Stars/Alpha Centauri B

This stunning view shows the bright Alpha Centauri A (on the left) and Alpha Centauri B (on the right). Credit: ESA/Hubble & NASA.{{free media}}

Alpha Centauri B, or "α Cen B (HD128621), a nearby K1V dwarf star [...] is part of a visual triple star system whose brightest component, α Cen A (HD128620), is a G2V dwarf."

Theoretical alpha Centauri b

"From a theoretical point of view, this star is important for various reasons. In particular, recent detection of solar-like oscillations in α Cen A and B (Bouchy & Carrier 2001, 2002; Carrier & Bourban 2003) have led several authors (e.g. Morel et al. 2000; Thévenin et al. 2002; Thoul et al. 2004; Eggenberger et al. 2004) to build evolution models of these two stars that are strongly constrained by the measured frequency spacings. The result is a better, but still debated, determination of the fundamental parameters of the system."

X-rays

Main resources: Radiation astronomy/X-rays and X-ray astronomy

File:X-ray and EUV.gif

The spectra of the binary system α Centauri are in the range from 1.0 to 18.0 nm, obtained by LETGS. Credit: A. J. J. Raassen, J.-U. Ness, R. Mewe, R. L. J. van der Meer, V. Burwitz, and J. S. Kaastra.{{fairuse}}

File:Spectra of representative features LETGS epochs.png

Spectra of representative features of α Cen B are from the Chandra X-ray Observatory three LETGS epochs. Credit: Thomas R. Ayres.{{fairuse}}

File:Time history α Cen coronal emissions.png

Time history of the α Cen coronal emissions are in the bottom panel, with cycles of the Sun in the upper panel for comparison. Credit: Thomas R. Ayres.{{fairuse}}

"A Chandra LETGS X-ray observation of α Centauri with an exposure time of 81.5 ks is presented [in the top image on the right] with the two components (K1V and G2V) spectrally resolved for the first time."

The "lines formed at lower temperatures (Fe IX, Fe X, C V, N VI, Si VIII, Si IX, and Si X) are stronger in the G2V star, while the hotter lines of Fe XI, N VII, O VII, O VIII, Si XI, and Si XII are stronger in the K1V star."

"The two stars [α Cen A and B] have coronal cycles in the X-ray and ultraviolet domains with periods around 19 and 8 years, respectively, as shown by Ayres (2014, 2015)."

"During 2008–2013, Chandra captured the rise, peak, and initial decline of B’s coronal luminosity. Together with previous high states documented by ROSAT and XMM-Newton, the long-term X-ray record suggests a period of 8.1 ± 0.2 yr, compared to 11 yr for the Sun, with a minimum-to-peak contrast of 4.5, about half the typical solar cycle amplitude."

"The light curves [...] indicate that B was experiencing heightened flare activity as it rose in X-ray luminosity between 2010 and 2013, as noted by Robrade et al. (2012)".

In the image on the right, "K star profiles are shaded yellow and outlined by red dots; the G star is represented by blue dots. Smoothed photometric errors (1σ ) are orange and green dot-dashed curves, respectively. Formation temperatures noted in the upper portion of the panel are "peak emissivity" temperatures: actual formation temperatures can differ significantly if the maximum of the emission measure distribution is at a much lower, or much higher, temperature. Note dramatic differences between the two stars at the shortest wavelengths (λ < 20 Å), whereas differences are hardly noticeable at the longest wavelengths (λ > 170Å). Nevertheless, if the comparison had been made in f_λ/f_bol, rather than f_λ, B would be systematically ∼3 times brighter at all the wavelengths."

In the image on the left, the upper panel shows the "solar 0.2–2 keV luminosities, 81 day averages (three rotations), over Cycles 23 and 24 (black dots) [where the] error bars indicate 1σ standard deviations of the daily measurements in each bin. [The three-cycle] average is shaded yellow, projected into [the] future as dashed curves and dark hatching (see Ayres 2009 for details). Cycle 23 showed an unusual extended minimum. Lower panel: post-2000 solid dots depict [High Resolution Camera] HRC fluxes of α Cen: blue for [the] solar-type primary; red for K-type secondary. Pre-2000 dots represent four ROSAT [High Resolution Imager] HRI epochs. Yellow dots mark [Low-Energy Transmission Grating Spectrometer] LETGS exposures. Asterisks are reported XMM-Newton X-ray luminosities of AB, scaled by ∼1.26 to match the apparent Chandra cycle of B. Dot-dashed curves, repeated in all panels, are schematic log-sinusoidal fits to Chandra and ROSAT for A, and including also scaled XMM-Newton for B."

The SIMBAD database contains 447 spectral type K1V stars, of these only 76 are confirmed X-ray sources. Although SIMBAD contains alf Cen B, it is not designated as an X-ray source contrary to the evidence shown for X-ray detection and 8.1 yr periodicity in the right and left images.

Ultraviolets

Main resources: Radiation astronomy/Ultraviolets and Ultraviolet astronomy

"FUSE Cycle 7/8 observations of the K1 V star alpha Cen B [indicate] (HD128621; V = 1.33; (B-V) = 0.88; tau = 5.5 Gyr). These recent observations are combined with the pre-existing Cycle 2 spectra to analyze temporal changes in its FUV emissions. We find that in this short span of five years alpha Cen B has exhibited significant variability in the key FUV fluxes, which may indicate an activity cycle."

Atmospheric astronomy

Main resources: Atmospheres/Astronomy, Atmospheric astronomy, and Radiation astronomy/Atmospheres

"The adopted atmospheric parameters are those of Morel et al. (2000), i.e. T_eff = 5260 K, log g = 4.51 and [Fe/H] = + 0.2."

We "adopt the following turbulence and metallicity parameters for α Cen A and B from Jofré et al. (2015) and Heiter et al. (2015): [...] T_eff[B]= 5231 ± 20 K, ξ_turb[B] = 0.99 ± 0.31 km s⁻¹, [Fe/H][B] = +0.22. These parameters are generally in good agreement with the following spectroscopic determination by Porto de Mello et al. (2008): [...] T_eff[B] = 5316 ± 28K, ξ_turb[B] = 1.28 ± 0.15 km s⁻¹, [Fe/H][B] = +0.25 ± 0.04. We adopt the masses determined by Kervella et al. (2016) as follows: [...] m_B = 0.9373 ± 0.0033 M_⊙. The surface gravity parameter of αCen A and B can be deduced from the combination of these masses and our radius measurements, and we obtain log g[A] = 4.3117 ± 0.0015 and log g[B] = 4.5431±0.0015, considering the IAU solar mass conversion constant of (GM)^N_⊙ = 1.3271244 10²⁰ m³ s⁻² (Prša et al. 2016). Our new values are within 1σ of the spectroscopic estimates from Porto de Mello et al. (2008) (log g[A] = 4.34 ± 0.12, log g[B] = 4.44±0.15) and in perfect agreement with the calibration of Gaia benchmark stars by Heiter et al. (2015) (log g[A] = 4.31 ± 0.01, log g[B] = 4.53 ± 0.03)."

Photospheric composition (%) of α Cen B compared to the Sun:

1. Hydrogen 69.4 73.7
2. Helium 27.7 24.5
3. Heavier elements 2.89 1.81

Trigonometric parallaxes

"Assuming the parallax value of Söderhjelm (1999), π = 747.1 ± 1.2 mas, we found a linear radius of 0.863 ± 0.003 R_⊙".

"The recently determined parallax of the αCen system by Kervella et al. (2016) of π = 747.17 ± 0.61 mas allows us to convert the measured LD angular diameters (using the power law LD) into linear radii. We adopt the IAU convention (Prša et al. 2016) for the nominal solar radius (R_⊙^N = 695 700 km) [...] for α Cen B, R_B = 0.8632 ± 0.0009 ± 0.0036 R_⊙ [± σ_stat ± σ_syst]".

"The VLTI measurements provided high-quality angular diameter values for both stars, 8.512 ± 0.022 milliarcsec and 6.002 ± 0.048 milliarcsec for A and B, respectively. With the distance measured earlier by the Hipparcos satellite of the European Space Agency (ESA), 4.36 light-years or 41 million million km, the true radii were then found to be 854,000 km and 602,000 km, or 1.227 ± 0.005 and 0.865 ± 0.007 times the radius of the Sun, respectively."

Stellar astrophysics

Main resource: Radiation astronomy/Astrophysics

"This slightly smaller linear radius obtained from 3D RHD simulations, compared with the one derived from 1D ATLAS model, supports the suggestion of a smaller mass (M = 0.907 M_⊙, Kervella et al. 2003) rather than the larger one (M = 0.934 ± 0.007 M_⊙) proposed by Pourbaix et al. (2002). However, stellar evolution models are sensitive to many parameters, and a smaller radius does not always lead to a smaller mass."

"Luminosity [=] 0.503 ± 0.007 L_⊙ [1.925 ± 0.026 × 10²⁶ W]"

"Luminosity [=] 0.500 L_⊙"

Orbital period (P yrs) = 79.929 ± 0.013

Semi-major axis (a") = 17.592 ± 0.013

Eccentricity (e) = 0.5208 ± 0.0011

Inclination (i deg) = 79.320 ± 0.011

Longitude of the node (Ω deg) = 205.064 ± 0.033

Periastron epoch (T₀ yrs) = 1955.604 ± 0.013

Argument of periastron (ω deg) = 232.006 ± 0.051

mass of B (m_B M_⊙) = 0.9373 ± 0.0033

Rotation (days) = 41, "The estimated equatorial rotation periods for α Cen A and B are 22 and 41 days respectively (Morel et al. 2000), bracketing the solar value."

Materials

Main resource: Chemicals/Materials

"Wood is natural; plastic and metal are artificial, also sterile. Plastic and metal, as is well known, do not naturally exist on earth at all, were first discovered by spectographic analysis of Alpha Centauri B, and have been produced on earth since 1914 by the smelting of meteorites."

Hypotheses

Main resource: Hypotheses

1. As the periastron between α Cen A & B occurs about every 80 yrs, the closeness of A to B is not the source for additional high-energy electron influx to produce coronal X-rays around component B.

See also

• Alpha Centauri
• Stars/Astronomy
• Stars/Betelgeuse
• Stars/Binaries
• Stars/Dwarfs
• Stars/Flares
• Stars/Nova-likes
• Stars/Novas
• Stars/Quasars
• Stars/Sirius
• Stars/Spots
• Stars/Sun
• Stars/Supernovas
• Stars/Vega
• Stars/Wolf-Rayets

External links

• The SAO/NASA Astrophysics Data System
• SIMBAD Astronomical Database