\documentstyle[epsf]{article}

\input ibvs.sty


\newcommand{\FeII}{Fe~{\sc ii}}
\newcommand{\kms}{km~s$^{-1}$}
\newcommand{\Fc}{$F/F_{\rm c}$}
\newcommand{\Ha}{H$\alpha$}
\newcommand{\NaI}{Na~{\sc i}}
\newcommand{\VR}{$V/R$}
\newcommand{\noi}{\noindent}
\newcommand{\Vhc}{$V_{\rm hc}$}
\newcommand{\DV}{$\Delta V$}

\begin{document}
\IBVShead{4258}{20 October 1995}

\centerline{\ft THE DETECTION OF NARROW ABSORPTION}
\centerline{\ft COMPONENTS  (NACS) IN FX Lib = 48 Lib\footnote{Based on observations obtained at the European
Southern Observatory, La Silla, Chile, and at the Observatoire de 
Haute-Provence (CNRS), St. Michel, France}}
\vskip 4mm

\begintext

 We report  on observations of optical shell lines in FX Lib = 
48 Lib = HR\,5941
(B3IV e-sh, $v \sin i = 400$ \kms),
a well-known equatorial shell star studied since decades
(see Guo 1994 for further references). 
 We have detected in some of
these shell lines satellite absorption features which, though quite 
conspicuous at
sufficiently high resolution, apparently have never  been observed before.

The observations were carried out in three runs in 
1995: March 07--10 (period I)
at the 1.4m Coud\'{e} Auxiliary Telescope of ESO at La Silla/Chile 
(observer: RWH).
Attached to the telescope was the 
Coud\'{e} Echelle Spectrograph CES, operated with the Long Camera,
 and a UV-coated LORAL 2048 $\times$ 2048 CCD 
 with 15$\mu$ pixel size. 
The  effective 
resolving power as determined from the width of
thorium emission lines  was $R=50~000-120~000$.

The second run  (period II) was on April 10--18 (observer: RWH) at the 1.52m telescope
of the Observatoire de Haute Provence at St. Michel/France, equipped with the
Coud\'{e} spectrograph {\em Aur\'{e}lie} and a one-column CCD (double
barette Thomson) with 2048 pixels of 13 $\mu$  size each. The 
effective  resolving power was
 $R \approx 45~000$.

The third run (period III) was on April 26 -- May 01, 1995 (observer: MV), 
again at the OHP 1.52m
telescope with the same instrumentation.

All data were reduced in the standard way and binned on a 
heliocentric radial velocity scale (\Vhc). 

We have measured the  spectral region of three important shell  lines in  48
Lib: \Ha, \FeII\ $\lambda5317$ (together with  several other fainter
\FeII\ lines including the $\lambda5276$ feature), and \NaI$-D_1,D_2$ including
the He {\sc i} $\lambda5876$ line. 

We have covered by our extensive spectroscopic survey timescales
between hours (in series of integrations with a single exposure 
time of 15--30 minutes) and 
%days (exposures in successive nights), one week (gap between
%periods II and III) and 
two months (gap between periods I and III).  

The \Ha\ profile (Figure 1a) 
consists of two asymmetric emission humps with 
ratio \VR\ = (2.65$-$1)/(4.22$-$1) = 0.51 (ratio of continuum-subtracted 
fluxes in violet peak and red peak, resp.),  separated by 
 a deep
central shell trough of residual intensity 0.08$F_{\rm c}$. 
There is no evidence for
any small-scale fine-structure in the line profile, 
contrary to the observation of Guo (1994) who reported
moving bumps at \Ha\ in April 1994.  


The \FeII\ lines offer a completely different picture.  
One such  $\lambda5317$ measurement from period I
is shown in Figure 1b, a larger subset on an extended scale in Figure 1c. 
The line structure is dominated
by an asymmetric blueshifted shell feature of typical width (FWHM) = 90 \kms,
centred at \Vhc\  = --52 \kms. 
This dominant absorption 
feature is flanked by  faint extended emission on its red 
 side.  
More conspicuous, however, are\break
 
\vfill \eject

\centerline{2}
\epsfysize=65mm
\centerline{\epsffile{4258-f1.ps}}
\small
{\bf Figure 1a}. \Ha\ profile of 48 Lib.
{\bf 1b}: \FeII\ $\lambda$5317 profile. 
{\bf 1c}: Close-up of some \FeII~profiles.
The NACs are numbered 1...5. We show four spectra out of 9 measured in 
period I.
\large

\epsfysize=61mm
\centerline{\epsffile{4258-f2.ps}}
\small
{\bf Figure 2}. Close-up of our \FeII\ $\lambda$5317 data from period III. 
{\bf 2a}: Line profiles. 
{\bf 2b}: ~normalized flux gradients
$(\Delta F/\Delta v)/F$ (where $F$ denotes the flux and $v$ the
radial velocity), chosen to enhance the visibility of the NACs.
The dotted line marks the stellar rest velocity.
\large

\epsfysize=61mm
\centerline{\epsffile{4258-f3.ps}}
\small
{\bf Figure 3}.
Gray-scaled plot of the NAC evolution during period I. We show
the flux gradient  on a linear timescale with one time-step
$\Delta t$  equivalent to 25 minutes. 
Lags between actual measurements are filled
by interpolation. The total time interval represented here is 100 min for 
March 9, and about 3 hours for March 10.
\large
\vfill \eject

\centerline{3}
\vskip 1mm
\noindent  several subordinate absorptions embedded
into the central part of the shell trough and its low-velocity flank (Figure 1c).
These will be 
called {\em narrow absorption components} (NACs) in the following.

They are characterized by the following properties:

{\em Number.} In the  spectra from period I, we count 5 NACs, while in the 
later periods only 4 are visible at any time with certainty (Figure 2).
They only show up in the subrange  \Vhc\ = --80 and --10 \kms\ of the shell 
trough.

{\em Width.}
Their  width decreases from the bluemost NAC to
the that one with the smallest velocity. 
 Typical values are about 10 \kms\ for the bluemost NAC.
The  NAC with lowest velocity 
has width \DV = 4 \kms\ (FWHM) in the CAT spectra with highest resolution
(period I, $R$ = 120~000 or instrumental FWHM = 2.5 \kms) and are well resolved.
In periods II and III, we measure \DV $\approx 7$ \kms\ which corresponds
to the instrumental FWHM, i.e. the NACs are unresolved then.

{\em Depth.}
The depth of the deepest resolved NACs from  period I is about 
0.11--0.13 continuum units.

{\em Time variability.}
In those nights with multiple exposure we find slow variability
of the NAC features both in RV and depth on a timescale of hours (see Figure 3). 
RV changes, if present, are always such that a feature moves {\em from
larger to smaller blueshifted 
velocities}. The bluemost NACs move fastest, while those with 
smallest 
radial velocity  are  stationary. Most rapid changes observed occur in 
period I, with $\ge 20$ \kms\ per day. ``Acceleration" is generally 
lower at later times. This rate is large enough to modify 
the overall appearance of the NACs from night to night considerably.
 
Beyond any doubt, the NACs are true spectroscopic structures
rather than noise since
their depth is considerable. Moreover
 we find the same features, with the same RV and relative depths, 
in the \FeII\ $\lambda$5276 line, as well as 
at \NaI. In the latter line, the NACs are even deeper than at \FeII.

In the stellar He {\sc i} profile, neither broad shell feature nor NACs
are visible. 

Due to their depth and to the fact that no small-scale structure is visible in 
 He~{\sc i}, we conclude that the NACs have nothing to do with 
profile fluctuations induced by non-radial pulsations (e.g., Baade \& Balona, 1994).
Furthermore their relatively small radial velocity, their drift to
{\em smaller} velocities, their extreme narrowness and 
their occurrence in optical lines of species with low ionization degree
(rather than in UV lines of  high ionization stages) 
clearly distinguishes them from the well-known DACs in stellar 
wind lines (Prinja 1994, Henrichs et al. 1994).

There are only a few earlier observations of 48 Lib which show  related
 behaviour:
Aydin \& Faraggiana (1978) report  night-to-night variability in the
\NaI-D lines in 1970--74.  Their photographic material, however, has
much lower S/N quality
than ours.  Guo (1994) reports \Ha\ variability in 1994, with not
fully resolved
spectra of unknown reliability.
Our own overview of earlier \Ha\ and \FeII\ data in 48 Lib
(Hanuschik et al., 1995)
does not show any  such fine structure
in 1987--1993.
In absence of any reliable  previous observation of NACs
we believe that we have found a new spectroscopic phenomenon in
Be star disks which occurs only rarely and transiently.


The NACs are likely to be caused by  regions of higher density,
or lower temperature,  as compared to the surrounding gas.
These regions must be very small in comparison to the disk dimensions,
$\Delta R/R_{\rm d} \ll 1$, 
since their  observed RV width of 4--10 \kms\ is   
only slightly more than the {\em thermal width} of
iron at $10^4$ K (2.9 \kms\ FWHM), and 
much less than the RV width of the 
shell volume (about 90 \kms).
\vfill \eject 

\centerline{4}
\vskip 1mm

However, if the NACs were simply caused by local clumps
moving 
across the stellar disk
in elliptical Keplerian orbits,
their observed RV change during a night 
would be  too small (typical time scale for
crossing the shell volume at 5 stellar radii is half a day).
It seems  more likely that they are due to
 higher-order components of the   density wave which
 causes the cyclically changing 
line profile
asymmetries in 48 Lib (see Hanuschik et al. 1995). 

The shell volume mainly traces the {\em radial component}
of the velocity field 
in a small part of the  circumstellar disk (Hanuschik, 1995).
The detection of NACs in 48 Lib therefore provides 
 highly valuable, otherwise unaccessible  
information about the structure of  
the  disk in 48 Lib. We strongly urge other observers with access
to a medium-sized telescope with  Coud\'{e}  spectrograph equipment 
to continue
observations of these conspicuous features,
which are best  
observable at \FeII\ $\lambda$5317.

\vskip 1cm
\beginauthors
\authorsrightcol
R.W. HANUSCHIK$^1$,
M. VRANCKEN$^2$
\endauthors

\vskip 2mm
\noindent $^1$Astronomisches Institut, Ruhr-Universit\"at,
Postfach 10~21~48, D-44780 Bochum, Germany

\noindent $^2$ Koninklijke Sterrenwacht van Belgi$\ddot{\rm e}$, Ringlaan 3, B-1180 Brussel,
Belgium
\vskip 9mm


\references

Aydin, C.,  Faraggiana, R., 1978, {\em A\&AS}, {\bf 34}, 51

Baade, D., Balona, L.A., 1994, {\it IAU Symp.}, {\bf 162}, 311, 
L.A. Balona et al. (eds.), Kluwer
%Changed: Tue Jun 14 15:38:26 MEST 2005

Guo Yulian, 1994, {\em Inf. Bull. Var. Stars}, No.\,4113

Hanuschik, R.W., 1995, {\em A\&A}, {\bf 295}, 423

Hanuschik, R.W., Hummel, W., Dietle, O., Sutorius, E., 1995, {\em A\&A}, {\bf 300}, 163

Henrichs, H.F., Kaper, L., Nichols, J.S., 1994, {\it IAU Symp.}, {\bf 162}, 517,
L.A. Balona et al. (eds.), Kluwer
%Changed: Tue Jun 14 15:38:26 MEST 2005

Prinja R.K., 1994, {\it IAU Symp.}, {\bf 162}, 507, 
L.A. Balona et al. (eds.), Kluwer
%Changed: Tue Jun 14 15:38:26 MEST 2005

\end{document}


