The First Ultraviolet Survey of Orion Nebula's Protoplanetary Disks and Outflows

ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN Several pointings were dropped in favor of better data quality. F373N filter was added (this was inadvertantly dropped from previous submission). The pointings for which there were no guide stars were changed. Pointings were optimized to maximize number of stars and outflows covered. **PC trying GSC1 for visits 07-09** 736640744 ----- Attempting Submission 2 (Thu Oct 31 20:50:42 GMT 2013) ----- ----- Attempting Submission 3 (Wed Dec 03 18:36:20 GMT 2014) ----- Will resolve during operational processing The First Ultraviolet Survey of Orion Nebula's Protoplanetary Disks and Outflows The Orion Nebula contains the nearest site of massive star-formation and the largest concentration of young stars, proto-planetary disks, and outflows in the Solar vicinity. Orion is a laboratory for addressing long-standing questions in the formation massive stars, star clusters, and the evolution of proto-planetary disks. A new WFC3 UV survey of the Nebula is proposed to resolve unprecedented detail in proto-planetary disks seen in silhouette, bright proplyds, and outflows, generating the first high-resolution UV images of a Galactic massive star-forming region. In the UV, nebular background provides a backlight 10 to 30 times brighter than at visual wavelengths. Low-mass young stars are much dimmer in the UV than in the visual, so disks previously hidden by stellar glare will be detected and characterized resulting in measurements of disk sizes. The color of radiation transmitted through the translucent dust in proplyds will probe grain growth. The UV MgII, violet [OII], and new H-alpha images will provide new constraints on disk mass-loss rates. By combining these measurements with our ALMA sub-mm determination of disk masses, disk life-times will be estimated. Outflows in the Nebula will be imaged for the first time in [OII] and [MgII] to probe the transition from neutral to ionized gas. Because of large proper motions, new WFC3 H-alpha images are required to register the MgII and [OII] data. The WFC3 H-alpha and coordinated parallel H-alpha imaging with ACS will be combined with archival HST images, to measure proper motions of nebular structures and outflows with unprecedented precision. UV imaging with WFC3 provides the highest angular resolution ever obtained with HST, ~ 0.035", a factor of two better than HST in H-alpha. Exposures will use sub- pixel dithering to recover the best resolution using multi-drizzle. Two broad-band filters are needed to determine the spectral index (colors) of the background light in the Nebula and the change in color caused by foreground dust associated with disks, proplyd disk-winds, and outflows. These data will be combined with archival longer wavelength HST images to determine grain properties such as grain sizes. While interstellar dust reddens background light, large grains produce grey, extinction nearly independent of wavelength. Most of the UV continuum is produced by normal ISM dust scattering in the ionization front at the rear of the nebula (e.g. Mathis et al. 1981). Nine fields will be observed with five WFC3 filters; F225W, F336W, F280N (MgII), F373N ([OII]), and F656N (H-alpha). Previous H-alpha studies have shown that outflow proper motions are detected on HST images obtained 1 to 2 years apart. New H-alpha images are needed for the fields being observed in the UV to measure the relative locations of the various emission regions in the current epoch. The WFC3 H-alpha images will require only a small fraction of the observing time. CR-SPLIT pairs of dithered images will be obtained to fill-in the inter-chip gaps. These observations will provide the first high-resolution and sensitive UV observations of a Galactic star forming region appropriate for determining the size distribution and mass-loss rates of proto-planetary disks and their outflows. Exposure times and Sensitivity of WFC3: HST/WFC3 has a throughput ranging from 8% in the F225W filter to about 18% in the F336W filter. Although this is a factor of three to four lower than the throughput of the filters around 600 nm, the order-of-magnitude increase of the background surface brightness more than compensates for this inefficiency. The UV continuum flux of the nebula has been measured with IUE (Mathis et al. 1981). From the Huygens core behind the Trapezium to the outer nebula 4' West and 3' North of the Trapezium, the measured continuum fluxes range from 7 to 90e-16 erg s^-1 arcsec^-2 A^-1 at 200 nm, to 4 to 50e16 erg s^-1 arcsec^-2 A^-1 at 300 nm. According to the WFC3 ETC, the photon count rates in the F225W and F336W filters will be 0.2 to 3.2 s^-1 per WFC3 pixel in the F225W filter to 0.4 to 5 s^-1 per pixel in the F336W filter, yielding S/N ratios of 10 to 70 with exposure times of 2 x 900 seconds. The violet [OII] lines have fluxes ranging from 1 to 10e-13 erg s^-1 cm^-2 arcsec^-2 (Rubin et al. 1998). The ETC indicates that a pair of 900 sec exposures will yield counts ranging from 2 to 25 photons per second in each WFC3 pixel, yielding a S/N of 40 to 140. MgII exposures will be comparable to [OII]. H-alpha exposures of 2 x 500 seconds are needed (as in previous HST programs). Coordinated Parallel observations with ACS: We propose to use the ACS WFC to observe portions of the Orion Nebula in coordinated parallel mode. Prior WFPC2 H-alpha images (GO 8324) and our Cycle 11 and 12 ACS programs (GO 9460 and GO 9825) show that everywhere in the proposed grid (Figure 4), half-orbit integrations in the F658N filter (which includes the H-alpha and 658 nm [NII] line) provide sufficient S/N to resolve proplyd structure, detect disks in silhouette against the background nebula, identify H-alpha bright shocks associated with outflows, and resolve their structure. The WFC3 grid will drive the ACS grid, which will be over-sampled. Since two orbits will be devoted to each field, we propose to use four ACS filters; F658N (H-alpha), F435W, F502N ([OIII]), & F660N ([NII]). This will be the first imaging of the outer Nebula in [OIII] and [NII]. The ACS fields are shown in red boxes in Figure 4. The orientations were chosen to maximize the coverage of the Nebula (the WFC3 and ACS fields cover about 70% of the grid observed in Cycles 12 and 13) and have at least a 5 week scheduling window according to the Visit Planner. 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