5,925 research outputs found
Light to Mass Variations with Environment
Large and well defined variations exist between the distribution of mass and
the light of stars on extragalactic scales. Mass concentrations in the range
10^12 - 10^13 M_sun manifest the most light per unit mass. Group halos in this
range are typically the hosts of spiral and irregular galaxies with ongoing
star formation. On average M/L_B ~ 90 M_sun/L_sun in these groups . More
massive halos have less light per unit mass. Within a given mass range, halos
that are dynamically old as measured by crossing times and galaxy morphologies
have distinctly less light per unit mass. At the other end of the mass
spectrum, below 10^12 M_sun, there is a cutoff in the manifestation of light.
Group halos in the range 10^11 - 10^12 M_sun can host dwarf galaxies but with
such low luminosities that M/L_B values can range from several hundred to
several thousand. It is suspected that there must be completely dark halos at
lower masses. Given the form of the halo mass function, it is the low relative
luminosities of the high mass halos that has the greatest cosmological
implications. Of order half the clustered mass may reside in halos with greater
than 10^14 M_sun. By contrast, only 5-10% of clustered mass would lie in
entities with less than 10^12 M_sun.Comment: 15 pages, 9 figures, 2 tables, Accepted Astrophysical Journal 619,
000, 2005 (Jan 1
Climate Modeling of a Potential ExoVenus
The planetary mass and radius sensitivity of exoplanet discovery capabilities
has reached into the terrestrial regime. The focus of such investigations is to
search within the Habitable Zone where a modern Earth-like atmosphere may be a
viable comparison. However, the detection bias of the transit and radial
velocity methods lies close to the host star where the received flux at the
planet may push the atmosphere into a runaway greenhouse state. One such
exoplanet discovery, Kepler-1649b, receives a similar flux from its star as
modern Venus does from the Sun, and so was categorized as a possible exoVenus.
Here we discuss the planetary parameters of Kepler-1649b with relation to Venus
to establish its potential as a Venus analog. We utilize the general
circulation model ROCKE-3D to simulate the evolution of the surface temperature
of Kepler-1649b under various assumptions, including relative atmospheric
abundances. We show that in all our simulations the atmospheric model rapidly
diverges from temperate surface conditions towards a runaway greenhouse with
rapidly escalating surface temperatures. We calculate transmission spectra for
the evolved atmosphere and discuss these spectra within the context of the
James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec)
capabilities. We thus demonstrate the detectability of the key atmospheric
signatures of possible runaway greenhouse transition states and outline the
future prospects of characterizing potential Venus analogs.Comment: 11 pages, 4 figures, 1 table, accepted for publication in the
Astrophysical Journal. The data from this paper are open source and are
available from the following data portals:
https://portal.nccs.nasa.gov/GISS_modelE/ROCKE-3D/Climate_Modeling_of_a_Potential_ExoVenus
https://archive.org/details/Climate_Modeling_of_a_Potential_ExoVenu
WQ 2059-247: An unusual high redshift X-ray cluster
X-ray, optical, and radio observations of a high redshift, Bautz-Morgan type I cluster of galaxies are reported. The cD galaxy contains a powerful, flat spectrum radio source coincident with the possibly stellar nucleus. The cluster is an extremely luminous X-ray source; however, unlike nearby luminous X-ray clusters the X-ray spectrum appears to be rather soft. Two possible interpretations of the soruces are suggested: either the intracluster gas is much cooler in high redshift clusters because they are less relaxed, or the X-ray and radio emissions from WQ 2059-247 are the result of a non thermal QSO/BL Lac type object in the nucleus of the cD
Climate Modeling of a Potential Exovenus
The planetary mass and radius sensitivity of exoplanet discovery capabilities has reached into the terrestrial regime. The focus of such investigations is to search within the Habitable Zone where a modern Earth-like atmosphere may be a viable comparison. However, the detection bias of the transit and radial velocity methods lies close to the host star where the received flux at the planet may push the atmosphere into a runaway greenhouse state. One such exoplanet discovery, Kepler-1649b, receives a similar flux from its star as modern Venus does from the Sun, and so was categorized as a possible exoVenus. Here we discuss the planetary parameters of Kepler-1649b in relation to Venus to establish its potential as a Venus analog. We utilize the general circulation model ROCKE-3D to simulate the evolution of the surface temperature of Kepler-1649b under various assumptions, including relative atmospheric abundances. We show that in all our simulations the atmospheric model rapidly diverges from temperate surface conditions toward a runaway greenhouse with rapidly escalating surface temperatures. We calculate transmission spectra for the evolved atmosphere and discuss these spectra within the context of the James Webb Space Telescope Near-Infrared Spectrograph capabilities. We thus demonstrate the detectability of the key atmospheric signatures of possible runaway greenhouse transition states and outline the future prospects of characterizing potential Venus analogs
Evaluation of radiography as a screening method for detection and characterisation of congenital vertebral malformations in dogs
Congenital vertebral malformations (CVM) are common in brachycephalic âscrew-tailedâ dogs; they can be associated with neurological deficits and a genetic predisposition has been suggested. The purpose of this study was to evaluate radiography as a screening method for congenital thoracic vertebral malformations in brachycephalic âscrew-tailedâ dogs by comparing it with CT. Forty-nine dogs that had both radiographic and CT evaluations of the thoracic vertebral column were included. Three observers retrospectively reviewed the images independently to detect CVMs. When identified, they were classified according to a previously published radiographic classification scheme. A CT consensus was then reached. All observers identified significantly more affected vertebrae when evaluating orthogonal radiographic views compared with lateral views alone; and more affected vertebrae with the CT consensus compared with orthogonal radiographic views. Given the high number of CVMs per dog, the number of dogs classified as being CVM free was not significantly different between CT and radiography. Significantly more midline closure defects were also identified with CT compared with radiography. Malformations classified as symmetrical or ventral hypoplasias on radiography were frequently classified as ventral and medial aplasias on CT images. Our results support that CT is better than radiography for the classification of CVMs and this will be important when further evidence of which are the most clinically relevant CVMs is identified. These findings are of particular importance for designing screening schemes of CVMs that could help selective breeding programmes based on phenotype and future studies
The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust
Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich
AGB stars. However, the main gas-phase precursors leading to the formation of
SiC dust have not yet been identified. The most obvious candidates among the
molecules containing an Si--C bond detected in C-rich AGB stars are SiC2, SiC,
and Si2C. We aim to study how widespread and abundant SiC2, SiC, and Si2C are
in envelopes around C-rich AGB stars and whether or not these species play an
active role as gas-phase precursors of silicon carbide dust in the ejecta of
carbon stars. We carried out sensitive observations with the IRAM 30m telescope
of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC,
and Si2C in the 2 mm band. We performed non-LTE excitation and radiative
transfer calculations based on the LVG method to model the observed lines of
SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We
detect SiC2 in most of the sources, SiC in about half of them, and do not
detect Si2C in any source, at the exception of IRC +10216. Most of these
detections are reported for the first time in this work. We find a positive
correlation between the SiC and SiC2 line emission, which suggests that both
species are chemically linked, the SiC radical probably being the
photodissociation product of SiC2 in the external layer of the envelope. We
find a clear trend in which the denser the envelope, the less abundant SiC2 is.
The observed trend is interpreted as an evidence of efficient incorporation of
SiC2 onto dust grains, a process which is favored at high densities owing to
the higher rate at which collisions between particles take place. The observed
behavior of a decline in the SiC2 abundance with increasing density strongly
suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes
around carbon stars.Comment: Published in A&A. 16 pages and 10 figure
Clues to NaCN formation
ALMA is providing us essential information on where certain molecules form.
Observing where these molecules emission arises from, the physical conditions
of the gas, and how this relates with the presence of other species allows us
to understand the formation of many species, and to significantly improve our
knowledge of the chemistry that occurs in the space. We studied the molecular
distribution of NaCN around IRC +10216, a molecule detected previously, but
whose origin is not clear. High angular resolution maps allow us to model the
abundance distribution of this molecule and check suggested formation paths. We
modeled the emission of NaCN assuming local thermal equilibrium (LTE)
conditions. These profiles were fitted to azimuthal averaged intensity profiles
to obtain an abundance distribution of NaCN. We found that the presence of NaCN
seems compatible with the presence of CN, probably as a result of the
photodissociation of HCN, in the inner layers of the ejecta of IRC +10216.
However, similar as for CH 3 CN, current photochemical models fail to reproduce
this CN reservoir. We also found that the abundance peak of NaCN appears at a
radius of 3 x 10 15 cm, approximately where the abundance of NaCl, suggested to
be the parent species, starts to decay. However, the abundance ratio shows that
the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE
assumption might result in NaCN abundances higher than the real ones. Updated
photochemical models, collisional rates, and reaction rates are essential to
determine the possible paths of the NaCN formation.Comment: 7 pages, 10 figures. Accepted for publication in A&A letter
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