A NEW RADIAL-VELOCITY CURVE FOR THE CEPHEID rj AQUILAE

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1 Publications of the Astronomical Society of the Pacific 93:48-485, August 98 A NEW RADIAL-VELOCITY CURVE FOR THE CEPHEID rj AQUILAE THEODOR S. JACOBSEN Astronomy Department, University of Washington, Seattle, Washington 9895 AND GEORGE WALLERSTEIN f Joint Institute for Laboratory Astrophysics, University of Colorado and National Bureau of Standards, Boulder, Colorado Received 98 February 28 A new radial-velocity curve based upon 2 0 Â mm - red spectrograms of the bright cepheid 7 Aql is presented. The shape of the velocity curve is unchanged since 923 but a small increase (0.4 km s - ) in amplitude may have occurred since 898. The systematic velocity appears to have decreased by 0.74 km s - during the same interval which, if real, may be associated with orbital motion around the companion recently discovered by Mariska et al. (980). The increase in period (since about 8) that had previously been noted by various authors is confirmed and extended. The Ha absorption velocity curve shows the same bump seen in metallic lines except about 0.6 cycle later. We interpret this in terms of an Ha absorption line that is formed near t if the metallic lines are formed near t = 0.0, and hence feels the bump at a later time. At minimum light, when the metallic-line velocities turn downward, the Ha curve becomes rapidly more positive until at phase 0.94 the Ha core is falling into the star at 48 km s - and a broad violet wing has appeared. It appears that up to the time at which the velocity of the metallic absorbing layer has been reversed, material high in the atmosphere is still falling inward. Key words: cepheid variable radial velocities velocity curve I. Introduction Radial-velocity curves of cepheid variables are useful in several ways. They serve to monitor changes in period as well as changes in the mean velocity of the star. Furthermore, they provide insight into the behavior of the atmosphere at various depths. Of particular use is the Ha line whose core is formed high in the atmosphere of the star (Wallerstein 979 and references cited therein). In that paper we discussed a new velocity curve for the five-day cepheid 8 Cephei. We now present a new curve for 77 Aquilae, whose period is near seven days, subsequent papers will deal with f Geminorum (0 days) and X Cygni (6 days). Twenty-one spectrograms of 7 Aql have been obtained with the.2-m coudé spectrograph at the Dominion Astrophysical Observatory. All were taken with the 0.8-m camera in the red spectral region on Kodak 098 plates with a grating that yields very nearly 0 Â mm -. All the exposures were made by G.W. except for three spectrograms taken by Carl Kaufman, four by Verne Smith, and one by Alan Batten. A string of seven consecutive clear nights in October 979 allowed one cycle to be completely covered. In addition two spectrograms each of a Aquarii and ß Aquarii (types G2 lb and GO lb, re- spectively) were taken as velocity standards. Their similarity of spectral type to 77 Aql and similar air mass during exposure makes them very valuable for standardization purposes. The spectra were measured by T.S.J. with the Gaertner measuring machine at the University of Washington. Three spectrograms taken near minimum light were underexposed below Ä6300 and one plate exposed near maximum showed slightly asymmetric lines. The four velocities from those plates were given one-half weight for purposes of discussion but appear to follow the velocity curves with no greater scatter than do the other points. Our velocities are presented in Table I. The same lines as were used in the 979 study of 8 Cep were also used for 77 Aql, a Aqr, and ß Aqr. The velocities of the two nonvariable supergiants deviate from their accepted velocities (Campbell 928) by +.0 ± 0.3 km s _. A further correction of 0.3 km s - was applied to the three spectrograms of limited spectral coverage since the average difference between velocities obtained from all lines and the 2 longward lines was +0. ± 0.4 km s -. The velocities are plotted in Figure where the Ha absorption velocities are shown separately. The continuous curve is the absorption-line velocity curve of (Jacobsen 9). On sabbatical leave from the Astronomy Department, University of Washington, Seattle, WA fguest investigator, Dominion Astrophysical Observatory, Herzberg Institute for Astrophysics, Victoria, British Columbia, Canada. 48 II. The Period and Center-of-Mass Velocity The period of 77 Aql has most recently been discussed by Schmidt (97) who noted that its well-known steady increase has been continuing. By combining 96 and

2 482 JACOBSEN AND WALLERSTEIN TABLE I Radial Vélocités of ri Aquilae ( ) Plate No. Wt. J.D Phase on 923 standard Plate velocity (measured Fe lines measured Ha velocity Observer curve + km s b / P AHB /2 /2 / , , Fig. The 979 velocity curve for 7) Aql. All velocities in Table I have been raised by km s" except for the half-weight spectra whose velocities have been raised by.3 km s data he derived a mean period for of days. By fitting our 979 data with that of Jacobsen (9) we find a mean period days for 95. One additional mean period may be found by combining the photometry of Pel (976) with that of Schmidt yielding days for the interval of 967 to Unfortunately we cannot combine our velocities with those of Evans (976) because the latter are not well distributed in phase. They are very useful to establish the systematic velocity, however (see below). The change in period over the past 200 years is shown in Figure 2. It will take at least another 50 years to establish whether the period is continuing to increase or is showing a sinusoidal behavior. If the period change between 900 and 980 is due to a changing orbital radial velocity the velocity difference should be about 30 km s -. We will next show that the data contradict this possibility completely. The discovery of a main-sequence companion to tj Aql (Mariska, Doschek, and Feldman 980) opens the possibility of determining both a visual and spectroscopic orbit leading to knowledge of the stellar masses and distance. Hence it is very important to reduce all velocities to the same system to see if real velocity changes have occurred during the interval of a little more than 80 years over which high-dispersion spectra of rj Aql have been obtained. Since the velocity curve of Jacobsen (9) is on the extremely well-defined Lick system of radial velocities (Campbell 928) we will use it for reference. Our observations of a and ß Aqr are shown in Table II. They

3 7} AQUILAE 483 were taken specifically to relate our 7 Aql velocities to the Lick system and yielded a correction of +.0 ± 0.3 km s -. This correction is uncertain because Ib supergiants may show atmospheric velocity fluctuations of the order of km s -. The most reliable systematic velocities and velocity amplitudes are listed in Table III. The velocities of 7 Aql by Evans (976) though based on only six spectrograms are important to eliminate any large-amplitude variations on a time scale that may fall between the long intervals in Table III. Using all six of Evans s spectrograms we fit her data to that of Jacobsen (9) with a systematic velocity of 4.88 ±.2 km s - for 970-7, which fits Table III very nicely. However the elimination of one spectrogram changes that to 4.04 ± 0.4 km s _, which is slightly discrepant. In any case the Evans velocities greatly reduce the chance that a period between five and ten years, which cannot be readily eliminated by the other data in Table III, is present. The systematic velocity data are indicative but by no means entirely convincing that a slow velocity change is taking place. As with the period change, another 50 years of observations will probably clarify this situation. We hope to report on it at a later time. YEAR FIG. 2 The changing period of r Aql. III. Differential Velocities A. The Metallic Lines A search for differential velocity effects among the metallic lines led to indefinite conclusions. The velocity difference of V Cai V Fei is near + km s - with no dependence on phase. For ionized lines (V SciI BaiI V Fei ) is near km s - throughout the cycle. The two high excitation Si lines, ÀX6347 and 637 show a positive residual of.5 km s - between phases 0. and 0.5 and becomes zero or perhaps.0 (if two discrepant plates are omitted) between phases 0.5 and Such a change could be due to minor blending as the star becomes cooler and the Si lines weaken. B. The Behavior of Ha The curve for metallic lines follows the curve almost exactly including the slight hump near phase 0.4 followed by the dip near phase The Ha curve deviates from the metallic absorption curve in three ways. It is too high from phases 0.45 to 0.65, too low from phases 0.65 to 0.82, and extraordinarily high from phase 0.82 (minimum light) to phase The first two of these are most simply interpreted as a manifestation of the same phenomenon as the bump in the metallic-line curve only appearing about 0.6 cycle later. We can employ this hypothesis to estimate the optical depth at which Ha is formed. If that layer feels the deceleration and reacceleration 0.6 cycle later we estimate its height above the metallic absorption layer as the velocity of sound times 0.6 cycle. For a sound velocity of 7 km s - the distance is 7 X 0 0 cm. We wish to know the optical depth 7 X 0 0 cm above the metallic absorption depth. Let us assume that the latter is t 5000 = 0.0. To select a model atmosphere we note that the effective temperature of a cepheid of seven days probably ranges from 5500 K at maximum light to about 4500 K at minimum. The surface gravity TABLE II Measurements of Standard Velocity Stars Star Plate Date 980 Measured Standard Standardvelocity velocity measure Averages Remarks a Aqr 3405 June 28 +6, a Aqr 348 June ß Aqr 3404 June ß Aqr 347 June wt* /2 Four plates: +.0

4 484 JACOBSEN AND WALLERSTEIN TABLE III Systemic Velocities and Total Amplitude of Four Radial Velocity Curves of n Aquilae Observer Year Systemic velocity Total amplitude km s * Wright, W. H. 898 Jacobsen, T. S. 923 Adams, W. S., et al. 940 Wallerstein, G. and Jacobsen, T. S * *Systemic velocity from Wright's "binary orbit" = -4.7 (899) probably lies between 0.5 and.0. Perhaps the lowergravity model should be used since the atmosphere may be nearly in free-fall during middeclining light. The Kurucz (979) models at T e = 5500 K show that for log g = 0.5 the layer 7 X 0 0 cm above t = 0.0 lies at t = while for logg =.0 the layer 7 X 0 0 cm above r= 0.0 lies at t = For T e = 4500 K the model of Bell et al. (976) for log g = 0.75 places the Ha formation layer at r = Such small optical depths are near the top of the photosphere but probably should not be described as in the lower chromosphere because Ha is seen in absorption rather than in emission. The departure of the Ha curve from the metallic absorption curve after minimum light is extremely large, reaching 47 km s - at phase 0.94 when the metallic absorbing layer is crossing the y axis. At this phase the fall of the metallic absorbing layer has been completely stopped by the rising gas and the Ha absorbing material is still falling into the star at nearly 50 km s - (times a projection factor which we will neglect since we are dealing with extremely high layers in the atmosphere). To some extent this is similar to the behavior of RR Lyrae stars (Preston and Paczynski 964; Preston, Smak, and Paczynski 965), however in the RR Lyrae stars the hydrogen velocities hold nearly constant near minimum light while the metallic-line velocities drop and a new component of hydrogen lines appears near minimum metallic velocity. The slope of the Ha curve from phase 0.83 to 0.94, yields an acceleration downward of 34 cm s -2 which is at least a factor of three larger than expected for the gravity of a seven-day cepheid with M v = 4.0 and a mass of 5 S DÎ. The downward acceleration of the metallic lines and Ha between phases 0.5 and 0.60 is 8.2 cm s -2, about as expected for a cepheid. There is no reason to expect material so high in the photosphere actually more likely in the lower chromosphere to be accelerated downward more rapidly than indicated by the mass and radius of the star. It seems that we are not looking at the same material as we follow Ha during the rapid rise Fig. 3 The profile of Ha in r Aql at phase This shows the position of the measured Ha core and the position Ha would have if it showed the same velocity as the metallic lines. The position V min is the center of Ha when it reaches its minimum value, i.e., at maximum light. of its velocity from phase 0.75 to 0.94 but rather at material that has been successively accelerated through the layer of most favorable physical conditions for Ha absorption. Such material must have risen substantially above the photosphere to have been so greatly accelerated. Hence we cannot simply integrate the velocity curve to learn the displacement of a discrete layer of gas. The behavior of Ha after minimum light is very similar to the observed velocities of the absorption core of the Ca il K line (Jacobsen 96). Unfortunately our Ha curve includes no points within 0.05 in phase from maximum light but our three points in the interval 0.07 to 0. do not show the negative displacements visible in the Ca il K at these phases. Perhaps the gas responsible for these absorption features does not provide the necessary excitation (be it radiative or collisional) to populate the n 2 level of hydrogen sufficiently. The positively displaced Ca n absorption cores also appear to last longer than the positively displaced Ha absorption line. An intensity tracing prepared by Verne Smith is shown in Figure 3. The core of Ha, the position that Ha would have if it followed the metallic lines, and the maximum violet displacement of Ha are shown. The violet wing could easily be a new component showing the velocity reached by the metallic lines and Ha at maximum light. The broad wing indicates a temperature of an F5 or somewhat earlier star. The Ha line does not show a hint of the red-displaced feature seen in T Monocerotis (Wallerstein 972) nor does its appearance indicate that the distinct core seen after minimum light is due to emission. Figure 3 is slightly deceptive in this regard. An examination of the original plate in the Grant oscilliscope machine shows that the small intensity maximum near

5 ï] AQUILAE 485 the line center is minute on a density scale but is very much enhanced when transformed to intensity because it lies on the toe of the calibration curve. Such transformations should not be entirely trusted. A full understanding of the behavior of Ha will require time-dependent, non-lte, radiative transfer calculations in the moving atmosphere of the cepheid. The excitation of the n 2 level of hydrogen by trapped La must be very important. We hope that our series of papers on Ha in cepheid variables will stimulate theoretical investigations of such effects. We are grateful to Director Sidney van den Bergh and the observing-time committee of the Dominion Astrophysical Observatory for assigning us the 48-inch telescope on so many clear nights and to Dr. Alan Batten for obtaining the very important plate at phase 0.94 for us. We also thank Verne Smith and Carl Kaufman for ob- taining several spectrograms. REFERENCES Bell, R. A., Erickson, K., Gustafsson, B., and Nordlund, A. 976, Astr. and Ap. Suppl 23, 37. Campbell, W. W. 928, Pub. Lick Obs. 6,. Evans, N. R. 976, Ap. J. Suppl. 32, 399. Jacobsen, T. S. 9, Lick Obs. Bull. 2, , Pub. A.S.P. 73, 6. Kurucz, R. L. 979, Ap. J. Suppl. 40,. Mariska, J. T., Doschek, G. A., and Feldman, U. 980, Ap. J. (Letters), 238, L87. Pel, J. W. 976, Astr. and Ap. Suppl. 24, 43. Preston, G. W., and Paczynski, B. 964, Ap. J. 40, 8. Preston, G. W., Smak, J., and Paczynski, B. 965, Ap. J. Suppl. 2, 99. Schmidt, E. G. 97, Ap. J. 65, 335. Wallerstein, G. 972, Pub. A.S.P. 84, , Pub. A.S.P. 9, 772. Wright, W. H. 899, Ap. J. 9, 59.