The Ages of Globular Clusters and the Population II Distance Scale
Globular clusters are the oldest objects in the universe whose age can be accurately determined. The dominant error in globular cluster age determinations is the uncertain Population II distance scale. We propose to use FGS 1r to obtain parallaxes with an accuracy of 0.2 milliarcsecond for 9 main sequence stars with [Fe/H] < -1.5. This will determine the absolute magnitude of these stars with accuracies of 0.04 to 0.06mag. This data will be used to determine the distance to 24 metal-poor globular clusters using main sequence fitting. These distances (with errors of 0.05 mag) will be used to determine the ages of globular clusters using the luminosity of the subgiant branch as an age indicator. This will yield absolute ages with an accuracy 5%, about a factor of two improvement over current estimates. Coupled with existing parallaxes for more metal-rich stars, we will be able to accurately determine the age for globular clusters over a wide range of metallicities in order to study the early formation history of the Milky Way and provide an independent estimate of the age of the universe.
The Hipparcos database contains only 1 star with [Fe/H] < -1.4 and an absolute magnitude error less than 0.18 mag which is suitable for use in main sequence fitting. Previous attempts at main sequence fitting to metal-poor globular clusters have had to rely on theoretical calibrations of the color of the main sequence. Our HST parallax program will remove this source of possible systematic error and yield distances to metal-poor globular clusters which are significantly more accurate than possible with the current parallax data. The HST parallax data will have errors which are 10 times smaller than the current parallax data. Using the HST parallaxes, we will obtain main sequence fitting distances to 11 globular clusters which contain over 500 RR Lyrae stars. This will allow us to calibrate the absolute magnitude of RR Lyrae stars, a commonly used Population II distance indicator.
COS FUV DCE Memory Dump
Whenever the FUV detector high voltage is on, count rate and current draw information is collected, monitored, and saved to DCE memory. Every 10 msec the detector samples the currents from the HV power supplies (HVIA, HVIB) and the AUX power supply (AUXI). The last 1000 samples are saved in memory, along with a histogram of the number of occurrences of each current value.
In the case of a HV transient (known as a "crackle" on FUSE), where one of these currents exceeds a preset threshold for a persistence time, the HV will shut down, and the DCE memory will be dumped and examined as part of the recovery procedure. However, if the current exceeds the threshold for less than the persistence time (a "mini-crackle" in FUSE parlance), there is no way to know without dumping DCE memory. By dumping and examining the histograms regularly, we will be able to monitor any changes in the rate of "mini-crackles" and thus learn something about the state of the detector.
CCD Daily Monitor (Part 3)
This program comprises basic tests for measuring the read noise and dark current of the ACS WFC and for tracking the growth of hot pixels. The recorded frames are used to create bias and dark reference images for science data reduction and calibration. This program will be executed four days per week (Mon, Wed, Fri, Sun) for the duration of Cycle 17. To facilitate scheduling, this program is split into three proposals. This proposal covers 308 orbits (19.25 weeks) from 21 June 2010 to 1 November 2010.
IR Dark Current Monitor
Analyses of ground test data showed that dark current signals are more reliably removed from science data using darks taken with the same exposure sequences as the science data, than with a single dark current image scaled by desired exposure time. Therefore, dark current images must be collected using all sample sequences that will be used in science observations. These observations will be used to monitor changes in the dark current of the WFC3-IR channel on a day-to-day basis, and to build calibration dark current ramps for each of the sample sequences to be used by Gos in Cycle 17. For each sample sequence/array size combination, a median ramp will be created and delivered to the calibration database system (CDBS).
UVIS Hot Pixel Anneal
The on-orbit radiation environment of WFC3 will continually generate new hot pixels. This proposal performs the procedure required for repairing those hot pixels in the UVIS CCDs. During an anneal, the two-stage thermo-electric cooler (TEC) is turned off and the four-stage TEC is used as a heater to bring the UVIS CCDs up to ~20 deg. C. As a result of the CCD warmup, a majority of the hot pixels will be fixed; previous instruments such as WFPC2 and ACS have seen repair rates of about 80%. Internal UVIS exposures are taken before and after each anneal, to allow an assessment of the procedure's effectiveness in WFC3, provide a check of bias, global dark current, and hot pixel levels, as well as support hysteresis (bowtie) monitoring and CDBS reference file generation. One IR dark is taken after each anneal, to provide a check of the IR detector.
Cycle 17: UVIS Bowtie Monitor
Ground testing revealed an intermittent hysteresis type effect in the UVIS detector (both CCDs) at the level of ~1%, lasting hours to days. Initially found via an unexpected bowtie-shaped feature in flatfield ratios, subsequent lab tests on similar e2v devices have since shown that it is also present as simply an overall offset across the entire CCD, i.e., a QE offset without any discernable pattern. These lab tests have further revealed that overexposing the detector to count levels several times full well fills the traps and effectively neutralizes the bowtie. Each visit in this proposal acquires a set of three 3x3 binned internal flatfields: the first unsaturated image will be used to detect any bowtie, the second, highly exposed image will neutralize the bowtie if it is present, and the final image will allow for verification that the bowtie is gone.
WFC3 UVIS CCD Daily Monitor
The behavior of the WFC3 UVIS CCD will be monitored daily with a set of full-frame, four-amp bias and dark frames. A smaller set of 2Kx4K subarray biases are acquired at less frequent intervals throughout the cycle to support subarray science observations. The internals from this proposal, along with those from the anneal procedure (Proposal 11909), will be used to generate the necessary superbias and superdark reference files for the calibration pipeline (CDBS).
To characterize the effects of the contamination (i.e., droplets) on the UVIS window, we will observe a star cluster in three wide band filters (F225W, F555W, and F814W) as well as a narrow band filter (F502N) and step the stars in the cluster across randomly located droplets. The step size is 20 pixels, and we execute a five point line dither for each filter. This should provide for observations both on and off the droplets, for the same star. Internal flat fields are also obtained, but, due to the high f/# of the internal calibration system, the flats will be of limited utility, but will serve to map and crudely track any changes in the droplets. The cluster needs to contain both hot and cool stars, and therefore we select NGC 6752, a nearby globular with a hot horizontal branch. Note, although the total population of HB stars may be larger in systems such as NGC 2419, NGC 6715, and NGC 2808, those clusters are much further away and will not provide a high density of stars over the global image (the droplets are located over the entire frame). There will be three visits (initial, 7 days later, and 30 days later), with each visit requiring 4 orbits. The total program thus requires 12 orbits total.
CCD Stability Monitor
This program will verify that the low frequency flat fielding, the photometry, and the geometric distortion are stable in time and across the field of view of the CCD arrays. A moderately crowded stellar field in the cluster 47 Tuc is observed with the ACS (at the cluster core) and WFC3 (6 arcmin West of the cluster core) using the full suite of broad and narrow band imaging filters. The positions and magnitudes of objects will be used to monitor local and large scale variations in the plate scale and the sensitivity of the detectors and to derive an independent measure of the detector CTE. The UV sensitivity for the SBC and ACS will be addressed in the UV contamination monitor program (11886, PI=Smith).
One additional orbit will be obtained at the beginning of the cycle will allow a verification of the CCD gain ratios for WFC3 using gain 2.0, 1.4, 1.0, 0.5 and for ACS using gain 4.0 and 2.0. In addition, one subarray exposure with the WFC3 will allow a verification that photometry obtained in full-frame and in sub-array modes are repeatable to better than 1%. This test is important for the ACS Photometric Cross-Calibration program (11889, PI=Bohlin) which uses sub-array exposures.
CCD Bias Monitor-Part 2
Monitor the bias in the 1x1, 1x2, 2x1, and 2x2 bin settings at gain=1, and 1x1 at gain = 4, to build up high-S/N superbiases and track the evolution of hot columns.
CCD Dark Monitor Part 2
Monitor the darks for the STIS CCD.
Probing Warm-Hot Intergalactic Gas at 0.5 < z < 1.3 with a Blind Survey for O VI, Ne VIII, Mg X, and Si XII Absorption Systems
Currently we can only account for half of the baryons (or less) expected to be found in the nearby universe based on D/H and CMB observations. This "missing baryons problem" is one of the highest-priority challenges in observational extragalatic astronomy. Cosmological simulations suggest that the baryons are hidden in low-density, shock-heated intergalactic gas in the log T = 5 - 7 range, but intensive UV and X-ray surveys using O VI, O VII, and O VIII absorption lines have not yet confirmed this prediction. We propose to use COS to carry out a sensitive survey for Ne VIII and Mg X absorption in the spectra of nine QSOs at z(QSO) > 0.89. For the three highest-redshift QSOs, we will also search for Si XII. This survey will provide more robust constraints on the quantity of baryons in warm-hot intergalactic gas at 0.5 < z < 1.3, and the data will provide rich constraints on the metal enrichment, physical conditions, and nature of a wide variety of QSO absorbers in addition to the warm-hot systems. By comparing the results to other surveys at lower redshifts (with STIS, FUSE, and from the COS GTO programs), the project will also enable the first study of how these absorbers evolve with redshift at z < 1. By combining the program with follow-up galaxy redshift surveys, we will also push the study of galaxy-absorber relationships to higher redshifts, with an emphasis on the distribution of the WHIM with respect to the large-scale matter distribution of the universe.
Snapshot Survey for Planetary Nebulae in Local Group Globular Clusters
Planetary nebulae (PNe) in globular clusters (GCs) raise a number of interesting issues related to stellar and galactic evolution. The number of PNe known in Milky Way GCs, four, is surprisingly low if one assumes that all stars pass through a PN stage. However, it is likely that the remnants of stars now evolving in galactic GCs leave the AGB so slowly that any ejected nebula dissipates long before the star becomes hot enough to ionize it. Thus there should not be ANY PNe in Milky Way GCs--but there are four! It has been suggested that these Pne are the result of mergers of binary stars within GCs, i.e., that they are descendants of blue stragglers. The frequency of occurrence of PNe in external galaxies poses more questions, because it shows a range of almost an order of magnitude.
I propose a SNAPshot survey aimed at discovering PNe in the GC systems of Local Group galaxies outside the Milky Way. These clusters, some of which may be much younger than their counterparts in our galaxy, might contain many more PNe than those of our own galaxy. I will use the standard technique of emission-line and continuum imaging, which easily discloses PNe. This proposal continues a WFPC2 program started in Cycle 16, but with the more powerful WFC3. As a by-product, the survey will also produce color-magnitude diagrams for numerous clusters for the first time, reaching down to the horizontal branch.
Infrared Survey of Star Formation Across Cosmic Time
We propose to use the unique power of WFC3 slitless spectroscopy to measure the evolution of cosmic star formation from the end of the reionization epoch at z>6 to the close of the galaxy- building era at z~0.3.Pure parallel observations with the grisms have proven to be efficient for identifying line emission from galaxies across a broad range of redshifts. The G102 grism on WFC3 was designed to extend this capability to search for Ly-alpha emission from the first galaxies. Using up to 250 orbits of pure parallel WFC3 spectroscopy, we will observe about 40 deep (4-5 orbit) fields with the combination of G102 and G141, and about 20 shallow (2-3 orbit) fields with G141 alone.
Our primary science goals at the highest redshifts are: (1) Detect Lya in ~100 galaxies with z>5.6 and measure the evolution of the Lya luminosity function, independent of of cosmic variance; 2) Determine the connection between emission line selected and continuum-break selected galaxies at these high redshifts, and 3) Search for the proposed signature of neutral hydrogen absorption at re-ionization. At intermediate redshifts we will (4) Detect more than 1000 galaxies in Halpha at 0.5< z<1.8 to measure the evolution of the extinction-corrected star formation density across the peak epoch of star formation. This is over an order-of-magnitude improvement in the current statistics, from the NICMOS Parallel grism survey. (5) Trace ``cosmic downsizing" from 0.5< z<2.2; and (6) Estimate the evolution in reddening and metallicty in star- forming galaxies and measure the evolution of the Seyfert population. For hundreds of spectra we will be able to measure one or even two line pair ratios -- in particular, the Balmer decrement and [OII]/[OIII] are sensitive to gas reddening and metallicity. As a bonus, the G102 grism offers the possibility of detecting Lya emission at z=7-8.8.
To identify single-line Lya emitters, we will exploit the wide 0.8--1.9um wavelength coverage of the combined G102+G141 spectra. All [OII] and [OIII] interlopers detected in G102 will be reliably separated from true LAEs by the detection of at least one strong line in the G141 spectrum, without the need for any ancillary data. We waive all proprietary rights to our data and will make high-level data products available through the ST/ECF.
Formation and Evolution of Massive Galaxies in the Richest Environments at 1.5 < z < 2.0
We propose to image seven 1.5< z<2 clusters and groups from the IRAC Shallow Cluster Survey with WFC3 and ACS in order to study the formation and evolution of massive galaxies in the richest environments in the Universe in this important redshift range. We will measure the evolution of the sizes and morphologies of massive cluster galaxies, as a function of redshift, richness, radius and local density. In combination with allocated Keck spectroscopy, we will directly measure the dry merger fraction in these clusters, as well as the evolution of Brightest Cluster Galaxies (BCGs) over this redshift range where clear model predictions can be confronted. Finally we will measure both the epoch of formation of the stellar populations and the assembly history of that stellar mass, the two key parameters in the modern galaxy formation paradigm.
Investigation Jet Rotation in Young Stars via High Resolution UV Spectra
In recent years we have successfully harnessed the high resolution of STIS in the optical to reveal asymmetries in Doppler shifts transverse to the flow direction in 8 T Tauri jets (Bacciotti ea 2002; Woitas ea 2005; Coffey ea 2004; 2007). We interpret the findings, just 100 AU above the disk, as signatures of jet rotation. The significance of these results is considerable. They form the only existing observational indications supporting the theory that jets extract angular momentum from star-disk systems. Furthermore, they hold the potential to discriminate between the main model contenders: X-wind and Disk-wind (Ferreira ea 2006). Although our results are encouraging, it is evident that we are only marginally resolving the effects of rotation because of the limiting resolution (spatially and spectrally) of STIS in the optical. Therefore, in Cycle 12 we proposed to extend this study into the near-ultraviolet (NUV), giving double the spatial and spectral resolution (proposal ID 9807). Unfortunately, only 3 targets in our survey were observed before the failure of STIS (Coffey ea 2007). Nevertheless, the results were very exciting. Agreement was found between the optical and NUV results in terms of the magnitude and sense of the Doppler shift gradient across the jet. Furthermore, the NUV lines indicated that the observed high velocity gas was launched from about 0.2-0.5 AU, compared to the lower velocity gas traced in optical lines which originates from as far as 2 AU. This puts a strong constraints on MHD launch models, and indeed holds the potential to differentiate between them. Given that the strength of a rotation argument lies in the survey nature of the findings, we need to resume this program in order to see if the same rotation signatures are commonly seen in the NUV, as they are in the optical. Furthermore, the higher spatial and spectral resolution of STIS in the NUV will allow us to more accurately quantify the variation in toroidal velocity as a function of distance from the jet axis. This study will provide an invaluable statistical argument to support the fact that we are indeed observing jet rotation. Such a conclusion is critical to providing observational backing to the widely accepted but untested theory of magnetocentrifugal ejection.
The Population of Compact Planetary Nebulae in the Galactic Disk
We propose to secure narrow- and broad-band images of compact planetary nebulae (PNe) in the Galactic Disk to study the missing link of the early phases of post-AGB evolution. Ejected AGB envelopes become PNe when the gas is ionized. PNe expand, and, when large enough, can be studied in detail from the ground. In the interim, only the HST capabilities can resolve their size, morphology, and central stars. Our proposed observations will be the basis for a systematic study of the onset of morphology. Dust properties of the proposed targets will be available through approved Spitzer/IRS spectra, and so will the abundances of the alpha- elements. We will be able thus to explore the interconnection of morphology, dust grains, stellar evolution, and populations. The target selection is suitable to explore the nebular and stellar properties across the galactic disk, and to set constraints on the galactic evolutionary models through the analysis of metallicity and population gradients.
Probing the Collimation of Pristine Post-AGB Jets with STIS
The shaping of planetary and protoplanetary nebulae (PNe and PPNe) is probably the most exciting yet least understood problem in the late evolution of ~1-8 solar mass stars. An increasing number of astronomers believe that fast jet-like winds ejected in the PPN phase are responsible for carving out the diverse shapes in the dense envelopes of the Asymptotic Giant Branch (AGB) stars. To date, the properties of these post-AGB jets have not been characterized and, indeed, their launching/collimation mechanism is still subject to controversial debate. This is due to the lack of the direct observations probing the spatio-kinematic structure of post-AGB winds in the stellar vicinity (~10e16cm), which is only possible with HST+STIS.
Recently, STIS observations have allowed us for the first time the DIRECT study of the structure and kinematics of the elusive post-AGB winds in one PPN, He3-1475 (Sanchez Contreras & Sahai 2001). Those winds have been discovered through H-alpha blue-shifted absorption features in the inner 0.3"-0.7" of the nebula. These STIS observations have revealed an ultra-fast collimated outflow relatively unaffected by the interaction with the AGB wind that is totally hidden in ground-based spectroscopic observations and HST images. The discovery of the pristine ultra-fast (~2300km/s) jet in He3-1475 is the first observational confirmation of the presence of collimated outflows as close as ~10e16cm from the central star. Most importantly, the spatio-kinematic structure of the ultra-fast jet clearly rules out hydrodynamical collimation alone and favors magnetic wind collimation. Therefore, STIS observations provide a unique method of probing the structure, kinematics, and collimation mechanism of the elusive post-AGB winds. We now propose similar observations for a sample of bipolar PPNe with ongoing post-AGB ejections in order to investigate the frequency of jets like those in He3-1475 in other PPNe and elucidate their nature and collimation mechanism. The observational characterization of these winds is indispensable for understanding this violent and important phase of post-AGB evolution.
The Disks, Accretion, and Outflows (DAO) of T Tau Stars
Classical T Tauri stars undergo magnetospheric accretion, power outflows, and possess the physical and chemical conditions in their disks to give rise to planet formation. Existing high resolution FUV spectra verify that this spectral region offers unique diagnostics of these processes, which have the potential to significantly advance our understanding of the interaction of a star and its accretion disk. To date the limited results are intriguing, with dramatic differences in kinematic structure in lines ranging from C IV to H2 among the few stars that have been observed. We propose to use HST/COS to survey the disks, outflows, and accretion (the DAO) of 26 CTTS and 6 WTTS in the FUV at high spectral resolution. A survey of this size is essential to establish how properties of accretion shocks, winds and disk irradiation depend on disk accretion rate. Specifically, our goals are to (1) measure the radiation from and understand the physical properties of the gas very near the accretion shock as a function of accretion rate using emission line profiles of hot lines (C IV, Si IV, N V, and He II); (2) measure the opacity, velocity, and temperature at the base of the outflow to constrain outflow models using wind absorption features; and (3) characterize the radiation incident on disks and protoplanetary atmospheres using H2 line and continuum emission and reconstructed bright Ly-alpha line emission.
Eta Carinae's Continuing Instability and Recovery - The 2009 Event
Eta Carinae is the only really observable example of structural recovery from a massive giant eruption, a "supernova imposter' event. Moreover it is the only well-observed star above 100 Msun, and its 5.5-year-recurrent spectroscopic events provide extraordinary clues to its surface instability. This truly unique combination of attributes makes it valuable for understanding the most massive stars. A fresh development arose a few years ago: the star has brightened much faster than before, and appears to have entered a rapid stage in its post-eruption recovery.
A spectroscopic event will occur at 2009.0, soon after the planned HST servicing mission. Because of the recent secular trend, this event is expected to differ from its well-observed 2003.5 predecessor. The differences will be very important, because they offer clues to very- massive-star structural instabilities that can't be observed in any other known way.
Some of the needed observations require HST's high spatial resolution and UV coverage. We propose an efficient, well-chosen set of STIS and ACS observations around the critical time. If the servicing mission is too late for the event, then a subset of the observations will still be merited.
Obtaining the Missing Links in the Test of Very Low Mass Evolutionary Models with HST
We are proposing for spatially resolved ACS+HRC observations of 11 very low mass binaries spanning late-M, L and T spectral types in order to obtain precise effective temperature measurements for each component. All of our targets are part of a program in which we are measuring dynamical masses of very low-mass binaries to an unprecedented precision of 10% (or better). However, without precise temperature measurements, the full scientific value of these mass measurements cannot be realized. Together, mass and temperature measurements will allow us to distinguish between brown dwarf evolutionary models that make different assumptions about the interior and atmospheric structure of these ultra-cool objects. While dynamical masses can be obtained from the ground in the near-IR, obtaining precise temperatures require access to optical data which, for these sub-arcsecond binaries, can only be obtained from space with Hubble.
A WFC3 Grism Survey for Lyman Limit Absorption at z=2
We propose to conduct a spectroscopic survey of Lyman limit absorbers at redshifts 1.8 < z < 2.5, using WFC3 and the G280 grism. This proposal intends to complete an approved Cycle 15 SNAP program (10878), which was cut short due to the ACS failure. We have selected 64 quasars at 2.3 < z < 2.6 from the Sloan Digital Sky Survey Spectroscopic Quasar Sample, for which no BAL signature is found at the QSO redshift and no strong metal absorption lines are present at z > 2.3 along the lines of sight. The survey has three main
observational goals. First, we will determine the redshift frequency dn/dz of the LLS over the column density range 16.0 < log(NHI) < 20.3 cm^-2. Second, we will measure the column density frequency distribution f(N) for the partial Lyman limit systems (PLLS) over the column density range 16.0 < log(NHI) < 17.5 cm^-2. Third, we will identify those sightlines which could provide a measurement of the primordial D/H ratio. By carrying out this survey, we can also help place meaningful constraints on two key quantities of cosmological relevance. First, we will estimate the amount of metals in the LLS using the f(N), and ground based observations of metal line transitions. Second, by determining f(N) of the PLLS, we can constrain the amplitude of the ionizing UV background at z~2 to a greater precision. This survey is ideal for a snapshot observing program, because the on-object integration times are all well below 30 minutes, and follow-up observations from the ground require minimal telescope time due to the QSO sample being bright.
Probing the Atomic and Molecular Inventory of a Beta-Pic Analog, the Young, Edge-On Debris Disk of HD32297
Edge-on, optically thin, debris disks provide unique opportunities to probe physical properties of the disk itself. Using the host star as the background source, trace atomic and molecular disk species can be detected in absorption. Redfield (2007) found that the recently discovered edge- on system, HD32297, has the strongest NaI absorption feature of any known debris disk, 5 times the level observed toward beta Pic, the canonical edge-on debris disk. Roberge et al. (2006) compiled the only comprehensive chemical inventory of a debris disk, using beta Pic, and found that carbon was surprisingly overabundant, which has important implications for the physical structure and support of a stable gas disk. What is severely lacking are comparison observations to determine if such an abundance pattern is typical of debris disk systems. HD32297 represents the best opportunity to make such a comparative study and perform a comprehensive gas inventory of a debris disk, due to its high NaI column density. The UV is critical for this work due to the large number of strong transitions (almost 50 ions and molecules are accessible) that are located in, and often only in, the UV. These observations will provide a much needed comparison dataset for addressing the gas chemistry of debris disk systems that are at the critical stage, near the end of planet formation, and in the process of clearing their interplanetary environments.