27 research outputs found
Discovery of Blue Luminescence in the Red Rectangle: Possible Fluorescence from Neutral Polycyclic Aromatic Hydrocarbon Molecules?
Here we report our discovery of a band of blue luminescence (BL) in the Red
Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the
brightest known sources of extended red emission as well as of unidentified
infra-red (UIR) band emissions. The spectrum of this newly discovered BL is
most likely fluorescence from small neutral polycyclic aromatic hydrocarbon
(PAH) molecules. PAH molecules are thought to be widely present in many
interstellar and circumstellar environments in our galaxy as well as in other
galaxies, and are considered likely carriers of the UIR-band emission. However,
no specific PAH molecule has yet been identified in a source outside the solar
system, as the set of mid-infra-red emission features attributed to these
molecules between the wavelengths of 3.3 micron and 16.4 micron is largely
insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs
is more specific as to size, structure, and charge state of a PAH molecule. If
the carriers of this near-UV/blue fluorescence are PAHs, they are most likely
neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene
(C14H10) and pyrene (C16H10). These small PAHs would then be the largest
molecules specifically identified in the interstellar medium.Comment: 4 pages, 4 figures, Accepted for publication in ApJL (LaTeX, uses
emulateapj.sty
A COMPLETE SPECTROSCOPIC MAP AND NARROW-BAND IMAGING OF SMALL PAHS IN THE RED RECTANGLE NEBULA
Author Institution: Ritter Astrophysical Research Center, Univeristy of Toledo, Toledo, OH 43606; University of Chicago, Chicago, IL 60637; NASA Goddard Space Flight Center, Greenbelt, MD 20771; McDonald Observatory, University of Texas at Austin, Austin, TX 78712Electronic flurescence spectra (peak 375 nm) in reflection nebulae have helped to identify the largest molecules that have been detected so far in the interstellar medium: 3-4 ringed polycyclic aromatic hydrocarbons (PAHs). This detection of blue luminescence (BL) by small, neutral PAHs was first made in the peculiar, proto-planetary nebula, the Red Rectangle. This first detection and subsequent observations in other reflection nebulae reveal spatial variations in the BL spectrum indicating a change in the size distribution/ionisation state of the emitters. Data from an ongoing, complete spectroscopic survey and narrow-band imaging of the Red Rectangle will be presented. This study sheds light on the spatial distribution, ionization state and the size distributions of the small PAHs in this nebula
The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons
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^{-1}\sun_{max} \ge 17,0002 -
5\times10^{-5}\sun^{-1}\sun$, about 5% of the
luminosity of the entire system. (abridged)Comment: Accepted for publication in Ap
The Dust in Lyman Break Galaxies
We present our analysis of UV attenuation by internal dust of a large sample
(N=906 galaxies) of Lyman Break Galaxies (LBGs). Using spectral energy
distributions (SEDs) from the P\'EGASE galaxy spectral evolution model we apply
dust attenuation corrections to the G-R colors using the Witt & Gordon (2000)
models for radiative transfer in dusty galactic environments to arrive at the
UV attenuation factors. We show that the dust in the LBGs exhibit SMC-like
characteristics rather than MW-like, and that the dust geometry in these
systems is most likely to be represented by a clumpy shell configuration. We
show that the attenuation factor exhibits a pronounced dependence on the
luminosity of the LBG, a_{1600}\propto (L/L_\sun)^\alpha, where
. The exponent depends on the initial parameters
of the stellar population chosen to model the galaxies and the dust properties.
We find that the luminosity weighted average attenuation factor is likely to be
in the range from , which is consistent with the upper limits to the
star formation rate at set by the FIR background. This implies that the
current UV/optical surveys do detect the bulk of the star formation during the
epoch , but require substantial correction for internal dust
attenuation.Comment: 17 pages, 12 figures, uses AASTEX, accepted for publication in the
Astrophysical Journa
Optical Emission Band Morphologies of the Red Rectangle
We present narrow-band images of the Red Rectangle (RR) nebula which reveal
the distinct morphologies of this intriguing nebula in different optical
emission bands. The morphology of the RR nebula in blue luminescence (BL) and
extended red emission (ERE) are almost mutually exclusive. We also present the
optical detection of the circum-binary disk of the RR in the light of the BL.
The total intensities from the two optical band emissions (BL and ERE) when
summed over the nebula are of comparable magnitude. Their spatial distributions
with respect to the embedded illumination sources lead us to suggest that they
may be attributed to different ionization stages of the same family of
carriers.Comment: Accepted to Ap
On the Origins of the High-Latitude H-alpha Background
The diffuse high-latitude H-alpha background is widely believed to be
predominantly the result of in-situ recombination of ionized hydrogen in the
warm interstellar medium of the Galaxy. Instead, we show that both a
substantial fraction of the diffuse high-latitude H-alpha intensity in regions
dominated by Galactic cirrus dust and much of the variance in the high-latitude
H-alpha background are the result of scattering by interstellar dust of H-alpha
photons originating elsewhere in the Galaxy. We provide an empirical relation,
which relates the expected scattered H-alpha intensity to the IRAS 100um
diffuse background intensity, applicable to about 81% of the entire sky. The
assumption commonly made in reductions of CMB observations, namely that the
observed all-sky map of diffuse H-alpha light is a suitable template for
Galactic free-free foreground emission, is found to be in need of
reexamination.Comment: 26 pages, 5 figures, Accepted for publication in Ap
The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing the
extended red emission (ERE) in many dusty interstellar environments remains
unidentified. Several competing models are more or less able to match the
observed broad, unstructured ERE band. We now constrain the character of the
ERE carrier further by determining the wavelengths of the radiation that
initiates the ERE. Using the imaging capabilities of the Hubble Space
Telescope, we have resolved the width of narrow ERE filaments appearing on the
surfaces of externally illuminated molecular clouds in the bright reflection
nebula NGC 7023 and compared them with the depth of penetration of radiation of
known wavelengths into the same cloud surfaces. We identify photons with
wavelengths shortward of 118 nm as the source of ERE initiation, not to be
confused with ERE excitation, however. There are strong indications from the
well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic
cirrus that the photon flux with wavelengths shortward of 118 nm is too small
to actually excite the observed ERE, even with 100% quantum efficiency. We
conclude, therefore, that ERE excitation results from a two-step process. While
none of the previously proposed ERE models can match these new constraints, we
note that under interstellar conditions most polycyclic aromatic hydrocarbon
(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV
and that the electronic energy level structure of PAH di-cations is consistent
with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations
deserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap
Variable Evolved Stars and YSOs Discovered in the Large Magellanic Cloud using the SAGE Survey
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 Galaxy's 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.Comment: Accepted for publication in A