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D. A. Semenov

Publications and source records attributed to D. A. Semenov.

35 records · Page 2Linked to original sources
arXiv2016

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 (CH$_3$OH), 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 CH$_3$OH abundances along with the CH$_3$OH 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 CH$_3$OH lines can be lower by factor of $>10$--$100$ 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 CH$_3$OH (sub-)millimetre lines that could be searched for with the Atacama Large Millimeter Array (ALMA) in nearby discs. The two best candidates are the CH$_{3}$OH $5_0-4_0~A^+$ (241.791 GHz) and $5_{-1}-4_{-1}~E$ (241.767 GHz) lines, which could possibly be detected with the $\sim5σ$ signal-to-noise ratio after $\sim3$ hours of integration with the full ALMA array.

astro-ph.SR↗
arXiv2016

A measurement method of a detector response function for monochromatic electrons based on the Compton scattering

In this paper we present a method of scintillation detector energy calibration using the gamma-rays. The technique is based on the Compton scattering of gamma-rays in a scintillation detector and subsequent photoelectric absorption of the scattered photon in the Ge-detector. The novelty of this method is that the source of gamma rays, the germanium and scintillation detectors are immediately arranged adjacent to each other. The method presents an effective solution for the detectors consisting of a low atomic number materials, when the ratio between Compton effect and photoelectric absorption is large and the mean path of gamma-rays is comparable to the size of the detector. The technique can be used for the precision measurements of the scintillator light yield dependence on the electron energy.

physics.ins-det↗
arXiv2015

New limit on the mass of 9.4-keV solar axions emitted in an M1 transition in $^{83}$Kr nuclei

A search for resonant absorption of the solar axion by $^{83}\rm{Kr}$ nuclei was performed using the proportional counter installed inside the low-background setup at the Baksan Neutrino Observatory. The obtained model independent upper limit on the combination of isoscalar and isovector axion-nucleon couplings $|g_3-g_0|\leq 1.69\times 10^{-6}$ allowed us to set the new upper limit on the hadronic axion mass of $m_{A}\leq 130$ eV (95\% C.L.) with the generally accepted values $S$=0.5 and $z$=0.56.

hep-ex↗
arXiv2014

Search for axioelectric effect of solar axions using BGO scintillating bolometer

A search for axioelectric absorption of solar axions produced in the $ p + d \rightarrow {^3\rm{He}}+γ~(5.5~ \rm{MeV})$ reactions has been performed with a BGO detector placed in a low-background setup. A model-independent limit on an axion-nucleon and axion-electron coupling constant has been obtained: $| g_{Ae}\times g_{AN}^3|< 1.9\times 10^{-10}$ for 90% confidence level. The constrains of the axion-electron coupling have been obtained for hadronic axion with masses in (0.1 - 1) MeV range: $|g_{Ae}| \leq (0.96 - 8.2)\times 10^{-8}$.

hep-ex↗
arXiv2014

First result of the experimental search for the 9.4 keV solar axion reactions with Kr-83 in the copper proportional counter

The experimental search for solar hadronic axions is started at the Baksan Neutrino Observatory of the Institute for Nuclear Researches Russian Academy of Science. It is assumed that axions are created in the Sun during M1-transition between the first thermally excited level at 9.4 keV and the ground state in Kr-83. The experiment is based on axion detection via resonant absorption process by the same nucleus in the detector. The big copper proportional counter filled with krypton is used to detect signals from axions. The experimental setup is situated in the deep underground low background laboratory. No evidence of axion detection were found after the 26.5 days data collection. Resulting new upper limit on axion mass is m_{A} < 130 eV at 95% C.L.

nucl-ex↗
arXiv2013

Search for solar axions produced by Compton process and bremsstrahlung using the resonant absorption and axioelectric effect

The search for resonant absorption of Compton and bremsstrahlung solar axions by $^{169}$Tm nuclei have been performed. Such an absorption should lead to the excitation of low-lying nuclear energy level: $A+^{169}$Tm $\rightarrow ^{169}$Tm$^*$ $\rightarrow ^{169}$Tm $+ γ$ (8.41 keV). Additionally the axio-electric effect in silicon atoms is sought. The axions are detected using a Si(Li) detectors placed in a low-background setup. As a result, a new model independent restrictions on the axion-electron and the axion-nucleon coupling: $g_{Ae}\times|g^0_{AN}+ g^3_{AN}|\leq 2.1\times10^{-14}$ and the axion-electron coupling constant: $|g_{Ae}| \leq 2.2\times 10^{-10}$ has been obtained. The limits leads to the bounds $m_{A}\leq$ 7.9 eV and $m_{A}\leq$ 1.3 keV for the mass of the axion in the DFSZ and KSVZ models, respectively ($90\%$ C.L.).

hep-ex↗
arXiv2013

New extended deuterium fractionation model: assessment at dense ISM conditions and sensitivity analysis

Observations of deuterated species are useful in probing the temperature, ionization level, evolutionary stage, chemistry, and thermal history of astrophysical environments. The analysis of data from ALMA and other new telescopes requires an elaborate model of deuterium fractionation. This paper presents a publicly available chemical network with multi-deuterated species and an extended, up-to-date set of gas-phase and surface reactions. To test this network, we simulate deuterium fractionation in diverse interstellar sources. Two cases of initial abundances are considered: i) atomic except for H2 and HD, and ii) molecular from a prestellar core. We reproduce the observed D/H ratios of many deuterated molecules, and sort the species according to their sensitivity to temperature gradients and initial abundances. We find that many multiply-deuterated species produced at 10 K retain enhanced D/H ratios at temperatures $\la 100$ K. We study how recent updates to reaction rates affect calculated D/H ratios, and perform a detailed sensitivity analysis of the uncertainties of the gas-phase reaction rates in the network. We find that uncertainties are generally lower in dark cloud environments than in warm IRDCs and that uncertainties increase with the size of the molecule and number of D-atoms. A set of the most problematic reactions is presented. We list potentially observable deuterated species predicted to be abundant in low- and high-mass star-formation regions.

astro-ph.SR↗
arXiv2011

Tracing the evolutionary stage of Bok globules: CCS and NH3

We pursue the investigation of a previously proposed correlation between chemical properties and physical evolutionary stage of isolated low-mass star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to be a potentially useful indicator for the evolutionary stage of cloud cores. We aim to study its applicability for isolated Bok globules. A sample of 42 Bok globules with and without signs of current star formation was searched for CCS(2-1) emission, the observations were complemented with NH3 measurements available in the literature and own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models. The NH3/CCS ratio could be assessed for 18 Bok globules and is found to be moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harbouring protostars of Class 0, Class I or later. Bok globules with extremely high CCS abundance analogous to carbon-chain producing regions in dark cloud cores are not found. The observed range of NH3/CCS hints towards a relatively evolved chemical state of all observed Bok globules.

astro-ph.GA↗
arXiv2011

Constraints on the axion-electron coupling for solar axions produced by Compton process and bremsstrahlung

The search for solar axions produced by Compton ($γ+e^-\rightarrow e^-+A$) and bremsstrahlung-like ($e^-+Z \rightarrow Z+e^-+A$) processes has been performed. The axion flux in the both cases depends on the axion-electron coupling constant. The resonant excitation of low-lying nuclear level of $^{169}\rm{Tm}$ was looked for: $A+^{169}$Tm $\rightarrow ^{169}$Tm$^*$ $\rightarrow ^{169}$Tm $+ γ$ (8.41 keV). The Si(Li) detector and $^{169}$Tm target installed inside the low-background setup were used to detect 8.41 keV $γ$-rays. As a result, a new model independent restriction on the axion-electron and the axion-nucleon couplings was obtained: $g_{Ae}\times|g^0_{AN}+ g^3_{AN}|\leq 2.1\times10^{-14}$. In model of hadronic axion this restriction corresponds to the upper limit on the axion-electron coupling and on the axion mass $g_{Ae}\times m_A\leq3.1\times10^{-7}$ eV (90% c.l.). The limits on axion mass are $m_A\leq$ 105 eV and $m_A\leq$ 1.3 keV for DFSZ- and KSVZ-axion models, correspondingly (90% c.l.).

hep-ex↗
arXiv2009

Search for Resonant Absorption of Solar Axions Emitted in M1 Transition in $^{57}$Fe Nuclei

A search for resonant absorbtion of 14.4 keV solar axions by $^{57}$Fe target was performed. The Si(Li) detector placed inside the low-background setup was used to detect the $γ$-quanta appearing in the deexcitation of 14.4 keV nuclear level: $A+\rm{^{57}Fe} \to \rm{^{57}Fe^{*}} \to \rm{^{57}Fe} + γ$. The new upper limit for the hadronic axion mass have been obtained: $m_{A} \leq 151$ eV (90% C.L.) ($S$=0.5, $z$=0.56).

hep-ex↗
arXiv2009

Search for Solar Axions Produced by Primakoff Conversion Using Resonant Absorption by $^{169}$Tm Nuclei

The search for resonant absorption of the Primakoff solar axions by $^{169}$Tm nuclei have been performed. Such an absorption should lead to the excitation of low-lying nuclear energy level: $A+^{169}$Tm $\to ^{169}$Tm$^*$ $\to ^{169}$Tm$ + γ$ (8.41 keV). The Si(Li) detector and $^{169}$Tm target placed inside the low-background setup were used for that purpose. As a result, a new restriction on the axion-photon coupling and axion mass was obtained: $g_{Aγ}({GeV}^{-1})\cdot m_A(eV)\leq1.36\cdot10^{-5}$ (90% c.l.). In model of hadronic axion this restriction corresponds to the upper limit on axion mass - $m_A\leq$ 191 eV for 90% c.l.

hep-ph↗
arXiv2008

A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. I. Testing Validity of Rate Equations in Molecular Clouds

In this study we demonstrate for the first time that the unified Monte Carlo approach can be applied to model gas-grain chemistry in large reaction networks. Specifically, we build a time-dependent gas-grain chemical model of the interstellar medium, involving about 6000 gas-phase and 200 grain surface reactions. This model is used to test the validity of the standard and modified rate equation methods in models of dense and translucent molecular clouds and to specify under which conditions the use of the stochastic approach is desirable. We found that at temperatures 25--30 K gas-phase abundances of H$_2$O, NH$_3$, CO and many other gas-phase and surface species in the stochastic model differ from those in the deterministic models by more than an order of magnitude, at least, when tunneling is accounted for and/or diffusion energies are 3x lower than the binding energies. In this case, surface reactions, involving light species, proceed faster than accretion of the same species. In contrast, in the model without tunneling and with high binding energies, when the typical timescale of a surface recombination is greater than the timescale of accretion onto the grain, we obtain almost perfect agreement between results of Monte Carlo and deterministic calculations in the same temperature range. At lower temperatures ($\sim10$ K) gaseous and, in particular, surface abundances of most important molecules are not much affected by stochastic processes.

astro-ph↗
arXiv2003

On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling

With the chemical reaction rate database UMIST95 (Millar et al. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of molecular abundances in the interstellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations are performed for dark and translucent molecular clouds where gas phase chemistry is adequate. Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in a molecule and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species.

astro-ph↗
arXiv2000

Modelling of Polarization Properties of Cometary Dust Grains

Observational data for dusty comets are summarized and systemized. The dependence of the linear polarization on various parameters (the phase angle, wavelength, etc.) is analyzed. An ensemble of aligned spheroidal particles having various sizes and chemical compositions is used to fit the linear and circular polarization observed for the comet Halley.

astro-ph↗
arXiv2000

The Temperature of Nonspherical Circumstellar Dust Grains

The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios {\small $a/b \le 10$} composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio {\small $T_{\rm d}({\rm spheroid})/T_{\rm d}({\rm sphere}) $} depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At {\small $a/b \approx 2$}, the temperature differences do not exceed 10 %. If {\small $a/b \ga 4$}, the temperatures can differ by 30-40 %. For a fixed dust mass in the medium, the fluxes at wavelengths {\small $λ\ga 100 \mkm$} are higher if the grains are nonspherical, which gives over estimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.

astro-ph↗
arXiv1999

The temperature of non-spherical interstellar grains

A model of spheroidal particles is used to calculate the steady-state temperature of dust grains immersed in the interstellar radiation field. It is found that the temperature of non-spherical grains with the aspect ratios a/b <= 2 deviates from that of spheres less than 10%. More elongated or flattened particles are usually cooler than spheres of the same mass and in some cases the temperatures may differ by even about a factor of 2. The shape effects increase with the infrared absorptivity of the grain material and seem to be more important in dark interstellar clouds.

astro-ph↗