UBVRI PHOTOMETRY II: THE COUSINS VRI SYSTEM, ITS TEMPERATURE AND ABSOLUTE FLUX CALIBRATION, AND RELEVANCE FOR TWO-DIMENSIONAL PHOTOMETRY.Bessell, M. S.
doi: 10.1086/130542pmid: N/A
The Cousins VRI system is proposed as an ideal system to adopt as the standard system for both Northern and Southern Hemisphere near-IR observations. Comparisons are made between (b-y), (G-R)6, (V-I)jJ, and (V-I)C color indices measured for a large sample of equatorial HR stars which illustrate the precision of the system and the excellent temperature sensitivity of the (V- I)C index. Transformations between the different systems including the Eggen-Kron system are derived, a temperature calibration of (b-y), (R-I)C, and (V-I)C is provided based on empirical temperature determinations, and an absolute flux calibration for UBVRI is derived.Photometry using photographic plates, electronic area detectors, and digital spectra is also discussed, and details given of specific photometric detector systems.
THE RECENT OPTICAL OUTBURST OF THE SEYFERT 1 GALAXY ARAKELIAN 120.Miller, H. R.
doi: 10.1086/130551pmid: N/A
The Seyfert 1 galaxy, Arakelian 120, has undergone an optical flare from October 1977 to September 1978. The most rapid change in brightness detected was a change of 0.24 m in 25 days. This variation implies an upper limit on the size of the optical source region of 0.0202 pc based on light-travel-time arguments (assuming no relativistic effects). The optical variability and color variations are consistent with a composite galaxy-nonthermal central source model in which the nonthermal central source varies both in luminosity and color. A second possible interpretation is that the variations in color and brightness are due to variability in both the broad emission lines which are present and in the nonthermal continuum.
THE CURIOUS VARIATION OF THE BLUE-TO-RED SUPERGIANT RATIO ACROSS THE FACE OF THE LARGE MAGELLANIC CLOUD.Cowley, A. P.; Dawson, P.; Hartwick, F. D. A.
doi: 10.1086/130552pmid: N/A
Using recent catalogs of blue and red supergiants in the LMC we have examined the variation in the B/R ratio over the face of the Large Cloud. We find that the variation is systematic across the galaxy in regions roughly parallel to the steep H I density gradient rather than radially from the center. Further we show from systematic changes in the mean spectral type of the blue stars in these zones, as well as from the oxygen abundance of the gas in H II regions, that the B/R variation is a sensitive indicator of metal abundance changes. The fact that the B/R zones and the H I density gradient are aligned strongly suggests the existence of an environmental influence on current star formation in the LMC.
DISCOVERY OF A RING GALAXY IN BOOTES.Hartl, H.; Weinberger, R.; Mross, R.; Dengel, J.
doi: 10.1086/130553pmid: N/A
A new ring galaxy was detected on the POSS. The object shows a distinct ring with an outer diameter of the major axis of about 33 arc sec and has a surface brightness of approximately 22.3 mag/sq arc sec and 23.8 mag/sq arc sec on the red and blue POSS prints, respectively. It lacks an easily discernible companion galaxy. Because of its well-defined ring structure and comparatively large angular size the galaxy can be placed along in a line with the handful of well-defined ring galaxies and may also serve as a prototype. The radio flux density at 6 cm is less than 7 mJy.
DOES NGC 5128 CONTAIN GLOBULAR CLUSTERS?van den Bergh, S.
doi: 10.1086/130555pmid: N/A
Counts of star-like images revealed that the elliptic galaxy NGC 5128 contains 15 + or - 60 m.e., globular clusters brighter than B approximately equal to 22.7. It is speculated that the extreme deficiency of globular clusters in NGC 5128 is related to the fact that this object is a field galaxy while the M 87 on the other hand is situated at the center of a rich cluster.
A REDDENING-FREE MAIN SEQUENCE FOR THE PLEIADES CLUSTER.Turner, D. G.
doi: 10.1086/130556pmid: N/A
The effects of differential reddening on the UBV colors of Pleiades members have been removed for stars earlier than (B-V)0 = 1.00m, and the dereddened colors have been found to exhibit an ultraviolet excess of delta(U-B) = +0.03m with respect to Hyades stars. A correction of +0.15m in mV at constant (B-V) has been derived for the Hyades main sequence to adjust it to Pleiades metallicity, and this has been fit to Pleiades late-type dwarfs in order to define a new photometric ZAMS for typical solar neighborhood stars of spectral types A, F, and G. A zero-point of 3.15 was adopted for the Hyades distance modulus based upon a review of available astrometric data, and the corresponding Pleiades distance modulus is 5.56.
THE RELATIONSHIP BETWEEN SPECTRUM AND LIGHT VARIATIONS IN THE MAGNETIC AP STAR HR 1217 (HD 24712).Bonsack, W. K.
doi: 10.1086/130557pmid: N/A
Measurements of equivalent widths and line-blocking coefficients have been made from 3850 Å to 5650 Å in the spectrum of HR 1217, using photographic data. The variation of the equivalent widths of the Eu II lines suggests a revised period of 12d.460 in order to phase the present data with those of Preston (1972). The variation of equivalent widths shows a dependence of amplitude on atomic number which resembles that found by Preston, but there are clear discontinuities in the relationship. The line blocking coefficient was measured in unweighted 50 Å bands throughout the spectral region, except between 4750 Å and 5200 Å, and weighted by the sensitivity functions of the υ, b, and y bands of the Strömgren photometric system. The line blocking variation in the filter bands is found to account for approximately one-half of the light variation observed by Wolff and Morrison (1973); the spectroscopy and photometry considered together demonstrate the need for continuous opacity variation in the filter bands to account for the light variation. Redistribution of flux from the ultraviolet appears to have no significant role in the light variation, which is not unexpected in this cool Ap star.