SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI DURING INGRESS AND TOTALITY, *

Transcription

1 SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI DURING INGRESS AND TOTALITY, * A. McKellar, K. O. Wright, and J. D. Francis Dominion Astrophysical Observatory Victoria, B.C. In a previous paper, 1 a description was given of the spectrum of VV Cephei as observed with high dispersion in April and May, 1956, and numerous references to previous work were listed. Total eclipse of the Be-type secondary by the supergiant M-type primary of this interesting eclipsing binary had been predicted to occur in August. The present note discusses some features of the high-dispersion (3.4 Â/mm) Victoria plates obtained from June 1956 to June 1957, with particular reference to the spectrographic evidence for onset of total eclipse. TIME OF ECLIPSE From the data on VV Cephei given by S. Gaposchkin, 2 the predicted times of first, second, third, and fourth contacts of the current eclipse were, respectively, August 9 and August 29, 1956, and November 12 and December 2, Spectrograms of VV Cephei were obtained at approximately weekly intervals from June to September, Reproductions of two regions of these series of spectrograms, kk and Xk , are shown in Plates I and II. From these reproductions and the intensity tracings of Figure 2, it is apparent that the spectrum changed quite markedly during July and that from August onward the changes were few and minor. In particular, the Ti ii lines at k 3759 and k 3761, each double through most* of July, showed a change wherein the redward absorption components disappeared and the regions between the components of the double lines emerged definitely as emission lines. The wavelength region at k 3900 is also worthy of comment. Here, until * Contributions from the Dominion Astrophysical Observatory, No. 58. Published by permission of the Deputy Minister, Department of Mines and Technical Surveys, Ottawa, Canada. A summary of this paper was presented at the June 1957 meeting of the Astronomical Society of the Pacific at Flagstaff, Arizona. 442

2 SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI 443 well on in July, the Ti n line (also double) at À was outstanding, but late that month faded out and was replaced by the nearby line of Ti i. This growth in intensity of the lines of neutral metals such as Til through June and July is clearly seen in Plate I. There is a decided difference between the spectrograms of July 7 and July 28, while those of July 15 and 21 are transitional (see Í.3901 and À 3913 on Plate I, X 3759 and X 3761 on Plate II). From July 28 onward, no marked changes are seen. This suggests that total eclipse (second contact) occurred between July 21 and 28, and that the gradual changes in spectrum that took place in the several preceding weeks marked the occurrence of partial eclipse, the interval between first and second contacts. This conclusion is corroborated by radial-velocity measurements. The upper section of Figure 1 shows the radial velocities found from lines of Fe i. The chromospheric lines during April and May, 1956, as previously reported, were double. 1 Thereafter, to July 20, while single, they were displaced by about 5 km/sec above the orbital radial-velocity curve given by Goedicke. 3 Between July 21 and July 28, however, there occurred a break, and from the latter date onward the points closely parallel the calculated radial-velocity curve. Here again the evidence points to totality occurring between July 21 and 28. In Bulletin No. 5 on W Cephei, dated November 7, 1956, F. Bradshaw Wood has reported on photoelectric observations made by G. Larsson-Leander. These observations apparently fix first contact within a few days of June 29 and second contact about July 29. Our spectrographic observations are in excellent accord with the photoelectric results and corroborate the date of second contact as July 29 or possibly a day or two earlier. The above date is one month before the predicted date of August 29. We may therefore expect the emergence from totality (third contact) to occur about mid-october, This conclusion rests on the assumption that any possible pulsation of the M-type supergiant does not appreciably change the diameter of the star or the regularity of occurrence of eclipse. THE CHROMOSPHERIC SPECTRUM BEFORE TOTALITY As previously mentioned, many of the stronger chromospheric lines were double during April and May, During

3 444 A. McKELLAR, K. O. WRIGHT, AND J. D. FRANCIS June, nearly all the lines became single, although those of Tin (ÀÀ3685, 3759, 3761, 3901, and 3913) still remained double. These points are illustrated in Plates I and II and also in Figures 1 and 2. In the earlier article, two possible explanations of the duplicity of the chromospheric lines were advanced. The one envisaged two 1956 APR. MAY JUNE JULY AUG. SER J.D Fig. 1. Radial-velocity measurements of VV Cephei. The upper section shows the data for Fe i lines ; note the change in velocity between July 20 and July 28. The lower section shows absorption and emission velocities for Ti n and Ca n lines. The dashed line represents the radial velocity calculated from Goedicke s orbit.

4 SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI APR , SEPT- 1957, MAY APR 13 JUNE 23-JULY 7 JULY 27-AUG. 8 AUG. 13- SEP 9 JULY 7-15 JULY JULY AUG.I3-SEP9 Fig. 2. Intensity tracings of the spectrum of VV Cephei, in the regions XX , , and The first section shows changes in the Ca i line X 4227 before and after totality, and also changes in the Fe ii line X The middle section shows how the emission at Ht, gradually decreased until August 8, but remained constant after August 13. The third section shows changes in the Ti n lines X 3759 and X 3761 during the eclipse. separate absorbing clouds in the atmosphere of the M-type star having radial velocities differing by over 20 km/sec and giving rise to the double lines. The other explanation considered the duplicity as arising from an emission line centered in a wide absorption line, similar to that described by Merrill and Wilson for

5 446 A. McKELLAR, K. O. WRIGHT, AND J. D. FRANCIS RW Cephei. 4 We had hoped that the question would be settled definitely by the spectra obtained during total eclipse, since, if the latter explanation were correct, one might expect to find emission lines in the spectrum during totality. Such emission, however, might be variable and, to judge by the disappearance of the duplicity of most lines during June, the emission might not have been expected by the time of total eclipse in late July. Since, as noted below, emission was found only in Fe n, Cr n, Ti n and in a few other isolated cases during totality, the emission hypothesis for the line doubling has not been fully and explicitly supported by observation. In the case of Ti n, particularly for À 3685, À 3759, and À 3761, where the doubling was much wider than for the Fei and other lines, there has developed definite evidence for an emission line (see Plate II and Figure 2). Such emission must have affected, at least to some extent, the positions of the absorption components of these Tin lines, and therefore plays a role in the doubling of some of the lines. Nevertheless, it is not considered that the explanation for the doubling is the same as for RW Cephei even for these Ti n lines. As noted later, there is evidence that the violet component of these three Tin lines arises in an expanding shell far out in the chromosphere of the primary star. For the great majority of the chromospheric lines (Fei, Mm, Cr I, etc.) that were double in April and May, 1956, we now consider that the two-cloud explanation is the more likely one. THE SPECTRUM IN TOTALITY Ultraviolet Region. Several grating spectrograms of moderately high dispersion (16Â/mm) have been obtained, which extend into the ultraviolet as far as À The first, on April 14, 1956, shows a great many chromospheric absorption lines on a continuous background dominated by the B-type star. Those obtained from August to November, 1956, and a recent one on June 10, 1957, are all very similar and show the spectrum only of the M-type primary plus any features (emission or absorption) arising from circumstellar or circumsystem gas. A feature of the latter plates is the occurrence in the ÀÀ region of numerous lines of Fe n, Cr n, and probably Mn ii, as emission lines, where essentially no continuous background is photographed.

6 SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI 447 The Fe ii lines (multiplets 1, 6, and 7) are included among those found by Herzberg to be in emission in the ultraviolet spectra of a Herculis and a Scorpii. 5 They give a mean radial velocity of 4-1 km/sec, which agrees with that given by the K-line emission (see Figure 1). Photographic Region. The spectrum is that of a supergiant star, with appreciably broadened lines, as illustrated in Plates I and II. The spectrum was compared with that of p Cephei (M2 la) and the two were found to be quite similar. Forbidden emission lines of Fen at À4244, À4277, etc., and the permitted Fen line at À4233 persist in the spectrum during totality. There is also a narrow emission line at X 3934 of Ca n and, as noted above, emission at À 3685, À 3759, and À 3761 of Ti ii. The behavior of the hydrogen emission is noted in a later paragraph. There seems fairly good evidence of the presence of circumstellar lines, indicating the existence of an expanding shell of material around the M-type star, perhaps similar to that described for a Herculis by Deutsch. 6 At the K line of Ca n, there is visible on sufficiently exposed plates a sharp absorption line giving a radial velocity of about 40 km/sec. There is also some evidence of a sharp violet-shifted core in the strong ultimate lines Caí À4227 (see the tracing in Figure 2) and Cri À4254, À4275, and X The circumstellar lines of VV Cephei indicate a velocity of expansion of about 20 km/sec, compared to the value of 10 km/sec for a Herculis. The comparatively sharp, violet component of the double Tin lines À3685, À3759, and À3761 remains the same before and after second contact (see Plate II) and it also gives a radial velocity of about 40 km/sec. The radial velocities of these lines of Ti ii and of the sharp K line of Can are plotted in the lower half of Figure 1. The excitation potential of these Tin lines is 0.60 electron volts. These lines do not appear to be quite as sharp as the component of the K line of Can. Moreover, since all circumstellar lines so far found have arisen from the ground state of the atom concerned, it is not likely that these particular Ti n lines are of true circumstellar origin. They must, however, arise in the very high chromosphere of the primary star. The similarity

7 448 A. McKELLAR, K. O. WRIGHT, AND J. D. FRANCIS in radial velocity between the Ti n lines and the circumstellar lines illustrates either how far out the Ti n absorption must take place or the similarity of the radial motion in the high chromosphere and circumstellar cloud, or perhaps both. It is difficult to define a border to distinguish lines of stellar and circumstellar origin ; it may be necessary later to distinguish different degrees of circumstellarity. Red Region. Grating spectrograms in the red (7.5 Â/mm) showing Ha were obtained on April 8, August 1, September 1, and November 23, 1956, and April 7, June 3, and July 12, The first plate, obtained before totality began, shows Ha as a strong absorption line flanked on both sides by broad emission wings. The remaining plates, taken during totality, are quite similar to each other and show Ha as a line with emission on each side of the central absorption, and a sharp, negatively-displaced absorption core. The plate of August 1, 1956, shows somewhat broader emission than the later plates but less broad than on April 8 of the same year. This probably verifies our suggestion that the region around the Be star producing hydrogen emission was not completely obscured by the beginning of August. The occurrence of the narrow hydrogen emission borders to the Ha line even at mid-totality suggests a source of hydrogen emission in the system other than that associated with the Be star. INTENSITY PROFILES Intensity profiles were derived for several spectral regions from the high-dispersion plates in the violet and near ultraviolet. As usual for late-type stars in the violet, it was extremely difficult to determine a satisfactory continuum, especially in the present case when there is emission that varies in intensity from time to time. The difficult problem cannot be discussed in detail in the present short communication. It is sufficient to state that every effort was made to determine the best continuum from peaks on the tracings between and À 4500 and, particularly, to maintain consistently the same continuum from plate to plate. Profiles for three regions are presented in Figure 2. The upper section of Figure 2 shows the region of the X4227 resonance line of Ca i about two and a half months before totality and during totality. The contrast between the chromospheric

8 SPECTROGRAPHIC OBSERVATIONS OF VV CEPHEI 449 absorption in the former case and the calcium line in the M-type spectrum above is quite evident. Another feature of interest is the circumstellar core of the X 4227 line displaced to shorter wavelengths in the M-type spectrum, as mentioned earlier. The Fen emission at X4233 is quite different in the two tracings. In Harper s earlier Victoria series of plates of VV Cephei, the emission at this wavelength also showed much variation in strength and structure. The middle section of Figure 2 exhibits intensity profiles of the region near HÇ for seven dates, three of the tracings shown being means of very similar profiles. It is apparent that as partial and total eclipse progressed the emission at HÇ decreased. A decided decrease occurred between August 8 and August 13, after which the profile remained fairly constant. The Michigan plates of the previous eclipse revealed the same phenomenon, which was interpreted as the gradual occulting of the outer atmosphere of the Be star where emission takes place. 3 On this interpretation, since partial eclipse lasted about 30 days, June 29 to July 29, 1956, and the emission fell off greatly between August 8 and 13, which is 10 to 15 days after totality, the diameter of the emitting sphere must be between 1.7 and 2.0 times that of the Be star. The third section of Figure 2 shows the interesting region near the Tin lines À3759 and À3761. The effects of the growth of emission at these wavelengths as the eclipse progressed from first to second contact and into totality, shown pictorially in Plate II, are here indicated quantitatively. The short-wavelength absorption component of the Ti n lines survives into the spectrum of total eclipse which has led us to conclude that it is a semi -circumstellar line. The long-wavelength component virtually disappears because it was a conventional chromospheric line. The process was hastened because it was being encroached upon by the emission. 1 K. O. Wright and A. McKellar, Pub. A.S.P., 68, 405, S. Gaposchkin, Harvard Circ., No. 421, V. Goedicke, Pub. Michigan Obs., 8, 1, P. W. Merrill and O. C. Wilson, Ap. /., 123, 392, G. Herzberg, Ap. /., 107, 94, A. J. Deutsch, Ap. /., 123,210,1956.