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.
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 angul
ar 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.
We present the first interferometric molecular line and dust emission maps for the Galactic Center (GC) cloud G0.253+0.016, observed using the Combined Array for Research in Millimeter--wave Astronomy (CARMA) and the Submillimeter Array (SMA). This c
loud is very dense, and concentrates a mass exceeding the Orion Molecular Cloud Complex (2x10^5 M_sun) into a radius of only 3pc, but it is essentially starless. G0.253+0.016 therefore violates star formation laws presently used to explain trends in galactic and extragalactic star formation by a factor ~45. Our observations show a lack of dense cores of significant mass and density, thus explaining the low star formation activity. Instead, cores with low densities and line widths 1km/s---probably the narrowest lines reported for the GC region to date---are found. Evolution over several 10^5 yr is needed before more massive cores, and possibly an Arches--like stellar cluster, could form. Given the disruptive dynamics of the GC region, and the potentially unbound nature of G0.253+0.016, it is not clear that this evolution will happen.
We present the results of combined NH3(1,1) and (2,2) line emission observed with the Very Large Array and the Effelsberg 100m telescope of the Infrared Dark Cloud G14.225-0.506. The NH3 emission reveals a network of filaments constituting two hub-fi
lament systems. Hubs are associated with gas of rotational temperature Trot sim 25 K, non-thermal velocity dispersion ~1.1 km/s, and exhibit signs of star formation, while filaments appear to be more quiescent (Trot sim 11 K, non-thermal velocity dispersion ~0.6 km/s). Filaments are parallel in projection and distributed mainly along two directions, at PA sim 10 deg and 60 deg, and appear to be coherent in velocity. The averaged projected separation between adjacent filaments is between 0.5 pc and 1pc, and the mean width of filaments is 0.12 pc. Cores within filaments are separated by ~0.33 pc, which is consistent with the predicted fragmentation of an isothermal gas cylinder due to the sausage-type instability. The network of parallel filaments observed in G14.225-0.506 is consistent with the gravitational instability of a thin gas layer threaded by magnetic fields. Overall, our data suggest that magnetic fields might play an important role in the alignment of filaments, and polarization measurements in the entire cloud would lend further support to this scenario.
We observed three high-mass star-forming regions in the W3 high-mass star formation complex with the Submillimeter Array and IRAM 30 m telescope. These regions, i.e. W3 SMS1 (W3 IRS5), SMS2 (W3 IRS4) and SMS3, are in different evolutionary stages and
are located within the same large-scale environment, which allows us to study rotation and outflows as well as chemical properties in an evolutionary sense. While we find multiple mm continuum sources toward all regions, these three sub-regions exhibit different dynamical and chemical properties, which indicates that they are in different evolutionary stages. Even within each subregion, massive cores of different ages are found, e.g. in SMS2, sub-sources from the most evolved UCHII region to potential starless cores exist within 30 000 AU of each other. Outflows and rotational structures are found in SMS1 and SMS2. Evidence for interactions between the molecular cloud and the HII regions is found in the 13CO channel maps, which may indicate triggered star formation.
We aim at investigating with high angular resolution the NH3/N2H+ ratio toward the high-mass star-forming region AFGL 5142 in order to study whether this ratio behaves similarly to the low-mass case, for which the ratio decreases from starless cores
to cores associated with YSOs. CARMA was used to observe the 3.2 mm continuum and N2H+(1-0) emission. We used NH3(1,1) and (2,2), HCO+(1-0) and H13CO+(1-0) data from the literature and we performed a time-dependent chemical modeling of the region. The 3.2 mm continuum emission reveals a dust condensation of ~23 Msun associated with the massive YSOs, deeply embedded in the strongest NH3 core (hereafter central core). The N2H+ emission reveals two main cores, the western and eastern core, located to the west and to the east of the mm condensation, and surrounded by a more extended and complex structure of ~0.5 pc. Toward the central core the N2H+ emission drops significantly, indicating a clear chemical differentiation in the region. We found low values of the NH3/N2H+ ratio ~50-100 toward the western/eastern cores, and high values up to 1000 in the central core. The chemical model indicates that density, and in particular temperature, are key parameters in determining the NH3/N2H+ ratio. The high density and temperature reached in the central core allow molecules like CO to evaporate from grain mantles. The CO desorption causes a significant destruction of N2H+, favoring the formation of HCO+. This result is supported by our observations, which show that N2H+ and HCO+ are anticorrelated in the central core. The observed values of the NH3/N2H+ ratio in the central core can be reproduced by our model for times t~4.5-5.3x10^5 yr (central) and t~10^4-3x10^6 yr (western/eastern). The NH3/N2H+ ratio in AFGL 5142 does not follow the same trend as in regions of low-mass star formation mainly due to the high temperature reached in hot cores.
Intermediate between the prestellar and Class 0 protostellar phases, the first core is a quasi-equilibrium hydrostatic object with a short lifetime and an extremely low luminosity. Recent MHD simulations suggest that the first core can even drive a m
olecular outflow before the formation of the second core (i.e., protostar). Using the Submillimeter Array and the Spitzer Space Telescope, we present high angular resolution observations towards the embedded dense core IRS2E in L1448. We find that source L1448 IRS2E is not visible in the sensitive Spitzer infrared images (at wavelengths from 3.6 to 70 um), and has weak (sub-)millimeter dust continuum emission. Consequently, this source has an extremely low bolometric luminosity (< 0.1 L_sun). Infrared and (sub-)millimeter observations clearly show an outflow emanating from this source; L1448 IRS2E represents thus far the lowest luminosity source known to be driving a molecular outflow. Comparisons with prestellar cores and Class 0 protostars suggest that L1448 IRS2E is more evolved than prestellar cores but less evolved than Class 0 protostars, i.e., at a stage intermediate between prestellar cores and Class 0 protostars. All these results are consistent with the theoretical predictions of the radiative/magneto hydrodynamical simulations, making L1448 IRS2E the most promising candidate of the first hydrostatic core revealed so far.
