Oral Presentation
Cloud-scale CO freeze-out in the giant molecular filament G214.5-1.8 as a result of low cosmic-ray ionisation rate
Presenter: Seamus Clarke (ASIAA)
Herschel has revealed that filaments across spatial scales are a prominent feature of the dense ISM and are intimately connected to the star formation process. Here we present 12CO (2-1) and (1-0), 13CO (1-0) and C18O (1-0) observations using the IRAM 30m of the outer Galaxy giant molecular filament (GMF) G214.5-1.8, an unusually cold, quiescent and potentially young GMF impacted by a HI superbubble. We use the 12CO data to construct excitation temperature maps, finding that the gas temperature is very low with a median of only 8.2 K. We use these excitation temperatures to determine the 13CO and C18O column densities; combined with the Herschel derived column densities, the abundance of these isotopologues are also found. A striking feature is that the entire 13 pc length spine of G214.5 is seen as a continuous region of low 13CO abundance. Radial slices show that this low abundance region is confined to the inner ~0.8 pc of the filament, where the total column density increases and the dust temperature drops noticeably. Because of this connection with increased density and decreased temperature, we attribute this drop in 13CO abundance to freeze-out, i.e. 13CO being deposited onto the dust grains as ice, making G214.5 one of the largest scale example of this phenomena more commonly seen in prestellar cores (~0.1 pc). We construct an axisymmetric model of the filament's radial H2 and CO profile using a simple freeze-out prescription, which we constrain using the filament's projected radial profiles. With this we find that due to the low central densities in the filament (~8000 cm-3), the cosmic-ray ionisation rate must also be very low, ~2e-18 /s, an order of magnitude lower than typically found. This low cosmic-ray ionisation rate is in agreement with the low gas and dust temperatures found too. Using timescale arguments, we posit that such a low ionisation rate may lead to ambipolar diffusion being an important physical process along G214.5's entire spine. We suggest that if low cosmic-ray ionisation rates are more common in the outer Galaxy, and other quiescent regions, cloud-scale CO freeze-out occurring at low column and number densities may also be more prevalent, having consequences for CO observations and their interpretation.
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