Examinando por Autor "Law, C. Y."

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ALMA–IRDC – II. First high-angular resolution measurements of the 14N/15N ratio in a large sample of infrared-dark cloud cores
(Oxford Academics: Oxford University Press, 2021-03-22) Fontani, F.; Barnes, A. T.; Caselli, P.; Henshaw, J. D.; Cosentino, G.; Jiménez Serra, I.; Tan, J. C.; Pineda, J. E.; Law, C. Y.; European Research Council (ERC); Agencia Estatal de Investigación (AEI)
The 14N/15N ratio in molecules exhibits a large variation in star-forming regions, especially when measured from N2H+ isotopologues. However, there are only a few studies performed at high-angular resolution. We present the first interferometric survey of the 14N/15N ratio in N2H+ obtained with Atacama Large Millimeter Array observations towards four infrared-dark clouds harbouring 3 mm continuum cores associated with different physical properties. We detect N15NH+ (1–0) in ∼20−40 per cent of the cores, depending on the host cloud. The 14N/15N values measured towards the millimetre continuum cores range from a minimum of ∼80 up to a maximum of ∼400. The spread of values is narrower than that found in any previous single-dish survey of high-mass star-forming regions and than that obtained using the total power data only. This suggests that the 14N/15N ratio is on average higher in the diffuse gaseous envelope of the cores and stresses the need for high-angular resolution maps to measure correctly the 14N/15N ratio in dense cores embedded in IRDCs. The average 14N/15N ratio of ∼210 is also lower than the interstellar value at the Galactocentric distance of the clouds (∼300–330), although the sensitivity of our observations does not allow us to unveil 14N/15N ratios higher than ∼400. No clear trend is found between the 14N/15N ratio and the core physical properties. We find only a tentative positive trend between 14N/15N and H2 column density. However, firmer conclusions can be drawn only with higher sensitivity measurements.
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ALMA–IRDC: dense gas mass distribution from cloud to core scales
(Oxford Academics: Oxford University Press, 2021-03-22) Barnes, A. T.; Henshaw, J. D.; Fontani, F.; Pineda, J. E.; Cosentino, G.; Tan, J. C.; Caselli, P.; Jiménez Serra, I.; Law, C. Y.; Avison, A.; Bigiel, F.; Feng, S.; Kong, S.; Longmore, S. N.; Moser, L.; Parker, R. J.; Sánchez Monge, Á.; Wang, K.; European Research Council (ERC); Agencia Estatal de Investigación (AEI); Deutsche Forschungsgemeinschaft (DFG); East Asia Core Observatories Association (EACOA); National Natural Science Foundation of China (NSFC); National Key Research and Development Program of China; Peking University; Avison, A. [0000-0002-2562-8609]
Infrared dark clouds (IRDCs) are potential hosts of the elusive early phases of high mass star formation (HMSF). Here, we conduct an in-depth analysis of the fragmentation properties of a sample of 10 IRDCs, which have been highlighted as some of the best candidates to study HMSF within the Milky Way. To do so, we have obtained a set of large mosaics covering these IRDCs with Atacama Large Millimeter/submillimeter Array (ALMA) at Band 3 (or 3 mm). These observations have a high angular resolution (∼3 arcsec; ∼0.05 pc), and high continuum and spectral line sensitivity (∼0.15 mJy beam−1 and ∼0.2 K per 0.1 km s−1 channel at the N2H+ (1 − 0) transition). From the dust continuum emission, we identify 96 cores ranging from low to high mass (M = 3.4−50.9 M⊙) that are gravitationally bound (αvir = 0.3−1.3) and which would require magnetic field strengths of B = 0.3−1.0 mG to be in virial equilibrium. We combine these results with a homogenized catalogue of literature cores to recover the hierarchical structure within these clouds over four orders of magnitude in spatial scale (0.01–10 pc). Using supplementary observations at an even higher angular resolution, we find that the smallest fragments (<0.02 pc) within this hierarchy do not currently have the mass and/or the density required to form high-mass stars. None the less, the new ALMA observations presented in this paper have facilitated the identification of 19 (6 quiescent and 13 star-forming) cores that retain >16 M⊙ without further fragmentation. These high-mass cores contain trans-sonic non-thermal motions, are kinematically sub-virial, and require moderate magnetic field strengths for support against collapse. The identification of these potential sites of HMSF represents a key step in allowing us to test the predictions from high-mass star and cluster formation theories.