1,633 research outputs found
Constraining the primordial initial mass function with stellar archaeology
We present a new near-field cosmological probe of the initial mass function
(IMF) of the first stars. Specifically, we constrain the lower-mass limit of
the Population III (Pop III) IMF with the total number of stars in large,
unbiased surveys of the Milky Way. We model the early star formation history in
a Milky Way-like halo with a semi-analytic approach, based on Monte-Carlo
sampling of dark matter merger trees, combined with a treatment of the most
important feedback mechanisms. Assuming a logarithmically flat Pop III IMF and
varying its low mass limit, we derive the number of expected survivors of these
first stars, using them to estimate the probability to detect any such Pop III
fossil in stellar archaeological surveys. Following our analysis, the most
promising region to find possible Pop III survivors is the stellar halo of the
Milky Way, which is the best target for future surveys. We find that if no
genuine Pop III survivor is detected in a sample size of () halo stars with well-controlled selection effects, then we can
exclude the hypothesis that the primordial IMF extended down below at a confidence level of 68% (99%). With the sample size of the
Hamburg/ESO survey, we can tentatively exclude Pop III stars with masses below
with a confidence level of 95%, although this is subject to
significant uncertainties. To fully harness the potential of our approach,
future large surveys are needed that employ uniform, unbiased selection
strategies for high-resolution spectroscopic follow-up.Comment: 19 pages, 14 figures, published in MNRA
Intracapillary leucocyte accumulation as a novel antihaemorrhagic mechanism in acute pancreatitis in mice
Background: Pancreatic infiltration by leucocytes represents a hallmark in acute pancreatitis. Although leucocytes play an active role in the pathophysiology of this disease, the relation between leucocyte activation, microvascular injury and haemorrhage has not been adequately addressed.Methods: We investigated intrapancreatic leucocyte migration, leucocyte extravasation and pancreatic microperfusion in different models of oedematous and necrotising acute pancreatitis in lys-EGFP-ki mice using fluorescent imaging and time-lapse intravital microscopy.Results: In contrast to the current paradigm of leucocyte recruitment, the initial event of leucocyte activation in acute pancreatitis was represented through a dose- and time-dependent occlusion of pancreatic capillaries by intraluminally migrating leucocytes. Intracapillary leucocyte accumulation (ILA) resulted in dense filling of almost all capillaries close to the area of inflammation and preceded transvenular leucocyte extravasation. ILA was also initiated by isolated exposure of the pancreas to interleukin 8 or fMLP, demonstrating the causal role of chemotactic stimuli in the induction of ILA. The onset of intracapillary leucocyte accumulation was strongly inhibited in LFA-1-/- and ICAM-1-/- mice, but not in Mac-1-/- mice. Moreover, prevention of intracapillary leucocyte accumulation led to the development of massive capillary haemorrhages and transformed mild pancreatitis into lethal haemorrhagic disease.Conclusions: ILA represents a novel protective and potentially lifesaving mechanism of haemostasis in acute pancreatitis. This process depends on expression of LFA-1 and ICAM-1 and precedes the classical steps of the leucocyte recruitment cascade
A unified model for the LISA measurements and instrument simulations
LISA is a space-based mHz gravitational-wave observatory, with a plannedlaunch in 2034. It is expected to be the first detector of its kind, and willpresent unique challenges in instrumentation and data analysis. An accuratepre-flight simulation of LISA data is a vital part of the development of boththe instrument and the analysis methods. The simulation must include a detailedmodel of the full measurement and analysis chain, capturing the main featuresthat affect the instrument performance and processing algorithms. Here, wepropose a new model that includes, for the first time, proper relativistictreatment of reference frames with realistic orbits; a model for onboard clocksand clock synchronization measurements; proper modeling of total laserfrequencies, including laser locking, frequency planning and Doppler shifts;better treatment of onboard processing and updated noise models. We thenintroduce two implementations of this model, LISANode and LISA Instrument. Wedemonstrate that TDI processing successfully recovers gravitational-wavesignals from the significantly more realistic and complex simulated data.LISANode and LISA Instrument are already widely used by the LISA community and,for example, currently provide the mock data for the LISA Data Challenges.<br
How an improved implementation of H2 self-shielding influences the formation of massive stars and black holes
High redshift quasars at z>6 have masses up to ~ M. One of the
pathways to their formation includes direct collapse of gas, forming a
supermassive star, precursor of the black hole seed. The conditions for direct
collapse are more easily achievable in metal-free haloes, where atomic hydrogen
cooling operates and molecular hydrogen (H2) formation is inhibited by a strong
external UV flux. Above a certain value of UV flux (J_crit), the gas in a halo
collapses isothermally at ~ K and provides the conditions for
supermassive star formation. However, H2 can self-shield, reducing the effect
of photodissociation. So far, most numerical studies used the local Jeans
length to calculate the column densities for self-shielding. We implement an
improved method for the determination of column densities in 3D simulations and
analyse its effect on the value of J_crit. This new method captures the gas
geometry and velocity field and enables us to properly determine the
direction-dependent self-shielding factor of H2 against photodissociating
radiation. We find a value of J_crit that is a factor of two smaller than with
the Jeans approach (~2000 J_21 vs. ~4000 J_21). The main reason for this
difference is the strong directional dependence of the H2 column density. With
this lower value of J_crit, the number of haloes exposed to a flux >J_crit is
larger by more than an order of magnitude compared to previous studies. This
may translate into a similar enhancement in the predicted number density of
black hole seeds.Comment: 14 pages, 12 figures, published in MNRA
On the Detection of Supermassive Primordial Stars. II. Blue Supergiants
Supermassive primordial stars in hot, atomically-cooling haloes at
15 - 20 may have given birth to the first quasars in the universe. Most
simulations of these rapidly accreting stars suggest that they are red, cool
hypergiants, but more recent models indicate that some may have been bluer and
hotter, with surface temperatures of 20,000 - 40,000 K. These stars have
spectral features that are quite distinct from those of cooler stars and may
have different detection limits in the near infrared (NIR) today. Here, we
present spectra and AB magnitudes for hot, blue supermassive primordial stars
calculated with the TLUSTY and CLOUDY codes. We find that photometric
detections of these stars by the James Webb Space Telescope (JWST) will be
limited to 10 - 12, lower redshifts than those at which red stars
can be found, because of quenching by their accretion envelopes. With moderate
gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope
(WFIRST) could detect blue supermassive stars out to similar redshifts in
wide-field surveys.Comment: 9 pages, 5 figures, accepted by MNRA
Predicting the locations of possible long-lived low-mass first stars: Importance of satellite dwarf galaxies
The search for metal-free stars has so far been unsuccessful, proving that if
there are surviving stars from the first generation, they are rare, they have
been polluted, or we have been looking in the wrong place. To predict the
likely location of Population~III (Pop~III) survivors, we semi-analytically
model early star formation in progenitors of Milky Way-like galaxies and their
environments. We base our model on merger trees from the high-resolution dark
matter only simulation suite \textit{Caterpillar}. Radiative and chemical
feedback are taken into account self-consistently, based on the spatial
distribution of the haloes. Our results are consistent with the non-detection
of Pop III survivors in the Milky Way today. We find that possible surviving
Population III stars are more common in Milky Way satellites than in the main
Galaxy. In particular, low mass Milky Way satellites contain a much larger
fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way
satellites are promising targets for future attempts to find Pop~III survivors,
especially for high-resolution, high signal-to-noise spectroscopic
observations. We provide the probabilities for finding a Pop~III survivor in
the red giant branch phase for all known Milky Way satellites to guide future
observations.Comment: 17 pages, 12 figures, 1 table, submitted to MNRA
Time Delay Interferometry combinations as instrument noise monitors for LISA
The LISA mission will likely be a signal dominated detector, such that one challenge is the separation of the different astrophysical sources, and to distinguish between them and the instrumental noise. One of the goals of LISA is to probe the early Universe by detecting stochastic GW backgrounds. As correlation with other detectors is not possible for LISA, discrimination of such a GW background from the instrumental noise requires a good estimate of the latter. To this purpose we have revisited Time Delay Interferometry (TDI) to look for new TDI signal combinations that fulfill the laser frequency noise suppression requirements. We illustrate that it is possible to do a linear combination of these TDI channels to find special null-combinations that suppress gravitational waves and mainly carry information about instrumental noise. We find that there exist many null-combinations that show different sensitivities to gravitational waves, some of which seem more suitable than the traditional T combination for estimating test-mass acceleration noise. In an idealised LISA configuration, they are all sensitive to a particular linear combination of the six test-masses acceleration, similar to a rigid rotation of the LISA triangle. In the following article, we illustrate what are the noise properties that can be extracted by monitoring these interferometry signals and discuss the implication of these findings for the detection of stochastic GW backgrounds
Is there a causal role for homocysteine concentration in blood pressure?:A Mendelian randomization study
Background: An understanding of whether homocysteine is a cause or a marker of increased blood pressure is relevant because blood homocysteine can be effectively lowered by safe and inexpensive interventions (e.g., vitamin B-6, B-9, and B-12 supplementation). Objective: The aim was to assess the causal influence of homocysteine on systolic and diastolic blood pressure (SBP and DBP, respectively) in adults with the use of Mendelian randomization (MR). Design: Data from the 1982 Pelotas Birth Cohort (Brazil) were used. A total of 4297 subjects were evaluated in 2004–2005 (mean age: 22.8 y). The association of homocysteine concentration with SBP and DBP was assessed by conventional ordinary least-squares (OLS) linear regression and 2-stage least-squares (2SLS) regression (MR analysis). The single nucleotide polymorphism (SNP) methylenetetrahydrofolate reductase (MTHFR) C677T (rs1801133) was used as proxy for homocysteine concentration. We also applied MR to data from the International Consortium for Blood Pressure (ICBP) genomewide association studies (>69,000 participants) using rs1801133 and additional homocysteine-associated SNPs as instruments. Results: In OLS regression, a 1-SD unit increase in log homocysteine concentration was associated with an increase of 0.9 (95% CI: 0.4, 1.4) mm Hg in SBP and of 1.0 (95% CI: 0.6, 1.4) mm Hg in DBP. In 2SLS regression, for the same increase in homocysteine, the coefficients were −1.8 mm Hg for SBP (95% CI: −3.9, 0.4 mm Hg; P = 0.01) and 0.1 mm Hg for DBP (95% CI: −1.5, 1.7 mm Hg; P = 0.24). In the MR analysis of ICBP data, homocysteine concentration was not associated with SBP (β = 0.6 mm Hg for each 1-SD unit increase in log homocysteine; 95% CI: −0.8, 1.9 mm Hg) but was positively associated with DBP (β = 1.1 mm Hg; 95% CI: 0.2, 1.9 mm Hg). The association of genetically increased homocysteine with DBP was not consistent across different SNPs. Conclusion: Overall, the present findings do not corroborate the hypothesis that homocysteine has a causal role in blood pressure, especially in SBP
Auditory coding of reaching space
Reaching movements are usually initiated by visual events and controlled visually and kinesthetically. Lately, studies have focused on the possible benefit of auditory information for localization tasks, and also for movement control. This explorative study aimed to investigate if it is possible to code reaching space purely by auditory information. Therefore, the precision of reaching movements to merely acoustically coded target positions was analyzed. We studied the efficacy of acoustically effect-based and of additional acoustically performance-based instruction and feedback and the role of visual movement control. Twenty-four participants executed reaching movements to merely acoustically presented, invisible target positions in three mutually perpendicular planes in front of them. Effector-endpoint trajectories were tracked using inertial sensors. Kinematic data regarding the three spatial dimensions and the movement velocity were sonified. Thus, acoustic instruction and real-time feedback of the movement trajectories and the target position of the hand were provided. The subjects were able to align their reaching movements to the merely acoustically instructed targets. Reaching space can be coded merely acoustically, additional visual movement control does not enhance reaching performance. On the basis of these results, a remarkable benefit of kinematic movement acoustics for the neuromotor rehabilitation of everyday motor skills can be assumed
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