126 research outputs found
Bayesian Analysis of Many-Pole Fits of Hadron Propagators in Lattice QCD
We use Bayes' probability theorem to analyze many-pole fits of hadron
propagators. An alternative method of estimating values and uncertainties of
the fit parameters is offered, which has certain advantages over the
conventional methods. The probability distribution of the parameters of a fit
is calculated. The relative probability of various models is calculated.Comment: 4 pages, 6 figures, Latex with espcrc2.sty, uuencoded compressed tar
file contains 7 Latex files: 1 with the paper and 6 with the figures. Talk
presented at LATTICE96(spectrum
Spectral Density on the Lattice
Spectral density in the pseudoscalar and vector channels is extracted from
the SU(2) lattice quenched data. It is shown to consist of three sharp poles
within the energy range accessible on the lattice.Comment: 38 pages, uuencoded tar-compressed ps-fil
Filtering of Signal Dependent Noise Applied to MIPS Data
Linear filtering and usual nonlinear median filtering are not effective for signal-dependent noise removal. We apply here an approximate decoupling of signal and noise by means of a nonlinear transform. The transform is followed by a linear filter and the corresponding inverse transform. This procedure allows us to mitigate the signal-dependent noise in the images obtained by the Multiband Imaging Photometer for Spitzer (MIPS), 70ÎŒm imaging band
Spitzer 24-micron Time-Series Observations of the Eclipsing M-dwarf Binary GU Bootis
We present a set of {\it Spitzer} 24m MIPS time series observations of
the M-dwarf eclipsing binary star GU Bo\"otis. Our data cover three secondary
eclipses of the system: two consecutive events and an additional eclipse six
weeks later. The study's main purpose is the long wavelength (and thus limb
darkening-independent) characterization of GU Boo's light curve, allowing for
independent verification of the results of previous optical studies. Our
results confirm previously obtained system parameters. We further compare GU
Boo's measured 24m flux density to the value predicted by spectral fitting
and find no evidence for circumstellar dust. In addition to GU Boo, we
characterize (and show examples of) light curves of other objects in the field
of view. Analysis of these light curves serves to characterize the photometric
stability and repeatability of {\it Spitzer's} MIPS 24\micron array over short
(days) and long (weeks) timescales at flux densities between approximately
300--2,000Jy. We find that the light curve root mean square about the
median level falls into the 1--4% range for flux densities higher than 1mJy.
Finally, we comment on the fluctuations of the 24\micron background on short
and long timescales.Comment: ApJ accepted. 10 pages, 12 figure
The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HII Region NGC 346
We use Spitzer Space Telescope observations from the Spitzer Survey of the
Small Magellanic Cloud (S3MC) to study the young stellar content of N66, the
largest and brightest HII region in the SMC. In addition to large numbers of
normal stars, we detect a significant population of bright, red infrared
sources that we identify as likely to be young stellar objects (YSOs). We use
spectral energy distribution (SED) fits to classify objects as ordinary (main
sequence or red giant) stars, asymptotic giant branch stars, background
galaxies, and YSOs. This represents the first large-scale attempt at blind
source classification based on Spitzer SEDs in another galaxy. We firmly
identify at least 61 YSOs, with another 50 probable YSOs; only one embedded
protostar in the SMC was reported in the literature prior to the S3MC. We
present color selection criteria that can be used to identify a relatively
clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the
infrared-bright YSOs in N66 have stellar masses ranging from 2 Msun to 17 Msun,
and that approximately half of the objects are Stage II protostars, with the
remaining YSOs roughly evenly divided between Stage I and Stage III sources. We
find evidence for primordial mass segregation in the HII region, with the most
massive YSOs being preferentially closer to the center than lower-mass objects.
Despite the low metallicity and dust content of the SMC, the observable
properties of the YSOs appear consistent with those in the Milky Way. Although
the YSOs are heavily concentrated within the optically bright central region of
N66, there is ongoing star formation throughout the complex and we place a
lower limit on the star formation rate of 3.2 x 10^-3 Msun/yr over the last ~1
Myr.Comment: 13 pages, 5 figures (3 in color), 2 tables. Accepted for publication
in Ap
Expansion Aspect of Color Transparency on the Lattice
The opportunity to observe color transparency (CT) is determined by how
rapidly a small-sized hadronic wave packet expands. Here we use SU(2) lattice
gauge theory with Wilson fermions in the quenched approximation to investigate
the expansion. The wave packet is modeled by a point hadronic source, often
used as an interpolating field in lattice calculations. The procedure is to
determine the Euclidean time (t), pion channel, Bethe-Salpeter amplitude
, and then evaluate . This quantity represents the soft interaction of a small-sized
wave packet with a pion. The time dependence of is fit as a
superposition of three states, which is found sufficient to reproduce a reduced
size wave packet. Using this superposition allows us to make the analytic
continuation required to study the wave packet expansion in real time. We find
that the matrix elements of the soft interaction between the excited
and ground state decrease rapidly with the energy of the excited state.Comment: 19 pages, latex, 4 figure
Spectral classification of the brightest objects in the galactic star forming region W40
We present high S/N, moderate resolution near-infrared spectra, as well as 10
micron imaging, for the brightest members of the central stellar cluster in the
W40 HII region, obtained using the SpeX and MIRSI instruments at NASA's
Infrared Telescope Facility. Using these observations combined with archival
Spitzer Space Telescope data, we have determined the spectral classifications,
extinction, distances, and spectral energy distributions for the brightest
members of the cluster. Of the eight objects observed, we identify four main
sequence (MS) OB stars, two Herbig Ae/Be stars, and two low-mass young stellar
objects. Strong HeI absorption at 1.083 micron in the MS star spectra strongly
suggests that at least some of these sources are in fact close binaries. Two
out of the four MS stars also show significant infrared excesses typical of
circumstellar disks. Extinctions and distances were determined for each MS star
by fitting model stellar atmospheres to the SEDs. We estimate a distance to the
cluster of between 455 and 535 pc, which agrees well with earlier (but far less
precise) distance estimates. We conclude that the late-O star we identify is
the dominant source of LyC luminosity needed to power the W40 HII region and is
the likely source of the stellar wind that has blown a large (~4 pc)
pinched-waist bubble observed in wide field mid-IR images. We also suggest that
3.6 cm radio emission observed from some of the sources in the cluster is
likely not due to emission from ultra-compact HII regions, as suggested in
other work, due to size constraints based on our derived distance to the
cluster. Finally, we also present a discussion of the curious source IRS 3A,
which has a very strong mid-IR excess (despite its B3 MS classification) and
appears to be embedded in a dusty envelope roughly 2700 AU in size.Comment: Accepted for publication in The Astronomical Journal. 29 pages, 10
figure
The Infrared Properties of Hickson Compact Groups
Compact groups of galaxies provide a unique environment to study the
mechanisms by which star formation occurs amid continuous gravitational
encounters. We present 2MASS (JHK), Spitzer IRAC (3.5-8 micron) and MIPS (24
micron) observations of a sample of twelve Hickson Compact Groups (HCGs 2, 7,
16, 19, 22, 31, 42, 48, 59, 61, 62, and 90) that includes a total of 45
galaxies. The near-infrared colors of the sample galaxies are largely
consistent with being dominated by slightly reddened normal stellar
populations. Galaxies that have the most significant PAH and/or hot dust
emission (as inferred from excess 8 micron flux) also tend to have larger
amounts of extinction and/or K-band excess and stronger 24 micron emission, all
of which suggest ongoing star formation activity. We separate the twelve HCGs
in our sample into three types based on the ratio of the group HI mass to
dynamical mass. We find evidence that galaxies in the most gas-rich groups tend
to be the most actively star forming. Galaxies in the most gas-poor groups tend
to be tightly clustered around a narrow range in colors consistent with the
integrated light from a normal stellar population. We interpret these trends as
indicating that galaxies in gas-rich groups experience star formation and/or
nuclear actively until their neutral gas consumed, stripped, or ionized. The
galaxies in this sample exhibit a ``gap'' between gas-rich and gas-poor groups
in infrared color space that is sparsely populated and not seen in the Spitzer
First Look Survey sample. This gap may suggest a rapid evolution of galaxy
properties in response to dynamical effects. These results suggest that the
global properties of the groups and the local properties of the galaxies are
connected.Comment: 34 pages, 26 figures, accepted for publication in AJ, higher quality
images available in publicatio
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