| Popis: |
Masers are known to occur in circumstellar envelopes of AGB stars that are long-period variables and have entered the TP-AGB phase. The maser radiation is intrinsically weak and detection limits prevent us from seeing maser stars much beyond the Galactic Center. Very deep searches have led to the detection of a handful of maser stars in the Large Magellanic Cloud. Some 1000 stellar masers are known. They trace a population of stars with main sequence masses between 1 and 6 M⊙ and ages between 2 and 8 Gyr, the same population that is traced by Cepheids. Their distribution in our Galaxy is the same as that of the red giants, and therefore the stars trace the galactic bar. An outstanding feature is a small disk of radius 150 pc around the galactic center. The disk is in rapid, solid body rotation and leads to a good measurement of the mass density within that distance for Sgr A. |
