Spatial Resolution of the R Aquarii Binary System
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THE ASTROPHYSICAL JOURNAL, 482 : L85–L88, 1997 June 10 q 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. 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 5 1, J 5 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 1188 H 28 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 Hal6563 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 Ha 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 Ha 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 Ha 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 5 1, J 5 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 s21). 2. OBSERVATIONS FIG. 2.—The v 5 1, J 5 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 beam21 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 23482165. Absolute flux density calibration operated in the 2AC spectral line observing mode with the was performed with scans of 07131438 (assumed 0.29 Jy) and on-line Hanning smooth option set and employed a 3.125 03191415 (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 21 the local standard of rest (Vlsr)is226.0 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 23482165 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 3 512 grid with 0"01 cell spacing, and the results are density for 23482165 (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 3 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 s 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 3 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.