Showing papers by "Mike Irwin published in 1990"

A blue stellar link between the Magellanic Clouds

Abstract: Three pairs of UK Schmidt plates of selected fields between the Magellanic Clouds have been processed using the APM facility in Cambridge. Color-magnitude diagrams obtained from the plate pairs revealed the presence of blue main-sequence stars between the wing of the SMC and the western halo of the LMC. The spatial distribution of these blue stars, B(J) - R less than 0.0, shows that many of them are in aggregates and newly identified clusters, stretching from an area surrounding and including the SMC wing tip all the way across to the LMC halo. Spectroscopic observations confirmed that the stars are indeed OB stars at a distance of about 50 kpc with radial velocities consistent with cloud membership. The number and distribution of horizontal branch stars identified in the same area also support the conclusion that the SMC wing is really the brightest section of the stellar link between the SMC and LMC. All of the individual blue stellar aggregates, including the region around the tip of the SMC wing, appear to have been formed about 10 to the 8th yr ago suggesting a common origin for all these features. 47 refs.

Automated galaxy surface photometry. V. Detection of very low surface brightness galaxies.

Abstract: It is shown that APM measurement of a standard deep UKSTU plate is equally as successful as photographic amplification in the detection of very low surface brightness galaxies. This method has the advantage of allowing the detection and quantitative measurement of images at the same time. Suitable median filtering techniques allow galaxies with scale sizes of a few arcseconds and central surface brightnesses as low as 26.5 B mag/sq arcsec to be detected. As suggested by previous work, detected galaxies occupy the whole of the parameter space which is accessible by the present method, again emphasizing the point that objectively determined selection boundaries are crucial in such studies. In particular there is no evidence for a correlation between galaxy scale size and surface brightness, and no sign of a cut-off in galaxy numbers as a function of surface brightness at these very faint levels.

Young stars between the Magellanic clouds. I. An association at 2 hrs 27 min, - 73 deg 59 min

Abstract: A CMD, based on deep CCD photometry of a poorly populated stellar association of about 85 pc in diameter and located nearly 7 deg to the east of the SMC and 14 deg from the LMC. An apparent distance modulus of 19.05 is determined from ZAMS fitting assuming that this is a system having Fe/H abundance = -0.65. The age of the association is estimated to be less than 100 million years. 38 refs.

An optical counterpart of the giant H I cloud H I 1225+0146

Abstract: An investigation based on deep U.K. Schmidt Telescope direct plates has discovered a previously uncataloged dwarf irregular galaxy which, it is proposed, is associated with the 1225 + 0146 extragalactic H I cloud. This new optical feature is characterized by a number of knots embedded in low surface brightness material about 1 arcmin in extent. The object is classified as an M(B) = -15.5 Magellanic type dwarf irregular galaxy about 5 kpc in extent. The brightest knot is considered to be an H II region. 25 refs.

Hot degenerates in the MCT survey. I. MCT 0130 - 1937, a new, color-selected PG 1159 object

Abstract: In the course of a survey for high-latitude, UV excess objects in the Southern Hemisphere, a new white dwarf star belonging to the PG 1159 spectroscopic class was discovered. A variety of spectroscopic observations show this object to be similar to the prototype PG 1159 - 035 in exhibiting He II, C IV, and O VI features in its optical spectrum, as well as a rich absorption spectrum in the ultraviolet. In addition, the slope of its energy distribution appears as steep as that of PG 1159 - 035 and is consistent with the Rayleigh-Jeans tail of a very hot star. White-light, high-speed photometry reveals that this object does not exhibit periodic variations in the range 10-2000 s with semiamplitudes larger than about 0.002 mag. The importance of such objects for current models of the spectral evolution of white dwarfs is emphasized. 37 refs.

A new satellite galaxy of the Milky Way in the constellation of Sextans.

Abstract: Une nouvelle galaxie elliptique naine de type Sculptor situee a une distance de 85 kpc a ete decouverte dans la constellation du Sextant, a partir des mesures APM effectuees sur les plaques photographiques du survey Schmidt UK.