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Aims.We aim to identify and characterize cores in the high-mass protocluster W49A, determine their evolutionary stages, and measure the associated lifetimes. Methods.We built a catalog of 129 cores extracted from an ALMA 1.3 mm continuum image at 0.26″ (2900 au) angular resolution. The association between cores and hypercompact or ultracompact HII(H/UC HII) regions was established from the analysis of VLA 3.3 cm continuum and H30αline observations. We also looked for emission of hot molecular cores (HMCs) using the methyl formate doublet at 218.29 GHz. Results.We identified 40 cores associated with an H/UC HIIregion and 19 HMCs over the ALMA mosaic. The 52 cores with an H/UC HIIregion and/or an HMC are assumed to be high-mass protostellar cores, while the rest of the core population likely consists of prestellar cores and low-mass protostellar cores. We found a good agreement between the two tracers of ionized gas, with 23 common detections and only four cores detected at 3.3 cm and not in H30α. The spectral indexes from 3.3 cm to 1.3 mm range from 1, for the youngest cores with partially optically thick free-free emission, to about −0.1, which is for the optically thin free-free emission obtained for cores that are likely more evolved. Conclusions.Using the H/UC HIIregions as a reference, we found the statistical lifetimes of the HMC and massive protostellar phases in W49N to be about 6 × 104yr and 1.4 × 105yr, respectively. We also showed that HMCs can coexist with H/UC HIIregions during a short fraction of the core lifetime, about 2 × 104yr. This indicates a rapid dispersal of the inner molecule envelope once the HC HIIis formed.
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Giant Postflare Loops in Active Regions with an Extremely Strong Coronal Magnetic Field
Abstract We report for the first time the detection of thermal free–free emission from post-flare loops at 34 GHz in images from the Nobeyama Radioheliograph. We studied eight loops, seven of which were from regions with an extremely strong coronal magnetic field reported by Fedenev et al. Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multitemperature structure and was associated with noise storm emission in metricλ. The comparison of the 17 GHz emission with that at 34 GHz, after a calibration correction of the latter, showed that the emission was optically thin at both frequencies. We describe the structure and evolution of the loops and we computed their density, obtaining values for the top of the loops between 1 and 6 × 1010cm−3, noticeably varying from one loop to another and in the course of the evolution of the same loop system; these values have only a weak dependence on the assumed temperature, 2 × 106K in our case, as we are in the optically thin regime. Our density values are above those reported from EUV observations, which go up to about 1010cm−3. This difference could be due to the fact that different emitting regions are sampled in the two domains and/or due to the more accurate diagnostics in the radio range, which do not suffer from inherent uncertainties arising from abundances and non-LTE excitation/ionization equilibria. We also estimated the magnetic field in the loop tops to be in the range of 10–30 G.
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- PAR ID:
- 10532575
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 971
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 122
- Size(s):
- Article No. 122
- Sponsoring Org:
- National Science Foundation
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