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Astron. Astrophys. 333, L51­L54 (1998)

Letter to the Editor 76 mas speckle-masking interferometry of IRC +10 216 with the SAO 6 m telescope: Evidence for a clumpy shell structure
¨ G. Weigelt1 , Y. Balega2 , T. Blocker1 , A.J. Fleischer3 , R. Osterbart1 , and J.M. Winters3
1 2 3

¨ ¨ Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchuk region, 35147 Karachai-Cherkesia, Russia ¨ ¨ Technische Universitat Berlin, Institut fur Astronomie und Astrophysik, Sekr. PN 8-1, Hardenbergstrasse 36, D-10623 Berlin, Germany

Received 18 December 1997 / Accepted 10 March 1998

Abstract. We present the first K -band image of the carbon star IRC +10 216 with 76 mas resolution. The diffraction-limited image was reconstructed from 6 m telescope speckle data using the speckle masking bispectrum method. The image shows that the dust shell of IRC +10 216 is extremely clumpy. Five individual clouds within a 0. 21 radius of the central star have been resolved for the first time. On the basis of consistent theoretical models we argue that these structures are produced by circumstellar dust formation. The fragmentation of the shell structure gives most likely direct evidence for an inhomogeneous massloss process which may be interpreted in terms of large-scale surface convection-cells (Schwarzschild 1975) being a common phenomenon for red giants. Key words: techniques: image processing ­ stars: carbon ­ circumstellar matter ­ stars: individual: IRC +10 216 ­ stars: mass-loss ­ stars: AGB, post-AGB

and visibility data. Consistent time­dependent models describing the circumstellar shells of dust forming long­period variables have been presented by Fleischer et al. (1992). A general result of these models is the formation of discrete dust layers, causing pronounced time­varying, step­like surface intensity distributions (Winters et al. 1995). In this Letter we present diffraction-limited 76 mas speckle masking observations of the clumpy dust shell of IRC +10 216. We speculate about the origin of these structures in the light of consistent time­dependent model calculations and discuss the red giants' large-scale surface convection (Schwarzschild 1975) as a possible mechanism for inhomogeneous mass loss. 2. Observations and speckle masking results The IRC +10 216 speckle interferograms were obtained with the 6 m telescope at the Special Astrophysical Observatory on April 3, 1996 (variability phase 0.10). The speckle interferograms were recorded with our NICMOS 3 camera through a standard K filter with a center wavelength of 2.17 µm and a bandwidth of 0.33 µm. 968 IRC +10 216 speckle interferograms and 1496 reference star interferograms (HIC 51133) were recorded. The observational parameters were as follows: exposure time per frame 70 ms, pixel size 14.6 mas, and seeing (K ) 2. 5. Fig. 1 shows the diffraction-limited K image of IRC +10 216 which was reconstructed from the speckle interferograms using the speckle masking bispectrum method (Weigelt 1977, Lohmann et al. 1983, Weigelt 1991; note that the image published by Weigelt et al. (1997) was actually a K image, not an H-band image as was incorrectly printed). In addition to the dominant central object A, three bright knots (called B, C, and D) can be identified. The separation of B, C, and D from the central object are 203 mas, 137 mas, and 136 mas, respectively. The position angles of B, C, and D are 21 , 339 , and 59 , respectively. The reconstructed image shows that the dust shell of IRC +10 216 is extremely clumpy. The asymmetry of the dominant central object suggests that there are at least two additional clouds E and F (not fully resolved) at separations much smaller than the separations of B, C, and D (i.e., at separation < 100 mas). The

1. Introduction IRC +10 216 (CW Leo) is the nearest and best­studied carbon star. It is a long­period variable star with a period of about 650 d and a spectral type of C9,5 (see e.g. Olofsson et al. 1982). Estimates of its distance range from 100 pc (Zuckerman et al. 1986) to 290 pc (Herbig & Zappala 1970). IRC +10 216 is surrounded by a dust shell which is expanding at vexp 15 kms-1 , thereby carrying a mass loss rate of M 2 - 510-5 M yr-1 (e.g. Loup et al. 1993). The first high-resolution IR observations of the dust shell of IRC +10 216 were reported by Toombs et al. (1972), McCarthy et al. (1980, 1990), Mariotti et al. (1983), Dyck et al. (1984, 1987, 1991), Ridgway & Keady (1988), Christou et al. (1990), Le Bertre et al. (1988), Danchi et al. (1994), Osterbart et al. (1997), and Weigelt et al. (1997). Detailed radiation transfer calculations for IRC +10 216 have been carried out, for instance, by Groenewegen (1997) using a large amount of spectroscopic
Send offprint requests to: G. Weigelt Correspondence to: weigelt@mpifr-bonn.mpg.de

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G. Weigelt et al.: Speckle-masking observations of IRC +10 216
1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0
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Fig. 2a­e Intensity cuts (solid lines) through the image in Fig. 1 in the indicated directions. The cut directions are shown by dotted lines in Fig. 1b. Dashed curves are cuts through the point spread function reconstructed from two different data sets of the reference star. In plot e the azimuthal averages of IRC +10 216 and the PSF are shown

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25 mas±8 mas which is an upper limit for the radius of the central star, as the observed central object A could consist of both the stellar disk plus nearby or foreground dust clouds.
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3. Discussion 3.1. Evolutionary status

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Fig. 1a and b Speckle masking observation of IRC +10 216: a K -band speckle masking reconstruction, b contour plot of a with the denotation of the components (the contour levels are plotted in steps of 0.3m ). The dotted lines indicate the cut directions taken for Figs. 2a­e

position angles of E and F are 122 and 200 , respectively. The objects A to F are located inside a larger nebulosity which looks like a bipolar, X­shaped nebula with an approximately NS polar axis (or position angle 10 to 20 , in agreement with Kastner & Weintraub 1994). At the 2% intensity level the nebulosity extends over 0. 8 in NS and 0. 6 in EW direction. The considerable brightness of cloud B ( 5% of the total flux) can, for instance, be explained if we assume that the central star is obscured by a dust disk seen nearly edge-on. The smallest radius of the dominant central object (A+E+F) was measured for position angle 21 (see Fig. 2a). At position angle 21 the radius of the central object was determined to be approximately

IRC +10 216 is in a very advanced stage of its AGB evolution due to its low effective temperature, long period and high massloss rates. Its carbon-rich chemistry indicates that a significant number of thermal pulses with corresponding dredge-up events did take place. The mass of the hydrogen-exhausted core, MH , can be expected to be already close to the later final mass, due to the high mass-loss rates and limited core growth-rates per pulse of only a few 10-3 M in its stage of evolution (depending on mass) partially compensated or even canceled by dredge-up episodes. The conjecture that IRC +10 216 has entered a phase immediately before moving off the AGB seems to be supported by its non-spherical appearance (Fig. 1; see also Kastner & Weintraub 1994). In contrast to their progenitors, AGB successors often expose prominent features of asphericities, mostly in axisymmetric geometry. Note, that IRC +10 216 is already considerably elongated in NS direction probably even with a bipolar structure (Fig. 1). The measured bolometric flux S at maximum light (S = 2.1 · 10-8 Wm-2 ; Sopka et al. 1985) leads to 11000 <


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L/L < 18900 for recent distance estimates of 130 pc < d < 170 pc (Le Bertre 1997, Winters et al. 1994a). Introducing these luminosities into the core-mass luminosity relation will give upper limits for MH since S requires corrections for the mean variability phase and the thermal-pulse cycle phase. ¨ Evolutionary models of Blocker (1995) give MH =0.65M for L = 10000L . Note, that the MH -L relation breaks down for ¨ ¨ massive AGB models (Blocker & Schonberner 1991) due to the penetration of the envelope convection into the hydrogenburning shell ("hot bottom burning", HBB). Accordingly, the upper luminosity value indicates MH < 0.8M and possibly HBB. Since the present core mass will not deviate much from the final mass, it can be applied in initial-final mass relations. ¨ Taking the AGB calculations of Blocker (1995) and Vassiliadis & Wood (1993), resp., we finally arrive at initial masses lower than 2.5 - 3 M for d = 130 pc and 4 - 4.5 M for d = 170 pc. The chemistry of the circumstellar envelope gives further constraints. Guelin et al. (1995) compared the observed isotopic abundance ratios with evolutionary models. Particularly the C, N and O isotopic ratios led to the conclusion that the initial mass ranges between 3 and 5M , and that moderate HBB has taken place, favouring an initial mass close to 4 M . 3.2. Discrete dust layers As a demonstrative example Fig. 3 shows a one­dimensional synthetic intensity profile resulting from a consistent time­ dependent model calculation for a carbon­rich circumstellar dust shell. These model calculations assume spherical symmetry and include a consistent treatment of time­dependent hydrodynamics, chemistry, dust formation, growth and evaporation and of the radiative transfer problem (Fleischer et al. 1992, Winters et al. 1994b). A general result of the calculations is the formation of discrete dust layers with the characteristic step­like intensity profile shown in Fig. 3 (top). The location and height of the steps vary in time since the dust layers are moving outwards and, thereby, become geometrically diluted (see Winters et al. 1995). In the calculation, these structures are produced by thermal dust emission which, via the dust opacity, depends on wavelength. Note also, that the steps are separated by only a few stellar radii (lower abscissa; the upper abscissa gives the angular extension assuming d = 170 pc). The bottom diagram of Fig. 3 shows the intensity profile convolved with the ideal point spread function (PSF) of the 6 m telescope (FWHM diameter 76 mas). At this resolution the step-like structures in the intensity profile disappear completely. Comparing this intensity profile (Fig. 3, bottom) with our measured one (same figure) shows that there is a good agreement, but the wings of the measured profile are slightly higher. The most striking structures in our image (Fig. 1) are the three knots B, C, and D. Assuming a typical stellar radius of the central source of R = 5 · 1013 cm and d = 170 pc (Winters et al. 1994a), the (tangential) separation of the knots from the central peak is 10 R (B) and 7 R (C,D). In terms of the models, this could be interpreted as knots B, C, and D being connected to an outer dust layer, whereas knots E and F belong to the next layer

Fig. 3. Synthetic intensity profile at = 2.4µm (upper panel) resulting from a model calculation (see text). The lower panel shows the convolution (solid line) of the synthetic profile with the PSF of the 6 m telescope, the PSF of the 6 m telescope (dashed line), and the azimuthal average of the measured intensity (dotted line)

inwards. This interpretation requires the fragmentation of inhomogeneous dust layers or that the knots result from spatially bounded separate dust formation events. Since dust nucleation is extremely sensitive to the local kinetic gas temperature, dust formation could be caused locally even by small temperature fluctuations. The radial velocity of the expanding dust shell is approximately 15 km s-1 . This corresponds to 3 AU/year or 18 mas/year at a distance of 170 pc for a movement perpendicular to the line­of­sight. Thus, if connected to this expansion, knot B should have formed at least 11.6 yr ago, while C and D would be 7 yr old. In terms of the pulsation period (P 650 d) this would correspond to a time scale for the formation of new dust layers (or knots) of t 2.5 P . Then, the structures E and F would have been formed 1.5 P ago. The formation of new dust layers on time scales longer than the pulsation period is a common phenomenon of the model calculations (e.g. Fleischer et al. 1992, Winters et al. 1994b, 1995). Therefore, future observations can be used to test this model and to determine the dust formation frequency and the tangential velocity of the structures. 3.3. Inhomogeneuos mass loss Since the present observations reveal that IRC +10 216's shell structure is highly fragmented in the immediate stellar vicinity, there seems to be evidence for an already inhomogeneous mass-loss process. Inhomogeneously outflowing matter implies corresponding stellar surface inhomogeneities which may be caused by magnetic activity, global pulsations or large-scale photospheric convection. In particular, the latter seems to be a common phenomenon of far-evolved stars. Schwarzschild (1975) showed that the typical horizontal size of a solar granule, xgran , is given by characteristic depth scales of the layers below the photosphere. With the pressure scale height, Hp , as the major depth scale and assuming that the ratio of xgran /Hp is constant he found that for red giants the dominant convective elements become so large that only a few of them can occupy the surface at any time leading to large

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temperature variations on the surface and concomitant brightness fluctuations. Due to the prominent temperature contrasts at the surface the emitted radiation is highly anisotropic leading to a polarization of the light scattered by circmstellar dust. Schwarzschild (1975) already supposed that mass ejection is triggered by photospheric convection and Dyck et al. (1987) outlined its possible importance for IRC +10 216. Indeed, based on a linear stability analysis of convective modes in the envelopes of red giants, Antia et al. (1984) found the pressure scale height to be the prevailing depth scale leading to dominant convective elements which are of comparable size to the stellar radius. More recently, Freytag et al. (1997) presented models for convection zones of main-sequence stars and subgiants with spectral type F to K based on 2D numerical radiation hydrodynamics calculations. They found a tight correlation between the characteristic photospheric scale height Hp0 = Rgas Teff /g and the size of the granules, xgran , viz. xgran 10Hp0 covering more than two orders of magnitudes in gravity. Due to this robustness a (cautious) extrapolation to the red giant regime seems to be justified. For IRC +10 216 (with Teff = 2200 K, R = 700R , and M = 0.8M ) this leads to a typical granule size of xgran 0.82R allowing the whole surface to be occupied by, at most, a few granules. The resulting temperature fluctuations can be expected to be in the range of up to several hundred Kelvin (Antia et al. 1984) being large enough to cause observable brightness fluctuations and to influence the formation of the shock- and dust-driven stellar wind and, therewith, the shape of the circumstellar shell. Thus, one further implication of large-scale surface inhomogeneities could be a corresponding large-scale fragmentation of the outflowing matter possibly leading to knots within the multiple shell structure as observed for IRC +10 216. 4. Conclusion Our speckle masking reconstruction of IRC +10 216 shows that the circumstellar dust shell of IRC +10 216 consists of at least five individual dust clouds B to F within a 0. 21 radius of the central star. With d = 170 pc and R = 5 · 1013 cm, the tangential separations of the clouds B, C, and D from the central star correspond to 34 AU 10 R (B) and 23 AU 7 R (C,D). Time-dependent calculations for circumstellar envelopes show the formation of multiple-shell structures, and the synthetic intensity profiles agree well with the measured ones. The prominent clumpiness of these very inner shells gives evidence for an already inhomogeneous mass loss which may be intimately linked with large-scale surface inhomogeneities possibly induced by giant surface convection-cells.
Acknowledgements. We thank B. Freytag for valuable discussions on surface convection zones. J.M.W. and A.J.F. thank the Konrad­Zuse­ ¨ ¨ Zentrum fur Informationstechnik Berlin and the HLRZ, Julich for a generous grant of computing time. A.J.F. was supported by the DFG (grant Se 420/8­1) and J.M.W. by the BMBF (grant 05 3BT13A 6).

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