STELLAR ACTIVITY
Beta Hydri
Beta Hydri (of the constellation Hydrus, not Hydra) is the closest
subgiant (G2 IV) and one of the oldest stars in the solar Galactic neighborhood.
Its position in the Hertzsprung-Russell diagram places it among those rather
few evolved field stars whose ages can be determined without ambiguity
from theoretical evolutionary tracks. It is close enough to the main
sequence to tell that it originated there from around G2 V, near the solar
position. At the same time, it is almost as old as a solar-mass star
can be, and still be possible to accurately date from stellar evolutionary
theory.
Beta Hydri is significantly older than the Sun. Previous age determinations,
based on groundbased parallax measurements gave age estimates around
9.5 Gy. The new Hipparcos
parallax of 133.78 ± 0.51 mas implies an absolute magnitude MV
= 3.43 ± 0.01, and new evolutionary calculations produce bestfit
models with ages around 6.7 Gy:
Figure: Stellar evolution, and the age of Beta Hydri. Post mainsequence tracks are shown for two representative models, passing through the postHipparcos position of Beta Hyi in the MV/logTeff plane (black diamond). The position based on the old groundbased parallax is also marked (gray diamond).
(D.Dravins, L.Lindegren & D.A.VandenBerg, Beta Hydri (G2
IV): A Revised Age for the Closest Subgiant, A&A
330, 1077-1079, 1998)
Photospheric processes
A relatively high lithium abundance of Beta Hydri may be a signature of
the early subgiant stage, when lithium that once diffused to beneath the
main-sequence convection zone, is dredged up to the surface as the convection
zone deepens. Numerical simulations of the three-dimensional photospheric
hydrodynamics show typical granules to be significantly larger (a factor
of 5) than solar ones. The photospheric pressure is smaller, thus
limiting the flux density to which photospheric magnetic fields can be
compressed. The lower surface gravity and density leads to greater
granular velocities (factor of 1.5- 2), since the same surface energy flux
as in the Sun must be carried by lower-density gas. Synthetic Fe
I photospheric line profiles are compared to those observed at very high
(200,000) spectral resolution. The line cores of Beta Hydri indicate
a stellar rotation V sini = 2 km/s, while the line wings suggest the presence
of trans- or supersonic horizontal motions in the more vigorous granulation
on this subgiant.
High-resolution Ca II H & K profiles show the emission in Beta Hyi
about half that for the Sun, but with the same sense of violet-red asymmetry
(the shortward peak the strongest). The emission's wavelength width
is slightly broader, consistent with the Wilson-Bappu relation for this
slightly more luminous star. Mg II h & k profiles also show an
emission weaker than in the Sun, but with the opposite sense of asymmetry.
The Mg II variability was monitored from the IUE satellite during
about 12 years, recording some 100 high-resolution line profiles. The emission
variations are small (30%) , and are characterized by smooth and systematic
changes, suggesting continuous changes in the chromospheric structure,
rather than the sudden emergence of active regions. These 12 years
embrace both an activity minimum and a maximum, consistent with an activity
cycle somewhat longer (15-18 years) than the solar one. Assuming
the Ca II emission variation to follow that of Mg II, the corresponding
peak-to-peak amplitude in the Mt.Wilson K-line index would be on the order
of 1%, making this the lowest-amplitude stellar activity cycle so far detected.
Ca II - PostScript figure (220 kb):
Figure: Ca II H and K lines in Beta Hydri (solid), compared to those in integrated sunlight (dotted), measured with identical resolution. For both H and K, the double-peaked chromospheric emission is clearly weaker in Beta Hyi than in the Sun, but all lines show the same sense of violet-red asymmetry. The emission's wavelength width is slightly broader, consistent with the Wilson-Bappu relation for its slightly greater luminosity. The bottom panels show an expanded view of the line cores.
Mg II - PostScript figure (210 kb):
Figure: Line profiles for Mg II h and k, compared to solar data. For Beta Hyi, the average profile over twelve years is shown (bold), while the dotted curve is the spectrum of integrated sunlight, as revealed by an IUE observation of the Moon with nominally identical instrumental settings. The expanded plots at bottom also compare with spatially resolved solar data with higher spectral resolution (90,000) for quiet regions at solar disk center (µ= 1; thin solid), and closer to the limb at µ= 0.23 (thin dotted).
(D.Dravins, P.Linde, K.Fredga & G.F.Gahm, The Distant
Future of Solar Activity A Case Study of Beta Hydri. II. Chromospheric
activity and variability, ApJ
403, 396-411, 1993).
Transition region and coronal emission seem to be closely connected to
magnetic fields. They are very pronounced in young stars, but decrease
rapidly with stellar age. Among all spectral types, solar-type stars show
the strongest age evolution of coronal emission. Thus, in very old solar-type
stars one could hope to identify some signatures of the "basal" (non-magnetic)
atmosphere, provided the magnetic fields have then largely decayed.
Indeed, the far-UV emission lines from the transition zone in Beta Hyi
are among the faintest so far seen in any solar-type star.
Figure: Photometrically processed images from the IUE low-dispersion camera illustrate the great range of transition-zone emission in G2 stars. Exposures of different length are combined, and the photospheric continuum subtracted. The emission features in Beta Hyi are among the faintest observed from the transition region of any solar-type star. Alpha Cen A is reminiscent of the Sun at its activity minimum, while the active bright giant Beta Dra displays a rich emission line spectrum.
(D.Dravins, P.Linde, T.R.Ayres, J.L.Linsky,
B.Monsignori-Fossi, T.Simon & F.Wallinder:
The Distant Future of Solar Activity - A Case Study of Beta Hydri. III.
Transition region, corona, and stellar wind, ApJ
403, 412-425, 1993 ).
Stellar coronae
The coronal soft X-ray spectrum of Beta Hydri was measured through different
filters on EXOSAT, and compared to simulated X-ray observations
of the Sun seen as a star. The flux is weaker than that from the
solar corona, and has a different spectrum. An emission-measure analysis
yields a temperature of either 5 105 or
4 106 K. The "cool" solution might
be physically stable due to a local minimum in the coronal radiative loss
function. It is conjectured that this could be an intermediate evolutionary
step to losing an observable corona altogether, as is the case in more
evolved cool giants. If the "cool" coronal solution is appropriate,
it could imply that a thermally driven stellar wind can no longer be supported,
removing the mechanism for further rotational braking of the star through
a magnetic stellar wind.
Other project activities
A major "spin-off" from the Beta Hydri program was the development at Lund
Observatory of new methods for IUE spectral image processing;
see e.g. the Appendix of: D.Dravins, P. Linde, K.Fredga
& G.F.Gahm, The Distant Future of Solar Activity A Case Study of
Beta Hydri. II. Chromospheric activity and variability, ApJ
403, 396-411, (1993). These methods were subsequently
adopted by ESA and NASA for the reprocessing of the more than 100,000 spectra
in the IUE data archive
(e.g., J.Nichols & J.L.Linsky: The Final Archive and Recalibration
of the International Ultraviolet Explorer (IUE) Satellite, AJ
111, 517-536, 1996 ).
Publications
Updated JD 2,451,700
