We present preliminary results from a high-resolution, high-sensitivity imaging survey of the northern galactic Hα emission. The survey is carried out using the Spectral Line Imaging Camera (SLIC) which incorporates a fast (f/1·2) lens attached to a cryogenic CCD in combination with a narrowband interference filter. The pixel size is 1·6 arcminutes and the diameter of each field is 10°. The fast optics, narrow bandpass (1·7 nm) filter, and high quantum-efficiency, low-noise CCD yield a high brightness sensitivity to Hα emission on arcminute scales. This gives an equivalent sensitivity to emission measure structure below 1 pc cm−6. Some faint features detected include a supershell connected with the star forming region W4 extending 7° above the galactic plane, and filaments possibly related to galactic loops II and III. In addition, we have carried out deep observations of fields in which anisotropies in the cosmic microwave background radiation have been detected. Our observations place stringent limits upon the contribution to the apparent microwave fluctuations from free–free emission in the galactic foreground.

From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10Њ diameter field centered on this region in the H␣ line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk. The shell appears to close 6Њ (or about 230 pc) above the star cluster, and at a Galactic latitude of 7Њ. The shell is quite elongated, with its major axis approximately perpendicular to the Galactic plane, as predicted for a superbubble formed in a stratified Galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl 352 (=2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.

We made a sensitive, wide-field H␣ image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the H␣ image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85Њ declination circle. We found no such irregularities that could be attributed to H␣ emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 K at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible. Subject headings: cosmic microwave background-instrumentation: miscellaneous-ISM: general

We have designed, constructed, and tested an optical polarimeter for use with the Virginia Military Institute (VMI) 0.5 m, f/13.5 Cassegrain telescope. Our instrument is based on the common dual-beam design that utilizes a rotatable half-wave plate and Wollaston prism to image starlight onto a CCD detector after it has passed through a broadband filter. The usable field of view is ≲10′′ and the operational range of the instrument is 400–700 nm. Measurements of unpolarized stars demonstrate that the instrumental polarization is ≲0.05%. Observations of seven standard stars were in agreement with their accepted values by an order of Δp(%) ≲ 0.23 for the degree of polarization and Δθ(°) ≲ 0.94 for the position angle.

Aims. NGC 1502 is located at the periphery of the Cam OB1 association and probably within the Orion Spur, with published distance estimates varying between 0.7 and 1.5 kpc. We combine new polarimetric observations and existing uvbyβ and UBV photometries to provide new estimates of the cluster’s parameters. Methods. We present new multi-wavelength polarization observations of 22 stars in the direction of the cluster and, using the wavelength of maximum polarization, calculate the total-to-selective extinction ratio for each of these stars. These are combined with homogenized uvbyβ and UBV photometries to obtain the individual stellar distances and study the structure of the field. Results. We found no evidence of significant extinction variation across the cluster and obtained an average color excess E(b − y) = 0.56 ± 0.02 mag, corresponding to E(B − V) = 0.76 mag. Given the uniformity of both pmax and λmax toward the cluster and the distribution of the color excess with distance, it seems likely that the polarization in this region arises from aligned dust grains in the foreground of the cluster, not farther than 500–600 pc. Based on 20 stars located beyond 500 pc within the studied field of view, we obtained an average polarization-based total-to-selective extinction ratio R = 2.83 ± 0.14 yielding visual absorption AV = 2.14 ± 0.16 mag. Using 11 stars with uvbyβ data that represent the main sequence of the cluster, we calculated a photometric distance of 1117−89+96 pc. This estimate is fully supported by the UBV -based analysis and the new individual membership probabilities that we derive. Assuming a solar metallicity yields an age of 5 Myr. Our analysis yields controversial results regarding the membership of the stars associated with the SZ Cam system (HD 25638 and HD 25639) to the cluster.

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NGC 457 is a moderately reddened and relatively young open cluster very rich in variable stars, particularly Be stars, but not associated with any prominent star-forming field. We combine new multi-wavelength polarization measurements with existing UBV and uvby photometries to obtain precise estimates of the total-to-selective extinction in the field of the cluster and re-evaluate its distance and age. The polarization measurements show a tight alignment of the polarization vectors with the Galactic plane and yield an average value of the total-to-selective extinction of 3.05±0.17 for the cluster's field. Using this value and the confirmed color excess E B V 0.500 0.030) mag, we obtain a distance modulus of 12.20±0.39, corresponding to 2.75±0.49 kpc and, assuming slightly sub-solar metallicity, an age of 15.8 Myr.