159 research outputs found
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
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
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
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
Supernova Remnants and Star Formation in the Large Magellanic Cloud
It has often been suggested that supernova remnants (SNRs) can trigger star
formation. To investigate the relationship between SNRs and star formation, we
have examined the known sample of 45 SNRs in the Large Magellanic Cloud to
search for associated young stellar objects (YSOs) and molecular clouds. We
find seven SNRs associated with both YSOs and molecular clouds, three SNRs
associated with YSOs but not molecular clouds, and eight SNRs near molecular
clouds but not associated with YSOs. Among the 10 SNRs associated with YSOs,
the association between the YSOs and SNRs can be either rejected or cannot be
convincingly established for eight cases. Only two SNRs have YSOs closely
aligned along their rims; however, the time elapsed since the SNR began to
interact with the YSOs' natal clouds is much shorter than the contraction
timescales of the YSOs, and thus we do not see any evidence of SNR-triggered
star formation in the LMC. The 15 SNRs that are near molecular clouds may
trigger star formation in the future when the SNR shocks have slowed down to
<45 km/s. We discuss how SNRs can alter the physical properties and abundances
of YSOs.Comment: 24 pages, 5 figures, 1 table, Accepted for publication in the August
2010 edition of the Astronomical Journa
Supersonic water masers in 30 Doradus
We report on extremely high velocity molecular gas, up to -80 km/s relative
to the ambient medium, in the giant star-formation complex 30 Doradus in the
Large Magellanic Cloud (LMC), as observed in new 22 GHz H2O maser emission
spectra obtained with the Mopra radio telescope. The masers may trace the
velocities of protostars, and the observed morphology and kinematics indicate
that current star formation occurs near the interfaces of colliding
stellar-wind blown bubbles. The large space velocities of the protostars and
associated gas could result in efficient mixing of the LMC. A similar mechanism
in the Milky Way could seed the galactic halo with relatively young stars and
gas.Comment: 11 pages plus 1 PS and 1 EPS figure, uses AASTeX preprint style;
accepted for publication in Astrophysical Journal Letter
Diversité des familles de radiolaires au cours du temps
Lâexamen de la biodiversitĂ© des radiolaires, au niveau de la famille au cours du PhanĂ©rozoĂŻque rĂ©vĂšle quelques
tendances gĂ©nĂ©rales connues chez dâautres groupes dâorganismes, surtout dans le plancton, alors que dâautres tendances
leur sont particuliĂšres. La crise permo-triasique, lâune des plus importantes dans lâĂ©volution des organismes
marins, est marquĂ©e chez les radiolaires par lâextinction de deux familles (Albaillellaria et Latentifistularia) vers la fin
du Permien, mais surtout par une énorme diversification des spumellaires et nassellaires au Trias inférieur et moyen. La
diversitĂ© des radiolaires sâaccroĂźt du Cambrien au Jurassique, reste relativement stable au CrĂ©tacĂ© et dĂ©croĂźt lĂ©gĂšremen
Textural variations in Neogene pelagic carbonate ooze at DSDP Site 593, southern Tasman Sea, and their paleoceanographic implications
Changes in Neogene sediment texture in pelagic carbonate-rich oozes on the Challenger Plateau, southern Tasman Sea, are used to infer changes in depositional paleocurrent velocities. The most obvious record of textural change is in the mud:sand ratio. Increases in the sand content are inferred to indicate a general up-core trend towards increasing winnowing of sediments resulting from increasing flow velocity of Southern Component Intermediate Water (SCIW), the forerunner of Antarctic Intermediate Water. In particular, the intervals c. 19-14.5 Ma, c. 9.5-8 Ma, and after 5 Ma are suggested to be times of increased SCIW velocity and strong sediment winnowing. Within the mud fraction, the fine silt to coarse clay sizes from 15.6 to 2 ”m make the greatest contribution to the sediments and are composed of nannofossil plates. During extreme winnowing events it is the fine silt to very coarse clay material (13-3 ”m) within this range that is preferentially removed, suggesting the 10 ”m cohesive silt boundary reported for siliciclastic sediments does not apply to calcitic skeletal grains. The winnowed sediment comprises coccolithophore placoliths and spheres, represented by a mode at 4-7 ”m.
Further support for seafloor winnowing is gained from the presence in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma where the sand content increases to c. 20% of the bulk sample. Associated with the condensed section is a 6 m thick orange unit representing sediments subjected to particularly oxygen-rich, late early to early middle Miocene SCIW. Together these are inferred to indicate increased SCIW velocity resulting in winnowed sediment associated with faster arrival of oxygen-rich surface water subducted to form SCIW. Glacial development of Antarctica has been recorded from many deep-sea sites, with extreme glacials providing the mechanism to increase watermass flow. Miocene glacial zones Mi1b-Mi6 are identified in an associated oxygen isotope record from Hole 593, and correspond with times of particularly invigorated paleocirculation, bottom winnowing, and sediment textural changes
The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005
The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting
more than 1 year and culminating in spring 2005. The maximum brightness
detected was R = 12.0, which represents the most luminous quasar state thus far
observed (M_B ~ -31.4). In order to follow the emission behaviour of the source
in detail, a large multiwavelength campaign was organized by the Whole Earth
Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was
performed in several bands. ToO pointings by the Chandra and INTEGRAL
satellites provided additional information at high energies in May 2005. The
historical radio and optical light curves show different behaviours. Until
about 2001.0 only moderate variability was present in the optical regime, while
prominent and long-lasting radio outbursts were visible at the various radio
frequencies, with higher-frequency variations preceding the lower-frequency
ones. After that date, the optical activity increased and the radio flux is
less variable. This suggests that the optical and radio emissions come from two
separate and misaligned jet regions, with the inner optical one acquiring a
smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index
behaviour (generally redder-when-brighter) during the outburst suggests the
presence of a luminous accretion disc. A huge mm outburst followed the optical
one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux
started to increase in early 2005 and reached a maximum at the end of our
observing period (end of September 2005). VLBA observations at 43 GHz during
the summer confirm theComment: 7 pages, 4 figures, to be published in A&
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