New Photometric Observations of Ori E

1
New Photometric Observations of ? Ori E
Mary Oksala Rich Townsend Bartol Research
Institute, University of Delaware
Introduction
The RRM Model

• The data were reduced using the IRAF software
  and PSF fitting was utilized for doing the
  photometry. We choose to use s Ori AB as the
  comparison star (See Fig. 1). The colors for
  this star are based on Echevarría et al. (1979,
  RMxAA, 4, 287). The resulting light curves can
  be found in Fig. 2.

• MHD simulations have shown that for stars with
  strong fields and low mass-loss rates, such as ?
  Ori E, that the magnetic field dominates the
  wind. In this case, the field lines act like
  rigid tubes, forcing plasma to flow along fixed
  lines. When the wind streams collide, the plasma
  settles toward minima in the effective potential.
  The minima are in stable equilibrium and the
  plasma accumulates until it forms a Rigidly
  Rotating Magnetosphere.

• The helium-strong star ? Ori E (B2 Vpe) is a
  singular object amongst magnetic early-type
  stars. The star exhibits modulations in its H?
  Balmer emission, its helium absorption-line
  strengths, its visible and near-IR photometric
  indices, its UV continuum flux and resonance-line
  strengths, its 6 cm radio emission, its linear
  polarization, and its longitudinal magnetic field
  strength - all varying on the same 1.19 day
  period identified with the stellar rotation
  cycle.
• The most current comprehensive photometric
  observations of the star are by Hesser et al.
  (1977, ApJ, 216, L31). Recently, Townsend et al.
  (2005, ApJ, 630, in press) have demonstrated that
  the so-called Rigidly Rotating Magnetosphere
  (RRM) model presented by Townsend Owocki (2004,
  MNRAS, 357, 251) can furnish a good fit to the
  Hesser et al. (1977) data.
• In this poster, we present fresh photometric
  observations of ? Ori E, obtained with the dual
  aims of further testing the RRM model and
  searching for any change in the stars
  circumstellar plasma distribution over the past
  three decades.

• Point 3 The model fits the u-band data
  accurately, however the U-band data shows that
  the secondary minimum has decreased in value.
  This change in the relative strength of the
  secondary minimum was thought to be from a change
  in the de-centering of the magnetic field, but we
  now think that it is a result of a difference in
  the degree of filling between the two clouds of
  plasma. The third panel of Fig. 4 shows again
  the plasma transiting the magnetic equator but
  less dense.
• Point 4 There is a discrepancy between both
  sets of data and the model. There appears to be
  an emission feature not predicted by the model
  present in both the 2004 and the 1977 data. The
  fourth panel of Fig. 4 shows the stars
  magnetosphere face-on.

• Fig. 3 shows the comparison between the RRM
  model and both the Hesser et al. (1977) data and
  our data. The Hesser et al. (1977) data is shows
  the Stromgren u-band light curve, while our data
  shows the Johnson U-band light curve. The model
  in the U-band has been adjusted for opacity.
• Point 1 Both primary minimum appear to have
  similar values and fit well with the model curve.
  This feature corresponds with the first panel of
  Fig. 4, where magnetospheric plasma is transiting
  across the stellar equator.
• Point 2 The model reproduced the u-band data
  well, but underestimates the U-band data. The
  second panel of Fig. 4 fits with this point,
  which shows the magnetosphere edge-on.

Evidence for Spindown
Observations

• Our new data was phased in Fig. 1 using a period
  of 1.190858 0.00001d. Hesser et al. claim a
  period of 1.19081 0.0001d. If this period is
  accurate to the error claimed, then we believe
  the increase in period may be due to a spindown
  of the star. However, we are still investigating
  the precision of this error which we believe may
  be too small.

• The data were acquired from the 0.9 m telescope
  at CTIO beginning on November 17, 2004 and
  proceeded for 14 nights until November 30, 2004.
  The observations were taken using the
  Johnson-Cousins photometric system of UBVRI
  filters. During each night, the pattern
  UVUVUVBRI demanded which observation filter was
  utilized.

Further Information

• See Rich Townsend for