Molecules in G1.6–0.025—“HOT” Chemistry in the Absence of Star Formation at the Periphery of the Galactic Center Region

Abstract

We present molecular line mapping of the Giant Molecular Cloud G1.6−0.025, which is located at the highlongitude end of the Central Molecular Zone (CMZ) of our Galaxy. We assess the degree of star formation activity in that region using several tracers, and find very little. We made a large-scale, medium (2 ) resolution map in the J = 2 − 1 transition of SiO for which we find clumpy emission over a ∼ 0.• 8 × 0.• 3 sized region stretching along the Galactic plane. Toward selected positions we also took spectra in the easy-to-excite J k = 2 k − 1 k quartet of CH 3 OH and the carbon monosulfide (CS) 2 − 1 line. Throughout the cloud these CH 3 OH lines are, remarkably, several times stronger than both the CS and the SiO lines. The large widths of all the observed lines, similar to values generally found in the Galactic center, indicate a high degree of turbulence. Several high LSR velocity clumps that have 50-80 km s −1 higher velocities than the bulk of the molecular cloud appear at the same projected position as "normal" velocity material; this may indicate cloud-cloud collisions. Statistical equilibrium modeling of the CH 3 OH lines observed by us and others yields relatively high densities and moderate temperatures for a representative dual velocity position. We find 8×10 4 cm −3 /30 K for material in the G1.6−0.025 cloud and a higher temperature (190 K), but a 50% lower density in a high-velocity clump projected on the same location. Several scenarios are discussed in which shock chemistry might enhance the CH 3 OH and SiO abundances in G1.6−0.025 and elsewhere in the CMZ.

EXPANDED VERY LARGE ARRAY DETECTION OF 44.1 GHz CLASS I METHANOL MASERS IN SAGITTARIUS A

“…These OH masers are collisionally pumped and are considered typical signposts of shock-excited material. Theoretical modeling indicates a similar range of physical properties (T K ∼ 80-100 K, n ∼ 10 4 -10 5 cm −3 ) for 36 GHz Class I methanol and 1720 MHz OH maser excitation (Morimoto et al 1985;Pratap et al 2008;Pihlström et al 2008;Menten et al 2009). That is, if there is 4 Also an Adjunct Astronomer at the National Radio Astronomy Observatory.…”

“…Perhaps this indicates that the methanol is located deeper in the cloud, where the line-of-sight LSR velocity is not as much disturbed by the SNR shock front as the region where the OH masers originate, and perhaps requiring less energetic shock excitation than OH. The location and physical properties of the gas giving rise to the 36 GHz methanol masers (T < 100 K and n ∼ 10 4 -10 5 cm −3 ; Morimoto et al 1985;Cragg et al 1992;Liechti & Wilson 1996;Menten et al 2009) also somewhat differ from the gas traced by 1720 MHz masers (T ∼75 K and n ∼ 10 5 cm −3 ; Pihlström et al 2008). We note that no bright 36 GHz masers were detected in the tangent points of the CND, at V LSR ∼ ±130 km s −1 , where 1720 MHz OH masers indicate that energetic shocks are present.…”

“…Because of the limited angular resolution, 36 GHz methanol modeling has not been heavily pursued (however see, e.g., Morimoto et al 1985;Cragg et al 1992;Liechti & Wilson 1996;Menten et al 2009, for individual modeling of the 36 GHz methanol line). Most authors have convincingly argued that the spectra indicate a broad thermal emission component with strong, narrow maser emission components superimposed.…”

A Comprehensive Survey of Hydrogen Chloride in the Galaxy

“…The source is a giant molecular cloud located in the Central Molecular Zone (CMZ) of the Galaxy, a region characterized by elevated gas temperature (typically ∼70 K), higher gas density ( 10 4 cm −3 ), and highly supersonic internal velocity dispersion (∼15-50 km s −1 ; Morris & Serabyn 1996). Dense cloud cores are seen in various molecular tracers (Gardner & Whiteoak 1981;Gardner & Boes 1987;Whiteoak & Peng 1989;Kuiper et al 1993;Menten et al 2009), yet there is little sign of apparent star-forming activity in the cloud. For the extended 50 km s −1 cloud of G1.6−0.025, we derive a C 18 O column density of 2 × 10 15 from the C 18 O J = 2-1 spectrum taken at the CSO, assuming LTE at a gas temperature of 30 K (Menten et al 2009).…”

“…A prime target would be M-0.02-0.07, the so-called 20 km s −1 cloud ∼2 northeast of Sgr A * . A common trait among the three clouds is the likely interaction with an SNR (Menten et al 2009). It is not clear though how such interaction would lead to relatively low production of HCl.…”