Sky & Telescope - March 2021 - 33

ESO

Hot RCBs
Most RCB stars are cool, with temperatures in the range of 5000 to
7000 kelvin. (By comparison, main
sequence stars range in temperature
from 2000K to 50,000K, with our
Sun registering a surface temperature of about 5800K.) However, four
RCBs are considerably hotter, with
temperatures ranging from 15,000K
to 25,000K. This quartet of hot RCB
stars (as they are collectively known)
consists of V348 Sagittarii, MV Sagittarii, DY Centauri (all three in our
galaxy), and HV 2671 (in the LMC).
In 2002, Orsola De Marco (now
at Macquarie University, Australia)
and her colleagues noted that the maximum brightness of the
three hot Milky Way RCBs has declined over the last century.
Five years ago, Bradley Schaefer (Louisiana State University)
extended the study to include HV 2671 in the LMC. Schaefer
also used photographic plates stored at Harvard College
Observatory (covering the period from 1896 to 1989) to compile more complete light curves for all four hot RCBs. He was
able to conﬁrm that the stars were fading at a rate of around
one magnitude per century.
De Marco also noted that the fading was related to the
stars evolving to hotter temperatures. This heating up has
been observed in DY Centauri, whose spectra show that the
star's surface temperature has increased from 19,400K in
1987 to 24,800K in 2010. Schaefer modeled DY Centauri's
evolution based on its changing brightness and determined
that in 1906 the star was a normal RCB with a temperature
of around 5800K. From there, its temperature soared to
its present value. Interestingly, DY Centauri hasn't experienced any brightness drops since 1934, suggesting that the
increased temperature shut down the mechanism that generates the star-dimming dust. Schaefer concludes that the time

W DUST PUFF In 2003, the European
Southern Observatory's Very Large Telescope observed the RCB star RY Sagittarii
at a wavelength of 4 microns to produce
this image showing large clouds in the
envelope of dust surrounding the star.
This study helped confirm the " Dust Puff
Theory " that explains the star's periodic
dimming as a result of material expelled by
the star itself.

required for a normal RCB to evolve
into a hot RCB is about one century,
which explains why so few hot RCB
stars are known.

R Coronae Borealis's
Recent Performance
Although observers have watched it
closely for more than two centuries, R Coronae Borealis can
still surprise. Following a period of relatively steady brightness, in 2007 the star began to fade in an unprecedented
manner. It ﬁrst dropped to its usual minimum of around 14th
magnitude, then hovered there for several months, before
dimming even further beginning in 2008. In early 2009 it
reached an all-time low of 15th magnitude - and it wasn't
done yet. The star's return to maximum was also unusual.
The star climbed to 12th magnitude in late 2011, before
dipping again to 14th magnitude. Then, as 2011 was ending,
the star resumed its recovery, this time nearly reaching 10th
magnitude in early 2013 before plummeting a few months
later to just a little brighter than 15th magnitude. R Coronae Borealis began to brighten yet again, this time punching
through to 10th magnitude in late 2014 for the ﬁrst time
since 2007. It ultimately brightened to just better than 7th
magnitude in 2015 before once again falling to 14th magnitude in early 2016. Soon, its long road to recovery began
in earnest and the star eventually reached 6th magnitude in
mid-2018 - some nine years after this incredible sequence of
events began.

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Sky & Telescope - March 2021

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