2 Current knowledge about EN Lac

EN Lac (HD 216916, 16 Lac, HR 8725, BD+40$^\circ$4949, SAO 52512, HIP 113281, ADS 16381A) is a well-known representative of the class of $\beta$ Cep stars and a member of the visual binary ADS 16381. Its spectral type is B2 IV. RV variations of EN Lac were discovered by Lee (1910). Struve & Bobrovnikoff (1925) demonstrated that the object is a single-lined spectroscopic binary for which they derived a period of 12.$^{\rm d}$3106 and the first set of orbital elements. Daniel obtained series of Allegheny spectra between 1912 and 1916 and considered two possible periods, either 12.$^{\rm d}$093 or its one-day alias of 1.$^{\rm d}$09 but his observations and conclusions have only been published by Beardsley (1969).

Only much later Walker (1951) found that EN Lac is a new $\beta$ Cep variable with light variations up to 0.$^{\rm d}$11 and RV variations of 40 kms^-1 (including orbital motion). For the short-term contributions he derived a preliminary period of 0.$^{\rm d}$16900. Walker (1952), Struve et al. (1952) and Miczaika (1952) published extended series of photoelectric and spectral observations and concluded that two short periods, 0.$^{\rm d}$169165 and 0.$^{\rm d}$170845, are excited which produce a beat period of about 17.$^{\rm d}$2. Moreover, Struve et al. (1952) improved the value of the orbital period to 12.$^{\rm d}$097. Fitch (1969) re-analyzed available photometric and RV observations and found that the rapid changes can be better described by three sinusoidal components with periods of 0.$^{\rm d}$16917, 0.$^{\rm d}$17086 and 0.$^{\rm d}$18182.

The interest in EN Lac was revived after Jerzykiewicz et al. (1978) discovered that EN Lac is an eclipsing system. Jarzebowski et al. (1979) analyzed new series of photoelectric observations and concluded that the previous authors identified one-year aliases of the two longer periods. According to them, the correct periods are 0.$^{\rm d}$169170, 0.$^{\rm d}$170793 and 0.$^{\rm d}$181712. Garrido et al. (1983) obtained new photometry, including one complete eclipse, and using also the new orbital elements derived by LeContel et al. (1983), they estimated the basic physical elements of the system. They also concluded that the amplitudes of the two shorter oscillation periods were secularly decreasing. Balona (1985) attempted to explain the amplitude variation by the precession in the binary system. Jerzykiewicz & Pigulski (1996) concluded that the amplitudes of the first two short periods vary in a cycle of about 50 years while the third one is in fact a close doublet with a beat period of 676d. For convenience, the exact values of the periods so far reported by various authors are summarized in Table 1.

 

 

Table 1: Short periods in days of EN Lac reported by various authors from photometry and from spectroscopy, as marked in columns $\triangle m$ and RV, respectively. The errors of the last digits of the periods, given by the respective authors, are in parentheses following the periods

Source	P1	P2	P3	P4	$\triangle m$	RV	Notes
A	.16900	-	-	-	*
B	.169165	.170845	-	-	*
C	.16916	.17085	-	-		*
D	.169168	.1707	-	-	*
E	.169166	.170857	.181821+.181885	-	*	*
F	.1691703(15)	.1707932(30)	.1817117(29)	-	*		1965 data
F	.169170(16)	.170770(30)	.181692(24)	-	*		1977 data
G	.169155(11)	.170765(20)	.181688(19)	-	*		1977 data
H	.169168	.170776	.181734	-		*
I	.169168	.170776	.181734	-	*		1977+1979 data
J	.16767	.17053	.18173	-	*	*	1977 data
J	.1691677	-	-	-	*		P variable?
K	.16917	.17079	.18171	6.05, .139005	*
L	.16915	.17077	.18171	-	*
M	-	-	.181732	-	*		P variable
N	-	-	.1816843+.18173312	-	*
N	62 yr	52 yr	676 d	-	*		modulation time scales
O	.1691778	.1707769	.1817331	-	*		misprint in P1?
P	-	-	.1814+.1817	-	*
Q	.1691651	0.1707775	.18173300(2)+.1816843(3)	-	*
Q	71 yr	50 yr	677.9(4)	-	*		modulation time scales

Details on sources of data and instruments used (column "Source") A: Walker (1951), B: Walker (1952), C: Struve et al. (1952), D: Miczaika (1952), E: Fitch (1969), F: Jarzebowski et al. (1979), G: Jarzebowski et al. (1980), H: LeContel et al. (1983), I: Chapellier et al. (1995), J: Sato & Hayasaka (1986), K: Jerzykiewicz (1993), L: Chapellier et al. 1995, M: Sato (1996), N: Jerzykiewicz & Pigulski (1996), O: González Bedolla et al. (1997), P: Sareyan et al. (1997), Q: Jerzykiewicz & Pigulski (1999).

Most accurate pulsation periods and modulation time scales of EN Lac should be that given by Jerzykiewicz & Pigulski (1999). Their values base on photometric data covering the period of 1950 to 1992.

There has been a continuing controversy on whether the binary nature of EN Lac plays an important role for the character of the $\beta$ Cep variations observed. Fitch (1967) and Fitch (1969) argued in favour of the presence of tidal effects in binaries with pulsating components, including EN Lac. For EN Lac, this possibility was refuted by Jarzebowski et al. (1979). Frolov et al. (1980) argued that the principal short periods of many $\beta$ Cep and other pulsating stars in binary systems are nearly integer submultiples of the orbital periods (or rotational periods). They obtained the period ratio of 71.5 for EN Lac, however. Recently, Chapellier et al. (1995) concluded on the basis of their new observations that the amplitude of the longest of the three short periods of light and RV variations varies with the phase of the orbital period (or rotational period of the primary). This finding was, however, challenged by Jerzykiewicz & Pigulski (1996).

Considering all these developments, we felt that a dedicated study of possible effects of the duplicity of the star on the character and time changes of the rapid variations observed is still desirable.