View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by The University of North Carolina at Greensboro Astronomy Reports, Vol. 48, No. 4, 2004, pp. 288–300. Translated from Astronomicheski˘ıZhurnal, Vol. 81, No. 4, 2004, pp. 319–332. Original Russian Text Copyright c 2004 by Klochkova, Panchuk, Yushkin, Miroshnichenko. Polarimetry of the Protoplanetary Nebula AFGL 2688 V. G. Klochkova1,V. E. Panchuk 1,M. V. Yushkin 1,andA.S.Miroshnichenko2,3 1Special Astrophysical Observatory, Russian Academy of Sciences, Nizhni ˘ı Arkhyz, Karacha ˘ı -Cherkessian Republic, 357147 Russia 2Main Astronomical Observatory, Russian Academy of Sciences, Pulkovskoe sh. 65, St. Petersburg, 196140 Russia 3Ritter Observatory, University of Toledo, Toledo, OH 43606–3390, USA Received September 10, 2003; in final form, November 10, 2003 Abstract—Our spectroscopic (with resolution R = 75 000) and spectropolarimetric (R = 15 000)obser- vations with the 6-m telescope of the Special Astrophysical Observatory have enabled us to distinguish photospheric and circumstellar features in the optical spectrum of the bipolar protoplanetary nebula AFGL 2688 for the first time. The linear polarization of the radiation was measured at 5000–6600 A.˚ The emission in the lines of the sodium doublet and the Swan bands is formed in the envelope, and the mechanism exciting the transitions is resonance fluorescence. We conclude that the circumstellar envelope has a low density. Features of the structure of the nebula are discussed based on published high-angular-resolution photometric and polarimetric (HST NICMOS) data. c 2004 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION polarimetric data and present the results of new high- angular-resolution polarimetric observations aimed Mass loss on and after the asymptotic giant at distinguishing the radiation of the central star from branch (the AGB and post-AGB stages) is accom- that of the circumstellar envelope of AFGL 2688. panied by the formation of a gas and dust envelope. We selected this object, known (from broadband The angular size of this envelope can be below the polarimetry) for its high degree of polarization, for the resolution limit for ground-based telescopes. In this development of our method. case, asymmetry of the envelope can be detected from the presence of radiation polarized by scattering 2. OBSERVATIONS AND REDUCTION on the envelope’s dust component. The degree of polarization depends on the scattering conditions Since these are the first high-spectral-resolution and the extent to which the star providing the spectropolarimetric observations of a post-AGB ob- unpolarized light is obscured. The protoplanetary ject, the observational method requires some dis- cussion. Medium resolution is preferable for most nebula AFGL 2688 is one of three bipolar post-AGB spectropolarimetric observations, with the exception structures resolved by ground-based telescopes [1, of studies of magnetic main-sequence stars, when 2]. Thirty years ago, the optical image of the source measurement of the four Stokes parameters within was thought to represent a pair of compact blue the profiles of magnetically sensitive lines makes it galaxies, IV Zw 67, at a distance of 30 Mpc [3]. possible to map the stellar magnetic fields [7, 8]. In During the flights of the AFCRL rockets in 1975, a other cases, the spectral resolution is chosen based on source that was unusually bright at 10 and 20 µmand the fact that, since the polarization of spectral features was not present in the 2.2 µm catalog of Neugebauer is low, it is necessary to obtain signal-to-noise ratios and Leighton [4] was detected. The nebula’s optical for such features appreciably higher than the values image consists of two elliptical lobes with different characteristic of spectrophotometric measurements brightnesses elongated approximately north–south. of unpolarized light. Within the possibilities provided The central object illuminating the lobes is hidden by a single telescope, this can usually be achieved from the observer by a dark bridge, thought to be by reducing the spectral resolution, which is often a dust disk or torus. High-angular-resolution HST done without taking into consideration the ratio of the observations [5] have increased the number of objects line’s width to the detector’s resolution. A more de- similar to AFGL 2688 to nine. The parameters and tailed consideration shows that the problem cannot be atmospheric chemical composition of the central star solved so trivially; as an example, we consider the es- were determined in [6]. Here, we discuss published timation of the uncertainties of equivalent widths, W . 1063-7729/04/4804-0288$26.00 c 2004 MAIK “Nauka/Interperiodica” PROTOPLANETARY NEBULA AFGL 2688 289 (a) (b) 25 20 , % 15 p 10 5 0 36 38 40 42 44 46 36 38 40 42 44 46 mm Fig. 1. (a) Instrumental polarization, p, measured from spectra of the zero-polarization standard ζ Peg in the center of each order with number m [11]. (b) Polarization as a function of m from spectral measurements of the polarization standard HD 204827. The instrumental polarization has been subtracted. For equal numbers of counts, N, detected during an echelle spectrograph [9] converted into a spectropo- exposure per unit wavelength (Angstrom),˚ the uncer- larimeter [11]. Observing at the prime focus makes tainty in the equivalent width is inversely proportional it possible to avoid the effects of instrumental polar- to the square root of the spectral resolution, R,when ization produced at the third (flat) mirror of the 6-m the light detector’s pixel width, s, is larger than the telescope. We simultaneously recorded wavelengths line’s width, l [9]. Thus, if s>l, it is advantageous of 4950–6630 A.˚ The width of a pixel of the light to increase the spectral resolution, R, and not N,if detector (a CCD chip with 1160 × 1040 elements) the latter must be achieved by increasing the total corresponded to a radial-velocity interval of 7.7 km/s. exposure time. This statement is correct if the readout A broadband (600–700 nm) image of the nebula is noise is negligible (as is true for spectrophotometric shown in Fig. 1 of [1]. We observed the nebula’s observations). Thus, the most “economical” spectral northern lobe, with the slit center placed on the main resolution (for measurements of W )istheR value axis of the nebula (the line connecting the centers of providing a linear resolution equal to the spectral line the optical lobes), 5 from the nebula’s center, which width (s = l). is hidden by a dark bridge. The effective magnitude of the part of the image that fit into the slit was Post-AGB stars are surrounded by cool envelopes 15.5m. During each of the exposures, we rotated s<l with narrow lines, and the condition is satisfied the spectrograph to compensate for the rotation of only if R>60 000. For this reason, it is desirable the field that is characteristic of telescopes with to approach such resolutions if one wishes to use altitude–azimuth mounts. During the spectropolari- spectropolarimetric observations to separate the light metric observations, we kept the projection of the emitted by the star and envelope. Below, we show that slit perpendicular to the nebula’s main axis during increasing the spectral resolution by more than an one exposure, and turned it counterclockwise by 45◦ order of magnitude (from R<1000 to R = 15 000) relative to the first position during the next exposure. made it possible to obtain new spectrophotometric Examples of the resulting spectropolarimetric echelle information and enabled us to improve the model of images are shown in Fig. 7 of [11]. In addition to the AFGL 2688. However, it is more useful to study the target object, we observed standard stars for which wavelength dependence of the polarization detected photometric data indicated either high polarization for a number of post-AGB objects over a wide spectral (several percent) or no polarization (zero-polarization range [10] using medium-resolution spectra, since standards). The position of the crystal optical axes of such spectra can also be used to detect (but not the analyzer relative to the spectrograph’s elements measure) polarization effects in individual spectral was always the same. features. We reduced the echelle spectrophotometric im- Our high-spectral-resolution (R = 15 000) linear- ages with the ECHELLE software package of the polarization measurements were obtained at the MIDAS system using original algorithms tested for prime focus of the 6-m telescope of the Special the reduction of observations of polarization stan- Astrophysical Observatory (SAO) with the PFES dards [11, 12]. In particular, these algorithms make ASTRONOMY REPORTS Vol. 48 No. 4 2004 290 KLOCHKOVA et al. 2 λ r 1 0 80 60 , % p 40 20 0 110° θ 100° 90° 5580° 5600° 5620° λ, Å Fig. 2. Fragment of the spectrum (R = 15 000) of the central part of the northern lobe of the AFGL 2688 nebula near the C2 (1; 2) 5585.2 A˚ and (0; 1) 5635.5 A˚ Swan bands and graphs of the linear polarization and polarization position angle. it possible to remove effects due to the fact that the of the lens components of the spectrograph optics, spectrograph is not rigid, which appear when the this leads to a complex wavelength dependence for projected position angle of the slit is changed by 45◦, the instrumental polarization. Figure 1a shows the i.e., during rapid repositioning of the spectrograph. instrumental polarization measured for the centers A characteristic feature of spectropolarimetric obser- of the echelle orders during observations of zero- vations with an echelle spectrograph is the complex polarization standards. An example of correction for relation between the instrumental polarization and this relation is presented in Fig.