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VLBA Observations of AU-scale HI Structures

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

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VLBA Observations of AU-scale HI Structures

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  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 Local bubble ~100 pc Cold HI scale height (2z) ~200 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) 3C147 Comparison of averages of “early” and “late” hour angles in two epochs Changing HI spectrum with hour angle indicates structure  Evidence for structures with size 3 x 10-4 pc (70 AU)

  4. Under Pressure…. • 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? What is measured: N(HI)/Ts (HI column density/spin temperature) and the  size scale L Typical Tiny HI: - N(HI)/Ts ~ 1 x 1019 cm-2 * K -  size scale of tiny HI ~ 50 AU - maximum deviation Dt ~ 0.5 - spin temperature Ts ~ 50 K  Density of tiny n(HI) ~ 3 x 105 cm-3  Pressure of tiny HI P/k ~ 107 cm-3 K Typical ISM - n ~ 50 cm-3 - P/k ~ 10(3-4) cm-3 K 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 105 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 104 cm-3

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

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

  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 Dt 5σ =5% Dt 5σ =7% • 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% Dt 5σ =10% Dt 5σ =11%

  13. Difference 2002 -1999 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!

  14. Is the Line of Sight to 3C 138 Special? VLBA HI Absorption Measurements toward Quasars  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

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

  16. What about Those Magnetic Fields?  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…. Zeeman effect Blos 3σ upper limits: Blos < 45 mG/pixel Blos < 20 mG for average Consistent with Heiles & Troland Arecibo detection of 5.6 ± 1.0 mG What does it mean? Assume magnetic and turbulent pressures equal: B = 0.4 DvNTn0.5 where DvNT is the non-thermal linewidth For n=50 cm-3 and DvNT =2.1 km/s; B = 6 mG For n=105 cm-3 and DvNT =2.1 km/s; B = 266 mG  Magnetic and turbulent pressures do not appear to be in equilibrium  Given flux freezing, how can density change by 103 and not produce appreciable increase in B?

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

  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?

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