! Proposal 6574, submission 1 ! PI: Jeff Hester ! Received Thu Feb 15 17:46:57 EST 1996 ! From: scowen@tycho.la.asu.edu Proposal_Information Title: Ionization Structure, Photoevaporation, and Star Formation in M17 Proposal_Category: GO Scientific_Category: INTERSTELLAR MEDIUM Cycle: 6 Investigators PI_name: Jeff Hester PI_Institution: Arizona State University CoI_Name: Paul Scowen CoI_Institution: Arizona State University Contact: CoI_Name: Ravi Sankrit CoI_Institution: Arizona State University Contact: Abstract: Photoionization of photoevaporative flows provides a paradigm that ties the density, ionization, and dynamical structure of H II regions together in a single physical description (Hester et al. 1991a; Hester 1991). Such a model in principal removes much of the ambiguity in modeling of H II regions by providing a single prescription for calculating both the density and ionization structure of the region of emission. Recently Hester et al. (1995) successfully applied this model to WFPC2 observations of the H II region / molecular cloud interface in M16. In addition, we discovered over 50 small star forming globules being uncovered as the molecular material around them was evaporated. These objects call into question the ``evaporating disk'' interpretation of similar objects in M42. They also provide evidence of the role of photoevaporation in establishing the IMF in regions surrounding massive stars. In this proposal we request WFPC2 images of M17 to allow us to continue this work. These data will allow us to test the photoevaporative flow picture in a very different environment than M16, as well as continue the investigation of the interplay between massive stars and star formation in surrounding regions. Questions Observing_Description: We propose to observe M17 in three lines and two continuum filters. The three lines to be observed are HAlpha (F656N), S II (F673N), and O III (F502N). As is clear from the data presented in the scientific justification, these three lines show very different structures, and probe very different regimes in the photoevaporative flow. Each is essential to the investigation. HAlpha provides a direct measure of n_e^2 l through the ionized volume, and when coupled with S II allows the total density profile to be inferred beyond the hydrogen ionization edge. (Just beyond the H ionization edge, S is still predominately S^+. As a result, while I_HAlpha \propto n_e n_p ~ n_e^2 in this region, I_SII \propto n_e n_tot, where n_tot is the total density. As a result, n_tot \propto I_SII/sqrtI_HAlpha in the region of S II emission beyond the H ionization edge.) It is essential to use S II (rather than O II or N II) as a tracer of the dense region beyond the ionization edge. The ionization potential of S is lower than that of H, and so S is still predominately S^+ at the point that H is still neutral. O and N, on the other hand, have ionization potentials that are greater than or equal to the ionization potential of H, and so are also mostly neutral at the point where H recombines. N II and O II are historically thought of as being concentrated toward the edge of the nebula because further from the cloud they are driven to N III and O III, not because they extend further into the molecular cloud than does H II. Unfortunately, the choice of N II rather than S II for imaging H II regions significantly hampers the utility of some archival data (e.g., extensive observations of M42) for this kind of work. Finally, O III is the obvious best tracer of the lower density / higher ionization parameter part of the flow. We will also observe M17 in F547M. This filter is relatively line free, and so provides us with a way of correcting the line frames for scattered starlight. The F547M image will also be used to look for young stellar objects associated with globules and other structures. Finally, an F814W image will be taken to get maximum sensitivity and color information about any embedded, highly reddened stellar objects associated with star forming globules. Intensities for all lines have been measured from calibrated ground based images. In an 1100 s exposure, S/N in a single WFC pixel will range from ~20 to 90 in HAlpha, from ~ 7 to 30 in O III, and from ~ 5 to 30 in S II (assuming that the S II peaks up by ~ * 3 in the higher resolution images). We request time for one CR-SPLIT orbit in HAlpha, and 1.5 CR-SPLIT orbits (3 exposures) in each of O III and S II to allow us to reach S/N ~ 10 in the fainter regions in the field in all lines. We also request a single CR-SPLIT orbit in F547M, that will allow us to detect stellar objects to m_V ~ 27. Finally, we request a single CR-SPLIT orbit in F814W. The total request is 6 orbits, which fit into a single visit. The HAlpha images will be taken in the first orbit, and shortened somewhat to allow for the extra GSACQ overhead in that orbit. Real_Time_Justification: The only special requirement is that we optimize the pointing and orientation to maximize the range of physical conditions covered in the field of view. We have a great deal of experience working with the scheduling personnel at the STScI to successfully schedule observations that are constrained by orientation. We have already obtained deep ground based line and continuum imaging of M17. These data will provide a physical context within which to place the more detailed view afforded by the WFPC2. Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: M17 Alternate_Names: Description: ISM, HII Region Position: RA=18H 20M 44.270S +/- 0.010S, DEC=-16D 09' 37.80" +/- 0.10" Equinox: J2000 Rv_or_z: Flux: SURF-LINE(6563)=2+/-1E-14 Comments: Visits Visit_Number: 01 Visit_Requirements: !ORIENT 354D TO 356D SCHED 70% Visit_Comments: Originally 174D to 176D. Removed orient to find guide stars Exposure_number: 10 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F656N Optional_Parameters: CLOCKS=NO, CR-SPLIT=DEF, ATD-GAIN=15 Number_of_iterations:1 Time_Per_Exposure: 2400S Special_Requirements: Exposure_number: 20 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F656N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=15 Number_of_iterations:1 Time_Per_Exposure: 200S Special_Requirements: Exposure_number: 22 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=15 Number_of_iterations:1 Time_Per_Exposure: 200S Special_Requirements: Exposure_number: 25 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F502N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 1800S Special_Requirements: Exposure_number: 30 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F502N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 1300S Special_Requirements: Exposure_number: 35 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F502N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 1200S Special_Requirements: Exposure_number: 40 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=15 Number_of_iterations:1 Time_Per_Exposure: 1300S Special_Requirements: Exposure_number: 45 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=15 Number_of_iterations:1 Time_Per_Exposure: 1200S Special_Requirements: Exposure_number: 50 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F673N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2600S Special_Requirements: Exposure_number: 60 Target_Name: M17 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F673N Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2600S Special_Requirements: Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS Recipient_Email: