237 research outputs found
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
A disk inside the bipolar planetary nebula M2-9
Bipolarity in proto-planetary and planetary nebulae is associated with events
occurring in or around their cores. Past infrared observations have revealed
the presence of dusty structures around the cores, many in the form of disks.
Characterising those dusty disks provides invaluable constraints on the
physical processes that govern the final mass expulsion of intermediate-mass
stars. We focus this study on the famous M2-9 bipolar nebula, where the moving
lighthouse beam pattern indicates the presence of a wide binary. The compact
and dense dusty core in the center of the nebula can be studied by means of
optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines
with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are
interpreted using a dust radiative transfer Monte Carlo code. A disk-like
structure is detected perpendicular to the lobes and a good fit is found with a
stratified disk model composed of amorphous silicates. The disk is compact,
2535 mas at 8, and 3746 mas at 13. For
the adopted distance of 1.2 kpc, the inner rim of the disk is 15 AU. The
mass represents a few percent of the mass found in the lobes. The compactness
of the disk puts strong constraints on the binary content of the system, given
an estimated orbital period 90-120yr. We derive masses of the binary components
between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an
evolved star. We present different scenarios on the geometric structure of the
disk accounting for the interactions of the binary system, which includes an
accretion disk as well.Comment: 9 figures, A&A accepte
Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
To study the effect of metallicity on the mass-loss rate of asymptotic giant
branch (AGB) stars, we have conducted mid-infrared photometric measurements of
such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies
with the 10-m camera VISIR at the VLT. We derive mass-loss rates for 29
AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated
from the and colours. Radiative transfer models are used to
check the consistency of the method. Published IRAS and Spitzer data confirm
that the same tight correlation between colour and dust mass-loss
rates is observed for AGB stars from galaxies with different metallicities,
i.e. the Galaxy, the LMC and the SMC.
The derived dust mass-loss rates are in the range 5 to
3 Myr for the observed AGB stars in Sgr dSph
and around 5 Myr for those in Fornax; while
values obtained with the two different methods are of the same order of
magnitude. The mass-loss rates for these stars are higher than the nuclear
burning rates, so they will terminate their AGB phase by the depletion of their
stellar mantles before their core can grow significantly. Some observed stars
have lower mass-loss rates than the minimum value predicted by theoretical
models.Comment: 12 pages, 9 figures, accepted for publication in MNRA
The Early Evolution of Massive Stars: Radio Recombination Line Spectra
Velocity shifts and differential broadening of radio recombination lines are
used to estimate the densities and velocities of the ionized gas in several
hypercompact and ultracompact HII regions. These small HII regions are thought
to be at their earliest evolutionary phase and associated with the youngest
massive stars. The observations suggest that these HII regions are
characterized by high densities, supersonic flows and steep density gradients,
consistent with accretion and outflows that would be associated with the
formation of massive stars.Comment: ApJ in pres
The trigger of the AGB superwind: the importance of carbon
The driving mechanism of the AGB superwind has become controversial in recent
years. The efficacy of dust-driven mass loss has been queried. Spitzer
observation of AGB stars in Local Group Galaxies show the surprising result
that at low metallicity, AGB mass loss occurs at low luminosity, possibly lower
than in the Galaxy, but only for carbon-rich stars. Oxygen-rich stars in the
Galaxy and in lower metallicity galaxies have similar mass-loss rates only at
high luminosities. To explain this dichotomy, we propose that the superwind has
a dual trigger. The superwind starts either when sufficient excess carbon
builds up for efficient formation of carbonaceous dust (which we propose occurs
when ), or when the luminosity reaches
a value sufficient for a silicate-dust-driven wind (proposed at . We show that this dual trigger fits the current
observational constraints: the luminosity at which the superwind begins, and
the predominance of carbon superwind star at low metallicity. We use stellar
evolution models to check the consistency of our explanations and present
detailed predictions of the luminosities at which the superwind is triggered
for different metallicities and initial stellar masses.Comment: accepted for publications in MNRAS Letters, 5 pages, 4 figure
On next-to-eikonal corrections to threshold resummation for the Drell-Yan and DIS cross sections
We study corrections suppressed by one power of the soft gluon energy to the
resummation of threshold logarithms for the Drell-Yan cross section and for
Deep Inelastic structure functions. While no general factorization theorem is
known for these next-to-eikonal (NE) corrections, it is conjectured that at
least a subset will exponentiate, along with the logarithms arising at leading
power. Here we develop some general tools to study NE logarithms, and we
construct an ansatz for threshold resummation that includes various sources of
NE corrections, implementing in this context the improved collinear evolution
recently proposed by Dokshitzer, Marchesini and Salam (DMS). We compare our
ansatz to existing exact results at two and three loops, finding evidence for
the exponentiation of leading NE logarithms and confirming the predictivity of
DMS evolution.Comment: 17 page
Circumstellar CO in metal-poor stellar winds: the highly irradiated globular cluster star 47 Tucanae V3
We report the first detection of circumstellar CO in a globular cluster.
Observations with ALMA have detected the CO J=3-2 and SiO v=1 J=8-7 transitions
at 345 and 344 GHz, respectively, around V3 in 47 Tucanae (NGC 104; [Fe/H] =
-0.72 dex), a star on the asymptotic giant branch. The CO line is detected at 7
sigma at a rest velocity v_LSR = -40.6 km/s and expansion velocity of 3.2 +/-
~0.4 km/s. The brighter, asymmetric SiO line may indicate a circumstellar
maser. The stellar wind is slow compared to similar Galactic stars, but the
dust opacity remains similar to Galactic comparisons. We suggest that the
mass-loss rate is set by the levitation of material into the circumstellar
environment by pulsations, but that the terminal wind-expansion velocity is
determined by radiation pressure on the dust: a pulsation-enhanced dust-driven
wind. We suggest the metal-poor nature of the star decreases the grain size,
slowing the wind and increasing its density and opacity. Metallic alloys at
high altitudes above the photosphere could also provide an opacity increase.
The CO line is weaker than expected from Galactic AGB stars, but its strength
confirms a model that includes CO dissociation by the strong interstellar
radiation field present inside globular clusters.Comment: 5 pages, accepted MNRAS Letter
Heart rate dynamics during cardio-pulmonary exercise testing are associated with glycemic control in individuals with type 1 diabetes
IntroductionThis study investigated the degree and direction (kHR) of the heart rate to performance curve (HRPC) during cardio-pulmonary exercise (CPX) testing and explored the relationship with diabetes markers, anthropometry and exercise physiological markers in type 1 diabetes (T1DM).Material and methodsSixty-four people with T1DM (13 females; age: 34 ± 8 years; HbA1c: 7.8 ± 1% (62 ± 13 mmol.mol-1) performed a CPX test until maximum exhaustion. kHR was calculated by a second-degree polynomial representation between post-warm up and maximum power output. Adjusted stepwise linear regression analysis was performed to investigate kHR and its associations. Receiver operating characteristic (ROC) curve was performed based on kHR for groups kHR 0.20 in relation to HbA1c.ResultsWe found significant relationships between kHR and HbA1c (β = -0.70, P < 0.0001), age (β = -0.23, P = 0.03) and duration of diabetes (β = 0.20, P = 0.04). Stepwise linear regression resulted in an overall adjusted R2 of 0.57 (R = 0.79, P < 0.0001). Our data revealed also significant associations between kHR and percentage of heart rate at heart rate turn point from maximum heart rate (β = 0.43, P < 0.0001) and maximum power output relativized to bodyweight (β = 0.44, P = 0.001) (overall adjusted R2 of 0.44 (R = 0.53, P < 0.0001)). ROC curve analysis based on kHR resulted in a HbA1c threshold of 7.9% (62 mmol.mol-1).ConclusionOur data demonstrate atypical HRPC during CPX testing that were mainly related to glycemic control in people with T1DM
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