The diameter of Betelgeuse (α Orionis) has been measured at a wavelength of 11.15 μm using the Infrared Spatial Interferometer over the past 15 years. During this 1993-2009 time period the star's size has decreased systematically by 15%

Hale, D. D. S.

Townes, C. H.

Walp, B.

Wishnow, E. H.

Caltech Authors - Main

The Astrophysical Journal, 697:L127–L128, 2009 June 1 doi:10.1088/0004-637X/697/2/L127
C© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
A SYSTEMATIC CHANGE WITH TIME IN THE SIZE OF BETELGEUSE
C. H. Townes1, E. H. Wishnow1, D. D. S. Hale2, and B. Walp1
1 Space Sciences Laboratory and Department of Physics, University of California, Berkeley, CA 94720, USA; cht@ssl.berkeley.edu, wishnow@ssl.berkeley.edu,
bernie@isi.mtwilson.edu
2 Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA; dhale@astro.caltech.edu
Received 2009 March 9; accepted 2009 April 20; published 2009 May 11
ABSTRACT
The diameter of Betelgeuse (α Orionis) has been measured at a wavelength of 11.15 μm using the Infrared Spatial
Interferometer over the past 15 years. During this 1993–2009 time period the star’s size has decreased systematically
by 15%.
Key words: stars: general – stars: individual (Betelgeuse) – techniques: interferometric
1. INTRODUCTION
Betelgeuse (α Orionis) was the first star to be measured in
size. Michelson & Pease (1921) reported a diameter of 0.′′047
at optical wavelengths, but in addition to a measurement uncer-
tainty of 10%, they estimated that its actual size was probably
10%–15% larger because of limb darkening. Since then it has
been measured from time to time by a number of interferometers
at various wavelengths, giving values that vary substantially with
wavelength. It has now been measured systematically over a pe-
riod of about 15 years with the Infrared Spatial Interferometer
(ISI). The ISI uses a narrow frequency range to avoid spectral
lines and mid-infrared (mid-IR) wavelengths which penetrate
the surrounding dust well. The diameters measured by the ISI
are not particularly inconsistent with previous measurements,
are rather accurate, and show clearly that the star has systemat-
ically decreased in size by about 15% over the past 15 years.
2. GENERAL DISCUSSION
Since all of the interferometric measurements made, except
those of the ISI, have used rather broad bandwidths it is
not surprising that there have been large variations in the
resulting apparent size. What most interferometers measure
is partly emission from spectral lines of gas surrounding the
star and partly absorption by spectral lines, as well as effects
of dust around the star, rather than the stellar continuum
directly. Variations in apparent size with wavelength have been
previously tabulated (Weiner et al. 2003; Perrin et al. 2004).
The variations in apparent diameters from visible wavelengths
to 3 μm are as large as about 50%.
The ISI operates at wavelengths near 11 μm, using hetero-
dyne detection with CO2 lasers as local oscillators. This con-
verts the infrared signal into a microwave frequency signal with
a bandwidth of about 5.4 × 109 Hz, or 0.18 cm−1. The sys-
tem is tunable so that wavelengths can be chosen to avoid any
spectral lines. Measurements reported here were thus made in a
narrow frequency range which is free of spectral lines, impor-
tantly including any substantial ones due to water vapor. The
radiation from the surrounding dust is easily detected, but at 11
μm wavelength the dust is also rather well penetrated. The dis-
tribution of dust and the stellar characteristics are hence easily
and clearly separated. Since this wavelength is not scattered as
much as shorter ones, the ISI views the stellar continuum rather
clearly and directly and gives a reliable measure of the real size
of the star, as defined by its effective continuum diameter. Inter-
Figure 1. Diameter of α Ori measured at 11.15 μm by the ISI over 15 years.
Values previously published are: 1994, Bester et al. (1996); 1999, 2000, and
2001, Weiner et al. (2003); 2006, Tatebe et al. (2007). Values from 2007 to 2009
are new measurements. The dashed line is a quadratic fit to the data.
ferometric measurements with the ISI began in 1993 with two
movable telescopes that can provide a wide variety of baselines.
A third telescope was added in 2002 so that stellar diameters
could be measured simultaneously from three directions and
point inversion asymmetries detected by phase closure, using
the phases of three simultaneous interference signals.
In 2006, the ISI detected a major hot spot on α Ori,
producing about 10% of the total flux at 11.15 μm, and
resulting in some asymmetry in the radiation intensity (Tatebe
et al. 2007). This discovery is consistent with theoretical
predictions by Schwarzschild (1975). Without correcting for
a hot spot, the apparent diameters measured by individual
differently oriented baselines differed substantially (Tatebe et al.
2007). But modeling the system as a uniform disk plus a hot spot
on one side of the star gives an effective circular diameter for the
star of 48.4 ± 1.4 mas. Other ISI measurements, between 1994
and 2009, did not indicate any large hot spots or asymmetries,
though these would not have been detected by the ISI before the
third telescope was added in 2002.
3. RESULTS AND CONCLUSIONS
Measurements of α Ori’s diameter over the period 1994–
2009 show a substantial and systematic change over this period,
L127
L128 TOWNES ET AL. Vol. 697
Table 1
ISI Diameter Measurements of Betelgeuse 1993–2009
Date (average) Diameter (mas) Reference
1993 Oct 30 56.0 ± 1.0 1
1994 Aug 8 56.0 ± 1.0 1
1999 Nov 15 54.9 ± 0.3 2
2000 Nov 5 53.4 ± 0.6 2
2000 Nov 28 55.8 ± 0.9 2
2000 Dec 21 54.8 ± 1.0 2
2001 Aug 20 53.4 ± 0.6 2
2001 Sep 30 52.9 ± 0.4 2
2001 Oct 29 53.5 ± 0.6 2
2001 Dec 19 52.7 ± 0.7 2
2006 Nov 29 48.4 ± 1.4 3
2007 Dec 6 50.0 ± 1.0
2008 Feb 3 49.0 ± 1.5
2008 Oct 29 47.0 ± 2.0
2008 Dec 5 47.0 ± 1.0
2009 Jan 17 48.0 ± 1.0
2009 Feb 2 48.0 ± 2.0
Reference. (1) Bester et al. 1996; (2) Weiner et al. 2003;
(3) Tatebe et al. 2007.
as may be seen in Figure 1 and Table 1. Data prior to 2007
have been previously reported and the data since this time are
new. The measurements were made with baselines of 32–59 m,
which allowed adequate resolution of the star. ISI measurements
with shorter baselines, sensitive to spatial frequencies related to
the dust shells, are not included in the figure. Variations in
the luminosity and diameters measured for α Ori have been
previously reported by White (1980), but the data are not good
enough to provide very clear-cut conclusions. Also, the modest
known variations in luminosity are probably not simply related
to size and the visible band photometric record of the AAVSO
does not resemble the size variation of Figure 1. Some of
the complexities of radiation from α Ori, particularly due to
surrounding molecular gas, are illustrated in the recent report
of ultraviolet, IR, and mm studies of the star by Harper et al.
(2009).
ISI measurements over the last 15 years clearly show a
systematic change in the diameter of α Ori. This change may
or may not be periodic; if it is, the period is likely rather long,
perhaps a few decades. One might attribute some of the change
in apparent size to a specific instability connected with the large
spot in 2006, but this would only be a possible conjecture.
It should be valuable to continue accurate measurements of
α Ori’s size and other characteristics in order to understand
the dynamics involved in this striking change, and to have
systematic long-term measurements of similar stars.
We thank W. Fitelson for his efforts in developing and keeping
the ISI operating during these measurements. We also thank
M. Bester, W. C. Danchi, and P. G. Tuthill, former staff members,
the former graduate students, E. A. Lipman, J. D. Monnier,
J. Wiener, and K. Tatebe, and the many undergraduate students
who have helped perform and analyze these measurements.
We acknowledge the use of the AAVSO database. This work
received support from the National Science Foundation, the
Office of Naval Research, and the Gordon and Betty Moore
Foundation.
Facility: ISI
REFERENCES
Bester, M., Danchi, W. C., Hale, D. D. S., Townes, C. H., Degiacomi, C. E.,
Mekarnia, D., & Geballe, T. R. 1996, ApJ, 463, 336
Harper, G. M., Carpenter, K. G., Ryde, N., Smith, N., Brown, J., Brown, A., &
Hinkle, K. H. 2009, Am. Inst. Phys. Conf. Ser., 1094, 868
Michelson, A. A., & Pease, F. G. 1921, PNAS, 7, 143
Perrin, G., Ridgeway, S. T., Coude´ du Foresto, V., Mennesson, B., Traub, W.
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Schwarzschild, M. 1975, ApJ, 195, 137
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A Systematic Change with Time in the Size of Betelgeuse

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A Systematic Change with Time in the Size of Betelgeuse

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