Model Line Profiles Research Articles

Wind signatures in the X-ray emission-line profiles of the late-O supergiant Orionis

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.

Observations of Global and Local Infall in NGC 1333

We report in the HCO+(1-0) and (3-2) lines toward NGC 1333, extended over ~0.39 pc2, a larger extent than has been reported before for any star-forming region. The infall asymmetry extends over a major portion of the star-forming complex, and is not limited to a single protostar, a single dense core, or a single spectral line. It seems likely that the infall asymmetry represents inward motions, and that these motions are physically associated with the complex. Both blue-asymmetric and red-asymmetric lines are seen, but in both the (3-2) and (1-0) lines of HCO+ the vast majority of the asymmetric lines are blue, indicating inward motions. The (3-2) line, tracing denser gas, has the spectra with the strongest asymmetry; these spectra are associated with the protostars IRAS 4A and 4B, which most likely indicates that a warm central source is affecting the line profiles. The (3-2) and (1-0) lines usually have the same sense of asymmetry in common positions, but their profiles differ significantly, and the (1-0) line appears to trace motions on much larger spatial scales than does the (3-2) line. Line profile models fit the spectra well, but do not strongly constrain their parameters. The mass accretion rate of the inward motions is of order 10-4 M☉ yr-1, similar to the ratio of stellar mass to cluster age.

VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure

Studies of the winds from single K and early M evolved stars indicate that these flows typically reach a significant fraction of their terminal velocity within the first couple of stellar radii. The most detailed spatially resolved information of the extended atmospheres of these spectral types comes from the ζ Aur eclipsing binaries. However, the wind acceleration inferred for the evolved primaries in these systems appears significantly slower than for stars of similar spectral type. Since there are no successful theories for mass loss from K and early M evolved stars, it is important to place strong empirical constraints on potential models and determine whether this difference in acceleration is real or an artifact of the analyses. We have undertaken a radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V) using the Very Large Array to test the wind density model of Baade et al. that is based on Hubble Space Telescope (HST) Goddard High Resolution Spectrograph ultraviolet spectra. ζ Aur was monitored at centimeter wavelengths over a complete orbital cycle, and flux variations during the orbit are found to be of similar magnitude to variations at similar orbital phases in the adjacent orbit. During eclipse, the flux does not decrease, showing that the radio emission originates from a volume substantially larger than R ~ (150 R⊙)3 surrounding the B star. Using the one-dimensional density model of the K4 Ib primary's wind derived from HST spectral line profile modeling and electron temperature estimates from previous optical and new HST studies, we find that the predicted radio fluxes are consistent with those observed. Three-dimensional hydrodynamic simulations indicate that the accretion flow perturbations near the B star do not contribute significantly to the total radio flux from the system, consistent with the radio eclipse observations. Our radio observations confirm the slow wind acceleration for the evolved K4 Ib component. ζ Aur's velocity structure does not appear to be typical of single stars with similar spectral types. This highlights the need for more comprehensive multiwavelength studies for both single stars, which have been sadly neglected, and other ζ Aur systems to determine if its wind properties are typical.

Emission-line profile modelling of structured T Tauri magnetospheres

We present hydrogen emission line profile models of magnetospheric accretion onto Classical T Tauri stars. The models are computed under the Sobolev approximation using the three-dimensional Monte Carlo radiative-transfer code TORUS. We have calculated four illustrative models in which the accretion flows are confined to azimuthal curtains - a geometry predicted by magneto-hydrodynamical simulations. Properties of the line profile variability of our models are discussed, with reference to dynamic spectra and cross-correlation images. We find that some gross characteristics of observed line profile variability are reproduced by our models, although in general the level of variability predicted is larger than that observed. We conclude that this excessive variability probably excludes dynamical simulations that predict accretion flows with low degrees of axisymmetry.

Molecular Line Profile Fitting with Analytic Radiative Transfer Models

We present a study of analytic models of starless cores whose line profiles have ``infall asymmetry,'' or blue-skewed shapes indicative of contracting motions. We compare the ability of two types of analytical radiative transfer models to reproduce the line profiles and infall speeds of centrally condensed starless cores whose infall speeds are spatially constant and range between 0 and 0.2 km s-1. The model line profiles of HCO+ (J=1-0) and HCO+ (J=3-2) are produced by a self-consistent Monte Carlo radiative transfer code. The analytic models assume that the excitation temperature in the front of the cloud is either constant (``two-layer'' model) or increases inward as a linear function of optical depth (``hill'' model). Each analytic model is matched to the line profile by rapid least-squares fitting. The blue-asymmetric line profiles with two peaks, or with a blue shifted peak and a red shifted shoulder, can be well fit by the ``HILL5'' model (a five parameter version of the hill model), with an RMS error of 0.02 km s-1. A peak signal to noise ratio of at least 30 in the molecular line observations is required for performing these analytic radiative transfer fits to the line profiles.

High‐Resolution X‐Ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cyg OB2 association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We obtained a Chandra High Energy Transmission Grating Spectrometer observation centered on Cyg OB2 No. 8A, the most luminous X-ray source in the association. Although our analysis focuses on the X-ray properties of Cyg OB2 No. 8A, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies of early-type stars, we find that the X-ray properties of Cyg OB2 No. 8A are very similar to those of other OB stars that have been observed using high-resolution X-ray spectroscopy. From analyses of the He-like ion fir emission lines (Mg XI, Si XIII, S XV, and Ar XVII), we derive radial distances of the He-like line emission sources and find that the higher energy ions have their lines form closer to the stellar surface than those of lower ion states. These fir-inferred radii are also found to be consistent with their corresponding X-ray continuum optical depth unity radii. Both of these findings are in agreement with previous O star studies and again suggest that anomalously strong shocks or high-temperature zones may be present near the base of the wind. The observed X-ray emission-line widths (HWHM ~ 1000 km s-1) are also compatible with the observations of other O star supergiants. Since Cyg OB2 No. 8A is similar in spectral type to ? Pup (the only O star that clearly shows asymmetric X-ray emission-line profiles with large blueshifts), we expected to see similar emission-line characteristics. Contrary to other O star results, the emission lines of Cyg OB2 No. 8A show a large range in line centroid shifts (~-800 to +250 km s-1). However, we argue that most of the largest shifts may be unreliable and the resulting range in shifts is much less than those observed in ? Pup. Although there is one exception, the H-like Mg XII line, which shows a blueshift of -550 km s-1, there are problems associated with trying to understand the nature of this isolated large blueshifted line. To address the degree of asymmetry in these line profiles, we present Gaussian best-fit line profile model spectra from ? Pup to illustrate the expected asymmetry signature in the ?2 residuals. Comparisons of the Cyg OB2 No. 8A best-fit line profile residuals with those of ? Pup suggest that there are no indications of any statistical significant asymmetries in these line profiles. Both the line shift characteristics and lack of line asymmetries are very puzzling results. Given the very high mass-loss rate of Cyg OB2 No. 8A (approximately 5 times larger than previous Chandra-observed O supergiants), the emission lines from this star should display a significant level of line asymmetry and blueshifts as compared to other OB stars. We also discuss the implications of our results in light of the fact that Cyg OB2 No. 8A is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.

Physical Parameters of Southern B- and Be-Type Stars

In this paper we present new results on stellar fundamental parameters for early B and Be field stars observed in the southern hemisphere: effective temperature, superficial gravity, and projected stellar rotation velocity. The estimation of their projected rotation velocities is made by two successive methods. We first obtain an initial value based on Fourier transforms of the He I λ4471 line for 34 B and Be field stars with magnitudes in the range 0.5 ≤ mv ≤ 10, followed by a more accurate fitting procedure of observed lines with non-LTE model line profiles. This procedure yields stellar rotation velocity estimates that are in agreement with those of the literature. We derive also Teff and log g values by fitting equivalent widths and profiles of NLTE model spectra to the observed ones. Finally, we give estimates of stellar ages, masses, and bolometric luminosities derived from interpolations in the evolutionary tracks calculated by Schaller.

Stellar Rotation in the Young Cluster M17

We constructed line profile models for young OB stars in M17 using TLUSTY and SYNSPEC, including the effects of rotational flattening and gravitational darkening. The values of v sin i for 8 member stars (with spectral classifications O4.5 V to O9 V) range from 7 km s–1 to 168 km s–1. The mean value of v sin i (102 km s–1) is lower than expected for very young clusters.

Constraints fromInfrared Space ObservatoryData on the Velocity Law and Clumpiness of WR 136

Observations with the Infrared Space Observatory (ISO) SWS spectrometer are used to constrain the velocity law and wind clumping of the well-studied Wolf-Rayet (W-R) star WR 136 (HD 192163, WN6). Because the free-free continuum opacity in W-R winds increases steadily with wavelength in the IR, each point in the continuous spectrum may be regarded as forming in a pseudophotosphere of larger radius for longer wavelength. Using this idea in combination with an analysis of the Doppler-broadened widths of several Heii recombination lines, we can derive information about the velocity law and clumpiness of the stellar wind of WR 136. The observed line emission emerges from the region exterior to the radius of optical depth unity in the free-free opacity, corresponding to ve0:3v1 for our shortest-wavelength line. The ISO observations provide the continuum shape, flux level, and seven fairly strong He ii emission profiles. Adopting a � -law distribution for the outflow velocity law, we compute the continuous energy distribution and line profiles. We find that there is a broad range of � -values consistent with the continuum data if we also allow the wind temperature to be a free parameter. Interestingly, the continuum data are found to constrain the wind to have fairly low clumping values for the IR-forming region. By using the continuum results in conjunction with line profile modeling, the observational constraints are best satisfied with a clumping factor of Dc ¼ � 2 hi =� hi 2 of 1–3 and a � -value of 2–3, although higher � -values are not strongly ruled out for a modest wind temperature. The wavelength range of our ISO data allows us to probe only the outer wind acceleration zone, but in combination with radio observations, our finding that the wind clumping is fairly small suggests that the clumping in the wind of WR 136 decreases with increasing radius. Subject headings: line: formation — stars: atmospheres — stars: individual (WR 136) — stars: mass loss — stars: Wolf-Rayet

High resolution measurements and modeling of auroral hydrogen emission line profiles

Abstract. Measurements in the visible wavelength range at high spectral resolution (1.3 Å) have been made at Longyearbyen, Svalbard (15.8 E,78.2 N) during an interval of intense proton precipitation. The shape and Doppler shift of hydrogen Balmer beta line profiles have been compared with model line profiles, using as input ion energy spectra from almost coincident passes of the FAST and DMSP spacecraft. The comparison shows that the simulation contains the important physical processes that produce the profiles, and confirms that measured changes in the shape and peak wave-length of the hydrogen profiles are the result of changing energy input. This combination of high resolution measurements with modeling provides a method of estimating the incoming energy and changes in flux of precipitating protons over Svalbard, for given energy and pitch-angle distributions. Whereas for electron precipitation, information on the incident particles is derived from brightness and brightness ratios which require at least two spectral windows, for proton precipitation the Doppler profile of resulting hydrogen emission is directly related to the energy and energy flux of the incident energetic protons and can be used to gather information about the source region. As well as the expected Doppler shift to shorter wavelengths, the measured profiles have a significant red-shifted component, the result of upward flowing emitting hydrogen atoms.Key words. Ionosphere (auroral ionosphere; particle precipitation) – Magnetospheric physics (auroral phenomena)

X-ray emission line profile modeling of hot stars

The launch of high-spectral-resolution x-ray telescopes (Chandra, XMM) has provided a host of new spectral line diagnostics for the astrophysics community. In this paper we discuss Doppler-broadened emission line profiles from highly supersonic outflows of massive stars. These outflows, or winds, are driven by radiation pressure and carry a tremendous amount of kinetic energy, which can be converted to x rays by shock-heating even a small fraction of the wind plasma. The unshocked, cold wind is a source of continuum opacity to the x rays generated in the shock-heated portion of the wind. Thus the emergent line profiles are affected by transport through a two-component, moving, optically thick medium. While complicated, the interactions among these physical effects can provide quantitative information about the spatial distribution and velocity of the x-ray-emitting and absorbing plasma in stellar winds. We present quantitative models of both a spherically symmetric wind and a wind with hot plasma confined in an equatorial disk by a dipole magnetic field.

Calculations of the Far‐Wing Line Profiles of Sodium and Potassium in the Atmospheres of Substellar‐Mass Objects

At the low temperatures achieved in cool brown dwarf and hot giant planet atmospheres, the less refractory neutral alkali metals assume an uncharacteristically prominent role in spectrum formation. In particular, the wings of the Na-D (5890 \AA) and K I (7700 \AA) resonance lines come to define the continuum and dominate the spectrum of T dwarfs from 0.4 to 1.0 \mic. Whereas in standard stellar atmospheres the strengths and shapes of the wings of atomic spectral lines are rarely needed beyond 25 \AA of a line center, in brown dwarfs the far wings of the Na and K resonance lines out to 1000's of \AA detunings are important. Using standard quantum chemical codes and the Unified Franck-Condon model for line profiles in the quasi-static limit, we calculate the interaction potentials and the wing line shapes for the dominant Na and K resonance lines in H$_2$- and helium-rich atmospheres. Our theory has natural absorption profile cutoffs, has no free parameters, and is readily adapted to spectral synthesis calculations for stars, brown dwarfs, and planets with effective temperatures below 2000 Kelvin.

Modeling Line Profiles of Protostellar Collapse Observed with High Angular Resolution

We investigated the line profile of moderately optically thick molecular transitions from collapsing protostellar envelopes. Using the inside-out collapse model, we simulated high angular resolution observations, which can probe the infall motion of the inner region better than low angular resolution observations. The degree of line asymmetry becomes substantially larger as the observing beam size becomes smaller. The self-absorption dip at the central velocity also develops a noticeable asymmetry. Therefore, suitability of collapse tracers depends on the angular resolution of the instrument used. Inclusion of the continuum emission from the central object produces another absorption dip on the redshifted side of the line; i.e., the line shows an inverse P Cygni profile. This effect is larger in the submillimeter band than in longer wavelength bands because of the bright continuum emission from dust grains. This redshifted dip is a good probe of the infall motion at the very inner part of the infalling envelope, and is also sensitive to other physical conditions, such as the depletion of molecules and the distribution of dust grains. We discuss how the line profile is affected by the variation of model parameters, such as molecular abundance, infall radius, and luminosity of the central object. In addition to the inside-out collapse model, we also discuss line profiles generated from Hunter's similarity solutions. Since high angular resolution can be achieved by aperture synthesis, its effect on the line profile was examined by simulating observations of the Sub-millimeter Array (SMA) toward B335, as an example. Only the central absorption dip part of the line profile is affected when the SMA is simulated. The simulated SMA line profile is weaker than the profile generated with a Gaussian beam because of the missing short-spacing flux. If the source is closer, or if the size of the element antenna of the interferometer is larger, even the line peaks can be affected.

Resonance Line Profile Calculations Based on Hydrodynamical Models of Cataclysmic Variable Winds

We describe a method of calculating synthetic line profiles using a generalized version of the Sobolev approximation. We apply this method to calculate line profiles predicted by the models of two-dimensional line-driven winds from luminous disks due to Proga, Stone, & Drew. We describe the main properties of the model line profiles and compare them with recent Hubble Space Telescope observations of the cataclysmic variable IX Vel. The model wind consists of a dense, slow outflow that is bounded on the polar side by a high-velocity stream. We find that these two wind components produce distinct spectral features. The fast stream produces profiles that show features consistent with observations. These include the appearance of the classical P Cygni shape for a range of inclinations, the location of the maximum depth of the absorption component at velocities less than the terminal velocity, and the transition from net absorption to net emission with increasing inclination. However, the model profiles have too little absorption or emission equivalent width compared to observed profiles. This quantitative difference between our models and observations is not a surprise because the line-driven wind models predict a mass-loss rate, mostly due to the fast stream, that is lower than the rate required by the observations. We note that the model profiles exhibit a double-humped structure near the line center that is not echoed in observations. We identify this structure with a nonnegligible redshifted absorption that is formed in the slow component of the wind where the rotational velocity dominates over expansion velocity. We conclude that the next generation of disk wind models, developed for application to cataclysmic variables, needs to yield stronger wind driving out to larger disk radii than do the present models.

The Effects of Noise and Sampling on the Spectral Correlation Function

The effects of noise and sampling on the spectral correlation function (SCF) introduced by Rosolowsky and coworkers are studied using observational data, numerical simulations of magnetohydrodynamic turbulence, and simple models of Gaussian spectral line profiles. The most significant innovations of this paper are (1) the normalization of the SCF based on an analytic model for the effect of noise and (2) the computation of the SCF as a function of the spatial lag between spectra within a map. A new definition of the quality of a spectrum, Q, is introduced, which is correlated with the usual definition of signal-to-noise ratio. The prenormalization value of the SCF is a function of Q. We derive analytically the effect of noise on the SCF, and then normalize the SCF to its analytic approximation. By computing the dependence of the SCF on the spatial lag, S0(Δr), we have been able to conclude the following: (1) S0(Δr) is a power law, with slope α, in the range of scales li < l < lo. (2) The correlation outer scale, lo, is determined by the size of the map, and no evidence for a true departure from self-similarity on large scales has been found. (3) The correlation inner scale, li, is a true estimate of the smallest self-similar scale in a map. (4) The spectral slope, α, in a given region, is independent of velocity resolution (above a minimum resolution threshold), spatial resolution, and average spectrum quality. (5) Molecular transitions that trace higher gas density yield larger values of α (steeper slopes) than transitions tracing lower gas density. (6) Nyquist sampling, bad pixels in detector arrays, and reference-sharing data acquisition need to be taken into account for a correct determination of the SCF at Δr = 1. The value of α, however, can be computed correctly without a detailed knowledge of observational procedures.

ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds

We compare the stellar wind features in far-UV spectra of Sk -67°111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67°111 must be due primarily to shocks in the wind.

Line Profile Modeling of Disks

AbstractIn this talk I give a short overview of models for spectral lines in Be stars disks which I think are quite instructive. I then examine the arguments for my own model assumptions and discuss selected topics concerning radiative line transfer calculations in three dimensions. We show that symmetric Hα emission lines of Be stars can be understood in terms of Keplerian disks. It is also demonstrated that one-armed global disk oscillations provide the best available solution to understand the long-term V/R variations in Be star emission lines.

Continuum Flux Excess in Be Stars Determined from Spectral Lines

AbstractWe study the continuum flux excess at λ4471 by comparing the observed He I λ4471 line profiles with non-LTE model line profiles. Assuming that emission of the Hγ line is formed nearly in the same regions of the circumstellar envelope as those where the visible continuum flux excess rises, we estimate the continuum opacity needed to account for the veiling of the He I λ4471 line. The flux excess so derived is then studied as a function of the stellar aspect angle in an attempt to determine the degree of incidence of the envelope geometry near the central star on the continuum flux of Be stars.

Fourier modelling of the anisotropic line broadening of X-ray diffraction profiles due to line and plane lattice defects

A new model of line-profile broadening due to the effect of linear and planar lattice defects has been incorporated into the conventional Rietveld algorithm for the structural refinement and whole-pattern fitting of powder data. The proposed procedure, applied to face-centred cubic (f.c.c.) structure materials, permits better modelling, even in the case of anisotropic line broadening and otherhkl-dependent effects that can be related to the presence of dislocations and planar defects (stacking faults and twinning). Besides better quality of the profile fitting, detailed information on the defect structure can be produced: dislocation density and cut-off radius, stacking- and twin-fault probabilities can be refined together with the structural parameters. For each phase (in different samples or in multi-phase samples) the appropriatesize–strainmodel can be selected. The Fourier formalism, which is the basis of the line-profile modelling, also permits an easy adaptation to different lattice-defect models. New approaches can be easily introduced and tested against or together with the existing ones. Finally, the devised program can also be used for the simulation of powder patterns for materials with different types and amounts of line and plane lattice defects.

A Contracting, Turbulent, Starless Core in the Serpens Cluster

We present combined single-dish and interferometric CS (2-1) and N2H+ (1-0) observations of a compact core in the northwest region of the Serpens molecular cloud. The core is starless according to observations from optical to millimeter wavelengths, but its lines have turbulent widths and asymmetry. Line profile modeling indicates supersonic inward motions vin 0.34 km s-1 over an extended region L > 12,000 AU. The high infall speed and large extent exceeds the predictions of most thermal ambipolar diffusion models and points to a more dynamical process for core formation. A short (dynamic) timescale, ~105 yr L/vin, is also suggested by the low N2H+ abundance, ~1 × 10−10.