455 research outputs found
The 11.2 m emission of PAHs in astrophysical objects
The 11.2 m emission band belongs to the family of the `Unidentified'
Infrared (UIR) emission bands seen in many astronomical environments. In this
work we present a theoretical interpretation of the band characteristics and
profile variation for a number of astrophysical sources in which the carriers
are subject to a range of physical conditions. The results of Density
Functional Theory (DFT) calculations for the solo out-of-plane (OOP)
vibrational bending modes of large polycyclic aromatic hydrocarbon (PAH)
molecules are used as input for a detailed emission model which includes the
temperature and mass dependence of PAH band wavelength, and a PAH mass
distribution that varies with object. Comparison of the model with astronomical
spectra indicates that the 11.2 m band asymmetry and profile variation can
be explained principally in terms of the mass distribution of neutral PAHs with
a small contribution from anharmonic effects.Comment: 13 pages, 10 figures, 3 table. Accepted for publication in MNRA
Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 {\mu}m band
In this Letter we report the results of density functional theory
calculations on medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH)
molecules with armchair edges. These PAH molecules possess strong C-H
stretching and bending modes around 3 {\mu}m and in the fingerprint region
(10-15 {\mu}m), and also strong ring deformation modes around 12.7 {\mu}m.
Perusal of the entries in the NASA Ames PAHs Database shows that ring
deformation modes of PAHs are common - although generally weak. We then propose
that armchair PAHs with NC >65 are responsible for the 12.7 {\mu}m Aromatic
Infrared Band in HII regions and discuss astrophysical implications in the
context of the PAH life-cycle.Comment: Minor editin
On the polarisation of the Red Rectangle optical emission bands
The origin of the narrow optical emission bands seen toward the Red Rectangle
is not yet understood. In this paper we investigate further the proposal that
these are due to luminescence of large carbonaceous molecules. Polarised
signals of several percent could be expected from certain asymmetric molecular
rotators. The ESPaDOnS echelle spectrograph mounted at the CFHT was used to
obtain high-resolution optical spectropolarimetric data of the Red Rectangle
nebular emission. The RRBs at 5800, 5850, and 6615 Angstrom are detected in
spectra of the nebular emission 7" and 13" North-East from the central star.
The 5826 and 6635 Angstrom RRB are detected only at the position nearest to the
central star. For both positions the Stokes Q and U spectra show no unambiguous
polarisation signal in any of the RRBs. We derive an upper limit of 0.02% line
polarisation for these RRBs. A tentative feature with peak polarisation of
0.05% is seen for the 5800 RRB at 7" offset. However, the Null spectra suggest
that this may be an instrumental artifact. The lack of a clear polarisation
signal for the five detected RRBs implies that, if the emission is due to
luminescence of complex organics, these gas-phase molecular carriers are likely
to have a high degree of symmetry, as they do not exhibit a Q-branch in their
rotational profile, although this may be modified by statistical effects.Comment: 5 pages, 4 figures, accepted for A&
Sequence structure emission in The Red Rectangle Bands
We report high resolution (R~37,000) integral field spectroscopy of the
central region (r<14arcsec) of the Red Rectangle nebula surrounding HD44179.
The observations focus on the 5800A emission feature, the bluest of the
yellow/red emission bands in the Red Rectangle. We propose that the emission
feature, widely believed to be a molecular emission band, is not a molecular
rotation contour, but a vibrational contour caused by overlapping sequence
bands from a molecule with an extended chromophore. We model the feature as
arising in a Polycyclic Aromatic Hydrocarbon (PAH) with 45-100 carbon atoms.Comment: 13 pages, 9 figures, accepted for publication in ApJ. A version of
the paper with full resolution figures is available at:
http://www.aao.gov.au/local/www/rgs/Sequence-Structure
Linear/circular spectropolarimetry of diffuse interstellar bands
Context. The identification of the carriers of diffuse interstellar bands
(DIBs) remains one of the long-standing mysteries in astronomy. The detection
of a polarisation signal in a DIB profile can be used to distinguish between a
dust or gas-phase carrier. The polarisation profile can give additional
information on the grain or molecular properties of the absorber. In order to
detect and measure the linear and circular polarisation of the DIBs we observed
reddened lines of sight showing continuum polarisation. For this study we
selected two stars HD 197770 and HD 194279. We used high-resolution (R~64.000)
spectropolarimetry in the wavelength range from 3700 to 10480 Angstrom with the
ESPaDOnS echelle spectrograph mounted at the CFHT.
Results. High S/N and high resolution Stokes V (circular), Q and U (linear)
spectra were obtained. We constrained upper limits by a factor of 10 for
previously observed DIBs. Furthermore, we analysed ~30 additional DIBs for
which no spectropolarimetry data has been obtained before. This included the
9577 A DIB and the 8621 A DIB.
Conclusions. The lack of polarisation in 45 DIB profiles suggests that none
of the absorption lines is induced by a grain-type carrier. The strict upper
limits, less than ~0.01%, derived for the observed lines-of-sight imply that if
DIBs are due to gas-phase molecules these carriers have polarisation
efficiencies which are at least 6 times, and up to 300 times, smaller than
those predicted for grain-related carriers.Comment: 6 pages + 13 pages online material, submitted to A&
Unusually Weak Diffuse Interstellar Bands toward HD 62542
As part of an extensive survey of diffuse interstellar bands (DIBs), we have
obtained optical spectra of the moderately reddened B5V star HD 62542, which is
known to have an unusual UV extinction curve of the type usually identified
with dark clouds. The typically strongest of the commonly catalogued DIBs
covered by the spectra -- those at 5780, 5797, 6270, 6284, and 6614 A -- are
essentially absent in this line of sight, in marked contrast with other lines
of sight of similar reddening. We compare the HD 62542 line of sight with
others exhibiting a range of extinction properties and molecular abundances and
interpret the weakness of the DIBs as an extreme case of deficient DIB
formation in a dense cloud whose more diffuse outer layers have been stripped
away. We comment on the challenges these observations pose for identifying the
carriers of the diffuse bands.Comment: 20 pages, 4 figures; aastex; accepted by Ap
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