Carbon-Oxygen White Dwarf Accreting CO-Rich Matter. II. Self-Regulating Accretion Process up to the Explosive Stage

We investigate the effect of rotation on the evolution of double-degenerate white dwarf systems, which are possible progenitors of Type Ia supernovae. We assume that prior to merging, the two white dwarfs rotate synchronously at the orbital frequency and that in the merger process, the lighter white dwarf is transformed into a thick disk from which the more massive white dwarf initially accretes at a very high rate (~10-5 M☉ yr-1). Because of the lifting effect of rotation, the accreting white dwarf expands until the gravitational acceleration and centripetal acceleration required for binding at the surface become equal, initiating a Roche instability. The white dwarf continues to accrete matter from the disk, but at a rate that is determined by the balance between two competing processes operating in outer layers: (1) heating, expansion, and spin-up due to accretion and (2) cooling and contraction due to thermal diffusion. The balance produces an accretion rate such that the angular velocity of the white dwarf ωWD and the break-up angular velocity ωcr remain equal. Because of the deposition of angular momentum by accreted matter and the contraction of the accreting star, ωWD increases continuously until the rotational energy reaches about 14% of the gravitational binding energy; then, another instability sets in: the structure is forced to adopt an elliptical shape and emit gravitational waves. Thereafter, a balance between the rate of deposition of angular momentum by accreted matter and the rate of loss of angular momentum by gravitational waves produces a nearly constant or "plateau" accretion rate of ~4 × 10-7 M☉ yr-1. The mass of the accreting white dwarf can increase up to and beyond the Chandresekhar mass limit for nonrotating white dwarfs before carbon ignition occurs. Independent of the initial value of the accretion rate, the physical conditions suitable for carbon ignition are achieved at the center of the accreting white dwarf and, because of the high electron degeneracy, the final outcome is an event of SN Ia proportions. Our results apply to merged binary white dwarf systems which, at the onset of explosive carbon ignition, have a total mass in the range 1.4-1.5 M☉

Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models

We investigate the lifting effect of rotation on the thermal evolution of CO WDs accreting CO-rich matter. We find that rotation induces the cooling of the accreting star so that the delivered gravitational energy causes a greater expansion with respect to the standard non-rotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting WD becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting WD and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting WD can attain the physical conditions for C-burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting WD undergoes a secular instability when its total mass approaches 1.4 Msun. At this stage, the ratio between the rotational and the gravitational binding energy of the WD becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via GWR. We find that, independent of the braking efficiency, the WD contracts and achieves the physical conditions suitable for explosive C-burning at the center so that a type Ia supernova event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical Journa

Faecal miRNA profiles associated with age, sex, BMI, and lifestyle habits in healthy individuals

Abstract For their stability and detectability faecal microRNAs represent promising molecules with potential clinical interest as non-invasive diagnostic and prognostic biomarkers. However, there is no evidence on how stool miRNA profiles change according to an individual’s age, sex, and body mass index (BMI) or how lifestyle habits influence the expression levels of these molecules. We explored the relationship between the stool miRNA levels and common traits (sex, age, BMI, and menopausal status) or lifestyle habits (physical activity, smoking status, coffee, and alcohol consumption) as derived by a self-reported questionnaire, using small RNA-sequencing data of samples from 335 healthy subjects. We detected 151 differentially expressed miRNAs associated with one variable and 52 associated with at least two. Differences in miR-638 levels were associated with age, sex, BMI, and smoking status. The highest number of differentially expressed miRNAs was associated with BMI (n = 92) and smoking status (n = 84), with several miRNAs shared between them. Functional enrichment analyses revealed the involvement of the miRNA target genes in pathways coherent with the analysed variables. Our findings suggest that miRNA profiles in stool may reflect common traits and lifestyle habits and should be considered in relation to disease and association studies based on faecal miRNA expression

Age-related DNA methylation changes are sex-specific: a comprehensive assessment

The existence of a sex gap in human health and longevity has been widely documented. Autosomal DNA methylation differences between males and females have been reported, but so far few studies have investigated if DNA methylation is differently affected by aging in males and females. We performed a meta-analysis of 4 large whole blood datasets, comparing 4 aspects of epigenetic age-dependent remodeling between the two sexes: differential methylation, variability, epimutations and entropy. We reported that a large fraction (43%) of sex-associated probes undergoes age-associated DNA methylation changes, and that a limited number of probes show age-by-sex interaction. We experimentally validated 2 regions mapping in FIGN and PRR4 genes and showed sex-specific deviations of their methylation patterns in models of decelerated (centenarians) and accelerated (Down syndrome) aging. While we did not find sex differences in the age-associated increase in epimutations and entropy, we showed that the number of probes having an age-related increase in methylation variability is 15 times higher in males compared to females. Our results can offer new epigenetic tools to study the interaction between aging and sex and can pave the way to the identification of molecular triggers of sex differences in longevity and age-related diseases prevalence

Fatality rate and predictors of mortality in an Italian cohort of hospitalized COVID-19 patients

Clinical features and natural history of coronavirus disease 2019 (COVID-19) differ widely among different countries and during different phases of the pandemia. Here, we aimed to evaluate the case fatality rate (CFR) and to identify predictors of mortality in a cohort of COVID-19 patients admitted to three hospitals of Northern Italy between March 1 and April 28, 2020. All these patients had a confirmed diagnosis of SARS-CoV-2 infection by molecular methods. During the study period 504/1697 patients died; thus, overall CFR was 29.7%. We looked for predictors of mortality in a subgroup of 486 patients (239 males, 59%; median age 71 years) for whom sufficient clinical data were available at data cut-off. Among the demographic and clinical variables considered, age, a diagnosis of cancer, obesity and current smoking independently predicted mortality. When laboratory data were added to the model in a further subgroup of patients, age, the diagnosis of cancer, and the baseline PaO2/FiO2 ratio were identified as independent predictors of mortality. In conclusion, the CFR of hospitalized patients in Northern Italy during the ascending phase of the COVID-19 pandemic approached 30%. The identification of mortality predictors might contribute to better stratification of individual patient risk