Download Photometric Surveys and Variable stars

Photometric Surveys and
Variable stars
Matthew Templeton, AAVSO
Presented at the USNO Flagstaff Station
February 24, 2006
Variable Stars and Astrophysics
• Stellar physics
• Stellar (& galactic)
evolution
• Extreme objects
• Cosmic Distance Scale
Astrophysics from variable stars:
Supernovae and cosmology
Courtesy of High-Z Supernova Team:
http://cfa-www.harvard.edu/oir/Research/supernova/home.html
Astrophysics from variable stars:
confirmation of second-overtone
Cepheids
(MACHO: Alcock et al. 1995)
Astrophysics from variable
stars:
The pulsation mode of Mira
stars
(MACHO: Wood et al. 1999)
Modern surveys
• Extragalactic surveys: Cepheids, Hi-z supernovae
• Microlensing surveys: MACHO, OGLE, EROS,
PLANET
• Large-area: Hipparcos, NSVS, ASAS
(MACHO)
(SuperMACHO)
(ASAS)
The Future...
• Amateurs!
• Harvesting archival data: MACHO/OGLE, HCO Plate
Stacks (for example), LONEOS
• NVO
• Palomar-QUEST, Pan-STARRS
• Kepler, Gaia
• LSST
The large surveys are looking for more than variable
stars -- everything from extrasolar planets to weak
lensing. Variable stars are a stated aim for most, but
secondary. (High-z SN are the only “variables”
mentioned on the LSST front page!)
Small surveys (like ASAS) are geared towards variable
stars. They’re cheap! But we are bumping into the
magnitude limits of these small telescopes.
Wish-list for Variable Star Surveys
It is very hard to design a survey that will satisfy
everyone’s needs. Variable stars exhibit such a
huge variety of behavior that one size can’t fit all.
• Photometry: multicolor, high-S/N, large dynamic range
• long time baseline
• dense temporal sampling
• Multi-site
• Wide area? Or not?
Surveys, in practice
• Telescope time /
resources
• Dynamic range
• Data pipeline capacity
• Analysis capacity (&
time, & interest...)
• Funding...
(Sloan 2.5-m)
Astrophysics from
cluster surveys: variability
• Intrinsic variability fraction
• Short-period pulsators, asteroseismology
• Pulsation-rotation connection
• Magnetic activity versus [age,rotation]
• Binaries, binary fraction, binary evolution
• Planet searching
Astrophysics from
cluster surveys: photometry
• H-R diagrams, stellar evolution studies
• cluster membership, Φ(L,M)
• astrometry
How the NGC 2301 variables project started…
“So anyway, we have this
data...”
NGC 2301
• In the plane, but low extinction (~820 pc)
• MSTO around A0 (~180 Myr)
• Near-solar abundances
NGC 2301 provides a good test of young stellar
evolution & isochrones, and searches for variability
in a coeval group of upper main sequence stars.
NGC 2301 data
(Tonry et al. 2005)
• UH 2.2-meter
• Two week span
• B-R calibration, Rband time-series
• ~2 dozen points per
night
Analysis challenges
• Time-series analysis -- CPU time
• Aperiodic/quasiperiodic stars
• “Difficult” lightcurves (binaries, spots)
• Spurious signals, bad data detection?
• Classification
Variable star data analysis is not always
straightforward; large-scale, automated analysis
design is non-trivial.
Sampling aliases!
What kinds of variables?
• Close EBs (EB/EW/KW?)
• pulsators (dSct, gDor, aCyg?, bCep?)
• RR/delta Cep (background, probably)
• rotating? (RS CVn...)
Because of the two-week data span, the data are
naturally biased towards the shortest period stars.
Thus, the close EBs dominate. Other types are hard
to study and firmly classify with this amount of data.
Upper MS pulsators
• alpha Cygni
• beta Cephei
• SPBs
• delta Scuti
• gamma Doradus
The MSTO of NGC 2301 lies around A0, so delta Scuti
and gamma Doradus stars should be present. Perhaps
some cluster Cepheids, too?
Gamma Doradus stars
• sp. type mid-late F, lum. class V, Pop I
• P: 8h - 3d (close to P(rot)!)
• sometimes multiperiodic
• g-mode pulsators (n < 0)
The gamma Doradus class is a new designation
(early 1990’s). Before they were discovered, they
were sometimes unknowingly used as comp
stars!
Interesting light curves
Blend
?
Wrong period?
Blend?
A background beta Cep?
• The star is a few
magnitudes fainter than the
MSTO
• (B-R) ~ +1.5
• But...
• The period is just right
• The field is in the Galactic
(Sterken & Jaschek 1996)
plane
• The light curve is beta Cep
Several other “interlopers”
are suspected in this cluster,
including background RR
Lyrae and Cepheid variables.
If this is a beta Cep star, it
suggests that classifications
based upon presumed HR
diagram positions must be
done with care.
(That
includes binaries.)
Variability study: in progress
• Common-envelope binaries are nearly sorted out, wider
ones and interacting ones not
• Pulsators? Have done two multiperiodic tests (out of
4000...)
• Rotating & Magnetic stars? (probably some alpha CVn
stars at the bright end -- spectra would be neat!)
• Background stars
• Bad data & alias rejections
The photometry was a “dry-run” for upcoming surveys.
The data analysis is becoming a dry-run for LONEOS...
Building a variable catalog
With 4000 stars, 3200 of which show some
hint of variability, the analysis, classification,
and study of this sample is proving a nontrivial task.
• “quick and dirty” time series
• sort by color, mag, period, amplitude
• visual inspection? computer methods?
This is ONLY 4000 stars -- what do you do with millions?
Learning from the past
Large-scale time-series databases like Hipparcos,
MACHO, OGLE, and ASAS provide reasonable
examples for how to deal with massive databases:
process them quickly, build a database, and either...
• Work on classes/clusters individually, and/or
• Release the data and let the community do the work
MACHO kept their data proprietary for a long time,
but still got lots of results. The others went public
sooner -- great data, but “less press”?
A work in progress...
• At the end, a detailed variable star census of NGC 2301
will provide us with some interesting astrophysics (net
variability fraction, variability versus [X])
• Asteroseismology? (At least we can find new
candidates for followup photometry)
• A good, small-scale project for building large-scale
analysis pipelines -- find what works, what doesn’t
• Archival data wants to be free! (And disks are only
getting cheaper!)
Matthew Templeton, AAVSO
http://www.aavso.org