1,515 research outputs found
The Planetary Nebula Luminosity Function: Pieces of the Puzzle
Extragalactic surveys in the emission line of [O III] 5007 have provided us
with the absolute line strengths of large, homogeneous sets of planetary
nebulae. These data have been used to address a host of problems, from the
measurement of the extragalactic distance scale, to the study of stellar
populations. I review our current understanding of the [O III] planetary nebula
luminosity function (PNLF), and discuss some of the physical processes that
effect its structure. I also describe the features of the H-alpha PNLF, a
function that, upon first glance, looks similar to the [O III] PNLF, but which
includes a very different set of objects. Finally, I discuss recent
measurements of alpha, the number of PNe found in a stellar population,
normalized to that population's bolometric luminosity. I show that, contrary to
expectations, the values of alpha found in actively star-forming spirals is
essentially the same as those measured in late-type elliptical and lenticular
systems. I discuss how this result sheds light on the physics of the planetary
nebula phenomenon.Comment: 7 pages, including 7 figures; presentation at the workshop on the
Legacies of the Macquarie/AAO/Strasbourg H-alpha Planetary Nebula project,
accepted for publication in PAS
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397
We have examined the radial distribution of white dwarfs over a single
HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations,
such as in a globular cluster, stellar velocity dispersion, and hence radial
distribution, is directly dependent on stellar masses. The progenitors of very
young cluster white dwarfs had a mass of ~0.8 solar masses, while the white
dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white
dwarfs to have a concentrated radial distribution (like that of their
progenitors) that becomes more extended over several relaxation times to mimic
that of ~0.5 solar mass main-sequence stars. However, we observe young white
dwarfs to have a significantly extended radial distribution compared to both
the most massive main sequence stars in the cluster and also to old white
dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in
the MNRAS Letter
Atmospheric dispersion and the implications for phase calibration
The success of any ALMA phase-calibration strategy, which incorporates phase
transfer, depends on a good understanding of how the atmospheric path delay
changes with frequency (e.g. Holdaway & Pardo 2001). We explore how the wet
dispersive path delay varies for realistic atmospheric conditions at the ALMA
site using the ATM transmission code. We find the wet dispersive path delay
becomes a significant fraction (>5 per cent) of the non-dispersive delay for
the high-frequency ALMA bands (>160 GHz, Bands 5 to 10). Additionally, the
variation in dispersive path delay across ALMA's 4-GHz contiguous bandwidth is
not significant except in Bands 9 and 10. The ratio of dispersive path delay to
total column of water vapour does not vary significantly for typical amounts of
water vapour, water vapour scale heights and ground pressures above Chajnantor.
However, the temperature profile and particularly the ground-level temperature
are more important. Given the likely constraints from ALMA's ancillary
calibration devices, the uncertainty on the dispersive-path scaling will be
around 2 per cent in the worst case and should contribute about 1 per cent
overall to the wet path fluctuations at the highest frequencies.Comment: 13 pages, 10 figures, ALMA Memo 59
Globular Clusters in NGC 1275
We present the results of a deep photometric study of the outer halo of NGC
1275, the highly active cD galaxy at the center of the Perseus cluster. We find
a modest excess of faint () starlike objects in its halo, indicating
a population of old-halo globular clusters. However, the total estimated
cluster population corresponds to a specific frequency of ,
no larger than that of normal giant ellipticals and three times lower than that
of other central cD galaxies such as M87. We discuss several ideas for the
origin of this galaxy. Our results reinforce the view that high (ie:
highly efficient globular cluster formation) is not associated with cooling
flows, or with recent starburst or merger phenomena.Comment: 25 pages, latex, postscript figures, tarred, Unix compressed,
postscript version of paper and figures available at
http://www.physics.mcmaster.ca/Grads/DKaisler/office.htm
Power-law nature of field-effect transistor experimental characteristics
In making experimental measurements of
field-effect transistor static drain characteristics
in the pinch-off region, determination
of the effective pinch-off voltage is not
possible by direct measurement because of
the presence of spurious drain current at and
beyond pinch-off. Further, indirect measurement
is hampered by the absence of a theoretical straight-line function from which the pinch-off voltage may be obtained as an intercept or a slope. In this communication
a power-law relation for the transfer characteristic
is assumed, from which values of both the pinch-off voltage and the exponent may be obtained directly from a straight-line plot of experimental quantities
Faint, moving objects in the Hubble Deep Field: components of the dark halo?
The deepest optical image of the sky, the Hubble Deep Field (HDF), obtained
with the Hubble Space Telescope (HST) in December 1995, has been compared to a
similar image taken in December 1997. Two very faint, blue, isolated and
unresolved objects are found to display a substantial apparent proper motion,
23+/-5 mas/yr and 26+/-5 mas/yr; a further three objects at the detection limit
of the second epoch observations may also be moving. Galactic structure models
predict a general absence of stars in the color-magnitude range in which these
objects are found. However, these observations are consistent with
recently-developed models of old white dwarfs with hydrogen atmospheres, whose
color, contrary to previous expectations, has been shown to be blue. If these
apparently moving objects are indeed old white dwarfs with hydrogen atmospheres
and masses near 0.5 M_Sun, they have ages of approximately 12 Gyr, and a local
mass density that is sufficient, within the large uncertainties arising from
the small size of the sample, to account for the entire missing Galactic
dynamical mass.Comment: 6 pages, using emulateapj, including 2 colour figures, accepted for
publication in ApJ Letter
[Authors' Comment on] Power-Law Nature of Field-Effect Transistor Characteristics
[no abstract
Measurement of Antenna Surfaces from In- and Out-Of-Focus Beam Maps using Astronomical Sources
We present a technique for the accurate estimation of large-scale errors in
an antenna surface using astronomical sources and detectors. The technique
requires several out-of-focus images of a compact source and the
signal-to-noise ratio needs to be good but not unreasonably high. For a given
pattern of surface errors, the expected form of such images can be calculated
directly. We show that it is possible to solve the inverse problem of finding
the surface errors from the images in a stable manner using standard numerical
techniques. To do this we describe the surface error as a linear combination of
a suitable set of basis functions (we use Zernike polynomials). We present
simulations illustrating the technique and in particular we investigate the
effects of receiver noise and pointing errors. Measurements of the 15-m James
Clerk Maxwell telescope made using this technique are presented as an example.
The key result is that good measurements of errors on large spatial scales can
be obtained if the input images have a signal-to-noise ratio of order 100 or
more. The important advantage of this technique over transmitter-based
holography is that it allows measurements at arbitrary elevation angles, so
allowing one to characterise the large scale deformations in an antenna as a
function of elevation.Comment: 6 pages, 5 figures (accepted by Astronomy & Astrophysics
Phase transitions in a gas of anyons
We continue our numerical Monte Carlo simulation of a gas of closed loops on
a 3 dimensional lattice, however now in the presence of a topological term
added to the action corresponding to the total linking number between the
loops. We compute the linking number using certain notions from knot theory.
Adding the topological term converts the particles into anyons. Using the
correspondence that the model is an effective theory that describes the
2+1-dimensional Abelian Higgs model in the asymptotic strong coupling regime,
the topological linking number simply corresponds to the addition to the action
of the Chern-Simons term. We find the following new results. The system
continues to exhibit a phase transition as a function of the anyon mass as it
becomes small \cite{mnp}, although the phases do not change the manifestation
of the symmetry. The Chern-Simons term has no effect on the Wilson loop, but it
does affect the {\rm '}t Hooft loop. For a given configuration it adds the
linking number of the 't Hooft loop with all of the dynamical vortex loops to
the action. We find that both the Wilson loop and the 't Hooft loop exhibit a
perimeter law even though there are no massless particles in the theory, which
is unexpected.Comment: 6 pages, 5 figure
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