1,244 research outputs found
The timing of cod spawning on the Scotian Shelf
We used results from the Scotian Shelf Ichtyoplankton Programme
to describe the spatial and temporal distributions of
cod eggs and larvae off the coast of Nova Scotia. Summer spawning
is restricted to the waters along the Laurentian Channel.
Eggs and/or larvae are found in the spring in all areas where
spawning activities are detected. Late fall - early winter
spawning takes place over several banks and along the southern
coast of Nova Scotia. A match between the seasonal blooms and
the time of spawning does not appear to be essential, although
it might exist in the spring. Larval drift from spawning
grounds to nursery grounds is apparently rare on the Scotlan
Shelf. These findings are discussed with reference to the
match-mismatch hypothesis of Cushing
Chandra Spectroscopy Of The Hot Star β Crucis And The Discovery Of A Pre-Main-Sequence Companion
In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, beta Crucis (beta Cru; B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3 million K, and that the emission lines are resolved but quite narrow, with half widths of 150 km s(-1). The forbidden-to-intercombination line ratios of Ne IX and Mg XI indicate that the hot plasma is distributed in the wind, rather than confined near the photosphere. It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars, primarily because of the narrow lines, but also because of the high X-ray production efficiency. A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data, but not very well motivated theoretically. It is possible that magnetic channelling could explain the X-ray properties, although no field has been detected on beta Cru. We detected periodic variability in the hard (h nu \u3e 1 keV) X-rays, modulated on the known optical period of 4.58 h, which is the period of the primary beta Cephei pulsation mode for this star. We also have detected, for the first time, an apparent companion to beta Cru at a projected separation of 4 arcsec. This companion was likely never seen in optical images because of the presumed very high contrast between it and beta Cru in the optical. However, the brightness contrast in the X-ray is only 3:1, which is consistent with the companion being an X-ray active low-mass pre-main-sequence star. The companion\u27s X-ray spectrum is relatively hard and variable, as would be expected from a post-T Tauri star. The age of the beta Cru system (between 8 and 10 Myr) is consistent with this interpretation which, if correct, would add beta Cru to the roster of Lindroos binaries - B stars with low-mass pre-main-sequence companions
The timing of cod spawning on the Scotian Shelf
We used results from the Scotian Shelf Ichtyoplankton Programme
to describe the spatial and temporal distributions of
cod eggs and larvae off the coast of Nova Scotia. Summer spawning
is restricted to the waters along the Laurentian Channel.
Eggs and/or larvae are found in the spring in all areas where
spawning activities are detected. Late fall - early winter
spawning takes place over several banks and along the southern
coast of Nova Scotia. A match between the seasonal blooms and
the time of spawning does not appear to be essential, although
it might exist in the spring. Larval drift from spawning
grounds to nursery grounds is apparently rare on the Scotlan
Shelf. These findings are discussed with reference to the
match-mismatch hypothesis of Cushing
Chandra Spectroscopy Of The Hot Star β Crucis And The Discovery Of A Pre-Main-Sequence Companion
In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, beta Crucis (beta Cru; B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3 million K, and that the emission lines are resolved but quite narrow, with half widths of 150 km s(-1). The forbidden-to-intercombination line ratios of Ne IX and Mg XI indicate that the hot plasma is distributed in the wind, rather than confined near the photosphere. It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars, primarily because of the narrow lines, but also because of the high X-ray production efficiency. A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data, but not very well motivated theoretically. It is possible that magnetic channelling could explain the X-ray properties, although no field has been detected on beta Cru. We detected periodic variability in the hard (h nu \u3e 1 keV) X-rays, modulated on the known optical period of 4.58 h, which is the period of the primary beta Cephei pulsation mode for this star. We also have detected, for the first time, an apparent companion to beta Cru at a projected separation of 4 arcsec. This companion was likely never seen in optical images because of the presumed very high contrast between it and beta Cru in the optical. However, the brightness contrast in the X-ray is only 3:1, which is consistent with the companion being an X-ray active low-mass pre-main-sequence star. The companion\u27s X-ray spectrum is relatively hard and variable, as would be expected from a post-T Tauri star. The age of the beta Cru system (between 8 and 10 Myr) is consistent with this interpretation which, if correct, would add beta Cru to the roster of Lindroos binaries - B stars with low-mass pre-main-sequence companions
A nearby young M dwarf with a wide, possibly planetary-mass companion
We present the identification of two previously known young objects in the solar neighbourhood as a likely very wide binary. TYC 9486-927-1, an active, rapidly rotating early-M dwarf, and 2MASS J21265040-8140293, a low-gravity L3 dwarf previously identified as candidate members of the 45 Myr old Tucana Horologium association (TucHor). An updated proper motion measurement of the L3 secondary, and a detailed analysis of the pair's kinematics in the context of known nearby, young stars, reveals that they share common proper motion and are likely bound. New observations and analyses reveal the primary exhibits Li 6708~\AA~absorption consistent with M dwarfs younger than TucHor but older than the 10 Myr TW Hydra association yielding an age range of 10-45 Myr. A revised kinematic analysis suggests the space motions and positions of the pair are closer to, but not entirely in agreement with, the 24 Myr old Pictoris moving group. This revised 10-45 Myr age range yields a mass range of 11.6--15 M for the secondary. It is thus likely 2MASS J21265040-8140293short is the widest orbit planetary mass object known (4500AU) and its estimated mass, age, spectral type, and are similar to the well-studied planet Pictoris b. Because of their extreme separation and youth, this low-mass pair provide an interesting case study for very wide binary formation and evolution.Peer reviewedFinal Accepted Versio
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
X-Ray Emission from Young Stars in the Massive Star Forming Region IRAS 20126+4104
We present a ks Chandra observation of the IRAS20126+4104 core
region. In the inner two X-ray sources were detected, which
are coincident with the radio jet source I20S and the variable radio source
I20Var. No X-ray emission was detected from the nearby massive protostar I20N.
The spectra of both detected sources are hard and highly absorbed, with no
emission below keV.
For I20S, the measured keV count rate was ctsks.
The X-ray spectrum was fit with an absorbed 1T APEC model with an energy of
kTkeV and an absorbing column of Ncm.
An unabsorbed X-ray luminosity of about ergs
was estimated. The spectrum shows broad line emission between 6.4 and 6.7\,
keV, indicative of emission from both neutral and highly ionized iron. The
X-ray lightcurve indicates that I20S is marginally variable; however, no flare
emission was observed.
The variable radio source I20Var was detected with a count rate of
ctsks but there was no evidence of X-ray variability. The
best fit spectral model is a 1T APEC model with an absorbing hydrogen column of
Ncm and a plasma energy of kT = 6.0keV.
The unabsorbed X-ray luminosity is about ergs.Comment: 17pages, 4 figures to appear in Astronomical Journa
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