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Optical Observations of High-Latitude Clouds. Adolf N. Witt University of Toledo. Collaborators: Steve Mandel, Hidden Valley Obs., Soquel, CA Thomas G. Dixon, Univ. of Hertfordshire, UK Paul H. Sell, Univ. of Toledo Karl D. Gordon, Univ. of Arizona Uma P. Vijh, STScI With support from:

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Optical observations of high latitude clouds

Optical Observations of High-Latitude Clouds

Adolf N. Witt

University of Toledo


Collaborators:

Steve Mandel, Hidden Valley Obs., Soquel, CA

Thomas G. Dixon, Univ. of Hertfordshire, UK

Paul H. Sell, Univ. of Toledo

Karl D. Gordon, Univ. of Arizona

Uma P. Vijh, STScI

With support from:

NSF Galactic Astronomy Program

RC Optical Systems

Santa Barbara Instrument Group, Inc.

Software Bisque

Astrodon



“Re-discovery” of HLCs by IRAS, F. J. Low et al. 1984, ApJ, 278, L19

Led to new designation as “IR Cirrus”

Followed by detailed studies at mm-wavelengths (CO) by Magnani et al.


  • High-Latitude Clouds are ideal for studying the morphology ApJ, 278, L19

  • of the diffuse interstellar medium.

  • Nearby, ~ 100 pc

  • Individual “clouds” seen in relative isolation

  • Little line-of-sight confusion

  • Free of star-formation effects

  • Wide range of optical depths

  • Atomic as well as molecular gas

  • Typical cloud “sizes” can be measured directly

  • We can see actual “clouds”


  • Advantages of Optical Imaging Approach ApJ, 278, L19

  • Combines high spatial resolution (arc sec) with large field of view

  • Sensitive to column density of dust ---> total gas column density

  • Insensitive to dust temperature

  • Independent of atomic or molecular state of the gas as long as

  • dust-to-gas ratio is constant

  • Offers ideal conditions for study of dust luminescence

  • Direct evidence for small-scale structure on few 102 AU scale

  • Low cost

  • Disadvantages (No such thing as a free lunch!!)

  • Low surface brightness (few % of that of the dark night sky)


Instrumentation
Instrumentation ApJ, 278, L19

Remotely operated, self-guided small telescopes, equipped with CCD cameras.

Detection of diffuse, extended sources depends only on f-ratio, not aperture.

Location: New Mexico Skies Altitude ~7300 ft, near Cloudcroft, NM

Primary Goal: Determination of the optical SEDs of HLCs


Optical bandpasses
Optical Bandpasses ApJ, 278, L19

Special BGRIHa filter set during Phase1 of our program

15-band BATC filter set during Phase 2 of our program. These filters allow the determination of a detailed cloud SED while avoiding the strongest emission features of the permanent airglow from the Earth atmosphere.


  • What makes a high-latitude cloud shine? ApJ, 278, L19

  • Scattering of the Galactic interstellar radiation field by dust.

  • Photoluminescence by nanoparticles, primarily Extended Red

  • Emission excited by far-UV photons from the ISRF.

  • H-alpha in emission, mostly from Galactic HII regions,

  • scattered by dust in the HLCs.

This is extended red emission ---->


  • How important is ERE? ApJ, 278, L19

  • about 30% of total surface brightness of HLCs

  • near 600 nm at intermediate latitudes

ERE is relatively important, because dust scattering is not very efficient at

high galactic latitudes.


Here s why
Here’s why: ApJ, 278, L19

  • Illumination of HLCs comes mainly from the Milky Way

  • Most of the light is scattered with scattering angles ~90 0

  • HG phase function for forward-scattering grains ---> highly inefficient at ~ 90 0

  • ERE is emitted isotropically, no dependence on direction of incoming illuminating radiation; depends only on density of UV radiation.



Mbm 30
MBM 30 ApJ, 278, L19


Mbm 32
MBM 32 ApJ, 278, L19


Focus small scale structure in mbm 12
Focus: Small-Scale Structure in ApJ, 278, L19MBM 12

Linear structures ~ 500 AU wide


Mbm 12
MBM 12 ApJ, 278, L19

Sharp edges


Mbm 121
MBM 12 ApJ, 278, L19


Mbm 301
MBM 30 ApJ, 278, L19


Focus: Small-Scale Structure in Absorption ApJ, 278, L19M 81; Image Credit: Tony Hallas http://www.astrophoto.com/ (with permission)


M81 detail
M81 detail ApJ, 278, L19

Linear structures again; nT ~ 106 K cm-3


Same M81 cirrus observed in CO by A. Heithausen ApJ, 278, L192006, A&A, 450, 193cirrus resolves into small molecular clumps



Combining optical structure data with IR and radio structure data can extend 2-D angular power spectrum (S. J. Gibson, 2006, SINS)

We are collaborating with Steven Gibson on the analysis of our data.


Conclusions
Conclusions data can extend 2-D angular power spectrum (S. J. Gibson, 2006, SINS)

  • Optical imaging data of high-latitude clouds reveal ISM morphology

  • and small-scale structures over 3 orders of magnitude of linear scale

  • Cloud morphology: Clumpy cores embedded in low-density envelope

  • Small-scale structure: linear strands of high-density gas, ~500 AU

  • Under special conditions (e.g. M81) structure in Galactic ISM can be observed

  • in absorption.

  • Illumination geometry of high-latitude clouds makes them ideal test beds

  • for studies of extended red emission (ERE) in the diffuse ISM.


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