274 research outputs found
Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system
M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting
a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main
Sequence companion in a slightly eccentric 3.45 day orbit. Although
post-main-sequence mass transfer explains the masses and tight orbit, it leaves
unexplained the observed X-Ray luminosity, star's underluminosity, black hole's
spin, and eccentricity. A common envelope phase, or rotational mixing, could
explain the orbit, but the former would lead to a merger and the latter to an
overluminous companion. A merger would also ensue if mass transfer to the black
hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a
primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit,
its observed properties can be consistently explained. In this model, the Main
Sequence primary transferred part of its envelope to the secondary and lost the
rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which
collapsed to a black hole (with or without an accompanying supernova). The
release of binding energy and, possibly, collapse asymmetries "kicked" the
nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray
luminosity, while the black hole spin can be natal.Comment: Manuscript: 18 pages, 2 tables, 2 figure. Supplementary Information:
34 pages, 6 figures. Advance Online Publication (AOP) on
http://www.nature.com/nature on October 20, 2010. To Appear in Nature on
November 4, 201
Manipulation of ionized impurity scattering for achieving high thermoelectric performance in n-type Mg
Achieving higher carrier mobility plays a pivotal role for obtaining potentially high thermoelectric performance. In principle, the carrier mobility is governed by the band structure as well as by the carrier scattering mechanism. Here, we demonstrate that by manipulating the carrier scattering mechanism in n-type Mg[subscript 3]Sb[subscript 2 ]-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved. In this work, Fe, Co, Hf, and Ta are doped on the Mg site of Mg[subscript 3.2]Sb[subscript 1.5]Bi[subscript 0.49]Te [subscript 0.01], where the ionized impurity scattering crosses over to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼16 to ∼81 cm 2 ·V[superscript −1]·s[superscript − 1] is obtained, thus leading to a notably enhanced power factor of ∼13 μW·cm [superscript −1]·K [superscript −2] from ∼5 μW·cm[superscript −1]·K[superscript −2]. A simultaneous reduction in thermal conductivity is also achieved. Collectively, a figure of merit (ZT) of ∼1.7 is obtained at 773 K in Mg[subscript 3.1]Co[subscript 0.1]Sb[subscript 1.5]Bi[subscript 0.49]Te [subscript 0.01]. The concept of manipulating the carrier scattering mechanism to improve the mobility should also be applicable to other material systems. Keywords: thermoelectric; carrier scattering mechanism; ionized impurity scattering; n-type; Mg[subscript 3]Sb[subscript 2]; defect
Phase-dependent study of near-infrared disk emission lines in LB-1
The mass, origin and evolutionary stage of the binary system LB-1 has been
the subject of intense debate, following the claim that it hosts an
70 black hole, in stark contrast with the expectations for
stellar remnants in the Milky Way. We conducted a high-resolution,
phase-resolved spectroscopic study of the near-infrared Paschen lines in this
system, using the 3.5-m telescope at Calar Alto Observatory. We find that
Pa and Pa (after proper subtraction of the stellar absorption
component) are well fitted with a standard double-peaked model, typical of disk
emission. We measured the velocity shifts of the red and blue peaks at 28
orbital phases: the line center has an orbital motion in perfect antiphase with
the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s
for different choices of lines and profile modelling. We interpret this curve
as proof that the disk is tracing the orbital motion of the primary, ruling out
the circumbinary disk and the hierarchical triple scenarios. The phase-averaged
peak-to-peak half-separation (proxy for the projected rotational velocity of
the outer disk) is 70 km s, larger than the stellar orbital
velocity and also inconsistent with a circumbinary disk. From those results, we
infer a primary mass 4--8 times higher than the secondary mass. Moreover, we
show that the ratio of the blue and red peaks (V/R intensity ratio) has a
sinusoidal behaviour in phase with the secondary star, which can be interpreted
as the effect of external irradiation by the secondary star on the outer disk.
Finally, we briefly discuss our findings in the context of alternative
scenarios recently proposed for LB-1. Definitive tests between alternative
solutions will require further astrometric data from .Comment: To be submitted to ApJ. Comments are welcom
Loss-of-Function Genomic Variants Highlight Potential Therapeutic Targets for Cardiovascular Disease
Pharmaceutical drugs targeting dyslipidemia and cardiovascular disease (CVD) may increase the risk of fatty liver disease and other metabolic disorders. To identify potential novel CVD drug targets without these adverse effects, we perform genome-wide analyses of participants in the HUNT Study in Norway (n = 69,479) to search for protein-altering variants with beneficial impact on quantitative blood traits related to cardiovascular disease, but without detrimental impact on liver function. We identify 76 (11 previously unreported) presumed causal protein-altering variants associated with one or more CVD- or liver-related blood traits. Nine of the variants are predicted to result in loss-of-function of the protein. This includes ZNF529:p.K405X, which is associated with decreased low-density-lipoprotein (LDL) cholesterol (P = 1.3 × 10−8) without being associated with liver enzymes or non-fasting blood glucose. Silencing of ZNF529 in human hepatoma cells results in upregulation of LDL receptor and increased LDL uptake in the cells. This suggests that inhibition of ZNF529 or its gene product should be prioritized as a novel candidate drug target for treating dyslipidemia and associated CVD
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