1,557 research outputs found
A New Look at Open Cluster NGC 6520
We use CCD and photoelectric photometry with Stromgren filters along with
medium resolution spectra to investigate NGC 6520, an open cluster very nearly
in the direction of the galactic center. We find an age of 60 Myr, a distance
of 2 kpc, and an average reddening E(b-y) = 0.295, but which increases toward
the south. The average heliocentric radial velocity of the B stars is -29
km/sec, while the velocity of the nearby Barnard 86 is about 0 (heliocentric,
-11 km/sec compared to the LSR). This velocity difference amounts to about 1.8
kpc since the cluster formed, implying that it is extremely doubtful NGC 6520
is related to Barnard 86.Comment: 19 pages, 6 figure
The Butcher-Oemler Effect in Abell 2317
This paper presents deep narrow band photometry of the cluster A2317
(z=0.211) carried out using KPNO 4 m and Steward 2.3 m telescopes. Using rest
frame Stromgren photometry, it is determined that A2317 has an unusually high
fraction of blue galaxies (the Butcher-Oemler effect) for its redshift
(f_B=0.35). We demonstrate that the ratio of blue to red galaxies has a strong
dependence on absolute magnitude such that blue galaxies dominate the top of
the luminosity function. Spectrophotometric classification shows that a
majority of the red galaxies are E/S0's, with a small number of reddened
starburst galaxies. Butcher-Oemler galaxies are shown to be galaxies with star
formation rates typical of late-type spirals and irregular. Starburst systems
were typically found to be on the lower end of the cluster luminosity function.
In addition, blue galaxies are preferentially found in the outer edges of the
cluster, whereas the red galaxies are concentrated in the cluster core.Comment: 23 pages including 1 table and 6 figures, AASTeX v4.0. Accepted by
Ap.J. Data, referee report and response are avaliable from
http://zebu.uoregon.edu/~j
Orbits and Pulsations of the Classical ζ Aurigae Binaries
We have derived new orbits for ζ Aur, 32 Cyg, and 31 Cyg with observations from the Tennessee State University (TSU) Automatic Spectroscopic Telescope, and used them to identify nonorbital velocities of the cool supergiant components of these systems. We measure periods in those deviations, identify unexpected long-period changes in the radial velocities, and place upper limits on the rotation of these stars. These radial-velocity variations are not obviously consistent with radial pulsation theory, given what we know about the masses and sizes of the components. Our concurrent photometry detected the nonradial pulsations driven by tides (ellipsoidal variation) in both ζ Aur and 32 Cyg, at a level and phasing roughly consistent with simple theory to first order, although they seem to require moderately large gravity darkening. However, the K component of 32 Cyg must be considerably bigger than expected, or have larger gravity darkening than ζ Aur, to fit its amplitude. However, again there is precious little evidence for the normal radial pulsation of cool stars in our photometry. Hα shows some evidence for chromospheric heating by the B component in both ζ Aur and 32 Cyg, and the three stars show among them a meager ~2-3 outbursts in their winds of the sort seen occasionally in cool supergiants. We point out two fundamental questions in the interpretation of these stars: (1) whether it is appropriate to model the surface brightness as gravity darkening and (2) whether much of the nonorbital velocity structure may actually represent changes in the convective flows in the stars\u27 atmospheres
New light curves and ephemeris for the close eclipsing binary V963 PER
We have obtained CCD photometry in 2010-11 of V963 Per (=GSC3355 0394), which
is a recently identified close binary star with unequal eclipse depths. The
seven new eclipse timings yield an improved ephemeris, but we caution that
secondary eclipse can be affected by variation of the light curve. This
variation seems to be on a monthly timescale at the few percent level.Comment: 6 pages, 3 figure
Cluster Populations in A115 and A2283
This paper presents four color narrow-band photometry of clusters A115
() and A2283 () in order to follow the star formation history
of various galaxy types. Although located at similar redshifts, the two
clusters display very different fractions of blue galaxies (i.e. the
Butcher-Oemler effect, for A115, for A2283). A system
of photometric classification is applied to the cluster members that divides
the cluster population into four classes based on their recent levels of star
formation. It is shown that the blue population of each cluster is primarily
composed of normal starforming (SFR < 1 M_{\sun} yrs) galaxies at the
high luminosity end, but with an increasing contribution from a dwarf starburst
population below . This dwarf starburst population appears to be
the same population of low mass galaxies identified in recent HST imaging (Koo
et al 1997), possible progenitors to present-day cluster dwarf ellipticals,
irregulars and BCD's. Deviations in the color-magnitude relationship for the
red galaxies in each cluster suggest that a population of blue S0's is evolving
into present-day S0 colors at this epoch. The radial distribution of the blue
population supports the prediction of galaxy harassment mechanisms for tidally
induced star formation operating on an infalling set of gas-rich galaxies.Comment: 28 pages including 2 tables and 9 figures, AASTeX v4.0. Accepted by
Ap.J. Data, referee report and response are avaliable from
http://zebu.uoregon.edu/~j
The Color-Magnitude Relation in Coma: Clues to the Age and Metallicity of Cluster Populations
We have observed three fields of the Coma cluster of galaxies with a narrow
band (modified Stromgren) filter system. Observed galaxies include 31 in the
vicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839 complete to
M_5500=-18 in all three subclusters. Spectrophotometric classification finds
all three subclusters of Coma to be dominated by red, E type (ellipticals/S0's)
galaxies with a mean blue fraction, f_B, of 0.10. The blue fraction increases
to fainter luminosities, possible remnants of dwarf starburst population or the
effects of dynamical friction removing bright, blue galaxies from the cluster
population by mergers. We find the color-magnitude (CM) relation to be well
defined and linear over the range of M_5500=-13 to -22. After calibration to
multi-metallicity models, bright ellipticals are found to have luminosity
weighted mean [Fe/H] values between -0.5 and +0.5, whereas low luminosity
ellipticals have [Fe/H] values ranging from -2 to solar. The lack of CM
relation in our continuum color suggests that a systematic age effect cancels
the metallicity effects in this bandpass. This is confirmed with our age index
which finds a weak correlation between luminosity and mean stellar age in
ellipticals such that the stellar populations of bright ellipticals are 2 to 3
Gyrs younger than low luminosity ellipticals.Comment: 26 pages AAS LaTeX, 6 figures, accepted for publication in A
Ages and Metallicities of Cluster Galaxies in a779 Using Modified Strömgren Photometry
In the quest for the formation and evolution of galaxy clusters, Rakos and co-workers introduced a spectrophotometric method using modified Strömgren photometry, but with the considerable debate toward the project's abilities, we re-introduce the system by testing for the repeatability of the modified Strömgren colors and compare them with the Strömgren colors, and check for the reproducibility of the ages and metallicities (using the Principle Component Analysis (PCA) technique and the GALEV models) for the six common galaxies in all three A779 data sets. As a result, a fair agreement between two filter systems was found to produce similar colors (with a precision of 0.09 mag in (uz - vz), 0.02 mag in (bz - yz), and 0.03 mag in (vz - vz)) and the generated ages and metallicities are also similar (with an uncertainty of 0.36 Gyr and 0.04 dex from PCA and 0.44 Gyr and 0.2 dex using the GALEV models). We infer that the technique is able to relieve the age-metallicity degeneracy by separating the age effects from the metallicity effects, but it is still unable to completely eliminate it.We further extend this paper to re-study the evolution of galaxies in the low mass, dynamically poor A779 cluster (as it was not elaborately analyzed by Rakos and co-workers in their previous work) by correlating the luminosity (mass), density, and radial distance with the estimated age, metallicity, and the star formation history. Our results distinctly show the bimodality of the young, low-mass, metal-poor population with a mean age of 6.7 Gyr (± 0.5 Gyr) and the old, high-mass, metal-rich galaxies with a mean age of 9 Gyr (± 0.5 Gyr). The method also observes the color evolution of the blue cluster galaxies to red (Butcher-Oemler phenomenon), and the downsizing phenomenon. Our analysis shows that modified Strömgren photometry is very well suited for studying low- and intermediate-z clusters, as it is capable of observing deeper with better spatial resolution at spectroscopic redshift limits, and the narrow-band filters estimate the age and metallicity with fewer uncertainties compared to other methods that study stellar population scenarios
- …