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Astron. Astrophys. 364, 232-236 (2000)

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1. Introduction

Interstellar masers are generally found in star-forming regions and are excellent signposts for both the low-mass and massive star formation. Observations have shown that most interstellar water masers are associated with the youngest objects, which usually have strong outflows. The flux density varies on time scales ranging from a few days to several months. The maser variability in certain sources is extraordinary. Detailed investigation enables us to understand not only the masers, but the relationship between the masers and their exciting centers.

W3(OH) region, at a distance of 2.2 pc (Humphreys 1978), contains at least two sites within a projected distance of 0.1 pc, in which massive stars are forming or have newly formed. Radio and IR observations indicate that the most prominent site is the ultra-compact HII region in the [FORMULA] size, consisting of a 3 Jy radio continuum source and OH and CH3OH masers (Reid et al. 1995). The OH maser spots are projected against the optically thick radio continuum source associated O star. Situated [FORMULA] 6 [FORMULA], or [FORMULA] 0.06 pc, to the east of the radio continuum source is a complex of strong H2O masers (Dreher & Welch 1981). The H2O masers are too far from the HII region to be excited by the O star. The presence of hot, dense HCN at the H2O position indicates a possible second source of luminosity (Turner & Welch 1984). A H2O flare was detected in the masing region by Haschick et al. (1977). The flux density of the feature rose for a time period of eight days and then decayed slowly over the following month. The flux density of some components reached more than 2000 Jy (Sullivan 1973; Moran et al. 1973; Little et al. 1977; Genzel et al. 1978). Based on VLBI observations, the relative proper motions of the H2O masers in the region outlined a bipolar outflow (Alcolea et al. 1993). The center of expansion appeared to be coincident with a dense, compact molecular clump detected in HCN emission (Turner & Welch 1984) and a nonthermal radio continuum emission (Reid et al. 1995).

NGC6334, at a distance of 1.74 kpc (Neckel 1978), is the largest and the most complex region of active star formation known in the Galaxy. The complex consists of three large HII region. Based on radio and IR observation, there are a large number of compact sources (Reid & Moran 1981; dePree et al. 1995; Tapia et al. 1996) in the region, such as active centers probably in different evolutionary stages and bipolar molecular outflows. NGC6334 has a variety of indicators of active star formation such as hot CO spots, far-infrared peaks, OH and H2O masers. Five sites of H2O masers have been found in the complex (Moran & Rodriguez 1980). Among them, NGC 6334C is peculiar because it has a large blueshift, [FORMULA] 80 km [FORMULA], while the other four masers all have velocity centroids within 10 km [FORMULA] of the cloud velocity. Moreover, the maser emission from NGC 6334C is two orders of magnitude more luminous than the other four masers. The spectacular water masers showed many features, in which seven main components had flux densities of more than 150 Jy, and the peak flux was about 1000 Jy. Because the ultra-compact HII region near NGC 6334C is optically thick at 4.9 GHz, the associated star has been inferred earlier than O 9.5 (Rodriguez et al. 1980).

With the objective of studying very short-time scale (days or hours) variations of the water maser emission at 22 GHz, a monitoring program of 14 sources was undertaken using the 13.7 m radio telescope in the Purple Mountain Observatory. Here we present the monitoring results of the water maser emission from W3(OH) and NGC 6334C.

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Online publication: December 15, 2000