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Search Results: 1 - 10 of 88441 matches for " Jun-Hui Zhao "
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The Structure and Variability of Sagittarius A*: Zooming in to the Supermassive Black Hole at the Galactic Center
Feng Yuan,Jun-Hui Zhao
Physics , 2002,
Abstract: The Galactic center provides a unique astrophysical laboratory for us to study various astrophysical processes. In this paper, we review and outline the latest results from observations of Sgr~A$^*$ in terms of source structure and variations in flux density. Sgr~A$^*$ phenomenon represents a typical case of low radiative efficiency accretion flow surrounding a supermassive black hole in low luminosity AGNs. Many pending astrophysical problems found from observations of Sgr A$^*$ have challenged the existing astrophysical theories. Current theoretical models of Sgr A$^*$ are also reviewed.
Nature of W51e2: Massive Cores at Different Phases of Star Formation
Hui Shi,Jun-Hui Zhao,J. L. Han
Physics , 2010, DOI: 10.1088/0004-637X/710/1/843
Abstract: 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.
A bipolar outflow from the massive protostellar core W51e2-E
Hui Shi,Jun-Hui Zhao,Jinlin Han
Physics , 2010, DOI: 10.1088/2041-8205/718/2/L181
Abstract: We present high resolution images of the bipolar outflow from W51e2, which are produced from the Submillimeter Array archival data observed for CO(3-2) and HCN(4-3) lines with angular resolutions of 0.8" x 0.6" and 0.3" x 0.2", respectively. The images show that the powerful outflow originates from the protostellar core W51e2-E rather than from the ultracompact HII region W51e2-W. The kinematic timescale of the outflow from W51e2-E is about 1000 yr, younger than the age (~5000 yr) of the ultracompact HII region W51e2-W. A large mass loss rate of ~1 x 10^{-3} M_sun yr^{-1} and a high mechanical power of 120 L_sun are inferred, suggesting that an O star or a cluster of B stars are forming in W51e2-E. The observed outflow activity along with the inferred large accretion rate indicates that at present W51e2-E is in a rapid phase of star formation.
Sagittarius B2 Main: A Cluster of Ultra-Compact HII Regions and Massive Protostellar Cores
Jun-Hui Zhao,M. C. H. Wright
Physics , 2011, DOI: 10.1088/0004-637X/742/1/50
Abstract: The ionized core in the Sgr B2 Main star-forming region was imaged using the Submillimeter Array archival data observed for the H26$\alpha$ line and continuum emission at 0.86 millimeter with an angular resolution 0.3\arcsec. Eight hyper-compact H26$\alpha$ emission sources were detected with a typical size in the range of 1.6--20$\times10^2$ AU and electron density of 0.3--3$\times10^7$ cm$^{-3}$, corresponding to the emission measure 0.4--8.4$\times10^{10}$ cm$^{-6}$ pc. The H26$\alpha$ line fluxes from the eight hyper-compact HII sources imply that the ionization for each of the sources must be powered by a Lyman continuum flux from an O star or a cluster of B stars. The most luminous H26$\alpha$ source among the eight detected requires an O6 star that appears to be embedded in the ultra-compact HII region F3. In addition, $\sim$ 23 compact continuum emission sources were also detected within the central 5\arcsec$\times$3\arcsec\,($\sim0.2$ pc) region. In the assumption of a power-law distribution for the dust temperature, with the observed brightness temperature of the dust emission we determined the physical properties of the submillimeter emission sources showing that the molecular densities are in the range of 1--10$\times10^8$ cm$^{-3}$, surface densities between 13 to 150 $g$ cm$^{-2}$, and total gas masses in the range from 5 to $\gtrsim$ 200 $M_\odot$ which are 1 or 2 orders of magnitude greater than the corresponding values of the Bonnor-Ebert mass. With a mean free-fall time scale of 2$\times10^3$ y, each of the massive protostellar cores are undergoing gravitational collapse to form new massive stars in the Sgr B2 Main core.
Outflow, Infall and Protostars in the Star Forming Core W3-SE
Lei Zhu,Jun-Hui Zhao,M. C. H. Wright
Physics , 2011, DOI: 10.1088/0004-637X/740/2/114
Abstract: We report new results on outflow and infall in the star forming cores W3-SE SMA-1 and SMA-2 based on analysis of $\sim2.5\arcsec$ resolution observations of the molecular lines HCN(3-2), HCO$^+$(3-2), N$_2$H$^+$(3-2) and CH$_3$OH(5$_{2,3}-4_{1,3}$) with the Submillimeter Array. A high-velocity bipolar outflow originating from the proto-stellar core SMA-1 was observed in the HCN(3-2) line, with a projected outflow axis in position angle 48$\arcdeg$. The detection of the outflow is confirmed from other molecular lines. An inverse P-Cygni profile in the HCN(3-2) line toward SMA-1 suggests that at least one of the double cores accretes matters from the molecular core. A filamentary structure in the molecular gas surrounds SMA-1 and SMA-2. Based on the SMA observations, our analysis suggests that the double pre-stellar cores SMA-1 and SMA-2 result from fragmentation in the collapsing massive molecular core W3-SE, and it is likely that they are forming intermediate to high-mass stars which will be new members of a star cluster in the W3-SE region.
Radio Detection of A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East
Jun-Hui Zhao,Mark R. Morris,W. M. Goss
Physics , 2013, DOI: 10.1088/0004-637X/777/2/146
Abstract: We report the VLA detection of the radio counterpart of the X-ray object referred to as the "Cannonball", which has been proposed to be the remnant neutron star resulting from the creation of the Galactic Center supernova remnant, Sagittarius A East. The radio object was detected both in our new VLA image from observations in 2012 at 5.5 GHz and in archival VLA images from observations in 1987 at 4.75 GHz and in the period from 1990 to 2002 at 8.31 GHz. The radio morphology of this object is characterized as a compact, partially resolved point source located at the northern tip of a radio "tongue" similar to the X-ray structure observed by Chandra. Behind the Cannonball, a radio counterpart to the X-ray plume is observed. This object consists of a broad radio plume with a size of 30\arcsec$\times$15\arcsec, followed by a linear tail having a length of 30\arcsec. The compact head and broad plume sources appear to have relatively flat spectra ($\propto\nu^\alpha$) with mean values of $\alpha=-0.44\pm0.08$ and $-0.10\pm0.02$, respectively; and the linear tail shows a steep spectrum with the mean value of $-1.94\pm0.05$. The total radio luminosity integrated from these components is $\sim8\times10^{33}$ erg s$^{-1}$, while the emission from the head and tongue amounts for only $\sim1.5\times10^{31}$ erg s$^{-1}$. Based on the images obtained from the two epochs' observations at 5 GHz, we infer the proper motion of the object: $\mu_\alpha = 0.001 \pm0.003$ arcsec yr$^{-1}$ and $\mu_\delta = 0.013 \pm0.003$ arcsec yr$^{-1}$. With an implied velocity of 500 km s$^{-1}$, a plausible model can be constructed in which a runaway neutron star surrounded by a pulsar wind nebula was created in the event that produced Sgr A East. The inferred age of this object, assuming that its origin coincides with the center of Sgr A East, is approximately 9000 years.
Radio Variability of Sagittarius A* - A 106 Day Cycle
Jun-Hui Zhao,G. C. Bower,W. M. Goss
Physics , 2000, DOI: 10.1086/318877
Abstract: We report the presence of a 106-day cycle in the radio variability of Sgr A* based on an analysis of data observed with the Very Large Array (VLA) over the past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of cycle frequency to frequency width f/Delta_f= 2.2+/-0.3. The periodic signal is also clearly observed at 2 cm. At 3.6 cm the detection of a periodic signal is marginal. No significant periodicity is detected at both 6 and 20 cm. Since the sampling function is irregular we performed a number of tests to insure that the observed periodicity is not the result of noise. Similar results were found for a maximum entropy method and periodogram with CLEAN method. The probability of false detection for several different noise distributions is less than 5% based on Monte Carlo tests. The radio properties of the pulsed component at 1.3 cm are spectral index alpha ~ 1.0+/- 0.1 (for S nu^alpha), amplitude Delta S=0.42 +/- 0.04 Jy and characteristic time scale Delta t_FWHM ~ 25 +/- 5 days. The lack of VLBI detection of a secondary component suggests that the variability occurs within Sgr A* on a scale of ~5 AU, suggesting an instability of the accretion disk.
A New Perspective of the Radio Bright Zone at The Galactic Center: Feedback from Nuclear Activities
Jun-Hui Zhao,Mark R. Morris,W. M. Goss
Physics , 2015,
Abstract: 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 $\mu$Jy/beam, and a dynamic range 100,000:1.The radio image is compared with X-ray, CN emission-line and Paschen-$\alpha$ 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-$\alpha$ 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.
Nonthermal Filamentary Radio Features Within 20 pc of the Galactic Center
Mark Morris,Jun-Hui Zhao,W. M. Goss
Physics , 2013, DOI: 10.1017/S1743921314000933
Abstract: Deep imaging of the Sgr A complex at 6 cm wavelength with the B and C configurations of the Karl G. Jansky VLA has revealed a new population of faint radio filaments. Like their brighter counterparts that have been observed throughout the Galactic Center on larger scales, these filaments can extend up to ~10 parsecs, and in most cases are strikingly uniform in brightness and curvature. Comparison with a survey of Paschen-alpha emission reveals that some of the filaments are emitting thermally, but most of these structures are nonthermal: local magnetic flux tubes illuminated by synchrotron emission. The new image reveals considerable filamentary substructure in previously known nonthermal filaments (NTFs). Unlike NTFs previously observed on larger scales, which tend to show a predominant orientation roughly perpendicular to the Galactic plane, the NTFs in the vicinity of the Sgr A complex are relatively randomly oriented. Two well-known radio sources to the south of Sgr A - sources E and F - consist of numerous quasi-parallel filaments that now appear to be particularly bright portions of a much larger, strongly curved, continuous, nonthermal radio structure that we refer to as the "Southern Curl". It is therefore unlikely that sources E and F are HII regions or pulsar wind nebulae. The Southern Curl has a smaller counterpart on the opposite side of the Galactic Center - the Northern Curl - that, except for its smaller scale and smaller distance from the center, is roughly point-reflection symmetric with respect to the Southern Curl. The curl features indicate that some field lines are strongly distorted, presumably by mass flows. The point symmetry about the center then suggests that the flows originate near the center and are somewhat collimated.
A New Perspective on the Radio Active Zone at The Galactic Center - Feedback from Nuclear Activities
Jun-Hui Zhao,Mark R. Morris,W. M. Goss
Physics , 2013, DOI: 10.1017/S1743921314000921
Abstract: Based on our deep image of Sgr A using broadband data observed with the Jansky VLA at 6 cm, we present a new perspective of the radio bright zone at the Galactic center. We further show the radio detection of the X-ray Cannonball, a candidate neutron star associated with the Galactic center SNR Sgr A East. The radio image is compared with the Chandra X-ray image to show the detailed structure of the radio counterparts of the bipolar X-ray lobes. The bipolar lobes are likely produced by the winds from the activities within Sgr A West, which could be collimated by the inertia of gas in the CND, or by the momentum driving of Sgr A*; and the poloidal magnetic fields likely play an important role in the collimation. The less-collimated SE lobe, in comparison to the NW one, is perhaps due to the fact that the Sgr A East SN might have locally reconfigured the magnetic field toward negative galactic latitudes. In agreement with the X-ray observations, the time-scale of ~ $1\times10^4$ yr estimated for the outermost radio ring appears to be comparable to the inferred age of the Sgr A East SNR.
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