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Cohen, M.

Publications and source records attributed to Cohen, M..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

The classification of stars from IRAS colors

IRAS stars with reliable fluxes at 12, 25, and 60 microns were investigated which fell into three main types: (1) bright; (2) O-rich; and (3) C-rich. The three star types had a tendency to separate in the IRAS color-color diagram such that almost nonoverlapping zones could be defined for the groups with about 70 percent of the stars included. The O-rich stars tended to be redder in (12) - (25) than bright stars, and C-rich stars tended to be redder in (25) - (60) than bright and O-rich stars. It is suggested that the flattening of the underlying continuum slope may be the dominant cause of the change in (12) - (25). Of the 4300 unassociated IRAS point sources considered, amost 50 percent are late-type stars.

Walker, H. J.↗
NASA NTRS1987

Airborne and groundbased spectrophotometry of comet P/Halley from 5-13 micrometers

Spectrophotometry of comet Halley from 5-13 microns was obtained from the Kuiper Airborne Observatory and from the Lick Observatory Nickel Telescope, revealing a strong broad emission band at 10 microns and a weak feature at 6.8 microns. The 10-micron band is identified with silicate materials, and the primary component of the silicate emission is suggested to be due to olivine. The 6.8 micron feature may be due either to carbonates or the C-H deformation mode in organic molecules. The data indicate that small particles are abundant in the coma and that the dust contains at least two physically separate components. Significant spatial and temporal variations are also noted in the spectrum.

Bregman, J. D.↗
NASA NTRS1987

New CO and HCN sources associated with IRAS carbon stars

Emission of CO and HCN was detected in 22 out of a sample of 53 IRAS sources classified as unidentified carbon-rich objects. The sample was selected according to the presence of the silicon carbide feature as revealed by low-resolution spectra. The molecular line widths indicate that the CO and HCN emission arises from the circumstellar envelopes of very highly evolved stars undergoing mass loss. The visible stars tend to be deficient in CO as compared with unidentified sources. Most the detected CO and HCN IRAS stars are distinct and thick-shelled objects, but their infrared and CO luminosities are similar to those of IRC + 102156 AFGL and IRC-CO evolved stars. The 12 micron flux seems to be a good indicator of the distance, hence a guide for molecular searches.

NGUYEN-Q-RIEU↗
NASA NTRS1987

The hydrogen coverage of interstellar PAHs

The rate at which the CH bond in interstellar Polycyclic Aromatic Hydrocarbons (PAHs) rupture due to the absorption of a UV photon has been calculated. The results show that small PAHs (less than or equal to 25 carbon atoms) are expected to be partially dehydrogenated in regions with intense UV fields, while large PAHs (greater than or equal to 25 atoms) are expected to be completely hydrogenated in those regions. Because estimate of the carbon content of interstellar PAHs lie in the range of 20 to 25 carbon atoms, dehydrogenation is probably not very important. Because of the absence of other emission features besides the 11.3 micrometer feature in ground-based 8 to 13 micrometer spectra, it has been suggested that interstellar PAHs are partially dehydrogenated. However, IRAS 8 to 22 micrometer spectra of most sources that show strong 7.7 and 11.2 micrometer emission features also show a plateau of emission extending from about 11.3 to 14 micrometer. Like the 11.3 micrometer feature, this new feature is attributed to the CH out of plane bending mode in PAHs. This new feature shows that interstellar PAHs are not as dehydrogenated as estimated from ground-based 8 to 13 micrometer spectra. It also constrains the molecular structure of interstellar PAHs. In particular, it seems that very condensed PAHs, such as coronene and circumcoronene, dominate the interstellar PAH mixture as expected from stability arguments.

Tielens, A. G. G. M.↗
NASA NTRS1986

Airborne spectrophotometry of Comet Halley from 5 to 9 microns

Spectrophotometry from 5 to 9 microns (resolution = 0.02) of comet Halley was obtained from the Kuiper Airborne Observatory on 1985 Dec. 12.1 and 1986 April 8.6 and 10.5 UT. Two spectral features are apparent in all the observations, one from 5.24 to 5.6 microns, and the silicate emission feature which has an onset between 7 and 8 microns. There is no evidence for the 7.5 microns feature observed by the Vega 1 spacecraft; the large difference between the areal coverage viewed from the spacecraft and the airplane may explain the discrepancy. Color temperatures significantly higher than a blackbody indicate that small particles are abundant in the coma. Significant spatial and temporal variations in the spectrum show trends similar to those observed from the ground.

Campins, H.↗
NASA NTRS1986

Infrared spectra of WC10 planetary nebulae nuclei

The 5.2 to 8.0 micron spectra are presented for two planetary nebulae nuclei Hen1044 (He2-113) and CPD-56 8032. The unidentified infrared (UIR) emission bands at 6.2 microns, 6.9 microns, 7.7 microns are present in the spectra of Hen1044 and in CPD-56 8032, and the 8.6 micron band is present in the long wavelength shoulder of the 7.7 micron band in the spectrum of CPD-56 8032. The 8 to 13 micron spectra of these two stars by Aitken et. al. clearly show the presence of the 8.6 micron band in He2-113 while weakly resolving this feature in the spectra of CPD-56 8032. In their spectra the 11.3 micron band is also clearly detected in both objects. The 6.2 micron and 7.7 micron bands are characteristic of the infrared active C-C stretching modes in polycyclic aromatic hydrocarbons (PAHs); the 3.3 micron, 8.6 micron, and 11.3 micron bands are respectively assigned to the in-plane stretching mode, the in-plane bending mode, and the out-of-plane bending mode of the aromatic CH bond. The weak 6.9 micron emission feature is attributed to the UIR spectrum by Bregman et. al. The IRAS LRS spectra of He2-113 (IRAS 14562-5406) and CPD-56 8032 (IRAS 17047-5650) are presented. Cohen et. al. identify the broad plateau from 11.3 to 13.0 microns in the spectrum of He2-113 with increased hydrogenation of PAHs. This broad plateau is not seen in the LRS spectrum of CPD-56 8032. Also, He2-113 has greater infrared excess emission in the 17-22 micron region than does CPD-56 8032.

Wooden, D. H.↗
NASA NTRS1986

Optical spectroscopy of HH-exciting stars from scattered light continua

Optical spectra of the reflected light continua visible in parts of several Herbig-Haro objects are presented. HH 100 unmistakably scatters the chromospheric spectrum of a strong emission-line T Tau star. HH 48S also reflects a T Tau stellar spectrum, as perhaps do HH 24 and HH 55, but less convincingly. HH 55 reveals photospheric absorption features too, corresponding to an M3.5 T Tau star. The continuum in HH 120 (= CG 30 HH) is unclassifiable while that in HH 46A has dimmed considerably (by a factor of order 20) although, only 7 yr ago, it clearly reflected a strong-line T Tau spectrum. It is concluded that at least some of the stars that excite Herbig-Haro nebulae, even when those stars are not directly visible, pass through a strong-line T Tau phase, and can undergo abrupt and dramatic changes in visual luminosity.

Cohen, M.↗
NASA NTRS1986

The infrared emission bands. I - Correlation studies and the dependence on C/O ratio

Airborne measurements obtained for the unidentified IR (UIR) 5-8 micron emission bands of eight planetaries, eight locations in five reflection nebulae, and seven locations in four H II regions (including the Orion Bar), are presently compared with existing and new ground-based observations of the 3.3, 8.7, and 11.3 micron bands. The good correlations found between the strengths of all pairs of bands lead to the conclusion that all seven UIR features form a 'generic spectrum', although there are significant variations in the relative strengths of the features among the sources. The fraction of total far-IR luminosity radiated by a planetary in the strongest UIR feature at 7.7 microns is strongly correlated with the nebular C/O ratio, strongly suggesting that hydrocarbons are the carriers of these features.

Cohen, M.↗
NASA NTRS1986

The exciting star of HH 57

The first spectral classification for the exciting star of HH 57 is provided. The star is an F8 III and suffers about 5 mag of visual extinction, far greater than that toward the HH 57A knot, only a few arcseconds away. This difference supports the previous identification of a flattened geometry for the circumstellar dust around this star. Using IRAS photometry, a distance of 940 pc to HH 57 is determined by bolometrically matching the observed luminosity of the star to those of FU Ori and V1057 Cyg.

Cohen, M.↗
NASA NTRS1986

The hydrogen coverage of interstellar PAHs

The rate at which the CH bond in interstellar Polycyclic Aromatic Hydrocarbons (PAHs) rupture due to the absorption of a UV photon has been calculated. The results show that small PAHs (less than or equal to 25 carbon atoms) are expected to be partially dehydrogenated in regions with intense UV fields, while large PAHs (greater than or equal to 25 atoms) are expected to be completely hydrogenated in those regions. Because estimate of the carbon content of interstellar PAHs lie in the range of 20 to 25 carbon atoms, dehydrogenation is probably not very important. Because of the absence of other emission features besides the 11.3 micrometer feature in ground-based 8 to 13 micrometer spectra, it has been suggested that interstellar PAHs are partially dehydrogenated. However, IRAS 8 to 22 micrometer spectra of most sources that show strong 7.7 and 11.2 micrometer emission features also show a plateau of emission extending from about 11.3 to 14 micrometer. Like the 11.3 micrometer feature, this new feature is attributed to the CH out of plane bending mode in PAHs. This new feature shows that interstellar PAHs are not as dehydrogenated as estimated from ground-based 8 to 13 micrometer spectra. It also constrains the molecular structure of interstellar PAHs. In particular, it seems that very condensed PAHs, such as coronene and circumcoronene, dominate the interstellar PAH mixture as expected from stability arguments.

Barker, J. R.↗
NASA NTRS1986

Narrow band imagery in the 8-14 micron spectral region

The problem of deconvolution of the point source response function (PSF) for two-dimensional imaging infrared arrays is addressed. A variety of phenomena can lead to PSF degradation, ranging from atmospheric turbulence to the array itself. The maximum entropy algorithm is used to demonstrate that substantial image quality improvements are possible in the case of a PSF which is degraded by electronic rather than optical smearing. This result implies that IR arrays need not have perfect or even very good pixel-pixel crosstalk rejection ratios for use in astronomy.

Goebel, J. H.↗
NASA NTRS1985

A new emission feature in IRAS spectra and the polycyclic aromatic hydrocarbon spectrum

IRAS spectra of those sources which show strong 7.7 and 11.3 micron emission features also show a plateau of emission extending from 11.3 to about 13.0 microns. Like the 11.3 micron feature, this new feature is attributed to the CH out-of-plane bending mode in polycyclic aromatic hydrocarbons (PAHs). Its discovery reinforces the identification of the 'unidentified infrared emission features' as emission from PAHs. The wavelength of this new feature suggests that interstellar PAHs are not as partially hydrogenated as hitherto thought. It also constrains their molecular structure.

Cohen, M.↗
NASA NTRS1985

Highly supersonic bipolar mass ejection from a red giant OH/IR source - OH 0739 - 14

From long-slit spectrophotometry of the bipolar nebula associated with the unusual OH source, OH 0739 - 14, the presence of a blue companion to the M9 III central star was shown and a Herbig-Haro-like knot beyond each nebular lobe was discovered. From differential colors of the lobes and from radial velocities of these knots it was demonstrated that the system inclines its northern lobe in the forward direction. It was also shown that the nebulous knots are shocks being driven into an extensive circumstellar envelope, and that this material is very overabundant in nitrogen, suggesting that it is matter lost from a star of mass greater than 3 solar masses. A model of biconical ejection from a central binary is consistent with the OH observations, and a possible relation of OH 0739 - 14 to the symbiotic stars and to bipolar planetary nebulae is suggested.

Cohen, M.↗
NASA NTRS1985

Optical spectroscopy of known and suspected Herbig-Haro objects

Optical spectra of a number of suspected Herbig-Haro objects are presented. From these, the nature of these nebulosities are determined. Several of the nebulae are of very high density, perhaps due to their extreme youth. Extinctions measured toward DG Tau HH and the L1551 IRS 5 optical jet are in each case substantially less than the stellar values. It is suggested that this phenomenon reflects the existence of appreciably thick circumstellar dust disks around these, and two additional, exciting stars. Shock model diagnostics suggest that the emission lines in these Herbig-Haro nebulae arise in modest velocity shocks with sizable preshock densities in several cases. Radial velocities enable lower limits to be placed on the mass loss rates of those stars that have been detected in the radio continuum.

Cohen, M.↗
NASA NTRS1985

Far-infrared observations of the exciting stars of Herbig-Haro objects. III - Circumstellar disks

Far-infrared observations of the exciting stars of Herbig-Haro objects are presented that (1) show these stars to be of low luminosity; (2) indicate that it is not usual for these objects themselves to be visible at far-infrared wavelengths; and (3) demonstrate the existence of spatially resolved, physically large, potentially disklike structures. These latter structures are resolved perpendicular to the directions of flow from the stars, but not parallel to the flows. In addition to these general properties, two new HH-exciting stars were discovered by searching along the extrapolated proper motion vectors for these HHs; and the jetlike object 'DG Tau B' was also detected.

Cohen, M.↗
NASA NTRS1985

The structure of Herbig-Haro object 43 and Orion dark cloud extinction

New ultraviolet and optical observations of Herbig-Haro object No. 43 are reported. Continuum and emission-line fluxes in the wavelength range 1250 to 7350 A have been measured. The continuum fluxes are best matched by an enhanced H exp 0 two-photon component added to H free-bound emission, assuming a Theta Ori extinction curve with E(B - V) = 0.2, R = 5. The structure and dynamics of three components within the object are discussed. The object has a radiative output of not less than 0.23 solar luminosities in ultraviolet and optical radiation combined. The energy requirements are discussed in terms of the production of shock waves by a collimated, supersonic mass outflow from a nearby infrared source.

Schwartz, R. D.↗
NASA NTRS1985

Spectrophotometry at 10 microns of T Tauri stars

New 8-13 micron spectra of 32 T Tau, or related young, stars are presented. Silicate emission features are commonly seen. Absorptions occur less frequently but also match the properties of silicate materials. The shape of the emission feature suggests that a more crystalline grain is responsible in the T Tau stars than those of the Trapezium region. The evolution of the silicate component of the circumstellar shell around T Tau stars, and its dependence upon stellar wind activity, visual linear polarization, and extinction are investigated. Several correlations suggest that the shells are likely to be flattened, disklike structures rather than spherical.

Cohen, M.↗