Publications of the Astronomical Society of the Pacific 97:138-150, February 1985

Publications of the Astronomical Society of the Pacific 97:138-150, February 1985 IUE AND OPTICAL SPECTRAL SCANS OF U SAGITTAL: AN ANALYSIS AND COMPARISON WITH U CEPHEI JAN J. DOBIAS AND MIREK J. PLAVEC0 Department of Astronomy, University of California, Los Angeles, California 90024 Received 1984 October 2 We have determined the effective temperatures and surface gravities of the components of U Sge, using a combination of optical spectral scans and IUE spectra. The primary component is found to be a B7.5 V star with Te{{ = 12,250 (± 250) Κ and logg = 3.9 ± 0.1. The secondary component is of spectral type G4 III-IV. The best value for distance is 295 ± 20 pc, and the color excess is E{B— V) = 0^06. Our determination of the slope of the radial curve of the secondary star at primary eclipse confirms the previous results (mainly by Tomkin) that the mass ratio is very close to 3. Com- bination with photometric solutions enables us to derive reliable system parameters, given in Table VII. One high-dispersion IUE spectrum has been studied in greater detail. We have been able to fit selected lines of Fell, Sill, and Sim assuming solar abundances. The resonance doublet of C π at 1336 Á could not be fitted and we are in- vestigating several possible explanations. The photospheric lines of the hotter component are broadened by rotation; we -1 find ürot 100 km s . We discovered emission lines of the W Serpentis type in an IUE spectrum taken in the total primary eclipse. Although the observed fluxes of these lines are very low, the powers emitted are half as strong as those seen in U Cep. This is rather surprising, since we did not find any other evidence of circumstellar matter or mass transfer activity. Key words: stars: abundance—stars: circumstellar shells—stars: eclipsing binaries—stars: emission-line—stars: individual (U Sge)—ultraviolet: spectra I. Importance of the System the Varo image tube attached to the coudé spectrograph The eclipsing binary U Sagittae (HD 181182 = of Lick Observatory. We will attempt to present a com- HR 7326 = BD+1903975) is the brightest semi- prehensive picture of the system and then discuss its re- detached eclipsing binary of the Algol type that displays lation to more active interacting binaries. total primary eclipses. It is also one of the simplest sys- II. A Review of Previous Work tems of this type, since the photometric and spectro- scopic perturbations are smaller than in most Algol vari- Although U Sge is rather free of circumstellar mate- ables. There appears to be so little circumstellar material rial, there does not exist yet a clearly defined picture of present in the system that it may be considered as nearly the system, and individual solutions differ. The first pho- dormant, i.e., probably near the end of the mass-transfer toelectric light curves were obtained in the blue and red phase. This gives an excellent opportunity to derive the colors by Irwin, but have never been published. They properties of the component stars with greater reliability were used by Kopal and Shapley (1956) to obtain the than in other Algol variables. On the other hand, an in- first good set of photometric elements. Later, the star teresting problem emerges when we compare U Sge was observed by Cester and Pucillo (1972) on the UBV with U Cephei. While the two systems are rather similar system, and by McNamara and Feltz (1976) on the in a number of characteristics, U Cep is much more ac- Strömgren four-color system. Both groups of observers tive than U Sge. Why? What parameter, or set of param- published solutions based on their observations. More- eters, causes this dichotomy? over, the same observations were subsequently redis- But perhaps the most important aspect of U Sge is cussed by Cester et al. (1977), Tsouroplis (1977), and Al- that it offers the opportunity to study the chemical com- Naimiy (1978). position of its atmosphere and to see if we can find evi- There exists a general consensus that the hotter star, of dence for abundance anomalies related to the past evolu- about the spectral type B7 V, has a fractional radius be- tion of the system, during which a large-scale mass tween or near the values 0.21-0.23, and that the frac- transfer should have occurred, if our current theories of tional radius of the cooler, G-type subgiant, lies between close-binary-star evolution are correct. or near the values 0.28-0.30. The system is totally eclips- For all these reasons, we have observed U Sge with ing and the inclination must be very near 90°; actually, the International Ultraviolet Explorer (IUE) satellite, some of the above solutions simply accept that value. with the ITS scanner of the Lick Observatory, and with The value most deviating from 90° is found in the Cester and Pucillo solution, and is 89? 3. Since the elements obtained by Cester et al. and by McNamara and Feltz "IUE Guest Investigator. agree very closely, we will adopt r(B) = 0.225 for the 138 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System U SAGITTAE 139 very nearly spherical primary component, and r(G) = metric investigations of the system by McNamara and 0.295 for the photometric radius of the secondary star. Feltz (1976) seemed to support Naftilan's result. Their The elements in both solutions agree closely except for value of the m1 index, if interpreted in the usual way, the U and u light curves. These near-ultraviolet light suggested that the secondary star in U Sge must be met- curves require systematically larger values of both frac- al-poor. Quantitatively, their result, [Fe/H] = —0.63, tional radii, and most likely indicate the presence of ad- agrees very well with Naftilan's. However, the much ditional weak radiation at the Balmer limit. The small more detailed study by Parthasarathy, Lambert, and uncertainty in inclination is not important for our work. Tomkin (1983) does not corroborate these results. They In spite of total eclipses and relatively high apparent applied spectrum synthesis to Reticon spectra obtained brightness of U Sge, the absolute parameters of the sys- near 630 nm with a much higher dispersion, 0.75 Â tem were poorly known until very recently. The primary mm-1, and found the metal abundances normal within component of U Sge is so much brighter than the sec- the observational errors; [Fe/H] = 0.0 ± 0.3. ondary star that for many years the only information on The important result of the work of Parthasarathy, the radial-velocity amplitude of the secondary star was Lambert, and Tomkin is the detection of carbon defi- the determination of the slope of this radial-velocity ciency and nitrogen overabundance in the secondary star curve at the primary eclipse by Joy (1930). Joy actually in U Sge. They applied spectrum synthesis to their Digi- gave the ratio of the slopes, expressed in terms of the con spectra of the CH lines at 4300 Â and CN lines at mass ratio: ^(B)/sJl)?(G) = 3.3. From a complete orbital 4215 Â and 3888 Â. They found that the secondary star solution for the primary star, he obtained K(B) = 67.9 is deficient in carbon, [C/Fe] = —0.5, and over- km s_1. This latter value was subsequently only slightly abundant in nitrogen, [N/Fe] = +0.5. This result not revised by McNamara (1951a) to 69.7 km s-1 (± 0.7 km only does not contradict the theory of the evolution of s-1). Thus, the implied value of K(G) would be about the Algol variables by mass transfer (Plavec 1968; Ziol- 230 km s-1. Plavec (1967a) rediscussed Joy's slope deter- kowski 1969), but actually strongly supports it. mination and concluded that the mass ratio is closer to While there is every good reason to believe that the 3.0, thereby revising K(G) down to 211 km s_1. Joy's present system of U Sge is a product of mass transfer, the spectrograms were taken with a reciprocal dispersion of present level of such activity in it is very low. In spite of only 75 Â mm-1; nevertheless, Plavec's rediscussion many attempts, emission in the optical spectrum was closely agrees with our new observations presented in seen only once: McNamara (1951b) obtained two spec- section VI, as well as with the recent accurate work of trograms on 1950 August 26, which show violet-dis- Tomkin (1979). placed emission components in the Balmer lines, in Mg π In an important development, Tomkin succeeded in 4481 Â, and Ca π Κ 3934 Â. McNamara (1951α) also no- obtaining a complete radial-velocity curve of the sec- ticed the typical asymmetries of the hydrogen absorption ondary star, using a Reticon. Tomkin measured the Na D lines, which are, however, much weaker than in U Ceph- lines, the infrared Ca π triplet, and the M g ι line at ei. Students of the light curves of U Sagittae point out 8806 Â. He derived K(G) = 209 km s_1 which, when that the secondary eclipse is anomalously deep, probably combined with McNamara's value for X(B), gives a mass because circumstellar material surrounding the primary ratio of almost exactly 3.0. Since our new slope determi- star causes additional loss of light of the secondary (Ces- nation, reported in section VI, corroborates this ratio, we ter and Puccillo 1972). The depth of the primary eclipse are confident that at long last, the absolute parameters is also variable, and McNamara and Feltz (1976) explic- of U Sge are known with an accuracy that is as good as is itly suggest the presence of a weak Balmer emisison.

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