1,294 research outputs found
Exotic Differentiable Structures and General Relativity
We review recent developments in differential topology with special concern
for their possible significance to physical theories, especially general
relativity. In particular we are concerned here with the discovery of the
existence of non-standard (``fake'' or ``exotic'') differentiable structures on
topologically simple manifolds such as , \R and
Because of the technical difficulties involved in the smooth case, we begin
with an easily understood toy example looking at the role which the choice of
complex structures plays in the formulation of two-dimensional vacuum
electrostatics. We then briefly review the mathematical formalisms involved
with differentiable structures on topological manifolds, diffeomorphisms and
their significance for physics. We summarize the important work of Milnor,
Freedman, Donaldson, and others in developing exotic differentiable structures
on well known topological manifolds. Finally, we discuss some of the geometric
implications of these results and propose some conjectures on possible physical
implications of these new manifolds which have never before been considered as
physical models.Comment: 11 pages, LaTe
Photometric Recovery of Crowded Stellar Fields Observed with HST/WFPC2 and the Effects of Confusion Noise on the Extragalactic Distance Scale
We explore the limits of photometric reductions of crowded stellar fields
observed with the Wide Field and Planetary Camera 2 on board the Hubble Space
Telescope. Two photometric procedures, based on the DoPHOT and DAOPHOT/ALLFRAME
programs are tested, and the effects of crowding, complex sky background and
cosmic-ray contamination are discussed using an extensive set of artificial
star simulations. As a specific application of the results presented in this
paper, we assess the magnitude of photometric biases on programs aimed at
finding Cepheids and determining distances. We find that while the photometry
in individual images can be biased too bright by up to 0.2 mag in the most
crowded fields due to confusion noise, the effects on distance measurements
based on Cepheid variables are insignificant, less than 0.02 mag (1% in
distance) even in the most problematic cases. This result, which is at odds
with claims recently surfaced in the literature, is due to the strict criteria
applied in the selection of the variable stars, and the photometric cross
checks made possible by the availability of multiple exposures in different
filters which characterizes Cepheid observations.Comment: Accepted for publication in PASP. 41 pages, 18 figures, 8 tables. The
figures included with this submission are very low quality bitmap postscript,
please see http://www.astro.ucla.edu/~laura/pub.htm for the full size image
The human CHRNA7 and CHRFAM7A genes: A review of the genetics, regulation, and function
The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is ubiquitously expressed in both the central nervous system and in the periphery. CHRNA7 is genetically linked to multiple disorders with cognitive deficits, including schizophrenia, bipolar disorder, ADHD, epilepsy, Alzheimer's disease, and Rett syndrome. The regulation of CHRNA7 is complex; more than a dozen mechanisms are known, one of which is a partial duplication of the parent gene. Exons 5–10 of CHRNA7 on chromosome 15 were duplicated and inserted 1.6 Mb upstream of CHRNA7, interrupting an earlier partial duplication of two other genes. The chimeric CHRFAM7A gene product, dupα7, assembles with α7 subunits, resulting in a dominant negative regulation of function. The duplication is human specific, occurring neither in primates nor in rodents. The duplicated α7 sequence in exons 5–10 of CHRFAM7A is almost identical to CHRNA7, and thus is not completely queried in high throughput genetic studies (GWAS). Further, pre-clinical animal models of the α7nAChR utilized in drug development research do not have CHRFAM7A (dupα7) and cannot fully model human drug responses. The wide expression of CHRNA7, its multiple functions and modes of regulation present challenges for study of this gene in disease
First Hubble Space Telescope observations of the brightest stars in the Virgo galaxy M100 = NGC 4321
As part of both the Early Release Observations from the Hubble Space Telescope and the Key Project on the Extragalactic Distance Scale, we have obtained multiwavelength BVR WFPC2 images for the face-on Virgo cluster spiral galaxy M100 = NGC 4321. We report here preliminary results from those observations, in the form of a color-magnitude diagram for -11,500 stars down to V ~ 27 mag and a luminosity function for the brightest blue stars which is found to have a slope of 0.7, in excellent agreement with previous results obtained for significantly nearer galaxies. With the increased resolution now available using WFPC2, the number of galaxies in which we can directly measure Population I stars and thereby quantify the recent evolution, as well as test stellar evolution theory, has dramatically increased by at least a factor of 100. Finally, we find that stars are present in M100 at the colors and luminosities expected for the brightest Cepheid variables in galaxies
The Luminosity of SN 1999by in NGC 2841 and the Nature of `Peculiar' Type Ia Supernovae
We present UBVRIJHK photometry and optical spectroscopy of the so-called
'peculiar' Type Ia supernova 1999by in NGC 2841. The observations began one
week before visual maximum light which is well-defined by daily observations.
The light curves and spectra are similar to those of the prototypical
subluminous event SN 1991bg. We find that maximum light in B occurred on 1999
May 10.3 UT (JD 2,451,308.8 +/- 0.3) with B=13.66 +/- 0.02 mag and a color of
B_max-V_max=0.51 +/- 0.03 mag. The late-time color implies minimal dust
extinction from the host galaxy. Our photometry, when combined with the recent
Cepheid distance to NGC 2841 (Macri et al. 2001), gives a peak absolute
magnitude of M_B=-17.15 +/- 0.23 mag, making SN 1999by one of the least
luminous Type Ia events ever observed. We estimate a decline rate parameter of
dm15(B)=1.90 mag, versus 1.93 for SN 1991bg, where 1.10 is typical for
so-called 'normal' events. We compare SN 1999by with other subluminous events
and find that the B_max-V_max color correlates strongly with the decline rate
and may be a more sensitive indicator of luminosity than the fading rate for
these objects. We find a good correlation between luminosity and the depth of
the spectral feature at 580 nm, which had been attributed solely to Si II. We
show that in cooler photospheres the 580 nm feature is dominated by Ti II,
which provides a simple physical explanation for the correlation. Using only
subluminous Type Ia supernovae we derive a Hubble parameter of H_0=75 +12 -11
km/s Mpc, consistent with values found from brighter events.Comment: 36 preprint pages including 18 figures. Near-IR photometry of the SN
has been added to the paper. Scheduled to appear in ApJ vol. 613 (September
2004). High-resolution version available from
http://www.nd.edu/~pgarnavi/sn99by/sn99by.p
High-resolution NMR studies of structure and dynamics of human ERp27 indicate extensive interdomain flexibility
ERp27 (endoplasmic reticulum protein 27.7 kDa) is a homologue of PDI (protein disulfide-isomerase) localized to the endoplasmic reticulum. ERp27 is predicted to consist of two thioredoxinfold domains homologous with the non-catalytic b and b domains of PDI. The structure in solution of the N-terminal blike domain of ERp27 was solved using high-resolution NMR data. The structure confirms that it has the thioredoxin fold and that ERp27 is a member of the PDI family. 15N-NMR relaxation data were obtained and ModelFree analysis highlighted limited exchange contributions and slow internal motions, and
indicated that the domain has an average order parameter S 2 of 0.79. Comparison of the single-domain structure determined in the present study with the equivalent domain within fulllength ERp27, determined independently by X-ray diffraction, indicated very close agreement. The domain interface inferred from NMR data in solution was much more extensive than that observed in the X-ray structure, suggesting that the domains flex independently and that crystallization selects one specific
interdomain orientation. This led us to apply a new rapid method to simulate the flexibility of the full-length protein, establishing that the domains show considerable freedom to flex (tilt and twist) about the interdomain linker, consistent with the NMR data
Color superconductivity in weak coupling
We derive perturbatively the gap equations for a color-superconducting
condensate with total spin J=0 in dense QCD. At zero temperature, we confirm
the results of Son for the dependence of the condensate on the coupling
constant, and compute the prefactor to leading logarithmic accuracy. At nonzero
temperature, we find that to leading order in weak coupling, the temperature
dependence of the condensate is identical to that in BCS-like theories. The
condensates for total spin J=1 are classified; to leading logarithmic accuracy
these condensates are of the same order as those of spin J=0.Comment: 30 pages, 3 figures, RevTeX, epsf and psfig style files require
A Database of Cepheid Distance Moduli and TRGB, GCLF, PNLF and SBF Data Useful for Distance Determinations
We present a compilation of Cepheid distance moduli and data for four
secondary distance indicators that employ stars in the old stellar populations:
the planetary nebula luminosity function (PNLF), the globular cluster
luminosity function (GCLF), the tip of the red giant branch (TRGB), and the
surface brightness fluctuation (SBF) method. The database includes all data
published as of July 15, 1999. The main strength of this compilation resides in
all data being on a consistent and homogeneous system: all Cepheid distances
are derived using the same calibration of the period-luminosity relation, the
treatment of errors is consistent for all indicators, measurements which are
not considered reliable are excluded. As such, the database is ideal for
inter-comparing any of the distance indicators considered, or for deriving a
Cepheid calibration to any secondary distance indicator. Specifically, the
database includes: 1) Cepheid distances, extinctions and metallicities; 2)
apparent magnitudes of the PNLF cutoff; 3) apparent magnitudes and colors of
the turnover of the GCLF (both in the V- and B-bands); 4) apparent magnitudes
of the TRGB (in the I-band) and V-I colors at and 0.5 magnitudes fainter than
the TRGB; 5) apparent surface brightness fluctuation magnitudes I, K', K_short,
and using the F814W filter with the HST/WFPC2. In addition, for every galaxy in
the database we give reddening estimates from DIRBE/IRAS as well as HI maps,
J2000 coordinates, Hubble and T-type morphological classification, apparent
total magnitude in B, and systemic velocity. (Abridged)Comment: Accepted for publication in the Astrophysical Journal Supplement
Series. Because of space limitations, the figures included are low resolution
bitmap images. Original figures can be found at
http://www.astro.ucla.edu/~laura/pub.ht
The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331
The distance to NGC 7331 has been derived from Cepheid variables observed
with HST/WFPC2, as part of the Extragalactic Distance Scale Key Project.
Multi-epoch exposures in F555W (V) and F814W (I), with photometry derived
independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a
total of 13 reliable Cepheids, with periods between 11 and 42 days. The
relative distance moduli between NGC 7331 and the LMC, imply an extinction to
NGC 7331 of A_V = 0.47+-0.15 mag, and an extinction-corrected distance modulus
to NGC 7331 of 30.89+-0.14(random) mag, equivalent to a distance of 15.1 Mpc.
There are additional systematic uncertainties in the distance modulus of +-0.12
mag due to the calibration of the Cepheid Period-Luminosity relation, and a
systematic offset of +0.05+-0.04 mag if we applied the metallicity correction
inferred from the M101 results of Kennicutt et al 1998.Comment: To be published in The Astrophysical Journal, 1998 July 1, v501 note:
Figs 1 and 2 (JPEG files) and Fig 7 (multipage .eps file) need to be
viewed/printed separatel
- …