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\IBVShead{5927}{8 March 2010}

\IBVStitletl{A Proposed Uniform Nomenclature for Pulsating}{ Hot Subdwarf Stars}

\IBVSauth{KILKENNY, D.$^1$; FONTAINE, G.$^2$; GREEN, E. M.$^3$; SCHUH, S.$^{4,5}$}

\IBVSinst{Department of Physics, University of the Western Cape, Private Bag
X17, Bellville 7535, South Africa.\\ 
e-mail: dkilkenny@uwc.ac.za}

\IBVSinst{D\'epartement de Physique, Universit\'e de Montr\'eal, Montr\'eal,
Qu\'ebec H3C 3J7, Canada.\\ 
e-mail: fontaine@astro.umontreal.ca}

\IBVSinst{Steward Observatory, University of Arizona, Tucson AZ85721, USA.\\ 
e-mail: bgreen@as.arizona.edu}

\IBVSinst{TEA Visiting Professor, Kepler Center for Astro and Particle Physics,
Institut f\"ur Astronomie und Astrophysik,
Eberhard-Karls-Universit\"at T\"ubingen, Sand 1, 72076 T\"ubingen, Germany.}

\IBVSinst{Georg-August-Universit\"at G\"ottingen, Institut f\"ur Astrophysik,  
37077 G\"ottingen, Germany.\\
e-mail: schuh@astro.physik.uni-goettingen.de}

\IBVSabs{The current status of the nomenclature for pulsating hot subdwarf}
\IBVSabs{stars is undecided and confusing. We propose a uniform and simple}
\IBVSabs{system analogous to that used for pulsating white dwarf stars and}
\IBVSabs{consistent with existing usage for other variable types.}         


\begintext

\vspace*{4mm}

The first rapidly-pulsating sdB stars were found accidentally in the
mid-1990s (Kilkenny et al., 1997 and following papers). At the same time --
and independently -- the Montreal group was showing that these stars should
pulsate (see the review by Charpinet~et~al.,~2001). Over 40 such stars are
now known; they are $p$-mode pulsators with periods~$\sim 2 - 5$~minutes,
though periods as long as 9 minutes are known. They can exhibit anywhere
from one to over 40 pulsation modes and occur amongst the hotter sdB stars
with \hbox{28000 $< T_{\rm eff} <$ 35000} and 5.2 $< {\rm log}~g <$ 6.1.

The slowly-pulsating sdB stars were also discovered serendipitously during a
search for sdB binaries by looking for eclipses, ellipsoidal and reflection
effects (Green et al.,~2003). Over 30 slow pulsators are now known, though it
is possible that a large fraction of the cooler sdB stars might pulsate.
They are $g$-mode pulsators and typically have \hbox{periods~$\sim 1 - 2$ hours.}
Like the rapid pulsators, they are multi-periodic but occur amongst sdB
stars with $T_{\rm eff} <$ 27000 and ${\rm log}~g$ $\sim$ 5.4, and there
appears to be a good separation between the rapidly- and slowly-pulsating
sdBs in a $T_{\rm eff}/{\rm log}~g$ diagram (see Fig.~3 in Schuh~et~al., 2006,
for example).

An exciting discovery was that some sdB stars exhibit {\it both} $p$- and
$g$-mode pulsations (Schuh et al., 2005; Oreiro et al., 2005). The
oddly-named Balloon 090100001 exhibits many modes; Baran et al. (2009)
recently listed 73 $p$-modes in the range $2800 - 5500 \mu$Hz, 24 $g$-modes
in the range $100 - 400 \mu$Hz, and 17 combination frequencies. A
handful of these stars are now known and all lie on the temperature boundary
between rapidly- and slowly- pulsating stars.

The first (and only known) variable helium-rich sdB star,
LSIV--14\deg116, was found by Ahmad \& Jeffery (2005). From the
discovery observations, these authors find two periods -- 1950s and 2900s
(amplitudes $\sim$ 0.004 mag) -- and suggest that these are $g$-modes. This
is in accord with the long periods, but the star has $T_{\rm eff}$ = 32500~K
which puts it in the rapidly-pulsating zone (for normal sdB stars). However,
recent sdB models indicate that $g$-modes should be stable at this
temperature (Fontaine et al., 2003).

Most recently, the discovery was announced of the first (and only known)
pulsating sdO star, SDSS~J160043.6+074802.9 (Woudt et al., 2006). Variability
was discovered (again serendipitously) during a search for new AM~CVn stars
amongst Sloan Digital Sky Survey stars of appropriate colour. This star
shows a strong 2 minute oscillation (of large amplitude $\sim$ 0.04 mag)
with a clear first harmonic near 1 minute. Woudt et al. (2006) find at least
another 8 frequencies between the main oscillation and its harmonic and show
that, spectroscopically, the star is a classical sdO star.

\vspace*{2mm}

The relatively rapid discovery of a new genus -- the pulsating hot subdwarfs
-- comprising several species, sdB (slow, fast and ``hybrid''), He-sdB and
sdO -- has resulted in a confusion over nomenclature. The rapidly-pulsating
sdB stars have been widely referred to as ``EC14026 stars'' after the
prototype, and ``sdBV stars''; the compilers of the General Catalogue of
Variable Stars tentatively labeled them ``RPHS'' (very rapidly pulsating
hot (subdwarf B) stars) but that name was never used -- and the GCVS had
already asked for suggestions for a better designation (Kazarovets, Samus \&
Durlevich, 2000). The slowly-pulsating stars have been called ``PG1716
stars'' after the prototype, or ``Betsy stars'' after the discoverer, Dr.
Elizabeth M. Green, and also ``lpsdBV'' stars -- where the ``long-period''
(lp) serves to separate these objects from the (rapidly-pulsating) sdBV
stars.  The sdB stars which show both rapid and slow pulsations -- all
fairly recent discoveries --have been widely referred to as ``hybrid''
pulsators. There is no clearly standardised nomenclature and this lack was
raised for discussion at the Vienna pulsation meeting (Kilkenny, 2007) and
the third and fourth meetings on pulsating hot subdwarfs (Fontaine et al.,
2008; Kilkenny, 2009).

Table 1 summarises the problem and suggests a solution. It is common usage
in variable star research to use ``prototype'' names -- Cepheids, Miras,
$\delta$ Scuti stars, and so on. In some cases, these are also the prototype
{\it variable} star name -- RR Lyrae stars, for example. Using prototype
names for the hot subdwarf variables would, in some cases, be terribly
unwieldy (e.g. ``SDSS~J160043.6+074802.6 stars'') and prototype variable
star names are less easy to remember in the cases of these late arrivals on
the variable star stage (e.g~``V1093~Her stars''). The informal names
(``EC14026'' and ``Betsy'' stars) have been a pleasant way of linking the
stars to their discoverers but perhaps should now be replaced by a more
systematic nomenclature.
 
By analogy with the white dwarf stars (DAV, DBV, etc.), we suggest that the
simplest expedient is to add ``V'' to the spectral designation to indicate 
a {\it photometric} variability. One problem
is that we have effectively three different types of pulsators within the
sdB class. It is here suggested that we add the subscripts ``r'',``s'' or
``rs'' -- for rapid, slow and hybrid pulsators. The subscripts need not be
added to the He-sdBV or sdOV designations unless new discoveries make this
useful. (We suggest subscripts, rather than straightforward letters because
these qualify the ``V'' designation, rather than the spectral type).

The use of ``r'' and ``s'' is, perhaps, not optimum; ``fast'' is a more
direct antonym of ``slow'' than ``rapid'' -- and ``s'' could be
misinterpreted as short (period) rather than slow (pulsation). But the
usage suggested in Table 1 already exists in variable star nomenclature
in the form of the rapidly-oscillating Ap stars (roAp) and the
slowly-pulsating B stars (spB) and is therefore more appropriate. 


\vspace*{5mm}

\centerline{Table 1. Photometrically variable Subdwarf Types.\label{tbl-1}}
\vskip 3mm
\begin{center}
\begin{tabular}{lllll}
\hline

Pulsator       &   Prototype  & Variable  & Informal  &  Proposed      \\
Type           &   Name       & Star Name & Usage     &  Nomenclature  \\
\hline
sdB (rapid)&  EC~14026-2647 \,\, = & V361 Hya  & EC14026   &  sdBV$_{\rm r}$ \\
($p$-mode) &                       &           & sdBV      &  \\
\hline
sdB (slow) &  PG~1716+426 \,\,\, = & V1093 Her & PG1716    &  sdBV$_{\rm s}$ \\
($g$-mode) &                       &           & ``Betsy'' &  \\
           &                       &           & lpsdBV    &  \\
\hline
sdB (both) &  HS 0702+6043 \,\,  = & DW Lyn    &``Hybrid'' &  sdBV$_{\rm rs}$\\
($p$ and $g$)&                     &           &           &  \\
\hline
He-sdB     &LSIV --14\deg116 &           &           &  He-sdBV    \\
($g$-mode ?)&                      &           &           &  \\
\hline
sdO        &SDSS J160043.6+074802.9&           &           &  sdOV       \\
($p$-mode) &                       &           &           &  \\
\hline
\end{tabular}
\end{center}

\vspace*{5mm}



{\bf Acknowledgment:} D.K. acknowledges support from the National Research
Foundation of South Africa.


\vskip 1cm

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\endreferences

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