258 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
Properties of dwarf stars in Cygnus OB2
We present the results of investigation of five stars, originally classified
as dwarfs, belonging to Cyg OB2 association, their stellar and wind properties.
Using both TLUSTY and CMFGEN codes we derived effective temperatures, surface
gravities, chemical abundances, mass-loss rates and projected rotation
velocities. Due to the fact that distance to the stars is well known, we were
able to estimate their luminosities. Using evolutionary models we estimated the
ages of these sample stars and find that lower mass ones - MT282 and MT343 -
belong to older population of the association. Their ages are greater than 10
Myr. The ages of three other stars - MT317, MT299, MT259 - are between 4-6 Myr.Comment: 16 pages, 10 figures, accepted for publication in PAS
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
On the methanol emission detection in the TW Hya disc: the role of grain surface chemistry and non-LTE excitation
The recent detection of gas-phase methanol (CHOH) lines in the disc of TW
Hya by Walsh et al. provided the first observational constraints on the complex
O-bearing organic content in protoplanetary discs. The emission has a ring-like
morphology, with a peak at au and an inferred column density of
cm. A low CHOH fractional abundance of (with respect to H) is derived, depending on the
assumed vertical location of the CHOH molecular layer. In this study, we
use a thermo-chemical model of the TW Hya disc, coupled with the ALCHEMIC
gas-grain chemical model, assuming laboratory-motivated, fast diffusivities of
the surface molecules to interpret the CHOH detection. Based on this disc
model, we performed radiative transfer calculations with the LIME code and
simulations of the observations with the CASA simulator. We found that our
model allows to reproduce the observations well. The CHOH emission in our
model appears as a ring with radius of au. Synthetic and observed line
flux densities are equal within the rms noise level of observations. The
synthetic CHOH spectra calculated assuming local thermodynamic equilibrium
(LTE) can differ by up to a factor of 3.5 from the non-LTE spectra. For the
strongest lines, the differences between LTE and non-LTE flux densities are
very small and practically negligible. Variations in the diffusivity of the
surface molecules can lead to variations of the CHOH abundance and,
therefore, line flux densities by an order of magnitude.Comment: Accepted for publication in MNRAS, 8 pages, 8 figure
Disorder-driven transition to tubular phase in anisotropic two-dimensional materials
We develop a theory of anomalous elasticity in disordered two-dimensional
flexible materials with orthorhombic crystal symmetry. Similar to the clean
case, we predict existence of infinitely many flat phases with anisotropic
bending rigidity and Young's modulus showing power-law scaling with momentum
controlled by a single universal exponent the very same as in the clean
isotropic case. With increase of temperature or disorder these flat phases
undergo crumpling transition. Remarkably, in contrast to the isotropic
materials where crumpling occurs in all spatial directions simultaneously, the
anisotropic materials crumple into tubular phase. In distinction to clean case
in which crumpling transition happens at unphysically high temperatures, a
disorder-induced tubular crumpled phase can exist even at room-temperature
conditions. Our results are applied to anisotropic atomic single layers doped
by adatoms or disordered by heavy ions bombarding.Comment: 18 LaTex pages, 6 figure
Towards detecting methanol emission in low-mass protoplanetary discs with ALMA:The role of non-LTE excitation
The understanding of organic content of protoplanetary discs is one of the
main goals of the planet formation studies. As an attempt to guide the
observational searches for weak lines of complex species in discs, we modelled
the (sub-)millimetre spectrum of gaseous methanol (CHOH), one of the
simplest organic molecules, in the representative T Tauri system. We used 1+1D
disc physical model coupled to the gas-grain ALCHEMIC chemical model with and
without 2D-turbulent mixing. The computed CHOH abundances along with the
CHOH scheme of energy levels of ground and excited torsional states were
used to produce model spectra obtained with the non-local thermodynamic
equilibrium (non-LTE) 3D line radiative transfer code LIME. We found that the
modelled non-LTE intensities of the CHOH lines can be lower by factor of
-- than those calculated under assumption of LTE. Though population
inversion occurs in the model calculations for many (sub-)millimetre
transitions, it does not lead to the strong maser amplification and noticeably
high line intensities. We identify the strongest CHOH (sub-)millimetre
lines that could be searched for with the Atacama Large Millimeter Array (ALMA)
in nearby discs. The two best candidates are the CHOH
(241.791 GHz) and (241.767 GHz) lines, which could possibly
be detected with the signal-to-noise ratio after hours of
integration with the full ALMA array.Comment: Accepted for publication in MNRAS, 15 figures, 3 table
Calculations of H2CO emission in massive accretion disks with considering levels of excited vibrational states
The work has been made in Ural Federal University with financial support from the Russian Science Foundation (project no. 15-12-10017)
Bayesian approach to estimations of star-formation regions physical parameters by few methanol lines
Показано, что использование Байесовского подхода позволяет оценить температуру газа и удельную лучевую концентрацию метанола в областях звездообразования по двум радиолиниям метанола, плотность газа по четырем. Полученные значения физических параметров хорошо согласуются с независимыми оценками.We show that Bayesian approach is useful for estimating kinetic temperatures and methanol specific column densities of starformation regions by only two methanol lines and hydrogen number densities by four lines. Calculated physical parameters’ values are in good agreement with values that were estimated in other ways.Работа выполнена при поддержке Правительства Российской Федерации (постановление № 211, контракт №02.A03.21.0006) и Министерства образования и науки Российской Федерации (базовая часть государственного задания, РК №AAAA-A17-117030310283-7)
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