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Register a new userHot Ammonia around Young O-type Stars. II. JVLA imaging of highly-excited metastable ammonia masers in W51-North
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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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To constrain theoretical models of high-mass star formation, observational signatures of mass accretion in O-type forming stars are desirable. Using the JVLA, we have mapped the hot and dense molecular gas in the hot core NGC7538 IRS1, with 0.2 angular resolution, in seven metastable (J=K) inversion transitions of ammonia: (J,K)=(6,6), (7,7), (9,9), (10,10), (12,12), (13,13), and (14,14). These lines arise from energy levels between ~400 K and ~1950 K above the ground state, and are observed in absorption against the HC-HII region associated with NGC7538 IRS1. With a 500 AU linear resolution, we resolve the elongated North-South ammonia structure into two compact components: the main core and a southernmost component. Previous observations of the radio continuum with a 0.08 (or 200 AU) resolution, resolved in turn the compact core in two (northern and southern) components. These features correspond to a triple system of high-mass YSOs IRS1a, IRS1b, and IRS1c identified with VLBI measurements of methanol masers. The velocity maps of the compact core show a clear velocity gradient in all lines, which is indicative of rotation in a (circumbinary) envelope, containing ~40 solar masses (dynamical mass). In addition, we derived physical conditions of the molecular gas: rotational temperatures ~280 K, ammonia column densities ~1.4-2.5 x 10^19 cm-2, H_2 volume densities ~3.5-6.2 x 10^10 cm-3, and a total gas mass in the range of 19-34 solar masses, for the main core. We conclude that NGC7538 IRS1 is the densest hot molecular core known, containing a rotating envelope which hosts a multiple system of high-mass YSOs, possibly surrounded by accretion disks. Future JVLA observations in the A-configuration are needed to resolve the binary system in the core and may allow to study the gas kinematics in the accretion disks associated with individual binary members.
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.
The circumstellar ammonia (NH$_3$) chemistry in evolved stars is poorly understood. Previous observations and modelling showed that NH$_3$ abundance in oxygen-rich stars is several orders of magnitude above that predicted by equilibrium chemistry. In this article, we characterise the spatial distribution and excitation of NH$_3$ in the O-rich circumstellar envelopes (CSEs) of four diverse targets: IK Tau, VY CMa, OH 231.8+4.2, and IRC +10420 with multi-wavelength observations. We observed the 1.3-cm inversion line emission with the Very Large Array (VLA) and submillimetre rotational line emission with the Heterodyne Instrument for the Far-Infrared (HIFI) aboard Herschel from all four targets. For IK Tau and VY CMa, we observed the rovibrational absorption lines in the $ u_2$ band near 10.5 $mu$m with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). We also attempted to search for the rotational transition within the $v_2=1$ state near 2 mm with the IRAM 30m Telescope towards IK Tau. Non-LTE radiative transfer modelling, including radiative pumping to the vibrational state, was carried out to derive the radial distribution of NH$_3$ in these CSEs. Our modelling shows that the NH$_3$ abundance relative to molecular hydrogen is generally of the order of $10^{-7}$, which is a few times lower than previous estimates that were made without considering radiative pumping and is at least 10 times higher than that in the C-rich CSE of IRC +10216. Incidentally, we also derived a new period of IK Tau from its $V$-band light curve. NH$_3$ is again detected in very high abundance in O-rich CSEs. Its emission mainly arises from localised spatial-kinematic structures that are probably denser than the ambient gas. Circumstellar shocks in the accelerated wind may contribute to the production of NH$_3$. (Abridged abstract)
We present the first detection of para-ammonia masers in NGC 7538: multiple epochs of observation of the 14NH3 (J,K) = (10,8) and (9,8) lines. We detect both thermal absorption and nonthermal emission in the (10,8) and (9,8) transitions and the absence of a maser in the (11,8) transition. The (9,8) maser is observed to increase in intensity by 40% over six months. Using interferometric observations with a synthesized beam of 0.25, we find that the (10,8) and (9,8) masers originate at the same sky position near IRS1. With strong evidence that the (10,8) and (9,8) masers arise in the same volume, we discuss the application of pumping models for the simultaneous excitation of nonmetastable (J > K) para-ammonia states having the same value of K and consecutive values of J. We also present detections of thermal absorption in rotational states ranging in energy from E/k_B ~ 200 K to 2000 K, and several non-detections in higher-energy states. In particular, we describe the populations in eight adjacent rotational states with K=6, including two maser transitions, along with the implications for ortho-ammonia pumping models. An existing torus model for molecular gas in the environment of IRS1 has been applied to the masers; a variety of maser species are shown to agree with the model. Historical and new interferometric observations of 15NH3 (3,3) masers in the region indicate a precession of the rotating torus at a rate comparable to continuum-emission-based models of the region. We discuss the general necessity of interferometric observations for diagnosing the excitation state of the masers and for determining the geometry of the molecular environment.
We report the discovery of maser emission in the two lowest rotational transitions of CS toward the high-mass protostar W51 e2e with ALMA and the JVLA. The masers from CS J=1-0 and J=2-1 are neither spatially nor spectrally coincident (they are separated by ~150 AU and ~30 km/s), but both appear to come from the base of the blueshifted outflow from this source. These CS masers join a growing list of rarely-detected maser transitions that may trace a unique phase in the formation of high-mass protostars.
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