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 M Sun Median luminosities: Alpha Per: log L x =29.00 erg/s Pleiades: log L x =29.03 erg/s Hyades: log L x =28.37 erg/s What about the evolution of magnetic field strength and filling factor? And the rotation – B field relation? (Randich 1999)

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

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=kλ 2 gB (note the dependence on λ 2 ) F(λ)=F B (λ)*f + F Q (λ)*(1-f)

8. Advantages of near-IR  Δλ B  λ 2  Δλ dopp  λ Higher accuracy, lower fields PropertyVisible1-2.5μ g max 33 g max λ~1.6~5.2 Complete splitting 1.5-2 KG0.4-0.6 KG Profile fits0.8-1 KG0.25-3 KG Line ratios 0.3-0.5 KG~0.1 KG  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 Saar (1988) **Resolution** B=1.5 KG: 2Δλ=0.1nm For the Fe I 1.565 μ **S/N** **Large spectral coverage ** M dwarfs in the Hyades: H~9-11; Field dwarfs later than M9: M H =10.9-12.1 (Zapatero Osorio et al. 1997)