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Register a new userExcited-state OH Mainline Masers in AU Geminorum and NML Cygni
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Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg outlined below), or possibly an artifact in the data, and that the excited state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.
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We present the results of a monitoring campaign using the KAT-7 and HartRAO 26m telescopes, of hydroxyl, methanol and water vapour masers associated with the high-mass star forming region G9.62+0.20E. Periodic flaring of the main line hydroxyl masers were found, similar to that seen in the 6.7 and 12.2 GHz methanol masers. The 1667 MHz flares are characterized by a rapid decrease in flux density which is coincident with the start of the 12.2 GHz methanol maser flare. The decrease in the OH maser flux density is followed by a slow increase till a maximum is reached after which the maser decays to its pre-flare level. A possible interpretation of the rapid decrease in the maser flux density is presented. Considering the projected separation between the periodic methanol and OH masers, we conclude that the periodic 12.2 methanol masing region is located about 1600 AU deeper into the molecular envelope compared to the location of the periodic OH masers. A single water maser flare was also detected which seems not to be associated with the same event that gives rise to the periodic methanol and OH maser flares.
Measurements by the U.C. Berkeley Infrared Spatial Interferometer at 11.15 micron have yielded strong evidence for multiple dust shells and/or significant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8-13 micron spectra taken from 1994-1995 and N band (10.2 micron) photometry from 1980-1992. These and past measurements are analyzed and fitted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both near- and mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 micron brightness distribution suggests a double shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth ($tau_{11 micron} sim 1.9$) plus an outer shell ($tau_{11 micron} sim 0.33$), is consistent not only with the 11 micron visibility data, but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 micron silicate feature. The outer shell, or large scale structure, is revealed only by long-baseline interferometry at 11 micron, being too cold ($sim$ 400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis (1992) proved superior to the Draine & Lee (1984) constants in fitting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of H$_2$O maser emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with the location of the two dust shells and provide further evidence for the two-shell model.
Aims: We have observed the 6030 and 6035 MHz transitions of OH in high-mass star-forming regions to obtain magnetic field estimates in both maser emission and absorption. Methods: Observations were taken with the Effelsberg 100 m telescope. Results: Our observations are consistent with previous results, although we do detect a new 6030 MHz maser feature near -70 km/s in the vicinity of W3(OH). In absorption we obtain a possible estimate of -1.1 +/- 0.3 mG for the average line-of-sight component of the magnetic field in the absorbing OH gas in K3-50 and submilligauss upper limits for the line-of-sight field strength in DR 21 and W3. Conclusions: These results indicate that the magnetic field strength in the vicinity of OH masers is higher than that of the surrounding, non-masing material, which in turn suggests that the density of masing OH regions is higher than that of their surroundings.
We present a new database of circumstellar OH masers at 1612, 1665, and 1667 MHz in the Milky Way galaxy. The database (version 2.4) contains 13655 observations and 2341 different stars detected in at least one transition. Detections at 1612,MHz are considered to be complete until the end of 2014 as long as they were published in refereed papers. Detections of the main lines (1665 and 1667 MHz) and non-detections in all transitions are included only if published after 1983. The database contains flux densities and velocities of the two strongest maser peaks, the expansion velocity of the shell, and the radial velocity of the star. Links are provided for about 100 stars ($<$5% of all stars with OH masers) to interferometric observations and monitoring programs of the maser emission published since their beginnings in the 1970s. Access to the database is possible over the Web (www.hs.uni-hamburg.de/maserdb), allowing cone searches for individual sources and lists of sources. A general search is possible in selected regions of the sky and by defining ranges of flux densities and/or velocities. Alternative ways to access the data are via the German Virtual Observatory and the VizieR library of astronomical catalogs.
Using the Very Large Array (VLA), we observed all three of the 6-cm lines of the doublet Pi 1/2, J=1/2 state of OH with sub-arcsecond resolution (about 0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6-cm lines are similar to those of the ground state (18-cm) OH lines, a large fraction of the total emission in all three 6-cm lines has large linewidths (about 5 -- 10 km/s) and is spatially-extended, very unlike typical ground state OH masers which typically are point-like at VLA resolutions and have linewidths less than 1 km/s. We find brightness temperatures of 5900 K, 4700 K, and greater than 730 K for the 4660-MHz, 4750-MHz, and 4765-MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are about 0.3, again very unlike the 18-cm lines which have gains greater than 10000. We compare the excited state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low-gain maser lines. We also comment on the relationship with the 18-cm masers
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