Triggered/sequential star formation? A multi-phase ISM study around the prominent IRDC G18.93-0.03

Context. Triggered star formation has been discussed for many years, and evidence for the formation of stars and cores triggered by HII regions is under debate. Aims. We investigate the imprints of an expanding HII region on a pre-existing starless clumps. Methods. We selected an infrared dark filament spanning 0.8 degrees. One portion of this filament, G18.93-0.03 is a prominent dust complex, with the molecular clump G18.93/m being IR dark from near-IR wavelength up to 160 mu m. Spitzer composite images show an IR bubble spatially associated with G18.93-0.03. We use GRS (CO)-C-13 and IRAM 30m (HCO+)-C-13 data to disentangle the large and small scale spatial structure of the region. From ATLASGAL submm data we calculate the gas mass, while we use the (HCO+)-C-13 line width to estimate its virial mass. To study the IR properties of G18.93/m we use Herschel data and produce temperature maps from fitting the spectral energy distribution. With the MAGPIS 20 cm and SuperCOSMOS H-alpha data we trace the ionized gas, and the VGPS Hi survey provides information on the atomic hydrogen gas. Results. We show that the bubble is spatially associated with G18.93, located at a kinematic near distance of 3.6 kpc. The total gas mass of similar to 870 M-circle dot splits up into 6 sub-clumps, of which G18.93/m is the most massive with 280 M-circle dot. The virial analysis shows that it may be gravitationally bound and has neither Spitzer young stellar objects nor mid-IR point sources within. Therefore we call it pre-stellar. Fitting the spectral energy distribution reveals a temperature distribution that decreases towards its center, but heating from the ionizing source puts it above the general ISM temperature. We find that the bubble is filled by HII gas, ionized by an O8.5 star. Between the ionizing source and the IR dark clump G18.93/m we find a layered structure of hydrogen phases, from ionized to atomic to molecular gas, revealing a photon dominated region. Furthermore, we identify an additional velocity component within the bubble's 8 mu m emission rim at the edge of the infrared dark cloud and speculate that it might be shock induced by the expanding HII region. Conclusions. While the elevated temperature allows for the build-up of larger fragments, and the shock induced velocity component may lead to additional turbulent support, the density profile of G18.93/m does not show signatures of the expanding bubble. While the first two signatures favor high-mass star formation, we do not find conclusive evidence that the massive clump G18.93/m is prone to collapse because of the expanding HII region.