10,153 research outputs found
Application of the Trend Filtering Algorithm on the MACHO Database
Due to the strong effect of systematics/trends in variable star observations,
we employ the Trend Filtering Algorithm (TFA) on a subset of the MACHO database
and search for variable stars. TFA has been applied successfully in planetary
transit searches, where weak, short-lasting periodic dimmings are sought in the
presence of noise and various systematics (due to, e.g., imperfect flat
fielding, crowding, etc). These latter effects introduce colored noise in the
photometric time series that can lead to a complete miss of the signal. By
using a large number of available photometric time series of a given field, TFA
utilizes the fact that the same types of systematics appear in several/many
time series of the same field. As a result, we fit each target time series by a
(least-square-sense) optimum linear combination of templates and
frequency-analyze the residuals. Once a signal is found, we reconstruct the
signal by employing the full model, including the signal, systematics and
noise. We apply TFA on the brightest ~5300 objects from subsets of each of the
MACHO Large Magellanic Cloud fields #1 and #79. We find that the Fourier
frequency analysis performed on the original data detect some 60% of the
objects as trend-dominated. This figure decreases essentially to zero after
using TFA. Altogether, We detect 387 variables in the two fields, 183 of which
would have remained undetected without using TFA. Where possible, we give
preliminary classification of the variables found.Comment: 12 pages, 15 figures, 3 tables with online material; to appear in
Astronomy and Astrophysic
Dynamics of the Young Binary LMC Cluster NGC 1850
In this paper we have examined the age and internal dynamics of the young
binary LMC cluster NGC 1850 using BV CCD images and echelle spectra of 52
supergiants. Isochrone fits to a BV color-magnitude diagram revealed that the
primary cluster has an age of Myr while the secondary member
has Myr. BV surface brightness profiles were constructed out
to R 40 pc, and single-component King-Michie (KM) models were applied. The
total cluster luminosity varied from L = 2.60 - 2.65
L\sol\ and L = 1.25 - 1.35 as the anisotropy radius
varied from infinity to three times the scale radius with the isotropic models
providing the best agreement with the data. Of the 52 stars with echelle
spectra, a subset of 36 were used to study the cluster dynamics. The KM radial
velocity distributions were fitted to these velocities yielding total cluster
masses of 5.4 - 5.9 M\sol\ corresponding to M/L =
0.02 M\sol/L\sol\ or M/L = 0.05 M\sol/L\sol.
A rotational signal in the radial velocities has been detected at the 93\%
confidence level implying a rotation axis at a position angle of 100\deg. A
variety of rotating models were fit to the velocity data assuming cluster
ellipticities of . These models provided slightly better
agreement with the radial velocity data than the KM models and had masses that
were systematically lower by a few percent. The preferred value for the slope
of a power-law IMF is a relatively shallow, x = 0.29 \pmm{+0.3}{-0.8}
assuming the B-band M/L or x = 0.71 \pmm{+0.2}{-0.4} for the V-band.Comment: 41 pages (figures available via anonymous FTP as described below
Beat Cepheid Period Ratios from OPAL Opacities
The discovery of a large number of beat Cepheids in the Large Magellanic
Cloud in the MACHO survey, provides an opportunity to compare the
characteristics of such Cepheids over a range of metallicities. We produced a
large grid of linear nonadiabatic pulsation models using the OPAL opacity
tables and with compositions corresponding to those of the Milky Way, and the
Large and Small Magellanic Clouds. Using the relationship between the period
ratio and the main pulsation period, we are able to define a range of models
which correspond to the observed beat Cepheids, and thereby constrain the
physical characteristics of the LMC beat Cepheids. We are also able to make
some predictions about the nature of the yet-to-be-discovered SMC beat
Cepheids.Comment: 20 pages, 6 embedded Postscript figures, uses aasms4.sty. Accepted
for publication in the 1997 September A
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