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Supernova Blastwaves and Pre-Supernova Winds: Their

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Supernova Blastwaves and Pre-Supernova Winds: Their SUPERNOVA BLASTWAVES AND PRESUPERNOVA WINDS THEIR COSMIC RAY CONTRIBUTION Peter L Biermann Max Planck Institut f urRadioastronomie Bonn Germany Sho cks in stellar winds can accelerate particles energetic particles are ob served through nonthermal radio emission in novae OB stars and Wolf Rayet stars Sup ernova explosions into predecessor stellar winds can lead to parti cle acceleration which we suggest can explain most of the observed cosmic rays of the nuclei of Helium and heavier elements from GeV in particle en 9 ergies up to near GeV as well as electrons ab ove ab out GeV We go through the following steps to make the case Using a p ostulate for an underlying principle that leads to transp ort co ecients in a turbulent plasma we derive the prop erties of energetic particles accelerated in spher ical sho cks in a stellar wind We suggest that a dynamo working in the inner convection zone of an upp er main sequence star can lead to high mag netic eld strengths which may b ecome directly observable in massive white dwarfs massive red giant stars and Wolf Rayet stars Such magnetic elds may put additional momentum into stellar winds from the pressure gradient of the toroidal eld with reduced angular momentum loss We use the statistics of Wolf Rayet stars and radiosup ernovae to derive a lower limit for the magnetic eld strengths This limit gives supp ort to the wind driv ing argument as well as the derivation of the maximum particle energy that can b e reached From a comparison of the radioluminosities of various stars radio sup ernovae as well as sup ernova remnants there app ears to b e a critical Alfvenic Machnumber for electron injection With this concept we prop ose an explanation for the observed protonelectron ratio in galactic cosmic rays at GeV energies We check the mo del prediction quantita tively on cosmic ray sp ectrum and chemical comp osition against airshower data from a Akeno b a world data set c Flys Eye and d against further cosmic ray data available from other exp eriments Finally we summarize various imp ortant caveats and outline imp ortant next steps as well as checks astro-ph/9501030 10 Jan 95 as regards the implications of these concepts for stars and stellar evolution I Introduction Most sup ernovae are explosions of massive stars see eg Wheeler often stars that have a stellar wind prior to the explosion Thus the physics of these stellar winds b ecomes imp ortant for a discussion of what happ ens when the star explo des and a sho ckwave travels down such a stellar wind Optical and Xray data have b een interpreted as due to sho ck structures and sho ckheating eg Owocki Obser PL BIERMANN vationally there is also evidence for sho ckwaves in stellar winds prior to the explosion such as suggested by nonthermal radio emission from OB and Wolf Rayet stars as well by the nonthermal radio emission from the nova GK Per Seaquist et al An interpretation of this nonthermal radio emission is an imp ortant test for any theory of particle acceleration in sho cks in stellar winds In this chapter we will review recent work on the acceleration pro cess in sho ck waves in stellar winds and argue that a large part of the observed cosmic rays can b e attributed to sup ernova sho cks in stellar winds A Stellar winds Stellar winds are observed in many cases low mass stars such as the Sun as well as high mass stars such as OB stars For the latter the wind driving can b e explained as an eect of radiation pressure Lucy Solomon Castor et al Pauldrach et al Owocki For Wolf Rayet stars the momentum in the wind is generally b elieved to b e to o large to b e explainable as due to radiation driving in the limit of single scattering and so as one p ossible solution multiple scattering mo dels have b een devised Magnetic elds have b een argued to contribute to the driving through the fast magnetic rotator concept Cassinelli this idea however has b een criticized on the basis that the corresp onding large angular momentum loss would lead to a severe selflimitation of the pro cess Nerney Suess and that therefore the pro cess could not b e general There is a mo died magnetic rotator mo del for which the Alfvenicsurface is close to the stellar surface and hence the angular momentum loss is strongly reduced Biermann Cassinelli In this case the lifetime is not limited by angular momentum loss but an initial driving of the wind is required and the pressure gradient of the tangential magnetic eld is argued to provide an amplication of the momentum of the wind B Cosmic Rays After the discovery of cosmic rays by Hess and Kohlhorster Baade Zwicky already prop osed that sup ernova explo sions pro duce cosmic rays Alfven argued early for a lo cal origin in our Galaxy which is conrmed by the age determinations of the cosmic rays GarciaMunoz et al Fermi prop osed the basic concept of acceleration still b eing used Shklovskii and Ginzburg made a convincing case for particle acceleration in sup ernova remnants Ginzburg already emphasized the inter esting role of novae and indeed the nova GK Per is a test case for the evolution of sho cks in winds and their particle acceleration Hayakawa prop osed that stellar evolution gives rise to an enhancement of heavy elements and p ointed out the imp ortance of spallation in the in STELLAR WINDS SNe and CRs terstellar medium and again the enrichment found in the cosmic ray contribution from sup ernova explosions into winds is indeed b elieved to day to b e due to just this enrichment And nally Co cconi al ready argued convincingly that the most energetic cosmic ray particles are from an extragalactic origin the GRO observations Sreekumar et al of a neighboring galaxy the Small Magellanic Cloud provided the last and a very strong argument that indeed the cosmic rays in the lower energy range are not universal and thus have to b e galactic An early seminal form of some of the ideas expressed in the following can b e found in Peters A brief historical review is given by Ginzburg The form of Fermis pro cess used to day was discov ered nearly simultaneously by Axford et al Bell a b Blandford Ostriker and Krymskii In this picture the main particle energy gain is from rep eated scattering o magnetic ir regularities on the two sides of a sho ck front Since those two sides can b e considered as a continuously contracting system relative to each other particles that remain in the system gain energy There are many imp ortant reviews and b o oks on cosmic ray phys ics We just mention the classical b o ok by Hayakawa the new b o ok by Berezinsky et al and the reviews by Drury Blandford Eichler and Jones Ellison Today there are only some well accepted arguments ab out the ori gins of cosmic rays a The cosmic rays b elow ab out GeV are b elieved to b e predominantly due to the explosion of stars into the normal interstellar medium Lagage Cesarsky b The cosmic rays from near GeV up to the knee at GeV are likely pre dominantly due to explosions of massive stars into their former stellar wind Volk Biermann c While this latter claim is not undis puted there has b een certainly no agreement yet on the origin of the cosmic rays of higher particle energy Direct observation of cosmic rays either from goundbased instru mentation from satellites or from ballo ons has b een the driving input for nearly all considerations in cosmic ray work These data demon strate that i the overall sp ectrum of cosmic rays is a p owerlaw up to an energy commonly referred to as the knee near GeV to continue with a steep er p owerlaw to the ankle near GeV with a slight turnup b eyond and an apparent cuto near GeV ii the sp ectra are ab out E for Hydrogen at mo derate energy and slightly atter for Helium and heavier elements while the electron sp ectrum is con sistent with the Hydrogen sp ectrum at low energy and then changes over to ab out E iii the chemical comp osition at low energy is crudely similar to that of the interstellar medium with Hydrogen and Helium underabundant relative to Silicon All such prop erties require explanation We will assume in the following that the correction from the ob PL BIERMANN served cosmic ray sp ectrum to the source sp ectrum is a change in sp ec tral slop e by exactly for relativistic particles see Biermann a a b c so that we are lo oking at source sp ectra of approximately E b elow the knee and of approximately E ab ove the knee This can b e argued on the basis of a Kolmogorov sp ectrum of the irregular ities of the interstellar magnetic eld from plasma simulations from analogies with in situ measurements of the solar wind and other ob servations of the interstellar medium see eg Biermann a It is clear however that a Kolmogorov sp ectrum of interstellar turbulence is insucient to explain in a straightforward manner eg the abun dance ratios in cosmic rays see eg GarciaMunoz et al and Biermann c Such prop erties of cosmic rays may require a deep er understanding of the interstellar medium than we currently have How ever we note that the secondary to primary ratio of spallation pro ducts in cosmic rays do es not simply yield a sp ectrum of interstellar turbu lence in a medium which varies on time scales equivalent to those of cosmic ray transp ort and which has known inhomogeneities of density contrasts of many p owers of ten C Sho cks in winds There are various kinds of evidence for the presence of sho ck struc tures in stellar winds from in situ observations in the solar wind to optical line proles to Xray emission from massive stars
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