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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
{\bf
We present preliminary results on an
XMM-Newton observation dedicated to 
Post-T-Tauri (PTT) stars. 
We observed  the visual binary 
2RE J0241-53 of the Horologium association of nearby PTT  stars. 
The XMM observations have been complemented with 
 ground-based optical spectroscopy and 
photometry. 
Many flares have been observed.
The  X-ray spectrum of the brightest star
during quiescence indicates a rather cool corona 
(with no emission at temperatures 
$\ge$ 10 MK) more similar to that 
of the Classical T-Tauri star TW Hya 
(observed with Chandra and XMM-Newton)
and at variance with that of another PTT Star (PZ Tel)
observed with Chandra.
Large Ne/Fe, N/Fe, C/Fe abundances 
(compared to solar photospheric values) are found.
}
\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.

{\bf 

In the recent years, XMM-Newton and Chandra observations have 
shown that a great number of stars have strong X-ray emission.
In particular, young pre-main-sequence (PMS) stars.
%
It is important to understand how the X-ray emission is formed,
and relate it to stellar characteristics such as age, rotation.

According to the current understanding of low-mass
star formation (cf. \citealt{feigelson_montmerle:99}), 
PMS stars go through several phases, from
infalling protostar (Class 0), to evolved protostar (Class I),
to Classical T-Tauri (CTT) star (class II or CTT), to Weak-lined
T-Tauri (WTT) star
(class III or WTT).
The distinction between CTT and WTT stars is normally
 based on the strength of chromospheric emission lines
(stronger in WTT) and 
on the infrared excess (strong in CTT).
However,  many  CTT and WTT stars are found to have the
same age.
It is usually assumed that the observed characteristics indicate
that CTT stars have accretion disks, which might have a role in
the X-ray emission, while WTT stars are disk-free.
Post-T-Tauri (PTT) stars were postulated by \cite{herbig:78}
to be  slightly older T Tauri stars that might be more
dispersed from their birth sites, hence far from star forming
regions. 
From the point of view of the physical
processes that might produce the 
X-rays (i.e. for the absence of a disk and the presence of
solar-type magnetic activity as opposed to accretion as is believed to occur
in CTT stars) PTT stars are similar to WTT stars.
However, PTT stars have physical characterisitcs very different from 
zero-age main-sequence stars (ZAMS) such as AB Dor. 


Only a few high-resolution X-ray
grating spectra of PMS stars have been obtained.
Some studies have recently been published, and many 
are in progress. 
These X-ray observations show a complex picture, in which the X-ray spectra
of stars of the same classes present different characteristcs. 

CTT stars  have been  observed  a few times
with high-resolution X-ray grating spectra. 
TW Hya was observed with Chandra 
\citep{kastner_etal:02} and XMM-Newton 
\citep{stelzer_schmitt:04}. 
The main findings were the lack of hot ($T >$  10MK)
emission, and anomalous chemical abundances.
Recently, XMM/RGS observations of another CTT star, SU Aur,
have been obtained  (cf Pallavicini et al., this volume),
which show instead  very hot emission, and suggest that
the X-ray spectrum of TW Hya may not be representative
for this class of stars.

A few observations of WTT stars are now available.
 HD 98800 observations are reported by 
\cite{kastner_etal:04}, while observations of other WTT
 stars are reported in Pallavicini et al. (this volume).
These stars have very hot coronae, with emission 
from plasma formed at $T >$  10MK.

In terms of WTT stars classified as  PTTS, 
high-resolution Chandra X-ray spectra of 
PZ Tel (a member of the Tucana association)
have been reported by \cite{argiroffi_etal:04}.
PZ Tel presented a very hot corona.
%
Here, we present preliminary results from an 
XMM-Newton observation (with RGS as a primary 
instrument) of two PTT stars, 2RE J0241-53.
We compare the main characteristics of the 
spectrum of the brighter star in terms of 
temperatures and chemical abundances with those
of PZ Tel and TW Hya.
Further in-depth analysis and details will 
be published in a forthcoming paper.


One of the most striking results from 
XMM-Newton and Chandra  observations of active stars are their 
(apparent) peculiar chemical abundances. 
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 (with 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. 
%

Active young stars present a complex variation of 
chemical abundances
(cf. the reviews of  
\citealt{guedel:04}, \citealt{audard_etal:03}).
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.

}

\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. 
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 successfully observed the stars  for 38ks between 
21:30 and 8:20 UT on  9-10 Feb 2004.
%
 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. 
{\bf We used different OM filters in succession: 
U (21:30-00:40 UT), UVW1 (00:45-04:34 UT), UVM2 (05:43-08:09 UT). }
%
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.
%

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 1.9m 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  0.5m 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.~1 (top) 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 night of 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 successive filters. }
\label{fig:epic_pn}
\end{figure}
%Fig.~\ref{fig:epic_pn}

\begin{figure}[!htbp]
\centerline{\epsfig{file=flare1_n.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,
{\bf due to the low-counts filter used then}.
%
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.
{\bf 
This increase in the X-ray emission in the southern component
is still under investigation. The increase was observed by 
all the XMM instruments. 
The spectral characteristics do not suggest this being a 
flat-top flare. 
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.).
}

%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, cf. \citealt{neupert:68}).
 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 flaring 
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=south_quiet.ps, width=5.5cm,angle=90 }
\epsfig{file=south_quiet_inset.ps, width=5.5cm,angle=90}}
\centerline{\epsfig{file=pz_tel_meg1_phot.ps, width=5.5cm,angle=90 }
\epsfig{file=tw_hya_meg1n_phot.ps, width=5.5cm,angle=90 }
}
\caption{Top: XMM-Newton RGS spectrum of 2RE J0241-53S 
during quiescence, with an enlarged part of the spectrum showing
the uncertainties related to the noise and background subtraction. 
Bottom: spectra of 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}

{\bf 
The XMM observation contains many interesting aspects.
A detailed analysis of the various features 
requires a discussion of atomic physics, line 
identifications and blending issues, and is left to 
a forthcoming paper.
Here, we present the averaged first-order RGS spectrum of the 
 southern component, extracted during 21:30--04:00 UT. 
During this period, the star was mainly in 
quiescence, with the exception of a small flare. 

The strongest lines are the oxygen and neon 
H- and He-like lines, with very little Fe~XVII emission.
Contrary to the spectra of other WTT stars, 
Fe~XVIII emission is weak, and hotter emission 
(Fe~XIX, Fe~XX, Fe~XXI and higher ionization stages) absent.

For comparison, the first-order Chandra  spectra of 
PZ Tel (with twice the exposure time -- 74 ks)  and of 
TW Hya (48 ks) as obtained from the Chandra archive are also shown
(for more details see \citealt{argiroffi_etal:04} and 
\citealt{kastner_etal:02}).
%
PZ Tel presents strong `hot' emission.
Compared to Fe~XVII, its spectrum shows strong Fe~XVIII,
Fe~XIX and Fe~XX emission.
Also, PZ Tel shows strong emission from temperatures
$T >$  10MK (Fe~XXIII and Fe~XXIV 
$n=3 \to n=2$ transitions). 
%
In  contrast, the spectrum of the CTT star TW Hya
does not have any significant emission at temperatures
higher than those where Fe~XVII is formed. 
%\cite{kastner_etal:02}


Coronal abundances can be obtained with emission
measure techniques. To obtain accurate values, 
a  thorough analysis 
which takes into account the many 
 uncertainties associated to the various methods and
assumptions (e.g. ionization equilibrium) used 
is needed (see, e.g., \citealt{delzanna02_aumic}, 
\citealt{delzanna01_ace}). This goes beyond
 the scope of this paper.
We have obtained approximate values 
assuming the ionization fractions of 
\cite{arnaud_rothenflug:85} and the CHIANTI atomic data 
\citep{young_etal:03}.
%
Compared to solar photospheric values, 
the southern star has enhanced Ne/Fe, O/Fe, N/Fe, C/Fe 
abundance ratios approximately 3, 1.5, 4, 4, respectively.
%
For example, Fig.~\ref{fig:emloci} shows the emission measure loci curves 
(cf. \citealt{delzanna02_aumic} and references therein for definitions) 
for a few of the prominent lines in the 
spectrum of the southern star.
%
For comparison, we
note that for TW Hya  \cite{kastner_etal:02} estimated
 enhanced Ne/Fe, O/Fe, N/Fe, by factors of 10, 1.5, 5,
while for PZ Tel \cite{argiroffi_etal:04} found
 Ne/Fe, O/Fe enhanced by factors of 2, 1.6.


\begin{figure}[!htbp]
\centerline{\epsfig{file=emloci.ps, width=7.5cm,angle=90 }}
\caption{Emission measure loci curves 
for a few of the prominent lines in the 
spectrum of the southern star,
calculated assuming ionization equilibrium.
The ordinate has an arbitrary logaritmic scale.
}
\label{fig:emloci}
\end{figure}
%Fig.~\ref{fig:emloci}




\section{Discussion and  conclusions}


X-ray spectral  observations of PTT 
are difficult to obtain because the stars are faint 
and require long exposure times.
However, a single spectrum is worth thousands of images or 
broad-band observations.  
%
The X-ray spectrum of 2RE J0241-53S 
 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 X-ray spectrum  shows a distinctive 
difference compared to the few other 
spectra of WTT stars (in particular the 
 PTT star PZ Tel), due to the  absence of any 
`hot' ($T \ge$ 10 MK) emission. 
In this respect, the  X-ray spectrum of 2RE J0241-53S 
is more similar to that  of the CTT star TW Hya.
It is interesting to note (cf. Pallaviciini et al, this issue)
 the large differences in the X-ray spectra of stars 
of the same class (TW Hya, SU Aur; PZ Tel, 2RE J0241-53S).

In terms of  coronal abundances, 
2RE J0241-53S appear to be closer to
 other PTT/WTT stars (such as PZ Tel), 
which show similar large Ne/Fe enhancements. 
We note, however, that coronal abundances 
should be compared to photospheric abundances of the 
same star.
In turn,  measurements of 
photospheric abundances of active stars are scarce, show large 
scatter, and are difficult 
to obtain to within a reasonable accuracy, in 
particular for the fast rotators, which are 
common in young stars.

%
PZ Tel is a member of the Tucana association, a local 
($\simeq$45 pc) group of young ($\simeq$20-30 Myr) stars,
while 2RE J0241-53 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. 
However, on average the 
 photospheric abundances are not too different from the 
solar ones, hence  the chemical anomalies that 
are found in these PTT stars are 
presumably real, despite all the cautionary remarks 
that can be made. 


Much of our understanding on PTT stars 
 could be improved with simultaneous 
 multi-wavelength observations, but unfortunatley these are 
difficult to obtain.
Also, accurate  knowledge of 
stellar characteristics such as age, rotation and
photospheric abundances of the stars would be required.
%
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.
%
This paper uses observations made from the South African Astronomical 
Observatory (SAAO).
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 would like to warmly thank for their participation 
in the multi-wavelength observing programme:
Carlos Alberto, Germano Quast 
and Ramiro de La Reza (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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\end{document}

