149 research outputs found
Towards a Continuous Record of the Sky
It is currently feasible to start a continuous digital record of the entire
sky sensitive to any visual magnitude brighter than 15 each night. Such a
record could be created with a modest array of small telescopes, which
collectively generate no more than a few Gigabytes of data daily.
Alternatively, a few small telescopes could continually re-point to scan and
reco rd the entire sky down to any visual magnitude brighter than 15 with a
recurrence epoch of at most a few weeks, again always generating less than one
Gigabyte of data each night. These estimates derive from CCD ability and
budgets typical of university research projects. As a prototype, we have
developed and are utilizing an inexpensive single-telescope system that obtains
optical data from about 1500 square degrees. We discuss the general case of
creating and storing data from a both an epochal survey, where a small number
of telescopes continually scan the sky, and a continuous survey, composed of a
constellation of telescopes dedicated each continually inspect a designated
section of the sky. We compute specific limitations of canonical surveys in
visible light, and estimate that all-sky continuous visual light surveys could
be sensitive to magnitude 20 in a single night by about 2010. Possible
scientific returns of continuous and epochal sky surveys include continued
monitoring of most known variable stars, establishing case histories for
variables of future interest, uncovering new forms of stellar variability,
discovering the brightest cases of microlensing, discovering new novae and
supernovae, discovering new counterparts to gamma-ray bursts, monitoring known
Solar System objects, discovering new Solar System objects, and discovering
objects that might strike the Earth.Comment: 38 pages, 9 postscript figures, 2 gif images. Revised and new section
added. Accepted to PASP. Source code submitted to ASCL.ne
Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journa
A Measurement of the Angular Power Spectrum of the Anisotropy in the Cosmic Microwave Background
We report on a measurement of the angular power spectrum of the anisotropy in
the Cosmic Microwave Background. The anisotropy is measured in 23 different
multipole bands from l=54 (~3 deg) to l=404 (~0.45 deg) and in 6 frequency
bands from 26 GHz to 46 GHz over three observing seasons. The measurements are
consistent from year to year. The frequency spectral index of the fluctuations
(measured at low l) is consistent with that of the CMB and inconsistent with
either dust or Galactic free-free emission. Furthermore, the observations of
the MSAM1-92 experiment (Cheng et al. 1994) are repeated and confirmed. The
angular spectrum shows a distinct rise from dT_l =
\sqrt{l(2l+1)/4pi} = 49(+8,-5) \uK at l=87 to dT_l=85(+10,-8) uK at
l = 237. These values do not include an overall +/-14\% (1sigma) calibration
uncertainty. The analysis and possible systematic errors are discussed.Comment: latex file is called spect.tex. 25 pages with 11 Postscript figures.
Uses aas2pp4.sty (included). Submitted to Ap
Detection of Six Rapidly Scintillating AGNs and the Diminished Variability of J1819+3845
The extreme, intra-hour and > 10% rms flux density scintillation observed in
AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has
been attributed to scattering in highly turbulent, nearby regions in the
interstellar medium. Such behavior has been found to be rare. We searched for
rapid scintillators among 128 flat spectrum AGNs and analyzed their properties
to determine the origin of such rapid and large amplitude radio scintillation.
The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two
hour intervals over 11 days. We detected six rapid scintillators with
characteristic time-scales of
10%. We found strong lines of evidence linking rapid scintillation to the
presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200
muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity
of rapid and large amplitude scintillators to the requirement of additional
constraints, including large source compact fractions. J1819+3845 was found to
display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11
day variations, suggesting that the highly turbulent cloud responsible for its
extreme scintillation has moved away, with its scintillation now caused by a
more distant screen ~ 50 to 150 pc away.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Hierarchical Star Formation: Stars and Stellar Clusters in the Gould Belt
We perform a study of the spatial and kinematical distribution of young open
clusters in the solar neighborhood, discerning between bound clusters and
transient stellar condensations within our sample. Then, we discriminate
between Gould Belt (GB) and local Galactic disk (LGD) members, using a previous
estimate of the structural parameters of both systems obtained from a sample of
O-B6 Hipparcos stars. Using this classified sample we analyze the spatial
structure and the kinematic behavior of the cluster system in the GB. The two
star formation regions that dominate and give the GB its characteristic
inclined shape show a striking difference in their content of star clusters:
while Ori OB1 is richly populated by open clusters, not a single one can be
found within the boundaries of Sco OB2. This is mirrored in the velocity space,
translating again into an abundance of clusters in the region of the kinematic
space populated by the members of Ori OB1, and a marginal number of them
associated to Sco OB2. In the light of these results we study the nature of the
GB with respect to the optical segment of the Orion Arm, and we propose that
the different content of star clusters, the different heights over the Galactic
plane and the different residual velocities of Ori OB1 and Sco OB2 can be
explained in terms of their relative position to the density maximum of the
Local Arm in the solar neighborhood. Although morphologically intriguing, the
GB appears to be the result of our local and biased view of a larger star
cluster complex in the Local Arm, that could be explained by the internal
dynamics of the Galactic disk.Comment: 23 pages, including 12 figures. Accepted for publication in MNRA
A Robotic Wide-Angle H-Alpha Survey of the Southern Sky
We have completed a robotic wide-angle imaging survey of the southern sky
(declination less than +15 degrees) at 656.3 nm wavelength, the H-alpha
emission line of hydrogen. Each image of the resulting Southern H-Alpha Sky
Survey Atlas (SHASSA) covers an area of the sky 13 degrees square at an angular
resolution of approximately 0.8 arcminute, and reaches a sensitivity level of 2
rayleigh (1.2 x 10^-17 erg cm^-2 s^-1 arcsec^-2) per pixel, corresponding to an
emission measure of 4 cm^-6 pc, and to a brightness temperature for microwave
free-free emission of 12 microkelvins at 30 GHz. Smoothing over several pixels
allows features as faint as 0.5 rayleigh to be detected.Comment: LATEX, 33 pages, 15 figures. Accepted for publication in PASP, 113,
November 2001. Further information at http://amundsen.swarthmore.edu/SHASSA
IR-correlated 31 GHz radio emission from Orion East
Lynds dark cloud LDN1622 represents one of the best examples of anomalous
dust emission, possibly originating from small spinning dust grains. We present
Cosmic Background Imager (CBI) 31 GHz data of LDN1621, a diffuse dark cloud to
the north of LDN1622 in a region known as Orion East. A broken ring with
diameter g\approx 20 arcmin of diffuse emission is detected at 31 GHz, at
\approx 20-30 mJy beam with an angular resolution of \approx 5 arcmin.
The ring-like structure is highly correlated with Far Infra-Red emission at
m with correlation coefficients of r \approx 0.7-0.8, significant
at . Multi-frequency data are used to place constraints on other
components of emission that could be contributing to the 31 GHz flux. An
analysis of the GB6 survey maps at 4.85 GHz yields a upper limit on
free-free emission of 7.2 mJy beam (\la 30 per cent of the observed
flux) at the CBI resolution. The bulk of the 31 GHz flux therefore appears to
be mostly due to dust radiation. Aperture photometry, at an angular resolution
of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of
IRAS maps and the {\it WMAP} 5-year W-band map at 93.5 GHz. A single modified
blackbody model was fitted to the data to estimate the contribution from
thermal dust, which amounts to \sim\sim100 \mu18.1\pm4.4 \mu^{-1}$, consistent with the values found for
LDN1622.Comment: 8 pages, 3 figures, 3 tables, submitted to MNRA
Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud
The Magellanic Mopra Assessment (MAGMA) is a high angular resolution CO
mapping survey of giant molecular clouds (GMCs) in the Large and Small
Magellanic Clouds using the Mopra Telescope. Here we report on the basic
physical properties of 125 GMCs in the LMC that have been surveyed to date. The
observed clouds exhibit scaling relations that are similar to those determined
for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO
luminosities than Galactic clouds of a similar size. The average mass surface
density of the LMC clouds is 50 Msol/pc2, approximately half that of GMCs in
the inner Milky Way. We compare the properties of GMCs with and without signs
of massive star formation, finding that non-star-forming GMCs have lower peak
CO brightness than star-forming GMCs. We compare the properties of GMCs with
estimates for local interstellar conditions: specifically, we investigate the
HI column density, radiation field, stellar mass surface density and the
external pressure. Very few cloud properties demonstrate a clear dependence on
the environment; the exceptions are significant positive correlations between
i) the HI column density and the GMC velocity dispersion, ii) the stellar mass
surface density and the average peak CO brightness, and iii) the stellar mass
surface density and the CO surface brightness. The molecular mass surface
density of GMCs without signs of massive star formation shows no dependence on
the local radiation field, which is inconsistent with the
photoionization-regulated star formation theory proposed by McKee (1989). We
find some evidence that the mass surface density of the MAGMA clouds increases
with the interstellar pressure, as proposed by Elmegreen (1989), but the
detailed predictions of this model are not fulfilled once estimates for the
local radiation field, metallicity and GMC envelope mass are taken into
account.Comment: 28 pages, 10 figures, accepted by MNRA
Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
Using precise full-sky observations from Planck, and applying several methods
of component separation, we identify and characterize the emission from the
Galactic "haze" at microwave wavelengths. The haze is a distinct component of
diffuse Galactic emission, roughly centered on the Galactic centre, and extends
to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining
the Planck data with observations from the WMAP we are able to determine the
spectrum of this emission to high accuracy, unhindered by the large systematic
biases present in previous analyses. The derived spectrum is consistent with
power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding
free-free emission as the source and instead favouring hard-spectrum
synchrotron radiation from an electron population with a spectrum (number
density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the
microwave haze morphology is consistent with that of the Fermi gamma-ray "haze"
or "bubbles," indicating that we have a multi-wavelength view of a distinct
component of our Galaxy. Given both the very hard spectrum and the extended
nature of the emission, it is highly unlikely that the haze electrons result
from supernova shocks in the Galactic disk. Instead, a new mechanism for
cosmic-ray acceleration in the centre of our Galaxy is implied.Comment: 15 pages, 9 figures, submitted to Astronomy and Astrophysic
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