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Brown, T. M.

Publications and source records attributed to Brown, T. M..

At least 19 records
All sourcesNASA NTRSDOE OSTI
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 NTRS2010

HST/ACS Observations of RR Lyrae Stars in Six Ultra-Deep Fields of M31

We present HST/ACS observations of RR Lyrae variable stars in six ultra deep fields of the Andromeda galaxy (M31), including parts of the halo, disk, and giant stellar stream. Past work on the RR Lyrae stars in M31 has focused on various aspects of the stellar populations that make up the galaxy s halo, including their distances and metallicities. This study builds upon this previous work by increasing the spatial coverage (something that has been lacking in previous studies) and by searching for these variable stars in constituents of the galaxy not yet explored. Besides the 55 RR Lyrae stars we found in our initial field located 11kpc from the galactic nucleus, we find additional RR Lyrae stars in four of the remaining five ultra deep fields as follows: 21 in the disk, 24 in the giant stellar stream, 3 in the halo field 21kpc from the galactic nucleus, and 5 in one of the halo fields at 35kpc. No RR Lyrae were found in the second halo field at 35kpc. The RR Lyrae populations of these fields appear to mostly be of Oosterhoff I type, although the 11kpc field appears to be intermediate or mixed. We will discuss the properties of these stars including period and reddening distributions. We calculate metallicities and distances for the stars in each of these fields using different methods and compare the results, to an extent that has not yet been done. We compare these methods not just on RR Lyrae in our M31 fields, but also on a data set of Milky Way field RR Lyrae stars.

Jeffery, E. J.↗
NASA NTRS2004

Analytical Modeling and Test Correlation of Variable Density Multilayer Insulation for Cryogenic Storage

A unique foam/multilayer insulation (MLI) combination concept for orbital cryogenic storage was experimentally evaluated using a large-scale hydrogen tank. The foam substrate insulates for ground-hold periods and enables a gaseous nitrogen purge as opposed to helium. The MLI, designed for an on-orbit storage period for 45 days, includes several unique features including a variable layer density and larger but fewer perforations for venting during ascent to orbit. Test results with liquid hydrogen indicated that the MLI weight or tank heat leak is reduced by about half in comparison with standard MLI. The focus of this effort is on analytical modeling of the variable density MLI (VD-MLI) on-orbit performance. The foam/VD-MLI model is considered to have five segments. The first segment represents the optional foam layer. The second, third, and fourth segments represent three different MLI layer densities. The last segment is an environmental boundary or shroud that surrounds the last MLI layer. Two approaches are considered: a variable density MLI modeled layer by layer and a semiempirical model or "modified Lockheed equation." Results from the two models were very comparable and were within 5-8 percent of the measured data at the 300 K boundary condition.

Hastings, L. J.↗
NASA NTRS2003

FUSE Observations of He-rich sdB Stars

Most subdwarf B stars are extremely deficient in helium and selected light elements, but a minority are helium-rich. New evolutionary calculations suggest that these helium-rich sdB stars are the result of a delayed helium-core flash on the white dwarf cooling curve, which leads to extensive mixing between the hydrogen envelope and helium core. Such mixed stars should show greatly enhanced helium and carbon with respect to the other heavy elements. We have recently obtained FUSE spectra of two helium-rich sdB stars, PG1544+488 and JL87, revealing huge C Ill lines at 977 and 1176 A. Our analysis shows that PG1544+488 has a surface composition of 97% He, 2% C, and 1% N, in agreement with the new evolutionary scenario. While JL87 also reveals a large enrichment in carbon and nitrogen (1.4% and 0.4%, respectively), there is still a significant amount of hydrogen in its atmosphere.

Swiegart, A. V.↗
NASA NTRS2001

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet color-magnitude diagrams of both omega Cen (DCruz et.al. 2000) and NGC 2808 (Brown et al. 2001). In order to investigate the origin of these subluminous stars, we have constructed a detailed set of evolutionary sequences that follow the evolution of low-mass stars continuously from the zero-age main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores (Castellani & Castellani 1993). Our results indicate that the location of the subluminous EHB stars, as well as the high temperature gap along the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the main helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed (Sweigart 1997). This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This "flash mixing" of the envelope during a late helium-core flash greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous increase in the HB effective temperature. We argue that the hot HB gap observed in NGC 2808 is associated with this theoretically predicted dichotomy in the HB properties. Using new helium- and carbon-rich stellar atmospheres, we show that the changes in the envelope abundances due to flash mixing will suppress the ultraviolet flux in the spectra of hot EHB stars. We suggest that such changes in the emergent spectral energy distribution are primarily responsible for explaining the hot subluminous EHB stars in omega Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations.

Sweigart, Allen V.↗
NASA NTRS2001

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Cen and NGC 2808

Ultraviolet observations of the globular clusters omega Cen and NGC 2808 have revealed an unexpected population of hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB), which axe not explained by canonical stellar models. In order to explore the evolutionary status of these stars, we have evolved a set of low-mass stars from the main sequence through the helium flash to the horizontal branch (HB) for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous stars can be explained if these stars undergo a late helium flash while descending the white dwarf cooling curve. Under these conditions the convection zone produced by the helium flash will penetrate into the stellar envelope, thereby mixing the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. Such "flashmixed" stars will have helium- and carbon-rich envelopes and will lie at higher effective temperatures than the hottest canonical (i.e., unmixed) EHB stars. Using new stellar atmospheres, we show that these changes in the envelope abundances will suppress the ultraviolet flux in the spectra of the flash-mixed stars by the amount needed to explain the hot subluminous stars in w Cen and NGC 2808. To test this evolutionary scenario, we have obtained medium resolution spectra of a sample of the hottest HB stars in omega Cen. We find that these stars axe indeed helium-rich compared to classical EHB stars and also considerably hotter than the hottest EHB models without flash mixing.

Sweigart, A. V.↗
NASA NTRS2000

Variable Density Multilayer Insulation for Cryogenic Storage

Two analytical models for a foam/Variable Density Multi-Layer Insulation (VD-MLI) system performance are discussed. Both models are one-dimensional and contain three heat transfer mechanisms, namely conduction through the spacer material, radiation between the shields, and conduction through the gas. One model is based on the methodology developed by McIntosh while the other model is based on the Lockheed semi-empirical approach. All models input variables are based on the Multi-purpose Hydrogen Test Bed (MHTB) geometry and available values for material properties and empirical solid conduction coefficient. Heat flux predictions are in good agreement with the MHTB data, The heat flux predictions are presented for the foam/MLI combinations with 30, 45, 60, and 75 MLI layers

Hedayat, A.↗
NASA NTRS2000

Ultraviolet Galaxy Counts From STIS Observations of The Hubble Deep Fields

We present galaxy counts in the near and far ultraviolet (NUV and FUV) obtained from Space Telescope Imaging Spectrograph (STIS) observations of portions of the Hubble Deep Field North, (HDFN), the Hubble Deep Field South, (HDFS) and a parallel field near the HDFN. All three fields have deep (AB>29) optical imaging, and we determine magnitudes by taking the ultraviolet flux detected within the limiting optical isophote. An analysis of the UV-optical colors of detected objects, combined with a visual inspection of the UV images, indicates that there are no detectable objects in the UV images which are not also detected in the optical. We measure the detection area and completeness as a function of magnitude by taking the size-magnitude distribution of galaxies in the entire HDFN WFPC2 V+I image, applying the measured UV-optical colors from the detected galaxies, and determining the total area over which each galaxy would have been detected in the UV images. The average area for the simulated galaxies in each UV magnitude bin, (including galaxies which would not be detected at all), provides the effective area and completeness for the bin. We test this procedure with Monte Carlo simulations. The galaxy counts reach to AB=29 in both the NUV and FUV; 1 magnitude fainter than the HDF F30OW counts, and 7 magnitudes fainter than balloon-based counts. We compare our measured counts to various models.

Gardner, J. P.↗
NASA NTRS1999

Reuse fo a Cold War Surveillance Drone to Flight Test a NASA Rocket Based Combined Cycle Engine

Plans for and early feasibility investigations into the modification of a Lockheed D21B drone to flight test the DRACO Rocket Based Combined Cycle (RBCC) engine are discussed. Modifications include the addition of oxidizer tanks, modern avionics systems, actuators, and a vehicle recovery system. Current study results indicate that the D21B is a suitable candidate for this application and will allow demonstrations of all DRACO engine operating modes at Mach numbers between 0.8 and 4.0. Higher Mach numbers may be achieved with more extensive modification. Possible project risks include low speed stability and control, and recovery techniques.

Brown, T. M.↗
NASA NTRS1998

Pressurization, Pneumatic, and Vent Subsystems of the X-34 Main Propulsion System

In pressurization systems, regulators and orifices are use to control the flow of the pressurant. For the X-34 Main Propulsion System, three pressurization subsystem design configuration options were considered. In the first option, regulators were used while in the other options, orifices were considered. In each design option, the vent/relief system must be capable of relieving the pressurant flow without allowing the tank pressure to rise above proof, therefore, impacts on the propellant tank vent system were investigated and a trade study of the pressurization system was conducted. The analysis indicated that design option using regulators poses least risk. Then, a detailed transient thermal/fluid analysis of the recommended pressurization system was performed. Helium usage, thermodynamic conditions, and overpressurization of each propellant tank were evaluated. The pneumatic and purge subsystem is used for pneumatic valve actuation, Inter-Propellant Seal purges, Engine Spin Start, and engine purges at the required interface pressures, A transient analysis of the pneumatic and purge subsystem provided helium usage and flow rates to Inter-Propellant Seal and engine interfaces. Fill analysis of the helium bottles of pressurization and pneumatic subsystems during ground operation was performed. The required fill time and the stored

Hedayat, A.↗
NASA NTRS1998

Propellant Management and Conditioning within the X-34 Main Propulsion System

The X-34 hypersonic flight vehicle is currently under development by Orbital Sciences Corporation (Orbital). The Main Propulsion ystem as been designed around the liquid propellant Fastrac rocket engine currently under development at NASA Marshall Space Flight Center. This paper presents analyses of the MPS subsystems used to manage the liquid propellants. These subsystems include the propellant tanks, the tank vent/relief subsystem, and the dump/fill/drain subsystem. Analyses include LOX tank chill and fill time estimates, LOX boil-off estimates, propellant conditioning simulations, and transient propellant dump simulations.

Brown, T. M.↗
NASA NTRS1998

Subsystem Analysis/Optimization for the X-34 Main Propulsion System

The Orbital Sciences Corporation X-34 vehicle demonstrates technologies and operations key to future reusable launch vehicles. The general flight performance goal of this unmanned rocket plane is Mach 8 flight at an altitude of 250,000 feet. The Main Propulsion System (MPS) supplies liquid propellants to the main engine, which provides the primary thrust for attaining mission goals. Major MPS design and operational goals are aircraft-like ground operations, quick turnaround between missions, and low initial/operational costs. Analyses related to optimal MPS subsystem design are reviewed in this paper. A pressurization system trade weighs maintenance/reliability concerns against those for safety in a comparison of designs using pressure regulators versus orifices to control pressurant flow. A propellant dump/feed system analysis weighs the issues of maximum allowable vehicle landing weight, trajectory, and MPS complexity to arrive at a final configuration for propellant dump/feed systems.

McDonald, J. P.↗
NASA NTRS1998

X-34 Main Propulsion System-Selected Subsystem Analyses

The X-34 hypersonic flight vehicle is currently under development by Orbital Sciences Corporation (Orbital). The Main Propulsion System (MPS) has been designed around the liquid propellant Fastrac rocket engine currently under development at NASA Marshall Space Flight Center. This paper presents selected analyses of MPS subsystems and components. Topics include the integration of component and system level modeling of the LOX dump subsystem and a simple terminal bubble velocity analysis conducted to guide propellant feed line design.

Brown, T. M.↗
NASA NTRS1993

Generation of artificial helioseismic time-series

We present an outline of an algorithm to generate artificial helioseismic time-series, taking into account as much as possible of the knowledge we have on solar oscillations. The hope is that it will be possible to find the causes of some of the systematic errors in analysis algorithms by testing them with such artificial time-series.

Schou, J.↗
NASA NTRS1991

The variability of the solar diameter

It is argued here that most of the variation in observations of the solar radius of over three centuries measure properties of the sun's limb darkening function and are affected by many sources of degradation of the solar image. They do not directly measure a true solar radius. From a long series of visual observations made by a single observer as well as recent photoelectric observations, evidence is found for periodicities in the apparent radius that occur in both modern and historical records. The magnitude of the observed variations is quite different in visual and photoelectric observations, suggesting that the process responsible for the periodicities is either one that modifies the solar limb darkening function or one that causes systematic variations in image blurring by the earth's atmosphere. A connection between solar magnetic activity an apparent radius seems likely, with evidence for such a relation dating back as fast as the Maunder minimum.

Ribes, E.↗
NASA NTRS1984

The Fourier Tachometer 2: an Instrument for Measuring Global Solar Velocity Fields

The High Altitude Observatory and Sacramento Peak Observatory have jointly constructed a second version of the Fourier Tachometer, which is now undergoing final integration and testing. This is an interferometric instrument for measuring the Doppler shift of solar spectrum lines. The principal features and performance goals of this instrument are: simultaneous velocity observations over a 2-dimensional, 100 x 100 pixel field of view; measurement of absolute Doppler shifts with 1 m/s accuracy; noise level for moderate-1 oscillation modes of 1 cm/s for a 1-day observing run; flexibility and ease of use. Early (though incomplete) testing suggests that these goals should be attainable with the current instrument.

Brown, T. M.↗
NASA NTRS1984

Analysis and Interpretation of Synthetic Time Strings of Oscillation Data

Artificial strings of solar oscillation data with gaps and noise, corresponding to the output of different spatial filter functions, were analyzed. Peaks in the power spectrum are identified for values of the degree l from 0 to 18, and rotational splitting is estimated. The filters prove effective in facilitating identification of essentially all the real peaks in the power spectrum. Estimates of peak frequencies and amplitudes and rotational splitting frequencies are in reasonably good agreement with the input values. Spurious peaks in autocorrelation spectra correspond to the frequency spacing between power peaks with the same order n, differing by one or two in the degree l.

Mihalas, B. W.↗