XMM-Newton observations of the sigma Ori cluster. II. Spatial and spectral analysis of the full EPIC field

Abstract

We present the results of an XMM-Newton observation of the young (~2-4 Myr) cluster around the hot star sigma Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars down to the substellar limit. We detected eleven new possible candidate members from the 2MASS catalogue. We find that late-type stars have a median log L_X/L_bol ~ -3.3, i.e. very close to the saturation limit. We detected significant variability in ~40% of late-type members or candidates, including 10 flaring sources; rotational modulation is detected in one K-type star and possibly in another 3 or 4 stars. Spectral analysis of the brightest sources shows typical quiescent temperatures in the range T_1 ~ 0.3-0.8 keV and T_2 ~ 1-3 keV, with subsolar abundances Z ~ 0.1-0.3 Z_sun, similar to what is found in other star-forming regions and associations. We find no significant difference in the spectral properties of classical and weak-lined T Tauri stars, although classical T Tauri stars tend to be less X-ray luminous than weak-lined T Tauri stars. Comment: 20 pages, 10 figures, 6 tables. Accepted by A&A

Full text

Page 1

arXiv:astro-ph/0510209v1 7 Oct 2005
Astronomy & Astrophysics manuscript no. aa3605
(DOI: will be inserted by hand later)
February 4, 2008
XMM-Newton observations of the σ Ori cluster.
II. Spatial and spectral analysis of the full EPIC field⋆
E. Franciosini1, R. Pallavicini1, and J. Sanz-Forcada2
1INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy
2Astrophysics Division – Research and Science Support Department of ESA, ESTEC, Postbus 299, NL-2200 AG Noordwijk,
The Netherlands
Received 9 June 2005/Accepted 15 September 2005
Abstract. We present the results of an XMM-Newton observation of the young (∼ 2 − 4 Myr) cluster around the hot star
σ Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the
results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster
members, including very low-mass stars down to the substellar limit. We detected eleven new possible candidate members
from the 2MASS catalogue. We find that late-type stars have a median logLX/Lbol ∼ −3.3, i.e. very close to the saturation
limit. We detected significant variability in ∼ 40% of late-type members or candidates, including 10 flaring sources; rotational
modulation is detected in one K-type star and possibly in another 3 or 4 stars. Spectral analysis of the brightest sources shows
typical quiescent temperatures in the range T1∼ 0.3−0.8 keV and T2∼ 1−3 keV, with subsolar abundances Z ∼ 0.1−0.3Z⊙,
similartowhat isfound inother star-formingregionsand associations. Wefindnosignificant difference inthespectral properties
of classical and weak-lined T Tauri stars, although classical T Tauri starstend tobe less X-rayluminous than weak-lined T Tauri
stars.
Key words. open clusters and associations: individual: σ Ori – stars: activity – stars: coronae – stars: pre-main sequence –
stars: late-type – X-rays: stars
1. Introduction
The σ Ori cluster was discovered by ROSAT (Wolk 1996;
Walter et al. 1997) around the O9.5V binary star σ Ori AB
(which results from two components separated by 0.2′′). It be-
longs to the Orion OB1b association and is located at a dis-
tance of 352+166
−85
pc (from Hipparcos, ESA 1997). In addi-
tion to several hot stars, it is known to contain more than 100
likely pre-main sequence (PMS) late-type stars within 30′of
σ Ori (Wolk 1996; Sherry et al. 2004), as well as several brown
dwarfs and planetary-mass objects (B´ ejar et al. 1999, 2001,
2004; Zapatero Osorio et al. 2000; Caballero et al. 2004). The
estimated age of the cluster is 2−4 Myr (Zapatero Osorio et al.
2002; Oliveira et al. 2002; Sherry et al. 2004).
We have observed the σ Ori cluster using XMM-Newton.
The observation was centered on the hot star σ Ori AB, in or-
der to obtain both a high-resolution RGS spectrum of the cen-
tral source and EPIC imaging data and low-resolution spec-
tra over the whole field. The analysis of the RGS and EPIC
spectra of the hot star σ Ori AB has been presented in a pre-
Send
francio@astropa.unipa.it
⋆Based on observations obtained with XMM-Newton, an ESA sci-
ence mission with instruments and contributions directly funded by
ESA Member States and NASA
offprint requeststo: E.Franciosini, e-mail:
vious paper (Sanz-Forcada et al. 2004, hereafter referred to as
Paper I), togetherwith the analysis of the EPIC spectra of other
three nearby bright sources that could potentially contaminate
the RGS spectrum of σ Ori AB. Of these, only the B2Vp star
σ Ori E was found to significantly affect the RGS spectrum: its
EPIC spectrum allowed us to correct for its contribution, and
therefore to derive the emission measure distribution and ele-
mental abundancesof σ Ori AB. We foundthat σ Ori AB has a
much softer spectrum than the other sources, consistently with
a wind origin, however the RGS spectrum shows no evidence
for line broadenings and shifts with velocities>∼800 km s−1,
as could be produced by strong winds; the low f/i line ratio
in the Ne and O He-like triplets indicates either high density
(i.e. magnetic confinement either close to or far from the star)
or, more likely, a strong UV radiation field (i.e. emission close
to the star where the wind is however too weak to produce
shocks).
In Paper I we also reported the detection of a strong flare
from the magnetic hot star σ Ori E, which is not expected from
models of X-ray emission from winds. Based on the character-
istics of the flare and of the quiescent emission, whose spec-
trum is harder than that of σ Ori AB and consistent with those
oflate-typestars,wearguedthattheflareandmostofthequies-
centemissionis mostlikelyduetoanunseenlate-typecompan-

Page 2

2E. Franciosini et al.: XMM-Newton observations of the σ Ori cluster. II.
Fig.1. Composite EPIC (MOS1+MOS2+PN) image of the
σ Ori cluster
ion, although emission from the magnetic hot star itself cannot
be excluded (see discussion in Paper I).
In this paper we present the analysis of the full EPIC field,
in order to derive the X-ray properties of the other members
or candidates of the σ Ori cluster. The paper is organized as
follows. X-ray observations and data analysis are described in
Sect. 2. In Sect. 3 we discuss the X-ray properties of cluster
membersandcompareourresultswith thoseofotherstar form-
ing regions and young open clusters. Spectral analysis of the
brightest sources is presented in Sect. 4. Discussion and con-
clusions are given in Sect. 5.
2. Observations and data analysis
XMM-Newton observations of the σ Ori cluster, centered
on the hot star σ Ori AB, were carried out as part of the
Guaranteed Time of one of us (R.P.) using both the EPIC MOS
and PN cameras and the RGS instrument. The observation (ID
0101440301)startedat 21:47UTonMarch23,2002andended
at 9:58 UT on March 24, 2002, for a total duration of 43 ks.
The EPIC cameras were operated in Full Frame mode using
the thick filter.
Data analysis was carried out using the standard tasks in
SAS v.5.4.1. The analysis of the RGS data has been discussed
in Paper I and will not be repeated here. EPIC calibrated and
cleaned event files were derived from the raw data using the
standard pipeline tasks  and  and then applying
the appropriate filters to eliminate noise and bad events. The
eventfiles have also been time filtered in orderto excludea few
short periods of high backgrounddue to proton flares; the final
effective exposuretime is 41 ks for each MOS and 36 ks for the
PN. We limited our analysis to the 0.3 − 7.8 keV energy band,
since events below 0.3 keV are mostly unrelated to bona-fide
X-rays, while above 7.8 keV only background is present. The
combined EPIC (MOS1+MOS2+PN) image in the 0.3 − 7.8
keV energy band is shown in Fig. 1. Exposure maps for each
instrument in the same energyband were created using the task
.
Source detection was performed both on the individual
datasets and on the merged MOS1+MOS2+PN dataset us-
ing the Wavelet Detection algorithm developed at INAF-
Osservatorio Astronomico di Palermo (Damiani et al. 1997,
Damiani et al., in preparation),adapted to the EPIC case. From
the comparison of the count rates of common sources detected
on the individual datasets, we derived a median ratio of PN to
MOS count rates of ∼ 3.2; this value was then used as a scaling
factor for the PN exposuremap in the detection on the summed
dataset, in order to take into account the different sensitivities
of the PN and MOS cameras. This implies a resulting MOS
equivalent exposure time of ∼ 200 ks for the merged dataset.
Count rates derived from the detection on the summed dataset
are MOS equivalent count rates.
We used a significance detectionthresholdof 5σ, chosenin
order to have at most one spurious detection, and determined
from a set of 100 simulations of pure backgrounddatasets with
the same number of counts as the observation. After removing
a few obviously spurious detections (due to hot pixels, to the
point spread function structure of the central bright source, and
to out-of-time events) we obtained a total of 175 sources, three
ofwhichweredetectedonlyona singleinstrumentdataset (two
on MOS and one on PN).
2.1. The optical catalogue
The σ Ori cluster has been the subject of several studies af-
ter its discovery by ROSAT (Walter et al. 1997). Wolk (1996)
performed follow-up UBVRI photometry down to V ∼ 18
within 30′from σ Ori, finding ∼ 130 candidate PMS stars, and
obtained medium-resolution spectroscopy for a few of them.
Recently, Sherry et al. (2004) extended the survey to an area
of ∼ 0.89 deg2around the hot star down to V ∼ 20. Several
deep optical and infrared surveys of the cluster, with limiting
magnitude I ∼ 22 − 24, have led to the discovery of several
very-low mass stars and brown dwarfs as well as of a num-
ber of planetary-mass objects (B´ ejar et al. 1999, 2001, 2004;
Zapatero Osorio et al. 2000; Caballero et al. 2004). Additional
photometric RI surveys down to I ∼ 18 − 19 have been
performed by Scholz & Eisl¨ offel (2004) and Kenyon et al.
(2005). Low- and medium-resolution spectroscopic observa-
tions are available for a subsample of low-mass stars and
brown dwarfs (B´ ejar et al. 1999; Barrado y Navascu´ es et al.
2001, 2003; Mart´ ın et al. 2001; Zapatero Osorio et al. 2002;
Kenyon et al. 2005; Burningham et al. 2005).
We constructed our optical catalogue by including all stars
from the above-mentioned studies, plus a few additional Hα
emission line stars from Weaver & Babcock (2004) and the
hot stars from the studies of the Orion OB association by
Warren & Hesser (1977) and Brown et al. (1994). After cross-
correlatingthe data fromthe available studies, we obtaina total
of 266 stars falling in the XMM field of view. About 40% of
the late-type stars in the catalogue have spectroscopic mem-

Page 3

E. Franciosini et al.: XMM-Newton observations of the σ Ori cluster. II.3
1234
V - I
20
18
16
14
12
10
V
0.5 1.0 1.5
R - I
2.0 2.5
22
20
18
16
14
12
10
8
I
Fig.2. V vs. (V − I) and I vs. (R − I) colour-magnitude di-
agrams for stars in the XMM-Newton field of view. Detected
cluster members and candidates are marked with filled circles,
while open circles indicate non detections. Crosses and dots
mark the position of detected andundetectednon-members,re-
spectively. The solid and dashed lines are the 5 Myr isochrones
from Siess et al. (2000) and Baraffe et al. (1998), respectively,
shifted for the distance and reddening of the σ Ori cluster
bership information based on radial velocity, lithium or other
youthness indicators. For the remaining stars only photometry
or at most Hα measurements are avalaible: in these cases, we
have assigned a photometric membership based on their posi-
tion in different colour-magnitudediagrams, rejecting as mem-
bers those stars falling below the 10 Myr isochrone in at least
twodiagrams.Forstarswithoutaspectroscopically-determined
spectraltype,we derivedspectraltypes fromtheir R−I,V−I or
I−J coloursusing the transformationsby Kenyon & Hartmann
(1995) and Leggett et al. (2001). In total, there are 218 proba-
ble or candidate members in our catalogue, including 8 early-
type (O-B-A) stars, two late F-G type stars, 143 K- and early
M-type stars (<∼M4), and 65 very-low mass stars and brown
dwarfs with spectral type later than ∼M51
Most of the stars in our optical catalogue have counterparts
in the 2MASS All-Sky Point Source Catalogue (Cutri et al.
2003): for these stars we used their 2MASS coordinates in or-
der to have accurate positions.
1Note that, at the age of σ Ori, the substellar limit lies around spec-
tral type ∼M5–M6, depending on the adopted model isochrones (see
e.g. Zapatero Osorio et al. 2002).
S Ori 68
SE 70
5′′
Fig.3. Close-up of the EPIC summed image at the position of
the X-ray source identified with SOri 68 and SE70. The posi-
tions of the two stars are marked with crosses. Contour levels
are at 0.2, 0.3, 0.4, 0.5 cts arcsec−2
2.2. Source identification
In order to find optical counterpartsto our X-ray source list, we
have determined the optimal search radius by constructing the
cumulative distribution of the offsets between X-ray and opti-
cal position, following Randich & Schmitt (1995). We chose a
search radius of 5′′, for which less than two spurious identi-
fications are expected. After correlating the X-ray source list
with the optical catalogue, we found a median offset of ∼ 1.4′′
in right ascension between the X-ray and optical positions; we
therefore corrected the X-ray positions, and repeated the iden-
tification process. We found88 sources with at least one cluster
member or candidate within 5′′(including two double identi-
fications); these sources are listed in Table A.1 together with
their optical properties. The position of detected members in
the V − (V − I) and I − (R − I) colour-magnitude diagrams
are shown in Fig. 2. X-ray detections delineate very clearly the
cluster sequence, confirming the importanceof X-ray emission
as a membership indicator for young stars.
Of the detected members, three are OB stars, i.e. σ Ori AB
(O9.5V), σ Ori E (B2Vp) and HD 294272 (B9.5III), and one
is an A8V star (HD 37564). We have detected the two FG-
type stars, ∼ 64% of K-type stars and ∼ 50% of early-M stars
(<M5). Seven sources (∼ 11%) have been identified with very
low-mass stars and brown dwarfs of spectral type later than
∼M5. However, in two cases, the identification is ambiguous,
since there is another early-M star falling inside the identifica-
tionradius.Oneofthesecases is sourceNX 67,whichis identi-
fied with SE70 (∼M4) at 2.6′′, and SOri 68, a planetary-mass
object of spectral type L5.0 (B´ ejar et al. 2001), at a distance
of 4.7′′. This source underwent a flare during the observation,
with an increase in the count rate by a factor of ∼ 4 and a total
duration of ∼ 10 ks (see Sect. 3.2, Fig. 7). Inspection of the X-
ray image (Fig. 3) shows that the bulk of the X-ray emission,
and therefore the flare, is associated with SE70, although we
cannot exclude the presence of a very weak contribution from
SOri 68 itself. Unfortunately the statistics outside of the flare
is too low to allow us to derive any information on the relative

Page 4

4E. Franciosini et al.: XMM-Newton observations of the σ Ori cluster. II.
Table 1. X-ray sources identified with possible cluster candidates from 2MASS. NX is a running identification number for the
X-ray sources. The column labeled “Sign.” indicates the significance of detection
NXRAX
DECX
Sign. Count rate
(cts/ks)
2MASS
∆r
(′′)
JJ − K
(J2000)
14
42
50
91
101
103
116
148
151
153
175
5:38:15.63
5:38:31.07
5:38:33.05
5:38:48.47
5:38:50.97
5:38:51.80
5:38:59.13
5:39:13.58
5:39:15.89
5:39:17.12
5:39:40.24
−2:42:05.5
−2:34:02.8
−2:39:27.2
−2:36:42.3
−2:27:44.4
−2:36:02.6
−2:34:15.5
−2:37:37.8
−2:36:49.7
−2:41:17.6
−2:43:07.9
12.9
7.0
8.2
6.6
6.4
28.2
7.7
10.4
7.4
22.7
7.5
2.67 ± 0.39
0.64 ± 0.13
0.53 ± 0.11
0.74 ± 0.14
0.70 ± 0.17
3.50 ± 0.25
0.63 ± 0.13
1.18 ± 0.18
0.74 ± 0.15
4.20 ± 0.37
1.94 ± 0.46
J05381552-0242051
J05383098-0234038
J05383302-0239279
J05384828-0236409
J05385101-0227456
J05385173-0236033
J05385884-0234131
J05391346-0237391
J05391582-0236507
J05391699-0241171
J05393998-0243097
0.36
1.00
1.30
1.86
2.35
0.84
3.71
1.31
1.15
0.59
3.04
16.33
14.92
14.59
12.04
14.28
12.91
15.86
13.41
13.25
14.29
10.65
0.91
0.89
0.90
0.95
0.88
0.97
0.91
1.03
0.92
1.12
Table 2. X-ray sources identified with probable cluster non-members or with stars without membership information. NX is a
running identification number for the X-ray sources. The column labeled “Sign.” indicates the significance of detection. Optical
identifications labeled 4771-... and r05... are from Wolk (1996); SWW222 is from Sherry et al. (2004)
NXRAX
DECX
Sign. Count rate
(cts/ks)
Optical ID
∆r
(′′)
IR − I
Notes
(J2000)
41
46
47
56
114
119
120
123
125
169
5:38:30.06
5:38:32.21
5:38:32.70
5:38:34.30
5:38:57.20
5:38:59.61
5:38:59.65
5:39:01.19
5:39:01.58
5:39:30.72
−2:23:36.6
−2:32:43.6
−2:31:16.1
−2:34:59.9
−2:31:25.5
−2:45:08.6
−2:35:27.9
−2:33:37.8
−2:38:57.0
−2:38:28.7
26.1
13.7
6.4
21.9
7.2
38.3
9.1
13.8
132.8
10.1
7.50 ± 0.59
1.14 ± 0.15
0.57 ± 0.13
3.52 ± 0.35
0.65 ± 0.13
9.36 ± 0.53
0.76 ± 0.16
1.42 ± 0.18
45.72 ± 0.91
1.77 ± 0.29
r053829-0223
GSC2 S02003215575
2MASS J05383268-0231156
r053834-0234
4771-1071
4771-0026
2MASS J05385930-0235282
GSC2 S02003215312
HD 37525
SWW 222
1.66
0.62
1.29
1.63
1.74
0.68
3.83
4.70
0.67
1.93
12.270.43
a
13.88
12.10
11.69
0.43
0.40
0.34
a
a
a
8.07
15.30
b
c
1.32
a: probable non member on the basis of photometry
b: non member from proper motion (Warren & Hesser 1977)
c: non member from radial velocity (Burningham et al. 2005)
contribution of the two objects to the quiescent X-ray source.
In the analysis of Sect. 3.1 we have therefore assigned all the
X-ray flux to SE70, while taking for SOri 68 the same value
as upper limit to its emission.
The othersource with two possible counterpartsis NX 167,
which has been identified with SOri J053926.8-022614, a
∼M6 star, at 3.9′′, and SE94 (∼M2) at 3.2′′. In this case, it
is not possible to identify which star is the most probable X-
ray emitting one; we have therefore equally divided the X-ray
flux between them.
The faintest star with a certain X-ray detection is the can-
didate brown dwarf SOri 25, which has a spectral type M7.5
(Barrado y Navascu´ es et al. 2003) and an estimated mass of
∼ 0.04 M⊙(Muzerolle et al. 2003). Its X-ray luminosity in the
0.3–8 keV band is LX∼ 3 × 1028erg s−1(see Sect. 3.1), cor-
responding to logLX/Lbol ∼ −3.3, i.e. close to the saturation
limit.
For the remaining 124 late-type (F to M) cluster mem-
bers and candidates with no associated X-ray source we de-
termined 3σ upper limits at the optical positions using the
Wavelet algorithm.TheirX-rayandopticalpropertiesaregiven
in Table A.2.
Six additional X-ray sources have been identified with
probable cluster non members from our optical catalogue, in-
cludinganearly-typestar(HD37525,B5V)whichwasrejected
as member of the OB association by Warren & Hesser (1977)
on the basis of proper motion. Interestingly, this source under-
went a flare at the end of the observation, after ∼ 10 hrs of
quiescent emission; an analysis of its quiescent PN spectrum
shows high temperatures (0.6 and 1.3 keV), similar to those
found for the B2Vp star σ OriE (Paper I) and for late-type
stars (see Sect. 4), suggesting that the emission might originate
from an unseen late-type companion. Of the other non mem-
bers, one (SWW222) has a radial velocity inconsistent with
cluster membership (Burningham et al. 2005), while the oth-
ers have been rejected on the basis of photometry only. These
X-ray sources are listed in Table 2.
In order to increase the number of identifications, we have
also cross-correlatedthe X-raysource list with the 2MASS and
the GSC2.2 catalogues, finding 13 counterparts from 2MASS

Page 5

E. Franciosini et al.: XMM-Newton observations of the σ Ori cluster. II.5

28
-1
29
30
31
32
log LX
0.0 0.5 1.01.5 2.02.5
R-I
-7
-6
-5
-4
-3
-2
log LX/Lbol
Fig.4. LX(top) and LX/Lbol(bottom) as a function of the (R −
I) colour for detected (dots) and undetected (open triangles)
cluster members and candidates
and 2 from GSC2. Eleven of the 2MASS counterparts have
JHK photometry consistent with cluster membership, and are
therefore considered as possible candidate members of the
cluster; we list them in Table 1. The other identified X-ray
sources are given in Table 2.
For the remaining 66 sources (listed in Table B.1) we did
not find any known counterpart in any astronomical catalogue.
This number is consistent with the expected number of extra-
galactic X-ray sources (∼ 60−80)in the directionof σ Ori, de-
rived using the logN−logS relations by Hasinger et al. (2001)
and Tozzi et al. (2001).
3. Results
3.1. X-ray luminosities
In order to deriveX-ray luminosities, we have computeda con-
version factor in the 0.3–8 keV band using the results of the
PN and MOS1 spectral fits of late-type stars (see Sect. 4). The
conversion factor has been determined by comparing the count
rates obtained from the Wavelet algorithm on the summed
dataset with the unabsorbed X-ray flux derived from the best-
fit models, excluding the few sources with absorption higher
than the mean value NH = 2.7 × 1020cm−2, derived from
the mean reddening E(B − V) = 0.05 (Lee 1968; Brown et al.
1994), and then taking the median value. The derived conver-
sion factor, valid for the summed and MOS datasets, is CF
= 6.6 × 10−12erg cm−2cnt−1, with an uncertainty of ∼ 15%
(1σ standard deviation). For the source detected only in the
PN we derived in the same way a median conversion factor
CF = 2.1 × 10−12erg cm−2cnt−1for the PN count rates. We
28.028.529.029.530.030.5 31.031.5
Log Lx
0.0
0.2
0.4
0.6
0.8
1.0
XLDF
K
M0-M4
Spectral type < M5
28 29 3031
Log Lx
0.0
0.2
0.4
0.6
0.8
1.0
XLDF
All spectral types
2829 3031
Log Lx






WTTS
CTTS
Fig.5. Upper panel: comparison of the XLDF for K-type (red
solid line) and early M-type (black dashed line) stars in σ Ori.
Lower panels: comparison of the XLDFs for WTTS (red solid
line) and CTTS (black dot-dashed line), considering only stars
earlier than ∼M5 (left) and all stars and candidate brown
dwarfs (right)
applied these factors to all stars for which no spectral analy-
sis can be performed, assuming that all of them have the mean
absorption, since individual absorption measures are generally
not available. For stars having NH∼ 1−2×1021cm−2, as found
for some sources, this will underestimate the X-ray luminosity
by a factor of ∼ 1.5. For the stars with spectral fits (Table 3 and
Paper I) we used instead the fluxes determined from the PN
or MOS1 (if PN is not available) best-fit models. Finally, X-
ray luminosities have been derived using the Hipparcos cluster
distance of 352 pc.
The sensitivity in the center of the field (from 3σ up-
per limits) is LX ∼ 2 × 1028erg s−1, and decreases to ∼
4 × 1028erg s−1at 13′offaxis, and to ∼ 7 × 1028erg s−1where
only MOS is present.
Fig. 4 shows LX and LX/Lbol as a function of the R −
I colour for cluster members and candidates. For early-
type stars, we find logLX/Lbol ∼ −6.6 for the O9.5 star
σ Ori AB, consistent with the typical value (∼ −7) found for
hot stars (Pallavicini et al. 1981; Bergh¨ ofer et al. 1997), while
logLX/Lbol∼ −5.5 for the other early-type stars and for the F7
star. For late-type stars there is a general trend of decreasing
LXwith increasing colour, although there is a scatter of more
than one order of magnitude at each colour. Correspondingly,
LX/Lbolis nearly constant, as observed in other young clus-
ters and star-forming regions (SFRs; e.g. Feigelson et al. 2002;
Flaccomio et al. 2003b,d). The median value is logLX/Lbol ∼
−3.3 for detected stars, and logLX/Lbol ∼ −3.4 taking into