171 research outputs found
The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
We present an examination of high resolution, ultraviolet spectroscopy from
Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the
massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based
optical spectra to determine the effective temperature and gravity of the O9.7
Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), line
blanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0
+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. The
optical spectrum is best fit with models that have enriched He and N
abundances. We fit the model spectral energy distribution for this temperature
and gravity to the UV, optical, and IR fluxes to determine the angular size of
and extinction towards the binary. The angular size then yields relations for
the stellar radius and luminosity as a function of distance. By assuming that
the supergiant rotates synchronously with the orbit, we can use the radius -
distance relation to find mass estimates for both the supergiant and black hole
as a function of the distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that limits the
solutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sun
for the supergiant and 11^{+5}_{-3} M_sun for the black hole.Comment: ApJ in pres
Tobacco\u27s Minor Alkaloids: Effects on Place Conditioning and Nucleus Accumbens Dopamine Release in Adult and Adolescent Rats
Tobacco products are some of the most commonly used psychoactive drugs worldwide. Besides nicotine, alkaloids in tobacco include cotinine, myosmine, and anatabine. Scientific investigation of these constituents and their contribution to tobacco dependence is less well developed than for nicotine. The present study evaluated the nucleus accumbens dopamine-releasing properties and rewarding and/or aversive properties of nicotine (0.2-0.8 mg/kg), cotinine (0.5-5.0 mg/kg), anatabine (0.5-5.0 mg/kg), and myosmine (5.0-20.0 mg/kg) through in vivo microdialysis and place conditioning, respectively, in adult and adolescent male rats. Nicotine increased dopamine release at both ages, and anatabine and myosmine increased dopamine release in adults, but not adolescents. The dopamine release results were not related to place conditioning, as nicotine and cotinine had no effect on place conditioning, whereas anatabine and myosmine produced aversion in both ages. While the nucleus accumbens shell is hypothesized to play a role in strengthening drug-context associations following initiation of drug use, it may have little involvement in the motivational effects of tobacco constituents once these associations have been acquired. Effects of myosmine and anatabine on dopamine release may require a fully developed dopamine system, since no effects of these tobacco alkaloids were observed during adolescence. In summary, while anatabine and myosmine-induced dopamine release in nucleus accumbens may play a role in tobacco dependence in adults, the nature of that role remains to be elucidated
Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910
As a result of the variability survey in Chi Persei and NGC6910, the number
of Beta Cep stars that are members of these two open clusters is increased to
twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational
properties, in particular the frequency spectra, of Beta Cep stars in both
clusters and explain the differences in terms of the global parameters of the
clusters. We also indicate that the more complicated pattern of the variability
among B type stars in Chi Persei is very likely caused by higher rotational
velocities of stars in this cluster. We conclude that the sample of pulsating
stars in the two open clusters constitutes a very good starting point for the
ensemble asteroseismology of Beta Cep-type stars and maybe also for other
B-type pulsators.Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife,
Lanzarote, Feb 2010, submitte
Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators
While the first binary post-AGB stars were serendipitously discovered, the
distinct characteristics of their Spectral Energy Distribution (SED) allowed us
to launch a more systematic search for binaries. We selected post-AGB objects
which show a broad dust excess often starting already at H or K, pointing to
the presence of a gravitationally bound dusty disc in the system. We started a
very extensive multi-wavelength study of those systems and here we report on
our radial velocity and photometric monitoring results for six stars of early F
type, which are pulsators of small amplitude. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. All of the six objects are binaries, with orbital periods ranging
from 120 to 1800 days. Five systems have non-circular orbits. The mass
functions range from 0.004 to 0.57 solar mass and the companions are likely
unevolved objects of (very) low initial mass. We argue that these binaries must
have been subject to severe binary interaction when the primary was a cool
supergiant. Although the origin of the circumstellar disc is not well
understood, the disc is generally believed to be formed during this strong
interaction phase. The eccentric orbits of these highly evolved objects remain
poorly understood. With the measured orbits and mass functions we conclude that
the circumbinary discs seem to have a major impact on the evolution of a
significant fraction of binary systems.Comment: 13 pages, 15 figures, accepted for Astronomy and Astrophysic
Structure-property relationships from universal signatures of plasticity in disordered solids
When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively
Analysing Dynamical Behavior of Cellular Networks via Stochastic Bifurcations
The dynamical structure of genetic networks determines the occurrence of various biological mechanisms, such as cellular differentiation. However, the question of how cellular diversity evolves in relation to the inherent stochasticity and intercellular communication remains still to be understood. Here, we define a concept of stochastic bifurcations suitable to investigate the dynamical structure of genetic networks, and show that under stochastic influence, the expression of given proteins of interest is defined via the probability distribution of the phase variable, representing one of the genes constituting the system. Moreover, we show that under changing stochastic conditions, the probabilities of expressing certain concentration values are different, leading to different functionality of the cells, and thus to differentiation of the cells in the various types
X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars
The winds of massive stars have an impact on stellar evolution and on the
surrounding medium. The maximum speed reached by these outflows, the terminal
wind speed, is a global wind parameter and an essential input for models of
stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a
legacy UV spectroscopic survey with HST, we have the opportunity to quantify
the wind speeds of massive stars at sub-solar metallicity (in the Large and
Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We
empirically quantify the wind speeds of a large sample of OB stars, including
supergiants, giants, and dwarfs at sub-solar metallicity. Using these
measurements, we investigate trends of terminal wind speed with a number of
fundamental stellar parameters, namely effective temperature, metallicity, and
surface escape velocity. We empirically determined the terminal wind speed for
a sample of 149 OB stars in the Magellanic Clouds either by directly measuring
the maximum velocity shift of the absorption component of the Civ 1548-1550
line profile, or by fitting synthetic spectra produced using the Sobolev with
exact integration method. Stellar parameters were either collected from the
literature, obtained using spectral-type calibrations, or predicted from
evolutionary models. We find strong trends of terminal wind speed with
effective temperature and surface escape speed when the wind is strong enough
to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a
metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03
when we compared our results to previous Galactic studies. Our results suggest
that effective temperature rather than surface escape speed should be used as a
straightforward empirical prediction of terminal wind speed and that the
observed metallicity dependence is steeper than suggested by earlier works.Comment: 21 pages, 16 figures, 8 tables. Accepted in A&
Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars
CONTEXT: Recent progress in the seismic interpretation of field beta Cep
stars has resulted in improvements of the physics in the stellar structure and
evolution models of massive stars. Further asteroseismic constraints can be
obtained from studying ensembles of stars in a young open cluster, which all
have similar age, distance and chemical composition.
AIMS: To improve our comprehension of the beta Cep stars, we studied the
young open cluster NGC 884 to discover new B-type pulsators, besides the two
known beta Cep stars, and other variable stars.
METHODS: An extensive multi-site campaign was set up to gather accurate CCD
photometry time series in four filters (U, B, V, I) of a field of NGC884.
Fifteen different instruments collected almost 77500 CCD images in 1286 hours.
The images were calibrated and reduced to transform the CCD frames into
interpretable differential light curves. Various variability indicators and
frequency analyses were applied to detect variable stars in the field. Absolute
photometry was taken to deduce some general cluster and stellar properties.
RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V,
6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the
amplitude spectra is 50 micromag in the V band. Our campaign confirms the
previously known pulsators, and we report more than one hundred new multi- and
mono-periodic B-, A- and F-type stars. Their interpretation in terms of
classical instability domains is not straightforward, pointing to imperfections
in theoretical instability computations. In addition, we have discovered six
new eclipsing binaries and four candidates as well as other irregular variable
stars in the observed field.Comment: Accepted for publication in Astronomy and Astrophysics, 21 pages, 14
figures, 4 tables. The full appendix is available at
http://www.ster.kuleuven.be/~sophies/Appendix.pdf (74 MB, 169 pages, 343
figures, 1 table
X-Shooting ULLYSES: Massive stars at low metallicity: I. Project description
Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESOa's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope spectra of the first stellar generations, the individual spectra of low-Z stars need to be understood, which is exactly where XShootU can deliver
X-shooting ULLYSES: massive stars at low metallicity
Context. The winds of massive stars have a significant impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed v∞, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with the Hubble Space Telescope, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5 Z⊙ and 0.2 Z⊙, respectively) at an unprecedented scale.
Aims. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of v∞ with a number of fundamental stellar parameters, namely effective temperature (Teff), metallicity (Z), and surface escape velocity vesc.
Methods. We empirically determined v∞ for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the C IV λλ1548–1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models.
Results. We find strong trends of v∞ with Teff and vesc when the wind is strong enough to cause a saturated P Cygni profile in C IV λλ1548–1550. We find evidence for a metallicity dependence on the terminal wind speed v∞ ∝ Z0.22±0.03 when we compared our results to previous Galactic studies.
Conclusions. Our results suggest that Teff rather than vesc should be used as a straightforward empirical prediction of v∞ and that the observed Z dependence is steeper than suggested by earlier works
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