| Autor: |
Lobanov, Andrei P., Zensus, J. Anton, Cesarsky, Catherine, Diamond, Phillip J., Slysh, V. |
| Zdroj: |
Exploring the Cosmic Frontier; 2006, p231-234, 4p |
| Abstrakt: |
Cosmic masers became a powerful tool for study of stellar evolution in the Galaxy, bursts of star-formation in external galaxies, and accretion disks around central black holes in active galaxies. Due to the small size and narrow line width of maser spots it is possible to measure transversal and radial velocity with high accuracy. Kinematics of the maser spots often reveals expansion and outflow of the matter from proto-stars and new-born stars, or rotation of circumstellar and circumnuclear disks. Stellar may disks contain proto-planets, and maser spots can trace their orbits. Parameters of the circumnuclear disks measured with maser spot motion are directly related to the mass of the central black hole. Another result of the study of maser kinematics is determination of distance to proto-stars and to galaxies. In the latter case the distance determination is independent of the red shift distance and may be used for the determination of the geometry of the Universe. The accuracy of the kinematic measurements of masers is limited by the available angular resolution, time span and sensitivity of VLBI systems used for such observations. The available time span is limited by the fast time variations of masers, especially variations in H2O masers. Many of the masers studied with VLBI have unresolved maser spots, even at the highest resolution. Examples of OH masers unresolved on the space-ground baselines of the Japanese interferometer HALCA are given in [1,2]. In H2O maser W3(OH) the fringe amplitude remains constant from zero baseline up to 635 M λ, which corresponds to the angular size of less than 0.06 milliarcsec [3]. High angular resolution images of methanol maser spots reveal presence of the position-velocity gradient across the spots from 3.3 to 50 AU/km s-1 . Bandwidth smearing of maser spot images may cause apparent increase of the size when measured with low spectral resolution. In NGC7538 0.1 km s-1 spectral resolution will cause increase of the angular size to 1.9 mas [4]. In W3(OH) the same spectral resolution will smear 12 GHz methanol maser spot images to the size from 0.15 to 2 mas [5]. Similar gradient may be present in OH and H2O masers. It is interesting that the position-velocity gradient of the same magnitude is required by OH maser pump models in order to provide non-local overlap of far-infrared rotational lines [6]. VLBI observations with high spectral resolution like those described in [5] show that when the spectral resolution is sufficient to avoid bandwidth smearing the maser spots remain unresolved. Therefore more angular resolution is needed in order to image maser spots. For the masers the only way to increase angular resolution is to increase the baseline, up to the space baselines. The imaging of the maser spots is one of the goals of the space-ground interferometer RADIOASTRON. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Supplemental Index |
| Externí odkaz: |
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