COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2704 Konkoly Observatory Budapest 9 April 1985 HU ISSN 0374 - 0676 CONFIRMATION OF THE REGULAR INTRINSIC VARIABILITY OF AU MONOCEROTIS A few years ago the eclipsing binary AU Mon was revealed to be an unusual object in view of its discovered photometric peculiarities (Lorenzi, 1980 a, b). The analysis carried out on the available data allowed to separate the geometrical effects due to the eclipses from the intrinsic variation. Here new normalized photoelectric observations of the system are presented. Their combination with previous data confirm the existence of the periodic intrinsic variation (P=411d) previously found. This confirmation is particularly meaningful in the context of recent IUE spectral analyses of AU Mon (Sahade and Ferrer, 1982, Peters and Polidan, 1984). Such spectral analyses may offer an explanation to the aforementioned intrinsic variation. During the period January-March 1983 and March 1984 new photoelectric observations in V light of AU Mon were obtained and combined in 82 normal points. The resulting normalized observations are listed in Table I a, b. They refer to the same comparison star and to the same technique which follow from the previous work (Lorenzi, 1980a,b). In Figure 1 these data show a rather smooth eclipsing light curve. In fact just at the end of January 83 and in March 84 the brightness minimum of the intrinsic variation occurred, being the cycle about 411 days long. Referring to the previous solution (Lorenzi, 1980b), the new normal points have also been transformed by the formula DeltamV(phi2)=DeltaVobs(phi1,phi2)+alpha(phi1) (1). phi1 and phi2 are the phases of the eclipsing and intrinsic variation, respectively, according to the ephemerides 2442801.3752 + 11.1130371 E, for the former, and 2443105 + 411 E, for the latter. The function alpha=alpha(phi1) represents the available mean light curve of the eclipsing variation, with its sign changed. Instead the expression (1) is Table Ia NR NP Delta phi1 J.D. phi1 phi2 DeltaV DeltaV alpha(phi1) DeltaV+alpha 60 13 .003 2445343.4114 .7436 .446 -.657 .002 +.108 -.55 61 11 .003 45346.3339 .0066 .453 +.100 .007 -.657 -.56 61 11 .002 .4262 .0149 .454 +.011 .003 -.518 -.51 62 9 .001 45351.4059 .4630 .466 -.622 .001 +.015 -.61 62 8 .001 .4202 .4643 .466 -.619 .003 +.015 -.60 63 19 .003 45353.3960 .6421 .471 -.646 .006 64 15 .001 45354.4253 .7345 .473 -.650 .002 +.108 -.54 65 13 .002 45355.3607 .8199 .475 -.652 .001 +.084 -.57 66 15 .001 45356.3518 .9089 .478 -.589 .001 +.040 -.55 67 12 .002 45357.2635 .9901 .480 +.081 .005 -.605 -.52 67 12 .001 .2823 .9918 .480 +.114 .005 -.635 -.52 67 11 .002 .3028 .9936 .480 +.128 .003 -.660 -.53 67 12 .001 .3205 .9952 .480 +.144 .003 -.670 -.53 67 11 .001 .3349 .9965 .480 +.143 .003 -.678 -.54 67 11 .001 .3471 .9976 .480 +.150 .003 -.685 -.54 67 11 .001 .3600 .9988 .480 +.151 .003 -.690 -.54 67 12 .002 .3739 .0000 .480 +.148 .002 -.692 -.54 67 11 .001 .3927 .0017 .480 +.155 .003 -.688 -.53 67 11 .001 .4045 .0028 .480 +.143 .003 -.683 -.54 67 11 .001 .4161 .0038 .480 +.132 .003 -.675 -.54 67 12 .001 .4287 .0049 .480 +.128 .003 -.670 -.54 67 11 .001 .4403 .0060 .480 +.118 .002 -.660 -.54 67 12 .001 .4565 .0074 .480 +.110 .002 -.644 -.53 67 11 .001 .4712 .0088 .480 +.098 .002 -.625 -.53 67 12 .001 .4851 .0100 .480 +.080 .002 -.605 -.53 67 11 .002 .5014 .0115 .481 +.067 .004 -.580 -.51 67 12 .002 .5234 .0135 .481 +.033 .003 -.545 -.51 67 11 .001 .5410 .0151 .481 +.012 .003 -.512 -.50 68 17 .002 45366.3099 .8041 .502 -.649 .002 +.090 -.56 69 15 .001 45367.3507 .8978 .505 -.618 .003 +.044 -.57 70 9 .001 45368.2710 .9806 .507 -.053 .004 -.445 -.50 70 9 .001 .2924 .9825 .507 -.025 .003 -.470 -.50 70 9 .001 .3144 .9845 .507 +.026 .005 -.505 -.48 70 9 .001 .3352 .9864 .507 +.059 .003 -.543 -.48 70 9 .001 .3589 .9885 .507 +.082 .003 -.580 -.50 70 9 .001 .3790 .9903 .507 +.107 .001 -.005 -.50 70 9 .001 .4041 .9926 .507 +.124 .003 -.648 -.52 70 9 .001 .4284 .9967 .507 +.137 .001 -.670 -.53 70 8 .001 .4385 .9957 .507 +.156 .002 -.675 -.52 70 9 .001 .4647 .9980 .507 +.184 .003 -.685 -.50 70 7 .001 .4766 .9991 .507 +.180 .002 -.690 -.51 Table Ib NR Np Deltaphi1 J.D. phi2 phi2 DeltaV sDeltaV alpha(phi1) DeltaV+alpha 70 9 .001 45368.4870 .0000 .507 +.183 .003 -.692 -.51 70 7 .001 .4991 .0011 .507 +.176 .003 -.690 -.51 70 7 .001 .5064 .0018 .507 +.188 .002 -.689 -.50 70 7 .001 .5137 .0024 .507 +.186 .003 -.682 -.50 71 15 .001 45369.3508 .0777 .509 -.585 .002 +.030 -.56 72 15 .001 45370.3481 .1675 .512 -.616 .003 +.082 -.53 73 9 .001 45377.3721 .7995 .529 -.640 .003 +.090 -.55 74 11 .001 45384.3068 .4236 .546 -.640 .002 +.015 -.62 75 10 .001 45385.2738 .5106 .548 -.587 .002 +.015 -.57 75 9 .001 .2816 .5115 .548 -.587 .001 +.015 -.57 75 10 .001 .2911 .5121 .548 -.587 .001 +.015 -.57 75 9 .001 .3613 .5184 .548 -.585 .002 +.015 -.57 75 10 .001 .3755 .5197 .548 -.590 .003 +.015 -.58 75 9 .001 .4429 .5258 .549 -.583 .002 +.015 -.57 75 10 .001 .4562 .5270 .549 -.588 .002 +.015 -.57 75 10 .001 .4697 .5282 .549 -.586 .002 +.015 -.57 76 11 .001 45396.2881 .5017 .575 -.587 .002 +.015 -.57 76 12 .001 .3032 .5030 .575 -.585 .002 +.015 -.57 Table Ib (cont.) NR Np Deltaphi1 J.D. phi1 phi2 DeltaV sDeltaV alpha(phi1) DeltaV+alpha 77 9 .001 45597.3484 .5971 .577 -.609 .007 77 7 .000 .3876 .6006 .575 -.626 .000 78 10 .001 45001.2917 .9519 .587 -.578 .002 -.007 -.59 78 10 .001 .3032 .9530 .587 -.578 .002 -.010 -.59 78 9 .001 .5213 .9546 .537 -.582 .001 -.022 -.60 78 9 .001 .3507 .9572 .597 -.542 .003 -.038 -.58 78 10 .001 .3648 .9585 .587 -.536 .001 -.046 -.58 78 10 .001 .3783 .9597 .587 -.524 .003 -.066 -.59 78 10 .001 .3897 .9608 .597 -.510 .004 -.030 -.59 73 10 .001 .4011 .9618 .597 -.488 .003 -.100 -.59 78 10 .001 .4118 .9627 .557 -.468 .002 -.117 -.59 78 10 .001 .4245 .9639 .587 -.448 .004 -.135 -.58 79 11 .001 45403.3170 .1342 .592 -.608 .003 +.065 -.54 80 9 .001 45766.3845 .8046 .475 -.643 .005 +.090 -.55 80 8 .001 .3982 .8058 .475 -.648 .003 +.090 -.56 81 13 .002 45778.2968 .8765 .504 -.629 .004 +.060 -.57 81 12 .002 .3189 .8795 .504 -.647 .013 +.060 -.59 82 11 .002 45789.3558 .8698 .531 -.639 .003 +.064 -.57 82 10 .001 .3531 .8714 .531 -.640 .003 +.064 -.58 83 9 .001 45790.3141 .9579 .534 -.522 .002 -.045 -.57 83 10 .002 .3302 .9593 .534 -.509 .006 -.055 -.56 83 9 .001 .3473 .9609 .534 -.473 .002 -.084 -.56 83 10 .001 .3609 .9621 .534 -.458 .004 -.100 -.56 [FIGURE 1] Figure 1 : Plot versus phi1 on new V normalized observations of AU Mon, carried out during January-February 1983 and March 1984, just in the period of the intrinsic variation cycle, around phi2=0.5 [FIGURE 2] Figure 2 : The light curve of the intrinsic variation by the old observations ([dot] 1976-1979) and by the new ones ([triangle] 1983-1984). The inserted normal points follow from the formula Delta MV (phi2) = Delta Vobs + alpha (phi1) representing the calculated mean light curve of the intrinsic variation. The new Delta m's, which follow from (1), appear to fit very well the above mentioned intrinsic light curve (see Figure 2), and this confirms strongly the regular behaviour of the intrinsic variability of the system. To emphasize this fact, we wish to remember that the light curve of Figure 2 results from the overlap of 7 cycles, more precisely a time interval 3000 days long. The physical scenario of such a binary system was recently enriched by extended spectral investigations (Sahade and Ferrer, 1982 - Peters and Polidan, 1984). From the photometric point of view, the author attempted an average photometric solution (Lorenzi, 1982a) and, in order to obtain more faithful and complete results, he suggested to work on a three-dimensional photometric representation of the involved light changes (Lorenzi, 1982b). In particular Peters and Polidan interpret their IUE observations in terms of a "high temperature accretion region" (HTAR) around the primary of AU Mon (B5 + F-G), as due to the existence of nonthermal sources of energy. Such HTAR shows to fade over a time scale of a few orbital cycles, while it seems to be present only when AU Mon is faint, that is during the intrinsic brightness decrease of the system. Possibly a change in the radius and / or stellar effective temperature induced by mass accretion is responsible. Alternatively, according to Peters and Polidan, an increase in the mass transfer rate could obscure more of the star and reduce the observed flux. Now the photoelectric observations presented in this paper, confirming the existence of the periodic intrinsic variation in AU Mon (P=411d), strengthen the suggestion that a positive correlation may be found between HTAR and the long brightness cycle. L. LORENZI Osservatorio Astronomico di Torino 10025 Pino Torinese Italy References: Lorenzi, L.: 1980a, Astron.Astrophys.Suppl.Ser. 40, 271 [BIBCODE 1980A&AS...40..271L ] Lorenzi, L.: 1980b, Astron.Astrophys. 85, 342 [BIBCODE 1980A&A....85..342L ] Lorenzi, L.: 1982a, Acta Astron. Vol. 32, No. 3-4 [BIBCODE 1982AcA....32..431L ] Lorenzi, L.: 1982b, Hvar Obs. Bull. 6, 1, 69 [BIBCODE 1982HvaOB...6...69L ] Peters, G.J., and Polidan, R.S.: 1984, Ap.J. 283, 745 [BIBCODE 1984ApJ...283..745P ] Sahade, J., and Ferrer, O.E.: 1982, Pub.A.S.P. 94, 113 [BIBCODE 1982PASP...94..113S ]