Soft X-ray surveys of star forming regions (SFRs), for example of
Chamaeleon, Lupus, Taurus and Orion, have revealed groups of low-mass
(M* )
active stars apparently associated with
those regions (e.g. Alcalá et al. 1995, 1996; Wichmann et al. 1996;
Krautter et al. 1997). These objects have late spectral types
(typically G0 or later) and are pre-main sequence (PMS) or young
(age
108 yr) main sequence (MS) stars. They are now
known as weak-line T Tauri stars (WTTS). WTTS are apparently
associated with SFRs and, assuming that their distances to the
observer are those of their associated regions,
one infers their youth from their positions
in the H-R diagram. Besides these morphological and photometric
properties, the WTTS also present key features in their spectra
that reveal us their young (age < 108) nature: they have
chromospheric activity (H
and CaII H+K lines in emission,
W(H
)
-10 Å), the photospheric line LiI
Å is conspicuously present in their spectra)
and the stars rotate fast (
km s-1 or more).
A necessary condition to classify these objects as PMS stars is the
presence of the Li I feature in excess in the W(Li) versus
plane when compared with the upper envelope of the Pleiades
stars (e.g. Fig. 12 of Alcalá et al. 1998, hereafter A98). In this
case, Lithium has not had enough
time to be destroyed in the deeper layers of the convection zone (cf.
Bodenheimer 1965). On the other hand, the WTTS are mildly masked,
if at all, by circumstellar material: they lack (strong) spectral
line and continuum (ultraviolet or infrared) emissions that
characterize the classical T Tauri stars (CTTS), which are
believed to be younger (age
yr). WTTS
are considered the descendants of CTTS. However, the criteria
for establishing the membership of the WTTS to an ongoing star
forming region should be taken with caution because of their large
spatial distribution (
0) and because their
distance to the observer is, in most cases, unknown.
Although the EINSTEIN satellite shed new light on the evolution of
these PMS objects, its observations were spatially biased towards the
denser parts of the SFRs, where CTTS predominate and follow a clumpy
pattern. Hence, little or no inference could be drawn about the history
of star formation at a cloud scale. The spatially unbiased ROSAT All-Sky
Survey (RASS) remedies this situation: it yields a spatially complete but
flux-limited sample of X-ray sources around a given cloud at about EINSTEIN's sensitivity. Contrary to the spatial distribution of the CTTS,
WTTS were found to be uniformly distributed over the whole observed area
and outnumber the preceding CTTS by, at least, a factor of 3. The location
of the WTTS in the evolutionary sequence has not yet been well established
(e.g. Montmerle et al. 1993; Chavarría-K et al. 1995), but in general,
stellar activity is considered a quantitative landmark of youth for this
type of objects. Since WTTS are less active in the uv, optical and infrared
spectral regions than CTTS, the former are
considered older than the latter. Consequently, WTTS had more time to
disperse from their birth sites, explaining, at least partially, their
broader and homogeneous spatial distribution. Despite the intrinsic X-ray
brightness of the WTTS, we are luminosity-limited in their detection.
At present, we are constrained to only nearby SFRs (d < 500 pc).
Many follow-up studies (optical, mostly spectroscopic) of the
EINSTEIN and ROSAT surveys of nearby SFRs such as Chameleon,
Taurus-Auriga, Lupus, Orion and Scorpius-Centaurus have recently
appeared in the literature (e.g. Alcalá et al. 1995; Wichmann et al. 1996, 1997;
Li & Hu 1998; Magazzù et al. 1997; Krautter et al. 1997; Alcalá et al. 1996, 1998;
Walter et al. 1988, 1994; Sciortino et al. 1998 and references therein).
Unfortunately, photometric data of many WTTS and WTTS candidates
associated with the regions of interest are lacking or the stars were
observed in an unsuitable photometric system (e.g. the
photographically-based GSC or satellite-born V magnitude estimates,
particularly for red
stars): the exceptions are the monitoring for light-variability of 58 WTTS
(and WTTS candidates) in Taurus by Bouvier et al. (1997) in Johnson's
B and V passbands, the uvby- photometry of WTTS and WTTS candidates
in Orion by Alcalá et al. (1996, hereafter A96) and the UBVRIJHKL
photometry of WTTS in Scorpius OB2-2 by Walter et al. (1994,
hereafter W94).
In this paper we report and discuss photometric data of X-ray flux
selected and confirmed WTTS and WTTS candidates
in the Taurus-Auriga SFR (uvby-and JHK, 58 and 20 stars,
respectively) and in Scorpius OB2-2 SFR (uvby-
,
18 stars). We also
extend the photometric
sample of WTTS in Orion reported by A96 (uvby-
,
40 stars). The
majority of stars are observed for the first time in this photometric
system. The data cover about 74% of the objects on the list by
Wichmann et al. (1996, hereafter Wi96), about 36% on the list by A96,
and about 46% from the list of WTTS and WTTS candidates in the
Upper Scorpius Association by Wa94, as well as six objects near the
runaway O star
Oph (Oph1 through Oph6, Walter 1986, see also
Terranegra et al. 1994).
In spite of previous spectroscopic, photometric and proper motion studies, the membership of WTTS to clusters with ongoing star formation is still an open matter. It is the purpose of this work to cover some of these deficiencies for future analysis and observations.
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