[asac] Enhanced sciences with ACA?

[Seiichi Sakamoto]

Dear ASAC members:

In the Santiago face-to-face meeting, relatively high priority was 
assigned to ACA based on rather vague feeling.  This is potentially 
of severe concern to ALMA, because the full scope of the enhancement 
may be threatened if we fail to present strong science cases for 
ALMA+SD+ACA that cannot be realized with ALMA+SD.  Here I summarize 
some of my comments/concerns and TBDs on the science cases for ACA 
in the following.  I hope this helps a bit to finalize the ASAC 
report.  Any comments and corrections are welcome.  

(1) General comments/concerns:
- Mass and motion of the objects are two of the most important 
  parameters in observational astrophysics.  How ACA is related to 
  these parameters?  Accurate imaging does not mean robust extraction 
  of mass distribution.  What is the importance of extremely high 
  fidelity images probed by molecules other than molecular hydrogen?  
- What fraction of ALMA users really takes advantages of the extremely 
  high fidelity of ALMA+SD+ACA?  Is it of higher priority than to 
  ensure routine high fidelity imaging with ALMA+SD to all users?  
- How easily will we lose the gain achieved with ACA by degrading the 
  SD observations or by inappropriate deconvolution algorisms?  

(2) Line ratios:
- In multi-line analyses, ambiguity of chemical abundances, complexity 
  of the objects, unmatched synthesized beams, inhomogeneity of 
  physical conditions in the synthesized beam, and robustness of the 
  model used (e.g., LVG) often limit the robustness of conclusions.  
  We should evaluate what level of fidelity is necessary/sufficient 
  for this kind of analyses, and what level of fidelity can be 
  achieved for realistic cases (noise for realistic integration times 
  and source strengths).
- We should explain why extremely high fidelity is preferred rather 
  than number of bands (or number of observable lines) that also 
  enhances ability of multi-line observations.

(3) Arm-to-interarm contrast:
- Fidelity of images at extremely high level may not be the limiting 
  factor that controls conclusions.  The arms of spiral galaxies are 
  not always well-defined, and identification of arms may limit the 
  robustness.  The situation becomes more difficult for flocculent 
  galaxies.  We should examine if different researchers independently 
  obtain consistent value of the arm-to-interarm contrast with sample 
  images to the accuracy of current interest.  
- Uncertainty due to ambiguous luminosity-mass conversion may also 
  play a major role.

(4) Mass spectrum of clouds and cores:
- Ken Tatematsu's analysis at the face-to-face meeting clearly 
  demonstrated that ALMA+SD already realizes enough fidelity for this 
  purpose.  In realistic cases, complexity of the objects (cloud/clump 
  overlapping, irregular structure, etc.) and robustness of simple 
  finding algorisms (e.g., CLUMPFIND) may limit the results.  
- Uncertainty due to ambiguous luminosity-mass conversion may also 
  play a role.  
- We should find other cases in which extremely high fidelity is 
  needed.  

(5) Polarization:
- Polarization study seems among which ACA might play a role.  We need
  examples that clearly demonstrate this.  We should be sure that ACA 
  supports polarization capability under severe budgetary constraints 
  if we are to stress this science case.  

(6) Submillimeter observations:
- Smaller diameter of ACA antenna may be suitable for submillimeter 
  observations.  However, current situation suggests that 
  submillimeter receivers that enable submillimeter observations may 
  be competitive with ACA.  We should justify why we prefer ACA rather 
  than Band 8 if we are to stress this science case.  
- If we are to stress this science case, optimization of the array 
  diameter, number of antenna, and configuration may be needed, 
  because the current optimization is based on imaging capability.  

With best,

Seiichi

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Seiichi Sakamoto
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ALMA-Japan Project Office
National Astronomical Observatory of Japan
Osawa, Mitaka, Tokyo 181-8588, Japan
phone:  +81-422-34-3843
fax:    +81-422-34-3764
e-mail: seiichi at nro.nao.ac.jp
URL:    http://www.nro.nao.ac.jp/~seiichi
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