Spectroscopic and Photometric Variability of Three Oxygen Rich Post-AGB “Shell” Objects

Abstract

Light, color, and radial velocity data (2007–2015) for HD 161796, V887 Her, and HD 331319, three oxygen-rich post-AGB stars, have thus far not provided direct support for the binary hypothesis to explain the shapes of planetary nebulae and severely constrain the properties of any such undetected companions. The light and velocity curves are complex, showing similar periods and variable amplitudes. Nevertheless, over limited time intervals, we compared the phasing of each. The color curves appear to peak with or slightly after the light curves, while the radial velocity curves peak about a quarter of a cycle before the light curves. Thus it appears that these post-AGB stars are brightest when smallest and hottest. The spectra of these objects are highly variable. The H$\alpha$ line has multiple, variable emission and absorption components. In these oxygen-rich post-AGB stars atmospheric lines, such as near-infrared Ca ii triplet and low-excitation atomic lines, also have multiple components and sometimes show line doubling, indicative of shocks induced by pulsation.

1. Introduction

Stars at the proto-planetary nebula (PPN) phase evolve from the Asymptotic Giant Branch towards the PN phase becoming hotter at almost constant luminosity. These low mass stars are surrounded by a circumstellar envelope ejected during the AGB phase. The details of the physical processes and stellar winds that shape these envelopes remain uncertain. The outstanding question in PN research is the role and importance of binarity in the shaping of PNe.

We selected seven bright (V < 10 mag) post-AGB stars observable with 1-m class telescopes from the northern hemisphere (Hrivnak et al. [1]). They all have F- and G spectral types. Of these 7 metal-poor PPNe, 3 are O-rich and 4 are C-rich. In this contribution, we will consider the 3 O-rich objects: HD 161796 (IRAS 17436+5003), HD 331319 (IRAS 19475+3119), and V887 Her (IRAS 18095+2704). The objects have spectral energy distributions typical for “shell” objects: they are double peaked, with a peak in the visible arising from the (reddened) photosphere and a second peak in the mid-infrared arising from re-radiation from cool (T ≤ 200 K) dust. However, they have no near-infrared excess, the presence of which is an indication of the presence of a disk. The disk-type post-AGB stars have been shown to be binaries (Manick et al. [2], Van Winckel et al. [3]).

HD 161796 is highly aspherical, and shows a low density polar outflow and a high density equatorial region (Min et al. [4]). HD 331319 is a quadrupolar nebula with two collimated bipolar lobes (Sahai et al. [5]). V887 Her is an extended source with a significant circumstellar shell (Ueta et al. [6]) and OH masers (Lewis et al. [7]).

To prove or exclude the presence of binaries in these shell objects is crucial to our understanding of the way(s) that bipolar PPNe and PNe form. If it turns out that these PPNe are not binaries, then it raises the possibility that there may be more than one way to produce bipolar PNe.

2. Observations and Analysis

High-resolution spectra were obtained at the Dominion Astrophysical Observatory 1.2 m telescope since 2007 and with the HERMES Spectrograph on the 1.2 m Mercator telescope (Raskin et al. [8], Van Winckel et al. [9]) since 2009. Radial velocities of the HERMES spectra were determined via the cross-correlation technique using a mask in the range 477–655 nm for F0-type stars and fitting the peak of the obtained correlation function with a gaussian (Hrivnak et al. [1]). Radial velocities of DAO spectra were also obtained via cross correlation with a set of bright IAU RV standards observed with the same instrumentation. In this case velocities were determined by fitting a parabola to the upper half of the cross correlation profiles (Hrivnak et al. [1]).

Photometric observations of these objects were obtained from 2007 to 2015 at the Valparaiso University Observatory with a 0.4 m telescope equipped with a CCD camera and filters standardized to the Johnson B and V and Cousins R systems. Differential photometry was employed, using an aperture of 11${}^{″}$ diameter (Hrivnak et al. [10]). For V887 Her ASAS-N data were available and used (Kochanek et al. [11]).

3. Results and Discussion

3.1. Radial Velocity and Photometry

The RV amplitudes are small (<4.0 km/s, Hrivnak et al. [12]) and the periods found in the RV data are in the range 35 to 103 days, similar to the pulsation periods of the objects. So far, no clear evidence for long-period RV variations due to binary companions has been found in any of these objects. This sets significant constraints on the properties of any undetected binary companion: they must be of low mass, ≤0.2 M${}_{\odot }$, or long period, longer than 30 years. Thus the present observations do not provide direct support for the binary hypothesis to explain the shapes of thes PPNe.

The light and velocity curves are complex with multiple periods and variable amplitudes. However, over limited time intervals during 2007–2015, we were able to identify dominant periods in the light, color, and velocity curves and compare the phasing of each. See figures in Hrivnak et al. [12] which shows the phased light, color, and velocity curves of the objects. The color curves appear to peak with or slightly after the light curves while the RV curves peak about a quarter of a cycle before the light curves. For all three objects plus two previously studies PPNe (Hrivnak et al. [13]), (a) the light and color curves are approximately in phase, with the suggestion that the color curve perhaps peaks slightly (∼0.05 P) after the light. (b) The RV curve is approximately −0.25 P out of phase with the light curve. Thus it appears that these PPNe are brightest when smallest and hottest. These results differ from those found for Cepheid variables, where the light and velocity differ by half a cycle, and are hottest at about average size and expanding. However, they do appear to have similar phasing to the larger amplitude pulsations seen in RV Tauri variables. Attempts at modeling pulsation in these post-AGB stars are few (Fokin et al. [14]) and the observed periods are longer than models indicate.

3.2. Spectroscopic Variability

The average H$\alpha$ profiles show broad wings with central emission and absorption components (Figure 1). The individual spectra show that these H$\alpha$ components vary in depth, in width, and position (Figure 2). The strength of the blue and red emission are changing alternately. The red emission and absorption component is generally weaker than the blue. In the inverse P-Cygni profile phase, the blue emission peak can be very strong.

The near-infrared Ca ii triplet and some low-excitation metal lines such as Ba ii and Fe ii also show strong variability (Figure 2). In these lines, line splitting in two can be observed sometimes. This splitting was observed in C-rich post-AGB stars (Klochkova [15]) with a complex circumstellar envelope, but here we confirm that it is also observed in such O-rich post-AGB stars. Further analysis will help clarify where these components originate in these stars. The observed splitting and short-term variability in low excitation atomic lines provide evidence that shock waves are present in the outer layers of the atmosphere or circumstellar envelope (Fokin et al. [14], Klochkova [15], Zacs et al. [16]). The lack of line splitting phenomena in the high excitation potential lines is most likely because these lines are formed deeper in the atmosphere.

The complex spectral variability and lack of stable periods in the light and velocity curves makes the temporal changes hard to phase. Provisionally, in Figure 3 we phased V887 Her to the period of 102.3 days and the same epoch as Hrivnak et al. [12]. We searched for evidence for a jet as seen in BD+46 442 (Bollen et al. [17]), in which a broad absorption with a velocity up to ∼−400 km/s is observed. In the phased spectra of V887 Her there is an extended blue absorption at phase 0.35. However the velocity at the blue edge of the absorption is only −50 km/s, no more than 2–3 times the typical expansion velocity of PNe and AGB stars, and lasts less than 0.10 phase cycle, while in BD+46 442 it is at least 0.25 phase cycle. These are not indications for a strong jet. These “Shell” objects have H$\alpha$ profiles similar to “disc” sources, but without the presence of strong jets.

The analysis of the high-resolution spectra of these post-AGB stars continues, in order to better investigate and understand the shocks, pulsation, and kinematics in these objects.