115 research outputs found
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
Effect of dexamethasone in patients with ARDS and COVID-19 - prospective, multi-centre, open-label, parallel-group, randomised controlled trial (REMED trial): A structured summary of a study protocol for a randomised controlled trial.
OBJECTIVES: The primary objective of this study is to test the hypothesis that administration of dexamethasone 20 mg is superior to a 6 mg dose in adult patients with moderate or severe ARDS due to confirmed COVID-19. The secondary objective is to investigate the efficacy and safety of dexamethasone 20 mg versus dexamethasone 6 mg. The exploratory objective of this study is to assess long-term consequences on mortality and quality of life at 180 and 360 days. TRIAL DESIGN: REMED is a prospective, phase II, open-label, randomised controlled trial testing superiority of dexamethasone 20 mg vs 6 mg. The trial aims to be pragmatic, i.e. designed to evaluate the effectiveness of the intervention in conditions that are close to real-life routine clinical practice. PARTICIPANTS: The study is multi-centre and will be conducted in the intensive care units (ICUs) of ten university hospitals in the Czech Republic. INCLUSION CRITERIA: Subjects will be eligible for the trial if they meet all of the following criteria: 1. Adult (≥18 years of age) at time of enrolment; 2. Present COVID-19 (infection confirmed by RT-PCR or antigen testing); 3. Intubation/mechanical ventilation or ongoing high-flow nasal cannula (HFNC) oxygen therapy; 4. Moderate or severe ARDS according to Berlin criteria: • Moderate - PaO2/FiO2 100-200 mmHg; • Severe - PaO2/FiO2 < 100 mmHg; 5. Admission to ICU in the last 24 hours. EXCLUSION CRITERIA: Subjects will not be eligible for the trial if they meet any of the following criteria: 1. Known allergy/hypersensitivity to dexamethasone or excipients of the investigational medicinal product (e.g. parabens, benzyl alcohol); 2. Fulfilled criteria for ARDS for ≥14 days at enrolment; 3. Pregnancy or breastfeeding; 4. Unwillingness to comply with contraception measurements from enrolment until at least 1 week after the last dose of dexamethasone (sexual abstinence is considered an adequate contraception method); 5. End-of-life decision or patient is expected to die within next 24 hours; 6. Decision not to intubate or ceilings of care in place; 7. Immunosuppression and/or immunosuppressive drugs in medical history: a) Systemic immunosuppressive drugs or chemotherapy in the past 30 days; b) Systemic corticosteroid use before hospitalization; c) Any dose of dexamethasone during the present hospital stay for COVID-19 for ≥5 days before enrolment; d) Systemic corticosteroids during present hospital stay for conditions other than COVID-19 (e.g. septic shock); 8. Current haematological or generalized solid malignancy; 9. Any contraindication for corticosteroid administration, e.g. • intractable hyperglycaemia; • active gastrointestinal bleeding; • adrenal gland disorders; • presence of superinfection diagnosed with locally established clinical and laboratory criteria without adequate antimicrobial treatment; 10. Cardiac arrest before ICU admission; 11. Participation in another interventional trial in the last 30 days. INTERVENTION AND COMPARATOR: Dexamethasone solution for injection/infusion is the investigational medicinal product as well as the comparator. The trial will assess two doses, 20 mg (investigational) vs 6 mg (comparator). Patients in the intervention group will receive dexamethasone 20 mg intravenously once daily on day 1-5, followed by dexamethasone 10 mg intravenously once daily on day 6-10. Patients in the control group will receive dexamethasone 6 mg day 1-10. All authorized medicinal products containing dexamethasone in the form of solution for i.v. injection/infusion can be used. MAIN OUTCOMES: Primary endpoint: Number of ventilator-free days (VFDs) at 28 days after randomisation, defined as being alive and free from mechanical ventilation. SECONDARY ENDPOINTS: a) Mortality from any cause at 60 days after randomisation; b) Dynamics of inflammatory marker (C-Reactive Protein, CRP) change from Day 1 to Day 14; c) WHO Clinical Progression Scale at Day 14; d) Adverse events related to corticosteroids (new infections, new thrombotic complications) until Day 28 or hospital discharge; e) Independence at 90 days after randomisation assessed by Barthel Index. The long-term outcomes of this study are to assess long-term consequences on mortality and quality of life at 180 and 360 days through telephone structured interviews using the Barthel Index. RANDOMISATION: Randomisation will be carried out within the electronic case report form (eCRF) by the stratified permuted block randomisation method. Allocation sequences will be prepared by a statistician independent of the study team. Allocation to the treatment arm of an individual patient will not be available to the investigators before completion of the whole randomisation process. The following stratification factors will be applied: • Age <65 and ≥ 65; • Charlson Comorbidity index (CCI) <3 and ≥3; • CRP <150 mg/L and ≥150 mg/L • Trial centre. Patients will be randomised in a 1 : 1 ratio into one of the two treatment arms. Randomisation through the eCRF will be available 24 hours every day. BLINDING (MASKING): This is an open-label trial in which the participants and the study staff will be aware of the allocated intervention. Blinded pre-planned statistical analysis will be performed. NUMBERS TO BE RANDOMISED (SAMPLE SIZE): The sample size is calculated to detect the difference of 3 VFDs at 28 days (primary efficacy endpoint) between the two treatment arms with a two-sided type I error of 0.05 and power of 80%. Based on data from a multi-centre randomised controlled trial in COVID-19 ARDS patients in Brazil and a multi-centre observational study from French and Belgian ICUs regarding moderate to severe ARDS related to COVID-19, investigators assumed a standard deviation of VFD at 28 days as 9. Using these assumptions, a total of 142 patients per treatment arm would be needed. After adjustment for a drop-out rate, 150 per treatment arm (300 patients per study) will be enrolled. TRIAL STATUS: This is protocol version 1.1, 15.01.2021. The trial is due to start on 2 February 2021 and recruitment is expected to be completed by December 2021. TRIAL REGISTRATION: The study protocol was registered on EudraCT No.:2020-005887-70, and on December 11, 2020 on ClinicalTrials.gov (Title: Effect of Two Different Doses of Dexamethasone in Patients With ARDS and COVID-19 (REMED)) Identifier: NCT04663555 with a last update posted on February 1, 2021. FULL PROTOCOL: The full protocol (version 1.1) is attached as an additional file, accessible from the Trials website (Additional file 1). In the interest of expediting dissemination of this material, the standard formatting has been eliminated; this Letter serves as a summary of the key elements of the full protocol
Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198 478
Context. Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind.
Aims. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind.
Methods. We modeled the H, He i, Si ii, and Si iii lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations.
Results. The Hα line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si ii and determine a multiperiodic oscillation in the He i absorption lines, with periods ranging from a few hours to 22.5 days.
Conclusions. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of α Cyg variables.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Science with a small two-band UV-photometry mission II: Observations of stars and stellar systems
We outline the impact of a small two-band UV-photometry satellite mission on
the field of stellar physics, magnetospheres of stars, binaries, stellar
clusters, interstellar matter, and exoplanets. On specific examples of
different types of stars and stellar systems, we discuss particular
requirements for such satellite missions in terms of specific mission
parameters such as bandpass, precision, cadence, and mission duration. We show
that such a mission may provide crucial data not only for hot stars that emit
most of their light in UV, but also for cool stars, where UV traces their
activity. This is important, for instance, for exoplanetary studies, because
the level of stellar activity influences habitability. While the main asset of
the two-band UV mission rests in time-domain astronomy, an example of open
clusters proves that such a mission would be important also for the study of
stellar populations. Properties of the interstellar dust are best explored when
combining optical and IR information with observations in UV. It is well known
that dust absorbs UV radiation efficiently. Consequently, we outline how such a
UV mission can be used to detect eclipses of sufficiently hot stars by various
dusty objects and study disks, rings, clouds, disintegrating exoplanets or
exoasteroids. Furthermore, UV radiation can be used to study the cooling of
neutron stars providing information about the extreme states of matter in the
interiors of neutron stars and used for mapping heated spots on their surfaces.Comment: Submitted to Space Science Review
Quick Ultra-VIolet Kilonova surveyor (QUVIK)
We present a near-UV space telescope on a ~70kg micro-satellite with a
moderately fast repointing capability and a near real-time alert communication
system that has been proposed in response to a call for an ambitious Czech
national mission. The mission, which has recently been approved for Phase 0, A,
and B1 study shall measure the brightness evolution of kilonovae, resulting
from mergers of neutron stars in the near-UV band and thus it shall distinguish
between different explosion scenarios. Between the observations of transient
sources, the satellite shall perform observations of other targets of interest,
a large part of which will be chosen in open competition.Comment: SPIE Astronomical Telescopes and Instrumentatio
X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars
The winds of massive stars have an impact on stellar evolution and on the
surrounding medium. The maximum speed reached by these outflows, the terminal
wind speed, is a global wind parameter and an essential input for models of
stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a
legacy UV spectroscopic survey with HST, we have the opportunity to quantify
the wind speeds of massive stars at sub-solar metallicity (in the Large and
Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We
empirically quantify the wind speeds of a large sample of OB stars, including
supergiants, giants, and dwarfs at sub-solar metallicity. Using these
measurements, we investigate trends of terminal wind speed with a number of
fundamental stellar parameters, namely effective temperature, metallicity, and
surface escape velocity. We empirically determined the terminal wind speed for
a sample of 149 OB stars in the Magellanic Clouds either by directly measuring
the maximum velocity shift of the absorption component of the Civ 1548-1550
line profile, or by fitting synthetic spectra produced using the Sobolev with
exact integration method. Stellar parameters were either collected from the
literature, obtained using spectral-type calibrations, or predicted from
evolutionary models. We find strong trends of terminal wind speed with
effective temperature and surface escape speed when the wind is strong enough
to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a
metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03
when we compared our results to previous Galactic studies. Our results suggest
that effective temperature rather than surface escape speed should be used as a
straightforward empirical prediction of terminal wind speed and that the
observed metallicity dependence is steeper than suggested by earlier works.Comment: 21 pages, 16 figures, 8 tables. Accepted in A&
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