184 research outputs found
Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation
We provide flux-limited and volume-limited galaxy group and cluster
catalogues, based on the spectroscopic sample of the SDSS data release 10
galaxies. We used a modified friends-of-friends (FoF) method with a variable
linking length in the transverse and radial directions to identify as many
realistic groups as possible. The flux-limited catalogue incorporates galaxies
down to m_r = 17.77 mag. It includes 588193 galaxies and 82458 groups. The
volume-limited catalogues are complete for absolute magnitudes down to M_r =
-18.0, -18.5, -19.0, -19.5, -20.0, -20.5, and -21.0; the completeness is
achieved within different spatial volumes, respectively. Our analysis shows
that flux-limited and volume-limited group samples are well compatible to each
other, especially for the larger groups/clusters. Dynamical mass estimates,
based on radial velocity dispersions and group extent in the sky, are added to
the extracted groups. The catalogues can be accessed via http://cosmodb.to.ee
and the Strasbourg Astronomical Data Center (CDS).Comment: 16 pages, 18 figures, 2 tables, accepted for publication in A&
Near-Infrared Spectroscopy of Carbon-Enhanced Metal-Poor Stars. I. A SOAR/OSIRIS Pilot Study
We report on an abundance analysis for a pilot study of seven Carbon-Enhanced
Metal-Poor (CEMP) stars, based on medium-resolution optical and near-infrared
spectroscopy. The optical spectra are used to estimate [Fe/H], [C/Fe], [N/Fe],
and [Ba/Fe] for our program stars. The near-infrared spectra, obtained during a
limited early science run with the new SOAR 4.1m telescope and the Ohio State
Infrared Imager and Spectrograph (OSIRIS), are used to obtain estimates of
[O/Fe] and 12C/13C. The chemical abundances of CEMP stars are of importance for
understanding the origin of CNO in the early Galaxy, as well as for placing
constraints on the operation of the astrophysical s-process in very
low-metallicity Asymptotic Giant Branch (AGB) stars.
This pilot study includes a few stars with previously measured [Fe/H],
[C/Fe], [N/Fe],[O/Fe], 12C/13C, and [Ba/Fe], based on high-resolution optical
spectra obtained with large-aperture telescopes. Our analysis demonstrates that
we are able to achieve reasonably accurate determinations of these quantities
for CEMP stars from moderate-resolution optical and near-infrared spectra. This
opens the pathway for the study of significantly larger samples of CEMP stars
in the near future. Furthermore, the ability to measure [Ba/Fe] for (at least
the cooler) CEMP stars should enable one to separate stars that are likely to
be associated with s-process enhancements (the CEMP-s stars) from those that do
not exhibit neutron-capture enhancements (the CEMP-no stars).Comment: 27 pages, including 5 tables, 6 figures, accepted for publication in
The Astronomical Journa
Variability in content and dissolution profiles of MDMA tablets collected in the UK between 2001 and 2018-A potential risk to users?
Official Student Newspaper
Issue is 12 pages long
Recovering 3D structural properties of galaxies from SDSS-like photometry
Because of the 3D nature of galaxies, an algorithm for constructing spatial
density distribution models of galaxies on the basis of galaxy images has many
advantages over surface density distribution approximations. We present a
method for deriving spatial structure and overall parameters of galaxies from
images and estimate its accuracy and derived parameter degeneracies on a sample
of idealised model galaxies. The test galaxies consist of a disc-like component
and a spheroidal component with varying proportions and properties. Both
components are assumed to be axially symmetric and coplanar. We simulate these
test galaxies as if observed in the SDSS project through ugriz filters, thus
gaining a set of realistically imperfect images of galaxies with known
intrinsic properties. These artificial SDSS galaxies were thereafter remodelled
by approximating the surface brightness distribution with a 2D projection of a
bulge+disc spatial distribution model and the restored parameters were compared
to the initial ones. Down to the r-band limiting magnitude 18, errors of the
restored integral luminosities and colour indices remain within 0.05 mag and
errors of the luminosities of individual components within 0.2 mag. Accuracy of
the restored bulge-to-disc ratios (B/D) is within 40% in most cases, and
becomes worse for galaxies with low B/D, but the general balance between bulges
and discs is not shifted systematically. Assuming that the intrinsic disc axial
ratio is < 0.3, the inclination angles can be estimated with errors < 5deg for
most of the galaxies with B/D < 2 and with errors < 15deg up to B/D = 6. Errors
of the recovered sizes of the galactic components are below 10% in most cases.
In general, models of disc components are more accurate than models of
spheroidal components for geometrical reasons.Comment: 15 pages, 13 figures, accepted for publication in RA
Variability in content and dissolution profiles of MDMA tablets collected in the UK between 2001 and 2018-A potential risk to users?
Rapid Quantitation of Flecainide in Human Plasma for Therapeutic Drug Monitoring Using Liquid Chromatography and Time-of-Flight Mass Spectrometry
Pulsational instability of yellow hypergiants
Instability of population I (X=0.7, Y=0.02) massive stars against radial
oscillations during the post-main sequence gravitational contraction of the
helium core is investigated. Initial stellar masses are in the range from
65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar
oscillations we assumed that energy transfer in the envelope of the pulsating
star is due to radiative heat conduction and convection. The convective heat
transfer was treated in the framework of the theory of time-dependent turbulent
convection. During evolutionary expansion of outer layers after hydrogen
exhaustion in the stellar core the star is shown to be unstable against radial
oscillations while its effective temperature is Teff > 6700K for
Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability
is due to the \kappa-mechanism in helium ionization zones and at lower
effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of
evolution does not exceed 200 day. Radial oscillations of the hypergiant resume
during evolutionary contraction of outer layers when the effective temperature
is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot.
Initially radial oscillations are due to instability of the first overtone and
transition to fundamental mode pulsations takes place at higher effective
temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for
Mzams=90M_\odot). The upper limit of the period of radial oscillations of
evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow
hypergiants are stable against radial stellar pulsations during the major part
of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter
Modeling the spectrum of V4334 Sgr (Sakurai's Object)
Theoretical spectral energy distributions were computed for a grid of
hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of
5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking
into account continuous, molecular band and atomic line absorption. These
energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's
object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1)
the shape of the theoretical spectra depends strongly on T(eff) but only very
weakly on the hydrogen abundance; (2) the comparison of the observed and
computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr
in April, 1997, and its interstellar reddening (plus a possible circumstellar
contribution) E(B-V) approximately 0.70.Comment: 7 pages, 8 figures, LaTeX, accepted by Astronomy and Astrophysic
The Kr85 s-process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.Comment: 54 pages + 6 figures + 6 tables. ApJ accepte
s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-element
content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass
s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are
derived using the spectral synthesis technique from high-resolution spectra.
The N-stars analyzed are of nearly solar metallicity and show moderate
s-element enhancements, similar to those found in S stars, but smaller than
those found in the only previous similar study (Utsumi 1985), and also smaller
than those found in supergiant post-AGB stars. This is in agreement with the
present understanding of the envelope s-element enrichment in giant stars,
which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the
AGB phase. We compare the observational data with recent -process
nucleosynthesis models for different metallicities and stellar masses. Good
agreement is obtained between low mass AGB star models (M < 3 M_o) and
s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction
is the main source of neutrons for the s-process; a moderate spread, however,
must exist in the abundance of 13C that is burnt in different stars. By
combining information deriving from the detection of Tc, the infrared colours
and the theoretical relations between stellar mass, metallicity and the final
C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this
work are intrinsic, thermally-pulsing AGB stars; their abundances are the
consequence of the operation of third dredge-up and are not to be ascribed to
mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap
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