Mendigutia I.,Autonomous University of Madrid |
Mora A.,Science Operations Center |
Montesinos B.,CSIC - National Institute of Aerospace Technology |
Eiroa C.,Autonomous University of Madrid |
And 3 more authors.
Astronomy and Astrophysics | Year: 2012
Context. The mass accretion rate (M acc) is a key parameter that has not accurately been determined for a wide sample of Herbig Ae/Be (HAeBe) stars until recently. Aims. We look for trends relating M acc and the stellar ages (t), spectral energy distributions (SEDs), and disk masses for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is ultimately to shed light on the timescale and physical processes that drive the evolution of intermediate-mass pre-main sequence objects. Methods. Mass accretion rates obtained by us in a previous work were related to several stellar and disk parameters: the age of the stars was compiled from the literature, SEDs were classified according to their shape and the wavelength where the IR excess starts, near-and mid-IR colour excesses were computed, and disk masses were estimated from mm fluxes. Results. M acc decreases with the stellar age, showing a dissipation timescale τ = 1.3 +1.0 -0.5 Myr from an exponential law fit, while a power law yields M acc(t) ∝ t -η, with η = 1.8 +1.4 -0.7 This result is based on our whole HAeBe sample (1-6 M·) but the accretion rate decline most probably depends on smaller stellar mass bins. The near-IR excess is higher and starts at shorter wavelengths (J and H bands) for the strongest accretors. Active and passive disks are roughly divided by ∼ 2 × 10 -7 M· yr -1. The mid-IR excess and the SED shape from the Meeus et al. classification are not correlated with M acc. Concerning disk masses, we find M acc ∝ M 1.1±0.3 disk Most stars in our sample with signs of inner dust dissipation typically show accretion rates ten times lower and disk masses three times smaller than the remaining objects. Conclusions. The trends relating M acc with the near-IR excess and M disk extend those found for T Tauri stars, and are consistent with viscous disk models. The differences in the inner gas dissipation timescale, and the relative position of the stars with signs of inner dust clearing in the M acc-M disk plane, could be suggesting a slightly faster evolution, and that a different process-such as photoevaporation-plays a more relevant role in dissipating disks in the HAeBe regime compared to T Tauri stars. Our conclusions must consider the mismatch between the disk mass estimates from mm fluxes and the disk mass estimates from accretion, which we also find in HAeBe stars. © 2012 ESO.
Page K.L.,University of Leicester |
Osborne J.P.,University of Leicester |
Evans P.A.,University of Leicester |
Wynn G.A.,University of Leicester |
And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010
We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the super-soft source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf (WD) is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic WD, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence time-scale estimates. © 2009 RAS.
James W.F.,University of Wisconsin - Stout |
Sorge P.W.,300 W Clairemont Ave |
Garrison P.J.,Science Operations Center
Lake and Reservoir Management | Year: 2015
Unsuccessful control of internal phosphorus (P) loading in Cedar Lake, Wisconsin, via artificially induced destratification led to reexamination of management strategies and linkages between profundal sediment diffusive P flux, P entrainment, and cyanobacterial bloom development. The destratification system, operated since 1991, was suspended in 2009-2010 and turned on in 2011 to evaluate P and phytoplankton dynamics under stratified and destratified conditions. In 2009-2010, the lake was susceptible to wind-induced mixing and P exchange into the epilimnion due to its shallow morphometry, long fetch, and low Osgood Index (2.7). Anaerobic diffusive P flux from bottom sediment was high at 12 mg/m2/d. While hypolimnetic soluble reactive P (SRP) exceeded 0.8 mg/L, dissolved iron (DFe) was low, resulting in a low DFe:SRP mass ratio of 0.9:1 despite high sediment Fe concentrations. Incomplete Fe binding control of P during entrainment events may have been due to Fe reaction to an inert form (chelation with organic carbon and reaction with sulfur) in sediment and removal from recycling. High anaerobic diffusive P flux coupled with entrainment resulted in potentially toxin-producing cyanobacteria blooms in excess of 50-100 g/L chlorophyll that were sustained from late August through October. Operation of the destratification system in 2011 did not prevent anoxia and actually exacerbated diffusive P flux, resulting in >0.1 mg/L SRP in the epilimnion and nitrogen-limited phytoplankton growth. Fe-P linkages and Fe:P ratio stoichiometry need to be considered in management and control of internal P loading. For Cedar Lake, internal P loading management should consider application of materials that can irreversibly bind sediment P instead of destratification. © Copyright by the North American Lake Management Society 2015.
Ellison K.S.,University of Wisconsin - Madison |
Ellison K.S.,World Wildlife Fund |
Hofmeister E.K.,U.S. Geological Survey |
Ribic C.A.,U.S. Geological Survey |
Sample D.W.,Science Operations Center
Journal of Wildlife Diseases | Year: 2014
Globally, Avipoxvirus species affect over 230 species of wild birds and can significantly impair survival. During banding of nine grassland songbird species (n = 346 individuals) in southwestern Wisconsin, USA, we noted species with a 2–6% prevalence of pox-like lesions (possible evidence of current infection) and 4–10% missing digits (potential evidence of past infection). These prevalences approach those recorded among island endemic birds (4–9% and 9–20% for the Galapagos and Hawaii, respectively) for which Avipoxvirus species have been implicated as contributing to dramatic population declines. Henslow's Sparrow Ammodramus henslowii (n = 165 individuals) had the highest prevalence of lesions (6.1%) and missing digits (9.7%). Among a subset of 26 Henslow's Sparrows from which blood samples were obtained, none had detectable antibody reactive to fowlpox virus antigen. However, four samples (18%) had antibody to canarypox virus antigen with test sample and negative control ratios (P/N values) ranging from 2.4 to 6.5 (median 4.3). Of four antibody-positive birds, two had lesions recorded (one was also missing a digit), one had digits missing, and one had no signs. Additionally, the birds with lesions or missing digits had higher P/N values than did the antibody-positive bird without missing digits or recorded lesions. This study represents an impetus for considering the impacts and dynamics of disease caused by Avipoxvirus among North American grassland bird species. © Wildlife Disease Association 2014.
Sembay S.,University of Leicester |
Guainazzi M.,Science Operations Center |
Plucinsky P.,Harvard - Smithsonian Center for Astrophysics |
Nevalainen J.,University of Helsinki
AIP Conference Proceedings | Year: 2010
The International Astronomical Consortium for High-Energy Calibration (IACHEC) aims to provide standards for high energy calibration and supervise cross-calibration between different X-ray and Gamma-ray observatories. This goal is reached through Working Groups, involving around 40 astronomers worldwide. In these Groups, IACHEC members co-operate to define calibration standards and procedures. Their scope is primarily a practical one: a set of astronomical sources, data and results (eventually published in refereed journals) will be the outcome of a co-ordinated and standardized analysis of reference sources ("high-energy standard candles"). We briefly describe here just two of the many studies undertaken by the IACHEC; a cross-calibration analysis of O and Ne line fluxes from the thermal SNR 1E0102.2-7219, and at higher energies a comparison study of a sample of cluster temperatures and fluxes. A more detailed picture of the activities of the IACHEC is available via the information portal at http://web.mit.edu/iachec/. © 2010 American Institute of Physics.