390 research outputs found
Nitrogen, Phosphorus, and Suspended Solids Concentrations in Tributaries to the Great Bay Estuary Watershed in 2013
Nitrogen, phosphorus, and sediment loads to the Great Bay Estuary are a growing concern. The Piscataqua Region Estuaries Partnership (PREP) calculates the nitrogen load from tributaries to the Great Bay Estuary for its State of Our Estuaries reports. Therefore, the purpose of this study was to collect representative data on nitrogen, phosphorus, and suspended sediment concentrations in tributaries to the Great Bay Estuary in 2013. The study design followed the tributary sampling design which was implemented by the New Hampshire Department of Environmental Services between 2001 and 2007 and by the University of New Hampshire between 2008 and 2012, so as to provide comparable data to the previous loading estimates
White Dwarf Cosmochronometry. I. Monte Carlo Simulations of Proper-Motion Ģ¶ and Magnitude-Limited Samples Using Schmidtās 1/Vmax Estimator
Observationally, white dwarf stars are a remarkably homogeneous class with a minimum observed Teff ~4000 K. Theoretically, the physics that determines their cooling timescales is relatively more straightforward than that which determines main-sequence evolutionary timescales. As a result, the white dwarf luminosity function has for the last decade been used as a probe of the age and star formation rate of the Galactic disk, providing an estimated local disk age of ~10 Gyr with estimated total uncertainties of roughly 20%. A long-standing criticism of the technique is that the reality of the reported downturn in the luminosity function (LF) hinges on just a handful of stars and on statistical arguments that fainter (older) objects would have been observed were they present. Indeed, the likely statistical variations of these small-number samples represent one of the primary uncertainties in the derived Galactic age, and the behavior of Schmidt\u27s 1/Vmax estimator in this limit is not well understood.
In this work, we explore these uncertainties numerically by means of a Monte Carlo population synthesis code that simulates the kinematics and relative numbers of cooling white dwarfs. The āobservationally selectedā subsamples are drawn using typical proper motion and V-magnitude limits. The corresponding 1/Vmax LFs are then computed and compared to the input-integrated LFs. The results from our (noise-free) data suggest that (1) Schmidtās 1/Vmax technique is a reliable and well-behaved estimator of the true space density with typical uncertainties of ~50% for 50 point samples and 25% for 200 point samples; (2) the age uncertainties quoted in previously published observational studies of the LF are consistent with uncertainties in the Monte Carlo results Ģ¶ specifically, there is a ~15% and ā¤10% observational uncertainty in the ages inferred from 50 point and 200 point samples, respectively; and (3) the large statistical variations in the bright end of these LFs Ģ¶ even in the large-N limit Ģ¶ preclude using the white dwarf LF to obtain an estimate of the recent star formation rate as a function of time
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
Temporal change in the contribution of immigration to population growth in a wild seabird experiencing rapid population decline
The sourceāsink paradigm predicts that populations in poorer-quality habitats (āsinksā) persist due to continued immigration from more-productive areas (āsourcesā). However, this categorisation of populations assumes that habitat quality is fixed through time. Globally, we are in an era of wide-spread habitat degradation, and consequently there is a pressing need to examine dispersal dynamics in relation to local population change. We used an integrated population model to quantify immigration dynamics in a long-lived colonial seabird, the black-legged kittiwake Rissa tridactyla, that is classified as globally āVulnerableā. We then used a transient life table response experiment to evaluate the contribution of temporal variation in vital rates, immigration rates and population structure to realised population growth. Finally, we used a simulation analysis to examine the importance of immigration to population dynamics. We show that the contribution of immigration changed as the population declined. This study demonstrates that immigration is unlikely to maintain vulnerable sink populations indefinitely, emphasising the need for temporal analyses of dispersal to identify shifts that may have dramatic consequences for population viability
Detection of an energetic flare from the M5V secondary star in the Polar MQ Dra
MQ Dra is a strongly magnetic Cataclysmic Variable whose white dwarf accretes material from its secondary star through a stellar wind at a low rate. TESS observations were made of MQ Dra in four sectors in Cycle 2 and show a short duration, high energy flare (similar to 10(35) erg) which has a profile characteristic of a flare from the M5V secondary star. This is one of the few occasions where an energetic flare has been seen from a Polar. We find no evidence that the flare caused a change in the light curve following the event and consider whether a coronal mass ejection was associated with the flare. We compare the frequency of energetic flares from the secondary star in MQ Dra with M dwarf stars and discuss the overall flare rate of stars with rotation periods shorter than 0.2 d and how such fast rotators can generate magnetic fields with low differential rotation rates
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