Planetary Nebula Central Stars and Symbiotic Stars in the MACHO Galactic Bulge Database

We found only one PNCS (Hf 2-2) that exhibited periodic behavior. Hf 2-2 had actually been studied by one of us before (Lutz et al. 1998), and the preliminary study is confirmed by our more detailed analysis. Three other PNCS appear to have irregular variations. All of these PNCS and their nebulae are discussed in the next section. Their variations are displayed in Figures 1 and 2 and in Table 1. The columns in Table 1 give the PN G number (or PK number in parenthesis), a common name for the PN, the MACHO FTS number (used to find the object in the MACHO database), the distance (in '') between the catalog coordinates of the PN and the MACHO object, the median V and R magnitudes for the object, as derived from the MACHO data (see § 2), and a note about the type of variability observed.

Most of the PNCS were not found to exhibit variability based upon the available MACHO data. A list of the PNCS that we were able to identify but were not found to be variable is given in Table 2, with a note explaining why the PNCS was not confirmed to belong in the variable category.

Out of 74 PNCS that we found in the MACHO database, 4 exhibited some type of variability, 47 exhibited no variability, and 17 had insufficient data for a judgment on their variability. The remaining 6 exhibited possible variability, noted as such in Table 2. A few PNCS that we had hoped to find in the MACHO database were not there. These are listed in Table 3 for completeness.

Hf 2-2 was discovered to be a variable PNCS in a preliminary survey of the MACHO database done by Lutz et al. (1998). Their finding of a period of 0.40 days, with an amplitude of 0.11 mag in blue and 0.13 mag in red, is confirmed by this study. The MACHO red and blue filter observations and the observations phased to a 0.39874 day period are shown in Figure 1. The variability likely comes from heating effects in a binary system.

Hf 2-2 is a particularly interesting PN. According to Liu et al. (2006), the optical spectrum of the PN has many strong heavy-metal recombination lines. The optical recombination lines appear to arise from regions in the nebula that are about 10 times cooler (about 900 K) than the regions where the collisionally-excited emission lines such as those of [O III] arise. The appearance of the nebula is unremarkable. It is round with a diameter of about 20'' in both [O III] and Hα (Schwarz et al. 1992).

H 1-54's central star shows a "dropout" variability in the MACHO red magnitude at the level of about 2 magnitudes, as shown in Figure 2. The star has a MACHO red magnitude of about 15 most of the time, but occasionally it dims suddenly. Unfortunately no data are available in the blue. No periodic behavior is evident in the variations. The nebula has a diameter of 2'', as measured from a Hubble Space Telescope (HST) Hα archival image (Bensby & Lundström 2001). The nebular abundances are typical for a bulge PN (Wang & Liu 2007; Chiappini et al. 2009). Kaler & Jacoby (1991) found a Zanstra temperature of 35,000 K for the central star. The nebula is low-excitation.

M 2-29 is a particularly interesting PN in several respects. Hajduk et al. (2008) reported an apparent dimming and rebrightening of the central star based upon OGLE data and confirmed by our MACHO observations. We reproduce the MACHO light curve in Figure 1, which is labeled M 2-29a. Hajduk et al. (2008) attribute the variation in the light curve to dust formation and dissipation. In addition to the object with the dramatic dimming in brightness, we found a very nearby "object" in the MACHO database which is labeled M 2-29b in Figure 1. This "object" is about 0.5'' from the central star and thus would be within the nebula, which has a diameter of about 4'', as seen on both Hα and [O III] HST images (Hajduk et al. 2008). M 2-29b also looks somewhat variable, and this variability may be related to the dust event that is seen so dramatically in the observations of the central star.

Miszalski et al. (2009) reached quite different conclusions about the nature of M 2-29. They found this object to be variable, but viewed it as a possible SS rather than a PN. If it is a SS, it is one of a very small number of these objects that has a nebula. They did not discuss the nature of the variability, as their article was on binary PNCS.

M 2-29 is a PN with extremely low abundances of C, N, O, Ne, Ar, and S (Pena et al. 1991; Exter et al. 2004). It is a medium-excitation PN with an atypically high electron temperature of 20,000 K (Exter et al. 2004).

The "Is it a PN or a SS?" conundrum is one that arises occasionally when dealing with these two types of objects (Lutz 1977; López et al. 2004; Arrieta et al. 2005). Miszalski et al. (2009) cite a paper by Gutierrez-Moreno et al. (1995) which attempts to distinguish between SS and PN by using the relative strengths of [O III] emission lines. There is ambiguity in using this criterion. High-density PN can overlap with low-density SS with regard to their line ratios.

However, colors from the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) of ambiguous objects can be helpful for some cases. The 2MASS source (18064090–2654562) closest to the coordinates of M 2-29 has J - H = 0.4 and H - K = 1.2, which places it in a realm of the J - H versus H - K color-color diagram occupied uniquely by planetary nebulae (Phillips 2007). The present study addresses only the optical variability of the central star of M 2-29, but we suspect it should be kept in the category of planetary nebula.

M 1-44 (PN G004.9-04.9) has been classified as a PN by some groups and as a SS by others. It is listed in the catalog of symbiotic stars compiled by Belczyński et al. (2000) under its alternate name of He 2-379. There is a nebula with a size of 6.0'' by 5.4'' (Tylenda et al. 2003) offset slightly from the red star that is the subject of the MACHO observations. Lutz & Kaler (1983) found a red star of spectral type K2 III offset by about 2'' from a low-excitation nebula. The spectrum of the nebula showed emission lines and very little continuum. Lutz and Kaler's hypothesis was that there are two separate objects which just happen to be close together in the sky: a low-excitation nebula and a K-type giant star. However, the image obtained by Schwarz et al. (1992) shows that the nebulosity encompasses two stars that are quite close together.

Further information about the nature of M 1-44 might be gleaned from investigating the 2MASS colors of the object (Skrutskie et al. 2006). The 2MASS source closest to the coordinates given for the PN (18161733–27040305) has colors (J - H = 0.7, H - K = 0.2) and thus falls in an area of the color-color diagram that is occupied both by late-type giant stars and by some PN (Phillips 2007). 2MASS sources that are quite nearby (18161686–2704379 and 18161773–2704253) also have J - H and H - K colors that place them in a similar area of the color-color diagram. Hence, the near-infrared colors do not provide any information that elucidates the nature of M 1-44.

According to Exter et al. (2004), the spectrum of the nebula exhibits emission lines of atoms with relatively low ionization levels such as strong [O II] and [N II] lines and no [O III]. They find a nebular electron density of 10^2.6 cm^-3. This result is in approximate agreement with that of 10^3.2 cm^-3 found from the [S II] lines by Stanghellini & Kaler (1989). Luna & Costa (2005) quoted the electron density of the nebula as 10⁸ cm^-3 based upon the absence of [O III] emission lines. However, since the [O III] emission lines 4959 Å and 5007 Å are not present in their table of spectral line intensities and the [N II] and [S II] lines are strong, the low densities quoted by other authors are likely valid. The low electron density is evidence that M 1-44 is a PN.

The MACHO data on M 1-44 indicate that the object is brighter in the red than in the blue, as shown in Figure 2. The red magnitude was generally about 12.6 and the blue magnitude about 13.2. It showed brightness excursions of about a magnitude in the red filter, but variations in the blue magnitude were observed only during the third MACHO observing season. No periodic behavior was found.

Our conclusion based upon the observations available so far is that M 1-44 is a PN with a late-type (and thus presumably binary) central star. Radial velocity observations of the nebula and the cool star would be desirable to establish association.

Symbiotic stars are binaries that pair a cool star (usually M-type, but some are spectral types F–K) with a hot star (usually a white dwarf). Sometimes the M-type star is a Mira variable. Depending upon the sizes of the stars and the characteristics of the orbit, there is a wide variety of possible interactions between the two stars. Some symbiotic stars exhibit emission line spectra with very high ionization levels, while others have only medium-excitation spectra. A few have spectra with a wide range of ionization potential emission lines. Some symbiotic stars show the variability of a Mira star, while others are not known to vary at all. The more active symbiotic stars can have sudden irregular outbursts of several magnitudes, whole others brighten and fade slowly but with no particular cadence. Symbiotic stars are rare. There are only 218 entries in the symbiotic stars catalog published by Belczyński et al. (2000) and 13 of them fall within the MACHO Galactic bulge fields.

In examining the MACHO Galactic bulge data on known symbiotic stars, we were looking for several possible types of variation: First, the variability characteristic of a Mira star, which occurs on timescales of months to years. Second, outbursts of several magnitudes that occur at visible (and other) wavelengths over timescales of a few months (e.g., RS Oph). Third, relatively slow (months to years) brightening and fading that does not look at all like Mira variability (e.g., BF Cyg). Fourth, erratic smaller outbursts over weeks to months such as occur in AG Dra. We found some Mira-type variability in symbiotic stars where it had not been documented before. We also found a couple of stars with irregular variability on the order of a magnitude or so. No major dramatic outbursts were found. The eight stars that showed variability will be discussed individually. Basic data for them are given in Table 4 and the light curves are given in Figures 3–10. We were also looking to confirm the extent to which symbiotic stars are "quiet," i.e., do not show optical variability. The long time frame of the MACHO monitoring provides a good indication that some symbiotic stars are indeed not variable on timescales of a few years. We found one star (AS 281) that had no apparent variability, two that might have some small irregular variability, and one that had insufficient data. These four objects are listed in Table 5. As with the PNCS, there were a few SS that we did not find in the MACHO database for reasons discussed in § 2. These are listed in Table 6.

HD 319167 (CnMy 17) has only one season of MACHO observations. The star is clearly variable in the blue, but it is impossible to tell whether or not it is periodic because of the limited data stream. In the red, the magnitudes are scattered. The object looks variable in the red, but the variations appear to be irregular.

The cool star in the system is an M3 giant (Zamanov et al. 2007). Luna & Costa (2005) derived N/O, Ne/O and Ar/O abundances from the emission line spectrum of HD 319167, so the spectrum must be fairly typical of a high-excitation symbiotic star. Van Winckel et al. (1993) observed only Hα emission in the spectrum of this SS in their study of line profile shapes, which included [O III] and He I lines. The line shape at the time of their observations was single-peaked.

SS73 129 exhibits irregular variability in both the blue and red filters. The MACHO blue magnitude is generally about 13.6, but dropped to as low as about 14.7 during the observation period. The red magnitude was usually at 12.2, but variations significantly above and below that magnitude were observed.

SS73 129 has only emission lines of H I, He I, and He II in its spectrum (Munari & Zwitter 2002), and the cool giant in the system varies in spectral type between K5 and M0 (Zamanov et al. 2007; Mikolajewska et al. 1997). It is a D-type symbiotic (Belczyński et al. 2000). The variability can only be characterized as "irregular" in both the blue and red magnitudes.

H 2-38 is a well-known D-type symbiotic star with very high-excitation emission lines, including strong [Fe VII] (Belczyński et al. 2000; Munari & Zwitter 2002). Gromadzki et al. (2009) recently published a pulsation period of 395 days for the Mira variable present in the system, based upon OGLE data. Figure 5 shows the MACHO blue and red light curves, as well as our best-fit period for the red light curve: 397 days. It is evident that the observed periodicity varies slightly from cycle to cycle. The blue light curve is phased to the period found in the red light curve. Observations near the brightness maximum are particularly scarce.

V2506 Sgr is a S-type symbiotic with a giant star varying between spectral types M0 and M5.5 and a very high-excitation emission line spectrum (Mikolajewska et al. 1997; Mürset & Schmid 1999). The infrared colors published by Phillips (2007) fall in a region of the symbiotic star J - H versus H - K diagram where Mira variables are usually found. The MACHO data are consistent with a 913 day overall period of variation with a shorter period variability of 90 days superimposed. The superimposed fluctuations occur over timescales of weeks to months. The MACHO blue light curve and the light curve phased to a period of 913 days are shown in Figure 6.

SS73 122 has a high-excitation emission line spectrum (Munari & Zwitter 2002) with a M7 star (Mürset & Schmid 1999). The 2MASS infrared colors put SS73 122 in the realm of the Mira variables (Phillips 2007). The variability of this S-type symbiotic system has not been studied previously. We find a long, possibly periodic variation which is shown in the plots in Figure 7.

Hen 2-376 has only a short stream of MACHO observations, which are shown in Figure 8. The star dims by about 0.3 mag and returns to its previous brightness in about 100 days. This S-type symbiotic star was observed to have an M0 giant and a medium-excitation ([O III] being the highest ionization potential species observed) emission line spectrum by Mikolajewska et al. (1997).

Hen 3-1591 is included among the variable SS because its MACHO blue light curve showed a downward trend over several observing seasons. In addition, there is considerably more scatter in the blue data as compared to the red data. Since the star is fairly bright, the scatter in the blue observations well exceeds the error bars. This sort of variability was found by Hedrick & Sokoloski (2004) in the Johnson B filter observations of Hen 3-1591. They observed variations on the order of 0.1 mag in B (but not in V), which they attribute to variations in the accretion disk of the symbiotic system. Hen 3-1591 has a K1 III star and is classed as both an S and a D' symbiotic (Belczyński et al. 2000). This means that the infrared shows signatures of both warm dust and the cool star. The emission line spectrum does not show extremely high excitation lines, but species such as [O III] and He II are present (Munari & Zwitter 2002).

Hen 3-1674 is a S-type symbiotic whose M5 star has the distinction of a very large rotational velocity for its spectral class—52 km s^-1 (Zamanov et al. 2007). The emission line spectrum has lines of a variety of excitation potentials (Munari & Zwitter 2002). The optical variability of this object has not been studied previously, although the 2MASS infrared colors of J - H = 1.1 and H - K = 0.5 indicate that a Mira variable is likely to be present (Phillips 2007). We find a period of approximately 1003 days from both the blue and red light curves. As was the case for V2506 Sgr, the light curve appears to have many minor wiggles superimposed on the main variation.