Observed B-fields in the ISM and their Roles in Forming Stars

1. Observed B-fields in the ISM and their Roles in Forming Stars Jongsoo Kim Korea Astronomy and Space Science Institute

2. Sunspot – Earth-sized Magnet • Photosphere (1st ) • ~ 100 Km above the photosphere (2nd) • Chromosphere; ~ 1000 Km above the photosphere (3th) Source: Astronomy Picture of the Day

3. How do astronomers measure magnetic fields in the interstellar medium? • Starlight (due to dust absorption) and IR (dust emission) polarizations • Faraday rotation • Synchrotron radiation (for external gals.) • Zeeman splitting

4. Dust Polarization dust emission dust absorption

5. Heiles & Crutcher 2005 Starlight polarization • The magnetic field is generally parallel to the plane of the Galaxy. • Polarization directions point to l~80 deg and l~260 deg, which is the orientation of the local spiral arm. • Bu/Br ~ 0.7 – 1.0

6. Crutcher et al. 2004 IR polarization • B=80mG estimated based a C-F method

7. Faraday Rotation linearly polarized EM wave = left-handed CP wave + right+handed CP wave Electrons gyrate with the Larmor frequency. e

8. Han et al. 1999 Bpara dist. from pulsar RMs • Local pitch angles: 18deg(stars), 13deg(gas), 8deg (B-field) • Reversals in the B field directions (underestimation of B-field) • Bpara ~ 1.4 mG +- 0.2mG near the Sun • Bu/Br ~ 0.3

9. Synchrotron Radiation • Synchrotron radiation at 408 MHz (Beuermann et al. 1985) • equipartition between B-field and CR energy densities • From the synchrotron polarization, Bu/Btot ~ 0.7 and Bu = 4mG, Bturb = 5mG 6 mG

10. Are magnetic fields dynamically important? Yes. • Sun: Most active phenomena are due to a • B-field in the Sun. • Stars: Magnetically controlled star formation; compact objects (neutron stars and accretion disks ...) • The ISM:Energy density of the B-field is comparable to those in other energy forms. (large-scale structure, CR generation, etc…) • The Galaxy: Dynamo vs. Primordial • Cosmology: Origin of the B-field

11. (Isothermal) MHD equations • Slow time variation • Small drift velocities between electrons and • ions • Ohm’s law; • Non-relativistic transform between the ion and the lab. rest frames

12. Jeans and Magnetically Critical Masses Scalar virial theorem 0.3 (Mouchovias & Spitzer 1974) 1/(4p2) (Nakano & Nakamura 1978)

13. Myers et al. 1986 • CO 2.6m, 150micron, 250micron, • 6cm radio continuum, • H 110alpha recombination • inner Galaxy, -1 deg < b <1 deg, • 12 deg < l < 60deg • 54 molecular cloud complexes • mean SFE = mean Ms/(Ms+Mc)=2%

14. Observed SFEs • Observed SFE = Ms/(Ms+Mc) is - 2-3% for the molecular cloud complexes in the inner Galaxy (e.g., Myers et al. 1986) - 10-30% for cluster-forming cores (e.g., Lada & Lada 2003) • SF theories should explain the low SFEs (Zuckerman & Evans 1974).

15. Two SF Theories ion neutral SF regulated by AD SF regulated by turbulence magnetically supercritical cloud. (B-field is not important ingredient.) magnetically subcritical cloud

16. subcritial supercritial Bourke et al. 2001 Criticality of MC cores • Almost all observed cores are magnetically supercritical if they have spherical geometry. • Even the case with the sheet geometry (Shu et al. 2001) the average normalized flux-to-mass ratio is 0.4, which is in the supercritical range. • More observations are needed in order to clarify the criticality of cores.

17. Conclusions • The range of the local B-field strength is from 1.4 mG (Faraday) to 4mG (synchrotron). The local B-field may be part of a magnetic arm between optical arms. • More observations on B-fields of MCs are needed to tell the criticality of them. • As we get more information on B-fields, the MHDs becomes more important in understanding the magnetized Universe.

18. 3D, self-gravitating, driven MHD simulations m =(M/F) /(M/F)c=0.9, 2.8, 8.8, infinite n = 500 cm-3 cs = 0.2 km s-1 L = 4pc B = 45, 15, 5, 0 mG Mtot = 2000 Msun periodic boundaries B uniform density turbulence is driven at a large scale around L/2 Mrms = 10 L=4LJ resolution: 2563 cells

19. 10 no 100 n0 1000 n0 dt_frame = 0.04Myr Magnetically supercritical case, m=2.8 • A few collapsing cores are formed. • First collapsing object goes from first appearance to a fully collapsed state in less than 1 Myr, twice of the local free-fall time.

20. Lifetime of starless Cores • A typical lifetime of starless cores ~ 0.3-1.6Myr estimated based on the number ratio, 0.3(94/306), of cores with embedded young stellar objects and starless cores (Lee & Myers 1999). • The lifetime is shorter than the one predicted by the AD models. • We will try to measure the lifetimes of cores formed in turbulent MCs and make a comparison with this observed value.

21. m=2.8 m=8.8 HD first core formation m=0.9, subcritical Time evolution of global maximum of density field • dmax < 100n0 for the subcritical case. • Within 0.5 tff, dmax ~104n0 (first cores are formed) for the supercritical cases. •  consequence of the production of locally gravitationally unstable objects by the turbulence

22. 10 no 30 n0 100 n0 dt_frame = 0.04Myr Magnetically subcritical case, m=0.9 • Most density peaks are transient with lifetimes at most 1.5Myr. • The AD timescale is comparable to the lifetimes of longest-lived clumps.  The cores may undergo AD-mediated evolution if AD is included even in a strongly turbulent, subcritical flow.

23. Hydrodynamic case, m=infinite 10 no 100 n0 1000 n0 dt_frame = 0.04Myr • Larger number of small collapsed cores are formed than in a MHD case. • The timescales from formation to full collapse cover a wider range, of 0.5 to 1 Myr.

24. Core Formation Efficiency (SFE) 0.12 0.04 M (n>500n0) 0.05 2.8 8.8 0.025 lifetime of cloud: 4Myr (e.g, Hartmann et al. 2001) • CFE is dependent on the seed for random driving • velocity fields (Heitsch et al 2001). • CFEs are lower than 10 % in most cases.

25. 2.8 8.8 Mass fraction of each core M (n>500n0) • As m increases, more low mass cores are formed. • Minimum masses of the collapsed objects increase monotonically with increasing field strength.