0 Assigned ID: 399 ----- Attempting Submission 1 (Fri Apr 08 06:28:13 GMT 2016) ----- HST Phase I Proposal 399 (phase1-GO.apt) successfully submitted. Receipt: # 399-1 ----- Attempting Submission 2 (Fri Apr 08 22:05:45 GMT 2016) ----- HST Phase I Proposal 399 (phase1-GO.apt) successfully submitted. Receipt: # 399-2 ----- Attempting Submission 3 (Fri Apr 08 23:39:53 GMT 2016) ----- 0 ----- Attempting Submission 1 (Tue Jul 19 21:51:35 GMT 2016) ----- Searching for the Peak of the Initial Mass Function in Galactic Center Star Clusters We propose to re-observe the Arches and Quintuplet clusters at the Galactic Center with WFC3-IR to measure the Initial Mass Function (IMF) down to 1 Msun using proper motions with 3x longer baseline and precision. These clusters are ideal targets as they probe one of the most extreme environments in which stellar populations can be spatially resolved and IMFs can be measured accurately. The proposed observations build on our previous two-year proper motion program where we demonstrated the ability to identify cluster members using proper motions down to 2.5 Msun (Hosek et al. 2015). In Clarkson et al. (2012), we used high-precision astrometry in the Arches cluster core to show that the cluster's dynamical mass is too low for a "universal" mass function, indicating that peak of the IMF is likely between 1-2 Msun. The proposed observations extend our cluster sample to 1 Msun, allowing us to directly search for the predicted peak while definitively establishing the high-mass slope of the IMF. The additional epoch of astrometry also makes it possible to measure the velocity dispersion profiles of the clusters, allowing us to constrain their orbits and model the impact of dynamical evolution on their observed mass functions. Ultimately, these observations will result in the deepest and cleanest measurement of the IMF in the Galactic Center environment. The goal of our observations is to obtain another epoch of high-precision astrometric measurements of the Arches and Quintuplet clusters. These clusters are optically obscured and have a large (~120") spatial extent. Therefore, all observations will be conducted with WFC3 in the near infrared (F153M). The specific requirements and exposure times for the observations are described below. Our group has significant experience in working with high precision astrometric and photometric data sets both in general and from HST. Astrometry: We will duplicate the observing conditions of program GO-12318 and observe with WFC3's longest-wavelength medium filter (F153M) to obtain astrometry. We use a many-step spiral dither pattern that fully samples the pixel phase in order to reduce the effects of the undersampled PSF. The total extent of the dither pattern is less than 20 pixels. To minimize the impact of any residual distortions, we request that the astrometric observations within each cluster have the SAME ORIENT and POS TARG offsets as our previous HST measurements (i.e., we want the stars in the exact same positions on the detector). We use the same filter as our previous observations to prevent any filter-dependent distortions from reducing the precision of our relative proper motions. We also obtain two short exposures of each cluster using the 512x512 subarray and RAPID read mode to provide much better coverage of saturated stars than can be achieved with the full-frame readout modes. This fills unused visibility at the end of each visit. Our observations will consist of a series of exposures with texp=349.2 to accommodate 12 MULTIACCUM readouts (STEP 50 readout pattern). This should achieve SNR ~ 100 for an H=21 star (~1.2 M_sun). Individual exposure times are limited by the effects of persistence from saturated sources, which can negatively impact astrometry. We chose a medium band filter for our astrometry rather than a wide filter in order to avoid saturating the brightest sources in the field (H=10) prior to the first detector read. Some saturated sources are inevitable since the brightest sources are H=10 in the Quintuplet and H=12 in the Arches. These sources will saturate in less than 3 s; however, the density of bright stars is low enough that hard-saturating all stars with H<15 (<100 stars in the 120" field of view) should produce persistence images with a low enough density that dithering can shift sources in subsequent exposures on to persistence-free portions of the detector. Many saturated sources will be recoverable from the earliest reads of the up-the-ramp samples used by WFC3. The maximum possible exposure time to avoid persistence images from stars fainter than H=15 is ~350 s, hence our texp of 349.2 s. Stellar Populations 12 false false false false false false SUPPORTED STELLAR CLUSTER NUCLEUS STAR FORMING REGION The V-magnitude is for the brightest optical star in the field of view J2000 J2000 J=11.5, H=10.5, K=10.2 STELLAR CLUSTER NUCLEUS STAR FORMING REGION The fluxes are for the brightest star in the WFC3 field of view. The brightest optical star is difference from the brightest near-infrared star J2000 J2000 J=11.8, H=8.9, K=7.29 WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M POS-TARG DITHER 0.42 LINE false false 21 15 SPIRAL 2 false /Users/mwhosek/Desktop/Thesis/Proposals/hst_cycle24_arches/phase2/OP-cache/HST_cycle24_phaseII-v01.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. Arches F153M Astrometry. We specify the required ORIENT to match our previous HST observations. This is necessary to achieve the astrometric precision we require. 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