400 research outputs found
Hydrogen in Type Ic Supernovae?
By definition, a Type Ic supernova (SN Ic) does not have conspicuous lines of
hydrogen or helium in its optical spectrum. SNe Ic usually are modelled in
terms of the gravitational collapse of bare carbon-oxygen cores. We consider
the possibility that the spectra of ordinary (SN 1994I-like) SNe Ic have been
misinterpreted, and that SNe Ic eject hydrogen. An absorption feature usually
attributed to a blend of Si II 6355 and C II 6580 may be produced by H-alpha.
If SN 1994I-like SNe Ic eject hydrogen, the possibility that hypernova (SN
1998bw-like) SNe Ic, some of which are associated with gamma-ray bursts, also
eject hydrogen should be considered. The implications of hydrogen for SN Ic
progenitors and explosion models are briefly discussed.Comment: Accepted by PASP. Several significant changes including one
additional figur
Determination of the Hubble Constant Using a Two-Parameter Luminosity Correction for Type Ia Supernovae
In this paper, we make a comprehensive determination of the Hubble constant
by using two parameters - the B-V color and the rate of decline - to simultaneously standardize the luminosities of all nearby
Cepheid-calibrated type Ia supernovae (SNe Ia) and those of a larger, more
distant sample of 29 SNe Ia. Each group is treated in as similar a manner as
possible in order to avoid systematic effects. A simultaneous
minimization yields a standardized absolute luminosity of the
Cepheid-calibrated supernovae as well as the Hubble constant obtained from the
more distant sample. We find and a standardized
absolute magnitude of -19.46. The sensitivity of to a metallicity
dependence of the Cepheid-determined distances is investigated. The total
uncertainty , dominated by uncertainties in the primary Cepheid
distance indicator, is estimated to be 5 km/s Mpc^{-1}.Comment: To appear in Ap
Spectral Consequences of Deviation from Spherical Composition Symmetry in Type Ia Supernovae
We investigate the prospects for constraining the maximum scale of clumping
in composition that is consistent with observed Type Ia supernova flux spectra.
Synthetic spectra generated without purely spherical composition symmetry
indicate that gross asymmetries make prominent changes to absorption features.
Motivated by this, we consider the case of a single unblended line forming in
an atmosphere with perturbations of different scales and spatial distributions.
Perturbations of about 1% of the area of the photodisk simply weaken the
absorption feature by the same amount independent of the line of sight.
Conversely, perturbations of about 10% of the area of the photodisk introduce
variation in the absorption depth which does depend on the line of sight. Thus,
1% photodisk area perturbations may be consistent with observed profile
homogeneity but 10% photodisk area perturbations can not. Based on this, we
suggest that the absence of significant variation in the depths of Si II 6355
absorption features in normal Type Ia spectra near maximum light indicates that
any composition perturbations in these events are quite small. This also
constrains future three-dimensional explosion models to produce ejecta profiles
with only small scale inhomogeneities.Comment: 11 pages, 6 figure
Supernova Resonance--scattering Line Profiles in the Absence of a Photosphere
In supernova spectroscopy relatively little attention has been given to the
properties of optically thick spectral lines in epochs following the
photosphere's recession. Most treatments and analyses of post-photospheric
optical spectra of supernovae assume that forbidden-line emission comprises
most if not all spectral features. However, evidence exists which suggests that
some spectra exhibit line profiles formed via optically thick
resonance-scattering even months or years after the supernova explosion. To
explore this possibility we present a geometrical approach to supernova
spectrum formation based on the "Elementary Supernova" model, wherein we
investigate the characteristics of resonance-scattering in optically thick
lines while replacing the photosphere with a transparent central core emitting
non-blackbody continuum radiation, akin to the optical continuum provided by
decaying 56Co formed during the explosion. We develop the mathematical
framework necessary for solving the radiative transfer equation under these
conditions, and calculate spectra for both isolated and blended lines. Our
comparisons with analogous results from the Elementary Supernova code SYNOW
reveal several marked differences in line formation. Most notably, resonance
lines in these conditions form P Cygni-like profiles, but the emission peaks
and absorption troughs shift redward and blueward, respectively, from the
line's rest wavelength by a significant amount, despite the spherically
symmetric distribution of the line optical depth in the ejecta. These
properties and others that we find in this work could lead to misidentification
of lines or misattribution of properties of line-forming material at
post-photospheric times in supernova optical spectra.Comment: 37 pages, 24 figures; accepted for publication in ApJ Supplement
Serie
The Probable Detection of SN 1923A: The Oldest Radio Supernova?
Based upon the results of VLA observations, we report the detection of two
unresolved radio sources that are coincident with the reported optical position
of SN 1923A in M83. For the source closest to the SN position, the flux density
was determined to be 0.30 +/- 0.05 mJy at 20 cm and 0.093 +/- 0.028 mJy at 6
cm. The flux density of the second nearby source was determined to be 0.29 +/-
0.05 at 20 cm and 0.13 +/- 0.028 at 6 cm. Both sources are non-thermal with
spectral indices of alpha = -1.0 +/- 0.30 and -0.69 +/- 0.24, respectively. SN
1923A has been designated as a Type II-P. No Type II-P (other than SN 1987A)
has been detected previously in the radio. The radio emission from both sources
appears to be fading with time. At an age of approximately 68 years when we
observed it, this would be the oldest radio supernova (of known age) yet
detected
A new species of rock-dwelling \u3cem\u3ePhyllodactylus\u3c/em\u3e (Squamata: Gekkonidae) from the Richtersveld, South Africa
A review of the Namaqua gecko, Pachydactylus namaquensis (Reptilia: Gekkonidae) from southern Africa, with the description of two new species
An analysis of morphological and allozyme variation in the Namaqua gecko, Pachydactylus namaquensis from southern Africa is presented. Three separately evolving lineages, well defined by morphology and allozyme variation, are identified. The isolated southern population, occurring on the southern escarpment and Cape Fold Mountains surrounding the western Little Karoo, is named P. kladamderma sp. nov., and is characterized by a slit-like ear opening, low number of granules bordering the mental (3-6) and mental and adjacent infralabials (513), the frequent (79%) occurence of the supralabial entering the nostril, and its drab brown base colouration. A northern population, occurring in southern Namibia and the Richtersveld is named P. haackei sp. nov., and is characterized by its more rounded or squared ear opening; high number of granules bordering the mental and adjacent infralabials (11-19), the general exclusion of the supralabial from the nostril (only 3.7% entry), and brighter, lighter colouration. It is further differentiated from P. kladaroderma on the basis of fixed differences at 11 allozyme loci. Both new species differ from P. namaquensis, which is mainly restricted to Little Namaqualand, but is sympatric with P. haackei in the Lower Orange River region, by their more heterogenous dorsal scalation, smaller cloacal spurs, lack of spine-like tubercles on the lateral surfaces of the tail, and more fragile skin. The type locality of P. namaquensis is restricted to ‘the vicinity of Springbok, Little Namaqualand, Northern Cape Province, South Africa’
Moderate Intensity Exercise Training Improves Cardiorespiratory Fitness in Women
Among women, there is an increased prevalence of sedentary lifestyle and less participation in physical activity at levels recommended by the Surgeon General. As a result, women have been identified as a target group in public health initiatives to increase physical activity. The health-related benefits of habitual, moderate intensity physical activity are well documented in the epidemiological literature, but less is known about the effect of such physical activity on cardiorespiratory fitness. Our hypothesis was that moderate and vigorous exercise training regimens of similar estimated energy expenditure would result in similar changes in cardiorespiratory fitness. Eighteen sedentary premenopausal women with the following baseline characteristics [x ± SE]: maximal oxygen consumption (VO2max) = 29.5 ± 1.5ml ● kg-1 ● min-1; age=33 ± 1 years; height=162.6 ± 0.9 cm; mass = 62.7 ± 2.3 kg, were randomly assigned to either vigorous (HI, 80% VO2max, n=10) or moderate intensity (MOD, 40% VO2max, n=8) cycle ergometer training groups. Exercise training was conducted 3-4 (3.37 ± 0.05) days/week for 12 weeks in a supervised and progressive manner, with estimated exercise energy expenditure equated across both training groups. VO2max and time to exhaustion increased significantly in both groups (p\u3c0.05), with no difference between groups. Both groups had lower (p\u3c0.05) posttraining submaximal heart rates (HR), respiratory exchange ratios (RER), and ratings of perceived exertion (RPE) during graded exercise testing, with no significant differences between the groups in posttraining values. Women participating in moderate intensity exercise training as recommended in basic public health guidelines demonstrate an increase in cardiorespiratory fitness similar to that elicited by vigorous training
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