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\begin{document}

\begin{frontmatter}

% Title, authors and addresses

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\title{X-ray emission from PTT stars}


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

\author[l1,l3]{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 the first 
XMM-Newton observation dedicated to 
Post-T-Tauri stars. 
We observed  the visual binary nicknamed 
`Horace' (2RE J0241-53). The two stars are 
active X-ray emitters, and are members of an association of
Post-T-Tauri stars. 
The XMM observations have been complemented with 
 ground-based optical spectroscopy and 
photometry. Many flares have been observed.
The average X-ray spectrum shows similarities to that 
of Classical T-Tauri stars, with high-temperature emission
non-existent, and strong neon and oxygen H- and He-like emission.
\end{abstract}

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

Post-T-Tauri stars \sep X-ray emission \sep Spectroscopy

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

\end{keyword}

\end{frontmatter}

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

%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.

In the recent years, XMM-Newton and Chandra observations have 
shown that a great number of stars have strong X-ray emission.
%
Only a few X-ray observations of young stars have been obtained.
Classical T-Tauri (CTT) stars  have been  observed  a few times, 
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 
\citealt{kastner_etal:02} for TW Hya, one of the best-known
classical T-Tauri stars).


Post-T-Tauri (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.
%
PTT stars do not have accretion disks, but are normally 
rapid rotators, and it is therefore possible that their 
X-ray emission is similar to that  of the solar 
corona with a stronger dynamo effect.
%, the main difference being the presence of a much 
%stronger dynamo effect producing stronger magnetic fields. 
%
In reality we are only now starting to study in some detail 
the X-ray emission from these stars. 
Indeed as far as we are aware, the study described here is the first XMM-Newton 
observation dedicated to the investigation of PTT stars.
The only other spectroscopic 
observation of a PTT star (PZ Tel) in the X-rays was 
recently presented by  \citet{argiroffi_etal:04}.


One of the most striking results from 
XMM-Newton and Chandra  observations of active stars are their 
(apparent) peculiar chemical abundances. 
Stellar coronal abundances have important 
 implications  for our understanding
of the stellar atmospheres, and yet are poorly known.
In the solar case,  coronal abundances
 appear to be  correlated with the 
first ionization potential (FIP) of the various elements.
The abundances of elements with low FIP ($<$ 10 eV, e.g. Fe)
 appear  enhanced (in respect to the photospheric
values) compared to those of the 
high FIP ($>$ 10 eV,  e.g.  Ne) elements. There are, 
however, variations depending on the solar 
feature observed, and possibly on its evolution. 
%
On the contrary, active stars appear to have a strong 
anti-FIP effect, i.e. elements such as iron are strongly 
depleted, compared to the solar photospheric values.
Furthermore, this anti-FIP 
effect  appears to be more pronounced 
in more active stars  and related to the stellar age
(see e.g.  \citealt{guedel:02}). 

Chemical abundance variations are important to study
because they must be associated with the 
physical processes that occur in the chromosphere. 
Ultimately, they could  reflect on the
origin of the heating mechanisms of the coronal plasma.
\citet{laming:04}  suggested that the FIP 
and anti-FIP effect are associated with the 
upward-propagating chromospheric waves impinging on the 
coronal structures.


In terms of broad characteristics, 
CTT stars such as TW Hya show a strong (factor of 5) anti-FIP 
effect, but do not show any emission from 
hot multi-million degrees plasma.
The only other PTT star that was observed, 
PZ Tel, showed instead the presence of hot 
(temperatures above 10$^7$ K) plasma, and a smaller 
(factor of 3) anti-FIP effect. 
It is therefore interesting to see first if 
 similar characteristics can be found in other 
PTT stars.



\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 by \citet{jeffries_etal:96}, 
who 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 separation 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.  \citealt{ball_bromage:95}).
The southern  star is
Li-rich.  ROSAT observations indicated that the 
 southern star was 3 times brighter than the 
northern one in the XUV.
Jeffries et al.
suggested that the system could represent a pair of PTT stars.
The PTT classification
was later  confirmed by \citet{torres_etal:00} on  
 kinematic grounds. 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}


 XMM-Newton is equipped with a suite of EPIC (PN and MOS) X-ray CCD cameras
 and two reflection grating spectrometers (RGS 1,2).
 XMM-Newton also has an optical telescope (OM) which allows 
broad-band photometry in the UV. 
We designed the observation to operate EPIC
 in the small window mode and  the OM in fast mode (1s time resolution)
 with two small windows centred on the two stars. 
The instruments performed extremely well, in particular the OM.
%
To separate the spectra of the two sources,
 the observation needed to be 
performed at specific roll angles.
XMM successfully observed the stars  for 38ks between 
21:30 and 8:20 UT on  9-10 Feb 2004.
%
In order to complement the XMM observations,
we coordinated  simultaneous optical
observations.
In theory the OM should provide continuous 
support of the X-ray observations, so ground-based photometry
should not be necessary. 
In reality this is not the case, 
because filter selection on the OM has some limitations, 
and because during ground station handovers the OM is switched off,
causing data gaps.
%

The scientific importance of having spectral information on 
the lower atmospheric layers such as the  chromosphere,
in particular for  flare stars, is self-evident.
However, organising simultaneous  ground-based observations
is particularly difficult, given the fact that 
normally XMM observations are scheduled only a few weeks in advance.
%
We were fortunate to obtain one week of photometric and spectral observations
at the South African Astronomical Observatory  (SAAO) around
the the time of 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  0.5-m telescope.
%
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   Cerro Tololo Interamerican Observatory 
 (CTIO, Chile) 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 weather was cloudy.


\section{Results}

Fig.~\ref{fig:saao} (left) shows a sample SAAO spectrum of the 
northern star. Strong chromospheric emission is present.
The southern star  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.~1.

\begin{figure}[!htbp]
\begin{center}
\centerline{\epsfig{figure=horace_fig1a.eps, width=6.5cm,angle=-90}}
%\epsfig{figure=north_night7.ps,width=7.0cm,angle=0}}
%\epsfig{figure=southern_star_label.eps,width=6.5cm,angle=0}
\centerline{\epsfig{figure=ew_n_9feb.ps,width=5.cm,angle=90}
}
\caption{ SAAO spectrum of the northern star (above) and 
equivalent widths during the XMM observation (below).
}
% - 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]
\centerline{\epsfig{file=epic_lc_n.ps, width=7.cm,angle=90 }}
\centerline{\epsfig{file=epic_lc_s.ps, width=7.cm,angle=90 }}
\caption{X-ray (EPIC PN)  and UV (OM) light curves of the 
XMM observation.
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 normalised 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=9.0cm,angle=90 }}
\caption{Light curves for the first large flare on the Northern star. 
See text for details. }
\label{fig:flare1}
\end{figure}
%Fig.~\ref{fig:flare1}


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 and constructed energy-resolved 
light-curves 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 are confirmed to be active, with 
strong variability and flares.
%
The northern  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  star was  in an unusually quiescent state
for most of the observation, with 
X-ray count rates only 2.6 times higher than those of 
the north component.
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 steady 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.


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 effect). The U-band emission is mainly dominated by 
 bremsstrahlung of non-thermal electrons 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 peak  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=6.5cm,angle=90 }}
\centerline{\epsfig{file=pz_tel_meg1_all.ps, width=6.5cm,angle=90 }}
\centerline{\epsfig{file=tw_hya_meg1n.ps, width=6.5cm,angle=90 }}
\caption{X-ray spectra of Horace (XMM-Newton, 38 ks),  
 PZ Tel (Chandra, 74 ks) and TW Hya (Chandra, 48 ks). 
The Chandra spectra are first-order counts for the total 
duration of each observation, rebinned to improve the 
signal. 
}
\label{fig:rgs2_all}
\end{figure}
%Fig.~\ref{fig:rgs2_all}

Fig.~\ref{fig:rgs2_all} shows the average RGS 2 
first order spectrum of Horace. 
The strongest lines are the oxygen and neon 
H- and He-like lines, with very little Fe~XVII emission.
For comparison, the Chandra  spectra of 
PZ Tel (with twice the exposure time -- 74 ks)  and of 
TW Hya (48 ks) are also shown.
The Chandra spectra are the un-calibrated total 
counts in one of the first-order channels, 
and have been obtained from the Chandra archive.
PZ Tel shows strong `hot' emission from  Fe~XXIII and Fe~XXIV 
$n=3 \to n=2$ transitions, and much stronger 
H-like (e.g. Ne~X) compared to He-like (Ne~IX) emission.  
Compared to other young stars and PZ Tel, 
Horace has no `hot' emission 
and its  total spectrum has more similarities to that one
of the CTT star TW Hya. 
Horace also appears to have a very strong anti-FIP effect,
with iron strongly depleted.

Fig.~\ref{fig:goft} shows the emissivities of some of the 
 prominent lines in the spectra of these young stars,
calculated assuming the ionization equilibrium of 
\cite{arnaud_rothenflug:85} and the CHIANTI atomic data 
\citep{young_etal:03}.
If  ionization equilibrium holds, then the 
Fe~XVII and Ne~IX  lines should be emitted by the same 
plasma (at the same temperature), and their intensity ratio would be directly 
proportional to the element abundance ratio. 
The relative intensity of the Fe~XVII vs. the Ne~IX  lines
can therefore be used as an approximate proxy of the FIP effect.
Note that in the Sun and in the case of young 
stars such as AB Dor the Fe~XVII lines are much more prominent,
compared to the Ne~IX lines. 

\begin{figure}[!htbp]
\centerline{\epsfig{file=horace_goft.ps, width=6.5cm,angle=90 }}
\caption{Emissivities (intensities per unit emission measure) 
of a few of the prominent lines in the Horace spectrum,
calculated assuming ionization equilibrium.
}
\label{fig:goft}
\end{figure}
%Fig.~\ref{fig:goft}




\section{Discussion and  conclusions}


X-ray spectral  observations of PTT 
are difficult to obtain and require long exposure times.
However, a single spectrum is worth thousands of images or 
broad-band observations.  Indeed the 
 Horace X-ray spectrum is largely different from 
what we could predict on the basis of the 
broad-band ROSAT and EUVE measurements, which were 
in a different spectral region.
The Horace  spectrum  shows a distinctive 
difference compared to the only other known 
spectrum of a PTT star, PZ Tel.
In particular the absence of any 
`hot' emission, despite the presence of 
multiple flares, is striking. 
Therefore, until more observations 
of PTT stars are available, we cannot assume that neither 
PZ Tel nor Horace's characteristics can be 
assumed to be `typical' for PTT stars.
Horace's spectra show characteristics more similar to those 
of the CTT star TW Hya.

Much of our understanding on PTT stars 
 could be improved with simultaneous 
 multi-wavelength observations, but unfortunatley these are 
difficult to obtain.
A detailed study of the Horace X-ray emission
 will require  an in-depth analysis 
of the flaring and more quiescent periods.
Also, a knowledge of 
stellar characteristics such as  rotation and
photospheric abundances of the stars.
A few comments can however be made.
The  FIP effect could  in principle 
be used as an additional diagnostic to help us 
understand the basic physical processes 
that occur in the stellar atmospheres.
%
In reality there are however various effects that 
complicate the analysis and interpretation 
of stellar coronal abundances.
For example, there are large uncertainties 
associated to the methods and the various 
assumptions (e.g. ionization equilibrium) used 
(see, e.g., \citealt{delzanna02_aumic}, \citealt{delzanna01_ace}).
%
Also, coronal abundances are rarely
compared to the photospheric abundances of the 
same star. In turn,  measurements of 
photospheric abundances of active stars are scarce, show large 
scatter, and are difficult (sometimes impossible)
to obtain to within a reasonable accuracy, in 
particular for the fast rotators, which are 
common in young stars.

Compared to the solar photospheric abundances,
the CTT star 
TW Hya showed an iron depletion by a factor of 5,
while the PTT star PZ Tel was found to have 
an iron abundance 0.3 times the solar photospheric one.
%
PZ Tel is a member of the Tucana association, a local 
($\simeq$45 pc) group of young ($\simeq$20-30 Myr) stars,
while Horace is a member of the Horologium association.
The Tucana and Horologium associations have similar 
characteristics. 
Recently, \citet{rojas_etal:04} provided measurements 
of photospheric abundances for 9 members of the former and 
3 members of the latter association.
A large scatter of values can be found, with variations 
up to factors of 3. These measurements stress the need 
to directly compare abundances of the same star.
However, on average the 
 photospheric abundances are not too different from the 
solar ones, hence  the anti-FIP effect 
is presumably real, despite all the cautionary remarks 
that can be made. 



Further X-ray 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.


% 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 
Universit\`a di Firenze (Italy) is acknowledged. \\
%
We thank the  XMM/SOC at Villafranca, Madrid (Spain), 
for their professional support.
%
Some of the data in this paper were obtained under the CTIO Research
Experiences for Undergraduates (REU) and Pr\'actica de Investigaci\'on en
Astronom\'ia (PIA) programs, funded by the U.S. National Science
Foundation. We also used observations made from the South African Astronomical
Observatory (SAAO).
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;
L. Schmidtobreick (ESO) and the 
CTIO REU program students M. Rice
(Wellesley College), R.  Barlow (Mt. Holyoke College), E. Knox (Humboldt
State University) and K.  Walker (Ohio State University).



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