289 research outputs found
Low Hubble Constant from Type Ia Supernovae by van den Bergh's Method
An interesting way to calibrate the absolute magnitudes of remote Type Ia
supernovae (SNe Ia) that are well out in the Hubble flow, and thus determine
the value of the Hubble constant, H_0, has been introduced by van den Bergh.
His approach relies on calculations of the peak absolute magnitudes and
broad--band colors for SN Ia explosion models. It does not require any
corrections for extinction by interstellar dust, and no SNe Ia are excluded on
grounds of peculiarity. Within the last few years distances have been
determined to the parent galaxies of six SNe Ia by means of Cepheid variables.
Cepheid--based distances also have become available for three other SNe Ia if
one is willing to use the distance to a galaxy in the same group in lieu of the
distance to the parent galaxy itself. Here we determine the value of H_0 in a
way that is analogous to that of van den Bergh, but now using Cepheid--based
distances instead of calculated light curves. We obtain H_0 = 55 km/s/Mpc. This
value, with Lambda=0 and Omega=1, corresponds to a cosmic expansion time of 12
Gyr, which is consistent with several recent determinations of the ages of
globular clusters.Comment: Latex, 4 pages, 1 table, 1 figure, Submitted to Nature March 28,
1996. PostScript version available at http://www.nhn.ou.edu/~nugent
On the High--Velocity Ejecta of the Type Ia Supernova 1994D
Synthetic spectra generated with the parameterized supernova
synthetic-spectrum code SYNOW are compared to spectra of the Type Ia SN 1994D
that were obtained before the time of maximum brightness. Evidence is found for
the presence of two-component Fe II and Ca II features, forming in high
velocity ( \kms) and lower velocity ( \kms) matter.
Possible interpretations of these spectral splits, and implications for using
early--time spectra of SNe Ia to probe the metallicity of the progenitor white
dwarf and the nature of the nuclear burning front in the outer layers of the
explosion, are discussed.Comment: 15 pages, 3 figures, 3 tables, Astrophysical Journal, in pres
Synthetic Spectra of Hydrodynamic Models of Type Ia Supernovae
We present detailed NLTE synthetic spectra of hydrodynamic SNe Ia models. We
make no assumptions about the form of the spectrum at the inner boundary. We
calculate both Chandrasekhar-mass deflagration models and sub-Chandrasekhar
``helium detonators.'' Gamma-ray deposition is handled in a simple, accurate
manner. We have parameterized the storage of energy that arises from the time
dependent deposition of radioactive decay energy in a reasonable manner, that
spans the expected range. We find that the Chandrasekhar-mass deflagration
model W7 of Nomoto etal shows good agreement with the observed spectra of SN
1992A and SN 1994D, particularly in the UV, where our models are expected to be
most accurate. The sub-Chandrasekhar models do not reproduce the UV deficit
observed in normal SNe Ia. They do bear some resemblance to sub-luminous SNe
Ia, but the shape of the spectra (i.e. the colors) are opposite to that of the
observed ones and the intermediate mass element lines such as Si II, and Ca II
are extremely weak, which seems to be a generic difficulty of the models.
Although the sub-Chandrasekhar models have a significant helium abundance
(unlike Chandrasekhar-mass models), helium lines are not prominent in the
spectra near maximum light and thus do not act as a spectral signature for the
progenitor.Comment: submitted to ApJ, 26 pages, 10 figures, uses LaTeX styles aasms4.sty
and natbib.sty Also available at: http://www.nhn.ou.edu/~baron
Results from the High-Z Supernova Search Team
We review the use of Type Ia supernovae for cosmological distance
determinations. Low-redshift SNe Ia () demonstrate that (a) the
Hubble expansion is linear, (b) (statistical) km s
Mpc, (c) the bulk motion of the Local Group is consistent with the COBE
result, and (d) the properties of dust in other galaxies are similar to those
of dust in the Milky Way. We find that the light curves of high-redshift SNe Ia
are stretched in a manner consistent with the expansion of space; similarly,
their spectra exhibit slower temporal evolution (by a factor of ) than
those of nearby SNe Ia. The luminosity distances of our 16 high-redshift SNe Ia
are, on average, 10--15% farther than expected in a low mass-density
() universe without a cosmological constant. Our analysis
strongly supports eternally expanding models with positive cosmological
constant and a current acceleration of the expansion. We address many potential
sources of systematic error; at present, none of them reconciles the data with
and . The dynamical age of the Universe is
estimated to be Gyr, consistent with the ages of globular star
clusters.Comment: 17 pages, latex, plus 2 figures, to appear in the Proceedings of the
3rd International Symposium on Sources and Detection of Dark Matter in the
Universe (DM98), Feb. 1998, ed. D. Clin
A structural perspective of how T cell receptors recognise the CD1 family of lipid antigen-presenting molecules
The CD1 family of antigen-presenting molecules adopt a Major Histocompatibility Complex class I (MHC-I) fold. Whereas MHC molecules present peptides, the CD1 family has evolved to bind self- and foreign-lipids. The CD1 family of antigen-presenting molecules comprises four members, CD1a, CD1b, CD1c, CD1d, that differ in their architecture around the lipid-binding cleft, thereby enabling diverse lipids to be accommodated. These CD1-lipid complexes are recognised by T cell receptors (TCRs) expressed on T cells, either through dual recognition of CD1 and lipid or in a new model whereby the TCR directly contacts CD1, thereby triggering an immune response. Chemical syntheses of lipid antigens, and analogues thereof, have been crucial in understanding the underlying specificity of T cell-mediated lipid immunity. This review will focus on our current understanding of how TCRs interact with CD1-lipid complexes, highlighting how it can be fundamentally different from TCR-MHC-peptide co-recognition
A structural perspective of how T cell receptors recognise the CD1 family of lipid antigen-presenting molecules
The CD1 family of antigen-presenting molecules adopt a Major Histocompatibility Complex class I (MHC-I) fold. Whereas MHC molecules present peptides, the CD1 family has evolved to bind self- and foreign-lipids. The CD1 family of antigen-presenting molecules comprises four members, CD1a, CD1b, CD1c, CD1d, that differ in their architecture around the lipid-binding cleft, thereby enabling diverse lipids to be accommodated. These CD1-lipid complexes are recognised by T cell receptors (TCRs) expressed on T cells, either through dual recognition of CD1 and lipid or in a new model whereby the TCR directly contacts CD1, thereby triggering an immune response. Chemical syntheses of lipid antigens, and analogues thereof, have been crucial in understanding the underlying specificity of T cell-mediated lipid immunity. This review will focus on our current understanding of how TCRs interact with CD1-lipid complexes, highlighting how it can be fundamentally different from TCR-MHC-peptide co-recognition
Analytic Inversion of Emission Lines of Arbitrary Optical Depth for the Structure of Supernova Ejecta
We derive a method for inverting emission line profiles formed in supernova
ejecta. The derivation assumes spherical symmetry and homologous expansion
(i.e., ), is analytic, and even takes account of occultation by
a pseudo-photosphere. Previous inversion methods have been developed which are
restricted to optically thin lines, but the particular case of homologous
expansion permits an analytic result for lines of {\it arbitrary} optical
depth. In fact, we show that the quantity that is generically retrieved is the
run of line intensity with radius in the ejecta. This result is
quite general, and so could be applied to resonance lines, recombination lines,
etc. As a specific example, we show how to derive the run of (Sobolev) optical
depth with radius in the case of a pure resonance scattering
emission line.Comment: 6 pages, no figures, to appear in Astrophysical Journal Letters,
requires aaspp4.sty to late
αÎČ T cell receptor recognition of self-phosphatidylinositol presented by CD1b
CD1 glycoproteins present lipid-based antigens to T-cell receptors (TCRs). A role for CD1b in T-cellâmediated autoreactivity was proposed when it was established that CD1b can present self-phospholipids with short alkyl chains (âŒC34) to T cells; however, the structural characteristics of this presentation and recognition are unclear. Here, we report the 1.9 Ă
resolution binary crystal structure of CD1b presenting a self-phosphatidylinositol-C34:1 and an endogenous scaffold lipid. Moreover, we also determined the 2.4 Ă
structure of CD1bâphosphatidylinositol complexed to an autoreactive αÎČ TCR, BC8B. We show that the TCR docks above CD1b and directly contacts the presented antigen, selecting for both the phosphoinositol headgroup and glycerol neck region via antigen remodeling within CD1b and allowing lateral escape of the inositol moiety through a channel formed by the TCR α-chain. Furthermore, through alanine scanning mutagenesis and surface plasmon resonance, we identified key CD1b residues mediating this interaction, with Glu-80 abolishing TCR binding. We in addition define a role for both CD1b α1 and CD1b α2 molecular domains in modulating this interaction. These findings suggest that the BC8B TCR contacts both the presented phospholipid and the endogenous scaffold lipid via a dual mechanism of corecognition. Taken together, these data expand our understanding into the molecular mechanisms of CD1b-mediated T-cell autoreactivity
αÎČ T cell receptor recognition of self-phosphatidylinositol presented by CD1b
CD1 glycoproteins present lipid-based antigens to T cell receptors (TCRs). A role for CD1b in T cell-mediated autoreactivity was proposed when it was established that CD1b can present self-phospholipids with short alkyl chains (âŒC34) to T cells; however, the structural characteristics of this presentation and recognition are unclear. Here we report the 1.9 Ă
-resolution binary crystal structure of CD1b presenting a self-phosphatidylinositol-C34:1 (PI) and an endogenous scaffold lipid. Moreover, we also determined the 2.4 Ă
structure of CD1b-PI complexed to an autoreactive αÎČ TCR, BC8B. We show the TCR docks above CD1b and directly contacts the presented antigen, selecting for both the phosphoinositol headgroup and glycerol neck region via antigen remodelling within CD1b and allowing lateral escape of the inositol moiety through a channel formed by the TCR α-chain. Furthermore, through alanine scanning mutagenesis and surface plasmon resonance, we identified key CD1b residues mediating this interaction, with Glu-80 abolishing TCR binding. We additionally define a role for both CD1b α1 and α2 molecular domains in modulating this interaction. These findings suggest the BC8B TCR contacts both the presented phospholipid and the endogenous scaffold lipid via a dual mechanism of co-recognition. Taken together, these data expand our understanding into the molecular mechanisms of CD1b-mediated T cell autoreactivity
Fourteen Months of Observations of the Possible Super-Chandrasekhar Mass Type Ia Supernova 2009dc
In this paper, we present and analyse optical photometry and spectra of the
extremely luminous and slowly evolving Type Ia supernova (SN Ia) 2009dc, and
offer evidence that it is a super-Chandrasekhar mass (SC) SN Ia and thus had a
SC white dwarf (WD) progenitor. Optical spectra of SN 2007if, a similar object,
are also shown. SN 2009dc had one of the most slowly evolving light curves ever
observed for a SN Ia, with a rise time of ~23 days and Delta m_15(B) = 0.72
mag. We calculate a lower limit to the peak bolometric luminosity of ~2.4e43
erg/s, though the actual value is likely almost 40% larger. Optical spectra of
SN 2009dc and SN 2007if obtained near maximum brightness exhibit strong C II
features (indicative of a significant amount of unburned material), and the
post-maximum spectra are dominated by iron-group elements. All of our spectra
of SN 2009dc and SN 2007if also show low expansion velocities. However, we see
no strong evidence in SN 2009dc for a velocity "plateau" near maximum light
like the one seen in SN 2007if (Scalzo et al. 2010). The high luminosity and
low expansion velocities of SN 2009dc lead us to derive a possible WD
progenitor mass of more than 2 M_Sun and a Ni-56 mass of about 1.4-1.7 M_Sun.
We propose that the host galaxy of SN 2009dc underwent a gravitational
interaction with a neighboring galaxy in the relatively recent past. This may
have led to a sudden burst of star formation which could have produced the SC
WD progenitor of SN 2009dc and likely turned the neighboring galaxy into a
"post-starburst galaxy." No published model seems to match the extreme values
observed in SN 2009dc, but simulations do show that such massive progenitors
can exist (likely as a result of the merger of two WDs) and can possibly
explode as SC SNe Ia.Comment: 30 pages, 16 figures, 8 tables, re-submitted to MNRA
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