Sara R. Heap

Goddard high-resolution spectrograph observations of the local interstellar medium and the deuterium/hydrogen ratio along the line of sight toward Capella

HST Goddard High-Resolution Spectrograph observations of the 1216, 2600, and 2800 A spectral regions are analyzed for the spectroscopic binary system Capella, obtained at orbital phase 0.26 with 3.27-3.57 km/s resolution and high SNR. The column densities of H I, D I, Mg II, and Fe II for the local interstellar medium along this 12.5 pc line of sight, together with estimates of the temperature and turbulent velocity are inferred. It is inferred that the atomic deuterium/hydrogen ratio by number is 1.65(+0.07, -0.18) x 10 exp -5 for this line of sight. Galactic evolution calculations indicate that the primordial D/H ratio probably lies in the range of (1.5-3) x (D/H)LISM. If H0 = 80 km/s Mpc, as recent evidence suggests, then the baryonic density in units of the Einstein-de Sitter closure density is 0.023-0.031. Thus the universe is argued to expand forever, unless nonbaryonic matter greatly exceeds the amount of baryonic matter.

A Pair of Compact Red Galaxies at Redshift 2.38, Immersed in a 100 Kiloparsec Scale Lyα Nebula*

We present Hubble Space Telescope (HST) and ground-based observations of a pair of galaxies at redshift 2.38, which are collectively known as 2142 4420 B1 (Francis et al. 1996). The two galaxies are both luminous extremely red objects (EROs), separated by 0.8 ′′ . They are embedded within a 100 kpc scale diffuse

The Ultraviolet and Optical Spectra of Metal‐deficient O Stars in the Small Magellanic Cloud1

ABSTRACT An ultraviolet and optical spectral atlas of 15 O stars in the Small Magellanic Cloud (SMC) is presented and described. The echelle data have resolving powers of order 104; they were obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph in the UV, and at the Anglo‐Australian Telescope or the European Southern Observatory 3.6 m in the optical. The ultimate objective is to develop metal‐deficient templates for the interpretation of distant starbursts, but here we discuss interesting new properties of the SMC stars themselves, revealed by the high quality of these data. The SMC metal deficiency produces anomalously weak stellar‐wind profiles along the entire O main sequence, as well as at intermediate luminosities; the first intermediate Si iv λ1400 wind profile in the SMC is shown. The second known Of star in the SMC displays wind peculiarities that are identical to those of its spectral classmate, again likely due to the low systemic metallicity. Several objects display mar...

Hubble Space Telescope Space Telescope Imaging System Observations of the He II Gunn-Peterson Effect toward HE 2347–4342*

We present an HST Space Telescope Imaging System (STIS) spectrum of the He II Gunn-Peterson effect toward HE 2347-4342. Compared to the previous HST Goddard High Resolution Spectrograph data obtained by Reimers et al. the STIS spectrum has a much improved resolution. The two-dimensional detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the He II Gunn-Peterson opacity τ in the high-opacity regions. We use the STIS spectrum together with a Keck-HIRES spectrum covering the corresponding H I Lyα forest to calculate a one-dimensional map of the softness S of the ionization radiation along the line of sight toward HE 2347-4342, where S is the ratio of the H I to He II photoionization rates. We find that S is generally large but presents important variations, from ~30 in the opacity gaps to a 1 σ lower limit of 2300 at z 2.86, in a region that shows an extremely low H I opacity over a 6.5 A spectral range. We note that a large softness parameter naturally accounts for most of the large Si IV/C IV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al., we propose that the large softness observed at z 2.86 is due to the presence of bright soft sources close to the line of sight, i.e., for which the ratio between the number of H I to He II ionizing photons reaching the intergalactic medium is large. We discuss these two models and suggest ways to discriminate between them.

An Empirical Isochrone of Very Massive Stars in R136a

We report on a detailed spectroscopic study of 12 very massive and luminous stars (M 35M?) in the core of the compact cluster R136a, near the center of the 30 Doradus complex. The three brightest stars of the cluster, R136a1, R136a2, and R136a3, have been investigated earlier by de Koter, Heap, & Hubeny. Low-resolution spectra (<200 km s-1) of the program stars were obtained with the GHRS and FOS spectrographs on the Hubble Space Telescope. These instruments covered the spectral range from 1200 to 1750 ? and from 3200 to 6700 ?, respectively. Fundamental stellar parameters were obtained by fitting the observations by model spectra calculated with the unified ISA-WIND code of de Koter et al. supplemented by synthetic data calculated using the program TLUSTY. We find that the stars are almost exclusively of spectral type O3. They occupy only a relatively narrow range in effective temperatures between 40 and 46 kK. The reason for these similar Teffs is that the isochrone of these very massive stars, which we determined to be at ~2 Myr, runs almost vertically in the H-R diagram. We present a quantitative method of determining the effective temperature of O3-type stars based on the strength of the O V ?1371 line. Present-day evolutionary calculations by Meynet et al. imply that the program stars have initial masses in the range of Mi ~ 37-76 M?. The observed mass-loss rates are up to 3 (2) times higher than is assumed in these evolution tracks when adopting a metallicity Z = 0.004 (0.008) for the LMC. The high observed mass-loss rates imply that already at an age of ~2 Myr the most luminous of our program stars will have lost a significant fraction of their respective initial masses. For the least luminous stars investigated in this paper, the observed mass loss agrees with the prediction by the theory of radiation-driven winds (Kudritzki et al.). However, for increasing luminosity the observed mass loss becomes larger, reaching up to 3-4 times what is expected from the theory. Such an increasing discrepancy fits in with the results of de Koter et al., where an observed overpredicted mass-loss ratio of up to 8 was reported for the brightest members of the R136a cluster, for which Mi ~ 100 M? was found. The failure of the theory is also present when one compares observed over predicted wind momentum as a function of wind performance number. This strongly indicates that the shortcoming of the present state of the theory is connected to the neglect of effects of multiple photon momentum transfer.

The Goddard High Resolution Spectrograph: In-orbit performance

The in-orbit performance of the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope (HST) is presented. This report covers the pre-COSTAR period, when instrument performance was limited by the effects of spherical aberration of the telescopes primary mirror. The digicon detectors provide a linear response to count rates spanning over six orders of magnitude, ranging from the normal background flux of 0.01 counts diode -1 s-1 to values larger than 104 counts diode-1 s-1. Scattered light from the first-order gratings is small and can be removed by standard background subtraction techniques. Scattered light in the echelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 A to 3300 A, at spectral resolutions as high as R = lambda/delta-lambda = 90,000. The wavelength scale is influenced by spectrograph temperature, outgassing of the optical bench, and interaction of the magnetic field within the detector with the earths magnetic field. Models of these effects lead to a default wavelength scale with an accuracy better than 1 diode, corresponding to 3 km s-1 in the echelle mode. With care, the wavelength scale can be determined to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity functions is tied into the HST flux calibration through observations of spectrophotometric standard stars. The measurements of vignetting and the echelle blaze function provide relative photometric precision to about 5% or better. The effects of fixed-pattern noise have been investigated, and techniques have been devised for recognizing and removing it from the data. The ultimate signal-to-noise ratio achievable with the spectrograph is essentially limited only by counting statistics, and values approaching 1000:1 have been obtained.

The Goddard High Resolution Spectrograph: Instrument, goals, and science results

The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range of 1150-3200A with spectral resolutions (lambda/delta-lambda) of approximately 2 X 103, 2 X 104, and 1 X 105. This paper describes the instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future.

Emission-Line Galaxies from the NICMOS/Hubble Space Telescope Grism Parallel Survey

We present the first results of a survey of random fields with the slitless G141 (λ_c = 1.5 μm, Δλ = 0.8 μm) grism on the near-IR camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope (HST). Approximately 64 arcmin^2 have been observed at intermediate and high Galactic latitudes. The 3 σ limiting line and continuum fluxes in each field vary from 7.5 × 10^(-17) to 1 × 10^(-17) ergs cm^(-2) s^(-1), and from H = 20 to 22, respectively. Our median and area-weighted 3 σ limiting line fluxes within a 4 pixel aperture are nearly identical at 4.1 × 10^(-17) ergs cm^(-2) s^(-1) and are 60% deeper than the deepest narrowband imaging surveys from the ground. We have identified 33 emission-line objects and derive their observed wavelengths, fluxes, and equivalent widths. We argue that the most likely line identification is Hα and that the redshift range probed is from 0.75 to 1.9. The 2 σ rest-frame equivalent width limits range from 9 to 130 A, with an average of 40 A. The survey probes an effective comoving volume of 10^5 h^(-3)_50 Mpc^3 for q_0 = 0.5. Our derived comoving number density of emission-line galaxies in the range 0.7 < z < 1.9 is 3.3 × 10^(-4) h^3_(50) Mpc^(-3), very similar to that of the bright Lyman break objects at z ~ 3. The objects with detected emission lines have a median F160W magnitude of 20.4 (Vega scale) and a median Hα luminosity of 2.7 × 10^(42) ergs s^(-1). The implied star formation rates range from 1 to 324 M_☉ yr^(-1), with an average [N II] λλ6583, 6548 corrected rate of 21 M_☉ yr^(-1) for H_0 = 50 km s^(-1) Mpc and q_0 = 0.5 (34 M_☉ yr^(-1) for q_0 = 0.1).

On the Age Estimation of LBDS 53W091

The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators has suggested that this red galaxy at z = 1.552 is at least 3.5 Gyr old. This imposes an important constraint on cosmology, suggesting that this galaxy formed at z ≳ 6.5, assuming recent estimates of cosmological parameters. While their analysis was heavily focused on the use of some UV spectral breaks as age indicators, we have performed χ2 tests to the continuum of this galaxy using its UV spectrum and photometric data (R, J, H, and K: 2000-9000 A in rest frame). We have used the updated Yi models that are based on the Yale tracks. We find it extremely difficult to reproduce such large age estimates, under the assumption of the most probable input parameters. Using the same configuration as in Spinrad et al. (conventional solar abundance models), our analysis suggests an age of approximately 1.4-1.8 Gyr. We have improved our models over conventional ones by taking into account convective core overshoot in the stellar model calculations and realistic metallicity distributions in the galaxy population synthesis. Overshoot affects the visible continuum normalized to the UV and raises the photometry-based age estimates by 25%. The use of metallicity mixtures affects the whole spectrum and raises all continuum-based age estimates by up to a factor of 2. If the mean metallicity of the stars in this galaxy is assumed to be twice solar, the models including these two effects match the UV spectrum and photometric data of LBDS 53W091 near the age of 1.5-2.0 Gyr. Our results cannot be easily reconciled with that of Spinrad et al. The discrepancy between Spinrad et al.s age estimate (based on the Jimenez models) and ours originates from the large difference in the model integrated spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual & Charlot models. We propose to apply some viable tests to them for verification and search for the origin of the difference through a more thorough investigation. Considering the significance of the age estimates of distant galaxies as probes of cosmology, it would be an urgent task.

Chemical enrichment and physical conditions in I Zw 18

Abridged. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The HI region provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. Our primary objective is to study the enrichment of the HI region and the interplay between star-formation history and metallicity evolution. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the abundances in the HII region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HI region through physical diagnostics drawn from the fine-structure level of C+. We find that HI region abundances are lower by a factor of ~2 as compared to the HII region. There is no differential depletion on dust between the HI and HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the HI envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the HI region by investigating the CII* absorption line. The cooling rate derived from CII* is consistent with collisions with H atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).