321 research outputs found
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
The large- and small-scale Ca IIK structure of the Milky Way from observations of Galactic and Magellanic sightlines
Aims: By utilising spectra of early-type stellar probes of known distances in
the same region of the sky, the large and small-scale (pc) structure of the
Galactic ISM can be investigated. This paper determines the variation in line
strength of CaII at 3933.661 A, as a function of probe separation for a sample
of stars, including many sightlines in the Magellanic Clouds. Methods:
FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic &
4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column
density and line width of IS Galactic Ca for a total of 657 stars, of which 443
are Magellanic sightlines. In each cluster there are 43-110 stars observed.
Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic
early-type stars are presented and combined with data from the literature to
study the Ca column density/parallax relationship. Results: For the four
Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K
EW over transverse scales from 0.05-9 pc is found to vary by factors of
1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in the
optically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, the
min and max reduced EW for MW gas is found to lie in the range 35-125 mA &
30-160 mA for CaII K and NaI D, respectively. The range is consistent with a
simple model of the ISM published by van Loon et al. (2009) consisting of
spherical cloudlets of filling factor 0.3, although other geometries are not
ruled out. Finally, the derived functional form for parallax and CaII column
density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived
parallax is 25 per cent lower than predicted by Megier et al. (2009) at a
distance of 100 pc and 15% lower at a distance of 200 pc, reflecting
inhomogeneity in the CaII distribution in the different sightlines studied.Comment: The full version including online material is available via the
Astronomy and Astrophysics website
http://www.aanda.org/articles/aa/olm/2015/10/aa25190-14/aa25190-14.htm
CaII K observations of QSOs in the line-of-sight to the Magellanic Bridge
We describe medium-resolution spectroscopic observations taken with the ESO
Multi-Mode Instrument in the CaII K line (3933.661 Angstroms) towards 7 QSOs
located in the line-of-sight to the Magellanic Bridge. At a spectral resolution
R = 6,000, five of the sightlines have a signal-to-noise ratio of 20 or higher.
Definite Ca absorption due to Bridge material is detected towards 3 objects,
with probable detection towards two other sightlines. Gas-phase CaII K Bridge
and Milky Way abundances or lower limits for the all sightlines are estimated
by the use of Parkes 21-cm HI emission line data. These data only have a
spatial resolution of 14 arcminutes compared with the optical observations
which have milli-arcsecond resolution. With this caveat, for the three objects
with sound CaII K detections, we find that the ionic abundance of CaII K
relative to HI, A=log(N(CaK)/N(HI)) for low-velocity Galactic gas ranges from
-8.3 to -8.8 dex, with HI column densities varying from 3-6x10^20 cm^-2. For
Magellanic Bridge gas, the values of A are 0.5 dex higher, ranging from -7.8 to
-8.2 dex, with N(HI)=1-5x10^20 cm^-2. Higher values of A correspond to lower
values of N(HI), although numbers are small. For the sightline towards
B0251--675, the Bridge gas has two different velocities, and in only one of
these is CaII K tentatively detected, perhaps indicating gas of a different
origin or present-day characteristics (such as dust content), although this
conclusion is uncertain and there is the possibility that one of the components
could be related to the Magellanic Stream. Higher signal-to-noise CaII K data
and higher resolution HI data are required to determine whether A changes with
N(HI) over the Bridge and if the implied difference in the metalicity of the
two Bridge components towards B0251-675 is real.Comment: Astronomy and Astrophysics, in pres
Making FORS2 fit for exoplanet observations (again)
For about three years, it was known that precision spectrophotometry with
FORS2 suffered from systematic errors that made quantitative observations of
planetary transits impossible. We identified the Longitudinal Atmospheric
Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged
in a project to exchange the LADC prisms with the uncoated ones from FORS1.
This led to a significant improvement in the depth of FORS2 zero points, a
reduction in the systematic noise, and should make FORS2 again competitive for
transmission spectroscopy of exoplanets.Comment: To appear in the March issue of the ESO Messenge
HI and star formation in the most metal-deficient galaxies
We present Giant Metrewave Radio Telescope (GMRT) observations for three
(viz., DDO 68, SDSS J2104-0035 and UGC 772) of the six most metal-deficient
actively star-forming galaxies known. Although there is a debate as to whether
these galaxies are undergoing their first episode of star formation or not,
they are `young' in the sense that their ISM is chemically unevolved. In this
regard, they are the nearest equivalents of young galaxies in the early
Universe. All three galaxies, that we have observed, have irregular HI
morphologies and kinematics, which we interpret as either due to tidal
interaction with neighbouring galaxies, or the consequences of a recent merger.
The remaining three of the six most metal-deficient galaxies are also known to
have highly disturbed HI distributions and are interacting. It is interesting
because these galaxies were chosen solely on the basis of their metallicity and
not for any particular signs of interaction. In this sense (i.e., their gas has
not yet had time to settle into a regular disc), one could regard these
extremely metal deficient (XMD) galaxies as `young'. The current star formation
episode is likely to have been triggered by interaction/merger. It is also
possible that the tidal interaction has lead to enhanced mixing with metal-poor
gas in outer disc, and hence to a low gas-phase metallicity in the central
star-forming regions. We also find that in general these galaxies do not show a
one-to-one correspondence between regions of high HI column density and regions
with current star formation. However, to the extent that one can define a
threshold density, its value (~10^{21} atoms cm^{-2}) is similar to that in
galaxies with much higher metallicity.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
Large Interstellar Polarisation Survey. LIPS I: FORS2 spectropolarimetry in the Southern Hemisphere
Polarimetric studies of light transmitted through interstellar clouds may
give constraints on the properties of the interstellar dust grains.
Traditionally, broadband linear polarisation (BBLP) measurements have been
considered an important diagnostic tool for the study of the interstellar dust,
while comparatively less attention has been paid to spectropolarimetric
measurements. However, spectropolarimetry may offer stronger constraints than
BBLP, for example by revealing narrowband features, and by allowing us to
distinguish the contribution of dust from the contribution of interstellar gas.
Therefore, we have decided to carry out a Large Interstellar Polarisation
Survey (LIPS) using spectropolarimetric facilities in both hemispheres. Here we
present the results obtained in the Southern Hemisphere with the FORS2
instrument of the ESO Very Large Telescope. Our spectra cover the wavelength
range 380--950\,nm at a spectral resolving power of about 880. We have produced
a publicly available catalogue of 127 linear polarisation spectra of 101
targets. We also provide the Serkowski-curve parameters, as well as the
wavelength gradient of the polarisation position angle for the interstellar
polarisation along 76 different lines of sight. In agreement with previous
literature, we found that the best-fit parameters of the Serkowski-curve are
not independent of each other. However, the relationships that we obtained are
not always consistent with what was found in previous studies.Comment: Accepted by A&A (replaced on 12 October 2017 simply to correct a
Metadata error
Molecular dark matter in galaxies
Clouds containing molecular dark matter in quantities relevant for star
formation may exist in minihaloes of the type of cold dark matter included in
many cosmological simulations or in the regions of some galaxies extending far
beyond their currently known boundaries. We have systematically explored
parameter space to identify conditions under which plane-parallel clouds
contain sufficient column densities of molecular dark matter that they could be
reservoirs for future star formation. Such clouds would be undetected or at
least appear by current observational criteria to be uninteresting from the
perspective of star formation. We use a time-dependent PDR code to produce
theoretical models of the chemistry and emission arising in clouds for our
chosen region of parameter space. We then select a subset of model clouds with
levels of emission that are low enough to be undetectable or at least
overlooked by current surveys. The existence of significant column densities of
cold molecular dark matter requires that the background radiation field be
several or more orders of magnitude weaker than that in the solar
neighbourhood. Lower turbulent velocities and cosmic ray induced ionization
rates than typically associated with molecular material within a kpc of the Sun
are also required for the molecular matter to be dark. We find that there is a
large region within the parameter space that results in clouds that might
contain a significant mass of molecular gas whilst remaining effectively
undetectable or at least not particularly noticeable in surveys. We note
briefly conditions under which molecular dark matter may contain a dynamically
interesting mass.Comment: 9 pages, 2 figures, accepted for publication in A&A; additional
concluding paragraph added at proof stag
Time-course of extracellular nicotine and cotinine levels in rat brain following administration of nicotine: effects of route and ethanol coadministration
RATIONALE:
Nicotine and ethanol are commonly coabused drugs, and nicotine-laced ethanol products are growing in popularity. However, little is known about time-course changes in extracellular nicotine and cotinine levels in rat models of ethanol and nicotine coabuse.
OBJECTIVES:
The objective of the present study was to determine the time-course changes in brain levels of nicotine and cotinine following subcutaneous (SC) and intragastric (IG) nicotine administration in alcohol-preferring (P) and Wistar rats.
METHODS:
In vivo microdialysis was used to collect dialysate samples from the nucleus accumbens shell (NACsh) for nicotine and cotinine determinations, following SC administration of (-)-nicotine (0.18, 0.35, and 0.70 mg/kg) in female P and Wistar rats or IG administration of (-)-nicotine (0.35 and 0.70 mg/kg) in 15 % (v/v) ethanol or water in female P rats.
RESULTS:
SC nicotine produced nicotine and cotinine dialysate levels as high as 51 and 14 ng/ml, respectively. IG administration of 15 % EtOH + 0.70 mg/kg nicotine in P rats resulted in maximal nicotine and cotinine dialysate levels of 19 and 14 ng/ml, respectively, whereas administration of 0.70 mg/kg nicotine in water resulted in maximal nicotine and cotinine levels of 21 and 25 ng/ml, respectively. Nicotine and cotinine levels were detectable within the first 15 and 45 min, respectively, after IG administration.
CONCLUSIONS:
Overall, the results of this study suggest that nicotine is rapidly adsorbed and produces relevant extracellular brain concentrations of nicotine and its pharmacologically active metabolite, cotinine. The persisting high brain concentrations of cotinine may contribute to nicotine addiction
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