COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1735

                                  Konkoly Observatory
                                  Budapest
                                  1980 January 30


    PHOTOMETRIC VARIABILITY OF THE SILICON Ap-STAR HD 43819  +)


In a larger photometric program of Ap stars I have included the object HD 43819
(=HR 2258) classified B9IIIp Si, (Cr) by Cowley (1972) in order to look for
possible photometric variability. The measurements were carried out at the ESO
1 m photometric telescope on La Silla (Chile) using a dry ice cooled EMI 6256
photomultiplier and a pulse counting device for the Stromgren and g_1g_2
(Maitzen, 1976) filter measurements. The latter were introduced for measuring
the lambda 5200 flux depression of Ap stars.

HD 43819 was measured against the comparison star HD 42784. The mean colours
of both stars as derived from my absolute photometry are:

              b-y    m_1    c_1   Delta a
HD 43819    -.053   .137   .744   .042
HD 42784    -.036   .099   .574   .000

The Delta a-parameter (which is a measure of the presence of the lambda
5200-depression as defined by Maitzen, 1976) of HD 43819, indicates a rather
pronounced Ap characteristic, while the comparison star shows up as a normal
late B type star using this criterion.

The absolute colours b-y and Delta a are slightly variable for HD 43819 with
amplitudes of about 0.01 mag. They vary in phase: the bluer b-y, the stronger
the Delta a-excess.

Based on the foregoing there seems to be sufficient evidence that the
variations displayed in Fig. 1 are caused by a photometric variability of the
Ap star HD 43819. Moreover, the wavelength dependence of the amplitude is
typical for a Silicon star - it decreases from u (0.04) to y (0.01). No final
determination of the period of the variations can be obtained from this data,
because the observational test for variability of the order of 1 day, i.e.
observations well displaced over the night, could not be performed due to the
northern position of the stars.

                           [FIGURE 1]

   Figure 1. Differential Stromgren photometry HD 43819 minus HD 42784.
   The precise time abscissae values are:
                a= JD 2441721.586         h= JD 2441730.530
                b=         22.588         i=         31.525
                c=         23.612         j=         33.557
                d=         24.551         k=         34.547
                e=         25.558         l=         35.545
                f=         28.534         m=         36.554
                g=         29.541         n=         37.542


The quicklook 14 days period as indicated by the run of the differential
photometry in Fig. 1 would be unusually large for an early type Ap star. It
can be safely ruled out, however, by its published rotational velocity v_e sin
i =55 km/s according to the catalog of Uesugi and Fukuda (1970). Such a fast
rotation would imply an impossible minimum radius of about 15 solar radii for
this star. Thus, the correct period will have to be found among the related
periods: +-1/P=+-n+1/14.

This points rather to 0.93 or 1.077 days than to 0.48 or 0.52 days because the
latter would require a nearly pole-on aspect (i=10deg for a 3 solar radii 
star) restricting strongly the observable variability.

In conclusion, future observers should concentrate their efforts to monitor the
photometric variability of HD 43819 from a more northern observatory throughout
the night in order to find the final period of this Silicon star.


HANS M. MAITZEN
Institut fur Astronomie der Universitat Wien
Turkenschanzstr. 17.
A-1180 Wien

References:

Cowley, A. 1972: Astron.J. 77, 750 [BIBCODE 1972AJ.....77..750C ]
Maitzen, H.M. 1976: Astron. and Astrophys. 51, 223 [BIBCODE 1976A&A....51..223M ]
Uesugi, A., Fukuda, I. 1970: Contr. Kwasan Obs. Univ. Kyoto No. 189 [BIBCODE 1970crvs.book.....U ]

+) Based on measurements collected at the European Southern Oservatory (ESO),
   La Silla - Chile.