The largest photometric campaign of Southern TTS is presented by
Covino et al. (1992), after monitoring a sample
of 30 stars in . They
confirm the previously reported period of CV Cha, Sz 6 and Sz 68 and
establish new periods for four other stars (AS 205, Wa Oph/2, WA Oph/3
and Wa CrA/2). They are unable to
confirm previously reported periods for
T Cha and S CrA or to find any periodical behavior for SZ Cha and GQ
Lup.
We analyze the light curves of the 26 T Tauri stars listed in Table 1 where the first 13 have only recently been identified as PMS stars.
Table 2 shows the periods determined in this work and the previously published
ones. We label each star in
Table 2 as weak or classical following the earlier criteria based solely on the
strength of the H emission: wTTS if EW
is less than
10 Å and cTTS if larger. However, the most complete set of criteria to
distinguish cTTS from wTTS is discussed by
Edwards et al. (1993). These are
the near infrared color excess, the optical continuum veiling and the forbidden
emission line of [OI] at
6300. Such data are presently lacking for
the majority of our targets.
PDS/HBC | w/c | H![]() | Lit. ![]() | ![]() | |||
PDS 1 | w | 0.21 | - | - | |||
45 | w | 2.01 | - | - | |||
50 | w | 12.5/8.01 | - | - | |||
59 | w | 5.01 | - | 11.73 | |||
66 | c | 471 | - | 5.75 | |||
70 | w | 2.01 | - | 5.10 | |||
77 | w | 2.01 | - | 2.50: | |||
81 | w | 0.71 | - | - | |||
82 | c/w | 29.0/9.01 | - | - | |||
83 | w | 10.01 | - | 5.52 | |||
89 | w | 2.21 | - | - | |||
99 | w | 5.01 | - | - | |||
101 | c | 521 | - | - | |||
HBC 565 | c | 492 | 7.63, 6.24, 6.15,6 | 5.97 | |||
567 | c | 262 | 8.63, 8.04, NP6 | NP | |||
570 | c | 562 | - | 9.86: | |||
578 | c | 712 | 74, 7.23 | 2.56 | |||
583 | c | 522 | - | 5.34 | |||
590 | c | 322 | - | 13.00: | |||
591 | c | 2.01 | 3.244,10, NP7 | - | |||
603 | c | 172 | - | NP | |||
605 | w | 7.02 | NP7 | NP | |||
620 | w | 0.52 | - | 4.05: | |||
656 | c | 548 | 3.12,NP | ||||
657 | c | 288 | 4.05 | ||||
663/664 | c | 102 | 5.2011, 4.839 | 5.15 |
The EW are taken from the HBC or PDS.
For a low mass PMS star, H
line strength indicates
(but not conclusively) the presence of
a circumstellar disk since important components of this line are formed either
in a wind or in an accretion column - both scenarios
requiring disks (Johns & Basri 1995).
Thus, the H
of Hen 892 and BZ Sgr suggest the presence of a disk.
All the remaining PDS stars have small
but overt EW
.
Further observations of these wTTS must be carried out, especially in
the near infrared, in order to constrain their circumstellar environments.
These objects were not observed with enough regularity to enable time series analysis. There are only 4 observations per star.
Hen 1 is not an IRAS source and does not indicate near infrared excess of
luminosity (GHETAL92). It is situated at high galactic latitude
and is not associated with any known molecular cloud.
The level of variability (as deduced solely from our 4 observations) is one
of the lowest in our sample - at a level which is
unusual for pre-Main Sequence stars.
PDS 89 is a serendipitous discovery of PDS since it lies outside of the error
box of an IRAS source. The source is actually associated with PDS 149 in the
small cloud L152 where HBC 651 is located
(Torres et al. 1995).
PDS 99 belongs to the CrA association and was previously identified as an
H emission line star by
Marraco & Rydgren (1981).
These two PDS stars together with PDS 54, PDS 55 and the cTTS TW Hya lie within
a circle of about 7 at high galactic latitude
.
de la
Reza et al. (1989) attempt to physically connect these objects as members of
a common parental cloud which has already dissipated.
Jensen et al. (1996) did
not detect PDS 45 nor Hen 600 at 800
m but did detect PDS 55, which
curiously is not an IRAS source. We have
less than 4 entries for PDS 54 and PDS 55, and they are not included in this
work.
Our photometry in the V-band of PDS 45 and 50 shows the low range of variability typically ascribed to solar-type active regions on the stellar surface (see Rodonò 1986). The amplitude of the variations reaches merely a few times the intrinsic errors in our measurements thereby preventing us from establishing a reliable rotational period.
PDS 77 has the spectroscopic characteristics of a wTTS and is a member of the
Lupus association. If the strength
of EW alone is the indicator of a circumstellar disk then PDS 77 is
a wTTS and probably devoid of an active disk. However,
Herbst et al. (1994) argue that wTTS generally have light fluctuations
less than 0.8 mag in V. Only accretion disks are capable of powering
light variations that are larger than this value in the V-band.
We have gathered a sparse and small number of data points (7). Nevertheless, we observed a significant range of variability in all colors (0.91 at V). Several possibilities are indicated by the DCDFT code and they all cluster about 2.50 days with mild acceptance. We suggest this as a preliminary result and we stress that additional observations are needed to confirm this.
These two stars are found in the same region of the Oph complex.
PDS 81 presents a range
of variability of 0.15 mag in all colors. It is a wTTS with
H
sometimes filled in. It is associated with
the cloud B40 for which
HBC lists 5 TTS, and the PDS finds
another 3. A total of 11 exposures in
were gathered in 1988/1989 (4/7) and the BSP algorithm does not
indicate any reliable period. The DCDFT method indicates a period
of 11.11 days but with a modest degree of confidence (62
in V)
which leads us to not include it in the pool of TTS with determined
rotational periods.
VV Sco is a binary system with a separation of 1.5''. It lies at the border of the complex and cannot be resolved photometrically. We search for periodical patterns in the light curve with no conclusive results.
The PMS nature of BZ Sgr was established quite recently (GHETAL92) but its
H profile in emission had been known since earlier prism surveys
(Stephenson & Sanduleak 1977). GHETAL92 associated this object with the
molecular cloud No. 159
(Magnani et al. 1985). We discover a very faint
companion less than 5'' apart. This possible companion has a firm emission in
H
but the noise prevents any conclusion as to the existence of the Li I
line.
This star presents remarkable variability and striking color-dependent
amplitudes that are typical of cTTS. The strength of its H also
leads us to believe that this object is a classical TTS.
Jensen et al. (1996) detect the star at 800
m which is additional
evidence for the presence of a circumstellar disk.
Neither algorithm was able to find any periodic pattern for BZ Sgr using our data set.
Previous periods of 8.6 and 8.0 days have been reported for TW Cha as well as a null result (see Table 2). TW Cha is a cTTS with a reportedly large degree of variability (Bouvier et al. 1986). This is confirmed here. In spite of the large amplitude, no convincing period-folded light curve presented itself. The reported periods of 8.0 and 8.6 days do not fold our data into a smooth light curve.
We gather 10 entries in 1995 distributed over two months for CT Cha.
It presents, in general, mild
variability. The BSP algorithm and the DCDFT code indicate two
periods with observations well distributed in phase and have large confidence
levels: 9.86d and 13.68d. Several other possible periods with acceptance larger
than 90 cluster at about the former value. We tentatively suggest 9.86d as
the computed period for the season.
The pre Main Sequence nature of T Cha is established by
Alcalá et al. (1993).
In spite of an EW typical of weak TTS, these authors
report evidence of disk accretion - as inferred from the inverse P Cygni
profile in the H
- and robust spectral variability. We also have
several
medium resolution data targeting the H
line that show strong variability (the PDS archives).
The line goes from absorption to emission on a time scale of days, as is also
reported in Alcalá et al. (1993).
Hoffmeister (1965) made extensive visual observations and proposed a period of 3.2436 days. A periodicity of 3.2d is also found by Mauder & Sosna (1975), although not confirmed by the more recent observations of Covino et al. (1992). This star consistently undergoes large photometric and spectroscopic variability (Covino et al. 1992; Alcalá et al. 1993).
Neither the BSP nor the DCDFT method reveal any conclusive period for our modest sample. Due in part to the variability revealed in the medium resolution data, we re-analyzed the data sets of Hoffmeister (1965) and Mauder & Sosna (1975) using the DCDFT method. We averaged their data into a maximum of 3 data points per night. Hoffmeister's observations give a period of 3.221 days with a false alarm probability of 10-8. No period was found for the Mauder & Sosna data set unless the observations taken after JD = 244141399 are omitted, in which case we are able to reproduce the period of 3.22 days. Our 7-exposure light curve does not fold smoothly into a 3.22 day period.
We hypothesize that T Cha is another case of a TTS that undergoes phases of periodical behavior (see for example Vrba et al. 1989) and phases of irregular variations.
These two stars are cTTS that show mild to low variability. Because of weather conditions we gathered a maximum of 10 points distributed over two consecutive years.
For SZ 45, the BSP periodogram method indicates two periods at 14.12 days and
14.75 days
with comparably low 's. The period-folded light curves, however, are
not well distributed in phase.
The DCDFT method computes several
possibilities, all clustering about a period of
13.00 days. We suggest the latter as a tentative value.
For SZ 108, both methods indicate similar solutions at about 4.05d with a high level of confidence. However, the data are not well distributed in phase and due to the modest number of entries we regard the result as tentative.
SZ 77 shows a moderate range of variability in the course of our campaign (Table 1). Analysis of the near infrared excess indicates active disk accretion in this object (Batalha & Basri 1993). Neither methods provide definitive period determinations.
SZ 82 was monitored by Covino et al. (1992) and no periodicity was found. It has signatures of disk accretion, with a mild near infrared excess indicative of reprocessed stellar light by overlying circumstellar dust. The ratio between the stellar and systemic luminosities tends to reinforce the presence of an active circumstellar disk (Batalha & Basri 1993).
Two possible periods are indicated by the BSP:
1.16d and 1.61d. The DCDFT method does not indicate a conclusive
solution for the V-band, nevertheless two maxima in the power spectra are
achieved at 7.42d and 1.55 d, both aliases. The confidence of both periods are
78 which leads us to conclude that no periodicity was established for SZ 82.
PDS 59 is situated close to T Cha and may have a faint companion at 12
arcsec. The EW (500 mÅ) and the EW
(less than 10 Å) are
typical of wTTS (GHETAL92).
We have a total of 10 data entries distributed over three consecutive years
(1989-1991) which is enough to cast suspicion on any result. Nevertheless, we
proceed with the analysis having in mind that wTTS's have, in general, stable
spot distributions (Herbst et al. 1994).
The BSP method indicates 14.07d and 11.73d as possible periods, with the
former folded light curve indicating a double wave shape. The DCDFT method
reveals
a peak in the power spectrum at 11.73d in all colors, except U, with a
modest confidence level of 30, 98
, 96
and
83
in the colors B, V, R and I respectively. The peak and formal
acceptance are significantly raised if all colors are included simultaneously.
We propose a tentative period of 11.73d and the folded
light curve is presented in Fig. 1 (click here).
Figure 1: Period-folded light curve for PDS 59
Hen 892 is a cTTS on the basis of its strong H, Li (370 mÅ) and
IRAS colors. The large EW
indicates accretion which implies
the presence of veiling and/or color excess, especially in the blue where these
effects are most strongly felt.
Nevertheless, the amplitude of variability of Hen 892 is merely 0.1 mag in all
colors. This variability is less than that found among active field stars
(Rodonò 1986) and is atypical of cTTS. This object is isolated from any
known star forming region.
We gathered a total of 9 entries in 1989/1990 (1/8).
The BSP method indicates two acceptable solutions in the colors V, R and
I: 4.67d and 5.71d. The DCDFT method indicates
5.75 0.03 days with a confidence of 97.2
and 4.70
0.02
with a confidence of 93.5
. The criteria to adopt 5.75 days as the
period for the season is based on the shape and the phase coverage
of the folded light curve (Fig. 2 (click here)).
Figure 2: The same as Fig. 1 for Hen 892
The H line of PDS 70 varies in between 2-4 Å.
It shows a low range of variability ( 0.1) which is independent of
color.
We gathered 9 entries during 1989/1990 (1/8). The BSP method fits
the data well at 5.1d or 5.6d, with comparable and low
and good phase
coverage.
The power spectrum from the DCDFT method peaks at 5.1d and 4.26d
with a large degree of confidence in the V-band. The confidence of the latter
value is not consistently
reproduced
in the other colors. Therefore, we suggest a
period of 5.1d during these two years (Fig. 3 (click here)).
Figure 3: The same as Fig. 1 for PDS 70
A steady increase in the amplitude towards the blue can be seen
in the light curves of
V896 Sco. Our data set is very modest with one entry taken in 1989 and
six others in the following year. The periodogram analysis done
with the BSP method indicates two
possible periods at 5.54 days and 7.93 days yielding good phase coverage
and low values.
The DCDFT method indicates 5.52 days with
a large confidence level after adding the magnitudes of all colors.
Thus, inspection of
both period-folded light curves
leads us to suggest 5.52 days as the rotational period of V896 Sco
(Fig. 4 (click here)).
Figure 4: The same as Fig. 1 for V896 Sco
SY Cha is an active T Tauri showing, at times, complex light curve behavior which suggests the presence of more than one dominant spot (Bouvier & Bertout 1989) and changes in the spot distribution. Periodicities have been reported for this object in Bouvier & Bertout (1989); Schaeffer (1983); Kappelman & Mauder (1981) and Mauder & Sosna (1975). These authors report 6.0, 6.129, 7.6 and 6.2d, respectively.
We gathered a total of 22 observations during 1994 and 1995 (6/16) and we
confirm the periodic behavior of SY Cha.
The BSP algorithm indicates
two periods at 5.97d and 6.07d. The
former gives more complete phase coverage.
The DCDFT method suggests 5.97 days as the most acceptable period for the season
with confidence levels of 99.99. We conclude that during our campaign SY Cha
had a period of 5.97d and
the final folded light curve is presented in Fig. 5 (click here).
Figure 5: The same as Fig. 1 for SY Cha
VZ Cha is a typical representative of cTTS, with very large EW and
IRAS fluxes. Studies based on spectroscopic observations indicate a period of
about 7 days
(Mauder & Schulz 1978). Another period of 7.2 days -
determined with sparser time sampling - is also pointed out by
Kappelman &
Mauder (1981).
The BSP method does not indicate any significant minimum.
The DCDFT algorithm presents peaks in the power spectrum at 2.577 days
(99.8 in I and 98.0
in V), and, with lower significance, 12.00 days.
We suggest 2.56 days as the final period, after inspecting the period-folded
light curve (Fig. 6 (click here)).
Figure 6: The same as Fig. 1 for VZ Cha
We have photometry of WY Cha taken during two seasons.
The BSP method indicates two acceptable periods of 5.43d and 2.10d,
with the former showing better phase coverage. The DCDFT method indicates the
period of 5.34
days with a very high level of acceptance.
The former period gives the minimum for the adjusted curve
and is chosen as the period for the season (Fig. 7 (click here)).
Figure 7: The same as Fig. 1 for WY Cha
Our photometric data of AS 216 - a PMS star not associated with any known
molecular
cloud (Quast et al. 1987) - have been gathered since 1985. The
EW
computed at medium resolution (GHETAL92) spans values between
40 and 70 Å which is indicative of disk-like activity. The photometric
variability of AS 216 shows no significant change in the first three years.
It reaches an amplitude of 0.15 mag in the V-band
during the interval
, rises up to 0.25 mag
during
, and then decreases again to 0.15 mag
. This behavior is consistent for all
the colors. In the last two years (1989/1990) AS 216 went through an overall
increase in variability, which is best observed in the U band.
Both algorithms indicate the periods 3.12d and 4.24d during the first three runs, independent of whether the data sets are used together or separately in the periodogram analysis. A slightly larger confidence is given to the 3.12d period leading us to adopt it as the photometric period of 1985/1986/1987. However, neither of these options reaches the minimum level of acceptance in the following two years (1989/1990). In fact, the BSP method gives a period of 6.69d which is not confirmed by the DCDFT method, in part because the period-folded light curve shape departs slightly from a sinusoidal one. We conclude that AS 216 is another case of a cTTS that undergoes periodical/aperiodical phases.
The folded light curve of the 3.12 day period (1985/1986/1987) is shown in Fig. 8 (click here). We note the occurrence of a maximum in brightness at phase 0.85 that is detected in the three colors. It is not an artifact of data reduction nor weather conditions. In fact, it indicates a significant increase of continuum emission which is not compatible with classical flare activity but with the optical veiling detected in PMS stars (Basri & Batalha 1990). We show in Fig. 9 (click here) the 1989/1990 photometry, folded into 3.12 days.
Figure 8: UBV period-folded light curve for AS 216 during 1985, 1986 and 1987
observing season
Figure 9: The 3.12 days period-folded light curve of AS 216 during the
1989/1990 season. Inspection in the Fig. 8 (click here)
show that AS 216 is a typical case
of cTTS that goes through phases of periodical/aperiodical behavior.
AS 218 has shown evidence of periodic or quasi-periodic modulation in several colors with possibly no connection to cold spots (Quast et al. 1987). It is also shown that this object is isolated from any molecular cloud (Quast et al. 1987).
The BSP method indicates periods of less than 2.0 days which are not included in our final analysis. The folded light curve with a period of 4.05 days obtained by the DCDFT method is indicated in Fig. 10 (click here).
Figure 10: The same as Fig. 1 for AS 218
FK Ser is classified as a post T Tauri Star by
Herbig (1973) and
a visual binary system with separation of 1.33'' (HBC 663/664). It presents
800 m
emission similar to that of PDS 55, and
Jensen et al. (1996) conclude that FK
Ser
has disk properties similar to that of a typical TTS.
Quast et al. (1987) suggest that this system is isolated from any known molecular cloud, and they note that the periodic modulation is incompatible with those of cool spots. Chugainov (1974) finds a sinusoidal light variation with a period of 5.20 days and an amplitude of 0.1 magnitudes. Observations of this star in B and V done in 1974 at CTIO result in a period of 4.83 days, erroneously printed as 4.53 days in Torres et al. (1983). The origin of the apparent discrepancy in the resulting periods may reside in the fact that FK Ser is a binary system. Each component has a similar spectral classification. Therefore, the continuum modulation of each component, if present, will necessarily add to the final light curve explaining the difference in periods.
We have 30 entries taken during 10 nights in the 1986 run
UBV and 17
taken during the 1988-1990 one . In 1986 FK Ser shows large
amplitude variations - a maximum in U. The period of 4.89d presents itself as
the best solution of the BSP algorithm for both runs. The DCDFT method finds
a period of 5.13d in the first run if the photometry of all colors are added
simultaneously. The
variability is conspicuous in the following run, but the indicated period of
5.15d has a lower confidence level than for the previous run. We suggest a
photometric period of 5.15d for FK Ser (Fig. 11 (click here)).
Figure 11: The same as Fig. 1 for FK Ser