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NASA NTRS2003

Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (greater than or approximately equal to 300 Solar Mass). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super- Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and their X-ray emission is from the slim disk shining at super-Eddington luminosities.

White, Nicholas E.↗
NASA NTRS2002

Young, Massive Stars at Low Metallicity

We report on an extensive analysis of O-type stars in the Small Magellanic Cloud that have been observed by HST and FUSE. The analysis, which involves detailed NLTE atmospheric models recently constructed by Lanz & Hubeny, has the following goals: (1) calibration of spectral properties in terms of fundamental parameters; (2) resolution of the discrepancy between spectro-scopic masses and evolutionary masses; and (3) evidence for mixing of nuclear-processed elements. We derive a significantly lower temperature scale for O-type stars than previously assumed; the lower temperatures help to resolve the mass discrepancy. We describe the results of our analysis and discuss their implications for stellar evolution at low metallicity.

Oegerle, William R.↗
NASA NTRS2003

What is Your Cosmic Connection to the Elements?

This booklet provides information and classroom activities covering topics in astronomy, physics, and chemistry. Chemistry teachers will find information about the cosmic origin of the chemical elements. The astronomy topics include the big bang, life cycles of small and large stars, supernovae, and cosmic rays. Physics teachers will find information on fusion processes, and physical principles important in stellar evolution. While not meant to replace a textbook, the information provided here is meant to give the necessary background for the theme of :our cosmic connection to the elements." The activities can be used to re-enforce the material across a number of disciplines, using a variety of techniques, and to engage and excite students about the topic. Additional activities, and on-line versions of the activities published here, are available at http://imagine.gsfc.nasa.gov/docs/teachers/elements/.

Lochner, J.↗
NASA NTRS2003

Multi-Temperature Emission and Abundances in the Hot Gaseous Halo

We present the results of XMM-Newton observations of NGC 507, a dominant elliptical galaxy in a small group of galaxies. After carefully considering various systematic effects on abundance measurements, we report 'super-solar' metal abundances (both Fe and a-elements) present in the hot ISM: ZFe = 2-3 times solar with an observational limit of as high as 4 times solar inside the D25 ellipse of NGC 507. This is the highest ZFe reported so far, and fully consistent with those expected by the stellar evolution models where heavy elements are enriched by both type II and Ia supernovae ejecta. No unusual constraint either on the SNe rate or IMF is required. Among various factors affecting the accurate abundance measurement, we find that selecting a proper emission model is most important. As opposed to the X-ray spectral data with limited s/n and poor spatial/spectral resolution obtained in the previous missions, the spatially resolved XMM spectra provide enough statistics to untie the model-Z degeneracy and statistically require at least 3 emission components in each concentric shell (2 thermal components representing a finite range of kT in the hot ISM + 1 hard LMXB component). We show that a simpler model (such as a two-component model) produce a much lower best-fit ZFe. The abundances of a-elements (most accurately determined by Si) is also found to be super-solar and its radio to Fe is close to the solar ratio, suggesting a considerably contribution of heavy elements from Type Ia SNe. We estimate approx. 70% of MFe in the hot ISM originate from Type Ia.

Kim, Dong-Woo↗
NASA NTRS2000

Laboratory investigation of the contribution of complex aromatic/aliphatic polycyclic hybrid molecular structures to interstellar ultraviolet extinction and infrared emission

We have demonstrated by experiment that, in an energetic environment, a simple polycyclic aromatic hydrocarbon (PAH) such as naphthalene will undergo chemical reactions that produce a wide array of more complex species (an aggregate). For a stellar wind of a highly evolved star (post-asymptotic giant branch [post-AGB]), this process would be in addition to what is expected from reactions occurring under thermodynamic equilibrium. A surprising result of that work was that produced in substantial abundance are hydrogenated forms that are hybrids of polycyclic aromatic and polycyclic alkanes. Infrared spectroscopy described here reveals a spectral character for these materials that has much in common with that observed for the constituents of circumstellar clouds of post-AGB stars. It can be demonstrated that a methylene (-CH2-) substructure, as in cycloalkanes, is the likely carrier of the 6.9 microns band emission of dust that has recently been formed around IRAS 22272+5433, NGC 7027, and CPD -56 8032. Ultraviolet spectroscopy previously done with a lower limit of 190 nm had revealed that this molecular aggregate can contribute to the interstellar extinction feature at 2175 angstroms. We have now extended our UV spectroscopy of these materials to 110 nm by a vacuum ultraviolet technique. That work, described here, reveals new spectral characteristics and describes how material newly formed during the late stages of stellar evolution could have produced an extinction feature claimed to exist at 1700 angstroms in the spectrum of HD 145502 and also how the newly formed hydrocarbon material would be transformed/aged in the general interstellar environment. The contribution of this molecular aggregate to the rise in interstellar extinction at wavelengths below 1500 angstroms is also examined. The panspectral measurements of the materials produced in the laboratory, using plasmas of H, He, N, and O to convert the simple PAH naphthalene to an aggregate of complex species, provide insight into possible molecular structure details of newly formed hydrocarbon-rich interstellar dust and its transformation into aged material that becomes resident in the interstellar medium. Specifically the presence of naphthalene-like and butadiene-like conjugated structures as chromophores for the 2175 angstroms ultraviolet extinction feature is indicated.

NASA Discipline Exobiology↗
NASA NTRS2002

The Very Slow Wind from the Pulsating Semiregular Red Giant, L2 Puppis

We have obtained 1 1.7 and 17.9 micron images at the Keck I telescope of the circumstellar dust emission from L(sub 2) Pup, which is one of the nearest ( D = 61 pc) mass-losing, pulsating red giants that has a substantial infra-red excess. We propose that the star is losing mass at a rate of approx.3 x 10(exp -7) Solar Mass/yr. Given its relatively low luminosity (approx. 1500 Solar Luminosity), relatively high effective temperature (near 3400 K), relatively short period (approx. 140 days), and inferred gas outflow speed of 3.5 km/s, standard models for dust-driven mass loss do not apply. Instead, the wind may be driven by the stellar pulsations, with radiation pressure on dust being relatively unimportant. as described in some recent calculations. L(sub 2) Pup may serve as the prototype of this phase of stellar evolution, in which a star could lose approx. 15% of its initial main-sequence mass. Subject headings: circumstellar matter - stars: individual (L2 Puppis) - stars: mass loss

Jura, M.↗
NASA NTRS2005

Online Tools for Astronomy and Cosmochemistry

Over the past year, the Webnucleo Group at Clemson University has been developing a web site with a number of interactive online tools for astronomy and cosmochemistry applications. The site uses SHP (Simplified Hypertext Preprocessor), which, because of its flexibility, allows us to embed almost any computer language into our web pages. For a description of SHP, please see http://www.joeldenny.com/ At our web site, an internet user may mine large and complex data sets, such as our stellar evolution models, and make graphs or tables of the results. The user may also run some of our detailed nuclear physics and astrophysics codes, such as our nuclear statistical equilibrium code, which is written in fortran and C. Again, the user may make graphs and tables and download the results.

Meyer, B. S.↗
NASA NTRS2004

Open and Globular Cluster Distances for Extragalactic, Galactic, and Stellar Astrophysics

One of the hallmarks of SIM's few-milliarcsecond astrometric precision is its ability to obtain accurate parallax measurements across more than half of the Galaxy. The "open and globular" project obtains parallax distances to a set of star clusters. One important, goal is to pinpoint the zeropoint of the distance scale for main-sequence fitting. Another goal is to improve stellar evolutionary isochrones and integrated light models. Another goal is to use the clusters themselves to address unsolved problems of late-stage stellar evolution and Galactic and extragalactic chemical evolution. The clusters to be observed are chosen to span the widest possible range of abundance and age, to be as rich as possible, and to be as well-studied as possible.

Worthey, Guy S.↗
NASA NTRS1997

The Boomerang Nebula - The Coldest Region of the Universe

In this letter, we report such observations of the Boomerang Nebula which show it to be a unique object, consisiting of an ultra-cold and extremely massive molecular envolope, expanding at very high speed. The extreeme physical characteristics of the Boomerang Nebula reported here have never been seen before in any AGB or post-AGB object, and should spur new theoretical and obesrvational efforts to understand the nature of the mass-loss processes occurring during later stellar evolution.

Boomerang Nebula↗
NASA NTRS2006

Deep X-ray and UV Surveys of Galaxies with Chandra, XMM-Newton, and GALEX

Only with the deepest Chandra surveys has X-ray emission from normal and star forming galaxies (as opposed to AGN, which dominate the X-ray sky) been accessible at cosmologically interesting distances. The X-ray emission from accreting binaries provide a critical glimpse into the binary phase of stellar evolution and studies of the hot gas reservoir constrain past star formation. UV studies provide important, sensitive diagnostics of the young star forming populations and provide the most mature means for studying galaxies at 2 < zeta < 4. This talk will review current progress on studying X-ray emission in concert with UV emission from normal/star-forming galaxies at higher redshift. We will also report on our new, deep surveys with GALEX and XMM-Newton in the nearby Coma cluster. These studies are relevant to DEEP06 as Coma is the nearest rich cluster of galaxies and provides an important benchmark for high-redshift studies in the X-ray and UV wavebands. The 30 ks GALEX (note: similar depth to the GALEX Deep Imaging Survey) and the 110 ks XMM observations provide extremely deep coverage of a Coma outskirts field, allowing the construction of the UV and X-ray luminosity function of galaxies and important constraints on star formation scaling relations such as the X-ray-Star Formation Rate correlation and the X-ray/Stellar Mass correlation. We will discuss what we learn from these deep observations of Coma, including the recently established suppression of the X-ray emission from galaxies in the Coma outskirts that is likely associated with lower levels of past star formation and/or the results of tidal gas stripping.

Hornschemeier, Ann↗
NASA NTRS2009

Presolar Organic Globules in Tagish Lake Meteorite, and other Astromaterials

Presolar grains were identified in meteorite residues 20 years ago based on their exotic isotopic compositions. Their study has provide new insights into stellar evolution and the first view of the original building blocks of the solar system. Organic matter in meteorites and IDPs is highly enriched in D/H and N-15/N-14 at micrometer scales, possibly due to presolar organic grains. These anomalies are ascribed to the partial preservation of presolar cold molecular cloud material. Identifying the carriers of these anomalies and elucidating their physical and chemical properties may give new views of interstellar chemistry and better understanding of the original components of the protosolar disk. However, identifying the carriers has been hampered by their small size and the inability to chemically isolate them. Thanks to immediate careful collection of Tagish Lake meteorite specimen, as well as major advances in nano-scale analytical techniques and advanced sample preparation, we were able to show that in the Tagish Lake meteorite, the principle carriers of these isotopic anomalies are sub-micrometer, hollow organic globules. The organic globules likely formed by photochemical processing of organic ices in a cold molecular cloud or the outermost regions of the protosolar disk. Organic globules with similar physical, chemical, and isotopic properties are also recently found from Bells CM2 carbonaceous chondrite, in IDPs and in the comet Wild-2 samples returned by Stardust. These results support the view that microscopic organic grains were widespread constituents of the protoplanetary disk. Their exotic isotopic compositions trace their origins to the outermost portions of the protosolar disk or a presolar cold molecular cloud.

Nakamura-Messenger, K.↗
NASA NTRS2010

The Fermi Gamma-Ray Space Telescope, Exploding Stars, Neutron Stars, and Black Holes

Since August, 2008, the Fermi Gamma-ray Space Telescope has been scanning the sky, producing a full-sky image every three hours. These cosmic gamma-rays come from extreme astrophysical phenomena, many related to exploding stars (supernovae) or what these explosions leave behind: supernova remnants, neutron stars, and black holes. This talk uses sample Fermi results, plus simple demonstrations, to illustrate the exotic properties of these endpoints of stellar evolution.

Thompson, David J.↗
NASA NTRS2009

Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen.

Brown, Thomas M.↗
NASA NTRS2011

New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

Brown, T. M.↗
NASA NTRS2012

Presolar Organic Globules in Astromaterials

Presolar grains were identified in meteorite residues 20 years ago based on their exotic isotopic compositions [1]. Their study has provide new insights into stellar evolution and the first view of the original building blocks of the solar system. Organic matter in meteorites and IDPs is highly enriched in D/H and N-15/N-14 at micron scales, possibly due to presolar organic grains [2-4]. These anomalies are ascribed to the partial preservation of presolar cold molecular cloud material. Identifying the carriers of these anomalies and elucidating their physical and chemical properties may give new views of interstellar chemistry and better understanding of the original components of the protosolar disk. However, identifying the carriers has been hampered by their small size and the inability to chemically isolate them. Thanks to major advances in nano-scale analytical techniques and advanced sample preparation, we were able to show that in the Tagish Lake meteorite, the principle carriers of these isotopic anomalies are sub-microns, hollow organic globules [5]. The organic globules likely formed by photochemical processing of organic ices in a cold molecular cloud or the outermost regions of the protosolar disk [5]. Organic globules with similar physical, chemical, and isotopic properties are also recently found from Bells CM2 carbonaceous chondrite, in IDPs [6] and in the comet Wild-2 samples returned by Stardust [7]. These results support the view that microscopic organic grains were widespread constituents of the protoplanetary disk. Their exotic isotopic compositions trace their origins to the outermost portions of the protosolar disk or a presolar cold molecular cloud.

Nakamura-Messenger, K.↗
NASA NTRS2011

An Analysis and Classification of Dying AGB Stars Transitioning to Pre-Planetary Nebulae

The principal objective of the project is to understand part of the life and death process of a star. During the end of a star's life, it expels its mass at a very rapid rate. We want to understand how these Asymptotic Giant Branch (AGB) stars begin forming asymmetric structures as they start evolving towards the planetary nebula phase and why planetary nebulae show a very large variety of non-round geometrical shapes. To do this, we analyzed images of just-forming pre-planetary nebula from Hubble surveys. These images were run through various image correction processes like saturation correction and cosmic ray removal using in-house software to bring out the circumstellar structure. We classified the visible structure based on qualitative data such as lobe, waist, halo, and other structures. Radial and azimuthal intensity cuts were extracted from the images to quantitatively examine the circumstellar structure and measure departures from the smooth spherical outflow expected during most of the AGB mass-loss phase. By understanding the asymmetrical structure, we hope to understand the mechanisms that drive this stellar evolution.

mass loss↗
NASA NTRS2012

Servicing Mission 4 and the Extraordinary Science of the Hubble Space Telescope

Just two years ago, NASA astronauts performed a challenging and flawless final Space Shuttle servicing mission to the orbiting Hubble Space Telescope. With science instruments repaired on board and two new ones installed, the observatory. is more powerful now than ever before. I will show the dramatic highlights of the servicing mission and present some of the early scientific results from the refurbished telescope. Its high sensitivity and multi-wavelength capabilities are revealing the highest redshift galaxies ever seen, as well as details of the cosmic web of intergalactic medium, large scale structure formation, solar system bodies, and stellar evolution. Enlightening studies of dark matter, dark energy, and exoplanet atmospheres add to the profound contributions to astrophysics that are being made with Hubble, setting a critical stage for future observatories such as the James Webb Space Telescope.

Wiseman, Jennifer J.↗