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Cosmic Ray & Star Formation

Marco Fatuzzo,Fred C. Adams, and Fulvio Melia 2006. Cosmic Ray & Star Formation. 1. CR enhancement in SNR/MC environments. 2.How CR from SNR/MC influence ionization rate. 3.How CR influence star formation. Introduction. Previous works:

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Cosmic Ray & Star Formation

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  1. Marco Fatuzzo,Fred C. Adams, and Fulvio Melia 2006 Cosmic Ray & Star Formation

  2. 1. CR enhancement in SNR/MC environments 2.How CR from SNR/MC influence ionization rate 3.How CR influence star formation Introduction

  3. Previous works: • Evidence for the hadronic process has been found through the association of at least 10 EGRET sources with SNRs expanding into MCs. • The EGRET SNRs have inferred gamma-ray luminosities spanning Lr ∼ 1034 to 4x1036 erg s-1(30 MeV–10 GeV). • Particle acceleration in the SNR/MC environments can significantly enhance the cosmic-ray (CR) density above that of the local background “sea” surrounding these regions (Aharonian & Atoyan 1996, hereafter AA96; Torres et al. 2003). COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (1)

  4. Diffusion of CR continues until they undergo p-p scattering with the ambient medium, and thereby experience catastrophic losses, on a timescale given by Setting the τp-p equal to the diffusion timescale COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (2)

  5. estimate the distance R(E) that particles will diffuse into the cloud before scattering and losing their energy Adopt energy-dependent diffusion coefficient from Ormes et al. 1988 The length scale is estimated to be COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (3)

  6. weakly dependent on energy for relativistic protons and is given by • (Mannheim & Schlickeiser 1994). • corresponding cooling time • The cooling time is considerably longer than p-p collision COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (4)

  7. The resulting energy density uCR within the MC during the SNR/MC interaction will be • This value is about 103 times larger than the energy density of the local cosmic rays (where uCR ∼ 0.5 eV cm-3) • a significant CR enhancement of cosmic-ray energy density can be achieved in molecular clouds through their interactions with SNRs. COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (5)

  8. The number density of ionized particles in a MC environment depends on • the ionization rate (due to cosmic rays) • a complex recombination process. • The ionization fraction? • Assuming ζ scales linearly with the cosmic-ray energy density How CR from SNR/MC influence ionization rate(1)

  9. Notice that the ionization rate depends on the number density of cosmic rays, rather than the energy density Means: Ionization enhancement will decay on a comparably short timescale, as soon as the CR enhancement is removed due to p-p scattering. How CR from SNR/MC influence ionization rate(2)

  10. 1.ONE the ambipolar diffusion rate (and hence the rate of core formation) 2.TWO action of magnetorotational instabilities (MRIs) in circumstellar disks (and hence the rate of disk accretion). What the ionization fraction controls Effects on star formation(1)

  11. Effects on star formation(2) • ambipolar diffusion rate 1. Standard considerations: the effective diffusion constant D~VA2 /(γCρ) ∝ ζ-1/2 2. Current observations indicate that the diffusion process is too slow to account for the observed statistics of starless molecular cloud cores (e.g., Jijina et al. 1999) this mechanism provides a channel for supernovae to inhibit star formation

  12. Effects on star formation(3) • MRI • disk accretion is produced by an effective viscosity • that is driven by turbulence, which in turn is driven by • MHD instabilities such as MRI (Balbus & Hawley 1991). • 2. In order for MRI to operate, the ionization fraction • must be sufficiently high so that the gas is well coupled • to the field • So, cosmic-ray flux can have a substantial impact on • disk accretion.

  13. CR unenhanced cosmic-ray flux that is enhanced by a factor F The attenuation column density for cosmic-ray ionization is ∑0 ≈ 100 g cm-2 (Umebayashi & Nakano 1981). only the uppermost 100 g cm-2 of the disk can experience enough cosmic-ray ionization for MRI to operate the disk will experience cosmic-ray ionization down to a larger column density ∑* ≈ (1+ln F) ∑0. Consider the disk models of Gammie (1996) Effects on star formation(4)

  14. 3. The ionization fraction increased with CR flux 2. These cosmic rays can diffuse a distance R ∼ 10 pc into the MC before undergoing p-p scattering leading to catastrophic energy losses. • SNRs interacting with MCs can produce • substantial enhancement of the flux of CRs in • The cloud 4. cosmic-ray enhancement may have a significant impact on star formation processes, summary

  15. 3. CR can inhibit the core formation and accelerate Disk accretion. However, triger SF or not? 2. Should consider the X-ray ionization? • The method to estimate CR enhancement is • Proper? questions

  16. Thanks

  17. If produced via a pion-decay mechanism the gamma-ray luminosity of SNR/MC environments would peak at the value η is the fraction of a relativistic proton’s energy that goes into the 0 decay photon channel, can be estimated using the approximation for the gamma-ray luminosity due to the decay of neutral pions COSMIC-RAY ENHANCEMENT IN SNR/MC ENVIRONMENTS (2)

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