Molecular Masers

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Radio Astronomy Department
Sternberg Astronomical Institute, Moscow University

This page is under construction

Cosmic masers are natural generators of high-intensity radio waves. They operate in region of interstellar space or circumstellar environment hosting certain molecular species and having appropriate physical conditions to excite ("pump") molecules. Molecules thus pumped amplify radio emission at frequencies intrinsic to energy transitions of these molecules. The pumping mechanism can be infrared radiation from the central stellar source or collisions of masering molecules with particles of the surrounding gas. For instance, favourable conditions for maser pumping can exist behind a shock front in circumstellar gas.

Masers are found in regions of active star formation, near protostellar or young stellar objects, as well as in gas-and-dust envelopes of late-type variable stars (Mira Ceti-type and semiregulars. Most widespread are masers on molecules of hydroxyl (OH), water vapour (H₂O), methanol (CH₃OH), silicon monoxide (SiO); rarer objects are masers on formaldehyde (H₂CO) and hydrogen cyanide (HCN). Hydroxyl, water-vapour and fomaldehyde masers are encountered both in star-forming regions and late-type stars; SiO and HCN masers are found in envelopes of late-type stars; while methanol masers exist only in star-forming regions.

This page is about the joint French-Russian project on the studies of molecular masers, mainly hydroxyl but also invoking data on other molecular masers detected in the same objects to get a more complete physical picture of the circumstellar environment of stars, young and old.

The following graph shows the structure of the energy levels of the lowest energy state of the hydroxyl radical. This lowest rotational state is split into two by so-called Lambda-doubling effect. In their turn, both Lambda-doubling sublevels are divided into two by hyperfine splitting due to interaction of the unpaired electron's spin with the magnetic moment of the hydrogen nucleus. Transitions between these four sublevels yield four spectral lines near the wavelength 18 cm. The two central lines near 1665 and 1667 MHz are the strongest, they are called main lines; the 1612 and 1720-MHz lines are weaker, they are called satellite lines. The main lines are strong in masers associated with star-forming regions. Often the 1665-MHz line, which in equilibrium conditions is weaker than 1667 MHz, exceeds it in intensity. Furthermore, the 1612-MHz line intensity in late-type stars is sometimes equal to or stronger than the main OH lines. Masers in the 1720-MHz line are rather rare, they were initially found near supernova remnants. However, recently some of them have been detected in star-forming regions.

Energy levels and transitions in the ground rotational state of the OH molecule.

Joint observations of French and Russian radio astronomers began in late 1960s on the Nancay Radio Telescope with the Russian-made spectral receiver for the wavelength 18 cm (lines of the hydroxyl molecule OH). The high performance parameters of the Nancay telescope enabled the researchers to obtain quality observations of masers in many star-forming regions. Later improvements in the instrumentation now allow us to measure all senses of polarisation of cosmic radiation, which largely broadens the capabilities of the instrument.

Up to now a great experience in spectral radio observations of molecular masers has been acquired by the French and Russian teams. Beginning from 1980 the Russian group has accumulated a large database of observations of variability of maser sources in star-forming regions in the H₂O line on the 22-meter radio telescope in Pushchino (Moscow Region, Russia); a database of such duration and completeness has no analogs in the practice of the world radio astronomy. A big deal of maser studies has been done on the Nancay telescope, including OH line observations of star-forming regions, stellar envelopes and comets. In 2007 French and Russian groups resumed joint observations of OH masers on the Nancay Radio Telescope. New promising data have already been obtained, including several detections of previously unknown masers, strong variability of some sources and complete pattern of polarisation of OH masers permitting to evaluate the magnetic field intensity near young stellar objects. In the meanwhile, the Russians have a possibility to continue the H₂O maser line monitoring of the same list of maser sources in Pushchino. Parallel monitoring of the masers in the OH and H₂O lines adds value to the radio study of the physics of circumstellar environment, since the lines are formed in the vicinity of the same stellar object, however in layers of material at different distances from the central star. Thus, tracing variations in both lines yields a cross-section of physical conditions near young stellar objects.

The list of the masers observed in the OH and H₂O lines is given here.

Below some sample profiles of the OH lines obtained on the Nancay Radio Telescope are shown. The Nancay Radio Telescope measures in every observational session of a maser source the emission of four kinds of polarisation: left-hand circular (LC), right-hand circular (RC) and two senses of linear, with position angles 0^o (vertical) and 90^o (horizontal) or 45^o and -45^o. Combining the spectra obtained in all these polarisations, we can get the spectra of the Stokes parameters:
I = LC + RC or I = L0^o + L(90^o) or I = L(45^o) + L(-45^o) - total intensity;
Q = L0^o - L(90^o);
U = L(45^o) - L(-45^o);
V = RC - LC.

The state of polarisation of the maser emission is an important features of maser sources. Hydroxyl masers very often radiate strongly polarised emision, sometimes having a 100 per cent circular polarisation. This tells about rather strong magnetic fields in masering regions, up to several milligauss, which is two to three orders of magnitude higher than in the general interstellar medium. And the magnetic field is an important factor at early stages of star formation. Thus, the maser emission is a useful tool for finding out physical conditions in the interiors of star-forming regions.

Figure 1. Profiles of the OH main lines 1665 and 1667 MHz of the OH emission toward the infrared source IRAS 05338-0624. Horizontal axis: radial velocity referred to the Local Standard of Rest; vertical axis: emission flux density in Janskys; 1 Jy = 10^-26 W m^-2Hz^-1. The three graphs in each column show Stokes parameters of the emission.

Figure 2. Probable fast variability of the source W75N.

Figure 3. Strong polarisation and time variations of the circumstellar OH maser in the semiregular variable star HU Puppis.

Existing publications on the subject

Joint publications:

M. I. Pashchenko, G. M. Rudnitskij, P. Colom. Fast variable maser source IRAS 05338-0624.
Astronomicheskij Zhurnal, 86, 587 (2009). In Russian.
Astronomy Reports, 53, 541 (2009). In English.

G. M. Rudnitskij, M. I. Pashchenko, P. Colom. Polarization Observations of Circumstellar OH Masers.
Astronomicheskij Zhurnal, 87, 444 (2010). In Russian.
Astronomy Reports, 54, 400 (2010). In English.

V. I. Slysh, M. I. Pashchenko, G. M. Rudnitskij, V. M. Vitrishchak, P. Colom. Polarization Observations of the OH Maser W75N on the Nancay Radio Telescope.
Astronomicheskij Zhurnal, 87, 655 (2010). In Russian.
Astronomy Reports, 54, 599 (2010). In English.

G. M. Rudnitskij, P. Colom, E. E. Lekht, M. I. Pashchenko, V. A. Samodurov, I. A. Subaev, A. M. Tolmachev.
Radio spectroscopy of circumstellar molecular masers.
IAU Symposium 280. Molecular Universe. Toledo, Spain, 30 May - 3 June 2011. Poster 72, Session 1.

P. Colom, E. E. Lekht, M. I. Pashchenko, G. M. Rudnitskij
The source of maser emission W33C (G12.8-0.2).
Astronomicheskii Zhurnal, 89, 811 (2012). In Russian.
Astronomy Reports, 56, 731 (2012). In English.
2012arXiv1208.0683

P. Colom, E. E. Lekht, M. I. Pashchenko, G. M. Rudnitskij.
A New Source of Maser Radio Emission in the 1720-MHz OH Line.
Astronomicheskii Tsirkulyar No. 1602, 2013 August 26.

P. Colom, E. E. Lekht, M. I. Pashchenko, G. M. Rudnitskij, and A. M. Tolmachev.
Investigation of the OH and H₂O maser emission from the semiregular variable HU Puppis.
Pis'ma v Astronomicheskii Zhurnal, 40, 241-250 (2014). In Russian.
Astronomy Letters, 40, 212-221 (2014). In English.

Colom P., Lekht E. E., Pashchenko M. I., and Rudnitskij G. M.
OH and H₂O maser variations in W33B.
Astronomy and Astrophysics, 575, A49 (2015)
2014arXiv1412.0462
Previous publications

French researchers

P. de Vicente, J. Martin-Pintado, R. Neri, P. Colom.
A ridge of recent massive star formation between Sgr B2M and Sgr B2N.
Astronomy and Astrophysics, 361, 1058-1072 (2000).

J. Crovisier, P. Colom, E. Gerard, D. Bockelee-Morvan, G. Bourgois.
Observations at Nancay of the OH 18-cm lines in comets. The data base. Observations made from 1982 to 1999.
Astronomy and Astrophysics, 393, 1053-1064 (2002).

J. Crovisier, P. Colom, E. Gerard, D. Bockelee-Morvan.
Observations of the OH radio lines of comets at Nancay - recent developments.
Earth, Moon, and Planets, 90, 369-370 (2002).

W.-L. Tseng, D. Bockelee-Morvan, J. Crovisier, P. Colom, W.-H. Ip.
Cometary water expansion velocity from OH line shapes.
Astronomy and Astrophysics, 467, 729-735 (2007).

J. Crovisier, N. Biver, D. Bockelee-Morvan, P. Colom.
Radio observations of Jupiter-family comets.
Planetary and Space Science, 57, 1162–1174 (2009).

J. Crovisier, P. Colom, N. Biver, D. Bockelee-Morvan, J. Boissier.
Observations of the 18-cm OH lines of Comet 103P/Hartley 2 at Nancay in support to the EPOXI and Herschel missions.
Icarus, 222, 679-683 (2013).

Russian researchers

G. M. Rudnitskij, E. E. Lekht, J. E. Mendoza-Torres, M. I. Pashchenko, I. I. Berulis.
Variability of the H₂O maser associated with U Orionis.
Astronomy and Astrophysics Supplement Series, 146, 385-395 (2000).

E. E. Lekht, J. E. Mendoza-Torres, G. M. Rudnitskij, A. M. Tolmachev.
Variability of the H₂O maser associated with the Mira variable RS Virginis.
Astronomy and Astrophysics, 376, 928-940 (2001).

E. E. Lekht, G. M. Rudnitskij, J. E. Mendoza-Torres, A. M. Tolmachev.
Variability of the H₂O maser associated with the M-supergiant S Persei.
Astronomy and Astrophysics, 437, 127-133 (2005).

G. M. Rudnitskij, M. I. Pashchenko, V. F. Esipov, V. A. Samodurov, I. A. Subaev, A. M. Tolmachev, E. E. Lekht.
Twenty-six-year monitoring of water masers, Astrophysical Masers and their Environments,
Proceedings of the International Astronomical Union, IAU Symposium, Volume 242, Cambridge University Press, 330-331 (2008).

A. M. S. Richards, S. Etoka, M. D. Gray, E. E. Lekht, J. E. Mendoza-Torres, K. Murakawa, G. M. Rudnitskij, J. A. Yates.
Evolved star water maser cloud size determined by star size.
Astronomy and Astrophysics, 546, A16 (2012).
2012arXiv1207.2583

See also about our project on Multiwavelength Studies of Mira-Ceti Type Variable Stars

How to reach us:

Postal Address:

Sternberg Astronomical Institute
Moscow State University
13 Universitetskij prospekt
Moscow V-234, 119234 Russia

Rudnitskij, Georgij M.

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