A&A 464, 211-227 (2007)
DOI: 10.1051/0004-6361:20066101
J. F. Albacete Colombo1 - E. Flaccomio1 - G. Micela1 - S. Sciortino1 - F. Damiani1
INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
Received 25 July 2006 / Accepted 10 October 2006
Abstract
Aims. The aim of this work is to identify the so far unknown low mass stellar population of the 2 Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members.
Methods. We analyzed a 97.7 ks Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWD ETECT code and were positionally correlated with optical and near-IR catalogs from the literature. Source events were extracted with the Acis E XTRACT package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models.
Results. We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams we estimate a typical absorption toward Cygnus OB2 of
7.0 mag. Although the region is young, very few stars (
4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in
13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at least 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typical X-ray spectral parameters are log
(cm-2) and
keV with 1
dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT= 3.3 and 3.8 keV, respectively. OB stars are typically softer (
keV). X-ray luminosities range between 1030 and 1031 erg s-1 for intermediate- and low-mass stars, and
and between
erg s-1 for OB stars.
Conclusions. The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is related to the magnetic activity of low-mass stars (
to 3.0) display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.
Key words: stars: formation - stars: early-type - stars: pre-main-sequence - X-rays: stars - Galaxy: open clusters and associations: individual: Cygnus OB2
The Cygnus OB2 association is one of the richest star forming regions in the Galaxy,
containing a large population of O- and B-type stars, some of which are among
the most massive stars known (Torres-Dodgen et al. 1991). For its richness Cygnus OB2
has been in the past considered a "young globular cluster''
(Knödlseder 2000), although uncertainties remain on the real size of
total cluster population (Hanson 2003). At a distance of 1.45
kpc (DM = 10.80, Hanson 2003), Cygnus OB2 lies behind the Great Cygnus
Rift and is affected by large and non-uniform extinction, :
5 to 15. Its
distance and absorption have so far hindered optical studies of intermediate-
and low- mass stars in the region. The position of known massive OB stars in
the HR diagram of Cygnus OB2 suggests an age between 1 and 3 Myr, with
2 Myr
as the most probable value (Massey & Thompson 1991; Knödlseder et al. 2002) .
The Cygnus OB2 region played a remarkable role in the history of stellar X-ray astronomy as an early X-ray observation, performed with the Einstein satellite and analyzed by Harnden et al. (1979), first revealed that early-type stars are intense X-ray emitters. This work was also the first to suggest the idea that X-rays in OB stars are produced in shocks within stellar winds, the starting point for the X-ray emission model of Lucy & White (1980). Subsequent ROSAT, ASCA, and Chandra-HETG observations of massive stars in the Cygnus OB2 region have improved our understanding of the mechanisms responsible for the X-ray emission of O-type stars (Waldron et al. 1998; Kitamoto & Mukai 1996; Waldron et al. 2004).
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Figure 1:
Upper panel:
color-coded ACIS-I image of the
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These latter studies of the region have focused on the four most known OB-type
stars of the region, Cyg#5, 8, 9, and 12, but have left the X-ray properties
of the other high-, intermediate-, and low-mass stars in the region unexplored.
Indeed the intermediate/low-mass population has not even been identified yet.
Because of the much higher X-ray luminosity of young stars (e.g.
erg/s, for M = 1.0-3.0
)
with respect
to older field stars (e.g. Getman et al. 2005a), X-ray observations
have proved an efficient tool to select likely members of young Star Forming
Regions (SFR). Thanks to its high sensitivity and spatial resolution, the
Advanced CCD Imaging Spectrometer (ACIS) camera on board the Chandra
satellite (Garmire et al. 2003) is particularly suited for this kind of
study in crowded stellar fields such as Cygnus OB2.
In this paper we present the analysis of a deep (97.7 ks) Chandra
ACIS observation of Cygnus OB2. In Sect. 2 we introduce our X-ray observation and
describe our data calibration and reduction procedure. In Sect. 3 we describe
source detection, photon extraction and first characterization of X-ray spectra
via hardness ratios. In Sect. 4 we characterize our sources by cross-identifying
them with available optical and near-IR catalogs. Section 5 deals with X-ray
variability while in Sect. 6 the results of the X-ray spectral analysis is
presented. In Sect. 7 we discuss out results concerning the X-ray characteristics
of high-, intermediate- and low-mass stars of the region. Finally, in Sect. 8 we
summarize our results and draw our conclusions.
Cygnus OB2 was observed with the ACIS detector on board the Chandra X-ray
Observatory (CXO) (Weisskopf et al. 2002) on 2004 January 16 (Obs.Id.
4511; PI: E. Flaccomio). The total exposure time was 97.7 ks. The data were acquired in VERY FAINT mode, to ease filtering of
background events, with six CCD turned on, the four comprising the ACIS-I array
[0, 1, 2, 3], plus CCDs 6 and 7, part of ACIS-S. However, data from the latter two CCDs
will not be used in the following because of the much degraded point spread
function (PSF) and reduced effective area resulting from their large distance
from the optical axis. The ACIS-I
field of view (FOV) is covered
by 4 chips each with
pixels (scale 0.49'' px-1). The
observation was pointed toward RA = 20
33
12.2
and Dec =+41
15'00.7'', chosen to maximize the number of stars in
the FOV and to keep two source-dense regions close to the optical axis, where
the PSF is sharper. Figure 1 shows Cygnus OB2 as seen in X-rays by our
ACIS-I observation.
Data reduction, starting with the Level 1 event list provided by the pipeline
processing at the CXO, was performed using CIAO 3.2.2 and the CALDB 3.1.0 set of
calibration files. We produced a level 2 event file using the ACIS_PROCESS_EVENT CIAO task, taking advantage of the VF-mode enhanced
background filtering, and retaining only events with grades = 0, 2, 3, 4, 6 and
status = 0. Photon energies were corrected for the time dependence of the energy
gain using the CORR_TGAIN CIAO task. Intervals of background flaring
were searched for, but none were found. We will hereafter assume a constant
background. To improve the sensitivity to faint sources, given the spectrum of the
background and that of typical sources, we filtered out events outside the
[500:8000] eV energy band.
An exposure map, needed by the source detection algorithm and to renormalizes
source count-rates, was calculated with the CIAO tool MKEXPMAP
assuming a monochromatic spectrum (kT=2.0 keV).
In this section we describe the first steps taken for the analysis of the ACIS-I data. We discuss the detection of X-ray point sources (Sect. 3.1), the definition of source and background event lists (Sect. 3.2), and a first spectral characterization of source spectra through hardness ratios (Sect. 3.3).
Source detection was performed with the Palermo Wavelet Detection code,
PWDetect
(Damiani et al. 1997b), on the level 2 event list restricted to the
[500:8000] eV energy band. PWDetect analyzes the data at different spatial
scales, allowing the detection of both point-like and moderately extended
sources, and efficiently resolving close sources pairs. The most important
input parameter is the detection threshold (SNR), which we establish from the
relationship between threshold, background level of the observation, and
expected number of spurious detections due to Poisson noise, as determined from
extensive simulations of source-free fields (cf. Damiani et al. 1997a).
The background level was determined with the BACKGROUND command in the
XIMAGE
package. The method amounts to dividing the image into equal-size boxes,
discarding those that, according to several statistical criteria, are
contaminated by sources, and finally computing the mean level of the remaining
ones. We obtain that our ACIS-I observation comprises a total of about
background photons. This background level translates into a
SNR threshold of 5.2 if we accept one spurious detection in the FOV,
or into
if we accept 10 spurious detections. The first
choice results in the detection of 868 sources and the second one in 1054
sources. By accepting an extra
9 spurious detections in the FOV we thus
gain about 177 new reliable sources. Considering moreover that 10 spurious
sources amounts to only
1% of the total number of detections, we decided
to adopt the second less conservative criterion.
After a careful visual inspection of the initial source list, we rejected a
total of 51 detections which we considered spurious: 39 were produced by
different instrumental artifacts (i.e. out-of-time events, CCD gaps,
detector edges, etc.) that were not included in the simulations of source-free
fields used to establish the SNR threshold, but that are easily recognized. The
remaining 12 were multiple detections of the same sources with different
spatial scales. In total, our final list of X-ray sources in the Cygnus OB2
region contains 1003 X-ray detections, 99% of which are expected to be real.
The first 7 columns of Table 1 list, for each source: a running
source number, name (according to CXC naming
convention
), sky position
(RA and Dec) with relative uncertainty, off-axis angle (
),
significance of the detection (Sig.).
For further analysis of the detected sources, we used ACIS
E XTRACT
(AE) v3.79 (Broos et al. 2002), an IDL based package that makes use of
TARA
, CIAO and
FTOOLS
.
The optimal extraction of point source photons is complicated by the
non-Gaussian shape of the PSF and by its strong non-uniformity across the ACIS
field of view. In particular the width and asymmetry of the PSF depend
significantly on the off-axis distance (). The PSF is narrow and nearly
circular in the inner
but becomes rapidly broader and
more asymmetric at larger off-axis, as demonstrated in the bottom panel of
Fig. 1 where we show sources detected in our data at four different
off-axis angles.
Extraction from circular regions containing a large fraction (e.g. 99%) of the PSF would guarantee to collect almost all source photons. However, given the extended wings of the PSF, very large regions would be needed, incurring in the risk of contamination from nearby sources, especially in crowded fields like ours. Moreover the resulting inclusion of a large number of background events would reduce the signal to noise of weak sources. On the other hand, extraction from regions that are too small may reduce the photon statistic for further spectral and timing analysis.
In order to tackle this problem, AE begins with calculating the shape of the
model PSF at each source position using the CIAO task MKPSF. It then
refines the initial source positions (in our case estimated by PWDetect
assuming a symmetric PSF) by correlating the source images with the model
PSFs. Following AE science
hints,
this last procedure was only used for those sources lying at off-axis larger
than 5 arcmin (464 sources), while for the rest of the source (539 sources) we
simply adopt data-mean positions. Coordinates listed in Table 1, as
well as their 1
uncertainties, are the result of this process. We
verified that these positions are an improvement over those computed by
PWDetect, especially at large off-axis angles, comparing the offsets between
X-ray sources and counterparts in the 2MASS catalog (see Sect. 4.2).
Table 1: Cygnus OB2 X-ray source catalog: first 35 rows. The complete table, containing 1003 rows is available in the electronic edition of the journal.
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Figure 2:
X-ray color-color diagrams for our ACIS sources. The three panels
refer to sources in three ranges of detected counts, as indicated in the
upper-left corners. Colors are defined for three energy bands: Soft
(0.5-1.7 keV), Medium (1.7-2.8 keV) and Hard (2.8-8.0 keV) keV.
The grids refer to predicted colors for Raymond-Smith thermal emission models
with different temperatures (kT = 0.5, 1.0, 2.0, 4.0, and 8.0 keV) and
affected by varying amounts of absorption,
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After re-computing positions, AE defines source extraction regions as polygonal
contours of the model PSF containing a specified fraction of source events
(
). Generally, we chose
,
and computed the
contours from the PSF for a mono-energetic source with E = 1.49 keV. For
8% of the sources in the denser parts of the Cygnus OB2 field this fraction
was reduced to avoid contamination with other nearby sources, in the most
extreme cases down to
.
The four panels at the bottom of
Fig. 1 show examples of extraction regions. Otherwise, we have
detected only six sources: Id.# 17, 60, 488, 544, 568, 729 that suffers of a
pile-up fraction (
)
of 3.96%, 17.7%, 11.6%, 2.39%, 15.6%
and 4.6%, respectively. In these cases, we extracted events in annular regions
that exclude the PSF cores for all sources that exhibits a
greater than 2%. The inner radii was fixed at 2
arcsec
,
while the outer circles were choosen as the smallest that inscribe the
contours.
Although the ACIS-I instrumental background level is spatially quite uniform, the actual observed background varies substantially across the crowded Cygnus OB2 field due to the extended PSF wings of bright sources and to their readout trails. The background was therefore estimated locally for each source, adopting once again the automated procedure implemented in AE, which defines background extraction regions as circular annuli with inner radii 1.1 times the maximum distance between the source and the 99% PSF contour, and outer radii defined so that the regions contain more than 100 "background'' events. In order to exclude contamination of the regions by nearby sources, background events are defined from a "swiss cheese'' image that excludes events within the inner annuli radii of all the 1003 sources.
Results of the photon extraction procedure are listed in Cols. 8-16 of Table 1. We give: the source extraction area (Col. 8); the PSF fraction
within the extraction area, assuming E=1.49 keV (9); the background-corrected
extracted source counts in the 0.5-8.0 keV band (10); the exposure time (11),
the count rates (CR) in four spectral bands computed as the ratio between the
source photons (corrected for
)
and the exposure time (12-15); the
median photon energy (
), (16).
In summary, our 1003 X-ray detections span a wide count range, from 4 to
15 000 photons. Most sources are faint (e.g. 42% have less than 20
photons). The lower envelope of the counts vs. off-axis plot (not shown)
indicates that the minimum number of photons necessary for detection ranges
from 4 on axis to
20 close to the detector corners (
arcmin).
In order to characterize the X-ray spectra of low-statistic sources it is common practice to use the ratios of source counts in different spectral bands, i.e. X-ray hardness ratios (XHR), or the logarithm of these values which can be considered "X-ray colors'' (Prestwich et al. 2003; Schulz et al. 1989).
We use this method, dividing the full energy range into three bands: Soft
(:
0.5-1.7 keV), Medium (
:
1.7-2.8 keV) and Hard (
:
2.8-8.0 keV). Figure 2 shows, separately for sources in
three detected counts ranges, the "Hard X-ray color'' (
)
vs. the "Soft X-ray color'' (
). For
reference we also plot the predicted loci for absorbed thermal sources with
plasma temperatures between 0.5 and 8.0 keV and
between 1020and 1023 cm-2. The grid was calculated with the Portable
Interactive Multi-Mission Simulator
(PIMMS
)
for a Raymond-Smith (RS) emission model (Raymond & Smith 1977).
A comparison of the three panels in Fig. 2 shows that the
position of sources with respect to the
grid is significantly
affected, other than by the source spectra, by the source statistic. The
positions of individual sources with less than 20 counts (left panel) are, for
example, considerably spread out because of large statistical uncertainties.
Sources with higher statistics (
,
central panel)
are more concentrated around
keV and
cm-2. Sources with more than 50
photons (right panel) are even more concentrated around these same values of
kT and
.
A separate group of soft-spectrum sources however becomes
visible in the upper part of the grid, with typical
keV. Most of
these sources are associated with known early type stars, indicated by squares
in Fig. 2. Others, with somewhat smaller
,
are likely
foreground stars as also argued in Sect. 4.2.
Table 2: Near-IR counterparts of Cygnus OB2 X-ray sources and near-IR stellar parameters: first rows. The complete version is available in the electronic edition of the journal.
Cygnus OB2 has been the target of numerous optical studies for more than 50 years
(Schulte 1958; Johnson & Morgan 1954). Massey & Thompson (1991)
identified 120 candidate massive members in an area of about 0.35 deg2, on
the basis of UBV photometry, and gave optical spectral classifications for
over 70 OB stars. More recently, Hanson (2003) published new
spectral classification for 14 more OB candidates, improving the massive star
census of Cygnus OB2, and the estimates for its distance (1450 pc) and age
(2 Myr). A total of 33 OB stars from the catalog of
Hanson (2003) lie in the 0.0823 deg2 FOV of our X-ray
observation and were thus cross identified with our X-ray source list (Sect. 3.2).
Due to the high and variable absorption in the Cygnus OB2 line of sight, and to the
relative shallowness of the available optical catalogs, the correlation between
X-ray sources and optical catalogs is of limited use for the study of
intermediate- and low-mass stars. We thus decided to base our characterization
of the X-ray sources mostly on near-IR data, for which the impact of dust
extinction is reduced and is comparable to that in the X-ray band. We
made use of J (1.25
m), H (1.65
m) and
(2.17
m)
photometry from the Two Micron All Sky Survey (2MASS) Point Source
Catalog (PSC)
. 2MASS is
complete to magnitudes of 15.8, 15.1 and 14.3 mag in the J, H and
bands,
respectively. We limited our analysis to 2MASS sources for which the quality
flag for at least one of the three magnitudes is equal to A, B, C or D (cf. the
2MASS All-Sky Data Release User's Guide). With this restriction 11 sources were
removed from our initial list of 5061 2MASS point sources in the ACIS FOV,
leaving a total of 5050 objects.
We began by cross identifying our X-ray source list with the 2MASS catalog.
Identification radii,
,
were chosen to limit the number of
spurious identifications due to chance alignments,
,
and at the
same time to include a large number of the true physical associations,
.
First we estimated, as a function of
,
the number of
chance identifications expected in a given sky area,
,
assuming
(i) uncorrelated NIR and X-ray positions and (ii) a uniform surface source
density of the
2MASS sources lying in the search area:
,
where
is the area of identification circles.
)
can instead be estimated from the observed total
number of identifications at any given
as
). We chose the identification radius as the
largest for which
.
Because the Chandra PSF, and therefore the position uncertainty of X-ray sources, depends mainly on the off-angles, we perform this analysis in four different ranges of off-angles: [0-2), [2-4), [4-7) and >7 arcmin. Our identification radii in these four regions are 1.0, 1.5, 2.1, and 2.7 arcsec, respectively.
Before performing the final identifications we searched for possible systematic
differences between the X-ray and 2MASS positions. We first performed a
preliminary cross-identification and compared the coordinates of identified
pairs. A small systematic offset between the two catalogs was found. We thus
shifted the Chandra coordinates and performed a new cross-identification,
repeating this process iteratively until the offset was reduced to 0.01 arcsec, i.e. much smaller than the statistical errors on source positions. In
the end the offset between the two catalogs was:
(RA
and
(Dec
.
The result of the final identification is
shown in Table 2, where we list, in the first 7 columns, the X-ray
and near-IR identifiers of cross-identified sources, the offset between the two
positions, and the J, H,
magnitudes from 2MASS. A total of 775 X-ray
sources out of the 1003 in our list were identified with 2MASS objects. Three
X-ray sources (#12, #148, and #449) were identified with two 2MASS objects
but, after checking the positions visually, we adopted the closer of the
counterparts. The fraction of identified X-ray sources appears to increase as
we move to larger off-angles. In the four annular regions defined above, these
fractions are: 76/116 (65%), 227/304 (74%), 294/374 (79%) and 178/209
(85%), in order of increasing off-angles. This trend can be attributed to the
already mentioned (3.2) dependence of the ACIS-I sensitivity
on the off-axis angle and to the fact that more intense X-ray sources are more
likely to have a near-IR counterpart than fainter ones.
With respect to the expected number of chance identifications we estimate, for
off-axis ranges [0:2), [2:4), [4:7), and >7 arcmin, and with the formula
given above, no more than 1.6, 7.9, 16.4, and 12.5, respectively for a total of
39. Note however that the assumption that X-ray and 2MASS source lists
are fully uncorrelated is not true (other than for the 10
expected spurious detections), so that this number should be considered as a
loose upper limit (cf. Damiani et al. 2003).
We estimate the expected number of extragalactic sources in our detection list
following Flaccomio et al. (2006). We consider the ACIS count-rates of
non-stellar sources in the Chandra Deep Field North
(CDFN, Barger et al. 2003; Alexander et al. 2003) and estimate
absorption corrected count-rates assuming
(from
= 7.0, see Sect. 4.2) using PIMMS and assuming power-law
spectra with index
.
We then compare these count rates with upper
limits taken at random positions in the ACIS FOV. For
between 1 and 2
we obtain 61 to 87 expected extragalactic sources. Given the intrinsic near-IR
fluxes of these sources and the absorption toward Cygnus OB2, they are expected
to be among the 228 without NIR counterparts
(cf. Flaccomio et al. 2006).
We identified our X-ray source list with the 33 early type stars listed by Hanson (2003) and lying in our ACIS FOV. Of these, all the 20 O-type stars associated with an X-ray source, while of the 13 B-type stars only 6 are detected.
We now investigate the NIR properties of the X-ray sources with 2MASS
counterparts. For this purpose we restrict our analysis to sources with high
quality photometry, i.e. those for which the quality flag (see 2MASS
documentation) is "AAA'', or for which uncertainties on the J, H and
magnitudes are all lower than 0.1 mag. With these requirements the total number
of IR sources in the ACIS FOV is reduced from 5050 to 2187. Counterparts of
X-ray sources were selected only on the basis of their magnitude errors (<0.1 mag), yielding 519 sources out of the original 775.
Figure 3 shows the J-H vs.
color-color (CC) diagram for all
the selected 2MASS objects in the ACIS FOV, both X-ray detected and undetected.
We also plot for comparison the MS (Kenyon & Hartmann 1995), the Classical
T-Tauri Stars (CTTS) locus of Meyer et al. (1997), and three reddening
vectors starting from these loci and with slope (
)
taken from the extinction law given by Hanson (2003). Cygnus OB2 members
with purely photospheric emission should lie in this reddening band. However,
as the line of sight toward Cygnus OB2 is not far from that to the Galactic center,
many field interlopers are also expected in the same band. Young stellar
objects (YSOs), such as Classical T Tauri and Herbig Ae/Be stars, because of
the NIR excess emission originating in the inner parts of their circumstellar
disks, are often found to the right of this band, i.e. in the CTTS locus.
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Figure 3:
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However, very few X-ray sources, i.e. likely Cygnus OB2 members,
have colors consistent with the (reddened) CTTS locus. If we neglect the
uncertainties on data points, 23 sources lie in the reddening band of the CTTS
locus. We do not include here the peculiar supergiant B5 Ie Cyg#12 and
discuss its nature in Sect. 7.1. A total of 23 stars in the CTTS
reddening band means a fraction of 23/
of all the
X-ray sources in the CC diagram. We compare this fraction with the one observed
in the ONC, adopting Chandra Orion Ultradeep Project (COUP) sources in the same
mass range
that we reach in Cygnus OB2 (i.e.
,
cf. Sect. 7): 19/
.
The difference
between the two fractions of stars with near-IR detected disks is significant,
a factor of
4.7. It could be for two different reasons:
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Figure 4:
CM diagrams of the Cygnus OB2 region. Symbols as in Fig. 3. The
two parallel curves indicate the expected cluster loci for the assumed distance
and for a mean reddening of
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Figure 4 shows the
vs.
color magnitude (CM)
diagram for the same stars plotted in Fig. 3. We also show for
reference the expected cluster locus: the intrinsic
magnitudes and
colors for stars earlier than B5V were taken from the MS
calibration of Knödlseder (2000) and Bessell & Brett (1989).
For later spectral types (masses between 0.1 and
), we
adopted the 2 Myr isochrone from Siess et al. (2000), converted to the
observational plane using the calibration given by Kenyon & Hartmann (1995).
The adopted MS and 2 Myr isochrone overlap satisfactorily.
In order to estimate the typical visual absorption of cluster members we
calculated the distance of each X-ray source to the cluster locus along the
reddening direction. Resulting values for individual sources are listed in
the last column of Table 2. Note that for
the absorption cannot be constrained because the reddening vector
intersects the cluster locus more than once. We computed median absorption
values in two luminosity ranges: for
we obtained
=7.0
mag, while for
,
considering only the known OB stars, we
obtained
= 5.63 mag. We note that the above estimations depend on the
reliability of the assumed cluster locus and on the assumption that the H and K magnitudes are not significantly affected by disk-induced excesses. As observed
above, given the paucity of stars with excesses, the latter appears to be a
good approximation for Cygnus OB2 stars. It is comforting that our estimate
for the OB type stars is in fairly good agreement with the median of the
extinctions computed by Hanson (2003) using photometric and
spectroscopic data:
= 5.7 mag. The lower extinction derived for high mass
stars than to lower mass ones may indicate that the strong winds and
radiation field of massive stars may have cleared their surrounding
environment.
In both the CC and CM diagrams, some X-ray sources, of the order of 15-20, lie close to the un-reddened cluster loci. These are likely to be foreground MS stars and thus to contaminate the sample of X-ray detected cluster members. This conclusion is corroborated by the X-ray hardness-ratio analysis: eight of these stars have soft and relatively unabsorbed spectra (the stars to the right of the OB stars, in the upper-right corner of Fig. 2-right), as expected from foreground field stars.
In the following (Sect. 7) we will correlate the X-ray properties
of our sources with stellar parameters derived from the available optical and
near-IR data. We obtain an estimate of stellar masses for 682 2MASS
counterparts from the J-band magnitudes (limited to quality flags "A''
to "D'') and the mass vs. J mag. relationship appropriate for the cluster age,
distance and extinction, obtained as described above for the cluster locus in
the H, H-K diagram. The relation is degenerate in two ranges of J, corresponding to
and
.
Four and 28 X-ray detected stars lie in
these ranges, respectively.
The temporal variability of young low mass sources is often complex: the most common phenomena are magnetic flares with rapid rise and slower decays, superposed on an apparently constant or, sometimes, rotationally modulated emission (Flaccomio et al. 2005; Wolk et al. 2005). Other forms of variability, with less clear physical origin, are often observed.
We first investigated X-ray variability using the non-binned one-sample
Kolmogorov-Smirnov (KS) test (Press et al. 1992). This test compares
the distribution of photon arrival times with that expected for a constant
source. The test was applied to photons in the source extraction regions, which
also contain background photons. Given that the background was found to be low
and constant (Sect. 2.1), the results, i.e. the confidence with which we can
reject the hypothesis that the flux was constant during our observation, can be
attributed to the source photons. Table 1, Col. 17, reports the
logarithm of the KS-test significance with values < -4 truncated at that
value: sources with log(
) < -3.0 can be considered almost
definitively variable as we expect at most one of the 1003 sources (i.e. 0.1%)
to be erroneously classified as variable. Eighty-five X-ray sources
(
8.5% of the total) fall in this category. Sources with
(
) < -3.0 can be considered as likely variable,
but up to 9 such sources (on average) might actually be constant. Forty-nine
Cygnus OB2 sources fall in this category.
These numbers of sources in which variability is detected are a lower limit to the total number of variable sources in the region for several reasons: i) most of the observed variability is in the form of flares, i.e. events that are shorter than our observation and with a duty-cycle that may be considerably longer (Wolk et al. 2005); ii) the sensitivity of statistical tests to time variability of a given relative amplitude depends critically on photon statistics. This is illustrated in Fig. 5, where we plot the fraction of variable sources as a function of source counts: the clear correlation between the two quantities is most likely due to this statistical bias even though we cannot exclude a real dependence.
![]() |
Figure 5:
Fraction of variable sources as a function of source counts.
Horizontal bars indicate the range of source counts. Filled circles indicate,
for each counts bin, the fraction of sources with log(
![]() ![]() |
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Next, we extracted binned light-curves for each of our Cygnus OB2 X-ray sources
adopting a bin length of 600 s, a compromise between bins that are long
enough to reach a good signal-to-noise ratio per bin for most sources and
sufficiently short to resolve the decay phase of typical flares. Since the
background of our observation is both low (negligible for many sources) and
constant, we did not apply any background subtraction. We inspected the 857
light-curves of sources with >10 photons, finding flare-like events,
qualitatively defined as a rapid rise and a slow decay, in 98 sources, i.e.
9.8% of the cases. These sources are indicated by a
in the
last column of Table 1. Of these 98 sources, 66 (
65%)
have a log(
)
-3.0, while 13 were classified as probably
variable (
). The remaining 19 sources
were not detected as variable by the KS test (log
)
and may
or may not be actually "flaring''. Figure 6 shows light-curves for 20
variable sources, 14 of which are classified as flaring. Some of these sources
(e.g. #1, #676 and #911) experience "impulsive'' flares with very quick
rises and decay phases of only a few hours. Others (e.g. #33, #172,
and #260) show longer (2 to 10 h) flares. In several instances a second
impulsive event is visible during the exponential decay of a previous flare
(e.g. sources #6, #52, #439, #600, #796, and #945). Other sources (e.g.
#555, #834 and #863) have variable light-curves that bear little resemblance
to typical flares and are instead characterized by slow continuous rises or
decays that might be explained by rotational modulation of non-homogeneously
distributed plasma (Flaccomio et al. 2005).
As expected if flares originate from magnetic
reconnection events (Favata & Micela 2003), the median photon energies
for flaring sources are generally higher than those of non-flaring sources: the
distribution of median energies for the flaring source peaks at
keV, with a 1
dispersion of 0.4 keV, while for
non-variable stars it peaks at
keV, with a
1
dispersion of 0.3 keV. A similar conclusion can be drawn from the
spectral modeling presented in the next section (Sect. 6):
flaring sources often require higher temperature models than non-variable ones.
The variability of massive O and early B-type stars is significantly different
from that of low mass members. Among the 26 OB stars detected in our FOV, 24
are classified as non-variable (log(
) > -2.0), in spite of their
higher than average statistics, having between
40 and 15 000 counts,
with a median of
111 counts. A comparison with Fig. 5 shows
that the variabile fraction of OB stars (7.7%) is significantly lower than
for the bulk of our sources with similar statistics. This finding agrees with
the common view of X-ray emission from O stars, which is believed to be
unrelated to solar-like magnetic activity, and rather is explained as the integrated
emission from a large number of small shocks occurring in the strong winds of
these stars (Feldmeier et al. 1997; Owocki & Cohen 1999). Interestingly
however, two of these sources, #60 and #979, are significantly variable, with
log(
)
values lower than -4. Figure 7 shows their
light-curves.
Source #60 (Cyg#12) is a B5 Ie star (Hanson 2003). The X-ray
light-curve shows a roughly linear decay of the ACIS count rate from
0.18 cnt s-1 to
0.16 cnt s-1. The star was
observed by Waldron et al. (1998) who did not report variability during
125 ks of non-continuous ROSAT PSPC observation. This kind of
variability may be similar to the rotational modulation observed on the O7 star
Ori C (Gagné et al. 2005) and is attributed to the
presence of a rigidly rotating magnetosphere (RRM), which may form in magnetic
early-type stars with misaligned magnetic and rotation axe
(Owocki et al. 2005). We defer further discussion of the nature of
Cyg #12 to Sect. 7.1.
Source #979 is identified as star #646 in Hanson (2003) and there is classified as a B1.5V + ? star, the question mark indicating the presence of an unresolved faint secondary. The observed variable X-ray emission is much fainter than in the previous case and might be due to the magnetic activity of the companion of the B1.5 V primary, presumably a lower mass star.
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Figure 6: Light curves (in the 0.5-8.0 keV band) for 20 sources with variable emission during our 97.7 ks Chandra observation. The bin length is 600 s. Source numbers are given in the upper-left corner of each panel, followed by an "f'' for sources classified as flaring. |
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![]() |
Figure 7:
Light curves in
the 0.5-8.0 keV band for the two variable early-type stars: Left: source
Id. #60, known as a B5Ie, has been recently classified a
![]() |
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In order to characterize the hot plasma responsible for the X-ray emission of
Cygnus OB2 stars, and to estimate their intrinsic X-ray luminosities, we analyzed the
ACIS spectra of each of our 1003 sources. Reduced source and background spectra
in the 0.5-8.0 keV band were produced with AE (see Sect. 3.2),
along with individual "redistribution matrices files'' (RMF) and "ancillary
response files'' (ARF). For model fitting, spectra were grouped so to have a
specified number of events in each energy bin. Grouping was tuned to the source
statistics and we chose 2, 5, 7, 10, and 60 counts per channel for sources with
net-counts in the following ranges: [0-40], [40-100], [100-200], [200-500], and
[500-16 000]. Spectral fitting of background-subtracted spectra was
performed with XSPEC v12.0 (Arnaud 2004) and our own shell and
TCL scripts to automate the process as described in
Flaccomio et al. (2006). Best fit parameters for the chosen models were
found by
minimization.
We fit our spectra assuming emission by a thermal plasma, in collisional
ionization equilibrium, as modeled by the APEC code
(Smith et al. 2001). Elemental abundances are not easily constrained
with low-statistic spectra and were fixed at
,
with solar
abundances taken from Anders & Grevesse (1989). The choice of sub-solar
abundances is suggested by several X-ray studies of star forming regions
(e.g. Preibisch 2003; Feigelson et al. 2002). Absorption was
accounted for using the WABS model, parametrized by the hydrogen column
density,
(Morrison & McCammon 1983).
![]() |
Figure 8:
Median ![]() ![]() ![]() ![]() |
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Except for X-ray sources associated with O- and early B-type stars, which are
discussed separately in Sect. 7.1, we fit source spectra with
one-temperature (1T) plasma models using an automated procedure. In order to
reduce the risk of finding a relative minimum in the
spaces, our
procedure chooses the best fit among several obtained starting from a grid of
initial values of the model parameters: log(
) = 21.0, 21.7, 22.0,
22.4, 22.7 and 23.0 cm-2 and kT = 0.5, 0.75, 1.0, 2.0, 5.0 keV. Best fit
values of log(
) < 20.8 cm-2 were truncated at 20.8 because,
in the 0.5-8.0 keV energy range, ACIS spectra are insensitive to lower column
densities. For the same reason, 154 best fit values of kT turned out to be
>10 keV and were truncated at that value.
As noted by many authors
(e.g. Getman et al. 2002,2005b; Flaccomio et al. 2006),
the spectral fitting of low statistic ACIS sources is problematic because of a
degeneracy between plasma temperature and absorption. The degeneracy also
results in a systematic bias: kT values are often underestimated by as much
as 50% while
values are overestimated. We investigated
this issue with our data by considering the distributions of the best fit
parameters for source in different count-statistic bins. Figure 8
shows the run of mean kT and
with source counts for spectra fitted
with 1T models. The systematic decrease of
and the increase of
kT with increasing source statistics are hardly explainable as physical effects
and rather indicate that the spectral parameters obtained for sources with less
than
20 photons are ill-constrained.
The uncertainty on the
is particularly serious as it implies large
and systematic uncertainties on the absorption-corrected X-ray luminosities. In
order to reduce the risk of erroneous results, we discarded from the following
analysis results for source with <20 net photons, a total of 423 sources.
Table 3 presents the results of the automated 1T spectral fits for
554 sources (the results of the spectral fitting for the 26 sources associated
with known OB stars will be presented in Sect. 7.1, Table 4).
We list source numbers (Col. 1) from Tables 1 and 2, background subtracted
counts in the spectra (2), the best fit hydrogen column densities and their
1
errors (3), the plasma temperatures and their 1
errors (4), and the emission measures (5). Columns (6) and (7) give the reduced
for the spectral fits and the relative degrees of freedom,
respectively.
Table 3: Results of X-ray spectral fits: first rows. The complete version is available in the electronic edition of the Journal.
The hydrogen column densities derived from X-ray spectral fitting depend of
the interstellar material in the line of sight to the Cygnus OB2 cloud and on the
location of stars within the cloud material of Cygnus OB2. Figure 9a
shows the distribution of log
values for the 580 sources with more
than 20 counts. They appear to be normally distributed with a median
log
(cm-2) and a FWHM of
0.21 dex.
The shaded area in Fig. 9a indicates an apparent excess of
relatively unabsorbed sources (log
cm-2) with respect to
the log-normal distribution of the bulk of the sources. These
23 sources
are likely associated with foreground stars. Most of these sources seem to be
spatially distributed uniformly in the FOV of the Cygnus OB2 region.
Another interesting observation is that the
distribution of the 106
X-ray sources without near-IR counterparts (dark gray histogram in
Fig. 9) seems to be skewed toward higher values than the
global distribution. For example, while only 11% of the X-ray sources with
counterparts have
,
this is true for 29 (27%) of the
unidentified sources.
According to the relationship between
and
,
cm-2 (Ryter 1996), the
median
of all detected sources with more than 20 counts corresponds
to
8.1
12.85.1 mag. If we only consider the sources with
near-IR counterparts (and > than 20 counts) we obtain log
,
which corresponds to
=7.7 mag, i.e. in reasonable agreement with the value
calculated from the NIR CMD (
7.0 mag).
Figure 9(b) shows the distribution of plasma temperatures: it
peaks at 1.35 keV, has a median
2.4 keV and shows an extended hard
tail. Variable sources (log
)
have harder spectra with median
kT=3.3 keV and flaring sources (see hatched histogram in Fig.
9b) are even harder with a median kT=3.75 keV.
We derived unabsorbed X-ray luminosities for each of our sources. For O
and B-type stars, discussed in detail below (Sect. 7.1), luminosities
were derived from individual spectral fits. For the other sources, given the
considerable uncertainties in the absorption estimates, especially at the faint
end, we preferred not to use the results of spectral fits individually. Rather,
we computed a single count-rate to
conversion factor that takes
into consideration the average intrinsic source spectrum and interstellar
absorption. This conversion factor, computed as the median ratio between the
individual unabsorbed X-ray luminosities (from the best fit spectral models)
and the source count-rates is
ergs.
Table 4: X-ray spectral parameters of the Cygnus OB2 OB-type stars.
![]() |
Figure 9:
a)
Distributions of absorbing columns, ![]() ![]() ![]() |
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We now discuss the implications of our data for the understanding of X-ray
emission from young stars and for the study of the Cygnus OB2 stellar population. We
split the discussion according to stellar mass and, correspondingly, expected
X-ray emission mechanism. We first discuss known O and early B (<B2) stars,
i.e. massive stars (
)
expected to generate X-rays in
their powerful stellar winds. Next, using our mass estimates based on the
J-band magnitude (Sect. 4.2), we define and study the properties of
two subsamples of intermediate and low-mass X-ray detected stars. We base the
distinction between the two groups on the model-predicted thickness of the
convective layer, whose presence has been often found to be correlated with
solar-like coronal activity. Stars with small or absent convective layers are
instead believed to be unable to sustain coronal activity, although the matter
is somewhat controversial. Adopting the 2 Myr isochrone from the SDF models, we
observe that the thickness of the convective layer relative to the stellar
radius,
,
decreases rapidly, from
5% to <1%, as the
stellar mass increases from 3 to
.
This rather sharp boundary
however lies in the
range in which our mass estimates
are degenerate and therefore unconstrained (see Sect. 4.2). In
order to resolve this ambiguity, we define the intermediate-mass range as
,
resulting in the selection of 57 stars
. Low mass stars are finally defined as stars with
and are predicted to have substantial convective envelopes (
). 578
counterparts to X-ray sources fall in this category. Because of the sensitivity
limit of 2MASS only 25(4) sources of these stars have masses below
0.5(0.4)
,
which we may consider as our approximate detection limit.
Our completeness limit is however probably higher, at
,
corresponding to J = 15.8, i.e. roughly the completeness limit of 2MASS. A
histogram of the logarithm of stellar masses also peaks at
,
confirming the above estimate given that the mass function usually increases
towards lower masses.
Figure 10 shows the -mass scatter plot for 683 X-ray sources in
all three mass ranges. We distinguish with different symbols stars of low- and
intermediate-mass, as determined from the J-band magnitude, and massive stars
with known O- and early B- spectral types
.
Note that stars in the
and 0.2-0.4
ranges are
plotted with horizontal error bars spanning the entire range, because the
J-band vs. mass relation was found to be degenerate. In the plot we also
indicate the position of 57 time-variable X-ray sources. They appear to be
brighter than other sources of the same mass, probably because variability is
more readily detected in sources with high photon statistics (see Sect. 5).
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Figure 10:
Log ![]() ![]() |
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The Cygnus OB2 region is one of the most massive SFR in the Galaxy, with
OB star members (Knödlseder 2000). The
presence of a variety of evolved stars suggests that the star formation process
was non-coeval (Massey & Thompson 1991; Hanson 2003). Out
of a total of 20 O-type and 13 B-type stars lying in our Cygnus OB2 ACIS FOV, 7
and 3, respectively, are evolved (luminosity class: I-III). While we
detect X-ray emission from all O stars
regardless of evolutionary status, we have only detected 6 B stars, among which
all the evolved ones. The detection fraction of evolved B stars is thus 100%
while it is only 33% (3 our of 9
) for un-evolved ones.
The most widely accepted explanation for the X-ray emission of single O and early B-type stars invokes multiple small-scale shocks in the inner layers of their radiation-driven stellar winds (e.g. Feldmeier et al. 1997). Recent theoretical and observational results however support, for several OB stars, another physical plasma heating model involving strong magnetic fields: the magnetically channeled wind shock (MCWS) model (Gagné et al. 2005; Schulz et al. 2003; Owocki et al. 2005). Moreover, binary systems in which both components are O- and early B-type stars can produce intense thermal X-ray emission from wind-wind interactions as well as non-thermal X-ray emission from Inverse Compton scattering.
We modeled the spectra of O and B stars with one- or two-temperature thermal
plasma emission models ( APEC), absorbed by neutral interstellar and
circumstellar material ( WABS). Because the dense stellar winds of evolved
OB stars are affected by the strong stellar UV/EUV ionizing radiation, the
additional absorption of X-rays by a partially ionized stellar also wind
should be considered. For evolved stars we then decided to model the combined
effect of ISM plus wind material with a warm absorption model (
WABS ABSORI, see e.g. Waldron et al. 1998). For sources #60,
#544, and #729, we found that the inclusion of a warm absorber in the
spectral model yields satisfactory fits of the soft part of the spectra
(1.2 keV), reducing the
from 2.1 to 1.4, from 2.7 to 1.3, and
from 2.2 to 1.2, for the three sources respectively. The spectra of other
evolved sources did not support the presence of the warm absorber,
although in many cases the spectra have too low statistics to be conclusive.
Metal abundances were fixed at
in fitting faint sources
(<100 ph.) and left as a free parameter for brighter sources. When the
best fit abundance was lower than
we repeated the fit with
Z fixed at this value.
Figure 12 shows three notable examples of spectral modeling and
the best-fit parameter values for the 26 detected OB stars are presented in
Table 4. We list: X-ray source numbers from Table 1 (Col. 1),
identification numbers and spectral types from Hanson (2003) (2
and 3), reduced
of the best fit (4), fraction of pile-up as reported
by AE (as discussed Sect. 3.2, if f(Pile-up) > 2% we used an
annular photon extraction region that excludes the peak of the PSF), the
of the cold and hot absorption components from WABS and
ABSORI, respectively (6 and 7), plasma temperatures of the two components (8
and 9), metal abundances (10). Column (11) finally gives the un-absorbed X-ray
flux in the 0.5-8.0 keV energy band.
The median absorption of the 26 OB stars is log
cm-2,
very similar but slightly lower than the median value of lower mass stars
(22.23, see Sect. 7.3). It is tempting to attribute the lower
absorption of O stars to the sweeping of the cloud material by strong winds. A
two-sided KS test comparing the distribution of
for O and B stars
with that of lower mass ones gives, however, a null result.
Most of the O- and early B- type stars are well fit by a soft thermal emission
model with kT ranging between 0.5 and 0.7 keV (5.8-8.1 MK), in agreement
with the predictions of the wind shock model
(Owocki et al. 1988; Lucy & White 1980). The spectra of some sources,
however, require a second (hard) thermal component, which is not predicted by
the wind shock model. The presence of this component seems to depend on the
stellar evolutionary status: out of the 10 evolved O- and early B-type stars
(spectral types B5 Ie, O4 III, O5 If, O3 If, O6 If +O5.5III(f)) 5 (50%)
show evidence of a hard thermal emission with kT roughly between 1.3 and 3.0
keV (14-35 MK), while among the 16 class V stars, the hot component is only
required in 3 cases (i.e. 23%). Several physical mechanisms have been proposed
to explain the hard emission observed in some of our OB stars:
The proportionality between
and
is retrieved, albeit
with a large dispersion. However, when we distinguish between main sequence
stars and evolved ones (i.e. giants and super-giants), we discover, for these
latter, an apparently tight
relationship which is
not a simple proportionality. For evolved stars, the
with respect to the
relation is 2.61, while
if we adopt the best-fit power law (with index
)
the
is reduced to
1.3. We test the statistical significance
of including this additional degree of freedom in the
relation. We used a F
-test as described in Bevington et al.
1969. We found that F
-test
0.6,
which does not indicate a significant improvement of the quality of the fit
(F
should be greater than 1.1), even when adopting a significance level
as high as 0.1. However, this result is affected by the small number of
evolved O and B stars in our sample. A similar analysis with more stars from
other regions will be needed to confirm, or disprove, this tentative result.
A more detailed study of the physical origins of the X-ray emission from OB stars and on its dependence on stellar wind parameters will be presented in forthcoming paper.
![]() |
Figure 11:
X-ray versus
bolometric luminosity for main-sequence and evolved OB-type stars (open
and filled symbols, respectively) in the Cygnus OB2 region. The dotted and
continuous lines indicate log
![]() ![]() |
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We now discuss the results of our analysis for three noteworthy OB stars in
the ACIS FOV that have been the subject of previous investigations.
![]() |
Figure 12:
ACIS-I spectra
of three OB stars. The data were fitted with absorbed two temperatures
optically thin thermal plasma models. Left: Cyg#12 (Src.Id. #60), showing a
prominent FeK 6.7 keV emission line, indicative of hot thermal plasma. Center:
Cyg#8 (Src.Id. #568), a colliding wind binary with a strong FeK![]() ![]() |
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The XMM-Newton EPIC spectrum of Cygnus OB2 #12 was also fitted by
Rauw et al. (2005), who find temperatures
and
keV, consistent with our own estimation (see
Fig. 12 and Table 4). The temperature of the hard
component is significantly higher than expected according to the wind shock
emission model, given the low velocity of the stellar wind,
150 km s-1. Emission from MCWS might better explain the spectrum (see above).
Such a scenario could also explain the observed X-ray variability (see Sect. 5) if the magnetosphere is tilted with respect to the rotation
axis as in the case of
Orionis C (Gagné et al. 2005).
Intermediate-mass stars (
)
are not expected to emit
X-rays because, unlike O- and early B stars, they do not drive strong stellar
winds nor possess outer convective zones that can sustain a dynamo mechanism
such as the one that is ultimately held responsible for X-ray activity in
low-mass stars. Several studies have however observed X-ray emission apparently
associated with late B- and A-type stars. Although its origin has been often
attributed to the coronal activity of unresolved late-type companions
(e.g. Stelzer et al. 2006a,b), the matter is
controversial. The Cygnus OB2 region has a large number of intermediate mass stars.
It is therefore an ideal target to check this hypothesis. We classify 66 ACIS
sources as intermediate-mass. The
vs. mass plot in Fig.
11 does not reveal any dramatic discontinuity at the boundaries with
low and high mass stars. Intermediate mass stars, however, do not seem to
follow the trend of increasing
with mass of lower mass stars, and
seem to have
values that span the whole range spanned by these
latter. This observation is consistent with the companion scenario.
As further supporting evidence for this conclusion we note that 8 of the X-ray sources associated with intermediate mass stars (about 12% of the sample) are variable, which is almost the same fraction found for low-mass stars (see Sect. 6.3). Moreover, their X-ray spectra are well represented by isothermal models with median kT=2.6 keV, also in agreement with values found for low mass stars (see Sect. 7.3).
At the distance of Cygnus OB2 an offset between IR and X-ray positions of 1'', i.e.
roughly the on-axis Chandra spatial resolution, corresponds to a
projected separation of 1450 AU. X-ray source positions can however be
determined with a precision of
(145 AU) or even better, depending on
the source statistics and off-angle. If the intermediate-mass primary of a
binary system is X-ray dark we should thus be able to distinguish, at least
statistically, whether the observed X-ray emission originates from the low mass
secondary in the case of wide binary systems, defined as systems with
separation >0.001 pc (206 AU) (cf. Close et al. 1990).
Figure 13 shows an example of an X-ray source associated with an
intermediate-mass counterpart, but with a significant spatial offset. A
comparison of the distributions of X-ray-NIR offsets for X-ray sources
associated with low and intermediate mass counterparts is however inconclusive.
If the companion hypothesis is correct, most X-ray emitting
low-mass companions are not in wide systems.
![]() |
Figure 13:
Example of a
possible misidentification of an X-ray source with a intermediate-mass near-IR
star. We show the X-ray and ![]() ![]() ![]() |
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The origin of X-ray activity in low-mass PMS stars has so far eluded full understanding. Although many aspects of MS coronal activity also are not understood, the X-ray activity of MS stars correlates well with stellar rotational and convection properties, thus supporting the picture of a solar-like corona that is ultimately powered by a stellar dynamo. For Pre-Main Sequence (PMS) stars, however, no such a correlation has been found and the picture appears somewhat complicated by the presence of accretion disks, which seem to play an important role in the X-ray emission. On one hand plasma heated by shocks due to the accretion process is likely to be a source of soft X-rays (cf. Favata et al. 2005; Stelzer & Schmitt 2004; Flaccomio et al. 2006; Kastner et al. 2002). On the other hand the presence of accretion disks seems to lower the overall activity level, although probably making the X-ray emission harder and more time variable (Flaccomio et al. 2006; Preibisch et al. 2005; Flaccomio et al. 2003).
We have defined the low mass range
,
based on the presence of a
significant convection envelope. The X-ray spectra and luminosities of low mass
stars in Cygnus OB2 are typical of T-Tauri stars in other regions: our spectral
analysis (Sect. 6) yields a median kT=2.38 keV and log
.
Figure 10 indicates that, although the spread of
at any given mass is of the order of 1 dex, a trend of increasing
with mass is observed. The same figure shows for comparison the nearly
complete sample of Orion Nebular Cluster (ONC) stars as observed by the Chandra
Orion Ultra-deep Project (COUP) (Getman et al. 2005b). The sensitivity
limits of the two observations are quite different because of the larger
distance of Cygnus OB2 (
1450 pc vs. 470 pc for the ONC), and the longer
exposure time of the COUP observation (
838 vs.
97 ks for our
data). However, as already discussed, the most severe limit to the
completeness of our sample of likely members with near-IR counterparts is
likely to be the sensitivity limit of the 2MASS data. The Cygnus OB2 stars in Fig. 10 have a well defined limit of mass, at
,
rather than X-ray luminosity.
Figure 10 shows that the luminosities of low mass stars in Cygnus OB2 and
in the ONC are compatible. The numbers of stars in the
mass range in the Cygnus OB2 and ONC regions differ considerably. In
Cygnus OB2 we count 334 such stars, about 10 times as many as the in a sample of 375
COUP-detected lightly absorbed (
< 2) ONC stars with mass estimates, in
which we find 34 stars in the same range (Getman et al. 2005b).
Assuming that the IMFs in the two regions are the same and that the considered
sample of lightly absorbed ONC stars is complete, we can estimate the total
number of stars in our Cygnus OB2 ACIS FOV as 375/
.
Considering that our FOV contains only one fourth of the 106 OB stars known in
Cygnus OB2, we can then scale this number by the number of OB stars to get the
total expected Cygnus OB2 population:
15 000 stars. The total stellar mass of
the cluster can be then estimated by multiplying this number by the average
stellar mass in the lightly absorbed COUP sample (0.84
):
,
somewhat lower than the estimate of
Knödlseder (2000), who derived
.
Deeper and more extended X-ray and near-IR surveys of Cygnus OB2 will be invaluable for the study of young stars of all masses with unprecedented statistic. Such observations has been recently obtained at the TNG observatory, and we will present such results in a forthcoming paper.
We have reported the results of a deep Chandra X-ray observation pointed
toward the 2 Myr old star forming region Cygnus OB2. Source detection was
performed using PWDdetect, a wavelet-based algorithm, supplemented by visual
inspection, identifying 1003 X-ray sources in the
ACIS-I field
of view. Data extraction was performed using the semi-automated IDL-based ACIS E
XTRACT package, which is well suited to the analysis of observations of
crowded fields such as ours.
The X-ray source list was cross-identified with optical and near-IR (2MASS)
catalogs: 26 X-ray sources were identified with optically characterized OB
members of Cygnus OB2 (out of the 33 lying in the FOV) and 775 with 2MASS sources.
Among these latter sources almost all are believed to be Cygnus OB2 members. About
80 X-ray sources without an optical/NIR counterpart are estimated to be
of extragalactic nature (AGNs) while the remaining X-ray sources with no
counterpart are likely associated with members that are fainter than the 2MASS
completeness limit.
In order to characterize the previously unidentified likely cluster members
with NIR counterparts, we placed them on NIR color-magnitude (
vs.
)
and color-color (
vs. J-H) diagrams. A
first estimate of interstellar extinction was obtained adopting a 2 Myr
isochrone for the low- and intermediate-mass stars and assuming that O- and
early B-type stars lie on the MS. We find a median visual absorption for OB
stars of
5.6 mag, while low mass likely members seem to be slightly
more absorbed,
7.0 mag. We also use the 2 Myr isochrone and the J magnitude to estimate masses of likely members assuming that they share the
same distance and absorption. Our sample of X-ray selected members with near-IR
counterparts reaches down to
,
and is likely complete down
to
1
.
From the
vs. J-H diagram we estimate the
fraction of low-mass stars with NIR excesses (23/519), finding it to be quite
low with respect to the 1 Myr old ONC region indicating that either (i) the
disk fraction decreases steeply with stellar age, or (ii) the intense
photo-evaporating UV-field due to the large population of O stars in the region
has reduced the disk lifetime.
At least 85 or 134 X-ray sources, i.e. 8.5% and 13% of the total, were
found to be variable within our observation with a confidence level of 99.9%
or 99%, respectively. The fraction of variable sources increases with
increasing source counts, likely as a result of a statistical bias. The
light-curves of 97 sources indicate an impulsive, flare-like, behavior while
the rest show more gradual variations of the X-ray emission. Only two of the 24
detected O- and early B-type stars are detected as variable during our 97 ks
observation, in spite of the high statistic of their light-curves. These
exceptions are the B5 Ie star Cygnus#12 (our source #60), showing a rather
linear decay of the count rate during the observation, and a B1.5 V+? star
(#979), which also shows a slow variability, possibly related to rotational
modulation. In this latter case the detected emission may originate from the
primary B1.5 component and/or from its presumably lower mass companion
indicated by the spectral type published by Hanson (2003).
We modeled the ACIS X-ray spectra of sources with more than 20 photons
assuming absorbed thermal emission models. Median log()
and kTvalues of the sources are 22.25 (cm-2) and 2.36 keV, respectively.
Variable sources,
have harder spectra (median kT: 3.3 keV)
than non-variable ones (median kT:2.1 keV) and flaring sources are even
harder (median kT:3.8). Sources associated with O- and early B-type stars are
instead quite soft (median kT: 0.75 keV). Absorption corrected X-ray
luminosities of OB stars were calculated from the best fit spectral models. For
the other, typically fainter, sources, we adopted a single count-rate to flux
conversion factor computed from the results of individual spectral fits. Low
mass stars have
ranging between 1030 and 1031 erg s-1 (median
). Variable low mass stars are on
average 0.4 dex brighter, probably because of a selection effect. These X-ray
luminosities are consistent with those of similar mass stars in the slightly
younger (1 Myr) ONC region. O and B-type stars are the most luminous, with
erg s-1. Their X-ray
and bolometric luminosities are in rough agreement with the relation
,
albeit with an order of magnitude dispersion. The
X-ray luminosities of the 10 evolved OB stars in our sample however, seem
to obey a better defined and steeper power-law relation with
:
). Further studies of a larger number of
evolved OB-type stars are needed to confirm or disprove this dependence.
Acknowledgements
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. J.F.A.C acknowledges support by the Marie Curie Fellowship Contract No. MTKD-CT-2004-002769 of the project "The Influence of Stellar High Energy Radiation on Planetary Atmospheres", and the host institution INAF - Osservatorio Astronomico di Palermo (OAPA). E. F., G. M., F. D. and S. S. acknowledge financial support from the Ministero dell'Universita' e della Ricerca research grants, and ASI/INAF Contract I/023/05/0.
Table 1: Cygnus OB2 source catalog.
Table 2: Near-IR counterparts of Cygnus OB2 X-ray sources and near-IR stellar parameters.
Table 3: X-ray source spectroscopy.