Review of scientific topics for Millimetron space observatory, Kardashev N.S., Novikov I.D., Lukash V.N., Pilipenko S.V., Mikheeva E.V., Bisikalo D.V., Wiebe D.Z., Doroshkevich A.G., Zasov A.V., Zinchenko I.I., Ivanov P.B., Kostenko V.I., Larchenkova T.I., Likhachev S.F., Malov I.F., Malofeev V.M., Pozanenko A.S., Smirnov A.V., Sobolev A.M., Cherepashchuk A.M., Shchekinov Yu.A.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We have conducted a survey of SiO emission towards galactic H2O and OH masers and ultracompact HII regions using the 15-m SEST and the 20-m Onsala telescope. With the SEST the transitions ($ \nu =0$, $J=2-1$) and ($\nu=0$,$J=3-2$) of SiO at 3 and 2 mm were measured simultaneously. With Onsala only the ($\nu=0$,$J=2-1$) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. The detection rate is highest towards the most intense H2O masers, which probably require powerful shocks to be excited. The SiO detection rate correlates also with the integrated far-infrared flux density and the FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from the central stellar source(s). For flux and luminosity limited samples the SiO detection rate is higher in the inner 7 kpc from the galactic centre than elsewhere. This suggests that dense cores belonging to the so called “molecular ring" provide particularly favourable conditions for the production of gaseous SiO. The full widths above $2_\sigma$ of the SiO($J=2-1$) lines, which are likely to be related to the associated shock velocities, range from 2 to 60 kms-1 except for the line in Ori KL which has a full width of about 100 kms-1. The median of our sample is 19 kms-1. The SiO lines are single-peaked and the peak velocities are always close to the ambient cloud velocity as determined from published CS observations. These line characteristics are compared with the predictions of kinematical bow-shock models. The SiO line shapes correspond with the model of [Raga & Cabrit (1993)] where the emission arises from turbulent wakes behind bow-shocks. However, the number of symmetric, relatively narrow profiles indicates that at least in some of the observed sources SiO emission arises also from the quiescent gas component. We suggest that this is due to evaporation of silicon compounds from grain mantles and their reprocessing to SiO in dense quiescent gas according to the model of [McKay (1995, 1996).] These reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO+ near Herbig-Haro objects has been explained in the model of [Wolfire & Königl (1993).] The excitation temperatures of SiO($J=2-1$) and ($J=3-2$) transitions were determined towards three strong sources using measurements in isotopically substituted SiO. In all three sources the transitions are clearly subthermally excited, implying moderate densities ($< 3~10^{6}$ cm-3) in the SiO emission regions.

Temporal photometric variations at near-infrared to submillimeter wavelengths have been found in low-mass young stellar objects. These phenomena are generally interpreted as accretion events of star-disk systems with varying accretion rates. There is growing evidence suggesting that similar luminosity flaring also occurs in high-mass star/cluster-forming regions. We report in this Letter the rise and fall of the 900 μm continuum emission and the newly found 349.1 GHz methanol maser emission in the massive star-forming region S255IR SMA1 observed with the Submillimeter Array and the Atacama Large Millimeter/submillimeter Array. The level of flux variation at a factor of ∼2 at the submillimeter band and the relatively short 2 years duration of this burst suggest that the event is probably similar to those milder and more frequent minor bursts seen in 3D numerical simulations.