30 research outputs found
RadioAstron space-VLBI project: studies of masers in star forming regions of our Galaxy and megamasers in external galaxies
Observations of the masers in the course of RadioAstron mission yielded
detections of fringes for a number of sources in both water and hydroxyl maser
transitions. Several sources display numerous ultra-compact details. This
proves that implementation of the space VLBI technique for maser studies is
possible technically and is not always prevented by the interstellar
scattering, maser beaming and other effects related to formation, transfer, and
detection of the cosmic maser emission. For the first time, cosmic water maser
emission was detected with projected baselines exceeding Earth Diameter. It was
detected in a number of star-forming regions in the Galaxy and megamaser
galaxies NGC 4258 and NGC 3079. RadioAstron observations provided the absolute
record of the angular resolution in astronomy. Fringes from the NGC 4258
megamaser were detected on baseline exceeding 25 Earth Diameters. This means
that the angular resolution sufficient to measure the parallax of the water
maser source in the nearby galaxy LMC was directly achieved in the cosmic maser
observations. Very compact features with angular sizes about 20 microarcsec
have been detected in star-forming regions of our Galaxy. Corresponding linear
sizes are about 5-10 million kilometers. So, the major step from milli- to
micro-arcsecond resolution in maser studies is done in the RadioAstron mission.
The existence of the features with extremely small angular sizes is
established. Further implementations of the space-VLBI maser instrument for
studies of the nature of cosmic objects, studies of the interaction of
extremely high radiation field with molecular material and studies of the
matter on the line of sight are planned.Comment: To be published in Astrophysical Masers: Unlocking the Mysteries of
the Universe, IAU Symposium 336, 201
RadioAstron probes the ultra-fine spatial structure in the HO maser emission in the star forming region W49N
HO maser emission associated with the massive star formation region W49N
were observed with the Space-VLBI mission RadioAstron. The procedure for
processing of the maser spectral line data obtained in the RadioAstron
observations is described. Ultra-fine spatial structures in the maser emission
were detected on space-ground baselines of up to 9.6 Earth diameters. The
correlated flux densities of these features range from 0.1% to 0.6% of the
total flux density. These low values of correlated flux density are probably
due to turbulence either in the maser itself or in the interstellar medium.Comment: Accepted for publication in Advances in Space Researc
Sun-Sized Water Vapor Masers in Cepheus A
We present the first VLBI observations of a Galactic water maser (in Chepeus
A) made with a very long baseline interferometric array involving the
RadioAstron Earth-orbiting satellite station as one of its elements. We
detected two distinct components at -16.9 and 0.6 km/s with a fringe spacing of
66 microarcseconds. In total power, the 0.6 km/s component appears to be a
single Gaussian component of strength 580 Jy and width of 0.7 km/s.
Single-telescope monitoring showed that its lifetime was only 8~months. The
absence of a Zeeman pattern implies the longitudinal magnetic field component
is weaker than 120 mG. The space-Earth cross power spectrum shows two
unresolved components smaller than 15 microarcseconds, corresponding to a
linear scale of 1.6 x 10^11 cm, about the diameter of the Sun, for a distance
of 700 pc, separated by 0.54 km/s in velocity and by 160 +/-35 microarcseconds
in angle. This is the smallest angular structure ever observed in a Galactic
maser. The brightness temperatures are greater than 2 x 10^14K, and the line
widths are 0.5 km/s. Most of the flux (about 87%) is contained in a halo of
angular size of 400 +/- 150 microarcseconds. This structure is associated with
the compact HII region HW3diii. We have probably picked up the most prominent
peaks in the angular size range of our interferometer. We discuss three
dynamical models: (1) Keplerian motion around a central object, (2) two chance
overlapping clouds, and (3) vortices caused by flow around an obstacle (i.e.,
von Karman vortex street) with Strouhal number of about~0.3.Comment: 15 pages, 9 figures. Accepted for publication in ApJ, February 16,
201
Brightness temperatures of galactic masers observed in the RadioAstron project
We present estimates of brightness temperature for 5 galactic masers in star-forming regions detected at space baselines. Very compact features with angular sizes of ~23-60 μas were detected in these regions with corresponding linear sizes of ~4-10×10 6 km. Brightness temperatures range from 10 14 up to 10 16 K. © International Astronomical Union 2018
The brightest OH maser in the sky: a flare of emission in W75 N
A flare of maser radio emission in the OH-line 1665 MHz has been discovered
in the star forming region W75 N in 2003, with the flux density of about 1000
Jy. At the time it was the strongest OH maser detected during the whole history
of observations since the discovery of cosmic masers in 1965. The flare
emission is linearly polarized with a degree of polarization near 100%. A
weaker flare with a flux of 145 Jy was observed in this source in 2000 - 2001,
which was probably a precursor of the powerful flare. Intensity of two other
spectral features has decreased after beginning of the flare. Such variation of
the intensity of maser condensation emission (increasing of one and decreasing
of the other) can be explained by passing of the magneto hydrodynamic shock
across regions of enhanced gas concentration.Comment: 9 pages with 2 figures, accepted for publication in Astronomy Letter
Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: II. Maser Properties and the Interpretation of Polarization
We analyze full-polarization VLBA data of ground-state, main-line OH masers
in 18 massive star-forming regions previously presented in a companion paper.
The OH masers often arise in the shocked neutral gas surrounding ultracompact
Hii regions. Magnetic fields as deduced from OH maser Zeeman splitting are
highly ordered, both on the scale of a source as well as the maser clustering
scale of ~10^15 cm. Results from our large sample show that this clustering
scale appears to be universal to these masers. OH masers around ultracompact
Hii regions live ~10^4 years and then turn off abruptly, rather than weakening
gradually with time. These masers have a wide range of polarization properties.
At one extreme (e.g., W75 N), pi-components are detected and the polarization
position angles of maser spots show some organization. At the other extreme
(e.g., W51 e1/e2), almost no linear polarization is detected and total
polarization fractions can be substantially less than unity. A typical source
has properties intermediate to these two extremes. In contrast to the well
ordered magnetic field inferred from Zeeman splitting, there is generally no
clear pattern in the distribution of polarization position angles. This can be
explained if Faraday rotation in a typical OH maser source is large on a maser
amplification length but small on a single (e-folding) gain length. Increasing
or decreasing Faraday rotation by a factor of ~5 among different sources can
explain the observed variation in polarization properties. We suggest that
almost all pi-components acquire a signficant amount of circular polarization
from low-gain stimulated emission of a sigma-component from OH appropriately
shifted in velocity and lying along the propagation path.Comment: AASTeX, 57 pages including 2 tables and 21 figures (1 color),
accepted for publication in ApJ
Parameters of Warm Molecular Clouds from Methyl Acetylene Observations
The results of a survey of 63 galactic star-forming regions in the 6_K-5_K and 5_K-4_K methyl acetylene lines at 102 and 85 GHz, respectively, are presented. Fourty-three sources were detected at 102 GHz, and twenty-five at 85 GHz. Emission was detected towards molecular clouds with kinetic temperatures 20-60 K (so-called ``warm clouds''). The CH3CCH abundances in these clouds appeared to be about several units X 10^(-9). Five mapped sources were analyzed using the maximum entropy method. The sizes of the mapped clouds fall within the range between 0.1 and 1.7 pc, virial masses - between 90-6200 Msun, and densities - between 6 X 10^4 and 6 X 10^5 cm^(-3). The CH3CCH sources spatially coincide with the CO and CS sources. Chemical evolution simulations showed that the typical methyl acetylene abundance in the observed clouds corresponds to an age of ~ 6 X 10^4 years