ASTEROSEISMOLOGY OF THE NEARBY SN II PROGENITOR RIGEL. II. Ε-Mechanism TRIGGERING GRAVITY-MODE PULSATIONS?

Abstract

The cores of luminous B and A-type (BA) supergiant stars are the seeds of later core collapse supernovae. Thus, constraining the near-core conditions in this class of stars can place tighter constraints on the size, mass and chemical composition of supernova remnants. Asteroseismology of these massive stars is one possible approach into such investigations. Recently, Moravveji et al. (2012, hereafter Paper I) extracted 19 significant frequencies from a 6-year radial velocity monitoring or Rigel (β Ori, B8 Ia). The periods they determined broadly range from 1.22 to 74.74 days. Based on our differentially rotating stellar structure and evolution model, Rigel, at its current evolutionary state, is undergoing core He burning and shell H burning. Linear fully non-adiabatic non-radial stability analyses result in the excitation of a dense spectrum of non-radial gravity-dominated mixed modes. The fundamental radial mode (ℓ = 0) and its overtones are all stable. When the hydrogen burning shell is located even partially in the radiative zone, a favorable condition for destabilization of g-modes through the so-called ǫ-mechanism becomes viable. Only those g-modes that have high relative amplitudes in the hydrogen burning (radiative) zone can survive the strong radiative damping. From the entire observed range of variability periods of Rigel (found in Paper I), and based on our model, only those modes with periods ranging between 21 to 127 days can be theoretically explained by the ǫ-mechanism. The origin of the short-period variations (found in Paper I) still remain unexplained. Because Rigel is similar to other massive BA supergiants, we believe that the ǫ-mechanism may be able to explain the long-period variations in α Cygni class of pulsating stars.

Stellar oscillations - II - The non-adiabatic case

“…Shibahashi and Osaki (1976) suggested the possible excitation of g-modes due to the ε-mechanism at the H-burning shell in post-main sequence massive stars. Recently, Moravveji et al (2012) proposed that excitation of a g-mode appearing in a B supergiant, Rigel, observed by MOST could be explained by this mechanism. Theoretical works have suggested that pre-white dwarfs also have a possibility to exhibit pulsations excited by the ε-mechanism at the He-burning shell (Kawaler et al, 1986;Gautschy, 1997).…”

Variability of massive stars with known spectral types in the Small Magellanic Cloud using 8 years of OGLE-III data

“…either the luminosity classes of these irregular supergiants may have been overestimated or a mechanism other than fast rotation must be invoked, such as dipole magnetic fields along with non-radial g-mode pulsations (Markova et al 2008;Saio 2011), which have been reported in the B supergiant region (Saio et al 2006;Lefever et al 2007;Moravveji et al 2012). Table 10 presents the light curves of all irregular and periodic variables discussed in the previous sections and is available in its entirety at the CDS.…”

The variable stellar wind of Rigel probed at high spatial and spectral resolution

“…Interestingly, the 2009−2010 AMBER observations performed from the modified Julian date (defined as Julian date -2 400 000.5, hereafter MJD) MJD 55 139 until MJD 55 300 partially overlap with the global Rigel monitoring campaignknown as the "Rigel-thon" -involving long-term spectroscopic monitoring, Microvariability and Oscillations in STars (MOST) space photometry, and spectropolarimetry (Moravveji et al 2012a;Shultz et al 2011). Owing to the brightness of the star, stringent constraints on the possible existence of magnetic field were provided up to a limit of the individual measurements as low as 13 G and exclude large dipolar fields in the range of 20−50 G. The MOST data also fostered some theoretical developments on the asteroseismology of this star (Moravveji et al 2012b). …”