Engineering topics

Cohen, Martin

Publications and source records attributed to Cohen, Martin.

62 records · Page 4

NASA NTRS1987

Observations of jets from low-luminosity stars. II - SVS 12, HH 30, near HL Tauri, and HH 34

Spatially resolved spectral scans were obtained for four Herbig-Haro jets emanating from low-luminosity premain-sequence stars. There appears to be a general tendency for the excitation and electron density to diminish along these jets. For three jets, the electron density dependence is close to 1/r. HH 30's scattered stellar continuum showed much weaker Fe II lines in 1985 than in 1979, indicative of variable stellar activity. The most distant knot in HH 34's jet has the lowest excitation of any known HH object, with S II-line/H-alpha = 12.

Cohen, Martin↗

NASA NTRS1987

Optical spectroscopy of IRAS sources with infrared emission bands - IRAS 21282+5050 and the diffuse interstellar bands

Spectroscopy of the starlike optical counterpart to IRAS 21282+5050, a source with the 'hydrocarbon' IR emission band spectrum, shows an O7(f)-(WC 11) planetary nebula nucleus suffering an extinction of 5.7 mag. Emission-line widths in the WC spectrum are only about 100 km/s, indicating a very slow stellar wind. Optical diffuse interstellar bands (DIBs) are prominent, and five DIBs are strongly enhanced. The presence of circumstellar hydrocarbon molecules may explain both the IR emission bands and the enhanced DIBs.

Cohen, Martin↗

NASA NTRS1987

IR excesses in Be stars

Infrared excesses are empirically derived for a sample of bright Be stars based on IRAS data and previously published near-infrared photometry. The excesses are compared with published H-alpha luminosities and are found to be correlated. The data are compared with the theoretically expected relation between the sum of free-free and free-bound continua and the H-alpha recombination-line intensity from a common envelope. The inclusion of temperature and optical-depth effects is necessary to produce agreement between observations and theory. No significant correlation is detected between L(IR)/L(H-alpha) ratios and rotational velocities.

Chokshi, Arati↗

NASA NTRS1987

IRAS observations of the exciting stars of Herbig-Haro objects

A list of the exciting stars of Herbig-Haro (HH) objects that have been detected by IRAS is presented. A critical comparison between IRAS and airborne observations clearly indicates that there are spatially extended far-infrared emitting structures associated with these HH-exciting stars. Estimates of bolometric luminosities from the IRAS measurements that show a range from 0.05 to almost 800 solar luminosities for 25 of these stars are derived. This range is not in conflict with the hypothesis that the onset of deuterium-burning terminates low-mass protostellar accretion and that the HH objects are generated at that time. The extended emission zones are identified with the infalling envelopes of these protostars.

Cohen, Martin↗

NASA NTRS1987

IRAS colors of normal stars

Using stars from the Bright Star Catalog, supplemented by cool dwarf stars from the Gliese catalog, that were detected by IRAS, the authors define empirically the median intrinsic visual-to-infrared color indices for 'normal' stars as a function of IRAS wavelength, spectral type and luminosity class. Anomalously red stars are discussed. Two otherwise undistinguished F giant stars are found with significant excesses at 12 microns. Be stars differ markedly from nonemission B stars in their V-(12) indices due to contamination of the former by free-free emission. Both B and Be stars show large dispersions in V-(25) colors that are associated with the heating of local, but strictly interstellar, dust clouds by some of the non-emission B stars. The derived sequences of stellar colors are closely approximated by either simple blackbody predictions or by model-atmosphere calculations.

Cohen, Martin↗

NASA NTRS1987

Optical spectroscopy of IRAS sources with infrared emission bands. 1: IRAS 21282+5050 and the diffuse interstellar bands

Spectroscopy of the starlike optical counterpart to IRAS 21282+5050, a source with the hydrocarbon infrared emission band spectrum, shows an 07(f)-(WC11) planetary nebula nucleus suffering an extinction of 5.7 mag. Emission line widths in the WC spectrum are only approx. 100 km/s, indicating a very slow stellar wind. Optical diffuse interstellar bands (DIBs) are prominent. Five DIBs are strongly enhanced, namely lamda lamda 5797, 6196, 6203, 6283, and 6613. The presence of circumstellar hydrocarbon molecules may explain both the infrared emission bands and the enhanced DIBs.

Cohen, Martin↗

NASA NTRS1986

Radio variability and structure of T Tauri stars

Observations of radio variability in V410 Tau and in HP Tau/G2 and /G3, and striking variations in the radio structure of DG Tau, are reported. The position of the radio peak of DG Tau has shown apparent motion between 1982 and 1985 along the flow axis from this star, while its morphology has varied from point-like to bipolar. These changes and the spectral index of 0.6 at high frequencies are interpreted as indicative of a variable, freely expanding radio jet in DG Tau.

Cohen, Martin↗

NASA NTRS1986

Observations of jets from low-luminosity stars - DG Tauri B

Low spectral resolution studies of DG Tau B, the faint system of knots south of the T Tauri star DG Tau, are described. The observations show this object to be bipolar, with the blueshifted lobe having extraordinarily low excitation. Infrared observations of the exciting star show it to be of very low luminosity, with a bolometric luminosity of 0.88 solar luminosity. The visual extinction indicates a highly nonspherical distribution of circumstellar dust around the exciting star. In spite of this lack of embedding within an obvious dark cloud, the system is identified as a young one.

Jones, B. F.↗