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Mid-Infrared Imaging of the Post-AGB Star AC Herculis with the MMT Adaptive Optics System

We utilized the MMT's unique deformable secondary adaptive optics system to produce high-resolution (FWHM=0.3"), very high Strehl mid-infrared (9.8, 11.7&18 micron) images of the post-AGB star AC Her. The very high (98+/-2%) Strehls achieved with Mid-IR AO led naturally to an ultra-stable PSF independent of airmass, seeing, or location on the sky. We find no significant difference between AC Her's morphology and our unresolved PSF calibration stars (mu UMa&alpha Her) at 9.8, 11.7,&18 microns. Our current observations do not confirm any extended Mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 micron images which suggested the presence of a resolved ~0.6"edge-on circumbinary disk. We conclude that AC Her has no extended Mid-IR structure on scales greater than 0.2"(R<75 AU). These first results of Mid-IR AO science are very encouraging for future high accuracy Mid-IR imaging with this technique.


introduction
Recent evidence suggests that at least a few post-main sequence giants have acquired long-lived orbiting disks of dust and molecular gas (Jura & Kahane 1999). It has been suggested that these dust disks around post-main sequence stars may possibly lead to planet formation (Jura et al. 2000). It would be quite revolutionary if the process of planet formation occurs at both the "pre" and "post" main sequence phases of a star's lifetime. Since post-main sequence stars are much more luminous (L ∼ 1000 − 10000L ⊙ ) than pre-main sequence stars and have stronger winds and outflows, there should be very interesting interactions between the disks and these outflows. Ultimately one might be able to learn more about the disk/planet formation process if these asymptotic giant branch (AGB) disks can be confirmed. Here we report observations of one of the most interesting of these post-AGB circumstellar disks: AC Her.
The post-AGB spectroscopic binary star AC Her may be the prototype of these AGB disk systems. AC Her is one of the most luminous (L∼1000 L ⊙ ) and closest (D∼ 750 pc; Shenton et al. (1992)) pulsational variables transiting from the AGB to the planetary nebula phase (an RV Tauri star). AC Her is a luminous Mid-IR source (F ν = 42 Jy at 12 µm from IRAS) indicating significant circumstellar dust. Shenton et al. (1992) find AC Her's optical and IR light-curves can be explained by circumstellar dust around a non-radial pulsator. The origin and nature of this circumstellar dust has been the subject of speculation. Jura & Kahane (1999) argue that the very narrow (FWHM ∼ 5 km/s) CO (2-1) emission line (Bujarrabal et al. 1988) in AC Her is a signature of a gravitationally bound (not outflowing) long-lived reservoir of orbiting gas and dust. Van Winckel et al. (1998) also argue that there is strong evidence that such a reservoir of dust may be long-lived and disk-like. Jura & Kahane (1999) further argue that grains in size from 0.2-20 µm are present and grain growth to planetesimal formation is possible. Encouraged by the sub-mm CO observations of Bujarrabal et al. (1988) Jura et al. (2000 obtained 11.7 and 18.7 µm images of AC Her at the 10m Keck telescope in May and August 1999. The resulting Keck PSF FWHM ∼ 0.35 − 0.45 ′′ (11.7 and 18.7 µm) images were the sharpest ever taken of AC Her.
The Keck images of Jura et al. (2000) suggest that AC Her is clearly resolved at 18.7 µm into north and south unresolved "points" separated by ∼ 0.6 ′′ (see our inset in the upper right of Figure 1). Jura et al. (2000) modeled this image as an edge-on ring of dust of radius 300 AU in orbit around the 1.39 AU binary AC Her. They speculated that a binary such as AC Her could produce a small circumbinary ring of dust which would expand to a radius of 300 AU over time. They argued that this dust ring would be primarily composed of long-lived ∼ 200 µm particles which could collide to create the IR emitting 1 µm grains observed in the mid-IR images. However, since their model relies on the morphology of the Mid-IR images, it is important to confirm this morphology at higher Strehls. Hence, we present here very high Strehl Mid-IR images of AC Her obtained with adaptive optics.

observations
We have utilized the University of Arizona adaptive secondary AO system to obtain high resolution images of AC Her and several PSF calibration stars. The 6.5 m MMT telescope has a unique adaptive optics system. To re-duce the aberrations caused by atmospheric turbulence all AO systems have a deformable mirror which is updated in shape at ∼ 500 Hz. Until now all adaptive optics systems have located this deformable mirror (DM) at a re-imaged pupil (effectively a compressed image of the primary mirror). To reimage the pupil onto a DM typically requires 6-8 additional warm optical surfaces which significantly increases the thermal background and decreases the optical throughput of the system (Lloyd-Hart 2000). The MMT utilizes a completely new type of DM, which serves as both the secondary mirror of the telescope and the DM of the AO system. In this design, there are no additional optics required in front of the science camera, the emissivity is lower and thermal IR AO imaging becomes feasible.
The DM consists of 336 voice coil actuators that drive 336 small magnets glued to the backsurface of a thin (2.0 mm thick) 642 mm aspheric ULE glass "shell" (for a detailed review of the secondary mirror see Brusa et al. (2003a,b)). Complete positional control of the surface of this reflective shell is achieved by use of a capacitive sensor feedback loop. This positional feedback loop allows one to position an actuator of the shell to within 4 nm rms (total surface errors amount to only 40 nm rms over the whole secondary). The AO system samples at 550 Hz using 108 active subapertures. For a detailed review of the MMT AO system see Wildi et al. (2003a,b) and references within.

MMT Mid-IR AO Observations
We observed AC Her on the night of 2003, May 13 (UT). The AO system corrected the lowest 52 system modes and was updated at 550 Hz guiding on AC Her itself (V=7.03 mag). The closed-loop -3 dB bandwidth was estimated at 30 Hz. At 1.65 µm (H band) this level of correction leads to Strehls of ∼ 0.20 (Close et al. 2003). Since the size of a coherent patch of air (r • ) increases with λ 6/5 , imaging without AO correction can obtain images close to diffraction-limited in FWHM once λ > 8µm. However such "seeing-limited" non-AO Mid-IR images only approach Strehls of ∼0.5 which can lead to significant instability in the PSF calibration (since approximately half the light is outside the telescope's diffraction pattern PSF). To increase the Mid-IR Strehl we used AO correction which vastly improved our AC Her images to nearly perfect Strehls (∼0.98±0.02).

The MIRAC3 Mid-IR Camera
We utilized the 128x128 SiAs BIB 2-20µm MIRAC3 camera (Hoffmann et al. 1998). The 0.088 ′′ /pixel scale was used with the 9.8, 11.7 and 18 µm 10% bandwidth filters. To remove thermal and detector instabilities we chopped at 1 Hz with an internal cold chopper in the interface dewar BLINC (Hinz et al. 2000) between the AO system and MIRAC3.
We observed with the AO system locked continuously on AC Her. The 15 • tilted BLINC dewar window is a high quality dichroic which reflected the visible light (λ < 1 µm) to the AO wavefront sensor and transmitted the IR through BLINC to MIRAC3. Since the internal chopper in BLINC was past the dichroic, continuous 1 Hz chopping did not affect the visible light beam and hence the AO lock was unaffected. To further calibrate the back-ground (in addition to chopping) we nodded ∼ 6 − 8 ′′ in the telescope's azimuth direction (the horizontal direction in Figure 1) every minute. The internal chopper was set to run in the altitude direction (the vertical direction) with a chop throw of ∼ 20 ′′ . The derotator was disabled during these observations to help minimize the residual background structure as well.

Reducing the Mid-IR AO images
For the 9.8, 11.7, and 18 µm filters we obtained 8x1 minute coadded chop differenced images (one image from each nod). Four of these were beam A nods interlaced with 4 beam B nods. We utilized a custom IRAF script to reduce this Mid-IR data (Biller et al. 2003). The script produced eight background subtracted images by subtracting nod B from the following nod A (and the A nods from the B nods). The resulting 8 images were bad pixel corrected and flat-fielded. The pipeline cross-correlates and aligns each individual nod image (to an accuracy of ∼ 0.02 pixels), then rotates each image (by 270 • minus the current parallactic angle) so north is up and east is to the left. However, there was ≤ 10 • net parallactic rotation for any one filter observation (over a period of ∼ 8 min.) hence non-rotated images were also processed on a parallel track. These final aligned images were median combined. Figure 1 illustrates the final AC Her images (non-rotated version).
The Mid-IR images of the PSF calibration stars (µ UMa and α Her; observed before and after AC Her, respectively) were obtained and reduced in an identical manner to AC Her. In Figure 1 we illustrate our reduced PSF and AC Her images.

The PSF Star µ UMa
µ UMa is a well known spectroscopic binary (SB) with a period of 230.089 days and a small eccentricity (ǫ =0.06; Batten et al. (1989)). At an Hipparcos distance of 76.3 pc this suggests an average separation of ∼ 1.2 AU. Hence this binary would only subtend a maximum angle of 0.02 ′′ on the sky. Since this is a factor of 10 less than our resolution limit, the spectroscopic binary µ UMa should appear insignificantly different from a point source with a 6.3m telescope in the Mid-IR. Hence µ UMa is perfectly reasonable PSF star for this paper. Moreover, are no reports of extended Mid-IR structures resolved around µ UMa to date.

The PSF Star α Her
We also utilized α Her as a PSF star. This star is part of a wide binary system with a fainter (SB) companion located ∼ 4.84 ′′ (567 AU) away (Fabricius et al. 2002). This companion is not Mid-IR luminous and was not in our FOV, hence it did not affect our PSF image of the α Her primary. However, in 1993 the α Her primary was observed by the ISI interferometer (Danchi et al. 1994) to have a 0.25 − 0.35 ′′ thin shell in the Mid-IR. We do not detect any evidence of such a shell around the α Her primary in our Mid-IR AO observations. This is not surprising given that more recent 1999-2001 ISI measurements also fail to detect any shell around α Her (S. Tevosian private communication). Hence, as noted by Weiner et al. (2003), this shell may have evolved since the 1993 ISI measurements of (Danchi et al. 1994). It is not clear how a R ∼ 0.25−0.35 ′′ (29-41 AU) shell could become undetectable to the ISI in a period of ∼ 6 years. Detailed discussion of the current lack extended structure in the recent ISI interferometric measurements is beyond the scope of this paper. For now we simply note that α Her appears to be currently unresolved (on scales > 0.2 ′′ ) and hence we will utilize it as a PSF star in this paper.

reductions
We found that the AC Her data appeared consistent with an unresolved point source. We measured the FWHM, ellipticity, and positional angle of any such ellipticity for all the images in Figure 1. In Figure 2 we plot the ellipticity and FWHM for our dataset. As is clear from Figure 2 our AC Her data is very consistent with the PSF stars. Moreover, DAOPHOT's PSF fitting routine ALL-STAR (Stetson 1987) found AC Her to be highly consistent (to within 0.5%; see Figure 3) with an unresolved point source (α Her). Hence, it appears that AC Her is point-like in our data.
We also deconvolved AC Her by both the PSF stars µ UMa and α Her. Due to the very high Strehl (∼0.98) and high signal-to-noise ratio in our PSF images we could detect low contrast structure on scales of ∼ 0.2 ′′ with the IRAF Lucy deconvolution task (Biller et al. 2003). Even at these small spatial scales we detected no significant extended structure in the deconvolved images.

analysis
As is clear from Figures 1 -3 AC Her is a point source and is incompatible with the resolved (0.6 ′′ double peaked) morphology previously observed at Keck and reported by Jura et al. (2000). We have confirmed that we indeed observed AC Her, since the telescope coordinates were checked twice by an offset from a nearby SAO star. The measured 11.7 µm flux of our object (∼35 Jy) was in agreement with that of AC Her measured by IRAS (42 Jy at 12 µm). The possibility of locking the AO system on another V=7 mag object with ∼ 35 Jy at 11.7 µm at the location of AC Her (18:30:16.2 +21:52:01 J2000) -where there are no other nearby 10 µm sources -is highly unlikely.
Hence, concluding that we did indeed observe AC Her, it appears impossible to explain how a long-lived, R∼300 AU circumbinary disk, could have disappeared since the 1999 observations of Jura et al. (2000). Our deep images (3σ ∼0.1 Jy) at 9.8 & 11.7 µm would have easily detected the 0.6 ′′ "double-peaked" structure reported by Jura et al. (2000). The BLINC dichroic and pupil lens unfortunately stops transmitting longwards of ∼18µm, consequently our 18 µm images have low throughput and are weighed towards the blue end of the 10% bandpass filter. However, we should have detected the equal magnitude spots even in our low S/N 18 µm image, but there is no sign of any 0.6 ′′ double-peaked structure at 18 µm either.
AC Her's lack of any extended structure subtending a FWHM angle (θ disk ) greater than 0. ′′ 2 allows one to place lower limits on the temperature of dust providing the 41 Jy of flux observed by IRAS at 12 µm. The low lineof-sight optical extinction (E(B-V)=0.17 mag) to the star and the narrow CO linewidth observations of Bujarrabal et al. (1988) strongly suggest that the IR flux is produced by a nearly face-on disk. Our images imply an upper limit to the disk diameter of 0. ′′ 2 (D disk 150 AU), which for optically thick emission implies a minimum average brightness temperature over the disk of ≥ 200 K to produce the 12 µm IRAS flux of 41 Jy. Simple blackbody emission is clearly inconsistent with the observed spectral energy distribution (SED). Our result for the apparent upper limit to the diameter of the disk at 12 µm poses a severe challenge to a satisfactory dust disk model. In particular, this size limit is incompatible with the dust model of Shenton et al (1992), who proposed dust shells of 0. ′′ 5 and 0. ′′ 9 diameter. Moreover, the ISO observations of sharp features in the mid-IR spectrum of AC Her as well as the time variability (Shenton et al. 1992) requires a significant population of small, warm grains, while the far-IR/mm spectral index is essentially Rayleigh-Jeans, implying emission from large cold grains (Jura et al. 2000). A possible model consistent with our upper limit on the size is a flared disk of large particles with a surrounding "halo" of small particles, as suggested by Jura et al. (2000), but either truncated or of such low surface brightness at 12 µm as to be undetected in our images beyond a diameter of 0. ′′ 2. A flared disk model, such as proposed by Jura (2003) for HD 233517, might offer an explanation for the SED as well as the time variable IR flux and spectral features. The interior of the disk will be a long-lived reservoir of large grains, which is surrounded by a halo of small grains. A detailed model calculation for the radiative transfer is required but beyond the scope of this paper.

conclusions
We find no morphological evidence of any resolved structure around AC Her. The combination of adaptive optics with a deformable secondary allows very high Strehl images and high PSF stability regardless of the seeing, airmass, or target brightness. We are confident that AC Her appears unresolved in the Mid-IR on scales of ≥ 0.2 ′′ . This conclusion may impact current theories about whether or how AGB binaries can produce large (R∼300 AU) longlived circumbinary disks since AC Her was the prototypical object. The hypothesis of a large R∼300 AU circumbinary ring around AC Her seems unlikely in light of these observations, however, a smaller R disk < 75 AU (D∼750 pc) circumbinary disk cannot be ruled out.
Adaptive optics at Mid-IR wavelengths appears to be a very promising new technique that allows for uniquely stable PSFs and high Strehls. A high degree of PSF stability will eliminate morphological ambiguities due to poor (seeing-limited) PSF calibrations. Mid-IR AO should have a significant impact on any field where Mid-IR imaging is possible. These MMT observations were made possible with the hard work of the entire Center for Astronomical Adaptive Optics (CAAO) staff at the University of Arizona. In particular, we would like to thank Tom McMahon, Kim Chapman, Doris Tucker, and Sherry Weber for their endless support of this project. The wide field AO CCD was installed by graduate student Nick Siegler. Dylan Curly helped develop the MMT AO system user interface. Graduate student Wilson Liu helped run the MIRAC3 camera during the run. The adaptive secondary mirror is a joint project of University of Arizona and the Italian National Institute of Astrophysics -Arcetri Observatory. We would also like thank the whole MMT staff for their excellent support and flexibility during our commissioning run at the telescope. Fig. 1.-The 9.8, 11.7, and 18 µm images of AC Her and PSF stars µ UMa and α Her as observed at the MMT. In the upper right we have inserted the published 18 µm Keck image of AC Her (in false color; Jura et al. (2000)). The box size of the MMT images is 1.5x1.0 ′′ , the effective scale of the Keck image is similar with a box size of ∼0.7x1.0 ′′ . Note how there is no sign of any extended structure in the MMT AC Her images in any of the filters. The faint point source in the lower left of each MMT image is a MIRAC3 ghost. Fig. 2.-The 9.8 and 11.7 µm FWHM and ellipticity of AC Her and the PSF stars µ UMa and α Her (the Gaussian fit FWHM are the upper star symbols and the enclosed FWHM are represented by the slightly lower circles; AC Her is the middle dataset in the 9.8 & 11.7 µm clusters). The location of the "double-peaked" morphology is estimated from the previous Keck image (FWHM ∼ 0.8 ′′ ; Jura et al. (2000)) to the upper right. Note that AC Her's morphology appears much more consistent with that of the PSF stars at 9.8 and 11.7 µm than an extended FWHM ∼ 0.8 ′′ disk. Fig. 3.-The 11.7 µm PSF of AC Her before (left) and after (right) PSF subtraction (using α Her as the PSF) with DAOPHOT's ALLSTAR task. The residual flux after PSF subtraction is < 0.5% of AC Her's original flux. Similar residuals resulted from PSF subtractions at 9.8 µm and 18 µm. Based on these excellent subtractions it appears AC Her is not detectably extended. Note that the small ghost image to the lower left in each frame is not subtracted to show that the vertical scales are the same for both images.