368 research outputs found
A spectroscopic study of LMC X-4
The orbital radial velocity semi-amplitude of the binary star system LMC X-4 primary was determined to be 37.9 + or - 2.4 km/s from measurements of the hydrogen absorption lines. The semi-amplitude of the He I and He II absorption lines are consistent with this, namely 44.9 + or - 5.0 and 37.3 + or - 5.3 km/s. The phase and shape of the radial velocity curves of the three ions are consistent with a circular orbit and an ephemeris based upon X-ray measurements of the neutron star, with the exception that the He II absorption line radial velocity curve has detectable shape distortion. Measurements of the He II LAMBOA 4686 emission line velocity are consistent with a phase shifted sine wave of semi-amplitude 535 km/s, a square wave of semi-amplitude 407 km/s, or high order harmonic fits. The spectral type was found to be 08.5 IV-V during X-ray eclipse. Variations to types as early as 07 occur, but not as a function or orbital phase. Absorption line peculiarities were noted on 6 of 58 spectra
The 125th anniversary of the first postulation of the soil origin of endophytic bacteria – a tribute to M.L.V. Galippe
In both managed and natural ecosystems, a wide range of various non-nodulating bacteria can thrive as endophytes in the plant interior, and some can be beneficial to their hosts (Hallmann and Berg 2007; Reinhold-Hurek and Hurek 2011). Colonizationmechanisms, the ecology and functioning of these endophytic bacteria as well as their interactions with plants have been investigated (Hardoim et al. 2008; Compant et al. 2010). Although the source of colonization can also be the spermosphere, anthosphere, caulosphere, and the phyllosphere,most endophytic bacteria are derived from the soil environment (Hallmann and Berg 2007; Compant et al. 2010)
Secretion dynamics of soyasaponins in soybean roots and effects to modify the bacterial composition
Soyasaponins are triterpenoid saponins widely found in legume plants. These compounds have drawn considerable attention because they have various activities beneficial for human health, and their biosynthesis has been actively studied. In our previous study, we found that legume plants including soybean secrete soyasaponins from the roots in hydroponic culture throughout the growth period, but the physiological roles of soyasaponins in the rhizosphere and their fate in soil after exudation have remained unknown. This study demonstrates that soyasaponins are secreted from the roots of field-grown soybean, and soyasaponin Bb is the major soyasaponin detected in the rhizosphere. In vitro analysis of the distribution coefficient suggested that soyasaponin Bb can diffuse over longer distances in the soil in comparison with daidzein, which is a typical isoflavone secreted from soybean roots. The degradation rate of soyasaponin Bb in soil was slightly faster than that of daidzein, whereas no soyasaponin Bb degradation was observed in autoclaved soil, suggesting that microbes utilize soyasaponins in the rhizosphere. Bacterial community composition was clearly influenced by soyasaponin Bb, and potential plant growth-promoting rhizobacteria such as Novosphingobium were significantly enriched in both soyasaponin Bb-treated soil and the soybean rhizosphere. These results strongly suggest that soyasaponin Bb plays an important role in the enrichment of certain microbes in the soybean rhizosphere
The Star Formation Region NGC 6530: distance, ages and Initial Mass Function
We present astrometry and photometry, down to , of the very
young open cluster NGC6530, obtained from observations taken with the Wide
Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the vs. and
the vs. color-magnitude diagrams (CMD) show the upper main sequence
dominated by very bright cluster stars, while, due to the high obscuration of
the giant molecular cloud surrounding the cluster, the blue envelopes of the
diagrams at are limited to the main sequence stars at the
distance of NGC6530. This particular structure of the NGC6530 CMD allows us to
conclude that its distance is about pc, significantly lower
than the previous determination of d=1800 pc. We have positionally matched our
optical catalog with the list of X-ray sources found in a Chandra-ACIS
observation, finding a total of 828 common stars, 90% of which are pre-main
sequence stars in NGC6530. Using evolutionary tracks of Siess et al. (2000)},
mass and age values are inferred for these stars. The median age of the cluster
is about 2.3 Myr; in the mass range (0.6--4.0), the Initial Mass
Function (IMF) shows a power law index , consistent with both
the Salpeter index (1.35), and with the index derived for other young clusters
; towards smaller masses the IMF shows a peak and then it starts to decrease.Comment: 32 pages, 13 ps figures, in press in Astronomy and Astrophysic
Discovery of a strong magnetic field in the rapidly rotating B2Vn star HR 7355
We report the detection of a strong, organized magnetic field in the
helium-variable early B-type star HR 7355 using spectropolarimetric data
obtained with ESPaDOnS on the 3.6-m Canada-France-Hawaii Telescope within the
context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is
both the most rapidly rotating known main-sequence magnetic star and the most
rapidly rotating helium-strong star, with = 300 15 km s
and a rotational period of 0.5214404 0.0000006 days. We have modeled our
eight longitudinal magnetic field measurements assuming an oblique dipole
magnetic field. Constraining the inclination of the rotation axis to be between
and , we find the magnetic obliquity angle to be
between and , and the polar strength of the magnetic
field at the stellar surface to be between 13-17 kG. The photometric light
curve constructed from HIPPARCOS archival data and new CTIO measurements shows
two minima separated by 0.5 in rotational phase and occurring 0.25 cycles
before/after the magnetic extrema. This photometric behavior coupled with
previously-reported variable emission of the H line (which we confirm)
strongly supports the proposal that HR 7355 harbors a structured magnetosphere
similar to that in the prototypical helium-strong star, Ori E.Comment: 6 pages, 3 figures. Accepted for publication in MNRAS Letter
An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with the
Short Wavelength Spectrometer (SWS) during the Post-Helium program of the
Infrared Space Observatory (ISO). This program is aimed at extending the Morgan
& Keenan classification scheme into the near-infrared. Later type stars will be
discussed in a seperate publication. The observations consist of 57 SWS
Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra
acquired during the nominal mission with a similar observational setting. For
B-type stars, this sample provides ample spectral converage in terms of subtype
and luminosity class. For O-type stars,the ISO sample is coarse and therefore
is complemented with 8 UKIRT L'-band observations. In terms of the presence of
diagnostic lines, the L'-band is likely the most promising of the near-infrared
atmospheric windows for the study of the physical properties of B stars.
Specifically, this wavelength interval contains the Brackett alpha, Pfund
gamma, and other Pfund lines which are probes of spectral type, luminosity
class and mass loss. Here, we present simple empirical methods based on the
lines present in the 2.4 to 4.1 microns interval that allow the determination
of: the spectral type of B dwarfs and giants to within two subtypes; the
luminosity class of B stars to within two classes; the mass-loss rate of O
stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
GS100-02-41: a new large HI shell in the outer part of the Galaxy
Massive stars have a profound effect on the surrounding interstellar medium.
They ionize and heat the neutral gas, and due to their strong winds, they swept
the gas up forming large HI shells. In this way, they generate a dense shell
where the physical conditions for the formation of new stars are given. The aim
of this study is to analyze the origin and evolution of the large HI shell
GS100-02-41 and its role in triggering star forming processes.To characterize
the shell and its environs, we carry out a multi-wavelength study. We analyze
he HI 21 cm line, the radio continuum, and infrared emission distributions. The
analysis of the HI data shows an expanding shell structure centred at (l, b) =
(100.6 deg, -2.04 deg) in the velocity range from -29 to -51.7 km/s.
We infer for GS100-02-41, a kinematical distance of 2.8 +/- 0.6 kpc. Several
massive stars belonging to Cep OB1 are located in projection within the large
HI, shell boundaries. The analysis of the radio continuum and infrared data
reveal that there is no continuum counterpart of the HI shell. On the other
hand, three slightly extended radio continuum sources are observed in
projection onto the dense HI shell. From their flux density determinations we
infer that they are thermal in nature. An analysis of the HI emission
distribution in the environs of these sources shows, for each of them, a region
of low emissivity having a good morphological correlation with the ionized gas
in a velocity range similar to the one where GS100-02-41 is detected. The
origin of GS100-02-41 could have been mainly due to the action of the Cep OB1
massive stars located inside the HI shell. The obtained age difference between
the HI shell and the HII regions, together with their relative location, led us
to conclude that the ionizing stars could have been created as a consequence of
the shell evolution.Comment: Accepted for publication in A&
A Sino-German 6 cm polarization survey of the Galactic plane II. The region from 129 degree to 230 degree longitude
Linearly polarized Galactic synchrotron emission provides valuable
information about the properties of the Galactic magnetic field and the
interstellar magneto-ionic medium, when Faraday rotation along the line of
sight is properly taken into account. We aim to survey the Galactic plane at 6
cm including linear polarization. At such a short wavelength Faraday rotation
effects are in general small and the Galactic magnetic field properties can be
probed to larger distances than at long wavelengths. The Urumqi 25-m telescope
is used for a sensitive 6 cm survey in total and polarized intensities. WMAP
K-band (22.8 GHz) polarization data are used to restore the absolute zero-level
of the Urumqi U and Q maps by extrapolation. Total intensity and polarization
maps are presented for a Galactic plane region of 129 degree < l < 230 degree
and |b| < 5 degree in the anti-centre with an angular resolution of 9'5 and an
average sensitivity of 0.6 mK and 0.4 mK Tb in total and polarized intensity,
respectively. We briefly discuss the properties of some extended Faraday
Screens detected in the 6 cm polarization maps. The Sino-German 6 cm
polarization survey provides new information about the properties of the
magnetic ISM. The survey also adds valuable information for discrete Galactic
objects and is in particular suited to detect extended Faraday Screens with
large rotation measures hosting strong regular magnetic fields.Comment: 17 pages, 20 figures, accepted by A&A. Resolutions of the figures
have been significantly reduced. For version with full resolution, see
http://159.226.88.6/zmtt/6cm/papers/gao.paper2.pd
Early-type stars in the young open cluster NGC 2244 and in the Mon OB2 association I. The multiplicity of O-type stars
Aims. We present the results obtained from a long-term spectroscopic campaign
devoted to the multiplicity of O-type stars in the young open cluster NGC2244
and in the Mon OB2 association. Methods. Our spectroscopic monitoring was
performed over several years, allowing us to probe different time-scales. For
each star, several spectral diagnostic tools are applied, in order to search
for line shifts and profile variations. We also measure the projected
rotational velocity and revisit the spectral classification. Results. In our
sample, several stars were previously considered as spectroscopic binaries,
though only a few scattered observations were available. Our results now reveal
a more complex situation. Our study identifies two new spectroscopic binaries
(HD46149 in NGC2244 and HD46573 in MonOB2). The first object is a long-period
double-lined spectroscopic binary, though the exact value of its period remains
uncertain and the second object is classified as an SB1 system with a period of
about 10.67 days but the time series of our observations do not enable us to
derive a unique orbital solution for this system. We also classify another star
as variable in radial velocity (HD46150) and we detect line profile variations
in two rapid rotators (HD46056 and HD46485). Conclusions. This spectroscopic
investigation places a firm lower limit (17%) on the binary fraction of O-stars
in NGC2244 and reveals the lack of short-period O+OB systems in this cluster.
In addition, a comparison of these new results with two other well-studied
clusters (NGC6231 and IC1805) puts forward possible hints of a relation between
stellar density and binarity, which could provide constraints on the theories
about the formation and early evolution of hot stars.Comment: 14 pages, 10 figures, 9 table
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