4,014 research outputs found
Rotation of Late-Type Stars in Praesepe with K2
We have Fourier analyzed 941 K2 light curves of likely members of Praesepe,
measuring periods for 86% and increasing the number of rotation periods (P) by
nearly a factor of four. The distribution of P vs. (V-K), a mass proxy, has
three different regimes: (V-K)<1.3, where the rotation rate rapidly slows as
mass decreases; 1.3<(V-K)<4.5, where the rotation rate slows more gradually as
mass decreases; and (V-K)>4.5, where the rotation rate rapidly increases as
mass decreases. In this last regime, there is a bimodal distribution of
periods, with few between 2 and 10 days. We interpret this to mean
that once M stars start to slow down, they do so rapidly. The K2 period-color
distribution in Praesepe (790 Myr) is much different than in the Pleiades
(125 Myr) for late F, G, K, and early-M stars; the overall distribution
moves to longer periods, and is better described by 2 line segments. For mid-M
stars, the relationship has similarly broad scatter, and is steeper in
Praesepe. The diversity of lightcurves and of periodogram types is similar in
the two clusters; about a quarter of the periodic stars in both clusters have
multiple significant periods. Multi-periodic stars dominate among the higher
masses, starting at a bluer color in Praesepe ((V-K)1.5) than in the
Pleiades ((V-K)2.6). In Praesepe, there are relatively more light curves
that have two widely separated periods, 6 days. Some of these could
be examples of M star binaries where one star has spun down but the other has
not.Comment: Accepted by Ap
Rotation of Low-mass Stars in Taurus with K2
We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young (~3 Myr) Taurus association, in addition to an older (~30 Myr) population of stars that is largely in the foreground of the Taurus molecular clouds. Out of 156 of the highest-confidence Taurus members, we find that 81% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all stars (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation toward both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than ~2 days. The overall amplitude of the LCs decreases with age, and higher-mass stars have generally lower amplitudes than lower-mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting LC morphologies are also different between these two classes
Transport Properties of a spinon Fermi surface coupled to a U(1) gauge field
With the organic compound -(BEDT-TTF)-Cu(CN) in mind, we
consider a spin liquid system where a spinon Fermi surface is coupled to a U(1)
gauge field. Using the non-equilibrium Green's function formalism, we derive
the Quantum Boltzmann Equation (QBE) for this system. In this system, however,
one cannot a priori assume the existence of Landau quasiparticles. We show that
even without this assumption one can still derive a linearized equation for a
generalized distribution function. We show that the divergence of the effective
mass and of the finite temperature self-energy do not enter these transport
coefficients and thus they are well-defined. Moreover, using a variational
method, we calculate the temperature dependence of the spin resistivity and
thermal conductivity of this system.Comment: 12 page
A multi-wavelength view of magnetic flaring from PMS stars
Flares from the Sun and other stars are most prominently observed in the soft
X-ray band. Most of the radiated energy, however, is released at optical/UV
wavelengths. In spite of decades of investigation, the physics of flares is not
fully understood. Even less is known about the powerful flares routinely
observed from pre-main sequence stars, which might significantly influence the
evolution of circumstellar disks. Observations of the NGC2264 star forming
region were obtained in Dec. 2011, simultaneously with three telescopes,
Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the
"Coordinated Synoptic Investigation of NGC2264" (CSI-NGC2264). Shorter Chandra
and CoRoT observations were also obtained in March 2008. We analyzed the
lightcurves to detect X-ray flares with an optical and/or mIR counterpart.
Basic flare properties from the three datasets, such as emitted energies and
peak luminosities, were then compared to constrain the spectral energy
distribution of the flaring emission and the physical conditions of the
emitting regions. Flares from stars with and without circumstellar disks were
also compared to establish any difference that might be attributed to the
presence of disks. Seventy-eight X-ray flares with an optical and/or mIR
counterpart were detected. Their optical emission is found to correlate well
with, and to be significantly larger than, the X-ray emission. The slopes of
the correlations suggest that the difference becomes smaller for the most
powerful flares. The mIR flare emission seems to be strongly affected by the
presence of a circumstellar disk: flares from stars with disks have a stronger
mIR emission with respect to stars without disks. This might be attributed to
the reprocessing of the optical (and X-ray) flare emission by the inner
circumstellar disk, providing evidence for flare-induced disk heating.Comment: 16 pages (36 including appendixes), 8 figures (main text), accepted
for publication by Astronomy & Astrophysics (section 8
Inhibition of TXNRD or SOD1 overcomes NRF2-mediated resistance to β-lapachone
Alterations in the NRF2/KEAP1 pathway result in the constitutive activation of NRF2, leading to the aberrant induction of antioxidant and detoxification enzymes, including NQO1. The NQO1 bioactivatable agent β-lapachone can target cells with high NQO1 expression but relies in the generation of reactive oxygen species (ROS), which are actively scavenged in cells with NRF2/KEAP1 mutations. However, whether NRF2/KEAP1 mutations influence the response to β-lapachone treatment remains unknown. To address this question, we assessed the cytotoxicity of β-lapachone in a panel of NSCLC cell lines bearing either wild-type or mutant KEAP1. We found that, despite overexpression of NQO1, KEAP1 mutant cells were resistant to β-lapachone due to enhanced detoxification of ROS, which prevented DNA damage and cell death. To evaluate whether specific inhibition of the NRF2-regulated antioxidant enzymes could abrogate resistance to β-lapachone, we systematically inhibited the four major antioxidant cellular systems using genetic and/or pharmacologic approaches. We demonstrated that inhibition of the thioredoxin-dependent system or copper-zinc superoxide dismutase (SOD1) could abrogate NRF2-mediated resistance to β-lapachone, while depletion of catalase or glutathione was ineffective. Interestingly, inhibition of SOD1 selectively sensitized KEAP1 mutant cells to β-lapachone exposure. Our results suggest that NRF2/KEAP1 mutational status might serve as a predictive biomarker for response to NQO1-bioactivatable quinones in patients. Further, our results suggest SOD1 inhibition may have potential utility in combination with other ROS inducers in patients with KEAP1/NRF2 mutations
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