# Direct Ultraviolet Imaging
and Spectroscopy of Betelgeuse

A. K. Dupree and R. P. Stefanik  Harvard-Smithsonian Center for Astrophysics,
60 Garden Street, Cambridge, MA 02138 USA

###### Abstract

Direct images of Betelgeuse were obtained over a span of 4 years with the
Faint Object Camera on the Hubble Space Telescope. These images reveal the
extended ultraviolet continuum emission ($\sim$2 times the optical diameter),
the varying overall ultraviolet flux levels and a pattern of bright surface
continuum features that change in position and appearance over several months
or less. Concurrent photometry and radial velocity measures support the model
of a pulsating star, first discovered in the ultraviolet from IUE. Spatially
resolved HST spectroscopy reveals a larger extention in chromospheric
emissions of Mg II as well as the rotation of the supergiant. Changing
localized subsonic flows occur in the low chromosphere that can cover a
substantial fraction of the stellar disk and may initiate the mass outflow.

††slugcomment: European Astronomical Society Publication Series, 2013,
Eds. P. Kervella, Th. Le Bertre & G. Perrin, in press

##  1 Introduction

Alpha Orionis (Betelgeuse) has been long and well-studied with a variety of
ground and space-based techniques as a prototypical supergiant. Ultraviolet
observations have been particularly useful because they probe the very outer
layers of this star and can pinpoint the onset of outflowing material and
indicate the driving mechanisms behind the mass loss from the star (Dupree
2010). In fact the monitoring of the flux from Betelgeuse shows that its
photometric behavior and its ’spottedness’ differ from the signals of magnetic
activity found in the Sun and active cool stars. The long-lived IUE satellite
clearly demonstrated the presence of periodic fluctuations and a traveling
disturbance in the outer atmosphere (Dupree et al. 1987). And the Hubble Space
Telescope with its Faint Object Camera acquired the first direct image of a
star other than the Sun (Gilliland & Dupree 1996) which in concert with
ground-based photometry and spectroscopy and further ultraviolet imaging
reveals the extent and characteristics of the supergiant’s variability.

Figure 1: Panels showing the V magnitude from the AAVSO photoelectric database
(Henden 2012), the UV continuum and the Mg II k-line flux measured from IUE
spectra, and the radial velocity. In the bottom panel, measurements denoted by
the plus-symbol (+) are taken from Smith et al. (1989); the open diamonds
($\diamond$) represent measures from Oak Ridge Observatory. The radial
velocity measures are generally made from photospheric neutral metal lines. A
discontinuity occurs between the two sets of radial velocity measures; this
discontinuity appears to be real, since no cause has been identified.

##  2 Spatially Unresolved Photometry and Spectroscopy

Over a time span of $\sim$ 16 years, photometry in the V-band and the
ultraviolet continuum ($\lambda\lambda$ 2950–3050Å), the Mg II k-line emission
flux, and the radial velocity are shown in Fig. 1. For $\sim$3 years (JD
6000-7400, mod JD2440000), the chromosphere displayed a brightness fluctuation
with a period of 420 days (Dupree et al. 1987) which later became
substantially weaker, and then disappeared entirely. The appearance of a
period suggests that the global brightness variations do not arise from the
appearance of convection cells on the surface which would not be expected to
be periodic. A period of $\sim$400 d is consistent with models of fundamental
pulsation modes of the star (Lovy et al. 1984, Stothers 2010). From Fig. 1, it
is apparent that the continuum brightenings are correlated with the high
chromospheric (Mg II) brightenings. This is also a clue that Betelgeuse’s
behavior is not the result of magnetically active ’star spots’. Cool stars
with magnetic activity are well-known to show an anti-correlation between
photometric brightenings and chromospheric activity. When classical star spots
are present, they are cooler and photometrically ’dark’, the continuum flux
decreases, and chromospheric emission lines become stronger as a result of
magnetically-associated chromospheric heating. The V-magnitude decreases
during three instances (JD7600, JD9100, JD9800, mod JD2440000) where the UV
continuum measures, observed simultaneously with V-band photometry exhibits a
decreased flux. Additionally, the flux modulation in Betelgeuse is substantial
… about a factor of 2 in the lines and continuum … and such an excursion
surpasses that found in low gravity magnetically active stars such as RS Cvn
binaries.

An indication of the presence of a travelling wave in the atmosphere comes
from measures of the B-magnitude variations and the flux in the chromospheric
lines reported in Dupree et al. (1987). On several occasions during the time
when the 420-day period was evident, the B magnitude became faint and then
recovered while, after a delay of 55 days, the Mg II h-line flux became faint
and subsequently recovered. If a propagating wave caused the decrease in
emission measure, followed by an increase, and this wave travelled at $\sim$2
km s-1, it would cover a reasonable distance of 0.1 R⋆ in 55 days, using the
larger distance of Betelgeuse (191 pc) suggested by Harper et al. (2008).
Observation of the variation of the Mg II h and k lines suggests a lag of
$\sim$70 days beween the h and k line variations. Because the opacity in the
k-line is larger than the h-line, the k-line is formed further out in the
atmosphere and the lag in flux variation between this two lines again is
consistent with a propagating disturbance.

The radial velocity measures might offer additional information. The values
displayed in Fig. 1 (lower panel) suggest a long-term variability ($\sim$13
yr) on which shorter variations ($\sim$400 d) are superposed. In a Cepheid
star, the light maximum is close to but does not always coincide with the
maximum velocity of approach (cf. Robinson & Hoffleit 1932; Bersier 2002). One
CS Mira star, R CMi, has shown light maximum after maximum velocity infall
(Jorissen 2004, and Lion et al. 2013). Detailed study shows the velocity
pattern in Miras is complex and varies with the line diagnostic (Hinkle et al.
1982). Inspection of Fig. 1 shows an inconsistent pattern at many light maxima
in Betelgeuse. For instance, the V-magnitude displays brightenings at JD8600,
JD 10800, JD 11200 (mod JD2440000) coincident with the radial velocity
corresponding to a local infall maximum - not subsequent to the infall maximum
as seen in a Mira variable. However, the data at JD9800 exhibit a minimum
V-magnitude brightness, yet the radial velocity measures signal maximum
inflow. Gaps in the observational measures obviously can compromise
conclusions here. The spatially resolved observations discussed later in this
paper appear to offer an explanation of the radial velocity behavior,
suggesting that a clean interpretation and seeking similarity with other
pulsating stars remains challenging.

In sum, the spatially unresolved measures suggest that pulsation phenomena
could dominate the photometric variability of Betelgeuse, but the details do
not consistently replicate in detail what is found in globally pulsating
Cepheid stars or in Mira giants.

##  3 Direct UV Imaging of Betelgeuse

The Faint Object Camera on HST was used (Gilliland & Dupree 1996) to image
Betelgeuse directly in the ultraviolet continuum ($\lambda$2550Å). This first
direct image of the surface of a star other than the Sun provided about 10
resolution elements on the ultraviolet disk (38 mas point spread function)
which has a diameter about 2.2 times larger than the optical diameter. This
image revealed a bright spot in the SW quadrant of the star which comprised
10% of the area and 20% of the flux from Betelgeuse at that time.
Subsequently, we followed up with similar ultraviolet images spanning 4.1
years. All of the images were obtained with a combination of filters: a
medium-band filter (F253M) was crossed with a second UV filter, F220W, and 4
magnitudes of neutral-density filter inserted also. These images are shown in
Fig. 2 where different scalings are used in the upper and lower panel set.
Each of these images contains the comparison star, HZ 4, taken during the
first visit of HST ($t=0$) demonstrating the extended nature of Betelgeuse in
the ultraviolet.

When the images are scaled to the same exposure time (Fig. 2, top panels), it
is obvious that the total ultraviolet flux not surprisingly varies on a time
scale of months. The lower panels in Fig. 2 show the same images scaled to the
brightest pixel. This figure reveals the changing brightness pattern across
the stellar chromosphere. The single bright area found at $t=0$ becomes
smaller and fainter over the next 2.6 yr, then appears to move to the north,
and becomes greatly extended in the 3.5 yr observation, approximately
’circling’ the spot in the original ($t=0$) image, before fading in the final
image at 4.1 yr. The bright spots seem to stay in approximately the same
position on the star. We find no large excursion to the stellar limb in the
position of the bright spot.

Characteristics of the UV images are shown in Fig. 3 with respect to the V
magnitude and the radial velocity. The mean UV flux from the HST images (third
panel) generally tracks the V magnitude. In fact, the faintest excursion in
magnitude is in harmony with the lowest value of the UV flux, and the times of
brightest optical magnitude generally agree with the brightest UV flux. The
relation beween UV flux and radial velocity is not so clear. The highest
infall velocity occurs twice during the HST observations, and at these times
the

Figure 2: Top 2 panels: UV images scaled to the same exposure time, 3559 s,
which corresponds to the longest summed exposure. Lower 2 panels: UV (F253M)
dithered image scaled to the brightest pixel.

mean UV flux is first at a minimum and then at a maximum value.

A Voigt profile was fit to the UV continuum images and the full width at half
maximum (FWHM) is also shown in Fig. 3 (bottom panel). There does not appear
to be a relationship between the diameter of the UV image and the photospheric
radial velocity. During the time span of the HST images, the V magnitude
displays a period of 366 days whereas the period found for the radial velocity
variation is 440 days. These periods were derived using the Lomb-Scargle
technique for irregularly-spaced data after removing a linear trend (Horne &
Baliunas 1986). The absence of correlations may be understood from the results
of the spatially resolved spectra discussed below.

Figure 3: These panels relate the magnitude and metal-line radial velocity to
the HST measures of flux and stellar diameter. The broken lines are meant to
guide the eye. Long term trends have been removed with a second-order
polynomial from the V magnitude and radial velocity values. Figure 4: UV image
of Betelgeuse with the sense of rotation shown as a wire frame (Uitenbroek et
al. 1998).

At the same time of the first UV image, spatially resolved UV spectra were
obtained (Uitenbroek et al. 1998) in the near ultraviolet region that included
both the resonance Mg II emission lines and several photospheric absorption
lines. The absorption lines from the spatial scan NW to SE across the stellar
disk displayed a systematic shift from negative to positive velocities with a
total amplitude $\sim$10 km s-1. This behavior is interpreted as due to the
rotation of the star. Uitenbroek et al. proposed that the bright spot
coincided with the pole of rotation (which is also consistent with the
measures of the angle of highest polarization), making the inclination of the
rotation axis of the star 20∘ from the line of sight (see Fig. 4). This
inclination, coupled with the measured 5 km s-1 rotation velocity suggests
that the deprojected radial velocity is 14.6 km s-1, and the rotational period
is 25.5 yr at a distance of 191 pc. Thus it appears plausible that the bright
spots shown in Fig. 2 emerge preferentially around the pole of the star.

Figure 5: The flow velocity in the low chromosphere, as inferred from modeling
of the Si I line, $\lambda$2516, observed with STIS (the 7 STIS aperture
positions are marked by broken lines) at time $\Delta t=4.1yr$. The length and
direction of the arrows indicate the magnitude of the velocity and the
direction of flow (Lobel & Dupree 2001).

##  4 Spatially Resolved Spectroscopy

STIS, the Space Telescope Imaging Spectrograph on HST possesses a narrow
aperture (25 $\times$ 100 mas) which offers true spatial resolution for the
ultraviolet emission lines of Betelgeuse since they have a diameter of
$\sim$270 mas or larger. In addition, line profiles of several neutral and
singly ionized species that occur in the near-ultraviolet exhibit centrally-
reversed emission which serves as a diagnostic of mass motions in the
atmosphere. Lobel & Dupree (2001) detected changes in many profiles with
spatial position on the disk that indicated both outflowing and inflowing
chromospheric material with velocities $\sim$2 km s-1. These velocities change
with position on the disk and also with time. Detailed non-LTE models in
spherical geometry were constructed to match the profiles of many lines
including Fe I, Fe II, Si I, and Al II. The 4 spectroscopic observations
spanned 1.3 yr with sampling of about 0.3 yr. These began at $t=2.8$ yr
corresponding to the times in Fig. 2. Beginning at $t=2.8$ yr, the flow
pattern in the low chromosphere changed from a global decelerating inflow to
outflow in one quadrant and subsequently the outflow extended to almost the
whole stellar hemisphere (Fig. 5). The spatially resolved spectroscopy reveals
that the outer atmosphere of Betelgeuse does not behave in a global fashion
but that asymmetric time dependent dynamics are present.

##  5 Conclusions

When observed as a star, the photosphere and chromosphere of Betelgeuse are
subject to a semi-periodic travelling oscillation with a period of $\sim$400
days. This period can be coherent for $\sim$4 years.

Spatially resolved imaging shows that the image size in the near ultraviolet
continuum ($\lambda$ 2550Å) exceeds the optical diameter (taken as 55 mas) by
about a factor of 2.2, and the chromospheric Mg II lines extend even further …
to a diameter $\sim$4 times that of the optical. Bright regions occur on the
ultraviolet disk that change in position and strength over a period of months.
They appear to be localized around the rotational pole of the star. Spatially
resolved spectroscopy demonstrates that the low chromosphere does not behave
uniformly, but that the dynamics are complex. We have discovered gradually
changing inflow and outflow patterns suggesting asymmetric mass motions. Such
behavior obviously complicates the interpretation of the spatially unresolved
radial velocity measures.

We acknowledge with thanks the variable star observations from the AAVSO
International Database contributed by observers worldwide and used in this
research.

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