Stability Criteria for Mass Transfer in Binary Stellar Evolution

The evolution of a binary star system by various analytic approximations of mass transfer is discussed, with particular attention payed to the stability of these processes against runaway on the thermal and dynamical timescales of the mass donating star. Mass transfer in red giant - neutron star binary systems is used as a specific example of such mass transfer, and is investigated. Hjellming and Webbink's (1987) results on the dynamic timescale response of a convective star with a core to mass loss are applied, with new results. It is found that mass transfer is usually stable, so long as the the wind's specific angular momentum does not exceed the angular momentum per reduced mass of the system. This holds for both dynamical and thermal timescales. Those systems which are not stable will usually transfer mass on the thermal timescale. Included are graphs illustrating the variation of the Roche radius exponent with mass ratio in the binary, for various parameters in the non-conservative mass transfer, as well as evolutionary paths of interacting red giant neutron star binaries.Comment: 18 pages, 11 figures, LaTeX, l-aa.sty and psfig.sty macros incl. to appear in Astronomy and Astrophysic

The Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star companion to investigate the formation of double neutron star systems.Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron-star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P<~0.1days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun). The existence of a short-period population of double neutron stars increases the predicted detection rate of inspiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as gamma-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592 n1 July 20, 200

Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer

We investigate the secular evolution of the orbital semi-major axis and eccentricity due to mass transfer in eccentric binaries, assuming conservation of total system mass and orbital angular momentum. Assuming a delta function mass transfer rate centered at periastron, we find rates of secular change of the orbital semi-major axis and eccentricity which are linearly proportional to the magnitude of the mass transfer rate at periastron. The rates can be positive as well as negative, so that the semi-major axis and eccentricity can increase as well as decrease in time. Adopting a delta-function mass-transfer rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution timescales ranging from a few Myr to a Hubble time or more, depending on the binary mass ratio and orbital eccentricity. Comparison with orbital evolution timescales due to dissipative tides furthermore shows that tides cannot, in all cases, circularize the orbit rapidly enough to justify the often adopted assumption of instantaneous circularization at the onset of mass transfer. The formalism presented can be incorporated in binary evolution and population synthesis codes to create a self-consistent treatment of mass transfer in eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa

The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems

With a view to understanding the formation of double neutron-stars (DNS), we investigate the late stages of evolution of helium stars with masses of 2.8 - 6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and spiral-in phase due to the development of a convective helium envelope. If the neutron star has sufficient time to complete the spiraling-in process before the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d) with a merger timescale of the order of 1 Myr or less. These systems would have important consequences for the detection rate of GWR and for the understanding of GRB progenitors. On the other hand, if the time left until the explosion is shorter than the orbital-decay timescale, the system will undergo a SN explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through CE evolution. The remnants of these massive helium stars are DNSs with periods in the range of 0.1 - 1 d. This suggests that this range of mass includes the progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12, respectively) must have been imparted at the birth of the pulsar's companion. The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced from helium star-neutron star binaries which avoid RLOF, with the helium star more massive than 2.5 Msun. For these systems the minimum kick velocities are 50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA

Type Ia Supernovae: An Examination of Potential Progenitors and the Redshift Distribution

We examine the possibility that supernovae type Ia (SN Ia) are produced by white dwarfs accreting from Roche-lobe filling evolved companions, under the assumption that a strong optically thick stellar wind from accretor is able to stabilize the mass transfer. We show that if a mass transfer phase on a thermal timescale precedes a nuclear burning driven phase, then such systems (of which the supersoft X-ray sources are a subgroup) can account for about 10% of the inferred SN Ia rate. In addition, we examine the cosmic history of the supernova rate, and we show that the ratio of the rate of SN Ia to the rate of supernovae produced by massive stars (supernovae of types II, Ib, Ic) should increase from about z = 1 towards lower redshifts.Comment: 29 pages, Latex, 6 figures, aasms4.sty, psfig.sty, to appear in The Astrophysical Journa

Prevalence and Predictors of Abnormal Cardiovascular Responses to Exercise Testing Among Individuals With Type 2 Diabetes: The Look AHEAD (Action for Health in Diabetes) study

OBJECTIVE We examined maximal graded exercise test (GXT) results in 5,783 overweight/obese men and women, aged 45–76 years, with type 2 diabetes, who were entering the Look AHEAD (Action for Health in Diabetes) study, to determine the prevalence and correlates of exercise-induced cardiac abnormalities. RESEARCH DESIGN AND METHODS Participants underwent symptom-limited maximal GXTs. Questionnaires and physical examinations were used to determine demographic, anthropometric, metabolic, and health status predictors of abnormal GXT results, which were defined as an ST segment depression ≥1.0 mm, ventricular arrhythmia, angina pectoris, poor postexercise heart rate recovery (<22 bpm reduction 2 min after exercise), or maximal exercise capacity less than 5.0 METs. Systolic blood pressure response to exercise was examined as a continuous variable, without a threshold to define abnormality. RESULTS Exercise-induced abnormalities were present in 1,303 (22.5%) participants, of which 693 (12.0%) consisted of impaired exercise capacity. ST segment depression occurred in 440 (7.6%), abnormal heart rate recovery in 206 (5.0%), angina in 63 (1.1%), and arrhythmia in 41 (0.7%). Of potential predictors, only greater age was associated with increased prevalence of all abnormalities. Other predictors were associated with some, but not all, abnormalities. Systolic blood pressure response decreased with greater age, duration of diabetes, and history of cardiovascular disease. CONCLUSIONS We found a high rate of abnormal GXT results despite careful screening for cardiovascular disease symptoms. In this cohort of overweight and obese individuals with type 2 diabetes, greater age most consistently predicted abnormal GXT. Long-term follow-up of these participants will show whether these abnormalities are clinically significant. Cardiovascular disease (CVD) risk factors improve with exercise in individuals with diabetes (1). Similarly, individuals with diabetes who are physically active or have higher fitness levels have reduced CVD incidence and mortality (2,3). Nevertheless, participation in exercise may involve risks for individuals with diabetes because of their high prevalence of CVD, including silent ischemia, and other comorbid conditions (4). Knowledge about the typical cardiovascular responses to exercise in individuals with type 2 diabetes has come mainly from small clinic-based studies (5) and a few larger studies in the general population (4,6,7). The Look AHEAD (Action for Health in Diabetes) study is a multicenter randomized clinical trial designed to evaluate the long-term effects of an intensive lifestyle intervention program of weight loss and physical activity on morbidity and mortality from CVD in overweight and obese individuals with type 2 diabetes (8). Graded exercise tests (GXTs) with electrocardiographic monitoring were conducted at baseline in 5,783 individuals, providing the largest sample of systematic stress testing ever conducted in individuals with type 2 diabetes. The aims of this analysis are to examine the range of abnormal exercise responses and to examine the degree to which demographic factors, health characteristics, and medication use associate with abnormal exercise responses. Identifying predictors from readily available demographic and clinical data may assist risk stratification before exercise testing or exercise prescription for individuals with diabetes

On the formation of neon-enriched donor stars in ultracompact X-ray binaries

We study the formation of neon-enriched donor stars in ultracompact X-ray binaries (orbital periods P<80 min) and show that their progenitors have to be low-mass (0.3 - 0.4 solar mass) ``hybrid'' white dwarfs (with CO cores and thick helium mantles). Stable mass transfer is possible if in the initial stages of mass exchange mass is lost from the system, taking away the specific orbital angular momentum of the accretor (``isotropic re-emission''). The excess of neon in the transferred matter is due to chemical fractionation of the white dwarf which has to occur prior to the Roche lobe overflow by the donor. The estimated lower limit of the orbital periods of the systems with neon-enriched donors is close to 10 min. We show that the X-ray pulsar 4U 1626-67, which likely also has a neon-enriched companion, may have been formed via accretion induced collapse of an oxygen-neon white dwarf accretor if the donor was a hybrid white dwarf.Comment: 6 pages, 3 figures, uses aa.cls 5.1 version class file, accepted for publication in Astronomy and Astrophysic

Dopa-Responsive Dystonia: Functional Analysis of Single Nucleotide Substitutions within the 5’ Untranslated GCH1 Region

Background: Mutations in the GCH1 gene are associated with childhood onset, dopa-responsive dystonia (DRD). Correct diagnosis of DRD is crucial, given the potential for complete recovery once treated with L-dopa. The majority of DRD associated mutations lie within the coding region of the GCH1 gene, but three additional single nucleotide sequence substitutions have been reported within the 5’ untranslated (5’UTR) region of the mRNA. The biologic significance of these 5’UTR GCH1 sequence substitutions has not been analyzed. Methodology/Principal Findings Luciferase reporter assays, quantitative real time PCR and RNA decay assays, combined with bioinformatics, revealed a pathogenic 5’UTR GCH1 substitution. The +142C>T single nucleotide 5’UTR substitution that segregates with affected status in DRD patients, substantially attenuates translation without altering RNA expression levels or stability. The +142C>T substitution disrupts translation most likely by creating an upstream initiation start codon (uAUG) and an upstream open reading frame (uORF). Conclusions/Significance: This is the first GCH1 regulatory substitution reported to act at a post-transcriptional level, increasing the list of genetic diseases caused by abnormal translation and reaffirming the importance of investigating potential regulatory substitutions in genetic diseases

An RGS4-Mediated Phenotypic Switch of Bronchial Smooth Muscle Cells Promotes Fixed Airway Obstruction in Asthma

Abstract In severe asthma, bronchodilator-and steroid-insensitive airflow obstruction develops through unknown mechanisms characterized by increased lung airway smooth muscle (ASM) mass and stiffness. We explored the role of a Regulator of Gprotein Signaling protein (RGS4) in the ASM hyperplasia and reduced contractile capacity characteristic of advanced asthma. Using immunocytochemical staining, ASM expression of RGS4 was determined in endobronchial biopsies from healthy subjects and those from subjects with mild, moderate and severe asthma. Cell proliferation assays, agonist-induced calcium mobilization and bronchoconstriction were determined in cultured human ASM cells and in human precision cut lung slices. Using gain-and loss-of-function approaches, the precise role of RGS proteins was determined in stimulating human ASM proliferation and inhibiting bronchoconstriction. RGS4 expression was restricted to a subpopulation of ASM and was specifically upregulated by mitogens, which induced a hyperproliferative and hypocontractile ASM phenotype similar to that observed in recalcitrant asthma. RGS4 expression was markedly increased in bronchial smooth muscle of patients with severe asthma, and expression correlated significantly with reduced pulmonary function. Whereas RGS4 inhibited G proteincoupled receptor (GPCR)-mediated bronchoconstriction, unexpectedly RGS4 was required for PDGF-induced proliferation and sustained activation of PI3K, a mitogenic signaling molecule that regulates ASM proliferation. These studies indicate that increased RGS4 expression promotes a phenotypic switch of ASM, evoking irreversible airway obstruction in subjects with severe asthma

Thioredoxin Interacting Protein Is Required for a Chronic Energy-Rich Diet to Promote Intestinal Fructose Absorption

Increased consumption of fats and added sugars has been associated with an increase in metabolic syndromes. Here we show that mice chronically fed an energy-rich diet (ERD) with high fat and moderate sucrose have enhanced the absorption of a gastrointestinal fructose load, and this required expression of the arrestin domain protein Txnip in the intestinal epithelial cells. ERD feeding induced gene and protein expression of Glut5, and this required the expression of Txnip. Furthermore, Txnip interacted with Rab11a, a small GTPase that facilitates the apical localization of Glut5. We also demonstrate that ERD promoted Txnip/Glut5 complexes in the apical intestinal epithelial cell. Our findings demonstrate that ERD facilitates fructose absorption through a Txnip-dependent mechanism in the intestinal epithelial cell, suggesting that increased fructose absorption could potentially provide a mechanism for worsening of metabolic syndromes in the setting of a chronic ERD