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MULTIEPOCH INTERFEROMETRIC STUDY OF MIRA VARIABLES II. Narrowband diameters of R Boo

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MULTI-EPOCH INTERFEROMETRIC STUDY OF MIRA VARIABLES II. Narrowband diameters ... are molecular absorptions in or above the photosphere of the oxygen-rich Miras. ... – PowerPoint PPT presentation

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Title: MULTIEPOCH INTERFEROMETRIC STUDY OF MIRA VARIABLES II. Narrowband diameters of R Boo


1
MULTI-EPOCH INTERFEROMETRIC STUDY OF MIRA
VARIABLES II. Narrowband diameters of R Boo
  • R. R. Thompson (JPL)
  • M. J. Creech-Eakman (JPL)
  • G. T. van Belle (IPAC/Caltech)
  • American Astronomical Society
  • Meeting 201, Seattle WA
  • 9 Jan 2003

2
Abstract
  • As part of the long-term monitoring of Mira
    variables at the Palomar Testbed Interferometer,
    we report high-resolution narrowband angular
    sizes of the oxygen-rich Mira R Bootis. The
    dataset spans five pulsation cycles for a total
    of 1496 25-sec observations), and represents the
    second study to correlate multi-epoch narrowband
    interferometric data of Mira variables. When the
    calibrated visibility data are fit using a
    uniform disk brightness model, differences are
    seen in their angular diameters as a function of
    wavelength within the K band (2.0 - 2.4 ?m) the
    source of which are molecular absorptions in or
    above the photosphere of the oxygen-rich Miras.
  • Using visible photometric data provided by the
    AFOEV, the continuum minimum size tracks the
    visual maximum brightness as found in our
    previous study (Paper I) for the oxygen-rich Mira
    S Lac. Based on the mean of the continuum angular
    diameter cycloid, basic stellar parameters are
    computed for R Boo, with this star showing
    maximum atmospheric extension with respect to the
    2.0 and 2.4 ?m diameters near phase 0.95. Using
    the mean value of the fitted cycloids, R Boo has
    a radius Rmean 216 ? 40 R? and a mean Teff
    2970 ? 43 K .The dominant source of error in the
    mean radius is the large uncertainty in the
    distance to this star.
  • 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
The Mira variable R Boo (HD 128609)
  • Chemical type Oxygen-rich
  • Spectral type M3 M8 IIIe
  • Pulsation period P 224.1 d
  • Visual magnitude 7.1 12.2 (AFOEV)
  • K-band magnitude 2.1 (Gezari et al, 1999)
  • Mean continuum angular diameter 3.31 ? 0.02 mas
    (from 2.2 ?m cycloid)
  • Bolometric flux 28.2 ? 1.6 x 10-8 erg/cm2/s
    (from photometry longward of 1 ?m )
  • Mean Teff 2970 ? 43 K
  • Mean linear radius 216 ? 40 R?

5
Distance Determination
  • The distance to R Boo as quoted herein is a mean
    value from four independent sources. The large
    uncertainty in distance results in similar
    uncertainty in the linear diameter.
  • 500 pc Bowers Hagen (1984)
  • 570 pc Wyatt Cahn (1983)
  • 600 pc Jura Kleinmann (1992)
  • 764 pc Whitelock Feast (2000), using
  • MK -7.32 and mK 2.10
  • 608 ? 112 pc mean distance (?/m 18)

6
Narrowband diameter cycloids
  • Normalized visibilities were fit to a
    uniformly-bright disk model (UD) diameters (?)
    for a projected baseline (B) and wavelength (?)
    such that
  • Each diameter point in Figure 1 represents a
    nightly ensemble mean measurement.
  • No conclusive evidence of departures from
    circular symmetry found to date.

7
Diameter cycloid best fits
  • Cycloids fit to data are of the form
  • Where a is the amplitude, b is the period (here,
    set to unity to normalize to 224.1d), ? is the
    visual phase, c is the diameter phase offset and
    d the mean linear offset.
  • Band a c d ?2?
  • 2.0 ?m 0.28?0.05 0.48?0.02
    3.68?0.03 77
  • 2.2 ?m 0.39?0.03 0.52?0.01
    3.31?0.02 28
  • 2.0 ?m 0.20?0.02 0.43?0.02
    4.04?0.02 21

8
Diameter ratios
  • Both the 2.0 and 2.4 ?m diameters were normalized
    to the center channel in the K-band (2.2 ?m).
    Figure 2 depicts these quantities, showing the
    effect of maximum opacity near phase 0.9. This is
    consistent with that found in the oxygen-rich
    Mira S Lac (Thompson et al. 2002).
  • The best-fit curves are depicted however, the
    2.0/2.2 ratio data is effected by lower SNR and
    atmospheric water in the 2.0 ?m channel.

9
Narrowband vs. synthetic wideband diameters
  • Visibility data from the spectrometer channels
    can also be used to synthesize a wideband
    visibility measurement which provides an improved
    SNR. From Colavita (1999), this is done using a
    photon-weighted average to the spectrometer
    visibilities such that
  • and the weighting function is
  • where cds represents correlated double
    sampling.

10
Narrowband vs. synthetic wideband diameters
  • Due to the changing stellar atmospheric opacity
    in both the 2.0 and the 2.4 ?m channels (H2O, CO,
    CO2), wideband diameters tend to overestimate the
    size of R Boo by as much as 5. This effect is
    greatest near phase 0.9, as seen in Figure 3.
    Minimum differences (NB-WB) are seen just before
    visual minimum, where R Boo is at its largest
    linear size.

11
Effective Temperature
  • The bolometric flux was computed from available
    photometry with ? gt 1 ?m. From this value, the
    effective temperature is calculated using the 2.2
    ?m narrowband angular diameters such that
  • Maximum Teff occurs just before visual maximum as
    seen in Figure 4, with a peak-to-peak temperature
    change of 360 K throughout R Boos cycle, and a
    relative temperature change of ?Tp-p / Tmean
    12.

12
Multi-wavelength Diameters
  • Observations of R Boo at both H-band (1.65 ?m)
    and K-band (2.2 ?m) were done only four days from
    each other (2 of visual phase period) in the
    2000 observing season. These data were converted
    to UD diameters, and are shown in Figure 5. The
    given K-band shape is representative of the O1
    class (Thompson, 2002), typical of oxygen-rich
    Miras of early spectral type (M2-M4).
  • Shown for comparison is a figure (Fig 6 herein)
    from Jacob and Scholz (2002), which represents a
    theoretical model (P series, Rp241 R?,
    Teff2860K) from Hofmann, Scholz and Wood (1998)
    which were transformed to narrowband angular
    diameters. (The dots/crosses are from Thompson et
    al. 2002 for the oxygen-rich Mira S Lac.)

13
Comparison with theory
  • From Jacob and Scholz (2002) dots/crosses are
    for S Lac (Thompson et al, 2002) at various Rp

14
Discussion
  • The Mira variable R Boo was observed over 5
    pulsation periods with the Palomar Testbed
    Interferometer. This oxygen-rich Mira compares
    well to another oxygen-rich Mira S Lac in Paper I
    of the PTI Mira series (Thompson et al, 2002).
    Maximum atmospheric extension (and hence maximum
    opacity effects) occurs just before visual
    maximum, and the multi-wavelength diameters lend
    themselves well to the P series models of
    Hofmann, Scholz Wood (1998) and Jacob Scholz
    (2002).
  • Departures from circular symmetry were
    statistically insignificant as evidenced in the
    use of two PTI baseline orientations. H2O maser
    emission about R Boo is minimal (Benson
    Little-Marenin 1996) as well as SiO and OH masers
    (refs therein), coinciding with low mass loss
    (0.1 x 10-6 M? / yr, Bowers Hagen 1984). As
    suggested by Meixner et al (1997), dusty mass
    loss during the pre-planetary nebula phase occurs
    in an axially-symmetric manner.

15
  • Since departures from circular symmetry have not
    been detected interferometrically, nor evidence
    of maser emission in the literature to date, nor
    a high mass loss rate, it is unlikely R Boo has
    developed the superwind needed to shed its
    outer atmosphere to create a PPN. Thus, R Boo
    represents a Mira in its younger stages, with its
    period around 224 d (mean for oxygen-rich Miras
    is 325 d), and its effective temperature very
    close to a theoretical parent star.
  • Photometry obtained from the AFOEV over the 5
    pulsation cycles observed at PTI show R Boo to
    pulsate in a fairly regular manner, as seen in
    Figure 7. This star also exhibits no evidence to
    be a symbiotic star (Kenyon Gallagher 1983) nor
    a binary system (Blazit et al 1987).

16
The five epochs of PTI observations
17
References
  • Benson Little-Marenin, 1996, ApJS, 106, 579
  • Blazit, Bonneau Foy, 1987, AAS, 71, 57
  • Bowers Hagen, 1984, ApJ, 235, 637
  • Colavita, PASP, 111, 111
  • Gezari et al, 1999, Catalog of Infrared
    Observations, 5th edition (available on
    CDSweb.u-strasborg.fr)
  • Hofmann, Scholz Wood, 1998, AA, 339, 346
  • Jacob Scholz, MNRAS, 336, 1377
  • Jura Kleinmann, 1992, ApJS, 79, 105
  • Kenyon Gallagher, 1983, AJ, 88, 666
  • Meixner et al, ApJ, 482, 897
  • Thompson, 2002, PhD thesis, University of Wyoming
  • Thompson, Creech-Eakman van Belle, 2002, ApJ,
    577,447
  • Whitelock Feast, 2000, MNRAS, 319, 759
  • Wyatt Kahn, 1983, ApJ275, 225
  • The authors gratefully acknowledge Kevin Rykoski
    and Jean Mueller of Palomar Observatory for their
    efforts in obtaining much of the data herein.
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