Sizes of Masing Parts of Massive Star Forming Regions
It is shown that the images, line profiles, and time evolution of different interstellar masing transitions can be explained by the hypothesis that the population inversions giving birth to masers take place in turbulent regions with extents that are orders of magnitude greater than the sizes of the...
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| Цитувати: | Sizes of masing parts of massive star forming regions / A.M. Sobolev, E.C. Sutton, W.D. Watson, A.B. Ostrovskii, O.V. Shelemei // Радиофизика и радиоастрономия. — 2008. — Т. 13, № 3. — С. 76-80. — Бібліогр.: 20 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859480198911623168 |
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| author | Sobolev, A.M. Sutton, E.C. Watson, W.D. Ostrovskii, A.B. Shelemei, O.V. |
| author_facet | Sobolev, A.M. Sutton, E.C. Watson, W.D. Ostrovskii, A.B. Shelemei, O.V. |
| citation_txt | Sizes of masing parts of massive star forming regions / A.M. Sobolev, E.C. Sutton, W.D. Watson, A.B. Ostrovskii, O.V. Shelemei // Радиофизика и радиоастрономия. — 2008. — Т. 13, № 3. — С. 76-80. — Бібліогр.: 20 назв. — англ. |
| collection | DSpace DC |
| description | It is shown that the images, line profiles, and time evolution of different interstellar masing transitions can be explained by the hypothesis that the population inversions giving birth to masers take place in turbulent regions with extents that are orders of magnitude greater than the sizes of the maser spots. It is shown that the images of methanol masers in the turbulent model persist within considerable time and do not prevent measurement of the annual parallaxes using data on the 12 GHz methanol maser positions.
Показано, что изображения, профили линий и временная эволюция различных переходов межзвездных мазеров могут быть объяснены в рамках гипотезы о том, что инверсия населенностей, приводящая к появлению мазеров, возникает в турбулентных районах с размерами, превышающими размеры мазерных пятен на порядки величины. Показано также, что изображения метанольных мазеров в турбулентной модели остаются неизменными в течение продолжительного времени, что позволяет проводить измерения годичных параллаксов по данным о положениях метанольных мазеров на 12 ГГц.
Показано, що зображення, профілі ліній та часову еволюцію різних переходів міжзоряних мазерів можна пояснити в межах гіпотези про те, що інверсія населеностей, котра призводить до появи мазерів, виникає в турбулентних районах з розмірами, вищими за такі мазерних плям на порядки величини. Показано також, що зображення метанольних мазерів у турбулентній моделі залишаються незмінними протягом довгого часу, що дозволяє вимірювати річні паралакси за даними про розташування метанольних мазерів на 12 ГГц.
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| first_indexed | 2025-11-24T11:53:39Z |
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Радиофизика и радиоастрономия, 2008, т. 13, №3, с. S76-S80
© A. M. Sobolev, E. C. Sutton, W. D. Watson, A. B. Ostrovskii, O. V. Shelemei, 2008
Sizes of Masing Parts of Massive Star Forming Regions
A. M. Sobolev, E. C. Sutton, W. D. Watson, A. B. Ostrovskii, O. V. Shelemei
The Ural State University,
Lenin Ave. 51, Ekaterinburg, 620053, Russia
E-mail: Andrey.Sobolev@usu.ru
It is shown that the images, line profiles, and time evolution of different interstellar masing transi-
tions can be explained by the hypothesis that the population inversions giving birth to masers take
place in turbulent regions with extents that are orders of magnitude greater than the sizes of the maser
spots. It is shown that the images of methanol masers in the turbulent model persist within consider-
able time and do not prevent measurement of the annual parallaxes using data on the 12 GHz methanol
maser positions.
Introduction
It is well known that clusters of interstellar maser spots are observed in those parts of
star forming regions exposed to ionization and shock front propagation. The time behavior
and the spatial distribution of the maser emission show that chaos plays a crucial role in
these regions. The influence of turbulence on various features of astrophysical masers has
been explored in a number of previous investigations, including [1-4] and references
therein.
This paper considers the case when population inversions of maser levels exist in turbu-
lent regions with sizes which are orders of magnitude greater than those of related maser
“hot” spots. Indeed, analysis of multi-transitional observations of a masing molecule such as
methanol (CH3OH) shows that the sizes of the masing regions (i. e., regions where maser
emission forms due to the existence of population inversions) can be comparable to those of
associated molecular cores (see, e. g., [5, 6]). Although only weak masing is possible on
such large scales, there are indications that strong masers also form in rather extended re-
gions. Firstly, interferometric observations show that the strongest H2O and CH3OH maser
images have a “core - extended halo” structure [7, 8], OH and CH3OH masers form large-
scale filaments [9]. Secondly, absorption lines are detected that correspond to transitions
which are considerably overcooled only under conditions where the masing transitions are
strongly inverted [10]. One of these was observed with VLA [11].
1. Sizes of masing regions in W3(OH)
The BIMA interferometer was used to observe 24 methanol lines toward W3(OH) with spectral
resolution better than 1.2 km/s. This source is a prototypical class II methanol maser source. We de-
tected emission in new maser transitions toward it [3]. Analysis of these observations has brought con-
siderable improvement to the model of the masing region in front of W3(OH) and has predicted strong
absorption in 6 methanol lines with frequencies 84.52121, 85.56807, 94.54181, 95.16952, 105.06376,
and 109.15321 GHz. The profiles of these lines can be well fitted with a superposition of 2 gaussians
corresponding to common emitting and absorbing regions. Detailed analysis is currently underway,
but it is clear that the depths of the absorptions are in accord with the model predictions. Most impor-
tantly, the maps in these lines at the maser velocities are spatially anticorrelated with the maser emis-
sion. That means that the size of the masing region is about the total spread of the 6.7 GHz class II
Sizes of Masing Parts of Massive Star Forming Regions
Радиофизика и радиоастрономия, 2008, т. 13, №3 S77
methanol maser spots. In the paper [10] it is shown that a comparison of the data and models for the
whole set of observed methanol lines at W3(OH) is in general agreement with the hypothesis that in
front of the ultracompact HII region (UCHII) are situated 2 methanol-rich regions: a rather hot metha-
nol emitting region with an angular size slightly exceeding that of the UCHII and a second region pro-
ducing the strong masers and absorptions. The latter has a size of a few arcseconds, which is about 3
orders of magnitude greater than that of individual maser hot spots and corresponds to the total extent
of the region where the strong class II methanol masers are distributed. These observational data agree
with the hypothesis that the maser spots form due to irregularities in a relatively extended turbulent
region with a population inversion. These irregularities can be represented by correlations in the Dop-
pler velocities, inhomogeneities of other physical parameters or combinations of these quantities. This
turbulent nature leads to a chaotic distribution of the spots over the face of the region. Hence, the
spread of maser spots reflects the extent of the turbulent masing region. The sizes of the methanol
masing regions under this hypothesis have values from a few 1016 cm for the 6.7 GHz masers in
W3(OH) to a few 1017 cm for the 25 GHz methanol masers in OMC-1.
2. Images and variability of the 25 GHz methanol masers
Because of the exponential sensitivity of maser radiation to optical depth, it is likely that small ir-
regularities in the velocities of the gas play a role in selecting the Doppler velocities at which strong
maser emission is observed. In the paper [1] we described basic principles for the formation of maser
images and spectra in a medium with a turbulent velocity field. A comparison of smoothed images and
integrated spectra of the turbulent layer models has shown that velocity distributions that are some-
what steeper than the Kolmogorov power law reproduce the characteristic features of the observational
data on the 25 GHz methanol masers in OMC-1 obtained by Johnston et al. [12].
After that Johnston et al. [13] published images at high angular resolution along with information
on the time variability of the 25 GHz maser spots in OMC-1. In addition, a computer code for the cal-
culation of time variations in the turbulent velocity field became available to us (see [14]). This and
increased (though still modest) computing power allowed us to make calculations with better angular
resolution and to investigate the characteristics of particular maser spots [2]. These computations
demonstrated that the basic properties of the images of the 25 GHz methanol masers in OMC-1 can be
reproduced by a model consisting of a turbulent layer of gas. These properties include: 1) features
which appear to be single spots with a 3'' beam are resolved into several spots in observations with a
0.07'' beam; 2) maser images show significant changes with frequency even for cases when the fre-
quencies are separated by less than a thermal breadth; 3) the average sizes of the spots increase as the
spots become weaker, while the relation between the peak intensity and the size of the spot shows sub-
stantial scatter; and 4) maser hot spots are surrounded by extended regions which emit at frequencies
close to those of the associated spots. Time-dependent computations show that the evolution of the
turbulent velocity field can explain the observational fact that individual maser spots can vary on a
time scale of about a year, whereas the maser spectrum integrated over the entire masing region re-
mains relatively constant. The computations predict that changes in the integrated maser spectrum
might become detectable over about ten years. Thus, there is evidence that the observational properties
of the 25 GHz methanol masers in OMC-1 can be understood through a consideration of the turbulent
velocities alone. We do realize that variations in other physical quantities can be important for astro-
physical masers. However, the goal of our investigation is to see how well turbulent velocity field
alone can reproduce the observed characteristics for a class of masers where there is evidence that
variations in the other physical quantities are small.
3. Images and variability of the water masers
The existence of extended regions with spectral properties similar to those of nearby 25 GHz
methanol maser spots is reminiscent of the halos found around water maser spots [7] and around the
spots of the brightest class II methanol masers [8]. The extended regions that appear in the above
A. M. Sobolev, E. C. Sutton, W. D. Watson, A. B. Ostrovskii, O. V. Shelemei
Радиофизика и радиоастрономия, 2008, т. 13, №3 S78
computations are considerably greater in size than these observed halos. A natural explanation for this
difference is in the much smaller amplification factors of the 25 GHz masers. An explanation in terms
of amplification factors is in agreement with the trend that the class II methanol maser halos are much
larger than those surrounding the much brighter water maser spots. In order to check whether halos
can form due to the turbulent velocity field, we performed calculations for a turbulent layer with char-
acteristics reproducing the general properties of water masers in W49A, as described in [15] and [16].
We found that the model with the turbulent spectrum b = 0.167, velocity dispersion σt ≈ 5 vth and τ0
corresponding to the brightest spot with 1014 K reproduces the following observational data: 1) spots
form clusters; 2) hot spot position can stay fixed or shift continuously with Doppler velocity; 3) halos
occur around numerous spots which have a wide range of peak fluxes; 4) spectra of the hot spots and
surrounding halos are often similar (see fig. 1); 5) fluxes of the spots vary on the timescale of weeks.
Fig. 1. Image of the water maser spot in the turbulent model and spectra of the spot and its halo. Similarity of
the spectra is seen in the wide dynamic range
4. Variability of the images of the strong methanol masers due to the source rotation
It is well established that the strong masers display time variability. The reasons for variability of
the strong methanol masers were discussed in several papers (see, e. g., [17, 18]). Here we consider
how the rotation of the turbulent slab can change the image of the masing region. In order to do that
we considered images of the slab with dimensions 128×512×512 seen from the different viewing an-
gles. For W3(OH) which is about 2 kpc from us [19], has angular size of about 2 arcsec and has char-
acteristic proper motions of masers about 20 km/s [20] within 1 year the change of the viewing angle
of about 0.001 rad can be realized.
The main results of our calculations of the influence of the slab rotation on the images of maser re-
gion in the methanol maser line at 12 GHz are presented in Fig. 2. We find that the spots in our models
display noticeable changes with rotational angle of about 0.005 rad. Main patterns of the variability of
the images of the spot features are shown in Fig. 2: spot 1 shows positional shift; group of spots 2 –
increase of the spot size and appearance of additional spots; spot 3 – weak variability of extended fea-
ture; spot 4 shows an example of disappearance of the bright maser spot. As it was shown above this
changes are likely to be realized in the time period exceeding 5 years. So, the visual motions of maser
spots caused by the turbulent velocity field do not prevent measurement of the distances to the objects
Sizes of Masing Parts of Massive Star Forming Regions
Радиофизика и радиоастрономия, 2008, т. 13, №3 S79
with the trigonometric parallax method used in [19, 20]. However, one has to be cautious using the
data with the difference of epochs exceeding 5 years.
Similar changes in the spot images of the 12 GHz methanol masers can be realized by the time evo-
lution of the turbulent velocity field but this requires greater periods of time.
1
2
3
41.0
0.5
0.0
Fig. 2. Images are shown in the vicinity of the brightest spot for different viewing angles of the slab. The size of
an individual panel is 32×200. Panels show the evolution of the images with rotational angle. Relative intensity
in logarithmic scale is shown in grayscale. Changes in the images with rotational angle of the slab are seen (see
description in section 4)
Conclusion
This paper shows that the data on the images, spectral profiles and time evolution of the 25 GHz
methanol masers in OMC-1, class II methanol masers and non-maser methanol lines in W3(OH), and
water masers in W49A can be explained under the hypothesis that the masing parts of massive star
forming regions are represented by turbulent zones with sizes that are orders of magnitude greater than
those displayed by the maser spots. It is shown that changes of the methanol maser images in the tur-
bulent model are quite slow and do not prevent measurement of the distances by trigonometric paral-
lax method using data on the 12 GHz methanol masers.
AMS, ABO and OVS were supported by RFBR grant (07-02-00628-a). ECS and WDW were sup-
ported by NSF Grants AST 02-28953 and AST 99-88104.
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Размеры мазерных областей в районах образования массивных звезд
А. М. Соболев, Е. C. Саттон, В. Д. Уотсон, А. Б. Островский, О. В. Шелемей
Показано, что изображения, профили линий и временная эволюция различных переходов
межзвездных мазеров могут быть объяснены в рамках гипотезы о том, что инверсия населенно-
стей, приводящая к появлению мазеров, возникает в турбулентных районах с размерами, пре-
вышающими размеры мазерных пятен на порядки величины. Показано также, что изображения
метанольных мазеров в турбулентной модели остаются неизменными в течение продолжитель-
ного времени, что позволяет проводить измерения годичных параллаксов по данным о положе-
ниях метанольных мазеров на 12 ГГц.
Розмір мазерних областей у районах утворення масивних зірок
А. М. Соболев, Е. C. Саттон, В. Д. Уотсон, А. Б. Островський, О. В. Шелемей
Показано, що зображення, профілі ліній та часову еволюцію різних переходів міжзоряних
мазерів можна пояснити в рамках гіпотези про те, що інверсія населеностей, котра призводить
до появи мазерів, виникає в турбулентних районах з розмірами вищими за такі мазерних плям
на порядки величини. Показано також, що зображення метанольних мазерів у турбулентній мо-
делі зостаются незмінними протягом довгого часу, що дозволяє вимірювати річні паралакси за
даними про розташування метанольних мазерів на 12 ГГц.
|
| id | nasplib_isofts_kiev_ua-123456789-8413 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1027-9636 |
| language | English |
| last_indexed | 2025-11-24T11:53:39Z |
| publishDate | 2008 |
| publisher | Радіоастрономічний інститут НАН України |
| record_format | dspace |
| spelling | Sobolev, A.M. Sutton, E.C. Watson, W.D. Ostrovskii, A.B. Shelemei, O.V. 2010-05-28T11:54:08Z 2010-05-28T11:54:08Z 2008 Sizes of masing parts of massive star forming regions / A.M. Sobolev, E.C. Sutton, W.D. Watson, A.B. Ostrovskii, O.V. Shelemei // Радиофизика и радиоастрономия. — 2008. — Т. 13, № 3. — С. 76-80. — Бібліогр.: 20 назв. — англ. 1027-9636 https://nasplib.isofts.kiev.ua/handle/123456789/8413 It is shown that the images, line profiles, and time evolution of different interstellar masing transitions can be explained by the hypothesis that the population inversions giving birth to masers take place in turbulent regions with extents that are orders of magnitude greater than the sizes of the maser spots. It is shown that the images of methanol masers in the turbulent model persist within considerable time and do not prevent measurement of the annual parallaxes using data on the 12 GHz methanol maser positions. Показано, что изображения, профили линий и временная эволюция различных переходов межзвездных мазеров могут быть объяснены в рамках гипотезы о том, что инверсия населенностей, приводящая к появлению мазеров, возникает в турбулентных районах с размерами, превышающими размеры мазерных пятен на порядки величины. Показано также, что изображения метанольных мазеров в турбулентной модели остаются неизменными в течение продолжительного времени, что позволяет проводить измерения годичных параллаксов по данным о положениях метанольных мазеров на 12 ГГц. Показано, що зображення, профілі ліній та часову еволюцію різних переходів міжзоряних мазерів можна пояснити в межах гіпотези про те, що інверсія населеностей, котра призводить до появи мазерів, виникає в турбулентних районах з розмірами, вищими за такі мазерних плям на порядки величини. Показано також, що зображення метанольних мазерів у турбулентній моделі залишаються незмінними протягом довгого часу, що дозволяє вимірювати річні паралакси за даними про розташування метанольних мазерів на 12 ГГц. AMS, ABO and OVS were supported by RFBR grant (07-02-00628-a). ECS and WDW were supported by NSF Grants AST 02-28953 and AST 99-88104. en Радіоастрономічний інститут НАН України Астрофизика Sizes of Masing Parts of Massive Star Forming Regions Размеры мазерных областей в районах образования массивных звезд Розмір мазерних областей у районах утворення масивних зірок Article published earlier |
| spellingShingle | Sizes of Masing Parts of Massive Star Forming Regions Sobolev, A.M. Sutton, E.C. Watson, W.D. Ostrovskii, A.B. Shelemei, O.V. Астрофизика |
| title | Sizes of Masing Parts of Massive Star Forming Regions |
| title_alt | Размеры мазерных областей в районах образования массивных звезд Розмір мазерних областей у районах утворення масивних зірок |
| title_full | Sizes of Masing Parts of Massive Star Forming Regions |
| title_fullStr | Sizes of Masing Parts of Massive Star Forming Regions |
| title_full_unstemmed | Sizes of Masing Parts of Massive Star Forming Regions |
| title_short | Sizes of Masing Parts of Massive Star Forming Regions |
| title_sort | sizes of masing parts of massive star forming regions |
| topic | Астрофизика |
| topic_facet | Астрофизика |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/8413 |
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