photometric surveys and variable stars

Photometric Surveys and Variable stars

Matthew Templeton, AAVSOPresented 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)

(MACHO: Wood et al. 1999)

Astrophysics from variable stars:

The pulsation mode of Mira stars

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

•Photometry: multicolor, high-S/N, large dynamic range

•long time baseline

•dense temporal sampling

•Multi-site

•Wide area? Or not?

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.

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 fromcluster surveys: photometry

•H-R diagrams, stellar evolution studies

•cluster membership, Φ(L,M)

•astrometry

“So anyway, we have this data...”

How the NGC 2301 variables project started…

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, R-band 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?

Blend?

Wrong period?

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 plane

•The light curve is beta Cep

(Sterken & Jaschek 1996)

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 rejectionsThe photometry was a “dry-run” for upcoming

surveys.

The data analysis is becoming a dry-run for LONEOS...

Building a variable catalog

•“quick and dirty” time series

•sort by color, mag, period, amplitude

•visual inspection? computer methods?

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.

This is ONLY 4000 stars -- what do you do with millions?

Learning from the pastLarge-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, AAVSOhttp://www.aavso.org