Superbubbles, Wolf-Rayet Stars, and the Origin of Galactic Cosmic Rays

W.R. Binns, M.H. Israel, L.M. Scott: Washington University

M.E. Wiedenbeck: Jet Propulsion Laboratory

A.C. Cummings, J.S. George, R.A. Leske, R.A. Mewaldt, E.C. Stone: Caltech

T.T. von Rosenvinge: Goddard Space Flight Center

M. Arnould, S. Goriely: Institut d’Astronomie et d’Astrophysique, Bruxelles

Outline

• Introduction—Cosmic Ray Source models» Superbubbles formed from OB associations as possible source of galactic

cosmic rays

» Wolf-Rayet (WR) Stars • as source of enhancement of certain isotopic ratios: e.g. 22Ne/20Ne, 58Fe/56Fe

• The CRIS experiment» Instrument

» Isotopic measurements

• WR component as tracer of galactic cosmic ray source» Comparison of data with WR model calculations

• Suggested scenario for cosmic ray origin• Conclusions

Cosmic Ray Source?

• Stellar atmosphere injection (e.g. Meyer, Shapiro)» Low-FIP elements enhanced (as in the solar corona).

• Interstellar grain source (Most recently Meyer et al.)» Refractory elements enhanced

» Mass dependence for volatile elements

• Acceleration of material in superbubbles by SN shocks

• Higdon et al. ApJ To be pub., Aug. 2005; ApJ 590 (2003) 822; ApJ 509 (1998) L33; Lingenfelter et al. ApJL 500 (1998) L153.

• Streitmatter et al. A&A 143 (1985) 249.

» Supernova material

» Wind material from massive stars

Superbubbles & Supernovae• Superbubbles blown by stellar winds & SN in OB associations• Superbubble size: ~100-1000 pc• The majority of core-collapse SN (80-90%) in our galaxy occur in superbubbles (Higdon & Lingenfelter).

• Mean time between SN within OB assoc.~106y• SN shocks accelerate ambient superbubble material

Superbubble in Perseus ArmSuperbubble (N 70) in the Large Magellanic Cloud (ESO-VLT image)

~100 pcdiameter

Wolf-Rayet Stars

• Evolutionary phase of massive O & B type stars exist primarily in OB associations

• WR Mass—15-45 M⊙ • High velocity surface winds (~1,000-

4,000 km/s) eject material into the ISM

• Often are dusty and ~>60% are binaries—puzzle how dust can exist in such a hot environment

• Two phases—WN and WC» WN--CNO processed material is

ejected with production of high 13C/12C and 14N/16O ratios

» WC--Wind enrichment of He-burning products: esp. C, O, and 22Ne through reaction 14N(,)18F(e+)18O(,)22Ne

WR-124 in Sagittarius—Hubble Image

WR-104 in Sagittarius—Keck Telescope Image

Diam~0.2pc

Diam~200au

• Evolution of surface abundances (mass fraction) with stellar mass for 60M⊙ star

(Meynet & Maeder, 2003)

Time evolution of WR abundancesNon-rotatingstar

RotatingStar300 km/sat equator •Top curve—total mass; Bottom

curve—convective core mass

•2D models—van Marle

Time evolution of mass

Non-rotatingStar

Rotating star

Cosmic Ray Isotope Spectrometer (CRIS)

• Large geometrical factor of CRIS (~50 x previous instruments)

• Excellent mass resolution enables precise identification of abundances.

• Statistical sample is large enough so that the energy spectra of the Neon isotopic ratios (important ratios as will be seen later) have been obtained

CRIS GCR Isotopic Measurements

Source Abundances & Tracer Isotopes

• To obtain source abundances from measured abundances, use “tracer” method (Wiedenbeck & Stone)

• Use secondary isotopes to “subtract” the secondary component of isotopes that are predominantly primary

•Two component models•Wolf-Rayet winds from stars with various initial masses, with and without rotation.•Adjust the WR fraction mixed with ISM to match CR 22Ne/20Ne.(Goriely, Arnould & MeynetModeling)

“Combined” data points (red) are mean values of ratios from Ulysses, Voyager, ISEE-3 and HEAO-3-C2

Model WR Fraction

M60-no rot 0.20

M85-no rot 0.12

M120-no rot 0.16

M40-rot 0.22

M60-rot 0.16

M85-rot 0.41

M120-rot 0.35

Fraction of WR materialmixed with ISM with solarsystem composition tonormalize to 22Ne/20Ne ratio

300 km/s

But what about the 14N/16O and N/Ne ratios???

Volatility & mass fractionated GCR source abundances

• Meyer et al., 1997 model—Refractory elements are enriched in GCRs since they sputter off accelerated dust grains preferential acceleration (~factor of 13 enhancement)» Additionally, even for volatile elements, there appears to be a mass bias for which

they estimate a mass dependency of A0.80.2

• Ratios need to be corrected for these effects.• Oxygen

» Volatile in elemental or molecular form» But 23% is estimated to reside in refractory compounds in the ISM (e.g. silicates)

(K. Lodders, 2003)

• Nitrogen» Exists primarily as a gas in space

• Carbon» Refractory in elemental form» But a poorly known fraction exists in volatile molecules (e.g. CO) in space.

• Neon» Entirely volatile

GCR source abundances compared with WR model corrected for volatility and mass fractionation (open symbols)

Suggested Scenario

• WR star ejecta, enriched in 22Ne and other neutron-rich isotopes, mixes within the superbubble (Higdon & Lingenfelter) with» Ejecta from core-collapse SN» Average ISM (represented by solar-system abundances)

• Refractory elements must exist mostly as grains and volatile elements mostly as gas.

• SN shocks accelerate mix of material in SB to cosmic ray energies» Grains are preferentially accelerated (Ellison et al.)

• Mean time between SN events in SB is ~3-35 x 105 y (Schaller et al. 1992)» Sufficient time for 59Ni to decay to 59Co

Summary

• CRIS measurements have led to an improved value 22Ne/20Ne, 58Fe/56Fe, and other isotope ratios useful for identifying a WR component in GCRs.

• Comparison of CRIS and other data show » the three isotope ratios predicted to be most enhanced in WR

models, 12C/16O, 22Ne/20Ne, and 58Fe/56Fe, are indeed enhanced in the cosmic rays.

» Those for which enhancement is not predicted are consistent with solar system abundances, provided volatility and mass fractionation corrections are applied

Summary (cont)

• We take agreement as evidence that WR star ejecta is likely an important component of cosmic-ray source material.

• Since most WR stars & core-collapse SN reside in SBs, then SBs must be the predominant site of injection of WR material and SN ejecta into the GCR source material.

• Picture that emerges is that SBs appear to be the site of origin and acceleration of at least a substantial fraction of GCRs.