% Template article for preprint document class `elsart'
% with harvard style bibliographic references
% SP 2001/01/05

\documentclass{elsart}

% Use the option doublespacing or reviewcopy to obtain double line spacing
% \documentclass[doublespacing]{elsart}

% the natbib package allows both number and author-year (Harvard)
% style referencing;
\usepackage{natbib}

% if you use PostScript figures in your article
% use the graphics package for simple commands
% \usepackage{graphics}
% or use the graphicx package for more complicated commands
% \usepackage{graphicx}
% or use the epsfig package if you prefer to use the old commands
 \usepackage{epsfig}

% The amssymb package provides various useful mathematical symbols
\usepackage{amssymb}

% Bibliographic references with the natbib package:
% Parenthetical: \citep{Bai92} produces (Bailyn 1992).
% Textual: \citet{Bai95} produces Bailyn et al. (1995).
% An affix and part of a reference:
%   \citep[e.g.][Ch. 2]{Bar76}
%   produces (e.g. Barnes et al. 1976, Ch. 2).

\begin{document}

\begin{frontmatter}

% Title, authors and addresses

% use the thanksref command within \title, \author or \address for footnotes;
% use the corauthref command within \author for corresponding author footnotes;
% use the ead command for the email address,
% and the form \ead[url] for the home page:
% \title{Title\thanksref{label1}}
% \thanks[label1]{}
% \author{Name\corauthref{cor1}\thanksref{label2}}
% \ead{email address}
% \ead[url]{home page}
% \thanks[label2]{}
% \corauth[cor1]{}
% \address{Address\thanksref{label3}}
% \thanks[label3]{}

\title{X-ray emission from PTT stars}


% use optional labels to link authors explicitly to addresses:
% \author[label1,label2]{}
% \address[label1]{}
% \address[label2]{}

\author[l1]{G. Del Zanna}
\author[l2]{H.L. Worters}
\author[l2]{G.E. Bromage}
\author[l3]{C.A. Foley}
\author[l1]{H.E. Mason}
\author[l4]{M. Landini}
\author[l5]{A. Whiting}


%\author[l1,l2,l3,l4,l5,l6,l7]{G. Del Zanna, H.L. Worters, G.E. Bromage, 
%C.A. Foley, \\ 
%H.E. Mason, M. Landini, A. Whiting }

\address[l1]{DAMTP, University of Cambridge, Cambridge UK}
\address[l2]{University of Central Lancashire, UK}
\address[l3]{MSSL, University College  London, UK}
\address[l4]{Universit\`a di Firenze, Italy}
\address[l5]{CTIO, Chile}


\begin{abstract}
% Text of abstract
We present preliminary results on new X-ray spectroscopic
XMM/RGS observations of the visual binary nicknamed 
`Horace Horologii' (2RE J0241-53). The two stars are strong and 
very active X-ray emitters, and are members of an association of
Post-T-Tauri stars. A detailed study (in particular in terms of 
chemical composition) is important for our understanding of the
evolution from the T-Tauri phase, and to explain the origin 
of the X-ray emission in very young stars.
The physical characteristics of the quiescent X-ray emission
are described, and related to  other stellar parameters
such as photospheric abundances and the rotational period.
Simultaneous ground-based optical spectroscopy and U-band
photometric monitoring observations were obtained at the South
African Astronomical Observatory.
\end{abstract}

\begin{keyword}
% keywords here, in the form: keyword \sep keyword

% PACS codes here, in the form: \PACS code \sep code

\end{keyword}

\end{frontmatter}

% main text
\section{Introduction}
\label{}

In this paper we present preliminary results on a multi-wavelength
campaign of observations of the 
 Post-T-Tauri (PTT) visual binary 2RE J0241-53.

%Our main scientific goal is to use the capabilities of XMM-Newton
%  to study the
%physical characteristics of the X-ray emission (during quiescent and flaring
%state) of the  Post-T-Tauri (PTT) 
%visual binary 2RE J0241-53.

PTT are young, pre-main sequence stars that have strong 
X-ray emission, as shown by the XUV all-sky survey of 
the ROSAT and EUVE satellites.
In the recent years, XMM-Newton and Chandra observations have 
shown that a great number of young stars have X-ray emission.

Classical T-Tauri stars  have been often observed, 
and since there is evidence for the presence of disks
around these stars, it is likely that their 
X-ray emission is due to accretion (as proposed by 
Kastner et al. 2002 for TW Hya, the prototype T-Tauri star).

PTT stars do not have accretion disks, but are normally 
rapid rotators, and is therefore likely that their 
X-ray emission is similar to that one of the solar 
corona, the main difference being the presence of a much 
stronger dynamo effect producing stronger magnetic fields. 

In reality we are only starting now to study in some detail 
the X-ray emission from these stars. 
Indeed that one described here is the first XMM-Newton 
observation dedicated to the study of these stars.
As far as we are aware, the only other spectroscopic 
observation of a PTT star (PZ Tel) in the X-rays was only 
recently presented by  Argiroffi et al. (2004).
Obviously, there still a long way to go.


Further observations of active stars with different ages 
are crucial for a better understanding of any correlation between
age and X-ray emission, and to identify the mechanisms that 
produce the X-ray emission in the first place.

XMM-Newton and Chandra grating observations of a few sample stars 
have already been obtained.
One of the most striking results from 
these observations is that  many 
 active stars  present a strong anti-FIP (first ionization potential) 
effect that appears to be more pronounced 
in more active stars  and related to the stellar age
(see e.g. Drake et al. 2001; Guedel, 2002). 
In the solar case,  coronal abundances
 appear to be  correlated with the 
FIP of the various elements
and probably reflect on the
origin of the heating mechanism of the coronal plasma.
The abundances of elements with low FIP ($<$ 10 eV, e.g. Fe)
 appear  enhanced compared to those of the 
high FIP ($>$ 10 eV,  e.g.  Ne) elements. 



The `FIP effect'  can therefore in principle 
be used as an additional diagnostic to help us 
understand the basic physical processes that occur in the stellar atmospheres.


However, most studies so far have focussed only on 
RS CVn binary systems, and in most cases 
the photospheric abundances of the stars were not known.


As in the more general case of active young stars, 
many basic questions on the X-ray emission from PTT 
stars  are still awaiting for answers.
For example: is the quiescent X-ray emission of these stars 
 mainly composed of a 
superposition of a continuous multitude of small solar-like flares ?
Is the anti-FIP effect real, and how does it correlate with 
the various stellar characteristics (e.g., age, rotation, 
photospheric abundances) ? 



\section{The target}

2RE J0241-53 (EUVE J0241-530) is  a bright XUV visual binary, discovered by 
the ROSAT (Pye et al., 1995) and EUVE  (Bowyer et al., 1996) all-sky
surveys. 
We nickname the stars `Horace'.

The stars were studied in detail by Jeffries et al. (1996).
They had ROSAT/EUVE count rates  in the XUV
  similar  to those of 
well-known flare stars such as  EQ Peg, AT Mic, AD Leo.
%
Jeffries et al. used  high-resolution ROSAT observations to 
show that both stars are soft X-ray emitters,
with the weaker component contributing to about 1/4 the total
emission.


The system is composed of two dK7e and dM3e young and fast 
rotating ($v$ sin $i$=12, 70 km/s) stars, with an angular spearation of
22''. Since the two stars are almost aligned along 
the N-S direction, we will refer to them as the northern and 
southern component.

Both stars have strong chromospheric emission
(large H$_\alpha$  and  Ca H+K equivalent 
width EW), and are very active.
%
Recurrent flares in both components have been reported
 (including large flares, cf.  Ball and Bromage 1995).
The brightest, southern  star is
Li-rich. Jeffries et al.
suggested that the system could represent a pair of PTT stars.
The PTT classification
was later  confirmed by Torres et al. (2000), on the basis 
of kinematic velocities. Torres et al. identified the 
binary as belonging to the nearby Horologium association of 
young stars, with an age of $\sim$ 30 Myr.



\section{Observations}


We plannned the XMM-Newton observations to consist of 40ks observations.
 XMM-Newton is equipped with a suite of EPIC (PN and MOS) X-ray CCD cameras
which allow good time and spatial  (and some energy) resolution, and two 
reflection grating spectrometers (RGS 1,2) that produce spectra in 
the 5-35~\AA\ range.
 XMM-Newton also has an optical telescope (OM) which allows 
broad-band photometry in the UV-visible. 
We designed the observation to operate the X-ray 
CCDs in the small window mode.
Also, to operate the OM in fast mode (1s time resolution)
 with two small windows centred on the two stars.

In order to be able to separate the spectra of the two sources,
it was necessary to ask for the observation to be 
performed at specific roll angles, so the stars would align 
normal to the dispersion direction. 
XMM succesfully observed the stars beween 
21:30 and 8:20 UT on the 19-20 Feb 2004.


In order to complement the XMM observations,
we coordinated  simultaneous optical
observations consisting of photometry at high time resolution and optical
 spectroscopy. 
In theory the OM should provide continuous photometry 
in support of the X-ray observations, so ground-based observations 
would not be necessary. 
In practice, filter selection on the OM is limited, 
and there was another problem 
related to  ground station handovers for the telemetry 
of the data. During these handovers, the OM 
is normally switched off, causing data gaps.
%
Aside from this problem, the  XMM/OM fast-mode windows worked very
well, and more of these type of observations should be ancouraged.


Having simultaneous ground-based spectral observations
that would for example provide information on the chromosphere,
for highly variable flare stars such as the target is 
an important plus for any scientific output.
However, organising simultaneous  ground-based observations
 proved particularly difficult, given the fact that 
normally XMM observations are scheduled only a few weeks in advance.

We obtained one week of photometric and spectral observations
at the SAAO, South Africa.
We were fortunate to obtain observations before and during the 
XMM observation.
Spectra were obtained using the SAAO 1.9-m grating spectrograph with 
1~\AA\ resolution in the  wavelength range: 3700 -- 4360 \AA\
on 6 consecutive nights.
UBV aperture photometry was carried out for 3 consecutive nights
with the SAAO 0.5-m telescope and the  Modular Photometer.
%
On the night of the XMM observation,
SAAO  covered the period 18:32 UT - 00:04 UT,
with 91 spectra of the Northern star and 17 of the Southern.
U-band monitoring was obtained during 19:11 --  22:40 UT.
The photometric observations were not sufficient to obtain 
information on the rotational period of the stars, and more 
observations have been planned for the future.


We were also able to obtain CCD  B- and U-band  photometry with the 0.9m 
and low-resolution (17.2~\AA\ FWHM) spectroscopy  on the 1.5m telescope 
at CTIO, Chile (telescopes operated by ???) covering part of 
the XMM observation, during 01:17--04:40 UT.
Other photometric observations were obtained at 
the Pico dos Dias Observatory (Brasil) during 23:00-00:30 UT,
courtesy of Carlos Alberto and Germano Quast. CASLEO (Argentina) 
also planned  photometry (courtesy of Federico Gonzalez),
but the wheather was cloudy.


\section{Results}

Fig.~\ref{fig:saao} shows a sample SAAO spectrum of the 
fainter northern star. Strong chomospheric emission is present.
The southern star also presents a similar spectrum.
The striking feature was the high variability of these lines 
even at the 90s time resolution of the observations, and during 
quiescence in the X-ray emission/ U-band.
 Equivalent widths were measured for the H$\delta$ and Ca~II K
lines. The values for the Northern star on the XMM night 
are shown in Fig.~\ref{fig:saao_ew}.

\begin{figure}[!htbp]
\begin{center}
\centerline{\epsfig{figure=n5_lab.eps, width=6.5cm,angle=0}
%\epsfig{figure=north_night7.ps,width=7.0cm,angle=0}}
\epsfig{figure=southern_star_label.eps,width=6.5cm,angle=0}
}
\caption{SAAO spectrum of the northern (left) and southern (right) star.}
% - 7th and 9th (XMM night)
% Feb 2004}
% and of the southern star - 7th Feb 2004.}
%variation in relative line strengths of the calcium and Balmer lines
\end{center}
\label{fig:saao}
\end{figure}
%Fig.~\ref{fig:saao}

\begin{figure}[!htbp]
\begin{center}
\centerline{\epsfig{figure=ew_n_9feb.ps,width=6.0cm,angle=90}}
\caption{Equivalent widths for the Northern star}
\end{center}
\label{fig:saao_ew}
\end{figure}
%Fig.~\ref{fig:saao_ew}


We performed relative aperture photometry on the 
XMM/OM fast-mode windows and on the CTIO B,U images.
The background regions were stationary and with low 
count rates, compared to the source regions.

We performed the standard pipeline processing of the 
XMM X-ray data, checked that no particle events affected
the observation, and constructed energy-resolved 
lightcurves of both sources.
Fig.~\ref{fig:epic_pn} gives an overview of the 
X-ray and U light curves of the two stars.
Both stars confirmed to be active, with 
strong variability and flares.

The northern weaker star had two large X-ray flares 
toward the end of the observations. 
The first flare was not observed by the OM, and 
only partly by CTIO. The second flare 
was observed by the OM but with low signal-to-noise.

The southern brighter star was for most of the 
observation in an unusually quiescent state, with 
X-ray count rates only 2.6 higher than those of 
the north component, i.e. lower than what was observed 
by ROSAT. 
A flare occurred at 01:10 UT, and the X-ray count rates
had a sudden increase toward the end of the observation.
This increase occurred over a period of 5-10m, and the 
star reached a quiescent state that lasted at least until the 
end of the observation, i.e. at least 2 hours.
This increase in the X-ray emission in the southern component
is still under investigation. 
An increase is also observed in 
the visible by the XMM/OM, but unfortunately we do not have 
other supporting observations.
XMM observations of flat-top flares have been recently observed 
(see, e.g.  ???), but they normally last for only tens of minutes.
The presence of a minor secondary component is not 
completely ruled out, since radial velocities for this 
star show a larger than normal scatter (Carlos Alberto, priv. comm.),
but is an unlikely possibility. 
Precise measurements are difficult to achieve because of the 
large rotational velocity of the star and the line broadening.
The rotation into view of a pre-existing flaring or active 
region of the star is another possibility that will 
be investigated once the rotational period is measured.


\begin{figure}[!htbp]
\centerline{\epsfig{file=epic_lc_n.ps, width=7.0cm,angle=90 }}
\centerline{\epsfig{file=epic_lc_s.ps, width=7.0cm,angle=90 }}
\caption{EPIC PN/OM light curves.
The upper points show the EPIC count rates for the 
two stars in the entire 0.2--12 keV energy band. 
The lower points show the normalized OM count rates 
in the various filters. }
\label{fig:epic_pn}
\end{figure}
%Fig.~\ref{fig:epic_pn}

\begin{figure}[!htbp]
\centerline{\epsfig{file=flare1_n_comp.ps, width=7.0cm,angle=90 }}
\caption{Flare on the Northern star. See text for details. }
\label{fig:flare1}
\end{figure}
%Fig.~\ref{fig:flare1}

The observed flares showed the usual characteristics, i.e.
a short-duration increase in the optical U-band, followed by a 
slower but longer-lasting increase in the X-rays  
(Neupert ???). The U-band emission is mainly dominated by 
non-thermal electron bremstrahlung probably directly related to 
the creation of a large number of energetic particles at the 
beginning of the energy release. Beams of fast particles would 
impinge on the chromosphere and induce chromospheric evaporation
of plasma filling the atmosphere in the form of post-flare 
X-ray loops or arcades of loops.
 
Fig.~\ref{fig:flare1} shows the EPIC count rates in two 
energy bands, both of which peaking approximately where 
the vertical line is located. 
Superimposed on the graph, we have the corresponding CTIO U-band 
and XMM/OM UVW1 count rates (normalised). 
The light curves in the U and UVW1 bands clearly peak a 
few minutes before the X-rays.


\begin{figure}[!htbp]
\centerline{\epsfig{file=rgs2_all.ps, width=6cm,angle=90 }
\epsfig{file=pz_tel_meg1_all.ps, width=6cm,angle=90 }}
\caption{Horace vs. PZ Tel }
\label{fig:rgs2_all}
\end{figure}
%Fig.~\ref{fig:rgs2_all}

Compared to other young stars and PZ Tel, 
these stars have no `hot' emission (more like CTTS)
and appear to have a very strong anti-FIP effect,
with very little iron emission.


\section{Conclusions}

A single spectrum is worth thousands of images or 
broad-band observations. 



% The Appendices part is started with the command \appendix;
% appendix sections are then done as normal sections
% \appendix

% \section{}
% \label{}


{\bf Acknowledgements}

Support from  PPARC (UK) and the 
Universita' di Firenze (Italy) is acknowledged. \\
%
We thank the  XMM/SOC at Villafranca, Madrid (Spain), 
for their professional support.
%
We would like to warmly thank for their participation 
to the multi-wavelength observing programme:
Carlos Alberto, Germano Quast 
and Ramiro Delareza (Brasil); 
Federico Gonzalez (CASLEO), Argentina;
staff and students at CTIO, Chile.



%\bibliographystyle{elsart-harv}
%\bibliography{bib}
%\bibliography{xmm}

% copy over 

\begin{thebibliography}{}
% \bibitem[Names(Year)]{label} or \bibitem[Names(Year)Long names]{label}.
% (\harvarditem{Name}{Year}{label} is also supported.)
% Text of bibliographic item
%\bibitem[]{}

\bibitem[]{}

\end{thebibliography}

\end{document}

