Millimeter-and submillimeter-wave astronomy

The list of astrophysical problems, which are solved by means of millimeter- and submillimeter-wave astronomy, includes studies of the Sun and the planets, the interstellar medium and star formation, activity of galaxies and quasars, the spectrum of and angular variations in the intensity of the cosmic microwave background, etc. The relevance of these problems stimulates a fast technical advance in this field.
The studies in the field of millimeter- and submillimeter-wave astronomy at IAP have long history. Jointly with many Russian and foreign organizations, IAP researchers are developing receiving equipment and perform astrophysical studies in this range. In recent years, main results have been achieved in the fields listed below.

Studies of the star formation regions.

Examples of maps of dense clumps in the regions of massive-star formation. Color means dust emission in continuum at a wavelength of 1.2 mm, solid blue lines are isophots of emission of the CS molecules (transition J = 5—4), and dashed yellow lines are isophots of emission of N2H+ (transition J = 1—0). Asterisks are IRAS sources

Observations in the millimeter and submillimeter wavelength bands are a unique instrument for studying internal regions of dense interstellar clouds which serve as "cradles" for new stars and are unaccessible for observations in other wavelength bands. The evolutionary pattern of star formation from protostellar condensations is still far from absolute clarity, especially for massive stars, which influence strongly the surrounding medium of the parent cloud. IAP researchers (I. I. Zinchenko, A. V. Lapinov, L. E. Pirogov, et al.) conduct systematic studies of star formation regions, with the purpose of, first, gaining statistical evaluations of their main physical characteristics and chemical composition and, second, examining typical objects in detail. Specifically, about 100 regions of highmass star formation in various galactic areas have been surveyed in the most informative radio lines of molecules and in continuum at millimeter and submillimeter waves. As a result, the main physical parameters of such objects were determined depending on their position in the galaxy, as well their internal structure and peculiarities of their chemical composition.

One of the results of these surveys was the discovery of regular chemical differentiation in these regsions, which differs significantly from that taking place in the regions of low mass star formation. An increase in the degree of ionization of dense molecular gas near young stars of high luminosity was detected. This can be the reason of the observed variations in the chemical composition. The detected ''ripples'' of the
J = 1—0 HCN emission lines in the regions of massive-star formation along with the observed features of the hyperfine structure of this line, indicates that the dense gas is characterized by significant small-scale inhomogeneity.
The purpose of multi-frequency observations of several such objects (S76, S255, W40) with the use of interferometers and single-dish antennas on the scales from several thousandths to several parsecs was to gain in-depth understanding of the conditions and processes that take place in the star formation regions. As a result, the data about the distribution and properties of dense molecular and ionized gases, characteristics of high-speed bipolar flows, etc. have been obtained. Great attention is given to the study of so-called infrared dark clouds which are assumed to be the earliest stage of formation of massive stars. Important data about their chemical composition and structure have been obtained.

Precision spectroscopy of molecules.

Galactic dark cold clouds, where low-mass stars are formed, are characterized by extremely narrow molecular emission lines. Observations of different lines allow one to conduct detail studies of relative motions of different components in the star formation regions (e.g., dense clumps in diffuse clouds, ions, neutrals, etc.). This requires knowing the frequencies of the corresponding transitions basing on the laboratory data with sufficient accuracy. Additionally, such observations allow one to check the theories, which predict variations of some fundamental constants (specifically, the ratio µ of the masses of the electron and the proton) as a function of the local barion density, with high accuracy that exceeds the accuracy of optical measurements (A. V. Lapinov). Numerous measurement errors have been found in the results obtained at largest radio telescopes, and the upper limits of possible µ variations have been improved significantly.

Example of precision measurements of several components in the hyperfine structure of the J = 2—1 transition of the HC3N molecule in the L1512 dark cloud

Molecules in the early Universe.

Chemistry of the Universe soon after formation of the first stars and galaxies is only starting to be studied now. The number of molecules detected at high redshifts
(at z > 2) is very small. One of the molecules, which are of the greatest interest for the studies of the early Universe is the HeH+ molecule, but it has not been found in space yet. A possible emission line (at the level of 3σ) near the expected frequency of the HeH+ transition in the spectrum of a distant quasar
(z = 6.42) has been registered recently (I. I. Zinchenko).

Technical developments.

IAP traditionally is the Russian leader in the field of development of radio astronomical receivers operated in the millimeter and submillimeter wavelength ranges. Notable recent developments include the HEMT-based 3-mm receiver for the 14-meter radiotelescope in Finland (V. F. Vdovin, A. M. Shtanyuk, O. S. Bol'shakov, et al.), the dual band radiometer for monitoring of atmosphere transparency at millimeter waves (V. I. Nosov, et al.). IAP is the leading organization responsible for the receiving equipment of the 70-meter radiotelescope which is currently under construction on the Suffa plateau, Uzbekistan.
IAP researchers take part also in many other Russian and international proejcts aimed at creating new high-efficiency radio astronomical instruments of the above-said range. Several versions of cooling systems for different temperature levels, both for radio astronomy and other applications, have been developed (V. F. Vdovin, I. V. Lapkin).

3 mm radio astronomical receiver on the base of HEMT amplifiers (left panel) and dual band radiometer (3 and 2 mm) for monitoring of atmosphere transparency