Molecular Line Widths Research Articles

DETECTION OF ATOMIC CARBON [C II] 158 μm AND DUST EMISSION FROM A z = 7.1 QUASAR HOST GALAXY

Using the IRAM Plateau de Bure Interferometer, we report the detection of the 158 micron [CII] emission line and underlying dust continuum in the host galaxy of the quasar ULAS J112001.48+064124.3 (hereafter J1120+0641) at z=7.0842+/-0.0004. This is the highest redshift detection of the [CII] line to date, and allows us to put first constraints on the physical properties of the host galaxy. The [CII] line luminosity is (1.2+/-0.2)x10^9 Lsun, which is a factor ~4 lower than observed in a luminous quasar at z=6.42 (SDSS J1148+5251). The underlying far-infrared (FIR) continuum has a flux density of 0.61+/-0.16 mJy, similar to the average flux density of z~6 quasars that were not individually detected in the rest-frame FIR. Assuming the FIR luminosity of L_FIR = 5.8x10^11-1.8x10^12 Lsun is mainly powered by star-formation, we derive a star-formation rate in the range 160-440 Msun/yr and a total dust mass in the host galaxy of 6.7x10^7-5.7x10^8 Msun (both numbers have significant uncertainties given the unknown nature of dust at these redshifts). The [CII] line width of sigma_V=100+/-15 km/s is among the smallest observed when compared to the molecular line widths detected in z~6 quasars. Both the [CII] and dust continuum emission are spatially unresolved at the current angular resolution of 2.0x1.7 arcsec^2 (corresponding to 10x9 kpc^2 at the redshift of J1120+0641).

Casimir-Polder interaction of fullerene molecules with surfaces

We calculate the thermal Casimir-Polder potential of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ fullerene molecules near gold and silicon nitride surfaces, motivated by their relevance for molecular matter-wave interference experiments. We obtain the coefficients governing the asymptotic power laws of the interaction in the thermal, retarded, and nonretarded distance regimes and evaluate the full potential numerically. The interaction is found to be dominated by electronic transitions and hence independent of the internal temperature of the molecules. The contributions from phonon transitions, which are affected by the molecular temperature, give rise to only a small correction. Moreover, we find that the sizable molecular line widths of thermal fullerenes may modify the nonretarded interaction, depending on the model used. Detailed measurements of the nonretarded potential of fullerene thus allow one to distinguish between different theories of incorporating damping.

Observational Studies of Pre-Stellar Cores and Infrared Dark Clouds

AbstractStars like our Sun and planets like our Earth form in dense regions within interstellar molecular clouds, called pre-stellar cores (PSCs). PSCs provide the initial conditions in the process of star and planet formation. In the past 15 years, detailed observations of (low-mass) PSCs in nearby molecular cloud complexes have allowed us to find that they are cold (T < 10K) and quiescent (molecular line widths are close to thermal), with a chemistry profoundly affected by molecular freeze-out onto dust grains. In these conditions, deuterated molecules flourish, becoming the best tools to unveil the PSC physical and chemical structure. Despite their apparent simplicity, PSCs still offer puzzles to solve and they are far from being completely understood. For example, what is happening to the gas and dust in their nuclei (the future stellar cradles) is still a mystery that awaits for ALMA. Other important questions are: how do different environments and external conditions affect the PSC physical/chemical structure? Are PSCs in high-mass star forming regions similar to the well-known low-mass PSCs? Here I review observational and theoretical work on PSCs in nearby molecular cloud complexes and the ongoing search and study of massive PSCs embedded in infrared dark clouds (IRDCs), which host the initial conditions for stellar cluster and high-mass star formation.

1H and13C NMR study of phase transition and molecular motion in ionic liquids forming lyotropic liquid-crystalline ionogels

The room temperature ionic liquid (RTIL) 1-decyl-3-methyl-imidazolium bromide [C10mim][Br], dissolved in water, was studied using 1H and 13C NMR spectroscopy. The manifestation of phase transitions and fine features of molecular motion in NMR spectra upon changing temperature and composition have been analyzed. The 1H NMR line shape typical for anisotropic fluids with zero biaxiality (asymmetry) of magnetic shielding and the chemical shift anisotropy (CSA) of ca 0.33 ppm was observed and attributed to water molecules. CSA values for 13C nuclei have been found in the range of 1.2–1.7 ppm. The difference between lyotropic liquid-crystalline (LC) ionogel phase and the solid one has been revealed, where the motions of RTIL and water molecules have been found to be dynamically segregated. The anisotropic 13C NMR signal shape at 16.89 ppm shows the difference between LC ionogel and the lamellar phases, where usually the decreasing order parameter moving along hydrocarbon chain from the polar head is observed. It indicates that the terminal –CH3 groups are more ordered and the supramolecular structures of [C10mim][Br], similar to some higher micellar RTIL aggregates are expected. In order to explain the experimental observations, the quantum chemistry DFT calculations of 1H and 13C magnetic shielding tensors of [C10mim][Br] and various H-bond structures of H2O were performed. Keywords: properties of molecules and molecular ions, line and bands widths, shapes and shifts, NMR and relaxation

Influence of the line-shape model on the spectroscopic determination of the Boltzmann constant

Recent high-resolution spectroscopic experiments demonstrated the possibility of measuring the Boltzmann constant directly from the Doppler width of atomic or molecular lines; however, both experimental setups and data analysis methods still need to be improved to compete with the best acoustic methods. In this paper, the influence of choice of spectral line-shape model for data analysis on systematical error of the Doppler-width determination is analyzed on the example of the oxygen line near $\ensuremath{\lambda}=687$ nm. Profile simulations were performed for many different models at pressures ranging from 0.012 Pa to 120 kPa, taking into account such line-shape effects as the speed dependence of collisional broadening and Dicke narrowing. We discuss limitations of an applicability of the Voigt profile for purposes of the Boltzmann constant determination. The influence of the model line-shape profile and the gas pressure range on the Doppler width extrapolated to the zero-pressure value is investigated.

Direct evidence for two-fluid effects in molecular clouds

We present a combination of theoretical and simulation-based examinations of the role of two-fluid ambipolar drift on molecular line widths. The dissipation provided by ion-neutral interactions can produce a significant difference between the widths of neutral molecules and the widths of ionic species, comparable to the sound speed. We demonstrate that Alfven waves and certain families of magnetosonic waves become strongly damped on scales comparable to the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show that the wave damping causes the power spectrum of the ion velocity to drop below that of the neutral velocity when measured on a relative basis. Following a set of motivational observations by Li & Houde (2008), we produce synthetic line width-size relations that shows a difference between the ion and neutral line widths, illustrating that two-fluid effects can have an observationally detectable role in modifying the MHD turbulence in the clouds.

Phase control over decaying molecular states in intense laser pulses

A time-dependent approach to study phase control over molecular photoabsorption, provided by intense laser pulses, is elaborate. The method allows for the decay linewidth of molecular states and frequency bandwidth of the controlling laser field, and can be applied in weak and strong laser fields where the perturbation theory is invalid. It is shown that a frequency mismatch between the fundamental laser wave and its third harmonic can destroy control. For the example of the one-photon versus three-photon control a simple picture of interference from two monochromatic absorption pathways is not enough to explain phase control and one needs to consider a nonlinear temporal interference of multiquantum transitions. In the perturbation-theory limit an elegant generalization of the famous Shapiro-Hepburn-Brumer equation for the one-photon versus three-photon control is derived. Various numerical calculations illustrate the dependence of phase control on molecular linewidth, fundamental laser wavelength, pulse duration, and peak intensity. It is obtained, that the one-photon versus three-photon control is productive if the molecular state populations, individually produced by each laser wave, have beats of approximately the same frequency. The calculations demonstrate that an enough intense optical pulse can suppress molecular decay and may be used in order to keep stable the state population of a decaying molecule for a long time. The available experimental results for the one-photon versus three-photon control over simple and large polyatomic molecules are analyzed and recommendations for the experimental improvement of control are formulated.

The Measurement of the Orientation of the Magnetic Field in Molecular Clouds

We discuss how the combination of polarimetry and ion-to-neutral molecular line width ratio measurements permits the determination of the magnitude and orientation of the magnetic field in the weakly ionized parts of molecular clouds. Polarimetry measurements give the field orientation in the plane of the sky and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. We show the first results obtained with this technique on the M17 and Orion A star-forming region using Hertz 350 µm polarimetry maps and HCO+-to-HCN molecular line width ratios to provide the first view of the spatial orientation of the magnetic field these molecular clouds.

The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud

We report sensitive, high-resolution molecular-line observations of the dark cloud Barnard 68 obtained with the IRAM 30 m telescope. We analyze spectral-line observations of C18O (1-0), C32S (2-1), C34S (2-1), and N2H+ (1-0) in order to investigate the kinematics and dynamical state of the cloud. We find extremely narrow line widths in the central regions of the cloud, ΔV = 0.18 ± 0.01 km s-1 and 0.15 ± 0.01 km s-1 for C18O and C34S, respectively. These narrow lines are consistent with thermally broadened profiles for the measured gas temperature of 10.5 K. We determine the thermal pressure to be a factor 4-5 times greater than the nonthermal (turbulent) pressure in the central regions of the cloud, indicating that thermal pressure is the primary source of support against gravity in this cloud. This confirms the inference of a thermally supported cloud drawn previously from deep infrared extinction measurements (Alves, Lada, & Lada). We also find the molecular line widths to systematically increase in the outer regions of the cloud, where we calculate the thermal pressure to be between 1-2 times greater than the turbulent pressure. We find the distribution of line-center radial velocities for both C18O and N2H+ to be characterized by systematic and well-defined linear gradients across the face of the cloud. The rotational kinetic energy is found to be only a few percent of the gravitational potential energy, indicating that the contribution of rotation to the overall stability of the cloud is insignificant. However, the C18O and N2H+ velocity gradients differ from each other in both magnitude and direction, suggesting that the cloud is differentially rotating, with the inner regions rotating slightly more slowly than the outer regions. Finally, our observations show that C32S line is optically thick and self-reversed across nearly the entire projected surface of the cloud. The shapes of the the self-reversed profiles are asymmetric and are found to vary across the cloud in such a manner that the presence of both inward and outward motions is observed within the cloud. Moreover, these motions appear to be globally organized in a clear and systematic alternating spatial pattern that is suggestive of a small-amplitude, nonradial oscillation or pulsation of the outer layers of the cloud about an equilibrium configuration.

On the Measurement of the Magnitude and Orientation of the Magnetic Field in Molecular Clouds

We demonstrate that the combination of Zeeman, polarimetry and ion-to-neutral molecular line width ratio measurements permits the determination of the magnitude and orientation of the magnetic field in the weakly ionized parts of molecular clouds. Zeeman measurements provide the strength of the magnetic field along the line of sight, polarimetry measurements give the field orientation in the plane of the sky and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. We apply the technique to the M17 star-forming region using a HERTZ 350 um polarimetry map and HCO+-to-HCN molecular line width ratios to provide the first three-dimensional view of the magnetic field in M17.

Helicoidal magneto-electron waves in interstellar molecular clouds

We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons — weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds.

Dissipative-free Models for Wave Dynamics of Interstellar Gas-dust Medium in Galactic Magnetic Fields

Current study of interstellar gas dynamics leads to the conclusion that large scale motions are dominated by strong coupling between gas-dust flows and Galactic magnetic fields; the widths of molecular lines is a direct evidence for such motions. However the absence of reliable information regarding microscopic composition and electrodynamical properties of interstellar matter can cause serious difficulties for an unambiguous theoretical interpretation of observable motions. We accentuate this uncertainty by three alternative models of dissipative-free wave motions owing their origin to the presence of uniform magnetic field. To wit, (i) the hydromagnetic waves in a non-magnetic perfectly conducting medium, (ii) the magneto-elastic waves in a non-conducting highly paramagnetic medium and (iii) whistlers - spiral magneto-electron waves. Emphasis is laid on two last wave processes, fairly fresh to interstellar gas dynamics, which are discussed in juxtaposition with well-studied hydromagnetic Alfvénic waves.

Velocities of Magneto-Elastic and Magneto-Electron Waves in Dark Molecular Clouds

We present analytic and numerical estimates for group velocity of wave motions in two models of cold interstellar medium presumably constituting the interior of cores of magnetically supported dark molecular clouds. Namely, in the model of gas-based ferrocolloidal soft matter and in the model of noncompensated electron magnetoplasma. The predictions of these models are given in juxtaposition with data on recent Zeeman measurements of the molecular linewidths detected from dark central regions of star-forming interstellar clouds.

The Most Massive C18O Molecular Complex in Centaurus and Star Formation Therein

Abstract New observations of the 12CO, 13CO, and $\mathrm{C}{{18\atop} \mathrm{O}}$ ($J = 1 \hbox{--} 0$) emission lines have been extensively made toward the Centaurus tangential region $({308\rlap {.}{}^{\mathrm {\circ }}5} \le l \le {310\rlap {.}{}^{\mathrm {\circ }}5}$ and $- {1\rlap {.}{}^{\mathrm {\circ }}0} \le b \le {1\rlap {.}{}^{\mathrm {\circ }}0})$ using the “NANTEN”telescope. These observations have revealed a massive molecular complex in $\mathrm{C}{{18\atop} \mathrm{O}}$ of $\sim 1.8 \times 10^{5}\,{{{M}_{\odot}}}$, the most massive one observed in $\mathrm{C}{{18\atop} \mathrm{O}}$ to date in the Galaxy. This complex is highly elongated with a size of $\sim 80 \,\mathrm{pc} \times \sim 25 \,\mathrm{pc}$ at $309^{\circ} \le l \le {309\rlap {.}{}^{\mathrm {\circ }}5}$ and $- {0\rlap {.}{}^{\mathrm {\circ }}9} \le b \le {0\rlap {.}{}^{\mathrm {\circ }}4}$, including 35 dense molecular clumps whose mass ranges from $4.7 \times 10^{2} \,{{{M}_{\odot}}}$ to $3.4 \times 10^{4} \,{{{M}_{\odot}}}$. Twenty-seven of the 35 clumps are associated with IRAS point sources whose luminosity ranges from $7.2 \times 10^{2} \,{{{L}_{\odot}}}$ to $3.6 \times 10^{5} \,{{{L}_{\odot}}}$. We find that the luminosity of the candidate protostellar IRAS sources is well correlated with the molecular line width, as fitted by a power law, $\log (L_\mathrm{IRAS}) = 1.1 + (9.2 \pm 0.6) \log (\Delta V)$. This suggests that the mass of the formed stars is determined by the internal velocity dispersion of the dense cores. A comparison with an H I distribution shows that the molecular gas forms a ring-like structure inside an H I hole of $\sim 55 \hbox{--} 70 \;\mathrm{pc}$ radius. Since the H I gas surrounding the hole shows a signature of expansion at several $\mathrm{km} \,\mathrm{s}^{-1}$, we suggest that the H I hole exhibits a shell created by some explosive events, like supernovae, during the last $\sim (6 \hbox{--} 8) \times 10^{6} \,\mathrm{yr}$. The ring-like structure of the CO distribution may be found due to the interaction between the expanding shell and the interstellar medium, leading to a trigger of massive star formation.

Radio-frequency Stark effect modulation of single-molecule lines

Stark effect modulation experiments were performed on single-molecule lines of terrylene in crystalline matrices of n-hexadecane and naphthalene at low temperatures. Modulation frequencies in the MHz range were used. The resulting fluorescence excitation spectra are distinctly different, depending on the relationship between the modulation frequency ω m and the molecular line width Γ. For ω m⪡ Γ, the single-molecule line exhibits a smooth broadening and apparent splitting into two components, whereas for ω m⪢ Γ it develops equidistant sidebands as in the case of phase modulation of an electromagnetic wave. It is further demonstrated that Stark effect modulation in the radio-frequency regime can be used to record absorption signals of single molecules with good signal-to-noise ratio.

Signal-to-Noise Ratio of FT-IR CO Gas Spectra

The minimum amount of a gaseous compound which can be detected and quantified with Fourier transform infrared (FT-IR) spectrometers depends on the signal-to-noise ratio (SNR) of the measured gas spectra. In order to use low-resolution FT-IR spectrometers to measure combustion gases like CO and CO2 in emission and transmission spectrometry, an investigation of the SNR in CO gas spectra as a function of spectral resolution has been carried out. We present a method to (1) determine experimentally the SNR at constant throughput, (2) determine the SNR on the basis of measured noise levels and Hitran simulated signals, and (3) determine the SNR of CO from high to low spectral resolutions related to the molecular linewidth and vibrational-rotational lines spacing. In addition, SNR values representing different spectral resolutions but scaled to equal measurement times were compared. It was found that the SNR was at a local minimum at a spectral resolution of 4 cm−1. As a result of the investigations, we suggest that the specific spectral resolution which smears out the vibrational-rotational line structure of the smaller molecules should be considered to be low (4 cm−1 for CO).

Ultrafine Structure in the λ5797 Diffuse Interstellar Absorption Band

High-resolution and high-S/N observations of the strong and relatively narrow λ5797 diffuse interstellar absorption band, recorded with the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Telescope and the 0.9 m coude feed of the Kitt Peak National Observatory, are reported. For selected lines of sight and using resolving powers of up to 600,000, a remarkable degree of intrinsic ultrafine structure is found within the band. There exists an almost exact correspondence between the structure in the spectra recorded toward μ Sgr, ζ Per, and ζ Oph, including one particularly narrow component with a width that is comparable to the widths of atomic and molecular absorption lines along these lines of sight. The results provide a new and extremely exacting test against which theoretical or laboratory candidates can be assessed, and they point to a new approach to studies of diffuse band carrier distributions through their velocity signatures.

High-Resolution Laser Excitation Spectroscopy of theB̃2Σ+–X̃2Σ+System of Jet-Cooled SrOD

The high-resolution laser excitation spectrum of the 001–000 and 000–000 bands of theB2Σ+–X2Σ+transition of SrOD was recorded. The SrOD molecules were made by pulsed laser ablation of Sr metal followed by reaction with D2O. Sub-Doppler resolution and rotational cooling were achieved by expansion into vacuum and excitation with a cw dye laser perpendicular to the molecular beam. A molecular linewidth of 200 MHz and the lack of spectral congestion allowed aQbranch to be detected. Spin–orbit mixing between theA2Π andB2Σ+states caused the unusual appearance of a moderate intensityQbranch in a nominally parallel2Σ+–2Σ+transition. From the relative intensities of the rotational lines, the ratio of the perpendicular to the parallel transition dipole moment was found to be 0.2 to 0.3.

Velocity correlations in turbulent molecular clouds

The observed widths of molecular lines indicate that molecular clouds are in a highly turbulent state with supersonic internal velocities. At the same time molecular clouds are highly fragmented into substructures exhibiting large density contrasts. These substructures seem to be arranged in a hierarchical order scheme where one can think of newly formed stars representing the smallest spatial scale of ordering. Stars, protostars and even dense clumps can be regarded as being hydrodynamically decoupled from the ambient gas. Thus, in an idealized way, one can think of molecular clouds as being a gravitationally coupled system consisting of gas (continuous component) and ‘point-masses’ (particle component) randomly moving through the gas. The random motion of these ‘point-masses’ is accompanied by spatial and temporal fluctuations of their gravitational potential which, in turn, induce velocity and density fluctuations of the gas.

Velocity correlations in turbulent molecular clouds

The observed widths of molecular lines indicate that molecular clouds are in a highly turbulent state with supersonic internal velocities. At the same time molecular clouds are highly fragmented into substructures exhibiting large density contrasts. These substructures seem to be arranged in a hierarchical order scheme where one can think of newly formed stars representing the smallest spatial scale of ordering. Stars, protostars and even dense clumps can be regarded as being hydrodynamically decoupled from the ambient gas. Thus, in an idealized way, one can think of molecular clouds as being a gravitationally coupled system consisting of gas (continuous component) and ‘point-masses’ (particle component) randomly moving through the gas. The random motion of these ‘point-masses’ is accompanied by spatial and temporal fluctuations of their gravitational potential which, in turn, induce velocity and density fluctuations of the gas.