Isabel Hawkins

The distribution of neutral hydrogen in the interstellar medium. 1: The data

We compile, from the existing literature, the largest sample to date (842 data points) of hydrogen column density measurements, N(H I), of the gas in the interstellar medium. We include only results obtained from absorption measurements toward individual stars (594 in our sample) in an effort to construct a three-dimensional picture of the interstellar gas. We derive hydrogen column densities toward a fraction of the stars in the sample from published column density measurements of metal ions. A three-dimensional physical model derived from this data set will be presented in a companion paper. The observed stars span distances from a few parsecs to a few thousand parsecs, and more than half of the sample serves to describe the local interstellar medium within a few hundred parsecs of the Sun. Hydrogen column densities range from 10(exp 17) to 10(exp 22)/sq cm. We describe here the various observational methods used to estimate the hydrogen column densities and present the table with the stellar and hydrogen column density data. The provided table is intended as a global reference work, not to introduce new results.

The abundance of interstellar oxygen toward Orion: Evidence for recent infall?

We present high S/N (greater than 800) Goddard High-Resolution Spectrograph (GHRS) observations of the weak interstellar O I lambda 1356 absorption in the low-density sight lines toward iota Ori and kappa Ori. By comparing these data with observations toward more reddened stars, we find no evidence of density-dependent depletion from the gas phase for oxygen. The derived total oxygen abundance (gas plus grains) towards iota Ori and kappa Ori is consistent with stellar and nebular determinations in Orion at a level that is one-half the solar value. We speculate that the O/H abundance ratio is lower in Orion compared to the Sun because the local Milky Way has suffered a recent infall of metal-poor material, perhaps from the Magellanic Stream.

Extreme Ultraviolet Explorer Bright Source List

Initial results from the analysis of the Extreme Ultraviolet Explorer (EUVE) all-sky survey (58-740 A) and deep survey (67-364 A) are presented through the EUVE Bright Source List (BSL). The BSL contains 356 confirmed extreme ultraviolet (EUV) point sources with supporting information, including positions, observed EUV count rates, and the identification of possible optical counterparts. One-hundred twenty-six sources have been detected longward of 200 A.

Interstellar cyanogen and the temperature of the cosmic microwave background radiation

We present the results of a recently completed effort to determine the amount of CN rotational excitation in five diffuse interstellar clouds for the purpose of accurately measuring the temperature of the cosmic microwave background radiation (CMBR). In addition, we report a new detection of emission from the strongest hyperfine component of the 2.64 mm CN rotational transition (N = 1-0) in the direction toward HD 21483. We have used this result in combination with existing emission measurements toward our other stars to correct for local excitation effects within diffuse clouds which raise the measured CN rotational temperature above that of the CMBR. After making this correction, we find a weighted mean value of T(CMBR) = 2.729 (+0.023, -0.031) K. This temperature is in excellent agreement with the new COBE measurement of 2.726 +/- 0.010 K (Mather et al., 1993). Our result, which samples the CMBR far from the near-Earth environment, attests to the accuracy of the COBE measurement and reaffirms the cosmic nature of this background radiation. From the observed agreement between our CMBR temperature and the COBE result, we conclude that corrections for local CN excitation based on millimeter emission measurements provide an accurate adjustment to the measured rotational excitation.

The C-12/C-13 isotope ratio of the interstellar medium in the neighborhood of the sun

Data from observations of Xi Per, P Cyg, 20 Tau, and 23 Tau, obtained at 4232 and 3957 A using a coude spectrograph and a 1872-element reticon on the 3-m Shane telescope at Lick Observatory during 1984-1985, are combined with data on Zeta Oph (Hawkins et al., 1985) and used to estimate the C isotope ratio of the ISM near the sun. The results are presented in extensive tables and graphs and characterized in detail. The (C-12)H(+)/(C-13)H(+) abundance ratios toward the five stars are found to agree to within 12 percent and shown to be representative of the C-12/C-13 ratios in the gas, strongly indicating that the local ISM is homogeneous. The difference between the ISM ratio (43 + or - 4) and the solar-system value (89) is attributed to the chemical evolution of the ISM in the 4.9 Gyr since the formation of the sun. 78 references.

The interstellar Li-7/Li-6 isotope ratio toward Zeta Ophiuchi and Zeta Persei

High S/N, high-resolution observations of the interstellar Li absorption lines toward the stars Zeta Ophiuchi and Zeta Persei are reported. Li I line profiles indicate the presence of both the Li-7 and Li-6 doublets in these two sightlines. Best-fit values for the interstellar Li-7/Li-6 isotope ratio are 6.8 (+1.4/-1.7) towards Zeta Ophiuchi and 5.5 (+ 1.3/-1.1) toward Zeta Persei. Measurement of 6.8 (+1.4/-1.7) for the interstellar Li-7/Li-6 isotope ratio towards Zeta Ophiuchi does not support the lower limit of 25 determined by Ferlet and Dennefeld (1984). The current value of the interstellar Li-7/Li-6 isotope ratio is the result of various lithium production and destruction processes involving stars, cosmic rays, and the big bang.

High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

Determination of (C-12)/(C-13) in the interstellar medium toward Zeta Ophiuchi and Xi Persei

The Lick Observatory Reticon (LOR) data presented in Hawkins et al. (1985) are reanalyzed in order to determine the cause of the significant discrepancy in (C-12)/(C-13) isotope ratios determined on the basis of observations of this isotope in the interstellar medium toward Zeta Ophiuchi and Xi Persei. A new set of Lick Observatory observations toward Zeta Oph are obtained using a CCD detector. The KPNO coude feed echelle spectrograph and a CCD detector are used to carry out high-resolution, high SNR ratio observations toward Zeta Oph and Xi Per. The LOR results for Zeta Oph are uncertain, ranging to 40 to 60, owing to the error in continuum placement. The KPNO results toward Zeta Oph are (C-12)H/(C-13)H(+)(0, 0) = 63 +/- 8, (C-12)H(+)/(C-13)H(+)(1, 0) = 67 +/- 19, and (C-12)N/(C-13)N = 100 + 88/- 33. Toward Xi Per, the KPNO measurements yield (C-12)H(+)/(C-13)H(+) = 49 +/- 15 from the 4232-A band, and a lower limit of 45 from the 3957-A observations.

Needling the early universe

The possibility that the whole microwave background can be produced by a bright population of pregalactic stars at a redshift of a few hundred is explored. The radiation is thermalized by a combination of amorphous silicate, amorphous carbon, graphite, and needle-shaped conducting grains which give rise to the opacity needed at wavelengths greater than 3 cm. The occurrence of distortion in a primordial microwave background spectrum due to its interaction with Population III stars and dust is investigated. The possibility of producing deviations small enough to be consistent with the best available observations, but still detectable by COBE, is considered. 65 references.

Increasing science literacy in optics and photonics through science centers, museums, and web-based exhibits

Exhibits and programs at hands-on science centers, museums, and on the web can be used to increase science literacy in optics and photonics. These informal science education efforts play a profound role in increasing the publics understanding of optics and photonics and its applications. Informal science education also plays a significant role in interesting young children in the photonics field. This paper presents a tour of the informal science education world and describes how scientists can work with science centers. It also describes how science centers and museums have made use of web broadcasts of special events such as eclipses to enhance public interest in science topics.