64,793 research outputs found
The pulsation modes, masses and evolution of luminous red giants
The period-luminosity sequences and the multiple periods of luminous red
giant stars are examined using the OGLE III catalogue of long-period variables
in the Large Magellanic Cloud. It is shown that the period ratios in individual
multimode stars are systematically different from the ratios of the periods at
a given luminosity of different period-luminosity sequences. This leads to the
conclusion that the masses of stars at the same luminosity on the different
period-luminosity sequences are different. An evolutionary scenario is used to
show that the masses of stars on adjacent sequences differ by about 16-26% at a
given luminosity, with the shorter period sequence being more massive. The mass
is also shown to vary across each sequence by a similar percentage, with the
mass increasing to shorter periods. On one sequence, sequence B, the mass
distribution is shown to be bimodal. It is shown that the small amplitude
variables on sequences A', A and B pulsate in radial and nonradial modes of
angular degree l=0, 1 and 2, with the l=1 mode being the most common. The stars
on sequences C' and C are predominantly radial pulsators (l=0). Matching period
ratios to pulsation models shows that the radial pulsation modes associated
with sequences A', A, B, C' and C are the 4th, 3rd, 2nd and 1st overtones and
the fundamental mode, respectively.Comment: 16 pages, 10 figures, 1 tabl
Evidence for mass ejection associated with long secondary periods in red giants
Approximately 30% of luminous red giants exhibit a Long Secondary Period
(LSP) of variation in their light curves, in addition to a shorter primary
period of oscillation. The cause of the LSP has so far defied explanation:
leading possibilities are binarity and a nonradial mode of oscillation. Here,
large samples of red giants in the Large Magellanic Cloud both with and without
LSPs are examined for evidence of an 8 or 24 m mid-IR excess caused by
circumstellar dust. It is found that stars with LSPs show a significant mid-IR
excess compared to stars without LSPs. Furthermore, the near-IR - color
seems unaffected by the presence of the 24 m excess. These findings
indicate that LSPs cause mass ejection from red giants and that the lost mass
and circumstellar dust is most likely in either a clumpy or a disk-like
configuration. The underlying cause of the LSP and the mass ejection remains
unknown.Comment: 6 pages, accepted for publication in Ap
Is there another major constituent in the atmosphere of Mars?
In view of the possible finding of several tens percent of inert gas in the atmosphere of Mars by an instrument on the descent module of the USSR's Mars 6 spacecraft, the likelihood of the correctness of this result was examined. The basis for the well-known fact that the most likely candidate is radiogenic argon is described. It is shown that, for the two important methods of investigating the atmosphere, earth-based CO2 is infrared absorption spectroscopy and S-band occultation, within the estimated 1 standard deviation uncertainties of these methods about 20% argon can be accommodated. Within the estimated 3 standard deviation uncertainties, more than 35% is possible. It is also stated that even with 35% argon the maximum value of heat transfer rate on the Viking 75 entry vehicle does not exceed the design value
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Pulsation and mass loss in Mira variables
The behavior of pulsation in the outer layers of a typical Mira variable was studied in the adiabatic and isothermal limits. A shock wave propagates outward once per period and the radial velocity obtained from observations of hydrogen emission lines is identified with the velocity of gas in the post shock region. In the adiabatic case, mass loss in the form of a steady stellar wind was produced. In the isothermal case, no continuous mass loss was produced but occasional ejection of shells occur. Pulsation introduced into a star undergoing steady mass loss as a result of radiation pressure acting on grains caused the mass loss rate to increase by a factor of approximately 40, while the terminal velocity of the flow was almost unaltered
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