The proto-planetary Red Rectangle nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year,
eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the H-alpha line profile around the orbital phase of superior conjunction reveal the secondary component to be the origin of the fast (max. v~560$ km s$^{-1}$) bipolar outflow in the Red Rectangle. The variation of total H-alpha flux from the central H II region with orbital phase also identifies the secondary or its surroundings as the source of the far-ultraviolet ionizing radiation in the system. The estimated mass of the secondary (~0.94 M$sun$) and the speed of the outflow suggest that this component is a main sequence star and not a white dwarf, as previously suggested. We identify the source of the Lyman/far-ultraviolet continuum in the system as the hot, inner region (T$_{max} ge 17,000$ K) of an accretion disk surrounding the secondary, fed by Roche lobe overflow from the post-AGB primary at a rate of about $2 - 5times10^{-5}$ M$sun$ yr$^{-1}$. The total luminosity of the accretion disk around the secondary is estimated to be at least 300 L$sun$, about 5% of the luminosity of the entire system. (abridged)
We present initial results and source lists of variable sources in the Large Magellanic Cloud (LMC) for which we detect thermal infrared variability from the SAGE (Surveying the Agents of a Galaxys Evolution) survey, which had 2 epochs of photometry
separated by three months. The SAGE survey mapped a 7 degree by 7 degree region of the LMC using the IRAC and the MIPS instruments on board Spitzer. Variable sources are identified using a combination of the IRAC 3.6, 4.5, 5.8, 8.0 micron bands and the MIPS 24 micron bands. An error-weighted flux difference between the two epochs is used to assess the variability. Of the ~ 3 million sources detected at both epochs we find ~ 2,000 variable sources for which we provide electronic catalogs. Most of the variable sources can be classified as asymptotic giant branch (AGB) stars. A large fraction (> 66%) of the extreme AGB stars are variable and only smaller fractions of carbon-rich (6.1%) and oxygen-rich (2.0%) stars are detected as variable. We also detect a population of variable young stellar object candidates.
Nearby interstellar clouds at high Galactic latitudes are ideal objects in which the interaction of interstellar dust with photons from the well-characterized interstellar radiation field can be studied. Scattering and UV-excited photoluminescence at
optical wavelengths as well as thermal emission at mid- and far-infrared wavelengths are observable manifestations of such interactions. Here we report initial results from an optical imaging survey of optically thin high-Galactic-latitude clouds, which is designed to study the surface brightness, structure, and spectral energy distribution of these objects. The primary aim of this paper is to study the extended red emission (ERE) that has been reported at high Galactic latitudes in earlier investigations and which is attributed to ultraviolet-excited photoluminescence of an as yet unidentified component of interstellar dust. We find strong evidence for dust emission in the form of a broad (>1000 A FWHM) ERE band with peak emission near 600 nm wavelength and peak intensity of ~ 5x10^-9 (erg cm^-2 s^-1 A^-1 sr^-1) in optically-thin clouds. This amounts to about 30% of the total optical surface brightness of these clouds, the remainder being consistent with expectations for dust-scattered light. This supports claims for the ubiquitous presence of the ERE carrier throughout the diffuse interstellar medium of the Milky Way Galaxy. We suggest that the ERE carrier is involved in the radiative processing of about 20% to 30% of the dust-absorbed UV/optical luminosity of the Milky Way galaxy, with the bulk of this energy being emitted in the near- to mid-infrared spectral regions.
