169 research outputs found
Structure of Supergiant Shells in the Large Magellanic Cloud
Nine supergiant shells (SGSs) have been identified in the Large Magellanic
Cloud (LMC) based on H-alpha images, and twenty-three SGSs have been reported
based on HI 21-cm line observations, but these sets do not always identify the
same structures. We have examined the physical structure of the optically
identified SGSs using HI channel maps and P-V diagrams to analyze the gas
kinematics. There is good evidence for seven of the nine optically identified
SGSs to be true shells. Of these seven H-alpha SGSs, four are the ionized inner
walls of HI SGSs, while three are an ionized portion of a larger and more
complex HI structure. All of the H-alpha SGSs are identified as such because
they have OB associations along the periphery or in the center, with younger OB
associations more often found along the periphery. After roughly 12 Myrs, if no
new OB associations have been formed a SGS will cease to be identifiable at
visible wavelengths. Thus, the presence and location of ionizing sources is the
main distinction between shells seen only in HI and those also seen in H-alpha.
Based on our analysis, H-alpha observations alone cannot unambiguously identify
SGSs, especially in distant galaxies.Comment: 26 pages, 16 figures, accepted for publication in the Astrophysical
Journal Supplemen
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Pliocene-Pleistocene radiolarian events and magnetostratigraphic calibrations for the tropical Indian Ocean
A composite of four piston cores from the Central Indian Basin and adjacent Ninetyeast Ridge has yielded a continuous magnetobiostratigraphic reference section for most of the Pliocene and the Pleistocene (0.0â4.5 Ma). We identified thirty-three radiolarian events (first- or last-occurrences), and precisely correlated each event to the Neogene geomagnetic polarity time scale. Thirteen of these events are based on revised taxonomic studies of the genera Anthocyrtidium and Pterocorys. Some events show significant departures from synchroneity: five of the radiolarian first-appearances and seven of the last-appearances are time-transgressive by 0.4 m.y. or greater. We here propose a revised, eleven-fold radiolarian zonation for the Pliocene-Pleistocene, using zonal boundaries defined by events which are easily recognized and are demonstrably synchronous in the tropical Indo-Pacific. The sequence of faunal and floral events reported in this paper will allow high-resolution biostratigraphic correlations within the tropical Indian Ocean; however, the same sequence of events is not necessarily applicable to other tropical or extratropical regions
The First Orbital Period for a Dwarf Nova in a Globular Cluster: V101 in M5
We report the first orbital period determination for a Dwarf Nova (DN) in a
glubular cluster: V101 in M5 has a period of 5.796 +- 0.036 hours. We derived
this period from I-band photometry acquired with the Calypso Observatory High
Resolution Camera operating with tip-tilt adaptive optics correction.
Observations from the South African Astronomical Observatory in the V-band were
also analyzed and exhibit a periodic signal of the same period. This orbital
period suggests that V101 has a secondary of mid to late K spectral type with
Mv = +8.2 +- 0.5. The predicted spectral type is consistent with previous
spectral observations in quiescence which show a fairly red continuum. From the
observed minimum brightness of V = 22.5, we derive a distance modulus of (m -
M)v = 14.3 +- 0.5 to the DN which supports V101's membership in the globular
cluster M5. Measurement of the ellipsoidality effect indicates that the orbital
plane of the V101 system is moderately inclined, but not enough to exhibit
eclipses.Comment: 28 pages, 12 figures, 3 table
Correlation between the Extraordinary Hall Effect and Resistivity
We study the contribution of different types of scattering sources to the
extraordinary Hall effect. Scattering by magnetic nano-particles embedded in
normal-metal matrix, insulating impurities in magnetic matrix, surface
scattering and temperature dependent scattering are experimentally tested. Our
new data, as well as previously published results on a variety of materials,
are fairly interpreted by a simple modification of the skew scattering model
An X-ray absorption analysis of the high-velocity system in NGC 1275
We present an X-ray absorption analysis of the high-velocity system (HVS) in
NGC 1275 using results from a deep 200 ks Chandra observation. We are able to
describe the morphology of the HVS in more detail than ever before. We present
an HST image for comparison, and note close correspondence between the deepest
X-ray absorption and the optical absorption. A column density map of the HVS
shows an average column density NH of 1x10^21 cm^-2 with a range from ~5x10^20
to 5x10^21 cm^-2. From the NH map we calculate a total mass for the absorbing
gas in the HVS of (1.32+-0.05)x10^9 solar masses at solar abundance. 75 per
cent of the absorbing mass is contained in the four regions of deepest
absorption. We examine temperature maps produced by spectral fitting and find
no direct evidence for shocked gas in the HVS. Using deprojection methods and
the depth of the observed absorption, we are able to put a lower limit on the
distance of the HVS from the nucleus of 57 kpc, showing that the HVS is quite
separate from the body of NGC 1275.Comment: 6 pages, 5 colour figures, accepted by MNRA
Spitzer 24 um Images of Planetary Nebulae
Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae
(PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective
temperatures of ~100,000 K or higher. Diffuse 24 um emission is detected in 28
of these PNe. The eight non-detections are angularly large PNe with very low
H-alpha surface brightnesses. We find three types of correspondence between the
24 um emission and H-alpha line emission of these PNe: six show 24 um emission
more extended than H-alpha emission, nine have a similar extent at 24 um and
H-alpha, and 13 show diffuse 24 um emission near the center of the H-alpha
shell. The sizes and surface brightnesses of these three groups of PNe and the
non-detections suggest an evolutionary sequence, with the youngest ones being
brightest and the most evolved ones undetected. The 24 um band emission from
these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emission and
dust continuum emission, but the relative contributions of these three
components depend on the temperature of the central star and the distribution
of gas and dust in the nebula.Comment: 24 pages, 8 figures, to appear in the Astronomical Journal, September
issue. Relace previous file; two references are added and typos are correcte
Discovery of an M8.5 Dwarf With Proper Motion mu=2.38 arcsec/yr
We report the discovery of LSR1826+3014, a very faint (V=19.36) star with a
very large proper motion (mu=2.38 arcsec/yr). A low resolution red spectrum
reveals that LSR1826+3014 is an ultra-cool red dwarf with spectral type M8.5 V
and with a radial velocity v_rad=+77+/-10 km/s. LSR1826+3014 is thus the
faintest red dwarf ever discovered with a proper motion larger than 2
arcsec/yr. Optical and infrared photometry suggest that the star is at a
distance d=13.9+/-3.5 pc from the Sun, which implies it is moving relative to
the local standard of rest with a total velocity of 175+/-25 km/s. Numerical
integration of its orbit suggests that LSR1826+3014 is on a halo-like galactic
orbit.Comment: 12 pages, including 1 table and 3 figures, accepted for publication
in The Astrophysical Journal Letter
Deep optical observations of the interaction of the SS 433 microquasar jet with the W 50 radio continuum shell
Four mosaics of deep, continuum-subtracted, CCD images have been obtained
over the extensive galactic radio continuum shell, W 50, which surrounds the
remarkable stellar system SS 433. Two of these mosaics in the Halpha+[N II] and
[O III] 5007 A emission lines respectively cover a field of ~2.3 x 2.5 degr^2
which contains all of W 50 but at a low angular resolution of 5 arcsec. The
third and fourth mosaics cover the eastern (in [O III] 5007 A) and western (in
Halpha+[N II]) filamentary nebulosity respectively but at an angular resolution
of 1 arcsec. These observations are supplemented by new low dispersion spectra
and longslit, spatially resolved echelle spectra. The [O III] 5007 A images
show for the first time the distribution of this emission in both the eastern
and western filaments while new Halpha+[N II] emission features are also found
in both of these regions. Approaching flows of faintly emitting material from
the bright eastern filaments of up 100 km/s in radial velocity are detected.
The present observations also suggest that the heliocentric systemic radial
velocity of the whole system is 56+-2 km/s. Furthermore, very deep imagery and
high resolution spectroscopy of a small part of the northern radio ridge of W
50 has revealed for the first time the very faint optical nebulosity associated
with this edge. It is suggested that patchy foreground dust along the ~5 kpc
sightline is inhibiting the detection of all of the optical nebulosity
associated with W 50. The interaction of the microquasar jets of SS 433 with
the W 50 shell is discussed.Comment: 19 pages, 13 figures, 2 tables. Accepted for pubication in MNRA
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