R. E. Schulte-Ladbeck

Tailored Analyses of the WN 8 Stars WR 40 and WR 16

We present the results of tailored spectral analyses of two WN 8 Wolf-Rayet stars: WR 40 (HD 96548) and WR 16 (HD 86161). These analyses were carried out utilizing line-blanketed non-LTE model atmospheres with provisions for a clumped wind. For the first time in WN analyses, stellar parameters are derived by attempting to match the entire observed spectrum from 900 to 35000 A, including the complex iron pseudocontinuum between 1200-2100 A. The inclusion of iron and other metals (O, Ne, Mg, Al, Si, S, Ar, and Ca) in the model atmosphere results in a decrease in derived stellar radii by ~20% from previous, nitrogen-line analyses, while mass-loss rates are decreased by a factor of ~3 owing to clumping. The abundance of iron and other metals are constrained with uncertainties of about 50%, demonstrating that Wolf-Rayet stars can be used to determine metallicities and metallicity gradients in other galaxies. We find the presence of metals significantly increases the radiative line force the wind experiences, allowing the outer wind to be driven to its theoretical velocity structure and coming within a factor of 5 of the needed line force in the inner region (an improvement over earlier radiative transfer models). Coupled with the substantial reduction in the derived performance numbers (factor of ~3), this indicates that radiation pressure alone may be sufficient to drive the winds of WN 8 stars. Our model parameters achieve good fits simultaneously to the H, He, and N III-IV features of the spectra, a feat that has eluded previous efforts. Comprehensive identification of lines and line complexes are provided, and the effects of the inclusion of metals on the strengths of diagnostic lines are discussed.

Star Formation Rates of Local Blue Compact Dwarf Galaxies. I. 1.4 GHz and 60 Micron Luminosities

We determine and examine the star formation rates of 50 well-known, local blue compact dwarf (BCD) galaxies based on their 1.4 GHz and 60 μm luminosities. We find that in cases for which both radio and far-infrared luminosities are available, the resulting star formation rates agree extremely well with one another. We determine that the star formation rates of the BCD galaxies in our sample span nearly 5 orders of magnitude, from approximately a few times 10-3 to several times 101 M⊙ yr-1, with a median SFR of about 0.3 M⊙ yr-1. We discuss trends of metallicity (primarily oxygen abundance) with star formation rate and explore the connections between SFR and galaxy mass estimates.

Modeling composite and fluffy grains: The effects of porosity

Recent studies of interplanetary and interstellar dust provide evidence that cosmic dust grains are fluffy, composite objects, highlighting the need for models of the electromagnetic scattering by these grains. Effective medium theory (EMT) with Mie-type series solutions has been used to explore the effects of porosity which would be important in composite dust particles. While this indirect approach is both flexible and computationally efficient, it is not necessarily a good approximation. The need for EMT and its rather restrictive assumptions may be circumvented through a direct computation of the scattering properties via finite element methods, such as the discrete dipole approximation (DDA). Recently, the utility of the DDA method has been advanced significantly through improvements in theory, in numerical algorithms, and in computer hardware. Extensive calculations with the DDA method are used here to examine more directly the effects of porosity. A particular emphasis is placed upon developing a valid methodology. For both solid and porous targets we establish both numerical and physical convergence properties over the range of size parameter that is required for our study. DDA cross sections for grains with a range of porosity are compared to those computed by the EMT/series expansion technique to examine the applicability of several mixing rules, including two extensions of the Bruggeman rule. We show that for particles with Rayleigh vacuum inclusions, the extension proposed by Rouleau & Martin is quite successful. We also investigate the effects of larger, non-Rayleigh vacuum inclusions for various levels of porosity and find that they can be significant.

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: The Wolf-Rayet Galaxy I Zw 36*

We report the results of near-IR imaging in J and H, of I Zw 36 (≈Z⊙/14) with the Hubble Space Telescope. Whereas imaging with the pre-COSTAR Faint Object Camera (FOC) previously resolved hot and massive stars in the near-UV, the NICMOS data furnish a census of the cool, intermediate- and low-mass stars. There clearly was star formation in I Zw 36 prior to the activity which earned it its blue compact dwarf/Wolf-Rayet galaxy classification. The detection of luminous, asymptotic giant branch stars requires that stars formed vigorously several hundred megayears ago. The well-populated red giant branch indicates stars with ages of at least 1–2 Gyr (and possibly older than 10 Gyr). We use the tip-of-the-red-giant-branch method to derive a distance of ≥5.8 Mpc. This is the third in a series of papers on near-IR–resolved blue compact dwarf galaxies. We notice that the color-magnitude diagrams of VII Zw 403, Mrk 178, and I Zw 36 do not exhibit the gaps expected from an episodic mode of star formation. Using simulated color-magnitude diagrams we demonstrate for I Zw 36 that star formation did not stop for more than a few 108 yr over the past 109 yr, and we discuss the implications of this result.

The axisymmetric stellar wind of AG Carinae

We present optical linear spectropolarimetry of the Luminous Blue Variable AG Carinae obtained after a recent visual brightness increase. The absence of He II lambda 4686 emission, together with the weakening of the He I spectrum and the appearance of Fe lines in the region around 5300 A, confirm that AG Car has started a new excursion across the HR diagram. The H alpha line profile exhibits very extended line wings that are polarized differently in both amount and position angle from either the continuum or the line core. The polarization changes across H alpha, together with variable continuum polarization, indicate the presence of intrinsic polarization. Coexistence of the line-wing polarization with extended flux-line wings evidences that both are formed by electron scattering in a dense wind. The position angle rotates across the line profiles, in a way that presently available models suggest is due to rotation and expansion of the scattering material. AG Car displays very large variations of its linear polarization with time, Delta P approximately 1.2%, indicating significant variations in envelope opacity. We find that the polarization varies along a preferred position angle of approximately 145 deg (with a scatter of +/- 10 deg) which we interpret as a symmetry axis of the stellar wind (with an ambiguity of 90 deg). This position angle is co-aligned with the major axis of the AG Car ring nebula and perpendicular to the AG Car jet. Our observations thus suggest that the axisymmetric geometry seen in the resolved circumstellar environment at various distances already exists within a few stellar radii of AG Car. From the H alpha polarization profile we deduce an interstellar polarization of Q = 0.31%, U = -1.15% at H alpha. The inferred interstellar polarization implies that the intrinsic polarization is not always of the same sign. This indicates either significant temporal changes in the envelope geometry, or it may arise from effects of multiple scattering in conjunction with density variations.

The first spectropolarimetric study of the wavelength dependence of interstellar polarization in the ultraviolet

The first UV spectropolarimetry along six lines of sight with significant interstellar polarization is reported. The observations were obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) during the Astro-1 mission. HD 37903, HD 62542 and HD 99264 show a wavelength dependence which follows the Serkowski relation extrapolated into the UV. HD 25443 and Alpha Cam have UV polarization well in excess of the Serkowski extrapolation. HD 197770 clearly shows a polarization bump which closely matches the 2175 A extinction feature. This bump polarization can be fitted by small aligned graphite disks. The differences along various lines of sight might be the result of differences in the environments which affect the size and alignment of the grains.

The asymmetric wind of R127

We present optical, linear polarimetry in broad-band UBVRI filters plus narrow-band filters centered on the emission lines of H-alpha and the red forbidden N II and spectropolarimetry in the wavelength range from 4120 A to 6770 A of the luminous blue variable R127 in the LMC. Both observations display a decrease of the percentage polarization across the H-alpha emission line with respect to the continuum. We assume that H-alpha is recombination-line dominated and thus intrinsically unpolarized, and we use the continuum subtracted line polarization to estimate the interstellar foreground polarization. The resulting amount of intrinsic continuum polarization of R127 is very large, of order 1-1.5 percent, implying both the presence of copious free electrons and a considerable asphericity in their distribution. The two data sets, taken two months apart, display significant variations in the continuum polarization, which confirms that the stellar-wind properties of R127 are time-dependent in the maximum state. We discuss several possible wind geometries and present arguments favoring a clumpy, axisymmetric outflow.

Contemporaneous Ultraviolet and Optical Observations of Direct and Raman-scattered O VI Lines in Symbiotic Stars

Symbiotic stars are binary systems consisting of a hot star, typically a white dwarf, and a cool giant companion. The wind from the cool star is ionized by the radiation from the hot star, resulting in the characteristic combination of sharp nebular emission lines and stellar molecular absorption bands in the optical spectrum. Most of the emission lines are readily identifiable with common ions. However, two strong, broad emission lines at 6825 and 7082 A defied identification with known atoms and ions. In 1989 Schmid made the case that these long unidentified emission lines resulted from the Raman scattering of the O VI resonance photons at 1032, 1038 A by neutral hydrogen. We present contemporaneous far-UV and optical observations of direct and Raman-scattered O VI lines for nine symbiotic stars obtained with the Hopkins Ultraviolet Telescope (Astro-2) and various ground-based optical telescopes. The O VI emission lines are present in every instance in which the λλ6825, 7082 lines are present, in support of the Schmid Raman-scattering model. We calculate the scattering efficiencies and discuss the results in terms of the Raman-scattering model. Additionally, we measure the flux of the Fe II fluorescence line at 1776 A, which is excited by the O VI line at 1032 A, and calculate the first estimates of the conversion efficiencies for this process.

Evidence for a Bipolar Geometry in R Coronae Borealis

The R Coronae Borealis (RCB) stars produce dust at irregular intervals, but the distribution of the circumstellar material is not known. We report spectropolarimetry of R Coronae Borealis (R CrB) itself, obtained in a deep decline with the Wisconsin-Indiana-Yale-NOAO telescope. The continuum is polarized ~1% in the red, rising to ~2% in the blue. There are significant polarization variations across the emission lines. The polarization of the Na I D lines indicates that light from the emission-line region is scattered but at a different angle from the stellar continuum. The position angle of the continuum polarization is almost constant from 1 μm to 7000 A but then changes rapidly, rotating by ~60° between 7000 and 4000 A. This behavior is strikingly similar to that produced in post-asymptotic giant branch (AGB) stars having an obscuring torus and bipolar dust lobes. These new data strengthen the earlier suggestion that there is a preferred direction to the dust ejections in R CrB. Dust ejections seem to occur predominantly along two roughly orthogonal directions consistent with a bipolar geometry. If confirmed, this finding will reinforce the relationship between the RCB stars and other post-AGB stars.

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS* Detection of Red Giant Stars in the Wolf-Rayet Galaxy Markarian 178

We observed the blue compact dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS camera aboard Hubble Space Telescope. The galaxy is well resolved into individual stars in the near-IR; photometry in J and H yields color-magnitude diagrams containing 791 individual point sources. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old stars. Mrk 178 is only the second blue compact dwarf galaxy in which the red giant branch has been resolved, indicating stars with ages of at least 1–2 Gyr. This allows us to derive a distance of ≥4.2(±0.5) Mpc. The near-IR color-magnitude diagram also exhibits an abundance of luminous, asymptotic giant branch stars. We find that this requires vigorous star formation several hundred Myr ago. Some candidate carbon stars are identified via their extreme near-IR color. We argue that Mrk 178 is fundamentally an old galaxy, based on the NICMOS detection of red giants underlying the blue, starburst core, and its extended, faint halo of redder color.