6,279 research outputs found
A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
Previously, the X-ray emission of zeta Puppis was found to be variable with
light curves harbouring "trends" with a typical timescale longer than the
exposure length. The origin of these changes was proposed to be linked to
large-scale structures in the wind, but further characterization of the
variability at high energies was needed. Since then, a number of new X-ray
observations have become available. Furthermore, a cyclic behaviour with a
1.78d period was identified in long optical photometric runs, which is thought
to be associated with the launching mechanism of large-scale wind structures.
We analysed these new X-ray data, revisited the old data, and compared X-ray
with optical data, including when simultaneous. We found that the behaviour in
X-rays cannot be explained in terms of a perfect clock because the amplitude
and shape of its variations change with time. For example, zeta Puppis was much
more strongly variable between 2007 and 2011 than before and after this
interval. Comparing the X-ray spectra of the star at maximum and minimum
brightness yields no compelling difference beyond the overall flux change: the
temperatures, absorptions, and line shapes seem to remain constant, well within
errors. The only common feature between X-ray datasets is that the variation
amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no
clear and coherent correlation can be found between simultaneous X-ray and
optical data. Only a subgroup of observations may be combined coherently with
the optical period of 1.78d, although the simultaneous optical behaviour is
unknown. The currently available data do not reveal any obvious, permanent, and
direct correlation between X-ray and optical variations. The origin of the
X-ray variability therefore still needs to be ascertained, highlighting the
need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.Comment: accepted for publication by A&
The Influence of Stellar Wind Variability on Measurements of Interstellar O VI Along Sightlines to Early-Type Stars
A primary goal of the FUSE mission is to understand the origin of the O VI
ion in the interstellar medium of the Galaxy and the Magellanic Clouds. Along
sightlines to OB-type stars, these interstellar components are usually blended
with O VI stellar wind profiles, which frequently vary in shape. In order to
assess the effects of this time-dependent blending on measurements of the
interstellar O VI lines, we have undertaken a mini-survey of repeated
observations toward OB-type stars in the Galaxy and the Large Magellanic Cloud.
These sparse time series, which consist of 2-3 observations separated by
intervals ranging from a few days to several months, show that wind variability
occurs commonly in O VI (about 60% of a sample of 50 stars), as indeed it does
in other resonance lines. However, in the interstellar O VI 1032
region, the O VI 1038 wind varies only in 30% of the cases. By
examining cases exhibiting large amplitude variations, we conclude that
stellar-wind variability {\em generally} introduces negligible uncertainty for
single interstellar O VI components along Galactic lines of sight, but can
result in substantial errors in measurements of broader components or blends of
components like those typically observed toward stars in the Large Magellanic
Cloud. Due to possible contamination by discrete absorption components in the
stellar O VI line, stars with terminal velocities greater than or equal to the
doublet separation (1654 km/s) should be treated with care.Comment: Accepted for publication in the Astrophysical Journal Lette
Time-series photometry of the O4 I(n)fp star zeta Puppis
We report a time-series analysis of the O4 I(n)fp star zeta Pup, based on
optical photometry obtained with the SMEI instrument on the Coriolis satellite,
2003--2006. A single astrophysical signal is found, with P = (1.780938 \pm
0.000093) d and a mean semi-amplitude of (6.9 \pm 0.3) mmag. There is no
evidence for persistent coherent signals with semi-amplitudes in excess of ca.
2~mmag on any of the timescales previously reported in the literature. In
particular, there is no evidence for a signature of the proposed rotation
period, ca. 5.1~days; zeta Pup is therefore probably not an oblique magnetic
rotator. The 1.8-day signal varies in amplitude by a factor ca. 2 on timescales
of 10--100d (and probably by more on longer timescales), and exhibits modest
excursions in phase, but there is no evidence for systematic changes in period
over the 1000-d span of our observations. Rotational modulation and
stellar-wind variability appear to be unlikely candidates for the underlying
mechanism; we suggest that the physical origin of the signal may be pulsation
associated with low-l oscillatory convection modes.Comment: MNRAS, in pres
Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph
We report a time-series analysis of satellite photometry of the non-radially
pulsating Oe star zeta Oph, principally using data from SMEI obtained
2003--2008, but augmented with MOST and WIRE results. Amplitudes of the
strongest photometric signals, at 5.18, 2.96, and 2.67/d, each vary
independently over the 6-year monitoring period (from ca. 30 to <2 mmag at
5.18/d), on timescales of hundreds of days. Signals at 7.19/d and 5.18/d have
persisted (or recurred) for around two decades. Supplementary spectroscopic
observations show an H-alpha emission episode in 2006; this coincided with
small increases in amplitudes of the three strongest photometric signals.Comment: MNRAS, in pres
Recommended from our members
Development of an Integrated Governance Strategy for the Voluntary and Community Sector
This report on governance provides a framework for thinking about how policy makers, funders,regulators and advisers can all work with Board members and staff to enhance the effectiveness of nonprofit organisations. It was commissioned by the Active Community Unit (ACU) of the Home Office, in parallel with other reviews designed to improve the capacity of the voluntary and community sector, at a time when the sector plays an increasingly important role in the delivery of services using public funds. That role has recently been investigated in two Government reports, the Cross Cutting Review carried out by the Treasury, and the Strategy Unit review of charities and nonprofits. Our report proposes actions of three types: some that can be taken immediately, some that require further discussion with key interests, and some integration with the other ACU reviews. Taken together they provide the starting point for an evolving strategy to improve governance across the sector. We recommend ACU chairs a group charged with the responsibility for planning and implementing this. Our focus is on governance as 'the systems and processes concerned with ensuring the overall direction, supervision and accountability of an organisation'. This is often taken to mean the way that a Board, management committee or other governing body steers the overall development of an organisation, where day-to-day management is in the hands of staff or volunteers. Sometimes, of course, the committee and volunteers are the same. They – like all governing bodies – have to balance the interests of the organisation and those they are trying to serve, while being conscious of financial and legal responsibilities, and the requirements of funders and other supporters
Effects of Metallicity on the Rotation Rates of Massive Stars
Recent theoretical predictions for low metallicity massive stars predict that
these stars should have drastically reduced equatorial winds (mass loss) while
on the main sequence, and as such should retain most of their angular momentum.
Observations of both the Be/(B+Be) ratio and the blue-to-red supergiant ratio
appear to have a metallicity dependence that may be caused by high rotational
velocities. We have analyzed 39 archival Hubble Space Telescope Imaging
Spectrograph (STIS), high resolution, ultraviolet spectra of O-type stars in
the Magellanic Clouds to determine their projected rotational velocities V sin
i. Our methodology is based on a previous study of the projected rotational
velocities of Galactic O-type stars using International Ultraviolet Explorer
(IUE) Short Wavelength Prime (SWP) Camera high dispersion spectra, which
resulted in a catalog of V sin i values for 177 O stars. Here we present
complementary V sin i values for 21 Large Magellanic Cloud and 22 Small
Magellanic Cloud O-type stars based on STIS and IUE UV spectroscopy. The
distribution of V sin i values for O type stars in the Magellanic Clouds is
compared to that of Galactic O type stars. Despite the theoretical predictions
and indirect observational evidence for high rotation, the O type stars in the
Magellanic Clouds do not appear to rotate faster than their Galactic
counterparts.Comment: accepted by ApJ, to appear 20 December 2004 editio
FUSE Observations of a Full Orbit of Hercules X-1: Signatures of Disk, Star, and Wind
We observed an entire 1.7 day orbit of the X-ray binary Hercules X-1 with the
Far Ultraviolet Spectroscopic Explorer (FUSE). Changes in the O VI 1032,1037
line profiles through eclipse ingress and egress indicate a Keplerian accretion
disk spinning prograde with the orbit. These observations may show the first
double-peaked accretion disk line profile to be seen in the Hercules X-1
system. Doppler tomograms of the emission lines show a bright spot offset from
the Roche lobe of the companion star HZ Her, but no obvious signs of the
accretion disk. Simulations show that the bright spot is too far offset from
the Roche lobe to result from uneven X-ray heating of its surface. The absence
of disk signatures in the tomogram can be reproduced in simulations which
include absorption from a stellar wind. We attempt to diagnose the state of the
emitting gas from the C III 977, C III 1175, and N III 991 emission lines. The
latter may be enhanced through Bowen fluorescence.Comment: Accepted for publication in The Astrophysical Journa
Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance
We have conducted a programme to determine the fundamental parameters of a
substantial number of eclipsing binaries of spectral types O and B in the Small
Magellanic Cloud. New spectroscopic data, obtained with the two-degree-field
multi-object spectrograph on the 3.9-m Anglo-Australian Telescope, have been
used in conjunction with photometry from the Optical Gravitational Lens
Experiment (OGLE-II) database of SMC eclipsing binaries. Previously we reported
results for 10 systems; in this second and concluding paper we present spectral
types, masses, radii, temperatures, surface gravities and luminosities for the
components of a further 40 binaries. The full sample of 50 OB-type eclipsing
systems is the largest single set of fundamental parameters determined for
high-mass binaries in any galaxy. We find that 21 of the systems studied are in
detached configurations, 28 are in semi-detached post-mass-transfer states, and
one is a contact binary.
Each system provides a primary distance indicator. We find a mean distance
modulus to the SMC of 18.91+/-0.03+/-0.1 (internal and external uncertainties;
D=60.6+/-1.0 kpc). This value represents one of the most precise available
determinations of the distance to the SMC.Comment: paper accepted on 22 November 2004 for publication by MNRAS; 26
pages, 6 tables, 12 figure
First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds
Massive O-type stars play a dominant role in our Universe, but many of their
properties remain poorly constrained. In the last decade magnetic fields have
been detected in all Galactic members of the distinctive Of?p class, opening
the door to a better knowledge of all O-type stars. With the aim of extending
the study of magnetic massive stars to nearby galaxies, to better understand
the role of metallicity in the formation of their magnetic fields and
magnetospheres, and to broaden our knowledge of the role of magnetic fields in
massive star evolution, we have carried out spectropolarimetry of five
extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We
have been able to measure magnetic fields with typical error bars from 0.2 to
1.0 kG, depending on the apparent magnitude and on weather conditions. No
magnetic field has been firmly detected in any of our measurements, but we have
been able to estimate upper limits to the field values of our target stars. One
of our targets, 2dFS 936, exhibited an unexpected strengthening of emission
lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period
with two photometric peaks and one spectroscopic maximum. The observed
strengthening of the emission lines of 2dFS 936, and the lack of detection of a
strong magnetic field in a star with such strong emission lines is at odd with
expectations. Together with the unusual periodic behaviour of BI 57, it
represents a challenge for the current models of Of?p stars. The limited
precision that we obtained in our field measurements (in most cases as a
consequence of poor weather) has led to field-strength upper limits that are
substantially larger than those typically measured in Galactic magnetic O
stars. Further higher precision observations and monitoring are clearly
required.Comment: Accepted by A&
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