The Disk and Environment of HD C.A. Grady (Eureka Scientific and GSFC), G. Schneider (U. of Arizona), G. Williger (JHU and U. Louisville), B. Woodgate (NASA’s GSFC), M. Silverstone (U. of Arizona) and the HST-GO Team 1. HD The Environment 3. System Age 5. The Inner Disk B-V0.26 TypeA5Ve (Dunkin et al. 1997)A7Ve (this study) E(B-V)0.10<0.04 Vsin i55± 2 (Dunkin et al. 1997)V eq =244 km/s (this study) Age4-15 Myr (van Boekel et al. 2005) 8 ±4 Myr (this study) Fig. 1: Despite a wealth of stars in the field of HD (left), only 3 objects have net H  emission (right), including the Herbig Ae star, a star at the bottom of the field and 2MASS , which lies 8” SW of HD Data obtained with the Goddard Fabry-Perot at the Apache Point Observatory 3.5m. Fig. 2: 2MASS is revealed as an M2.5Ve star in Dual Imaging Spectrograph data from APO. Fig. 3: 2MASS is resolved into at least 2 sources separated by 0.11” in both NICMOS observations (right). Fig. 4: Using the distance to HD (145± 43 pc), and assigning the spectral type to the brighter dMe star allows us to place the pair in the HR diagram. Following Weinberger et al. (2000) in T eff calibration, and using the models of Baraffe et al. (1998), we find 2MASS is coeval with HR 4796 A. The estimated age agrees with independent estimates for HD A. The presence of 3 PMS stars within 1160 AU of each other is consistent with an aggregate similar to HD The Disk of HD A: Fig. 5:NICMOS camera 2 (FOV = 19.2” x 19.3”, scale: ~ 75.8 mas/pixel) direct and coronagraphic imagery of HD A and its nearby environment was obtained in a single HST orbit on 2005 April 30. Each observation set consisted of (a) two short F171M (1.71  m) target acquisition images, (b) three deep F110W (1.1  m) coronagraphic images (with the target obscured by the r = 0.3” coronagraphic hole [red circles in 7”x7” fields above]), and (c) two shallow direct (unocculted) F110W images. The spacecraft orientation was changed by 29.9° between the observation sets to permit discriminating against roll-invariant imaging artifacts. After subtracting a suite of appropriately scaled and registered PSF template star observations (spectral types A0-A8; J-H ± 0.3 of HD A), non-instrumentally scattered circumstellar light is seen in all PSF subtracted images where the underlying PSF structure is well matched to the HD A observations. The circumstellar light presents as an azimuthally-symmetric region which can be traced to (at least) ~ 1.3”. The PSF- subtracted images at both field orientations are consistent with a face-on viewing geometry as originally suggested by Dunkin et al. (1997) and found by Kuhn et al. (2001) with simultaneous dual beam polarimetric imaging in H-band. As a null test, paired subtractions of the template PSF stars from each other were found to closely match expectations of PSF-PSF images (Schneider et al, 2001) with small color and “breathing” (PSF stability) variations. Fig. 6, 7: The azimuthally-medianed data for both visits can be fit by single power laws. However, we consider the fit for the first visit more robust as the HST PSF was more stable and better matched to our reference PSFs. From those data, we find a peak surface brightness at r = 0.57" of 0.5 mJy/arcsec 2, with the disk surface brightness declining by r -3. This is a comparatively steep radial surface brightness power law, only slightly less steep than for  Pic, and comparable to that of HD (Grady et al. 2001; Augereau et al. 2001). The disk is intermediate in brightness between HD A or TW Hya and faint disks like HD (Grady et al.2000). While neither the NICMOS data nor the earlier data of Kuhn et al. (2001) sample r≤0.3" without saturation, more recent work by Habart et al. (2005) resolves the mid-IR PAH emission. The extent of the PAH emission is similar to HD , where an eccentric cavity is marginally resolved in fluorescent H 2 (Grady et al. 2005). Both PAH and H 2 emission require direct illumination by the star. The r -3 radial surface brightness profile for HD is consistent with grain growth and settling in the disk (Dullemond & Dominik 2004), resulting in a comparatively flat disk which can only be directly illuminated if there is a deficit of material in the inner disk. The low NIR excess, absence of emission from small, warm silicate grains (van Boekel et al. 2005; Meeus et al. 2001; Sylvester et al. 1996). the absence of [O I] emission (Acke et al. 2005), and the 3.3  m PAH profile (Habart & Natta 2005) all suggest a deficit of material at r ≤20 AU relative to other, face- on Herbig Ae disks. 6. Summary HD has newly identified T Tauri companions which allow us to confirm that this system is an older Herbig Ae star, comparable in age to HR 4796 A. The circumstellar disk can be traced out to 1.4” (200 AU) and exhibits an r -3 radial surface brightness power law, indicative of dust settling. The presence of extended PAH emission is expected in such a comparatively flat disk if there is a central cavity in the disk. The presence of a cavity is supported by the low level of the UV excess light, the weak Mg II and H  emission and the absence of a high velocity wind/jet component similar to that seen toward other Herbig Ae stars. Despite the presence of a large dust disk with abundant molecular gas (Dent et al. 2005; Raman et al. 2005), the UV data suggest that comparatively little material currently reaches the star. The central portion of this disk would therefore appear to be a promising environment, coupled with the face-on geometry, for near-stellar brown dwarf or young exo-planet searches. This study made use of data obtained as part of HST-GO The Goddard Fabry-Perot was supported under TBD. Apache Point Observatory Goddard Fabry-Perot observations were made through a grant of Director's Discretionary Time. Apache Point Observatory is operated by the Astrophysical Research Consortium. This studyKuhn et al. (2001)Raman et al. (2005) size1.4”1.5”1.6” inclination<20”13º±2 structurenoneSpiral feature to N Radial surface brightnessr -3 Fig. 8: HD has a UV spectrum in good agreement with Altair (A7V). Any UV excess is at least a factor of 5 fainter, relative to the star than for the accreting system HD Fig. 9: HD also lacks the strong Mg II emission and wind features characteristic of accreting Herbig Ae stars. Raman et al. (2005) find an accretion rate of M  yr -1. The available UV data suggest that this material is not reaching the star, and must be being intercepted either by a brown dwarf or planetary mass body.