4,388 research outputs found
Empirical logic of finite automata: microstatements versus macrostatements
We compare the two approaches to the empirical logic of automata. The first,
called partition logic (logic of microstatements), refers to experiments on
individual automata. The second one, the logic of simulation (logic of
macrostatements), deals with ensembles of automata.Comment: late
Young red supergiants and the near infrared light appearance of disk galaxies
Disk galaxies often show prominent nonaxisymmetric features at near-infrared
wavelengths. Such features may indicate variations in the surface density of
stellar mass, contributions from young red supergiants in star forming regions,
or substantial dust obscuration. To distinguish among these possibilities, we
have searched for spatial variations in the 2.3 micron photometric CO index
within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This
index measures the strength of the absorption bands of molecular CO in stellar
atmospheres, and is strong in cool, low surface-gravity stars, reaching the
largest values for red supergiants. We observe significant spatial CO index
variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant
stellar population in the near-infrared is not everywhere the same. Central CO
index peaks are present in two galaxies; these could be due to either
metallicity gradients or recent star formation activity. In addition,
significant azimuthal CO index variations are seen in NGC 278. Because strong
azimuthal metallicity gradients are physically implausible in disk galaxies,
these features are most naturally explained by the presence of a young stellar
population. The fraction of 2 micron light due to young stellar populations in
star forming regions can be calculated from our data. Overall, young stellar
populations can contribute ~3% of a (normal) galaxy's near infrared flux.
Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the
total near infrared flux, but can be locally dominant in star forming regions,
and can bias estimates of spiral arm amplitude or other nonaxisymmetric
structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg
format. Uses AASTeX. Accepted for publication in The Astronomical Journa
Surface Brightness Gradients Produced by the Ring Waves of Star Formation
We compute surface brightness profiles of galactic disks for outwardly
propagating waves of star formation with a view to investigate the stellar
populations in ring galaxies. We consider two mechanisms which can create
outwardly propagating star forming rings in a purely gaseous disk --- a
self-induced wave and a density wave. We show that the surface brightness
profiles produced by both scenarios of ring formation are similar and are
strongly sensitive to the velocity of the wave. The results of our computations
are compared with the observational quantities sensitive to the young and old
stellar populations in the ring galaxies A0035-335 (the Cartwheel galaxy) and
VIIZw466. The best fit to the observed radial H_alpha surface brightness
distribution in the Cartwheel galaxy is obtained for a wave velocity of about
90 km/s. The red continuum brightness of the ring can be fully explained by the
evolving stars present in the trailing part of the wave. However the red
continuum brightness in regions internal to the ring indicates that the wave of
star formation propagates in a pre-existing stellar disk in the Cartwheel. The
H_alpha and K-band surface brightness profiles in VIIZw466 match the values
expected from stellar populations produced by a wave of star formation
propagating in a purely gaseous disk very well. We conclude that VIIZw466 is
probably experiencing the first event of star formation in the disk.Comment: Uses aas2pp4.sty and epsfig.sty, 15 pages To appear in Astrophysical
Journal, March 10, 199
Type Ia Supernova Scenarios and the Hubble Sequence
The dependence of the Type Ia supernova (SN Ia) rate on galaxy type is
examined for three currently proposed scenarios: merging of a
Chandrasekhar--mass CO white dwarf (WD) with a CO WD companion, explosion of a
sub--Chandrasekhar mass CO WD induced by accretion of material from a He star
companion, and explosion of a sub--Chandrasekhar CO WD in a symbiotic system.
The variation of the SNe Ia rate and explosion characteristics with time is
derived, and its correlation with parent population age and galaxy redshift is
discussed. Among current scenarios, CO + He star systems should be absent from
E galaxies. Explosion of CO WDs in symbiotic systems could account for the SNe
Ia rate in these galaxies. The same might be true for the CO + CO WD scenario,
depending on the value of the common envelope parameter. A testable prediction
of the sub--Chandrasekhar WD model is that the average brightness and kinetic
energy of the SN Ia events should increase with redshift for a given Hubble
type. Also for this scenario, going along the Hubble sequence from E to Sc
galaxies SNe Ia events should be brighter on average and should show larger
mean velocities of the ejecta. The observational correlations strongly suggest
that the characteristics of the SNe Ia explosion are linked to parent
population age. The scenario in which WDs with masses below the Chandrasekhar
mass explode appears the most promising one to explain the observed variation
of the SN Ia rate with galaxy type together with the luminosity--expansion
velocity trend.Comment: 16 pages uuencoded compressed Postscript, 2 figures included. ApJ
Letters, in pres
Evolution of the microstructure of cobalt during diffusionless transformation cycles
Differential scanning calorimetry and transmission electron microscopy have been used to study thermal fatigue due to diffusionless phase transformation cycling in pure cobalt. Thermal cycling through the allotropic (hcp ↔ fcc) transformation results in a temperature shift of the calorimetric peaks, which means a delay of the transformation. In addition, the transformation enthalpy, which is greater on heating than on cooling, diminishes when the number of transformation cycles increases. This is interpreted as being due to an evolution of the microstructure. Transmission electron microscopy shows the appearance of transformation-induced defects, which are mainly sessile dislocations. We can interpret the calorimetry results (enthalpy evolution and transformation delay) as due to the interactions between interface dislocations and these sessile dislocation
Discovery of an Unbound Hyper-Velocity Star in the Milky Way Halo
We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a
heliocentric radial velocity of +853+-12 km/s, the largest velocity ever
observed in the Milky Way halo. The star is either a hot blue horizontal branch
star or a B9 main sequence star with a heliocentric distance ~55 kpc. Corrected
for the solar reflex motion and to the local standard of rest, the Galactic
rest-frame velocity is +709 km/s.
Because its radial velocity vector points 173.8 deg from the Galactic center,
we suggest that this star is the first example of a hyper-velocity star ejected
from the Galactic center as predicted by Hills and later discussed by Yu &
Tremaine. The star has [Fe/H]~0, consistent with a Galactic center origin, and
a travel time of <80 Myr from the Galactic center, consistent with its stellar
lifetime. If the star is indeed traveling from the Galactic center, it should
have a proper motion of 0.3 mas/yr observable with GAIA. Identifying additional
hyper-velocity stars throughout the halo will constrain the production rate
history of hyper-velocity stars at the Galactic center.Comment: 4 pages, submitted to ApJ Letter
The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches
We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be
detected in supernova searches as a function of limiting flux. Although GRB
afterglows are rarer than supernovae, they are detectable at greater distances
because of their higher intrinsic luminosity. Assuming that GRBs trace the
cosmic star formation history and that every GRB gives rise to a bright
afterglow, we find that the average detection rate of supernovae and afterglows
should be comparable at limiting magnitudes brighter than K=18. The actual rate
of afterglows is expected to be somewhat lower since only a fraction of all
gamma-ray selected GRBs were observed to have associated afterglows. However,
the rate could also be higher if the initial gamma-ray emission from GRB
sources is more beamed than their late afterglow emission. Hence, current and
future supernova searches can place strong constraints on the afterglow
appearance fraction and the initial beaming angle of GRB sources.Comment: 13 pages, submitted to ApJ
The Galactic 26Al Problem and The Close Binary SNIb/c Solution?
The origin of the long-lived radioactive 26Al, which has been observed in the
Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission,
has been a persistent problem for over twenty years. Wolf-Rayet (WR) winds were
thought to be the most promising source, but their calculated 26Al yields are
not consistent with recent analyses of the 1.809 MeV emission from the nearest
WR star and nearby OB associations. The expected 26Al yield from the WR star
exceeds by as much as a factor of 3, that set by the 2-sigma upper limit on the
1.809 MeV emission, while the WR yields in the OB associations are only about
1/3 of that required by the 1.809 MeV emission. We suggest that a solution to
these problems may lie in 26Al from a previously ignored source: explosive
nucleosynthesis in the core collapse SNIb/c supernovae of WR stars that have
lost most of their mass to close binary companions. Recent nucleosynthetic
calculations of SNIb/c suggest that their 26Al yields depend very strongly on
the final, pre-supernova mass of the WR star, and that those with final masses
around 6 to 8 solar masses are expected to produce as much as 0.01 solar masses
of 26Al per supernova. Such binary SNIb/c make up only a small fraction of the
current SNIb/c and only about 1% of all Galactic core collapse supernovae. They
appear to be such prolific sources that the bulk of the present 26Al in the
Galaxy may come from just a few hundred close binary SNIb/c and the intense
1.809 MeV emission from nearby OB associations may come from just one or two
such supernova.Comment: Accepted for publication in Astrophysical Journal Letters, 611,10
August 200
The Stellar Content of Obscured Galactic Giant HII Regions V: G333.1--0.4
We present high angular resolution near--infrared images of the obscured
Galactic Giant HII (GHII) region G333.1--0.4 in which we detect an OB star
cluster. For G333.1--0.4, we find OB stars and other massive objects in very
early evolutionary stages, possibly still accreting. We obtained --band
spectra of three stars; two show O type photospheric features, while the third
has no photospheric features but does show CO 2.3 m band--head emission.
This object is at least as hot as an early B type star based on its intrinsic
luminosity and is surrounded by a circumstellar disc/envelope which produces
near infrared excess emission. A number of other relatively bright cluster
members also display excess emission in the --band, indicative of
disks/envelopes around young massive stars. Based upon the O star photometry
and spectroscopy, the distance to the cluster is 2.6 0.4 kpc, similar to
a recently derived kinematic (near side) value. The slope of the --band
luminosity function is similar to those found in other young clusters. The mass
function slope is more uncertain, and we find - for stars with M M where the upper an lower limits are
calculated independently for different assumptions regarding the excess
emission of the individual massive stars. The number of Lyman continuum photons
derived from the contribution of all massive stars in the cluster is 0.2
. The
integrated cluster mass is 1.0
.Comment: 31 pages, including 12 figures and 3 tables. Accepted for publication
in the A
The Stellar Content of Obscured Galactic Giant H II Regions III.: W31
We present near infrared (J, H, and K) photometry and moderate resolution
(lambda/Deltalambda = 3000) K-band spectroscopy of the embedded stellar cluster
in the giant H II region W31. Four of the brightest five cluster members are
early O--type stars based on their spectra. We derive a spectro--photometric
distance for W31 of 3.4 +/- 0.3 kpc using these new spectral types and infrared
photometry. The brightest cluster source at K is a red object which lies in the
region of the J - H vs. H - K color--color plot inhabited by stars with excess
emission in the K-band. This point source has an H plus K-band spectrum which
shows no photospheric features, which we interpret as being the result of
veiling by local dust emission. Strong Brackett series emission and permitted
FeII emission are detected in this source; the latter feature is suggestive of
a dense inflow or outflow. The near infrared position of this red source is
consistent with the position of a 5 GHz thermal radio source seen in previous
high angular resolution VLA images. We also identify several other K-band
sources containing excess emission with compact radio sources. These objects
may represent stars in the W31 cluster still embedded in their birth cocoons.Comment: LaTeX2e/aastex, 29 pages including 9 figures, 3 table
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