124 research outputs found
BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution
We present a radiative-transfer calculation which reproduces the infrared
spectrum of the planetary nebula BD~+303639. We calculate the
transfer process through absorption and scattering in a spherical-symmetric
multi-grain dust shell. The emission of transiently heated particles is taken
into account, as well as polycyclic aromatic hydrocarbons. We obtain an
acceptable fit to most of the spectrum, including the PAH infrared bands. At
submillimetre wavelengths the observed emission is larger than the model
predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be
needed as an additional constituent. The fit favours a distance of kpc, which implies that BD~+303639 has evolved from a massive
progenitor of several solar masses. A low dust-to-gas mass ratio is found in
the ionised region. The calculations yield an original mass-loss rate of
2\times10^{-4} \msolar \peryr on the Asymptotic Giant Branch. Using this
mass-loss rate, we calculate how the infrared spectrum has evolved during the
post-AGB evolution. We show in particular the evolution of the IRAS colours
during the preceding post-AGB evolution.Comment: accepted for publication in MNRAS. LaTeX, 15 pages, hardcopy and 8
figures available from [email protected] or [email protected]
Radio continuum monitoring of the extreme carbon star IRC+10216
We describe Very Large Array observations of the extreme carbon star
IRC+10216 at 8.4, 14.9, and 22.5 GHz made over a two year period. We find
possible variability correlated with the infrared phase and a cm- to
sub-millimeter wavelength spectral index very close to 2. The variability,
observed flux densities, and upper limit on the size are consistent with the
emission arising from the stellar photosphere or a slightly larger radio
photosphere.Comment: 9 pages, incl. 4 figures. To appear in Astronomy & Astrophysic
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
7âhydroxymitragynine is an active metabolite of mitragynine and a key mediator of its analgesic effects
Mitragynina speciosa, more commonly known as kratom, is a
plant native to Southeast Asia, the leaves of which have been used
traditionally as a stimulant, analgesic, and treatment for opioid addiction. Recently,
growing use of the plant in the United States and concerns that kratom
represents an uncontrolled drug with potential abuse liability, have
highlighted the need for more careful study of its pharmacological activity. The
major active alkaloid found in kratom, mitragynine, has been reported to have
opioid agonist and analgesic activity in vitro
and in animal models, consistent with the purported effects of kratom leaf in
humans. However, preliminary research has provided some evidence that
mitragynine and related compounds may act as atypical opioid agonists, inducing
therapeutic effects such as analgesia, while limiting the negative side effects
typical of classical opioids. Here we report evidence that an active metabolite
plays an important role in mediating the analgesic effects of mitragynine. We
find that mitragynine is converted in
vitro in both mouse and human liver preparations to the much more potent
mu-opioid receptor agonist 7-hydroxymitragynine, and that this conversion is
mediated by cytochrome P450 3A isoforms. Further, we show that 7-hydroxymitragynine
is formed from mitragynine in mice and that brain concentrations of this
metabolite are sufficient to explain most or all of the opioid-receptor-mediated
analgesic activity of mitragynine. At the same time, mitragynine is found in the
brains of mice at very high concentrations relative to its opioid receptor
binding affinity, suggesting that it does not directly activate opioid
receptors. The results presented here provide a metabolism-dependent mechanism
for the analgesic effects of mitragynine and clarify the importance of route of
administration for determining the activity of this compound. Further, they
raise important questions about the interpretation of existing data on
mitragynine and highlight critical areas for further research in animals and
humans.</p
OH Satellite Line Masers and an AGN Candidate in M82
We report the detection of OH satellite line masers at 1720 MHz and 1612 MHz
from the nuclear region of the starburst galaxy M82. The observations were
aimed at detecting 1720 MHz maser emission from the known radio emitting SNR in
the nuclear region. At 1720 MHz we detect six features above the 5-sigma limit
set by noise, four in emission and two in absorption. Three of the emission
features appear closely associated with known discrete continuum radio sources,
and one of the absorption features is precisely coincident with the discrete
continuum source 44.01+59.6. The latter source also exhibits strong 1612 MHz
emission at the same velocity. No other 1612 MHz features were detected. All of
the 1720 MHz emission features are consistent with masers pumped by collisions
with molecular hydrogen at densities between 10^3 cm^{-3} and 10^5 cm^{-3}, and
T_k between 50 K and 250 K. The absorption and emission associated with the two
satellite lines in 44.01+59.6, together with other evidence, points to the
possibility that this source is the AGN in M82.Comment: accepted ApJ
High-excitation OH and H_2O lines in Markarian 231: the molecular signatures of compact far-infrared continuum sources
The ISO/LWS far-infrared spectrum of the ultraluminous galaxy Mkn 231 shows
OH and H_2O lines in absorption from energy levels up to 300 K above the ground
state, and emission in the [O I] 63 micron and [C II] 158 micron lines. Our
analysis shows that OH and H_2O are radiatively pumped by the far-infrared
continuum emission of the galaxy. The absorptions in the high-excitation lines
require high far-infrared radiation densities, allowing us to constrain the
properties of the underlying continuum source. The bulk of the far-infrared
continuum arises from a warm (T_dust=70-100 K), optically thick
(tau_100micron=1-2) medium of effective diameter 200-400 pc. In our best-fit
model of total luminosity L_IR, the observed OH and H2O high-lying lines arise
from a luminous (L/L_IR~0.56) region with radius ~100 pc. The high surface
brightness of this component suggests that its infrared emission is dominated
by the AGN. The derived column densities N(OH)>~10^{17} cm^{-2} and
N(H_2O)>~6x10^{16} cm^{-2} may indicate XDR chemistry, although significant
starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 micron,
and [O I] 63 micron lines arise from a more extended (~350 pc) starburst
region. We show that the [C II] deficit in Mkn 231 is compatible with a high
average abundance of C+ because of an extreme overall luminosity to gas mass
ratio. Therefore, a [C II] deficit may indicate a significant contribution to
the luminosity by an AGN, and/or by extremely efficient star formation.Comment: 16 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Cold and warm dust along a merging galaxy sequence
We investigate the cold and warm dust properties during galaxy interactions
using a merging galaxy sample ordered into a chronological sequence from pre-
to post-mergers. Our sample comprises a total of 29 merging systems selected to
have far-infrared and sub-millimeter observations. The sub-millimeter data are
mainly culled from the literature while for 5 galaxies (NGC 3597, NGC 3690, NGC
6090, NGC 6670 and NGC 7252) the sub-millimeter observations are presented here
for the first time. We use the 100-to-850 micron flux density ratio,
f_{100}/f_{850}, as a proxy to the mass fraction of the warm and the cold dust
in these systems. We find evidence for an increase in f_{100}/f_{850} along the
merging sequence from early to advanced mergers and interpret this trend as an
increase of the warm relative to the cold dust mass. We argue that the two key
parameters affecting the f_{100}/f_{850} flux ratio is the star-formation rate
and the dust content of individual systems relative to the stars. Using a
sophisticated model for the absorption and re-emission of the stellar UV
radiation by dust we show that these parameters can indeed explain both the
increase and the observed scatter in the f_{100}/f_{850} along the merging
galaxy sequence. We also discuss our results under the hypothesis that
elliptical galaxies are formed via disc galaxy mergers.Comment: Accepted for publication in MNRA
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