COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3672

                                                Konkoly Observatory
                                                Budapest
                                                11 October 1991

                                                HU ISSN 0374 - 0676


               Further Observations of 9 Aurigae


Krisciunas and Guinan (1990) recently found that the F0 V star 9 Aurigae is
variable, with an amplitude of Delta V ~= 0.06 mag and a suggested period of
36-39 days. There was some evidence of short term variations (< 1 hr), which
would be attributable to pulsations of the star, though it is to be noted that
9 Aur lies to the right of the instability strip (Breger 1979). In this paper
we present results that refute the notion that 9 Aur might be a pulsating star.

The photometry discussed in this paper can be found in IAU file 238 of
unpublished photometry of variable stars (Breger et al. 1990). This amounts
to 182 V-band points by Skillman, obtained on 10 nights with his 32-cm
automatic photoelectric telescope (APT) (see Skillman 1981), and 143 V-band and
142 B-band points by Guinan, obtained on 4 nights with a 25-cm APT at Mt.
Hopkins, Arizona. The comparison star was BS 1561. Data previously made
available in IAU file 218 show, using BS 1568 as a check star on 52 nights,
that BS 1561 exhibits no suspected variability.

Figs. 1 and 2 demonstrate that 9 Aur can vary linearly by |Delta V/dt| ~=0.01
mag/hour over the course of a few hours. (The differential brightness of 9 Aur
vs. BS 1561 showed no significant temporal trend on 9 of the 14 nights on
which the two stars were monitored by Guinan and Skillman.) Frequency analysis
was carried out using the Lomb-Scargle algorithm (see Press and Teukolsky 1988)
to produce power spectra. Analysis of individual nights' data yields no
significant short term (< 1 hr) variations. Combining several nights' data
gives peaks approximately equal to 0.73, 2, 3, and 4 cycles per day, which
would most likely be artifacts of aliasing (demonstrating only that the data
were obtained at about the same time each night).


                                   [FIGURE 1]

 Fig. 1 - V-band differential photometry of 9 Aurigae by Guinan.
          The data were obtained over 96 minutes.

                                   [FIGURE 2]

 Fig. 2 - V-band differential photometry of 9 Aurigae by Skillman.
          The data were obtained over 5 hours and 35 minutes.

                                   [FIGURE 3]

 Fig. 3 - Radial velocity measurements of 9 Aurigae, using BS 7560
          as the reference star. Data by Abt.


File 238 also contains 28 radial velocities of 9 Aur vs. BS 7560, obtained by
Abt on the night of 29/30 Nov 1990 over a two hour time span. Abt's radial
velocities were obtained at Kitt Peak with a coude spectrograph giving a
dispersion of 15 angstrom/mm. Fig. 3 shows Abt's radial velocity data. A
linear least-squares fit gives a slope that is non-zero only at the 2.3-sigma
level. The scatter about the mean is only +-0.18 km/sec, which is no more than
one would expect for the equipment and line width. Frequency analysis
indicates no periodicity in the data. Thus on the basis of the photometry and
radial velocity data, there is no evidence that 9 Aur is a pulsating star with
a period comparable (tens of minutes) to those of stars with similar mass and
size.

Abt (unpublished) has also obtained a new value of v sin i = 20 km/sec for 9
Aur. (The non-definitive value given in the Bright Star Catalogue is 14
km/sec.) Given an assumed size of 1.35 R_Sun for an F0 V star, this means that
the maximum rotational period of 9 Aur (setting i = 90deg) is about 3.4 days.
We note that a reanalysis of Guinan's data given in IAU file 218, using the
Lomb-Scargle algorithm, gives a power spectrum whose highest peak corresponds
to P = 2.888 days for the V-band and B-band data. Could this be the true
rotational period of 9 Aur?  If the variability of the star is other than
purely sinusoidal and is somehow related to such a rotational period, a
definitive light curve could only be obtained by means of coordinated
photometry carried out with a number of telescopes distributed around the
Earth. Even then we need to explain how such variability results on a star
with no reported spectroscopic anomalies (e.g. evidence of magnetic fields).


K. KRISCIUNAS
Joint Astronomy Centre
665 Komohana Street
Hilo, Hawaii 96720 USA

D. R. SKILLMAN
9517 Washington Avenue
Laurel, Maryland 20723 USA

E. F. GUINAN
Villanova University
Department of Astronomy
Villanova, PA 19085

H. A. ABT
National Optical Astronomy
Observatories
P. O. Box 26732
Tucson, Arizona 85726 USA

References:

Breger, M. 1979, Publ. Astr. Soc. Pacific 91, 5. [BIBCODE 1979PASP...91....5B ]
Breger, M., Jaschek, C., and Dubois, P. 1990, IBVS 3422.
Krisciunas, K. and Guinan, E. 1990, IBVS 3511.
Press, W. H. and Teukolsky, S. A. 1988, Computers in Physics 
  2, No. 6 (Nov/Dec), 77. 
Skillman, D. R. 1981, Sky and Telescope 61, 71. [BIBCODE 1981S&T....61R..71S ]


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