Multi-Epoch BVRI Photometry of Luminous Stars in M31 & M33
OverviewIn 2012 we began surveying the two nearby galaxies M31 and M33 to keep tabs on the most luminous stars in those galaxies. The evolutionary state of these stars is clearly post hydrogen fusing main sequence. They may become supernovae or supernovae imposters sometime in the near future. But otherwise their path from main-sequence to terminal state (death) is unclear. This survey endeavors to develop a baseline of data complimentary to other surveys that can be used to disentangle the evolutionary state and history of the most luminous stars.
It would be difficult to do a similar survey to any level of completeness in our own galaxy. Nearby face-on spiral galaxies, M31 and M33 are oriented in a way where we can study most of the galaxy unobscured and use what we learn to build a holistic picture of a galactic population of luminous stars. This can in turn help astronomers better interpret the stellar population in the Milky Way.
Our targets fall into four general categories:
Massive stars that are candidates to become supernovae and/or supernova impostors.
Stars with strong IR excesses
Classical LBVs designated by other surveys.
Cadence and Fields Imaged
Each year between August and February we have used the 20-inch telescope to take images of a number of overlapping fields in each galaxy. This is a targeted survey so the fields have been positioned to cover the most targets in the fewest fields.
We are using standard Johnson-Cousins high-throughput filters manufactured by AstroDon. Every epoch is imaged fully in V and either B or R. Later epochs are imaged in B, V, R, and Ic. Photometric comparison stars are selected from APASS. K corrections and standard photometric color transforms are determined for the instrumental system through imaging of M67.
Right now John Martin is using a custom DAOphot pipeline to calibrate measure the changing brightness of designated targets across epochs. Our goal is 5-10% photometry down to 20th magnitude (some what relaxed from other more comprehensive surveys of M31 and M33). We are using PSF fitting photometry because crowding (particularly in M33) makes it difficult to do uncontaminated aperture photometry for the fainter targets. We have performed tests on our pipeline to show that we are able to perform PSF fitting photometry that is consistent with aperture photometry down to the 5-10% level.
The photometric solutions are computed for each image using a list of comparison stars selected from APASS DR10. The details of how those stars were selected is outlined in Martin (2020). For more information about the comparison and check stars used follow this link.
Last Updated 2020 May 21