The IMF and Internal Kinematics of the Massive Young Star Cluster, Westerlund 1The most massive young star cluster known in the Milky Way, Westerlund 1, represents a far more extreme environment for star formation than nearby, well-studied, and lower-mass star forming regions such as Taurus and Orion. We propose to construct a complete photometric and kinematic census of Westerlund 1 in order to identify cluster members down to 0.1 solar masses, precisely determine the initial mass function (IMF), and measure the internal kinematic structure of the cluster. With these measurements, we will test whether the IMF is universal, as may be the case for nearby lower-mass star forming regions, or favors high-mass star formation, as has been suggested theoretically and from some observational results. We will observe Wd 1 with WFC3-IR, which is the only instrument capable of delivering high spatial resolution, a well-characterized and stable PSF, and a wide field of view at infrared wavelengths. We exploit WFC3's capabilities to cover the full extent of the cluster with photometry, to correct for variable extinction and derive stellar masses, and with proper motions, to distinguish between cluster members and contaminating field stars. Our proposed observations of Westerlund 1 will help determine whether the star formation process, and the emergent stellar mass distribution, varies with initial cloud conditions.Our program to measure the IMF and internal kinematics of the massive young cluster, Westerlund 1, requires both astrometric and near-infrared photometric observations. Archival HST data sets of Wd 1 include multi-band WFC3-IR imaging of the full extent of the cluster, which we will use as our first epoch of observation when establishing membership probabilities down to the lowest stellar masses. New observations in cycle 20 and cycle 22 would be obtained with WFC3 in the near infrared to provide additional astrometric epochs. A 2x2 (4'x4') mosaic is needed to cover the full cluster extent.
We will obtain 2 additional observations using WFC3-IR in cycle 20 and cycle 22 specifically optimized for astrometry. Our target goal is to identify cluster members down to the limits of the existing WFC3-IR data at 0.1 solar masses, which corresponds to H=20.5. In order to effectively select the field stars, we require a 1 sigma proper motion precision of <0.3 mas/yr. We will use existing data from 2010 and new observations in cycles 20 (2013) and 22 (2015) with a total time baseline of 5 years to achieve our desired proper motion errors. We will measure the astrometry with WFC3's longest-wavelength filter (F160W) to minimize the impact of extinction. Therefore, we need a total SNR=160 on the stars of interest to achieve an astrometric precision of 1 mas and a final velocity precision of 0.3 mas/yr. Additionally, to minimize the impact of geometric distortions, we request that the astrometric observations have the SAME ORIENT with small POSTARG offsets as the 2010 observations.
Our observations require a high dynamic range as we are interested in both the higher mass, brighter stars (typically H=0) and the lower mass, fainter stars (typically H=20.5) in the cluster. As a result we must limit our integration time per exposure so as to avoid saturating too many stars, such that the resulting persistence images would not impact the astrometric and photometric measurements. We also adopt a minimum integration time that still allows us to dump the WFC3 buffer in parallel (249 s for a STEP50 readout pattern with 10 MULTIACCUM reads). Integration times for observations of Wd 1 in the F160W filter were calculated with the WFC3 exposure time calculator to achieve SNR=160 for an H=20.5 star with a late M dwarf spectral type and an E(B-V) = 3.5. For each position in our proposed 2x2 mosaic of Wd 1, we require 14 exposures for a total of 3486 s of integration time. We also require additional short exposures in sub-arrayed mode to capture the most massive stars that were saturated in the longer exposures. This amounts to 7 orbits to complete the entire mosaic in cycle 20. The observations will be repeated in cycle 22 with an additional 7 orbits.
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The archival observations have the PID 11708 and the cycle 20 observations have the PID 13044. We request that our new observations have the same orientation and detector position as the F160W images from the archived data set. The relevant data set numbers are ib5w02, ib5w03, ib5w04, and ib5w05.STAR FORMATION12falsefalse0.00.0falsefalsefalseSUPPORTEDSTELLAR CLUSTEROB ASSOCIATIONOPEN CLUSTERSTAR FORMING REGIONYOUNG ASSOCIATIONThis object was generated by the targetselector and retrieved from the SIMBAD database.J2000J2000H=9-20WFC3/IRImagingNoNoF160WWFC3/IRImagingNoNoF160WPOS-TARGDITHER3.26LINEfalsefalse1414SPIRAL1falsePOS-TARGDITHER3.26LINEfalsefalse1314SPIRAL2falseTiming link suitability includes constraints ([02 After 01 by at most 30.0 days])The scheduling data was generated by CASM 22.2.CASM was run in Phase II mode./private/var/folders/x_/1qv46zj1749bb9m0x2z0fjfm0001xf/T/OPVisit1793061889902897249.xml(Tic-Visit
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