Person: Zhao, Jun-Hui
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Zhao
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Jun-Hui
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Zhao, Jun-Hui
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Publication The High-Density Ionized Gas in the Central Parsec of the Galaxy(American Astronomical Society, 2010) Zhao, Jun-Hui; Blundell, Raymond; Moran, James; Downes, Dennis; Schuster, Karl F.; Marrone, Daniel P.We report a study of the \(H30\alpha\) line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2'' over a field of 60'' (2 pc) and a velocity range of \(-360 to +345 km s^{–1}\). This field encompasses most of the Galactic center's "minispiral." With an isothermal homogeneous H II model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the \(H30\alpha\) line data along with Very Large Array data from the \(H92\alpha\) line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are \(3-21×104 cm^{–3}\) and 5000-13,000 K, respectively. The \(H30\alpha\) and \(H92\alpha\) line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a three-dimensional model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the "Bar" region, which is located 0.1-0.2 pc south of and behind Sgr A*. Finally, a total Lyman continuum flux of \(3 × 10^{50}\) photons \(s^{–1}\) is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to ~250 O9-type zero-age-main-sequence stars.Publication An Unambiguous Detection of Faraday Rotation in Sagittarius A*(IOP Publishing, 2006) Marrone, Daniel P.; Moran, James; Zhao, Jun-Hui; Rao, RamprasadThe millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on time scales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be ( − 5 . 6 ± 0 .7) × 10 5 rad m − 2; the measurements are consistent with a constant value. We conservatively assign a 3 σ upper limit of 2 × 10 5 rad m − 2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2 × 10 − 7 M⊙ yr − 1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 ×10 − 9 M⊙ yr − 1 holds even for a sub-equipartition, disordered, or toroidal field. The mean intrinsic position angle is 167 ◦ ± 7 ◦ and we detect variations of 31+18 −9 degrees. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.Publication The flare activity of Sagittarius A*(EDP Sciences, 2006) Eckart, A.; Baganoff, F. K.; Schödel, R.; Morris, M.; Genzel, R.; Bower, G. C.; Marrone, D.; Moran, James; Viehmann, T.; Bautz, M. W.; Brandt, W. N.; Garmire, G. P.; Ott, T.; Trippe, S.; Ricker, G. R.; Straubmeier, C.; Roberts, D. A.; Yusef-Zadeh, F.; Zhao, Jun-Hui; Rao, RamprasadContext. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the SgrA* counterpart associated with the massive 3–4×106M⊙ black hole at the Galactic Center. Aims. The main aim is to investigate the physical processes responsible for the variable emission from SgrA*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Large Telescope⋆ and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA⋆⋆ on Mauna Kea, Hawaii, and the Very Large Array⋆⋆⋆ in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2 - 8 keV luminosity of about 33×1033 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 minutes. For 4 flares simultaneous infrared/X-ray observations are available. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub-millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straight forward explanation of the variable and red spectral indices of NIR flares.Publication Submillimeter Array 440 μm/690 GHz Line and Continuum Observations of Orion KL(IOP Publishing, 2006) Beuther, H.; Zhang, Qizhou; Reid, Mark; Hunter, T. R.; Gurwell, Mark; Wilner, David; Zhao, Jun-Hui; Shinnaga, H.; Keto, Eric; Ho, P. T. P.; Moran, James; Liu, S.‐Y.Submillimeter Array observations of Orion-KL at ∼ 1′′ resolution in the 440 µm/690 GHz band reveal new insights about the continuum and line emission of the region. The 440 µm continuum flux density measurement from source I allows us to differentiate among the various proposed physical models: Source I can be well modeled by a “normal” protostellar SED consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The non-detection of SMA1 at cm and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion-KL region. The molecular line maps show emission mainly from the sources I, SMA1 and the hot core peak position. An analysis of the CH 3CN(37 K − 36 K) K-ladder ( K = 0...3) indicates a warm gas component of the order 600 ± 200K. In addition, we detect a large fraction ( ∼ 58%) of unidentified lines and discuss the difficulties of line identifications at these frequencies.