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The HubbleObserver Corner for February 2012

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Proposal ID = 12446
Principle Investigator =  Michael Shara  - American Museum of Natural History
Title = "Ionization and Light Echoes in the T Pyxidis Nebula"
Time = Feb 22, 2012 17:10:20 - 19:08:47
Target =  T Pyxidis
Instrument = WFC3/UVIS

Background:

Just as sound bounces off canyon walls causing an echo, light from a stellar outburst bounces off the debris around it and can be observed well after the original outburst as a "light echo". The most famous case is V838 Monocerotis. Another example is Eta Carinae, where the echos of the 1837 outburst are still reaching us over 170 years later! The recurrent nova T Pyxidis is a particularly good example of a light echo because of the richness of the debris field around it. Just like a sound echo, the reflections come from nearly 360 degrees around the central object (as long as there is material there to reflect off), providing a three-dimensional record of light as emitted by the star in all directions. Similarly, the time delay between the original outburst and the reflected echo, when matched with the projected angle on the sky of the material in the light echo from the central star, can be used to get a precise measurement of the distance to the star. This technique also requires polarization observations to provide information about the geometric configuration of the material with respect to the central object.

Paraphrasing from the abstract:

Contrary to published predictions, the famous prototype recurrent nova T Pyxidis unexpectedly commenced a new outburst on 2011 April 14 UT --- its first outburst since 1966. T Pyx is unique among recurrent novae in being surrounded by a nebula, representing material ejected during previous eruptions, which is well resolved into thousands of sub-arcsecond features in HST images. The outburst offers the opportunity to make two unique measurements. (1) A direct geometric determination of the distance to T Pyx (a highly controversial subject), based on imaging polarimetry of the knots --- a technique already validated for the light echo around V838 Monocerotis. (2) A determination of the 3D structure of the nebula, based on the ionization echo that will sweep out into the nebula, causing each individual knot to develop strong H-alpha emission when first illuminated by the nova. Our proposed observations are essential for determining if T Pyx (and other recurrent novae) will exceed the Chandrasekhar limit and erupt as a type Ia Supernova. The wave of illumination and ionization will pass through the entire nebula within the next few months, so it is crucial to begin the HST observations as soon as possible. We request the first Halpha observation to be made as soon as possible, nominally about 7-10 days after the outburst begins.

(Editors Note: - The first Hubble observation in this series was taken April 18, 2011, 4 days after the outburst. This week's observations was the 6th in the time series, with the 7th and last scheduled for May 14, 2012. This is an example of a Director's Discretionary, Target of Opportunity observation.)

You can find most of this information and more on the HST Homepage by entering "12446" in the Prop. ID box.


Proposal ID = 12528
Principle Investigator =  Philip Massey  -  Lowell Observatory 
Title = "Probing the Nature of LBVs in M31 and M33: Blasts from the Past"
Time = Feb 14, 2012 04:05:39 - 05:31:15
Target =  LGGSJ013416.07+303642.1
Instrument = STIS/CCD

Background:

Most stars, like our sun, last billions of years. These are relatively low mass stars, so their central temperatures are modest and they therefore burn their fuel slowly and last long periods of time. However, there are also rare high mass stars, with more than 100 times the mass of the sun. The temperature of these stars are much higher, resulting in a blue appearance. These stars burn their fuel thousands of times faster than our sun and hence only last several million rather than billions of years. These stars , which lie near the theoretical upper mass limits for stars, undergo large variations in luminosity as they try to find a balance between their own gravity, trying to hold the star together, and radiation pressure from their tremendous luminosity, trying to blow the star apart. Hence the name Luminous Blue Variables. The fact that high mass stars are very rare, combined with the fact that their outbursts last only a short period of time, make LBVs a very rare type of star. In this proposal, rather than wait to see an eruption, the proposers are looking for spectroscopic evidence of past eruptions in the form of emission-line nebulae around candidate LBVs in M31 and M33, the two nearest spiral galaxies to our Milky Way galaxy.

Paraphrasing from the abstract:

Luminous Blue Variables (LBVs) are a short-lived, but critical stage in the evolution of the most massive stars. Episodic outbursts during the LBV phase may provide the dominant mass-loss mechanism for evolution to the Wolf-Rayet stage. However, these large mass-loss outbursts (accompanied by large changes in the visual magnitude) take place on timescales of order 1000 years or so: the archetypical LBVs P Cyg and Eta Car had their last major outbursts in the 17th and 19th centuries, respectively. Were these stars located in nearby galaxies would we know of them today? Only six LBVs have been confirmed in M31 and M33 through detection of outbursts, although 175 stars have now been identified as LBV "candidates" in these galaxies. These LBV candidates are spectroscopically indistinguishable from the known LBVs, but no large-scale (>2 mag) photometric outbursts have been found, although many of the candidates do show smaller photometric variability and/or spectroscopic variability. Rather than wait 1000 years for an outburst, we instead here propose to look for signatures of past outbursts in the form of ejecta nebulae close to the stars. The high spatial resolution of STIS will be sensitive to nebulae with radii larger than 0.3-0.4 pc, which corresponds to ages of >500-1000 yrs. These data will allow us to determine the frequency of ejecta among our LBV candidates compared to those of the known LBVs, and determine the physical characteristics of these past mass-loss events. Ultimately, this will help us constrain the lifetime and total mass lost in the LBV phase.

You can find most of this information and more on the HST Homepage by entering "12528" in the Prop. ID box.


Proposal ID = 12585
Principle Investigator =  Sara Petty  - University of California - Los Angeles
Title = "Unveiling the Physical Structures of the Most Luminous IR Galaxies 
      Discovered by WISE at z > 1.6"
Time = Feb 5, 2012 09:17:46 - 10:07:28
Target =  WISEPC183013.51+650420.4
Instrument = WFC3/IR

Background:

The launch of the Infrared Astronomical Satellite (IRAS) in 1983 provided the first all-sky survey of the infrared universe and opened the door to the systematic study of large numbers of star-bursting galaxies. The brightest of these are called ULIRGS (Ultra Luminous Infrared Galaxies), and can have star formation rates that are hundreds or even thousands of times larger than our own Milky Way Galaxy. In 2009, the Wide-field Infrared Survey Explorer (WISE) was launched, providing a much deeper all-sky infrared survey. The purpose of this proposal is to use Hubble to study a sample of 53 WISE-discovered ULIRGS with redshifts greater than z > 1.6.

Stars form in dense cold clouds of gas and dust. The remaining dust around a star is heated by the star until it is "warm", and hence emits most of it's light in the infrared part of the spectrum, This is why infrared telescopes are so efficient at finding starbursting galaxies. Unfortunately, the longer wavelengths also result in poor spatial resolution; roughly 20 arcsec compared to the 0.1 arcsec resolution of Hubble. The proposers plan to use Hubble's much better spatial resolution to determine the morphology of the ULIRGS.

Paraphrasing from the abstract:

WISE recently completed the first all-sky infrared survey since IRAS, pushing hundreds of times deeper. The search for the most extreme and rare ULIRGs has been successful, providing a unique opportunity to study the astrophysical origin and structural environments at the extreme end of the luminosity function. Extreme ULIRGs, such as the WISE color-selected sample, are powered by rapid star formation and/or AGN heavily obscured by dust, and are likely in a late stage merger process. WISE ULIRG color-selections have resulted in a sample of 53 confirmed z>1.6 galaxies that do not fit standard dusty luminous-IR SEDs. Their structural properties are also unknown, due to a lack of high-resolution near-IR imaging. We propose a SNAP proposal to obtain WFC3/F160W images of 53 z>1.6 WISE color-selected ULIRGs The 53 WISE-selected ULIRGs are distributed throughout the sky, providing a convenient Snapshot target list. A significant result from WFC3/F160W imaging is the quantitative measurements of the relative fraction of AGN vs starburst powered ULIRGs, lending insight into the co-evolution of black holes and galaxies in the Universe and constraining the dynamics and structures of extreme phases in galaxy evolution.

You can find most of this information and more on the HST Homepage by entering "12585" in the Prop. ID box.


Proposal ID = 12504
Principle Investigator =  Michael Liu  -  University of Hawaii 
Title = "Bridging the Brown Dwarf/Jupiter Temperature Gap with a 
     Very Cold Brown Dwarf"
Time = Feb 4, 2012 00:44:01 - 01:45:51
Target =  CFBDSIR1458+1013AB
Instrument = ACS/WFC, WFC3/IR, WFC3/UVIS

Background:

Most stars shine because gravitational contraction of their core raises the central temperature above the limit required to ignite nuclear fusion processes. Planets, on the other hand, shine by reflected light from their central stars. The middle ground between these two types of objects is partially filled in by Brown Dwarfs. These objects are massive enough so that gravitational contraction significantly heats their core, but not to the point that nuclear fusion begins. Hence Brown Dwarfs shine because they are hot, but without nuclear fusion to keep them hot, they rapidly cool. Hence the lower mass "cold" brown dwarfs that bridge the gap to the planets do not last very long (on astronomical time scales at least). They are extremely faint and hence hard to find. One trick is to look for a binary system, where at least one of the objects is brighter and hence easier to find. This approach led to the discovery of CFBDSIR1458+1013AB, currently the coolest, and lowest mass brown dwarf known. The low surface temperature of these objects leads to some interesting physics which is relevant to the study of gas giant exoplanets.

Paraphrasing from the abstract:

Residing at the extremes of low mass, luminosity and temperature, brown dwarfs serve as laboratories for understanding gas-giant extrasolar planets. Still, until a few months ago, the coolest brown dwarf known was ~4 times warmer than Jupiter. We have now identified the nearby T9.5 dwarf CFBDSIR J1458+10 as a 0.11" physical binary. As established by our near-IR parallax to the system, the very blue secondary component is the coldest and least luminous object outside the solar system directly imaged. With an estimated temperature of ~350-400 K, it is the coolest known brown dwarf by ~150 K and the least luminous by a factor of 4-5. As such, CFBDSIR J1458+10B provides a gateway for measuring the properties of substellar objects at previously unexplored extremes.

We propose to use HST to obtain far-red and near-IR medium-band photometry of CFBDSIR J1458+10B and to measure its 0.8-1.6 micron spectral energy distribution. Theoretical models predict this wavelength range to be highly sensitive to completely new physical processes not yet seen in brown dwarfs, including the formation of photospheric water clouds and the disappearance of the very broad potassium line that dominates the far-red spectra of T dwarfs. The impact of these changes on the emergent spectrum, however, depends on very uncertain input physics. Our observations will sensitively probe these processes.

You can find most of this information and more on the HST Homepage by entering "12504" in the Prop. ID box.


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