A&A 454, 595-607 (2006)
DOI: 10.1051/0004-6361:20054350
P. Gondoin
European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands
Received 13 October 2005 / Accepted 15 March 2006
Abstract
Aims. In this paper, I present analysis results of an
observation of the Lupus 3 region that contains a high proportion of young low mass (M < 0.3
)
T Tauri stars in the Lupus star-forming complex.
Methods. The detection of X-ray sources in 0.5 to 4.5 keV images of the Lupus 3 core was performed using the standard source detection method of the
Science Analysis Software. The detected sources were correlated with a list of Herbig-Haro objects and H
emission stars that contains mainly classical T Tauri stars, with a catalogue of weak-line T Tauri Stars and with a recent list of new low-mass members of the Lupus 3 dark cloud found in a visible-light spectroscopic survey at the center of the Lupus 3 star-forming core. The light curves and spectra of the brightest X-ray sources with known T Tauri star counterparts were analysed.
Results. One hundred and two X-ray sources were detected in the 30 diameter field-of-view of the EPIC cameras, of which 25 have visible or near-IR counterparts that are known as pre-main sequence stars. Their X-ray luminosity ranges from 3
1028 to 3
1030 erg s-1. Two of these objects with mass estimates lower than 0.075
have an X-ray luminosity of about 4-7
1028 erg s-1, comparable with that of flaring young brown dwarfs. A linear correlation is found between the X-ray luminosity and the mass or volume of the stars that is qualitatively expected from some models of distributed turbulent dynamos. The EPIC spectra of the X-ray brightest sources can be fitted using optically thin plasma emission models with two components at temperatures in the ranges 3-9
106 K and 1-50
107 K, respectively. The large emission measure of hot plasma may be caused by disruptions of magnetic fields associated with an intense flaring activity, while the X-ray emission from the "cool'' plasma components may result from solar-type active regions. The emission measures of the plasma components are of the order of 1052 cm-3, typical of the values expected from coronal plasmas in T Tauri stars, post-T Tauri stars, and active late-type dwarfs in close binary systems. One property of the X-ray brightest stars in Lupus 3 that seems common among pre-main sequence stars is the low abundance of Fe.
Key words: stars: activity - stars: atmospheres - stars: coronae - stars: evolution - stars: low-mass, brown dwarfs - stars: late-type
The Lupus star-forming complex is one of the belts of low mass
star-forming regions within 200 pc from the Sun, together with those
of Chameleon, Ophiuci, R Coronae Australis, and Taurus. Its
population of T Tauri-stars is concentrated in four small subgroups
(Lupus 1-4) that are distinct, loosely connected entities scattered
over a large area of the sky and approximately at the same distance
(Hughes et al. 1993). A survey of the pre-main sequence population
associated with the Lupus clouds has been carried out by Schwartz
(1977), who extended the list of H
-emitting members previously
identified by Henize (1954) and The (1962). A detailed spectroscopy
study of many of the members identified in Schwartz's work is reported
by Hughes et al. (1994). These studies were performed on a survey
sample of "Classical'' T Tauri Stars (CTTS), and not on the total
population of pre-main sequence stars (PMS). The known PMS population
in Lupus was significantly increased when "Weak-line'' T Tauri Stars
(WTTS) were detected by
.
Identification of
sources in the area of the Lupus star-forming region has led to the
discovery of 136 new T Tauri stars (Krautter et al. 1997; Wichmann et al. 1997a), of which the large majority are WTTS. These stars are most
easily detected and identified as WTTS in the X-ray band. Due
to their lack of strong emission lines in the optical spectral range,
most of the WTTS are not detected by optical objective prism surveys
like those carried out by The (1962) and Schwartz (1977). The
X-ray survey of Krautter et al. (1997) excludes all X-ray sources without
a stellar counterpart of R magnitude
15, which
converts to
16 for an M2V star. The survey of
Wichmann et al. (1997a) was limited to
All Sky Survey
(RASS) sources with GSC counterparts that are rarely fainter than
14. For comparison, some of the H
survey sources identified by Schwartz (1977) reach a V magnitude
17.5 (Hughes et al. 1994).
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Figure 1:
Combined MOS1, MOS2, and PN image of the Lupus 3 region
in the 0.5 to 4.5 keV band. The bright sources in the center of the
field of view are HR 6000 (A1.5 III) and the ![]() |
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Recently, special attention was paid to the Lupus 3 cloud, the one
containing the richest stellar aggregate. Nakajima et al. (2000)
identified new faint, moderately obscured members with estimated masses
near the sub-stellar limit by means of near-infrared imaging. This
confirmed the unusual mass spectrum of stars in this region which
exhibits a higher proportion of very low mass stars than that found in
other star-forming regions (Hughes et al. 1994). I report on
the analysis results of an
observation of the Lupus 3
region performed in 2003. Section 2 describes the X-ray observations
and the data reduction procedure, while Sect. 3 presents the analysis
results. Section 4 describes correlations that were found
between the X-ray emission of the stars and their main physical
parameters. The study results are summarized in Sect. 5.
The Lupus 3 region (see Fig. 1) was observed by the
space observatory (Jansen et al. 2001) in revolution 685, on 6 September 2003 (see Table 1). The satellite observatory uses
three grazing incidence telescopes that provide an effective area
higher than 4000 cm2 at 2 keV and 1600 cm2 at 8 keV (Gondoin
et al. 2000). One CCD EPIC pn camera (Strüder et al. 2001) and two EPIC MOS cameras (Turner et al. 2001) at the prime focus of the
telescopes provide imaging in a 30 arcmin field of view and broadband
spectroscopy with a resolving power of between 10 and 60 in the energy
band 0.3 to 10 keV. Two identical RGS reflection grating spectrometers
behind two of the three X-ray telescopes in front of the MOS cameras
allow higher resolution (
to 500) measurements of
bright sources in the soft X-ray range (den Herder et al. 2001). The
observations of the Lupus 3 region were conducted with the EPIC camera
operating in full frame mode (Ehle et al. 2001). The EPIC pn and MOS observations were performed with exposure times of 21 ks and 22.6 ks,
respectively. A "thick'' aluminum filter was used in front of the
EPIC cameras to reject visible light from the stars.
Table 1:
Lupus 3 observation log during
revolution 685.
Table 2: Counterparts to X-ray sources detected in the EPIC field of view. Column [1] gives the identifying numbers of the sources. Columns [2] and [3] provide the identifiers of IR counterparts from the IRAS and 2MASS catalogues. Columns [4]-[6] provide the identifiers of visible counterparts (with prefix "[KWS97]TTS'' or "Lupus 3'' from Krautter et al. 1997; with prefix "SZ'' from Schwartz 1977; with prefix "[CFB2003]Par-Lup'' from Comeron et al. 2003; with prefix [GSC] from the Guide Star Catalogues; with prefix [LEM]Lup from Lopez Marti et al. 2005). The identifiers of previous X-ray detections are indicated under Col. [7]. The B, V magnitudes (Ochsenbein 1974; Mermilliod 1987) and spectral types (Houk & Cowley 1975; Jaschek 1978) of the counterparts are given in Cols. [8]-[10], respectively.
Individual PN, MOS1, and MOS2 images of the Lupus 3 region were built
in the 0.5 to 4.5 keV band using all events registered during the
observation. The source detection was performed on the individual
images, using the standard source detection method of the
Science Analysis Software (SAS) that operates in
four steps. In a first step, source candidates are identified by a sliding cell method using the SAS package "eboxdetect''. A small cell surrounded by a larger cell is moved over the X-ray images. The background counts are estimated from the area surrounding the small
cell. If the counts in the small cell are significantly larger than in
the background, the cell is considered to have excess counts. The
background rate was about 0.015 s-1 and 0.05 s-1 on average
with a maximum value during short time intervals of 0.15 s-1 and
0.5 s-1 for the MOS and PN, respectively. After sliding the cell
over the whole images, these zones of excess counts are considered as
source candidates. In a second step, a local background image is
created by the SAS package "esplinemap''. The source candidates
obtained in step 1 are subtracted from the X-ray image, and a spline
fit is applied to the resulting image to obtain the background count
distribution. In a third step, the sliding cell method is performed
again with the SAS package "eboxdetect'' to obtain reliable source
candidates. This time, only the small size cell is moved over the
X-ray images. The local background counts are estimated locally using
the background count distribution obtained in the second step. In the
last step, a two-dimensional fit is performed on the spatial count
distribution of the source candidates by the SAS package "emldetect''
using the model of the telescope point spread function and of the
background from step 2 and taking into account contamination by nearby
sources. The task checks the detection significance of the candidate
sources and calculates the count rates and their uncertainties.
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Figure 2:
Light curves of the X-ray brightest TTS detected in the
Lupus 3 subgroup. The curves represent the count rates in the 0.5 to 4.5 keV
band. The X-axis is the time since the beginning of the
observation. Events are binned in 900 s time intervals for SZ 96, TTS 106,
TTS 110, and TTS 116 and in 1800 s time intervals for the other
sources. Bars represent a ![]() ![]() |
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Figure 3:
Best-fit models to the EPIC PN spectra of the X-ray brightest
sources in the Lupus 3 subgroup. The names of the stars are
indicated on the top of each graph. The EPIC data (crosses) and
spectral fit (solid line) are shown in the upper panel of each graph
and the ![]() |
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As a result of this procedure, 102 X-ray sources were detected with a likely threshold above 12, corresponding to a significance of 4.4
for a Gaussian statistic, within the 30
diameter of the EPIC PN, MOS1, and MOS2 fields of view. Hence, the
probability is low of a single spurious detection among the 102 sources. The coordinates and count rates corrected for the effective
exposure time of the 102 X-ray detected sources are given in Table 6 (in the on-line version only). A significant number of
sources are not detected in the MOS1 or MOS2 cameras due to their
lower sensitivities and a field dependant vignetting effect of the
telescope exit pupils by the Reflection Grating Spectrometers. The
source S1 is detected by the MOS cameras, but falls outside of the PN camera field of view. The source HR 6000 (A1.5III) subject of the
initial PI observation proposal is not included in Table 6. The
detected sources were correlated with the list of Herbig-Haro
objects and H
emission stars from Schwartz (1977)
that contains mainly Lupus 3 CTTS, with the catalogue of weak-line
TTS in Lupus (Krautter et al. 1997), and with a recent list of new
low-mass members of the Lupus 3 dark cloud found in a visible-light
spectroscopic survey at the center of the Lupus 3 star-forming core
(Comeron et al. 2003). The correlation radius was 4
based
on the astrometric accuracy of the
detections. Twenty-four counterparts were found (see Table 2)
including two possible candidates Par-Lup3-1 and its companion
Par-Lup3-1/cc1 for one single X-ray detection. IR counterparts were
searched for in the IRAS (Kleinmann et al. 1986) and in the Two Micron All-Sky Survey (2MASS) catalogues. Only one new counterpart
was found; it is known as Lup 608s, a low-mass member of the Lupus 3
cloud recently detected in a multi-band optical survey for very
low-mass stars and brown dwarfs in the Lupus clouds (Lopez Marti et al. 2005). Correlation with visible catalogues including the Guide
Star Catalog (Lasker et al. 1990) lead to one last counterpart
identification (GSC 07851-01158) not listed as a member of the
region, although its magnitude, color index, and proper motion are not
inconsistent with Lupus membership. Only 6 sources were detected
previously in X-ray, namely TTS 106, TTS 107, Lupus 3 31, TTS 110, TTS 111, and TTS 116 (Krautter et al. 1997). Of special interest are the 14 stars (Sz 94, Sz 95, Sz 96, Sz 98, Sz 100, Sz 101, Sz 102, Sz 107, Sz 108, Sz 109, Sz 110, Sz 112, Sz 114, and Sz 115) studied in detail by
Hughes et al. (1994) and the four low-mass members (Par-Lupus3-1,
Par-Lupus3-1/cc1, Par-Lupus3-2, and Par-Lupus3-3) recently discovered
by Comeron et al. (2003).
The light curves and spectra (see Figs. 2 and 3) of the brightest
X-ray sources with known TTS counterparts (see Table 2) were built
from photons detected with the PN camera within windows of
about 30
diameter around the targets. The background was
estimated on the same CCD chips as the sources, within windows of the
same sizes offset from the source positions in empty field regions. The
light curves were accumulated in the 0.5 to 4.5 keV band. To
remove variable contributions from the background, the background
light curves were subtracted from the light curves of the sources
using the "lcmath'' program of the XRONOS general purpose timing
analysis package (Johnson 2004). The calibrated light curves were then
tested for variability using the XRONOS task "lcstats'' that performs a Kolmogorov-Smirnov test on binned light curves. None of the X-ray bright sources displayed a light curve compatible with that of a constant source.
Table 3: Best-fit parameters to the EPIC spectra using a two components MEKAL model (Mewe et al. 1985) with variable abundance and with a hydrogen column density derived from the visual extinction.
The Pulse-Invariant (PI) spectra of the brightest X-ray sources were
rebinned such that each resulting channel had at least 10 to 20 counts per bin depending on source counts. An EPIC response matrix and an ancillary response file that includes an aperture correction were generated by the SAS task "rmfgen'' and "arfgen'' for each
individual source. All fits were performed using the XSPEC package
(Arnaud & Dorman 2001), and minimization was used for
spectral fitting. The spectral fitting was performed in the 0.3-4.5 keV spectral band with a MEKAL optically thin plasma emission model
(Mewe et al. 1985). The WABS model for photoelectric absorption was
used to take extinction by the interstellar medium into
account. The neutral hydrogen column density
was estimated
from the visual extinction
using the relation
(cm-2)/
= 1.774
1021 following the
detailed investigation of absorption in the local environment of T Tauri stars by Paresce (1984) and Vrba & Rydgren (1985). The visual
extinctions were obtained from Hughes et al. (1994) for Sz 96, Sz 98,
Sz 108, and Sz 114, from Krautter et al. (1997) for TTS 106, TTS 110,
and TTS 116 and from Comeron et al. (2003) for Par-Lup3-1/cc1. No
single temperature plasma model that assumes either solar photospheric
(Anders & Grevesse 1989) or non-solar abundances can fit the spectra
of Sz 96, Sz 98, Sz 108, Sz 114, TTS 106, TTS 110, and TTS 116, as
unacceptably large values of
are obtained. A MEKAL plasma
model with two components at different temperatures, but having the
same metallicity, proves acceptable for all data. The best-fit parameters
of the two temperature components plasma model are given in Table 3. The emission measures of the different plasma components in the
X-ray brightest sources of the Lupus 3 region are of the order of 1052 cm-3, in agreement with the emission measure values of coronal
plasma observed in some T Tauri stars stars (Argiroffi et al. 2005;
Stelzer & Schmitt 2004) for which high resolution X-ray spectra could
be obtained because they are bright and close enough. A comparison
with the much larger sample of PMS stars detected in the Orion Nebula
Cluster by the COUP project (Getman et al. 2005; Feigelson et al. 2005) suggests that these objects are not representative of the
general PMS population. The emission measure of the X-ray bright Lupus 3 stars is intermediate between that of the bulk of the COUP sources
and the brightest in the COUP sample, likely because this latter is
much larger than that of the Lupus 3 region. Emission measure values
similar to those measured on the X-ray brightest Lupus 3 TTS are found
in post-T Tauri stars (Argiroffi et al. 2004) and in late-type dwarfs
that are members of close binary systems (e.g., Gondoin 2004a,b).
Spectral fitting of EPIC data yields flux measurements in the
0.3-2 keV and 2-4.5 keV bands. These measurements were converted
into X-ray luminosities
and
using a distance d= 140 pc (Hughes et al. 1993). Table 4 gives the
X-ray luminosities of the stars and the hardness ratios hr of their
X-ray emission defined as
.
The X-ray luminosities,
plasma temperatures, and emission measures in Lupus 3 X-ray brightest TTS are comparable to those of rapidly rotating main-sequence stars in
close binary systems. Isolated main-sequence stars generally have
lower activity levels.
Table 4: X-ray luminosities of the X-ray brightest T Tauri stars detected in the Lupus 3 "star-forming'' region in the 0.3-2.0 keV and 2-4.5 keV energy bands corrected for interstellar absorption.
One property of the X-ray brightest stars in Lupus 3 that distinguish
them from normal late-type stars is the low abundance of Fe obtained
where abundance is left as a free parameter in the spectral
fitting. The peculiar chemical compositions and in particular low Fe abundance noticed in some T Tauri stars may be due to the
composition of the original molecular cloud that formed the stars or
to a settling of dust in a circumstellar disk (Argiroffi et al. 2005;
Stelzer & Schmitt 2004). Dust continuum emission at mm wavelength
from Lupus 3 TTS has been reported (Nürnberger et al. 1997) and
strong H
and CaII infrared triplet emission in the
spectra of many Lupus 3 stars is interpreted as a tracer of accretion
processes (Comeron et al. 2003). Low coronal abundances seems to be
the norm rather than the exception for PMS stars (e.g., Imanishi et al. 2001a; Getman et al. 2005) and, in general, little relation has
been found between abundances and accretion. The spectral fits of the
X-ray brightest TTS in Lupus 3 do not support such a relation,
given that the star with the lowest abundances is a WTTS (SZ 96; see
Table 4). Moreover, low abundances are also found in more evolved, but
still very active 40-100 Myr old stars that do not accrete any longer,
e.g., in the Pleiades (Briggs et al. 2003), in Bianco 1 (Pillitteri et al. 2004), and in IC 2391 (Marino et al. 2005).
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Figure 4: Energy conversion factor in the 0.5-4.5 keV energy range for the EPIC pn camera equipped with a thick filter versus the hydrogen column density. The energy conversion factor was calculated using an emission model of optically thin plasmas with temperatures ranging from 106.6 to 107.4 K. |
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Most X-ray bright sources in Lupus 3 exhibit a hard X-ray spectrum
with a flux above 2.0 keV that sometimes exceeds 20% of the flux in
the 0.3-2.0 keV band. Hot plasmas at temperatures in the range (1-50)
107 K contribute to most of the luminosity above 2 keV. Their emission measure is comparable and sometimes larger that
those of the cool plasma components at (3-10)
106 K. Although accreting material may provide a heating mechanism for
the emitting plasma, shock-heated plasma cannot attain temperatures
higher than a few MK. The evidence for hot plasma components with
large emission measures indicates that accretion shocks provide
at most a moderate contribution to the X-ray emission of the
brightest Lupus 3 stars. In active stellar coronae, it has been
proposed that the peak in emission measure around 107 K is due to
flaring activity (Drake et al. 2000; Sanz-Forcada et al. 2002). The
large emission measure of hot (T > 107 K) plasma in Lupus 3
TTS may thus be caused by disruptions of magnetic fields associated
with an intense flaring activity, while the X-ray emission from the
"cool''
3-9
106 K plasma components may
result from solar-type active regions. As commonly interpreted for WTTS, the
origin of the X-ray emission in the X-ray brightest Lupus 3 TTS could
be a scaled-up solar-type magnetic activity.
To estimate the X-ray luminosities of the faintest sources from
their count rates, I calculated energy conversion factors for
the EPIC pn camera (see Fig. 4) using the Portable, Interactive,
Multi-Mission Simulator (PIMMS; Mukai 1993) in the 0.5-4.5 keV range
for optically thin plasmas with temperatures comparable to those found
in the spectral fitting of the brightest sources (see Table 3). For
absorbing hydrogen column densities lower than 1021 cm-2,
the energy conversion factor in the 0.5-4.5 keV band is flat and
well-approximated by ECF = 3.7
1011 counts erg-1 cm2 for plasma temperatures in the range (4-25)
106 K. For converting the count rates of faint sources that could not be
studied spectroscopically into X-ray luminosities, a 107 K
optically thin plasma was assumed that is absorbed by an hydrogen
column density derived from the visual extinction
(Hughes
et al. 1994; Krautter et al. 1997; Comeron et al. 2003; see
Sect. 3.1). This procedure was also used to estimate the upper limits
of the X-ray luminosities of known low-mass stars in the EPIC field of
view that were not detected in X-rays. The count rate upper limits of
these sources were derived as follows. First, event numbers
were estimated within extraction windows of radius rcentered on the x, y detector position of each undetected source. For
each value
,
the approximate formula
=
(Gehrels 1986) was used to calculate
the 99.9989% confidence level upper limit
that
corresponds to the S = 4.4
in Gaussian statistics selected
for the source detection algorithm. For each position of undetected
source, the local average background
per window of radius r was
then estimated, assuming a uniform background. Finally, the upper limit
of the source count rate was calculated as follows:
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(1) |
Table 5:
Properties of known low-mass TTS in the EPIC field of view. Column [2] is the source name (with prefix "SZ'' from Schwartz 1977, "TTS'' from Krautter et al. 1997, and "Par-Lup'' from Comeron et al. 2003). Column [3] gives the spectral type. Columns [10] and [11] provide the the H
equivalent width and the
(H-K) color excess from Hughes et al. (1994), Wichmann et al. (1997b), and Comeron et al. (2003).
The effective temperatures and bolometric luminosities derived from
the catalogues of Hughes et al. (1994), Krautter et al. (1997), and Comeron et al. (2003), were used to place the stars in an HR diagram
(see Fig. 5) and to compare their positions with evolutionary models
of solar metallicity low-mass stars computed by Baraffe et al. (1998; hereafter B98). The comparison yields estimates of masses
and ages. The median mass is about 0.2
and the mass vs. age
diagram (see Fig. 6) shows that all stars more massive than 0.5
were detected in X-rays. X-ray emission was detected from
stars with high bolometric luminosities, i.e., preferably from the
more massive or from the youngest stars. Three objects in the
observed Lupus 3 region (SZ 107, SZ 109, and Par-Lup3-1) have mass
estimates lower than the brown dwarf limit of about 0.075
for solar metallicity. These mass estimates are, however,
uncertain since the HR diagram positions of these candidate brown
dwarfs fall outside the B98 evolutionary tracks. Remarkably, two of
these young (age < 1 Myr) and low-mass objects, namely SZ 107 and
SZ 109, are emitting X-rays with a luminosity of about 4-7
1028 erg s-1. Young brown dwarfs are known to emit in
X-rays (Neuhäuser & Comeron 1998; Neuhäuser et al. 1999;
Imanishi et al. 2001a,b; Preibisch & Zinnecker 2001; Mokler &
Stelzer 2002; Feigelson et al. 2002; Tsuboi et al. 2003). From the
first spectral analysis of a flaring brown dwarf, Ozawa et al. (2005) derived an X-ray luminosity of
= 19
1028 erg s-1, comparable with the X-ray luminosity
estimates of SZ 107 and SZ 109.
This section describes the X-ray properties as a function of
stellar parameters of a sample of stars within the EPIC field of view
that were known prior the
observation. The sample
(see Table 5) consists of all stars in the EPIC field of view known
from the catalogues of Schwartz (1977), Krautter et al. (1997), and
Comeron et al. (2003). It is biased towards CTTS since 66% of the
stars were identified in the H
survey performed by Schwartz
(1977). Some of the H
survey sources identified by this
author reach a V magnitude
17.5 (Hughes et al. 1994). The X-ray survey of Krautter et al. (1997) excludes
all X-ray sources without a stellar counterpart of R magnitude
15, which converts to
16 for an M2V star.
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Figure 5:
H-R diagram of low mass stars in the EPIC field of view
compared to evolutionary tracks (Baraffe et al. 1998). The solid
lines from right to left describe the evolutionary tracks of 0.05 ![]() ![]() ![]() ![]() ![]() |
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Figure 6: Mass vs. age diagram of known low mass stars in the EPIC field of view. Black circles mark stars that have been detected in X-rays. Ages marked as 1 Myr are actually upper limits. |
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Figure 7: X-ray luminosities vs. bolometric luminosities of known low mass stars in the EPIC field of view. Black circles mark X-ray detections and triangles mark upper limits. The thick gray line shows the EM algorithm regression fit computed with ASURV (see text). |
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Figure 7 shows a plot of the X-ray luminosities including upper limits
versus the bolometric luminosities of the Lupus 3 stars. All stars
with bolometric luminosities greater than 0.13
were detected
in X-rays. The X-ray detected stars have log(
) > -4.2 and are therefore more X-ray active than the Sun for which
log(
)
-6.5 is an average during the
course of the solar cycle. For the most active stars (e.g., TTS 106,
TTS 110, and TTS 116), log(
)
-3, which
is known as the "saturation limit'' for coronally active stars
(Fleming et al. 1989; Patten & Simon 1996; Randich et al. 2000;
Pizzolato et al. 2003). Only one star (SZ 96; M1.5) exceeds this
saturation limit, possibly because it was flaring during the
observation, as suggested by its light-curve variability. A correlation between
and
is noticeable in
Fig. 7 with, however, a large scatter. A linear regression to the
vs.
plot for X-ray detected sources
(i.e., excluding upper limits) yields log(
[erg s-1]) = 30.0(
0.2) + 0.9(
0.2)
log(
/
)
with a standard deviation of 0.58 in log(
). This relation is similar to the relation found for other young
clusters (see Feigelson & Montmerle 1999) and is consistent with a linear relation between X-ray and bolometric luminosity characterized
by
log(
/
)
= -3.5
0.4. However, the
/
upper limits for the Lupus stars that were not
detected in X-rays are most often lower than this value. Hence, I used
the ASURV analysis package (Feigelson & Nelson 1985; Isobe et al. 1986;
LaValley et al. 1990) for a statistical investigation of the relation
between
and
,
taking into account not only
X-ray detections, but also upper limits of the X-ray luminosity. The
ASURV software provides the maximum-likehood estimator of the censored
distribution, several two-sample tests, correlation tests, and linear
regressions. The linear regression fit with the parametric estimation
maximization (EM) algorithm in ASURV yields log(
[erg s-1]) = 30.4(
0.2) + 1.7(
0.3)
log(
/
), suggesting that the relation between
and
is no longer linear when X-ray faint objects are
taken into account. It is worth noting that this relation is much
steeper than the linear regression fit to log(
) vs.
log(
)
(slope = 0.42
0.05) found by Preibisch et al. (2005) for a sample of nearby G-, K-, and M-type field
stars. The relation is also steeper than the linear regression fit to
log(
) vs. log(
)
(slope = 1.04
0.06) found by the same authors for the Orion Nebula Cluster stars
that are similar in age to Lupus 3 members. The difference could
result from the bias in the selection of the Lupus sample that is
based for the most part on an H
survey. This bias is
especially important among stars with X-ray upper limits (see Table 5)
that are responsible for increasing the slope from 0.9 to 1.7 (see
Fig. 7). This also suggests a difference between accreting and
non-accreting stars with a steeper slope for the accreting stars that
dominate the Lupus 3 sample. Preibisch et al. (2005) find two different relations for accretors and non-accretors, but with an opposite effect on the log(
) vs. log(
) relation since the non-accretors in the ONC show a steeper power-law
slope. However, these authors noted that the accreting stars show a scatter in log(
) vs. log(
) correlation
considerably larger than expected from X-ray variability. They argue
that some fraction of this scatter may be due to the fact that the
more rapidly accreting stars may have large errors in stellar
luminosity and effective temperature values due to the effect of
accretion on the observables that lead to these quantities.
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Figure 8: X-ray luminosities vs. masses ( left) estimated from B98 evolutionary models and versus volumes ( right) for known low-mass stars in the EPIC field of view. Black circles represent X-ray detection and triangles indicate upper limits. The thick gray lines show the EM algorithm regression fit computed with ASURV (see text). |
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The X-ray surface fluxes (i.e., X-ray luminosities divided by the
stellar surface area) of the Lupus 3 stars are included between 105 and 108 erg s-1 cm-2. These values are
comparable with the range of X-ray fluxes found for different
structures in the solar corona. Coronal holes show X-ray fluxes around
104 erg s-1 cm-2, while active regions show fluxes of
up to 108 erg s-1 cm-2. The similarity of the X-ray surface
flux range for late-type stars and for different constituents of the
solar corona have already been noted, e.g., in Schmitt (1997) and in
Peres et al. (2004). The X-ray surface fluxes strongly decrease at
effective temperatures below 3500 K, corresponding to M-type
stars. This effect results from the dependence =
/
,
in
which
is the Stefan-Boltzmann constant and
/
does not vary much with
.
Mass estimates from PMS evolutionary models are subject to significant
uncertainties (Baraffe et al. 2002). Different models or temperature
scales can lead to differences of as much as a factor of 2 in the mass
estimates (e.g., Luhman 1999; Hillenbrand & White 2004). As a test to what
extent the vs. mass relation is dependant on the choice of
the model, I compared the relation found for the masses derived from
the B98 models to those based on stellar masses estimated by Hughes et al. (1994) from D'Antona & Mazzitelli (1994; DM94 hereafter)
evolutionary tracks that used the Canuto & Mazzitelli (1992) convection
model and opacities from Alexander et al. (1991). Mass estimates from
DM94 tracks (Hughes et al. 1994) are only available for the
Schwartz (1977) sample stars. A clear correlation is found between
X-ray luminosities and masses (see Fig. 8, left) estimated from either
the DM94 or B98 evolutionary tracks. The B98 model leads to an EM linear regression fit of log(
[erg s-1]) = 29.6(
0.2) + 0.9(
0.3)
log(M/
)
with a standard deviation of 0.60, while the DM94 yields a relation
log(
[erg s-1]) = 29.7(
0.6) + 1.3(
0.8)
log(M/
)
with a standard deviation of 0.67. The B98 and DM94 models thus give
consistent
vs. mass dependences with power-law slopes
similar to those found for TTS in the Orion Nebula Cluster and for
nearby G-, K-, and M-type field stars (Preibisch et al. 2005). These
slopes of 0.9 and 1.3 are lower than those found for the Chameleon
star-forming region (slope = 3.6
0.6 in the mass range 0.6-2
;
Feigelson et al. 1993), for the very young stellar
cluster IC 348 (slope = 2.0
0.2 in the mass range 0.1-2
;
Preibisch & Zinneker 2002), or for that derived for
M-type field stars (slope = 2.5
0.5 in the mass range 0.15-0.6
;
Fleming et al. 1988). The differences in slopes are in
part due to differences in the considered mass ranges and in the
methods used to estimate stellar masses. Also, studies prior to
and
had to deal with large numbers of
X-ray upper limits, which perhaps caused the typical X-ray luminosities
of very low-mass stars to be underestimated (Preibisch et al. 2005).
The X-ray luminosities of the Lupus TTS also show a correlation with
the stars' internal volumes (see Fig. 8, right). The EM linear regression
fit gives log([erg s-1]) = 28.9(
0.1) + 0.9(
0.3)
log(V/
)
with a standard deviation of 0.59. This suggests a magnetic flux
dependence on mass and volume. Although the dependence on mass may be
unexplained, the importance of volume is qualitatively expected from
some models of distributed turbulent dynamos, where the magnetic field is both
generated and transported in the convection zone. Such a dynamo could
be an important source of magnetic activity in Lupus 3 TTS that are
most likely fully convective, and for which the standard solar-like
dynamo, which is anchored at the boundary between the
convective envelope and the inner radiative core, should not
work. Several studies, however, indicate that accretion could change
the stellar structure (Prialnik & Livio 1985; Wuchterl & Tscharnuter
2003) so that even for moderate accretion rates, the stars are no
longer fully convective. Quantitative relationships between magnetic
field generation and stellar parameters for the candidate dynamo
mechanisms would be needed to understand the meaning of
vs.
mass or volume correlation in Lupus 3 TTS.
X-ray activity is known to be present during the evolution of pre-main
sequence stars both in the initial evolutionary stage of a "classical
T Tauri Star'' (CTTS), during which the star has an accretion disk
that surrounds it, and in the subsequent "weak line T Tauri Star''
(WTTS) phase in which the star has no accreting material and is
approaching the zero-age main sequence (Feigelson & Montmerle
1999). The origin of the X-ray emission from WTTS is
commonly interpreted in terms of scaled-up solar-type magnetic
activity, but the situation is less clear for the accreting
CTTS. While nothing should prevent the formation of magnetic fields
and ensuing coronae in CTTS, X-rays could also be produced above the
circumstellar disk or at the star-disk interface, e.g., above the hot
spot where a magnetically funneled accretion flow impacts on the
stellar surface (Lamzin et al. 1996). One way to separate CTTS from
WTTS consists of using the H
emission and the IR excess
emission
(H - K) calculated by de-reddening the observed colors
and subtracting the color of the photosphere of the correct spectral
type (Hughes et al. 1994). The infrared excess emission is a tracer of
circumstellar material, while H
line emission is
thought to be a tracer of accretion. CTTS are often defined as having
an H
equivalent width greater than 10 Å. Among the
18 known CTTS in the EPIC field of view, 12 were detected in
X-rays. Figure 9 plots the logarithm of the H
equivalent width against the IR excess emission
(H - K) of
known low-mass stars in the EPIC field of view. Figure 9 shows that most of
the known TTS in the EPIC field of view have considerable H
emission since they were discovered by Schwartz (1977)
in a deep red prism objective survey aiming to detect the prominent
red emission line. Hence, the sample of Lupus 3 TTS described in
Table 5 is most likely incomplete for WTTS that are often
found in star-forming regions through X-ray observations
(Neuhäuser 1997). Some of the 75 objects detected in
X-rays with EPIC and that have no visible or near-infrared
counterpart (see Table 6) could belong to this category [WTTS].
This bias in the study sample limits the significance of any
statement on the X-ray properties's dependence on accretion or on
the presence of a disk. The influence of accretion disks surrounding
young PMS stars on their observed X-ray activity level is
debated. Many studies found no indication that the presence of an accretion disk modifies activity levels in those stars (e.g., Lawson
et al. 1996; Grosso et al. 2000; Preibisch & Zinnecker 2001, 2002; Feigelson et al. 2002; Getman et al. 2002). On the other hand, CTTS
belonging to different clusters or associations are reported to be
sub-luminous in the X-ray band with respect to WTTS
(e.g., Neuhäuser et al. 1995; Stelzer & Neuhäuser 2001;
Flaccomio et al. 2003b,c).
![]() |
Figure 9:
Logarithm of the H
![]() ![]() ![]() ![]() ![]() |
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![]() |
Figure 10:
X-ray luminosities vs. H
![]() ![]() ![]() ![]() ![]() |
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Figure 9 shows that some of the stars with large H
emission have little or no IR emission above the photosphere. This is
quite puzzling since accretion obviously requires the presence of a disk. One explanation is that the
(H - K) color excess does
not discriminate well between cool low-mass stars with and without
optically thick inner disks since the K-band excess traces only the
hottest dust in the innermost region of the central star. Actually,
many stars with circumstellar material show significant excess
emission only at longer wavelengths (Haisch et al. 2001). This could
explain why no correlation is found between the X-ray luminosities
and the
(H - K) color excesses.
The Cox proportional hazard model in the ASURV software package shows
the existence of a relationship between the logarithms of the
H
equivalent width (EW
)
and the
X-ray luminosity. The EM linear regression (i.e., including upper
limits) to the log(
) vs. log(EW
)
curve yields log(
[erg s-1]) = 29.5(
0.3) - 0.5(
0.2)
log(
)
with a standard deviation of 0.6 (see Fig. 10,
left). This relation, that indicates a decreasing X-ray luminosity with
increasing H
equivalent width, supports the view that
CTTS are sub-luminous in the X-ray band with respect to WTTS. The
dispersion of the measurements around the best linear fit could
result from non-simultaneous measurements of the X-ray luminosities
and H
equivalent widths. Damiani & Micela (1995)
argued that, in the case of CTTS, the H
equivalent
width may not be a meaningful indicator since it measures the ratio
between line and adjacent continuum emission that, unlike normal
stars, does not arise from contiguous atmospheric layers. Indeed, CTTS
H
line profiles indicate plasma velocities greater than
a 100 km s-1 (e.g., Mundt 1984), with signatures of winds or
sometimes infalls (e.g., Hartmann & Kenyon 1990). For this reason, a quantity of more direct physical significance than the equivalent
width is the actual luminosity in the line above the continuum
level. The luminosity in the H
line has been measured
by Hughes et al. (1994) on a significant number of Lupus 3 stars (see
Fig. 10, right). No correlation was found between the logarithms of
the H
intensity and the X-ray luminosities. For this reason
and because of the sample bias towards CTTS, the present study is
unable to make any conclusive statement on the X-ray property
dependence of Lupus 3 stars with accretion or with the presence of disks.
![]() |
Figure 11: X-ray luminosities vs. ages. Black circles represent X-ray detections and triangles indicate upper limits, as explained in the legend. |
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Analysis of
observations of young stellar clusters
suggested that magnetic activity measured in the units
increases during the first few Myr to the saturation
level log(
)
-3 as the circumstellar
disk disappears and declines on timescales of 107-108 yr
in a mass dependant fashion (Flaccomio et al. 2003). This conclusion
does not result from the study of the X-ray luminosity evolution
within any given star-forming region, but from a comparison of
several star-forming regions with different mean ages. Such a systematic
behavior is not apparent for Lupus 3 stars (see Fig. 11) although
statistical tests indicate a correlation between the X-ray luminosity
and age of the individual stars. Some of the young TTS with age of a few Myr have an X-ray-to-bolometric-luminosity-ratio one order of
magnitude below the saturation limit. On the contrary, a few objects
(e.g., TTS 107 and TTS 111) with similar masses (0.60
M
0.90
), but ages over 20 Myr, are still
emitting in X-rays at a significant fraction of the saturation
limit. One problem in studying the X-ray luminosity vs. age
dependence for any given star-forming region comes from the large
uncertainties on the PMS age determination (see, e.g., Hartmann
2001). Age estimates are affected by the choice of the evolutionary
tracks and the empirical determination of the bolometric luminosities
and effective temperatures. Bolometric luminosities have several
potential source of errors, including intrinsic photometric
variability that can be very large for accreting systems, error in the
estimation of extinction that can be affected by circumstellar
material, and the possible presence of unresolved binary
companions. The evolution of X-ray emission in young stars in a mass
dependant fashion is an additional complication. Using mass-stratified
sub-samples of PMS in the Orion Nebula Cluster, Preibisch & Feigelson
(2005) found a mild decrease in X-ray luminosity over the 1-100 Myr
range for ONC stars with 0.1
< M < 0.4
that contrasts with a faster decline for stars in the 0.5
< M < 1.2
mass range. In view of
the limited number of stars in the present sample, a mass dependant
evolution of X-ray emission cannot be investigated in the Lupus 3 region.
The
observation of the Lupus 3 subgroup in the Lupus
dark cloud complex leads to the detection of 102 sources in the EPIC field of view, of which 96 are new X-ray detections and 77 have no visible or near-IR counterparts. The detection of this large number of new X-ray sources is an interesting result since these are candidate
members of the Lupus 3 star-forming region. Their characterization
will allow a better understanding of the Lupus 3 region by the
determination of its initial mass function, star-forming rate, and
X-ray luminosity function. The EPIC pn, MOS1, and MOS2 source list was correlated with a list of known objects, mostly pre-main sequence stars (Schwartz 1977; Krautter et al. 1997; Comeron et al. 2003), located in a 15
radius field of view around the
pointing direction. Visible and near-IR counterparts were found for
25 sources including 12 CTTSs (i.e., with an H
equivalent
width greater than 10 Å; see Table 2) and three recently
discovered low-mass members. For converting the count rates of faint
sources that could not be studied spectroscopically into X-ray
luminosities, a 107 K optically thin plasma was assumed that is
absorbed by a hydrogen column density derived from the visual
extinction. This procedure was also used to estimate the upper limits
of the X-ray luminosities of known low-mass stars in the EPIC field of
view that were not detected in X-rays.
A sub-sample of 29 stars in the EPIC field of view, including 22 X-ray
sources and 7 non-detections with known effective temperatures and
bolometric luminosities was studied in detail. A comparison of their HR diagram positions with evolutionary models of Baraffe et al. (1998)
yields estimates of their masses and ages. The study indicates that
X-ray emission is detected from stars with the highest bolometric
luminosities, i.e., preferably from the more massive or younger
stars. The stars detected in X-rays are more X-ray active than the
Sun. Their X-ray luminosities range between 3 1028 and 3
1030 erg s-1. One star
exceeds the saturation limit log(
)
-3.0 and was presumably flaring during the observation. Two young
(age < 1 Myr) objects with mass estimates lower than the brown dwarf
limit were detected at an X-ray luminosity level of 4-7
1028 erg s-1. The X-ray detected stars have X-ray surface
fluxes included between 105 and 108 erg s-1 cm-2that are comparable with the range of X-ray surface fluxes found for
different structures in the solar corona. A linear correlation was
found between the X-ray luminosity and the mass or the volume of the
stars. Although the dependence on mass may be unexplained, a correlation with the stars' volume is qualitatively expected from some
models of distributed turbulent dynamos, where the magnetic field is
both generated and transported in the convection zone.
A temporal and spectral analysis was conducted on the 8 X-ray
brightest sources of the sample. All these sources displayed a significant X-ray variability during the 22 ks observation. Their
X-ray spectra could be fitted using an optically thin plasma emission
model with two components at temperatures in the range 3-9 106 K and 1-50
107 K, respectively. Although
accreting material may provide a heating mechanism for the emitting
plasma, shocks heated plasma cannot attain temperatures higher than a few MK. Hot (T > 107 K) plasma components in Lupus 3 TTS may rather be caused by disruptions of magnetic fields associated with an intense flaring activity. The temperatures
3-9
106 K of the "cool'' plasma components are reminiscent of solar
type active regions. The emission measures of the plasma components
are of the order of 1052 cm-3, in agreement with the values
expected from coronal plasma in T Tauri stars, post-T Tauri stars, and active
late-type dwarfs. One property of the X-ray brightest stars in Lupus 3
that is common among PMS is the low abundance of Fe. Dust continuum
emission at mm wavelength from Lupus 3 TTS has been reported
(Nürnberger et al. 1997) and strong H
and CaII infrared triplet emission in the spectra of many Lupus 3 stars is interpreted as a tracer of accretion processes (Comeron et al. 2003). The low Fe abundance does not seem to be related to accretion.
Acknowledgements
I am grateful to the anonymous referee for the helpful comments that allowed me to improve the paper.