Engineering topics

Henry, Gregory W.

Publications and source records attributed to Henry, Gregory W..

At least 19 records

NASA NTRS2005

A Search for Transiting Neptune-Mass Extrasolar Planets in High-Precision Photometry of Solar-Type Stars

Tennessee State University operates several automatic photometric telescopes (APTs) at Fairborn Observatory in southern Arizona. Four 0.8 m APTs have been dedicated to measuring subtle luminosity variations that accompany magnetic cycles in solar-type stars. Over 1000 program and comparison stars have been observed every clear night in this program for up to 12 years with a precision of approximately 0.0015 mag for a single observation. We have developed a transit-search algorithm, based on fitting a computed transit template for each trial period, and have used it to search our photometric database for transits of unknown companions. Extensive simulations with the APT data have shown that we can reliably recover transits with periods under 10 days as long as the transits have a depth of at least 0.0024 mag, or about 1.6 times the scatter in the photometric observations. Thus, due to our high photometric precision, we are sensitive to transits of possible short-period Neptune-mass planets that likely would have escaped detection by current radial velocity techniques. Our search of the APT data sets for 1087 program and comparison stars revealed no new transiting planets. However, the detection of several unknown grazing eclipsing binaries from among our comparison stars, with eclipse depths of only a few millimags, illustrates the success of our technique. We have used this negative result to place limits on the frequency of Neptune-mass planets in close orbits around solar-type stars in the Sun's vicinity.

Henry, Stephen M.↗

NASA NTRS2001

Techniques for Automated Single-Star Photometry

Tennessee State University operates several automatic photoelectric telescopes (APTs) located at Fairborn Observatory in the Patagonia Mountains of southern Arizona. The APTs are dedicated to photometric monitoring programs that would be expensive and difficult to accomplish without the advantages of automation. I review the operation of two of the telescopes (a 0.75- and 0.80-meter (m) APT) and the quality-control techniques that result in the routine acquisition of single-star differential photometry with a precision of 0.001 mag for single observations and 0.0001 - 0.0002 mag for seasonal means. I also illustrate the capabilities of the APTS with sample results from a program to measure luminosity cycles in sun-like stars and a related program to search for the signatures of extrasolar planets around these stars.

Henry, Gregory W.↗

NASA NTRS1998

Automatic Telescope Search for Extrasolar Planets

We are using automatic photoelectric telescopes at the Tennessee State University Center for Automated Space Science to search for planets around nearby stars in our galaxy. Over the past several years, wc have developed the capability to make extremely precise measurements of brightness changes in Sun-like stars with automatic telescopes. Extensive quality control and calibration measurements result in a precision of 0.l% for a single nightly observation and 0.0270 for yearly means, far better than previously thought possible with ground-based observations. We are able, for the first time, to trace brightness changes in Sun-like stars that are of similar amplitude to brightness changes in the Sun, whose changes can be observed only with space-based radiometers. Recently exciting discoveries of the first extrasolar planets have been announced, based on the detection of very small radial-velocity variations that imply the existence of planets in orbit around several Sun-like stars. Our precise brightness measurements have been crucial for the confirmation of these discoveries by helping to eliminate alternative explanations for the radial-velocity variations. With our automatic telescopes, we are also searching for transits of these planets across the disks of their stars in order to conclusively verify their existence. The detection of transits would provide the first direct measurements of the sizes, masses, and densities of these planets and, hence, information on their compositions and origins.

Henry, Gregory W.↗

NASA NTRS1998

Brightness Changes in Sun-like Stars

Does the Sun's energy output vary with time? Are observable climatic changes on the earth caused by changes in the Sun? Can we gain greater insight into this relation-ship by studying other stars with properties similar to the Sun's? In recent years, satellite observations have shown that the solar irradiance varies in phase with the 1 l-year sunspot cycle. The Sun is brighter by about O.l% at the peak of the sunspot cycle when solar magnetic activity is at its maximum. Over longer intervals, changes in the cart h's climate and solar magnetic activity seem to be correlated. We are using automatic photoelectric telescopes to measure brightness changes in a sample of 150 Sun-like stars. Lowell Observatory astronomers have also observed about 30 of these same stars with a manual telescope in a program that began 10 years before ours. Since these two data sets were acquired with different instruments and so have significant systematic differences, we developed software to combine them accurately and, therefore, extend our observational time coverage. We show sample results of brightness variations over 14 years in several Sun-like stars with different ages. Longitudinal studies like these, combined with cross-sectional studies of the larger sample of stars, may eventually allow us to infer with confidence the Sun's long-term brightness history and its impact on the earth's climate.

Henry, Stephen M.↗

NASA NTRS1998

The Associate Principal Astronomer for AI Management of Automatic Telescopes

This research program in scheduling and management of automatic telescopes had the following objectives: 1. To field test the 1993 Automatic Telescope Instruction Set (ATIS93) programming language, which was specifically developed to allow real-time control of an automatic telescope via an artificial intelligence scheduler running on a remote computer. 2. To develop and test the procedures for two-way communication between a telescope controller and remote scheduler via the Internet. 3. To test various concepts in Al scheduling being developed at NASA Ames Research Center on an automatic telescope operated by Tennessee State University at the Fairborn Observatory site in southern Arizona. and 4. To develop a prototype software package, dubbed the Associate Principal Astronomer, for the efficient scheduling and management of automatic telescopes.

Henry, Gregory W.↗

NASA NTRS1995

Starspot evolution, differential rotation, and magnetic cycles in the chromospherically active binaries lambda andromedae, sigma Geminorum, II Pegasi, and V711 Tauri

We have analyzed 15-19 yr of photoelectric photometry, obtained manually and with automated telescopes, of the chromospherically active binaries lambda And, sigma Gem, II Peg, and V711 Tau. These observations let us identify individual dark starspots on the stellar surfaces from periodic dimming of the starlight, follow the evolution of these spots, and search for long-term cyclic changes in the properties of these starspots that might reveal magnetic cycles analogous to the Sun's 11 yr sunspot cycle. We developed a computer code to fit a simple two-spot model to our observed light curves that allows us to extract the most easily determinable and most reliable spot parameters from the light curves, i.e., spot longitudes and radii. We then used these measured properties to identify individual spots and to chart their life histories by constructing migration and amplitude curves. We identified and followed 11 spots in lambda And, 16 in sigma Gem, 12 in II Peg, and 15 in V711 Tau. Lifetimes of individual spots ranged from a few months to longer than 6 yr. Differential rotation coefficients, estimated from the observed range of spot rotation periods for each star and defined by equation (2), were 0.04 for lambda And, 0.038 for sigma Gem, 0.005 for II Peg, and 0.006 for V711 Tau, versus 0.19 for the Sun. We searched for cyclic changes in mean brightness, B-V color index, and spot rotation period as evidence for long-term cycles. Of these, long-term variability in mean brightness appears to offer the best evidence for such cycles in these four stars. Cycles of 11.1 yr for lambda And, 8.5 yr for sigma Gem, 11 yr for II Peg, and 16 yr V711 Tau are implied by these mean brightness changes. Cyclic changes in spot rotation period were found in lambda And and possibly II Peg. Errors in B-V were too large for any long-term changes to be detectable.

Henry, Gregory W.↗

NASA NTRS1995

A spectroscopic and photometric study of 12 BM Camelopardalis

Radial velocities from 1916.95 to 1991.95 and photometry from l979.25, both published and new in this paper, are presented and analyzed. A new solution of the radial velocity curve reveals a new period of 80.90 days and an eccentricity of e = 0.05 +/- 0.02, both very different from the 80.17 days and 0.35 found by Abt et al. (1969). An alternative solution with e = 0 is given because we cannot decide firmly whether or not the small eccentricity is real. We find V sin i = 11.3 +/- 0.3 km/s from Maidanak and 10 unequal depth. 0.048 mins and 0.026 mins. The orbital ephemeris for conjunction (K gisnt behind) is JD(hel.) 2,448,111.1 (+/- 0.4 days ) + 80.898 days (+/- 0.004 days ) E, consistent with both the radial velocities and the photometry. With the ellipticity effect removed, the light curve shows residual variability which we fit with a two-spot model. During the 13 years covered by photometry there were nine different starspots, the largest one producing a light loss of 0.19 mins. Rotation periods for the nine spots ranged from 78.6 +/- 0.5 days to 83.7 +/- 0.4 days from which we concluded that the K giant does rotate synchronously but with a differential rotation coefficient of k = 0.06 +/- 0.01. Lifetimes for the nine spots ranged from 1.1 to greater than 4.2 yr and were consistent with the empirical spot lifetime laws of Hall & Henry (1994). Use of the mass function, the orbital period, the V sin i, the two different ellipticity effect amplitudes, and various logical constraints led to ranges of possible masses, radii and inclinations. The most believable solution was around i = 90 deg, R(sub 1) = 24 solar radii, M(sub 1) = 1.1 solar mass, and M(sub 2) = 0.6 solar mass. THe Rossby number for the K giant is 0.48, small enough compared to the critical value of 0.65 to explain why, though rotating 'slowly', it does have large spots.

Hall, Douglas S.↗

NASA NTRS1995

Starspots found on the ellipsoidal variable V350 lacertae = HR 8575

It has been a puzzle why this chromospherically active, strong-dynamo K2 IV-III star is not known to have the large starspots characteristic of other such stars. Published individual radial velocities, which had never been analyzed, are used to derive an orbital solution. Combined with the one older existing orbital solution, this yields an improved orbital ephemeris: time of conjunction (K star behind) = JD 2445255.47 +/- 0.11 days and period = 17.75346 +/- 0.00016 days. All available photoelectric photometry, from 1970.9 to 1992.5, is collected A cos 2 theta fit of the ellipticity effect yields JD 2445255.60 +/- 0.06 days for a time of conjunction, 17.7523 +/- 0.0005 days for the period, and 0.084 mins for the peak-to-peak amplitude in V. With the ellipticity effect removed, the light curve does show measurable starspot variability in 15 of 16 data groups, the starspot wave amplitudes ranging between 0.03 mins and 0.08 mins. Ten starspots are identified and their rotation periods determined, the mean being 17.70 +/- 0.03 days (confirming synchronous rotation) and the range being Delta P/P = 0.017 +/- 0.006 (indicating differential rotation). There is a slow variation in mean brightness, almost 0.1 min in range and at least 2 decades in length.

Crews, Lionel J.↗

NASA NTRS1994

1988-1991 photometry of the chromospherically active double-lined binary system HD 163621 = V835 Herculis

Photometry in V and B with a 16-inch automatic telescope from 1988 to 1991 confirms the synchronous rotation suspected earlier and reveals variability up to 0.1M in the B bandpass at one epoch. Five spots formed with lifetimes ranging from 0.15 to 0.77 years. As with other rapid rotators, the differential rotation coefficient (k = 0.03) is significantly smaller than that of the Sun (k = 0.19).

Hall, Douglas↗

NASA NTRS1994

Chromospherically active stars. 12: ADS 11060 C: A double lined K dwarf binary in a quintuple system

ADS 11060 C is a double lined spectroscopic binary with a period of 25.7631 days and an eccentricity of 0.565. Spectral types of the two stars are estimated as K7 V and MO V with a magnitude difference of about 0.55 mag in V. The stars appear to be somewhat metal rich with respect to the Sun. Despite the relatively large masses of 0.53 and 0.51 solar mass, our photometric observations find no evidence for eclipses and we estimate an inclination of 77 deg plus or minus 11 deg. ADS 11060 C is, however, photometrically variable with a period of 9 plus or minus 1 day and an amplitude of 0.05 mag in V. Thus, it is a newly identified BY Draconis variable. The center-of-mass velocity of ADS 11060 C and an estimated parallax of 0.030 sec support its physical association with ADS 11060 AB, making this a quintuple system. The projected separation of the AB-C system is nearly 1200 AU. Although the log lithium abundances of the two components of ADS 11060 C are only upper limits, less than or equal to -0.14, lithium abundances of the AB-C components appear to be consistent with those of similar stars in the alpha Persei and Pleiades clusters, suggesting an age of about 70 Myr for ADS 11060 AB-C. The system is a possible member of the Pleiades moving group. Listed as an optical counterpart to a source in the ROSAT Wide Field Camera extreme-ultraviolet bright source catalog, both ADS 11060 AB and C may contribute to the observed flux.

Fekel, Francis C.↗

NASA NTRS1994

Optical variability in the unusual K5 V infrared-excess star HD 98800

The dusty infrared-excess star HD 98800 (K5 V) was observed for several weeks in the spring of 1993 by the Vanderbilt/Tennessee State 0.4 m automatic photoelectric telescope. It was found to be a variable star with an amplitude of 0.07 mag in V and a period of 14.7 days. We show, by comparison with other chromospherically active variable stars and constant stars with good observational histories, that the Rossby number for HD 98800, determined to be 0.30, places it well within the regime of stars whose convective envelopes and rotation rates combine to drive a magnetic dynamo strong enough to generate photometrically observable starspots. The light curve suggests at least two large spots at somewhat different longitudes on HD 98800, one of which could be as large as 16 deg in radius.

Henry, Gregory W.↗

NASA NTRS1994

The two variables in the triple system HR 6469 = V819 Her: One eclipsing, one spotted

A complete BV light curve, from 14 nights of good data obtained with the Vanderbilt University-Tennessee State University (VU-TSU) automatic telescope, are presented and solved with the Wilson-Devinney program. Third light is evaluated, with the companion star brighter by 0.58(sup m) in V and 0.11(sup m) in B. The eclipses are partial. Inferred color indices yield F2 V and F8 V for the eclipsing pair and G8 IV-III for the distant companion star. After removing the variability due to eclipses, we study the residual variability of the G8 IV-III star over the ten years 1982 to 1992. Each yearly light curve is fit with a two-spot model. Three relatively long-lived spots are identified, with rotation periods of 85.9(sup d), 85.9(sup d), and 86.1(sup d). The weak and intermittent variability is understood because the G8 IV-III star has a Rossby number at the threshold for the onset of heavy spottedness.

Van Hamme, Walter V.↗

NASA NTRS1993

Chromospherically active stars. 11: Giant with compact hot companions and the barium star scenario

We have determined spectroscopic orbits for three chromsopherically active giants that have hot compact companions. They are HD 160538 (KO III + wd, P = 904 days), HD 165141 (G8 III + wd, P approximately 5200 days), and HD 185510 (KO III + sdB, P = 20.6619 days). By fitting an IUE spectrum with theoretical models, we find the white dwarf companion of HD 165141 has a temperature of about 35,000 K. Spectral types and rotational velocities have been determined for the three giants and distances have been estimated. These three systems and 39 Ceti are compared with the barium star mass-transfer scenario. The long-period mild barium giant HD 165141 as well as HD 185510 and 39 Ceti, which have relatively short periods and normal abundance giants, appear to be consistent with this scenario. The last binary, HD 160538, a system with apparently near solar abundances, a white dwarf companion, and orbital characteristics similar to many barium stars, demonstrates that the existence of a white dwarf companion is insufficient to produce a barium star. The paucity of systems with confirmed white dwarf companions makes abundance analyses of HD 160538 and HD 165141 of great value in examining the role of metallicity in barium star formation.

Fekel, Francis C.↗

NASA NTRS1993

Spots on RS CVn from spectroscopy and photometry

We have used contemporaneous spectra and V light curves to form spot models for RS CVn in 1991 and 1992. More than two spots are needed to fit all the properties of the observations. In fact, moderately small spots (22 x 28 deg in latitude and longitude) having only a slight effect on the rotational light curve were eclipsed in both years, and we find that a collection of 6-8 such moderate spots is required to fit the line profiles in each year. These groups of spots also account naturally for a difference in level of light between the two years. There is no evidence for polar spots larger than 18 degrees in radius. We have also derived new orbits from radial velocities of the stars, which give the mass ratio Mc/Mh = 1.04 +/-0.02.

Eaton, Joel A.↗

NASA NTRS1993

Chromospherically active stars. X - Spectroscopy and photometry of HD 212280

The system HD 212280 is a chromospherically active double lined spectroscopic binary with an orbital period of 45.284 days and an eccentricity of 0.50. The spectrum is composite with spectral types of G8 IV and F5-8 V for the components. An estimated inclination of 78 +/- 8 deg results in masses of 1.7 and 1.4 solar mass for the G subgiant and mid-F star, respectively. The distance to the system is estimated to be 112 pc. Photometric observations obtained between 1987 November and 1992 June reveal that HD 212280 is a newly identified variable star with a V amplitude of about 0.15 mag and a mean period of 29.46 days. Our V data were divided into 11 sets and in all but one case two spots were required to fit the data. Lifetimes of 650 days and a minimum of 1350 days have been determined for two of the four spots. The differential rotation coefficient of 0.05 is relatively small. The age of the system is about 1.9 X 10 exp 9 yrs. The G subgiant is rotating slower than pseudosynchronously while the F-type star is rotating faster.

Fekel, Francis C.↗

NASA NTRS1993

Observations of the Ca II infrared triplet in chromospherically active single and binary stars

Spectroscopic observations of the Ca II infrared triplet (8498, 8542, 8662 A) have been obtained for 45 stars which are known or suspected to be chromospherically active. The sample includes both single and binary stars of spectral types from F2 to M5 spanning luminosity classes III, IV, and V. Several different types of activity diagnostics were measured, and their relative merits are discussed. Dependence of chromospheric emission upon rotation period, luminosity, temperature, and duplicity are analyzed. Synchronous binaries show a slight trend of increased emission with decreasing period while the asynchronous binaries show abnormally high activity levels for their rotation periods. Several stars exhibit rotationally modulated emission which is anticorrelated with the stellar brightness. Finally, estimates of chromospheric energy losses are presented with the result that the total loss in the infrared triplet is about twice that of the H and K lines.

Dempsey, Robert C.↗

NASA NTRS1993

Chromospherically active stars. 6: Giants with compact hot companions and the barium star scenario

We have determined spectroscopic orbits for three chromospherically active giants that have hot compact companions. They are HD 160538 (K0 III + wd, P = 904 days), HD 165141 (G8 III + wd, P approximately 5200 days), and HD 185510 (K0 III + sdB, P = 20.6619 days). By fitting an IUE spectrum with theoretical models, we find the white dwarf companion of HD 165141 has a temperature of about 35000 K. Spectral types and rotational velocities have been determined for the three giants and distances have been estimated. These three systems and 39 Ceti are compared with the barium star mass-transfer scenario. The long-period mild barium giant HD 165141 as well as HD 185510 and 39 Ceti, which have relatively short periods and normal abundance giants, appear to be consistent with this scenario. The last binary, HD 160538, a system with apparently near solar abundances, a white dwarf companion, and orbital characteristics similar to many barium stars, demonstrates that the existence of a white-dwarf companion is insufficient to produce a barium star. The paucity of systems with confirmed white-dwarf companions makes abundance analyses of HD 160538 and HD 165141 of great value in examining the role of metallicity in barium star formation.

Fekel, Francis C.↗

NASA NTRS1992

Line profile asymmetries in chromospherically active stars

A powerful, new probe of chromospheric activity, cross-correlation, has been developed and applied to a variety of stars. In this particular application, an entire CCD spectrum of an active star is correlated with the spectrum of a narrow-line, inactive star of similar spectral type and luminosity class. Using a number of strong lines in this manner enables the detection of absorption profile asymmetries at moderate resolution (lambda/Delta lambda about 40,000) and S/N 150:1. This technique has been applied to 14 systems mostly RS CVn's, with 10 not greater than nu sin i not greater than 50 km/s and P not less than 7 d. Distortions were detected for the first time in five systems: Sigma Gem, IM Peg, GX Lib, UV Crb, and Zeta And. Detailed modeling, incorporating both spectral line profiles and broad-band photometry, is applied to Sigma Gem. Profile asymmetries for this star are fitted by two high-latitude spots covering 5 percent of the stellar surface. The derived spot temperature of 3400 K is lower than found in previous studies. In addition, two well-known systems have been studied: HD 199178 and V711 Tau. Polar spots are found on both.

Dempsey, Robert C.↗