Engineering topics

Kulkarni, Shrinivas R.

Publications and source records attributed to Kulkarni, Shrinivas R..

26 records · Page 2

NASA NTRS1991

On the detectability of pulsars in close binary systems

The limitations of standard pulse searches in close binary systems are quantitively investigated. An efficiency factor gamma is introduced to describe the ease with which binary pulsars can be found, and the results of calculations of gamma as a function of pulsar spin period and binary orbital period are presented for both standard pulse searches and for searches using sophisticated acceleration codes. Two strategies are suggested which maximize the probability of finding pulsars in close binaries, and empirical relations between the optimum observation time and the parameters of a binary orbit are given.

Johnston, Helen M.↗

NASA NTRS1990

The interstellar halo of spiral galaxies: NGC 891

Researchers have detected the Warm Ionized Medium (WIM) phase in the galaxy NGC 891. They found that the radial distribution of the WIM follows the molecular or young star distribution - an expected dependence. The amount of the WIM in this galaxy exceeds that in our Galaxy. The major surprize is the large thickness of the WIM phase - about 9 kpc instead 3 kpc as in our Galaxy. Clearly, this is the most significant result of the observations. The presence of low ionization gas at high z as well as at large galactocentric radii (where young stars are rare) is an important clue to the origin of the halo and observations such as the one reported here provide important data on this crucial question. In particular, the ionization of gas at high absolute z implies that either the UV photons manage to escape from the disk of the galaxy or that the extragalactic UV background plays an important role. The bulk of the WIM in spiral galaxies is a result of star-formation activity and thus these results can be understood by invoking a high star formation rate in NGC 891. Only the concerted action of supernovae can get the gas to the large z-heights as is observed in this galaxy. Support for this view comes from our detection of many worms i.e., bits and pieces of supershells in the form of kilo-parsec long vertical filaments. Researchers also saw a 600-pc size supershell located nearly one kpc above the plane of the galaxy.

Kulkarni, Shrinivas R.↗

NASA NTRS1990

Deep narrow band imagery of the diffuse ISM in M33

Very deep narrow band images were obtained for several fields in the local group spiral galaxy M33 using a wide field reimaging Charge Coupled Device (CCD) camera on the 1.5 m telescope at Palomar Observatory. The reimaging system uses a 306 mm collimator and a 58 mm camera lens to put a 16 minute by 16 minute field onto a Texas Instruments 800 x 800 pixel CCD at a resolution of 1.2 arcseconds pixel (-1). The overall system is f/1.65. Images were obtained in the light of H alpha (S II) lambda lambda 6717, 6731, (O III) lambda 5007, and line-free continuum bands 100A wide, centered at 6450A and 5100A. Assuming a distance of 600 kpc to M33 (Humphreys 1980, Ap. J., 241, 587), this corresponds to a linear scale of 3.5 pc pixel (-1), and a field size of 2.8 kpc x 2.8 kpc. Researchers discuss the H alpha imagery of a field centered approx. equal to 8 minutes NE of the nucleus, including the supergiant HII region complex NGC 604. Two 2000 second H alpha images and two 300 second red continuum images were obtained of two slightly offset fields. The fields were offset to allow for discrimination between real emission and possible artifacts in the images. All images were resampled to align them with one of the H alpha frames. The continuum images were normalized to the line images using the results of aperture photometry on a grid of stars in the field, then the rescaled continuum data were directly subtracted from the line data.

Hester, J. Jeff↗

NASA NTRS1990

M51: Molecular spiral arms, GMAs and superclouds

Researchers present an aperture synthesis image of M51 in the CO 1 to 0 line at 9 seconds x 7 seconds resolution made with the Owens Valley Millimeter Interferometer. The image is a mosaic of 30 one-arcminute fields. The image shows narrow spiral arms which are coincident with the optical dust lanes and non-thermal radio emission, but are offset from the ridges of H alpha emission. Many dense concentrations of CO emission, termed Giant Molecular Associations (GMAs), are seen both along and between the arms. The typical GMA mass is about 3 times 10(exp 7) solar mass. Most of the on-arm GMAs appear to be gravitationally bound. These GMAs consist of several spectral components (Molecular Superclouds) with typical mass 10(exp 7) solar mass, which also appear to be bound. The observed streaming motions in the GMAs are consistent with density wave theory. The interarm GMAs are not gravitationally bound, and are likely to be due to a secondary compression of the density wave.

Rand, Richard J.↗

NASA NTRS1990

The distribution of warm ionized gas in NGC 891

Narrow-band imaging is presented of the edge-on spiral NGC 891 in the H-alpha and S II 6716, 6731 A forbidden lines. Emission from H II regions confined to the plane of the galaxy and from diffuse gas up to about 4 kpc off the plane is readily detected. The full radial extent of the diffuse emission in the plane is about 30 kpc. NGC 891 is found to have a surface density of diffuse ionized gas twice the Galactic value, a thicker ionized gas layer, and a larger surface density of ionized gas relative to neutral gas. These are interpreted as consequences of a relatively high level of star formation in this galaxy. Other star formation tracers indicate the same conclusion. Many vertical H-alpha filaments, or 'worms,' extending to over 2 kpc off the plane of the galaxy are seen. These worms are interpreted in terms of chimney models for the interstellar media of spirals.

Rand, Richard J.↗

NASA NTRS1990

Star formation and the distribution of HI and infrared emission in M51

H I, infrared, CO, H alpha and beta band observations of M51, the prototypical grand-design spiral galaxy, are used to study the consequences of star formation for the distribution of H I and dust. Using the H I and CO data sets new tests of the idea that the H I is largely a dissociation product in star-forming regions were performed. It was confirmed that the H I spiral arms are generally coincident with the H II region arms, and offset downstream from the CO arms. The radial distributions of total gas, H alpha and H I surface density have a simple explanation in the dissociation picture. The distributions also demonstrate how the surface density of H I might be related to the star formation efficiency in molecule-rich galaxies. The large width of the H I regions along the arms compared to that of the H II regions can be understood in terms of a simple Stroemgren sphere calculation. The longer lifetime of the stars producing dissociating radiation vs. those producing ionizing radiation will also contribute to the greater width of the H I arms if stars are continuously forming on the arms. The lack of detailed coincidence of the H I and H II regions along the inner arms has a variety of possible explanations within the dissociation scenario. Two simple tests to probe the origin of the IRAS emission in M51 were performed.

Rand, Richard J.↗

NASA NTRS1989

Optical imagery and spectrophotometry of CTB 80

Narrow-band imagery and spectrophotometry of the central region of CTB 80 are presented. The images show weak forbidden O III and ubiquitous filamentary forbidden S II and H-alpha emission from the extended radio lobes in which the core is embedded. The data indicate that the extended component is shock heated. Balmer line-dominated emission is observed around the perimeter of the core. Assuming that the volume of the radio shell is similar to the volume of the thermal shell, it is found that a magnetic field of about 600 microG and a cosmic-ray proton-to-electron ratio of about 200 are required to explain the pressure and synchrotron volume emissivity in the radio shell. It is suggested that the optical emission form the core of CTB 80 arises behind shocks which are being driven into a magnetized thermal plasma by the confined relativistic wind from PSR 1951+32.

Hester, J. Jeff↗

NASA NTRS1988

The origin and energetics of CTB 80

Deep narrow-band images of the core of the unusual supernova remnant CTB 80 are presented, and the origin and energetics of the remnant are discussed. Based on a description of the core as a wind-blown bubble or bow shock around a pulsar, a scenario is presented relating the core and extended components to a single supernova explosion about 100,000 yr ago. The physical nature of the interface between the pulsar wind and the ambient thermal plasma is considered, and it is concluded that the pulsar is probably moving through shock-compressed material associated with the evolved remnant. It is argued that the core geometry is probably a bow shock viewed partially face-on.

Hester, J. Jeff↗