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\IBVShead{5964}{20 January 2011}

\IBVStitletl{Photometric Variability of the Chemically Peculiar}{ Hot Subdwarf LS\,IV$-$14\deg116}

\IBVSauth{Jeffery, C. Simon}

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

\SIMBADobjAlias{LS IV -14 116}{ALS 11634}

\IBVSabs{We present follow-up photometry of the variable helium-rich and}
\IBVSabs{extremely chemically peculiar subdwarf B star LS IV-14deg 116.}
\IBVSabs{These data confirm the discovery observations, and point to multiple}
\IBVSabs{periodicity with principle periods around 1900 and 3000 s.}
\IBVSabs{Further data are urgently required to secure these results. }


\begintext

\vspace*{-0.8cm}

\noindent  LS IV$-$14\deg116 was first catalogued as a luminous
star by Nassau \& Stephenson (1963). Kilkenny \& Pauls (1990) classified it 
as an O-type subdwarf (sdO), while Viton et al. (1991) described it 
as an He-rich sdO star.  Ahmad \& Jeffery (2005)
found the star to be photometrically variable with
periods in the 30 -- 90 minute range, and amplitudes of $\approx
0.003$ mag. The star currently constitutes the sole member of the class of
He-sdBVs (Kilkenny et al., 2010). Its variability is not reconciled
with any theory of pulsational instability; g-modes would be indicated
by the period, but are inconsistent with theoretical models of 
g-mode excitation in hot subdwarfs (Jeffery \& Saio, 2006). 

Although showing significantly more helium ($n_{\rm He} = 0.21$, Ahmad
\& Jeffery, 2003) than the majority of sdB stars,  LS IV$-$14\deg116
is not as extremely helium-rich as a few.
Analysis of the spectrum at high-resolution has revealed a 
super-abundance ($\approx
4$ dex) of zirconium, strontium, yttrium and germanium (Naslim et
al., 2011), suggesting a heavily stratified atmosphere in which these
particular elements have accumulated in the line-forming region. 
The question arises as to whether the photometric variability is in
any way connected to the extreme chemical peculiarity of this star.

%----------------------------
\begin{table}[h]
\caption{Photometric observations of LS\,IV$-$14\deg116.}
\label{t_obs_ls4}
\begin{center}
\begin{tabular}{ccccrr}
\hline
\hline
Night & Date	    &UT Start & $t_{\rm exp}$ & $N_{\rm obs}$ \\
\hline
N1 & 2005 06 15 &  23:00:43 & 10 & 1505 \\ %% 53536.9588310185 1575-70
N2 & 2005 06 16 &  22:21:49 & 10 & 2093 \\ %% 53537.9318171296 2163-70
N3 & 2005 06 17 &  22:19:18 & 15 &  336 \\ %% 53538.9300752315  406-70
N4 & 2005 06 19 &  22:42:46 & 15 & 1459 \\ %% 53540.9463715278 1529-70
\hline
\end{tabular}
\end{center}
\end{table}
%-----------------------------

\vspace*{-0.2cm}

The very low-amplitude variations detected in the discovery 
data demanded confirmation. Additional observations were
obtained with the South African Astronomical Observatory 1.0m
telescope in 2005 June, using the University of Cape Town (UCT)
high-speed CCD camera operated in `frame-transfer' mode.  
Although the weather was not perfect, approximately 12.5 hours of data, 
and 5393 images  were obtained (Table 1). The field size was the same as 
reported by Ahmad \& Jeffery (2005), so that only one star was available as
a useful comparison  (GS2.2: S331330313746, R=14.1). A third 
star was too faint to provide a satisfactory check of the photometry. 
Regrettably, an unmarked improvement in seeing led to 
approximately 1300 frames obtained on the night of  2005~June~16 
being saturated. These frames were
easy to identify and discard during analysis of the light curve. 
The observer was duly chastened.

\IBVSfig{10.8cm}{5964-f1.eps}{Differential photometry of LS IV$-$14\deg116 
from June 2005.}
\IBVSfigKey{5964-f1.eps}{LS IV-14deg 116}{light curve}

The data were reduced using the {\sc ultracam} data reduction pipeline software
(Dhillon \& Marsh, 2001), extracted to differential magnitudes, and normalised ({\it
  i.e.} corrected such that $\langle V-C\rangle =0$). Because the comparison star is
substantially redder then LS~IV$-$14\deg116, 
differential extinction is significant ($>0.04$ mag/airmass), so data for each night were
detrended by subtracting a third order polynomial fit.  
The final differential light curve is shown in Fig.~1, 
where variations of up to $\pm0.01$mag. are clearly visible. 

\IBVSfig{8cm}{5964-f2.eps}{Scargle periodogram and window function (inset)
  for LS IV$-$14\deg116 from June 2005.}
\IBVSfigKey{5964-f2.eps}{LS IV-14deg 116}{power spectrum}

\begin{table}[!ht]
\vspace*{-1cm}
\caption{Frequencies}
\begin{center}
\begin{tabular}{cccc}
\hline
$f$/mHz & $a$/mag & ($f_{2004}$) & ($a_{2004}$) \\
0.2908 & 0.0027 &  & \\
0.2011 & 0.0018 &  & \\
0.3368 & 0.0019 &  0.3484 & 0.0019 \\
0.5203 & 0.0018 &  0.5119 & 0.0021 \\
\hline
\end{tabular}
\end{center}
\end{table}

The Scargle power spectrum and window function for the entire dataset 
are shown in Fig.~2. The power spectrum resembles that obtained in 2004 (Ahmad \& Jeffery, 2005) in
so far as there is power at around 0.34 mHz and at 0.52 mHz in both
cases. Any power at $f<0.05$mHz has been removed by the detrending procedure. 
Best-fit frequencies and semi-amplitudes obtained using the period analysis
software {\sc PERIOD04} (Lenz \& Breger, 2005) are shown in Table
2. Errors are $\pm 0.0116$ mHz in frequency ({\it i.e.} at least one
cycle per day) and $\pm0.0005$ mag. in
semi-amplitude.  Allowing for such errors, we note that 
three frequencies might be construed as an harmonic series 
of 0.17, 0.34 and 0.51 mHz. 

Ahmad \& Jeffery (2005) suggested that the periodic variability in
LS\,IV$-$14\deg116 could be due to pulsation. This view has
been difficult to reconcile with the effective temperature and
surface gravity of LS\,IV$-$14\deg116, 
and the known instability mechanisms for subdwarf B 
stars (Jeffery \& Saio, 2007). The discovery of extreme chemical
peculiarity (Naslim~et al.,~2011) suggests at least one
alternative; namely that the stellar surface could be chemically 
inhomogeneous and that the
surface flux might be modulated by rotation as in the
strongly-magnetic Bp(He) stars. Arguing against such a proposition is the
projected rotation velocity which is less than 2\,km\,s$^{-1}$. 

Given the quantity and quality of the 2005 photometric data, there is 
substantially little new information to be extracted from the
light curve.  The persistence of power at 0.34 and 0.52 mHz from 2004 to 2005
suggests that the underlying mechanism is physically robust, and not
due to some stochastic process. Substantially better data are needed to 
establish the power spectrum more securely. 



\references

Ahmad, A. \& Jeffery, C. S.,\ 2003, {\it A\&A}, {\bf 402}, 335 

Ahmad, A. \& Jeffery, C. S.,\ 2005, {\it A\&A}, {\bf 437}, L51
   
Dhillon, V. \& Marsh, T.,\ 2001, {\it New Astronomy Reviews}, {\bf 45}, 91
%\BIBCODE{2001NewAR..45...91D}

Jeffery, C. S. \& Saio, H.,\ 2006, {\it MNRAS}, {\bf 372}, L48  
%\BIBCODE{2006MNRAS.372L..48J}

Jeffery, C. S. \& Saio, H.,\ 2007, {\it MNRAS}, {\bf 378}, 379.  
%\BIBCODE{2007MNRAS.378..379J}

Lenz, P. \& M. Breger, M.,\ 2005, {\it Comm. in Asteroseismology}, {\bf 146}, 53

Kilkenny, D. \& Pauls, L.,\ 1990, {\it MNRAS}, {\bf 244}, 133 

Kilkenny, D., Fontaine, G., Green, E. M. \& Schuh, S.,\ 2010, {\it IBVS}, 5927
%\BIBCODE{2010IBVS.5927....1K}

Naslim, N., Jeffery, C.~S., Behara, N.~T., \& Hibbert, A.\ 2011, {\it MNRAS} in press. arXiv:1010.5146

Nassau, J. J. \& Stephenson, C. B.,\ 1963, {\it Luminous Stars}, Hamburger Sternw., Warner \& Swasey Obs., 4
\BIBCODE{1963LS....C04....0N}
%Changed: Fri Jan 28 09:26:21 CET 2011
 
Viton, M., Deleuil, M., Tobin, W., Pr\'evot, L. \& Bouchet, P.,\ 1991, {\it A\&A}, {\bf 242}, 175


\endreferences

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