YHGs
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Yellow HyperGiants (Definition: see wiki)

　

General properties of YHGs

see the review of YHGs by de Jager, 1998A&ARv...8..145D.
YHGs are massive stars with 10 Msun < M < 50 Msun that are either evolving towards RSG phase after they have exited the main sequence or evolved off RSGs towards WR stars. They are located in a narrow temperature range in the HR diagram between luminous blue variables (LBVs) and RSGs. There is a region between YHGs and LBVs in the HR diagram called yellow void, because massive stars only fast cross this region in the end of their YHG evolution. The evolution from RSG to WR stars is typically a few handred to a few thousand year. (More massive stars >50 Msun will directly go through BSG or LBV to WR stars.)
The heavy mass loss of YHGs are episodic: When a star evolve off RSG and its surface temperature reach ~ 7000 K, it abruptly ejects mass at a very high rate due to thermal instability and forms an extended cool pseudo-photosphere. The star turns back to RSG direction. Then the ejected mass dissipate into space and the cool pseudo-photosphere is cleared, the star turns back to evolve bluewards. This is the so-called 'bouncing against the Yellow Void'. After several cycles of bouncing back, the star eventually evolves into LBV and WR star.
It is not easy to observationally tell pre-RSG from post-RSG YHG stars. Some helpful criteria are: post-RSGs will have their surface enriched by CNO processed material, often also traced by their Na overabundance; thay have H deficient photosphere; they have the remnent of RSG mass loss shell.
Mass loss rate can be as high as 10-4 to 10-3 Mo/yr, usually much larger than that of AGB stars and they present significant variations on time a scale of ~1000 yr. (Castro-Carrizo et al., 2007A&A...465..457C)
Some other interesting phenomena could be related to YHGs: 1) ring nebula around WR stars; 2) aspherical Supernovae ejecta; 3) beamed Gamma Ray Bursts.

Examples of YHGs

  1. AFGL 2343 -- 
  2. HD 179821 -- It is one of the two YHGs showing large IR excess.
  3. HR 8752 -- a YHG showed two bounces across the Yellow Void in HR diagram.
  4. HR 5171A -- well known YHG.
  5. IRC +10420 -- a YHG showing thick extended bipolar CSE. It is one of the two YHGs showing large IR excess.
  6. rho Cas -- a famous YHG showing fast change of temperature from 7000K to 4000K within a few hundred days and back to 7000K in roughly the same amount of time. (from Lobel et al., 2003ApJ...583..923L)
  7. Var A -- a YHG showing fast variation of spectral type from F to M and back to F in 45 years in the galaxy M33. (Humphreys et al., 2006AJ....131.2105H)

　

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