Modulation Time-scale Research Articles

Fast and artifact-free circular dichroism measurement of solid-state structural changes.

A novel chiroptical spectrophotometer, Multichannel (MC)-circular dichroism (CD), which does not require a wavelength scan and provides artifact-free CD spectra in seconds, was developed by solving the incompatibility problem between data acquisition and modulation timescales. The design and instrumentation are compared with the Universal Chiroptical Spectrophotometer (UCS)-1, which can measure artifact-free CD spectra in the solid state. Enantiomeric single crystals composed of achiral components, α-Ni(H2 O)6 ・SO4 , and achiral films with substantial macroscopic anisotropies were measured on both MC-CD and UCS-1 and compared.

Stochastic star formation in early galaxies: Implications for the James Webb Space Telescope

Contact. The star formation rate (SFR) in high-redshift galaxies is expected due to competing physical processes. This stochastic variability might boost the luminosity of galaxies and might explain the over-abundance seen at z ≳ 10 by the James Webb Space Telescope. Aims. We quantify the amplitude and timescales of this variability and identify the key physical processes. Methods. We selected 245 z = 7.7 galaxies with stellar mass 5 × 106 ≲ M⋆/M⊙ ≲ 5 × 1010 from SERRA, which is a suite of high-resolution radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, ⟨SFR⟩, we quantified the time-dependent variation, δ(t)≡log[SFR/⟨SFR⟩], for each system and performed a periodogram analysis to search for periodicity modulations. Results. We find that δ(t) is distributed as a zero-mean Gaussian, with standard deviation σδ ≃ 0.24 (corresponding to a UV magnitude s.d. σUV ≃ 0.61) that is independent of M⋆. However, the modulation timescale increases with stellar mass: tδ ∼ (9, 50, 100) Myr for M⋆ ∼ (0.1, 1, 5)×109 M⊙, respectively. These timescales are imprinted on the SFR by different processes: (i) photoevaporation, (ii) supernova explosions, and (iii) cosmological accretion/merging dominating in low-, intermediate-, and high-mass systems, respectively. Conclusions. The predicted SFR variations cannot account for the required z ≳ 10 UV luminosity function boost. Other processes, such as radiation-driven outflows clearing the dust, must then be invoked to explain the enhanced luminosity of super-early systems.

Variable Hard X-Ray Emission from the Central Star of the Eskimo Nebula

Abstract The central star of NGC 2392 shows the hardest X-ray emission among central stars of planetary nebulae (CSPNe). The recent discovery of a spectroscopic companion with an orbital period of 1.9 days could provide an explanation for its hard X-ray emission, as well as for the collimation of its fast outflow. Here, we analyze the available Chandra and XMM-Newton X-ray observations to determine accurately the spectral and temporal variation properties of the CSPN of NGC 2392. The X-ray emission can be described by an absorbed thermal plasma model with temperature MK and X-ray luminosity (8.7 ± 1.0) × 1030 erg s−1. No long-term variability is detected in the X-ray emission level, but the Chandra light curve is suggestive of short-term variations with a period ∼0.26 days. The possible origins of this X-ray emission are discussed. X-ray emission from the coronal activity of a companion or shocks in the stellar wind can be ruled out. Accretion of material from an unseen main-sequence companion onto the CSPN or from the CSPN wind onto a white dwarf companion are the most plausible origins for its hard X-ray emission, although the mismatch between the rotational period of the CSPN and the modulation timescale of the X-ray emission seems to preclude the former possibility.

Long-term evolution of CFS-unstable neutron stars and the role of differential rotation on short time-scales

Abstract I consider differential rotation, associated with radiation-driven Chandrasekhar–Friedman–Schutz (CFS) instability, and respective observational manifestations. I focus on the evolution of the apparent spin frequency, which is typically associated with the motion of a specific point on the stellar surface (e.g. polar cap). I start from long-term evolution (on the time-scale when instability significantly changes the spin frequency). For this case, I reduce the evolution equations to one differential equation and I demonstrate that it can be directly derived from energy conservation law. This equation governs the evolution rate through a sequence of thermally equilibrium states and it provides linear coupling for the cooling power and rotation energy losses via gravitational wave emission. In particular, it shows that differential rotation does not affect long-term spin-down. In contrast, on short time-scales, differential rotation can significantly modify the apparent spin-down, if we examine a strongly unstable star with a very small initial amplitude for the unstable mode. This statement is confirmed by considering a Newtonian non-magnetized perfect fluid and dissipative stellar models as well as a magnetized stellar model. For example, despite the fact that the widely applied evolution equations predict effective spin to be constant in the absence of dissipation, the CFS-unstable star should be observed as spinning-down. However, the effects of differential rotation on apparent spin-down are negligible for realistic models of neutron star recycling, unless the neutron star is non-magnetized, the r-mode amplitude is modulated faster than the shear viscosity dissipation time-scale, and the amplitude is large enough that spin-down can be measured on a modulation time-scale.

Identify the nonlinear wave‐particle interaction regime in rising tone chorus generation

AbstractNonlinear wave‐particle interaction during chorus wave generation was assumed to be in the adiabatic regime in previous studies, i.e., the particle phase‐space trapping timescale (τtr) is considered to be much smaller than the nonlinear dynamics timescale τNL. In this work, we use particle‐in‐cell simulations to demonstrate that τtr∼τNL, i.e., the interaction regime during chorus generation is in the nonadiabatic regime. The timescale for nonlinear evolution of resonant particle phase‐space structures is determined by making the time‐averaged power exchange plot, which clearly demonstrates that particles with pitch angle near 80° make the most significant contribution to wave growth. The phase‐space trapping timescale is also comparable to the amplitude modulation timescale of chorus, suggesting that chorus subpackets are formed because of the self‐consistent evolution of resonant particle phase‐space structures and spatiotemporal features of the fluctuation spectrum.

Acoustic-Emergent Phonology in the Amplitude Envelope of Child-Directed Speech.

When acquiring language, young children may use acoustic spectro-temporal patterns in speech to derive phonological units in spoken language (e.g., prosodic stress patterns, syllables, phonemes). Children appear to learn acoustic-phonological mappings rapidly, without direct instruction, yet the underlying developmental mechanisms remain unclear. Across different languages, a relationship between amplitude envelope sensitivity and phonological development has been found, suggesting that children may make use of amplitude modulation (AM) patterns within the envelope to develop a phonological system. Here we present the Spectral Amplitude Modulation Phase Hierarchy (S-AMPH) model, a set of algorithms for deriving the dominant AM patterns in child-directed speech (CDS). Using Principal Components Analysis, we show that rhythmic CDS contains an AM hierarchy comprising 3 core modulation timescales. These timescales correspond to key phonological units: prosodic stress (Stress AM, ~2 Hz), syllables (Syllable AM, ~5 Hz) and onset-rime units (Phoneme AM, ~20 Hz). We argue that these AM patterns could in principle be used by naïve listeners to compute acoustic-phonological mappings without lexical knowledge. We then demonstrate that the modulation statistics within this AM hierarchy indeed parse the speech signal into a primitive hierarchically-organised phonological system comprising stress feet (proto-words), syllables and onset-rime units. We apply the S-AMPH model to two other CDS corpora, one spontaneous and one deliberately-timed. The model accurately identified 72–82% (freely-read CDS) and 90–98% (rhythmically-regular CDS) stress patterns, syllables and onset-rime units. This in-principle demonstration that primitive phonology can be extracted from speech AMs is termed Acoustic-Emergent Phonology (AEP) theory. AEP theory provides a set of methods for examining how early phonological development is shaped by the temporal modulation structure of speech across languages. The S-AMPH model reveals a crucial developmental role for stress feet (AMs ~2 Hz). Stress feet underpin different linguistic rhythm typologies, and speech rhythm underpins language acquisition by infants in all languages.

Structure and clumping in the fast wind of NGC 6543

Far-ultraviolet spectroscopy from the FUSE satellite is analysed to uniquely probe spatial structure and clumping in the fast wind of the central star of the H- rich planetary nebula NGC 6543 (HD 164963). Time-series data of the unsaturated Pv lambda lambda 1118, 1128 resonance line P Cygni profiles provide a very sensitive diagnostic of variable wind conditions in the outflow. We report on the discovery of episodic and recurrent optical depth enhancements in the Pv absorption troughs, with some evidence for a similar to 0.17-d modulation time-scale. Empirical line-synthesis modelling is used to derive physical properties, including the optical depth evolution of individual 'events'. The characteristics of these features are essentially identical to the ` discrete absorption components' (DACs) commonly seen in the ultraviolet lines of massive OB stars. We have also employed the unified model atmosphere code CMFGEN to explore spectroscopic signatures of clumping, and report, in particular, on the clear sensitivity of the Pv lines to the clump volume filling factor. The results presented here have implications for the downward revision of mass-loss rates in planetary nebula central stars. We conclude that the temporal structures seen in the Pv lines of NGC 6543 likely have a physical origin that is similar to that operating in massive, luminous stars, and may be related to near-surface perturbations caused by stellar pulsation and/or magnetic fields.

Charge dynamics through pi-stacked arrays of conjugated molecules: effect of dynamic disorder in different transport/transfer regimes

We provide further computational evidence that the electronic coupling between pi-stacked molecules is strongly modulated by the thermal motions at room temperature, not only in supramolecular flexible systems (like DNA) but also in molecular crystals. The effect of this modulation on the charge dynamics is different for different transfer/transport mechanisms and depends on the modulation timescale. In the case of charge transfer (CT) between a donor and an acceptor, the effect of electronic coupling fluctuations introduces a corrective term in the expression of the rate constant (different for adiabatic and non-adiabatic CT). For the transport in molecular crystals, this fluctuation can be the limiting factor for the charge mobility. Although the fluctuation of the electronic coupling is similar in magnitude for all systems containing molecular pi-stacking, its importance for the charge dynamics increases with the decrease of the reorganization energy.

The spectroscopic signature of hot Jupiters in FU Orionis objects

We show that if FU Orionis objects harbour hot Jupiters embedded in their discs, the resulting non-axisymmetric dissipation profile in the disc would be manifest as time-dependent distortions in the absorption-line profiles of these objects. In order to affect the infrared line profiles, planets must lie within ∼0.5 au of the central star, whereas only planets within ∼0.1 au would influence the optical line profiles. The time-scale for modulation of the line profiles is relatively short (months) in each case, so that the effect could not have been discovered from published spectra (which combine data taken in different observing seasons). The detection of hot Jupiters in FU Orionis objects would be in line with the expectations of tidal migration theories (which predict a high incidence of close planets around young stars), and would also lend support to models that link the triggering of rapid-rise FU Orionis events to the existence of a close massive planet.

The Discovery of Time Variability in OH Megamasers

We report the discovery of variability in the OH megamaser in IRAS 21272+2514 at z = 0.15. This is the first OH megamaser observed to vary in time. The variation is broadband, spanning 0.5-1.5 MHz or 100-300 km s-1 in the rest frame, and strong, showing rms modulations of 10%-15% while the largest change in flux density between extremal epochs is 34%. Timescales sampled range from 39 to 821 days, and the characteristic modulation timescale is likely to be less than the shortest time baseline. The source of modulation is currently ambiguous, although we favor interstellar scintillation. Best estimates of the size scales of the 1667 MHz OH line features obtained from refractive interstellar scintillation models constrain variable features to be smaller than 2 pc and quiescent features to be larger than a few parsecs. Compact masers account for roughly 27%-58% of the total OH emission in this source. Accelerations in spectral lines are generally constrained to be less than 3 km s-1 yr-1, but acceleration of this magnitude is suggested in one of the quiescent spectral components and merits further study.

Fuel‐Supply–limited Stellar Relaxation Oscillations: Application to Multiple Rings around Asymptotic Giant Branch Stars and Planetary Nebulae

We describe a new mechanism for pulsations in evolved stars: relaxation oscillations driven by a coupling between the luminosity-dependent mass-loss rate and the H fuel abundance in a nuclear-burning shell. When mass loss is included, the outward flow of matter can modulate the flow of fuel into the shell when the stellar luminosity is close to the Eddington luminosity LEdd. When the luminosity drops below LEdd, the mass outflow declines and the shell is resupplied with fuel. This process can be repetitive. We demonstrate the existence of such oscillations and discuss the dependence of the results on the stellar parameters. In particular, we show that the oscillation period scales specifically with the mass of the H-burning relaxation shell (HBRS), defined as the part of the H-burning shell above the minimum radius at which the luminosity from below first exceeds the Eddington threshold at the onset of the mass-loss phase. For a stellar mass M* ~ 0.7 M☉, luminosity L* ~ 104 L☉, and mass-loss rate || ~ 10-5 M☉ yr-1, the oscillations have a recurrence time of ~1400 yr ~ 57τfsm, where τfsm is the timescale for modulation of the fuel-supply in the HBRS by the varying mass-loss rate. This period agrees very well with the ~1400 yr period inferred for the spacings between the shells surrounding some planetary nebulae. We also find the half-width of the luminosity peak to be ~0.39 times the oscillation period; for comparison, the observational shell thickness of ~1000 AU corresponds to ~0.36 of the spacing between pulses. We find oscillations only for models in which the luminosity of the relaxation shell is ~10%-15% of the total stellar luminosity and for which energy generation occurs through the p-p chain. We suggest this mechanism as a natural explanation for the circumnebular shells surrounding some planetary nebulae, which appear only at the end of the AGB phase.

ISOCAM Observations of Galactic Globular Clusters: Mass Loss along the Red Giant Branch

Deep images in the 10 micron spectral region have been obtained for five massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (omega Cen), NGC 6388, NGC 7078 (=M15) and NGC 6715 (=M54) in the Sagittarius Dwarf Spheroidal using ISOCAM in 1997. A significant sample of bright giants have an ISOCAM counterpart but only < 20% of these have a strong mid-IR excess indicative of dusty circumstellar envelopes. From a combined physical and statistical analysis we derive mass loss rates and frequency. We find that i) significant mass loss occurs only at the very end of the Red Giant Branch evolutionary stage and is episodic, ii) the modulation timescales must be greater than a few decades and less than a million years, and iii) mass loss occurrence does not show a crucial dependence on the cluster metallicity.

Extended optical spectroscopic monitoring of wind structure in HD 152408

New perspectives are provided on signicant spatial structure and temporal variability in the near- star wind regions (i.e. < 3 R?) of the massive luminous star HD 152408 (classied as O8:Iafpe or WN9ha). This study is primarily based on the analysis of high-quality echelle spectra secured during 21 nights between 1999 July to August, using the Landessternwarte-developed (bre-fed) FEROS instrument on the ESO 1.52-m telescope. These extended time-series data, with a total simultaneous wavelength coverage of 3600 9200 A, were exploited to monitor absorption and emission fluctuations (of5{10% of the line flux) in several He i and Balmer lines, together with more deep-seated (near-photosphere) disturbances in weaker metallic emission and absorption lines. Organised large-scale wind structure in HD 152408 is principally betrayed by sequential episodes of discrete absorption and emission features, which migrate from near zero velocity to almost the wind terminal velocity. This evolution is extremely slow, however, typically spanning 4 days for an individual episode. We demonstrate that the blue-shifted absorption episodes in He i are very closely mirrored (in velocity and time) by absorption features (i.e. reduced not enhanced flux) in the blue wings of the mainly recombination formed broad H emission line. The implication is that there is detailed balancing between ground state photoionization and recombination in the substantially optically thick Balmer lines. Surprisingly, the velocity behaviour of the red-ward and blue-ward migrating features is highly asymmetric, such that the mean acceleration of the former is less than 50% of the latter. Fourier analysis reveals a modulation time-scale for the wind activity of7.7 days, plus its harmonic at 3.9 days. The longer period is28 times greater than the characteristic radial wind flow time of HD 152408. We also detect a1.5 day periodic variation in the radial velocity of the weak C iv 5801, 5812 absorption lines, which are the closest approximation to \pure photospheric lines in the optical spectrum of HD 152408. The wind-formed optical lines of HD 152408 are also aected by fluctuations in the central peak emission, particularly evident in H where the equivalent width may vary by up to 20%. Data secured between 1995 and 1999 reveal, however, that the epoch-to-epoch mean proles are remarkably similar. Non-LTE steady- state stellar atmosphere models are used to synthesis proles to match representative H and He i 5876 line proles. Only a slow (tailored) velocity law (compared to =1 ) provides ag ood match to the H emission peak and wings, but the models predict excess He i absorption. The observed extreme H emission variations can be reproduced by the synthetic proles with an implied 10% variation in mass-loss rate. The results on optical line prole variability in HD 152408 are discussed in the context of models for co-rotating interaction regions (CIRs) in the wind. Several constraints are provided that argue against simple velocity elds in such streams, including (i) the slow acceleration of features to high velocities, within3 R?, (ii) the strong asymmetry in projected acceleration of the approaching and receding stream material, (iii) Balmer line absorption eects in the approaching material, (iv) the rise of localised features from very low velocities, and (v) the stability of the large-scale CIRs against turbulent small-scale wind structure. We suggest that it may be worth exploring hydrodynamic simulations of CIRs that incorporate dierent velocity elds on the leading (faster accelerating; blue-ward absorption) and trailing (slower accelerating; red-ward emission) edges of the spiral structures.

Repetitive structure in the stellar wind of HD 93843: a normal O-type star

We present the results from a 28-day IUE time-series campaign monitoring the stellar wind of the O5-type giant HD 93843. The principal aim was to study variability in the wind of a star with a normal projected rotation velocity. Systematic changes are identified, amidst continuous line-profile variability, in the absorption troughs of the Si iv and N v resonance lines. The patterns observed have characteristic time-scales of several days and are mimicked by fluctuations (of several 100 km s−1) in the blue wings of the saturated C iv P Cygni profile. Fourier analysis provides support for the repeatability of wind structures in HD 93843 on a 7.1-d ‘period’. Power at this frequency is evident only at intermediate and high velocities (i.e., above ∼0.3 of the terminal velocity). The long modulation time-scale suggests that changes in the star itself probably provide the physical source for triggering the onset of wind structure. Unfortunately the rotational, photometric, pulsational and magnetic properties of HD 93843 are too poorly constrained or known to permit a more detailed interpretation of the 7.1-d wind modulation in terms of potential inhomogeneities at the stellar surface. Nevertheless, our study demonstrates that the incidence of cyclic, possibly regular, stellar-wind variability is not restricted to rapid rotators. Comparisons with other OB stars which have exhibited repetitive wind changes on ‘periods’ of several days suggest that the time-dependent UV properties of HD 93843 are more akin to those of the O4-type supergiant ζ Puppis.

Near‐inertial oscillations over the Texas‐Louisiana shelf

Wind‐induced, near‐inertial oscillations over the Texas‐Louisiana shelf in spring and summer 1992 are described using the current and wind observations taken during the first year of the Texas‐Louisiana Shelf Circulation and Transport Processes Study (LATEX A). Rotary spectral analysis shows clockwise‐rotating energy peaks at near‐inertial frequencies for records from all current meter moorings after the suppression of principal tidal signals. The vertical structure of near‐inertial oscillations is characterized by a first baroclinic mode with a near 180° phase difference between the upper mixed layer and the lower stratified layer. The oscillations are intermittent with a modulation timescale of about 5–10 days. They are surface‐intensified and have maximum values near the shelf break, decaying gradually toward the coast but rapidly offshore. Near‐inertial oscillations appear to accompany a sudden change of the wind stress during frontal passages. Diagnostic analysis suggests that the large near‐inertial oscillations over the LATEX shelf are mainly generated by high‐frequency (near‐inertial) variation of the wind stress accompanying the passage of atmospheric fronts. When the downward transfer of the near‐inertial energy to the deep stratified layer is small, a simple mixed layer model forced by the observed wind stress provides a reasonable prediction of the near‐inertial currents in the mixed layer.