A&A 372, 145-151 (2001)
DOI: 10.1051/0004-6361:20010444
A. Henden1 - U. Munari2
1 - Universities Space Research Association/U. S. Naval Observatory
Flagstaff Station, PO Box 1149, Flagstaff AZ 86002-1149, USA
2 -
Osservatorio Astronomico di Padova, Sede di Asiago, 36012 Asiago (VI), Italy
Received 15 February 2001 / Accepted 9 March 2001
Abstract
We present accurate
photometric sequences for an additional
20 symbiotic stars. As for the 20 targets of Paper I, the sequences extend
over wide brightness and color ranges, and are suited to cover both
quiescence and outburst phases. The sequences are intended to assist both
present time photometry as well as measurement of photographic plates from
historical archives.
Key words: catalogs - binaries: symbiotic
In Paper I (Henden & Munari 2000) we discussed the need for
extended, accurate and homogeneous photometric sequences around symbiotic stars
and how their lack has contributed to our currently poor photometric knowledge
of these interacting binaries. We also reviewed the basic types of
variability for symbiotic stars and their causes, and provided
sequences for a first sample of 20 objects.
This Paper II presents accurate photometric comparison sequences for an
additional 20 symbiotic stars using observing strategies, reduction
methodologies and presentation layouts strictly similar to those of Paper I
(to which the reader is referred for all details). The sequences are
intended to allow a general observer to capture on a single CCD frame or to
have in the same eyepiece field of view when inspecting archival
photographic plates: (a) enough stars to cover the whole range of known or
expected variability for the given symbiotic star, (b) stars of enough
different colors to be able to calibrate the instrumental color equations
and therefore reduce to the standard
system the collected
data, and (c) stars well separated from surrounding ones to avoid blending
at all but the shortest telescope focal lengths. As for Paper I, all
observations have been made with the 1.0-m Ritchey-Chrétien telescope of
the U. S. Naval Observatory, Flagstaff Station with a Tektronix/SITe
1024x1024 thinned, backside-illuminated CCD and Johnson UBV and
Kron-Cousins RI filters.
The sequences around the 20 symbiotic stars of Paper I have already been
used by Munari et al. (2001a) and Jurdana & Munari (2001) to investigate
their long term photometric behavior on the rich collection of Asiago
historical photographic plates. With other observatories with historical
plate archives joining the effort, the community could soon
assemble complete, century-long lightcurves for a significant fraction of
the symbiotic stars. Given their complex, non-periodic and usually slow
photometric evolution with major outbursts appearing erratically and lasting
for years or decades, the understanding of the symbiotic stars and their
role in the general astrophysical context cannot progress much without a
detailed knowledge of their photometric histories and habits.
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The program symbiotic stars are listed in Table 1. An average of 11 stars
around each program star have been selected to form the comparison
sequences, which are given in Table 2. The stars included in the sequences
have been chosen and ordered on the basis of the B band magnitude, because
the latter is the band closest to the
of the historical
photographic observations and is the better suited to investigate the
variability of symbiotic stars (cf. Kenyon 1986 and Paper I). The stars
included in the comparison sequences have been checked on at least three
different nights for variability (cf. Col. N of Table 2).
For 10 objects (V471 Per, GH Gem, UKS Ce1, UU Ser, DT Ser, DQ Ser,
V4368 Sgr, NSV 11776, StH 164 and LL Cas) the symbiotic star and the
comparison sequence both lie inside a 6.0
6.0 arcmin field (which
match in dimension the Allen 1984 finding charts) and are plotted in
Fig. 1. For 8 brighter objects (V694 Mon, TX CVn, IV Vir, V503 Her,
ER Del, StH
149, StH
180 and StH
190) the comparison
sequences are distributed over a
arcmin area and are
presented in Fig. 2. The remaining two program stars (BX Mon and YY Her)
are hybrid cases having their fainter comparison stars identified in
Fig. 1 and the brighter, more distant ones in Fig. 2.
Finally, for three objects (TX CVn, ER Del and StH 190) there are no
stars in the explored 11.4
11.4 arcmin fields brighter than the
symbiotic star itself (particularly important for estimates on photographic
plates). For these program stars we have selected from the Hipparcos/Tycho Catalogue
suitable, nearby and non-variable stars to complement
our comparison sequences. The selected Hipparcos/Tycho stars are listed in Table 3,
where their
magnitudes have been transformed to the standard
Johnson's UBV system following the transformation equations provided
in the explanatory notes of the Hipparcos/Tycho Catalogue:
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= | ![]() |
(1) |
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= | ![]() |
(2) |
A few individual notes follow on the basic photometric behavior of the program symbiotic stars. They are intended as simple guidelines for observers planning an observational campaign and cannot by any means be considered as an exhaustive review. A collected history of many symbiotic stars and a compilation of photometric information available in literature has been presented by Kenyon (1983, 1986).
While calibrating the photometric comparison sequences for this paper we
have also collected data on the program symbiotic stars. These
data will be presented and discussed elsewhere together with similar
data for more than 130 symbiotic stars observed from ESO and Asiago. From
such a
survey of symbiotic stars (hereafter indicated as
MHZ: Munari, Henden and Zwitter, in preparation), we report in the following
average V and B-V values for the year 2000 for the reader's benefit.
Another large multi-epoch
and JHKL photometric survey
has been presented by Munari et al. (1992).
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Figure 1:
Finding charts for the UBV(RI)![]() ![]() ![]() |
Open with DEXTER |
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Figure 2:
Several of program symbiotic stars reach maximum magnitudes
which impose looking for suitably bright comparison stars over a wider
field. The symbols are the same as in Fig. 1, with
an imaged field of view of about
![]() ![]() ![]() |
Open with DEXTER |
V471 Per. Discovery and numbering of variable stars in the Perseus constellation has not yet progressed far enough to reach entry V741 Per with which this variable has been erroneously confused so frequently in literature, probably because of a mis-print in the Allen (1984) catalogue of symbiotic stars.
There are not much informations available on the photometric behavior of the system, which is usually observed around V=13.0/13.4 and B-V=+0.90/+1.00. A moderate activity phase was observed in the 1990s, when on 1992 the object started a slow rise reaching V=12.4 in 1994-95 and declined to V=13.3/13.4 by 1996. The orbital period is unknown. MHZ lists V=13.10 and B-V=+1.00.
GH Gem. Proposed by Kenyon (1986) as a possible symbiotic
star, it has been observed by amateurs since then. VSNET and VSOLJ databases
show the star stable at
from 1986 to 1990, when the star entered
a gradual brightness decrease to
(reached by late 1993) and
recovering to
by 1995. Descending slowly to
by 1999,
in early 2000 it rapidly dropped to
.
Not much else is known
about the photometric behavior of this star. MHZ reports V=14.01 and
B-V=+1.00. A spectrum for GH Gem from the 3200-9100 spectral atlas of
symbiotic stars of Munari & Zwitter () show the continuum of a K-type
giant with a weak H
in emission.
BX Mon. Its orbital period is around 1400 days (Mayall 1940;
Dumm et al. 1998). The lightcurve is varied, with relatively quite phases
()
interspersed with periods of rapid and large variability
(
). A fast rise, large amplitude event occurred in
1999, when BX Mon rose to V=9.9 in a pattern resembling an outburst. The
photometric behavior of this bright object remain poorly known and
understood. It could be an eclipsing object according to Kenyon (1986).
MHZ reports B=12.36 and B-V=+1.13.
V694 Mon. The object attracted much attention when hugely
blushifted absorption components (up to -6000 km s-1, Tomov et al.
1990) were discovered to flank the emission line spectrum during the 1990
bright phase, when the star rose to mag. Doroshenko et al.
(1993) have discovered a
mag sinusoidal
variation that they attribute to manifestations of a reflection effect
following the ephemeris:
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(3) |
TX CVn. This bright, isolated object has a relatively flat
quiescence at .
Moderate amplitude outbursts have been reported for
1920 (7 years to return to quiescence), 1945 (4 years duration), 1952, 1962
and 1986 (4 years rising, 5 years descent). At maximum the object may reach
mag. MHZ reports V=10.10 and B-V=+0.69 (it is worth noticing that
such a moderate B-V color suggests that the zero point of the
magnitude scale adopted by Mumford (1956) could be too faint by 0.7 mag
compared to modern B values). Apart from the frequent outbursts, not much
is known about the other photometric properties of TX CVn. Kenyon & Garcia
(1989) have spectroscopically determined an orbital period of 199 days, the
shortest known among symbiotic stars.
IV Vir. The orbitally modulated lightcurve (P=282 days) of this bright
symbiotic star ()
is quite interesting: at longer
wavelengths it is dominated by the ellipsoidal distortion of the cool giant
filling its Roche lobe (
mag, with the
classical pattern of two-maxima/two-minima per period; Niehues et al. 1992),
while at shorter wavelengths the reflection effect takes over
(
mag, with a sinusoidal pattern; Smith et al. 1997).
The ephemeris for maxima in u is
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(4) |
UKS Ce1. The photometric properties and history of this carbon
symbiotic stars are unknown. MHZ reports V=15.89 and B-V=+1.88.
A field star of similar brightness lies 5.9 arcsec from UKS Ce1 toward the SSW,
at
(
0.055 arcsec) and
(
0.177 arcsec), with magnitudes V=16.732 (
0.023),
B-V=+0.840 (
0.033),
U-B=+0.286 (
0.045),
V-R=+0.606 (
0.030)
and
R-I=+0.482 (
0.042).
V503 Her. In the GCVS it is reported as a long period
variable varying between
.
Kenyon (1986) suggested its
possible symbiotic nature but Bond (1978) and Munari & Zwitter (2001)
observed an early M spectrum without emission lines, so its nature remains
unclear. The VSOLJ data for 1988-1990 shows a relatively constant brightness
at V=12.3, with a slightly brighter phase at V=11.7 in 1987 and a few
doubtful reports about very faint states. The more recent VSNET lightcurve
reports a
in 1997 and 1999,
in 1998 and
in 2000. MHZ lists V=12.67 and B-V=+1.28.
UU Her. After the initial discovery by Reinmuth (1926)
reporting the object to vary between 16.0
,
not much
else has appeared in literature. The photometric properties of this
symbiotic star remain essentially unknown. MHZ gives V=15.47 and
B-V=+1.79.
DT Ser. This is an interesting case. The GCVS lists the
variability range as
.
Cieslinski et al. (1997)
have shown that the symbiotic star is indeed a faint star (B= 15.91,
V=15.40) about 5 arcsec from a much brighter field star (that we have
measured at
,
,
,
and
).
The B=13.55 of the nearby field star (not variable between the three
observations we have obtained in separate nights) well matches the mean
brightness of DT Ser listed in the GCVS. Thus, we believe that GCVS data
refer to the combined light of (a) the much brighter, probably not
variable field star, and (b) the faint, nearby symbiotic star that
cannot be easily resolved under normal seeing conditions and with small
telescopes. If the nearby field star is not itself variable, than DT Ser
must vary by a large amount to account for the observed
mag, as if undergoing outbursts, and indeed the slow, long term
variability exhibited by the pair combined light in the VSOLJ data
(up-and-down time of
8 years) could support this interpretation. MHZ
lists V=16.23 and B-V=+0.37.
YY Her. The complete photometric history of YY Her over
1890-1996 has been reconstructed by Munari et al. (1997). The quiescent
lightcurve (at V=13.2) is modulated by a
sinusoidal
reflection effect, with minima given by the ephemeris
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(5) |
StH 149. In spite of its brightness (MHZ lists
V=12.06 and B-V=+1.55), it is photometrically unknown.
DQ Ser. This moderately bright symbiotic star has been
so far largely ignored by observers and its photometric properties
are unknown. The GCVS gives a variability range
.
Cieslinski et al. (1997) list V=14.5 and B-V=+1.23, while MHZ reports
V=14.94 and B-V=+1.34.
V4368 Sgr. This probable symbiotic nova was discovered
at V=10.7 in 1994 by M. Wakuda. The progenitor is not visible in ESO/SERC
photographic atlas (limiting magnitude 21.5), and searches in the Harvard
and Sonnerberg plate archives have failed to reveal anything at this position
since the earliest
images obtained in 1888 (Grebel et al. 1994; Hazen 1994). Since then the
object has remained close to maximum brightness, in a photometric and
spectroscopic pattern reminiscent of PU Vul, another symbiotic nova (cf.
Grebel et al. 1994; Bragaglia et al. 1995). According to the VSOLJ database
V4368 Sgr has slowly risen to V=10.2 by 1997 and descent to V=10.5 by
1999. MHZ reports V=10.57 and B-V=+0.63.
NSV 11776. Scanty photometric information exists for this
relatively bright symbiotic star. Cieslinski et al. (1994) reported about
photometry secured in 1991-1992 that did not
revealed variability or the presence of flickering activity. MHZ lists V=13.47and B-V=+0.92, which are identical to the mean values V=13.46B-V=+0.92 of Cieslinski et al. for 1991-92, which suggests limited
variability for NSV 11776. Such a tight photometric stability is surprising
in view of the very intense, high ionization emission lines indicating the
presence of a very hot and luminous accreting white dwarf in the system.
StH 164. The photometric history, type of variability,
presence of outbursts and orbital period are unknown. MHZ reports B=14.48and B-V=+2.03.
StH 180. Another example of a bright symbiotic star
completely unknown in its photometric properties. MHZ lists V=12.68 and
B-V=+1.40.
ER Del. ER Del is one of the rare symbiotics whose cool giant
is an S star. It attracted attention when IUE spectra showed a high
ionization emission line spectrum, revealing the symbiotic nature (as much
as it happens with EG And). Little is known about its photometric
properties. The poorly sampled VSNET lightcurve over the last six years may
be described as a slow, linear descent to
to
,
with
fluctuations overimposed. MHZ reports V=10.33 and B-V=+1.78.
StH 190. The Hipparcos/Tycho did not detect
variability for this object which is however close to the sensitivity limit,
so a limited variability (some tenths of a magnitude) can surely be
accommodated by the noise in the data. The Tycho
and
when transformed to the standard UBV system (V=10.23, B-V=+0.68) are
close to the values reported by MHZ, namely V=10.50 and B-V=+0.84.
Stephenson (1986) discovered StH
190 at
,
Robertson &
Jordan (1989) re-discovered it (as RJH
120) at
and
Downes & Keyes (1988) measured it at V=10.5. Thus over the last 15 years
the system mean brightness has remained fairly constant, and similarly went
for the optical spectra very similar one to the other (Downes & Keyes 1988;
Whitelock et al. 1995; Munari & Zwitter 2001), with no report of
outbursts. Variability has been instead firmly established in the infrared
by Whitelock et al. (1995) who reported
.
Munari et al. (2001b) have recently discovered a fast
evolving and complex mixture of spectral signatures produced by high
variable bipolar jets and P-Cyg profiles, with indication of a high orbital
inclination and a possible 171 day orbital period. Therefore, searches for flickering
and eclipses in the bluest photometric bands could pay dividends over a single
observing season.
LL Cas. Originally listed among the planetary nebulae
it has been later reclassified as an Me star by Sabbadin et al. (1987)
and finally as a symbiotic star by Kondrateva (1992) that reports about
a
mag variability with a period P=286.6 days.