99 research outputs found
The fluorine abundance in a Galactic Bulge AGB star measured from CRIRES spectra
We present measurements of the fluorine abundance in a Galactic Bulge
Asymptotic Giant Branch (AGB) star. The measurements were performed using high
resolution K-band spectra obtained with the CRIRES spectrograph, which has been
recently installed at ESO's VLT, together with state-of-the-art model
atmospheres and synthetic spectra. This represents the first fluorine abundance
measurement in a Galactic Bulge star, and one of few measurements of this kind
in a third dredge-up oxygen-rich AGB star. The F abundance is found to be close
to the solar value scaled down to the metallicity of the star, and in agreement
with Disk giants that are comparable to the Bulge giant studied here. The
measurement is of astrophysical interest also because the star's mass can be
estimated rather accurately (1.4 \lesssim M/\mathrm{M}_{\sun} \lesssim 2.0).
AGB nucleosynthesis models predict only a very mild enrichment of F in such low
mass AGB stars. Thus, we suggest that the fluorine abundance found in the
studied star is representative for the star's natal cloud, and that fluorine
must have been produced at a similar level in the Bulge and in the Disk.Comment: 11 pages, 1 figure, accepted for publication by Ap
Chemical evolution of the Galactic Center
In recent years, the Galactic Center (GC) region (200 pc in radius) has been
studied in detail with spectroscopic stellar data as well as an estimate of the
ongoing star formation rate. The aims of this paper are to study the chemical
evolution of the GC region by means of a detailed chemical evolution model and
to compare the results with high resolution spectroscopic data in order to
impose constraints on the GC formation history.The chemical evolution model
assumes that the GC region formed by fast infall of gas and then follows the
evolution of alpha-elements and Fe. We test different initial mass functions
(IMFs), efficiencies of star formation and gas infall timescales. To reproduce
the currently observed star formation rate, we assume a late episode of star
formation triggered by gas infall/accretion. We find that, in order to
reproduce the [alpha/Fe] ratios as well as the metallicity distribution
function observed in GC stars, the GC region should have experienced a main
early strong burst of star formation, with a star formation efficiency as high
as 25 Gyr^{-1}, occurring on a timescale in the range 0.1-0.7 Gyr, in agreement
with previous models of the entire bulge. Although the small amount of data
prevents us from drawing firm conclusions, we suggest that the best IMF should
contain more massive stars than expected in the solar vicinity, and the last
episode of star formation, which lasted several hundred million years, should
have been triggered by a modest episode of gas infall/accretion, with a star
formation efficiency similar to that of the previous main star formation
episode. This last episode of star formation produces negligible effects on the
abundance patterns and can be due to accretion of gas induced by the bar. Our
results exclude an important infall event as a trigger for the last starburst.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Matter-wave interferometer for large molecules
We demonstrate a near-field Talbot-Lau interferometer for C-70 fullerene
molecules. Such interferometers are particularly suitable for larger masses.
Using three free-standing gold gratings of one micrometer period and a
transversally incoherent but velocity-selected molecular beam, we achieve an
interference fringe visibility of 40 % with high count rate. Both the high
visibility and its velocity dependence are in good agreement with a quantum
simulation that takes into account the van der Waals interaction of the
molecules with the gratings and are in striking contrast to a classical moire
model.Comment: revtex4, 4 pages, 3 figure
The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
Context: Stars on the asymptotic giant branch (AGB) show broad evidence of
inhomogeneous atmospheres and circumstellar envelopes. These have been studied
by a variety of methods on various angular scales. In this paper we explore the
envelope of the well-studied carbon star TX Psc by the technique of
spectro-astrometry. Aims: We explore the potential of this method for detecting
asymmetries around AGB stars. Methods:We obtained CRIRES observations of
several CO v=1 lines near 4.6 m and HCN lines near 3 m in
2010 and 2013. These were then searched for spectro-astrometric signatures. For
the interpretation of the results, we used simple simulated observations.
Results: Several lines show significant photocentre shifts with a clear
dependence on position angle. In all cases, tilde-shaped signatures are found
where the positive and negative shifts (at PA 0deg) are associated with blue
and weaker red components of the lines. The shifts can be modelled with a
bright blob 70 mas to 210 mas south of the star with a flux of several percent
of the photospheric flux. We estimate a lower limit of the blob temperature of
1000 K. The blob may be related to a mass ejection as found for AGB stars or
red supergiants. We also consider the scenario of a companion object.
Conclusions: Although there is clear spectro-astrometric evidence of a rather
prominent structure near TX Psc, it does not seem to relate to the other
evidence of asymmetries, so no definite explanation can be given. Our data thus
underline the very complex structure of the environment of this star, but
further observations that sample the angular scales out to a few hundred
milli-arcseconds are needed to get a clearer picture
Understanding AGB evolution in Galactic bulge stars from high-resolution infrared spectroscopy
An analysis of high-resolution near-infrared spectra of a sample of 45
asymptotic giant branch (AGB) stars towards the Galactic bulge is presented.
The sample consists of two subsamples, a larger one in the inner and
intermediate bulge, and a smaller one in the outer bulge. The data are analysed
with the help of hydrostatic model atmospheres and spectral synthesis. We
derive the radial velocity of all stars, and the atmospheric chemical mix
([Fe/H], C/O, C/C, Al, Si, Ti, and Y) where possible. Our ability
to model the spectra is mainly limited by the (in)completeness of atomic and
molecular line lists, at least for temperatures down to K. We find that the subsample in the inner and intermediate
bulge is quite homogeneous, with a slightly sub-solar mean metallicity and only
few stars with super-solar metallicity, in agreement with previous studies of
non-variable M-type giants in the bulge. All sample stars are oxygen-rich,
C/O1.0. The C/O and carbon isotopic ratios suggest that third dredge-up
(3DUP) is absent among the sample stars, except for two stars in the outer
bulge that are known to contain technetium. These stars are also more
metal-poor than the stars in the intermediate or inner bulge. Current stellar
masses are determined from linear pulsation models. The masses, metallicities
and 3DUP behaviour are compared to AGB evolutionary models. We conclude that
these models are partly in conflict with our observations. Furthermore, we
conclude that the stars in the inner and intermediate bulge belong to a more
metal-rich population that follows bar-like kinematics, whereas the stars in
the outer bulge belong to the metal-poor, spheroidal bulge population.Comment: 21 pages, 13 figures, 6 tables (incl. appendix), years of work,
published in MNRA
Meandering periods and asymmetries in light curves of Miras: Observational evidence for low mass-loss rates
Some Miras -- long-period variables in late evolutionary stages -- have
meandering pulsation periods and light curve asymmetries, the causes of which
are still unclear. We aim to understand better the origin of these phenomena by
investigating a sample of solar-neighbourhood Miras. We characterised this
group of stars and related their variability characteristics to other stellar
parameters. We analysed observations from several databases to obtain light
curves with maximum time span and temporal coverage for a sample of 548 Miras.
We determined their pulsation period evolution over a time span of many
decades, searched for changes in the periods, and determined the amplitude of
the period change. We also analysed the Fourier spectra with respect to
possible secondary frequency maxima. The sample was divided into two groups
with respect to the presence of light curve bumps. IR colours and indicators of
the third dredge-up were collected to study the sample stars' mass-loss and
deep mixing properties. Our analysis revealed one new star, T~Lyn, with a
continuously changing period. The group of Miras with meandering period changes
is exclusively made up of M-type stars. The Fourier spectra of the meandering
period Miras have no prominent additional peaks, suggesting that additional
pulsation modes are not the cause of the meandering periods. We confirm that
bumps are more common among S and C Miras and show, for the first time, that
Miras with bumps have lower mass-loss rates than those with regular, symmetric
light curves. Also Miras with meandering period changes have relatively little
mass loss. We conclude that Miras with strongly changing periods or asymmetries
in their light curves have relatively low dust mass-loss rates. Meandering
period changes and light curve asymmetries could be connected to He-shell
flashes and third dredge-up episodes.Comment: 13 pages (plus 13 pages Appendix), 14 Figures, accepted for
publication in A&
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