Title Envelope Structure of Intermediate-Mass YSOs Pi J. Di Francesco Time 4850 hrs 1. Name of program and authors Envelope Structure of Intermediate-Mass YSOs James Di Francesco (NRC-HIA) 2. One short paragraph with science goal(s) The goal of this project is to probe the density and thermal structure of envelopes surrounding intermediate-mass young stellar objects by imaging the submillimeter continuum emission of selected Herbig Ae/Be stars. Herbig Ae/Be stars are young stellar objects of 2-10 Msun, which may have 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. Early work on this subject was performed by Mannings (1994; MNRAS, 271, 587) based on continuum measurements made from the JCMT. (Similar, recent studies of low-mass embedded objects have been done by Shirley et al. (2000; ApJS, 131, 249) and Young et al. (2003; ApJS, 145, 111)) The low-resolution data of Mannings, however, could not distinguish well the relative contributions of submillimeter continuum emission from disks and envelopes around the Herbig Ae/Be stars studied. ALMA will provide very sensitive, high-resolution submillimeter continuum data of the envelopes surrounding these stars. Data from 3 well-spaced bands will allow these structures to be well characterized using modern, multidimensional radiative transfer codes. 3. Number of sources 12 4. Coordinates: 4.1. Rough RA and DEC Targets are distributed at low galactic latitudes across the sky. 4.2. Moving target: No. 4.3. Time critical: No. 4.4. Scheduling constraints: (optional) None. 5. Spatial scales: 5.1. Angular resolution (arcsec): 0.50" (all bands) 5.2. Range of spatial scales/FOV (arcsec): Wide-field mosaics over a range of spatial scales from 0.5-30" (all bands) 5.3. Required pointing accuracy: (arcsec) 0.1" (**justifiable?**) 6. Observational setup 6.1. Single dish total power data: Beneficial. Observing modes for single dish total power: OTF 6.2. Stand-alone ACA: Required. 6.3. Cross-correlation of 7m ACA and 12m baseline-ALMA antennas: Beneficial. 6.4. Subarrays of 12m baseline-ALMA antennas: No. 7. Frequencies: 7.1. Receiver band: Bands 3, 6, 9 7.2. Lines and Frequencies (GHz): n/a 7.3. Spectral resolution (km/s): n/a 7.4. Bandwidth or spectral coverage (km/s or GHz): n/a 8. Continuum flux density: (values below listed for Bands 3, 6 and 9 respectively) 8.1. Typical value (Jy): 0.01, 0.1, 1.0 Jy 8.2. Required continuum rms (Jy or K): 0.000002, 0.00002, 0.0002 Jy 8.3. Dynamic range within image: 500, 500, 500 8.4. Calibration requirements: absolute: 5%, 10%, 10% repeatability: 5%, 10%, 10% relative: 5%, 10%, 10% 9. Line intensity: 9.1. Typical value (K or Jy): n/a 9.2. Required rms per channel (K or Jy): n/a 9.3. Spectral dynamic range: n/a 9.4. Calibration requirements: absolute ( n/a ) repeatability ( n/a ) relative ( n/a ) 10. Polarization: yes/no (optional) No. 10.1. Required Stokes parameters: No. 10.2. Total polarized flux density (Jy): n/a 10.3. Required polarization rms and/or dynamic range: n/a 10.4. Polarization fidelity: n/a 10.5. Required calibration accuracy: n/a 11. Integration time for each observing mode/receiver setting (hr): 121, 81, 768 hrs 12. Total integration time for program (hr): 970 hr (+ 3880 hrs ACA time to get same sensitivity in low uv-spacing data) 13. Comments on observing strategy : (optional) 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 11 hrs, 6.75 hrs, and 64 hrs in Bands 3, 6, and 9 respectively. Note 2: Given the difficulties in observing at Band 9, Band 8 data could be substituted for Band 9 data in the above program. In this case, the required sensitivities could be very similar to those for Band 6, i.e., maybe an rms of 0.0001 Jy would be sufficient. In this instance, 5x5 pointing mosaics for 12 sources would be 375 hrs, and the total integration time would be 577 hrs (+ 2308 hrs 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. Note 6: Determination of radial structure of envelopes (e.g., "p", the exponent of a radial power-law function) is more dependent upon the observed radial profile of the emission than the absolute flux values. The low-mass embedded object studies described above by Shirley et al. and Young et al. used 1-D RT codes to model continuum emission from various Class 0 and I sources and fit "p" through *normalized* angularly-averaged radial intensity profiles. These studies found that simultaneous SED fitting was less influential in determining "p". Their methods could fit "p" within +/- 0.2, although other sources of un- certainty (ISRF strength, presence of a disk) could modify "p" dramatically , e.g., ~0.5. Their JCMT data had flux uncertainties of ~10% at 850 microns and ~40% at 450 microns. We note, however, that their determinations of the envelope mass and source luminosities are dependent upon the SED. Improvements to the accuracies of low and high frequency flux data by factors of only 2-4 (e.g., 5% at Band 3 and 10% for Bands 6 and 9) for the case of ALMA data of intermediate-mass YSO envelopes would likely improve determinations of envelope mass, source luminosities and envelope "p" - the main science goals. Note 7: These objects are not expected to have submillimeter or millimeter fluxes that are variable on short (monthly) timescales, so the precision of the observations need only match the relatively low accuracy quoted above. Note 8: Again, since the SED (i.e., relative fluxes) is less important than radial intensity distributions in determining "p" through models, relative calib- ration accuracy that is superior to absolute calibration accuracy is not necessary. If, however, one wanted to find variations of dust opacities within the sources, higher relative calibration accuracy may be needed, but the requirements for that are not well known. Note 9: This program depends on making mosaics of extended submillimeter and millimeter emission. Such mosaics are made generally by stepping through a series of pre-defined sky positions separated by the Nyquist sampling distance or less. Gain stability *over the time required to make such a mosaic* (20-30 minutes?) is crucial to obtain accurate visibilities across the field. Since this project depends more on radial intensity profiles and not absolute fluxes, gain instabilities over the timescale of a pass through a mosaic could impact the science. Such time-related requirements are not specifically questioned in this survey, but it is hoped that the gain stability can be specified within some limits in the future. Moreover, Young et al. described how the shape of the JCMT beam over time affected their 450 micron observations due to thermal variations of the telescope (pointing and beam size). It is likely that the ALMA telescopes will also encounter this problem, with higher frequencies affected more than lower frequencies. Since this DRSP program involves making mosaics at high frequencies, untracked variations of the pointing and beam size will have a dramatic effect on the science. It is hoped that pointing and beam size will be monitored regularly by the ALMA Observatory. -------------------------------------------------- Review v2.0: init_9 = 2.1.9 1. Name: Envelope Structure of Intermediate-Mass YSOs ============================================ DiFrancesco Unchanged w.r.t. DRSP1.1. This is a large program requiring lots of ACA time. Would it be useful to also record the 7m x 12m correlations? Answer: Since this project involves probing the extended envelopes of intermediate mass stars, correlations of the 7m x 12m antennas may be useful. First, the added spatial frequency coverage would be welcome, provided the ACA is situated near the center of the compact ALMA 50-m configuration. Otherwise, I suspect the extended structure would be resolved out along ALMA/ACA baselines. Second, the added sensitivity would be welcome, given the same concern as to spatial frequency coverage.