arXiv:astro-ph/0310580v1 20 Oct 2003 oapa nteNvme 0 03iseo h Astrophysical the of L issue using 2003 typeset 20, Preprint November the in appear To Institution. 1 nA e sasgaueo rvttoal on (not bound gravitationally a of signature (FWHM a narrow is very Her the AC that in Jura argue cir- (1999) speculation. this ∼ of of subject nature Kahane the and & been origin has IR The dust and cumstellar pulsator. optical around Her’s dust non-radial circumstellar AC a by find explained (1992) be can al. light-curves et Shenton dust. 12 at h rttp fteeABds ytm.A e sone is Her (L AC luminous systems. most disk the AGB of these of prototype the post- these of Her. interesting AC most disks: report the circumstellar we AGB of Here one confirmed. gi- of be about asymptotic observations can more these disks learn (AGB) if branch to process ant able formation out- be disk/planet very might these the be one and Ultimately should disks there the flows. between outflows, interactions and interesting winds stronger have otmi eunesasaemc oelmnu (L luminous Since more and much lifetime. “pre” are ’s the a pro- ∼ both the of sequence at if phases post–main revolutionary occurs sequence quite main formation be “post” planet would (Jura of It has formation cess planet It 2000). to al. lead 1999). se- et possibly post-main may around Kahane disks stars dust & quence these (Jura that suggested gas of been disks molecular orbiting and long-lived dust acquired have giants sequence tr.A e salmnu i-Rsuc ( source Mid-IR luminous a Tauri is RV (an Her phase AC transiting planetary star). variables the to pulsational AGB (1992)) the from al. et Shenton pc; h eut rsne eemd s fteo M Observatory MMT of the of use made here presented results The h otABsetocpcbnr trA e a be may Her AC star binary spectroscopic post-AGB The eeteiec ugssta tlatafwpost-main few a least at that suggests evidence Recent ms O(-)eiso ie(uarble l 1988) al. et (Bujarrabal line emission (2-1) CO km/s) 5 1000 µ rmIA)idctn infiatcircumstellar significant indicating IRAS) from m − trA e.Tevr ih(98 high very The Her. AC star eouin(FWHM=0.3 resolution S needn faras eig rlcto ntesy efidn ( find stars We calibration sky. PSF the unresolved on our location and or morphology seeing, Her’s airmass, AC of independent PSF bevtosaei oflc ihpeiul eotdKc (seeing- resolved Keck a reported of presence previously the with suggested M conflict which extended any in confirm are not do observations observations current Our microns. a oetne i-Rsrcueo clsgetrta 0 than greater scales on structure Mid-IR extended no has Osineaevr norgn o uuehg cuayMdI im Mid-IR accuracy high future headings: for Subject encouraging very are science AO euiie h nqe65 M eombescnayaatv optics adaptive secondary deformable MMT 6.5m unique the utilized We 10000 I-NRRDIAIGO H OTABSA CHRUI IHT WITH HERCULIS AC STAR POST-AGB THE OF IMAGING MID-INFRARED ar .Close M. Laird rnosWildi Francois L ⊙ A 1. T hnpemi eunesasand stars sequence pre-main than ) E tl mltajv 25/04/01 v. emulateapj style X introduction ∼ omto tr:AB rt-lntr Nebulae Proto-Planetary AGB, stars: — formation oapa nteNvme 0 03iseo h Astrophysical the of issue 2003 20, November the in appear To 00L 1000 ntuetto:aatv pis—bnre:gnrl—sas evo stars: — general binaries: — optics adaptive instrumentation: 1 ehBiller Beth , 2 1 rer bevtr,Uiest el td i iez,I Firenzi, di, Studi degli Universita Observatory, Arcetri ihe Lloyd-Hart Michael , ′′ 1 ⊙ ,vr ihSrh i-nrrd(.,1. 18 & 11.7 (9.8, mid-infrared Strehl high very ), twr bevtr,Uiest fAioa usn Z857 AZ Tucson, Arizona, of University Observatory, Steward n lss (D closest and ) ± % tel civdwt i-RA e aual oa ultra-stable an to naturally led AO Mid-IR with achieved Strehls 2%) DPIEOTC SYSTEM OPTICS ADAPTIVE 1 ila .Hoffmann F. William , F ν 2Jy 42 = [email protected] ∼ ∼ oe Angel Roger ora Letters Journal 0 ABSTRACT 750 . 6 1 aiiyjityoeae yteUiest fAioaan Arizona of University the by operated jointly facility a , ud Brusa Guido , ′′ deo icmiayds.W ocueta CHer AC that conclude We disk. circumbinary edge-on 1 atce hc ol old ocet h Reitn 1 emitting IR the create to collide µm could which particles mgso CHrotie ihaatv optics. Mid-IR adaptive Strehl with high obtained very Her AC here higher im- of at present Mid-IR images morphology we the this Hence, confirm of to Strehls. morphology important the is it on ages, relies model their eecp a nqeaatv pissse.T re- To system. MMT optics m adaptive 6.5 unique The a stars. has calibration AC of PSF telescope images several resolution and high obtain Her to system AO ondary ta.(00 band1. n 18.7 Jura and 11.7 (1988) obtained al. (2000) et the al. Bujarrabal by et of Encouraged observations CO possible. sub-mm is formation planetesimal to n iklk.Jr aae(99 ute ru that argue further long-lived (1999) 0.2-20 be from may Kahane size strong in dust & grains is Jura of there reservoir disk-like. that a and argue such also dust. that (1998) and evidence al. gas et orbiting Winckel of Van reservoir long-lived outflowing) tte1mKc eecp nMyadAgs 99 The 1999. August and May FWHM PSF in Keck telescope resulting Keck 10m the at µm e sceryrsle t18.7 at resolved clearly is Her neovd“ons eaae by separated “points” unresolved aiso 0 Uoe ie hyage htti dust this that long-lived argued of They composed primarily a time. be to over would They expand AU a ring would 300 produce which Her. could of dust 300 AC Her radius of AC radius binary ring as of AU circumbinary mod- such dust small 1.39 binary (2000) of a the al. that ring around et speculated edge-on Jura an in 1). as AU Figure image of this right eled upper the in ehv tlzdteUiest fAioaaatv sec- adaptive Arizona of University the utilized have We h ekiae fJr ta.(00 ugs htAC that suggest (2000) al. et Jura of images Keck The . 1 mgswr h hretee ae fA Her. AC of taken ever sharpest the were images ) 2 risosre ntemdI mgs oee,since However, images. mid-IR the in observed grains ′′ ( 1 < R hlM Hinz M. Phil , 1 , 2 o Fisher Don , dI tutr rudA e.These Her. AC around structure id-IR µ 5A) hs rtrslso Mid-IR of results first These AU). 75 1 iie)1. n 8mco images micron 18 and 11.7 limited) gn ihti technique. this with aging M & UMa 515Italy -50125 infiatdffrnebetween difference significant o ora Letters Journal µ 2. )iae ftepost-AGB the of images m) 1 21 observations α onH Bieging H. John , 1 ytmt rdc high- produce to system ogMiller Doug , µm e)a .,1.,&18 & 11.7, 9.8, at Her) ∼ r rsn n ri growth grain and present are uin—stars: — lution 0 µm . 35 EMMT HE − ∼ µm nonrhadsouth and north into 0 0 . 45 . 1 mgso CHer AC of images 6 , ′′ ′′ 1 h Smithsonian the d , 1. n 18.7 and (11.7 seorinset our (see ∼ 200 µm 2 Close et al. duce the aberrations caused by atmospheric turbulence all ground (in addition to chopping) we nodded ∼ 6 − 8′′ AO systems have a deformable mirror which is updated in in the telescope’s azimuth direction (the horizontal direc- shape at ∼ 500 Hz. Until now all adaptive optics systems tion in Figure 1) every minute. The internal chopper was have located this deformable mirror (DM) at a re-imaged set to run in the altitude direction (the vertical direction) pupil (effectively a compressed image of the primary mir- with a chop throw of ∼ 20′′. The derotator was disabled ror). To reimage the pupil onto a DM typically requires during these observations to help minimize the residual 6-8 additional warm optical surfaces which significantly in- background structure as well. creases the thermal background and decreases the optical The 0.505′′ PA=269◦ Washington Double Star catalog throughput of the system (Lloyd-Hart 2000). The MMT astrometric binary WDS 02589+2137 BU was observed utilizes a completely new type of DM, which serves as both earlier (2003, November 25 UT) and used to calibrate the the secondary mirror of the telescope and the DM of the camera’s orientation and its 0.088′′/pixel platescale. AO system. In this design, there are no additional optics required in front of the science camera, the emissivity is 2.1.2. Reducing the Mid-IR AO images lower and thermal IR AO imaging becomes feasible. For the 9.8, 11.7, and 18 µm filters we obtained 8x1 The DM consists of 336 voice coil actuators that drive minute coadded chop differenced images (one image from 336 small magnets glued to the backsurface of a thin (2.0 each nod). Four of these were beam A nods interlaced mm thick) 642 mm aspheric ULE glass “shell” (for a de- with 4 beam B nods. We utilized a custom IRAF script tailed review of the secondary mirror see Brusa et al. to reduce this Mid-IR data (Biller et al. 2003). The script (2003a,b)). Complete positional control of the surface of produced eight background subtracted images by subtract- this reflective shell is achieved by use of a capacitive sen- ing nod B from the following nod A (and the A nods from sor feedback loop. This positional feedback loop allows the B nods). The resulting 8 images were bad pixel cor- one to position an actuator of the shell to within 4 nm rected and flat-fielded. The pipeline cross-correlates and rms (total surface errors amount to only 40 nm rms over aligns each individual nod image (to an accuracy of ∼ 0.02 the whole secondary). The AO system samples at 550 Hz pixels), then rotates each image (by 270◦ minus the cur- using 108 active subapertures. For a detailed review of the rent parallactic angle) so north is up and east is to the MMT AO system see Wildi et al. (2003a,b) and references left. However, there was ≤ 10◦ net parallactic rotation within. for any one filter observation (over a period of ∼ 8 min.) hence non-rotated images were also processed on a parallel 2.1. MMT Mid-IR AO Observations track. These final aligned images were median combined. We observed AC Her on the night of 2003, May 13 (UT). Figure 1 illustrates the final AC Her images (non-rotated The AO system corrected the lowest 52 system modes and version). was updated at 550 Hz guiding on AC Her itself (V=7.03 The Mid-IR images of the PSF calibration stars (µ UMa mag). The closed-loop -3 dB bandwidth was estimated and α Her; observed before and after AC Her, respectively) at 30 Hz. At 1.65 µm (H band) this level of correction were obtained and reduced in an identical manner to AC leads to Strehls of ∼ 0.20 (Close et al. 2003). Since the Her. In Figure 1 we illustrate our reduced PSF and AC 6/5 size of a coherent patch of air (r◦) increases with λ , Her images. imaging without AO correction can obtain images close The PSF Star µ UMa to diffraction-limited in FWHM once λ > 8µm. How- 2.2. ever such “seeing-limited” non-AO Mid-IR images only µ UMa is a well known spectroscopic binary (SB) with approach Strehls of ∼0.5 which can lead to significant a period of 230.089 days and a small eccentricity (ǫ =0.06; instability in the PSF calibration (since approximately Batten et al. (1989)). At an Hipparcos distance of 76.3 half the light is outside the telescope’s diffraction pattern pc this suggests an average separation of ∼ 1.2 AU. Hence PSF). To increase the Mid-IR Strehl we used AO correc- this binary would only subtend a maximum angle of 0.02′′ tion which vastly improved our AC Her images to nearly on the sky. Since this is a factor of 10 less than our resolu- perfect Strehls (∼0.98±0.02). tion limit, the spectroscopic binary µ UMa should appear insignificantly different from a point source with a 6.3m 2.1.1. The MIRAC3 Mid-IR Camera telescope in the Mid-IR. Hence µ UMa is perfectly rea- We utilized the 128x128 SiAs BIB 2-20µm MIRAC3 sonable PSF star for this paper. Moreover, are no reports camera (Hoffmann et al. 1998). The 0.088′′/pixel scale of extended Mid-IR structures resolved around µ UMa to was used with the 9.8, 11.7 and 18 µm 10% bandwidth date. filters. To remove thermal and detector instabilities we 2.3. The PSF Star α Her chopped at 1 Hz with an internal cold chopper in the in- terface dewar BLINC (Hinz et al. 2000) between the AO We also utilized α Her as a PSF star. This star is part system and MIRAC3. of a wide binary system with a fainter (SB) companion We observed with the AO system locked continuously on located ∼ 4.84′′ (567 AU) away (Fabricius et al. 2002). AC Her. The 15◦ tilted BLINC dewar window is a high This companion is not Mid-IR luminous and was not in our quality dichroic which reflected the visible light (λ < 1 FOV, hence it did not affect our PSF image of the α Her µm) to the AO wavefront sensor and transmitted the IR primary. However, in 1993 the α Her primary was observed through BLINC to MIRAC3. Since the internal chopper by the ISI interferometer (Danchi et al. 1994) to have a in BLINC was past the dichroic, continuous 1 Hz chop- 0.25 − 0.35′′ thin shell in the Mid-IR. We do not detect ping did not affect the visible light beam and hence the any evidence of such a shell around the α Her primary in AO lock was unaffected. To further calibrate the back- our Mid-IR AO observations. This is not surprising given Mid-IR AO Imaging of AC Her 3 that more recent 1999-2001 ISI measurements also fail to Jy of flux observed by IRAS at 12 µm. The low line- detect any shell around α Her (S. Tevosian private com- of-sight optical extinction (E(B-V)=0.17 mag) to the star munication). Hence, as noted by Weiner et al. (2003), this and the narrow CO linewidth observations of Bujarrabal shell may have evolved since the 1993 ISI measurements of et al. (1988) strongly suggest that the IR flux is produced (Danchi et al. 1994). It is notclear howa R ∼ 0.25−0.35′′ by a nearly face-on disk. Our images imply an upper limit ′′ (29-41 AU) shell could become undetectable to the ISI in to the disk diameter of 0. 2 (Ddisk . 150 AU), which for a period of ∼ 6 . Detailed discussion of the current optically thick emission implies a minimum average bright- lack extended structure in the recent ISI interferometric ness temperature over the disk of ≥ 200 K to produce the measurements is beyond the scope of this paper. For now 12 µm IRAS flux of 41 Jy. Simple blackbody emission is we simply note that α Her appears to be currently unre- clearly inconsistent with the observed spectral energy dis- solved (on scales > 0.2′′) and hence we will utilize it as a tribution (SED). Our result for the apparent upper limit to PSF star in this paper. the diameter of the disk at 12 µm poses a severe challenge to a satisfactory dust disk model. In particular, this size 3. reductions limit is incompatible with the dust model of Shenton et al ′′ ′′ We found that the AC Her data appeared consis- (1992), who proposed dust shells of 0. 5 and 0. 9 diameter. tent with an unresolved point source. We measured the Moreover, the ISO observations of sharp features in the FWHM, ellipticity, and positional angle of any such ellip- mid-IR spectrum of AC Her as well as the time variabil- ticity for all the images in Figure 1. In Figure 2 we plot ity (Shenton et al. 1992) requires a significant population the ellipticity and FWHM for our dataset. As is clear from of small, warm grains, while the far-IR/mm spectral index Figure 2 our AC Her data is very consistent with the PSF is essentially Rayleigh-Jeans, implying emission from large stars. Moreover, DAOPHOT’s PSF fitting routine ALL- cold grains (Jura et al. 2000). A possible model consistent STAR (Stetson 1987) found AC Her to be highly con- with our upper limit on the size is a flared disk of large sistent (to within 0.5%; see Figure 3) with an unresolved particles with a surrounding “halo” of small particles, as point source (α Her). Hence, it appears that AC Her is suggested by Jura et al. (2000), but either truncated or of such low surface brightness at 12 µm as to be undetected point-like in our data. ′′ We also deconvolved AC Her by both the PSF stars µ in our images beyond a diameter of 0. 2. A flared disk UMa and α Her. Due to the very high Strehl (∼0.98) and model, such as proposed by Jura (2003) for HD 233517, high signal-to-noise ratio in our PSF images we could de- might offer an explanation for the SED as well as the time tect low contrast structure on scales of ∼ 0.2′′ with the variable IR flux and spectral features. The interior of the IRAF Lucy deconvolution task (Biller et al. 2003). Even disk will be a long-lived reservoir of large grains, which is at these small spatial scales we detected no significant ex- surrounded by a halo of small grains. A detailed model cal- tended structure in the deconvolved images. culation for the radiative transfer is required but beyond the scope of this paper. 4. analysis 5. conclusions As is clear from Figures 1 – 3 AC Her is a point source and is incompatible with the resolved (0.6′′ double peaked) We find no morphological evidence of any resolved struc- morphology previously observed at Keck and reported by ture around AC Her. The combination of adaptive optics Jura et al. (2000). We have confirmed that we indeed with a deformable secondary allows very high Strehl im- observed AC Her, since the telescope coordinates were ages and high PSF stability regardless of the seeing, air- mass, or target brightness. We are confident that AC Her checked twice by an offset from a nearby SAO star. The ′′ measured 11.7 µm flux of our object (∼35 Jy) was in agree- appears unresolved in the Mid-IR on scales of ≥ 0.2 . This ment with that of AC Her measured by IRAS (42 Jy at 12 conclusion may impact current theories about whether or µm). The possibility of locking the AO system on another how AGB binaries can produce large (R∼300 AU) long- V=7 mag object with ∼ 35 Jy at 11.7 µm at the location lived circumbinary disks since AC Her was the prototypical of AC Her (18:30:16.2 +21:52:01 J2000) – where there are object. The hypothesis of a large R∼300 AU circumbinary no other nearby 10 µm sources – is highly unlikely. ring around AC Her seems unlikely in light of these obser- Hence, concluding that we did indeed observe AC Her, vations, however, a smaller Rdisk < 75 AU (D∼750 pc) it appears impossible to explain how a long-lived, R∼300 circumbinary disk cannot be ruled out. AU circumbinary disk, could have disappeared since the Adaptive optics at Mid-IR wavelengths appears to be 1999 observations of Jura et al. (2000). Our deep images a very promising new technique that allows for uniquely (3σ ∼0.1 Jy) at 9.8 & 11.7 µm would have easily detected stable PSFs and high Strehls. A high degree of PSF sta- the 0.6′′ “double-peaked” structure reported by Jura et al. bility will eliminate morphological ambiguities due to poor (2000). The BLINC dichroic and pupil lens unfortunately (seeing-limited) PSF calibrations. Mid-IR AO should have stops transmitting longwards of ∼18µm, consequently our a significant impact on any field where Mid-IR imaging is 18 µm images have low throughput and are weighed to- possible. wards the blue end of the 10% bandpass filter. However, we should have detected the equal magnitude spots even These MMT observations were made possible with the in our low S/N 18 µm image, but there is no sign of any hard work of the entire Center for Astronomical Adap- 0.6′′ double-peaked structure at 18 µm either. tive Optics (CAAO) staff at the University of Arizona. In AC Her’s lack of any extended structure subtending a particular, we would like to thank Tom McMahon, Kim ′′ FWHM angle (θdisk) greater than 0. 2 allows one to place Chapman, Doris Tucker, and Sherry Weber for their end- lower limits on the temperature of dust providing the 41 less support of this project. The wide field AO CCD was 4 Close et al. installed by graduate student Nick Siegler. Dylan Curly The secondary mirror development could not have been helped develop the MMT AO system user interface. Grad- possible without the support of the Air Force Office of Sci- uate student Wilson Liu helped run the MIRAC3 camera entific Research under grant AFOSR F49620-00-1-0294. during the run. The adaptive secondary mirror is a joint LMC acknowledges support from NASA Origins grant project of University of Arizona and the Italian National NAG5-12086 and NSF SAA grant AST0206351. JHB ac- Institute of Astrophysics - Arcetri Observatory. We would knowledges support from NSF grants AST-9987408 and also like thank the whole MMT staff for their excellent AST-0307687. We thank the anonymous referee for helpful support and flexibility during our commissioning run at comments about this paper and our PSF stars. We thank the telescope. Mike Jura for helpful discussions and insightful comments.

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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. 6 Close et al.

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. Mid-IR AO Imaging of AC Her 7

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.