Physical Sciences and Mathematics Commons^™

Post-Outburst Infrared Spectra Of V1647 Ori, The Illuminating Star Of Mcneil's Nebula, Erika Gibb, T W. Rettig, S D. Brittain, D Wasikowski, Theodore Simon, William D. Vacca, Michael C. Cushing

Erika Gibb

The Polarization Signature From Microlensing Of Circumstellar Envelopes In Caustic Cossing Events., R. Ignace, J. E. Bjorkman, H. M. Bryce

Richard Ignace

In recent years, it has been shown that microlensing is a powerful tool for examining the atmospheres of stars in the Galactic bulge and Magellanic Clouds. The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarization signal: arising from differential magnification across the otherwise symmetric source. Earlier work has addressed the signal from a scattering photosphere for both point mass lenses and caustic crossings. In a previous paper, …

Reflections On The Growth Of Astronomy At The University Of Massachusetts And The Five College Astronomy Department, William M. Irvine

William M. Irvine

No abstract provided.

First Glimpse Results On The Stellar Structure Of The Galaxy., R. A. Benjamin, E. Churchwell, B. L. Babler, R. Indebetouw, M. R. Meade, B. A. Whitney, C. Watson, M. G. Wolfire, M. J. Wolff, R. Ignace, T. M. Bania, S. Bracker, D. P. Clemens, L. Chomiuk, M. Cohen, J. M. Dickey, J. M. Jackson, H. A. Kobulnicky, E. P. Mercer, J. S. Mathis, S. R. Stolovy, B. Uzpen

Richard Ignace

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~ 30 million mid-infrared sources towards the inner Galaxy, 10 < |l| < 65 degrees and |b| < 1 degree, was used to determine the distribution of stars in Galactic longitude, latitude, and apparent magnitude. The counts versus longitude can be approximated by the modified Bessel function N=N_0*(l/l_0)*K_1(l/l_0), where l_0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l_0= 17-30 degrees with a best fit value in the the 4.5 micron band of l_0=24 +/- 4 degrees. Modeling the source distribution as an exponential disk yields a radial scale length of H= 3.9 +/- 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| < 30 degrees, with ~ 25% more stars in the north. For l=10-30 degrees, there is a strong enhancement of stars of m= 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R_bar=4.4 +/- 0.5 kpc, tilted by phi=44 +/- 10 degrees to the Sun-Galactic Center line, provides the simplest interpretation of this data. We examine the possibility that enhanced source counts at l=26-28 degrees, 31.5-34 degrees, and 306-309 degrees are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m_K-m_[8.0] >3) peak. In these areas, the counts are reduced by extinction due to molecular gas and/or high diffuse backgrounds associated with star formation.

The Large Millimeter Telescope: Neighbors Explore The Cosmos, William M. Irvine, E Carrasco, I Aretxaga

William M. Irvine

No abstract provided.

Optically Thick Clumps – Not The Solution To The Wolf-Rayet Wind Momentum Problem?, J. C. Brown, J. P. Cassinelli, Q. Li, A. F. Kholtygin, R. Ignace

Richard Ignace

The hot star wind momentum problem eta = M-upsiloninfinity/(L/c) much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate. M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather …

Free-Free Spectral Energy Distributions Of Hierarchically Clumped Hii Regions., Richard Ignace, Ed Churchwell

Richard Ignace

In an effort to understand unusual power-law spectral slopes observed in some hypercompact HII regions, we consider the radio continuum energy distribution from an ensemble of spherical clumps. An analytic expression for the free-free emission from a single spherical clump is derived. The radio continuum slope (with F_\nu \nu^\alpha) is governed by the population of clump optical depths N(tau), such that (a) at frequencies where all clumps are thick, a continuum slope of +2 is found, (b) at frequencies where all clumps are optically thin, a flattened slope of -0.11 is found, and (c) at intermediate frequencies, a power-law segment …

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes Iv. Application To Polarized P Cygni Wind Lines., R. Ignace, K. H. Nordsieck, J. P. Cassinelli

Richard Ignace

The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1-300 G range. In this paper we compute the polarized P-Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the ``last scattering approximation''. For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized; while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. …

H2s (22,0-21,1) Observations Toward The Sgr B2 Region, Young C. Minh, William M. Irvine, S J. Kim

William M. Irvine

The H2S 22,0-21,1 line emission is observed to be strongly localized toward Sgr B2(M), and emissions from other positions in the more extended SgrB2 region are almost negligible. H2S is thought to form effectively by the passage of the C-type shocks but to be quickly transformed to SO2 or other sulfur species (Pineau des Forêts et al. 1993). Such a shock may have enhanced the H2S abundance in Sgr B2(M), where massive star formation is taking place. But the negligible emission of H2S from other observed positions may indicate that these positions have not been affected by shocks enough to …

Constraints From Infrared Space Observatory Data On The Velocity Law And Clumpiness Of Wr 136., R. Ignace, M. F. Quigley, J. P. Cassinelli

Richard Ignace

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 (WR) star WR 136 (HD 192163) (WN6). Because the free-free continuum opacity in WR winds increases steadily with wavelength in the IR, each point in the continuous spectrum may be regarded as forming in a pseudo-photosphere of larger radius for longer wavelength. Using this idea in combination with an analysis of the Doppler-broadened widths of several \ion{He}{2} recombination lines, we can derive information about the velocity law and clumpiness of the stellar wind of WR 136. Adopting …

Xmm-Newton Observations Of The Nitrogen-Rich Wolf-Rayet Star Wr 1., R. Ignace, L. M. Oskinova, J. C. Brown

Richard Ignace

We present XMM-Newton results for the X-ray spectrum from the N-richWolf-Rayet (WR) star WR1. The EPIC instrument was used to obtain a medium-resolution spectrum. The following features characterize this spectrum: ( a) significant emission "bumps" appear that are coincident with the wavelengths of typical strong lines, such as MgXI, SiXIII, and SXV; (b) little emission is detected above 4 keV, in contrast to recent reports of a hard component in the stars WR 6 and WR 110 which are of similar subtype; and ( c) evidence for sulfur K-edge absorption at about 2.6 keV, which could only arise from absorption …

The Zeeman Effect In The Sobolev Approximation: Applications To Spherical Stellar Winds., R. Ignace, K. G. Gayley

Richard Ignace

Modern spectropolarimeters are capable of detecting subkilogauss field strengths using the Zeeman effect in line profiles from the static photosphere, but supersonic Doppler broadening makes it more difficult to detect the Zeeman effect in the wind lines of hot stars. Nevertheless, the recent advances in observational capability motivate an assessment of the potential for detecting the magnetic fields threading such winds. We incorporate the weak-field longitudinal Zeeman effect in the Sobolev approximation to yield integral expressions for the flux of circularly polarized emission. To illustrate the results, two specific wind flows are considered: (i) spherical constant expansion with v(r) = …

The Conspicuous Absence Of X-Ray Emission From Carbon-Enriched Wolf-Rayet Stars., L. M. Oskinova, R. Ignace, W.-R. Hamann, A. M. T. Pollock, J. C. Brown

Richard Ignace

The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of L-X less than or similar to 2.5 x 10(30) erg s(-1) and to the X-ray to bolometric luminosity ratio of L-X/L(bo)l less than or similar to 4 x 10(-9). This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and Chandra observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven …

Microlensing Of Circumstellar Envelopes Ii. Emission Lines From Radial And Azimuthal Flow During Fold Caustic Crossings., H. M. Bryce, R. Ignace, M. A. Hendry

Richard Ignace

This paper examines the line profile evolution due to bulk motion in circumstellar envelopes during microlensing fold caustic crossing events. These events have recently been shown to be a sensitive probe of stellar surface brightness profiles, thus providing a means - through both photometric and spectroscopic observations - to constrain and test stellar atmosphere models. Here it is demonstrated, through the examination of simplified line profiles, that spectroscopic studies of fold caustic crossings could also prove to be a powerful diagnostic of bulk motion in circumstellar envelopes.

Polarization From Microlensing Of Spherical Circumstellar Envelopes By A Point Lens., J. F. L. Simmons, J. E. Bjorkman, R. Ignace, I. J. Coleman

Richard Ignace

We discuss the flux and polarization signals obtained from the microlensing of stars with extended circumstellar envelopes by a single point-mass lens. A previous paper considered the case of main-sequence stars, and showed that microlensing of the stellar photosphere could produce a small net polarization (up to 0.1 per cent). In this paper, we show that stars with extensive envelopes will show a much higher level of variable polarization (up to 10 per cent), even if they are spherically symmetric. Since the stellar envelopes most likely to be lensed are produced by red giant winds, we also investigate the effects …

Profile Shapes For Optically Thick X-Ray Emission Lines From Stellar Winds., R. Ignace, K. G. Gayley

Richard Ignace

We consider the consequences of appreciable line optical depth for the profile shape of X-ray emission lines formed in stellar winds. The hot gas is thought to arise in distributed wind shocks, and the line formation is predominantly via collisional excitation followed by radiative decay. Such lines are often modelled as optically thin, but the theory has difficulty matching resolved X-ray line profiles. We suggest that for strong lines of abundant metals, newly created photons may undergo resonance scattering, modifying the emergent profile. Using Sobolev theory in a spherically symmetric wind, we show that thick-line resonance scattering leads to emission …

Spectral Energy Distribution Signatures Of Jovian Planets Around White Dwarf Stars., R. Ignace

Richard Ignace

The problem of detecting Jovian-sized planets orbiting white dwarf stars is considered. Significant IR excesses result from warm Jupiters orbiting a white dwarf of T_eff = 10,000 K at a distance of ~10³ white dwarf radii (corresponding to ~10² Jupiter radii or a few tenths of an AU) with an orbital period of ~100 days. Such a planet will have a 10 µm flux density at its Wien peak that is comparable to the emission of the white dwarf at that wavelength. Although the white dwarf is much hotter than the planet, the planet will have peak …

Line Forces In Keplerian Circumstellar Disks And Precession Of Nearly Circular Orbits., K. G. Gayley, R. Ignace, S. P. Owocki

Richard Ignace

We examine the effects of optically thick line forces on orbiting circumstellar disks, such as occur around Be stars. For radially streaming radiation, line forces are only effective if there is a strong radial velocity gradient, as occurs, for example, in a line-driven stellar wind. However, within an orbiting disk, the radial shear of the azimuthal velocity leads to strong line-of-sight velocity velocity gradients along nonradial directions. As such, in the proximity of a stellar surface extending over a substantial cone angle, the nonradial stellar radiation can impart a significant line force, even in the case of purely circular orbits. …

Terminal Speeds And Ion Fractions From [Caiv] 3.207 Μm In Three Single Wn Stars., Richard Ignace

Richard Ignace

We report on the forbidden emission lines of [CaIV] at 3.207 μm and [NeIII] at 15.56 μm based on Infrared Space Observatory observations. The data set consists of three single Wolf-Rayet stars (WR 1, WR 134, and WR 136) and three binary systems (WR 11, WR 146, and WR 147). For the single stars, only the [CaIV] line was observed. Our primary objectives are to determine the wind terminal speed from the emission-line widths, derive ion number fractions, and discuss the relation between emission profile shapes and wind flow geometry. Compared to previous determinations of the terminal speed, we find …

Modelling X-Ray Variability In The Structured Atmospheres Of Hot Stars., L. M. Oskinova, R. Ignace, J. C. Brown, J. P. Cassinelli

Richard Ignace

We describe X-ray production in the atmospheres of hot, early-type stars in the framework of a "stochastic shock model". The extended envelope of a star is assumed to possess numerous X-ray emitting "hot" zones that are produced by shocks and embedded in the ambient "cold" medium in dynamical equilibrium. It is shown that the apparent lack of X-ray variability on short (similar to hours) timescales do not contradict a shock model for X-ray production. The character of the X-ray variability is found to depend on the frequency with which hot zones are generated, the cool wind opacity to X-rays, and …

Theoretical Profile Shapes For Optically Thin X-Ray Emission Lines From Spherical Stellar Winds., R. Ignace

Richard Ignace

One of the major outstanding problems in hot star wind theory is an understanding of the observed X-ray emissions from the early-type B, O, and Wolf-Rayet (WR) stars. The latest X-ray satellites Chandra and XMM-Newton are providing key new observations of resolved emission profiles to advance that understanding. This study presents a derivation of the expected emission-line profiles, assuming optically thin line emission and spherical symmetry, with a proper treatment of the attenuation of X-rays by the dense cool wind component. Examples of line profile variability for a narrow outflowing shell are presented. Then the case of embedded hot gas …

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes Iii. Including The Finite Star Depolarization Effect., R. Ignace

Richard Ignace

The Hanle effect is a relatively new magnetic diagnostic in stellar astrophysics. Although a substantial literature exists for applications of the Hanle effect in solar studies, the Hanle effect is only a fledgling subject in stellar astrophysics, with previous work focusing on simplistic cases to isolate the magnetic effects on polarized resonance scattering line profiles. In particular, applications to stars have treated the star as a point source of illumination. This paper carries the work forward by considering the consequences of finite stellar size for the line polarization. An approach based on intensity moments is derived. For optically thin line …

Resonance Line Scattering Polarization In Optically Thin Planar Equatorial Disks., Richard Ignace

Richard Ignace

This paper is the third in a series on the anisotropic scattering by optically thin resonance lines in extended stellar envelopes. Considered here is the polarization arising from resonance line scattering in equatorial disks. The shape of the polarized line profile is analytically derived under simplifying conditions of constant expansion or rotation for thin lines, with stellar occultation and finite star depolarization effects also included. The polarized profiles for the two cases are radically different. Moreover owing to the symmetries, rotation leads to profiles in both Q_ν and U_ν, whereas only a Q_ν profile survives for …

Exospheric Models For The X-Ray Emission From Single Wolf-Rayet Stars., R. Ignace, L. M. Oskinova

Richard Ignace

We review existing ROSAT detections of single Galactic Wolf-Rayet (WR) stars and develop wind models to interpret the X-ray emission. The ROSAT data, consisting of bandpass detections from the ROSAT All-Sky Survey (RASS) and some pointed observations, exhibit no correlations of the WR X-ray luminosity (L_X) with any star or wind parameters of interest (e.g. bolometric luminosity, mass-loss rate or wind kinetic energy), although the dispersion in the measurements is quite large. The lack of correlation between X-ray luminosity and wind parameters among the WR stars is unlike that of their progenitors, the O stars, which show …

Analytic Inversion Of Emission Lines Of Arbitrary Optical Depth For The Structure Of Supernova Ejecta., R. Ignace, M. A. Hendry

Richard Ignace

We derive a method for inverting emission line profiles formed in supernova ejecta. The derivation assumes spherical symmetry and homologous expansion (i.e., v(r)∝r), is analytic, and even takes account of occultation by a pseudo-photosphere. Previous inversion methods have been developed which are restricted to optically thin lines, but the particular case of homologous expansion permits an analytic result for lines of arbitrary optical depth. In fact, we show that the quantity that is generically retrieved is the run of line intensity Iλ with radius in the ejecta. This result is quite general, and so could be applied to resonance lines, …

Hot Star Polarimetric Variability And The Nature Of Wind Inhomogeneities., J. C. Brown, R. Ignace, J. P. Cassinelli

Richard Ignace

The problem is addressed of how much hot star polarisation variability can result from density redistribution processes within the wind as opposed to localised enhancement of stellar mass loss rate, such as ejections of wind inhomogeneities. For optically thin electron scattering, we present a theory for the relative polarisation arising from particle redistribution and consider several specific cases relevant to interpreting observations of wind variability. It is concluded that, allowing for partial cancellation of the contribution from compressed and evacuated regions, density redistribution internal to the wind can produce significant polarisation but only for processes that redistribute wind material over …

Trajectory Mapping: A Tool For Validation Of Trace Gas Observations, Gary A. Morris, James F. Gleason, Jerald Ziemke, Mark R. Schoeberl

Gary A. Morris

We investigate the effectiveness of trajectory mapping(TM) as a data validation tool. TM combines a dynamical model of the atmosphere with trace gas observations to provide more statistically robust estimates of instrument performance over much broader geographic areas than traditional techniques are able to provide. We present four detailed case studies selected so that the traditional techniques are expected to work well. In each case the TM results are equivalent to or improve upon the measurement comparisons performed with traditional approaches. The TM results are statistically more robust than those achieved using traditional approaches since the TM comparisons occur over …

Wolf-Rayet Wind Structure And Optical Variability., Q. Li, J. C. Brown, R. Ignace, J. P. Cassinelli, L. M. Oskinova

Richard Ignace

Results are presented on the expected variability of Wolf-Rayet (WR) stars in broad-band optical polarimetry and photometry, and in emission line profiles, due to an inhomogeneous random distribution of blobs in spherical geometry. Time dependent 3-D simulations are carried out with blob ejection random in time and direction, and the radiation properties are evaluated in the optically thin limit. In contrast with previous purely statistical analyses, inclusion in the present treatment of a beta velocity law and stellar occultation effects can yield results consistent with observations of the mean polarisation bar p and the ratio R=sigma_p /sigma_phot of polarimetric to …

An Explanation Of Observed Trends In The X-Ray Emission From Single Wolf-Rayet Stars., R. Ignace, L. M. Oskinova

Richard Ignace

The O and early B star winds show empirical correlations between X-ray (L_x) and Bolometric (L_Bol) luminosity as well as wind properties such as wind momentum and wind kinetic energy. Wolf-Rayet stars do not. We discuss scaling relations to qualitatively explain this lack of correlation among the WR winds and to quantitatively reproduce the observed ratio of X-ray luminosities between the N-rich WN types and C-rich WC types. If (a) the filling factor of hot X-ray emitting gas varies as (M⊙/v_\infty)(-1) for stars of different mass loss and terminal speed and (b) the ambient Wolf-Rayet wind component is optically thick …

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes Ii. Some Theoretical Results For Resolved Line Profiles., R. Ignace, J. P. Cassinelli, K. H. Nordsieck

Richard Ignace

A magnetic field diagnostic of stellar winds that uses the Hanle effect is discussed. This diagnostic pertains to the modification of resonance-line-scattering polarization in the presence of magnetic fields. The case for resolved polarized profiles of optically thin emission lines is considered, and some analytic results for an expanding equatorial disk are derived. Numerical results for a dipole magnetic field embedded in a spherical outflow are also presented. Although the considerations are somewhat simplified, the primary conclusion is that the modification or creation of line profile polarization by the Hanle effect can be used to discriminate between different magnetic field …