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Magnetic fields in cool objects

Understand the significance of magnetic fields in stellar physics, focusing on non-thermal radio emission, jets in young stellar objects, and solar activity. Learn about magnetic field evolution and technical requirements for measurements.

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Magnetic fields in cool objects

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  1. Magnetic fields in cool objects Sofia Randich INAF-Osservatorio di Arcetri

  2. OUTLINE • Why magnetic fields? • Some scientific cases • How – Why near-IR? • Previous work • Technical requirements

  3. Why magnetic fields? Crucial role in stellar physics across the entire HR diagram • Non-thermal radio emission in early-type stars • Jets in young stellar objects • Activity in the Sun, solar-type, and lower mass stars (heating of the upper atmospheres) • Mass loss and angular momentum evolution • Li depletion/preservation? (D’ Antona et al. 2000)

  4. Coronal activity vs. age M dwarfs with Mass > 0.3 MSun Median luminosities: Alpha Per: log Lx=29.00 erg/s Pleiades: log Lx=29.03 erg/s Hyades: log Lx=28.37 erg/s (Randich 1999) What about the evolution of magnetic field strength and filling factor? And the rotation – B field relation?

  5. Saturation Hyades M dwarfs Stauffer et al (1997b) Saturation of B? Saturation of f? Why two branches? Stauffer et al. (1997a)

  6. Activity in very cool objects Rotation-activity relation appears to break (and even to reverse) for very late-M and L-type dwarfs (Basri 2000, Gizis et al. 2000), though with exceptions (e.g., Berger 2002; Schmitt & Liefke 2002; Liebert et al. 2003) • Insufficient conductivity due to low ionization level in the photoshere? • Too low Rossby numbers (dynamo unable to operate)? • Large scale, relatively stable field?

  7. How can one measure B fields? Zeeman effect: 1) Circular polarization of magnetically sensitive lines 2) Zeeman broadening of magnetically sensitive lines: Split of σ components: Δλ=kλ2gB (note the dependence on λ2) F(λ)=FB(λ)*f + FQ(λ)*(1-f)

  8. Advantages of near-IR Higher accuracy, lower fields • ΔλB λ2 • Δλdopp  λ • Continuous opacity lowest in H band Lines form deeper in the atmosphere Stronger B • Line density lower Near-IR mgnetically sensitive lines: Fe I @ 1.565 μ, Ti I @ 2.2 μ, For VLMs and BDs FeH most promising candidate (but Lande factors Need to be empirically calibrated)

  9. Previous work Very little (for cool stars): • First measurement AD Leo – B=3.8±0.3 KG, f=77 (Saar & Linsky 1986) • A few dMe stars (mostly from optical spectra) with very strong B (a few KG) (Johns-Krull 2000 and ref. therein) • Ε Eri (near-IR, Johns-Krull & Valenti 1996) • A few PMS stars (Johns-Krull 2000 and ref. therein) • A K3 Pleiades member (near IR – Valenti & Johns-Krull 2003, 2004)

  10. Technical requirements **S/N** **Resolution** B=1.5 KG: 2Δλ=0.1nm For the Fe I 1.565 μ **Large spectral coverage ** Saar (1988) M dwarfs in the Hyades: H~9-11; Field dwarfs later than M9: MH=10.9-12.1 (Zapatero Osorio et al. 1997)

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