26 research outputs found
The Brightening of Re50N: Accretion Event or Dust Clearing?
The luminous Class I protostar HBC 494, embedded in the Orion A cloud, is
associated with a pair of reflection nebulae, Re50 and Re50N, which appeared
sometime between 1955 and 1979. We have found that a dramatic brightening of
Re50N has taken place sometime between 2006 and 2014. This could result if the
embedded source is undergoing a FUor eruption. However, the near-infrared
spectrum shows a featureless very red continuum, in contrast to the strong CO
bandhead absorption displayed by FUors. Such heavy veiling, and the high
luminosity of the protostar, is indicative of strong accretion but seemingly
not in the manner of typical FUors. We favor the alternative explanation that
the major brightening of Re50N and the simultaneous fading of Re50 is caused by
curtains of obscuring material that cast patterns of illumination and shadows
across the surface of the molecular cloud. This is likely occurring as an
outflow cavity surrounding the embedded protostar breaks through to the surface
of the molecular cloud. Several Herbig-Haro objects are found in the region.Comment: 8 pages, accepted by Ap
Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?
A leading formation scenario for R Coronae Borealis (RCB) stars invokes the
merger of degenerate He and CO white dwarfs (WD) in a binary. The observed
ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the
solar value of ~500. In this paper, we investigate whether such a low ratio can
be obtained in simulations of the merger of a CO and a He white dwarf. We
present the results of five 3-dimensional hydrodynamic simulations of the
merger of a double white dwarf system where the total mass is 0.9 Mdot and the
initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations
with a feature around the merged stars where the temperatures
and densities are suitable for forming 18O. However, more 16O is being
dredged-up from the C- and O-rich accretor during the merger than the amount of
18O that is produced. Therefore, on a dynamical time scale over which our
hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is
found. If the conditions found in the hydrodynamic simulations persist for 10^6
seconds the oxygen ratio drops to 16 in one case studied, while in a hundred
years it drops to ~4 in another case studied, consistent with the observed
values in RCB stars. Therefore, the merger of two white dwarfs remains a strong
candidate for the formation of these enigmatic stars.Comment: 42 pages, 19 figures. Accepted for publication in the Astrophysical
Journa
Solar System Objects Observed in the SDSS Commissioning Data
We discuss measurements of the properties of about 10,000 asteroids detected
in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data.
The moving objects are detected in the magnitude range 14 < r < 21.5, with a
baseline of 5 minutes. Extensive tests show that the sample is at least 98%
complete, with the contamination rate of less than 3%.
We find that the size distribution of asteroids resembles a broken power-law,
independent of the heliocentric distance: D^{-2.3} for 0.4 km < D < 5 km, and
D^{-4} for 5 km < D < 40 km. As a consequence of this break, the number of
asteroids with r < 21.5 is ten times smaller than predicted by extrapolating
the power-law relation observed for brighter asteroids (r < 18). The observed
counts imply that there are about 530,000 objects with D>1 km in the asteroid
belt, or about four times less than previous estimates.
The distribution of main belt asteroids in the 4-dimensional SDSS color space
is bimodal, and the two groups can be associated with S (rocky) and C
(carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric
distribution of asteroids and suggests the existence of two distinct belts: the
inner rocky belt, about 1 AU wide (FWHM) and centered at R~2.8 AU, and the
outer carbonaceous belt, about 0.5 AU wide and centered at R~3.2 AU. The colors
of Hungarias, Mars crossers, and near-Earth objects are more similar to the
C-type than to S-type asteroids, suggesting that they originate in the outer
belt. (abridged).Comment: 89 pages, 31 figures, submitted to A
A Survey of z>5.8 Quasars in the Sloan Digital Sky Survey I: Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z~6
We present the results from a survey of i-dropout objects selected from ~1550
deg^2 of multicolor imaging data from the Sloan Digital Sky Survey, to search
for luminous quasars at z>5.8. Objects with i*-z*>2.2 and z*<20.2 are selected,
and follow-up J band photometry is used to separate L and T type cool dwarfs
from high-redshift quasars. We describe the discovery of three new quasars, at
z=5.82, 5.99 and 6.28, respectively. Their spectra show strong and broad Ly
alpha+NV emission lines, and very strong Ly alpha absorption, with a mean
continuum decrement D_A > 0.90. The ARC 3.5m spectrum of the z=6.28 quasar
shows that over a range of 300 A immediately blueward of the Ly alpha emission,
the average transmitted flux is only 0.003 +/-0.020 times that of the continuum
level, consistent with zero flux, and suggesting a tentative detection of the
complete Gunn-Peterson trough. The existence of strong metal lines suggests
early chemical enrichment in the quasar enviornment. The three new objects,
together with the previously published z=5.8 quasar form a complete
color-selected flux-limited sample at z>5.8. We estimate that at , the
comoving density of luminous quasars at M_1450 < -26.89 (h=0.5, Omega=1)is
1.1x10^-9 Mpc^-3. This is a factor of ~2 lower than that at z~5, and is
consistent with an extrapolation of the observed quasar evolution at low-z. We
discuss the contribution of quasars to the ionizing background at z~6. The
luminous quasars discussed in the paper have central black hole masses of
several times 10^9 M_sun by the Eddington argument. Their observed space
density provides a sensitive test of models of quasar and galaxy formation at
high redshift. (Abridged)Comment: AJ in press (Dec 2001), 40 pages, 10 figures. Updated following
referee report; minor change
Virology under the microscope—a call for rational discourse
Viruses have brought humanity many challenges: respiratory infection, cancer, neurological impairment and immunosuppression to name a few. Virology research over the last 60+ years has responded to reduce this disease burden with vaccines and antivirals. Despite this long history, the COVID-19 pandemic has brought unprecedented attention to the field of virology. Some of this attention is focused on concern about the safe conduct of research with human pathogens. A small but vocal group of individuals has seized upon these concerns – conflating legitimate questions about safely conducting virus-related research with uncertainties over the origins of SARS-CoV-2. The result has fueled public confusion and, in many instances, ill-informed condemnation of virology. With this article, we seek to promote a return to rational discourse. We explain the use of gain-of-function approaches in science, discuss the possible origins of SARS-CoV-2 and outline current regulatory structures that provide oversight for virological research in the United States. By offering our expertise, we – a broad group of working virologists – seek to aid policy makers in navigating these controversial issues. Balanced, evidence-based discourse is essential to addressing public concern while maintaining and expanding much-needed research in virology
LCROSS (Lunar Crater Observation and Sensing Satellite) Observation Campaign: Strategies, Implementation, and Lessons Learned
Thirty years of H-3(+) astronomy
This review covers the work of the three decades since the first spectroscopic identification of the H3+ molecular ion outside of the laboratory in 1988, in the auroral atmosphere of the giant planet Jupiter. These decades have seen the astronomy related to this simple molecular ion expand to such an extent that a summary and evaluation of some 450 refereed articles is provided in the review. This enormous body of work has revealed surprises and illuminated the extensive role played by H3+ in astrophysical environments in our Solar System and beyond. At the same time the physical chemistry and chemical physics of the molecule that has been revealed and studied during this time has proved to be fascinating and enabled high-resolution spectroscopy to benchmark its achievements against equally high-precision calculations. This review includes a brief look at some of the key foundational articles from before the original 1988 Jupiter detection (including the original 1911 ion discharge tube detection by J. J. Thomson and the key laboratory spectroscopy and quantum mechanics calculations on H3+ structure and spectrum). The review explains the original detection and its serendipitous nature and looks at the astronomy that followed, all the way up to the latest results from NASA's Juno mission. Also covered are the major advances in our understanding of the interstellar medium (known as ISM) that have resulted from the detection of H3+ absorption lines there in 1996. The review closes by examining claims for the ion's presence in other astrophysical environments and its potential role in the atmospheres of exoplanets and brown dwarfs