Atmospheric Velocity Field Research Articles

Discovery of cycle-to-cycle modulated spectral line variability and velocity gradients in long-period Cepheids

This work reports the discovery of cycle-to-cycle modulated spectral line and atmospheric velocity gradient variability in long-period Cepheids based on 925 high-resolution optical spectra of $\ell$ Carinae (P $\sim$ 35.5 d) recorded during three heavy duty-cycle monitoring campaigns (in 2014, 2015, and 2016). Spectral line variability is investigated via cross-correlation functions (CCFs) computed using three sets of spectral lines (weak, solar, strong). A metallic line velocity gradient, {\delta}v$_r$ (t), is computed as the difference between weak and strong-line RVs. CCF shape indicators BIS (asymmetry), FWHM, and depth all exhibit clear phase-dependent variability patterns that differ from one pulsation cycle to the next. Weak-line CCFs exhibit these effects more clearly than strong-line CCFs. BIS exhibits the most peculiar modulated variability and can be used to identify the presence of cycle-to-cycle modulated line profile variations. {\delta}v$_r$ (t) clearly exhibits cycle-to-cycle differences that correlate very closely with modulated BIS variability, suggesting perturbations of the atmospheric velocity field as the cause for modulated spectral line variability. These perturbations are most significant during contraction and are not in phase with the pulsation, transmitting information between consecutive pulsation cycles. This work shows RV curve modulation to be a consequence of atmospheric velocity gradient perturbations. Possible origins of these perturbations and their impact on Cepheid RV measurements as well as the projection factor used in Baade-Wesselink-type distance determinations are discussed.

Atmospheric velocity fields in tepid main sequence stars

Context. The line profiles of the stars with ve sin i below a few km s −1 can reveal direct signatures of local velocity fields such as convection in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars. Aims. This paper reports observations of main sequence B, A and F stars (1) to identify additional stars with suffi ciently low values of ve sin i to search for spectral line profile signatures of local veloc ity fields, and (2) to explore how the signatures of the local v elocity fields in the atmosphere depend on stellar parameters such as effective temperature and peculiarity type. Methods. We have carried out a spectroscopic survey of B and A stars of low ve sin i at high resolution. Comparison of model spectra with those observed allows us to detect signatures of the loc al velocity fields such as asymmetric excess line wing absorp tion, best-fit ve sin i parameter values that are found to be larger for strong lines than for weak lines, and discrepancies between observed and modelled line profile shapes. Results. Symptoms of local atmospheric velocity fields are always det ected through a non-zero microturbulence parameter for main sequence stars having Te below about 10000 K, but not for hotter stars. Direct line profile tracers of the atmospheric velocity field are found in six very sharp-lined stars in addition to the three r eported earlier. Direct signatures of velocity fields are fo und to occur in A stars with and without the Am chemical peculiarities, although the amplitude of the effects seems larger in Am stars. Velocity fields are also directly detected in spectral line profiles of A and e arly F supergiants, but without significant line asymmetrie s. Conclusions. We confirm that several atmospheric velocity field signature s, particularly excess line wing absorption which is strong er in the blue line wing than in the red, are detectable in the spe ctral lines of main sequence A stars of suffi ciently low ve sin i. We triple the sample of A stars known to show these effects, which are found both in Am and normal A stars. We argue that the observed line distortions are probably due to convective motions reachin g the atmosphere. These data still have not been satisfactor ily explained by models of atmospheric convection, including numerical simulations.

Three-dimensional simulations of the atmosphere of an AGB star

Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary (“piston”) simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres – convection and pulsations – are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.

Autonomous Control of Micro Aircraft Vehicles Falling Through an Atmospheric Boundary Layer

A trajectory control problem for simple micro air vehicles (MAVs) is introduced and studied. The MAVs are endowed with the ability to estimate their position and velocity and can control their drag coefficient. The objective of this work is to design a control law for the drag coefficient in order to land the MAV close to a desired streamwise location, after being dropped from an airborne carrier at low altitude. We use a simple model-predictive control law, relying on a two-dimensional time-averaged atmospheric velocity field, which is validated on an unsteady three-dimensional representation of an unstratified atmospheric boundary layer. The simulation results show that the proposed control law results in a significant (10-fold) improvement in the accuracy of the streamwise landing point location, with respect to the uncontrolled case. Similar results are obtained when errors are considered in the location of the release point and in the estimate of the mean flowfield.

Observational signatures of atmospheric velocity fields in Main Sequence stars

In stars with sufficiently small projected rotational velocities (less than a few ), it is often possible to detect signatures of the atmospheric velocity field in line profiles. These signatures may be as subtle as small asymmetries in the profile (“line bisector curvature”) or as obvious as profile shapes that strongly depart from those predicted by simple microturbulence models. We have recently carried out a high resolution survey of sharp-line stars to search for these symptoms of local velocity fields. We report the first results of a comparison of models with the observed profiles.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Errors for Space-Based Doppler Lidar Wind Measurements: Definition, Performance, and Verification

Abstract Verification of space-based wind measurements will be difficult because of the random variations of the atmospheric velocity field over the measurement volume. The definition of accuracy requires a definition of “truth.” For this work, truth is defined as the spatial average of the random velocity over the measurement volume. The bias of space-based velocity measurements is dominated by the pointing errors of the lidar beam. The bias and random errors of Doppler lidar measurements produced by the estimation algorithm and the random atmospheric parameters are determined with computer simulations for both the sampling errors of the lidar scanning pattern and the estimation error of the radial velocity measurements. The results are compared with the error produced by ideal rawinsonde measurements and aircraft winds. A simple method to verify the random error of Doppler lidar radial velocity measurements using only multiple-shot lidar data (no in situ data are required) is compared with results from ...

Classical rotational broadening of spectral lines

view Abstract Citations (77) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Classical Rotational Broadening of Spectral Lines Collins, George W., II ; Truax, Ryland J. Abstract In this paper we investigate the extent to which the classical approach to stellar rotation can yield reliable estimates of the projected stellar rotational velocity. We compare the classical picture of stellar rotation devloped by Carroll, Struve, Unsold, and others with ab initio models of rotating stellar atmospheres having known characteristics. We estimate the extend to which the differences between the simple model and the more physically motivated model can be attributed to various assumptions of the simple model. In addition to the simplest model envisioned by early investigators, we consider the use of an empirically determined 'sharp-line profile'in conjuction with the classical model, and the extent to which this improved the results. Particular attention is paid to the effects of limb darkening both in the line and in the continuum. To accomplish this comparison between these two classes of models, we employ two traditional methods of analysis for rotationally broadened line profiled, the relation of the half-width of the line, and the zeros of the Fourier transform of the line as measures of the projected rotational velocity of the star. This analysis is carried out both for the early assumption that the local stellar line profile can be represented by a strong, arbitrarily sharp line and for the more common case where the local line profile can be approximated by a flux proflie appropriate for a similar star that shows no rotation. We find that star exhibiting rapid rotation are poorly described by the classical model in any form. What is somewhat surprising is that even early-type stars showing moderate rotation may suffer significant departures from the classical model. Inappropriate treatment of the limb darkening seems to be a central reason for the breakdown of the classical model at modest speeds. Applications of the classical model as a probe of the more sophisticated aspects of the atmospheric velocity field, such as differential rotation, zonal wind flow, and turbulance, are discussed. As expected because of the departure of the projected rotational speed of the classical model from that of more complete models, the use of the classical treatment for more subtle aspects of the stellar velocity field is problematic at best. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175225 Bibcode: 1995ApJ...439..860C Keywords: Limb Darkening; Rotational Spectra; Spectral Line Width; Stellar Atmospheres; Stellar Models; Stellar Rotation; Doppler Effect; Fourier Transformation; Spectrum Analysis; Velocity Distribution; Astrophysics; LINE: PROFILES; STARS: ATMOSPHERES; STARS: ROTATION full text sources ADS |

Formation Depths of Spectral Lines in Cepheids

There has been a number of observational programmes that have endeavoured to investigate the atmospheric velocity fields in Cepheids (e.g., Sanford 1956, Wallerstein et al. 1992, Butler 1993). These studies measured the radial velocities of lines of different strength, excitation and ionisation potential as these provide an indication of line formation at different levels in the atmosphere. From these measurements, the presence of velocity gradients can be inferred, but determination of the magnitude of such gradients requires knowledge of the spectral line depth of formation. Through dynamical modelling we are endeavouring to ascertain what is actually being measured in the above observational programmes.

Stellar atmospheric velocity fields: The Beta Cephei variables γ Peg and β Cep

The shape parameters of a number of selected ultraviolet lines in BUSS-spectra of the Beta Cephei stars γ Peg and β Cep have been analyzed to determine the principal parameters of the atmospheric velocity field. We find for both stars a fairly high value (∼5 km s−1) for the microturbulent line-of-sight velocity component, which confirms an earlier result based on lower resolution UV spectra. Macroturbulent and rotational velocities are virtually zero in the atmosphere of γ Peg; for β Cep we findvrotsini=40 km s−1.

Curves of growth and line profiles in expanding and rotating atmospheres

This paper studies the problem of diagnosing velocity fields in expanding and rotating stellar atmospheres. Line profiles are calculated by solving for the source function in the comoving fluid frame, and computing emergent intensities using a formal solution in the observer's frame. The computations are done for schematic model atmospheres over a wide range of line strengths. Results are presented for equivalent widths, line depth, line shift, line width, and asymmetry parameters. Various methods of estimating the characteristics of the atmospheric velocity field are examined. It is shown that the standard curve of growth cannot yield unique physically meaningful results. Line-shape parameters, when studied systematically for a range of line strengths, do appear to provide sufficient information to estimate at least the broad features of the flow. It is shown that a fundamental assumption upon which all present Fourier methods are based is invalid; possible approaches for overcoming the problem are suggested.