3,754 research outputs found

    Nature of W51e2: Massive Cores at Different Phases of Star Formation

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    We present high-resolution continuum images of the W51e2 complex processed from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3 mm and the Very Large Array (VLA) at 7 and 13 mm. We also made line images and profiles of W51e2 for three hydrogen radio recombination lines (H26\alpha, H53\alpha, and H66\alpha) and absorption of two molecular lines of HCN(4-3) and CO(2-1). At least four distinct continuum components have been detected in the 3" region of W51e2 from the SMA continuum images at 0.85 and 1.3 mm with resolutions of 0.3"x0.2" and 1.4"x0.7", respectively. The west component, W51e2-W, coincides with the UC HII region reported from previous radio observations. The H26\alpha line observation reveals an unresolved hyper-compact ionized core (<0.06" or <310 AU) with a high electron temperature of 1.2x10^4 K, with corresponding emission measure EM>7x10^{10} pc cm^{-6} and electron density N_e>7x10^6 cm^{-3}. The inferred Lyman continuum flux implies that the HII region W51e2-W requires a newly formed massive star, an O8 star or a cluster of B-type stars, to maintain the ionization. The east component, W51e2-E, has a total mass of ~140 M_{\sun} according to our SED analysis and a large infall rate of > 1.3x10^{-3} M_{\sun}yr^{-1} inferred from the absorption of HCN. W51e2-E appears to be the accretion center in W51e2 and to host one or more growing massive proto-stars. Located 2" northwest from W51e2-E, W51e2-NW is not detected in the continuum emission at \lambda>=7 mm. Along with the maser activities previously observed, our analysis suggests that W51e2-NW is at an earlier phase of star formation. W51e2-N is located 2" north of W51e2-E and has only been detected at 1.3 mm with a lower angular resolution (~1"), suggesting that it is a primordial, massive gas clump in the W51e2 complex.Comment: 10 pages, 5 figures, 3 table, accepted for publication in Ap

    A Nonthermal Radio Filament Connected to the Galactic Black Hole?

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    Using the Very Large Array, we have investigated a non-thermal radio filament (NTF) recently found very near the Galactic black hole and its radio counterpart, SgrA*. While this NTF -- the Sgr A West Filament (SgrAWF) -- shares many characteristics with the population of NTFs occupying the central few hundred parsecs of the Galaxy, the SgrAWF has the distinction of having an orientation and sky location that suggest an intimate physical connection to SgrA*. We present 3.3 and 5.5 cm images constructed using an innovative methodology that yields a very high dynamic range, providing an unprecedentedly clear picture of the SgrAWF. While the physical association of the SgrAWF with SgrA* is not unambiguous, the images decidedly evoke this interesting possibility. Assuming that the SgrAWF bears a physical relationship to SgrA*, we examine the potential implications. One is that SgrA* is a source of relativistic particles constrained to diffuse along ordered local field lines. The relativistic particles could also be fed into the local field by a collimated outflow from SgrA*, perhaps driven by the Poynting flux accompanying the black hole spin in the presence of a magnetic field threading the event horizon. Second, we consider the possibility that the SgrAWF is the manifestation of a low-mass-density cosmic string that has become anchored to the black hole. The simplest form of these hypotheses would predict that the filament be bi-directional, whereas the SgrAWF is only seen on one side of SgrA*, perhaps because of the dynamics of the local medium.Comment: 9 pages, 4 figures, accepted for ApJ Letter

    A New Perspective of the Radio Bright Zone at The Galactic Center: Feedback from Nuclear Activities

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    New observations of Sgr A have been carried out with the VLA using the broadband (2 GHz) continuum mode at 5.5 GHz, covering the central 30 pc region of the RBZ at the Galactic center. Using the MS-MFS algorithms in CASA, we have imaged Sgr A with a resolution of 1", achieving an rms 8 μ\muJy/beam, and a dynamic range 100,000:1.The radio image is compared with X-ray, CN emission-line and Paschen-α\alpha images obtained using Chandra, SMA and HST/NICMOS, respectively. We discuss several prominent radio features. The "Sgr A West Wings" extend 5 pc from the NW and SE tips of the ionized "Mini-spiral" in Sgr A West to positions located 2.9 and 2.4 arc min to the NW and SE of Sgr A*, respectively. The NW wing, along with several other prominent features, including the "NW Streamers", form an elongated radio lobe (NW lobe), oriented nearly perpendicular to the Galactic plane. This radio lobe, with a size of 14.4 pc x 7.3 pc, has a known X-ray counterpart. A row of three thermally emitting rings is observed in the NW lobe. A field containing numerous amorphous radio blobs extends for a distance of ~2 arc min beyond the tip of the SE wing; these features coincide with the SE X-ray lobe. Most of the amorphous radio blobs in the NW and SE lobes have Paschen-α\alpha counterparts, suggesting that a shock interaction of ambient gas concentrations with a collimated nuclear wind (outflow) that may be driven by radiation force from the central star cluster within the CND. Finally, we remark on a prominent radio feature located within the shell of the Sgr A East SNR. Because this feature -- the "Sigma Front" -- correlates well in shape and orientation with the nearby edge of the CND, we propose that it is a reflected shock wave resulting from the impact of the Sgr A East blast wave on the CND.Comment: 18 pages, 9 figures, ApJ accepte

    Mobile Communication, Public Participation and E-Governance in China:A case study of Xiamen Anti-PX Demonstration

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    China has become the biggest mobile communication carrier in the world since 2001. Advanced technologies create a communication revolution, and the individual, through the advent of mobile media, has become an active participant in this process. This study investigates the mobile phone’s impact upon the developments of public participation, social inclusion and citizenship through the case study of Xiamen PX demonstration. In terms of local environmental activism, the Xiamen residents shared information with the help of wireless communication about the alleged misdeeds of party officials and took various civic actions again them. A rare sense of participation in public affairs is fostered through the use of mobile communication technology. The government must figure out how to improve the effective and regular information exchange and feedback top down and bottom up to raise the awareness and understanding among higher decision making agencies, government and the public