This is a resubmission of the Phase II for 11336 and includes use of a DITHER pattern for the subexposures. ----- Attempting Submission 1 (Tue Oct 16 18:10:55 GMT 2007) ----- Phase II Proposal 11336 (GMAURcyc16_PHII.apt) successfully submitted. ----- Attempting Submission 2 (Thu Oct 25 22:40:11 GMT 2007) ----- Transitional disks are one of the most crucial and important stages of the evolution of pre-main-sequence stars and protoplanetary systems. These disks have transformed most of the dust and gas in their inner regions into planetesimals and show clear inner ``holes'' that almost certainly harbor infant planetary systems. We propose to observe 3 young stars with transitional disks (T Cha, GM Aur, LkHa 330) using ACIS-S to measure their X-ray radiation field and coronal emission properties and HST ACS-SBC to image the fluorescently excited molecular hydrogen in these systems to determine if the gas distribution follows the behavior seen in the dust distribution. [Only observations of GM Aur were approved. ]Our broader goal is to model the X-ray/UV photoionization and photoexcitation of the disks and study their photochemistry.

We shall obtain ACS-SBC imaging of the circumstellar H_2 emission surrounding the pre-main-sequence star GM Aur. Based on our previous UV spectroscopic observations there is strong evidence that extended H_2 emission is present associated with the star's circumstellar disk and that the SBC spatial resolution can provide spatially-resolved images of the emission. The STIS G140L UV spectrum of GM Aur was observed by Bergin et al. (2004, ApJ, 614, L133). The star will be imaged using the ACS SBC detector and the F140LP and F165LP filters. The ACS exposures will require three HST orbits to obtain the SBC longpass images of GM Aur. This observing procedure is identical to that which we used to observe the young star T Tau in Cycle 14 (GO 10489), except that a 4-point dither pattern has been used. Three orbits provide cumulative exposures of 5.0 ksec and 2.5 ksec for F140LP and F165LP respectively. When an F165LP image is subtracted from an F140LP image, a well-defined, effective passband of 1350 - 1650 A results, and the off-star signal is dominated by H_2 emission. The F165LP image is an important control to characterize the presence of any reflection nebulosity and other confusing (non-H_2) features. Use of the F140LP and F165LP filters maximizes the signal-to-noise because the H Ly alpha and O I geocoronal emission is suppressed, while the selected passband contains the strongest H_2 lines. The F165LP passband contains only a small amount of H_2 flux. At the 140 pc distance of the Taurus-Auriga star formation region [GM Aur] the 0.032 arcsec SBC pixels have a size equivalent to 4.5 AU; thus the ACS-SBC offers the possibility to explore detailed spatial structure very close to a PMS star, that is difficult to observe any other way. The PSF is measured to be 0.12 arcsec FWHM in our T Tau images, which is small compared to structures such as the 0.3 arcsec hole (Calvet et al. 2005, ApJ, 630, 185) in the GM Aur disk. GM Aur should not violate the ACS MAMA bright limits even for the pixels containing the stellar point source. The nearest other comparable star is located 28 arcsec away. We are able to derive reliable signal-to-noise estimates for GM Aur, because it already has suitable UV spectra. The on-source image will have 16 ct/sec/pixel for F140LP and 5 ct/sec/pixel for F165LP; such count rates are at levels of less that 1/3 of the local bright limit. The observed spectral flux from GM Aur is almost identical to that of T Tau (actually a few percent lower), so we expect no saftety issues based on our previous successful imaging of T Tau. STAR FORMATION 12 false 0.0 false false false false false false SUPPORTED STAR K V-IV PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK T TAURI STAR Coordinates taken from "Pre-main Sequence star proper Motion Catalogue" (Ducorant et al. 2005, A&A, 438, 769). J2000 04 55 10.9800 +30 21 59.40 J2000 ACS/SBC Imaging No No F165LP false ACS/SBC Imaging No No F140LP POS-TARG DITHER 0.179 LINE 0.116 false false 63.650002 4 20.02 ACS-SBC-DITHER-BOX 1 false true C:\Documents and Settings\ab\My Documents\HST\OP-cache\GMAURcyc16_PHII-v01.xml 1193350992032 (Tic-Visit :proposal-id 11336 :visit-id "01" :proposal-version -1 :trans-version "20070710" :date "25-Oct-2007 16:23:10.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "28" :si-bright-earth-limb-angle 20.0 :data-volume 222160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11336 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11336 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2789 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11336 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2788 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11336 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2792 :al-overhead-at-end 0 :internal-dur-at-end 281 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) true 3 0 true C:\Documents and Settings\ab\My Documents\HST\OP-cache\GMAURcyc16_PHII-v01.xml 1192558228440