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Enhancing the Legacy of HST Spectroscopy
Workshop Presentations

Listing of Talk Abstracts

To Boldly Go Where No UV Echelle Spectrograph Has Gone Before: Post-Processing and Pipeline Enhancements for STIS High-Resolution Spectroscopy
Dr.  Thomas Ayres (University of Colorado (CASA))
Two recent HST programs -- one AR (Cycle 14 : “StarCAT”), the other GO/LTP (Cycle 18: “Advanced Spectral Library: Cool Stars”) -- have fostered the development of specialized tools to optimally process STIS echelle pipeline x1d files into coherent 1D traces; and further to coadd traces from the same mode/setting and splice those from neighboring settings; to ultimately yield a broad coverage spectrum of enhanced S/N with a high-precision wavelength scale. Part of the key mitigation of detector fixed pattern noise comes from the design of the original observing plan (as in the ASTRAL project), while part of the enhanced wavelength precision comes from application of a post-facto distortion correction, modeled for each of the supported STIS echelle settings individually, to compensate for small-scale errors in the pipeline-assigned wavelength scales. These result from lack of higher order terms in the calstis dispersion polynomials, or simply poorly defined polynomial coefficients. The distortions, which can reach 2 km/s in amplitude in the worst cases compared with a measurement precision of 300 m/s or better, were identified by processing STIS wavecal spectra through the pipeline and comparing empirical centroids of the Pt/Ne/Cr emission lines against laboratory wavelengths. Several past HST programs were involved: “Deep Lamp” (Cycle 13), “Ironing out the Wrinkles in STIS” (Cycle 17), and “Deep Lamp Too” (Cycle 18). An advanced dispersion model was developed as part of Wrinkles and DLToo. It contains additional terms beyond those incorporated in the present calstis, a new wavelength “template” that corrects a few anomalies in the current pipeline version, and a freshly and consistently derived set of polynomial coefficients for all 44 supported echelle settings. The new dispersion model still is accompanied by a post-facto distortion correction, to account for residual wavelength shifts that the dispersion polynomials are unable to compensate (without a massive overhaul in the way that the dispersion solution is applied), but the new maps are far milder than their cousins utilized to correct contemporary calstis x1d files. It is hoped that a beta version of the new calstis dispersion model will be fully tested by the time of the workshop. This, and the ASTRAL processing tools, have implications for the future of the STIS echelle archive, as well as future extensions of the ASTRAL project to other parts of the H-R diagram.
Multiple and Extensive Uses of Ultraviolet Quasar Spectra
Jane Charlton (Penn State)
A single ultraviolet quasar spectra can often be used to study the Milky Way galaxy, to study other galaxies, to study the intergalactic medium, and to study the environment of the quasar itself. Because of this, there are many fundamental topics in galaxy evolution that are advancing rapidly due to the increasing COS database. High S/N, high resolution, and large wavelength coverage are essential. Because the environments of galaxies are complicated and highly varied, and because there is substantial evolution, as large a database as possible is needed to sample all of the processes and epochs. The higher redshift universe can be studied with optical spectra, but the majority of the information to be gained about the last half of cosmic time can only be gained in the UV.
Enhancing the HST Legacy for Spectroscopic Studies of Exoplanets and Circumstellar Disks
Dr.  Carol  Grady (Eureka Scientific and GSFC)
Over the past decade, studies of circumstellar material, including disks around both young and old stars, accretion and mass loss, and studies of exoplanets have become a major component of HST's science portfolio. In some cases, such studies build on the classical legacy of high S/N spectroscopy, particularly in the UV, and have yet to explore the time domain, or do so on timescales of months to years. In other cases, such as exoplanet transit science, the observations explore time-domain spectroscopy at a level not typically sampled on other HST programs, and are equally sensitive to stellar variability and time-dependent instrument sensitivity effects. Other studies exploit HST's angular resolution and provide geometrical and compositional data for understanding where and when giant planets are formed in circumstellar disks. Such studies are closely related to imaging studies and require knowledge of both the instrumental line spread function and point spread functions. Recent spectroscopic exoplanet and circumstellar disk studies with HST are reviewed, with an emphasis on the requirements such studies place on the calibration of HST's spectroscopic modes, and the need for new visualization tools in public domain archives such as the Hubble Legacy archive.
Under Construction: The Next Generation Spectral Library
Dr.  Michael Gregg (University of California, Davis)
In HST Cycles 10, 12, and 13, the Next Generation Spectral Library obtained data for 380 stars before being ended short of its goal of 600 targets by the failure of STIS. The NGSL spectra cover stars of four metallicities: very low ([Fe/H] < -1.5), low (-1.5 < [Fe/H] < -0.5), near-solar (-0.3 < [Fe/H] < 0.1), and super-solar ([Fe/H] > 0.2), well-sampling the entire HR-diagram in each bin. The spectra cover the CCD low resolution modes of STIS, from 1650 to 10000 angstroms. The NGSL is envisioned as a resource for galaxy spectral synthesis work. To enhance this resource, work is now being done to create high level science products from the NGSL database. Our main goal is to develop code to use the NGSL as a basis for interpolating spectra of arbitrary atmospheric parameters in effective temperature, surface gravity, and metallicity. A secondary goal is extension of the library spectra to the near-IR and to echelle resolution; much of this needed ground-based data has already been obtained. These extensions will enhance the value of the NGSL for use in modeling the integrated light of galaxies and star clusters. We will make the results available to the community via MAST and a public website.
HST/COS observations of weak intergalactic HI absorbers at z ~ 0.06
Dr.  Tae-Sun Kim (Univ. Wisconsin-Madison, Dept. Astronomy)
The shape of density distribution of neutral hydrogen (HI) absorption lines imprinted in background AGN spectra constrains the baryon density of the intergalactic medium (IGM). Since weak IGM absorbers are much more numerous, they could contribute to the baryon density significantly, if the steep power-law nature of density distribution continues at log N(HI) < 13. Unfortunately, absorption lines at 12 < log N(HI) < 13 in high S/N HST/COS spectra suffer from contamination by the instrumental fixed pattern noise. We developed a custom-made IDL program to remove the fixed pattern noise and to re-calibrate the CalCOS wavelength at the ~5 km/sec accuracy in high S/N COS spectra. Simulations show that as much as 50% of weak absorption lines are missed due to noise and the fixed pattern noise tends to increase the measured HI column density by ~0.2 dex at log N(HI) < 13. Using the high S/N (100-150 per resolution element) COS spectra of 3C273 and Mrk876 at z~0.06, we have found the density distribution function shows the continous single power law down to log N(HI) ~ 12.5, with no evolution from z=3 to z=0.06. The HI baryon density of 0.023 x 10^-6 h^-1 at 12.5 < log N(HI) < 13.5 does not show any redshift evolution.
UV Spectroscopic Diagnostics of Flares on Low Mass Stars
Dr.  Adam Kowalski (NASA/GSFC)
I will review the spectroscopic diagnostics from the FUV to the NUV that have been used to understand flare processes on nearby super-active, cool stars. I'll show results of recent optical observations and radiative-hydrodynamic flare models in order to highlight missing observational constraints in the UV.
What Multidimensional Numerical Modeling of HST/STIS Observations Can Tell Us: The Spectacular Example of Eta Carinae
Dr.  Thomas Madura (ORAU/GSFC)
The Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS), with its high spatial and moderate spectral resolutions, is an ideal instrument for performing dynamical studies of complex astrophysical systems. One such system is the extremely luminous, highly eccentric massive colliding wind binary Eta Carinae. HST/STIS observations of Eta Car reveal spatially-extended (0''.1 – 0''.7), time-varying forbidden line emission from several low- ([Fe II]) and high- ([Fe III], [Ne III]) ionization species. Mapping observations taken with HST/STIS of Eta Car over the past two years are the first to fully image such emission in a massive colliding wind binary. Until very recently, our understanding of this emission was limited by the lack of proper numerical models, which require a full three-dimensional (3D) treatment since orbital motion, especially during periastron, greatly affects the shape and dynamics of the wind-wind collision region formed between the stars. In this talk, I discuss how state-of-the-art 3D hydrodynamical simulations of Eta Car's colliding winds, coupled with multidimensional radiative transfer codes, have been used to create synthetic HST/STIS observations for comparison to the available data, leading to key insights into how/where the forbidden line emission forms and the first tight constraints on the 3D orientation of the binary orbit. I further discuss how the models, together with HST/STIS spectral observations, have led to a fundamental rethinking of how the binary companion affects the extended optically-thick wind photosphere of the LBV primary. Future HST/STIS observations and improved 3D models will be used to help constrain the number of ionizing photons from the as-yet unseen companion star and place limits on its temperature, luminosity, and evolutionary state. I conclude by detailing how these modeling and observational techniques go well beyond studying Eta Car and can be applied, for example, to dust formation in colliding wind binaries and the search for supermassive black holes.
What Multidimensional Numerical Modeling of HST/STIS Observations Can Tell Us: The Spectacular Example of Eta Carinae
Dr.  Thomas Madura (ORAU/GSFC)
The Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS), with its high spatial and moderate spectral resolutions, is an ideal instrument for performing dynamical studies of complex astrophysical systems. One such system is the extremely luminous, highly eccentric massive colliding wind binary Eta Carinae. HST/STIS observations of Eta Car reveal spatially-extended (0''.1 – 0''.7), time-varying forbidden line emission from several low- ([Fe II]) and high- ([Fe III], [Ne III]) ionization species. Mapping observations taken with HST/STIS of Eta Car over the past two years are the first to fully image such emission in a massive colliding wind binary. Until very recently, our understanding of this emission was limited by the lack of proper numerical models, which require a full three-dimensional (3D) treatment since orbital motion, especially during periastron, greatly affects the shape and dynamics of the wind-wind collision region formed between the stars. In this talk, I discuss how state-of-the-art 3D hydrodynamical simulations of Eta Car's colliding winds, coupled with multidimensional radiative transfer codes, have been used to create synthetic HST/STIS observations for comparison to the available data, leading to key insights into how/where the forbidden line emission forms and the first tight constraints on the 3D orientation of the binary orbit. I further discuss how the models, together with HST/STIS spectral observations, have led to a fundamental rethinking of how the binary companion affects the extended optically-thick wind photosphere of the LBV primary. Future HST/STIS observations and improved 3D models will be used to help constrain the number of ionizing photons from the as-yet unseen companion star and place limits on its temperature, luminosity, and evolutionary state. I conclude by detailing how these modeling and observational techniques go well beyond studying Eta Car and can be applied, for example, to dust formation in colliding wind binaries and the search for supermassive black holes.
Spectral Analysis in the 21st Century
Dr.  Thomas Rauch (IAAT, Eberhard Karls University Tübingen)
Spectral analysis by means of non-LTE model-atmosphere techniques has arrived at a high level of sophistication: fully line-blanketed model atmospheres consider opacities of species from H to trans-iron elements. We present examples of recent spectral analyses based on high-resolution and high-S/N ultraviolet observations and demonstrate the necessity to use state-of-the-art non-LTE models for the reliable flux calibration of astronomical observations.
Interstellar Abundances of Neutron-Capture Elements from HST/STIS Spectroscopy
Dr.  Adam Ritchey (University of Washington)
We present the latest results of our archival HST program (AR 12123) on the abundances of neutron-capture elements in the diffuse interstellar medium (ISM). Observations of r and s-process elements in interstellar gas provide an opportunity to explore the relative contributions of high and low-mass stars to nucleosynthesis beyond the Fe-peak. However, the depletion characteristics of the elements involved must be well understood if one is to distinguish between the effects of nucleosynthesis and those of dust grain depletion. We employ high and medium-resolution HST/STIS spectra of over 50 Galactic sight lines to extract gas-phase abundances from resonance lines of the dominant ions of Ga, Ge, As, Kr, Cd, Sn, and Pb. Our survey increases substantially the number of sight lines with secure detections of these elements and allows their depletion characteristics to be studied in greater detail than had been possible before. We also report the first detection in the ISM of the Pb II line at 1203.6 Angstroms, which is found along multiple lines of sight, nearly doubling the number of existing measurements for interstellar Pb. Of all the elements surveyed, Sn exhibits the largest amount of scatter in gas-phase abundance among sight lines that show similar degrees of overall depletion. Since a dominant contribution to the production of Sn comes from the main s-process in low-mass stars, the observed scatter may be evidence for real intrinsic scatter in the abundance of Sn due to s-process enrichment and incomplete mixing in the ISM.
Magellanic Cloud Planetary Nebulae: the low-metallicity environment actors and probes
Dr.  Letizia Stanghellini (NOAO )
The study of old stellar populations and cosmic recycling in the Magellanic Clouds has been enhanced in the recent past by the wealth of HST spectroscopic observations of planetary nebulae (PNe). Our group acquired large data sets with various techniques and HST instruments to measure the most important physical parameters of these targets, including their shape through various emission lines and their elemental content. The HST, together with Spitzer/IRS and ground-based, spectroscopy of Magellanic Cloud PNe offered a deep insight on the structure and evolution of these important probes, and showed the link between gas, dust, and stellar evolution in a low-metallicity environment.