arXiv subjects

Sergei N. Yurchenko

Publications and source records attributed to Sergei N. Yurchenko.

139 records · Page 8Linked to original sources

arXiv2014

ExoMol line lists VII: The rotation-vibration spectrum of phosphine up to 1500 K

A comprehensive hot line list is calculated for $^{31}$PH$_3$ in its ground electronic state. This line list, called SAlTY, contains almost 16.8 billion transitions between 7.5 million energy levels and it is suitable for simulating spectra up to temperatures of 1500~K. It covers wavelengths longer than 1~$μ$m and includes all transitions to upper states with energies below $hc \cdot 18\,000$~cm$^{-1}$ and rotational excitation up to $J=46$. The line list is computed by variational solution of the Schrödinger equation for the rotation-vibration motion employing the nuclear-motion program TROVE. A previously reported {\it ab initio} dipole moment surface is used as well as an updated `spectroscopic' potential energy surface (PES), obtained by refining an existing \textit{ab initio} surface through least-squares fitting to the experimentally derived energies. Detailed comparisons with other available sources of phosphine transitions confirms SAlTY's accuracy and illustrates the incompleteness of previous experimental and theoretical compilations for temperatures above 300 K. Atmospheric models are expected to severely underestimate the abundance of phosphine in disequilibrium environments, and it is predicted that phosphine will be detectable in the upper troposphere of many substellar objects. This list is suitable for modelling atmospheres of many astrophysical environments, namely carbon stars, Y dwarfs, T dwarfs, hot Jupiters and solar system gas giant planets. It is available in full as supplementary data to the article and at \url{www.exomol.com}.

astro-ph.EP↗

arXiv2014

The spectrum of hot methane in astronomical objects using a comprehensive computed line list

Hot methane spectra are important in environments ranging from flames to the atmospheres of cool stars and exoplanets. A new spectroscopic line list, 10to10, for $^{12}$CH$_4$ containing almost 10 billion transitions is presented. This comprehensive line list covers a broad spectroscopic range and is applicable for temperatures up to 1 500 K. Previous methane data are incomplete leading to underestimated opacities at short wavelengths and elevated temperatures. Use of 10to10 in models of the bright T4.5 brown dwarf 2MASS 0559-14 leads to significantly better agreement with observations and in studies of the hot Jupiter exoplanet HD 189733b leads to up to a twentifold increase in methane abundance. It is demonstrated that proper inclusion of the huge increase in hot transitions which are important at elevated temperatures is crucial for accurate characterizations of atmospheres of brown dwarfs and exoplanets, especially when observed in the near-infrared.

astro-ph.SR↗

arXiv2014

The Status of Spectroscopic Data for the Exoplanet Characterisation Missions

The status of laboratory spectroscopic data for exoplanet characterisation missions such as EChO is reviewed. For many molecules (eg H2O, CO, CO2, H3+, O2, O3) the data are already available. For the other species work is actively in progress constructing this data. Much of the is work is being undertaken by ExoMol project (www.exomol.com). This information will be used to construct and EChO-specific spectroscopic database.

astro-ph.EP↗

arXiv2014

ExoMol line lists IV: The rotation-vibration spectrum of methane up to 1500 K

A new hot line list is calculated for $^{12}$CH$_4$ in its ground electronic state. This line list, called 10to10, contains 9.8 billion transitions and should be complete for temperatures up to 1500 K. It covers the wavelengths longer than 1 $μ$m and includes all transitions to upper states with energies below $hc \cdot 18\,000$ cm$^{-1}$ and rotational excitation up to $J=39$. The line list is computed using the eigenvalues and eigenfunctions of CH$_4$ obtained by variational solution of the Schrödinger equation for the rotation-vibration motion of nuclei employing program TROVE. An ab initio dipole moment surface and a new 'spectroscopic' potential energy surface are used. Detailed comparisons with other available sources of methane transitions including HITRAN, experimental compilations and other theoretical line lists show that these sources lack transitions both higher temperatures and near infrared wavelengths. This line list is suitable for modelling atmospheres of cool stars and exoplanets. It is available from the CDS database as well as at www.exomol.com.

astro-ph.EP↗

arXiv2013

ExoMol line lists II: The ro-vibrational spectrum of SiO

Accurate rotation-vibration line lists are calculated for silicon monoxide. Line lists are presented for the main isotopologue, $^{28}$Si$^{16}$O, and for four monosubsituted isotopologues ($^{29}$Si$^{16}$O, $^{30}$Si$^{16}$O, $^{28}$Si$^{18}$O and $^{28}$Si$^{17}$O), in their ground electronic states. These line lists are suitable for high temperatures (up to 9000 K) including those relevant to exoplanetary atmospheres and cool stars. A combination of empirical and \textit{ab initio} methods is used: the potential energy curves are determined to high accuracy by fitting to extensive data from the analysis of both laboratory and sunspot spectra; a high quality {\it ab initio} dipole moment curve is calculated at the large basis set, multi-reference configuration interaction level. A partition function plus full line lists of rotation-vibration transitions are made available in an electronic form as supplementary data to this article and at \url{www.exomol.com}.

astro-ph.SR↗

arXiv2013

A Computed Room Temperature Line List for Phosphine

An accurate and comprehensive room temperature rotation-vibration transition line list for phosphine (31PH3) is computed using a newly refined potential energy surface and a previously constructed ab initio electric dipole moment surface. Energy levels, Einstein A coefficients and transition intensities are computed using these surfaces and a variational approach to the nuclear motion problem as implemented in the program TROVE. A ro-vibrational spectrum is computed, covering the wavenumber range 0 to 8000 cm-1. The resulting line list, which is appropriate for temperatures up to 300 K, consists of a total of 137 million transitions between 5.6 million energy levels. Several of the band centres are shifted to better match experimental transition frequencies. The line list is compared to the most recent HITRAN database and other laboratorial sources. Transition wavelengths and intensities are generally found to be in good agreement with the existing experimental data, with particularly close agreement for the rotational spectrum. An analysis of the comparison between the theoretical data created and the existing experimental data is performed, and suggestions for future improvements and assignments to the HITRAN database are made.

astro-ph.EP↗

arXiv2013

An ab initio variationally computed room-temperature line list for SO3

{\it Ab initio} potential energy and dipole moment surfaces are computed for sulfur trioxide (SO$_3$) at the CCSD(T)-F12b level of theory and appropriate triple-zeta basis sets. These surfaces are fitted and used, with a slight correction for the equilibrium S--O distance, to compute pure rotational and rotation-vibraton spectra of \sothree\ using the variational nuclear motion program TROVE. The calculations considered transitions in the region 0--4000 \cm\ with rotational states up to J=85. The resulting line list of 349~348~513 transitions is appropriate for modelling room temperature SO3 spectra. Good agreement is found with the observed infrared absorption spectra and the calculations are used to place the measured relative intensities on an absolute scale. A list of 10~878 experimental transitions is provided in a form suitable for inclusion in standard atmospheric and planetary spectroscopic databases.

astro-ph.EP↗

arXiv2013

Vibrational transition moments of CH$_4$ from first principles

A new nine-dimensional (9D), \textit{ab initio} electric dipole moment surface (DMS) of methane in its ground electronic state is presented. The DMS is computed using an explicitly correlated coupled cluster CCSD(T)-F12 method in conjunction with an F12-optimized correlation consistent basis set of the TZ-family. A symmetrized molecular bond representation is used to parameterise the 9D DMS in terms of sixth-order polynomials. Vibrational transition moments as well as band intensities for a large number of IR-active vibrational bands of $^{12}$CH$_4$ are computed by vibrationally averaging the \textit{ab initio} dipole moment components. The vibrational wavefunctions required for these averages are computed variationally using the program TROVE and a new `spectroscopic' $^{12}$CH$_4$ potential energy surface. The new DMS will be used to produce a hot line list for $^{12}$CH$_4$.

astro-ph.SR↗

arXiv2013

Variational calculation of highly excited rovibrational energy levels of H2O2

Results are presented for highly accurate ab initio variational calculation of the rotation - vibration energy levels of H2O2 in its electronic ground state. These results use a recently computed potential energy surface and the variational nuclear-motion programmes WARV4, which uses an exact kinetic energy (EKE) operator, and TROVE, which uses a numerical expansion for the kinetic energy. The TROVE calculations are performed for levels with high values of rotational excitation, $J$ up to 35. The purely \ai\ calculations of the rovibrational energy levels reproduce the observed levels with a standard deviation of about 1 \cm, similar to that of the $J = 0$ calculation as the discrepancy between theory and experiment for rotational energies within a given vibrational state is substantially determined by the error in the vibrational band origin. Minor adjustments are made to the ab initio equilibrium geometry and to the height of the torsional barrier. Using these and correcting the band origins using the error in $J = 0$ states lowers the standard deviation of the observed $-$ calculated energies to only 0.002 \cm\ for levels up to $J = 10$ and 0.02 \cm\ for all experimentally know energy levels, which extend up to $J = 35$.

astro-ph.EP↗

arXiv2012

Temperature-dependent molecular absorption cross sections for exoplanets and other atmospheres

Exoplanets, and in particular hot ones such as hot Jupiters, require a very significant quantities of molecular spectroscopic data to model radiative transport in their atmospheres or to interpret their spectra. This data is commonly provided in the form of very extensive transition line lists. The size of these line lists is such that constructing a single model may require the consideration of several billion lines. We present a procedure to simplify this process based on the use of cross sections. Line lists for water, H$_3^+$, HCN /HNC and ammonia have been turned into cross sections on a fine enough grid to preserve their spectroscopic features. Cross sections are provided at a fixed range of temperatures and an interpolation procedure which can be used to generate cross sections at arbitrary temperatures is described. A web-based interface (www.exomol.com/xsec) has been developed to allow astronomers to download cross sections at specified temperatures and spectral resolution. Specific examples are presented for the key water molecule.

astro-ph.EP↗

arXiv2012

The rovibrational spectrum of BeH, MgH and CaH at high temperatures in the $X\,{}^2Σ^+$ state: a theoretical study

Accurate line lists for three molecules, BeH, MgH and CaH, in their ground electronic states are presented. These line lists are suitable for temperatures relevant to exoplanetary atmospheres and cool stars (up to 2000K). A combination of empirical and \textit{ab initio} methods is used. The rovibrational energy levels of BeH, MgH and CaH are computed using the programs Level and DPotFit in conjunction with `spectroscopic' potential energy curves (PECs). The PEC of BeH is taken from the literature, while the PECs of CaH and MgH are generated by fitting to the experimental transition energy levels. Both spin-rotation interactions (except for BeH, for which it is negligible) and non-adiabatic corrections are explicitly taken into account. Accurate line intensities are generated using newly computed \textit{ab initio} dipole moment curves for each molecule using high levels of theory. Full line lists of rotation-vibration transitions for $^9$BeH, $^{24}$MgH, $^{25}$MgH, $^{26}$MgH and $^{40}$CaH are made available in an electronic form as supplementary data to this article and at \url{www.exomol.com}.

astro-ph.SR↗

arXiv2012

ExoMol: molecular line lists for exoplanet and other atmospheres

The discovery of extrasolar planets is one of the major scientific advances of the last two decades. Hundreds of planets have now been detected and astronomers are beginning to characterise their composition and physical characteristics. To do this requires a huge quantity of spectroscopic data most of which is not available from laboratory studies. The ExoMol project will offer a comprehensive solution to this problem by providing spectroscopic data on all the molecular transitions of importance in the atmospheres of exoplanets. These data will be widely applicable to other problems and will be used for studies on cool stars, brown dwarfs and circumstellar environments. This paper lays out the scientific foundations of this project and reviews previous work in this area. A mixture of first principles and empirically-tuned quantum mechanical methods will be used to compute comprehensive and very large rotation-vibration and rotation-vibration-electronic (rovibronic) line lists. Methodologies will be developed for treating larger molecules such as methane and nitric acid. ExoMol will rely on these developments and the use of state-of-the-art computing.

astro-ph.EP↗

arXiv2010

The discovery of a very cool, very nearby brown dwarf in the Galactic plane

We report the discovery of a very cool, isolated brown dwarf, UGPS 0722-05, with the UKIDSS Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 um. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as d=4.1{-0.5}{+0.6} pc, making it the closest known isolated brown dwarf. With the aid of Spitzer/IRAC we measure H-[4.5] = 4.71. It is the coolest brown dwarf presently known -- the only known T dwarf that is redder in H-[4.5] is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 0722-05. Our measurement of the luminosity, aided by Gemini/T-ReCS N band photometry, is L = 9.2 +/- 3.1x10^{-7} Lsun. Using a comparison with well studied T8.5 and T9 dwarfs we deduce Teff=520 +/- 40 K. This is supported by predictions of the Saumon & Marley models. With apparent magnitude J=16.52, UGPS 0722-05 is the brightest T dwarf discovered by UKIDSS so far. It offers opportunities for future study via high resolution near-infrared spectroscopy and spectroscopy in the thermal infrared.

astro-ph.SR↗