The Variables of ESSENCE: a Search for Variable Stars in the ESSENCE Database Rachel Anderson1

1
The Variables of ESSENCE a Search for Variable
Stars in the ESSENCE DatabaseRachel Anderson1
Stella Kafka2 Armin Rest3 1McMaster
University, Hamilton ON, participant at the 2007
REU program of CTIO, 2SSC/Caltech, 3CTIO/AURA
We report the results of a variable star search
in the fields observed by the ESSENCE
collaboration. This data was obtained with the
MOSAIC imager on the 4m CTIO telescope, covering
a 10 10 region looking outside the galactic
plane. With data spanning 5 years, we provide
information for long-term as well as short-term
(lt 1day) variability in a number of sources.
Among the 1119 variable candidates, 52 had been
classified as type Ia Sne by the ESSENCE
collaboration. From the remaining candidates, our
preliminary classification includes 154 long
period variables, 61 outbursting systems, and 12
short period variables. Many of the remaining
candidates are possible Active Galactic Nuclei.
In this presentation we also discus the methods
the used to discover our candidates, as well as
our plans for future study of the sources.
Results
Goals and Motivation
Classification of Variable Sources

• Our goal is to detect variable stars in the
  ESSENCE survey and classify these stars in an
  online catalog in order to learn of the origin of
  their variations. Through this, we hope to aid in
  the understanding of fundamental questions such
  as
• Is the density of intergalactic variable stars
  different from the density inside the galaxy?
• Are there the same phases as in intragalactic
  star evolution?
• How do variable stars come to exist in
  intergalactic space?
• Finally, with the arrival of the Large Synoptic
  Survey Telescope (LSST), our research hopes to
  provide the groundwork for an intergalactic
  variable star search by not only providing
  possible candidates for future study, but
  allowing for some predictions to be made with the
  results.

To help separate intergalactic sources from
galactic, we used color-color plots and
color-magnitude diagrams which were added to the
web pages. We had V filter images for 9 of the
32 fields. The plots for stars WVAR-0019 and
WVAR-0207 are shown in figure 2.
Preliminary Classification Quantity
Active Galactic Nuclei (AGN) 814
Long Period Variables 154
Outbursts 61
Supernovae 52
Short Period Variables 12
Unknown 26
Total Variable Stars 1119
Table 1 The distribution of our preliminary
classification of variable star candidates. We
continue to include AGNs as we have yet to
confirm this classification. Many of the stars
classified as long period variables could also be
AGNs. Those classified as outbursts could be
supernovae or cataclysmic variables. The 52
supernovae are all supernovae that have been
confirmed by the ESSENCE collaboration. The
short period variables have all been fitted to a
period and folded into a phase diagram.
Variable Stars and the Equation of State Super
Novae Trace Cosmic Expansion (ESSENCE) Survey
? R filter ? I filter
ESSENCE an NOAO survey program which looked for
supernovae type Ia outside the galactic plane
with the goal to confirm or reject the existence
of the cosmological constant by measuring the
history of cosmic expansion over the past 5
billion years.  Our research aims to compliment
this survey by the addition of the intergalactic
variable star population in the same 10??10?
field (Wood-Vasey et al. 2006).
Figure 2 Clockwise from top, color-color
diagram, and two CMD diagrams for V-R and R-I
colors for variable source 0019. This source was
determined to be a long period variable star.
Stars closest to the detections are marked by
green triangles.
The ESSENCE survey had already detected the type
Ia SN in the field 4, so we were able to
immediately classify 52 of our candidates as
such. Next we undertook the classification of
the periodic variables, using two programs called
Periodogram and LC Phase. Periodogram also uses
Fourier analysis to compute a period for the
variable star. The main page of Periodogram is
shown below in figure 3.
Data and Data Reduction
The ESSENCE data is from the CTIO Blanco 4 m
prime-focus MOSAIC II 64 Megapixel CCD camera. We
had a total of 32 fields with 16 amplifiers all
at declination 0 to -10. The data was taken
every other night of dark and gray time in both
the R and I filter (with some data in the V
filter) in the months of October, November, and
December from 2002-2007, with a revisit cadence
of every 4 nights (Miknaitis et al. 2007). This
data covered about 10 square degrees, and an
estimated 41,000 stars.
Figure 5 Light curves (left) and phase diagrams
(right) of two short period variable stars,
WVAR-0207 and WVAR-0366 with periods of 0.1138599
days and 2.0742142 days respectfully. The R
filter is shown in red and the I filter in blue.
Finding Variable Star Candidates
? R filter ? I filter
As a first step, we used difference imaging to
detect variable sources. We inspected each
source visually and rejected those that were
falsely determined to be a variable source due to
a cosmic ray, bad pixel, or bad difference
imaging. An internet page 3 was created for
the resulting 1119 variable star candidates
detected in each field and amplifier. An example
is shown in figure 1.
Figure 3 Main page from Periodogram showing the
light curve (top) and periodogram (bottom) of
variable source WVAR-0075 in the I filter. This
page gives a suggested period of 0.1111605 days.
We then use LC Phase and the calculated period
from Periodogram to fold the light curve into a
phase diagram (figure 4). If the phase diagram
shows periodic variations, then we will classify
it as either a long period variable (gt 365 days)
or a short period variable. Otherwise these
candidates will be classified visually along with
the others already determined to be non-periodic,
as outbursts or other'' for later examination.
Figure 6 Light curves for two long period
variables (WVAR-0541 and WVAR-0703, top) and two
outbursts (WVAR-1440 and WVAR-1066, bottom).
Conclusion
Using the ESSENCE survey data, we created a list
of variable stars in a 10??10? region outside
the galactic plane. Out of an estimated 41,000
stars, we found 1,119 variable sources. By
period folding and color-color plots, we did a
preliminary classification of the sources.
Future plans include acquiring spectroscopy for
the outbursts, and the completion of the on-line
variable star catalog.
Acknowledgments
I would like to thank Stella Kafka and Armin Rest
for their guidance, support, and encouragement.
This work was supported by the National Science
Foundation under grant no. 0353843.  Any
opinions, findings and conclusions or
recommendations expressed in this material are
those of the authors and do not necessarily
reflect the views of the National Science
Foundation.
References 1 Miknaitis, G., et al. 2007. The
ESSENCE Supernova Survey Survey Optimization,
Observations, and Supernova Photometry. 2
Wood-Vasey, W. M., et al. 2006. Observational
Constraints on the Nature of Dark Energy First
Cosmological Results from the ESSENCE Supernova
Survey. 3 http//ctiokw.ctio.noao.edu/sm/wvar/2
006/alerts/group.html 4http//www.ctio.noao.edu/
essence/
Figure 1 An example of an internet page made for
the classification of each variable source in the
ESSENCE fields. Each page includes the
difference imaging (upper right corner), forced
photometry light curve for 2006 (top left),
photometry swarped light curve for all epochs,
and detection plots from the 2006. This specific
variable star candidate was determined to be a
short period variable star by methods described
later..
Figure 4 Period diagram from LC Phase. This is
of variable source WVAR-0075 folded to the period
calculated by Periodogram of 0.1111605 days.