Spatial Resolution of the R Aquarii Binary System

SPATIAL RESOLUTION OF THE R AQUARII BINARY SYSTEM

J. M. HOLLIS Space Data and Computing Division, Code 930, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

J. A. PEDELTY Biospheric Sciences Branch, Code 923, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

AND

R. G. LYON University of Maryland, Center of Excellence in Space Data and Information Sciences, Code 930.5, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 Received 1997 February 26; accepted 1997 March 24

ABSTRACT We report continuum VLA observations at 7 mm that have resolved the stellar components in the R Aqr binary system. R Aqr was simultaneously probed in both a 50 MHz bandpass containing lineless continuum emission associated with the hot companion/accretion disk and a 3.125 MHz bandpass containing the spectral line v ϭ 1, J ϭ 1– 0, SiO maser emission associated with the long-period–variable (LPV) envelope. The offset between the two stars is 55 H 2 mas with a position angle of 118Њ H 2Њ relative to the LPV, providing the first data point for a subsequent monitoring program to determine precisely the binary orbit that is suspected to be highly elliptical and have a period of 144 yr. We evaluate these first observations in the context of constraints placed on the orbital geometry of the system and obtain a geometrical distance of 1200 pc to R Aqr. We also report spectral line VLA observations at this same epoch that confirm that the SiO maser spots have a ringlike morphology, as previously reported by other investigators using the VLBA. Subject headings: astrometry — binaries: symbiotic — H II regions — masers — stars: variables: other — techniques: interferometric

1. INTRODUCTION since disappeared (Wallerstein & Greenstein 1980). In recent R Aqr is a symbiotic stellar system composed of a mass- years, the hot companion and its accretion disk have been losing 11–2 M Mira-like long-period variable (LPV) with a inferred from ultraviolet observations of strong, hot nebular J lines in the system, since no significant ultraviolet continuum 387 day period and a 11.0 MJ hot companion/accretion disk that is believed to give rise to the symmetrical jet seen at emission is present (Kafatos, Michalitsianos, & Hollis 1986). ultraviolet, optical, and radio wavelengths (e.g., see Hollis et In an attempt to resolve the binary system by detecting both al. 1991; Solf & Ulrich 1985; and Hollis et al. 1985; respec- stellar components simultaneously, Hege, Allen, & Cocke tively, and references therein). A review of various attempts to (1991) used speckle interferometry in a 1.8 nm bandpass constrain or derive an orbit for the R Aqr system is contained centered on H␣␭6563 on 1983 October 16. A three-compo- in Hinkle et al. (1989), who conclude that the orbit is at best nent image at 45 mas resolution was obtained. The weakest very uncertain. The geometry of the binary system has been of H␣ component detected was diffuse and extended, consistent much debate, particularly since the jet was first observed in the with component C2 of the radio jet (Hollis et al. 1986) which optical circa 1977 (Wallerstein & Greenstein 1980; Herbig is 10"5 removed from the central source(s). The strongest H␣ 1980), because the interaction of the system components is component detected was easily identified with the hot com- relevant to the jet formation mechanism. panion/accretion disk, consistent with the component C1, Based on an analysis of the R Aqr visible light curve from which delineates the central H II region of the radio jet (Hollis 1811 through 1979 (Mattei & Allen 1979), Willson, Garnavich, et al. 1986). The third H␣ component detected had no radio & Mattei (1981) suggested that the R Aqr system undergoes counterpart and was designated C3 by Hege et al. (1991), who eclipse with a period of 144 yr; these light-curve data show speculated that this emission could arise in the LPV envelope that the LPV pulsational variations nearly ceased during the or simply be another knot in the inner jet. Hege et al. (1991) epochs 1928–1934 and 1974–1980. The jet probably under- surmised that if C3 were associated with the LPV, it would be goes episodic refueling and subsequent increased activity at the first spatial resolution of the stellar components in the periastron, and a similar period of 144 yr has also been system. obtained by proper motion analyses of discrete ejected radio The recent advent of VLA Q-band receivers permits simul- components (Lehto & Johnson 1992; Hollis & Michalitsianos taneous probes of weak H II regions and strong SiO maser 1993) and ultraviolet components (Hollis et al. 1997), which emission regions in close proximity. The bright SiO maser can comprise the strong northeast jet. Thus, a binary period of be used to self-calibrate the phase and amplitude of the 144 yr has strong circumstantial evidence, even though direct continuum emanating from the weak H II region. Thus, we observations of the hot companion in the system are problem- were motivated to resolve the R Aqr binary system because atical. For example, the hot component in the system became SiO is associated with the LPV envelope and the weak H II as bright as mv 1 8 during the interval of 1928–1934 but has region presumably surrounds the hot star/accretion disk. L85 L86 HOLLIS, PEDELTY, & LYON Vol. 482

FIG. 1.—The v ϭ 1, J ϭ 1– 0, SiO maser spectral line proﬁle toward R Aqr on 1996 November 20. The 64 channel spectrum has spacings of 48.828 kHz (10.34 km sϪ1).

2. OBSERVATIONS FIG. 2.—The v ϭ 1, J ϭ 1– 0, SiO spatial structure summed over all The R Aqr system was observed at 43 GHz with 13 antennas velocity channels from spectral line observations (see Fig. 1) toward R Aqr on of the NRAO1 Very Large Array on 1996 November 19–20 1996 November 20. Contour levels are 3%, 5%, 10%, and 50% of the peak contour summed flux of 215 Jy beamϪ1 with a gray-scale background. from 2300 to 0700 UT. The antenna spacings effectively sampled the full range of the standard A configuration. total of 15 123 minute scans of R Aqr were alternated with 12 For the spectral line observations the VLA correlator was minute scans of 2348Ϫ165. Absolute flux density calibration operated in the 2AC spectral line observing mode with the was performed with scans of 0713ϩ438 (assumed 0.29 Jy) and on-line Hanning smooth option set and employed a 3.125 0319ϩ415 (measured as 9.8H0.4 Jy) as a consistency check. MHz bandwidth centered on the maser rest frequency of The amplitudes and phases of the 50 MHz continuum data 43122.08 MHz, assuming the source velocity with respect to were calibrated using the strong maser emission in the BD IFs Ϫ1 the local standard of rest (Vlsr)isϪ26.0 km s . Three 5 minute following Reid & Menten (1997). Both the narrow- (maser spectral line scans of R Aqr were performed. Contemporane- emission from the LPV) and wide-band (continuum emission ous 2 minute scans of 2348Ϫ165 were made to calibrate the from the H II region) data were mapped and CLEANed on a spectral bandpass, and absolute flux calibration used the flux 512 ϫ 512 grid with 0"01 cell spacing, and the results are density for 2348Ϫ165 (1.48 Jy) determined using the contin- shown in Figure 3. The resolution obtained with uniform uum data. Standard spectral line processing techniques, in- weighting is approximately 55 ϫ 41 mas. Using AIPS task cluding self-calibration using the “channel 0” data and IMFIT, elliptical Gaussians were fit to the two components in CLEANing of the individual channels (Clark 1981), were used Figure 3 that provide the VLA data entry with 1 ␴ errors in to produce a cube containing the spectrally resolved total Table 1; these results were confirmed within the formal fit intensity (Stokes I) SiO maser emission. Figure 1 shows the errors of IMFIT by means of task MAXFIT, which fits a spatially average maser profile produced using the Astronom- quadratic function for position determination of an emission ical Image Processing System (AIPS) task POSSM. Interactive peak. visual inspection of the cube clearly shows that the peak of the maser emission moves clockwise in an approximate circular 3. DISCUSSION ring as velocity decreases. The diameter of this ring is compa- Table 1 VLA data provide the first in a series of apparent rable to the nominal resolution of 61 ϫ 42 mas, but, given the orbit data that will permit an unambiguous determination of high flux densities of the maser emission, we used CLEAN to the true orbit for the R Aqr binary system. However, by using superresolve the system. Figure 2 shows the sum of the the constraint that this first set of data affords and some CLEANed maps for channels 7–50 after they were restored further assumptions, we can already make a preliminary with circular Gaussian beams having 10 mas FWHM. This (albeit crude) estimate of the true orbit for R Aqr. image confirms the ringlike morphology of the R Aqr maser Foremost, there is no set of spectroscopic data on the R Aqr spots as observed with the Very Long Base Array (VLBA) system through one or more cycles of the orbital phase that (Boboltz, Diamond, & Kemball 1996). would unambiguously determine the period, P. Even if there Continuum observations were made with the intermediate were, the situation is complicated by the fact that only the LPV frequencies (AC IFs) set to 43164.9 MHz with a 50 MHz is spectroscopically observable, and the small-velocity semiam- bandwidth while the BD IFs were tuned to 43121.7 MHz with plitude is contaminated by the pulsations of the LPV (Hinkle a 3.125 MHz bandwidth to observe the SiO maser (43122.08 et al. 1989). However, there is ample circumstantial evidence MHz rest frequency corrected to the topocentric frame at the that P 1 44 yr because of such an observed periodicity in the Ϫ1 middle of the observations assuming Vlsr ϭϪ26 km s ). A Mira brightness (Willson et al. 1981) and a similar periodicity 1 The National Radio Astronomy Observatory is operated by Associated seen in ejection events inherent in the radio and ultraviolet Universities, Inc., under cooperative agreement with the National Science manifestations of the jet (Lehto & Johnson 1992; Hollis & Foundation. Michalitsianos 1993; Hollis et al. 1997). No. 1, 1997 SPATIAL RESOLUTION OF R AQR BINARY SYSTEM L87

Therefore, we adopt a mass range of 11.5–2.0 MJ for the LPV, giving a total binary mass range of 2.5–3.0 MJ, which corresponds to a 12.54 ϫ 1014–2.69 ϫ 1014 cm. Mass transfer from the LPV to the secondary is necessary for the formation of an accretion disk and jet and is facilitated if the orbital eccentricity, e, is large enough to cause Roche lobe overﬂow. The criterion in such a binary system is

a(1 Ϫ e) 1 2 RLPV (Haynes, Lerche, & Wright 1980). Assum- ing that a radius for an LPV is 1300 RJ, then e Ն 0.8 ensures Roche lobe mass transfer occurs at periastron (see also Kafatos et al. 1986; and Hinkle et al. 1989). Bulk changes in the pulsational cycle of the Mira occur periodically (Mattei & Allen 1979). Such changes probably occur at periastron, when the mass transfer influences the atmosphere of the Mira (Hinkle et al. 1989). Such light-cycle events occurred in 1928–1934 and 1974–1980 (Mattei & Allen 1979). Hence, following Hinkle et al. (1989), we arbitrarily fix the last time of periastron passage, T, at 1974 February 21 (JD 2,442,100.0), and it may be in error by 1–2 yr (Fekel 1997). The morphology of two large ejected shells of material surrounding the R Aqr system indicate a basic plane of symmetry that has been suggested to be identical with the orbital plane of the binary system (Solf & Ulrich 1985). This FIG. 3.—Simultaneous continuum observations of the v ϭ 1, J ϭ 1–0, SiO observed projected plane has an axial ratio of 3 (approximately maser and adjacent lineless continuum toward R Aqr on 1996 November 20. The SiO emission in a 3.125 MHz bandwidth is pointlike, representative of the east-west) to 1 (approximately north-south), and if the mate- emission centroid of maser spots in Fig. 2, and is shown as four thin black rial in this plane has circular symmetry, then this suggests that contour levels that are 50%, 70%, 90%, and 98% of the SiO peak flux value of the orbital plane is inclined by 120Њ with respect to the line of 11.1 Jy beamϪ1. The lineless continuum emission in a 50 MHz bandwidth is sight (LOS) along the approximate north-south direction. extended and shown as eight thick gray contour levels in multiples (2, 3, 4, 6, 8, 12, 18, and 23.7) of 2.7 ϫ 10Ϫ4 Jy beamϪ1, which represents the rms map noise; Therefore, the angle between the orbit plane and the plane at the peak flux is 6.5 mJy beamϪ1. Assuming the SiO emission emanates from the right angles to the LOS is the inclination angle, i ϭ 70Њ, LPV and the adjacent lineless continuum emission emanates from the H II implying that the system is seen nearly edge-on. region surrounding the hot companion/accretion disk, the offset in peak In comparing our VLA observational epoch in Table 1 with positions suggests the binary system is unambiguously resolved. our adopted T (time of periastron passage), the VLA observations occurred when the system is almost exactly at apastron. Assuming P 1 44 yr, the semimajor axis, a, can be esti- Figure 3 and Table 1 VLA data show that the secondary is mated by Kepler’s third law with some reasonable assumptions observed approximately north of the LPV at apastron, and the concerning the total systemic mass. The LPV primary is a Mira separation between the two stars represents the foreshortened variable, while the secondary is generally regarded to be a apastron distance. Moreover, orbital symmetry would have white dwarf whose average mass MS 1 1 MJ (Allen 1973). placed the secondary in close proximity but approximately due Regarding the LPV mass, Mira variables are the endpoints in south of the LPV at time T. Thus, with a value of i ϭ 70Њ, the asymptotic giant branch evolution just prior to planetary foreshortened major axis of the orbit lies in the approximate nebula formation, and, hence, the initial mass for such stars is north-south direction. Since i ϭ 90Њ for an edge-on orbit, this just a few solar masses (see Whitelock 1990). Moreover, the suggests that our LOS is approximately along the major axis of space distribution of symbiotics is concentrated toward the the orbit. Recalling the projected orbital plane axial ratio of 3 Galactic plane, similar to distributions seen in planetary (east-west) to 1 (north-south) and that the foreshorten major nebulae, suggesting that these objects belong to an old disk axis lies north-south, then the latus rectum of the orbit lies population. Thus, the space distribution for symbiotic stars east-west and is not foreshortened. Consequently, the position favors a total binary mass of Ն2–3 MJ (Kenyon 1986). angle of the line of nodes, ⍀,is190Њ. Therefore, the angle in the plane of the orbit between the line of nodes and the major TABLE 1 axis, ␻ is 1H90Њ; here the sign ambiguity arises due to the fact SUMMARY OF ATTEMPTS TO OBSERVE THE APPARENT ORBIT OF RAQR that we do not yet know whether the secondary moves RELATIVE TO THE LPV counterclockwise or clockwise in the sky. Thus, our VLA observations provide the missing piece of geometrical infor- Interferometry Julian ␳ ␪ Technique Date (arcsec) (deg) Reference mation that relates the orbit of the double star system to the observational LOS. Specklea,b ...... 2,445,623.5 0.124 H 0.002 46.4 H 0.3 1 c Our preliminary orbital elements, which are summarized in VLA ...... 2,450,407.5 0.055 H 0.002 17.9 H 2.0 2 Table 2, can be used with the current observations to deter- a Speckle results are not reconcilable with orbital parameters in Table 2 (see mine the distance to R Aqr. The foreshortened apastron § 2). distance a(1 ϩ e) * cos i is 11.56 ϫ 1014–1.65 ϫ 1014 cm, and b Hege et al. (1991) least-squares analysis of H␣ imagery with 1 ␴ errors on Table 1 data show we have observed this distance to be 155 mas, the fit. 1 c Results from fitting elliptical gaussians to Fig. 3 VLA imagery with 1 ␴ implying that the distance is 195–206 pc. A review of the errors on the fit. literature on R Aqr suggests that the distance as determined by a REFERENCES.—(1) Hege et al. 1991; (2) this work. variety of methods falls within the range 180–260 pc (180 pc by L88 HOLLIS, PEDELTY, & LYON

TABLE 2 determine a reliable spectroscopic orbit for this system (see PRELIMINARY ORBITAL ELEMENTS FOR RAQR Hinkle et al. 1989 and references therein). If our orbital parameters in Table 2 apply, we ﬁnd that the Ms ϩ MLPV Hege et al. (1991) result in Table 1 is inconsistent with

ELEMENT VALUE 2.5 MJ 3.0 MJ detecting the stellar components in the R Aqr binary system— the 124 mas separation between the LPV and the hot second- P...... 44yr ...... a...... 2.54 ϫ 1014 cm 2.69 ϫ 1014 cm ary is too large by a factor of 12 for the Hege et al. epoch of e ...... 0.8 ...... observation. Hence, we are led to suggest that the feature T...... 2,442,100.0 JD ...... Hege et al. observed and designated as C3 is not the LPV but ...... i...... 70Њ an H␣ emission knot in the southwest counterjet. ⍀ ...... 190Њ ...... ␻...... H90Њ ...... a ... Ϫ1 Ϫ1 KLPV .... 7.2 km s 6.4 km s 4. SUMMARY a M 1 M Assumes S 1 J (see eq. [1]). With the VLA at Q band in the continuum mode of Solf & Ulrich 1985; 181 pc by Lepine, Le Squeren, & Scalise operation, we simultaneously probed the central H II region in 1978; 250 pc by Whitelock 1987; and 260 pc by Baade 1943, 1944). the 7 mm continuum emission (associated with the hot com- Further, from Table 2 orbital elements, we can predict the panion/accretion disk) as well as the 7 mm, v ϭ 1, J ϭ 1–0, semiamplitude for spectroscopic observations and when best SiO maser line emission (associated with the LPV envelope) to observe orbital radial velocity extrema. Since our line of and resolved the R Aqr binary system for the first time. These nodes nearly coincides with the latus rectum of the orbit, one excellent relative position determinations provide the first set of observations of the apparent orbit in a subsequent moni- can calculate the LPV velocity semiamplitude, KLPV, from the expression toring program to determine precisely the true orbit itself. We developed a preliminary set of orbital parameters that show Ϫ1 2 Ϫ0.5 KLPV ϭ 2␲aP ͑1 Ϫ e ͒ sin i * MS/͑MLPV ϩ MS͒ , (1) why a spectroscopic orbit determination for this system is difficult at best and that show why the speckle interferometry where we assume M 1 1 M and M 1 1.5–2 M , which S J LPV J result of Hege et al. (1991) probably did not detect the LPV in yields K 1 7.2–6.4 km sϪ1. These derived K values are LPV LPV the system but, rather, an H␣ emission knot in the southwest consistent with the well-determined lower limit established by counterjet; we obtained a geometrical distance of 1200 pc to Solf & Ulrich (1985), who observed a 13 km sϪ1 difference in R Aqr. Our VLA spectral line results also confirm VLBA centroid velocities of the two expanding shells surrounding R observations that the SiO maser spots have a ring-like mor- Aqr, which they attribute to the effects of orbital motion. phology. Observed radial velocity extrema will occur at the nodal points, which are approximately coincident with the endpoints of the latus rectum of the orbit. Our Table 2 preliminary We thank Greg Taylor, Mark Reid, and Farhad Yusef- orbital parameters predict that these radial velocity extrema Zadeh for help with the Observe file and data analysis, Phil occur 1.1 yr in advance of periastron and 1.1 yr following Diamond for graciously providing VLBA results prior to periastron. Such a narrow time frame, combined with the 11.1 publication, Judy Laue for help with graphics, and an anony- yr pulsational cycle of the LPV, would account for the mous referee for a timely report. J. M. H. and J. A. P. received difficulties previous investigators have had in attempts to support from NASA RTOP 344-02-03-01.

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