4,340 research outputs found
Light and circadian regulation of clock components aids flexible responses to environmental signals
The circadian clock measures time across a 24h period, increasing fitness by phasing biological processes to the most appropriate time of day. The interlocking feedback loop mechanism of the clock is conserved across species; however, the number of loops varies. Mathematical and computational analyses have suggested that loop complexity affects the overall flexibility of the oscillator, including its responses to entrainment signals. We used a discriminating experimental assay, at the transition between different photoperiods, in order to test this proposal in a minimal circadian network (in Ostreococcus tauri) and a more complex network (in Arabidopsis thaliana). Transcriptional and translational reporters in O.tauri primarily tracked dawn or dusk, whereas in A.thaliana, a wider range of responses were observed, consistent with its more flexible clock. Model analysis supported the requirement for this diversity of responses among the components of the more complex network. However, these and earlier data showed that the O.tauri network retains surprising flexibility, despite its simple circuit. We found that models constructed from experimental data can show flexibility either from multiple loops and/or from multiple light inputs. Our results suggest that O.tauri has adopted the latter strategy, possibly as a consequence of genomic reduction
Yarkovsky Drift Detections for 247 Near-Earth Asteroids
The Yarkovsky effect is a thermal process acting upon the orbits of small
celestial bodies, which can cause these orbits to slowly expand or contract
with time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter
object) and is thus generally difficult to measure. We analyzed both optical
and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of
detecting and quantifying the Yarkovsky effect. We present 247 NEAs with
measured drift rates, which is the largest published set of Yarkovsky
detections. This large sample size provides an opportunity to examine the
Yarkovsky effect in a statistical manner. In particular, we describe two
independent population-based tests that verify the measurement of Yarkovsky
orbital drift. First, we provide observational confirmation for the Yarkovsky
effect's theoretical size dependence of 1/D, where D is diameter. Second, we
find that the observed ratio of negative to positive drift rates in our sample
is 2.34, which, accounting for bias and sampling uncertainty, implies an actual
ratio of . This ratio has a vanishingly small probability of
occurring due to chance or statistical noise. The observed ratio of retrograde
to prograde rotators is two times lower than the ratio expected from numerical
predictions from NEA population studies and traditional assumptions about the
sense of rotation of NEAs originating from various main belt escape routes. We
also examine the efficiency with which solar energy is converted into orbital
energy and find a median efficiency in our sample of 12%. We interpret this
efficiency in terms of NEA spin and thermal properties.Comment: 27 pages, 9 figures, published in the Astronomical Journal, 159, 92,
202
Stellar Populations in the Phoenix Dwarf (dIrr/dSph) Galaxy as Observed by HST/WFPC2
We present HST/WFPC2 photometry of the central regions of the Phoenix dwarf.
Accurate photometry allows us to: 1) confirm the existence of the horizontal
branch previously detected by ground-based observations, and use it to
determine a distance to Phoenix, 2) clearly detect the existence of multiple
ages in the stellar population of Phoenix, 3) determine a mean metallicity of
the old red giant branch stars in Phoenix, and suggest that Phoenix has evolved
chemically over its lifetime, 4) extract a rough star formation history for the
central regions which suggests that Phoenix has been forming stars roughly
continuously over its entire lifetime.Comment: Accepted by AJ, 22 pages including 6 figures + 1 figure in JPEG
forma
The Star Formation History of LGS 3
We have determined the distance and star formation history of the Local Group
dwarf galaxy LGS 3 from deep Hubble Space Telescope WFPC2 observations. LGS 3
is intriguing because ground-based observations showed that, while its stellar
population is dominated by old, metal-poor stars, there is a handful of young,
blue stars. Also, the presence of HI gas makes this a possible ``transition
object'' between dwarf spheroidal and dwarf irregular galaxies. The HST data
are deep enough to detect the horizontal branch and young main sequence for the
first time. A new distance of D=620+/-20 kpc has been measured from the
positions of the TRGB, the red clump, and the horizontal branch. The mean
metallicity of the stars older than 8 Gyr is Fe/H = -1.5 +/- 0.3. The most
recent generation of stars has Fe/H ~ -1. For the first few Gyr the global star
formation rate was several times higher than the historical average and has
been fairly constant since then. However, we do see significant changes in
stellar populations and star formation history with radial position in the
galaxy. Most of the young stars are found in the central 63 pc (21''), where
the star formation rate has been relatively constant, while the outer parts
have had a declining star formation rate.Comment: To appear in The Astrophysical Journal, 26 pages, 14 figures, uses
AASTe
Age-dependent changes in clock neuron structural plasticity and excitability are associated with a decrease in circadian output behaviour and sleep
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordAgeing has significant effects on circadian behaviour across a wide variety of species, but the underlying mechanisms are poorly understood. Previous work has demonstrated the age-dependent decline in behavioural output in the model organism Drosophila. We demonstrate this age-dependent decline in circadian output is combined with changes in daily activity of Drosophila. Ageing also has a large impact on sleep behaviour, significantly increasing sleep duration whilst reducing latency. We used electrophysiology to record from large ventral lateral neurons (l-LNv) of the Drosophila circadian clock, finding a significant decrease in input resistance with age, but no significant changes in spontaneous electrical activity or membrane potential. We propose this change contributes to observed behavioural and sleep changes in light-dark conditions. We also demonstrate a reduction in the daily plasticity of the architecture of the small ventral lateral neurons (s-LNv), likely underlying the reduction in circadian rhythmicity during ageing. These results provide further insights into the effect of ageing on circadian biology, demonstrating age-related changes in electrical activity in conjunction with the decline in behavioural outputs.Wellcome TrustLeverhulme TrustEngineering and Physical Sciences Research Council (EPSRC
Tidal dwarfs in the M81 group: the second generation?
We derive quantitative star formation histories of the four suspected tidal
dwarf galaxies in the M81 group, HolmbergIX, BK3N, Arp-loop (A0952+69), and
Garland, using Hubble Space Telescope/Wide Field Planetary Camera2 images in
F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the
Local Universe. We consider the spatial distribution and ages of resolved
stellar populations in these dwarf irregular galaxies. We use synthetic
color-magnitude diagrams to derive the ages of the major star formation
episodes, star formation rates, and approximate metallicity ranges. All the
galaxies show evidence of continuous star formation between about 20 and 200
Myr ago with star formation rates in the range 7.5*10^(-3)- 7.6*10^(-4)
M(sun)/yr. The metallicity of the detected stars spans a wide range, and have
lower than solar abundance. A possible scenario is that all four dwarf galaxies
were formed from material in the metal-poor outer part of the giant spiral
galaxy M81 after the tidal interaction between M81, M82, and NGC3077 about 200
Myr ago. While we do not directly detect pronounced old stellar populations,
the photometric limits of our data are such that the presence of such a
population is not entirely ruled out
The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy
We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in
the V and I passbands of the recently discovered Local Group dwarf spheroidal
galaxy in Cetus. Our color-magnitude diagram extends from above the first
ascent red giant branch (RGB) tip to approximately half a magnitude below the
horizontal branch (HB). Adopting a reddening of E(B-V) = 0.03, the magnitude of
the RGB tip yields a distance modulus of (m-M)o = 24.46 +/- 0.14. After
applying the reddening and distance modulus, we have utilized the color
distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7
on the Zinn & West scale with an intrinsic internal abundance dispersion of
+/-0.2 dex. An indirect calculation of the HB morphology of Cetus based on the
mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 +/- 0.09, which
represents an HB that is redder than what can be attributed solely to Cetus'
metal abundance. As such, Cetus suffers from the `second parameter effect' in
which another parameter besides metallicity is controlling the HB morphology.
If we adopt the conventional `age hypothesis' explanation for the second
parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic
globular clusters at its metallicity.Comment: 13 pages, 8 figures, Accepted for publication in the March 10, 2002
Ap
Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime
The Be X-ray binary EXO 2030+375 was in an extended low luminosity state
during most of 2016. We observed this state with NuSTAR and Swift, supported by
INTEGRAL observations as well as optical spectroscopy with the NOT. We present
a comprehensive spectral and timing analysis of these data here to study the
accretion geometry and investigate a possible onset of the propeller effect.
The H-alpha data show that the circumstellar disk of the Be-star is still
present. We measure equivalent widths similar to values found during more
active phases in the past, indicating that the low-luminosity state is not
simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV
luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by
standard accreting pulsar models, such as an absorbed power-law with a
high-energy cutoff. We find that pulsations are still clearly visible at these
luminosities, indicating that accretion is continuing despite the very low mass
transfer rate. In phase-resolved spectroscopy we find a peculiar variation of
the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period.
This variation is similar to that observed with XMM-Newton at much higher
luminosities. It may be connected to the accretion column passing through our
line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s
during which the data quality did not allow us to search for pulsations, but
the spectrum is much softer and well described by either a blackbody or soft
power-law continuum. This softer spectrum might be due to the fact that
accretion has been stopped by the propeller effect and we only observe the
neutron star surface cooling.Comment: 11 pages, 6 figures, accepted for publication in A&A (v2 including
language edits
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