| 1. |
|
|
| 2. |
|
|
| 3. |
- Dischler, Johann, et al.
(författare)
-
Predicted properties of eclipsing binaries observable by Gaia
- 2005
-
Ingår i: European Space Agency, (Special Publication) ESA SP. - 1609-042X .- 0379-6566. ; 576, s. 569-572
-
Konferensbidrag (refereegranskat)abstract
- Using a population synthesis model we estimate the fraction of stars of different types that are likely to show eclipses, as a function of the period. The population model is based on the bse-code from Hurley et al. (2002) which is a rapid binary evolution code that include all common effects in close binaries such as mass transfer, tidal locking, wind etc. We use this to evolve millions of systems from original distributions in separation, mass and eccentricity, with ages between zero and 12 Gyr, to get a population of systems representative of our Galaxy. The binary data from our model are then analysed in order to, statistically, see how many eclipsing systems we should have. This is done with a simple model where we neglect limb-darkening and other complicating effects. Assuming a random distribution of the inclination angle the probability of an eclipse of a given depth, Am, can easily be calculated. Adding a reasonable fraction of true single stars, we can finally estimate the fraction of eclipsing binaries in limited areas of the HR-diagram, as a function of the period. A first comparison with observational data from the Hipparcos mission shows quite satisfying agreement, and extrapolation to Gaia should thus be a natural application. We find that Gaia will observe about 500 000 eclipsing binaries, this (surprisingly) small number arises from the fact that many eclipsing systems will not be detected by Gaia.
|
|
| 4. |
- Dorch, S.B.F., et al.
(författare)
-
Dynamo action in M-dwarfs
- 2005
-
Ingår i: ESA SP (Special Publication). - 1609-042X .- 0379-6566. ; 560, s. 515-517
-
Konferensbidrag (refereegranskat)
|
|
| 5. |
- Dravins, Dainis, et al.
(författare)
-
Wavelength shifts in solar-type spectra
- 2005
-
Ingår i: European Space Agency, (Special Publication) ESA SP. - 1609-042X .- 0379-6566. ; 560, s. 113-119
-
Konferensbidrag (refereegranskat)abstract
- Spectral-line displacements away from the wavelengths naively expected from the Doppler shift caused by stellar radial motion may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Absolute lineshifts, in the past studied only for the Sun, are now accessible also for other stars thanks to astrometric determination of stellar radial motion, and spectrometers with accurate wavelength calibration. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves). Work is in progress to survey the spectra of the Sun and several solar-type stars for "unblended" photospheric lines of most atomic species with accurate laboratory wavelengths available. One aim is to understand the ultimate information content of stellar spectra, and in what detail it will be feasible to verify models of stellar atmospheric hydrodynamics. These may predict bisectors and shifts for widely different classes of lines, but there will not result any comparison with observations if such lines do not exist in real spectra, or are too blended for meaningful measurement. An important near-future development to enable a further analysis of stellar surface structure will be the study of wavelength variations across spatially resolved stellar disks, e.g., the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability.
|
|
| 6. |
|
|
| 7. |
- Johansson, Anders J
(författare)
-
Performance measures of implant antennas
- 2006
-
Ingår i: European Space Agency, (Special Publication) ESA SP. - 1609-042X .- 0379-6566. ; 626 SP
-
Konferensbidrag (refereegranskat)abstract
- Modern electronic medical implants have reached a high degree of complexity. This has increased the demands on the communication link with the implant, both regarding the bandwidth and the communication distance. A medical communication system at RF frequencies has been standardised, the Medical Implant Communication System (MICS), which use a frequency allocation of 402 - 405 MHz. This frequency band is allocated for implant use both and the US and in the EU. The EIRP is limited to -16 dBm in order to reduce the interference to existing users of the same frequency band. This low EIRP makes it necessary to have reasonable effective antennas in the implants in order to get a benefit form the switch from the classical inductive link to RF. The normal performance measures of antennas have to be modified when applied to implant antennas. The reflection coefficient S11 and the VSWR are straightforward to use also in the implant case. But the gain definition is only valid in a lossless medium. This is not a problem for the implanted antenna, as it is placed in a finite body, i.e. the patient. The implant and the body carrying it will act as one larger antenna, and will have a measurable gain according to the classic definition. The drawback is that the gain will depend heavily on the size and shape of the body, which makes it hard to give a generic value for the gain from a certain antenna. The type of antenna and the amount of isolation around the antenna will influence the amount of nearfield losses, and thus the efficiency. There is a modification of the efficiency measure which solves the problem of the gain definition in an infinite lossy medium [the]. This efficiency measure is a candidate for a quality measure of implanted antennas. The efficiency of an antenna in an infinite lossy material is evaluated by calculating the integral of the Poynting vector over a closed surface in the far zone of the antenna.
|
|
| 8. |
- Jørgensen, Bjarne Rosenkilde, et al.
(författare)
-
Determination of star formation histories from gaia-type photometric and astrometric survey data
- 2005
-
Ingår i: European Space Agency, (Special Publication) ESA SP. - 0379-6566 .- 1609-042X. ; 576, s. 171-173
-
Konferensbidrag (refereegranskat)abstract
- To understand the evolution of the Milky Way Galaxy requires detailed knowledge of the star formation history of various populations. The vast amounts of photometric and astrometric data provided by the Gaia mission give unprecedented opportunities in this area. The relationships between the observed data and the ages of stars are however complex and highly non-linear and great care must be taken in analyzing the data. We describe a Bayesian approach to calculate the star formation rate (SFR) from astrophysical data, using a genetic algorithm to solve the basic integral equation. We present simulations showing that the method is capable of resolving structures in the SFR that cannot be seen from a distribution of the individually estimated stellar ages.
|
|
| 9. |
|
|
| 10. |
|
|
