https://nova.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:33124 −1 spectral resolution. In this paper, we present data on the inner ~250 pc (1°4) between Sgr C and Sgr B2. We focus on the hyperfine structure of the metastable ammonia inversion lines (J, K) = (1, 1)–(6, 6) to derive column density, kinematics, opacity, and kinetic gas temperature. In the CMZ molecular clouds, we find typical line widths of 8–16 km s⁻¹ and extended regions of optically thick (τ > 1) emission. Two components in kinetic temperature are detected at 25–50 K and 60–100 K, both being significantly hotter than the dust temperatures throughout the CMZ. We discuss the physical state of the CMZ gas as traced by ammonia in the context of the orbital model by Kruijssen et al. that interprets the observed distribution as a stream of molecular clouds following an open eccentric orbit. This allows us to statistically investigate the time dependencies of gas temperature, column density, and line width. We find heating rates between ~50 and ~100 K Myr⁻¹ along the stream orbit. No strong signs of time dependence are found for column density or line width. These quantities are likely dominated by cloud-to-cloud variations. Our results qualitatively match the predictions of the current model of tidal triggering of cloud collapse, orbital kinematics, and the observation of an evolutionary sequence of increasing star formation activity with orbital phase.]]> Wed 04 Sep 2019 10:06:16 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:52899 Tue 31 Oct 2023 15:46:12 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:34083 ⊙), cold (12 K), and 3.6–70 μm IR dark clump (MM1) that has the potential to form high-mass stars. We observed this prestellar clump candidate with the Submillimeter Array (~3".5 resolution) and Jansky Very Large Array (~2farcs1 resolution) in order to characterize the early stages of high-mass star formation and to constrain theoretical models. Dust emission at 1.3 mm wavelength reveals five cores with masses ≤15 M_⊙. None of the cores currently have the mass reservoir to form a high-mass star in the prestellar phase. If the MM1 clump will ultimately form high-mass stars, its embedded cores must gather a significant amount of additional mass over time. No molecular outflows are detected in the CO (2-1) and SiO (5-4) transitions, suggesting that the SMA cores are starless. By using the NH₃ (1, 1) line, the velocity dispersion of the gas is determined to be transonic or mildly supersonic (ΔV_nt/ΔV_th ~ 1.1–1.8). The cores are not highly supersonic as some theories of high-mass star formation predict. The embedded cores are four to seven times more massive than the clump thermal Jeans mass and the most massive core (SMA1) is nine times less massive than the clump turbulent Jeans mass. These values indicate that neither thermal pressure nor turbulent pressure dominates the fragmentation of MM1. The low virial parameters of the cores (0.1–0.5) suggest that they are not in virial equilibrium, unless strong magnetic fields of ~1–2 mG are present. We discuss high-mass star formation scenarios in a context based on IRDC G028.23-00.19, a study case believed to represent the initial fragmentation of molecular clouds that will form high-mass stars.]]> Tue 03 Sep 2019 18:23:32 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:34924 10⁴ M_⊙), remains an open problem, largely because they are so rare that examples of their cold, dense, molecular progenitors continue to be elusive. The molecular cloud G337.342−0.119, the "Pebble," is a candidate cold progenitor. Although G337.342−0.119 was originally identified as four separate ATLASGAL clumps, the similarities in their molecular line velocities and line widths in the MALT90 data set demonstrate that these four clumps are in fact one single, coherent cloud. This cloud is unique in the MALT90 survey for its combination of both cold temperatures (T _dust ~ 14 K) and large line widths (ΔV ~ 10 km s−1). The near/far kinematic distance ambiguity is difficult to resolve for G337.342−0.119. At the near kinematic distance (4.7 kpc), the mass is 5000 M_⊙ and the size is 7 × 2 pc. At the far kinematic distance (11 kpc), the mass is 27,000 M_⊙ and the size is 15 × 4 pc. The unusually large line widths of G337.342−0.119 are difficult to reconcile with a gravitationally bound system in equilibrium. If our current understanding of the Galaxy's Long Bar is approximately correct, G337.342−0.119 cannot be located at its end. Rather, it is associated with a large star-forming complex that contains multiple clumps with large line widths. If G337.342−0.119 is a prototypical cold progenitor for a high-mass cluster, its properties may indicate that the onset of high-mass star cluster formation is dominated by extreme turbulence.]]> Tue 03 Sep 2019 17:58:48 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:34952 ⊙), cold (14 K) 3.6–70 μm dark IRDC, G331.372-00.116. This infrared dark cloud (IRDC) has the potential to form high-mass stars, and given the absence of current star formation signatures, it seems to represent the earliest stages of high-mass star formation. We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.1 and 1.3 mm in dust continuum and line emission. The dust continuum reveals 22 cores distributed across the IRDC. In this work, we analyze the physical properties of the most massive core, ALMA1, which has no molecular outflows detected in the CO (2–1), SiO (5–4), and H₂CO (3–2) lines. This core is relatively massive (M = 17.6 M _⊙), subvirialized (virial parameter α _vir = M_vir/M = 0.14), and is barely affected by turbulence (transonic Mach number of 1.2). Using the HCO⁺ (3–2) line, we find the first detection of infall signatures in a relatively massive, prestellar core (ALMA1) with the potential to form a high-mass star. We estimate an infall speed of 1.54 km s⁻¹ and a high accretion rate of 1.96 × 10⁻³ M _⊙ yr⁻¹. ALMA1 is rapidly collapsing, out of virial equilibrium, which is more consistent with competitive accretion scenarios rather than the turbulent core accretion model. On the other hand, ALMA1 has a mass ~6 times larger than the clumps Jeans mass, as it is in an intermediate mass regime (M_J = 2.7 ⊙), contrary to what both the competitive accretion and turbulent core accretion theories predict.]]> Tue 03 Sep 2019 17:56:50 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:52118 45 K). The detection rate of the N2D+ emission toward the protostellar cores is 38%, which is higher than 9% for the prestellar cores, indicating that N2D+ does not exclusively trace prestellar cores. The detection rates of the DCO+ emission are 35% for the prestellar cores and 49% for the protostellar cores, which are higher than those for N2D+, implying that DCO+ appears more frequently than N2D+ in both prestellar and protostellar cores. Both the N2D+ and DCO+ abundances appear to decrease from the prestellar to the protostellar stage. The DCN, C2D, and 13CS emission lines are rarely seen in the dense cores of early evolutionary phases. The detection rate of the H2CO emission toward dense cores is 52%, three times higher than that for CH3OH (17%). In addition, the H2CO detection rate, abundance, line intensities, and line widths increase with the core evolutionary status, suggesting that the H2CO line emission is sensitive to protostellar activity.]]> Thu 28 Sep 2023 15:04:02 AEST ]]>