Brian Murray Lewis emeritus, Arecibo Observatory 2010- Head, Radio Astronomy Group (2006-2009) Arecibo Spectrum Manager (2000 - 2010)
I joined the AO staff in February 1982. My principal interests are in single-dish research utilizing radio frequency spectroscopy. In the past this was concerned with the study of neutral hydrogen in galaxies. More recently, I have been concerned with observing masers located in the circumstellar shells generated by red giant stars in their mass-loss phase, and in what we can therefore learn about the late stages of stellar evolution.
Many of the problems posed by thick circumstellar shells about asymptotic giant branch stars become far more tractable once most shells are considered to be transient, less than 1000 year phenomena, rather than longer term (>5000 year) features. In particular I find that the 1612 MHz masing properties of the shells of high-latitude OH/IR stars are only explicable if they are both transient and cyclical features of the AGB phase of stellar evolution. Whereupon once in every thermal pulse cycle the gas density at the radius of dust formation rises past the threshold that suffices to slow dust grains down, even though they are being accelerated by stellar radiation pressure, so that photon momentum is suddenly coupled efficiently to the gas. Both its expansion velocity and the rate of mass-loss dramatically increase for ~500 years while the excess luminosity from the thermal pulse lasts. However the molecules in a newly-expanding thick shell then lose their dust protection against interstellar UV until a new denser layer of dust is again in place to protect them, so the shell loses its masers for a time. This scenario provides an explanation for the existence of most mimics (shells with the IR colors of OH/IR stars that lack 1612 MHz masers), which thus become the immediate precursors to OH/IR stars. I have seen more than 20 mimics from our 1986-1989 survey turn into OH/IR stars, aka the BIRTH of OH/IR stars, and found that five of the OH/IR stars from the Arecibo survey with a first epoch peak intensity >100 mJy are now undetectable after ~12 years, aka the DEATH of an OH/IR star or its cyclical evolution back to a Mira variable.
Secondary interests are in the field of the precision of measurements, particularly with respect to radial velocities deriving from radio-frequency spectra. One aspect of this concerns the use of better algorithms for excising rfi.
Lewis, B. M.
Direct Detection of Dust Formation Events about OH/IR Stars
in Why Galaxies Care About AGB Stars II, ASP Conf.Ser. 445, ed. Kerschbaum, Lebzelter, \& Wing 2011, p 629.
Gray, M. D., Howe, D. A., & Lewis, B. M.
Saturation and Spectral Asymmetry in 1612MHz Shell masers
MNRAS 391, 334 (2008).
Lewis, B. M.
On Modeling the NIR Two-color Locus of OH/IR Stars with a Constant dM/dt
AJ 132, 489-496 (2006).
Gray, M. D., Howe, D. A., & Lewis, B. M.
Evolution of 1612 MHz Maser emission in Expanding Circumstellar Shells
MNRAS 364, 783-795 (2005).
Lewis, B. M.
Identifying the Youngest Proto Planetary Nebulae
in "Planetary Nebulae in our Galaxy and Beyond", Proc. IAU Symposium No 234,
ed M.J.Barlow & R.H.Mendez (Cambridge: Cambridge University Press),
Deshpande, A. A., & Lewis, B. M.
Iridium Satellite Signals: A case study in interference characterization
and mitigation for Radio Astronomy observations
The physical understanding from first principles of mass-loss from AGB stars is currently a major unsolved problem in stellar evolution theory: stellar codes bypass it using ad hoc mass-loss prescriptions, which adds uncertainty to the results of their calculations concerning all subsequent stages of stellar evolution. While radiation pressure on dust is widely understood to provide the outwardly directed momentum flux needed to complete the removal of mass from the environs of a star, there is no unanimity about how to raise mass from a photosphere out to the radius where dust can form, nor, in particular, is there any agreed limit on the mass flux that a star can support. This problem provides the context of my current astrophysical work, which has two aspects; (i) the gleaning of inferences about the nature of mass loss from AGB stars afforded by observations of their masers; and (ii) the development of a physical mechanism to underwrite and specify the maximum sustainable mass flux that an AGB star can sustain.
The masers of high-latitude OH/IR stars offer some clues. They differ from the classic OH/IR stars in having a much wider distribution in galactic latitude, and so come from stars with a much lower progenitor mass. Their IR colors are in general bluer, indicating that their shells are more translucent, and their mass-loss rates are at least an order of magnitude smaller. And the statistics on the number of high-latitude OH/IR stars versus the number of associated proto planetary nebulae and mimics shows that they only have thick circumstellar shells for a few hundred years, though these recur as a cyclical phase in the evolution of these stars. Inferentially, their copious mass-loss phase is linked to the only other similarly transient development in these stars, the rise and decline in their luminosity following after a thermal pulse. So the copious mass loss mechanism is limited to the conditions that arise in the wake of a thermal pulse, higher-than-average luminosity, a longer pulsation period, and, as I believe, the occurrence only at that time of sufficient gas density in the shell to ensure that photon momentum is coupled to the shell. These matters are covered in my two transience papers.
If many OH/IR stars are transient phases in the evolution of a star, then some stars should be observed to lose their 1612 MHz masers, in effect to "die", while new OH/IR stars should emerge from shells that previously had no masers, in effect to be "born". I have found instances of both.
What of a fundamental momentum source for lifting mass away from the photosphere? AGB stars have electron-degenerate cores, with an annular shell of H-burning about them, so the core must increase in mass as it accumulates He ashes from H-burning. Since degenerate objects contract under mass accretion, this process releases both an excess of gravitational contraction energy, and an associated, spherically-symmetric, momentum flux. I find that this momentum flux is sufficient to support the largest observed rates of AGB mass-loss.
last updated: 17 January 2018