ALMA Atacama Compact Array Tiger Team

Teleconference, 5 October 2001 1415 UT

Draft Minutes

Participants: P. Cox, R. Crutcher, S. Guilloteau, F. Gueth, M. Gurwell, M. Holdaway, J. Mangum, Koh-Ichiro Morita, J. Pety, A. Wootten, E. van Dishoeck, Michiel Hogerheidje, M. Ishiguro, S. Sakamoto. T. Hasegawa, K. Tatematsu, M. Gurwell, R. Crutcher and J. Welch

1. Progress Reports. The ASAC is in the process of writing up the science case for the ACA based upon the material which the Tiger Team has furnished. One new set of images has been simulated by the IRAM team, based upon a polarization set of I, Q, U images from Crutcher. Results can be downloaded at here.

Stephane gave a short summary of these results:

  • No high frequency information on spatial scales greater than 100m
  • These being BIMA images, the simulations are complicated by the lack of fine spatial structure.
  • ALMA + SD had some difficulty in giving good simulations
  • ALMA + SD + ACA reduced errors by a factor of three.
  • A simple analysis suggests errors in position angle might be as much as 30 degrees without the ACA but were reduced to 5 degrees with it.

    This simulations shows the impact of the compact array quite well.

    Cox, struggling with the science case, agreed. It was discussed whether this simulation could be done with the MEM reconstruction, since the image contains both positive and negative numbers. Holdaway mentioned that maximum emptiness could be used. It was included within the SDE package.

    Cox discussed the science case, difficult to summarize concisely and compellingly, but the most convincing cases are the HCO+ disk and the polarization case, potentially. Bryan's analysis of the debris disk suggests it is not so compelling.

    Holdaway discussed his simulations, which include the illumination offset. This limits quality below about 500 GHz. Most of this problem, perhaps 80 %, may be corrected by taking out the illumination offset with a correction to (u,v,w). He will redo the simulations, though only one computer is currently available. The cluster model is now being done. MEM does a poor job for not completely understood reasons with the HCO+ disk; with pointing errors the results are similar for 115 and 850 GHz.

    Crutcher joined and the polarization case was discussed an additional amount.

    Hasegawa commented that the Q map indeed looks different. Indeed in the protostellar disk model, the line ratio became unrepresentative in the outer parts of the disk. The cases where the compact array adds robustness are those in which high dynamic range can be achieved. For other science targets, it may not add so much.

    Wootten commented that one reason he is interested in ALMA is to reach high dynamic range, to understand the details of the final few percent of structure in an image. For this reason, ALMA has been constructed with great sensitivity and care for excellent imaging.

    Holdaway commented that although peak flux in some outlying parts of images such as the protostellar disk image, may be small, it is spread over a substantial area, thus contributing a substantial part of the flux. So while it may be the final per cent of a large dynamic range, it is important for understanding global parameters.