Methanol masers and star formation

Methanol masers which are traditionally divided into two classes provide possibility to study important parts of the star forming regions: Class II masers trace vicinities of the massive YSOs while class I masers are likely to trace more distant parts of the outflows where newer stars can form. There are many methanol transitions which produce observed masers. This allows to use pumping analysis for estimation of the physical parameters in the maser formation regions and its environment, for the study of their evolution. Extensive surveys in different masing transitions allow to conclude on the values of the temperatures, densities, dust properties, etc. in the bulk of masing regions. Variability of the brightest masers is monitored during several years. In some cases it is probably caused by the changes of the dust temperature which follow variations in the brightness of the central YSO reflecting the character of the accretion process. A unified catalogue of the class II methanol masers consisting of more than 500 objects is compiled. Analysis of the data shows that: physical conditions within the usual maser source vary considerably; maser brightness is determined by parameters of some distinguished part of the object - maser formation region; class II methanol masers are formed not within the outflows but in the regions affected by their propagation. It is shown that the "near" solutions for the kinematic distances to the sources can be used for statistical analysis. The luminosity function of the 6.7 GHz methanol masers is constructed. It is shown that improvement of the sensitivity of surveys can increase number of detected maser sources considerably. The distribution of class II methanol masers in the Galaxy is constructed on the basis of estimated kinematic distances. It is shown that most of the sources are located in the Molecular Ring and that the dependence of the number of sources on the distance from the Galactic Center has significant peaks at the positions corresponding to the spiral arms. A survey of CS(2-1) line emission tracing dense gas is performed at Mopra toward the positions of the brightest class II methanol masers. Velocity correlations between the maser and CS lines are analyzed. It is shown that the sources with 1 from 320 to 350 deg in which the masers are relatively blue-shifted, form a group which is located in the region of the Scutum-Centaurus spiral arm. This can reflect existence of a grand design, i.e., grouping of the sources with similar peculiarity of morphology or evolutionary stage of the massive star forming regions. © 2005 International Astronomical Union

Langmuir wave linear evolution in inhomogeneous nonstationary anisotropic plasma

Equations describing the linear evolution of a non-dissipative Langmuir wave in inhomogeneous nonstationary anisotropic plasma without magnetic field are derived in the geometrical optics approximation. A continuity equation is obtained for the wave action density, and the conditions for the action conservation are formulated. In homogeneous plasma, the wave field E universally scales with the electron density N as E ~ N^{3/4}, whereas the wavevector evolution varies depending on the wave geometry

Імпедансна спектроскопія ниткоподібних кристалів Sі в області переходу метал-діелектрик

Introduction. The paper deals with investigation of impedance spectroscopy of Si whiskers with doping concentration in the vicinity to metal-insulator transition in the region of low (4,2 – 70 K) temperature and frequency range 0,01 – 250x103 Hz. Experimental results. The silicon whiskers were grown by chemical vapour deposition method in closed bromine system. The whiskers of 40x10-6 m in diameter have boron concentration from 2×1018 to 2×1019 сm-3. The impedance spectroscopy of Si whiskers was investigated with use of Lock-in amplifier in the region of low (4,2 – 70 K) temperature and frequency range 0,01 – 250 x 103 Hz. The investigations showed that in the range of zone conductance (T= 30 – 70 K) the whisker impedance has inductive character, while at the range of impurity conductance (T = 4,2 – 20 K) impedance changes to capacity character. The reduction of impurity concentration at dielectric side of metal-insulator transition leads to decrease of the whisker capacity in the range of low temperatures. Discussion. An inductive character of whisker impedance is explained skin-effect of conductance in thin whisker, while capacity character of the whisker impedance is connected with hopping conductance on impurity band. Conclusions. Character of changes of Si whisker impedance conductance is discussed in the frame of hopping conductance on impurity band, which takes place at low temperatures.В результате выполненных исследований электропроводности нитевидных кристаллов Si с диаметрами 40x10-6 м, легированных примесью бора до концентраций вблизи перехода металл-изолятор (ρ300К =0,94 - 1,68x10-4 Ом м) в температурном интервале 4,2-70 К, частотном диапазоне 0,01-250x103 Гц обнаружено, что импеданс образцов в зависимости от температуры имеет емкостный (4,2 – 20 К) и индуктивный (30-70 К) характер, величина которого зависит от концентрации легирующей примеси. Исходя из результатов исследований кристаллов методом импедансной спектроскопии обсуждены отличия поведения импенданса образцов с различной концентрацией легирующей примеси.На основі дослідження електропровідності ниткоподібних кристалів Si з діаметрами 40х10-6 м, легованих домішкою бору до концентрацій поблизу переходу метал-діелектрик (ρ300К = 0,94-1,68х10-4 Ом•м) у температурному інтервалі 4,2-70 К, частотному діапазоні 0,01-250 х103 Гц виявлено, що імпеданс зразків в залежності від температури має ємнісний (4,2К – 20 К) та індуктивний (30 – 70 К) характер, величина якого залежить від концентрації легуючої домішки. На основі досліджень кристалів методом імпедансної спектроскопії обговорено відмінність поведінки імпендансу зразків з різною концентрацією легуючої домішки

Modulational instability in periodic quadratic nonlinear materials

We investigate the modulational instability of plane waves in quadratic nonlinear materials with linear and nonlinear quasi-phase-matching gratings. Exact Floquet calculations, confirmed by numerical simulations, show that the periodicity can drastically alter the gain spectrum but never completely removes the instability. The low-frequency part of the gain spectrum is accurately predicted by an averaged theory and disappears for certain gratings. The high-frequency part is related to the inherent gain of the homogeneous non-phase-matched material and is a consistent spectral feature.Comment: 4 pages, 7 figures corrected minor misprint

How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?

General characteristics of methanol (CH3OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact HII regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified HII regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region hcHII with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable amplification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.Comment: 8 pages, 4 figures, invited presentation at IAU242 "Astrophysical Masers and their environments

Beat-wave generation of plasmons in semiconductor plasmas

It is shown that in semiconductor plasmas, it is possible to generate large amplitude plasma waves by the beating of two laser beams with frequency difference close to the plasma frequency. For narrow gap semiconductors (for example n-type InSb), the system can simulate the physics underlying beat wave generation in relativistic gaseous plasmas.Comment: 11 pages, LaTex, no figures, no macro