Re: Spectral class Y star ?

On Sep 18, 3:04 pm, Gautam Majumdar <gmajum...@xxxxxxxxxx> wrote:
What is the spectral class Y star ?
I read about classes L & T which are at a lower temperature than M.
What is the temperature range of Y class stars ?
How many have been found so far ?
I assume they are brown dwarfs - is that correct ?

Yes. Actually, the spectral class Y is at the moment
a class waiting for actual stars to fall into it. In an article
published in Annual Reviews of Astronomy and Astrophysics
in 2005, Kirkpatrick suggested that at temperatures below
750 K, the character of spectra might change from the
typical "class T" spectrum. Here's a quotation from
his paper:

Brown dwarfs of lower temperature than the coolest known T dwarf
undoubtedly exist. Spectroscopic models by Burrows, Sudarsky, & Lunine
(2003) suggest that at temperatures somewhat below the current floor
of ?750 K (see Table 3) several physical processes occur that alter
the emergent spectra. The strong optical lines of Na I and K I
disappear around 500 K, water clouds form around 400 K-500 K, the J -
K color reverses (to the red) between 300 K and 400 K, the position of
the M-band peak shifts at these same low temperatures, and ammonia
clouds form below ?160 K. Counterintuitively, though, the formation of
water and ammonia clouds has little effect on the spectral energy
distribution, and it might be one of the other effects that provides a
natural spectroscopic division between spectral classes. If, after
studying the empirical data, a new spectral type beyond T is
warranted, Kirkpatrick et al. (1999) have suggested "Y" as the new
letter designation. It is worth noting that there is no reason for the
T class to end at T9.5, either. The data may show that more T dwarf
subdivisions are needed before another natural break point is reached,
and having the T sequence extend to T11 or T15 or later before the
next new type ("Y0") is used is perfectly legitimate. Nature will let
us know.


So, at the moment, it's just an idea.





.

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