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Register a new userExtended Red Emission in High-Galactic Latitude Interstellar Clouds
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A. N. Witt
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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.
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Interstellar dust in nebulae and in the diffuse interstellar medium of galaxies contains a component which responds to illumination by ultraviolet photons with efficient luminescence in the 500 nm to 1000 nm spectral range, known as Extended Red Emission (ERE). We review the techniques of detection and the observational characteristics of the ERE in a wide range of astrophysical environments. We then discuss results of the analysis of ERE observations, leading to a set of specific constraints that any proposal for the ERE carrier must confront. Finally, we review specific models that have been advanced over the past two decades to explain the ERE phenomenon. Despite promising progress on several fronts, no completely satisfactory model for the ERE carrier/process exists at this time.
In this chapter we review the young stars and molecular clouds found at high Galactic latitudes $(|b| ge 30^circ)$. These are mostly associated with two large-scale structures on the sky, the Gould Belt and the Taurus star formation region, and a handful of molecular clouds including MBM 12 and MBM 20 which, as a population, consist of the nearest star formation sites to our Sun. There are also a few young stars that are found in apparent isolation far from any molecular cloud. The high latitude clouds are primarily translucent molecular clouds and diffuse Galactic cirrus with the majority of them seen at high latitude simply due to their proximity to the Sun. The rare exceptions are those, like the Draco and other intermediate or high velocity clouds, found significantly above or below the Galactic plane. We review the processes that result in star formation within these low density and extraplanar environments as well as the mechanisms for production of isolated T Tauri stars. We present and discuss the known high-latitude stellar nurseries and young stellar objects.
Based on the accurate color excess $E_{rm G_{BP},G_{RP}}$ of more than 4 million stars and $E_{rm NUV,G_{BP}}$ of more than 1 million stars from citet{2021ApJS..254...38S}, the distance and the extinction of the molecular clouds in the MBM catalog at $|b|>20^{circ}$ are studied in combination with the distance measurement of emph{Gaia}/EDR3. The distance as well as the color excess is determined for 66 molecular clouds. The color excess ratio $E_{rm G_{BP},G_{RP}}/E_{rm NUV,G_{BP}}$ is derived for 39 of them, which is obviously larger and implies more small particles at smaller extinction. In addition, the scale height of the dust disk is found to be about 100 pc and becomes large at the anticenter direction due to the disk flaring.
PASIPHAE (the Polar-Areas Stellar Imaging in Polarization High-Accuracy Experiment) is an optopolarimetric survey aiming to measure the linear polarization from millions of stars, and use these to create a three-dimensional tomographic map of the magnetic field threading dust clouds within the Milky Way. This map will provide invaluable information for future CMB B-mode experiments searching for inflationary gravitational waves, providing unique information regarding line-of-sight integration effects. Optical polarization observations of a large number of stars at known distances, tracing the same dust that emits polarized microwaves, can map the magnetic field between them. The Gaia mission is measuring distances to a billion stars, providing an opportunity to produce a tomographic map of Galactic magnetic field directions, using optical polarization of starlight. Such a map will not only boost CMB polarization foreground removal, but it will also have a profound impact in a wide range of astrophysical research, including interstellar medium physics, high-energy astrophysics, and evolution of the Galaxy. Taking advantage of the novel technology implemented in our high-accuracy Wide-Area Linear Optical Polarimeters (WALOPs) currently under construction at IUCAA, India, we will engage in a large-scale optopolarimetric program that can meet this challenge: a survey of both northern and southern Galactic polar regions targeted by CMB experiments, covering over 10,000 square degrees, which will measure linear optical polarization of over 360 stars per square degree (over 3.5 million stars, a 1000-fold increase over the state of the art). The survey will be conducted concurrently from the South African Astronomical Observatory in Sutherland, South Africa in the southern hemisphere, and the Skinakas Observatory in Crete, Greece, in the north.
We have found a class of circular radio objects in the Evolutionary Map of the Universe Pilot Survey, using the Australian Square Kilometre Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs, about one arcmin diameter, that are unlike other objects previously reported in the literature. We explore several possible mechanisms that might cause these objects, but none seems to be a compelling explanation.
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