Gary L. Grasdalen

The nature of the Herbig Ae- and Be-type stars associated with nebulosity.

A quantitative spectrophotometric study of the Herbig Ae and Be stars associated with nebulosity demonstrates that the majority have surface gravities appropriate to pre-main-sequence stars while the remainder have surface gravities characteristic of the zero-age main seee. This evidence, along with their gross spectroscopic and pc similarity to T Tauri-like stars and the fact that all these stars appear to be surrounded by circumstellar dust, places them among the youngest known stellar objects. The geometry of the circumstellar shells is discussed and the evidence seems to favor a nonspherical, possibly disklike shape. Finally, photographs taken with a 1-micron image-tube system reveal a large number of red stellar objects surrounding many of these stars. This suggests that the Herbig Ae and Be objects may be the brightest representatives of very recently formed stellar groups.-

A study of the stellar population in the Lynds 1641 dark cloud. I - The IRAS catalog sources

The character of the sources identified in the IRAS Point Source Catalog and located within the boundaries of the nearest giant molecular cloud, Lynds 1641 is discussed. New optical and near-infrared photometry are combined to provide spectral energy distributions (SEDs) for these objects divided into three classes: class I objects with flat or rising spectra, class II objects with spectra intermediate in slope between a flat and blackbody spectrum, and class III objects with spectra similar to those of blackbodies. It is found that L1641 contains a much larger percentage of class I sources than does the nearby Taurus-Auriga star-forming complex. Spectral energy distributions for the IRAS-selected sample are examined and compared with SEDs for young stellar objects (YSOs) located in Taurus-Auriga. The IRAS-selected sources having optical counterparts in the H-R diagram are identified and discussed along with the distribution of masses and ages for these YSOs.

The evolution of the dust shell of Nova Serpentis 1978

Photometric measurements of Nova Serpentis 1978 from V to 19.5 microns covering the first 240 days of the novas development are reported. A free-free expansion phase was followed by the condensation of a dust shell that became optically thick at both visible and infrared wavelengths. Carbon grains grew to radii of approximately 0.3 microns during the condensation period. The grains appeared to sputter or evaporate for a period of approximately 100 days following infrared maximum. It is found that the condensed carbon comprised 2% of the ejected mass, which implies that carbon abundance was enhanced in the nova shell. The angular expansion rate of the shell yields a distance of approximately 5 kpc and a luminosity of approximately 3000 solar luminosities for Nova Serpentis 1978.

The infrared coronal lines of recent novae

IR coronal line emissions are reported in the novae V1819 Cyg and V827 Her; high-resolution near-IR spectra of coronal line emission in the nova QU Vul have also been obtained, increasing the number of known IR coronal line novae from two to four. Each of the four is found to be characterized by approximately the same Si VI/Si VII line-intensity ratio, which indicates a common coronal-zone electron temperature of 300,000 K. The coronal-line novae are noted to possess a remarkably similar near-IR spectrum, suggesting that their coronal line emission will only occasionally be observed at optical wavelengths. 51 refs.

An optical and infrared study of the region surrounding Herbig-Haro objects 1 and 2

Optical and near-IR observations of the environs of H-H objects 1 and 2 provide the basis for identifying a recently discovered radio continuum as the star responsible for powering the highly collimated mass outflow traced by these objects. Polarimetric observations show that the source (designated as VLA 1), located at the midpoint between HH 1 and 2, illuminates a biconical reflection nebula. It is suggested that VLA 1 is probably surrounded by an optically thick disk viewed edge on; an optical jet emanates from the vicinity of HH 1 and is directed along the axis of the putative disk toward HH 1; its spectrum resembles that of HH 1 and other Herbig-Haro objects. 37 references.

ABSOLUTE gf-VALUES FOR Fe I AND Fe II LINES.

Abstract : Absolute gf-values for 34 lines of Fe I and 12 lines of Fe II were determined along with Van der Waals broadening coefficients. The relative and absolute scales of published gf-values are considered. (Author)

An infrared study of Orion Molecular Cloud-2 (OMC-2)

This paper reports 1.2-23 micron photometry for 11 discrete sources in Orion Molecular Cloud-2 (OMC-2). These data, combined with H and K photometric and K polarimetric images, are used to model the cluster sources. Most appear to be young stars of roughly solar mass. Some have circumstellar dust reradiation or reflection nebulosity. A model based on single scattering of light from an exciting star explains some features of the IRS 1 nebula, the largest reflection nebula in OMC-2. However, the red colors and high surface brightness of the IRS 1 nebula require a cool excitation source that is more luminous than far-infrared observations would indicate. 34 refs.

Images of star-forming regions. II. The circumstellar environment of L1551 IRS 5

The circumstellar environment of L 1551 IRS 5 is investigated based on high-resolution 8000 A (i) and 9000 A (z) broadband CCD images. A small conical reflection nebula extending from a bright semistellar knot located near the cusp of the nebula is noted. It is suggested that the point-like structure at lambda of less than about 3.7 microns may represent a bright knot of dust-scattered light located on the inner surface of a flattened circumstellar disk surrounding the radio source. Evidence is found of sharp changes in the position angle of the jet emanating from IRS 5, possibly resulting from precession of the jet nozzle provided by a dense inner disk which has dynamically decoupled from the much larger flattened molecular structure. 35 references.

The mysterious 10 micron emission feature in the spectrum of Nova Aquilae 1982

Broad-band 1.2-23 micron photometry and 1 percent resolution 7-14 micron spectrophotometry of Nova Aql 1982 are presented. This nova developed similarly to other dusty novae in that the appearance of thermal infrared emission from a circumstellar dust shell was coincident with a downward transition at visual wavelengths. A strong 10 micron emission feature appeared in the spectrum of Nova Aql 1982 during the dust shell development phase. This feature was not entirely similar to emission features previously observed in other stars with circumstellar shells. The contrast of the novas 10 micron feature was larger than those observed in the SiC-rich shells of carbon stars. On the other hand, although the nova emission feature was reminiscent of the 10 micron silicate emission feature observed in oxygen-rich supergiants, it peaked at a slightly longer wavelength and there was no corresponding 20 micron emission feature. The contrast of the feature increased with time. It is concluded that an anomalous dust grain component, tentatively identified as SiC, nucleated and grew in the circumstellar shell of Nova Aql 1982.

Massive star formation in NGC 6946

We present H..cap alpha.. measurements of the late-type spiral galaxy NGC 6946. Assuming an initial mass function, we use the H..cap alpha.. data to calculate the total star formation rate and efficiency of massive star formation as a function of galactocentric radius. We then attempt to determine the factors influencing these quantities by comparing them to the surface densities of H I and H/sub 2/ and the degree of compression suffered by the gas in spiral density wave theory. The rate of star formation is very closely correlated with the first power of the surface density of H/sub 2/, and somewhat less correlated with the degree of compression of the gas predicted by density wave theory. In contrast, the efficiency of massive star formation is roughly constant across the face of the galaxy. These results are consistent with results from previous studies of our own and other galaxies.