lots of dust from massive galactic wr stars tony moffat – univ. de montréal sergey marchenko –...
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Lots of Dust from Massive Galactic WR
StarsTony Moffat – Univ. de Montréal
Sergey Marchenko – Science Systems and Applications Inc., Lanham, MD
M1-67/WR124
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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)
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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 (?)
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WR140 (WC7pd + O5.5fc) = the Rosetta Stone of massive binary systems (P = 8 ans, e = 0.9) with strong colliding winds
Marchenko et al. (2003)
Fahed et al. (2011)
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Episodic dust formation during periastron passage of WR140
Marchenko & Moffat (2007)
Williams et al. (1997)
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Optical light curves of WR140:
o quiescence
rapid var.
arrow dust <a> = 0.07 m
Marchenko et al. 2003
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Among WC9 stars, direct orbital motion is rarely seen. The link between WC9d stars and binarity often comes from ``pinwheel``, images, e.g.:
WR104, WC9d, P = 220 d (Tuthill et al. 1999)
WR112, WC9d, P = 12 a (Marchenko et al. 2002, 2007)
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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)
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… and more pinwheels elsewhere, too, e.g. here in the Quintuplet Cluster near the Galactic center (Tuthill et al. 2006)
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… another example: WR48a, WC8d, Gemini/S MIR – a rather spectacular case (Marchenko & Moffat 2007):
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Recent analysis of WR48a IR light-curves (Williams et al. 2011) Evolution of dust emission:
- Rel. slow variation with P ~ 32 a- Secondary short episodes (no periodicity)- Rate of fall faster for shorter (as WR140) formation & cooling of dust
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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, process poorly understood)
2. Growth of grains by accretion of 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
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SED model (opt. thin) of the mini-eruption 1994-5:
4 d^2 F_ = M_d _ B(, T_g), where:
B = Planck functionT_g = grain temperature_ = grain emissivityd = our distanceM_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
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Mean of 5 pinwheels (WR48a, 98a, 104, 112, 118)
PAH template (high-ionization bar in the Orion HIIR)
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.)
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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)
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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.)
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Just a small aside about …
The Humble Space Telescope
... not to be confused with another HST (m b)
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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!
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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:
1.Shara et al. (2009, 2012) – using narrow-band line photometry2.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
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Conventional sources of stars dust in the Galaxy (Dweck 1985)
AGB
RG ~ 10-3 M/yr each
Novae
SNe
PNe ~ 10-4 M/yr each
Protostars
PN: Egg nebula
Conclusion
… and now add WCd stars with similar contributions!
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… 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