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 (< 200 d), long period (> 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).

6. 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)

8. 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) X Del 0.18 1.00 (0.02) 0.98 (0.02) 0.89 (0.02) 0.86 (0.03) 0.86 (0.02) (b/a) ------ 269 (4) 275 (16) 268 (12) 270 (20) PA (deg) R Gem 0.37 -------- 0.96 (0.02) 0.96 (0.02) 0.95 (0.03) 0.96 (0.02) (b/a) -------- 8 (1) 36 (9) 1 (19) 18 (13) 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.

11. 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??

12. 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.