2,326 research outputs found
The Rising Light Curves of Type Ia Supernovae
We present an analysis of the early, rising light curves of 18 Type Ia
supernovae (SNe Ia) discovered by the Palomar Transient Factory (PTF) and the
La Silla-QUEST variability survey (LSQ). We fit these early data flux using a
simple power-law to determine the time of first
light , and hence the rise-time from first light to
peak luminosity, and the exponent of the power-law rise (). We find a mean
uncorrected rise time of days, with individual SN rise-times
ranging from to days. The exponent n shows significant
departures from the simple 'fireball model' of (or ) usually assumed in the literature. With a mean value of , our data also show significant diversity from event to event. This
deviation has implications for the distribution of 56Ni throughout the SN
ejecta, with a higher index suggesting a lesser degree of 56Ni mixing. The
range of n found also confirms that the 56Ni distribution is not standard
throughout the population of SNe Ia, in agreement with earlier work measuring
such abundances through spectral modelling. We also show that the duration of
the very early light curve, before the luminosity has reached half of its
maximal value, does not correlate with the light curve shape or stretch used to
standardise SNe Ia in cosmological applications. This has implications for the
cosmological fitting of SN Ia light curves.Comment: 19 pages, 19 figures, accepted for publication in MNRA
Lung region segmentation based on computer tomographic images
The article is written for the benefit of hospitals in order to identify segments of the lungs and thereby help in the process of bisecting lungs according to their respective segments during surgery. Further research in this area can also help in identification of various abnormalities related to each segment and also identify abnormal walls of unhealthy lungs. Segmentation of the lungs has not been implemented in reality, as the lung, being an organ with dynamic chest volumes during respiratory cycles, makes it impossible to address changes during respiration unlike fixed structures like the brain. Further, abnormalities situated on the walls of lung segments, make it more difficult to address volume changes concerned with the lungs. The author of this paper has provided a more effective method to identify various lung segments by using various well known segmentation techniques combined together to identify moving lung image segments more effectively
IMPACT OF THERMAL STRESS AND HIGH VPD ON GAS EXCHANGE AND CHLOROPHYLL FLUORESCENCE OF CITRUS GRANDIS UNDER DESERT CONDITIONS
Additional index words: photosynthesis, photoinhibition, heat stress, temperature Abstract The photosynthetic response of Citrus grandis to high light intensities, low air humidity and high temperature stress was investigated under desert conditions in the southern AravaValley (Israel). During summer, a typical midday stomatal closure was observed even in well-watered trees due to the dramatic increase of the leaf-to-air water vapour deficit. As a result of the reduced transpirational cooling, leaf temperatures increased up to 11 °C above ambient air temperature. The combination of heat stress and photoinhibition resulted in a reversible decrease of photosynthetic activity of Citrus grandis under the extreme summer conditions
Structural Properties, Order-Disorder Phenomena and Phase Stability of Orotic Acid Crystal Forms
Orotic acid (OTA) is reported to exist in the anhydrous (AH), monohydrate (Hy1) and dimethylsulfoxide monosolvate (SDMSO) forms. In this study we investigate the (de)hydration/desolvation behavior, aiming at an understanding of the elusive structural features of anhydrous OTA by a combination of experimental and computational techniques, namely, thermal analytical methods, gravimetric moisture (de)sorption studies, water activity measurements, X-ray powder diffraction, spectroscopy (vibrational, solid-state NMR), crystal energy landscape and chemical shift calculations. The Hy1 is a highly stable hydrate, which dissociates above 135°C and loses only a small part of the water when stored over desiccants (25°C) for more than one year. In Hy1, orotic acid and water molecules are linked by strong hydrogen bonds in nearly perfectly planar arranged stacked layers. The layers are spaced by 3.1 Å and not linked via hydrogen-bonds. Upon dehydration the X-ray powder diffraction and solid-state NMR peaks become broader indicating some disorder in the anhydrous form. The Hy1 stacking reflection (122) is maintained, suggesting that the OTA molecules are still arranged in stacked layers in the dehydration product. Desolvation of SDMSO, a non-layer structure, results in the same AH phase as observed upon dehydrating Hy1. Depending on the desolvation conditions different levels of order-disorder of layers present in anhydrous OTA are observed, which is also suggested by the computed low energy crystal structures. These structures provide models for stacking faults as intergrowth of different layers is possible. The variability in anhydrate crystals is of practical concern as it affects the moisture dependent stability of AH with respect to hydration
NGC 2770 - a supernova Ib factory?
NGC 2770 has been the host of three supernovae of Type Ib during the last 10
years, SN 1999eh, SN 2007uy and SN 2008D. SN 2008D attracted special attention
due to the serendipitous discovery of an associated X-ray transient. In this
paper, we study the properties of NGC 2770 and specifically the three SN sites
to investigate whether this galaxy is in any way peculiar to cause a high
frequency of SNe Ib. We model the global SED of the galaxy from broadband data
and derive a star-formation and SN rate comparable to the values of the Milky
Way. We further study the galaxy using longslit spectroscopy covering the major
axis and the three SN sites. From the spectroscopic study we find subsolar
metallicities for the SN sites, a high extinction and a moderate star-formation
rate. In a high resolution spectrum, we also detect diffuse interstellar bands
in the line-of-sight towards SN 2008. A comparison of NGC 2770 to the global
properties of a galaxy sample with high SN occurance (at least 3 SN in the last
100 years) suggests that NGC 2770 is not particularly destined to produce such
an enhancement of observed SNe observed. Its properties are also very different
from gamma-ray burst host galaxies. Statistical considerations on SN Ib
detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe
detected in 10 years. The high number of rare Ib SNe in this galaxy is
therefore likely to be a coincidence rather than special properties of the
galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is
highly starforming, has a very low mass and one of the lowest metallicities
detected in the nearby universe as derived from longslit spectroscopy. In the
most metal poor part, we even detect Wolf-Rayet features, against the current
models of WR stars which require high metallicities.Comment: 15 pages, 10 figures, submitted to Ap
Effects of a localized beam on the dynamics of excitable cavity solitons
We study the dynamical behavior of dissipative solitons in an optical cavity
filled with a Kerr medium when a localized beam is applied on top of the
homogeneous pumping. In particular, we report on the excitability regime that
cavity solitons exhibits which is emergent property since the system is not
locally excitable. The resulting scenario differs in an important way from the
case of a purely homogeneous pump and now two different excitable regimes, both
Class I, are shown. The whole scenario is presented and discussed, showing that
it is organized by three codimension-2 points. Moreover, the localized beam can
be used to control important features, such as the excitable threshold,
improving the possibilities for the experimental observation of this
phenomenon.Comment: 9 Pages, 12 figure
Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?
The stars that end their lives as supernovae (SNe) have been directly
observed in only a handful of cases, due mainly to the extreme difficulty in
identifying them in images obtained prior to the SN explosions. Here we report
the identification of the progenitor for the recent Type II-plateau
(core-collapse) SN 2005cs in pre-explosion archival images of the Whirlpool
Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). From high-quality ground-based images of the SN from the
Canada-France-Hawaii Telescope, we precisely determine the position of the SN
and are able to isolate the SN progenitor to within 0".04 in the HST/ACS
optical images. We further pinpoint the SN location to within 0".005 from
HST/ACS ultraviolet images of the SN, confirming our progenitor identification.
From photometry of the SN progenitor obtained with the pre-SN ACS images, and
also limits to its brightness in pre-SN HST/NICMOS images, we infer that the
progenitor is a red supergiant star of spectral type K0--M3, with initial mass
7--9 Msun. We also discuss the implications of the SN 2005cs progenitor
identification and its mass estimate. There is an emerging trend that the most
common Type II-plateau SNe originate from low-mass supergiants 8--15 Msun.Comment: Submitted to ApJ. A high resolution version can be found at
http://astron.berkeley.edu/~weidong/sn05cs.p
- …