5 Notes on individual symbiotic stars

A few individual notes follow on the photometric behavior of the program symbiotic stars, to the aim of assisting the interested reader in planning an observing strategy. An inspiring reading would also be the collected history of symbiotic stars assembled by Kenyon (1986).

While calibrating the photometric comparison sequences for this paper we have also collected data on the program symbiotic stars. These $UBV(RI)_{\rm C}$ data will be presented and discussed elsewhere together with similar data for more than another 100 symbiotic stars observed from ESO and Asiago. To the reader's benefit we report in this section mean B and B-Vvalues for 1999 from the $UBV(RI)_{\rm C}$ survey (hereafter indicated as MHZ: Munari, Henden and Zwitter, in preparation).

  

Table 2: The comparison sequences around the 20 program symbiotic stars. Positions for J2000 equinox and a mean epoch 1999.5 are given (errors in arcsec are derived from different exposures in different bands), together with magnitudes and colors (errors in magnitudes). The stars in each sequence are ordered according to fainter B magnitudes. N is the number of observing nights. The sequences are given in the same order as in Table 1 and Figs. 1 and 2. The comparison stars laying outside the field of view of Figs. 1 and 2 and plotted in Fig. 3 are given at the bottom of each sequence, separated by an empty line

 

Table 2: continued

 

Table 2: continued

 

Table 2: continued

Draco C-1. This carbon symbiotic star belongs to the Draco dwarf galaxy (Aaronson et al. 1982). Infrared photometry by Munari (1991a) proves C-1 to be at the tip of the Draco AGB with very blue IR colors for a carbon star, probably caused by the low metal content of the parent galaxy (Munari 1991b). No outburst has been so far recorded and the orbital period is unknown. BVI photometry by Munari (1991c) seems to support a variability of the carbon giant with a period of $\sim$ 55 days. If confirmed, this would be among the shortest period known for carbon pulsating variables (cf. Claussen et al. 1987). MHZ report B=18.6 and B-V=+1.5 mag. ALS 2. Its symbiotic nature has been discovered by Acker et al. (1988). MHZ lists B=16.2 and B-V=+1.9 mag. The orbital period, type of variability and presence of historical outburst are unknown.

FG Ser. After the 1988-1994 outburst when it rose to B=10.4 and B-V=+1.1, it is now back toward the quiescent B=13.8and B-V=+2.0 mag values. MHZ list B=13.5 and B-V=+1.7 mag. Munari et al. (1992b) discovered eclipses during the outburst phase (of amplitude $\bigtriangleup V=1.4$, $\bigtriangleup B$ = 1.9, $\bigtriangleup U$ = 2.3 mag and 120 days between first and fourth contact). From three consecutive mimina Munari et al. (1995) derived the following ephemeris

$$T(\min) = 2448492 (\pm 4) \ + \ 658 (\pm 4) \times E$$ (6)

where 658 is the period in days, as usual. From 250 archive blue plates covering the period 1949-1987, Kurockin (1993) found a large and sinusoidal variability at quiescence ( $\bigtriangleup B=1.5$ mag), following the ephemeris

$$T(\min) = 2446591 \ + \ 630 \times E$$ (7)

which could be interpreted in terms of a reflection effect. The difference between the two periods (both should trace the orbital period) has to be investigated. It should also be noted that the cool component does not show intrinsic or ellipsoidal variability in excess of 0.1 mag. The eclipses have not yet been searched for during quiescence. Their detection and monitoring would be of interest to measure the size, temperature and luminosity of the white dwarf now that it is returning back to quiescence dimensions.

V443 Her. No outburst has ever been recorded from this fairly bright symbiotic star. Its behavior in quiescence has been investigated by Kolotilov et al. (1995) who found a lightcurve dominated by a reflection effect of $\bigtriangleup U$ = 0.9, $\bigtriangleup B$ = 0.4 and $\bigtriangleup V$ = 0.1 mag amplitude. The minima follow the ephemeris

$$T(\min) = 2443660 (\pm 30) \ + \ 594 (\pm 3) \times E.$$ (8)

There seems to be another periodicity of no easy interpretation at 430 days. The mean values in quiescence are B = 12.5 and B-V=+1.0. Limited infrared observations by Kolotilov et al. (1998) seems to argue against variability of the cool giant or an ellipsoidal distortion of it.

K 3-9. Originally classified among the planetary nebulae, its symbiotic star nature has been discovered by Acker et al. (1983). According to Ivison & Seaquist (1995) K 3-9 is among the brightest symbiotic radio sources, and could harbor a Mira variable and a WD locked in a permanent outburst state. A thick dust cocoon should encircle the binary system, and a huge external ionized nebular material completely dominates the optical spectra. The photometric properties, history and orbital period are unknown. MHZ report B = 18.3 and B-V=+1.3.

MWC 960. This is a bright symbiotic star neglected by the observers. Munari et al. (1992a) report B = 13.6 and B-V=+1.5 and MHZ list B = 13.8 and B-V=+1.6. The photometric properties, history and orbital period are unknown.

AS 323. Another object originally classified as a planetary nebula which later turned out to be a symbiotic star (Sabbadin 1986; Acker et al. 1983). MHZ report B = 15.2 and B-V=+1.0. The photometric properties, history and orbital period are unknown.

FN Sgr. Another bright symbiotic star that has been overlooked by most observers even though reports of large variability date back to Ross (1926). Outbursts have been recorded in 1924-1926 and 1936-1941. The brightness in quiescence seems to vary by a large amplitude ( $\bigtriangleup m \sim$ 2 mag) with possible periodicities between 1 and 3 years (cf. Kenyon 1986 and references therein). Amateur visual observations over the last few years show a pattern reminiscent of an eclipsing binary following the ephemeris (Munari et al., in preparation):

$$T(\min) = 2451410 (\pm 15) \ + \ 1120 (\pm 20) \times E.$$ (9)

The mean brightness is $V \sim$13.5 in eclipse and $V \sim$11.0 outside. Next minimum is scheduled for mid September 2002. MHZ list B = 12.7 and B-V=+0.7.

V919 Sgr. Another bright object ignored by observers. According to literature review and new observations by Ivison et al. (1993), V919 Sgr varies between B = 12 and $B \geq$ 14.2 mag. Its cool giant is definitively variable in the infrared by at least $\bigtriangleup K$ = 0.7 mag. The announcement of an outburst was made in 1991, but it is not yet proven it actually occurred (see Ivison et al. 1993). MHZ report B = 14.2and B-V=+1.2. The past photometric history and orbital period are unknown.

CM Aql. Another relatively bright symbiotic star that has been overlooked by most observers. Its range of variability extend from B = 13.0 to B = 16.5. Outbursts have been reported for 1914, 1925, 1934, 1950. CM Aql also attracted some attention in late 1992 when from the usual V=13.2 it rose for a short period to $V\sim 12$ mag. The orbital period is unknown. MHZ report B = 14.6 and B-V=+1.3.

V1413 Aql. The star erupted into a symbiotic nova in late 1981, and has not yet returned to quiescence conditions. According to Munari (1992), V1413 Aql presented in quiescence one of the largest known reflection effects (B varying between 16.5 and 14.0 mag). When the star erupted into outburst, deep eclipses appeared perfectly in phase with the minima of the reflection effect according the ephemeris

$$T(\min) = 2446650 (\pm 15) \ + \ 434.2 (\pm 0.2) \times E.$$ (10)

At outburst maximum the star peaked at B=11.2 and B-V=+0.7(compared to B = 15.5 and B-V=+1.5 in quiescence). The decline has been very slow but smooth until late 1992 when V1413 Aql went back on the rise and returned to peak brightness (V = 10.5) by summer of 1995 and started to decline again in a very smooth way (Munari 1996). The eclipses have always been visible during the whole outburst phases since 1982, and at minimum the star shines at $V \sim$15.0. Outside eclipses the star is currently at V = 13.1 and B-V=+0.9. If the present rate of decline of $\bigtriangleup V$ = 0.56 mag yr^-1 will be maintained in the future, the star should return to quiescence brightness by late 2002. A detailed multi-band monitoring of successive eclipses would be of great interest to model the radius, temperature and luminosity of the outbursting component while it is returning back to quiescence conditions.

Ap 3-1. Another example of an object originally classified as a planetary nebula, and which later turned out to be a symbiotic star (Allen 1984). Its photometric properties are unknown. MHZ list B = 19.1 and B-V=+2.1.

ALS 1. Its symbiotic nature has been discovered by Acker et al. (1988), who report V = 14.8 mag. MHZ lists V = 13.5 and B-V=+1.4 mag. The photometric properties, history and orbital period are unknown.

V335 Vul. A symbiotic nature for this carbon star has been suggested only recently (Munari et al. 1999a). The star colors are very red, with MHZ giving V = 12.9 mag and B-V=+5.1 for quiescence. Munari et al. (1999b) caught the star on the rising branch of an apparent outburst, with V = 11.3 and B-V=+3.1 and a ten-fold increase in the intensity of emission lines. The orbital period is unknown. According to Dahlmark (1993) the carbon star is a Mira variable of 10.5 < V < 13.2 range, and maxima given by the ephemeris

$$T(\max) = 2446740 \ + \ 342 (\pm 0.2) \times E.$$ (11)

The possible outburst reported by Munari et al. (1999b) for the end of 1999 happened at the time of maximum brightness for the Mira variable (O. Pejcha and P. Sobotka, private communication). The puzzling coincidence of the two events should be studied in more details.

QW Sge. Examining 438 archive blue plates covering the period 1960-1992, Kurockin (1993) has discovered two outbursts: one extending from July 1962 to March 1972 with B = 11.5 at maximum, the other from May 1982 to September 1989 with a much more complex lightcurve and a peak brightness B = 12.0. Outside outburst phases the star is since first observations in 1898 at $B\sim 13.1\div 13.3$. MHZ lists B = 13.2 and B-V=+0.81 mag. QW Sge has an optical companion 3.5 arcsec to the north, that Munari & Buson (1991) classified as an F0 V star with B = 13.59 and B-V=+0.45. Our photometry gives different values, B = 13.18 and B-V=+0.83, with a large scatter of 0.25 mag between three different measurements (compared to the few millimag for nearby stars of similar brightness). All this suggests that the optical companion is itself a variable star, and this complicates the interpretation of photometry made with moderate or short focus telescopes that are not able to separate QW Sge from the close optical companion (as it is the case for most of the archive photographic plates). It seems relevant to observe QW Sge with enough spatial resolution to avoid contamination from the nearby companion and to characterize the type and amplitude of variability of the latter. If the companion should turn out to be a moderate-amplitude variable and/or of a predictable type (like an eclipsing system), it would be possible to remove its contribution from the photographic photometry collected on QW Sge over the last century. No orbital period has been determined for QW Sge.

LT Del. A large reflection effect ( $\bigtriangleup U$ = 1.6, $\bigtriangleup B$ = 0.5 and $\bigtriangleup V$ = 0.2 mag) following the ephemeris

$$T(\min) = 2445910 (\pm 5) \ + \ 478.5 (\pm 2) \times E$$ (12)

has been discovered by Arkhipova et al. (1995), who lists B = 14.4 and B-V=+1.3 mag as mean values for the quiescence. MHZ report B = 14.3 and B-V=+1.3. The only recorded outburst of LT Del has been discovered in the summer of 1994 by Passuello et al. (1994), when the star rose to B = 12.8 and B-V=+0.5. The star has returned to quiescence by early 1998.

Hen 2-468. The photometric properties, history and orbital period are unknown. MHZ list B = 16.6 and B-V=+1.8 mag.

V407 Cyg. Discovered as Nova Cyg 1936, it was found by Meinunger (1966) to harbor a Mira variable with one of the longest pulsation period known and maxima following the ephemeris

$$T(\max) = 2429710 \ + \ 745 \times E.$$ (13)

While reconstructing the historical lightcurve, Munari et al. (1990) discovered that brightness at the maximum of the pulsation cycle is strongly modulated by a sinusoidal variation with a possible period around 43 years and extrema at $B_{\max}=13.3$ and $B_{\max}=17.0$. The 43 year periodicity was interpreted as the orbital period of the system. V407 Cyg was discovered again in outburst in the summer of 1994 by Munari et al. (1994), when it rose to B=14.0 and B-V=+1.0 (same value as in the 1936 outburst) at a time when contemporaneous infrared photometry confirmed that the Mira was at a minimum in its pulsation cycle. According to VSNET databank the last minimum of the Mira has been in the very early 1998 (V > 16.0) and the last maximum in June 1999 ($V \sim$ 11.2). The lightcurve is however quite complicated with humps as large as one magnitude superimposed to the much smoother lightcurve of the Mira. The humps are possibly connected with the current enhanced activity phase of the white dwarf and would deserve close monitoring over the whole $UBV(RI)_{\rm C}$ range. MHZ list B = 13.2 and B-V=+1.5.

V627 Cas. Originally classified among the T Tau pre-main sequence variables, its symbiotic star nature was discovered by Kolotilov (1988). Kolotilov et al. (1996) has summarized the optical and infrared photometric properties of V627 Cas. The cool component seems to be a M2 supergiant in a post-AGB phase, pulsating with a 466 day period. The hot component presents flickering activity superimposed onto several different types of variability, including a secular dimming by $\bigtriangleup B$ = 2.0 mag in 60 years. This is a peculiar type of symbiotic star which needs more effort to be better characterized from a photometric point of view. To null the effect of flickering, the star should be observed more times per night and over a few consecutive nights in all the $UBV(RI)_{\rm C}$ bands.

StH$\alpha$ 32. Its symbiotic star nature has been discovered by Downes & Keyes (1988). The photometric properties are unknown. MHZ list B = 14.2 and B-V=+1.4.