262 research outputs found
The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies
Silicates are an important component of interstellar dust and the structure
of these grains -- amorphous versus crystalline -- is sensitive to the local
physical conditions. We have studied the infrared spectra of a sample of
ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow
absorption features at 11, 16, 19, 23, and 28 microns, characteristic of
crystalline silicates, superimposed on the broad absorption bands at 10 and 18
microns due to amorphous silicates in a subset of this sample. These features
betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of
the olivines. Previously, crystalline silicates have only been observed in
circumstellar environments. The derived fraction of forsterite to amorphous
silicates is typically 0.1 in these ULIRGs. This is much larger than the upper
limit for this ratio in the interstellar medium of the Milky Way, 0.01. These
results suggest that the timescale for injection of crystalline silicates into
the ISM is short in a merger-driven starburst environment (e.g., as compared to
the total time to dissipate the gas), pointing towards massive stars as a
prominent source of crystalline silicates. Furthermore, amorphization due to
cosmic rays, which is thought to be of prime importance for the local ISM, lags
in vigorous starburst environments.Comment: 7 pages, 5 figures, accepted for publication in Ap
Mid-Infrared interferometry of dust around massive evolved stars
We report long-baseline interferometric measurements of circumstellar dust
around massive evolved stars with the MIDI instrument on the Very Large
Telescope Interferometer and provide spectrally dispersed visibilities in the
8-13 micron wavelength band. We also present diffraction-limited observations
at 10.7 micron on the Keck Telescope with baselines up to 8.7 m which explore
larger scale structure. We have resolved the dust shells around the late type
WC stars WR 106 and WR 95, and the enigmatic NaSt1 (formerly WR 122), suspected
to have recently evolved from a Luminous Blue Variable (LBV) stage. For AG Car,
the protoypical LBV in our sample, we marginally resolve structure close to the
star, distinct from the well-studied detached nebula. The dust shells around
the two WC stars show fairly constant size in the 8-13 micron MIDI band, with
gaussian half-widths of ~ 25 to 40 mas. The compact dust we detect around NaSt1
and AG Car favors recent or ongoing dust formation.
Using the measured visibilities, we build spherically symmetric radiative
transfer models of the WC dust shells which enable detailed comparison with
existing SED-based models. Our results indicate that the inner radii of the
shells are within a few tens of AU from the stars. In addition, our models
favor grain size distributions with large (~ 1 micron) dust grains. This
proximity of the inner dust to the hot central star emphasizes the difficulty
faced by current theories in forming dust in the hostile environment around WR
stars. Although we detect no direct evidence for binarity for these objects,
dust production in a colliding-wind interface in a binary system is a feasible
mechanism in WR systems under these conditions.Comment: 21 pages, 4 tables, 13 figures. Accepted for publication in the
Astrophysical Journa
Spectroscopy of the candidate luminous blue variable at the center of the ring nebula G79.29+0.46
We report optical and near-infrared spectra of the central star of the radio
source G79.29+0.46, a candidate luminous blue variable. The spectra contain
numerous narrow (FWHM < 100 km/s emission lines of which the low-lying hydrogen
lines are the strongest, and resemble spectra of other LBVc's and B[e]
supergiants. A few prominent infrared lines are unidentified. The terminal wind
speed is determined from H alpha to be 110 km/s. The strength of H alpha
implies the presence of a very dense wind. Extended emission from H alpha and
[N II] was detected but appears to be associated with the Cygnus X region
rather than the radio source. Both diffuse interstellar bands and interstellar
absorption lines are present in the optical spectrum of the central star,
suggesting that there are both diffuse and molecular cloud components to the
extinction and implying a minimum distance of 1 kpc and minimum luminosity of
\~10^5 L(sun) for the star. The new spectra and their analysis indicate a low
excitation, dense, and slowly expanding wind and support the LBVc
classification.Comment: Accepted for publication in Astronomy and Astrophysics, 7 PostScript
figures, 5 table
On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars
We suggest that the mass lost during the evolution of very massive stars may
be dominated by optically thick, continuum-driven outbursts or explosions,
instead of by steady line-driven winds. In order for a massive star to become a
WR star, it must shed its H envelope, but new estimates of the effects of
clumping in winds indicate that line driving is vastly insufficient. We discuss
massive stars above roughly 40-50 Msun, for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our clearest
example of this phenomenon is the 19th century outburst of eta Car, when the
star shed 12-20 Msun or more in less than a decade. Other examples are
circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova
impostors''), and massive shells around SNe and GRBs. We do not yet fully
understand what triggers LBV outbursts, but they occur nonetheless, and present
a fundamental mystery in stellar astrophysics. Since line opacity from metals
becomes too saturated, the extreme mass loss probably arises from a
continuum-driven wind or a hydrodynamic explosion, both of which are
insensitive to metallicity. As such, eruptive mass loss could have played a
pivotal role in the evolution and fate of massive metal-poor stars in the early
universe. If they occur in these Population III stars, such eruptions would
profoundly affect the chemical yield and types of remnants from early SNe and
hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter
AG Carinae: a Luminous Blue Variable with a high rotational velocity
We report the detection of broad absorptions due to Si IV 4088-4116 A in the
Luminous Blue Variable (LBV) AG Carinae during its last hot phase (2001-2003).
Our NLTE spectral analysis, with the radiative transfer code CMFGEN, revealed
the photospheric nature of these lines predicting, however, much narrower and
deeper absorption profiles than observed. Using a recently-developed code to
compute synthetic spectra in 2D geometry allowing for the effects of rotation,
we could match the broad absorptions with a high projected rotational velocity
of 190 +/- 30 km/s on 2001 April. Analysis of spectra obtained on 2002 March
and 2003 January, when the star was cooling, yielded to a projected rotational
velocity of 110 +/- 10 km/s and 85 +/- 10 km/s, respectively. The derived
rotational velocities are proportional to R^-1, as expected from angular
momentum conservation. We discuss the effects of such high rotation on the
spectral analysis of AG Car, and on the wind terminal velocity. Our results
show direct spectroscopic evidence, for the first time, that a LBV may rotate
at a significant fraction of its break-up velocity. Thus, AG Car (and possibly
other LBVs) is indeed close to the Gamma-Omega limit, as predicted by
theoretical studies of LBVs.Comment: 15 pages, 4 figures, submitted to ApJ Letter
The Structure of the Homunculus. II. Modeling the physical conditions in Eta Car's molecular shell
We present models that reproduce the observed double-shell structure of the
Homunculus Nebula around eta Carinae, including the stratification of infrared
H2 and [FeII] emission seen in data obtained with the Phoenix spectrograph on
Gemini South, as well as the corresponding stratified grain temperature seen in
thermal-infrared data. Tuning the model to match the observed shell thickness
allows us to determine the threshold density which permits survival of H2. An
average hydrogen density of n_H=(0.5-1)x10^7 cm-3 in the outer zone is required
to allow H2 to exist at all latitudes in the nebula, and for Fe+ to recombine.
This gives independent confirmation of the very large mass of the Homunculus,
indicating a total of roughly 15--35 Msun (although we note reasons why the
lower end of this range is favored). At the interface between the atomic and
molecular zones, we predict a sharp drop in the dust temperature, in agreement
with the bimodal dust color temperatures observed in the two zones. In the
outer molecular shell, the dust temperature drops to nearly the blackbody
temperature, and becomes independent of grain size because of self-shielding at
shorter UV wavelengths and increased heating at longer wavelengths. This
relaxes constraints on large grain sizes suggested by near-blackbody color
temperatures. Finally, from the strength of infrared [FeII] emission in the
inner shell we find that the gas-phase Fe abundance is roughly solar. This is
astonishing in such a dusty object, where one normally expects gaseous iron to
be depleted by two orders of magnitude.Comment: 18 pages, 5 figures. Accepted by Ap
Ejection Fraction, Biomarkers, and Outcomes and Impact of Vericiguat on Outcomes Across EF in VICTORIA
Background: Vericiguat reduced the risk of cardiovascular death (CVD) or hospitalization for heart failure (HF) in patients with worsening HF and reduced left ventricular ejection fraction (LVEF).Objectives: The authors assessed the association of LVEF with biomarker levels, risk of outcome, and whether the effect of vericiguat was homogeneous across LVEF in the VICTORIA (Vericiguat Global Study in Subjects with Heart Failure With Reduced Ejection Fraction) trial.Methods: Patients were grouped by LVEF tertiles (â€24%, 25%-33%, and >33%). Patient characteristics, clinical outcomes, and efficacy and safety of vericiguat were examined by tertile. Prespecified biomarkers including N-terminal proâB-type natriuretic peptide, cardiac troponin T, growth differentiation factor 15, interleukin 6, high-sensitivity C-reactive protein, and cystatin C were examined.Results: The mean LVEF was 29% ± 8% (range: 5%-45%). A pattern of higher N-terminal proâB-type natriuretic peptide, high-sensitivity C-reactive protein, and interleukin 6 was evident in patients in the lowest LVEF tertile vs the other tertiles. Patients with lower LVEF experienced higher rates of the composite outcome (41.7%, 36.3%, and 33.4% for LVEF â€24, 25-33, and >33; P < 0.001). There was no significant treatment effect heterogeneity of vericiguat across LVEF groups (adjusted HR from lowest to highest tertiles: 0.79 [95% CI: 0.68-0.94]; 0.95 [95% CI: 0.82-1.11]; 0.94 [95% CI: 0.79-1.11]; P for interaction = 0.222), although the HR was numerically lower in the lowest tertile. There was also no heterogeneity of effect for CVD and HF hospitalization individually (P interaction for CVD = 0.964; HF hospitalization = 0.438). Discontinuation of treatment because of adverse events, symptomatic hypotension, or syncope was consistent across the range of LVEF.Conclusions: Patients with lower LVEF had a distinctive biomarker profile and a higher risk for adverse clinical outcomes vs those with a higher LVEF. There was no significant interaction for the benefit of vericiguat across LVEF tertiles, although the largest signal for benefit in both the primary outcome and HF hospitalizations was noted in tertile 1 (LVEF â€24%).</p
Effects of serelaxin in acute heart failure patients with renal impairment:results from RELAX-AHF
Serelaxin showed beneficial effects on clinical outcome and trajectories of renal markers in patients with acute heart failure. We aimed to study the interaction between renal function and the treatment effect of serelaxin.In the current post hoc analysis of the RELAX-AHF trial, we included all patients with available estimated glomerular filtration rate (eGFR) at baseline (n = 1132). Renal impairment was defined as an eGFR <60 ml/min/1.73 m(2) estimated by creatinine.817 (72.2 %) patients had a baseline eGFR <60 ml/min/1.73 m(2). In placebo-treated patients, baseline renal impairment was related to a higher 180 day cardiovascular (HR 3.12, 95 % CI 1.33-7.30) and all-cause mortality (HR 2.81, 95 % CI 1.34-5.89). However, in serelaxin-treated patients, the risk of cardiovascular and all-cause mortality was less pronounced (HR 1.19, 95 % CI 0.54 -2.64; p for interaction = 0.106, and HR 1.15 95 % CI 0.56-2.34 respectively; p for interaction = 0.088). In patients with renal impairment, treatment with serelaxin resulted in a more pronounced all-cause mortality reduction (HR 0.53, 95 % CI 0.34-0.83), compared with patients without renal impairment (HR 1.30, 95 % CI 0.51-3.29).Renal dysfunction was associated with higher cardiovascular and all-cause mortality in placebo-treated patients, but not in serelaxin-treated patients. The observed reduction in (cardiovascular) mortality in RELAX-AHF was more pronounced in patients with renal dysfunction. These observations need to be confirmed in the ongoing RELAX-AHF-2 trial.</p
Angiotensin-converting enzyme 2 (ACE2), SARS-CoV-2 and the pathophysiology of coronavirus disease 2019 (COVID-19)
Angiotensinâconverting enzymeâ2 (ACE2) has been established as the functional host receptor for severe acute respiratory syndrome coronavirus 2 (SARSâCoVâ2), the virus responsible for the current devastating worldwide pandemic of coronavirus disease 2019 (COVIDâ19). ACE2 is abundantly expressed in a variety of cells residing in many different human organs. In human physiology, ACE2 is a pivotal counterâregulatory enzyme to ACE by the breakdown of angiotensin II, the central player in the reninâangiotensinâaldosterone system (RAAS) and the main substrate of ACE2. Many factors have been associated with both altered ACE2 expression and COVIDâ19 severity and progression, including age, sex, ethnicity, medication and several coâmorbidities, such as cardiovascular disease and metabolic syndrome. Although ACE2 is widely distributed in various human tissues and many of its determinants have been well recognised, ACE2âexpressing organs do not equally participate in COVIDâ19 pathophysiology, implying that other mechanisms are involved in orchestrating cellular infection resulting in tissue damage. Reports of pathologic findings in tissue specimens of COVIDâ19 patients are rapidly emerging and confirm the established role of ACE2 expression and activity in disease pathogenesis. Identifying pathologic changes caused by SARSâCoVâ2 infection is crucially important as it has major implications for understanding COVIDâ19 pathophysiology and the development of evidenceâbased treatment strategies. Currently, many interventional strategies are being explored in ongoing clinical trials, encompassing many drug classes and strategies, including antiviral drugs, biological response modifiers and RAAS inhibitors. Ultimately, prevention is key to combat COVIDâ19 and appropriate measures are being taken accordingly, including development of effective vaccines. In this review, we describe the role of ACE2 in COVIDâ19 pathophysiology, including factors influencing ACE2 expression and activity in relation to COVIDâ19 severity. In addition, we discuss the relevant pathological changes resulting from SARSâCoVâ2 infection. Finally, we highlight a selection of potential treatment modalities for COVIDâ19
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