Departures from Spherical Symmetry in Mira Variables at PTI

1
Departures from Spherical Symmetry in Mira
Variables at PTI

• R. R. Thompson (JPL/U Wyoming)
• M. J. Creech-Eakman (Caltech/JPL)
• G. T. van Belle (JPL)
• American Astronomical Society
• Meeting 198, 4 June 2001

2
Abstract

• We report evidence of departures from spherical
  symmetry in a sub-sample of the Mira dataset
  taken with the Palomar Testbed Interferometer
  (PTI). The criteria for testing such departures
  is two-fold first, that the visibility data
  using the N-S and N-W baselines be taken within
  0.1 of a Mira's pulsation phase (to rule out
  dramatic size changes) and second, that
  extensive hour coverage be taken on a single
  baseline.
• Of the 11 stars which meet this criteria, 9 show
  evidence of a departure from spherical symmetry
  when fit using a uniform elliptical disk model.
  It must be stressed that this "ellipticity" may
  only be apparent in nature, as atmospheric
  structures such as starspots, limb-darkening,
  binarity and non-radial pulsation induced
  outflows can influence apparent geometries of
  these stars. Axial ratios (2b/2a) for the model
  ellipses range between 0.60 and 0.90 for the
  eight stars.
• We also report wideband (2.2 um) models of R Tri.
  These models - which rely upon Fast Fourier
  Transforms and Monte Carlo techniques - support a
  geometry of a slightly elliptical star with a
  circumstellar region of gas/dust in emission.
• The work performed here was conducted at the Jet
  Propulsion Laboratory, California Institute of
  Technology, under contract with the National
  Aeronautics and Space Administration.

3
The Palomar Testbed Interferometer

• Maximum projected baselines N-S 110m, N-W 85m
• Resolution range 1.0 - 5.5 mas
• 40 cm collecting apertures, K 5 fringe
  tracking, V 12 angle
  tracking
• Visibilities in H-band (4 chan) and K-band (5
  chan) R 22 - 50

4
Miras as post-AGB stars

• Pulsation driven shockwaves.
• Outer layers are convective to carry away energy
  production.
• Outer layers lifted by shockwave(s).
• Mass loss due to a superwind, creating
  circumstellar shells, dM/dt 10 -7 to 10 -4 Msol
  / yr

5
Mira Variables

• Pulsation periods 120 - 500 days.
• Short period (lt 200 d), long period (gt 400 d).
• Classed with long period variables (LPV)
• DVmag 5 - 10, or 100 - 10,000 x brightness
  amplitude
• Teff 2800 - 3500 K, but dust can make them
  appear cooler ( down to 1800K).
• L / Lsol 1000 - 10,000
• R / Rsol 100 - 800
• Evidence for jets, circumstellar shells, dusty
  regions.
• Convective hot spots theorized.
• Departures from spherically-symmetric geometry.
• Chemical taxonomy oxygen-rich (M), intermediate
  (S), carbon-rich (C).

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Departures from Spherical Symmetry

• Asymptotic giant branch (AGB) and post-AGB stars
  are characterized by their extended atmospheres.
  As such, departures from spherical symmetry of
  these outer atmospheres may occur as a result of
  non-radial pulsation, stellar hotspots due to
  large-scale convection and asymmetric mass
  outflows. Spatially-resolved studies of using
  various techniques depict evolved supergiants,
  semi-regular pulsators and Mira stars as having
  geometric asymmetries in their outer atmospheres.
• a Ori mid-IR interferometry (Weiner et al.,
  2000)
• R Leo near -IR aperture masking (Tuthill et
  al., 1994)
• R Leo, W Hya FGS, HST (Lattanzi et al.,
  1997)
• R Aqr mid-IR interferometry (Tuthill et
  al., 2000)
• o Cet direct imaging, HST (Karovska et
  al., 1999)
• R Cas speckle masking (Hofmann et al.,
  2000)
• (7 Mira stars) near-IR interferometry (Thompson
  et al., 2000)

7
Results from near-infrared long baseline
interferometry

• Using two-baseline uniform disk angular sizes, or
  single baseline FFT fits to visibilities, we have
  detected departures from spherical symmetry in 9
  Mira stars. Axial ratios (b/a) of simple uniform
  ellipse fits and corresponding position angles
  vary with wavelength. The following stars were
  tested.
• V CVn (M6e) R Gem (Se) RZ Peg (Ce)
• R Boo (M4e) T Lyn (Ne)
• R Del (M5e)
• R Vir (M4e)
• RS Her (M5e)
• S Gem (M6e)
• S Lac (M5e)
• T And (M3e)
• TU And (M5e)
• X Del (M6e)
• R Tri (M4e)

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Axial Ratios and Postion Angles

• Phase 2.0mm 2.1mm 2.2mm 2.3mm
  2.4mm
• V CVn ? 0.76 (0.04) 0.92 (0.09)
  0.86 (0.04) 0.87 (0.02) ------- (b/a)
• 219 (3)
  241 (42) 241 (14) 230 (4)
  ------- PA (deg)
• R Vir 0.96 --------
  0.99 (0.02) 0.92 (0.05) 0.93 (0.08)
  0.78 (0.14) (b/a)
• ------- ------ 315 (9)
  7 (51) 318 (7) PA (deg)
• RS Her 0.90 ------- 0.73 (0.12) 0.72 (0.16)
  0.78 (0.19) 0.80 (0.09) (b/a)
• 327 (4) 337 (10) 342 (34) 328 (7) PA
  (deg)
• S Gem 0.23 0.88 (0.02) 0.83 (0.04) 0.93 (0.01)
  0.90 (0.02) 0.86 (0.03) (b/a)
• 1 (9) 22 (6) 4 (7) 17 (3) 21 (7) PA (deg)
• T And 0.78 -------- ---------
  0.92 (0.07) 0.94 (0.08) 0.99 (0.05) (b/a)
• --------
  --------- 335 (5) 344
  (10) 327 (3)
  PA (deg)
• TU And 0.19 0.89 (0.20) 0.92 (0.09) 0.95
  (0.06) 0.84 (0.06) 0.96 (0.05) (b/a)
• 340 (62) 347 (60) 13 (61) 27 (19) 337
  (40) PA (deg)

9
Time-dependent changes, R Del

• Uniform disk angular sizes from 2 baselines (NS,
  NW) show R Del maintaining axial ratios near 0.9
  but whose position angles changed over the course
  of one pulsation cycle.

10
Time-dependent changes, R Boo

• Uniform disk angular sizes from 2 baselines (NS,
  NW) show R Boo changing from an ellipse just past
  maximum light, to a more spherical shape near
  phase 0.4 with non-static position angles.

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The Enigmatic R Tri

• R Triangulum oxygen-rich Mira, P 266 d
• Single baseline, extensive hour angle coverage
  between phases 0.84 and 0.95.
• When data is fit to a uniformly-bright sphere
  (UD) c2 300!
• An event at phase 0.93??

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Models tested on R Tri

• Using FFTs on calibrated wideband visibilities
  three models fit the dataset better than a
  uniform spherical or elliptical disk. Monte Carlo
  techniques were utilized.
• In each case, a secondary structure brighter than
  the stellar atmosphere is found between PA 15 -
  25 degrees.