20 research outputs found
Period -- mass-loss rate relation of Miras with and without technetium
Aims: We report the discovery that Mira variables with and without absorption
lines of the element technetium (Tc) occupy two different regions in a diagram
of near- to mid-infrared colour versus pulsation period. Tc is an indicator of
a recent or ongoing mixing event called the third dredge-up (3DUP), and the
near- to mid-IR colour, such as the (K-[22]) colour where [22] is the the 22
micron band of the WISE space observatory, is an indicator of the dust
mass-loss rate of a star. Methods: We collected data from the literature about
the Tc content, pulsation period, and near- and mid-infrared magnitudes of more
than 190 variable stars on the asymptotic giant branch (AGB) to which Miras
belong. The sample is naturally biased towards optical AGB stars, which have
low to intermediate (dust) mass-loss rates. Results: We show that a clear
relation between dust mass-loss rate and pulsation period exists if a
distinction is made between Tc-poor and Tc-rich Miras. Surprisingly, at a given
period, Tc-poor Miras are redder in (K-[22]) than are Tc-rich Miras; i.e. they
have higher mass-loss rates than the Tc-rich Miras. A few stars deviate from
this trend; physical explanations are given for these exceptions, such as
binarity or high mass. Conclusions: We put forward two hypotheses to explain
this dichotomy and conclude that the two sequences formed by Tc-poor and
Tc-rich Miras are probably due to the different masses of the two groups. The
pulsation period has a strong correlation with the dust-mass loss rate,
indicating that the pulsations are indeed triggering a dust-driven wind. The
location in the (K-[22]) vs. period diagram can be used to distinguish between
pre- and post-3DUP Miras, which we apply to a sample of Galactic bulge AGB
stars. We find that 3DUP is probably not common in AGB stars in the inner
bulge.Comment: Accepted for publication in A&A (5 pages, 2 figures, 1 on-line
table); final version (language corrected
Two barium stars in the Galactic bulge
Barium stars conserve important information on the s-process and the third
dredge-up in intermediate mass stars. Their discovery in various environments
is therefore of great help to test nucleosynthesis and mixing models. Our aim
is to analyse two stars with a very strong barium line detected in a large
survey of red giants in the Galactic bulge. Abundance analysis was done
comparing synthetic model spectra based on the COMARCS code with our medium
resolution spectra. Abundances of Ba, La, Y, and Fe were determined. Beside the
two main targets, the analysis was also applied to two comparison stars. We
confirm that both stars are barium stars. They are the first ones of this kind
identified in the Galactic bulge. Their barium excesses are among the largest
values found up to now. The elemental abundances are compared with current
nucleosynthesis and mixing models. Furthermore, we estimate a frequency of
barium stars in the Galactic bulge of about 1%, which is identical to the value
for disc stars.Comment: 4 pages, accepted for publication in A&
Interplay between pulsation, mass loss, and third dredge-up: More about Miras with and without technetium
We follow-up on a previous finding that AGB Mira variables containing the
3DUP indicator technetium (Tc) in their atmosphere form a different sequence of
K-[22] colour as a function of pulsation period than Miras without Tc. A near-
to mid-infrared colour such as K-[22] is a good probe for the dust mass-loss
rate of the stars. Contrary to what might be expected, Tc-poor Miras show
redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at
a given period. Here, the previous sample is extended and the analysis is
expanded towards other colours and dust spectra. The most important aim is to
investigate if the same two sequences can be revealed in the gas mass-loss
rate. We analysed new optical spectra and expanded the sample by including more
stars from the literature. Near- and mid-IR photometry and ISO dust spectra of
our stars were investigated. Literature data of gas mass-loss rates of Miras
and semi-regular variables were collected and analysed. Our results show that
Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR,
suggesting that the previous finding based on the K-[22] colour is not due to a
specific dust feature in the 22 micron band. We establish a linear relation
between K-[22] and the gas mass-loss rate. We also find that the 13 micron
feature disappears above K-[22]~2.17 mag, corresponding to \dot{M}_{\rm
g}\sim2.6\times10^{-7}M_{\sun}yr^{-1}. No similar sequences of Tc-poor and
Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most
probably owing to limitations in the available data. Different hypotheses to
explain the observation of two sequences in the P vs. K-[22]
explain the observation of two sequences in the P vs. K-[22 diagram are
discussed and tested, but so far none of them convincingly explains the
observations. Nevertheless, we might have found an hitherto unknown but
potentially important process influencing mass loss on the TP-AGB.Comment: 16 pages, 15 figures, 2 online tables, accepted for publication in
A&
Constraining the structure and formation of the Galactic bulge from a field in its outskirts. FLAMES-GIRAFFE spectra of about 400 red giants around (l,b)=(0{\deg},-10{\deg})
The presence of two stellar populations in the Milky Way bulge has been
reported recently. We aim at studying the abundances and kinematics of stars in
the outer bulge, thereby providing additional constraints on models of its
formation. Spectra of 401 red giant stars in a field at
(l,b)=(0{\deg},-10{\deg}) were obtained with FLAMES at the VLT. Stars of
luminosities down to below the two bulge red clumps (RCs) are included. From
these spectra we measure general metallicities, abundances of Fe and the
alpha-elements, and radial velocities (RV) of the stars. These measurements as
well as photometric data are compared to simulations with the Besancon and
TRILEGAL models of the Galaxy. We confirm the presence of two populations among
our sample stars: i) a metal-rich one at [M/H] ~+0.3, comprising about 30% of
the sample, with low RV dispersion and low alpha-abundance, and ii) a
metal-poor population at [M/H] ~-0.6 with high RV dispersion and high
alpha-abundance. The metal-rich population could be connected to the Galactic
bar. We identify this population as the carrier of the double RC feature. We do
not find a significant difference in metallicity or RV between the two RCs, a
small difference in metallicity being probably due to a selection effect. The
RV dispersion agrees well with predictions of the Besancon Galaxy model, but
the metallicity of the "thick bulge" model component should be shifted to lower
metallicity by 0.2 to 0.3dex to well reproduce the observations. We present
evidence that the metallicity distribution function depends on the evolutionary
state of the sample stars, suggesting that enhanced mass loss preferentially
removes metal-rich stars. We also confirm the decrease of \alpha-element
over-abundance with increasing metallicity.Comment: 19 pages (excluding on-line table), 21 figures, accepted for
publication in A&
Mass loss and luminosities of S and C AGB stars with and without Li
AbstractWe present the preliminary results of an analysis performed on two samples of thermally pulsing Asymptotic Giant Branch stars from our Galaxy, the first made of carbon-rich sources and the second of S-type stars. We have estimated their absolute luminosities and updated rates of the stellar winds through methods based on their infrared spectrophotometry and on updated estimates of their variability and distance.We then focus on those sources in our database showing Li in their spectra looking for correlations between the Li abundance and the other physical parameters, in the aim of establishing observational criteria for understanding the conditions for the occurrence of the deep mixing phenomena to which the production of Li is currently attributed
Elemental abundances in AGB stars and the formation of the Galactic bulge
We obtained high-resolution near-IR spectra of 45 AGB stars located in the
Galactic bulge. The aim of the project is to determine key elemental abundances
in these stars to help constrain the formation history of the bulge. A further
aim is to link the photospheric abundances to the dust species found in the
winds of the stars. Here we present a progress report of the analysis of the
spectra.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand, France, 17-22
April 2011, European Physical Journal, editors C. Reyl\'e, A. Robin and M.
Schulthei
Collisional decoherence observed in matter wave interferometry
We study the loss of spatial coherence in the extended wave function of
fullerenes due to collisions with background gases. From the gradual
suppression of quantum interference with increasing gas pressure we are able to
support quantitatively both the predictions of decoherence theory and our
picture of the interaction process. We thus explore the practical limits of
matter wave interferometry at finite gas pressures and estimate the required
experimental vacuum conditions for interferometry with even larger objects.Comment: 4 pages, 3 figure
The wave nature of biomolecules and fluorofullerenes
We demonstrate quantum interference for tetraphenylporphyrin, the first
biomolecule exhibiting wave nature, and for the fluorofullerene C60F48 using a
near-field Talbot-Lau interferometer. For the porphyrins, which are
distinguished by their low symmetry and their abundant occurence in organic
systems, we find the theoretically expected maximal interference contrast and
its expected dependence on the de Broglie wavelength. For C60F48 the observed
fringe visibility is below the expected value, but the high contrast still
provides good evidence for the quantum character of the observed fringe
pattern. The fluorofullerenes therefore set the new mark in complexity and mass
(1632 amu) for de Broglie wave experiments, exceeding the previous mass record
by a factor of two.Comment: 5 pages, 4 figure