In Fig. 10, we compare the complete log N-log S curve
with those for stellar, extra-galactic, and unidentified sources.
At a PSPC count rate threshold of 0.1 cts
,
of the
RASS sources have optical counterparts, most of them being stars.
At lower count rates the fraction of unidentified sources
increases from
at 0.07 cts
to more than
at 0.02 cts
,
a level at which all curves flatten because of
the incompletness of the RASS itself.
Above a PSPC count rate of 0.03 cts s-1 we have optically
identified 75 among 136 RASS X-ray sources (i.e.
), most of them
being stars. The small number of extra-galactic object identified at
this threshold (only two) is simply due to our follow-up observation
strategy. According to the results of Guillout (1996) and Zickgraf
et al. (1997), we conclude that the extra-galactic population in the R CrA
region is likely to account for about
of the unidentified X-ray
sources detected above 0.03 cts s-1, the rest (i.e.
)
probably being optically faint active K- and M-type stars.
We now focus on the identified stellar population and define two
sub-regions within our field, namely the on-cloud region
(from
= 18 h 56 m to 19 h 24 m and from
to
,
i.e. 14 deg2)
and the off-cloud region
(complementary to the on-cloud region, i.e. 112 deg2).
We have plotted in Fig. 11 the observed on-cloud and off-cloud stellar log N-log S curves as well as the predictions of
the stellar X-ray population model from Guillout et al. (1996).
Computations were run for |b| = 15
and l = 180
although the galactic longitude is irrelevant at the RASS sensitivity.
Results are summarized in Table 7.
S |
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0.10 | 0.179 | 0.167 | 0.113 | 1.58 | 1.47 |
0.05 | 0.626 | 0.414 | 0.273 | 2.29 | 1.51 |
0.03 | 1.073 | 0.682 | 0.482 | 2.22 | 1.41 |
First we note that at any PSPC count rate
the on-cloud stellar density is significantly higher (by a
factor 2) compared to the model predictions, as expected for
a region with ongoing star formation. On the other hand,
we expect that the off-cloud log N-log S curve lies within
of the model prediction, which is clearly not the case.
In order to check the relevance of the model prediction, we compare with
the so-called RasTyc sample (Guillout et al. 1999), a sample of all objects
included in both RASS and Tycho, i.e. the largest sample of stellar X-ray
sources with homogeneous and accurate data constructed so far. In order
to account for the magnitude and X-ray flux limited biases of of RasTyc,
we ran a specific model
optically limited at 10.5 mag (plus
of stars down to 11.5 mag).
We then compared the expected number of stars per deg2 with the observed
one computed in two regions extending
wide all around the
sky and centered at |b| = 15
.
At a PSPC count rate threshold
of 0.03 cts s-1, there are 1819 RasTyc stars detected within these
two regions amounting to 6946 deg2, i.e. 0.26 stars per deg2.
At this level, the model predicts a stellar surface density of
0.23 stars per deg2, in very good agreement with the observations.
We are thus confident that the theoretical log N-log S curves plotted
in Fig. 11 give a good estimation of the ambient galactic plane
stellar population at the R CrA cloud galactic latitude.
We then conclude that in the region surrounding the CrA molecular
cloud our observations reveal
excess of stellar
X-ray sources with respect of a "pure'' galactic plane population
(see Table 7).
According to the expected contribution of extra-galactic sources to
the unidentified population,
is a lower limit on the excess.
Such excesses were also detected around other star forming
regions (see Neuhäuser 1997 and references therein).
However, contrary to some other star forming regions like Lup-Sco-Cen
(Guillout et al. 1998a,b), the Gould Belt can hardly be an explanation
because of the position of the CrA molecular cloud projected well below the
Gould Belt plane.
Also around the Chamaeleon clouds (Alcála et al. 1995) and south of the Taurus clouds (Neuhäuser et al. 1997), many new pre-MS stars were found, although there is no Gould Belt in that directions. As far as the Chamaeleon off-cloud TTS are concerned, Mizuno et al. (1998) found new, previously unknown, small cloud-lets near one third of the off-cloud TTS, which may be the birth places of those seemingly off-cloud TTS. If one can explain off-cloud TTS around the CrA and Cha clouds by cloud-lets rather than by the Gould Belt, at least some of the Lup-Sco-Cen, and Orion off-cloud TTS may also have originated in such small cloud-lets, as originally proposed for the Chamaeleon off-cloud TTS by Feigelson (1996).
The question now is whether we found all young, i.e. coronally active stars (inside and) around the CrA dark cloud. This can be investigated by optical follow-up observations of additional unidentified X-ray sources found in deep ROSAT PSPC and HRI pointed observations (Walter et al., in preparation).
Whether all young stars were found among all RASS sources
can be investigated in the following way:
Sterzik et al. (1995) have shown that it is possible to pre-select TTS
candidates from the RASS using four criteria,
namely the two hardness ratios, the X-ray count rate, and the optical
magnitude of the nearest (if any) counterpart (within, say,
).
Then, TTS candidates are those RASS sources which resemble best
previously known RASS-detected bona-fide TTS according
to the same properties. The parameter which describes how well
a particular RASS source resembles the typical TTS properties
is called discrimination probability P,
described in detail in Sterzik et al. (1995).
In Fig. 12, we plot the number of CrA RASS sources per
discrimination probability P, namely for PMS stars, otherwise
active stars, other objects, and unidentified RASS sources.
If we would have pre-selected TTS candidates using the
Sterzik et al. (1995) method, i.e. if we would have done
optical follow-up observations only for RASS sources with a
discrimination probability of, say,
,
we would have
obtained a high success rate by loosing only one TTS.
Now, for a discrimination probability of, say, P=0.5,
the reliability (rel) of the TTS candidate selection
is 0.45, based on the classified sub-sample.
The reliability number gives the fraction of real TTS
(real according to our spectroscopy) among those X-ray
sources with discrimination probability above some
threshold, e.g. .
The fraction of lost unidentified TTS is 0.17,
which is the number of real TTS to be expected (according
to their discrimination probability values) among those
X-ray sources not observed by optical spectroscopy.
Because there is a total of N = 46 sources with
and 160 below this threshold, the expected number of
TTS hidden in the RASS sample is
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