High-excitation CO lines toward southern low-mass protostars

Coordinator: T. van Kempen, J. Jorgensen, E. van Dishoeck, M. Hogerheijde,R. Guesten, P. Schilke


Program is available and data products can be downloaded

Scientific justification:

CO and the less abundant isotopes 13CO, C18O and/or C17O are often used as tracer molecules for the temperature and abundance distributions of the gas around young-stellar objects (YSOs). Observations of many low-mass YSOs (Jorgensen et al. 2002, A&A389, 908; 2005, A&A 435, 177) have shown that the 3-2 and 2-1 lines trace the cold part of the envelope further away from the central source. Higher excitation lines are needed to trace the dense and warm inner parts of the circumstellar envelopes. Despite many attempts, such observations have not proven possible with the JCMT for low-mass YSOs; one of the few sets of published CO J=6-5 data is that of Hogerheijde et al. (1998, ApJ 502, 315) obtained with the CSO.
As a test case for APEX, we propose to observe the J=8-7, 7-6, 4-3 and 3-2 lines of the different isotopes of CO in two low-mass YSOs. The combination of these data can test the physical and chemical structure of the envelope, and the high spectral resolution can distinguish the quiescent envelope and outflow components. The results will demonstrate to the community the feasibility of such high-J observations and will give an indication of the expected line strengths, important for planning future CHAMP+ observations.
The proposed sources are IRAS 12496-7650 and HH 46 (daytime permitting). Both are typical low-mass protostars in the southern hemisphere, which cannot be observed from the Northern hemisphere (see HDO proposal for a detailed description of HH46). Spectrally unresolved high-J CO has been detected for IRAS12496 by Lorenzetti et al. (1999, A&A 346, 604), but their origin is unclear (core vs. outflow). The line profiles will be essential to distinguish between the scenarios. Detailed radiative transfer models are available for these objects and predict line intensities for CO 7-6 of at least 0.5 K for the main isotope. The lower-J data are needed for comparison to determine line ratios and optical depths of the main isotope lines. C18O 3-2 lines are typically ~1 K, 12CO 3-2 lines of order 10 K.

Observational details

HH46IRS 08 25 43.9 (J2000) -51 00 36 (J2000) -4.0 km/s (TBC)
IRAS 12496 12 49 38.9 (B1950) -76 50 39 (B1950) - 4 km/s (TBC)

Receivers: Apex-2a, FLASH
Backends: 256 MHz bandwidth; FFTS-1GHz
Central frequencies:

12CO 3-2 345.795
13CO 3-2 330.587
C18O 3-2 329.33
12CO 4-3 461.040
12CO 7-6 806.651
13CO 8-7 881.273

Observing mode: Beam switching by 180" for isotopes; position switching by 900" for main isotope
Required rms:
0.1 K (T_A*) in 0.5 km/s bin for APEX-2a
0.3 K (T_A*) in 0.5 km/s bin for FLASH-low
0.3 K (T_A*) in 0.5 km/s bin for FLASH-high
Required integration time:
10 min/line/source APEX-2a (on+off) => 1hr total (assuming T_sys=400 K)
15 min/line/source FLASH low => 30 min total (assuming T_sys=2000 K)
30 min/line/source FLASH high => 120 min total (assuming T_sys=4000 K)
Total: 3.5 hrs excluding overheads
Note: if lines are stronger than expected, integration times can be shortened; good line profiles for the strongest lines are essential for the science to distinguish core and outflow components