Local Sound Speed Research Articles

Rotation of the asymptotic giant branch star R Doradus

High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 × 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of ωR sin i = (3.5 ± 0.3) × 10−9 rad s−1, which indicates a rotational velocity of |υrot sin i| = 1.0 ± 0.1 km s−1 at the stellar surface (R* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Φ = 7 ± 6°. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.

A physical model of mass ejection in failed supernovae

During the core collapse of massive stars, the formation of the protoneutron star is accompanied by the emission of a significant amount of mass-energy ($\sim 0.3 \, M_{\odot}$) in the form of neutrinos. This mass-energy loss generates an outward-propagating pressure wave that steepens into a shock near the stellar surface, potentially powering a weak transient associated with an otherwise-failed supernova. We analytically investigate this mass-loss-induced wave generation and propagation. Heuristic arguments provide an accurate estimate of the amount of energy contained in the outgoing sound pulse. We then develop a general formalism for analyzing the response of the star to centrally concentrated mass loss in linear perturbation theory. To build intuition, we apply this formalism to polytropic stellar models, finding qualitative and quantitative agreement with simulations and heuristic arguments. We also apply our results to realistic pre-collapse massive star progenitors (both giants and compact stars). Our analytic results for the sound pulse energy, excitation radius, and steepening in the stellar envelope are in good agreement with full time-dependent hydrodynamic simulations. We show that {prior} to the sound pulses arrival at the stellar photosphere, the photosphere has already reached velocities $\sim 20-100 \%$ of the local sound speed, thus likely modestly decreasing the stellar effective temperature prior to the star disappearing. Our results provide important constraints on the physical properties and observational appearance of failed supernovae.

A Green’s function parabolic equation description of infrasound propagation in an inhomogeneous and moving atmosphere

Accurate atmospheric infrasound propagation models must account for variations in local sound speed and ambient flow. This paper describes and demonstrates a method to extend the Green’s function parabolic equation (GFPE) to account for 3D high Mach number ambient flow in addition to an inhomogeneous atmosphere. Predictions of infrasonic propagation using the resulting GFPE model are then compared with predictions using other models.

Time-domain modeling of the wind effects on the nonlinearity and absorption of sonic booms

This paper discusses the derivation and solution, in the time domain, of a Burgers’ equation for sonic boom propagation in a windy atmosphere. As the wind effects become important with increasing wind speed, the proper treatment of wind is necessary for sonic booms propagating through a jet stream as well as for those propagating to the stratosphere or thermosphere where stronger winds typically exist. The effects of wind are quantified in terms of the Doppler shift and of a parameter which measures the strength of the convection. The latter, referred to as the convective index, is a ratio of the local sound speed to the magnitude of the ray velocity and is unity in the absence of wind. Both Doppler shift and convective index are larger than unity if there is a component of the wind velocity opposite to the direction of the acoustic propagation. Consequently, the effective nonlinearity coefficient and effective absorption coefficients are larger for rays propagating opposite the wind direction than for rays propagating in the same direction. To demonstrate the significance of the derived Burgers’ equation, the effect of a jet stream with a typical strength and profile on nominal sonic booms will be described.

The FRIGG project: From intermediate galactic scales to self-gravitating cores

Context. Understanding the detailed structure of the interstellar gas is essential for our knowledge of the star formation process.Aim. The small-scale structure of the interstellar medium (ISM) is a direct consequence of the galactic scales and making the link between the two is essential.Methods. We perform adaptive mesh simulations that aim to bridge the gap between the intermediate galactic scales and the self-gravitating prestellar cores. For this purpose we use stratified supernova regulated ISM magneto-hydrodynamical simulations at the kpc scale to set up the initial conditions. We then zoom, performing a series of concentric uniform refinement and then refining on the Jeans length for the last levels. This allows us to reach a spatial resolution of a few 10−3 pc. The cores are identified using a clump finder and various criteria based on virial analysis. Their most relevant properties are computed and, due to the large number of objects formed in the simulations, reliable statistics are obtained.Results. The cores’ properties show encouraging agreements with observations. The mass spectrum presents a clear powerlaw at high masses with an exponent close to ≃−1.3 and a peak at about 1–2 M⊙. The velocity dispersion and the angular momentum distributions are respectively a few times the local sound speed and a few 10−2 pc km s−1. We also find that the distribution of thermally supercritical cores present a range of magnetic mass-to-flux over critical mass-to-flux ratios, typically between ≃0.3 and 3 indicating that they are significantly magnetized. Investigating the time and spatial dependence of these statistical properties, we conclude that they are not significantly affected by the zooming procedure and that they do not present very large fluctuations. The most severe issue appears to be the dependence on the numerical resolution of the core mass function (CMF). While the core definition process may possibly introduce some biases, the peak tends to shift to smaller values when the resolution improves.Conclusions. Our simulations, which use self-consistently generated initial conditions at the kpc scale, produce a large number of prestellar cores from which reliable statistics can be inferred. Preliminary comparisons with observations show encouraging agreements. In particular the inferred CMFs resemble the ones inferred from recent observations. We stress, however, a possible issue with the peak position shifting with numerical resolution.

Radiation Hydrodynamical Turbulence in Protoplanetary Disks: Numerical Models and Observational Constraints

Abstract Planets are born in protostellar disks, which are now observed with enough resolution to address questions about internal gas flows. Magnetic forces are possibly drivers of the flows, but ionization state estimates suggest that much of the gas mass decouples from magnetic fields. Thus, hydrodynamical instabilities could play a major role. We investigate disk dynamics under conditions typical for a T Tauri system, using global 3D radiation-hydrodynamics simulations with embedded particles and a resolution of 70 cells per scale height. Stellar irradiation heating is included with realistic dust opacities. The disk starts in joint radiative balance and hydrostatic equilibrium. The vertical shear instability (VSI) develops into turbulence that persists up to at least 1600 inner orbits (143 outer orbits). Turbulent speeds are a few percent of the local sound speed at the midplane, increasing to 20%, or 100 m s−1, in the corona. These are consistent with recent upper limits on turbulent speeds from optically thin and thick molecular line observations of TW Hya and HD 163296. The predominantly vertical motions induced by the VSI efficiently lift particles upward. Grains 0.1 and 1 mm in size achieve scale heights greater than expected in isotropic turbulence. We conclude that while kinematic constraints from molecular line emission do not directly discriminate between magnetic and nonmagnetic disk models, the small dust scale heights measured in HL Tau and HD 163296 favor turbulent magnetic models, which reach lower ratios of the vertical kinetic energy density to the accretion stress.

Various Local Heating Events in the Earliest Phase of Flux Emergence

Abstract Emerging flux regions (EFRs) are known to exhibit various sporadic local heating events in the lower atmosphere. To investigate the characteristics of these events, especially to link the photospheric magnetic fields and atmospheric dynamics, we analyze Hinode, Interface Region Imaging Spectrograph (IRIS), and Solar Dynamics Observatory data of a new EFR in NOAA AR 12401. Out of 151 bright points (BPs) identified in Hinode/SOT Ca images, 29 are overlapped by an SOT/SP scan. Seven BPs in the EFR center possess mixed-polarity magnetic backgrounds in the photosphere. Their IRIS UV spectra (e.g., Si iv 1402.8 Å) are strongly enhanced and red- or blueshifted, with tails reaching , which is highly suggestive of bi-directional jets; each brightening lasts for 10–15 minutes, leaving flare-like light curves. Most of this group show bald patches, the U-shaped photospheric magnetic loops. Another 10 BPs are found in unipolar regions at the EFR edges. They are generally weaker in UV intensities and exhibit systematic redshifts with Doppler speeds up to , which could exceed the local sound speed in the transition region. Both types of BPs show signs of strong temperature increase in the low chromosphere. These observational results support the physical picture that heating events in the EFR center are due to magnetic reconnection within cancelling undular fields like Ellerman bombs, while the peripheral heating events are due to shocks or strong compressions caused by fast downflows along the overlying arch filament system.

An experimental study of sheet to cloud cavitation

A 2D convergent-divergent test section was built to study experimentally sheet cavitation followed by bubble cloud formation. Flow visualizations and pressure measurements enabled correlating high speed photography observations with the pressures on the cavitating surface. These indicate that the frequency of the recurring sheet cavity decreases with increased inlet flow velocity. As the inlet velocity increases, the flow structure changes from vortex shedding with entrapped thin cavities, to a sheet cavity with a reentrant jet producing bubble cloud shedding, to a shock dominant cavity collapse flow regime. The two-phase bubbly flow shock front moves upstream at a speed higher than the local sound speed, creating a pressure surge clearly measured as the shock front passes over a pressure gauge. The sheet cavity breakdown during collapse leaves behind vortical bubble clouds.

Nonlinear acoustic waves in the viscous thermosphere and ionosphere above earthquake

AbstractThe nonlinear behavior of acoustic waves and their dissipation in the upper atmosphere is studied on the example of infrasound waves generated by vertical motion of the ground surface during the Mw 8.3 earthquake that occurred about 46 km from Illapel, Chile on 16 September 2015. To conserve energy, the amplitude of infrasound waves initially increased as the waves propagated upward to the rarefied air. When the velocities of air particles became comparable with the local sound speed, the nonlinear effects started to play an important role. Consequently, the shape of waveform changed significantly with increasing height, and the original wave packet transformed to the “N‐shaped” pulse resembling a shock wave. A unique observation by the continuous Doppler sounder at the altitude of about 195 km is in good agreement with full wave numerical simulation that uses as boundary condition the measured vertical motion of the ground surface.

The depth dependence of ambient noise in deep ocean trenches

A series of experiments aimed at measuring the power spectrum and vertical and horizontal noise coherence (directionality) in the deep ocean were carried out between 2009and 2015 using a family of autonomous instrument platforms named “Deep Sound.” Deep Sound is a free-falling acoustic recorder designed to descend from the ocean’s surface to a pre-assigned depth where it drops an iron weight and returns to the surface under its own buoyancy, while recording pressure time series on four hydrophones with vertical and horizontal spacing over the bandwidth 5 Hz—30 kHz. The complete vertical noise profile was recorded from the surface to 6 km in the Philippine Sea, 8.5 km in the Tonga Trench, and 9 km in the Serena and Challenger Deeps in the Mariana Trench. Generally, the vertical noise coherence is well described by the Cron & Sherman surface noise model with some depth-dependence explained by seawater attenuation and local sound speed variations. Below the reciprocal or critical depth (depth at which the so...

Source level of fin whale calls from the Equatorial Pacific Ocean

Knowledge of source levels is essential for determining the range of effective fin whale communication and for estimating fin whale density from passive acoustic recordings. Source level calculations were performed on vocalizations automatically detected from spectrograms of Comprehensive Nuclear Test-Ban Treaty Organization data recorded at Wake Island in the Equatorial Pacific Ocean. Received levels were calculated, and transmission loss (TL) was determined using a season- specific OASIS Peregrine parabolic equation model for a 20 Hz signal. The model incorporated the location of the sensor in the deep sound channel, bathymetry of the area, and local sound speed profiles. TL was modeled for 360 bearings with a 1o resolution. TL values between the sensor and source were found for individual vocalizations using ranges and bearings calculated through hyperbolic localization. The exact whale depths were unknown but assumed to be 50 m. Over 7500 localizations of the fin whale 20 Hz call were identified from 2007 to 2009 with an average source level of 184.8 dB + /- 8.6 dB. When the 7500 calls were subsampled at 6 hour intervals to reduce the bias of loud, persistent singers, the average source level was 168.5 dB + /- 4.4 dB (n = 174). [Work supported by ONR.]

REFLECTION OF PROPAGATING SLOW MAGNETO-ACOUSTIC WAVES IN HOT CORONAL LOOPS: MULTI-INSTRUMENT OBSERVATIONS AND NUMERICAL MODELING

ABSTRACT Slow MHD waves are important tools for understanding coronal structures and dynamics. In this paper, we report a number of observations from the X-Ray Telescope (XRT) on board HINODE and Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA) of reflecting longitudinal waves in hot coronal loops. To our knowledge, this is the first report of this kind as seen from the XRT and simultaneously with the AIA. The wave appears after a micro-flare occurs at one of the footpoints. We estimate the density and temperature of the loop plasma by performing differential emission measure (DEM) analysis on the AIA image sequence. The estimated speed of propagation is comparable to or lower than the local sound speed, suggesting it to be a propagating slow wave. The intensity perturbation amplitude, in every case, falls very rapidly as the perturbation moves along the loop and eventually vanishes after one or more reflections. To check the consistency of such reflection signatures with the obtained loop parameters, we perform a 2.5D MHD simulation, which uses the parameters obtained from our observation as inputs, and perform forward modeling to synthesize AIA 94 Å images. Analyzing the synthesized images, we obtain the same properties of the observables as for the real observation. From the analysis we conclude that a footpoint heating can generate a slow wave which then reflects back and forth in the coronal loop before fading. Our analysis of the simulated data shows that the main agent for this damping is anisotropic thermal conduction.

Modelling the performance of fish tag monitoring stations on the Scotian Shelf

The Ocean Tracking Network operates and maintains a continental shelf scale array of 256 bottom mounted fish-tag monitoring stations, spanning from the entrance of Halifax harbour to the Scotian Shelf break. These stations detect tagged keystone, commercially important, and endangered species as they migrate across this acoustic curtain, known as the Halifax line. The detection performance of each station is dependent on the local bathymetry, oceanography (sound speed profile variability), ambient noise level, and source depth distribution. At each station, local sound speed profiles from archived glider data were collected and sorted into representative groups. An Nx2D ray-trace model was used to calculate the transmission loss and relative strength of the noise field at fish tag frequencies (69 kHz) for each of the representative sound speed profiles at a number of stations across the array. The performance variability at each station and between stations is presented and compared to real detection data.

WEAK TURBULENCE IN THE HD 163296 PROTOPLANETARY DISK REVEALED BY ALMA CO OBSERVATIONS

Turbulence can transport angular momentum in protoplanetary disks and influence the growth and evolution of planets. With spatially and spectrally resolved molecular emission line measurements provided by (sub)millimeter interferometric observations, it is possible to directly measure non-thermal motions in the disk gas that can be attributed to this turbulence. We report a new constraint on the turbulence in the disk around HD 163296, a nearby young A star, determined from ALMA Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions). The different optical depths for these lines permit probes of non-thermal line-widths at a range of physical conditions (temperature and density) and depths into the disk interior. We derive stringent limits on the non-thermal motions in the upper layers of the outer disk such that any contribution to the line-widths from turbulence is <3% of the local sound speed. These limits are approximately an order of magnitude lower than theoretical predictions for full-blown MHD turbulence driven by the magneto-rotational instability, potentially suggesting that this mechanism is less efficient in the outer (R>30AU) disk than has been previously considered.

On the Generation of Hydrodynamic Shocks by Mixed Beams and Occurrence of Sunquakes in Flares

Observations of solar flares with sunquakes by space- and ground-based instruments reveal essentially different dynamics of seismic events in different flares. Some sunquakes are found to be closely associated with the locations of hard X-ray (HXR) and white-light (WL) emission, while others are located outside either of them. In this article we investigate possible sources causing a seismic response in a form of hydrodynamic shocks produced by the injection of mixed (electron plus proton) beams, discuss the velocities of these shocks, and the depths where they deposit the bulk of their energy and momentum. The simulation of hydrodynamic shocks in flaring atmospheres induced by electron-rich and proton-rich beams reveals that the linear depth of the shock termination is shifted beneath the level of the quiet solar photosphere on a distance from 200 to 5000 km. The parameters of these atmospheric hydrodynamic shocks are used as initial condition for another hydrodynamic model developed for acoustic-wave propagation in the solar interior (Zharkov, Mon. Not. Roy. Astron. Soc. 431, 3414, 2013). The model reveals that the depth of energy and momentum deposition by the atmospheric shocks strongly affects the propagation velocity of the acoustic-wave packet in the interior. The locations of the first bounces from the photosphere of acoustic waves generated in the vicinity of a flare are seen as ripples on the solar surface, or sunquakes. Mixed proton-dominated beams are found to produce a strong supersonic shock at depths 200 – 300 km under the level of the quiet-Sun photosphere and in this way produce well-observable acoustic waves, while electron-dominated beams create a slightly supersonic shock propagating down to 5000 km under the photosphere. This shock can only generate acoustic waves at the top layers beneath the photosphere since the shock velocity very quickly drops below the local sound speed. The distance \(\Delta\) of the first bounce of the generated acoustic waves is discussed in relation to the minimal phase velocities of wave packets defined by the acoustic cutoff frequency and the parameters of atmospheric shock termination beneath the photosphere.

Instability of the Superfluid Flow as Black-Hole Lasing Effect.

We show that the critical velocity of a superfluid flow through a penetrable barrier coincides with the onset of the analog black-hole lasing effect. This dynamical instability is triggered by modes resonating in an effective cavity formed by two horizons enclosing the barrier. The location of the horizons is set by v(x)=c(x), with v(x),c(x) being the local fluid velocity and sound speed, respectively. We compute the critical velocity analytically and show that it is univocally determined by the configuration of the horizons. In the limit of broad barriers, the continuous spectrum at the origin of the Hawking-like radiation and of the Landau energetic instability is recovered.

THE ROLE OF NUCLEAR STAR CLUSTERS IN ENHANCING SUPERMASSIVE BLACK HOLE FEEDING RATES DURING GALAXY MERGERS

During galaxy mergers the gas falls to the center, triggers star formation, and feeds the rapid growth of supermassive black holes (SMBHs). SMBHs respond to this fueling by supplying energy back to the ambient gas. Numerical studies suggest that this feedback is necessary to explain why the properties of SMBHs and the formation of bulges are closely related. This intimate link between the SMBH's mass and the large scale dynamics and luminosity of the host has proven to be a difficult issue to tackle with simulations due to the inability to resolve all the relevant length scales simultaneously. In this paper we simulate SMBH growth at high-resolution with {\it FLASH}, accounting for the gravitational focusing effects of nuclear star clusters (NSCs), which appear to be ubiquitous in galactic nuclei. In the simulations, the NSC core is resolved by a minimum cell size of about 0.001 pc or approximately $10^{-3}$ of the cluster's radius. We discuss the conditions required for effective gas funneling to occur, which are mainly dominated by a relationship between NSC velocity dispersion and the local sound speed, and provide a sub-grid prescription for the augmentation of central SMBH accretion rates in the presence of NSCs. For the conditions expected to persist in the centers of merging galaxies, the resultant large central gas densities in NSCs should produce drastically enhanced embedded SMBH accretion rates - up to an order of magnitude increase can be achieved for gas properties resembling those in large-scale galaxy merger simulations. This will naturally result in faster black hole growth rates and higher luminosities than predicted by the commonly used Bondi-Hoyle-Lyttleton accretion formalism.

Array shape calibration using long-term ambient noise records

When using an acoustic array to determine the bearing of a source, errors in the sensor positions severely reduce the accuracy of any bearing measurements. This issue is particularly problematic for three sensor arrays, designed to work in two dimensions, because of the lack of redundancy built into the array. This paper presents a method for correcting errors in sensor positions. It is mathematically straightforward to determine the bearing of a far-field impulsive signal by determining the time difference of arrival between sensors by cross-correlation. These time delays can also be used to estimate the sound speed in the vicinity of the array. In an isotropic acoustic medium, the local sound speed is expected to be independent of the source bearing. If the sensor positions used to determine the sound speed and bearing are incorrect, the resulting sound speed measurements will be bearing-dependent. Using an analytically derived function, the correct array shape (with only translational and rotational ambiguity) can be backed out from the directional sound speed distribution. This method will be demonstrated using long-term ambient noise records from the Comprehensive Nuclear-Test Ban Treaty Organization’s (CTBTO) hydrophone arrays.

SSX MHD plasma wind tunnel

A new turbulent plasma source at the Swarthmore Spheromak Experiment (SSX) facility is described. The MHD wind tunnel configuration employs a magnetized plasma gun to inject high-beta plasma into a large, well-instrumented, vacuum drift region. This provides unique laboratory conditions approaching that in the solar wind: there is no applied background magnetic field in the drift region and has no net axial magnetic flux; the plasma flow speed is on the order of the local sound speed (M ~ 1), so flow energy density is comparable to thermal energy density; and the ratio of thermal to magnetic pressure is of order unity (plasma β ~ 1) so thermal energy density is also comparable to magnetic energy density. Results presented here and referenced within demonstrate the new capabilities and show how the new platform is proving useful for fundamental plasma turbulence studies.

Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation★

High resolution spectroscopy with the Subaru High Dispersion Spectrograph, and Swift ultraviolet photometry are presented for the pulsating extreme helium star V652\,Her. Swift provides the best relative ultraviolet photometry obtained to date, but shows no direct evidence for a shock at ultraviolet or X-ray wavelengths. Subaru has provided high spectral and high temporal resolution spectroscopy over 6 pulsation cycles (and eight radius minima). These data have enabled a line-by-line analysis of the entire pulsation cycle and provided a description of the pulsating photosphere as a function of optical depth. They show that the photosphere is compressed radially by a factor of at least two at minimum radius, that the phase of radius minimum is a function of optical depth and the pulse speed through the photosphere is between 141 and 239 km/s (depending how measured) and at least ten times the local sound speed. The strong acceleration at minimum radius is demonstrated in individual line profiles; those formed deepest in the photosphere show a jump discontinuity of over 70 km/s on a timescale of 150 s. The pulse speed and line profile jumps imply a shock is present at minimum radius. These empirical results provide input for hydrodynamical modelling of the pulsation and hydrodynamical plus radiative transfer modelling of the dynamical spectra.