APEX SV projects

H2D+ in the Prestellar Core Barnard 68

Coordinator: M. Hogerheijde, J. Alves, P. Caselli

Abstract:

Data:
Program is available and data products can be downloaded

Scientific Background:

The chemistry in prestellar cores is dominated by freeze out of molecules onto dust grains in the cold and dense interiors. Under these conditions, deuterium-carrying isotopomers of common species are significantly enhanced, through the exchange reaction H3+ + HD <-> H2D+ + H2, which mainly runs forward at low T.
Ortho-H2D+ has its 1_1,0 -- 1_1,1 transition at 372 GHz, at the edge of an atmospheric absorption band, and can only be observed under excellent opacity condtions. This line has been detected in a few objects, including the prestellar core L1544 (Caselli et al. 2003), with a strength of 1 K.
One of the best studied prestellar cores is Barnard 68 (B68). Its density structure was determined by Alves et al. (2001), and the depletion investigated by Bergin et al. (2002). It is an excellent target to try and detect H2D+, and we propose to observe this transition with APEX on 5 positions along a 80 arcsec strip across its center. These observations will reveal the H2D+ abundance profile across the region of maximum depletion, and will provide direct insight into the depletion and deuteration in this core.
These observations will demonstrate that APEX can make use of the excellent atmospheric conditions above Chajnantor and successfully detect lines at the edges of the atmospheric windows. It will also represent the largest angular extent over which the H2D+ emission has been measured in any object (the Caselli et al. data on L1544 go out to 20"). B68 is also well positioned in the sky for observations in July. It is likely that a publication can be based on these observations.

Feasability
Under the best weather conditions on Mauna Kea a Tsys of 1500 K can be achieved at 372 GHz. Assuming that a similar Tsys can be achieved during the observing period on Chajnantor, and using the 250 MHz bandwidth backend setting, 45 min (on+off) integration will yield a noise rms of 100 mK. If the line strength at the center of the core is similar to that observed in L1544 (1 K), a strip of 5 positions spaced by 20 arcsec can be observed in 5 hours including overheads. If the emission is less strong (the density in B68 is a factor 6 lower than in L1544), more time can be spent on getting a detection at the center position + one or two offset positions.

Feasability
Under the best weather conditions on Mauna Kea a Tsys of 1500 K can be achieved at 372 GHz. Assuming that a similar Tsys can be achieved during the observing period on Chajnantor, and using the 250 MHz bandwidth backend setting, 45 min (on+off) integration will yield a noise rms of 100 mK. If the line strength at the center of the core is similar to that observed in L1544 (1 K), a strip of 5 positions spaced by 20 arcsec can be observed in 5 hours including overheads. If the emission is less strong (the density in B68 is a factor 6 lower than in L1544), more time can be spent on getting a detection at the center position + one or two offset positions.

Target
Barnard 68 RA=17:22:35 DEC=-23:49:30 J2000
5 positions:
dRA = -40", -20", 0", +20", +40"
dDEC = 0", 0", 0", 0", 0"

Line
H2D+ 1_1,0--1_1,1 372.42134 GHz
Integration time per position: 45 min, 100 mK rms noise target
Total, incl overheads: 5 hours