1,940 research outputs found
Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b
Gliese 436 is an M dwarf with a mass of 0.45 Msun and hosts the extrasolar
planet GL 436b [3, 6, 7, 2], which is currently the least massive transiting
planet with a mass of ~23.17 Mearth [10], and the only planet known to transit
an M dwarf. GL 436b represents the first transiting detection of the class of
extrasolar planets known as "Hot Neptunes" that have masses within a few times
that of Neptune's mass (~17 Mearth) and orbital semimajor axis <0.1 AU about
the host star. Unlike most other known transiting extrasolar planets, GL 436b
has a high eccentricity (e~0.16). This brings to light a new parameter space
for habitability zones of extrasolar planets with host star masses much smaller
than typical stars of roughly a solar mass. This unique system is an ideal
candidate for orbital perturbation and transit-time variation (TTV) studies to
detect smaller, possibly Earth-mass planets in the system. In April 2008 we
began a long-term intensive campaign to obtain complete high-precision light
curves using the Apache Point Observatory's 3.5-meter telescope, NMSU's 1-meter
telescope (located at APO), and Sommers Bausch Observatory's 24" telescope.
These light curves are being analyzed together, along with amateur and other
professional astronomer observations. Results of our analysis are discussed.
Continued measurements over the next few years are needed to determine if
additional planets reside in the system, and to study the impact of other
manifestations on the light curves, such as star spots and active regions.Comment: 4 pages, 3 figures. To appear in "Proceedings of the 15th Cambridge
Workshop on Cool Stars, Stellar Systems and the Sun", 2009, AIP Conference
Proceedings vol. 1094, ed. Eric Stempel
Neutron beam imaging with micromegas detectors in combination with neutron time-of-flight at the n_tof facility at CERN
A bulk micromegas detector with the anode segmented in 2 orthogonal directions and equipped with a neutron/charged particle converter is employed at the neutron time-of-light (nTOF) facility at CERN to determine the incident neutron beam profile and beam interception factor as a function of the neutron energy determined by the time of flight. Discrepancies between experimental results and simulations in the values of the beam interception factor range up to 12 % and are to be ascribed to a defect in the mesh of the bulk. Nevertheless the detector proved to be really useful for checking the alignment of the neutron beam optics of the facility. Measurements with a new pixelized bulk detector for the determination of the beam interception factor are forseen before the end of 2012Postprint (published version
Comparative Population Genetics and Evolutionary History of Two Commonly Misidentified Billfishes of Management and Conservation Concern
Background: Misidentifications between exploited species may lead to inaccuracies in population assessments, with potentially irreversible conservation ramifications if overexploitation of either species is occurring. A notable showcase is provided by the realization that the roundscale spearfish (Tetrapturus georgii), a recently validated species, has been historically misidentified as the morphologically very similar and severely overfished white marlin (Kajikia albida) (IUCN listing: Vulnerable). In effect, no information exists on the population status and evolutionary history of the enigmatic roundscale spearfish, a large, highly vagile and broadly distributed pelagic species. We provide the first population genetic evaluation of the roundscale spearfish, utilizing nuclear microsatellite and mitochondrial DNA sequence markers. Furthermore, we re-evaluated existing white marlin mitochondrial genetic data and present our findings in a comparative context to the roundscale spearfish.
Results: Microsatellite and mitochondrial (control region) DNA markers provided mixed evidence for roundscale spearfish population differentiation between the western north and south Atlantic regions, depending on marker-statistical analysis combination used. Mitochondrial DNA analyses provided strong signals of historical population growth for both white marlin and roundscale spearfish, but higher genetic diversity and effective female population size (1.5-1.9X) for white marlin.
Conclusions: The equivocal indications of roundscale spearfish population structure, combined with a smaller effective female population size compared to the white marlin, already a species of concern, suggests that a species-specific and precautionary management strategy recognizing two management units is prudent for this newly validated billfish
The proinflammatory cytokine interleukin 18 regulates feeding by acting on the bed nucleus of the stria terminalis
The proinflammatory cytokine IL-18 has central anorexigenic effects and was proposed to contribute to loss of appetite observed during sickness. Here we tested in the mouse the hypothesis that IL-18 can decrease food intake by acting on neurons of the bed nucleus of the stria terminalis (BST), a component of extended amygdala recently shown to influence feeding via its projections to the lateral hypothalamus (LH). We found that both subunits of the heterodimeric IL-18 receptor are highly expressed in the BST and that local injection of recombinant IL-18 (50 ng/ml) significantly reduced c-fos activation and food intake for at least 6 h. Electrophysiological experiments performed in BST brain slices demonstrated that IL-18 strongly reduces the excitatory input on BST neurons through a presynaptic mechanism. The effects of IL-18 are cell-specific and were observed in Type III but not in Type I/II neurons. Interestingly, IL-18-sensitve Type III neurons were recorded in the juxtacapsular BST, a region that contains BST-LH projecting neurons. Reducing the excitatory input on Type III GABAergic neurons, IL-18 can increase the firing of glutamatergic LH neurons through a disinhibitory mechanism. Imbalance between excitatory and inhibitory activity in the LH can induce changes in food intake. Effects of IL-18 were mediated by the IL-18R because they were absent in neurons from animals null for IL-18R\u3b1 (Il18ra-/-), which lack functional IL-18 receptors. In conclusion, our data show that IL-18 may inhibit feeding by inhibiting the activity of BST Type III GABAergic neurons
Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS
One third of present-day spirals host optically visible strong bars that
drive their dynamical evolution. However, the fundamental question of how bars
evolve over cosmological times has yet to be addressed, and even the frequency
of bars at intermediate redshifts remains controversial. We investigate the
frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the
GEMS survey, which provides morphologies from HST ACS two-color images, and
highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies
using the Sersic index, concentration parameter, and rest-frame color. We
characterize bars and disks by fitting ellipses to F606W and F850LP images,
taking advantage of the two bands to minimize bandpass shifting. We exclude
highly inclined (i>60 deg) galaxies to ensure reliable morphological
classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More
than 40% of the bars that we detect have semi major axes a<0.5" and would be
easily missed in earlier surveys without the small PSF of ACS. The bars that we
can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in
the range ~1.2-13 kpc. We find that the optical fraction of such strong bars
remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr
(z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic
decline at z>0.7. Our findings of a large and similar bar fraction at these
three epochs favor scenarios in which cold gravitationally unstable disks are
already in place by z~1, and where on average bars have a long lifetime (well
above 2 Gyr). The distributions of structural bar properties in the two slices
are, however, not statistically identical and therefore allow for the
possibility that the bar strengths and sizes may evolve over time.Comment: Accepted by ApJ Letters, to appear in Nov 2004 issue. Minor
revisions,updated reference
Broad Geographic Distribution of Roundscale Spearfish (Tetrapturus georgii) (Teleostei, Istiophoridae) in the Atlantic Revealed by DNA Analysis: Implications for White Marlin and Roundscale Spearfish Management
The recent validation of the roundscale spearfish (Tetrapturus georgii) within the western North Atlantic has introduced new complexities in the management of the overfished white marlin (Kajikia albida) in this region due to historical and contemporary misidentification between the two morphologically similar species. Compounding the management challenge for white marlin, which is currently assessed as a single Atlantic-wide stock, is an unclear picture of the extent of the roundscale spearfish\u27s overall Atlantic distribution. By using genetic tools (mitochondrial DNA ND4L-ND4 locus sequences) for species identification, we confirm that the roundscale spearfish has a much broader distribution than previously known, including the central North Atlantic and much of the western South Atlantic to at least 28°52âČS. This much wider Atlantic distribution of the roundscale spearfish sympatric with its morphologically similar congeners, the white marlin and longbill spearfish (Tetrapturus pfluegeri), raises further management complexities: it increases the geographic scale for species misidentification in catch records that form the basis for stock assessments and uncertainty in currently accepted white marlin biological parameters. Additional vigilance in obtaining accurate species identification by improved fishery onboard observer training and incorporation of genetic tools is recommended for informing management of white marlin, longbill spearfish and roundscale spearfish throughout the Atlantic
Interacting Galaxies in the A901/902 Supercluster with STAGES
We present a study of galaxy mergers and the influence of environment in the
Abell 901/902 supercluster at z~0.165. We use HST ACS F606W data from the
STAGES survey, COMBO-17, Spitzer 24um, and XMM-Newton X-ray data. Our analysis
utilizes both a visual classification system, and quantitative CAS parameters
to identify systems which show evidence of a recent or ongoing merger of mass
ratio >1/10. Our results are: (1) After visual classification and minimizing
the contamination from false projection pairs, we find that the merger fraction
f_merge is 0.023+/-0.007. The estimated fractions of likely major mergers,
likely minor mergers, and ambiguous cases are 0.01+/-0.004, 0.006+/-0.003, and
0.007+/-0.003, respectively. (2) The mergers lie outside the cluster core of
radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large
galaxy velocity dispersion in the core. Mergers populate the region (0.25 Mpc <
R <= 2 Mpc) between the core and outskirt. In this region, the estimated
frequency of mergers is similar to those seen at typical group overdensities.
This suggests ongoing growth of the clusters via accretion of group and field
galaxies. (3) We compare our observed merger fraction with those reported in
other clusters and groups out to z~0.4. Existing data points on the merger
fraction for L<= L* galaxies in clusters allow for a range of evolutionary
scenarios. (4) The fraction of mergers, which lie on the blue cloud is
80%+/-18% versus 34%+/-7% for non-interacting galaxies, implying that
interacting galaxies are preferentially blue. (5) The average SFR, based on UV
or UV+IR data, is enhanced by a factor of ~1.5 to 2 in mergers compared to
non-interacting galaxies. However, mergers in the clusters contribute only a
small fraction (between 10% and 15%) of the total SFR density.(Abridged)Comment: Accepted for publication in ApJ. 34 pages, 16 figures. Version with
full resolution figures available at: http://www.as.utexas.edu/~alh/apj/int/
; updated abridged abstrac
Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions
Both observations and simulations show that major tidal interactions or
mergers between gas-rich galaxies can lead to intense bursts of starformation.
Yet, the average enhancement in star formation rate (SFR) in major mergers and
the contribution of such events to the cosmic SFR are not well estimated. Here
we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24
micron SFRs from Spitzer and morphologies from two deep HST cosmological survey
fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a
function of projected galaxy separation. We apply two-point projected
correlation function techniques, which we augment with morphologically-selected
very close pairs (separation <2 arcsec) and merger remnants from the HST
imaging. Our analysis confirms that the most intensely star-forming systems are
indeed interacting or merging. Yet, for massive (M* > 10^10 Msun) star-forming
galaxies at 0.4<z<0.8, we find that the SFRs of galaxies undergoing a major
interaction (mass ratios <1:4 and separations < 40 kpc) are only 1.80 +/- 0.30
times higher than the SFRs of non-interacting galaxies when averaged over all
interactions and all stages of the interaction, in good agreement with other
observational works.
We demonstrate that these results imply that <10% of star formation at 0.4 <
z < 0.8 is triggered directly by major mergers and interactions; these events
are not important factors in the build-up of stellar mass since z=1.Comment: Submitted to ApJ. 41 pages, 11 figure
- âŠ