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All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

WFPC2 Observations of the URSA Minor Dwarf Spheroidal Galaxy

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F555W (approximately V) and F814W (approximately I) of the central region of the Ursa Minor dwarf spheroidal galaxy. The V versus V - I color-magnitude diagram features a sparsely populated blue horizontal branch, a steep thin red giant branch, and a narrow subgiant branch. The main sequence reaches approximately 2 magnitudes below the main-sequence turnoff (V(sup UMi, sub TO) approximately equals 23.27 +/- 0.11 mag) of the median stellar population. We compare the fiducial sequence of the Galactic globular cluster M92 (NGC 6341). The excellent match between Ursa Minor and M92 confirms that the median stellar population of the UMi dSph galaxy is metal poor ([Fe/H](sub UMi) approximately equals [Fe/H](sub M92) approximately equals -2.2 dex) and ancient (age(sub UMi)approximately equalsage(sub M92) approximately equals 14 Gyr). The B - V reddening and the absorption in V are estimated to be E(B - V) = 0.03 +/- 0.01 mag and A(sup UMi, sub V) = 0.09 +/- 0.03 mag. A new estimate of the distance modulus of Ursa Minor, (m - M)(sup UMi, sub 0) = 19.18 +/- 0.12 mag, has been derived based on fiducial-sequence fitting M92 [DELTA.V(sub UMi - M92) = 4.60 +/- 0.03 mag and DELTA(V - I)(sub UMi - M92) = 0.010 +/- 0.005 mag] and the adoption of the apparent V distance modulus for M92 of (m - M)(sup M92, sub V) = 14.67 +/- 0.08 mag (Pont et al. 1998, A&A, 329, 87). The Ursa Minor dwarf spheroidal galaxy is then at a distance of 69 +/- 4 kpc from the Sun. These HST observations indicate that Ursa Minor has had a very simple star formation history consisting mainly of a single major burst of star formation about 14 Gyr ago which lasted approximately < 2 Gyr. While we may have missed minor younger stellar populations due to the small field-of-view of the WFPC2 instrument, these observations clearly show that most of the stars in the central region Ursa Minor dwarf spheroidal galaxy are ancient. If the ancient Galactic globular clusters, like M92, formed concurrently with the early formation of the Milky Way galaxy itself, then the Ursa Minor dwarf spheroidal is probably as old as the Milky Way.

Mighell, Kenneth J.↗
NASA NTRS2004

On the Measurement of Elemental Abundance Ratios in Inner Galaxy H II Regions

Although abundance gradients in the Milky Way Galaxy certainly exist, details remain uncertain, particularly in the inner Galaxy, where stars and H II regions in the Galactic plane are obscured optically. In this paper we revisit two previously studied, inner Galaxy H II regions: G333.6-0.2 and W43. We observed three new positions in G333.6-0.2 with the Kuiper Airborne Observatory and reobserved the central position with the Infrared Space Observatory's Long Wavelength Spectrometer in far-infrared lines of S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 microns to the suite of lines measured in W43 by Simpson et al.. The measured electron densities range from approx. 40 to over 4000 per cu cm in a single HII region, indicating that abundance analyses must consider density variations, since the critical densities of the observed lines range from 40 to 9000 per cu cm. We propose a method to handle density variations and make new estimates of the S/H and N/H abundance ratios. We find that our sulfur abundance estimates for G333.6-0.2 and W43 agree with the S/H abundance ratios expected for the gradient previously reported by Simpson et al., with the S/H values revised to be smaller owing to changes in collisional excitation cross sections. The estimated N/H, S/H, and N/S ratios are the most reliable because of their small corrections for unseen ionization states (< or approx. 10%). The estimated N/S ratios for the two sources are smaller than what would be calculated from the N/H and S/H ratios in our previous paper. If all low excitation H II regions had similar changes to their N/S ratios as a result of adding measurements of N+ to previous measurements of N++, there would be no or only a very small gradient in N/S. This is interesting because nitrogen is considered to be a secondary element and sulfur is a primary element in galactic chemical evolution calculations. We compute models of the two H II regions to estimate corrections for the other unseen ionization states. We find, with large uncertainties, that oxygen does not, have a high abundance, with the result that the N/O ratio is as high (approx. 0.35) as previously reported. The reasons for the uncertainty in the ionization corrections for oxygen are both the non-uniqueness of the H II region models and the sensitivity of these models to different input atomic data and stellar atmosphere models. We discuss these predictions and conclude that only a few of the latest models adequately reproduce H II region observations, including the well-known, relatively-large observed Ne++/O++ ratios in low- and moderate-excitation H II regions.

Simpson, Janet P.↗
NASA NTRS1993

Leo I - The youngest Milky Way dwarf spheroidal galaxy?

Deep CCD photometry of about 16,000 stars in the Milky Way's Leo I spheroidal galaxy satellite is reported. An account is given of the features observed in the color-magnitude diagrams (CMDs) derived therefrom. A very blue and well-defined red giant branch (RGB) is noted. The CMDs of Leo I shows about 50 anomalous Cepheid candidates; there are another 50 or so asymptotic giant branch stars above the tip of the RGB, including 15 known carbon stars. The mean color of the RGB is estimated at M sub I = -3.5 mag.

Lee, Myung G.↗
NASA NTRS1982

Faint-object spectrograph optical bench

The Faint-Object Spectrograph (FOS) is one of five scientific instruments under development for use with the Optical Telescope Assembly (OTA) of NASA's Space Telescope. It is a dual-channel spectrograph operating with two independent 512-channel pulse-counting Digicons. The FOS will be employed in connection with the study of scientific questions associated with quasars, active galaxies, normal distant and local group galaxies, a wide class of objects within the Milky Way Galaxy and neighboring galaxies, and objects within the solar system. The FOS contains an optical bench which supports all optical elements. Dimensional stability was the primary design requirement for the optical bench. This led to the selection of a graphite/epoxy structure using laminates with very low coefficients of thermal expansion and high stiffness. The technical requirements are considered and details of fabrication are discussed.

Toth, J. M., Jr.↗
NASA NTRS1987

An astrometric facility for planetary detection on the space station

An Astrometric Telescope Facility (ATF) for planetary detection is being studied as a potential space station initial operating capability payload. The primary science objective of this mission is the detection and study of planetary systems around other stars. In addition, the facility will be capable of other astrometric measurements such as stellar motions of other galaxies and highly precise direct measurement of stellar distance within the Milky Way Galaxy. The results of a recently completed ATF preliminary systems definition study are summarized. Results of this study indicate that the preliminary concept for the facility is fully capable of meeting the science objective without the development of any new technologies. A simple straightforward operations approach was developed for the ATF. A real-time facility control is not normally required, but does maintain a near real-time ground monitoring capability for the facility and science data stream on a full-time basis. Facility observational sequences are normally loaded once a week. In addition, the preliminary system is designed to be fail-safe and single-fault tolerant. Routine interactions by the space station crew with the ATF will not be necessary, but onboard controls are provided for crew override as required for emergencies and maintenance.

Nishioka, Kenji↗
NASA NTRS1987

The gas content in starburst galaxies

The results from two large and homogeneous surveys, one in H I, the other in CO, are used for a statistical review of the gaseous properties of bright infrared galaxies. A constant ratio between the thermal FIR radiation and nonthermal radio emission is a universal property of star formation in spiral galaxies. The current rate of star formation in starburst galaxies is found to be 3-20 times larger than in the Milky Way. Galaxies with the higher FIR luminosities and warmer dust, have the larger mass fractions of molecular to atomic interstellar gas, and in some instances, striking deficiencies of neutral hydrogen are found. A statistical blueshift of the optical systemic velocities relative to the radio systemic velocities, may be due to an outward motion of the optical line-emitting gas. From the high rates of star formation, and from the short times required for the depletion of the interstellar gas, it is concluded that the most luminous infrared galaxies represent a brief but important phase in the evolution of some galaxies, when two galaxies merge changing substantially their overall properties.

Mirabel, I. F.↗
NASA NTRS1979

Columbia/Einstein observations of extragalactic X-ray sources

Results are presented of the analysis of data from observations of extragalactic objects with the imaging proportional counter on board the Einstein Observatory. Surveys of normal galaxies, radio galaxies, active galaxies, quasars and BL Lacs, and clusters of galaxies were studied in order to improve the understanding of the origin of the Milky Way Galaxy.

Ku, W. H. M.↗
NASA NTRS1979

A limit to the X-ray luminosity of nearby normal galaxies

The hypothesis that normal galaxies are on the average more luminous in the X-ray region than the Milky Way galaxy or M31 and therefore are possible candidates for the low-luminosity sources of the 2 to 60 keV extragalactic diffuse background is tested. Data from the A-2 detectors on the HEAO-1 spacecraft were examined for emission from positions coincident with 76 selected normal galaxies, and upper limits to the average galactic luminosity for various luminosity distributions resulting in the observed count rate distribution were determined. For uniform and exponential galactic luminosity distributions, limits of 2.7 x 10 to the 38th erg/sec and 3.4 x 10 to the 38th erg/sec, respectively, at the 90% confidence level were obtained. It is shown that the Hubble-constant-independent upper limit to galactic emissivity is less than 1% of the diffuse background emissivity, indicating that normal galaxies are not responsible for the diffuse X-ray background and have luminosities comparable to that of the Galaxy.

Worrall, D. M.↗
NASA NTRS1982

Distribution of gas in the halo of the galaxy

Results are presented from a high-resolution, ultraviolet study of interstellar gas situated away from the plane of the Milky Way Galaxy, using the nuclei of Seyfert galaxies Mkn 509 and F9 as background probes. In these directions, low-velocity, galactic gas were detected as well as two extragalactic clouds, one probably associated with the Magellanic Stream and the other with Mkn 509. These data were combined with results from other lines of sight to show that the ultraviolet species extend about 10 kpc from the plane, assuming the high-latitude gas corotates with the galactic disk. Complimentary observations of the optical Ca II and Na I species suggests that these do not extend as far - perhaps 2 to 3 kpc from the plane. Further, the exceedingly complex velocity structure found only in Magellanic Cloud directions suggests that these sight-lines are not typical of high-latitude gas in general.

Blades, J. C.↗
DOE OSTI2023

A variational encoder–decoder approach to precise spectroscopic age estimation for large Galactic surveys

Constraints on the formation and evolution of the Milky Way Galaxy require multidimensional measurements of kinematics, abundances, and ages for a large population of stars. Ages for luminous giants, which can be seen to large distances, are an essential component of studies of the Milky Way, but they are traditionally very difficult to estimate precisely for a large data set and often require careful analysis on a star-by-star basis in asteroseismology. Because spectra are easier to obtain for large samples, being able to determine precise ages from spectra allows for large age samples to be constructed, but spectroscopic ages are often imprecise and contaminated by abundance correlations. Here we present an application of a variational encoder–decoder on cross-domain astronomical data to solve these issues. The model is trained on pairs of observations from APOGEE and Kepler of the same star in order to reduce the dimensionality of the APOGEE spectra in a latent space while removing abundance information. The low dimensional latent representation of these spectra can then be trained to predict age with just ∼1000 precise seismic ages. We demonstrate that this model produces more precise spectroscopic ages (∼ 22 per cent overall, ∼ 11 per cent for red-clump stars) than previous data-driven spectroscopic ages while being less contaminated by abundance information (in particular, our ages do not depend on [α/M]). We create a public age catalogue for the APOGEE DR17 data set and use it to map the age distribution and the age-[Fe/H]-[α/M] distribution across the radial range of the Galactic disc.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2004

Anchoring the Population II Distance Scale: Accurate Ages for Globular Clusters

The metal-poor stars in the halo of the Milky Way galaxy were among the first objects formed in our Galaxy. These Population II stars are the oldest objects in the universe whose ages can be accurately determined. Age determinations for these stars allow us to set a firm lower limit, to the age of the universe and to probe the early formation history of the Milky Way. The age of the universe determined from studies of Population II stars may be compared to the expansion age of the universe and used to constrain cosmological models. The largest uncertainty in estimates for the ages of stars in our halo is due to the uncertainty in the distance scale to Population II objects. We propose to obtain accurate parallaxes to a number of Population II objects (globular clusters and field stars in the halo) resulting in a significant improvement in the Population II distance scale and greatly reducing the uncertainty in the estimated ages of the oldest stars in our galaxy. At the present time, the oldest stars are estimated to be 12.8 Gyr old, with an uncertainty of approx. 15%. The SIM observations obtained by this key project, combined with the supporting theoretical research and ground based observations outlined in this proposal will reduce the estimated uncertainty in the age estimates to 5%).

Chaboyer, Brian C.↗
NASA NTRS2004

Theoretical Studies of Gamma-Ray Bursts with Swift

Graduate student L.J. Gou , P. Meszaros, T. Abel and B. Zhang investigated the detectability of long GRB afterglows from very high redshifts, where bright reverse shock emission last longer in the observer frame, and its importance for detection and analysis purposes relative to the forward shock increases. They consider two different models for the GRB environment, based on current ideas about the redshift dependence of gas properties in galaxies and primordial star formation. They calculate the observed flux as a function of the redshift and observer time for typical GRB afterglows, taking into account intergalactic photoionization and Lyman-alpha absorption opacity as well as extinction by the Milky Way Galaxy. The fluxes in the X-ray and near IR bands are compared with the sensitivity of different detectors such as Chandra, Swift and JWST. They find that Chandra and Swift can potentially detect GRBs out to very high redshifts z above 13 and 30, respectively. In the K and M bands, the JWST and ground-based telescopes are potentially able to detect GRBs even one day after the trigger out to z approximately 16 and 33, if present. While the X-ray band is insensitive to the external density and to reverse shocks, the near IR bands provides a sensitive tool for diagnosing both the environment and the reverse shock components.

Meszaros, Peter↗
NASA NTRS2000

A Tethered Formation Flying Concept for the SPECS Mission

The Sub-millimeter Probe of the Evolution of Cosmic Structure (SPECS) is a bold new mission concept designed to address fundamental questions about the Universe, including how the first stars formed from primordial material, and the first galaxies from pre-galactic structures, how the galaxies evolve over time, and what the cosmic history of energy release, heavy element synthesis, and dust formation is. Half of the luminosity and 98% of the post Big-Bang photons exit in the sub-millimeter range. The spectrum of our own Milky Way Galaxy shows this, and many galaxies have even more pronounced long-wavelength emissions. There can be no doubt that revolutionary science will be enabled when we have tools to study the sub-millimeter sky with Hubble- Space-Telescope-class resolution and sensitivity. Ideally, a very large telescope with an effective aperture approaching one kilometer in diameter would be needed to obtain such high quality angular resolution at these long wavelengths. However, a single aperture one kilometer in diameter would not only be very difficult to build and maintain at the cryogenic temperatures required for good seeing, but could actually turn out to be serious overkill. Because cosmic sub-millimeter photons are plentiful and the new detectors will be sensitive, the observations needed to address the questions posed above can be made with an interferometer using well established aperture synthesis techniques. Possibly as few as three 3-4 meter diameter mirrors flying in precision formation could be used to collect the light. To mitigate the need for a great deal of propellant, tethers may be needed as well. A spin-stabilized, tethered formation is a possible configuration requiring a more advanced form of formation flying controller, where dynamics are coupled due to the existence of the tethers between nodes in the formation network. The paper presents one such concept, a proposed configuration for a mission concept which combines the best features of structure, tethers and formation flying to meet the ambitious requirements necessary to make a future SPECS mission a success.

Quinn, David A.↗
NASA NTRS1990

The space distribution of AGB stars

The AGB stars can be classified into three main types on the basis of their atmospheric composition: carbon-rich, oxygen-rich, and S-type. The carbon-rich stars typically have 1.5-solar-mass main sequence stars as progenitors. There are about 40 of these stars per sq kpc projected onto the plane of the Milky Way galaxy in the neighborhood of the sun with an exponential scale height above the galactic disk of about 200 pc. Contrary to the general distribution of mass, there is no decrease with galactocentric radius of the surface density of these stars for at least 3 kpc beyond the solar circle. The S-type stars appear to have the same spatial distribution as the carbon stars; there are about 1/3 as many 'pure' S stars as there are luminous carbon stars. One major class of oxygen-rich AGB stars is the Mira variables. There are approximately between 1 and 2 times as many of these stars per sq kpc projected onto the galactic plane of the Milky Way as there are carbon stars.

Jura, M.↗
NASA NTRS2001

Dips in X-Ray Flux Associated with Superluminal Ejections in the Radio Galaxy 3C 120

We compare the RXTE X-ray light curve of 3C 120 from our late 1997 to early 2000 observations, plus early 1997 archival data, with both the cm-wave light curves (University of Michigan Radio Astronomy Observatory) and the times of ejections of apparent superluminal components (from VLBA observations). There is no correlation between the X-ray and radio light curves. However, the epochs of all four observed superluminal ejections correspond, to within the errors, with pronounced dips in the X-ray light curves (photon energies in the range 2-20 keV). This behavior is similar to that of the binary-system 'microquasar' GRS1915+105 in the Milky Way galaxy, although in 3C 120 there is no evidence for rapid, quasi-periodic fluctuations in X-ray flux. These observations therefore support the notion that in active galactic nuclei eruptions of energetic outflow are related to excess accretion onto a supermassive black hole.

Marscher, A. P.↗
NASA NTRS1984

Detection of far-infrared forbidden O I and forbidden O III emission from the galaxy M82

Observations in M82 of the lowest-lying transitions of neutral and doubly ionized oxygen, forbidden O I 63.2 microns and forbidden O III 88.4 microns, which have wavelengths at which the extinction toward M82 is negligible, are reported. For O III, the +220 km/s velocity of the line center with respect to the local standard of rest, the 300 km/s intrinsic line width, and the asymmetry of the profile are consistent with the systemic velocity and rotational broadening seen in other ionic lines in the nucleus of M82. The fraction of oxygen that is present in doubly ionized form, indicates that the ionization state is similar to that found in H II regions in the disk of the Milky Way Galaxy. The central velocity of the O I line is +130 km/s, and no broadening in excess of instrumental resolution is seen. This result appears to rule out association of this emission with the periphery of the central H II region.

Watson, D. M.↗
NASA NTRS1992

Propagation of cosmic-ray nuclei in a diffusing galaxy with convective halo and thin matter disk

A diffusion model for cosmic-ray propagation in the galaxy that includes the effects of convection in the halo is presented. Calculations are made for 13 primary and secondary nuclei with rigidities between 1 and 1000 GV using interaction loss rates, secondary production rates, and radioactive decay on the basis of recent new cross-section measurements. It is found that, in order to fit the rather weak radial dependence of cosmic-ray protons derived from gamma-ray data, the radial profile of the cosmic-ray sources must also have a weak radial dependence. It is suggested that convection perpendicular to the disk of the Milky Way Galaxy may not be important even at rigidities less than a few GV. The obtained limits on halo thicknesses are consistent with what can be determined for the distribution of cosmic-ray electrons in the halo based on the distribution of radio synchrotron emission in this and other galaxies.

Webber, W. R.↗
NASA NTRS1984

Interstellar absorption lines in the spectrum of supernova Evans in M83 (NGC 5236)

It is pointed out that supernovae in other galaxies create spontaneous opportunities to probe spectroscopically the gaseous material in galactic halos. Because of their brightness, supernovae offer an unparalleled chance to obtain spectra with high wavelength resolution and/or signal-to-noise ratio. The present investigation is concerned with observations of visual interstellar lines in the spectrum of a supernova discovered on July 3, 1983, and identified as Type I. The supernova was located 124 arc sec south and 122 arc sec west of the nucleus of M83. Evidence is obtained that for velocities near that of M83, the line of sight toward the supernova contains a complex array of absorption features of Ca II. In both Ca II and Na I, the absorptions associated with M83 are far more prominent than those halfway through the disk (and halo) of the Milky Way Galaxy. It is felt that many of the displaced velocity components in the M83 system arise from a gaseous halo.

Rodgers, A. W.↗