91 research outputs found
The diffusion-induced nova scenario. CK Vul and PB 8 as possible observational counterparts
We propose a scenario for the formation of DA white dwarfs with very thin
helium buffers. For these stars we explore the possible occurrence of
diffusion-induced CNO- flashes, during their early cooling stage. In order to
obtain very thin helium buffers, we simulate the formation of low mass remnants
through an AGB final/late thermal pulse (AFTP/LTP scenario). Then we calculate
the consequent white dwarf cooling evolution by means of a consistent treatment
of element diffusion and nuclear burning. Based on physically sounding white
dwarf models, we find that the range of helium buffer masses for these
diffusion-induced novas to occur is significantly smaller than that predicted
by the only previous study of this scenario. As a matter of fact, we find that
these flashes do occur only in some low-mass (M < 0.6M) and low metallicity
(Z_ZAMS <0.001) remnants about 10^6 - 10^7 yr after departing from the AGB. For
these objects, we expect the luminosity to increase by about 4 orders of
magnitude in less than a decade. We also show that diffusion-induced novas
should display a very typical eruption lightcurve, with an increase of about a
few magnitudes per year before reaching a maximum of M_V ~ -5 to -6. Our
simulations show that surface abundances after the outburst are characterized
by logNH/NHe ~ -0.15...0.6 and N>C>O by mass fractions. Contrary to previous
speculations we show that these events are not recurrent and do not change
substantially the final H-content of the cool (DA) white dwarf. (Abridged)Comment: 16 pages, 8 figures, 3 tables. Replaced to match the final version
published by MNRAS. The definitive version is available at
http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291365-296
The born again (VLTP) scenario revisited: The mass of the remnants and implications for V4334 Sgr
We present 1-D numerical simulations of the very late thermal pulse
(VLTP) scenario for a wide range of remnant masses. We show that by taking
into account the different possible remnant masses, the observed evolution of
V4334 Sgr (a.k.a. Sakurai's Object) can be reproduced within the standard
1D-MLT stellar evolutionary models without the inclusion of any
reduced mixing efficiency. Our simulations hint at a consistent picture with
present observations of V4334 Sgr. From energetics, and within the standard MLT
approach, we show that low mass remnants \hbox{(\msun)} are
expected to behave markedly different than higher mass remnants
\hbox{(\msun)} in the sense that the latter are not expected to
expand significantly as a result of the violent H-burning that takes place
during the VLTP. We also assess the discrepancy in the born again times
obtained by different authors by comparing the energy that can be liberated by
H-burning during the VLTP event.Comment: Submitted to MNRAS. In includes an appendix regarding the treatment
of reduced convective motions within the Mixing Length Theor
A Population- and Hospital-based Cross-sectional Study of Renal Function in Hidradenitis Suppurativa
Planetary Nebulae in the Magellanic Clouds: II) Abundances and element production
We present the second part of an optical spectroscopic study of planetary
nebulae in the LMC and SMC. The first paper, Leisy & Dennefeld (1996),
discussed the CNO cycle for those objects where C abundances were available. In
this paper we concentrate more on other elemental abundances (such as O, Ne, S,
Ar) and their implications for the evolution of the progenitor stars. We use a
much larger sample of 183 objects, of which 65 are from our own observations,
where the abundances have been re-derived in a homogeneous way. For 156 of
them, the quality of data is considered to be satisfactory for further
analysis. We confirm the difficulty of separating Type-I and non-type-I objects
in the classical He-N/O diagram, as found in Paper I, a problem reinforced by
the variety of initial compositions for the progenitor stars. We observed
oxygen variations, either depletion via the ON cycle in the more massive
progenitor stars, or oxygen production in other objects. Neon production also
appears to be present. These enrichments are best explained by fresh material
from the core or from burning shells, brought to the surface by the 3rd
dredge-up, as reproduced in recent models, some including overshooting. All the
effects appear stronger in the SMC, suggesting a higher efficiency in a low
metallicity environment. Neither oxygen nor neon can therefore be used to
derive the initial composition of the progenitor star: other elements not
affected by processing such as sulfur, argon or, if observed, chlorine, have to
be preferred for this purpose. Some objects with very low initial abundances
are detected, but on average, the spatial distribution of PNe abundances is
consistent with the history of star formation (SF) as derived from field stars
in both Clouds.Comment: 22 pages, 28 figures, Accepted for publication in Astronomy &
Astrophysic
Asteroseismological constraints on the coolest GW Vir variable star (PG 1159-type)PG 0122+200
We present an asteroseismological study on PG 0122+200, the coolest known
pulsating PG1159 (GW Vir) star. Our results are based on an augmented set of
the full PG1159 evolutionary models recently presented by Miller Bertolami &
Althaus (2006). We perform extensive computations of adiabatic g-mode pulsation
periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to
0.741 Msun. We derive a stellar mass of 0.626 Msun from a comparison between
the observed period spacing and the computed asymptotic period spacing, and a
stellar mass of 0.567 Msun by comparing the observed period spacing with the
average of the computed period spacing. We also find, on the basis of a
period-fit procedure, an asteroseismological model representative of PG
0122+200 which is able to reproduce the observed period pattern with an average
of the period differences of 0.88 s. The model has an effective temperature of
81500 K, a stellar mass of 0.556 Msun, a surface gravity log g= 7.65, a stellar
luminosity and radius of log(L/Lsun)= 1.14 and log(R/Rsun)= -1.73,
respectively, and a He-rich envelope thickness of Menv= 0.019 Msun. We derive a
seismic distance of about 614 pc and a parallax of about 1.6 mas. The results
of the period-fit analysis carried out in this work suggest that the
asteroseismological mass of PG 0122+200 could be 6-20 % lower than thought
hitherto and in closer agreement (to within 5 %) with the spectroscopic mass.
This result suggests that a reasonable consistency between the stellar mass
values obtained from spectroscopy and asteroseismology can be expected when
detailed PG1159 evolutionary models are considered.Comment: 10 pages, 6 figures. To be published in Astronomy & Astrophysic
The population of close double white dwarfs in the Galaxy
We present a new model for the Galactic population of close double white
dwarfs. The model accounts for the suggestion of the avoidance of a substantial
spiral-in during mass transfer between a giant and a main-sequence star of
comparable mass and for detailed cooling models. It agrees well with the
observations of the local sample of white dwarfs if the initial binary fraction
is close to 50% and an ad hoc assumption is made that white dwarfs with mass
less than about 0.3 solar mass cool faster than the models suggest. About 1000
white dwarfs brighter than V=15 have to be surveyed for detection of a pair
which has total mass greater than the Chandrasekhar mass and will merge within
10 Gyr.Comment: 15 pages, 7 figures, to appear in Proc. ``The influence of binaries
on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren
ed.
On the robustness of H-deficient post-AGB tracks
We analyze the robustness of H--deficient post--AGB tracks regarding previous
evolution of their progenitor stars and the constitutive physics of the
remnants. Our motivation is a recent suggestion of Werner & Herwig (2006) that
previous evolution should be important in shaping the final post--AGB track and
the persisting discrepancy between asteroseismological and spectroscopical mass
determinations. This work is thus complementary to our previous work (Miller
Bertolami & Althaus 2006) and intends to shed some light on the uncertainty
behind the evolutionary tracks presented there. We compute full evolutionary
models for PG1159 stars taking into account different extramixing
(overshooting) efficiencies and lifetimes on the TP-AGB during the progenitor
evolution. We also assess the effect of possible differences in the opacities
and equation of state by artificially changing them before the PG1159 stage.
Also comparisons are made with the few H-deficient post--AGB tracks available
in the literature. Contrary to our expectations, we found that previous
evolution is not a main factor in shaping H--deficient post--AGB tracks.
Interestingly enough, we find that only an increase of in the
intershell opacities at high effective temperatures may affect the tracks as to
reconcile spectroscopic and asteroseismologic mass determinations. This forces
us to conclude that our previous tracks (Miller Bertolami & Althaus 2006) are
robust enough as to be used for spectroscopic mass determinations, unless
opacities in the intershell region are substantially different. Our results,
then, call for an analysis of possible systematics in the usually adopted
asteroseismological mass determination methods.Comment: Accepted for publication in Astronomy and Astrophysc
Dairy Intake and Acne Vulgaris:A Systematic Review and Meta-Analysis of 78,529 Children, Adolescents, and Young Adults
A meta-analysis can help inform the debate about the epidemiological evidence on dairy intake and development of acne. A systematic literature search of PubMed from inception to 11 December 2017 was performed to estimate the association of dairy intake and acne in children, adolescents, and young adults in observational studies. We estimated the pooled random effects odds ratio (OR) (95% CI), heterogeneity (I2-statistics, Q-statistics), and publication bias. We included 14 studies (n = 78,529; 23,046 acne-cases/55,483 controls) aged 7–30 years. ORs for acne were 1.25 (95% CI: 1.15–1.36; p = 6.13 × 10−8) for any dairy, 1.22 (1.08–1.38; p = 1.62 × 10−3) for full-fat dairy, 1.28 (1.13–1.44; p = 8.23 × 10−5) for any milk, 1.22 (1.06–1.41; p = 6.66 × 10−3) for whole milk, 1.32 (1.16–1.52; p = 4.33 × 10−5) for low-fat/skim milk, 1.22 (1.00–1.50; p = 5.21 × 10−2) for cheese, and 1.36 (1.05–1.77; p = 2.21 × 10−2) for yogurt compared to no intake. ORs per frequency of any milk intake were 1.24 (0.95–1.62) by 2–6 glasses per week, 1.41 (1.05–1.90) by 1 glass per day, and 1.43 (1.09–1.88) by ≥2 glasses per day compared to intake less than weekly. Adjusted results were attenuated and compared unadjusted. There was publication bias (p = 4.71 × 10−3), and heterogeneity in the meta-analyses were explained by dairy and study characteristics. In conclusion, any dairy, such as milk, yogurt, and cheese, was associated with an increased OR for acne in individuals aged 7–30 years. However, results should be interpreted with caution due to heterogeneity and bias across studies
Observations of V838 Mon in the CO rotational lines
We investigate the structure of a field around the position of V838 Mon as
seen in the lowest CO rotational transitions. We also measure and analyse
emission in the same lines at the position of V838 Mon.Observations have
primarily been done in the CO J = 2-1 and J = 3-2 lines using the KOSMA
telescope. A field of 3.4 squared degrees has been mapped in the on-the-fly
mode in these transitions. Longer integration spectra in the on-off mode have
been obtained to study the emission at the position of V838 Mon. Selected
positions in the field have also been observed in the CO J = 1-0 transition
using the Delingha telescope.In the observed field we have identified many
molecular clouds. They can be divided into two groups from the point of view of
their observed radial velocities. One, having V(LSR) in the range 18-32 km/s,
can be identified with the Perseus Galactic arm. The other one, having V(LSR)
between 44-57 km/s, probably belongs to the Norma-Cygnus arm. The radial
velocity of V838 Mon is within the second range but the object does not seem to
be related to any of the observed clouds. We did not find any molecular buble
of a 1 degree dimension around the position of V838 Mon claimed in van Loon et
al. An emission has been detected at the position of the object in the CO J =
2-1 and J = 3-2 transitions. The emission is very narrow (FWHM ~ 1.2 km/s) and
at V(LSR) = 53.3 km/s. Our analysis of the data suggests that the emission is
probably extended.Comment: paper accepted in A&
Thermohaline mixing and the photospheric composition of low-mass giant stars
We compute full evolutionary sequences of red giant branch stars close to the
luminosity bump by including state of the art composition transport
prescriptions for the thermohaline mixing regimes. In particular we adopt a
self-consistent double-diffusive convection theory, that allows to handle the
instabilities that arise when thermal and composition gradients compete against
each other, and a very recent empirically motivated and parameter free
asymptotic scaling law for thermohaline composition transport. In agreement
with previous works, we find that during the red giant stage, a thermohaline
instability sets in shortly after the hydrogen burning shell (HBS) encounters
the chemical discontinuity left behind by the first dredge-up. We also find
that the thermohaline unstable region, initially appearing at the exterior wing
of the HBS, is unable to reach the outer convective envelope, with the
consequence that no mixing of elements that produces a non-canonical
modification of the stellar surface abundances occurs. Also in agreement with
previous works, we find that by artificially increasing the mixing efficiency
of thermohaline regions it is possible to connect both unstable regions, thus
affecting the photospheric composition. However, we find that in order to
reproduce the observed abundances of red giant branch stars close to the
luminosity bump, thermohaline mixing efficiency has to be artificially
increased by about 4 orders of magnitude from that predicted by recent 3D
numerical simulations of thermohaline convection close to astrophysical
environments. From this we conclude the chemical abundance anomalies of red
giant stars cannot be explained on the basis of thermohaline mixing alone.Comment: 7 pages, 6 figures, accepted for publication in A&
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