Diffusion of Neon in White Dwarf Stars

Sedimentation of the neutron rich isotope $^{22}$Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for $^{22}$Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants $D_i$ from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that $D_i$ in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter $\Gamma>$ few), $D_i$ has a modest dependence on the charge $Z_i$ of the ion species, $D_i \propto Z_i^{-2/3}$. However $D_i$ depends more strongly on $Z_i$ for weak coupling (smaller $\Gamma$). We conclude that the self-diffusion constant $D_{\rm Ne}$ for $^{22}$Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in $D_{\rm Ne}$ should be unimportant for simulations of white dwarf cooling.Comment: 6 pages, 5 figures, minor changes, Phys. Rev. E in pres

Direct molecular dynamics simulation of liquid-solid phase equilibria for two-component plasmas

We determine the liquid-solid phase diagram for carbon-oxygen and oxygen-selenium plasma mixtures using two-phase molecular dynamics simulations. We identify liquid, solid, and interface regions using a bond angle metric. To study finite-size effects, we perform 27 648- and 55 296-ion simulations. To help monitor nonequilibrium effects, we calculate diffusion constants ${D}_{i}$. For the carbon-oxygen system we find that ${D}_{\text{O}}$ for oxygen ions in the solid is much smaller than ${D}_{\text{C}}$ for carbon ions and that both diffusion constants are 80 or more times smaller than diffusion constants in the liquid phase. There is excellent agreement between our carbon-oxygen phase diagram and that predicted by Medin and Cumming. This suggests that errors from finite-size and nonequilibrium effects are small and that the carbon-oxygen phase diagram is now accurately known. The oxygen-selenium system is a simple two-component model for more complex rapid proton capture nucleosynthesis ash compositions for an accreting neutron star. Diffusion of oxygen, in a predominantly selenium crystal, is remarkably fast, comparable to diffusion in the liquid phase. We find a somewhat lower melting temperature for the oxygen-selenium system than that predicted by Medin and Cumming. This is probably because of electron screening effects

Risk and protective factors for mental health morbidity in a community sample of female-to-male trans-masculine adults

Abstract Background Trans-masculine (TM) individuals, who are assigned female sex at birth and identify along the masculine gender spectrum, face mental health disparities relative to cisgender people. Limited research has sought to explore the multi-level risk and protective factors associated with mental health morbidity for TM populations. Methods Between August 2015–September 2016, 150 TM adults were enrolled in a one-time biobehavioral health study. A survey assessed socio-demographics, past 12-month everyday discrimination, lifetime intimate partner violence (IPV), resilience (using the Brief Resilience Scale), and other factors. Bivariate and multivariable logistic regression analyses examined associations between participant characteristics and four mental health statuses: post-traumatic stress disorder (PTSD), depression, anxiety, and non-suicidal self-injury (NSSI). Results In this sample (76.7% had a binary gender identity, i.e., man or transgender man; 74.7% were white, 70.0% were under age 30 years), 42.2% had PTSD based on past 30-day symptoms; 25.7% had depression based on past 7-day symptoms; 31.1% had anxiety based on past 7-day symptoms; and 31.3% had engaged in NSSI within the past 12-months. Results from multivariable models: 1) PTSD: unemployment, lifetime IPV and past 12-month discrimination were each associated with increased odds of PTSD, while having a partner was associated with the reduced odds of PTSD. 2) Depression: lower educational attainment and past 12-month discrimination were each associated with the increased odds of depression, while greater resilience was associated with the reduced odds of depression. 3) Anxiety: low annual household income and past 12-month discrimination were each associated with the increased odds of anxiety, while resilience was associated with the reduced odds of anxiety. 4) NSSI: past 12-month discrimination was associated with the increased odds of past 12-month NSSI, while higher age and greater resilience was associated with the reduced odds of NSSI (all p-values < 0.05). Conclusions Unemployment, low income, limited education, everyday discrimination, and violence were risk factors for poor mental health, while being in a relationship, higher age, and personal resilience were protective against mental health morbidity. Findings highlight the need for interventions to address the individual, interpersonal, and societal factors that may be driving poor mental health in this population

Syndemics and gender affirmation: HIV sexual risk in female-to-male trans masculine adults reporting sexual contact with cisgender males

Female-to-male trans masculine adults who have sex with cisgender (non-transgender) males (TMSM) represent an understudied population in relation to HIV/STI risk. This study examined the role of syndemic conditions and social gender affirmation processes (living full-time in one’s identified gender) in potentiating sexual risk among TMSM adults in Massachusetts. Cross-sectional data were restricted to TMSM who reported lifetime sexual behaviour with a cisgender male (n = 173; mean age = 29.4, SD = 9.6; 18.5% people of colour; 93.1% non-heterosexual identity; 56.1% hormones/surgery). Sexual risk outcomes were: lifetime STI diagnoses, three or more past-6-month sexual partners, and condomless anal/vaginal sex at last encounter with a cisgender male. Age- and survey mode-adjusted logistic regression models regressed sexual risk outcomes on the main effect of syndemics (six indicators summed: binge drinking, substance use, depression, anxiety, childhood abuse, intimate partner violence), followed by the interaction of syndemics and social gender affirmation. Syndemics were associated with increased odds of all sexual risk indicators (adjusted odds ratios (aORs) = 1.32–1.55; p < 0.0001). Social gender affirmation moderated the association between syndemics and condomless anal/vaginal sex at last encounter with a cisgender male (p < 0.0001). Syndemics were associated with sexual risk in TMSM who had socially affirmed their gender (aOR = 1.79; 95% CI = 1.42–2.25; p < 0.001), but not among those TMSM who had not (aOR = 0.86; 95% CI = 0.63–1.19; p = 0.37). Findings suggest that syndemic pathways to sexual risk are similar for TMSM who have socially gender affirmed as for cisgender MSM. Integration of syndemics and gender affirmation frameworks is recommended in interventions to address TMSM sexual risk