2,888 research outputs found
On the Class II Methanol Maser Periodic Variability due to the Rotating Spiral Shocks in the Gaps of Disks Around Young Binary Stars
We argue that the periodic variability of Class II methanol masers can be
explained by variations of the dust temperature in the accretion disk around
proto-binary star with at least one massive component. The dust temperature
variations are caused by rotation of hot and dense material of the spiral shock
wave in the disk central gap. The aim of this work is to show how different can
be the Class II methanol maser brightness in the disk during the Moment of
Maximum Illumination by the Spiral Shock material (hereafter MMISS) and the
Moment when the disk is Illuminated by the Stars Only (MISO). We used the code
CLOUDY (v13.02) to estimate physical conditions in the flat disk in the MISO
and the MMISS. Model physical parameters of the disk were then used to estimate
the brightness of 6.7, 9.9, 12.1 and 107 GHz masers at different impact
parameters using LVG approximation. It was shown that the strong masers
experience considerable brightness increase during the MMISS with respect to
MISO. There can happen both flares and dips of the 107 GHz maser brightness
under the MMISS conditions, depending on the properties of the system. The
brightest 9.9 GHz masers in the MMISS are situated at the greater than the
strong 6.7, 12.1 and 107 GHz masers that are situated at AU. The
brightness of 9.9 GHz maser in the MMISS suppressed at AU and increase
at AU.Comment: Accepted for publication in MNRAS, 9 figure
A family of anisotropic integral operators and behaviour of its maximal eigenvalue
We study the family of compact integral operators in
with the kernel K_\beta(x, y) = \frac{1}{\pi}\frac{1}{1 +
(x-y)^2 + \beta^2\Theta(x, y)}, depending on the parameter , where
is a symmetric non-negative homogeneous function of degree
. The main result is the following asymptotic formula for the
maximal eigenvalue of : M_\beta = 1 - \lambda_1
\beta^{\frac{2}{\gamma+1}} + o(\beta^{\frac{2}{\gamma+1}}), \beta\to 0, where
is the lowest eigenvalue of the operator . A central role in the proof is played by the fact that
is positivity improving. The case has been studied earlier in the literature as a simplified model
of high-temperature superconductivity.Comment: 16 page
The Spectral Type of the Ionizing Stars and the Infrared Fluxes of HII Regions
The 20 cm radio continuum fluxes of 91 HII regions in a previously compiled
catalog have been determined. The spectral types of the ionizing stars in 42
regions with known distances are estimated. These spectral types range from
B0.5 to O7, corresponding to effective temperatures of 29 000-37 000 K. The
dependences of the infrared (IR) fluxes at 8, 24, and 160 m on the 20 cm
flux are considered. The IR fluxes are used as a diagnostic of heating of the
matter, and the radio fluxes as measurements of the number of ionizing photons.
It is established that the IR fluxes grow approximately linearly with the radio
flux. This growth of the IR fluxes probably indicates a growth of the mass of
heated material in the envelope surrounding the HII region with increasing
effective temperature of the star.Comment: 16, pages, 10 figures, published in Astronomy Report
Gas kinematics in massive star-forming regions from the Perseus spiral arm
We present results of a survey of 14 star-forming regions from the Perseus
spiral arm in CS(2-1) and 13CO(1-0) lines with the Onsala Space Observatory 20
m telescope. Maps of 10 sources in both lines were obtained. For the remaining
sources a map in just one line or a single-point spectrum were obtained. On the
basis of newly obtained and published observational data we consider the
relation between velocities of the "quasi-thermal" CS(2-1) line and 6.7 GHz
methanol maser line in 24 high-mass star-forming regions in the Perseus arm. We
show that, surprisingly, velocity ranges of 6.7 GHz methanol maser emission are
predominantly red-shifted with respect to corresponding CS(2-1) line velocity
ranges in the Perseus arm. We suggest that the predominance of the "red-shifted
masers" in the Perseus arm could be related to the alignment of gas flows
caused by the large-scale motions in the Galaxy. Large-scale galactic shock
related to the spiral structure is supposed to affect the local kinematics of
the star-forming regions. Part of the Perseus arm, between galactic longitudes
from 85deg to 124deg, does not contain blue-shifted masers at all. Radial
velocities of the sources are the greatest in this particular part of the arm,
so the velocity difference is clearly pronounced. 13CO(1-0) and CS(2-1)
velocity maps of G183.35-0.58 show gas velocity difference between the center
and the periphery of the molecular clump up to 1.2 km/s. Similar situation is
likely to occur in G85.40-0.00. This can correspond to the case when the
large-scale shock wave entrains the outer parts of a molecular clump in motion
while the dense central clump is less affected by the shock.Comment: accepted by Astronomy Report
Molecular Emission in Dense Massive Clumps from the Star-Forming Regions S231-S235
The article deals with observations of star-forming regions S231-S235 in
'quasi-thermal' lines of ammonia (NH), cyanoacetylene (HCN) and maser
lines of methanol (CHOH) and water vapor (HO). S231-S235 regions is
situated in the giant molecular cloud G174+2.5. We selected all massive
molecular clumps in G174+2.5 using archive CO data. For the each clump we
determined mass, size and CO column density. After that we performed
observations of these clumps. We report about first detections of NH and
HCN lines toward the molecular clumps WB89 673 and WB89 668. This means
that high-density gas is present there. Physical parameters of molecular gas in
the clumps were estimated using the data on ammonia emission. We found that the
gas temperature and the hydrogen number density are in the ranges 16-30 K and
2.8-7.2 cm, respectively. The shock-tracing line of CHOH
molecule at 36.2 GHz is newly detected toward WB89 673.Comment: 16 pages, 4 figure
Study of variable stars associated with maser sources: G025.65+1.05
We report variation of K-band infrared (IR) emission in the vicinity of the
G025.65+1.05 water and methanol maser source. New observational data were
obtained with 2.5m telescope of the Caucasian Mountain Observatory (CMO) of
Moscow State University on 2017-09-21 during the strong water maser flare. We
found that the IR source situated close to the maser position had decreased
brightness in comparison to archive data. This source is associated with a
massive young stellar object (MYSO) corresponding to the compact infrared
source IRAS 18316-0602 (RAFGL 7009S). Similar decrease in K-brightness of the
IR source close to the maser position was observed in March~2011 when the water
maser activity was increased. The dips in MYSO brightness can be related to the
maser flare phases. Maser flares that are concurrent with dips of the IR
emission can be explained if the lower IR radiation field enables more
efficient sink of the pumping cycle by allowing IR photons to escape the maser
region.Comment: Accepted for publication in RA
Star formation in the S233 region
The main objective of this paper is to study the possibility of triggered
star formation on the border of the HII region S233, which is formed by a
B-star. Using high-resolution spectra we determine the spectral class of the
ionizing star as B0.5 V and the radial velocity of the star to be -17.5(1.4)
km/s. This value is consistent with the velocity of gas in a wide field across
the S233 region, suggesting that the ionizing star was formed from a parent
cloud belonging to the S233 region. By studying spatial-kinematic structure of
the molecular cloud in the S233 region, we detected an isolated clump of gas
producing CO emission red-shifted relative to the parent cloud. In the UKIDSS
and WISE images, the clump of gas coincides with the infrared source containing
a compact object and bright-rimmed structure. The bright-rimmed structure is
perpendicular to the direction of the ionizing star. The compact source
coincides in position with IRAS source 05351+3549. All these features indicate
a possibility of triggering formation of a next-generation star in the S233
region. Within the framework of a theoretical one-dimensional model we conclude
that the "collect-and-collapse" process is not likely to take place in the S233
region. The presence of the bright-rimmed structure and the compact infrared
source suggest that the "collapse of the pre-existing clump" process is taking
place.Comment: 12 pages, 10 figure
Relation between parameters of dust and parameters of molecular and atomic gas in extragalactic star-forming regions
The relationships between atomic and molecular hydrogen and dust of various
sizes in extragalactic star-forming regions are considered, based on
observational data from the Spitzer and Herschel infrared space telescopes, the
Very Large Array (atomic hydrogen emission) and IRAM (CO emission). The source
sample consists of approximately 300 star-forming regions in 11 nearby
galaxies. Aperture photometry has been applied to measure the fluxes in eight
infrared bands (3.6, 4.5, 5.8, 8, 24, 70, 100, and 160m), the atomic
hydrogen (21cm) line and CO (2--1) lines.
The parameters of the dust in the starforming regions were determined via
synthetic-spectra fitting, such as the total dust mass, the fraction of
polycyclic aromatic hydrocarbons (PAHs), etc. Comparison of the observed fluxes
with the measured parameters shows that the relationships between atomic
hydrogen, molecular hydrogen, and dust are different in low- and
high-metallicity regions. Low-metallicity regions contain more atomic gas, but
less molecular gas and dust, including PAHs. The mass of dust constitutes about
of the mass of molecular gas in all regions considered. Fluxes produced
by atomic and molecular gas do not correlate with the parameters of the stellar
radiation, whereas the dust fluxes grow with increasing mean intensity of
stellar radiation and the fraction of enhanced stellar radiation. The ratio of
the fluxes at 8 and 24m, which characterizes the PAH content, decreases
with increasing intensity of the stellar radiation, possibly indicating
evolutionary variations of the PAH content. The results confirm that the
contribution of the 24m emission to the total IR luminosity of
extragalactic star-forming regions does not depend on the metallicity.Comment: Published in Astronomy Reports, 2017, vol. 61, issue
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
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