3,156 research outputs found
A holistic approach to carbon-enhanced metal-poor stars
By considering the various CEMP subclasses separately, we try to derive, from
the specific signatures imprinted on the abundances, parameters (such as
metallicity, mass, temperature, and neutron source) characterizing AGB
nucleosynthesis from the specific signatures imprinted on the abundances, and
separate them from the impact of thermohaline mixing, first dredge-up, and
dilution associated with the mass transfer from the companion.To put CEMP stars
in a broad context, we collect abundances for about 180 stars of various
metallicities, luminosity classes, and abundance patterns, from our own sample
and from literature. First, we show that there are CEMP stars which share the
properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars).
We also show that there is a strong correlation between Ba and C abundances in
the s-only CEMP stars. This strongly points at the operation of the 13C neutron
source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with
elements from both the s- and r-processes), the correlation of the N abundances
with abundances of heavy elements from the 2nd and 3rd s-process peaks bears
instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs
stars exhibit O and Mg enhancements, we conclude that extremely hot conditions
prevailed during the thermal pulses of the contaminating AGB stars. Finally, we
argue that most CEMP-no stars (with no overabundances for the neutron-capture
elements) are likely the extremely metal-poor counterparts of CEMP
neutron-capture-rich stars. We also show that the C enhancement in CEMP-no
stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2).
This trend is not predicted by any of the current AGB models.Comment: 27 pages, 24 figures, accepted for publication in A&
Barium & related stars and their white-dwarf companions I. Giant stars
This paper provides long-period and revised orbits for barium and S stars
adding to previously published ones. The sample of barium stars with strong
anomalies comprise all such stars present in the Lu et al. catalogue. We find
orbital motion for all barium and extrinsic S stars monitored. We obtain the
longest period known so far for a spectroscopic binary involving an S star,
namely 57 Peg with a period of the order of 100 - 500 yr. We present the mass
distribution for the barium stars, which ranges from 1 to 3 Msun, with a tail
extending up to 5 Msun in the case of mild barium stars. This high-mass tail
comprises mostly high-metallicity objects ([Fe/H] >= -0.1). Mass functions are
compatible with WD companions and we derive their mass distribution which
ranges from 0.5 to 1 Msun. Using the initial - final mass relationship
established for field WDs, we derived the distribution of the mass ratio q' =
MAGB,ini / MBa (where MAGB, ini is the WD progenitor initial mass, i.e., the
mass of the system former primary component) which is a proxy for the initial
mass ratio. It appears that the distribution of q' is highly non uniform, and
significantly different for mild and strong barium stars, the latter being
characterized by values mostly in excess of 1.4, whereas mild barium stars
occupy the range 1 - 1.4. We investigate as well the correlation between
abundances, orbital periods, metallicities, and masses (barium star and WD
companion). The 105 orbits of post-mass-transfer systems presented in this
paper pave the way for a comparison with binary-evolution models.Comment: This version 2 is the one accepted by A&A, after language edition.
Paper II about dwarf-Ba and subgiant-CH orbits by Escorza et al. is
arXiv:1904.0409
Rivals for the crown: Reply to Opthof and Leydesdorff
We reply to the criticism of Opthof and Leydesdorff [arXiv:1002.2769] on the
way in which our institute applies journal and field normalizations to citation
counts. We point out why we believe most of the criticism is unjustified, but
we also indicate where we think Opthof and Leydesdorff raise a valid point
Towards a new crown indicator: Some theoretical considerations
The crown indicator is a well-known bibliometric indicator of research
performance developed by our institute. The indicator aims to normalize
citation counts for differences among fields. We critically examine the
theoretical basis of the normalization mechanism applied in the crown
indicator. We also make a comparison with an alternative normalization
mechanism. The alternative mechanism turns out to have more satisfactory
properties than the mechanism applied in the crown indicator. In particular,
the alternative mechanism has a so-called consistency property. The mechanism
applied in the crown indicator lacks this important property. As a consequence
of our findings, we are currently moving towards a new crown indicator, which
relies on the alternative normalization mechanism
Control of inflorescence architecture in tomato by BTB/POZ transcriptional regulators
Plant productivity depends on inflorescences, flower-bearing shoots that originate from the stem cell populations of shoot meristems. Inflorescence architecture determines flower production, which can vary dramatically both between and within species. In tomato plants, formation of multiflowered inflorescences depends on a precisely timed process of meristem maturation mediated by the transcription factor gene TERMINATING FLOWER (TMF), but the underlying mechanism is unknown. We show that TMF protein acts together with homologs of the Arabidopsis BLADE-ON-PETIOLE (BOP) transcriptional cofactors, defined by the conserved BTB (Broad complex, Tramtrack, and Bric-a-brac)/POZ (POX virus and zinc finger) domain. TMF and three tomato BOPs (SlBOPs) interact with themselves and each other, and TMF recruits SlBOPs to the nucleus, suggesting formation of a transcriptional complex. Like TMF, SlBOP gene expression is highest during vegetative and transitional stages of meristem maturation, and CRISPR/Cas9 elimination of SlBOP function causes pleiotropic defects, most notably simplification of inflorescences into single flowers, resembling tmf mutants. Flowering defects are enhanced in higher-order slbop tmf mutants, suggesting that SlBOPs function with additional factors. In support of this, SlBOPs interact with TMF homologs, mutations in which cause phenotypes like slbop mutants. Our findings reveal a new flowering module defined by SlBOP-TMF family interactions that ensures a progressive meristem maturation to promote inflorescence complexity
Hertzsprung-Russell diagram and mass distribution of barium stars
With the availability of parallaxes provided by the Tycho-Gaia Astrometric
Solution, it is possible to construct the Hertzsprung-Russell diagram (HRD) of
barium and related stars with unprecedented accuracy. A direct result from the
derived HRD is that subgiant CH stars occupy the same region as barium dwarfs,
contrary to what their designations imply. By comparing the position of barium
stars in the HRD with STAREVOL evolutionary tracks, it is possible to evaluate
their masses, provided the metallicity is known. We used an average metallicity
[Fe/H] = -0.25 and derived the mass distribution of barium giants. The
distribution peaks around 2.5 Msun with a tail at higher masses up to 4.5 Msun.
This peak is also seen in the mass distribution of a sample of normal K and M
giants used for comparison and is associated with stars located in the red
clump. When we compare these mass distributions, we see a deficit of low-mass
(1 - 2 Msun) barium giants. This is probably because low-mass stars reach large
radii at the tip of the red giant branch, which may have resulted in an early
binary interaction. Among barium giants, the high-mass tail is however
dominated by stars with barium indices of less than unity, based on a visual
inspection of the barium spectral line; that is, these stars have a very
moderate barium line strength. We believe that these stars are not genuine
barium giants, but rather bright giants, or supergiants, where the barium lines
are strengthened because of a positive luminosity effect. Moreover, contrary to
previous claims, we do not see differences between the mass distributions of
mild and strong barium giants.Comment: 14 pages, 17 figure
Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae
We report on the discovery of an infrared emission band in the Spitzer
spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in
the circumstellar environment. We modelled the observed emission to derive the
temperature of the TiO molecules (\approx 600 K), an upper limit on the column
density (\approx 10^17.25 cm^{-2}) and a lower limit on the spatial extent of
the layer that contains these molecules. (\approx 4.6 stellar radii). This is
the first time that this TiO emission band is observed. A search for similar
emission features in the sample of S-type stars yielded two additional
candidates. However, owing to the additional dust emission, the identification
is less stringent. By comparing the stellar characteristics of NP Aur to those
of the other stars in our sample, we find that all stars with TiO emission show
large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These
characteristics might be necessary requirements for a star to show TiO in
emission, but they are not sufficient.Comment: 4 pages, 4 figures, letter to the edito
Binary properties of CH and Carbon-Enhanced Metal-Poor stars
The HERMES spectrograph installed on the 1.2-m Mercator telescope has been
used to monitor the radial velocity of 13 low-metallicity carbon stars, among
which 7 Carbon-Enhanced Metal-Poor (CEMP) stars and 6 CH stars. All stars but
one show clear evidence for binarity. New orbits are obtained for 8 systems.
The sample covers an extended range in orbital periods, extending from 3.4 d
(for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26,
the longest known among barium, CH and extrinsic S stars). Three systems
exhibit low-amplitude velocity variations with periods close to 1 yr
superimposed on a long-term trend. In the absence of an accurate photometric
monitoring of these systems, it is not clear yet whether these variations are
the signature of a very low-mass companion, or of regular envelope pulsations.
The period - eccentricity (P - e) diagram for the 40 low-metallicity carbon
stars with orbits now available shows no difference between CH and CEMP-s stars
(the latter corresponding to those CEMP stars enriched in s-process elements,
as are CH stars). We suggest that they must be considered as one and the same
family and that their different names only stem from historical reasons.
Indeed, these two families have as well very similar mass-function
distributions, corresponding to companions with masses in the range 0.5 - 0.7
Msun, indicative of white-dwarf companions, adopting 0.8 - 0.9 Msun for the
primary component. This result confirms that CH and CEMP-s stars obey the same
mass-transfer scenario as their higher-metallicity analogs, the barium stars.
The P - e diagrams of barium, CH and CEMP-s stars are indeed very similar. They
reveal two different groups of systems: one with short orbital periods (P <
1000 d) and mostly circular or almost circular orbits, and another with
longer-period and eccentric (e > 0.1) orbits.Comment: Accepted in Astronomy & Astrophysic
Effect of smoke and clouds on the transmissivity of photosynthetically active radiation inside the canopy
Biomass burning activities emit high concentrations of aerosol particles to the atmosphere. Such particles can interact with solar radiation, decreasing the amount of light reaching the surface and increasing the fraction of diffuse radiation through scattering processes, and thus has implications for photosynthesis within plant canopies. This work reports results from photosynthetically active radiation (PAR) and aerosol optical depth (AOD) measurements conducted simultaneously at Reserva Biológica do Jaru (Rondonia State, Brazil) during LBA/SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia/ Smoke, Aerosols, Clouds, Rainfall, and Climate) and RaCCI (Radiation, Cloud, and Climate Interactions in the Amazon during the Dry-to-Wet Transition Season) field experiments from 15 September to 15 November 2002. AOD values were retrieved from an AERONET (Aerosol Robotic Network) radiometer, MODIS (Moderate Resolution Spectroradiometer) and a portable sunphotometer from the United States Department of Agriculture – Forest Service. Significant reduction of PAR irradiance at the top of the canopy was observed due to the smoke aerosol particles layer. This radiation reduction affected turbulent fluxes of sensible and latent heats. The increase of AOD also enhanced the transmission of PAR inside the canopy. As a consequence, the availability of diffuse radiation was enhanced due to light scattering by the aerosol particles. A complex relationship was identified between light availability inside the canopy and net ecosystem exchange (NEE). The results showed that the increase of aerosol optical depth corresponded to an increase of CO<sub>2</sub> uptake by the vegetation. However, for even higher AOD values, the corresponding NEE was lower than for intermediate values. As expected, water vapor pressure deficit (VPD), retrieved at 28m height inside the canopy, can also affect photosynthesis. A decrease in NEE was observed as VPD increased. Further studies are needed to better understand these findings, which were reported for the first time for the Amazon region under smoky conditions
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