planets (?) END

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