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High-resolution near-infrared speckle interferometry and radiative transfer modeling of the OH/IR star OH 104.9+2.4
D. Riechers1 , T. Drieb e2 , Y. Y. Balega3 , K.-H. Hofmann2 , A. B. Men'shchikov4 , and G. Weigelt2
1 2 3 4

Max Planck Institute for Astronomy, Heidelberg Max Planck Institute for Radioastronomy, Bonn Special Astrophys. Observatory, Nizhnij Arkhyz, Karachaevo-Cherkesia, Russia Institute for Computational Astrophysics, Saint Mary's Univ., Halifax, Canada

Abstract We present near-infrared sp eckle interferometry of the OH/IR star OH 104.9+2.4 in the K' band obtained with the 6m telescop e of the Sp ecial Astrophysical Observatory (SAO) in Sep. 2002 and Oct. 2003. At a wavelength of = 2.13 µm the diffraction-limited resolution of 74 mas was attained. The reconstructed visibility reveals a spherically symmetric, circumstellar dust shell (CDS) surrounding the central star. The visibility function shows that the stellar contribution to the total flux at = 2.13 µm is less than 30% at all phases, indicating a rather large optical depth of the CDS. To determine the structure and the prop erties of the CDS of OH 104.9+2.4, radiative transfer calculations using the co de DUSTY [1] were p erformed to simultaneously mo del its visibility and the sp ectral energy distribution (SED). Since OH 104.9+2.4 is highly variable, the observational data taken into consideration for the mo deling corresp ond to different phases of the object's variability cycle. This offers the p ossibility to derive several physical parameters of the central star and its CDS as a function of phase.

1 Observations
The ma jority of OH/IR stars are long-p erio d variables (LPVs) of variability typ e Me, extending the sequence of optical Mira variables towards longer p erio ds, larger optical depths, and higher mass-loss rates. As a consequence of their high mass loss, OH/IR stars are surrounded by massive, optically and geometrically thick circumstellar envelop es comp osed of gas and dust. For OH 104.9+2.4, a highly dust-enshrouded OH/IR typ e I I-A class star, we obtained visibilities from sp eckle-interferometric observations with the SAO 6 m telescop e on Sep 22, 2002 and Oct 11, 2003 by applying the sp eckle interferometry metho d [2]. The measurements were accomplished with a K band filter at = 2.13 µm (F W H M = 0.11 µm). Although obtained at different ep o chs, b oth visibilities exhibit striking similarity and reveal that
contact: riechers@mpia-hd.mpg.de


2

speckle interferometry of OH 104.9+2.4

the CDS is fully resolved by our measurements (see Fig. 1a). From the 2Dvisibilities no ma jor deviation of the CDS from spherical symmetry could b e detected.
1.0 0.8 visibility 0.6 0.4 0.2 0.0 0 2
SAO vis., 22-09-2002 SAO vis., 11-10-2003 model, 2002 data model, 2003 data

-10 -11 log(F [W/m2]) -12 -13 -14 -15
ISO SWS+LWS model

4

6

8

10

12

14

10 [µm]

100

spatial frequency [cycles/arcsec]

Fig. 1. Comparison of K -band visibility (left) and SED (right) from our bestfitting model (solid lines) to the SAO (left) and ISO (right) measurements of OH 104.9+2.4. Different bolometric flux values have been used for the SED and visibility model in order to account for the different epochs/phases of the observations. For more details on the model parameters see [4].

2 Results
Our goal was to simultaneously mo del the K -band visibilities and the SED of OH 104.9+2.4 measured at different ep o chs to determine the temp oral change of some physical parameters of the CDS. To accomplish this goal, we used the 1D radiative transfer co de DUSTY [1] and calculated several 10 5 mo dels to scan large fractions of the corresp onding parameter space. According to our final mo del (see also Fig. 1) the effective temp erature of the central star increases from Teff = 2250 K at minimum phase ( = 0.5) to Teff = 3150 K at maximum phase ( = 0.0), while the stellar radius decreases from R = 730 R at = 0.5 to 675 R at = 0.0. For the CDS, we found that the inner b oundary of the dust shell is lo cated at 8.3 R at minimum phase and approximately a factor of two further away at maximum phase (Rin /R = 17.5). The optical depth at 2.2 µm decreases from 8.5 to 3.5 b etween minimum and maximum phase. Our detailed analysis demonstrates the p otential of dust shell mo deling constrained by b oth the SED and visibilities obtained from interferometric measurements. For further details on the mo deling the reader is kindly refered to [3] and [4].

References
1. 2. 3. 4. Z. A. D. D. Iveziґ M. c, Labeyrie: Riechers, Riechers, Elitzur: ApJ 445, 415 (1995) A&A 6, 85 (1970) Y. Y. Balega, T. Driebe et al.: A&A 424, 165 (2004) Y. Y. Balega, T. Driebe et al.: A&A in press, (2005)