387 research outputs found
A Stellar Model-fitting Pipeline for Solar-like Oscillations
Over the past two decades, helioseismology has revolutionized our
understanding of the interior structure and dynamics of the Sun.
Asteroseismology will soon place this knowledge into a broader context by
providing structural data for hundreds of Sun-like stars. Solar-like
oscillations have already been detected from the ground in several stars, and
NASA's Kepler mission is poised to unleash a flood of stellar pulsation data.
Deriving reliable asteroseismic information from these observations demands a
significant improvement in our analysis methods. We report the initial results
of our efforts to develop an objective stellar model-fitting pipeline for
asteroseismic data. The cornerstone of our automated approach is an
optimization method using a parallel genetic algorithm. We describe the details
of the pipeline and we present the initial application to Sun-as-a-star data,
yielding an optimal model that accurately reproduces the known solar
properties.Comment: 5 pages, 2 figs, Stellar Pulsation: Challenges for Theory and
Observation (proceedings to be published by AIP
Separation of Mixed Phase Signals by Zeros of the Z-transform:A Reformulation of Complex Cepstrum Based Separation by Causality
Potential for sound sensitivity in cephalopods
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Advances in Experimental Medicine and Biology 730 (2012): 125-128, doi:10.1007/978-1-4419-7311-5_28.Hearing is a primary sense in many marine animals and we now have a reasonable understanding of what stimuli generate clear responses, the frequency range of sensitivity, expected threshold values and mecha-nisms of sound detection for several species of marine mammals and fishes (Fay 1988; Au et al. 2000). For marine invertebrates, our knowledge of hearing capabilities is relatively poor and a definition or even certainty of sound detection is not agreed upon (Webster et al. 1992) despite their magnitude of biomass and often central role in ocean ecosystems. Cephalopods (squid, cuttlefish, octopods and nautilus) are particularly interesting subjects for inver-tebrate sound detection investigations for several reasons. Ecologically, they occupy many of the same niches as sound-sensitive fish (Budelmann 1994) and may benefit from sound perception and use for the same reasons, such as to detect predators, navigate, or locate conspecifics. Squid, for example, are often the prey of loud, echolocating marine mammals (Clarke 1996), and may therefore be expected to have evolved hearing to avoid predators. Anatomically, squid have complex statocysts that are considered to serve primarily as vestibular and acceleration detectors (Nixon and Young 2003). However, statocysts may also be analogs for fish otolithic organs, detecting acoustic stimuli (Budelmann 1992). Previous studies have debated the subject of squid hearing and recently there has been a revival of research on the subject. Here, we briefly review what is known about squid sound detection, revisit hearing definitions, discuss potential squid susceptibility to anthropogenic noise and suggest potential future research direc-tions to examine squid acoustic sensitivity.2013-01-2
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in the dark
Plankton respiration rate is a major component of global CO2 production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (Rlight) and in the dark (Rdark) increased as the 2/3 power of Rlight so that the Rlight:Rdark ratio increased from an average value of 1.37 at the median Rlight measured here (3.62 µmol O2 L-1 d-1) to an average value of 17.56 at the highest Rlight measured here (15.8 µmol O2 L-1 d-1). The role of respiratory processes as a source of CO2 in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest
Solar-like oscillations in the G2 subgiant beta Hydri from dual-site observations
We have observed oscillations in the nearby G2 subgiant star beta Hyi using
high-precision velocity observations obtained over more than a week with the
HARPS and UCLES spectrographs. The oscillation frequencies show a regular comb
structure, as expected for solar-like oscillations, but with several l=1 modes
being strongly affected by avoided crossings. The data, combined with those we
obtained five years earlier, allow us to identify 28 oscillation modes. By
scaling the large frequency separation from the Sun, we measure the mean
density of beta Hyi to an accuracy of 0.6%. The amplitudes of the oscillations
are about 2.5 times solar and the mode lifetime is 2.3 d. A detailed comparison
of the mixed l=1 modes with theoretical models should allow a precise estimate
of the age of the star.Comment: 13 pages, 14 figures, accepted by ApJ. Fixed minor typo (ref to Fig
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