1 Introduction

The carbon star IRC +10216 is a long-period variable star evolving along the Asymptotic Giant Branch (AGB). It is the nearest carbon star known (distance $\sim$110-150 pc; Crosas & Menten 1997; Groenewegen et al. 1998) and the brightest 12 $\mu$m object outside the solar system (IRAS 1986). A strong stellar wind has led to an almost complete obscuration of the star by dust. The mass-loss rate as measured in CO rotational lines amounts to 2- $5\times 10^{-5}~M_{\odot}$/yr (Loup et al. 1993). Detailed two-dimensional (2D) radiative transfer modeling shows that IRC +10216 had recently suffered from an even higher mass-loss rate of $\sim$ $10^{-4}~M_{\odot}$/yr (Men'shchikov et al. 2001). Based on the high mass-loss rate, long period of P=649 d (Le Bertre 1992), and carbon-rich chemistry of the dust-shell, IRC +10216 is obviously in a very advanced stage of its AGB evolution (see, e.g., Blöcker 1999).

 

 

Table 1: Observational parameters. JD refers to the Julian date and $\Phi$ to the photometric phase. Phase 0 (maximum light) corresponds to JD = 2 449 430 (Mar. 18, 1994) as extrapolated from the light curve of Le Bertre (1992) with P=649 d. $\lambda _{\rm c}$ is the central wavelength and $\Delta \lambda$the FWHM bandwidth of the filters. $N_{\rm T}$ and $N_{\rm R}$are the numbers of IRC +10216 speckle interferograms and reference-star speckle interferograms, respectively. T is the exposure time per frame, S is the seeing (FWHM), p is the pixel size, and R is the resolution. In the last column, the reference stars are given. The observations of 1995-1998 were already presented in Paper II and are given here for completeness.

	Date	JD	$\Phi$	$\lambda _{\rm c}$	$\Delta \lambda$	$N_{\rm T}$	$N_{\rm R}$	T	S	p	R	Ref. star
				[$\mu$m]	[$\mu$m]			[ms]	[ $^{\prime\prime}$]	[mas]	[mas]
J	2 Apr. 1996	2 450 176	1.15	1.24	0.28	1196	981	200	1.2	14.6	149	HIP 51133
	10 Mar. 2001	2 451 979	3.93	1.24	0.14	1042	783	160	1.0	13.3	50	HD 83871
H	23 Jan. 1997	2 450 472	1.61	1.64	0.31	1665	2110	100	1.5	19.8	70	HIP 52689
	10 Mar. 2001	2 451 979	3.93	1.65	0.32	607	915	30	1.0	20.1	56	HD 83871
K	8 Oct. 1995	2 449 999	0.88	2.12	0.02	251	266	100	1.5	31.5	92	SAO 116569
	3 Apr. 1996	2 450 177	1.15	2.17	0.33	1403	1363	70	2.5	14.6	82	HIP 51133
	23 Jan. 1997	2 450 472	1.61	2.19	0.41	2165	1539	50	0.9	30.6	87	HIP 52689
	14 Jun. 1998	2 450 979	2.39	2.17	0.33	800	571	50	1.6	30.6	87	HIP 50792
	3 Nov. 1998	2 451 121	2.61	2.20	0.20	1087	842	40	1.3	27.2	75	HIP 49583
	24 Sep. 1999	2 451 446	3.11	2.12	0.21	2702	1383	80	0.9	26.4	73	HIP 49637
	15 Oct. 2000	2 451 833	3.70	2.09	0.02	1740	2091	30	1.3	26.8	73	HIP 49637
	9 Mar. 2001	2 451 978	3.93	2.09	0.02	390	777	20	1.0	27.0	73	31 Leo

Interferometric near-infrared imaging of IRC+10216 with angular resolutions of better than 100 mas has revealed that its dust shell is clumpy and bipolar, and is changing on a time scale of only $\sim$1 yr (Weigelt et al. 1997; Weigelt et al. 1998 [hereafter Paper I], Haniff & Buscher 1998; Osterbart et al. 2000 [hereafter Paper II], Tuthill et al. 2000). In 1996, four components were identified in the inner dust shell of IRC +10216 within a radius of 200 mas (Weigelt et al. 1997, Paper I, Haniff & Buscher 1998) and were denoted as A, B, C, and D in order of decreasing brightness (see Fig. 1). On larger scales the envelope of IRC +10216 appears to be spherically symmetric (Mauron & Huggins 1999,2000). Since most dust shells around AGB stars are known to be spherically symmetric on larger scales, whereas most proto-planetary nebulae (PPN) appear in axisymmetric geometry (Olofsson 1996), it is likely that IRC +10216 has already entered the transition phase to PPN. This suggests that the break of symmetry already takes place at the end of the AGB evolution. So far, only a few AGB objects are known to show prominent asphericities of their dust shells in the near-infrared, and are therefore believed to have entered this transition phase at the end of their AGB life. This includes, for instance, the carbon star CIT 6 (Monnier et al. 2000), and the oxygen-rich stars AFGL 2290 (Gauger et al. 1999) and CIT 3 (Hofmann et al. 2001).

Figure 1: K-band speckle reconstructions of IRC +10216 for 8 epochs from 1995 to 2001. The total area is 1 $\hbox {$^{\prime \prime }$ }\times$1 $\hbox {$^{\prime \prime }$ }$. All images are normalized to the brightest pixel and are presented with the same color table. North is up and east is to the left.

Figure 2: J-, H- and K-band speckle reconstructions of IRC +10216 in March 2001. The total area is $1.6\hbox {$^{\prime \prime }$ }\times$1.6 $\hbox {$^{\prime \prime }$ }$. All images are normalized to the brightest pixel and are presented with the same color table. North is up and east is to the left. The tick marks indicate the likely position of the central star.

Recently, we have developed a 2D radiative transfer model of IRC +10216 (Men'shchikov et al. 2001; hereafter Paper III) which can explain many aspects of the nebula. In this model, the star is located at component B and is surrounded by an optically thick shell with bipolar cavities. The brightest southern component A is identified with radiation emitted and scattered in the optically thinner southern cavity of the dense shell. However, Weigelt et al. (1998), Haniff & Buscher (1998), and Tuthill et al. (2000) argued that the star is located at the position of component A and that components B, C, and D are dust clouds. In the present paper, we attempt to explain the recent changes in the nebula in light of the 2D model of Paper III, and will comment on the alternative interpretations only in passing.

High-resolution near-infrared monitoring of the components A, B, C, and D has already revealed that the dust shell of IRC +10216 is rapidly evolving. The 6 m telescope speckle observations presented in Paper II cover five epochs between 1995 and 1998 and show that the separations between the different components had steadily been increasing. For example, the distance between the initially brightest components A and B increased by 36% during 1995-1998. These results are in very good agreement with Keck telescope K-band observations obtained with the highest resolution to date at 7 epochs between 1997 and 1999 by Tuthill et al. (2000). Such direct observations of the dust-shell evolution offer an ideal opportunity to study the mass-loss process in the late stages of AGB evolution, revealing details of the dust formation process as well as the geometry and clumpiness of the stellar wind.

This paper presents new near-infrared bispectrum speckle interferometry monitoring of IRC+10216 obtained with the SAO 6 m telescope in 1999-2001, adding 3 new epochs to the already available data of dust-shell evolution. The present observations show that the appearance of the dust shell has considerably changed compared to the epochs of 1995 to 1998. This paper is organized as follows. In Sect. 2, K-band observations of the dust-shell evolution from 1999 to 2001, a comparison of J-, H-, and K-band images of 1996/97 and 2001, and J-H, J-K, and H-K color images are presented. In Sect. 3, these observations are discussed on the basis of the general morphology, 2D radiative transfer models, and dust-formation models. Conclusions are given in Sect. 4.