A. Pereyra

Polarimetry toward the Musca Dark Cloud. I. The Catalog

We have used CCD imaging polarimetry to obtain linear polarization measurements of background stars toward the filamentary Musca dark cloud. We present a catalog of 2497 objects with polarization signal-to-noise ratio larger than 5. This allows us to build polarization maps to infer the detailed geometry of the local magnetic field. We show composite polarization maps along the cloud and explore general correlations of the polarimetric data with the morphology of the region. We find the overall field to be strikingly aligned with the projected small axis of the filamentary cloud. We detect a lower limit for the polarization across the cloud of ~2%, with an enhanced polarization of 6%-7% in the central region. We find evidence that the polarization pattern is altered in the inner regions, those associated with higher extinction.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. I. The Ground-based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS 2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superposed on longer timescale variations. Rapid variations of ~0.01 mag minute-1, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wave bands (UBVRI) track each other well over the period of observation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular, B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence (polarization increases toward the blue, PU/PI = 1.31) and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the UV flux. For the fastest variability timescale observed, we estimate a central black hole mass of 1.5 × 109(δ/10) M☉, consistent with UV and X-ray constraints and smaller than previously calculated for this object.

THE ROLE OF EVOLUTIONARY AGE AND METALLICITY IN THE FORMATION OF CLASSICAL BE CIRCUMSTELLAR DISKS II. ASSESSING THE EVOLUTIONARY NATURE OF CANDIDATE DISK SYSTEMS

We present the first detailed imaging polarization observations of six SMC and six LMC clusters, known to have large populations of B-type stars that exhibit excess Hα emission from 2-CD photometric studies, to constrain the evolutionary status of these stars and hence better establish links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. We parameterize and remove the interstellar polarization (ISP) associated with each line of sight, thereby isolating the presence of any intrinsic polarization. We use the wavelength dependence of this intrinsic polarization to discriminate pure gas disk systems, i.e., classical Be stars, from composite gas-plus-dust disk systems, i.e., Herbig Ae/Be or B[e] stars. Our intrinsic polarization results, along with available near-IR color information, support the suggestion of Wisniewski et al. that classical Be stars are present in clusters of age 5-8 Myr and contradict assertions that the Be phenomenon only develops in the second half of a B stars main-sequence lifetime, i.e., no earlier than 10 Myr. The prevalence of polarimetric Balmer jump signatures decreases with metallicity; we speculate that either it is more difficult to form large disk systems in low-metallicity environments or that average disk temperatures are higher in low-metallicity environments. The polarimetric signatures of ~25% of our sample appear unlikely to arise from true classical Be star disk systems, suggesting 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.

Polarimetry toward the IRAS Vela Shell. II. Extinction and Magnetic Fields

We explore correlations between visual extinction and polarization along the western side of the Infrared Astronomical Satellite (IRAS) Vela Shell using a published polarimetric catalog of several hundred objects. Our extinction maps along this ionization front (I-front) find evidence of clumpy structure with typical masses between 1.5 and 6 M? and a mean length scale L ~ 0.47 pc. The polarimetric data allowed us to investigate the distribution of the local magnetic field in small (~parsec) scales across the I-front. Using the dispersion of polarization position angles, we find variations in the kinetic-to-magnetic energy density ratio of at least 1 order of magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. These findings suggest that the magnetic component makes a significant contribution to the dynamical balance of this region. Along the I-front, the mean magnetic field projected on the sky is 0.018 ? 0.013 mG. The polarization efficiency seems to change along the I-front. We attribute high polarization efficiencies in regions of relatively low extinction to an optimum degree of grain alignment. Analysis of the mass-to-magnetic flux ratio shows that this quantity is consistent with the subcritical regime (? < 1), showing that magnetic support is indeed important in the region. Our data extend the overall ?-N(H2) relation toward lower density values and show that such a trend continues smoothly toward low N(H 2) values. This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage.

Near infrared polarimetry of a sample of YSOs

Aims. Our goal is to study the physical properties of the circumstellar environment of young stellar objetcs (YSOs). In particular, the determination of the scattering mechanism can help us to constrain the optical depth of the disk and/or envelope in the near infrared. Methods. We used the IAGPOL imaging polarimeter along with the CamIV infrared camera at the LNA observatory to obtain near infrared polarimetry measurements in the H band of a sample of optically visible YSOs, namely, eleven T Tauri stars and eight Herbig Ae/Be stars. An independent determination of the disk (or jet) orientation was obtained for twelve objects from the literature. The circumstellar optical depth could then be estimated by comparing the integrated polarization position angle (PA) with the direction of the major axis of the disk projected onto the plane of the sky. Optically thin disks have, in general, a polarization PA that is perpendicular to the disk plane. In contrast, optically thick disks have polarization PAs parallel to the disks. Results. Among the T Tauri stars, three are consistent with having optically thin disks (AS 353A, RY Tau and UY Aur) and five with optically thick disks (V536 Aql, DG Tau, DO Tau, HL Tau and LkHα 358). Among the Herbig Ae/Be stars, two stars exhibit evidence of optically thin disks (Hen 3-1191 and VV Ser) and two of optically thick disks (PDS 453 and MWC 297). Our results seem consistent with optically thick disks at near infrared bands, which are more likely to be associated with younger YSOs. Marginal evidence of polarization reversal is found in RY Tau, RY Ori, WW Vul, and UY Aur. In the first three cases, this feature can be associated with the UXOR phenomenon. Correlations with the IRAS colors and the spectral index yielded evidence of an evolutionary segregation in which the disks tend to be optically thin when they are older.

Polarimetry toward the IRAS Vela Shell. I. The Catalog

We have obtained linear polarization measurements of stars along the western side of the IRAS Vela Shell toward HD 62542. From 16 CCD fields distributed along the ionization front (I-front) we have built a catalog of 856 objects with polarization signal-to-noise ratio larger than 10. We detect very significant levels of polarization and hence an appreciable magnetic field throughout the region. Composite polarization maps around the I-front are shown. In some regions the polarization vectors are parallel to the I-front, but a perpendicular trend is also evident along parts of the front. In addition, the polarization pattern seems to be affected by gas streaming inside the cloud.

The Magnetic Field Structure of the LMC 2 Supershell: NGC 2100

We present U,B,V,R,I imaging polarimetry of NGC 2100 and its surrounding environment, which comprise a part of the LMC 2 supershell. The morphology of the observed position angle distribution provides a tracer of the projected magnetic field in this environment. Our polarization maps detail regions exhibiting similarly aligned polarization position angles, as well as more complex position angle patterns. We observe regions of coherent fields on spatial scales of 42 x 24 pc to 104 x 83 pc, and infer projected field strengths of

Optical polarimetric monitoring of the type II-plateau SN 2005af

\sim14-30\mu

Hα spectropolarimetry of the B[e] supergiant GG Carinae

G. We propose that the superposition of global outflows from the LMC 2 environment, as well as outflows created within NGC 2100, produce the unique field geometry in the region.

Polarimetry of Li-rich giants

Aims. Core-collapse supernovae may show significant polarization that implies non-spherically symmetric explosions. We observed the type II-plateau SN 2005af using optical polarimetry in order to verify whether any asphericity is present in the supernova temporal evolution. Methods. We used the IAGPOL imaging polarimeter to obtain optical linear polarization measurements in