Engineering topics
Publications and source records attributed to Noah, P. V..
Analysis of IRAS data of the thermal zodiacal emission reveals an out-of-plane dust distribution near the Earth which is well-represented by a Lorentzian function. We suggest a possible explanation for the similarity as near-Earth gravitational perturbations of dust grains. Such perturbations are expected to be chaotic and will randomize any residual band structure near the Earth.
The results of an extensive study of the eighth-magnitude G star HD 8358 employing optical photometry and spectroscopy, as well as UV observations with the IUE satellite, are presented. The star is found to be an active chromosphere binary with orbital and photometric period of 0.516 days. It exhibits photometric variability of 0.1-0.2 mg in V, due to starspots. At times the light curve is stable for several months, indicating that the spots persist essentially unchanged for more than 200 rotations. At other times, the spot configuration changes in a month or less. HD 8358 is an unusual member of the 'short-period' group of active chromosphere binaries due to its high space velocity and its very broad and highly variable H-alpha emission.
New optical spectroscopic and photometric data are presented for the active chromosphere FK Com candidate UZ Lib. The star is shown to have an extremely large photometric amplitude in V of 0.35 mag, and its rotation period is established as 4.75 + or - 0.01 days. The optical spectrum is that of an early K giant, broadened by a rotation velocity of approximately 65 km/s. H-alpha is visible as a very broad emission feature, with a profile resembling that seen in FK Com. The emission intensity and profile are variable over the rotation period, with the strongest emission present at photometric minimum, in accord with dark starspot models. The photospheric absorption line profiles show variable asymmetries and distortions which are interpreted as due to the effects of the dark starspot rotating across the line of sight. New radial velocity measures are combined with published data to demonstrate the UZ Lib is a member of a binary system in synchronous rotation with a secondary of mass approximately 0.5 solar masses. This information is considered in light of the conflicting models for the origin of the optical and spectral variability of the FK Com stars, as well as their uncertain evolutionary status.
It is pointed out that spectroscopic signatures of stellar chromospheric activity are readily observable. The present study is concerned with new photometric and spectroscopic observations of active-chromosphere RS CVn, BY Dra, and FK Com stars. Attention is given to the first results of a synoptic monitoring program of many active chromosphere stars. During the time from 1980 to 1982, photometric and spectroscopic observations of 10 known or suspected active-chromosphere objects were made. The results regarding the individual stars are discussed. Seven stars observed with the International Ultraviolet Explorer (IUE) are all spectroscopic binaries.
In September 1981 R Sct H-alpha declined in brightness to V about 8.5, an exceptionally faint minimum. A 15 A/mm spectrogram of the H-alpha region of R Sct two days past minimum brightness, when the visual magnitude was about 8.3, was obtained. This is apparently the first spectral observation of R Sct close to such a deep minimum. Contrary to spectroscopic observations during 'ordinary' minima of V about 6-7, at this deep minimum the expected G-K Ia absorption spectrum is not seen. Instead, a fantastic array of dozens of strong, narrow emission features and strong TiO absorption bands are present. The emission lines are due to neutral metals, and closely resemble a reversed photospheric spectrum. The deep minimum spectrum is apparently purely chromospheric, and is not produced in a stellar wind or extended atmosphere. The spectroscopic behavior of R Sct at deep minimum, as well as a number of other kinematic and photometric properties, is surprisingly similar to that of the R Coronae Borealis variables.
Spectroscopic and photometric observations of the K2III star HD 185151 are reported which establish it as a chromospherically active binary with an orbital period of 40.1 days. Reticon observation of the hydrogen alpha region shows a partially filled profile, owing presumably to chromospheric emission, as well as a blue emission wing extending out about 100 km/s. The light curve is effectively a double sine curve, and can be modeled by two starspots separated by almost exactly 180 deg in longitude.
The discovery of BY Draconis variables was efficiently accomplished by a spectroscopic survey of dK-M stars for weak H-alpha emissions, using 1-2 A resolution. The four BY Dra variables discovered are all spectroscopic binaries with P values lower than about 10 d, in light of which, it is noted that the onset of high surface activity and appearance of H-alpha emission occur sharply at v(equator) of approximately 5 km/sec. At v(equator) of about 3 km/sec, dK-M stars have low levels of surface activity. It is found that while there is a range of Ca II emission strength, and only the strongest emitters of this line are BY Dra and/or flare stars, the H-alpha feature changes abruptly to an emission feature signaling the onset of flaring and/or the BY Dra syndrome. An increase of the rotation rate above v(equator) 5 km/sec does not appear to increase the level of surface activity.