750 research outputs found
The spectroscopic indistinguishability of red giant branch and red clump stars
Stellar spectroscopy provides useful information on the physical properties
of stars such as effective temperature, metallicity and surface gravity (log
g). However, those photospheric characteristics are often hampered by
systematic uncertainties. The joint spectro-seismo project (APOKASC) of field
red giants has revealed a puzzling offset between the log g determined
spectroscopically and those determined using asteroseismology, which is largely
dependent on the stellar evolutionary status. Therefore, in this letter, we aim
to shed light on the spectroscopic source of the offset using the APOKASC
sample. We analyse the log g discrepancy as a function of stellar mass and
evolutionary status and discuss the impact of He and carbon isotopic ratio.
We first show that for stars at the bottom of the red giant branch, the
discrepancy between spectroscopic and asteroseismic log g depends on stellar
mass. This indicates that the discrepancy is related to CN cycling. We
demonstrate that the C isotopic ratio () has the largest
impact on the stellar spectrum. We find that this log g discrepancy shows a
similar trend in mass as the ratios expected by stellar
evolution theory. Although we do not detect a direct signature of ,
the data suggests that the discrepancy is tightly correlated to the production
of . Moreover, by running a data-driven algorithm (the Cannon) on a
synthetic grid trained on the APOGEE data, we quantitatively evaluate the
impact of various ratios. While we have demonstrated that
impacts all parameters, the size of the impact is smaller than the
observed offset in log g. If further tests confirm that is not the
main element responsible of the log g problem, the number of spectroscopic
effects remaining to be investigated is now relatively limited. [Abridged]Comment: 4 Pages, 6 Figures. Accepted for publication in A&
IP Eri: A surprising long-period binary system hosting a He white dwarf
We determine the orbital elements for the K0 IV + white dwarf (WD) system IP
Eri, which appears to have a surprisingly long period of 1071 d and a
significant eccentricity of 0.25. Previous spectroscopic analyses of the WD,
based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a
mass of only 0.43 M, implying it to be a helium-core WD. The orbital
properties of IP Eri are similar to those of the newly discovered long-period
subdwarf B star (sdB) binaries, which involve stars with He-burning cores
surrounded by extremely thin H envelopes, and are therefore close relatives to
He WDs. We performed a spectroscopic analysis of high-resolution spectra from
the HERMES/Mercator spectrograph and concluded that the atmospheric parameters
of the K0 component are K, , [Fe/H] = 0.09
and km/s. The detailed abundance analysis focuses on C, N, O
abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process
(Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree
with those of normal field stars of the same metallicity. The long period and
non-null eccentricity indicate that this system cannot be the end product of a
common-envelope phase; it calls instead for another less catastrophic
binary-evolution channel presented in detail in a companion paper (Siess et al.
2014).Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&A
(Update of Table 3, Fig. 8 and text in Sect. 5.1, 5.3 and 6 due to minor
corrections on N and Y II
New determination of abundances and stellar parameters for a set of weak G-band stars
Weak G-band (wGb) stars are very peculiar red giants almost devoided of
carbon and often mildly enriched in lithium. Despite their very puzzling
abundance patterns, very few detailed spectroscopic studies existed up to a few
years ago, preventing any clear understanding of the wGb phenomenon. We
recently proposed the first consistent analysis of published data for 28 wGb
stars and identified them as descendants of early A-type to late B-type stars,
without being able to conclude on their evolutionary status or the origin of
their peculiar abundance pattern.
We used newly obtained high-resolution and high SNR spectra for 19 wGb stars
in the southern and northern hemisphere to homogeneously derive their
fundamental parameters, metallicities, as well as the spectroscopic abundances
for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution
models that we used to determine the masses and to investigate the evolutionary
status and chemical history of the stars in our sample. We confirm that the wGb
stars are stars in the mass range 3.2 to 4.2 M. We suggest that a large
fraction could be mildly evolved stars on the SGB currently undergoing the 1st
DUP, while a smaller number of stars are more probably in the core He burning
phase at the clump. After analysing their abundance pattern, we confirm their
strong N enrichment anti-correlated with large C depletion, characteristic of
material fully processed through the CNO cycle to an extent not known in other
evolved intermediate-mass stars. However, we demonstrate here that such a
pattern is very unlikely due to self-enrichment. In the light of the current
observational constraints, no solid self-consistent pollution scenario can be
presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy &
Astrophysic
Deductive synthesis of recursive plans in linear logic
Linear logic has previously been shown to be suitable for describing and deductively solving planning problems involving conjunction and disjunction. We introduce a recursively defined datatype and a corresponding induction rule, thereby allowing recursive plans to be synthesised. In order to make explicit the relationship between proofs and plans, we enhance the linear logic deduction rules to handle plans as a form of proof term
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