Equatorial Density Enhancement Research Articles

THEORETICAL AND OBSERVATIONAL CONSEQUENCES OF STELLAR ROTATION AND MAGNETIC FIELDS FOR STELLAR WINDS

ABSTRACT My dissertation concerns the study of stellar wind structure from (1) the theoretical modeling of stellar rotation and magnetic fields and (2) the development of observational diagnostics. First, I have investigated the effects of stellar rotation for the wind structure of stars across the H-R Diagram using principles from the Wind Compressed Disk (WCD) model. Relative to a spherical wind, the effect of rotation is to increase the wind density at the equator while decreasing the density near the poles. An investigation of mild equatorial density enhancements has led to the development of the Wind Compressed Zone (WCZ) model. The WCZ model predicts that equatorial wind compressions are most likely to occur for stellar winds with low terminal speeds and/or radial velocity distributions that increase gradually from the base of the wind. In favorable cases stellar rotation can produce significant equatorial density enhancements in the winds of Wolf-Rayet stars, B~supergiants, Asymptotic Giant Branch stars, and even some novae at moderate stellar rotations of 20-30% break-up. A spectral line diagnostic based on multiline observations at Infrared wavelengths has been developed to measure the velocity law in the Wolf-Rayet winds, to determine whether they are likely susceptible to wind compression effects. The second major part of the thesis relates to the result that the WCZ model predicts the magnetic field structure in the wind, if the field strength is relatively weak. However, there are generally no good spectral diagnostics of sub-kiloGauss stellar magnetic fields. Magnetic fields at these low strengths are difficult to measure with the Zeeman effect, because the Zeeman line splitting is significantly smaller than the Doppler broadening. Thus, I have explored applications of the Hanle effect, that is sensitive to weaker magnetic fields than is the Zeeman effect, for probing the magnetic properties of stellar winds. The Hanle effect concerns the modification of resonance line polarization by the magnetic field. It has been used in studies of the Solar atmosphere, but not in other stars. Solutions for the Hanle effect in optically thin axisymmetric extended stellar envelopes have been derived. Relative to the zero field case, the Hanle effect can result in significant changes of the total line polarization. Consequences of the Hanle effect for the polarization of line profiles is also investigated, and analytic results are presented for a few special cases. A magnetic field can cause strong variations of the polarized line profile shape as compared to the case without a magnetic field. With multiline observations the Hanle effect will be a valuable diagnostic of stellar magnetic fields in the range 1-1000 Gauss.

Circumstellar molecular envelopes of AGB and post-AGB objects

This review presents the physical and chemical structure of circumstellar envelopes (CSEs) around stars on, and beyond, the asymptotic giant branch (AGB). It focusses on information that is relevant to the interpretation of the molecular line emission from these envelopes. The mass loss rate estimate from CO radio line emission is briefly discussed. It continues with an overview of the morphology and kinematics of AGB and post-AGB CSEs, based on the observational evidence obtained from molecular line emission. Also the small-scale structure of the circumstellar medium is discussed. Finally, a possible scenario for the puzzling, drastic change (at least apparently) from largely spherical AGB-CSEs to the prominent equatorial density enhancements that dominate in CSEs around late post-AGB objects, e.g., planetary nebulae (PNe), is put forward.

Winds from rotating Wolf-Rayet stars: the wind-compressed zone model

Theoretical models for Wolf-Rayet winds that include the effects of rotation and magnetic fields are discussed. These are of two types: magnetic rotator models and wind compression models. The magnetic rotator models are examples of equatorial expulsion models, typically requiring large rotation rates to produce significant equatorial density enhancements. Wind- compression models are introduced here as an application of the two-dimensional Wind-Compressed Disk (WCD) model that has recently been developed for Be stars. An equatorial disk forms because of the supersonic confluence of the flow from the upper and lower hemispheres of the star. In the application to WR stars, we suggest that disk formation is not required, and instead a less extreme example that we call a wind-compressed zone (WCZ) may suffice. Because the winds of WR stars have a geometrically extended acceleration region, or a “slow” velocity law, there can be compressions by an order of magnitude, even with moderate stellar rotation rates of about 16% critical. Magnetic flux conservation is then used to estimate the enhancement of the field strength that occurs because of the equatorial wind-compression. We consider an application of the model to explain the occurrence of polarization and dust formation among only some WR stars. Finally, we consider the WR + compact companion system WR 147, and suggest that the enhanced accretion in a WCZ could help explain the large X-ray emission.

CO Interferometric Maps of CIT 6 and CRL 618

We present CO (1−0) interferometer maps of carbon star, CIT 6, and carbon rich proto-planetary nebula, CRL618. Both objects were mapped with the Berekeley-Illinois-Maryland millimeter array (BIMA). The resulting resolution is ∼ 4″ and structure larger than ∼1′ is completely filtered out. Two velocity resolutions were employed; 0.4 km s−1 and 0.8 km s−1 permitting a velocity coverage of 52 and 104 km s−1. We find evidence for non-spherical expanding envelopes in both objects. The interferometer detected all the single dish CO flux in CRL618 (840 Jy km s−1), and 70% of the CO flux in CIT 6 (or 1030 Jy km s−1). For CIT 6, we obtained single dish maps at the NRAO 12m to measure the zero spacing flux distribution. We measure an envelope diameter of 90″ (2.6 × 1017 at 190 pc) for CIT 6 and 15″ (4.5 × 1017 at 2 kpc) for CRL 618 and expansion velocities of 17 km s−1 for both. The small scale structure seen by the interferometer in CIT 6 has a core and an elongation to the SW. We interpret this elongation as the beginning of an equatorial density enhancement and predict that that CIT 6 will evolve into a bipolar planetary nebula. In CRL 618, the elongated structure seen in CO is perpendicular to the optical reflection nebula, which has an east-west orientation. Hence the Asymptotic Giant Branch (AGB) wind appears as an expanding waist band around this bipolar nebula giving credence to the idea that equatorial density enhancements in AGB winds produce bipolar planetary nebulae (Balick 1987).

Molecular-Line Observations of the Remnant AGB Envelopes Around Planetary Nebulae

We present recent results from a “search and mapping” program of molecular line emission (mainly CO) from remnant AGB envelopes around planetary nebulae (PNe), using the SEST (La Silla, Chile). New detections in CO J=2−1 include NGC2899 (0.02K), NGC6369 (0.14K) & NGC7009 (0.08K). In many of the detected PNe, notably NGC3132, IC4406, NGC6302, M1-16, and CPD-56°8032, the molecular envelopes contain 2 kinematically distinct outflows. Mapping of the strongest of these shows (1) that the fast (e.g. Vexp≳40–60 km s−1 in NGC3132, IC4406) outflows have bipolar spatial structure, and (2) there exists an equatorial density enhancement in the slower, more massive [· (M⊙yr−1)>5 10−6(NGC3132), >2 10−5(IC4406)] outflows, which presumably collimates the fast outflow (e.g. Sahai et al. 1990, A & A, 234, L1; Sahai et al. 1992, A & A, 251, 560). The fractional CO abundance in the envelope, f(CO), is probably rather low (<10−4), as a result of photodissociation by the stellar and interstellar UV radiation [e.g. f(CO)≲10−5 in IC4406]. HCN, HCO+, and 13CO have also been detected in several PNe, and sensitive upper limits set on CS, C18O, & C17O (in M1-16), and SO (in NGC3132). Some results are tabulated below, and calculations to estimate the molecular masses, mass-loss rates and molecular abundances are in progress.

A NEW MID-INFRARED CAMERA FOR GROUND-BASED ASTRONOMY AND AN INFRARED STUDY OF PLANETARY NEBULAE

ABSTRACT The dissertation is composed of two parts. The first part is a description of the Mid-Infrared Array Camera (MIRAC), a new camera for ground-based astronomy. The second part of the dissertation is an infrared study of planetary nebulae utilizing observations with the new camera. MIRAC is a collaborative effort between the University of Arizona, Smithsonian Astrophysical Observatory, and Naval Research Laboratory. It currently utilizes a Hughes 20 X 64 Si:As impurity-band-conduction (IBC) detector array with a capacitive transimpedance amplifier (CTIA) readout, which is sensitive to IR radiation from 2 to 26 microns. The camera is equipped with 10% bandiwidth filters at 2.2, 3.8, 4.6, 8.8, 9.8, 11.7, and 12.5 microns, and a wide-band 8.0-12.8 micron (N-band) filter. There is also a 20% filter at 20.5 microns and an 8-14 micron CVF with a resolution of 1.8%. An adjustable slide with several pupil stops allows the camera to be configured for telescopes with focal ratios ranging from 15 to 45. Two externally selectable magnifications are provided by the fully reflective optics, giving plate scales of 0.35" and 0.7" pixel-1 on the NASA IRTF. The MIRAC electronics provides timing signals and coadds successive frames at a maximum rate of 10 kHz for the full array, and higher rates for a partial array readout. the data are transferred via a serial interface to a PC for real-time display, storage, and further processing. MIRAC recently achieved a NEFD of 0.010 Jy arcsec-2 at 8.8, 11.7, and 12.5 microns for a 900-s on-source integration on the Steward Observatory 1.5-m telescope. Planetary nebulae (PNe) are formed from circumstellar material ejected during the late stages of stellar evolution. These nebulae are an important source of enrichment for the interstellar medium. In many PNe, there is an excess of emission in the IR, indicating the presence of dust. There are several different components seen in the IR emission, including a family of unidentified IR (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 microns. The source morphology provides information on the structure of the nebula as well as the carriers of the emission features. Images in the near- and/or mid-IR are presented for the following PNe: IC 418, BD+30degree3639, J900, NGC 2392, NGC 6543, AFGL 2688, and M 2-9. The mid-IR images were obtained using MIRAC, and the images at J, H, and K were obtained using the Steward Observatory 64 X 64 Hg:Cd:Te array camera. In BD+30degree3639, the UIR emission is seen to be spatially distinct from the IR continuum and the ionized gas as traced by the 12.8 micron [Ne II] line emission. The average temperature across the nebula of the dust responsible for the mid-IR continuum emission is 172 K. In IC 418, the size of the nebula increases as a function of wavelength in the near-IR images at J, H, and K. Emission from the halo is detected in the H and K images. In NGC 2392 and NGC 6543, evidence for excess emission is seen in the distribution of the near-IR flux. The distribution of IR emission differs significantly from the emission observed at optical wavelengths. In the bipolar nebulae AFGL 2688 and M 2-9, structures in the IR emission are seen that could be related to the equatorial density enhancements that have caused the bipolar morphology.

Ponderomotive mass transport in the magnetosphere

It has recently been suggested that the nonlinear ponderomotive force of hydromagnetic waves in the magnetosphere may cause significant mass transport along the ambient magnetic field. To investigate this further, a two‐dimensional nonlinear hydromagnetic computer code is applied to box magnetospheres with two different radial variations of β, the ratio of plasma pressure to magnetic pressure. It is found that the rate at which density structure develops is dependent on the local sound speed. For fundamental standing waveforms along the magnetic field both the size of maximum equatorial density enhancements and the time for enhancements to develop decrease with increasing sound speed. Density enhancements are limited at each radial position by the propagation of density fronts from the equator toward the ionospheres. The propagation is confined to a given field line, supporting the idea that the fronts propagate in the slow magnetosonic mode. Simple predictors Dsm (for the maximum relative density change) and tsm (for the time to reach maximum density) derived from second‐order theory are tested against the numerical simulations and are found to give quite good results. A dipolelike magnetosphere model is then set up with plausible plasma and magnetic field parameters. Radial variations of Dsm and tsm are obtained for observationally reasonable models of cavity modes and Alfvén resonances. It is found that maximum density enhancements in the inner plasmasphere and in the plasma sheet are relatively small. However, large density enhancements are obtained in the outer plasmasphere and plasma trough. The size of these enhancements depends on the mean ion temperature and the effective ion mass in that region and is also likely to depend on how strongly slow magnetosonic waves are Landau damped.

Model for Rotating and Nonuniform Planetary Exospheres

The model neutral exosphere with a uniformly rotating exobase is generalized by allowing variations in exobase density and temperature which characterize the thermosphere just below the base. The corresponding velocity distribution function, satisfying the collisionless Boltzmann equation, is constructed and used to form a general expression for the velocity moments. Resulting density profiles of rotating exospheres with nonuniform densities and temperatures on the exobase are compared with corresponding nonrotating exospheres. Density enhancements due to rotation are found to be greatest above regions of exobase density or temperature minima. Equatorial density enhancements of terrestrial hydrogen, resulting from rotation, are estimated to be 15%-17% at altitudes of 10-20 earth radii. Corresponding increases in terrestrial helium are 30%-50% on the equator at altitudes of 0.7 to 1 earth radii even when there is a polar density bulge in the barosphere.