Periodic Variability of the Barium Central Star of the Planetary Nebula Abell 70

The central stars of planetary nebulae (PNe) are hot objects—the cores of AGB stars that have recently ejected their outer layers. However, a rare class of "Abell 35-type" PN nuclei (PNNi) have optical spectra dominated by cool stars (Bond et al. 1993). These are likely binary companions of the visually fainter hot cores. In the first three known A 35-type nuclei (A 35 itself, LoTr 1, LoTr 5), the late-type giant or subgiant is rapidly rotating.

Several known wide binaries contain hot white dwarfs and cool, rapidly rotating companions, which are probably descendants of A 35-type PNNi. Jeffries & Stevens (1996) proposed that these systems result from an AGB star in a wide binary developing a dense stellar wind, part of which accretes onto a late-type companion, spinning it up to faster rotation.

If the outer layers of the AGB star contain nuclearly processed material (carbon, s-process elements), the surface of the wind-accreting companion will become contaminated. Such a process accounts for the origin of late-type "barium stars," first identified by Bidelman & Keenan (1951).

Support for this scenario came from the discovery (Bond et al. 2003) that the PNN of WeBo 1 is a cool barium star, with enhanced carbon and s-process elements. UV photometry by the Neil Gehrels Swift Observatory confirmed that WeBo 1 has a hot companion (Siegel et al. 2012). The cool component is chromospherically active, with an apparent rotation period of 4.7 days, based on periodic photometric variability and the presence of starspots.

Recently, several more cool Ba stars have been found in A 35 PNe (e.g., Miszalski et al. 2013; Tyndall et al. 2013; Jones & Boffin 2017). In addition to WeBo 1, rotation periods of a few days have been found for the cool components of LoTr 1 (5.95 days; Aller et al. 2018), LoTr 5 (6.4 days; Tyndall et al. 2013), and Hen 2-39 (∼5.5 days; Miszalski et al. 2013). The binary orbital periods, when known, are much longer, e.g., ∼2700 days for LoTr 5.

The central star of the "diamond-ring" PN Abell 70 (hereafter A 70) was found to be a cool barium-rich star by Miszalski et al. (2012, hereafter M12). Its optical spectrum shows a G8 IV-V subgiant, but UV data confirm a hot PNN in the binary. In order to investigate whether the Ba star in A 70 exhibits a short rotation period, we obtained optical photometry during 2010 and 2011, and report the results here.

Observations were made by Chilean service personnel, using the 1.3 m SMARTS Consortium³ telescope at CTIO and its ANDICAM CCD camera. Data were obtained on 25 nights between 2010 October 26 and November 28 (I band only), and 79 nights between 2011 July 30 and November 29 (B, V, and I bands). The frames were bias-subtracted and flat-fielded in the SMARTS pipeline at Yale University. We then carried out photometry, using standard IRAF tasks, as described in Bond et al. (2016), to determine relative magnitudes of A 70 and four nearby comparison stars. Finally we calculated the magnitude difference between A 70 and the sum of the fluxes of the comparison stars. On two photometric nights, we determined approximate calibrated magnitudes for A 70 by reference to a standard field of Landolt (1992), obtaining B = 18.5, V = 17.8, and I = 16.9, in good agreement with M12. The recent Gaia DR2 parallax (Gaia Collaboration et al. 2018) of 0.3829 ± 0.1657 mas gives an approximate absolute magnitude of M_V ≃ +5.7, consistent with a late G star near the main sequence.

Our 2010 data show a 2.061 days, periodic variation, as shown in the phased I-band magnitudes at the top of Figure 1. Because this period is so close to 2 days, an alias period of 1.939 days, fits the data nearly as well. However, when the observations resumed in 2011, this variation had become undetectable, as shown in the BVI light curves labeled "2011a" in Figure 1. By about 2011 August 20, the variations reappeared, at the same period but with lower amplitude, as shown in the curves labelled "2011b." This behavior is consistent with an origin in starspots, whose amount of coverage varies with time as the activity level changes. (The rotation period could, of course, be twice the value given, if there are, for example, two starspots.)

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A 70 thus joins other A 35-type PNNi in having a rotating, late-type spotted companion of the hot central star. It would be of interest to determine the orbital period of the system, based on long-term radial-velocity monitoring.