27 research outputs found
The Dunhuang chinese sky: a comprehensive study of the oldest known star atlas
This paper presents an analysis of the star atlas included in the medieval
Chinese manuscript (Or.8210/S.3326), discovered in 1907 by the archaeologist
Aurel Stein at the Silk Road town of Dunhuang and now held in the British
Library. Although partially studied by a few Chinese scholars, it has never
been fully displayed and discussed in the Western world. This set of sky maps
(12 hour angle maps in quasi-cylindrical projection and a circumpolar map in
azimuthal projection), displaying the full sky visible from the Northern
hemisphere, is up to now the oldest complete preserved star atlas from any
civilisation. It is also the first known pictorial representation of the
quasi-totality of the Chinese constellations. This paper describes the history
of the physical object - a roll of thin paper drawn with ink. We analyse the
stellar content of each map (1339 stars, 257 asterisms) and the texts
associated with the maps. We establish the precision with which the maps are
drawn (1.5 to 4 degrees for the brightest stars) and examine the type of
projections used. We conclude that precise mathematical methods were used to
produce the atlas. We also discuss the dating of the manuscript and its
possible author and confirm the dates 649-684 (early Tang dynasty) as most
probable based on available evidence. This is at variance with a prior estimate
around +940. Finally we present a brief comparison with later sky maps, both in
China and in Europe.Comment: 19 pages, 5 Tables, 8 Figure
The CNO problem in magnetic cataclysmic variables
Some magnetic CVs like BY Cam are characterized by unusual CNO line ratios
compared to other polars and non-solar abundances have been suggested to
explain this anomaly. We present here a first attempt to constrain the
elemental abundances in these systems by applying a specific ionisation model
combined with a geometrical description of the accretion column where these
lines are thought to be formed. The line luminosities have been computed using
the CLOUDY plasma code for different ionisation spectra and column extension.
We show here selected results and compare to the values observed in "peculiar"
magnetic CVs. The model applied to BY Cam confirms that ionization models with
solar abundances fail to reproduce the observed line intensity ratios. Assuming
the model to be valid, the induced best abundances imply an overabundance of N
(x25), underabundance of C (:8) and nearly solar O (:2), in line with CNO
reprocessing.Comment: 7 pages, 5 figures, To appear in `Magnetic Cataclysmic Variables',
IAU Col. 190, Capetown, Eds.: M. Cropper & S. Vrielman
Search for companions around Sirius
Since the discovery of Sirius-B about 130 yr ago, there have been several
claims of a possible second companion around the brightest star Sirius-A. Such
a companion could, in particular, be responsible of the suspected colour change
of the star, now strongly suggested from two independent historical sources. We
reported here on a new observation of the sky region around Sirius, to search
for such a companion, using a coronographic device.
By comparison of the new stellar field with a similar image obtained by us
13 yr ago and using the Sirius proper motion, we are able to eliminate
the most obvious companion candidates down to a magnitude m17 in a
field from 30 arcsec to 2.5 arcmin of the central star. None of the visible
stars appears consistent in magnitude and colours with what expected from
current theoretical models and observations of low-mass stars.
From the study of the same field, it is also shown that the Sirius companion,
consistently reported by observers during the years 1920-1930, is most probably
an unrelated m12 background star, now 1 arcmin away but
located precisely on the Sirius proper motion trajectory. The closest apparent
conjunction with Sirius was realized in 1937 with a minimum angular distance of
6.9 arcsec, of the same order than the Sirius A-B binary separation.
The reported observations do not eliminate the possibility of a second
companion but now confined the search to the more central 30 arcsec region
around Sirius. In particular, the existence of a long period companion cannot
definitively be ruled out since the arbitrary orientation of the orbit can
yield an observed projected position on sky inside this more central region.Comment: 6 pages, 4 figures, published in Astronomy and Astrophysic
Two new intermediate polars with a soft X-ray component
Aims. We analyze the first X-ray observations with XMM-Newton of 1RXS J070407.9+262501 and 1RXS 180340.0+401214, in
order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as intermediate polars.
Methods. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2–10.0 keV range to characterize the emission properties of both sources.
Results. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3–5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 h that is longer than previously estimated. Both systems show complex spectra with a hard (temperature up to 40 keV) optically thin and a soft (kT ∼ 85–100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources.
Conclusions. Our observations confirm the two systems as intermediate polars and also add them as new members of the growing group of “soft” systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation
Maths Express au carrefour des cultures
International audienceCette brochure a été réalisée à l'occasion de la quinzième édition du Salon Culture et Jeux mathématiques. Elle a pour objectif de montrer que les mathématiques sont au coeur de toutes les activités humaines qu'elles soient sociales, techniques, scientifiques, artistiques ou ludiques. Elles sont nées et se sont développées au rythme des sociétés humaines
Far-UV FUSE spectra of peculiar magnetic cataclysmic variables
We present far-UV spectra of the three magnetic cataclysmic variables (MCVs)
BY Cam, V1309 Ori and AE Aqr obtained with the FUSE satellite. These MCVs have
revealed strongly unusual NV and CIV UV resonance lines. The FUSE spectra
exhibit broad OVI lines as well as a strong NIII line at 991A, while the CIII
1175A line is nearly absent, supporting non-solar CNO abundances of the
accreting matter in these sources. The spectrum of BY Cam shows molecular H2
lines which might be of circumstellar nature. The flaring activity of AE Aqr is
also observed in the far-UV range. The radial velocities of the broad OVI
components in AE Aqr are orbitally modulated and would indicate an emission
region close to the magnetosphere.Comment: 7pages, 4 figures, To appear in `Magnetic Cataclysmic Variables', IAU
Col. 190, Capetown, Eds.: M. Cropper & S. Vrielmann, uses newpasp.st
The Large Observatory For X-ray Timing: LOFT
LOFT, the Large Observatory for X-ray Timing, is a new space mission concept devoted to observations of Galactic and extra-Galactic sources in the X-ray domain with the main goals of probing gravity theory in the very strong field environment of black holes and other compact objects, and investigating the state of matter at supra-nuclear densities in neutron stars. The instruments on-board LOFT, the Large area detector and the Wide Field Monitor combine for the first time an unprecedented large effective area (~10 m2 at 8 keV) sensitive to X-ray photons mainly in the 2-30 keV energy range and a spectral resolution approaching that of CCD-based telescopes (down to 200 eV at 6 keV). LOFT is currently competing for a launch of opportunity in 2022 together with the other M3 mission candidates of the ESA Cosmic Vision Progra
The Diffuse Light of the Universe
International audienceIn 1965, the discovery of a new type of uniform radiation, located between radiowaves and infrared light, was accidental. Known today as Cosmic Microwave background (CMB), this diffuse radiation is commonly interpreted as a fossil light released in an early hot and dense universe and constitutes today the main ’pilar’ of the big bang cosmology. Considerable efforts have been devoted to derive fundamental cosmological parameters from the characteristics of this radiation that led to a surprising universe that is shaped by at least three major unknown components: inflation, dark matter and dark energy. This is an important weakness of the present consensus cosmological model that justifies raising several questions on the CMB interpretation. Can we consider its cosmological nature as undisputable? Do other possible interpretations exist in the context of other cosmological theories or simply as a result of other physical mechanisms that could account for it? In an effort to questioning the validity of scientific hypotheses and the under-determination of theories compared to observations, we examine here the difficulties that still exist on the interpretation of this diffuse radiation and explore other proposed tracks to explain its origin. We discuss previous historical concepts of diffuse radiation before and after the CMB discovery and underline the limit of our present understanding