Email: 
Jun-Hui

Filename: 
apjlaas.tex

Reference: 
 ApJL 1998 in press 

Title:
Radio Continuum Structure of  IRS 13 and Proper Motions of\\
     Compact HII Components at the Galactic Center\\
 
Author:
Jun-Hui Zhao
  \gcbrk W. M. Goss
 
Address:
(1) Harvard-Smithsonian Center for Astrophysics,
     60 Garden St, MS 78,Cambridge, MA 02138
 (2) NRAO-AOC, P. O. Box O, Socorro, NM 87801
 
Abstract:
 
 A compact HII region, IRS 13, is located 3\arcsec to the SW of Sgr A*, 
 the compact radio source at the Galactic Center.
 The source is double, IRS 13E and 13W, at 7 and 13 mm
 as observed with resolutions of 0\farcs06.
 A rising spectrum  with spectral index of $\alpha=0.9$  
 is observed for IRS 13E probably arising from 
 a high density, ionized  stellar wind. IRS 13W  has 
 a flat spectrum; in addition, 
 the radio morphology  suggests that the emitting  material in IRS 13W 
 has been swept off from the stellar envelope
 associated with IRS 13E by a strong wind and/or an 
 outflow emanating from the IRS 16 complex and/or Sgr A*. 
 Based on two epoch observations between 1991 and 1996, 
 IRS 13E shows no significant proper motion
 $<4$ mas y$^{-1}$ (3$\sigma$)
 while IRS 13W shows a proper motion
 $\mu_\alpha=0.4\pm0.7$, $\mu_\delta=6.2\pm1.1$ mas y$^{-1}$,
 with respect to Sgr A*. In addition, proper motions of
 fifty-seven compact HII components at 
 the Galactic Center are determined. The proper motion observations suggest that
 the minicavity is expanding with a speed of 200 km s$^{-1}$.
 The mean transverse velocity of 
 700 km s$^{-1}$ is inferred for the IRS 9 complex
 which is located at the SW rim of the Eastern cavity.  
 Large proper motions (up to 30 mas y$^{-1}$, 
 or  $\sim$1200 km s$^{-1}$) are 
 found for a number of HII components associated with filamentary 
 structures, such as the Bullet and the Loop.
 The large proper motion measured from the compact HII components
 can be explained by motions of clumpy HII gas accelerated
 by either strong winds and outflows or explosive events at the 
 Galactic Center. Large pattern speeds due to wave motions  
 in the circumnuclear medium at the Galactic Center 
 can also be responsible for some components of the
 observed proper motions.
 
 
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