2,056 research outputs found
Astrometry and Photometry with Coronagraphs
We propose a solution to the problem of astrometric and photometric
calibration of coronagraphic images with a simple optical device which, in
theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier
optics. Placing a periodic grid of wires (we use a square grid) with known
width and spacing in a pupil plane in front of the occulting coronagraphic
focal plane mask produces fiducial images of the obscured star at known
locations relative to the star. We also derive the intensity of these fiducial
images in the coronagraphic image. These calibrator images can be used for
precise relative astrometry, to establish companionship of other objects in the
field of view through measurement of common proper motion or common parallax,
to determine orbits, and to observe disk structure around the star
quantitatively. The calibrator spots also have known brightness, selectable by
the coronagraph designer, permitting accurate relative photometry in the
coronagraphic image. This technique, which enables precision exoplanetary
science, is relevant to future coronagraphic instruments, and is particularly
useful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure
Direct Detection of Galactic Halo Dark Matter
The Milky Way Galaxy contains a large, spherical component which is believed
to harbor a substantial amount of unseen matter. Recent observations indirectly
suggest that as much as half of this ``dark matter'' may be in the form of old,
very cool white dwarfs, the remnants of an ancient population of stars as old
as the Galaxy itself. We conducted a survey to find faint, cool white dwarfs
with large space velocities, indicative of their membership in the Galaxy's
spherical halo component. The survey reveals a substantial, directly observed
population of old white dwarfs, too faint to be seen in previous surveys. This
newly discovered population accounts for at least 2% of the halo dark matter.
It provides a natural explanation for the indirect observations, and represents
a direct detection of Galactic halo dark matter.Comment: 13 pages, 4 figures, 1 table. Note added after Science Express online
publication: This text reflects the correction of a few typographical errors
in the online version of the table. It also includes the new constraint on
the calculation of d_max which accounts for the fact that the survey could
not have detected stars with proper motions below 0.33 arcseconds per year.
Published online at ScienceExpress www.sciencemag.org 22 March 2001;
10.1126/science.1059954; To appear in Science 27 April 200
A New Method for Characterizing Very-Low-Mass Companions with Low Resolution Near-Infrared Spectroscopy
This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.We present a new and computationally efficient method for characterizing very low-mass companions using low-resolution (R ~ 30), near-infrared (YJH) spectra from high-contrast imaging campaigns with integral field spectrograph (IFS) units. We conduct a detailed quantitative comparison of the efficacy of this method through tests on simulated data comparable in spectral coverage and resolution to the currently operating direct-imaging systems around the world. In particular, we simulate Project 1640 data as an example of the use, accuracy, and precision of this technique. We present results from comparing simulated spectra of M, L, and T dwarfs with a large and finely sampled grid of synthetic spectra using Markov-chain Monte Carlo techniques. We determine the precision and accuracy of effective temperature and surface gravity inferred from fits to PHOENIX dusty and cond, which we find reproduce the low-resolution spectra of all objects within the adopted flux uncertainties. Uncertainties in effective temperature decrease from ± 100-500 K for M dwarfs to as small as ± 30 K for some L and T spectral types. Surface gravity is constrained to within 0.2-0.4 dex for mid-L through T dwarfs, but uncertainties are as large as 1.0 dex or more for M dwarfs. Results for effective temperature from low-resolution YJH spectra generally match predictions from published spectral type-temperature relationships except for L-T transition objects and young objects. Single-band spectra (i.e., narrower wavelength coverage) result in larger uncertainties and often discrepant results, suggesting that high-contrast IFS observing campaigns can compensate for low spectral resolution by expanding the wavelength coverage for reliable characterization of detected companions. We find that S/N ~ 10 is sufficient to characterize temperature and gravity as well as possible given the model grid. Most relevant for direct-imaging campaigns targeting young primary stars is our finding that low-resolution near-infrared spectra of known young objects, compared to field objects of the same spectral type, result in similar best-fit surface gravities but lower effective temperatures, highlighting the need for better observational and theoretical understanding of the entangled effects of temperature, gravity, and dust on near-infrared spectra in cool low-gravity atmospheres.This research was supported in part by the American Astronomical Societyâs Small Research Grant Program, NASA Astrophysics Data Analysis Program (ADAP) award 11- ADAP11-0169, and by the National Science Foundation under Grant No. 1211568. A portion of this work was supported by NASA Origins of the Solar System Grant No. NMO7100830/102190. A portion of the research in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) and was funded by internal Research and Technology Development funds. In addition, part of this work was performed under a contract with the California Institute of Technology (Caltech) funded by NASA through the
Sagan Fellowship Program. The members of the Project 1640 team are also grateful for support from the Cordelia Corporation, Hilary and Ethel Lipsitz, the Vincent Astor Fund, Judy Vale, Andrew Goodwin, and an anonymous donor. This research has made use of the IRTF Spectral Library, the
SIMBAD database, operated at CDS, Strasbourg, France, and NASAâs Astrophysics Data System
Rotating Neutron Stars in a Chiral SU(3) Model
We study the properties of rotating neutron stars within a generalized chiral
SU(3)-flavor model. The influence of the rotation on the inner structure and
the hyperon matter content of the star is discussed. We calculate the Kepler
frequency and moments of inertia of the neutron star sequences. An estimate for
the braking index of the associated pulsars is given.Comment: 14 pages, 9 figure
The Flux Auto- and Cross-Correlation of the Lyman-alpha Forest. II. Modelling Anisotropies with Cosmological Hydrodynamic Simulations
The isotropy of the Lyman-alpha forest in real-space uniquely provides a
measurement of cosmic geometry at z > 2. The angular diameter distance for
which the correlation function along the line of sight and in the transverse
direction agree corresponds to the correct cosmological model. However, the
Lyman-alpha forest is observed in redshift-space where distortions due to
Hubble expansion, bulk flows, and thermal broadening introduce anisotropy.
Similarly, a spectrograph's line spread function affects the autocorrelation
and cross-correlation differently. In this the second paper of a series on
using the Lyman-alpha forest observed in pairs of QSOs for a new application of
the Alcock-Paczynski (AP) test, these anisotropies and related sources of
potential systematic error are investigated with cosmological hydrodynamic
simulations. Three prescriptions for galactic outflow were compared and found
to have only a marginal effect on the Lyman-alpha flux correlation (which
changed by at most 7% with use of the currently favored variable-momentum wind
model vs. no winds at all). An approximate solution for obtaining the zero-lag
cross-correlation corresponding to arbitrary spectral resolution directly from
the zero-lag cross-correlation computed at full-resolution (good to within 2%
at the scales of interest) is presented. Uncertainty in the observationally
determined mean flux decrement of the Lyman-alpha forest was found to be the
dominant source of systematic error; however, this is reduced significantly
when considering correlation ratios. We describe a simple scheme for
implementing our results, while mitigating systematic errors, in the context of
a future application of the AP test.Comment: 20 page
Observations of Ultracool White Dwarfs
We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object
Resolution of Cosmological Singularities
We show that a class of 3+1 dimensional Friedmann-Robertson-Walker
cosmologies can be embedded within a variety of solutions of string theory. In
some realizations the apparent singularities associated with the big bang or
big crunch are resolved at non-singular horizons of higher-dimensional
quasi-black hole solutions (with compactified real time); in others plausibly
they are resolved at D-brane bound states having no conventional space-time
interpretation.Comment: 11 pages, latex. Two references added, one typo correcte
The nature of submillimetre galaxies in cosmological hydrodynamic simulations
We study the nature of rapidly star-forming galaxies at z= 2 in cosmological hydrodynamic simulations, and compare their properties to observations of submillimetre galaxies (SMGs). We identify simulated SMGs as the most rapidly star-forming systems that match the observed number density of SMGs. In our models, SMGs are massive galaxies sitting at the centres of large potential wells, being fed by smooth infall and gas-rich satellites at rates comparable to their star formation rates (SFRs). They are not typically undergoing major mergers that significantly boost their quiescent SFR, but they still often show complex gas morphologies and kinematics. Our simulated SMGs have stellar masses of M*⌠1011â11.7 Mâ, SFRs of âŒ180â500 Mâ yrâ1, a clustering length of âŒ10 hâ1 Mpc and solar metallicities. The SFRs are lower than those inferred from far-infrared (far-IR) data by âŒĂ3, which we suggest may owe to one or more systematic effects in the SFR calibrations. SMGs at z= 2 live in âŒ1013 Mâ haloes, and by z= 0 they mostly end up as brightest group galaxies in âŒ1014 Mâ haloes. We predict that higher M* SMGs should have on average lower specific SFRs, less disturbed morphologies and higher clustering. We also predict that deeper far-IR surveys will smoothly join SMGs on to the massive end of the SFRâM* relationship defined by lower mass z⌠2 galaxies. Overall, our simulated rapid star-formers provide as good a match to available SMG data as merger-based scenarios, offering an alternative scenario that emerges naturally from cosmological simulations
Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf
The discovery of the first methane brown dwarf provides a framework for
describing the important advances in both fundamental physics and astrophysics
that are due to the study of companions of stars. I present a few highlights of
the history of this subject along with details of the discovery of the brown
dwarf Gliese 229B. The nature of companions of stars is discussed with an
attempt to avoid biases induced by anthropocentric nomenclature. With the newer
types of remote reconnaissance of nearby stars and their systems of companions,
an exciting and perhaps even more profound set of contributions to science is
within reach in the near future. This includes an exploration of the diversity
of planets in the universe and perhaps soon the first solid evidence for
biological activity outside our Solar System.Comment: 31 pages, 13 figure
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