3,160 research outputs found
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
Far-field Unlabelled Super-Resolution Imaging with Superoscillatory Illumination
Unlabelled super-resolution is the next grand challenge in imaging. Stimulated emission depletion and single-molecule microscopies have revolutionised the life sciences but are still limited by the need for reporters (labels) embedded within the sample. While the Veselago-Pendry âsuper-lensâ using a negative-index metamaterial is a promising idea for imaging beyond the diffraction limit, there are substantial technological challenges to its realisation. Another route to far-field subwavelength focusing is using optical superoscillations: engineered interference of multiple coherent waves creating an, in principle, arbitrarily small hotspot. Here we demonstrate microscopy with superoscillatory illumination of the object and describe its underlying principles. We show that far-field images taken with superoscillatory
illumination are themselves superoscillatory and hence can reveal fine structural details of the object that are lost in conventional far-field imaging. We show that the resolution of a superoscillatory microscope is determined by the size of the hotspot, rather than the bandwidth of the optical instrument. We demonstrate high-frame-rate polarisation-contrast imaging of unmodified living cells with resolution significantly exceeding that achievable with conventional instruments. This non-algorithmic, low-phototoxicity imaging technology is a powerful tool both for biological research and for super-resolution imaging of samples that do not allow labelling, such as the interior of silicon chips
A Cepheid Distance to NGC 4603 in Centaurus
In an attempt to use Cepheid variables to determine the distance to the
Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space
Telescope on 9 epochs using WFPC2 and the F555W and F814W filters. This galaxy
has been suggested to lie within the ``Cen30'' portion of the cluster and is
the most distant object for which this method has been attempted. Previous
distance estimates for Cen30 have varied significantly and some have presented
disagreements with the peculiar velocity predicted from redshift surveys,
motivating this investigation. Using our observations, we have found 61
candidate Cepheid variable stars; however, a significant fraction of these
candidates are likely to be nonvariable stars whose magnitude measurement
errors happen to fit a Cepheid light curve of significant amplitude for some
choice of period and phase. Through a maximum likelihood technique, we
determine that we have observed 43 +/- 7 real Cepheids and that NGC 4603 has a
distance modulus of 32.61 +0.11/-0.10 (random, 1 sigma) +0.24/-0.25
(systematic, adding in quadrature), corresponding to a distance of 33.3 Mpc.
This is consistent with a number of recent estimates of the distance to NGC
4603 or Cen30 and implies a small peculiar velocity consistent with predictions
from the IRAS 1.2 Jy redshift survey if the galaxy lies in the foreground of
the cluster.Comment: Accepted for publication in the Astrophysical Journal. 17 pages with
17 embedded figures and 3 tables using emulateapj.sty. Additional figures and
images may be obtained from http://astro.berkeley.edu/~marc/n4603
New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
Discoveries of two new white dwarf plus M star binaries with striking optical
cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to
six the total number of X-ray faint, magnetic accretion binaries that accrete
at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in
cataclysmic variables. This fact, coupled with donor stars that underfill their
Roche lobes and very cool white dwarfs, brand the binaries as post
common-envelope systems whose orbits have not yet decayed to the point of
Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems
exhibit spin/orbit synchronism and apparently accrete by efficient capture of
the stellar wind from the secondary star, a process that has been dubbed a
``magnetic siphon''. Because of this, period evolution of the binaries will
occur solely by gravitational radiation, which is very slow for periods >3 hr.
Optical surveys for the cyclotron harmonics appear to be the only means of
discovery, so the space density of pre-Polars could rival that of Polars, and
the binaries provide an important channel of progenitors (in addition to the
asynchronous Intermediate Polars). Both physical and SDSS observational
selection effects are identified that may help to explain the clumping of all
six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap
A Strategy for Finding Near Earth Objects with the SDSS Telescope
We present a detailed observational strategy for finding Near Earth Objects
(NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate
strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3,
which affects the sky coverage rate and the limiting apparent magnitude. We
present results from 1 month, 3 year and 10 year simulations of such surveys.
For each cadence and binning mode, we evaluate the possibility of achieving the
Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for
an albedo of 0.1). We find that an unbinned survey is most effective at
detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the
Spaceguard Goal after only seven years of operation. As the proposed large
survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an
SDSS NEO survey could make a significant contribution to the detection and
photometric characterization of the NEO population.Comment: Accepted by AJ -- 12 pages, 11 figure
Are the singularities stable?
The spacetime singularities play a useful role in gravitational theories by
distinguishing physical solutions from non-physical ones. The problem, we
studying in this paper is: are these singularities stable? To answer this
question, we have analyzed the general problem of stability of the family of
the static spherically symmetric solutions of the standard Einstein-Maxwell
model coupled to an extra free massless scalar field. We have obtained the
equations for the axial and polar perturbations. The stability against axial
perturbations has been proven.Comment: 13 pages, LaTeX, no figure
Planning, implementation, and first results of the Tropical Composition, Cloud and Climate Coupling Experiment (TC4)
The Tropical Composition, Cloud and Climate Coupling Experiment (TC4), was based in Costa Rica and Panama during July and August 2007. The NASA ER-2, DC-8, and WB-57F aircraft flew 26 science flights during TC4. The ER-2 employed 11 instruments as a remote sampling platform and satellite surrogate. The WB-57F used 25 instruments for in situ chemical and microphysical sampling in the tropical tropopause layer (TTL). The DC-8 used 25 instruments to sample boundary layer properties, as well as the radiation, chemistry, and microphysics of the TTL. TC4 also had numerous sonde launches, two ground-based radars, and a ground-based chemical and microphysical sampling site. The major goal of TC4 was to better understand the role that the TTL plays in the Earth's climate and atmospheric chemistry by combining in situ and remotely sensed data from the ground, balloons, and aircraft with data from NASA satellites. Significant progress was made in understanding the microphysical and radiative properties of anvils and thin cirrus. Numerous measurements were made of the humidity and chemistry of the tropical atmosphere from the boundary layer to the lower stratosphere. Insight was also gained into convective transport between the ground and the TTL, and into transport mechanisms across the TTL. New methods were refined and extended to all the NASA aircraft for real-time location relative to meteorological features. The ability to change flight patterns in response to aircraft observations relayed to the ground allowed the three aircraft to target phenomena of interest in an efficient, well-coordinated manner
A New Giant Stellar Structure in the Outer Halo of M31
The Sloan Digital Sky Survey has revealed an overdensity of luminous red
giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we
have called Andromeda NE. The line-of-sight distance to Andromeda NE is within
approximately 50 kpc of M31; Andromeda NE is not a physically unrelated
projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central
surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of
magnitude more diffuse than any known Local Group dwarf galaxy at that
luminosity. Based on its distance and morphology, Andromeda NE is likely
undergoing tidal disruption. Andromeda NE's red giant branch color is unlike
that of M31's present-day outer disk or the stellar stream reported by Ibata et
al. (2001), arguing against a direct link between Andromeda NE and these
structures. However, Andromeda NE has a red giant branch color similar to that
of the G1 clump; it is possible that these structures are both material torn
off of M31's disk in the distant past, or that these are both part of one
ancient stellar stream.Comment: 11 pages, 3 figures; ApJ Letters accepted versio
Deep Multi-object Spectroscopy to Enhance Dark Energy Science from LSST
Community access to deep (i ~ 25), highly-multiplexed optical and
near-infrared multi-object spectroscopy (MOS) on 8-40m telescopes would greatly
improve measurements of cosmological parameters from LSST. The largest gain
would come from improvements to LSST photometric redshifts, which are employed
directly or indirectly for every major LSST cosmological probe; deep
spectroscopic datasets will enable reduced uncertainties in the redshifts of
individual objects via optimized training. Such spectroscopy will also
determine the relationship of galaxy SEDs to their environments, key
observables for studies of galaxy evolution. The resulting data will also
constrain the impact of blending on photo-z's. Focused spectroscopic campaigns
can also improve weak lensing cosmology by constraining the intrinsic
alignments between the orientations of galaxies. Galaxy cluster studies can be
enhanced by measuring motions of galaxies in and around clusters and by testing
photo-z performance in regions of high density. Photometric redshift and
intrinsic alignment studies are best-suited to instruments on large-aperture
telescopes with wider fields of view (e.g., Subaru/PFS, MSE, or GMT/MANIFEST)
but cluster investigations can be pursued with smaller-field instruments (e.g.,
Gemini/GMOS, Keck/DEIMOS, or TMT/WFOS), so deep MOS work can be distributed
amongst a variety of telescopes. However, community access to large amounts of
nights for surveys will still be needed to accomplish this work. In two
companion white papers we present gains from shallower, wide-area MOS and from
single-target imaging and spectroscopy.Comment: Science white paper submitted to the Astro2020 decadal survey. A
table of time requirements is available at
http://d-scholarship.pitt.edu/36036
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