Publications of the Astronomical Society of the Pacific 104: 1173-1176, 1992 December Variation of the Radial Velocity of Epsilon Cygni A R. S. McMillan, P. H. Smith, T. L. Moore, and M. L. Perry Lunar & Planetary Laboratory, Space Sciences Building, University of Arizona, Tucson, Arizona 85721 Electronic mail: [email protected] Received 1992 August 4; accepted 1992 September 16 ABSTRACT. A series of 217 measurements of the radial velocity of the KO— III star Epsilon Cygni A were made between 1987 May 16 and 1992 May 17 with an uncertainty per observation of ±12 m s_1. The results are inconsistent with the listed status of Epsilon Cygni A as a spectroscopic binary with an amplitude of 5 km s_1 and a period of 20 days. Instead, they indicate a sustained drift of —60 m s_ 1 yr-1. If the companion inducing this deceleration is in a circular orbit, the period must be at least 15 yr. Lower limits on the companion's mass are presented as functions of period and primary mass. The companion is probably more massive than a planet. 1. INTRODUCTION files by Gray ( 1982), who did not report evidence for dou- bled spectral lines or temporal changes of line asymmetry We are conducting an observing program to detect large that would indicate a spectrum of a companion. However, planets orbiting stars by measuring the periodic variations Gray was not specifically looking for a secondary spec- such companions induce in the star's Doppler shifts. We trum, especially not one at nearly the same radial velocity started observing Epsilon Cygni A in 1985 to demonstrate as Epsilon Cyg A. lanna and Culver's (1985) trigonomet- our capability to improve the orbit of a spectroscopic bi- ric parallax and the visual magnitude in Blanco et al. nary using a schedule of frequent, accurate observations. (1970) indicate an intrinsic luminosity more than a mag- Epsilon Cygni A [=HR 7949A=ADS 14274A=HD nitude fainter than that normally expected for an early Κ 197989 = Gliese (1969) 806.1 A] is of spectral class K0— giant star (Allen 1973). The angular diameter was mea- III (Keenan and Pitts 1980). Accurate radial-velocity ob- sured by Koechlin and Rabbia (1985) and Mozurkewich servations of it are feasible because it is bright, has many et al. ( 1991 ), with results that were reasonable for a single sharp lines, and is conveniently placed for observing from Κ giant at that distance. Kyrolainen et al. (1986) list sur- our latitude for a large part of the year. The companion Β face gravity and effective temperature within the normal is "optical," that is, not gravitationally associated (Hoffleit range for a star of this spectral type. Therefore, a close and Jaschek 1982). The latter authors also list a compan- companion to A, if present, must have a comparatively ion "C" with a proper motion similar to that of "A" at a faint luminosity. separation of 78 arcsec and a visual magnitude of 13. Cur- tis and Burns (quoted by Campbell and Moore 1906) first 2. OBSERVATIONS announced that A's radial velocity was variable. Harper Descriptions of our instrument and technique are given (1920) analyzed 68 measurements of radial velocity, 50 of by McMillan et al. (1985, 1986, 1990, 1993), McMillan which were made by him, and derived a tentative period of -1 and Smith (1987), and Smith et al. (1987). A brief sum- 20 days and an amplitude of 5 kms from the best 41 mary of the instrument's precision and accuracy is given measurements that spanned 1.3 yr. Harper's data show a -1 below. probable error of ±1.5 kms , which according to him Epsilon Cyg A was observed 246 times between 1985 was unexpectedly high for "...the good quality of the October 22 and 1992 May 17 UT inclusive, with the 0.9-m lines." A phase diagram of his data folded modulo his telescope of the Steward Observatory on Kitt Peak. Expo- period (not reproduced here) admits a curve with a semi- 1 sure times ranged from 15 min to 1 h. The observation amplitude of 1 km s" drawn through considerable scatter. series was interrupted in 1986 to upgrade the instrument. Campbell and Moore (1928) listed Epsilon Cyg A in a This created a boundary in the time series across which compilation of stellar radial velocities, showing a full span -1 data cannot be compared easily. Therefore we present two of 8 kms among 12 additional measurements spread separate series of observations with different instrumental over 23 years. Woolley et al. (1960) and Jones and Wool- "zero points." ley (1961) list a few measurements of its radial velocity In Table 1, 29 observations are given for 18 nights, 15 of and acknowledge the above studies that had "well estab- which were consecutive. The data span 33 days. The ve- lished" the radial-velocity variation. However, our inspec- locities are referred to a "zero" arbitrarily assigned to one tion of Woolley et al.'s few measurements does not support of the observations. The standard deviation of the data Harpers' claim. during the first 15 consecutive nights is ±30 m s-1. (All Many studies have been made since then of the proper- error estimates in this contribution correspond to the stan- ties of the atmosphere of the star, some of them with very dard deviation per sample.) The observations in Table 1 high spectroscopic-resolving power and signal-to-noise ra- are illustrated in Fig. 1. This star could not have been tio. One of the most relevant to the question of whether varying in the manner described by Harper when we ob- star A is binary was the high-resolution study of line pro- served it. Therefore, Harper's variation, if real, could not 1173 © 1992. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 1174 MCMILLAN ET AL. Table 1 Table 2 Differential Radial Velocities of Epsilon Cyg; 1985 Differential Radial Velocities of Epsilon Cygni; 1987-1992 V -1 V V V Time (ms ) Time (m s_1) MJD (ms-1) MJD (ms-1) 60.247 -11 69.082 + 86 46931.4556 61.093 + 9 69.165 +91 -28.76 47691.4173 -141.45 61.126 -23 70.149 + 54 46931.4688 -15.76 47691.4389 -146.93 61.158 +72 71.102 + 53 46931.4833 -16.86 47691.4600 -153.36 61.194 +46 72.098 + 56 46945.4306 -14.42 47695.4433 -171.97 62.109 + 14 72.142 +43 46945.4465 -0.12 47695.4644 -179.52 63.158 +47 73.100 +20 46947.4583 -14.05 47696.3864 -170.88 63.201 46957.4667 -5.95 47696.4205 -150.97 + 19 73.142 +49 46959.4681 0.00 47790.2349 64.131 + 12 74.195 + 11 -172.77 64.169 +23 91.069 -47 46974.4451 -25.53 47790.2565 -184.17 65.163 -2 92.109 -19 46977.3465 + 16.44 47790.2787 -160.76 66.184 +0 (ref.) 92.156 -22 46977.3764 -10.97 47794.1075 -149.19 67.192 -8 93.112 -146 46977.4178 + 1.97 47794.1285 -151.40 68.091 +40 93.158 -102 46978.4694 -35.32 47795.1238 -150.80 68.157 + 19 47073.0932 -57.83 47795.1894 -156.31 47073.1161 -64.21 47798.1043 -151.25 "Time"=Julian Date minus 2,446,300.5. 47073.1386 -64.95 47809.0820 -94.08 47073.1611 -76.31 47809.0960 -109.90 47073.1829 -75.91 47809.1100 -103.11 47073.2050 -84.77 47810.0782 -143.27 have been due to orbital motion. Figure 1 also suggests a 47088.1463 -60.48 47810.0922 -134.26 deceleration of the star along the line of sight. However, 47088.1688 -62.31 47810.1063 -139.74 considering the primitive state of our equipment at that 47088.1965 -64.79 47812.1358 -141.26 time we do not attach great significance to the difference of 47089.1118 -70.84 47812.1568 -141.81 47107.0794 -88.66 47814.0666 -152.15 velocities between the two observing runs in 1985. 47107.1089 -88.52 47814.0878 -160.29 In Table 2, 217 observations span 5.0 yr. The much- 47107.1466 -88.33 47838.0719 -140.32 improved instrument was not modified during this interval 47108.1062 -69.83 47838.0931 -138.39 and calibrations were rigorous. The stability of the velocity 47111.0913 -78.37 47843.0792 -124.56 metric over this interval is presented and analyzed by Mc- 47139.0668 -60.49 47843.1011 -110.62 Millan et al. (1993). No celestial sources are used for cal- 47277.4843 -67.89 47845.0601 -150.20 47278.4650 -56.62 47845.0812 -144.46 ibration or velocity correction. Calibrations of the interfer- 47280.4708 -62.94 47988.5253 -196.11 ometer are based on measurements of emission lines from 47281.4646 -80.44 47989.5291 -200.28 an Fe-Ar hollow cathode lamp. The properties of the in- 47284.4767 -82.77 47991.5131 -218.53 terferometer are determined in absolute physical units, 47285.4036 -78.61 47994.5000 -178.12 which provide absolute vacuum wavelengths of the inter- 47285.4348 -84.91 47996.5202 -202.66 47286.4436 -58.13 48017.5030 -197.98 ference orders constructively transmitted by the interfer- 47287.4064 -90.70 48018.4741 -194.83 ometer. The short-term consistency of the individual cali- 47287.4486 -89.34 48020.4649 -194.91 brations made immediately before and after each setting on 47288.4687 -78.17 48022.4707 -184.83 a star is ±3-4 ms_1 (McMillan et al.