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Note: By "progenitor", I mean "the star as it was when it was still main sequence". Please correct me if I've got the terminology wrong.

Most B-type subdwarfs have mass roughly half that of the Sun. Prior to becoming blue-white subdwarfs, they went through the red giant phase, so their main sequence masses would have been between $0.3$ and $8M_{\odot}$. Since they went on to fuse helium, we can raise that lower bound to $0.5 M_{\odot}$. Since helium fusion in these stars begins with a "helium flash", then they would be $< 2M_{\odot}$ during the main sequence.

Is anything more known about the masses of main-sequence stars that can become B-type subdwarfs? The narrow range of masses observed for the subdwarfs derives from the narrow range of viable core masses. However, the papers I have read indicate that the progenitor stars may have masses in a much wider range than that of their cores. So the $0.5M_{\odot} \leq M \leq 2M_{\odot}$ range given above may be the tightest-known bounds for the mass of such a star, but if anyone is able to provide a reliable source for tighter bounds, I would be very interested.

Source for the $0.5M_{\odot}$ lower bound:

Laughlin, G., Bodenheimer, P., & Adams, F. C. (1997). The end of the main sequence. The Astrophysical Journal, 482(1), 420.

Source for the $2M_{\odot}$ upper bound:

Harpaz, A. (1993). Stellar evolution. AK Peters/CRC Press.

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  • $\begingroup$ I realise this may be a bit of a difficult question, as there doesn't seem to be much information about this in the scientific literature. (Well, not as far as I can tell.) But if someone knows the answer in the specific case of a one-solar-mass star (e.g. our Sun) I'd be particularly interested in that. $\endgroup$
    – Astrid_Redfern
    Commented Jan 28, 2020 at 8:27
  • $\begingroup$ I've been reading a paper called "The Origin of Subdwarf B Stars (I) - the Formation Channels" (Han et al., 2002, very heavily cited). In it, there are evolutionary diagrams for sdB stars with various different zero-age main sequence (ZAMS) masses. These are: 1M⊙ (becoming an 0.496M⊙ sdB), 0.8M⊙, 1.26M⊙, 1.6M⊙ and 1.9M⊙. Many actual observed sdB stars were shown to be in graph positions that might correspond to these evolutionary tracks. So I now have a sub-range of viable main sequence masses! Maybe enough to post an answer. $\endgroup$
    – Astrid_Redfern
    Commented Feb 7, 2020 at 22:13

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