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Hubble Space Telescope
The HubbleObserver Corner for December 2011

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Proposal ID = 12071
Principle Investigator = Dr. Julianne Dalcanton, University of Washington
Title = "A Panchromatic Hubble Andromeda Treasury"
Time = Dec 17, 2011 02:24:06 - 06:56:20
Target =  M31-B12-F013 and M31-B12-F016 in parallel
Instrument = ACS/WFC, WFC3/UVIS, WFC3/UVIS

Background:

The Panchromatic Hubble Andromeda Treasury (PJHAT) project is one of three Multiple Cycle Treasury Programs (MCTPs) that were defined in Cycle 18 and will be taking data through Cycle 20. The program has been awarded 834 orbits over a 3 year period; roughly 350 orbits of data have already been taken. The target, M31 (the Andromeda galaxy), is the nearest neighboring giant spiral galaxy to our own Milky Way galaxy, and has just come out from behind the sun (i.e., there is a 50 degree from sun restriction on HST observations) following the last set of observations back in July. The data for the MCTPs are available immediately for public access. In 1923, the discovery of Cepheid variables in M31 played a key roll in the recognition that there were many other galaxies ("island universes") besides our own (see http://hubblesite.org/newscenter/archive/releases/2011/15/), hence expanding the scale of the known universe.

Their PHAT website can be accessed at: http://www.astro.washington.edu/groups/phat/Home.html

Paraphrasing from the abstract:

We propose to image the north east quadrant of M31 to deep limits in the UV, optical, and near-IR. HST imaging should resolve the galaxy into more than 100 million stars, all with common distances and foreground extinctions. UV through NIR stellar photometry (F275W, F336W with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/NIR) will provide effective temperatures for a wide range of spectral types, while simultaneously mapping M31's extinction. Our central science drivers are to: understand high-mass variations in the stellar IMF as a function of SFR intensity and metallicity; capture the spatially-resolved star formation history of M31; study a vast sample of stellar clusters with a range of ages and metallicities. These are central to understanding stellar evolution and clustered star formation; constraining ISM energetics; and understanding the counterparts and environments of transient objects (novae, SNe, variable stars, x-ray sources, etc.). As its legacy, this survey adds M31 to the Milky Way and Magellanic Clouds as a fundamental calibrator of stellar evolution and star-formation processes for understanding the stellar populations of distant galaxies.

You can find most of this information and more on the HST Homepage by entering "12071" in the Prop. ID box.


Proposal ID = 12257
Principle Investigator = Leo Girardi,  Osservatorio Astronomico di Padova
Title = "The Nature of Multiple Main Sequence Turn-offs and Dual Red 
      Clumps in Magellanic Cloud Star Clusters"
Time = Dec 2, 2011 21:14:40 - 23:06:52
Target(s) = NGC 1718
Instrument(s) = WFC3/UVIS, ACS/WFC 

Background:

Until a few years ago, globular clusters where considered to be the ideal location to study a "simple stellar population", i.e., one where all the stars have the same age and metallicity. However, the excellent photometric precision provided by the Advanced Camera for Surveys (ACS) has been revealing two, three, and even more intertwined populations along the main sequence, the main-sequence turn-off, the red giant branch, and the "red clump". These features are distinctive regions in the color-magnitude diagram (CMD) that have been used for decades to study populations of stars. While one or two globular clusters in our own Milky Way galaxy were known to have these "abnormalities in the past (notably Omega Cen - which is believed to be the remains of the nucleus of a nearby dwarf galaxy that was captured by our own galaxy), the surprise was that nearly every globular cluster with adequate photometry was now showing these multiple populations. This poses a major challenge to our understanding of how stars are formed in clusters. This proposal moves the venue to the Large and Small Magellanic Clouds (LMC and SMC), nearby companions to our own Milky Way galaxy where similar multiple populations have also recently been found.

Paraphrasing from the abstract:

Recently, deep images from the ACS camera aboard HST provided conclusive evidence that several massive intermediate-age star clusters in the LMC and SMC present multiple main-sequence turn-offs (MMSTO), and in some cases also dual red clumps. These observations challenge the notion that star clusters are simple stellar populations, and pose serious questions regarding the mechanisms responsible for star cluster formation. In this project, we propose to collect WFC3 imaging that should lead to an understanding of the nature of the MMSTO phenomenon. We will perform deep F475W and F814W imaging for a complete sample of star clusters covering well-defined intervals of age and total mass. The sample comprises star clusters spanning a wide variety of concentrations and locations within the LMC and SMC, so as to allow a study of the physical properties causing the MMSTO phenomenon. The data will allow us to derive detailed star formation histories via CMD reconstruction methods, and locate the targets in the robust absolute age scale provided by clusters with dual red clumps. The underlying field population will be characterized by means of ACS images of nearby areas obtained in parallel. Thanks to its photometric depth and accuracy, the resulting dataset will constitute an invaluable resource for studies of stellar evolution and dynamics in star clusters.

You can find most of this information and more on the HST Homepage by entering "12257" in the Prop. ID box.


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