Photometric Surveys and Variable stars

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Photometric Surveys and Variable stars

• Matthew Templeton, AAVSO
• Presented at the USNO Flagstaff Station
• February 24, 2006

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Variable Stars and Astrophysics

• Stellar physics
• Stellar ( galactic) evolution
• Extreme objects
• Cosmic Distance Scale

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Astrophysics from variable stars Supernovae and
cosmology
Courtesy of High-Z Supernova Team http//cfa-www.
harvard.edu/oir/Research/supernova/home.html
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Astrophysics from variable stars confirmation
of second-overtone Cepheids
(MACHO Alcock et al. 1995)
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Astrophysics from variable stars The pulsation
mode of Mira stars
(MACHO Wood et al. 1999)
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Modern surveys

• Extragalactic surveys Cepheids, Hi-z supernovae
• Microlensing surveys MACHO, OGLE, EROS, PLANET
• Large-area Hipparcos, NSVS, ASAS

(ASAS)
(MACHO)
(SuperMACHO)
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The Future...

• Amateurs!
• Harvesting archival data MACHO/OGLE, HCO Plate
  Stacks (for example), LONEOS
• NVO
• Palomar-QUEST, Pan-STARRS
• Kepler, Gaia
• LSST

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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. Theyre cheap! But we are
bumping into the magnitude limits of these small
telescopes.
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Wish-list for Variable Star Surveys
It is very hard to design a survey that will
satisfy everyones needs. Variable stars exhibit
such a huge variety of behavior that one size
cant fit all.

• Photometry multicolor, high-S/N, large dynamic
  range
• long time baseline
• dense temporal sampling
• Multi-site
• Wide area? Or not?

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Surveys, in practice

• Telescope time / resources
• Dynamic range
• Data pipeline capacity
• Analysis capacity ( time, interest...)
• Funding...

(Sloan 2.5-m)
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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

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Astrophysics fromcluster surveys photometry

• H-R diagrams, stellar evolution studies
• cluster membership, F(L,M)
• astrometry

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How the NGC 2301 variables project started
So anyway, we have this data...
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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.
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NGC 2301 data(Tonry et al. 2005)

• UH 2.2-meter
• Two week span
• B-R calibration, R-band time-series
• 2 dozen points per night

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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.
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Sampling aliases!
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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.
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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?
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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 lt 0)

The gamma Doradus class is a new designation
(early 1990s). Before they were discovered,
they were sometimes unknowingly used as comp
stars!
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Interesting light curves
Blend?
Wrong period?
Blend?
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A background beta Cep?
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• 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 plane
• The light curve is beta Cep

(Sterken Jaschek 1996)
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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.)
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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...
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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 non-trivial
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?
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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?
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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 doesnt
• Archival data wants to be free! (And disks are
  only getting cheaper!)

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Matthew Templeton, AAVSO http//www.aavso.org