The present multicolor photometry has been used to investigate the light
curve evolutions, to search for the presence of photospheric solar-like
activity cycles and, using the method described by
Cutispoto et al. (1996), to
infer or further constrain the spectral type and luminosity class of program
stars (see Table 4 (click here)). Color indices of active stars have to be taken prudently
when used for spectral classification, as the presence of activity
phenomena can modify them by an unknown amount. In a recent paper
Amado &
Byrne (1997) have investigated the effects of stellar activity on photometric
colors, showing that active stars have a bluer U-B index compared to non-active
stars of the same spectral type. These new data have been now taken into
account computing the spectral type listed in Table 4 (click here). In most cases the
spectral types obtained from color indices agree very well with the
spectral classification, when available, as also shown for a larger
sample of late-type stars by Metanomski et al. (1997).
The results for the individual stars are now discussed.
HD 32918 = YY Men is an active giant with the typical
characteristics of FK Com stars (see Cutispoto et al. 1992 and references
therein). It has shown two of the most intense and longest duration radio
flares (Slee et al. 1987a; Bunton et al. 1989) and probably the longest optical
flare ever recorded for any class of active stars (Cutispoto et al. 1992).
The data obtained over the 15-29 Mar. interval are shown in Fig. 1 (click here), where phases
are reckoned from the 9.5476-day photometric period computed by
Collier (1982a). The asymmetric V-band light curve is single-peaked and
its maximum results about 0.045 magnitudes fainter than the brighest values
observed to date (cf. Fig. 7 (click here) in Cutispoto & Leto 1997).
Remarkable color variations, well correlated with the V-band modulation,
show the star to be redder at light minimum.
HD 36705 = AB Dor is one of the most studied active stars
as can be inferred from the papers listed in the SIMBAD database (see, among
the most recent, Mewe et al. 1996;
Rucinski et al. 1995; Cutispoto 1995;
Unruh et al. 1995; Kürster et al. 1994, 1997
and references therein). The
data obtained over the 15-28 Mar. interval are plotted in Fig. 2 (click here), where phases
are reckoned from the 0.51423 day photometric period computed by Pakull (1981).
The low amplitude V-band light curve exhibits two close maxima at phases
and 
, respectively. The weak color variations, better
visible for the U-B and V-I indices, are in phase with the V
modulation. However, it is interesting to note that the primary maximum for the
U-B color curve corresponds to the maximum in the V curve at phase
, while the primary maximum for the V-I color curve
corresponds to the phase 
0.26 maximum in the V curve. Finally,
the shape of the light curve is quite different from that observed in February
1991 by Anders et al. (1992), when only one maximum was present and the
peak-to-peak amplitude was smaller. The colors agree fairly well with those of
an active K0 IV/V star. From the 
value computed by
Randich et al. (1993) a minimum stellar radius of about 0.86 
is inferred. This
is in agreement with the conclusion, reached by Rucinski (1985)
and Innis et al. (1986), that HD 36705 has not arrived to the mean sequence yet.
HD 37847 = TW Lep is a SB2 spectroscopic binary with an
orbital period of 28.344 days (Balona 1987) that has been recentely observed
by several authors (see, among others, Strassmeier & Fekel 1990;
Strassmeier et al. 1993a; Fox et al. 1994;
Cutispoto 1995 and references
therein). It was detected by the ROSAT satellite during the all-sky EUV survey
(Pye et al. 1995). The photometric variability, with a period of 28.22 days,
was first reported by Henry et al. (1982). The observations presented here were
carried out over the 15-31 Mar. interval with the 0.5 m ESO. Three additional
points were obtained with the 1.0 m ESO on 9, 10 and 12 April. The data are
folded by using the 28.22-day photometric period and are shown in Fig. 3 (click here). The
V-band light curve is single-peaked and the maximum luminosity is close
to that observed during the Jan.-Mar. 1989 period (Cutispoto 1992). The behavior
of the color curves is rather peculiar. With respect to the V-band
modulation the U-B and the B-V color curves are clearly
anticorrelated, while the V-R and V-I color curves are well
correlated. This behavior is qualitatively the same exibited by TW Lep during
Jan.-Mar. 1989 (Cutispoto 1992), but is remarkably different from that observed
in Dec. 1989 (Cutispoto 1995) when the U-B and B-V color curves
appeared rather flat. Strassmeier & Fekel (1990) report the spectral
classification K2 III + F6 IV. The observed colors are reproduced very well by
those of a K2/3 III + F6 IV system with an active K-type giant component.
HD 39917 = SZ Pic is an SB2 spectroscopic binary, with an
orbital period of about 4.96 days (Kürster 1997), showing strong Ca II
H&K emission from both components (Andersen et al. 1980;
Henry et al. 1996),
very low Li abundance (Pallavicini et al. 1992;
Randich et al. 1993) and
photometric variability (Andersen et al. 1980;
Bell et al. 1983;
Cutispoto 1995). Moreover, it has been detected by the ROSAT satellite and by
the VLA (Fox et al. 1994). It was observed over the 15-31 Mar. interval and
these data, folded by using the 4.905-day period inferred by Cutispoto (1995),
are presented in Fig. 4 (click here). The V-band light curve, though incomplete,
appears quite different from the one obtained in Dec. 1989 and the color
variations are marginal. Assuming both components to be active evolved stars,
see Cutispoto (1995) and references therein, the spectral classification
K0/1 IV + G5 IV gives the best fit of the observed colors.
HD 61245 = V 344 Pup is an SB1 spectroscopic binary that
shows weak Ca II H&K emission lines and H
absorption (Bopp &
Hearnshaw 1983). The optical variability was discovered by
Lloyd-Evans &
Koen (1987), further photometric observations were acquired by
Cutispoto
(1992, 1995). The data presented in this paper, obtained over the 15-27 Mar.
interval, are shown in Fig. 5 (click here). The 11.761-day orbital period inferred
by Balona (1987) has been used for phase computation. The V-band light
curve is double-peaked while the weak color variations appear to be in phase
with the V-band modulation. A remarkable evolution of the HD 61245 light
curve is clear when comparing the present data with previuos observations
(Cutispoto 1992, 1995). HD 61245 was classified as K1 III and as K2 III by
Bopp
& Hearnshaw (1983) and Houk (1978), respectively. In order to fit the peculiar
U-B color Cutispoto (1992, 1995) had to assume the presence of an F8 IV
companion. From the results of Amado & Byrne (1997) it is now clear that
the peculiar value of the U-B can be entirely ascribed to the presence of
photospheric activity. The observed colors are indeed very well matched by
those of a single active K1 III star.
HD 81410 = IL Hya is an SB1 spectroscopic binary that has been
observed by several authors at different wavelenghts (see
Strassmeier et al. 1993a; Cutispoto 1993, 1995
and references therein). More recently, it has
been detected by the ROSAT and EUVE satellites
(Pye et al. 1995; Bowyer et al. 1996;
Mitrou et al. 1997). The data presented in this paper were obtained over
the 15-28 Mar. interval and are shown in Fig. 6 (click here), where phases are reckoned from
the 12.86833-day orbital period computed by Raveendran et al. (1982). The
V-band light curve is double-peaked and very clear color variations,
showing the star redder at both light minima, are present. Comparing the present
data with those obtained about 15 months earlier (Cutispoto 1995) it is clear
that the photospheric spotted regions have undergone a remarkable evolution.
The collection of the available photometry of IL Hya, shown in Fig. 7 (click here),
is indicative of the presence of long-term variability of the global degree
of spottedness. The data presented in this paper show HD 81410 to be close to
the maximum luminosity observed in late 1989. Quite different spectral
classifications of IL Hya are reported in the literature. The radius has been
computed in the 
range (see discussion in
Cutispoto
1993, 1995). In order to fit the peculiar U-B color,
Cutispoto (1995)
assumes the presence of a G8 V companion. From the computation of Amado &
Byrne (1997) it is now clear that the peculiar U-B can be entirely
ascribed to the presence of photosperic activity. The observed colors are
indeed very well matched with those of a single and active K2 IV or K0/1 III
star. The subgiant classification, that agrees better with the results obtained
by Houk & Smith-Moore (1988) and Eggen (1973), implies a value for the
inclination angle close to 90 degrees, that is consistent with the very large
amplitudes of the light curve observed for this star in the past (cf. Fig. 7 (click here)).
HD 82558 = LQ Hya is a very active rapidly rotating single
star. It has been recently observed at different wavelengths by several
authors and classified as a very young or even as a PMS star (see
Vilhu et al. 1991;
Strassmeier et al. 1990; Fekel et al. 1986a
and references
therein). A very good photometric coverage has been obtained from late 1983
(see Jetsu 1993; Strassmeier et al. 1993b;
Cutispoto 1996; Alekseev & Gershberg
1996 and references therein). HD 82558 has been detected by the ROSAT
(Pye et al. 1995) and EUVE (Bowyer et al. 1996) satellites. The data obtained over the
15-28 Mar. interval are shown in Fig. 8 (click here), where phases have been reckoned from
the 1.5978-day photometric period computed by
Strassmeier & Hall (1988). The
light curve is single-peaked and the color variation appears in phase with the
V-band modulation, showing the star redder at light mimimum. The colors
are consistent with those of an active K2 V star.
HD 86005 was reported as a possible very active star by
Bopp &
Hearnshaw (1983) because of H and Ca II H&K emission lines and by
Verma et al. (1983) because of a strong infrared excess.
Scott et al. (1992)
gave evidence for optical variability with a period of about 89 days.
The present observations were obtained over the 15-25 Mar. interval, so that
they cover only a small part of the light curve. The mean magnitude in the
V-band results 
, with the maximum and minimum luminosity
differing in 0.014 magnitudes. Houk (1978) classified HD 86005 as K2 III and
noticed the possibility that the star is a close double.
Pallavicini et al.
(1992) and Randich et al. (1993) reported a very low Li abundance and a
in the 
range. This leads to compute a minimum
radius in the 
range, a value that appears too high even for a
luminosity class III star. Actually, the power spectrum presented by
Scott et al. (1992) shows a significant second armonic, that corresponds to a period of
44.57 days, i.e. to a stellar radius in the 
range. Moreover,
the lines profiles in the Li I 6708 Å region are consistent with the star
being a close binary (Randich 1997). In this case the high value of the
could be due to a partial merging of the lines of the two
components. There are several spectral combinations that can reproduce the
observed colors of HD 86005, but only the systems K4 III + K1/2 III and K4 III
+ G3 IV/III could give origin to a composite spectrum.
HD 98712 = SZ Crt is the visual binary ADS 8138. The primary
component shows a variable H that has been observed sporadically
in emission (Torres et al. 1985;
Bopp 1987). The secondary component shows
strong H emission (Torres et al. 1985). The presence of optical
variability, with a photometric period of 11.58 days, was first reported by
Torres et al. (1985), further observations have been obtained by
Cutispoto
(1993, 1996). The observations presented in this paper were obtained over the
16-27 Mar. interval and are shown in Fig. 9 (click here), where phases have been reckoned
from the 11.58-day period computed by Torres et al. (1985). Due to the
angular distance of about 5.1 arcsec both components were always observed
simultaneously at the 0.5 m ESO. The light curve is single-peaked and shows
the largest amplitude and the brightest maximum luminosity observed to date
for this star. The V-I index shows clear correlation with the
V-band modulation. Separate magnitudes for the two components (see Table 4 (click here))
were obtained with the 1.0 m ESO telescope on the night of 10 April 1991.
HD 101309 = V 829 Cen is a SB2 system with an orbital period
of 11.71 days (Balona 1987) that shows Ca II H&K and H emission lines
(Collier et al. 1982) and has been detected during a microwave survey of
active stars (Slee et al. 1987b). A rather high Li abundance in both components
has been reported by Randich et al. (1993). The photometric variability,
discovered by Lloyd Evans & Koen (1987), has been later investigated by
Collier Cameron (1987) and by Cutispoto (1993, 1996).
The present observations
were collected over the 15-28 Mar. interval and are presented in Fig. 10 (click here), where
the 11.65-day photometric period inferred by Cutispoto (1993) was used for
phase computation. The light curve is single-peaked and clear V-I color
variation in phase with V-band modulation are present; the
U-B data are more scattered but they seem to be anticorrelated with the
V-band data. A collection of the available photometric data of HD 101309,
shown in Fig. 11 (click here), gives evidence for sizeable changes of the global
degree of spottedness. In particular, during the time interval
covered by the present observations HD 101309 was seen at the brightest
luminosity ever observed. The colous are consistent with those of a
K1/2 IV + G5 V active system, in very good agreement with the spectral
classification inferred by Collier et al. (1982).
HD 102077 = V 838 Cen is a close visual binary
(sep. 
arcsec) consisting of two almost identical components
(Turon et al. 1992) that
show moderate Ca II H&K emission lines and a filled-in H line
(Bopp et al. 1986). Its photometric variability was first reported by
Udalski
& Geyer (1985a). Further photometry has been collected by
Bopp et al. (1986),
Cutispoto (1990, 1993, 1996) and by Anders et al. (1991).
The observations
presented in Fig. 12 (click here) were obtained over the 15-29 Mar. interval. Phases have been
computed by using the 
day photometric period inferred from
a Fourier analysis of VRI data. The light curve is double-peaked
and the weak color variations show the system to be redder at light minimum.
Houk (1978) lists a K0/1Vp spectral type for the composite spectrum. The
observed colors fit those of a K1/2 V + K2 V system with active components.
For such a system the luminosity difference between the two components
(
) results in agreement with the observations
(Turon et al. 1992).
HD 106225 = HU Vir is a SB1 spectroscopic binary with an
orbital period of 10.38758 days (Strassmeier 1994) showing very
strong Ca II H&K emission lines (Montes et al. 1996) and a variable H
line (Strassmeier 1994). It has been detected in a radio survey of RS Cvn-type
systems (Morris & Mutel 1988; Drake et al. 1989)
and a huge X-ray flare,
lasting about two days, was observed by the ROSAT satellite
(Endl et al. 1995).
For a detailed study of the physical parameters of HD 106225 see
Strassmeier
(1994). The optical variability has been studied by several authors in recent
years (see Strassmeier et al. 1997, 1993a;
Cutispoto 1996; Hall & Henry 1992 and
references therein). The observations presented here were obtained over the
15-30 Mar. interval and are shown in Fig. 13 (click here), where phases are computed by using
the 10.353-day photometric period inferred by Strassmeier et al. (1997).
The light curve is double-peaked and large amplitude color variations, showing
the star redder at both light minima, are present. The colors agree well with
those of an active K1 IV or K1 IV/III star, in good agreement with the results
of Strassmaier (1994). Photometric periods ranging from 10.102 to 10.6 days
(Strassmeier et al. 1997; Fekel et al. 1986b) and values in the
range (Randich et al. 1994;
Fekel et al. 1986b) have been
observed for HD 106225. Moreover, for the inclination angle "i",
Strassmeier (1994) deduced values in the 52-78 degrees range. These results
lead to a stellar radius in the 
range, in better agreement
with a luminosity class IV/III.
HD 119285 = V 851 Cen is an SB1 system with an orbital period of
11.9886 days (Saar et al. 1990) that has been studied by several authors in
recent years (see Saar et al. 1990; Cutispoto 1993,
1996 and references
therein). It has been also detected by ROSAT satellite (Pye et al. 1995) and
low Li abundance has been reported by Randich et al. (1993). The optical
variability was first discovered by Udalki & Geyer (1984) and by
Lloyd Evans
& Koen (1987). The data presented in Fig. 14 (click here) were collected over the
15-27 Mar.
interval. The 12.05-day photometric period reported by Lloyd
Evans & Koen
(1987) was used for phase computation. The light curve is single-peaked and the
low-amplitude color variations are in phase with the V-band modulation.
From the collection of the available photometry of HD 119285 shown in Fig. 15 (click here),
it seems that the V-band mean stellar luminosity is decreasing after the
maximum observed in early 1990 (Cutispoto 1996). A luminosity class IV or IV/V
was deduced by Cutispoto (1996) for the primary component of HD 119285. The
colors are consistent with those of an active K2/3 IV star, in good agreement
with the K2 IV classification reported by Saar et al. (1990).
HD 127535 = V 841 Cen is an SB1 system, with an orbital period
of 5.998 days (Collier 1982b), showing strong Ca II H&K and H
emission lines (Houk & Cowley 1975;
Weiler & Stencel 1979; Collier et al.
1982). It was detected at radio (Innis et al. 1985;
Slee & Stewart 1989) and
EUV (Pye et al. 1995) wavelengths. Low Li abundance was
reported by Randich et al. (1993). The photometric variability was discovered
by Collier (1982a) and Udalski & Geyer (1984). The observations obtained over
the 15-31 Mar. interval are shown in Fig. 16 (click here), where the 5.929-day photometric
period computed by Cutispoto (1990) was used for phase computation. The light
curve is quite peculiar, as it shows a well pronounced minimum at about phase
0.2 while it is almost flat in the 0.35-1.0 interval. The low amplitude
color variations show the star redder at light mimimum. Moreover, it is clear
the presence of a flare event lasting at least one day. HD 127535 was flaring
the first and second night of observations, as can be easily inferred from the
color curves. There was no evidence of flare activity during any of the other
15 nights, that span over almost three complete rotational cicles. The length
of the observed flare cannot be estimated from the present data. The
collection of the available photometry of HD 127535, shown in Fig. 17 (click here), confirms
the occurrence of a minimum luminosity during the 1985-87 interval, while
the most recent data (Strassmeier et al. 1994a) indicate a much lower degree of
spottedness. Cutispoto (1996) inferred a luminosity class IV for the primary
component. The colors are consistent with those of an active K2/3 IV star, in
reasonable agreement with the K1 IV spectral classification given by
Collier et al. (1982).
HD 136905 = GX Lib is an SB1 binary with an orbital period of
11.13448 days (Kaye et al. 1995). It shows moderate Ca II H&K emission
(Bidelman & MacConnell 1973; Strassmeier et al. 1994b), strong H
absorption (Eker et al. 1995) and very low Li abundance (Randich et al. 1993).
It was detected by the Einstein (Fleming et al. 1989; Gioia et al. 1990)
and by the ROSAT (Dempsey et al. 1993) satellites. Its photometric variability
is thought to be mainly due to the ellipticity effect, where distortion due to
starspots is superimposed (Burke et al. 1982; Cutispoto 1993, 1996;
Kaye et al.
1995 and references therein). In this paper the observations obtained over the
15-29 Mar. interval are presented. The data are shown in Fig. 18 (click here), where the
11.13448-day spectroscopic period deduced by Kaye et al. (1995) was used for
phase computation. The light curve is double-peaked and clear color variations
showing the star redder at minimum luminosity are present. Comparing these
data with previous observations the evolution of the light curve appears
evident, thus confirming the presence of evolving spotted regions. Moreover,
from the collection of the available photometry of HD 136905, shown in Fig. 19 (click here),
a weak variation of the mean stellar brightness can be inferred. In particular,
the present observations show the faintest stellar luminosity ever observed
for this star. From the v sin i values computed by Randich et al. (1993)
and by Favata et al. (1995) the minimum stellar radius results in the
range. The spectral classification of HD 136905 is given as
K1 III + [G-K V] by Strassmeier et al. (1993a) and as K1 III + F by
Bidelman
& MacConnell (1973). The two best fits of the observed colors are obtained by
assuming K1 III + G5 IV or K1/2 III + G0 IV systems with active
components.
HD 139084 = V 343 Nor is a very active single (Balona 1987)
star belonging to the Pleiades group (Anders et al. 1991). It shows Ca II
H&K emission (Bidelman & MacConnell 1973) and a filled H line
(Bopp
& Hearnshaw 1983). A very high Li abundance was observed by
Anders et al.
(1991) and Randich et al. (1993). HD 139084 was detected during microwave
(Slee et al. 1987b) and EUV (Pye et al. 1995;
Bowyer et al. 1996) surveys. The
optical variability was discovered by Udalski & Geyer (1985b). The
observations carried out over the 15-28 Mar. interval are presented in Fig. 20 (click here),
where phases have been reckoned from the 
day photometric period
obtained from a Fourier analysis of the VRI data. The light curve is
single-peaked and the very strong color variations show the star redder at
light minimum. An active K0 IV/V star fits well the observed colors, in
agreement with the minimum stellar radius that results in the 
range (Cutispoto 1996). This could imply, taking also into
account the very high Li abundance, that HD 139084 has not arrived to the main
sequence yet.
HD 155555 = V 824 Ara is a rapidly rotating SB2 binary that
has been classified as a pre-main sequence star by Pasquini et al. (1991)
and by Martin & Brandner (1995). Detection at EUV wavelengths has been
obtained by the ROSAT (Pye et al. 1995) and EUVE (Bowyer et al. 1996)
satellites. Photometric observations have been carried out from late 1979 by
several authors (see Cutispoto & Leto 1997; Cutispoto 1996 and references
therein). The observations presented in Fig. 21 (click here) were acquired over the 15-31
Mar. interval. Any light contribution from the M-type optical companion
LDS587 B, that lies 32.6 arcsec apart, was avoided. Phases have been reckoned
from the 1.694 
0.010 day photometric period computed by a Fourier analysis
of the VRI data. The light curve is rather complicated and has a
different shape and a smaller amplitude compared to the light curve
obtained about six months before (Cutispoto & Leto 1997). The low-amplitude
color variations are in phase with the V-band modulation and show the
system to be redder at minimum luminosity.
HD 174429 = PZ Tel is a single (Balona 1987; Innis et al. 1988)
rapidly rotating active star that has been classified as a very young or
as a pre-main sequence object (Innis et al. 1986). It shows a filled in
H (Innis et al. 1988) and emission Ca II H&K lines (Bidelman &
MacConnell 1973; Houk 1978; Innis et al. 1988). HD 174429 has
been detected during a microwave survey of southern active stars (Slee et al.
1987b) and shows a very high Li abundance (Randich et al. 1993). The optical
variability was discovered by Coates et al. (1980), further photometric data
can be found in Bopp et al. (1986), Innis et al. (1990) and
Cutispoto & Leto
(1997). The observations presented in Fig. 22 (click here) were obtained over the 15-31 Mar.
interval, phases have been computed by using the 
day
photometric period inferred from a Fourier analysis of the VRI data.
The light curve is double-peaked, weak
color variations, showing the star redder at the primary minimum, are
present. The maximum luminosity is the same obtained about six months before
(Cutispoto & Leto 1997), but the light curve has a larger amplitude and a
quite different shape. Houk (1978) classified PZ Tel as a K0 Vp and the
observed colors are indeed in good agreement with those of an active G9/K0
dwarf or G9 IV/V stars. Randich et al. (1993) computed a of the
order of 
, which leads to infer a minimum stellar radius of
about 
. This radius is too high for a luminosity class V star,
confirming that
HD 174429 is probably still approaching the main sequence.
HD 197481 = AU Mic is a flare star that is also a member of the
BY Dra-type variables. It shows the typical spectral features indicative of the
presence of an active atmosphere (Linsky et al. 1982;
Quin et al. 1993;
Houdebine & Doyle 1994; Monsignori-Fossi et al. 1996 and references therein).
X-ray and EUV emission has been revealed by the Einstein (Golub 1983),
ROSAT (Pye et al. 1995) and EUVE (Bowyer et al. 1996) satellites. The optical
variability was discovered by Torres et al. (1972). The observations presented
here, obtained over the 20-31 Mar. interval, are shown in Fig. 23 (click here), where phases
are reckoned from the 4.865-day photometric period computed by Torres et al.
(1972). The light curve is double-peaked and the V-R and V-I
colors show the star to be redder at light minima. The U-B and B-V
are not well correlated with the V-band modulation, a circumstance that
could be due to the presence of micro-flaring activity. From the collection of
the available photometry of HD 197481 shown in Fig. 24 (click here), in which remarkable
variations of the mean V magnitude and light curve amplitude can be seen,
it is clear that sizeable modifications of the spotted areas and spot
distribution do occur. The colors are consistent with the M1 Ve spectral
classification reported by Houk (1982).
Acknowledgements
Stellar activity research at Catania Astrophysical Observatory is supported by the Italian "Ministry for Universities and Research" (MURST) and the "National Research Council" (Gruppo Nazionale di Astronomia) that are gratefully acknowledged. The author thank Ms. L. Santagati for revising the manuscript. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. A special thank is also due to the ESO staff for the collaboration and technical support during the observations.