1,417 research outputs found
Cool carbon stars in the halo: new very red or distant objects
The goal of this paper is to present and analyse a new sample of cool carbon
(C)stars located in the halo. Twenty three new C stars were discovered. Spectra
are typical of N-type stars with C2 and CN bands and sometimes Halpha in
emission. ... Four objects are particularly red with J-K > 3, with 2 located at
more than 5 kpc. from the Galactic plane. Eight additional objects with similar
properties are found in the literature and our previous works. These 12 C stars
could be useful to study mass loss at low metallicity. Two objects are at
distances of 95 and 110 kpc. They are located in the region with galactocentric
Z < -60 kpc in which the model of Law et al. predicts the Sgr stream to have a
loop. (Abstact abridged)Comment: 16 pages, 12 figures, accepted by A
Рациональный компромисс целей субъектов дистанционного образования
На базе системной методологии исследованы механизмы согласования деятельности субъектов в дистанционном образовании для нахождения рационального компромисса целей субъектов обучения, учебных заведений и агентов рынка труда. На схемах взаимосвязей компонентов системы образования в процессе обучения и на схеме взаимодействия учебных заведений проиллюстрированы преимущества такого подхода. Предложена математическая постановка задачи обеспечения рационального компромисса целей. Разработан алгоритм ее решения.На основі системної методології досліджено механізми погодження діяльності суб’єктів у дистанційній освіті з метою знаходження раціонального компромісу цілей суб’єктів навчання, навчальних закладів та агентів ринку праці. На схемах взаємозв’язків компонентів системи освіти у процесі навчання та на схемі взаємодії навчальних закладів проілюстровано переваги такого підходу. Запропоновано математичну модель задачі знаходження раціонального компромісу цілей та наведено алгоритм її розв’язку.Coordination mechanisms for agents purposes in the distance education have been studied on the basis of system methodology. The finding of rational compromise for purposes of learners, educational institutions and labour market agents is proposed. Benefits of the approach are illustrated by the scheme of education system components interconnection during the learning process and the scheme of educational institutions interaction. A mathematical model and an algorithm for finding the rational compromise of purposes are given
Use of tunable nanopore blockade rates to investigate colloidal dispersions
Tunable nanopores in elastomeric membranes have been used to study the
dependence of ionic current blockade rate on the concentration and
electrophoretic mobility of particles in aqueous suspensions. A range of
nanoparticle sizes, materials and surface functionalities has been tested.
Using pressure-driven flow through a pore, the blockade rate for 100 nm
carboxylated polystyrene particles was found to be linearly proportional to
both transmembrane pressure (controlled between 0 and 1.8 kPa) and particle
concentration (between 7 x 10^8 and 4.5 x 10^10 mL^-1). This result can be
accurately modelled using Nernst-Planck transport theory. Using only an applied
potential across a pore, the blockade rates for carboxylic acid and amine
coated 500 nm and 200 nm silica particles were found to correspond to changes
in their mobility as a function of the solution pH. Scanning electron
microscopy and confocal microscopy have been used to visualise changes in the
tunable nanopore geometry in three dimensions as a function of applied
mechanical strain. The pores observed were conical in shape, and changes in
pore size were consistent with ionic current measurements. A zone of inelastic
deformation adjacent to the pore has been identified as critical in the tuning
process
Mid-IR period-magnitude relations for AGB stars
Asymptotic Giant Branch variables are found to obey period-luminosity
relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24
microns where emission from circumstellar dust is expected to be dominant.
Their loci in the M, logP diagrams are essentially the same for the LMC and for
NGC6522 in spite of different ages and metallicities. There is no systematic
trend of slope with wavelength. The offsets of the apparent magnitude vs. logP
relations imply a difference between the two fields of 3.8 in distance modulus.
The colours of the variables confirm that a principal period with log P > 1.75
is a necessary condition for detectable mass-loss. At the longest observed
wavelength, 24 microns, many semi-regular variables have dust shells comparable
in luminosity to those around Miras. There is a clear bifurcation in LMC
colour-magnitude diagrams involving 24 micron magnitudes.Comment: 5 pages, 4 figure
Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
The main aim is to derive reliable mass-loss rates and circumstellar SiO
abundances for a sample of 40 S-type AGB stars based on new multi-transitional
CO and SiO radio line observations. In addition, the results are compared to
previous results for M-type AGB stars and carbon stars to look for trends with
chemical type. The circumstellar envelopes are assumed to be spherically
symmetric and formed by a constant mass-loss rate. The mass-loss rates are
estimated from fitting the CO observations using a non-local, non-LTE radiative
transfer code. Once the physical properties of the circumstellar envelopes are
determined, the same radiative transfer code is used to model the observed SiO
lines in order to derive circumstellar abundances and the sizes of the SiO
line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars
and find that the derived mass-loss rates have a distribution that resembles
those previously derived for similar samples of M-type AGB stars and carbon
stars. The estimated mass-loss rates also correlate well with the corresponding
expansion velocity. In all, this indicates that the mass loss is driven by the
same mechanism in all three chemical types of AGB stars. In addition, we have
estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of
the sample S-type AGB stars. The derived SiO abundances are, on average, about
an order of magnitude higher than predicted by stellar atmosphere thermal
equilibrium chemistry, indicating that non-equilibrium chemical processes
determines the abundance of SiO in the circumstellar envelope. Moreover, a
comparison with the results for M-type AGB stars and carbon stars show that for
a certain mass-loss rate, the circumstellar SiO abundance seems independent
(although with a large scatter) of the C/O-ratio.Comment: 24 pages, 11 figure
System evolution by migration coordination
Collaborations between components can bemodeled in the coordination language Paradigm[3]. A collaboration solution is specified by loosely coupling component dynamics to a protocol via their roles. Not only regular, foreseen collaboration can be specified, originally unforeseen collaboration can be modeled too [4]. To explain how, we first look very briefly at Paradigm’s regular coordination specification. Component dynamics are expressed by state-transition diagrams (STDs), see Figure 1(a) for a mock-up STD MU in UML style. MU contributes to a collaboration via a role MU(R). Figure 1(b) specifies MU(R) through a different STD, whose states are so-called phases of MU: temporarily valid, dynamic constraints imposed on MU. The figure mentions four such phases, Clock, Anti, Inter and Small. Figure 1(c) couplesMU and MU(R). It specifies each phase as part of MU, additionally decorated with one or more polygons grouping some states of a phase. Polygons visualize so-called traps: a trap, once entered, cannot be left as long as the phase remains the valid constraint. A trap having been entered, serves as a guard for a phase change. Therefore, traps label transitions in a role STD, cf. Figure 1(b). Single steps from different roles, are synchronized into one protocol step. A protocol step can be coupled to one detailed step of a so-called manager component, driving the protocol. Meanwhile, local variables can be updated. It is through a consistency rule, Paradigm specifies a protocol step: (i) at the left-hand side of a ?? the one, driving manager step is given, if relevant; (ii) the right-hand side lists the role steps being synchronized; (iii) optionally, a change clause [2] can be given updating variables, e.g. one containing the current set of consistency rules. For example, a consistency rule without change clause, MU2:A!B ?? MU1(R):Clock triv ! Anti, MU3(R): Inter toSmall ! Small where a manager step ofMU2 is coupled to the swapping ofMU1 from circling clockwise to anti-clock-wise and swapping MU3 from intermediate inspection into circling on a smaller scale
The mass-loss return from evolved stars to the Large Magellanic Cloud III. Dust properties for carbon-rich asymptotic giant branch stars
We present a 2Dust model for the dust shell around a LMC long-period variable
(LPV) previously studied as part of the OGLE survey. OGLE LMC LPV 28579 (SAGE
J051306.40-690946.3) is a carbon-rich asymptotic giant branch (AGB) star for
which we have photometry and spectra from the Spitzer SAGE and SAGE-Spec
programs along with UBVIJHK_s photometry. By modeling this source, we obtain a
baseline set of dust properties to be used in the construction of a grid of
models for carbon stars. We reproduce its spectral energy distribution using a
mixture of AmC and SiC (15% by mass). The grain sizes are distributed according
to the KMH model. The best-fit model has an optical depth of 0.28 for the shell
at the peak of the SiC feature, with R_in~1430 R_sun or 4.4 R_star. The
temperature at this inner radius is 1310 K. Assuming an expansion velocity of
10 km s^-1, we obtain a dust mass-loss rate of 2.5x10^-9 M_sun yr-1. We
calculate a 15% variation in this rate by testing the fit sensitivity against
variation in input parameters. We also present a simple model for the molecular
gas in the extended atmosphere that could give rise to the 13.7 \mu m feature
seen in the spectrum. We find that a combination of CO and C_2H_2 gas at an
excitation temperature of about 1000 K and column densities of 3x10^21 cm^-2
and 10^19 cm^-2 respectively are able to reproduce the observations. Given that
the excitation temperature is close to T_dust(R_in), most of the molecular
contribution probably arises from the inner shell region. The luminosity
corresponding to the first epoch of SAGE observations is 6580 L_sun. For an
effective temperature of about 3000 K, this implies a stellar mass of 1.5-2
M_sun and an age of 1-2.5 Gyr. For a gas:dust ratio of 200, we obtain a gas
mass-loss rate of 5.0x10^-7 M_sun yr^-1, consistent with the gas mass-loss
rates estimated from the period, color and 8 \mu m flux of the source.Comment: 14 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud
Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium.
Aims: This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).
Methods: The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2-1 line.
Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC.
Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models
ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris
[abridged] The 12CO/13CO ratio is often used as a measure of the 12C/13C
ratio in the circumstellar environment, carrying important information about
the stellar nucleosynthesis. External processes can change the 12CO and 13CO
abundances, and spatially resolved studies of the 12CO/13CO ratio are needed to
quantify the effect of these processes on the globally determined values.
Additionally, such studies provide important information on the conditions in
the circumstellar environment. The detached-shell source R Scl, displaying CO
emission from recent mass loss, in a binary-induced spiral structure as well as
in a clumpy shell produced during a thermal pulse, provides a unique laboratory
for studying the differences in CO isotope abundances throughout its recent
evolution. We observed both the 12CO(J=3-2) and the 13CO(J=3-2) line using
ALMA. We find significant variations in the 12CO/13CO intensity ratios and
consequently in the abundance ratios. The average CO isotope abundance ratio is
at least a factor three lower in the shell (~19) than that in the present-day
(60). Additionally, variations in the ratio of more
than an order of magnitude are found in the shell itself. We attribute these
variations to the competition between selective dissociation and isotope
fractionation in the shell, of which large parts cannot be warmer than ~35 K.
However, we also find that the 12CO/13CO ratio in the present-day mass loss is
significantly higher than the 12C/13C ratio determined in the stellar
photosphere from molecular tracers (~19). The origin of this discrepancy is
still unclear, but we speculate that it is due to an embedded source of
UV-radiation that is primarily photo-dissociating 13CO. This radiation source
could be the hitherto hidden companion. Alternatively, the UV-radiation could
originate from an active chromosphere of R Scl itself....Comment: 6 pages, 5 figures, online data available at
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/A+A/556/L
The distance to the Pleiades: Main sequence fitting in the near infrared
Hipparcos parallax measurements of stars in the Pleiades notoriously result
in a cluster distance of 118 pc; ~10% shorter than the `classical' results of
MS-fitting. In an earlier paper we developed a purely empirical MS-fitting
method to address this problem, which produced conflicting results between the
(B-V) and (V-I) colour indices, indicating that the cluster's photometric
metallicity is substantially lower than its (~solar) spectroscopic metallicity.
We were able to reconcile the discrepancy by assuming [Fe/H]=-0.4 (as indicated
from (B-V)/(V-I) colour-colour plots), and the distance moduli obtained from
the 2 indices were in agreement with the Hipparcos result, within the 1sig
errors. With the release of the 2MASS All Sky Catalogue, we now apply our
MS-fitting method to the Pleiades using the infrared colours, in order to test
the plausibility of our earlier result. Fitting in the V/(V-K) and K/(J-K)
colour planes, we find that assuming a subsolar metallicity does not produce
distances in agreement with the (B-V) and (V-I) results. However the infrared
plus (V-I) distances are in mutual agreement when adopting the spectroscopic
metallicity. By considering only stars with Mv<=6, the infrared and optical
colour indices all yield consistent distances using the spectroscopic [Fe/H].
The concordant distances thus obtained from the (B-V), (V-I), (V-K) and (J-K)
indices yield a mean of 133.8+/-3 pc, in excellent agreement with the
pre-Hipparcos MS-fitting results, and the most recent determinations from other
methods. We conclude that there are two distinct issues affecting the Pleiades:
the Hipparcos parallax is in error by ~10%, as previously claimed; the (B-V)
colours of the lower MS are anomalous, and we caution against their use for
MS-fitting to the Pleiades and similarly young open clusters. (Abridged)Comment: 8 pages. A&A accepte
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