Title: "Globular Cluster Variable Stars - Atlas and Coordinate Improvement"
Proposers: Doug Welch and Arne Henden
The variable stars of globular clusters have played and continue to
play a significant role in our understanding of certain classes of
variable stars. Since all stars associated with a cluster have the same
age, metallicity, distance and usually very similar (if not identical
reddenings), such variables can produce uniquely powerful constraints
on where certain types of pulsation behaviors are excited.
While 158 globular clusters are known to be associated with the Milky Way,
only 103 currently have been surveyed for variable stars (Samus et al, 2009;
Harris 1996; Clement 2013). This proposal is restricted to producing
an atlas for those 103 clusters.
Advanced amateur astronomers are increasingly well-positioned to provide
long-term CCD monitoring of globular cluster variable star but are
hampered by a long history of poor or inaccessible finder charts
and coordinates. Many of variable-rich clusters have published
photographic finder charts taken in relatively poor seeing with
blue-sensitive photographic plates. While useful signal-to-noise
ratios are relatively straightforward to achieve for RR Lyrae, Type 2
Cepheids, and red giant variables, correct identification remains
a difficult issue - particularly when images are taken at V or
We propose to remedy this situation by producing an atlas of all
globular clusters with known variable stars in V and I with exposures
tailored to the apparent magnitudes of the horizontal branch of
the clusters. Furthermore, by visiting each globular cluster field
five times, we can independently verify variability with AAVSOnet
Samus et al (2009) provided greatly improved positions for globulars
- significantly reducing the probability of mis-identification -
there are no corresponding finder charts available. One would imagine
that positions for the relatively bright Type 2 Cepheids would at
least be well-determined, but the attached images for M19 shows that
this is not the case for V4.
The field of view of the imagers on TMO61 and OC61 are well-matched
to this project. Large-format CCDs are greatly superior to mosaics
of smaller chips (with their numerous chip gaps) for this purpose.
We also note that many of the lower-RA globulars are in a position
in the sky where RA-pressure on AAVSOnet telescopes is low.
While most globular clusters can be imaged with a single pointing,
several will require additional non-central pointings to
completely map the cluster and its variable star population. In
the north, the 30x40 arcmin field of SRO can be used to survey the
entire cluster simultaneously, while leaving the most crowded
central region for the superior image scale of TMO61.
NGC 5904 (M5)
NGC 6121 (M4)
NGC 6205 (M13)
NGC 6218 (M12)
NGC 6254 (M10)
NGC 6333 (M9)
NGC 6402 (M14)
NGC 7078 (M15)
NGC 7089 (M2)
3x3 (overlap of 5% of field edges requested)
NGC 104 (47 Tuc)
NGC 5139 (omega Cen)
2x2 plus central (overlap of 5% of field edges requested)
NGC 6273 (M19)
NGC 6809 (M55)
Proposed telescope: TMO61 and OC61 (and SRO)
Cadence: 5 epochs in (V or SG) and (I or SI) each
exposed to obtain a SNR of 10 at 0.5 mag fainter than
the horizontal branch.
Reduction and Analysis: One of us (DLW) will be employing
a summer student whose sole responsibility will be the
processing and production of the atlas and coordinate
improvement as observations of clusters become complete.
While the project won't be complete by Sept 2014, the
process of producing the final product of images and
coordinates and making those publicly available should
be very refined after the first dozen clusters are
"Catalogue of Variable Stars in Galactic Globular Clusters"
Clement, C. 2013
"Catalogue of Parameters for Milky Way Globular Clusters: The Database"
Harris, W.E. 1996, AJ, 112, 1487
"A Catalog of Accurate Equatorial Coordinates for Variable Stars in Globular Clusters"
Samus, N.N. et al 2009, PASP, 121, 1378
Current major use of AAVSOnet resources:
- One of us (DLW) has an on-going program with Ken Mogul to acquire photometry on three dozen
field Type 2 Cepheids to assess what evidence might exist for binarity in their lightcurves
(combined with +/-2 km/s radial velocities of the brighter objects from the DAO 1.2m telescope).
Originally on K28, the continuity of this program was interrupted by the decision of T. Kracji
to suspend data-taking and then many-month gaps between new data from W30 and then its successor
Coker30. The program is still underway, but will require at least one more year of data acquisition
on Coker30 to complete the original science goals. Since the data were taken in Sloan griz, calibration
and reduction is most efficient once APASS DR8 is available within VPHOT. Sloan standards
are available in VPHOT at present.
- One of us (DLW) has just been allocated AAVSOnet time to study the variable stars in M56.