Dissipation Efficiency in Turbulent Convective Zones in Low Mass Stars

We extend the analysis of Penev et al. (2007) to calculate effective viscosities for the surface convective zones of three main sequence stars of 0.775Msun, 0.85Msun and the present day Sun. In addition we also pay careful attention to all normalization factors and assumptions in order to derive actual numerical prescriptions for the effective viscosity as a function of the period and direction of the external shear. Our results are applicable for periods that are too long to correspond to eddies that fall within the inertial subrange of Kolmogorov scaling, but no larger than the convective turnover time, when the assumptions of the calculation break down. We find linear scaling of effective viscosity with period and magnitudes at least three times larger than the Zahn (1966, 1989) prescription.Comment: 13 pages, 3 figures Effective viscosity scaling changed by a factor of ~100. More details provided for the numerical model

MOST photometry of the enigmatic PMS pulsator HD 142666

We present precise photometry of the pulsating Herbig Ae star HD 142666 obtained in two consecutive years with the MOST (Microvariability & Oscilations of STars) satellite. Previously, only a single pulsation period was known for HD 142666. The MOST photometry reveals that HD 142666 is multi-periodic. However, the unique identification of pulsation frequencies is complicated by the presence of irregular variability caused by the star's circumstellar dust disk. The two light curves obtained with MOST in 2006 and 2007 provided data of unprecedented quality to study the pulsations in HD 142666 and also to monitor the circumstellar variability. We attribute 12 frequencies to pulsation. Model fits to the three frequencies with the highest amplitudes lie well outside the uncertainty box for the star's position in the HR diagram based on published values. The models suggest that either (1) the published estimate of the luminosity of HD 142666, based on a relation between circumstellar disk radius and stellar luminosity, is too high and/or (2) additional physics such as mass accretion may be needed in our models to accurately fit both the observed frequencies and HD 142666's position in the HR diagram.Comment: 10 pages, 11 figures, accepted for publication by Astronomy and Astrophysic

Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry

We have simultaneously monitored the photometric and polarimetric variations of the Classical T Tauri star AA Tau during the fall of 2002. We combine these data with previously published polarimetric data covering two earlier epochs. The phase coverage is complete, although not contiguous. AA Tau clearly shows cyclic variations coupled with the rotation of the system. The star-disk system produces a repeatable polarisation curve where the polarisation increases with decreasing brightness. The data fit well with the model put forward by Bouvier et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively dumping material onto the central star via magnetospheric accretion. The inner edge of the disk is deformed by its interaction with the tilted magnetosphere, producing eclipses as it rotates and occults the photosphere periodically. From the shape of the polarisation curve in the QU-Plane we confirm that the accretion disk is seen at a large inclination, almost edge-on, and predict that its position angle is PA~90 deg., i.e., that the disk's major axis is oriented in the East-West direction.Comment: Astron. Astrophys., in pres

Simulating the outer layers of Procyon A: a comparison with the Sun

Compared to the Sun, the atmospheric structure and convective flow in Procyon A exhibit the following characteristics: (1) the highly superadiabatic transition layer (SAL) is located at much shallower optical depth; it is in a dynamically active region, and its outer region is located part of the time in the optically thin atmosphere; (2) the outer region of the SAL moves from an optically thin region to thick region and back again over a time of 20-30 minutes. This motion, which is driven by the granulation, takes place in a time approximately half the turnover time of the largest granules; The main reason for the radically different radiative-convective behaviour in Procyon A compared to the Sun is the role played by turbulent eddies in determining the overall flow/thermal structure. The turbulent pressure and turbulent kinetic energy can exceed 50 % of the local gas pressure (compared to about 10-20 % in the Sun). The Procyon A simulation thus reveals two distinct timescales - the autocorrelation time of the vertical velocity and the characteristic timescale of the SAL which is tied to granulation. Just below the surface the autocorrelation decay time is about 5 minutes in Procyon A, and the SAL motion timescale is 20-30 mins. When the SAL penetrates the optically thin region there are efficient radiative losses and the peak of the SAL is low. We speculate that these losses damp out the relative amplitudes in luminosity (temperature fluctuations) compared to velocity (Doppler). Although this will not affect the frequencies of the peaks in the power spectrum, it will probably lower the average amplitude of the peaks relative to the noise background.Comment: 18 pages 10 figure

Preoperative Cognitive Impairment and Postoperative Delirium Predict Decline in Activities of Daily Living after Cardiac Surgery-A Prospective, Observational Cohort Study.

Cardiac surgery and subsequent treatment in the intensive care unit (ICU) has been shown to be associated with functional decline, especially in elderly patients. Due to the different assessment tools and assessment periods, it remains yet unclear what parameters determine unfavorable outcomes. This study sought to identify risk factors during the entire perioperative period and focused on the decline in activity of daily living (ADL) half a year after cardiac surgery. Follow-ups of 125 patients were available. It was found that in the majority of patients (60%), the mean ADL declined by 4.9 points (95% CI, -6.4 to -3.5; p < 0.000). In the "No decline" -group, the ADL rose by 3.3 points (2.0 to 4.6; p < 0.001). A multiple regression analysis revealed that preoperative cognitive impairment (MMSE ≤ 26; Exp(B) 2.862 (95%CI, 1.192-6.872); p = 0.019) and duration of postoperative delirium ≥ 2 days (Exp(B) 3.534 (1.094-11.411); p = 0.035) was independently associated with ADL decline half a year after the operation and ICU. Of note, preoperative ADL per se was neither associated with baseline cognitive function nor a risk factor for functional decline. We conclude that the preoperative assessment of cognitive function, rather than functional assessments, should be part of risk stratification when planning complex cardiosurgical procedures