Identifying Long Period Variable LPV Stars Using Images from the Star Dial Observatory

1
Identifying Long Period Variable (LPV) Stars
Using Images from the Star Dial Observatory
Vanessa Friedman1, Jim Bedient2, Pebble
Richwine3, Tim Slater4 1STARBASE DOD (Tucson, AZ)
2U. Hawaii 3Riverwatch Middle School (Suwanee,
GA) 4U. Wyoming
Introduction Variable stars are unique stars
that periodically change in brightness over
time.The magnitude of a star is a number assigned
to a stars brightness where the brightest stars
are known as magnitude 1 stars and the dimmest
stars that can be seen by the unaided eye are
magnitude 6 therefore, the brighter the star,
the smaller its numerical magnitude. The
magnitudes of variable stars can change from a
thousandth of a magnitude to as much as twenty
magnitudes over periods of a fraction of a second
to several years, depending on the type of
variable star. The type of variable stars being
studied in this project are long period variables
(LPV) and are called so because their periodicity
is longer than 80 days. There are a number of
reasons why variable stars change in magnitude.
Long period variable (LPV) stars are pulsating
stars and they swell and shrink due to internal
forces and their brightness changes with their
change in size. Over 30,000 variable stars are
known and catalogued, and many thousands more are
suspected to be variable. Variable stars need to
be systematically observed over decades in order
to determine the long-time behavior of a star,
provide professional astronomers with data
needed to analyze variable star behavior, to
schedule observations of certain stars, to
correlate data from satellite and ground-based
observations, and to make computerized
theoretical models of variable stars. Research
on variable stars is important because it
provides information about stellar properties,
such as mass, radius, luminosity, temperature,
internal and external structure, composition, and
evolution. This information can then be used to
understand other stars. Professional astronomers
have neither the available time nor the unlimited
telescope access needed to gather data on the
nightly brightness changes of thousands of
variable stars. Thus, it is amateur astronomers
utilizing various techniques who are making a
real and highly useful contribution to science by
observing variable stars and submitting their
observations to the AAVSO International Database.
The aim of this project is to measure LPV star
magnitudes for several stars and to submit data
obtained to the international variable star
database in the hopes of helping astronomers
learn more about objects in the sky.
Figure 2. A section of the night sky known as
1930. 1930 describes the Right Ascension (RA)
location of this section of the sky. The negative
image is backwards from what is usually seen at
night (black sky with white stars). This is for
the purpose of blinking two images and trying to
identify variable stars.
Results All four variable stars were analyzed
using the aforementioned methodology and the
photometric light curves seen above were created.
The important piece of data that can be
extracted from the photometric light curve is the
periodicity of the variable star. The
periodicity of any variable star describes the
time it takes to change from least magnitude to
greatest magnitude and back to least magnitude
again. Using the data from the sky region along
the ecliptic of Right Ascension 19hrs 30 min, the
measured periodicity of all four variable stars
are listed in the table below. These numbers were
determined by estimating the number of days
between several different peaks and taking the
average. The estimated periodicity values
compare favorably to the published periodicity
values as shown in the table. The published
values were found in both the ASAS Catalog of
Variable Stars, Version 2005-09-05 and the
Combined General Catalog of Variable Stars Vol.
I-III. The percent error between the published
values and the estimated values can also been
seen below. All error values are judged to be in
the acceptable range indicating that the Star
Dial Observatory data is reliable for determining
the pulsation period for newly identified LPV
stars.
Figure 1. Photometric light curves for variable
stars V0820 Aql, V1414 Aql, BZ Aql and V1541 Aql
in RA 1930.
Methods
Observations of variable stars have been
facilitated by the use of an automated telescope,
called Star Dial that takes nightly images of the
sky. The subsequent CCD images are then analyzed
with accompanying computer software to obtain
precise and accurate magnitude measurements over
time. The first step is to collect these images
from Star Dial. Next, two images of the sky from
two different nights need to be aligned and
compared to determine if any stars are changing
in brightness. This method is known as blinking
and is done using a program called Astrometrica.
Using this method, it is possible to see changes
in magnitude of several stars. After these stars
have been characterized by their Right Ascension
and Declination, an Internet database is used to
further research these stars for possible
variable characteristics.
After several stars have been identified as
possible variables using the information gathered
from these databases, it is necessary to measure
the changing brightness of a star for as many
nights as possible. This methodology is known as
aperture photometry. During this time, comparison
stars, whose brightness does not vary, are also
identified. Comparison stars are stars of known
magnitude that are used to compare the magnitude
of the variable stars and calibrate the images.
We then use this comparison to determine the
relationship between brightness and time by
creating a photometric light curve. Finally, it
is essential to contribute the light curve
results to the larger international database of
variable star observations at the American
Association of Variable Star Observers.
This work was funded in part by the UA LAPLACE
Astrobiology Center.