12 mm line survey of the dense molecular gas towards the W28 field TeV gamma-ray sources

Abstract

We present 12 mm Mopra observations of dense molecular gas towards the W28 supernova remnant (SNR) field. The focus is on the dense molecular gas towards the TeV gamma‐ray sources detected by the HESS telescopes, which likely trace the cosmic rays from W28 and possibly other sources in the region. Using the NH3 inversion transitions we reveal several dense cores inside the molecular clouds, the majority of which coincide with high‐mass star formation and H ii regions, including the energetic ultracompact H ii region G5.89−0.39. A key exception to this is the cloud north‐east of W28, which is well known to be disrupted as evidenced by clusters of 1720 MHz OH masers and broad CO line emission. Here we detect broad NH3, up to the (9,9) transition, with linewidths up to 16 km s−1. This broad NH3 emission spatially matches well with the TeV source HESS J1801−233 and CO emission, and its velocity dispersion distribution suggests external disruption from the W28 SNR direction. Other lines are detected, such as HC3N and HC5N, H2O masers, and many radio recombination lines, all of which are primarily found towards the southern high‐mass star formation regions. These observations provide a new view on to the internal structures and dynamics of the dense molecular gas towards the W28 SNR field and, in tandem with future higher‐resolution TeV gamma‐ray observations, will offer the chance to probe the transport of cosmic rays into molecular clouds.

Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants

“…Still, recent observations hint at enhanced ionization rates in molecular clouds associated with SNRs, e.g. Nicholas et al (2011) and Ceccarelli et al (2011), in support of the presented model.…”

“…The W28 SNR is a prominent example of an association of partly resolved HESS γ-ray sources and dense, shocked molecular gas, as revealed by the molecular line observations of Nicholas et al (2011Nicholas et al ( , 2012. In particular, the cm-wave inversion transitions of ammonia (NH 3 ) show peak intensities that coincide with the positions of peaks in γ-ray emission and radio synchrotron emission.…”

“…The high abundance and high excitation of NH 3 observed in molecular Core 2 by Nicholas et al (2011) could be explained as the result of rapid formation in an ion-driven chemistry. As discussed by Nicholas et al, the NH 3 might result from shockdriven chemistry, which would also fit well with the other tracers of molecular shocks that they describe.…”

“…5. Dense molecular gas associated with the W28 SNR shows evidence of heating and chemistry driven by shock waves (Nicholas et al 2011(Nicholas et al , 2012, but the high excitation of ammonia molecules in one molecular core can also be interpreted in terms of ion chemistry driven by an enhanced rate of ionization as outlined in the next subsection. A derivation of the ionization rate is given in Sect.…”

“…Conventionally NH 3 emission is thought to probe dense molecular gas and the relative intensities of the inversion transitions are considered to be good indicators of kinetic temperature. Nicholas et al (2011) identified a molecular cloud Core 2 that is associated with the γ-ray source HESS J1801−233 and where the (J, K) = (3, 3) inversion line of NH 3 is unusually intense compared with the (1, 1) and (2, 2) lines of lower excitation. Moreover, they clearly detected emission in the (6, 6) line and weakly in the (9, 9) line at the same position.…”