APEX SV projects

H3O+ in the Galactic Center

Coordinator: F. van der Tak, A. Belloche & P. Schilke

Abstract:
We propose to observe the 364 GHz line of H3O+ towards selected positions in Sgr A and B, including a map around Sgr B2(OH). Together with existing H2O and H+ observations, the new APEX data will constrain the ionization rate of molecular gas in the Galactic Center.

Data:
Program is available and data products can be downloaded

Scientific justification:

Hydronium (H3 O+ ) is the key ion for the oxygen chemistry of dense clouds. Its submillimeter rotation-inversion transitions may be used to measure the ionization rate of molecular gas, especially if information is available from H2O and/or H+ observations. This potential has gone largely untapped so far, as the lines are hard to observe even from Mauna Kea, which explains the lack of H3O+ observations since the pioneering work by Wootten et al (1991) and Phillips et al (1992). Lacking lower-frequency lines, H3 O+ is an excellent candidate for the science verification phase of APEX.
In warm, dense molecular clouds such as occur in the Galactic Center, the strongest line of H3O+ is the one at 364797.4 MHz. The 307 GHz line benefits from better atmospheric transmission, but it is weaker, and it may be blended with a methanol line.
With T_mb ~1 K and deltaV~16 km s-1 , the Sgr B2(OH) position is the strongest detection of the H3 O+ 364 GHz line to date (Phillips et al 1992). The emission is reported as extended by Wootten et al (1991) although these authors do not show a map. The source Sgr B2(OH) lies at an offset of (8,-27) arcsec from the Sgr B2(M) core, which is the largest mass of molecular gas in the Galaxy. In this region, strong H2 O absorption was detected by Cernicharo et al (1997) with ISO-LWS, which is extended over several arc minutes. Absorption of H3O+ far-infrared lines is seen toward Sgr B2(M), both local and in the line-of-sight clouds (Goicoechea & Cernicharo 2001). Emission in the 22 GHz line of H2 O is also widespread (McGrath et al 2004).
The sources GCS 3-2 and GC IRS3 are the strongest H+ absorbers in the Galactic Center (Geballe et al 1999). While GCS 3-2 is located close to the Quintuplet cluster, GC IRS3 is associated with the Sgr A cloud. Observations at these positions will measure the ionization rate in a variety of environments within the Galactic Center.
Recently, Goto et al (2004) observed strong mid-infrared absorption of H+3 towards many sources all over the Galactic Center region. They infer an enhancement of the cosmic-ray ionization rate by factors of 10 - 100 compared with the Solar neighbourhood. One question is if this enhancement is confined to the diffuse gas that the H+ observations probe, or also holds for the dense gas probed with H2O and H3O+ .

Observing strategy and time estimate
For Sgr B2(OH), a signal to noise ratio of 10 on 1 km s-1 channels, with Tsys 250 K, is reached in 2 minutes integration (on+off) per point. We plan to start with a nine-point map, which would take an hour to make with a factor of 3 overhead. Depending on the result, we may extend the map probably to the North, toward the M and ultimately the N core, which lies (+3,+43) arc- sec from the N core. Encompassing all three cores at 8 spacing (half-beam) would mean making a map of 10×5 points, directed almost North-South. To accommodate the full velocity range where H2 O absorption is seen (Neufeld et al 2000) we will use a correlator bandwidth of 512 MHz, which gives a velocity coverage of 500 km s-1 at 0.25 km s-1 resolution. Total time request in- cluding pointing, calibration etc is 6 hours. If time permits we will move on to the sources in Sgr A and the Quintuplet cluster. If their H3 O+ emission is strong, we will give them a nine-point too. Note that the strong continuum of Sgr B2(M) can be used as pointing source. Including these sources, the request is 8 hours.

Source	RA(2000)	DEC(2000)	V_LSR
Sgr B2(OH)	17 47 20.8	-28 23 32	+62
GCS 3-2	17 46 14.9	-28 49 43	-70
GC IRS3	17 45 39.6	-29 00 24	+30

References

Cernicharo et al 1991: A&A 323, L25
Geballe et al 1999: ApJ 510, 251
Goto et al 2002: PASJ 54, 951
Goicoechea & Cernicharo 2001: ApJ 554, L213
McGrath et al 2004: ApJS 155, 577
Neufeld et al 2000: ApJ 539, L111
Phillips et al 1992: ApJ 399, 533
Wootten et al 1991: ApJ 380, L79