Title  Ionized Gas Accretion in Hypercompact HII Regions
Pi     S. Kurtz
Time   6 hrs
DRSP 2.0 - section 2.2 proposal

1. Name of program and authors

Ionized Gas Accretion in Hypercompact HII Regions

S. Kurtz

2. One short paragraph with science goal(s)

Hypercompact HII regions have emerged as one of the earliest
observable stages of a young massive star.  On these very small scales
(tens of milliparsec) the ionized gas is extremely dense, and radio
recombination lines suffer substantial impact broadening and non-LTE
effects.  The dynamics of the ionized gas on these scales are not at
all clear.  Due to the high over-pressure within the HII region,
outward, expansive flows might be expected.  On the other hand, models
for massive star formation suggest that ionized in-fall may be
occurring, as the star continues to accrete matter from the parental
cloud.  It is probable that the stellar gravity maintains a steep
density gradient within the ionized gas, further contributing to the
complicated problem of separating radiative transfer effects from gas
dynamics.  Here, we propose to observe a sample of HC HII regions in
several millimeter radio recombination line tracers, to map the gas
flows within the region, and distinguish the contribution from non-LTE
effects from that of actual gas dynamics.  These observations will
help to distinguish in a detailed way between several of the currently
proposed mechanisms for massive star formation.


3. Number of sources 
   
Six presumed hypercompact HII regions


4. Coordinates:

4.1. Rough RA and DEC 

     6 "galactic plane" objects (18-19 hours RA, -20 to +15 Dec)

4.2. Moving target: yes/no (e.g. comet, planet, ...)

     no

4.3. Time critical: yes/no (e.g. SN, GRB, ...)

     no

4.4. Scheduling constraints: (optional)

     none

5. Spatial scales:

5.1. Angular resolution (arcsec):

     0.1

5.2. Range of spatial scales/FOV (arcsec):

     single field, LAS 1 arcsec

5.3. Required pointing accuracy: (arcsec)

     10% of primary beam

6. Observational setup

6.1. Single dish total power data: no/beneficial/required

     no

6.2. Stand-alone ACA: no/beneficial/required

     no

6.3. Cross-correlation of 7m ACA and 12m baseline-ALMA antennas: 
     no/beneficial/required

     no

6.4. Subarrays of 12m baseline-ALMA antennas: yes/no

     no

7. Frequencies:

7.1. Receiver band: Band 3, 4, 5, 6, 7, 8, or 9

     Band 4 

7.2. Lines and Frequencies (GHz): 

     H36a  135.286
     H45b  135.249
     He36a 135.341

7.3. Spectral resolution (km/s):

     0.1 km/s

7.4. Bandwidth or spectral coverage (km/s or GHz):

     80 km/s

8. Continuum flux density:

8.1. Typical value (Jy):

     1 Jy

8.2. Required continuum rms (Jy or K):

     0.001 Jy

8.3. Dynamic range within image:

     500:1

8.4. Calibration requirements: 
     absolute      ( 1-3% / 5% / 10% / n/a ) 10%
     repeatability ( 1-3% / 5% / 10% / n/a ) 5%
     relative      ( 1-3% / 5% / 10% / n/a ) 5%

9. Line intensity:

9.1. Typical value (K or Jy):

     1 Jy

9.2. Required rms per channel (K or Jy):

     0.004 Jy

9.3. Spectral dynamic range:

     200:1

9.4. Calibration requirements: 
     absolute      ( 1-3% / 5% / 10% / n/a ) 10%
     repeatability ( 1-3% / 5% / 10% / n/a ) 5%
     relative      ( 1-3% / 5% / 10% / n/a ) 5%

10. Polarization: yes/no (optional)

     no

10.1. Required Stokes parameters:

     I

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):

     1 hr 

12. Total integration time for program (hr):

     6 hr + calibration overhead

13. Comments on observing strategy : (optional)

     none
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