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تسجيل مستخدم جديدHot ammonia around young O-type stars. III. High-mass star formation and hot core activity in W51~Main
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This paper is the third in a series of ammonia multilevel imaging studies in well-known high-mass star forming regions. Using the JVLA, we have mapped the hot and dense molecular gas in W51 Main, with about 0.2 arcsec angular resolution, in five highly-excited metastable inversion transitions of ammonia (ammonia): (J,K)=(6,6), (7,7), (9,9), (10,10), and (13,13). We have identified and characterised two main centers of high-mass star formation in W51-Main: the W51e2 complex and the W51e8 core (6 arcsec southward of W51e2). The former breaks down into three further sub-cores: W51e2-W, which surrounds the well known HC HII region, where hot ammonia is observed in absorption, and two additional dusty cores, W51e2-E (~0.8 to the East) and W51e2-NW (~1 to the North), where hot ammonia is observed in emission. The velocity maps towards the HC HII region show a clear velocity gradient that may indicate rotation, though we do not directly observe a Keplerian velocity profile. The absence of outflow and/or maser activity and the low amount of molecular gas available for accretion (~5 solar masses) with respect to the mass of the central YSO (>20 solar masses), both indicate that the central YSO has already accreted most of its final mass. On the other hand, in the nearby W51e2-E object, the relatively large amount of hot molecular gas available for accretion (~20 solar masses, within about half an arcsecond or 2500 AU), along with strong outflow and maser activity, indicates that the main accretion center in the W51e2 complex is W51e2-E rather than W51e2-W. Finally, W51e2-NW and W51e8, although less dense, are also hot cores and contain a significant amount of molecular gas (~30 and 70 solar masses, respectively). We speculate that they may host high-mass YSOs either at a previous evolutionary stage or with lower mass than W51e2-E and W51e2-W.
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We have used the JVLA at the 1 cm band to map five highly-excited metastable inversion transitions of ammonia, (J,K)=(6,6), (7,7), (9,9), (10,10), and (13,13), in W51 IRS2 with ~0.2 angular resolution. We present detections of both thermal (extended)
ammonia emission in the five inversion lines, with rotational states ranging in energy from about 400 to 1700 K, and point-like ammonia maser emission in the (6,6), (7,7), and (9,9) lines. The thermal ammonia emits around a velocity of 60 km/s, near the clouds systemic velocity, is elongated in the east-west direction across 4 and is confined by the HII regions W51d, W51d1, and W51d2. The ammonia masers are observed in the eastern tip of the dense clump traced by thermal ammonia, offset by 0.65 to the East from its emission peak, and have a peak velocity at ~47.5 km/s. No maser components are detected near the systemic velocity. The ammonia masers are separated by 0.65 (3500 AU) from the (rare) vibrationally-excited SiO masers, excited by the deeply-embedded YSO W51-North. This excludes that the two maser species are excited by the same object. Interestingly, the ammonia masers originate at the same sky position as a peak in a submm line of SO2 imaged with the SMA, tracing a face-on circumstellar disk/ring around W51-North. In addition, the thermal emission from the most highly excited ammonia lines, (10,10) and (13,13), shows two main condensations, the dominant one towards W51-North with the SiO/H2O masers, and a weaker peak at the ammonia maser position. We propose a scenario where the ring seen in SO2 emission is a circumbinary disk surrounding (at least) two high-mass YSOs, W51-North (exciting the SiO masers) and a nearby companion (exciting the ammonia masers), separated by 3500 AU. This finding indicates a physical connection (in a binary) between the two rare SiO and ammonia maser species.
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