Dust Formation, Propagation and Survival in the ISM. Ant Jones. Institut d Astrophysique Spatiale Orsay, FRANCE

Transcription

1 Dust Formation, Propagation and Survival in the ISM Ant Jones Institut d Astrophysique Spatiale Orsay, FRANCE

2 INTRODUCTION

3 The Life-cycle of Interstellar Dust

4 DUST (1) formation in CS environments (2) formation in the ISM (3) survival in the ISM (4) propagation in the ISM

5 Presolar grains from meteorites

6 composition typical sizes abundance stellar source nanodiamond 3 nm 1400 ppm type II SNe? SiC (mainstream) µm 14 ppm C-rich AGB stars graphite µm 10 ppm type II SNe, W-R stars corundum (Al 2 O 3 ) µm 0.1 ppm RGB/AGB stars SiC (X grains) µm 0.1 ppm type II SNe Si 3 N 4 ~1 µm 10 ppb type II SNe spinel (MgAl 2 O 4 ) 1-5 µm 3 ppb RGB/AGB stars hibonite (CaAl 12 O 19 ) 1-5 µm 1 ppb RGB/AGB stars amorphous silicate ( GEMS-like ) crystalline silicate ( forsterite, olivine, pyroxene) µm µm 5 grains 8 grains RGB/AGB stars RGB/AGB stars Messenger et al. (2003); Mostefaoui & Hoppe (2004)

7 A comparison of IS, CS and presolar grains

8 interstellar aromatic hydrocarbons aliphatic hydrocarbons amorphous silicates [Mg & Fe oxides] circumstellar aromatic hydrocarbons aliphatic hydrocarbons [nano]diamond amorphous silicates [aluminosilicate] Mg-rich crystalline silicates ( forsterite, Mg 2 SiO 4 enstatite, MgSiO 3 diopside, CaMgSi 2 O 6 ) oxides (Fe 0.9 Mg 0.1 O, spinel, MgAl 2 O 4 corundum, Al 2 O 3 ) β-sic pre-solar [aromatic hydrocarbons] graphite Nanodiamond amorphous silicates (GEMS) 5 grains known Mg-rich crystalline silicate ( forsterite, Mg 2 SiO 4 only 1 grain! ) olivine (3 grains) pyroxene (4 grains) crystalline oxides ( spinel, MgAl 2 O 4 hibonite, CaAl 12 O 19, corundum, Al 2 O 3 ) β-sic TiC (as inclusions in graphite) Si 3 N 4

9 (1) DUST FORMATION IN CS ENVIRONMENTS STARDUST

10 Jones et al. (1997) Sources of stardust Stardust is primarily formed around stars in their dying throes

11 Stardust formation timescales Red giant/asymptotic giant branch star winds total mass input to the ISM ~ 0.5 M Sun yr -1 if 1% of this mass is in the form of dust time to replenish all ISM dust ~ (5 x 10 9 M Sun x 0.01) / (0.5 M Sun x 0.01) ~ yr SN ejecta assuming dust forms here! typical SN 4 M Sun of heavy elements ISM assuming (i) 25% O + requisite Si, Mg & Fe silicates (ii) dust formation is 50% efficient (iii) SN rate ~ 1/30 yr -1 time to replenish all ISM dust ~ (5 x 10 9 M Sun x 0.01) / (4 M Sun x 0.25 x 0.01)/30 ~ 3 x 10 9 yr

12 Stardust formation timescales Stardust formation around evolved stars, including efficient supernova dust formation, indicates a dust formation time scale of t formation 3 x 10 9 yr (Jones & Tielens 1994, Dwek 1998)

13 THE DETAILS OF DUST FORMATION

14 HST: Sahai et al. (1998) HST: Balick et al. (2001) Circumstellar shells AGB stars proto-planetary Nebulae Planetary Nebulae

15 after Patzer (2004) Stardust formation in CS shells ambient ISM PLASMA COOLING FLOW cooling & expansion PLASMA dust clusters molecules atoms ions DUST FORMATION WINDOW DUST NUCLEATION & GROWTH ambient ISM

16 Dust formation in circumstellar environments Dust condenses in the gas phase at low pressure gas density ~ cm -3 ( 1 atm. ~ cm -3 ) in the expanding and cooling gas shells around stars velocity ~ 10 km/s, T ~ K The dust composition is determined by the C/O ratio C/O > 1 carbon-rich grain materials C/O < 1 oxygen-rich grain materials Homogeneous nucleation theory does not apply Condensation is driven by critical nucleii formation Ionisation-driven processes may drive grain nucleation

17 Elemental abundances C, O > Mg, Si, Fe > Na, Al, Ca, Ni Palme & Jones (2004)

18 Dust condensation as a function of C/O N.B. No amorphous materials Courtesy of Denton Ebel (published in JGR 105, A5, 2000)

19 O-rich dust sources

20 AGB star - disc sources Thermal emission from dust

21 AGB star - outflow sources Thermal emission from dust

22 AFGL 4106 an OH/IR star supergiant in a binary system dust continuum Thermal emission from dust Molster et al. (1999)

23 AFGL 4106 an OH/IR star supergiant in a binary system dust continuum continuum-subtracted thermal emission Molster et al. (1999)

24 AFGL 4106 an OH/IR star supergiant in a binary system dust continuum olivine pyroxene Molster et al. (1999)

25 Silicate dust around OH/IR stars forsterite diopside enstatite + H 2 O (s) forsterite diopside enstatite + H 2 O (s) Demyk et al. (2000)

26 Oxide/silicate dust condensation sequence Demyk et al. (2000)

27 Dust composition as a function of mass loss rate around O-rich O evolved stars (RGB/AGB/PN) stellar mass loss rate [ M _Sun yr -1 ] dust composition oxides (Fe 0.9 Mg 0.1 O, spinel, MgAl 2 O 4 corundum, Al 2 O 3 ), amorphous silicates amorphous silicates after Waters (2004) a minor component observed in emission 10-5 > 10-5 PNe (e.g. NGC 6302) amorphous silicates, crystalline silicates (olivine & pyroxene) H 2 O ice diopside (CaMgSi 2 O 6 ), calcite (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ) self-absorbed Fe-poor 5-15 % of dust mass also in RL OH/IR stars Origin??? also observed < 1 % of cold dust mass

28 C-rich dust sources

29 HST view of the dust shells around the C-rich C AGB star - IRC Mauron & Huggins (2000)

30 after Waters (2004), Jura (2004) Dust composition around C-rich C evolved stars ( e.g. IRC ) V` = 15 km s 1 M = 3 x 10 5 M Sun yr 1 grain radius generally ²0.1 µm Dust composition around Carbon stars SiC 11.3 µm band (²10 % of the dust mass) MgS `30 µm band a-c & a-c:h (HAC), PAHs bands in the 3-15 µm region (aliphatic/aromatic mix) SiS 2, TiC, PAHs, nanodiamonds source of the `21 µm band?

31 Mixed messages!

32 Mixed O-rich O and C-rich C dust in the same object Appears to be relatively common Cause? - rapid stellar chemical evolution - binarity

33 (2) DUST FORMATION IN THE ISM

34 Elemental depletions in the ISM Palme & Jones (2004)

35 Is accretion in the ISM selective? We would, naively (?), expect an accreted material in the ISM to be a messy amalgam of all atoms! Nature does not always work as expected; she seems to have found a way to selectively accrete There seems to be some chemically selective process at work in the ISM leading to the re-formation of separate carbon-rich and oxygen/metal-rich silicates/oxides, e.g., photo- or chemi-sputtering of heteroatoms (Draine, 1990)

36 AN EXAMPLE?

37 The accretion of carbon in the ISM PROCESSED H-poor INITIAL H-rich GAS n³1,m³0 HAC (hydrogenated amorphous carbon) a-c:h Jones et al. (1990)

38 Jones et al. (1990) The structure of hydorgenated amorphous carbon [ a-c:h a ] H/(C+H) = 0.35 ( ) H/C = 0.54 sp 2 /sp 3 = 0.37

39 a-c:h in the ISM? lab. data SST lab. data Galactic centre VLT-ISAAC ISOPHOT a-c:h formed via low-temperature deposition of CH 4 with accompanying photolysis. The a-c:h is H-rich - H/(C+H) > 0.5 Dartois et al. (2004)

40 (3) DUST SURVIVAL IN THE ISM

41 The effects of shocks on dust Acceleration of gas and dust. Energetic atom/ion-grain and grain-grain collisions. Acceleration opposed by collisional and plasma drag: drag (a ) -1 i.e., small and low-density/porous grains stop fast. Dust destruction and dust size distribution changes.

42 Post-shock shock grain size distribution Grain volume / H nucleus initial MRN size distribution 50 km/s 100 km/s 150 km/s 200 km/s 5 nm 250 nm Jones (2004)

43 Grain survival in IS shocks Fraction of dust surviving grains a > 100 nm silicate carbon total dust mass Shock velocity [ km s -1 ] after Jones et al. (1996)

44 Grain survival in IS shocks Fraction of dust surviving grains a > 100 nm grains a > 50 nm silicate carbon total dust mass Shock velocity [ km s -1 ] after Jones et al. (1996)

45 (4) DUST PROPAGATION IN THE ISM

46 The effects of shocks on dust Acceleration of gas and dust Energetic atom/ion-grain and grain-grain collisions Acceleration opposed by collisional and plasma drag Dust destruction and dust size distribution changes Dust destruction is < 100% efficient, therefore a significant fraction of dust survives and is transported through the ISM by SN-generated shock waves The transported grains are small ( < 50 nm )

47 CONCLUSIONS

48 In the Galaxy stardust formation is dominated by the input from evolved stars ( RGB/AGB, & SNe? ) Stardust includes: aromatic and aliphatic hydrocarbons, amorphous hydrocarbons, graphite, (nano)diamond, amorphous and crystalline silicates, oxides, carbides and a nitride dust propagates into the ISM via stellar winds and is propagated through the ISM on large scales by SNe The dust destruction time scale is shorter than the dust injection time scale and the grain fragmentation time scale is even shorter. Dust is re-formed in the ISM by selective accretion and large grains must be re-assembled from smaller fragments.

49 THE END THE END of stars is the beginning of stardust