5,945 research outputs found
Ancient eruptions of Eta Carinae: A tale written in proper motions
We analyze eight epochs of Hubble Space Telescope H+[N II] imaging of
Eta Carinae's outer ejecta. Proper motions of nearly 800 knots reveal that the
detected ejecta are divided into three apparent age groups, dating to around
1250 A.D., to around 1550 A.D., and to during or shortly before the Great
Eruption of the 1840s. Ejecta from these groups reside in different locations
and provide a firm constraint that Eta Car experienced multiple major eruptions
prior to the 19th century. The 1250 and 1550 events did not share the same
axisymmetry as the Homunculus; the 1250 event was particularly asymmetric, even
one-sided. In addition, the ejecta in the S ridge, which have been associated
with the Great Eruption, appear to predate the ejection of the Homunculus by
several decades. We detect essentially ballistic expansion across multiple
epochs. We find no evidence for large-scale deceleration of the observed knots
that could power the soft X-ray shell by plowing into surrounding material,
suggesting that the observed X-rays arise instead from fast, rarefied ejecta
from the 1840s overtaking the older dense knots. Early deceleration and
subsequent coasting cannot explain the origin of the older outer
ejecta---significant episodic mass loss prior to the 19th century is required.
The timescale and geometry of the past eruptions provide important constraints
for any theoretical physical mechanisms driving Eta Car's behavior.
Non-repeating mechanisms such as the merger of a close binary in a triple
system would require additional complexities to explain the observations.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Proper motions of collimated jets from intermediate-mass protostars in the Carina Nebula
We present proper motion measurements of 37 jets and HH objects in the Carina
Nebula measured in two epochs of H images obtained yrs apart
with HST/ACS. Transverse velocities in all but one jet are faster than km s, confirming that the jet-like H features identified by
Smith et al. (2010) trace outflowing gas. Proper motions constrain the location
of the jet-driving source and provide kinematic confirmation of the
intermediate-mass protostars that we identify for 20/37 jets. Jet velocities do
not correlate with the estimated protostar mass and embedded driving sources do
not have slower jets. Instead, transverse velocities (median km
s) are similar to those in jets from low-mass stars. Assuming a constant
velocity since launch, we compute jet dynamical ages (median yr).
If continuous emission from inner jets traces the duration of the most recent
accretion bursts, then these episodes are sustained longer (median
yr) than the typical decay time of an FU Orionis outburst. These jets can carry
appreciable momentum that may be injected into the surrounding environment. The
resulting outflow force, , lies between that measured in low- and
high-mass sources, despite the very different observational tracers used.
Smooth scaling of the outflow force argues for a common physical process
underlying outflows from protostars of all masses. This latest kinematic result
adds to a growing body of evidence that intermediate-mass star formation
proceeds like a scaled-up version of the formation of low-mass stars.Comment: accepted to MNRAS, 29 pages, 30 figures, 3 table
Proper motions of five OB stars with candidate dusty bow shocks in the Carina Nebula
We constrain the proper motions of five OB stars associated with candidate
stellar wind bow shocks in the Carina Nebula using HST ACS imaging over 9--10
year baselines. These proper motions allow us to directly compare each star's
motion to the orientation of its candidate bow shock. Although these stars are
saturated in our imaging, we assess their motion by the shifts required to
minimize residuals in their Airy rings. The results limit the direction of each
star's motion to sectors less than 90 degrees wide. None of the five stars are
moving away from the Carina Nebula's central clusters as runaway stars would
be, confirming that a candidate bow shock is not necessarily indicative of a
runaway star. Two of the five stars are moving tangentially relative to the
orientation of their candidate bow shocks, both of which point at the OB
cluster Trumpler 14. In these cases, the large-scale flow of the interstellar
medium, powered by feedback from the cluster, appears to dominate over the
motion of the star in producing the observed candidate bow shock. The remaining
three stars all have some component of motion toward the central clusters,
meaning that we cannot distinguish whether their candidate bow shocks are
indicators of stellar motion, of the flow of ambient gas, or of density
gradients in their surroundings. In addition, these stars' lack of outward
motion hints that the distributed massive-star population in Carina's South
Pillars region formed in place, rather than migrating out from the
association's central clusters.Comment: 13 pages, 5 figures, accepted for publication in MNRA
Resolving the H-alpha-emitting Region in the Wind of Eta Carinae
The massive evolved star Eta Carinae is the most luminous star in the Milky
Way and has the highest steady wind mass-loss rate of any known star. Radiative
transfer models of the spectrum by Hillier et al. predict that H-alpha is
mostly emitted in regions of the wind at radii of 6 to 60 AU from the star (2.5
to 25 mas at 2.35 kpc). We present diffraction-limited images (FWHM ~25 mas)
with Magellan adaptive optics in two epochs, showing that Eta Carinae
consistently appears ~2.5 to 3 mas wider in H-alpha emission compared to the
adjacent 643 nm continuum. This implies that the H-alpha line-forming region
may have a characteristic emitting radius of 12 mas or ~30 AU, in very good
agreement with the Hillier stellar-wind model. This provides direct
confirmation that the physical wind parameters of that model are roughly
correct, including the mass-loss rate of 10^-3 M_sun/yr, plus the clumping
factor, and the terminal velocity. Comparison of the H-alpha images
(ellipticity and PA) to the continuum images reveals no significant asymmetries
at H-alpha. Hence, any asymmetry induced by a companion or by the primary's
rotation do not strongly influence the global H-alpha emission in the outer
wind.Comment: Published in ApJ
Government Payments: Economic Impact on Southeastern Peanut Farms
Southeastern peanut farms with diversified field crops utilize government payments to supplement market receipts. Production in 2002 represented growing conditions under adverse weather, while 2003 represented optimal conditions. Representative farm analysis provides insight into allocation of market receipts and government payments for meeting variable costs and fixed costs.Crop Production/Industries,
Substituting Otoliths for Chemical Analyses: Does Sagitta = Lapillus?
Chemical analysis of fish otoliths has become an important technique in fisheries science with widespread applications. Most research up to this point has focused predominantly on sagittal otoliths, but the underlying assumptions may also apply to lapilli. The goal of this study was to determine whether lapilli and sagittae have the same otolith chemistry and whether one can be substituted for the other for solution-based chemical analysis in wild-captured fish. We compared the stable isotope chemistry (δ13C and δ18O) of paired sagittae and lapilli of juvenile spotted seatrout Cynoscion nebulosus collected from Chesapeake Bay seagrass beds in 2002. Stable isotopic signatures were significantly different in both carbon and oxygen for paired sagittae and lapilli. Both δ13C and δ18O values were higher in the sagitta relative to the lapillus. As a correlate to isotopic analysis, shape measurements were taken on paired sagittae and lapilli for growth comparison. While the 2 otoliths are a similar size at formation, the sagitta grows faster, quickly outgrowing the lapillus during the juvenile stage. We expected that after detrending isotope data to account for fish length, the relation with otolith growth would become clear. Surprisingly, this was not true for either δ13C or δ18O. Carbon isotopic differences appear to be strongly influenced by metabolism, and the relationship of δ18O with otolith growth is obscured by a variable environment. Our results clearly show that sagittae is not equal to lapilli for solution-based whole otolith analysis. Moreover, while the sagitta reflects the juvenile stage environment with greater fidelity, the lapillus reflects the larval stage more strongly
Systematic Blueshift of Line Profiles in the Type IIn Supernova 2010jl: Evidence for Post-Shock Dust Formation?
Type IIn SNe show spectral evidence for strong interaction between their
blast wave and dense circumstellar material (CSM) around the progenitor star.
SN2010jl was the brightest core-collapse SN in 2010, and it was a Type IIn
explosion with strong CSM interaction. Andrews et al. recently reported
evidence for an IR excess in SN2010jl, indicating either new dust formation or
the heating of CSM dust in an IR echo. Here we report multi-epoch spectra of
SN2010jl that reveal the tell-tale signature of new dust formation:
emission-line profiles becoming systematically more blueshifted as the red side
of the line is blocked by increasing extinction. The effect is seen clearly in
the intermediate-width (400--4000 km/s) component of H beginning
roughly 30d after explosion. Moreover, we present near-IR spectra demonstrating
that the asymmetry in the hydrogen-line profiles is wavelength dependent,
appearing more pronounced at shorter wavelengths. This evidence suggests that
new dust grains had formed quickly in the post-shock shell of SN 2010jl arising
from CSM interaction. Since the observed dust temperature has been attributed
to an IR echo and not to new dust, either (1) IR excess emission at m is not a particularly sensitive tracer of new dust formation in SNe, or
(2) some assumptions about expected dust temperatures might require further
study. Lastly, we discuss one possible mechanism other than dust that might
lead to increasingly blueshifted line profiles in SNeIIn, although the
wavelength dependence of the asymmetry argues against this hypothesis in the
case of SN2010jl.Comment: 6 pages, 4 figures, submitted to A
Crop Rotations and Dynamic Analysis of Southeastern Peanut Farms
Agricultural policy objectives provide green payment incentives for farmers to initiate practices with environmental benefits. Velvet beans planted as a cover crop offer an alternative for southeastern peanut farmers to control nematodes without chemicals, while increasing soil fertility. Commodity programs provide government payments that are essential to rural economies of the southeast.Environmental Economics and Policy,
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