\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{5817}{7 March 2008}

\IBVStitle{Variable Star Designations for Extreme Helium Stars}

\IBVSauth{Jeffery, C. Simon}

\IBVSinsto{Armagh Observatory, College Hill, Armagh BT61 9DG,
  N. Ireland, UK}

\IBVSabs{We discuss the classifications used by the GCVS for}
\IBVSabs{pulsating hydrogen-deficient supergiants. We argue that}
\IBVSabs{the current PV Tel designation is misleading,}
\IBVSabs{and that it should be replaced by three new classes --}
\IBVSabs{V652 Her, V2076 Oph and FQ Aqr.}
\IBVSabs{The characteristics and membership of each class are well defined.}


\begintext

\noindent Variability in hydrogen-deficient Bp supergiants was first
established in BD$+13\deg3224$ = V652\,Her and HD\,160641 = V2076\,Oph 
by Landolt (1975). 
Reports of variability in other hydrogen-deficient Bp supergiants followed.
Amongst these was the detection of small-amplitude
variations in HD\,168476 = PV\,Tel by Walker \& Hill (1985). 
After establishing that several such stars were variable, 
the GCVS identified a new class with PV\,Tel as the
prototype and a definition: ``a helium supergiant Bp 
star with weak hydrogen lines and enhanced lines of He and C.
They pulsate with periods of approximately 0.1 to 1 days, or vary
in brightness with an amplitude of 0.1 mag in V during a time
interval of about a year.'' (Kholopov et al., 1985--88).
The GCVS contains twelve confirmed PV\,Tel variables, three
unconfirmed PV\,Tel variables and V652\,Her, which
``resembles PV\,Tel type stars but is a helium rich subdwarf.''

The PV\,Tel designation is useful because it identifies A- and B-type
hydrogen-deficient stars which are also variable. It presents problems
because it misrepresents the types of variability observed. 
We propose a revision of the PV\,Tel designation.\\

\noindent{\bf Why should the designation be changed?}
First, observations of the helium-rich Bp supergiant PV\,Tel (Walker \& Hill,
1985) showed evidence for light and radial velocity changes over
intervals of the order of a year. There is no doubt that 
the star is variable. However the Walker \& Hill data are also
consistent with variability on shorter timescales. Subsequent
observations have shown PV\,Tel to vary quasi-periodically
with an amplitude of about 0.1 mag on a timescale of 8 -- 10 days
(Jones et al., 1989; Lawson et al., 1993). 
Hence PV\,Tel is not a PV\,Tel variable -- according to the
definition. 

Second, the inclusion of periods of approximately 0.1 to
1 d followed the detection of variability in V652\,Her (P=0.108d,
Landolt, 1975), V2076\,Oph 0.7 - 1.1 d (Landolt, 1975; Lynas-Gray et al., 1987) 
and BD+10\deg 2179 = 0.16 d (Bartolini et al., 1982).  
The light curves of V652\,Her and V2076\,Oph are 
easily distinguished from one another, the former is strictly regular, the
latter is either multiperiodic, quasiperiodic or irregular. Which one
is not yet clear. BD+10\deg 2179
was shown to be non-variable (Hill et al., 1984; Grauer et al., 1984). 
Hence, simply changing the definition of the PV\,Tel class 
({\it e.g.} by extending the period range) fails to use available
information. A corollary would be the need for two classes of
pulsating subdwarf B star, namely EC14026=V361\,Hya and PG1716=V1093\,Her
variables, with periods of $\sim100$\,s and $\sim3000$\,s, respectively.

\newpage

\begin{table}[!ht]
\caption{Types of light variation in hydrogen-deficient supergiants.}
\begin{center}
\begin{tabular}{lp{75pt}ccp{200pt}}
\hline
V* & Other & $P[d]$ &  & Light curve \\
\hline
\multicolumn{4}{l}{\bf PV\,TEL\,I}\\
FQ\,Aqr    & BD$+1\deg$4381               &19--22&& Jeffery \& Malaney, 1985 \\
NO\,Ser    & BD$-1\deg$3438               &5--8&& Jeffery et al., 1985 \\
PV\,Tel    & HD\,168476                   &8--10&& Walker \& Hill, 1985; Jones et al., 1989 \\
V354\,Nor  & CPD$-48\deg$7730, =LSS\,3378 &10-15 && Lawson et al., 1993  \\
V2244\,Oph & LS\,IV$-1\deg$2              &10--11      && Morrison, 1987; Jones et al., 1989 \\
V4732\,Sgr & LS\,IV$-14\deg$109           & $\sim25$ &&  Lawson et al., 1993 \\
V1920\,Cyg & HD\,225642, =LS\,II$+33\deg$5& 3--4 && Morrison \& Willingale, 1987 \\
           & LSS\,4357                    & ? & & Lawson \& Kilkenny, 1998 \\
           & CoD$-46\deg11775$, =LSE\,78  & ? & & Lawson \& Kilkenny, 1998  \\[1mm]
%\multicolumn{3}{l}{UPS\,SGR}\\
Ups\,Sgr   & HD\,181616                   & $\sim21$ & SB & Malcom \& Bell, 1986 \\
KS\,Per    & HD\,30353                    & $\sim30$ & SB & Osawa et al., 1963; Morrison \& Willingale, 1987 \\
V426\,Car  & CPD$-58\deg$2721, =LSS\,1922 & $\sim20$ & SB & Morrison et al., 1987 \\
V1037\,Sco & HD\,320156, =LSS\,4300       & 15--20 & SB & Jones et al., 1989; Frame et al., 1995 \\[1mm]
\multicolumn{4}{l}{\bf PV\,TEL\,II}\\
V2076\,Oph & HD\,160641                   & 0.7--1.1 &&  Landolt, 1975; Lynas-Gray et al., 1987; Wright et al., 2005 \\
V2205\,Oph & BD$-9\deg$4395               & 3--9 && Jeffery et al., 1985, Jeffery \& Heber, 1992 \\
V5541\,Sgr & LSS\,5121                    & ? && Lawson \& Kilkenny, 1998; Woolf et al., 2001 \\[1mm]
\multicolumn{4}{l}{\bf BX\,CIR}\\
V652 Her   & BD$+13\deg$3224              & 0.1 & & Landolt, 1975 \\
BX\,Cir    & LSS\,3184                    & 0.1 & & Kilkenny \& Koen, 1995 \\[1mm]
%\multicolumn{3}{l}{Hot RCB}\\
\multicolumn{4}{l}{\bf not pulsating}\\
(V821\,Cen)& HD\,124448                   & -- & & Jeffery \& Lynas-Gray, 1990 \\
(DN\,Leo)  & BD$+10\deg2179$              & -- & & Hill et al., 1984; Grauer et al., 1984 \\
           & HD\,144941                   & -- & & Jeffery \& Hill, 1996 \\
MV\,Sgr    &                              & -- & RCB & De Marco et al., 2002 \\
DY\,Cen    &                              & -- & RCB & De Marco et al., 2002 \\[1mm]
\multicolumn{4}{l}{\bf not known}\\
           & LS\,IV$+6\deg2$              &  & & Lawson \& Kilkenny, 1998 \\
           & LSS\,99                      &  & & \\
           & BD$+37\deg442$               & & & \\
           & BD$+37\deg1977$              & & & \\
           & LSE 153                      & & & \\
           & LSE 259                      & & & \\
           & LSE 263                      & & & \\
\hline
\end{tabular}
\end{center}
\end{table}

\newpage 

\noindent{\bf Proposed variability classes.}
The variability types exhibited by helium-rich Bp
supergiants (referring to luminosity 
class, since these are all low-mass stars) can
be quite clearly divided into sub-types. For continuity, we 
propose to preserve the PV\,TEL moniker for two of these, simply adding 
the principal spectral type for each group: \\

\noindent{\bf PV\,TEL\,I: } Hydrogen-deficient A or late-B supergiants
showing low-amplitude quasi-periodic light variations on a timescale
of 5 -- 30 days; radial velocity variations are also seen. 

Theoretically, these variations are interpreted as due to radial
pulsations driven by strange-mode instability (Saio \& Jeffery, 1988). The
prototype FQ\,Aqr = BD$+1\deg4381$ (Jeffery \& Malaney 1985) is the coolest EHe,
other recognised members of the class include NO\,Ser, V354\,Nor,
V2244\,Oph, V4732\,Sgr, V1920\,Cyg, up to the hottest PV\,Tel =
HD\,168476 itself.  LSE\,78 and LSS\,4357 were reported variable by
(Lawson \& Kilkenny, 1998).

This proposed class definition also includes the hydrogen-deficient
binaries $\upsilon$\,Sgr, KS\,Per, V426\,Car and V1037\,Sco (Table 1). These
single-lined spectroscopic binaries are easily distinguished from EHes
by other means. However, all show low-amplitude light variations
in the 15 -- 30 days range.

\noindent{\bf PV\,TEL\,II:} Hydrogen-deficient O or early-B supergiants
showing low-ampli\-tude quasi-periodic light variations on a timescale
of 0.5 -- 5 days; radial velocity and line-profile variations are also
seen.  

Theoretically, these variations are interpreted as
non-radial g-mode pulsations driven by strange-mode instability
(Saio \& Jeffery, 1988), since ``periods'' are much longer than the dynamical
timescales in these stars. The prototype V2076\,Oph = HD\,160641
(Lynas-Gray et al., 1987) is the hottest EHe, other members of the class are
V5541\,Sgr (Lawson \& Kilkenny, 1998; Woolf et al., 2001) and V2205\,Oph = BD$-9\deg$4395
(Jeffery et al., 1985).\\

The third sub-type is quite distinct, and its designation should reflect this:\\

\noindent{\bf BX\,CIR:} Hydrogen-deficient B stars showing
low-amplitude variations in light (0.1 mag in V) and radial velocity
with a unique and regular period of a few hours. 

Theoretically, we ascribe the variation to radial pulsations
driven by the $\kappa$ mechanism through Z-bump instability (Saio,
1993). 
The class comprises V652\,Her and BX\,Cir. Unlike the PV\,TEL variables
described above, the pulsations are so regular that both stars can
be used as clocks (Kilkenny et al., 2005). 

V652\,Her was originally classified as a helium-rich subdwarf; in fact its
gravity is lower than that of the main-sequence (Jeffery et al., 2001), 
so it is a  helium-rich Bp giant, with a well-defined period 
(as well as $\dot P$ and $\ddot P$, Kilkenny et al., 1982, 1984, 1996) 
and an amplitude of 0.1 mag. \\


A minority of EHes are established as {\it not} varying on short
timescales, including BD$+10\deg2179$
(=DN\,Leo), HD\,124448 (=V821\,Cen)  and the metal-poor HD144941
(Grauer et al., 1984; Hill et al., 1984; Jeffery \& Lynas-Gray, 1990; 
Jeffery \& Hill, 1996). These stars lie below the
instability boundary for the appropriate metallicity (Fig.\,1)
(Jeffery \& Saio, 1999).  Variability in LS\,IV$+6\deg2$ and LSS\,99 has not
been confirmed. The hot R Coronae Borealis (RCB) 
stars DY\,Cen and MV\,Sgr strongly
resemble EHe stars LSE\,78 and HD\,124448 respectively
(Jeffery \& Heber, 1993; Jeffery et al., 1988). 
They showed strong RCB-like variability in
the past, but are not currently known to vary on short timescales
(De Marco et al., 2002).

Table 1 summarises the principal variability characteristics for all
known EHe stars. Approximate timescales $P$ are given for PV\,TEL 
variables, but the term is used loosely. 
Major references are given for the definition of the 
light curve types and the estimate of periods.
The proposed classes are sensible for several reasons. They 
can be simply established from the timescale,
light curve and amplitude of the variation and from the spectral 
class of the variable (which was already required for the PV\,TEL 
classification). Figure 1 shows the $log\ g-log\ T_{\rm eff}$ diagram for 
extreme helium stars, including the
position of the Eddington limit (assuming Thomson scattering: dashed)
and the loci of stars with given luminosity-to-mass ratios (solar
units: dotted). Stars above the boundaries shown for metallicities
$Z=0.004, 0.01, 0.03$ (dot-dash) are predicted to be unstable to
pulsations (Jeffery \& Saio, 1999). 
Ellipses (coloured in electronic version) identify three groups of
pulsating helium stars. In the
electronic version, PV\,TEL\,I variables are shown in purple and red 
(H-def binaries), PV\,TEL\,II variables in blue, and BX\,CIR variables in 
green. Non-variables are black. The values and sources for $T_{\rm eff}$
and $log\ g$ 
are given in Table 2. There has to be some question about the high
value of $\log\ g$ for NO\,Ser.  

\begin{table}[!ht]
\caption{Surface properties for O, B and A type extreme-helium stars}
\begin{center}
\begin{tabular}{lp{75pt}ccl}
\hline
V* & Other & $T_{\rm eff}$ & $\log g$ & Reference \\
\hline
\multicolumn{5}{l}{\bf PV\,TEL\,I}\\
FQ\,Aqr    & BD$+1\deg$4381               &  8750$\pm$300 &  0.30$\pm$0.30&  Pandey et al., 2006 \\
V4732\,Sgr & LS\,IV$-14\deg$109           &  9500$\pm$250 &  1.00$\pm$0.20&  Pandey et al., 2006 \\
V354\,Nor  & CPD$-48\deg$7730, =LSS\,3378 & 10400$\pm$500 & *1.00$\pm$0.50&  Pandey \& Reddy, 2006\\
NO\,Ser    & BD$-1\deg$3438               & 11750$\pm$250 &  2.30$\pm$0.40&  Pandey et al., 2001 \\
V2244\,Oph & LS\,IV$-1\deg$2              & 12750$\pm$250 &  1.75$\pm$0.25&  Pandey et al., 2001 \\
PV\,Tel    & HD\,168476                   & 13750$\pm$400 &  1.60$\pm$0.25&  Pandey et al., 2006 \\
           & LSS\,4357                    & 16130$\pm$500 &  2.00$\pm$0.25&  Jeffery et al., 1998 \\
V1920\,Cyg & HD\,225642, =LS\,II$+33\deg$5& 16300$\pm$900 &  1.70$\pm$0.35&  Pandey et al., 2006 \\
           & CoD$-46\deg11775$, =LSE\,78  & 18300$\pm$400 &  2.20$\pm$0.20&  Pandey et al., 2006 \\[1mm]
%\multicolumn{3}{l}{UPS\,SGR}\\
Ups\,Sgr   & HD\,181616                   & 11750$\pm$750 &  1.5 $\pm$0.5 &  Dudley, 1992 \\
KS\,Per    & HD\,30353                    &	          &		  &  \\
V426\,Car  & CPD$-58\deg$2721, =LSS\,1922 & 14000$\pm$800 &  1.25$\pm$0.5 &  Morrison, 1987 \\ % PhD thesis
V1037\,Sco & HD\,320156, =LSS\,4300       & 14500$\pm$800 &  1.4 $\pm$0.5 &  Sch\"onberner \& Drilling, 1984\\[1mm] %ApJ...276..229S
\multicolumn{5}{l}{\bf PV\,TEL\,II}\\   
V2205\,Oph & BD$-9\deg$4395               & 22700$\pm$1200&  2.55$\pm$0.10&  Jeffery and Heber, 1992 \\
V5541\,Sgr & LSS\,5121                    & 29800$\pm$1830& *3.00$\pm$0.50&  Jeffery et al., 2001 \\
V2076\,Oph & HD\,160641                   & 34000         &  2.80         &  Rauch, 1996 \\[1mm]
\multicolumn{5}{l}{\bf BX\,CIR}\\
BX\,Cir    & LSS\,3184                    & 23390$\pm$90  &  3.38$\pm$0.02&  Woolf and Jeffery, 2002 \\
V652 Her   & BD$+13\deg$3224              & 24550$\pm$500 &  3.68$\pm$0.05&  Jeffery et al., 1999 \\[1mm]
%\multicolumn{3}{l}{Hot RCB}\\
\multicolumn{5}{l}{\bf not pulsating}\\
MV\,Sgr    &                              & 16000$\pm$500 &  2.48$\pm$0.30&  Jeffery et al., 1988 \\
(V821\,Cen)& HD\,124448                   & 16100$\pm$300 &  2.30$\pm$0.25&  Pandey et al., 2006 \\
(DN\,Leo)  & BD$+10\deg2179$              & 16900$\pm$500 &  2.55$\pm$0.20&  Pandey et al., 2006 \\
DY\,Cen    &                              & 19500$\pm$500 &  2.15$\pm$0.10&  Jeffery \& Heber, 1993 \\
           & HD\,144941                   & 23200$\pm$500 &  3.9 $\pm$0.1 &  Harrison \& Jeffery, 1997 \\[1mm]
\multicolumn{5}{l}{\bf not known}\\
           & LSS\,99                      & 15330$\pm$500 &  1.90$\pm$0.25&  Jeffery et al., 1998 \\
           & LS\,IV$+6\deg2$              & 31800$\pm$800 &  4.05$\pm$0.10&  Jeffery, 1997 \\
           & BD$+37\deg442$               & 53000         &  4.0          &  Heber et al., 1987 \\
           & BD$+37\deg1977$              & 56000         &  4.1          &  Darius et al., 1979 \\ %IUE conference
           & LSE\,153                     & 70000$\pm$2000&  4.75$\pm$0.15&  Husfeld et al., 1989 \\
           & LSE\,259                     & 70000$\pm$3000&  4.9 $\pm$0.25&  Husfeld et al., 1989 \\
           & LSE\,263                     & 70000$\pm$7000&  4.4 $\pm$0.3 &  Husfeld et al., 1989 \\
\hline
*: estimated\\
\end{tabular}
\end{center}
\end{table}


\IBVSfig{12cm}{5817-f1.eps}{$log\ g-log\ T_{\rm eff}$ diagram for H-deficient  variable stars}
\IBVSfigKey{5817-f1.eps}{}{}

Each type of variable
has a different physical mechanism, and occupies a separate region of the
Hertzsprung-Russell or $log\ g-log\ T_{\rm eff}$ diagram. 
Since these classes have been
well established for over a decade (Saio \& Jeffery, 1988; Saio, 1993), 
a revision to the single PV\,TEL designation is overdue. The suggested division 
into classes PV\,TEL types I and II and BX\,CIR reflects current knowledge.

\newpage
\noindent{\bf Cool H-deficient variables.}
We conclude with a remark on designations for some related variables.
Theoretically speaking (Saio \& Jeffery, 1988), there is a probable physical connection 
between PV\,TEL-type variations described above and the $\sim40$\,d pulsations 
observed in cool RCB variables such as RY\,Sgr (Alexander et al., 1980) and the late-type carbon-rich 
hydrogen-deficient giants such as HM\,Lib = HD\,137613 (Kilkenny et al., 1988; Jones et al., 1989). 
RCB-type minima are definitive and, coupled with an appropriate spectral type, 
the designation needs little modification. Indeed, the RCB definition includes ``cyclic 
pulsations with amplitudes up to several tenths of a magnitude and periods in the range
30-100 days''. 
Of those hydrogen-deficient giants which do not show RCB-type minima, 
the GCVS tentatively describes HM\,Lib as a semi-regular variable (``SR:''). Three other variables 
are LV\,Tra = HD\,148839, V4152\,Sgr = HD\,175893, and HD\,173409 (Kilkenny et al., 1988; Jones et al., 1989). 
Only HD\,182040 has {\it not} been proven to vary on these timescales (ibid.). 
Radial velocity variations have been observed on the same timescales (Lawson \& Cottrell, 1997).
While LV\,TrA is classified ``RCB:'', it is not clear that it has ever shown a deep RCB-type minimum.
Meanwhile, there is increasing evidence for 
an evolutionary connection between all of these groups of stars (Saio
\& Jeffery, 2002; Pandey et al., 2006; Clayton, 2008).  Perhaps the PV\,TEL 
designation should be further extended to include these cool giants:\\[1mm]
\noindent{\bf PV\,TEL\,III: } Hydrogen-deficient and carbon-rich F or G supergiants
showing low-amplitude quasi-periodic light variations on a timescale
of 30 -- 100 days; radial velocity variations are also seen.

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\end{document}

