Engineering topics

Megeath, S. T.

Publications and source records attributed to Megeath, S. T..

3 records

NASA NTRS2006

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope

We presented the results of an infrared imaging survey of Tr 37 and NGC 7160 using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 microns, allowing us to detect disk emission over a typical range of radii 0.1 to 20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths i 4.5 microns but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6-8 microns than at 24 microns, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 micron excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provided new constraints on disk evolution through an important age range.

Sicilia-Aguilar Aurora↗

NASA NTRS1990

Infrared and radio measurements of the density structure of compact H II regions

The density structure of compact H II regions is studied by comparing the ratio of the S III forbidden 33.47-micron and 18.71-micron emission lines and 5-GHz continuum maps. Electron densities are computed for DR 22, G29.9 - 0.0, G75.84 + 0.4, M8, M42, S158, W3, and W33. For DR 22, M42, and W3, the electron densities determined from the IR data are consistent with those derived from radio observations. The densities for S158 and M8 are quite low, implying that the S III emission is dominated by the diffuse ionized gas observed in the radio continuum. For G29.9 - 0.0, G75.84 + 0.4, and W33, the S III data give higher densities than predicted from radio observations. This implies that either these nebulae are filled with dense clumps of ionized gas which are not resolved in the radio maps, or that the S III zone is confined to the small, dense cores found in the radio maps of all three nebulae.

Megeath, S. T.↗

NASA NTRS1989

Si II mapping of the neutral gas ring in the Galactic center - Evidence for dust destruction

The distribution of Si II emission in the Galactic center has been measured and found to extend beyond 4 pc from the center. The observed forbidden Si II 34.8 micron line, which is thermally excited by H I and H2 collisions, originates from the photodissociation region formed in the neutral gas ring surrounding the ionized core. Two peaks in the Si II emission are found, one lying 20 arcsec northeast of Sgr A-asterisk along the Galactic plane and the other located about 80 arcsec north of Sgr A-asterisk. The latter peak is located beyond the northern arm seen in the ionized gas. The ratio of Si II to dust mass density varies in the ring, indicating changes in the gas-to-dust ratio. The mass density of Si II and dust are anticorrelated, indicating that Si II tracks the destruction of dust.

Herter, T.↗