H3O+ in the Galactic Center

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

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

Program is available and data products can be downloaded

Scientific justification:

Hydronium (H3 O+ ) is the key ion for the oxygen chemistry of denseclouds.Its submillimeter rotation-inversion transitions may be used to measuretheionization rate of molecular gas, especially if information isavailable from H2O and/or H+ observations. This potential has gonelargely untapped so far, as the lines are hard to observe even fromMauna Kea, which explains the lack of H3O+ observations since thepioneering work by Wootten et al (1991) and Phillips etal (1992). Lacking lower-frequency lines, H3 O+ is an excellentcandidate for the science verification phase of APEX.
In warm, dense molecular clouds such as occur in the Galactic Center, thestrongest line of H3O+ is the one at 364797.4 MHz. The 307 GHz line benefitsfrom better atmospheric transmission, but it is weaker, and it may be blendedwith a methanol line.
With T_mb ~1 K and deltaV~16 km s-1 , the Sgr B2(OH) position isthe strongestdetection of the H3 O+ 364 GHz line to date (Phillips et al 1992). Theemissionis reported as extended by Wootten et al (1991) although these authorsdo notshow a map. The source Sgr B2(OH) lies at an offset of (8,-27) arcsecfromthe Sgr B2(M) core, which is the largest mass of molecular gas in theGalaxy.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 ofH3O+far-infrared lines is seen toward Sgr B2(M), both local and in theline-of-sight clouds (Goicoechea & Cernicharo 2001). Emission inthe 22 GHz line of H2 O isalso widespread (McGrath et al 2004).
The sources GCS 3-2 and GC IRS3 are the strongest H+ absorbers inthe Galactic Center (Geballe et al 1999). While GCS 3-2 is locatedclose to the Quintuplet cluster, GC IRS3 is associated with the Sgr Acloud. Observations at these positions will measure the ionization ratein a variety of environments within the Galactic Center.
Recently, Goto et al (2004) observed strong mid-infraredabsorption of H+3 towards many sources all over the Galactic Centerregion. They infer an enhancement of the cosmic-ray ionization rate byfactors of 10 - 100 compared with the Solar neighbourhood. One questionis if this enhancement is confined to the diffuse gas that the H+observations probe, or also holds for the dense gas probed with H2O andH3O+ .

Observing strategy and time estimate
For Sgr B2(OH), a signal to noise ratio of 10 on 1 km s-1 channels, withTsys 250 K, is reached in 2 minutes integration (on+off) per point. We planto start with a nine-point map, which would take an hour to make with a factorof 3 overhead. Depending on the result, we may extend the map ­ probably tothe 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. Toaccommodate the full velocity range where H2 O absorption is seen (Neufeld etal 2000) we will use a correlator bandwidth of 512 MHz, which gives a velocitycoverage 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 onto the sources in Sgr A and the Quintuplet cluster. If their H3 O+ emission isstrong, we will give them a nine-point too. Note that the strong continuum ofSgr B2(M) can be used as pointing source. Including these sources, the requestis 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


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