Engineering topics

Magalhaes, A. M.

Publications and source records attributed to Magalhaes, A. M..

17 records

NASA NTRS2007

The Role of Evolutionary Age and Metallicity in the Formation of Classical BE Circumstellar Disks II. Assessing the True Nature of Candidate Disk Systems

Photometric 2-color diagram (2-CD) surveys of young cluster populations have been used to identify populations of B-type stars exhibiting excess Ha emission. The prevalence of these excess emitters, assumed to be "Be stars". has led to the establishment of links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. We have obtained imaging polarization observations of six SMC and six LMC clusters whose candidate Be populations had been previously identified via 2-CDs. The interstellar polarization (ISP) associated with these data has been identified to facilitate an examination of the circumstellar environments of these candidate Be stars via their intrinsic polarization signatures, hence determine the true nature of these objects. We determined that the ISP associated with the SMC cluster NGC 330 was characterized by a modified Serkowski law with a lambda(sub max) of approx. 4500Angstroms, indicating the presence of smaller than average dust grains. The morphology of the ISP associated with the LMC cluster NGC 2100 suggests that its interstellar environment is characterized by a complex magnetic field. Our intrinsic polarization results confirm the suggestion of Wisniewski et al. that a substantial number of bona-fide classical Be stars are present in clusters of age 5-8 Myr. Hence, our data contradict recent assertions that the Be phenomenon develops in the second half of a B star's main sequence lifetime, i.e. no earlier than 10 Myr. These data imply that a significant number of B-type stars must emerge onto the zero-age-main-sequence rotating at near-critical rotation rates, although we can not rule out the possibility that these data instead reveal the presence of a sub-group of the Be phenomenon characterized by sub-critically rotating objects. Comparing the polarimetric properties of our dataset to a similar survey of Galactic classical Be stars, we find that the prevalence of polarimetric Balmer jump signatures decreases with metallicity. We speculate that these results might indicate that either it is more difficult to form large disk systems in low metallicity environments, or that the average disk temperature is higher in these low metallicity environments. We have characterized the polarimetric signatures of all candidate Be stars in our data sample and find approx. 25% are unlikely to arise from true classical Be star-disk systems. This detection of such a substantial number "contaminants" suggests one should proceed with caution when attempting to determine the role of evolutionary age and/or metallicity in the Be phenomenon purely via 2-CD results.

Wisniewski, J. P.↗

NASA NTRS2001

An AGN Identification for 3EG J2006-2321

We present a multiwavelength analysis of the high-energy gamma-ray source 3EG J2006-2321 (l = 18 deg.82, b = -26 deg.26). The flux of this source above 100 MeV is shown to be variable on time scales of days and months. Optical observations and careful examination of archived radio data indicate that its most probable identification is with PMN J2005-2310, a flat-spectrum radio quasar with a 5GHz flux density of 260 mJy. Study of the V = 19.3 optical counterpart indicates a redshift of 0.833 and variable linear polarization. No X-ray source has been detected near the position of PMN J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper limit provides for a spectral energy distribution with global characteristics similar to those of known gamma-ray blazars. Taken together, these data indicate that 3EG J2006-2321, listed as unidentified in the 3rd EGRET Catalog, is a member of the blazar class of AGN. The 5-GHz radio flux density of this blazar is the lowest of the 68 EGRET-detected AGN. The fact that EGRET has detected such a source has implications for unidentified EGRET sources, particularly those at high latitudes (absolute value of b greater than 30 deg), many of which may be blazars.

Wallace, P. M.↗

NASA NTRS2001

Supernova 1998bw In ESO 184-G82

Over the wavelength range 390-750 nm, we measure intrinsic linear polarization of 0.53 +/- 0.08 percent at position angle 49 +/- 3 deg, after correcting for Galactic interstellar polarization using the star HD 184100, which has polarization of 0.75 +/- 0.01 percent at p.a. 176.5 +/- 2.5 deg. This measured interstellar polarization is consistent with the Galactic extinction in this direction, estimated to be E(B-V) = 0.059 from IRAS maps, or E(B-V) = 0.079 from 21-cm H I. Interstellar polarization in the host galaxy ESO 184-G82 is expected to be negligible based on the relative absence of Na I D absorption at z = 0.00841 +/- 0.00005, the redshift of the environment of the supernova from narrow H II region emission lines in its spectrum. Polarization appears highest in between emission features in the total flux spectrum, which strengthens the interpretation of the polarization as intrinsic to the supernova. This modest polarization is less than that of some type-II supernovae, but greater than that of type-Ia supernovae, which are generally unpolarized. This supports the interpretation of SN 1998bw, a peculiar type-Ic supernova, as a core-collapse event in which the observed polarization is due to moderate asymmetry in either the photosphere of the ejecta or an overlying scattering envelope. However, this result does not strongly constrain arguments about whether some supernovae emit gamma-ray bursts, since such emission may come from a mildly relativistic shock associated with the radio emission and above the optical photosphere, without any requirements on beaming or orientation." A. V. Filippenko, University of California at Berkeley, comments on the total flux spectrum obtained above: "The spectrum most closely resembles those of the peculiar SN 1997ef, but perhaps evolving more slowly. It is not typical of type-Ic supernovae; indeed, the spectrum does not match any of the known spectral classes, but perhaps 'peculiar type Ic' is the best choice at this time. The object appears to be starting a transition to the nebular phase, but identifications of the broad emission features are uncertain. The feature at 650 nm is probably [Fe III], as is the one at 455 nm (perhaps with some contribution from Mg I]). That at 590 nm may be a mixture of [Co III] and Na D. The strongest feature, at 545 nm, may consist of Fe, Co, and Ti lines."

Kay, L. E.↗

NASA NTRS1996

Dust in the small Magellanic cloud. 1: Interstellar polarization and extinction data

The typical extinction curve for the Small Magellanic Cloud (SMC), in contrast to that for the Galaxy, has no bump at 2175 A and has a steeper rise into the far ultraviolet. For the Galaxy the interpretation of the extinction and, therefore, the dust content of the interstellar medium has been greatly assisted by measurements of the wavelength dependence of the polarization. For the SMC no such measurements existed. Therefore, to further elucidate the dust properties in the SMC we have for the first time measured linear polarization with five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. We also attempt to correlate the SMC extinction and polarization data. The main results are: the wavelength of maximum polarization, lambda(sub max), in the SMC is typically smaller than that in the Galaxy; however, AZC 456, which shows the UV extinction bump, has a lambda(sub max) typical of that in the Galaxy, but its polarization curve is narrower and its bump is shifted to shorter wavelengths as compared to the Galaxy; and from an analysis of both the extinction and polarization data it appears that the SMC has typically smaller grains than those in the Galaxy. The absence of the extinction bump in the SMC has generally been thought to imply a lower carbon abundance in the SMC compared to the Galaxy. We interpret our results to mean that te size distribution of the interstellar grains, and not only the carbon abundance, is different in the SMC as compared to the Galaxy. In Paper 2 we present dust model fits to these observations.

Magalhaes, A. M.↗

NASA NTRS1995

Dust in the Small Magellanic Cloud

We discuss simultaneous dust model fits to our extinction and polarization data for the Small Magellanic Cloud (SMC) using existing dust models. Dust model fits to the wavelength dependent polarization are possible for stars with small lambda(sub max). They generally imply size distributions which are narrower and have smaller average sizes compared to those in the Galaxy. The best fits for the extinction curves are obtained with a power law size distribution. The typical, monotonic SMC extinction curve can be well fit with graphite and silicate grains if a small fraction of the SMC carbon is locked up in the grains. Amorphous carbon and silicate grains also fit the data well.

Rodrigues, C. V.↗

NASA NTRS1995

Dust in the small Magellanic Cloud. 2: Dust models from interstellar polarization and extinction data

We study the dust in the Small Magellanic Cloud using our polarization and extinction data (Paper 1) and existing dust models. The data suggest that the monotonic SMC extinction curve is related to values of lambda(sub max), the wavelength of maximum polarization, which are on the average smaller than the mean for the Galaxy. On the other hand, AZV 456, a star with an extinction similar to that for the Galaxy, shows a value of lambda(sub max) similar to the mean for the Galaxy. We discuss simultaneous dust model fits to extinction and polarization. Fits to the wavelength dependent polarization data are possible for stars with small lambda(sub max). In general, they imply dust size distributions which are narrower and have smaller mean sizes compared to typical size distributions for the Galaxy. However, stars with lambda(sub max) close to the Galactic norm, which also have a narrower polarization curve, cannot be fit adequately. This holds true for all of the dust models considered. The best fits to the extinction curves are obtained with a power law size distribution by assuming that the cylindrical and spherical silicate grains have a volume distribution which is continuous from the smaller spheres to the larger cylinders. The size distribution for the cylinders is taken from the fit to the polarization. The 'typical', monotonic SMC extinction curve can be fit well with graphite and silicate grains if a small fraction of the SMC carbon is locked up in the grain. However, amorphous carbon and silicate grains also fit the data well. AZV456, which has an extinction curve similar to that for the Galaxy, has a UV bump which is too blue to be fit by spherical graphite grains.

Rodrigues, C. V.↗

NASA NTRS1993

Ultraviolet spectropolarimetry of the Be star PP Carinae with the Wisconsin Ultraviolet Photo-Polarimeter Experiment

We present the first ultraviolet spectropolarimetric observations of the Be star PP Car, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro 1 mission. Usable polarization data were obtained from 1400 to 2330 A, along with a good spectrum from 1400 to 3200 A. These data show a lower polarization shortward of the Balmer jump than had been predicted by standard models, and a broad UV polarization dip around 1900 A is seen. These results are in agreement with those found from the WUPPE observations of two other Be stars, Xi Tau and Pi Aqr, which were published earlier. All these observations are an important probe of the Be circumstellar envelopes and demonstrate the need for the inclusion of metal-line effects in circumstellar disk models of Be star UV polarization.

Bjorkman, K. S.↗

NASA NTRS1993

Dust in the Small Magellanic Cloud

Observations of reddened stars in the Small Magellanic Cloud (SMC) indicate that the interstellar grains in that galaxy may show distinct optical properties from those in the Galaxy. In a careful study of three SMC objects, Prevot showed that the UV extinction law in the SMC is almost linear with inverse wavelength and the 2200A feature is generally absent. The first results of a program to determine the wavelength dependence of the interstellar optical polarization in the SMC indicate that highly polarized objects are scarce. Our study has uncovered, however, several objects with optical polarization greater than around 1 percent: AZV 126, AZV 211, AZV 221, AZV 398, and AZV 456. The latter two have already had their UV extinction law determined. Our aim was to obtain International Ultraviolet Explorer (IUE) data and determine the UV extinction law also for AZV 126, AZV 211, and AZV 221. AZV 456, which presents a 'galactic' extinction law, has a 'normal' value for its wavelength of maximum polarization, lmax, while AZV 398, which shows a 'typical' SMC extinction curve, shows a somewhat smaller value for such wavelength. AZV 126, AZV 211, and AZV 221 all present extreme small values of lmax but had not had its extinction curve in the W determined yet. We therefore aimed at ultimately determining the extinction law in the direction of these three objects. Such results, in combination with the optical polarization data, have an important bearing on constraining the composition and size distribution of the interstellar dust in the SMC. In the last report, the images gathered with IUE, their processing, and the extinction curves derived from them were described. Such extinction curves and the theoretical models developed to interpret the SMC extinction and polarization data are discussed. Details are presented in an enclosed preprint. The activities in our ongoing polarimetric program of determining the magnetic field structure of the SMC and the images collected at Cerro Tololo Interamerican Observatory during the period are also briefly described. Other activities are also described.

Magalhaes, A. M.↗

NASA NTRS1992

Evidence for a bipolar nebula around the peculiar B(e) star HD 45677 from ultraviolet spectropolarimetry

We report the first ultraviolet spectropolarimetry of the peculiar B-type emission-line star with infrared excess HD 45677. The observations were obtained during the 1990 December Astro-l space shuttle mission with the Wisconsin Ultraviolet Photo-Polarimeter Experiment, and cover the spectral range 1400-3220 A. We also present the first optical spectropolarimetry, extending from the atmospheric cutoff to about 7600 A. The observed UV/optical linear polarization displays a strong increase toward shorter wavelengths indicative of scattering by circumstellar dust. The position angle of the intrinsic polarization flips by 90 deg in the near-UV as expected from a bipolar reflection nebula.

Schulte-Ladbeck, R. E.↗

NASA NTRS1992

Polarization and the envelopes of B(e) supergiants in the Magellanic Clouds

We report optical linear polarization observations of nine B(e) supergiants in the Magellanic Clouds. Several of them have large intrinsic polarizations. The data are consistent with nonspherically symmetric envelopes, around the B(e) stars, that possess a range of intrinsic polarizations. Comparison of the polarimetric data with the viewing aspect, inferred from the spectroscopy of the individual objects, agrees with this interpretation. The polarization correlates best with the infrared excess due to dust, suggesting the latter as the cause of most of the polarization. We cannot find, using the available data, a statistically significant difference between the polarization distributions of Galactic and Magellanic B(e) stars. Comparison of our data with previous polarimetric data, recovered from the literature for four stars, seems to indicate that the envelopes are stable.

Magalhaes, A. M.↗

NASA NTRS1992

The first linear polarization spectra of Wolf-Rayet stars in the ultraviolet - EZ Canis Majoris and Theta Muscae

During the 1990 December Astro-1 Space Shuttle mission, spectropolarimetry was conducted in the wavelength region from 1400 to 3200 A of the Wolf-Rayet stars EZ CMa (WN5) and Theta Mus (WC6 + O9.5I) with the Wisconsin Ultraviolet Photo-Polarimeter Experiment. The UV polarization of EZ CMa displays features which correspond to emission lines. This indicates a large, about 0.8 percent, intrinsic UV-continuum polarization, and provides further evidence that the wind of EZ CMa is highly distorted. The polarization of Theta Mus does not change across emission lines, or the strong interstellar 2200 A feature. The polarization decreases smoothly to shorter wavelengths, at constant position angle. The combined UV-optical polarization spectrum of Theta Mus can be described well with interstellar polarization following a Serkowski law.

Schulte-Ladbeck, R. E.↗

NASA NTRS1992

The wind geometry of the Wolf-Rayet star HD 191765

A time-dependent spectropolarimetric data set of HD 191765 in the wavelength range 3159-7593 A is presented. At all epochs the present observations display a large and strongly wavelength-dependent continuum polarization and reduced levels of polarization across the emission lines. The data imply a significant intrinsic continuum polarization which requires a general deviation of the electron distribution from spherical symmetry. The global shape is quite stable as a function of time; small fluctuations may arise from localized density/temperature changes. The line polarizations are consistent with an axisymmetric wind geometry and ionization stratification. A qualitative model for polarization in a Wolf-Rayet atmosphere is developed. It is argued that the blueward rise of the continuum polarization in HD 191765 can be explained if the density in the wind is high, resulting in a competition of thermal and electron-scattering continuum opacity in the vertical.

Schulte-Ladbeck, R. F.↗

NASA NTRS1991

First ultraviolet spectropolarimetry of Be stars from the Wisconsin Ultraviolet Photo-Polarimeter Experiment

The first UV spectropolarimetric observations of Be stars are presented. They were obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro-1 mission. WUPPE data on the Be stars Zeta Tau and Pi Aqr, along with near-simultaneous optical data obtained at the Pine Bluff Observatory (PBO). Combined WUPPE and PBO data give polarization as a function of wavelength across a very broad spectral region, from 1400 to 7600 A. Existing Be star models predicted increasing polarization toward shorter wavelengths in the UV, but this is not supported by the WUPPE observations. Instead, the observations show a constant or slightly declining continuum polarization shortward of the Balmer jump, and broad UV polarization dips around 1700 and 1900 A, which may be a result of Fe-line-attenuation effects on the polarized flux. Supporting evidence for this conclusion comes from the optical data, in which decreases in polarization across Fe II lines in Zeta Tau were discovered.

Bjorkman, K. S.↗

NASA NTRS1991

First ultraviolet spectropolarimetry of hot supergiants

UV spectropolarimetric observations of the early-type supergiant stars P Cygni and Kappa Cassiopeiae obtained with the Wisconsin UV Photo-Polarimeter Experiment (WUPPE) are presented. Contrary to model predictions, the intrinsic polarization of P Cyg remains constant in the Balmer continuum except for a broad dip between 2600 and 3000 A. This decrease in polarization is discussed in terms of possible Fe line blanketing effects. The intrinsic position angle is essentially constant in the UV except for a rotation across the feature between 2600 and 3000 A. The intrinsic polarization of Kappa Cas is small; most of the observed polarization is interstellar in nature. The Serkowski curve extrapolated into the UV is found to fall consistently below the observed polarization. A slight rotation in position angle from optical to UV wavelengths is noted. Very good agreement is found between the mean continuum polarization obtained by WUPPE and the broadband UV polarization measurement obtained by Gehrels (1968).

Taylor, M.↗

NASA NTRS1991

On the wind geometry of the Wolf-Rayet star EZ Canis Majoris

Recent models of Wolf-Rayet star winds have been tailored to EZ CMa, and make predictions of the envelope structure and location of line-emitting regions. It is discussed how the wind structure of EZ CMa can be probed observationally through electron distribution integrals as measured by spectropolarimetry, and then present, analyze, and interpret a time-dependent spectropolarimetric data set of EZ CMa. The observations further the view of an electron-scattering wind that is axisymmetric, rotating, and expanding, with a variable mass-loss rate being responsible for the quasi-periodic polarimetric variability. It is demonstrated that the emission lines of EZ CMa are partially polarized, indicating that line photons are electron-scattered in the wind. The polarization in N V lambda 4945 and N IV lambda 4058 is observed to be larger than that of He II lambda 4686 and He I lambda 5876, as expected from ionization stratification.

Schulte-Ladbeck, R. E.↗

NASA NTRS1990

A rotating, expanding disk in the Wolf-Rayet star EZ Canis Majoris?

The discovery of linear polarization changes across the extended wings of He II lines, mainly the strong 4-3 transition at 4686 A, in the WN5 star EZ CMa, is reported. When the polarization across the line profiles is plotted in the Stokes parameters plane, it traces loops clockwise from the blue wing through line center to the red, rather than straight lines. Such polarization loops are reminiscent of what is observed in the Balmer lines of Be stars. The continuum polarization in EZ CMa can be understood by an axisymmetric, electron-scattering envelope, with the decrease in polarization in He II being caused by an increase in absorptive opacity in the lines and dilution by unpolarized line emission, while the variations in position angle are due to the Doppler-shifted absorptive opacity and/or scattered line photons. As the sense of rotation in the loops is also independent of phase of this alleged Wolf-Rayet + compact binary, the polarized line profiles are the signature of a rotating, expanding wind geometry around a single star.

Schulte-Ladbeck, R. E.↗

NASA NTRS1990

A polarimetric survey of symbiotic stars

Optical and near-infrared polarization observations of 24 symbiotic stars, 14 observed with polarimetry for the first time are presented. In combination with published data, it is found that about 50 percent of the symbiotics observed polarimetrically show evidence for intrinsic polarization. The results are discussed in the light of previous observations, and comments are made on the temporal variability and wavelength dependence of the polarization. Dust scattering is identified as the dominant mechanism producing polarization in symbiotic stars. While it cannot be excluded that some symbiotic systems are completely engulfed in their dust shells, the data indicate that the H-alpha emission line may originate from outside of the dust-scattering envelopes in some systems.

Schulte-Ladbeck, R. E.↗