2,544 research outputs found
Microlensing Surveys of M31 in the Wide Field Imaging Era
The Andromeda Galaxy (M31) is the closest large galaxy to the Milky Way, thus
it is an important laboratory for studying massive dark objects in galactic
halos (MACHOs) by gravitational microlensing. Such studies strongly complement
the studies of the Milky Way halo using the the Large and Small Magellanic
Clouds. We consider the possibilities for microlensing surveys of M31 using the
next generation of wide field imaging telescopes with fields of view in the
square degree range. We consider proposals for such imagers both on the ground
and in space. For concreteness, we specialize to the SNAP proposal for a space
telescope and the LSST proposal for a ground based telescope. We find that a
modest space-based survey of 50 visits of one hour each is considerably better
than current ground based surveys covering 5 years. Crucially, systematic
effects can be considerably better controlled with a space telescope because of
both the infrared sensitivity and the angular resolution. To be competitive, 8
meter class wide-field ground based imagers must take exposures of several
hundred seconds with several day cadence.Comment: 10 pages, 4 figures, 2 table
The Unusual Variability of the Large Magellanic Cloud Planetary Nebula RPJ 053059-683542
We present images and light curves of the bipolar Planetary Nebula RPJ
053059-683542 that was discovered in the Reid-Parker AAO/UKST H-alpha survey of
the Large Magellanic Cloud (LMC). The emission from this object appears
entirely nebular, with the central star apparently obscured by a central band
of absorption that bisects the nebula. The light curves, which were derived
from images from the SuperMACHO project at CTIO, showed significant, spatially
resolved variability over the period 2002 January through 2005 December.
Remarkably, the emission from the two bright lobes of the nebula vary either
independently, or similarly but with a phase lag of at least one year. The
optical spectra show a low level of nebular excitation, and only modest N
enrichment. Infrared photometry from the 2MASS and SAGE surveys indicates the
presence of a significant quantity of dust. The available data imply that the
central star has a close binary companion, and that the system has undergone
some kind of outburst event that caused the nebular emission to first brighten
and then fade. Further monitoring, high-resolution imaging, and detailed IR
polarimetry and spectroscopy would uncover the nature of this nebula and the
unseen ionizing source.Comment: Accepted for ApJ Letters; 6 page
Public Participation in Contaminated Communities
The present study examines seven current, ongoing cases of public participation across a broader spectrum of communities. In contrast to earlier notorious historical failures, such as those at Love Canal, Woburn, and Times Beach, the cases in this study explore experiences considered relatively successful by both the agencies and the communities. The study sought to better understand the determinants of successful public involvement in contaminated communities where: (1) site characterization, cleanup options, and economic redevelopment were issues of concern and conflict; (2) more than one federal agency was involved; (3) state and local agencies were also
involved; and (4) environmental justice was often an issue. The purposes of the study were to: (1) identify those factors most important to, and essential for, successful community involvement, (2) evaluate or suggest initiatives to further enhance successful public participation, and (3) identify options for more successful interaction and coordination of federal, state, and local agencies in their efforts to promote environmental and public health goals in contaminated communities.The study focused on initiatives which: enhance communication, outreach, and learning in the community; build skills and capability in the community; and provide for increased community participation in, and access to, government decisions. Special attention was paid to public participation problems in economically disadvantaged and minority communities with disproportionate environmental burdens (i.e., environmental justice communities), and to mechanisms for improving interagency coordination at all levels of government
Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption
Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky
Way. It became the second-brightest star in the sky during its mid-19th century
"Great Eruption," but then faded from view (with only naked-eye estimates of
brightness). Its eruption is unique among known astronomical transients in that
it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3
kpc away, spatially resolved studies of the nebula have constrained the ejected
mass and velocity, indicating that in its 19th century eruption, Eta Car
ejected more than 10 M_solar in an event that had 10% of the energy of a
typical core-collapse supernova without destroying the star. Here we report the
discovery of light echoes of Eta Carinae which appear to be from the 1838-1858
Great Eruption. Spectra of these light echoes show only absorption lines, which
are blueshifted by -210 km/s, in good agreement with predicted expansion
speeds. The light-echo spectra correlate best with those of G2-G5 supergiant
spectra, which have effective temperatures of ~5000 K. In contrast to the class
of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption,
the effective temperature of its outburst is significantly cooler than allowed
by standard opaque wind models. This indicates that other physical mechanisms
like an energetic blast wave may have triggered and influenced the eruption.Comment: Accepted for publication by Nature; 4 pages, 4 figures, SI: 6 pages,
3 figures, 5 table
A Method for Extracting Light Echo Fluxes Using the NN2 Difference Imaging Technique
Light echoes are interesting because of the wealth of information they offer
about their progenitors and the reflecting dust. Due to their faint surface
brightnesses, difference imaging is necessary to separate most light echoes
from the sky background. However, difference images reveal only the relative
fluxes between two epochs. Obtaining absolute fluxes for individual epochs has
traditionally relied on a single template image that is free of light echoes.
Since such an observation is normally unavailable, a light echo-free template
must be constructed by a complicated and usually subjective process. Here we
present an application of the NN2 method of Barris et al. to extract the
relative fluxes of light echoes across a range of epochs directly from a series
of difference images. This method requires no privileged image and makes
maximal use of the observational data. Statistical methods to estimate the
zero-flux level and thus the absolute flux are also presented. The efficacy of
the technique is demonstrated by an application to the light echoes around SN
1987A. The resulting images reveal new detail and faint light echo structures.
This method can be adapted and applied to other extended variable light
sources, such as stellar outflows and supernova remnants.Comment: To be published in PASP 118 (Oct. 2006
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