Engineering topics

Cohen, M.

Publications and source records attributed to Cohen, M..

At least 55 records · Page 3

NASA NTRS1985

The binary system MWC 349

The ionized zone surrounding MWC 349 has been resolved using the VLA. On the basis of a radio continuum map, it is concluded that there is a physical interaction between the two stellar components of MWC 349, and that the system is therefore a binary. The B component can be classified as a B0 III star, yielding a distance of about 1.2 kpc for the binary. The low ratio of infrared to short-wavelength luminosity in the A component suggests a circumstellar dusty region of special geometry. It is proposed that an appreciable dust and gas disk surrounds the star, modifying the symmetry of its H II region, and yielding the low infrared luminosity and perhaps its extra circumstellar extinction.

Cohen, M.↗

NASA NTRS1985

Multifrequency radio images of L1551 IRS 5

L1551 IRS 5 has been observed at 15 GHz, 5 GHz, and 1.5 GHz with the VLA, and the findings are discussed. At 15 GHz, the central core is well resolved into two pointlike sources surrounded by weaker extended emission. The northern, brighter source is coincident with the peak at 5 GHz. The jet seen at 5 GHz is barely detected at 15 GHz. There is clearcut evidence for a much flatter spectral index in the extended (jet) structure than in either of the two point sources. The observed length-to-width ratio in the jet is greater than ten. It is suggested that L1551 IRS 5 is a binary system whose orbital plane coincides with the CS molecular toroid. A model of the jet is summarized in which the jet contains small clumps which dominate the 15 GHz emission.

Bieging, J. H.↗

NASA NTRS1985

Young stellar objects and their circumstellar dust - An overview

The basic observational classes of young stellar objects are briefly reviewed and some of the theoretical and observational problems in premain sequence stellar evolution are discussed. The evidence which indicates that the infrared excesses in recently formed stars are primarily due to thermal emission from circumstellar dust is summarized. Various indirect lines of evidence on the spatial distribution of the circumstellar dust are examined, and it is concluded that the concentration of the dust in a large disk structure is the most viable model at this time.

Rydgren, A. E.↗

NASA NTRS1985

Bipolar nebulae and mass loss from red giant stars

Observations of several bipolar nebulae are used to learn something of the nature of mass loss from the probable red-giant progenitors of these nebulae. Phenomena discussed are: (1) probable GL 2688's optical molecular emissions; (2) newly discovered very high velocity knots along the axis of OH 0739 - 14, which reveal evidence for mass ejections of + or 300 km/s from the M9 III star embedded in this nebula; (3) the bipolar structure of three extreme carbon stars, and the evidence for periodic mass ejection in IRC + 30219, also at high speed (about 80 km/s); and (4) the curious cool TiO-rich region above Parsamian 13, which may represent the very recent shedding of photospheric material from a cool, oxygen-rich giant. Several general key questions about bipolar nebulae that relate to the process of mass loss from their progenitor stars are raised.

Cohen, M.↗

NASA NTRS1984

Circumstellar Dust Disks Around Young Stars

Kuiper Airborne Observatory observations of several resolved far-infrared structures that are likely to represent disks formed during the process of the collapse of a cloud to form a star are discussed. An infrared radiation nebula, star HH57, and SVS13 were studied. Forty seven and 95 micron maps of far infrared radiation are given.

Cohen, M.↗

NASA NTRS1984

The proper motion of Herbig-Haro Objects. IV - RNO 40 and 43

Proper motions and spectra of the Herbig-Haro Objects RNO 40 and 43 have been obtained. The proper-motion vectors point away from far-infrared sources. The spectra were used to infer shock velocities. These shock velocities agree with the tangential motion in the case of RNO 40, but are smaller than the tangential motion in the case of RNO 43. RNO 43 appears to be associated with an entire family of comoving knots.

Jones, B. F.↗

NASA NTRS1984

The dust content of the planetary nebula IC 3568

Far-infrared photometry of IC 3568 shows a substantial excess that is not suggested by 10 micron data. A recently constructed model for the nebula permits the temperature profile for dust to be computed. The calculated infrared spectrum reproduces the observations well and requires that the grains have absorption properties like those of amorphous carbon. The dust coexists in the ionized gas but is heated directly by stellar radiation; most of the Ly-alpha radiation escapes from the nebula.

Cohen, M.↗

NASA NTRS1984

VLA observations of T Tauri stars. II - A luminosity-limited survey of Taurus-Auriga

The results of a 4.885-GHz VLA survey of 46 T Tau stars and Herbig-Haro objects with log L 0.2 or greater in the Taurus-Auriga dark clouds, performed in February 1982, with 40-60-min observation times, are presented in tables and contour maps and discussed. Upper limits are given for objects not detected at 4.885 GHz; additional observations at 1.465, 14.965, and 22.485 GHz are included for the objects detected; and spectral indices are derived for 10 objects. The individual objects are characterized in detail, and it is concluded that detectable radio emission is more likely for T Tau stars with associated Herbig-Haro objects than for those without. The implications of the nonthermal spectra identified in several objects for theories of theoretical models of radio emission are indicated.

Bieging, J. H.↗

NASA NTRS1984

Far-infrared observations of southern Herbig-Haro objects

Ground-based and airborne infrared observations are presented of the vicinities of several HH objects. New photographs of these HH nebulae have made it possible to locate the exciting stars of HH 46/47 and HH 57 from their infrared peak positions. Bolometric luminosities of the exciting stars lie in the range 4-12, consistent with those for low-mass accreting protostars.

Cohen, M.↗

NASA NTRS1984

Far-infrared studies of Herbig-Haro objects and their exciting stars

Far-infrared (40-160 micron) observations of Herbig-Haro objects and their probable exciting stars are presented. The importance of these far-infrared observations is epitomized by the fact that, on average, 70 percent of the bolometric luminosity of a typical candidate star is radiated beyond the longest detected ground-based wavelength, even for those objects observed out to 20 micron from the ground. Extended far-infrared emission characterizes some of these regions. Some Herbig-Haro nebulae are themselves conspicuous 100 micron peaks (e.g., HH 1 and HH 25). A bipolar structure is found at 100 micron in the vicinity of the Cohen-Schwartz star. All the exciting stars must be low-mass objects (approximately less than 3 solar mass). It is argued that they might be approximately 1 solar mass protostars still in their accretion phase.

Cohen, M.↗

NASA NTRS1984

Ultraviolet spectra of planetary nebulae. X - Physical conditions in the compact planetary nebula Sw St 1

Photoelectric visual and ultraviolet observations of the compact planetary nebula Sw St 1 are analyzed. The electron density, determined from the C III 1907/1909 A line ratio, is N(e) = (1.1 + or - 0.1) x 10 to the 5th/cu cm, consistent with the high emission measure and high critical frequency determined from observations of the thermal radio emission. The C/O abundance ratio in the nebula is found to be N(C)/N(O) = 0.72 + or - 0.1, i.e. the envelope is oxygen-rich, as suggested by the identification of the silicate feature in the 8-13 micron infrared spectrum. Difficulties remain in accurately determining the reddening constant to the nebula and its electron temperature.

Flower, D. R.↗

NASA NTRS1984

A study of extreme carbon stars. I - Silicon carbide emission features

10-micron spectra of many extreme carbon stars reveal a prominent emission feature near 11 microns. This is compared with laboratory spectra of SiC grains. Two distinct types of features are found, perhaps indicative of different mechanisms of grain formation in different stars. Estimates are made of probable column densities and total masses of SiC in the circumstellar shells.

Cohen, M.↗

NASA NTRS1984

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 range 1250 A equal to or less than lambda equal to less than 7350A have been measured. The continuum fluxes are best matched by an enhanced H two photon component added to H free bound emission, assuming theta Ori extinction curve with E(B-V) = 0.2, R = 5. The strucutre and dynamics of three components within the object are discussed. The object has a radiative output of equal to or greater than 0.23 infrared luminosity 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 NTRS1983

The unique cometary nebula Parsamian 13

Parsamian 13 is found to be a cometary nebula, with a deeply embedded star suffering strong absorption by silicate grains. A feature near 2.7 microns, attributed to absorption by terminal OH groups, may represent the first detection in an astrophysical environment of water ice grains diluted by another molecule, for example by CO. The bolometric luminosity suggests that this star is either an extremely young T Tauri star or an evolved low-mass star, now a red giant, high on its convective track. Most unusual is the presence of cold (50 K) TiO gas in sufficient abundance to show in absorption in the optical and very near-infrared regions; this material probably represents very recently expelled photospheric layers.

Cohen, M.↗

NASA NTRS1983

HL Tauri and its circumstellar disk

New far infrared observations of HL Tau which support the identification of an edge-on disk surrounding the star are presented. A bolometric luminosity for the star of 7.2 solar luminosities and a ratio of infrared to optical luminosity of 630 are indicated. A circumstellar A(V) of about 7.0 mag is produced, consistent with the silicate optical depth to the star. Data on HL Tau's effective temperature and radius and its position on the HR diagram suggest that the star has recently completed its accretion phase and is only 100,000 yr old. The column masses of ice and silicates are combined with the disk dimensions to build a simple model of the disk for comparison with the primitive solar nebula. Estimates of the far-infrared emitting mass provide independent probes of the mass in larger grains around HL Tau.

Cohen, M.↗

NASA NTRS1983

The exciting stars of Herbig-Haro objects

Accurate astrometry and infrared photometry are presented for 16 objects that are believed to represent the exciting stars for Herbig-Haro (HH) objects in their vicinities. These identifications are made on the bases of steeply rising energy distributions, proper motions of HH objects, alignment of sources with HH objects, or far-infrared emission. Exciting stars for the following HH objects are newly discovered or recognized: HH 17; HH 31; HH 32; HH 33 and 40; HH 34; HH 38 and 43; HH group 14 in NGC 2264; RNO 40; RNO 43; and GGD 33a, 33b, and 35. Geometrical alignments of some sources with HH objects support the view that young stars are capable of channeling kinetic energy into small solid angles, as does the fact that some sources are slightly displaced from the apices of fan nebulae (HH 17, R Mon, Haro 6-10). These exciting stars span a wide range of bolometric luminosity, although all are likely to be very young.

Cohen, M.↗

NASA NTRS1983

Recent work on bipolar nebulae

The results of recent studies of bipolar nebulae (BPN) using nebular-polarization mapping, spectropolarimetry, near-IR spectroscopy, far-IR photometry, and radio-maser and continuum observations are surveyed. The characteristics of several BPN of different evolutionary types are discussed and illustrated with spectra, model drawings, and maps. As shown in a Hertzsprung-Russell diagram of 19 BPN, this morphological class includes pre-main-sequence stars, red giants in transition to ordinary planetary nebulae, pre-white-dwarfs, a dust-shrouded carbon star, and a visual binary with a type-O primary. VLA 6-cm observations of the latter object, MWC 349, reveal a morphology similar to the optical structure of the Red Rectangle illuminated by HD 44179: it is suggested that equatorial dusty tori may occur commonly at different phases of stellar evolution, and hence that BPN may be relatively abundant, although short-lived, phenomena.

Cohen, M.↗

NASA NTRS1982

Infrared observations of low-mass star formation in Orion - HH objects

The results of a preliminary analysis of IR data on Herbig-Haro objects in the Orion nebula are reported. The observations were made with the high angular resolution IR photometry equipment on the NASA Kuiper Airborne Observatory and the NASA facility on Mauna Kea, HI. Data were taken in the 1-200 microns region with 40, 6, and 8 arcsec resolution. Attention was focused on NGC 1999 (HH1-3) and M78 (HH24-25) and the determination of absolute luminosities of the exciting stars. Measurements were also made of the IR energy distribution in the thermally emitting dust clouds and the point sources. Herbig-Haro objects featured compact and far IR sizes and large visual extinction, in addition to a steeply rising energy distribution up to 50-100 microns, where the luminosity emitted was concentrated.

Harvey, P. M.↗