Engineering topics

Harrison, Thomas E.

Publications and source records attributed to Harrison, Thomas E..

6 records

NASA NTRS2011

Galex and Optical Observations of GW Librae during the Long Decline from Superoutburst

The prototype of accreting, pulsating white dwarfs (GW Lib) underwent a large amplitude dwarf nova outburst in 2007. We used ultraviolet data from Galaxy Evolution Explorer and ground-based optical photometry and spectroscopy to follow GW Lib for three years following this outburst. Several variations are apparent during this interval. The optical shows a superhump modulation in the months following outburst, while a 19 minute quasi-periodic modulation lasting for several months is apparent in the year after outburst. A long timescale (about 4 hr) modulation first appears in the UV a year after outburst and increases in amplitude in the following years. This variation also appears in the optical two years after outburst but is not in phase with the UV. The pre-outburst pulsations are not yet visible after three years, likely indicating the white dwarf has not returned to its quiescent state.

NOVAE↗

NASA NTRS2009

Astrometry With the Hubble Space Telescope: Trigonometric Parallaxes of Planetary Nebula Nuclei NGC 6853, NGC 7293, ABELL 31, and DeHt 5

We present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae NGC 6853 (The Dumbbell), NGC 7293 (The Helix), Abell 31, and DeHt 5. This paper details our reduction and analysis using DeHt 5 as an example. We obtain these planetary nebula nuclei (PNNi) parallaxes with astrometric data from Fine Guidance Sensors FGS 1r and FGS 3, white-light interferometers on the Hubble Space Telescope. Proper motions, spectral classifications and VJHKT2M and DDO51 photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each PNN. Weighted averaging with previous independent parallax measurements yields an average parallax precision, sigma (sub pi)/ pi = 5%. Derived distances are: d(sub NGC6853) = 405(exp +28 sub -25) pc, d(sub NGC7293) = 216(exp +14 sub -12) pc, d(sub Abell31) = 621(exp +91 sub -70) pc, and d(sub DeHt5) = 345(exp +19 sub -17) pc. These PNNi distances are all smaller than previously derived from spectroscopic analyses of the central stars. To obtain absolute magnitudes from these distances requires estimates of interstellar extinction. We average extinction measurements culled from the literature, from reddening based on PNNi intrinsic colors derived from model SEDs, and an assumption that each PNN experiences the same rate of extinction as a function of distance as do the reference stars nearest (in angular separation) to each central star. We also apply Lutz-Kelker bias corrections. The absolute magnitudes and effective temperatures permit estimates of PNNi radii through both the Stefan-Boltzmann relation and Eddington fluxes. Comparing absolute magnitudes with post-AGB models provides mass estimates. Masses cluster around 0.57 solar Mass, close to the peak of the white dwarf mass distribution. Adding a few more PNNi with well-determined distances and masses, we compare all the PNNi with cooler white dwarfs of similar mass, and confirm, as expected, that PNNi have larger radii than white dwarfs that have reached their final cooling tracks.

STARS↗

NASA NTRS1994

Optical studies of gamma-ray burst fields. 3: GRB 790613 and GRB 781124

An optical photometric and proper motion study of the fields of the gamma-ray bursts GRB 790613 and GRB 781124 is presented. No objects were visible on our Charge Coupled Devices (CCD) images inside the GRB 790613 error box. About 15 objects are visible on our CCD images inside the GRB 781124 error box. Two of these objects were found to be faint galaxies. Two stars with sizeable proper motions were identified near, but outside of the GRB 781124 error box. No large proper motion objects were found in the area surrounding the GRB 790613 error box.

Harrison, Thomas E.↗

NASA NTRS1994

Optical studies of gamma-ray bursts. 2: The superbowl burst GRB 930131

We report Charge Couple Devices (CCD) observations of the central region of the Compton Telescope (COMPTEL) error box for the 31 Jan 1993 gamma-ray burst. One optical transient was found. Its distance from the IPN track and Energy Gamma Ray Experiment Telescope (EGRET) error box make the association of this object with GRB 930131 doubtful. We present new CCD observations of two objects identified by Schaefer et al. (1993) that are located within a few arcminutes of the interplanetary network (IPN) that were reported to be possibly variable. We also report observations on the ROSAT All-sky Survey Extreme Ultraviolet (EUV) source noted by those investigators and the RS CVn star HD 106255. The ROSAT All-Sky Survey EUV source is suggested to arise from a faint, but relatively rich cluster of galaxies. HD 106255 is located approximately 7 min from the IPN. Our CCD results show that this star was bluer than previously reported, but these observations were obtained significantly after the occurrence of GRB 930131. It is not possible to estimate the color of HD 106255 when the burst occurred.

Mcnamara, Bernard↗

NASA NTRS1994

Optical studies of gamma-ray burst fields, 1: GRB 790329

An optical photometric and proper motion survey of a approximately = 160 sq arcmin region centered on an interplanetary network 41 sq arcmin GRB 790329 error box is presented. No obvious counterparts to the gamma-ray burst were identified. The distribution of stellar colors in the survey region was that expected for the galactic latitude of the burst field. Six stars with significant proper motions were identified. One of these, Gl 397, was found to be a double star. The companion has the color and luminosity of an M4/5 dwarf. We note that associating the gamma-ray burst with the Gl 397 system seems to require energies beyond that seen in the flares of other late-type dwarfs. A small number of galaxies were identified, one of which (object #4), was the bluest object found in the survey (V - I = 0.49).

Harrison, Thomas E.↗

NASA NTRS1990

Photometry of variable AFGL sources

Results of infrared photometric observations of 63 AFGL sources over the past 9 years are presented. Using these data together with previous measurements of these stars, pulsation periods and mean photometric characteristics are determined. These stars are found to lie midway between optically identified Mira variables and the radio-luminous OH/IR stars in their period distribution and photometric properties. For the sample as a whole, there is no evidence for sudden or transient behavior such as a switch in pulsation mode. Rather, these stars suggest rapid, but continuous, evolution from shorter period Miras with weak mass loss to longer periods and larger mass-loss rates. The carbon-rich stars in the sample have the same period distribution as the oxygen-rich stars. None of the carbon stars have periods as long as those of the very long period radio-luminous OH/IR stars.

Jones, Terry Jay↗