Search for: All records

We have used ALMA imaging (resolutions 0.1–0.4 arcsec) of ground and vibrationally excited lines of HCN and HC3N toward the nucleus of NGC 4945 to trace the protostellar phase in super star clusters (proto-SSC). Out of the 14 identified SSCs, we find that eight are in the proto-SSC phase showing vibrational HCN emission with five of them also showing vibrational HC3N emission. We estimate proto-SSC ages of 5–9.7 × 104 yr. The more evolved ones, with only HCN emission, are close to reach the zero-age main sequence (ZAMS; ages ≳105 yr). The excitation of the parental cloud seems to be related to the SSC evolutionary stage, with high (∼65 K) and low (∼25 K) rotational temperatures for the youngest proto and ZAMS SSCs, respectively. Heating by the H ii regions in the SSC ZAMS phase seems to be rather local. The youngest proto-SSCs are located at the edges of the molecular outflow, indicating SSC formation by positive feedback in the shocked regions. The proto-SSCs in NGC 4945 seem to be more evolved than in the starburst galaxy NGC 253. We propose that sequential SSC formation can explain the spatial distribution and different ages of the SSCs in both galaxies.

Using high angular resolution ALMA observations (0.02 arcsec ≈ 0.34 pc), we study the thermal structure and kinematics of the proto-super star cluster 13 in the central region of NGC 253 through their continuum and vibrationally excited HC3N emission from J = 24−23 and J = 26−25 lines arising from vibrational states up to v4 = 1. We have carried 2D-LTE and non-local radiative transfer modelling of the radial profile of the HC3N and continuum emission in concentric rings of 0.1 pc width. From the 2D-LTE analysis, we found a Super Hot Core (SHC) of 1.5 pc with very high vibrational temperatures (>500 K), and a jump in the radial velocity (21 km s−1) in the SE-NW direction. From the non-local models, we derive the HC3N column density, H2 density, and dust temperature (Tdust) profiles. Our results show that the thermal structure of the SHC is dominated by the greenhouse effect due to the high dust opacity in the IR, leading to an overestimation of the LTE Tdust and its derived luminosity. The kinematics and Tdust profile of the SHC suggest that star formation was likely triggered by a cloud–cloud collision. We compare proto-SSC 13 to other deeply embedded star-forming regions, and discuss the origin of the $L_\text{IR}/M_{\text{H}_2}$ excess above ∼100 L⊙ M$_\odot ^{-1}$ observed in (U)LIRGs.

Using 0.2 arcsec (∼3 pc) ALMA images of vibrationally excited HC3N emission (HC3N*) we reveal the presence of eight unresolved Super Hot Cores (SHCs) in the inner 160 pc of NGC 253. Our LTE and non-LTE modelling of the HC3N* emission indicate that SHCs have dust temperatures of 200–375 K, relatively high H2 densities of (1−6) × 106 cm−3 and high IR luminosities of (0.1–1) × 108 L⊙. As expected from their short-lived phase (∼104 yr), all SHCs are associated with young super star clusters (SSCs). We use the ratio of luminosities from the SHCs (protostar phase) and from the free–free emission (ZAMS star phase), to establish the evolutionary stage of the SSCs. The youngest SSCs, with the larges ratios, have ages of a few 104 yr (proto-SSCs) and the more evolved SSCs are likely between 105 and 106 yr (ZAMS-SSCs). The different evolutionary stages of the SSCs are also supported by the radiative feedback from the UV radiation as traced by the HNCO/CS ratio, with this ratio being systematically higher in the young proto-SSCs than in the older ZAMS-SSCs. We also estimate the SFR and the SFE of the SSCs. The trend found in the estimated SFE ($\sim 40{{\ \rm per\ cent}}$ for proto-SSCs and $\gt 85{{\ \rm per\ cent}}$ for ZAMS-SSCs) and in the gas mass reservoir available for star formation, one order of magnitude higher for proto-SSCs, suggests that star formation is still going on in proto-SSCs. We also find that the most evolved SSCs are located, in projection, closer to the centre of the galaxy than the younger proto-SSCs, indicating an inside-out SSC formation scenario.

We present airborne observations of the vertical gradient of atmospheric oxygen (δ(O₂/N₂)) and carbon dioxide (CO₂) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O₂/N₂Ratio and CO₂Airborne Southern Ocean Study, from 15 January to 29 February 2016. Gradients were predominately anticorrelated, with excesses ofδ(O₂/N₂) and depletions of CO₂found within the boundary layer, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model, we found that the main driver of gradients was air‐sea exchange of O₂and CO₂driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air‐sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45°S, 75°S, 100°W, and 45°W, we show that the sampling density of the campaign was such that the observed mean GR (± standard error), −4.0± 0.8 mol O₂per mol CO₂, was a reasonable proxy for both the mean GR and the mean molar ratio of air‐sea fluxes of O₂and CO₂during the O₂/N₂Ratio and CO₂Airborne Southern Ocean Study.