We have been performing daily VLBI monitoring of the ux density of Sagittarius (Sgr) A* at 22 GHz from February 2013 to August 2014 using a sub-array of the Japanese VLBI Network (JVN). The purpose of this monitoring is to explore the ux density variability at daily time resolution for a period longer than one year with the G2 cloud approaching. The ux density of Sgr A* is basically stable during the observational period, though there are some small variations. The average and scattering range are consistent with the previously observed values. We have observed no strong are of Sgr A* although it is near the expected peri-center passing.

The 2-1 and 5-4 transitions of SiO have been observed toward the Sgr B2 region, including the Principal Cloud(the GMC containing Sgr B2(M)) and its surroundings. The morphology and velocity structure of the SiO emission show a close resemblance with the HNCO Ring feature, identified by Minh & Irvine(2006), of about 10 pc in diameter, which may be expanding and colliding with the Principal Cloud. Three SiO clumps have been found around the Ring, with total column densities Nsio ~1x1014 cm-2 at the peak positions of these clumps. The fractional SiO abundance relative to H2 has been estimated to be ~(0.5-1)X10-9, which is about two orders of magnitude larger than the quiet dense cloud values. Our SiO observational result supports the existence of an expanding ring, which may be triggering active star formations in the Principal Cloud.

This paper reviews the progress in the VLBI (Very Long Baseline Interferometry) studies of Sgr A*, the best known supermassive black hole candidates with a dark mass concentration of 4x106 M⊙ at the center of the Milky Way. The emphasis is on the importance of the millimeter and sub-millimeter VLBI observations in the detection of Sgr A*'s intrinsic structure and search for the structural variation.

The H2S 22,0 - 21,1 line emission is observed to be strongly localized toward Sgr B2(M), and emissions from other positions in the more extended SgrB2 region are almost negligible. H2S is thought to form effectively by the passage of the C-type shocks but to be quickly transformed to SO2 or other sulfur species (Pineau des Forets et al. 1993). Such a shock may have enhanced the H2S abundance in Sgr B2(M), where massive star formation is taking place. But the negligible emission of H2S from other observed positions may indicate that these positions have not been affected by shocks enough to produce H2S, or if they have experienced shocks, H2S may have transformed already to other sulfur-containing species. The SO2 222,20 - 221,21 line was also observed to be detectable only toward the (M) position. The line intensity ratios of these two molecules appear to be very similar at Sgr B2(M) and IRAS 16239-2422, where the latter is a region of low-mass star formation. This may suggest that the shock environment in these two star-forming regions is similar and that the shock chemistry also proceeds in a similar fashion in these two different regions, if we accept shock formation of these two species.

We observed the thermal transitions of SiO (J=I-0, 2-1) and 29SiO (J=l-O) toward the Sgr A molecular clouds. The distribution and the velocity structure of SiO are very similar to previous results for 'quiet' interstellar molecules. We think· that the SiO has been well mixed with other molecules such as H2 which may indicate that the formation of Sgr A molecular clouds was affected by the activities, such as shock waves or energetic photons, from the Galactic center in large scales. The total column density of SiO is about 4.1×1014cm−2 and the fractional abundance SiO/H2 appears to be about 10 times larger than those of other clouds in the central region of our galaxy. The derived values are thought to be lower limits since the optical depths of the observed SiO lines are not very thin. The formation of SiO has been known to be critically related to shocks, and our results provide informative data on the environment of our Galactic center.

We have observed the 10-9 transitions of HC3N and its 13C substitutes (H13CCCN,HC13CCN, and HCC13CN), and the vibration ally excited 12-11 (vr=1) HC3N transition toward the Sgr B2 molecular cloud. The observed HC3N emission shows an elongated shape around the Principal Cloud (~4.5 pc in R.A. × 7.4 pc in Decl.). The optically thin H13CCCN line peaks around the (N) core and we derive the total column density N(H13CCCN) = 4 ×10 13 cm-2 at this position. Toward the 2' N cloud which shows the peculiar chemistry, the HC3N lines show enhancements compared to the extended envelope. The shocks of the 2' N may have resulted in the enhancement of HC3N. The hot component of HC3N is strongly concentrated around the (N) core and its HPW is ~0.9 pc in diameter. We derive the lower limit of the abundance ratio N(HC3N)/N(H13CCCN) to be larger than 40 in most regions except the (M) and (N) cores. The fractionation processes of 13C at this region may not be as effective as previously reported.