Lots of Dust from Massive Galactic WR

StarsTony Moffat – Univ. de Montréal

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

• 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)

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 (?)

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)

Episodic dust formation during periastron passage of WR140

Marchenko & Moffat (2007)

Williams et al. (1997)

Optical light curves of WR140:

o quiescence

rapid var.

arrow dust <a> = 0.07 m

Marchenko et al. 2003

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)

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)

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

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

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

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

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

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.)

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)

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.)

Just a small aside about …

The Humble Space Telescope

... not to be confused with another HST (m b)

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!

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

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!

… 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