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RE: Alpha Centauri
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Title: Models of alpha Centauri A and B with and without seismic constraints: time dependence of the mixing-length parameter
Authors: M. Yildiz

The alpha Cen binary system is a well-known stellar system with very accurate observational constraints to structure of its component stars. In addition to the classical non-seismic constraints, there are also seismic constraints for the interior models of alpha Cen A and B. These two types of constraint give very different values for the age of the system. While we obtain 8.9 Gyr for the age of the system from the non-seismic constraints, the seismic constraints imply that the age is about 5.6-5.9 Gyr. There may be observational or theoretical reasons for this discrepancy, which can be found by careful consideration of similar stars. The alpha Cen binary system, with its solar type components, is also suitable for testing the stellar mass dependence of the mixing-length parameter for convection derived from the binaries of Hyades. The values of the mixing-length parameter for alpha Cen A and B are 2.10 and 1.90 for the non-seismic constraints. If we prioritise to the seismic constraints, we obtain 1.64 and 1.91 for alpha Cen A and B, respectively. By taking into account of these two contrasting cases for stellar mass dependence of the mixing-length parameter, we derive two expressions for its time dependence, which are also compatible with the mass dependence of the mixing-length parameter derived from the Hyades stars. For assessment, these expressions should be tested in other stellar systems and clusters.

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Title: Deep imaging survey of the environment of Alpha Centauri - I. Adaptive optics imaging of Alpha Cen B with VLT-NACO
Authors: Pierre Kervella (LESIA), Frédéric Thévenin (OCA), Vincent Coudé Du Foresto (LESIA), François Mignard (OCA)

Context: Alpha Centauri is our closest stellar neighbour, at a distance of only 1.3 pc, and its two main components have spectral types comparable to the Sun. This is therefore a favourable target for an imaging search for extrasolar planets. Moreover, indications exist that the gravitational mass of Alpha Cen B is higher than its modelled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses.
Aims: We searched for faint comoving companions to Alpha Cen B. As a secondary objective, we built a catalogue of the detected background sources.
Methods: We used the NACO adaptive optics system of the VLT in the J, H, and Ks bands to search for companions to Alpha Cen B. This instrument allowed us to achieve a very high sensitivity to point-like sources, with a limiting magnitude of mKs ~ 18 at 7" from the star. We complemented this data set with archival coronagraphic images from the HST-ACS instrument to obtain an accurate astrometric calibration.
Results: Over the observed area, we did not detect any comoving companion to Alpha Cen B down to an absolute magnitude of 19-20 in the H and Ks bands. However, we present a catalogue of 252 background objects within about 15" of the star. This catalogue fills part of the large void area that surrounds Alpha Cen in sky surveys due to the strong diffused light. We also present a model of the diffused light as a function of angular distance for the NACO instrument, that can be used to predict the background level for bright star observations.
Conclusions: According to recent numerical models, the limiting magnitude of our search sets the maximum mass of possible companions to 20-30 times Jupiter, between 7 and 20 AU from Alpha Cen B.

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-- Edited by Blobrana at 00:29, 2006-08-26

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Proxima Centauri
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Title: Are Proxima and Alpha Centauri Gravitationally Bound?
Authors: Jeremy G. Wertheimer, Gregory Laughlin

Using the most recent kinematic and radial velocity data in the literature, we calculate the binding energy of Proxima Centauri relative to the centre of mass of the Alpha Centauri system. When we adopt the centroids of the observed data, we find that the three stars constitute a bound system, albeit with a semi-major axis that is on order the same size as Alpha Centauri AB's Hill radius in the galactic potential. We carry out a Monte Carlo simulation under the assumption that the errors in the observed quantities are uncorrelated. In this simulation, 44% of the trial systems are bound, and systems on the 1-3 sigma tail of the radial velocity distribution can have Proxima currently located near the apastron position of its orbit. Our analysis shows that a further, very significant improvement in the characterisation of the system can be gained by obtaining a more accurate measurement of the radial velocity of Proxima Centauri.

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-- Edited by Blobrana at 10:09, 2006-07-19

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Alpha Centauri B
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Astronomers have used ESO's Very Large Telescope in Chile and the Anglo-Australian Telescope in eastern Australia as a 'stellar stethoscope' to listen to the internal rumblings of the nearby star Alpha Centauri B. The data collected with the VLT have a precision better than 1.5 cm/s, or less than 0.06 km per hour!

By observing the star with two telescopes at the same time, the astronomers have made the most precise and detailed measurements to date of pulsations in a star similar to our Sun. They measured the rate at which the star's surface is pulsing in and out, giving clues to the density, temperature, chemical composition and rotation of its inner layers - information that could not be obtained in any other way.
The astronomers from Denmark, Australia, and the USA used Kueyen, one of the four 8.2-m Unit Telescopes of ESO's Very Large Telescope (VLT) at Cerro Paranal in Chile, and the 3.9-m Anglo-Australian Telescope (AAT) in New South Wales (Australia), to study the star which is about 4.3 light-years away.


This wide-field image is of a region around the Southern Cross. Alpha Centauri is the bright yellowish star seen at the middle left, one of the "Pointers" to the star at the top of the Southern Cross.

Alpha Centauri also called Rigel Kentaurus B, is the brighter of the two 'Pointers' to the Southern Cross. Alpha Centauri itself is a triple system and Alpha Centauri B is an orange star, a little cooler and a little less massive than the Sun. Alpha Centauri A and B orbit each other at a distance of about 3.6 billion kilometres.

* Parallax: 0.742 arcsecs
* Spectral type: K0-1 V
* Radial velocity: -21 km/s
* Proper Motion: 3.724 arcsecs/year
* Apparent Visual Magnitude: 1.35
* Absolute Visual Magnitude: 5.70
* Luminosity: 0.5 Solar Luminosities
* Diameter: 0.865 times that of the Sun
* Angular diameter: 6.002 milliarcseconds


Churning gas in the star's outer layers creates low-frequency sound waves that bounce around the inside of the star, causing it to ring like a bell. This makes the star's surface pulsate in and out by very tiny amounts - only a dozen metres or so every four minutes. Astronomers can detect these changes by measuring the small, associated wavelength shifts.
The researchers sampled the light from Alpha Centauri B for seven nights in a row, making more than 5 000 observations in all. At the VLT, 3379 spectra were obtained with typical exposure times of 4 seconds and a median cadence of one exposure every 32 seconds! At the AAT 1642 spectra were collected, with typical exposures of 10 s, taken every 90 s.

"From this unique dataset, we were able to determine as many as 37 different patterns (or modes) of oscillation" - Hans Kjeldsen, University of Aarhus (Denmark) and lead author of the paper describing the results.

The astronomers also measured the mode lifetimes (how long the oscillations last), the frequencies of the modes, and their amplitudes (how far the surface of the star moves in and out). Such measurements are a huge technical challenge. Indeed, the star' surface moves slowly, at the tortoise-like speed of 9 cm a second, or about 300 metre an hour. The astronomers borrowed their high-precision measurement technique from the planet-hunters, who also look for slight Doppler shifts in starlight.

"So much of what we think we know about the universe rests on the ages and properties of stars. But there is still a great deal we don't know about them" - Tim Bedding, University of Sydney and co-author of the study.

By using two telescopes at different sites the astronomers were able to observe the Alpha Centauri B as continuously as possible.

"That's a huge advantage, because gaps in the data introduce ambiguity. The success of the observations also depended on the very stable spectrographs attached to the two telescopes - UVES at the VLT and UCLES at the AAT - which analysed the star's light" - Tim Bedding.

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Alpha Centauri
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Two XMM EPIC MOS images of Alpha Centauri A+B, taken in March 2003 (left) and Feb. 2005 (right). Alpha Centauri is our nearest stellar systemconsisting of a G2V (A) and a K1V (B) star at a distance of about four light-years, the M dwarf star Proxima Centauri is not in the field of view.
Alpha Cen B is the X-ray brighter object at lower right and exhibits a comparable X-ray luminosity in both exposures. In contrast Alpha Cen A, a star very similar to our Sun, is only visible in the left image. It has faded in X-rays by at least an order of magnitude, a behaviour never observed before despite several observations of the Alpha Centauri system over the last 25 years.
Is this an irregular event or do we see a coronal activity cycle?

Our Sun, a relatively inactive star, exhibits a well-known activity cycle with a period of 11 years. While chromospheric activity cycles on low-activity stars are established from optical measurements of Ca II emission lines, coronal X-ray activity cycles are known for very few objects.
A long-term XMM-Newton monitoring programme of solar-like stars, including Alpha Centauri A+B, was initiated to put some more light on this topic. No activity cycle was ever detected on a component of Alpha Centauri. Since also no chromospheric data of Alpha Centauri is available and all previous resolved X-ray observations (Einstein, ROSAT, Chandra) revealed a similar situation as this March 2003 XMM-Newton observation, a definite explanation of this astonishing finding can only be given by future observations.

Credits: ESA/Jan Robrade

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