1. VLBA Observations of AU- scale HI Structures Crystal Brogan (NRAO/NAASC) W. M. Goss (NRAO), T. J. W. Lazio (NRL) SINS Meeting, Socorro, NM, May 21, 2006

2. Probing Galactic HI through Absorption Cold HI scale height (2z) ~200 pc Local bubble ~100 pc ~500 pc G187.4-11.3

3. First Evidence for Small HI Structures Hat Creek – Owens Valley interferometer with a fringe spacing of 0.09” Dieter, Welch, & Romney (1976) Changing HI spectrum with hour angle indicates structure Comparison of averages of “early” and “late” hour angles in two epochs 3C147  Evidence for structures with size 3 x 10 -4 pc (70 AU)

4. Under Pressure…. What is measured: N(HI)/Ts (HI column density/spin temperature) and the  size scale L Typical Tiny HI: - N(HI)/Ts ~ 1 x 10 19 cm -2 * K -  size scale of tiny HI ~ 50 AU - maximum deviation  ~ 0.5 - spin temperature Ts ~ 50 K  Density of tiny n(HI) ~ 3 x 10 5 cm -3  Pressure of tiny HI P/k ~ 10 7 cm -3 K Typical ISM - n ~ 50 cm -3 - P/k ~ 10 (3-4) cm -3 K What is the nature of the TSAS? How long does it live? How common is it? Is it really a “structure” or just a statistical phenomenon? Solutions: - Skinny (Heiles 1997) - Cold (Heiles 1997) - Temporary (Jenkins 2004) - Statistical (Deshpande 2000)

5. Quest for Better S/N Lovell, Effelsberg, & Westerbork VLBI with a resolution of 0.05” Diamond et al. (1989)  Evidence for structures with size ~25 AU and density 10 5 cm -3

6. First Imaging of Small Scale HI MERLIN + EVN with a resolution of 0.1” Davis, Diamond & Goss (1996)  Evidence for structures with size ~70 AU and density 10 4 cm -3

7. Imaging with the VLBA (i.e. Epoch I) 3C138 (20 mas) 0404+786 (10 mas) Faison et al. (1998, 2001)

8. Multi-epoch VLBA Study Toward 3c138 Epochs: 1995, 1999, 2002 Resolution: 20 mas = 10AU at 500 pc  Superior dynamic range to any previous study  First attempt to study variabilty Difference 2002 -1999 Average 2002 HI optical depth

9. Position-Velocity Diagram

10. Cross-Cuts Shaded lines are ± 1σ Apparent sizescale of features is 50mas or ~ 25 AU

11. Epoch to Epoch Opacity Changes

12. Percentage of “Significantly” Deviant Pixels   5σ =7%   5σ =5%   5σ =11%   5σ =10% At typical ISM densities of ~50 cm -3 all of the observed HI column would fit into a cloud only 3.5 pc in diameter  The filling factor of the CNM itself is low (<1% in this direction)  Thus the filling factor of TSAS must be < 0.1%

13. Could Optical Depth Changes be Due to a Change in Temperature? Arecibo (Heiles & Troland 2003) effective resolution ~800 mas For a temperature drop from 50 K to 15 K the line width should decrease by ~0.7 km/s! Difference 2002 -1999

14. Is the Line of Sight to 3C 138 Special?  If the typical scale size of tiny scale HI is ~50mas, and the “covering fraction” is low, then the best chance of seeing it is toward a large source VLBA HI Absorption Measurements toward Quasars

15. Pulsar Observation Simulation  from 120 simulated pulsar observations with epochs separated by 50 mas = typical size of HI variations Probability of landing on variation with a 1-D sampling method is very low x x

16. What about Those Magnetic Fields? Zeeman effect Blos 3σ upper limits: Blos < 45  G/pixel Blos < 20  G for average Consistent with Heiles & Troland Arecibo detection of 5.6 ± 1.0  G What does it mean? Assume magnetic and turbulent pressures equal: B = 0.4  v NT n 0.5 where  v NT is the non-thermal linewidth For n=50 cm -3 and  v NT =2.1 km/s; B = 6  G For n=10 5 cm -3 and  v NT =2.1 km/s; B = 266  G  Magnetic and turbulent pressures do not appear to be in equilibrium  Given flux freezing, how can density change by 10 3 and not produce appreciable increase in B?  MHD waves likely mediate magnetic field/turbulent pressure balance  MHD waves with frequencies larger than the ion-neutral collision frequency cannot propagate: For a parent cloud with size ~ 3.5 pc ionization fraction 10 -4 Alfven speed = 2.1 km/s the cutoff wavelength is on the order of 10AU Simulations are needed….

17. Preliminary Results for 3C 147 Special thanks to E. Fomalont, V. Dhawan, C. Walker D= 100 – 1000 pc 10mas ~ 5 AU at 500 pc G161.7+10.3 S/N of Deviation

18. Summary of VLBA Tiny HI Results Distinct structures with typical sizes of ~25AU are observed significant changes on few year timescales are observed The line widths of these features rule out the significantly cooler gas scenario The covering fraction of these features is only about 10% The filling factor is very tiny < 0.1% The magnetic fields of these features are not significantly enhanced Perhaps MHD waves can’t propagate? These features are sufficiently rare that the probability of observing them is low unless the search region is large Could explain the low rate of return from pulsar observations and small quasars What is the nature of the TSAS? How long does it live? How common is it? Is it really a “structure” or just a statistical phenomenon?