06/28/2013:
Hi again, Terry . . . OK, the SN origin of the TiO abundances is clear now. I figured your ‘M’ identification probably referred to Class VII dwarfs and not the similarly coloured Class I to II supergiant RGs. The sawtooth-shaped diffuse lines and lower luminosity than the K star pointed to a dim M dwarf, not an M supergiant 6+ mags brighter and sporting a multitude of crisp skinny lines.
Alas, I now have even more questions than I started with. (I understand this business of one answer leading to two questions started around the time of Archimedes and has led mainly to the building of ever-larger telescopes.) If Ti and other heavy metals, e.g., VaO, are abundant via accretion from earlier SN ejecta, do these metals convect into the M star’s interior? I ask because M stars are wholly convective <0.35 Msolar, which lifts He as high as the photosphere and would show up as He lines in your spectrum. Given the line broadening I can’t identify any, compared with the Lalande 21185 and Wolf 359 sequences in the 3900-7100 nm range. If accreted metals are convected inward the same way He is convected to the surface, the cores of such stars would expand as opacity and temp increases, thereby shutting off the transport mechanism. Would you happen to have your M dwarf’s B-V and/or U-B color indexes? I am curious about the star’s He lines because if they are present it would point to a Msol of <0.25 and it would end up a He dwarf.
Unfortunately the 04/2013 Walker ‘Guide to Spectral Classes’ in your link provides two misleading spectra under the rubric ‘Spectral Class M’ starting p.87. The text refers to stars Msol 0.5 down to 0.08. but the spectra in Table 60 are two red supergiants, Antares and Alpha Herc. Walker doesn’t provide a spectrum for an M dwarf; I looked one up in
Rojas-Ayala et al 2011.
On the other hand, the TiO bands in the Alpha Herc spread between 4000 and 7000 point to TiO’s impressive ubiquity. To look at the r-hand side of Chart 60, one could conclude the whole dadgum star is hot TiO.
All this is fascinating. Thanks for the tip that led to the quest. Somehow Archimedes doesn’t seem quite so ancient any more.
=Dana in SA