1,163 research outputs found
The detectability of eccentric binary pulsars
Studies of binary pulsars provide insight into various theories of physics.
Detection of such systems is challenging due to the Doppler modulation of the
pulsed signal caused by the orbital motion of the pulsar. We investigated the
loss of sensitivity in eccentric binary systems for different types of
companions. This reduction of sensitivity should be considered in future
population synthesis models for binary pulsars. This loss can be recovered
partially by employing the `acceleration search' algorithm and even better by
using the `acceleration-jerk search' algorithm.Comment: 2 pages. To appear in the proceeding of "The Metrewavelength Sky
Conference" held at NCRA-TIFR, Pune, from December 9-13 2013, to mark 50
years of radio astronomy research at the Tata Institute of Fundamental
Research (TIFR), as well as 10 years of operation of the Giant Metrewave
Radio Telescope (GMRT) as an international observator
Neutral Hydrogen in the Local Group and around the Milky Way
Galaxies in our universe must acquire fresh gas to continue forming new stars. A likely source of this material may be the gas that resides between galaxies. We do not, however, have a clear understanding of the specifics, such as its distribution. The first claimed detection of this cosmic web of material directly in emission was published a decade ago using the Westerbork Synthesis Radio Telescope in the Netherlands while surveying neutral hydrogen in the Local Group of galaxies. Later evidence, in the form of stellar surveys and test particle simulations, showed that a tidal origin of the gas was another possibility.;More recent survey work of the Local Group, specifically between the galaxies M31 and M33, motivated us to map a section of the Westerbork emission using the Robert C . Byrd Green Bank Telescope (GBT). Our survey covers a 12 square degree area between M31 and M33, in which we reach 21 cm column density sensitivities of 1017.2 cm-2 after 400 hours of observations. These observations provide more than a factor of five better spatial resolution, and better than a factor of three in velocity resolution. Not only do we confirm the emission seen in the Westerbork data, we find that the hydrogen gas is composed of clouds a few kiloparsecs across, with properties suggesting they are a unique population to the Local Group. We conclude that the clouds are likely transient condensations from an intergalactic filament of gas, although a tidal feature cannot currently be ruled out. We also conducted GBT pointings to the northwest of M31 to search for the extended emission seen in the Westerbork data as well. What detections we find appear to be more related to the high velocity cloud population of M31. We are continuing to map other regions around M31 to search for more diffuse emission.;We also present southern sky maps of the high velocity and intermediate velocity clouds around our own Milky Way, using 21 cm survey data from the Parkes telescope in Australia. The existence of these objects have been known for over 50 years, yet there is no general consensus as to their origins. The maps we have produced are the most detailed to date, with high spatial and velocity resolution and good sensitivity. By using a model of Milky Way rotation, we more effectively filter out foreground emission from our own Galaxy to produce these maps. We also discuss the basic global properties of this gas and the features that are seen. Apart from the Magellanic system and Galactic warp, most of the emission is in the form of small clouds. Some of the emission seen in these data may be representative of the eventual fate of the M31-M33 clouds. Work with this survey is ongoing
The Evolution of Luminous Compact Blue Galaxies: Disks or Spheroids?
Luminous compact blue galaxies (LCBGs) are a diverse class of galaxies
characterized by high luminosity, blue color, and high surface brightness that
sit at the critical juncture of galaxies evolving from the blue to the red
sequence. As part of our multi-wavelength survey of local LCBGs, we have been
studying the HI content of these galaxies using both single-dish telescopes and
interferometers. Our goals are to determine if single-dish HI observations
represent a true measure of the dynamical mass of LCBGs and to look for
signatures of recent interactions that may be triggering star formation in
LCBGs. Our data show that while some LCBGs are undergoing interactions, many
appear isolated. While all LCBGs contain HI and show signatures of rotation,
the population does not lie on the Tully-Fisher relation nor can it evolve onto
it. Furthermore, the HI maps of many LCBGs show signatures of dynamically hot
components, suggesting that we are seeing the formation of a thick disk or
spheroid in at least some LCBGs. There is good agreement between the HI and
H-alpha kinematics for LCBGs, and both are similar in appearance to the H-alpha
kinematics of high redshift star-forming galaxies. Our combined data suggest
that star formation in LCBGs is primarily quenched by virial heating,
consistent with model predictions.Comment: 4 pages, 2 figures, to appear in the proceedings of IAU Symposium
277, "Tracing the Ancestry of Galaxies on the Land of our Ancestors", eds. C.
Carignan, K.C. Freeman, and F. Combe
Sensitive 21 cm observatios of neutral hydrogen in the local group near M31
Very sensitive 21 cm H i measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope at an angular resolution of 9.ā²1, with a 5Ļ detection level of N{sub H} {sub i} = 3.9 Ć 10{sup 17} cm{sup ā2} for a 30 km s{sup ā1} line. Most of the H i in a 12 square-degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and a H i mass of 10{sup 5}M{sub ā}. Their velocities in the Local Group Standard of Rest lie between ā100 and +40 km s{sup ā1}, comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total H i mass of all nine clouds is 1.4 Ć 10{sup 6}M{sub ā} for an adopted distance of 800 kpc, with perhaps another 0.2 Ć 10{sup 6}M{sub ā} in smaller clouds or more diffuse emission. The H i mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and H i mass of dwarf galaxies, the cloudsmore Ā» are unlikely to be part of the satellite system of the Local Group, as they lack stars. To the north of M31, sensitive H i measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud (HVC) population to projected distances of ā¼100 kpc. An extension of the M31 HVC population at a similar distance to the southeast, toward M33, is not observed.Ā« les
Interstratal dewatering origin for polygonal patterns of sand-filled cracks: a case study from late Proterozoic metasediments of Islay, Scotland
Sand-filled cracks from the Lower Fine-grained Quartzite of Dalradian (late Proterozoic) age on the Island of Islay, western Scotland, may be divided into two main types, both of which
form orthogonal and non-orthogonal closed patterns on bedding surfaces. Type 1 cracks are short and lenticular in cross-section, contain sand which had been injected downwards, and are found on the
bottoms of cross-laminated sandstone beds. Type 2 cracks cut several beds and preserve evidence of upward flow of water-saturated sand. Both types of crack developed through the interstratal
intrusion of water-saturated sand into shrinkage cracks in mud or muddy sand, not, as previously thought, as a result of sub-aerial desiccation, or sub-aqueous cracking of the sediment surface
(synaeresis). These cracks likely resulted from layer-parallel contraction caused by compaction of mudstone layers during burial. Seismic shock may have provided the trigger for the preferential
development of polygonal crack patterns in these layers instead of the more usual small-scale dewatering structures. From a detailed comparison with published descriptions of filled cracks from a
number of different geological environments, it is concluded that interstratal cracking is a mechanism which rivals sub-aerial desiccation in importance, and is more common in the geological record
than is currently realized
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Delayed chilling appears to counteract flowering advances of apricot in southern UK
Temperatures are rising across the globe, and the UK is no
exception. Spring phenology of perennial fruit crops is to a large extent
determined by temperature during effective chilling (endo-dormancy) and
heat accumulation (eco-dormancy) periods. We used the apricot flowering
records of the UK National Fruit Collections (NFC) to determine the
influence of temperature trends over recent decades (1960 to 2014) on
apricot (Prunus armeniaca L.) flowering time. Using Partial Least Squares
(PLS) regression, we determined the respective periods for calculating
chill and heat accumulation. Results suggested intervals between
September 27th and February 26th and between December 31st and April 12th
as the effective chilling and warming periods, respectively. Flowering
time was correlated with temperature during both periods, with warming
during chilling corresponding to flowering delays by 4.82 dĀ°C-1, while
warming during heat accumulation was associated with bloom advances by
9.85 dĀ°C-1. Heat accumulation started after accumulating 62.7 Ā± 5.6 Chill
Portions, and flowering occurred after a further 3744 Ā± 1538 Growing
Degree Hours (above a base temperature of 4Ā°C, with optimal growth at
26Ā°C). When examining the time series, the increase in temperature during
the chilling period did not appear to decrease overall chill accumulation
during the chilling period but to delay the onset of chill accumulation
and the completion of the the average chill accumulation necessary to
start heat accumulation. The resulting delay in heat responsiveness
appeared to weaken the phenology-advancing effect of spring warming.
These processes may explain why apricot flowering time remained
relatively unchanged despite significant temperature increases. A
consequence of this may be a reduction of frost risk for early flowering
crops such as apricot in the UK
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