1. Name: Envelope Structure of Intermediate-Mass YSOs
          ============================================
    (This is a revised version of DRSP 2.1.9 from DRSP 1.0)
    The main differences are:
    1) the use of mosaics at higher frequency bands so the same
       fields are covered
    2) the elimination of the use of band 7
    3) a reduction of the total number of objects from 12 to 8

    Author: J. Di Francesco (NRC-HIA)

 2. Science goal: Probe the density and thermal structure of envelopes
    surrounding intermediate-mass young stellar objects by imaging
    submillimetre continuum emission of selected Herbig Ae/Be stars.
    This program follows work done by Mannings (1994; MNRAS, 271, 587)
    based on continuum measurements made from the JCMT.  Herbig Ae/Be
    stars are young stellar objects of 2-10 Msun, with large infrared
    excesses due to reprocessing of stellar photons by dust in
    circumstellar disks and envelopes.  Such objects may or may not have
    followed the same formation path as low-mass stars.  Comparing the
    circumstellar structures of Herbig Ae/Be stars with those of low-mass
    objects should reveal the similarities or differences in their
    formation.  The low-resolution data of Mannings, however, could not
    distinguish well the relative contributions of submillimetre continuum
    emission from disks and envelopes around the Herbig Ae/Be stars
    studied.  ALMA will provide very sensitive, high-resolution
    submillimetre continuum data of the envelopes surrounding these stars.
    Data from 3 well-spaced bands will allow these structures to be well
    characterized using modern radiative transfer codes.


3. Number of sources: 12

4. Coordinates: 

  4.1. 12 sources distributed at low galactic latitudes over sky 
       (RA=any, DEC=any visible)

  4.2. Moving target: no

  4.3. Time critical: no


5.  Spatial scales:

  5.1. Angular resolution: 0.50" (all bands)

  5.2. Range of spatial scales/FOV: 0.5-30" (all bands)

  5.3. Single dish: yes (all bands, if ACA unavailable)

  5.4. ACA: yes (all bands, preferred)

  5.5. Subarrays: no


6. Frequencies:

  6.1. Receiver band: Bands 3, 6, 9

  6.2. Line: n/a
       Frequency: n/a

  6.3. Spectral resolution (km/s): n/a

  6.4. Spectral coverage (km/s or GHz): n/a


7. Continuum flux density:

  7.1. Typical value: O.01, 0.1, 1.0 Jy

  7.2. Continuum peak value: 0.001, 0.01, 0.1 Jy

  7.3. Required continuum rms: 0.000002, 0.00002, 0.0002 Jy

  7.4. Dynamic range in image: 500, 500, 500


8. Line intensity:

  8.1. Typical value: n/a

  8.2. Required rms per channel: n/a

  8.3. Spectral dynamic range: n/a


9. Polarization: no


10. Integration time per setting: 84, 30, 276 hrs 

11. Total integration time for program: 390 hr (+ 1560 hours ACA time to get
    same sensitivity in low uv-spacing data)

Note 1: To sample well the envelopes of these objects, mosaicked observations
will be necessary at the higher bands.  For example, the Band 3 observations 
will sample the sky within 30" radius of the phase centre beam in one pointing 
but the Band 6 and Band 9 observations will require respectively 3x3 and 8x8 
pointing mosaics to sample the same field.  To get the required sensitivities 
across the respective fields, each object will have to be observed over 7 hrs, 
2.5 hrs, and 23 hours in Bands 3, 6, and 9.

Note 2: Given the difficulties in observing at high frequencies, Band 7 data
could be substituted for Band 9 data in the above program.  In this case, the
required sensitivities would be very similar to those for Band 6, although the
time to make 5x5 pointing mosaics for 12 sources would be 336 hours, and the
total integration time would be 450 hours (+ 1800 hours ACA time).

Note 3: The dynamic range goal is flexible with a minimum of 100 preferred.
Integration times may be scaled down significantly to meet this minimum.

Note 4: Simultaneous observations of 12CO/13CO transitions may be useful 
to trace gas dynamics in the envelopes, although depletion and outflows 
could make interpreting these data difficult.

Note 5: Higher angular resolution observations will begin to probe the disks
of these stars.  Since this is an interesting topic of its own, it would be 
useful to define a DSRP for such a project in the disks sub-theme and link 
this project to it.

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Review John Richer: see program 2.1.1.