Periodic photometric variability of the brown dwarf Kelu-1

We have detected a strong periodicity of 1.80+/-0.05 hours in photometric observations of the brown dwarf Kelu-1. The peak-to-peak amplitude of the variation is ~1.1% (11.9+/-0.8 mmag) in a 41nm wide filter centred on 857nm and including the dust/temperature sensitive TiO & CrH bands. We have identified two plausible causes of variability: surface features rotating into- and out-of-view and so modulating the light curve at the rotation period; or, elliposidal variability caused by an orbiting companion. In the first scenario, we combine the observed vsin(i) of Kelu-1 and standard model radius to determine that the axis of rotation is inclined at 65+/-12 degrees to the line of sight.Comment: 7 pages, 9 figures. Accepted for publication in MNRA

Global environmental effects of impact-generated aerosols: Results from a general circulation model

Cooling and darkening at Earth's surface are expected to result from the interception of sunlight by the high altitude worldwide dust cloud generated by impact of a large asteroid or comet, according to the one-dimensional radioactive-convective atmospheric model (RCM) of Pollack et al. An analogous three-dimensional general circulation model (GCM) simulation obtains the same basic result as the RCM but there are important differences in detail. In the GCM simulation the heat capacity of the oceans, not included in the RCM, substantially mitigates land surface cooling. On the other hand, the GCM's low heat capacity surface allows surface temperatures to drop much more rapidly than reported by Pollack et al. These two differences between RCM and GCM simulations were noted previously in studies of nuclear winter; GCM results for comet/asteroid winter, however, are much more severe than for nuclear winter because the assumed aerosol amount is large enough to intercept all sunlight falling on Earth. In the simulation the global average of land surface temperature drops to the freezing point in just 4.5 days, one-tenth the time required in the Pollack et al. simulation. In addition to the standard case of Pollack et al., which represents the collision of a 10-km diameter asteroid with Earth, additional scenarios are considered ranging from the statistically more frequent impacts of smaller asteroids to the collision of Halley's comet with Earth. In the latter case the kinetic energy of impact is extremely large due to the head-on collision resulting from Halley's retrograde orbit

Student Evaluation of Universities

The evaluation of higher education has been the subject of many recent studies. With few exceptions, student input into these studies has been minimal, if at all present. What has arisen out of these evaluation studies is a need for student input, particularly in the subject area of rating universities. This study was an attempt to obtain needed information about student evaluation and ranking of universities. Students were viewed as the primary consumers of university products and thus their perceptions of ratings and quality of their university and other universities was important. Because of the increasing competitiveness for quality students by universitiess and the increasing concern of students for quality education, the perceived rating of universities and their components may become increasingly important for students, universities, and future employers

IN-SYNC. VIII. Primordial Disk Frequencies in NGC 1333, IC 348, and the Orion A Molecular Cloud

In this paper, we address two issues related to primordial disk evolution in three clusters (NGC 1333, IC 348, and Orion A) observed by the INfrared Spectra of Young Nebulous Clusters (IN-SYNC) project. First, in each cluster, averaged over the spread of age, we investigate how disk lifetime is dependent on stellar mass. The general relation in IC 348 and Orion A is that primordial disks around intermediate mass stars (2--5$M_{\odot}$) evolve faster than those around loss mass stars (0.1--1$M_{\odot}$), which is consistent with previous results. However, considering only low mass stars, we do not find a significant dependence of disk frequency on stellar mass. These results can help to better constrain theories on gas giant planet formation timescales. Secondly, in the Orion A molecular cloud, in the mass range of 0.35--0.7$M_{\odot}$, we provide the most robust evidence to date for disk evolution within a single cluster exhibiting modest age spread. By using surface gravity as an age indicator and employing 4.5 $\mu m$ excess as a primordial disk diagnostic, we observe a trend of decreasing disk frequency for older stars. The detection of intra-cluster disk evolution in NGC 1333 and IC 348 is tentative, since the slight decrease of disk frequency for older stars is a less than 1-$\sigma$ effect.Comment: 25 pages, 26 figures; submitted for publication (ApJ

The Mass-Radius Relation Of Young Stars. I. Usco 5, An M4.5 Eclipsing Binary In Upper Scorpius Observed By K2

We present the discovery that UScoCTIO 5, a known spectroscopic binary in the Upper Scorpius star-forming region (P = 34 days, M-tot sin(i) = 0.64M(circle dot)), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M-A = 0.329 +/- 0.002 M-circle dot, R-A = 0.834 +/- 0.006 R-circle dot, M-B = 0.317 +/- 0.002 M-circle dot, and R-B = 0.810 +/- 0.006 R-circle dot. The radii are broadly consistent with pre-main-sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25%-50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications to mass-dependent outcomes of star and planet formation. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-T-eff scale. The simplest modification to the models (changing T-eff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (tau similar to 11 Myr).NASA Science Mission directorateW. M. Keck FoundationAstronom

M Dwarfs in SDSS Stripe 82: Photometric Light Curves and Flare Rate Analysis

We present a flare rate analysis of 50,130 M dwarf light curves in SDSS Stripe 82. We identified 271 flares using a customized variability index to search ~2.5 million photometric observations for flux increases in the u- and g-bands. Every image of a flaring observation was examined by eye and with a PSF-matching and image subtraction tool to guard against false positives. Flaring is found to be strongly correlated with the appearance of H-alpha in emission in the quiet spectrum. Of the 99 flare stars that have spectra, we classify 8 as relatively inactive. The flaring fraction is found to increase strongly in stars with redder colors during quiescence, which can be attributed to the increasing flare visibility and increasing active fraction for redder stars. The flaring fraction is strongly correlated with |Z| distance such that most stars that flare are within 300 pc of the Galactic plane. We derive flare u-band luminosities and find that the most luminous flares occur on the earlier-type M dwarfs. Our best estimate of the lower limit on the flaring rate (averaged over Stripe 82) for flares with \Delta u \ge 0.7 magnitudes on stars with u < 22 is 1.3 flares hour^-1 square degree^-1 but can vary significantly with the line-of-sight.Comment: 44 pages, 13 figure

Precipitation-climate sensitivity to initial conditions in an atmospheric general circulation model

Atmospheric climate, in contrast to weather, is traditionally considered to be determined by boundary conditions such as sea surface temperature (SST). To test this hypothesis, we examined annual mean precipitation from an ensemble of 20 general circulation model (GCM) simulations. Ensemble members were forced with identical 10-year series of SST and sea ice, but they began with slightly differing initial conditions. A surprisingly small proportion of the variance in the output is attributable to the effects of boundary forcing. This result-and similar evidence from smaller ensembles of other GCM simulations-implies that long-term precipitation variations are mostly unpredictable, even if SST forecasts are `perfect.

Overview of the Coupled Model Intercomparison Project (CMIP)

The Coupled Model Intercomparison Project (CMIP) involves study and intercomparison of multimodel simulations of present and future climate. The simulations of the future use idealized forcing in which CO, increase is compounded 1% yr(-1) until it doubles (near year 70) with global coupled models that contain, typically, components representing atmosphere, ocean, sea ice, and land surface. Results from CMIP diagnostic sub-projects were presented at the Second CMIP Workshop held at the Max Planck Institute for Meteorology in Hamburg, Germany, in September 2003. Significant progress in diagnosing and understanding results from global coupled models has been made since the time of the First CMIP Workshop in Melbourne, Australia, in 1998. For example, the issue of flux adjustment is slowly fading as more and more models obtain stable multicentury surface climates without them. El Nino variability, usually about half the observed amplitude in the previous generation of coupled models, is now more accurately simulated in the present generation of global coupled models, though there are still biases in simulating the patterns of maximum variability. Typical resolutions of atmospheric component models contained in coupled models are now usually around 2.5degrees latitude-longitude, with the ocean components often having about twice the atmospheric model resolution, with even higher resolution in the equatorial Tropics. Some new-generation coupled models have atmospheric resolutions of around 1.5degrees latitude - longitude. Modeling groups now routinely run the CMIP control and 1% CO2 simulations in addition to twentieth- and twenty-first-century climate simulations with a variety of forcings e.g., volcanoes, solar variability, anthropogenic sulfate aerosols, ozone, and greenhouse gases, with the anthropogenic forcings for future climate as well. However, persistent systematic errors noted in previous generations of global coupled models are still present in the current generation (e.g., overextensive equatorial Pacific cold tongue, double ITCZ). This points to the next challenge for the global coupled climate modeling community. Planning and commencement of the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) has prompted rapid coupled model development, which is leading to an expanded CMIP-like activity to collect and analyze results for the control, 1% CO2, and twentieth-, twenty-first, and twenty-second-century simulations performed for the AR4. The international climate community is encouraged to become involved in this analysis effort